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Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals across a chemical synapse, from a neuron (or from a neuron to a muscle or gland) to a target cell, enabling communication throughout the nervous system.^[1] They are synthesized within neurons and stored in synaptic vesicles at the presynaptic terminal, where they are released in response to an action potential via calcium-dependent exocytosis.^[1] Upon release into the synaptic cleft, neurotransmitters diffuse across the narrow gap and bind to specific receptors on the postsynaptic membrane, either directly altering ion channel permeability (ionotropic receptors) or triggering intracellular signaling cascades (metabotropic receptors), thereby modulating the target cell's electrical activity or biochemical state.^[2] This process underlies essential neural functions, including excitation, inhibition, and modulation of synaptic transmission.^[3] The major classes of neurotransmitters are broadly categorized by chemical structure and function, with amino acids, monoamines, peptides, and others playing distinct roles in brain activity.^[1] Excitatory neurotransmitters, such as glutamate—the principal excitatory transmitter in the central nervous system—promote neuronal depolarization and are crucial for synaptic plasticity, learning, and memory formation.^[1] In contrast, inhibitory neurotransmitters like gamma-aminobutyric acid (GABA), which accounts for about 40% of inhibitory neurotransmission in the brain, and glycine, the primary inhibitory transmitter in the spinal cord, hyperpolarize postsynaptic neurons to dampen activity and maintain neural balance.^[1] Modulatory neurotransmitters, including monoamines such as dopamine (involved in reward, motivation, and motor control), norepinephrine (regulating arousal, attention, and stress responses), and serotonin (influencing mood, sleep, and appetite), often act via G-protein-coupled receptors to fine-tune broader neural circuits.^[1] Acetylcholine serves diverse roles, from facilitating muscle contraction at neuromuscular junctions to modulating cognition and autonomic functions.^[2] Neurotransmitters are essential for coordinating complex processes like sensory processing, motor control, emotional regulation, and cognitive functions, with their dysregulation implicated in various neurological conditions.^[3] After release, they are rapidly cleared from the synaptic cleft through reuptake into the presynaptic neuron, enzymatic degradation, or diffusion, ensuring precise temporal control of signaling.^[2] More than 100 neurotransmitters have been identified, though the core ones—glutamate, GABA, dopamine, serotonin, norepinephrine, and acetylcholine—dominate synaptic communication in the mammalian brain.^[1] Their discovery and study, beginning with acetylcholine in the early 20th century, have revolutionized neuroscience, highlighting their role in both normal physiology and therapeutic targets for disorders like Parkinson's disease and depression.^[2]

Definition and Fundamentals

Chemical Composition

Neurotransmitters are endogenous chemicals that transmit signals across a chemical synapse, from a presynaptic neuron to a postsynaptic neuron or target cell such as a muscle or gland cell.^[2] These signaling molecules enable communication in the nervous system by being released into the synaptic cleft and binding to specific receptors on the target cell.^[1] Neurotransmitters are broadly classified into several chemical categories based on their molecular structures and properties, including small-molecule neurotransmitters (such as amino acids, monoamines, acetylcholine, and purines), neuropeptides, and gaseous molecules like nitric oxide.^[4] Small-molecule neurotransmitters typically have low molecular weights (around 100–200 Da) and include amino acids like glutamate (C₅H₉NO₄), the principal excitatory neurotransmitter, and gamma-aminobutyric acid (GABA, C₄H₉NO₂), a major inhibitory neurotransmitter derived from glutamate.^[4] Monoamines, another subgroup of small molecules, encompass catecholamines such as dopamine (C₈H₁₁NO₂) and norepinephrine (C₈H₁₁NO₃), which feature a characteristic benzene ring with hydroxyl groups derived from tyrosine, and indolamines like serotonin (C₁₀H₁₂N₂O), derived from tryptophan.^[4] Neuropeptides are larger, consisting of short chains of 3 to 36 amino acids, such as substance P or the endorphins, with molecular weights often exceeding 3000 Da.^[4] Gaseous neurotransmitters, exemplified by nitric oxide (NO, molecular weight 30 Da), are unconventional due to their instability and lack of vesicular storage.^[4] The physicochemical properties of neurotransmitters, including solubility and lipophilicity, critically influence their diffusion across the synaptic cleft and interaction with receptors.^[5] Most small-molecule neurotransmitters, such as amino acids and monoamines, are polar and water-soluble, enabling their dissolution in the aqueous environment of the synapse and storage in synaptic vesicles.^[6] Their moderate lipophilicity, arising from aromatic rings in monoamines, facilitates binding to lipid-embedded receptors while preventing excessive membrane penetration.^[5] In contrast, gaseous neurotransmitters like nitric oxide are highly lipophilic, allowing rapid diffusion through cell membranes without reliance on transporters or vesicles.^[4] Neuropeptides, being peptides, exhibit amphiphilic properties with hydrophilic backbones and variable side-chain interactions that affect their solubility and receptor affinity.^[4] For classical neurotransmitters, a substance must meet specific criteria related to its localization and functional role in synaptic transmission.^[2] It must be synthesized or actively taken up by the presynaptic neuron and stored in synaptic vesicles.^[2] Upon presynaptic depolarization, it is released in a calcium-dependent manner into the synaptic cleft.^[2] When applied exogenously to the postsynaptic cell, it must mimic the natural response by binding to specific receptors and altering the target's electrical or biochemical properties.^[2] Finally, mechanisms must exist to inactivate or remove the substance from the synapse after signaling.^[2] Unconventional neurotransmitters, such as gaseous molecules, fulfill analogous roles through modified mechanisms, including on-demand synthesis and diffusion without vesicular storage.^[4]

Functions in Neural Signaling

Neurotransmitters serve as chemical messengers that bridge the gap between presynaptic neurons and postsynaptic cells at chemical synapses, enabling the transmission of signals across the synaptic cleft through their release and binding to receptors on the target cell. This process is triggered by action potentials arriving at the presynaptic terminal, leading to calcium influx and vesicular exocytosis of neurotransmitters, which then diffuse to activate postsynaptic responses.^[1] In this way, neurotransmitters facilitate the propagation of electrical signals in the form of action potentials from one neuron to another or to effector cells like muscle or gland cells.^[1] The effects of neurotransmitters on postsynaptic cells can be classified as excitatory, inhibitory, or modulatory, depending on the type of receptor activated and the resulting change in membrane potential. Excitatory neurotransmitters, such as glutamate, typically open ion channels that allow influx of cations like sodium, causing depolarization and increasing the likelihood of action potential generation in the postsynaptic neuron.^[1] Inhibitory neurotransmitters, including gamma-aminobutyric acid (GABA) and glycine, promote hyperpolarization by facilitating chloride influx or potassium efflux, thereby reducing neuronal excitability and preventing excessive firing.^[1] Modulatory neurotransmitters, often monoamines like dopamine or serotonin, exert longer-lasting influences by altering the excitability of neural circuits through second-messenger systems, contributing to processes such as learning and emotional regulation without directly driving rapid synaptic transmission.^[1] Beyond immediate synaptic transmission, neurotransmitters play crucial roles in synaptic integration, where multiple inputs are summed spatially and temporally to determine whether a postsynaptic neuron fires an action potential, and in network oscillations that synchronize activity across brain regions for coordinated functions like attention and memory.^[1] For instance, GABAergic inhibition helps generate rhythmic oscillations in cortical networks, while cholinergic modulation can enhance theta rhythms in the hippocampus.^[7] These mechanisms ensure efficient information processing and maintain the balance of excitation and inhibition in neural circuits.^[1] In addition to synaptic actions, neurotransmitters participate in non-synaptic transmission, such as volume transmission, where they diffuse over larger distances to influence multiple target cells via paracrine signaling, particularly for neuromodulators like monoamines and neuropeptides.^[8] This mode of signaling, distinct from the precise, point-to-point synaptic release, allows for broader regulation of neural activity and is mediated by extrasynaptic receptors, enabling slower, more diffuse effects on circuit dynamics.^[8] The fundamental roles of neurotransmitters in neural signaling exhibit remarkable evolutionary conservation, appearing in simple invertebrates such as nematodes, where acetylcholine functions as an excitatory transmitter at neuromuscular junctions, and extending to complex mammalian systems with conserved pathways for glutamate, GABA, and biogenic amines across bilaterians.^[9] This conservation underscores their ancient origins in secretory signaling predating synaptic structures, adapting over time to support diverse neural functions from basic locomotion to higher cognition.^[9]

Biosynthetic and Metabolic Processes

Synthesis Mechanisms

Neurotransmitters are primarily synthesized from amino acid precursors in the cytoplasm of presynaptic neurons or at their terminals, where specific enzymes catalyze the conversion processes. These syntheses occur in specialized neuron types, such as dopaminergic neurons for catecholamines or serotonergic neurons for serotonin, ensuring targeted production for synaptic transmission. The enzymes involved are typically transported from the cell body to the terminals via axonal transport, allowing for on-demand synthesis close to release sites.^[10] For catecholamines like dopamine and norepinephrine, synthesis begins with the amino acid tyrosine, which is hydroxylated to L-3,4-dihydroxyphenylalanine (L-DOPA) by the enzyme tyrosine hydroxylase (TH), followed by decarboxylation of L-DOPA to dopamine by aromatic L-amino acid decarboxylase (AADC, also known as DOPA decarboxylase). Dopamine is then further converted to norepinephrine by dopamine β-hydroxylase in noradrenergic neurons. This pathway is localized in catecholaminergic neurons, such as those in the substantia nigra for dopamine.^[10]^[11] Serotonin (5-hydroxytryptamine) is synthesized from the essential amino acid tryptophan, first hydroxylated to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase (TPH), and then decarboxylated to serotonin by AADC. TPH exists in two isoforms, TPH1 and TPH2, with TPH2 being the predominant form in the brain's serotonergic neurons of the raphe nuclei.^[10]^[12] The inhibitory neurotransmitter γ-aminobutyric acid (GABA) is produced from the excitatory amino acid glutamate through decarboxylation by glutamic acid decarboxylase (GAD), which has two isoforms: GAD65 and GAD67. This synthesis is confined to GABAergic neurons throughout the central nervous system, with GAD67 primarily responsible for basal GABA production in the cytoplasm.^[10]^[13] Acetylcholine synthesis involves the esterification of choline with acetyl-coenzyme A (acetyl-CoA) by the enzyme choline acetyltransferase (ChAT), occurring in cholinergic neurons such as those in the basal forebrain and motor neurons. Acetyl-CoA is generated from glucose metabolism and requires ATP for its formation via pyruvate dehydrogenase in mitochondria.^[10]^[14] Synthesis rates are tightly regulated to match neuronal activity and demand. Tyrosine hydroxylase serves as the rate-limiting enzyme in catecholamine production, subject to feedback inhibition by end-product catecholamines like dopamine, which bind to its regulatory domain to reduce activity. Similarly, TPH is rate-limiting for serotonin synthesis and experiences feedback inhibition by serotonin. GAD activity for GABA is regulated post-translationally, including through phosphorylation, while ChAT levels are controlled transcriptionally in response to neuronal signaling. These processes require specific cofactors, such as tetrahydrobiopterin (BH4) for TH and TPH in monoamine synthesis, which acts as a hydrogen donor and is essential for enzymatic function. Localization to particular neuron types ensures specificity, with deficiencies in these pathways linked to neurological disorders. Following synthesis, neurotransmitters are briefly stored in vesicles prior to release.^[10]^[11]^[12]^[13]

Storage and Release Dynamics

Neurotransmitters are stored in synaptic vesicles within the presynaptic terminal, where they are concentrated against their electrochemical gradients by specific vesicular transporters powered by a proton electrochemical gradient (ΔμH⁺) generated by vacuolar H⁺-ATPases. For monoamines such as dopamine, norepinephrine, and serotonin, the vesicular monoamine transporters (VMAT1 and VMAT2) facilitate uptake primarily driven by the pH gradient (ΔpH), exchanging two protons for one monoamine molecule, achieving concentrations up to 100,000-fold higher than in the cytoplasm.^[15] Glutamate, the primary excitatory neurotransmitter, is loaded into vesicles via vesicular glutamate transporters (VGLUT1-3), which rely mainly on the membrane potential component (Δψ) of the gradient, with chloride ions modulating activity to optimize filling.^[15] Inhibitory neurotransmitters like GABA and glycine are transported by the vesicular GABA transporter (VGAT), utilizing both ΔpH and Δψ, often with chloride cotransport, ensuring efficient packaging for release.^[15] Synaptic vesicles exist in distinct types tailored to their cargo. Small synaptic vesicles (40-60 nm in diameter), which appear clear in electron micrographs, primarily store classical small-molecule neurotransmitters like glutamate, GABA, and monoamines, enabling rapid, high-frequency release at active zones.^[16] In contrast, large dense-core vesicles (90-250 nm), identifiable by their electron-dense cores, package neuropeptides such as substance P or enkephalins, often alongside small molecules in some neurons, and support slower, activity-dependent release from extrasynaptic sites.^[16] These differences in size and composition influence release kinetics, with small vesicles recycling quickly via clathrin-mediated endocytosis to sustain ongoing signaling.^[16] Upon arrival of an action potential, neurotransmitter release is triggered by calcium influx through voltage-gated calcium channels (e.g., P/Q- or N-type) clustered at the active zone, raising local Ca²⁺ concentrations to micromolar levels within microseconds.^[17] This Ca²⁺ binds to synaptotagmin-1, the primary Ca²⁺ sensor, inducing a conformational change that promotes assembly of the SNARE complex—comprising syntaxin-1, SNAP-25 on the plasma membrane, and synaptobrevin-2 (VAMP2) on the vesicle—driving rapid exocytosis through membrane fusion.^[17] Release occurs in quanta, with each vesicle representing one quantum; spontaneous miniature end-plate potentials (MEPPs) reflect single-vesicle fusion events, while evoked release involves multiple quanta whose number follows Poisson statistics.^[18] The probability of release is modulated by readily releasable, recycling, and reserve vesicle pools, with Ca²⁺ enhancing recruitment from these pools to fine-tune synaptic strength.^[18] Co-release of multiple neurotransmitters from the same vesicle adds complexity to signaling, particularly in neurons packaging a fast-acting small molecule with a modulatory peptide. For instance, central excitatory neurons often co-store glutamate and neuropeptides like cholecystokinin in small synaptic vesicles or dense-core vesicles, allowing simultaneous release to elicit both rapid ionotropic and prolonged metabotropic effects.^[19] This co-packaging is facilitated by compatible vesicular transporters and gradients, enabling diverse postsynaptic responses without requiring separate vesicle populations.^[19]

Inactivation and Clearance

After neurotransmitter release into the synaptic cleft, rapid inactivation and clearance are essential to terminate signaling, prevent receptor overstimulation, and maintain synaptic homeostasis.^[20] Enzymatic degradation represents a primary mechanism for terminating the action of several neurotransmitters. For acetylcholine (ACh), acetylcholinesterase (AChE) hydrolyzes it into choline and acetate within milliseconds in the synaptic cleft.^[21] Monoamine neurotransmitters, such as dopamine and serotonin, undergo oxidative deamination by monoamine oxidase (MAO), primarily within presynaptic terminals or glial cells, converting them into inactive metabolites.^[22] Reuptake via specific transporter proteins provides another key clearance pathway, allowing neurotransmitters to be recycled back into the presynaptic neuron. The dopamine transporter (DAT) facilitates rapid uptake of dopamine from the synaptic cleft, while the serotonin transporter (SERT) performs a similar function for serotonin.^[23]^[24] These sodium-dependent transporters operate on timescales of milliseconds to seconds, depending on concentration gradients and transporter density. For gaseous neurotransmitters like nitric oxide (NO), clearance occurs primarily through passive diffusion away from the synapse due to its lipophilic nature and lack of specific transporters or enzymes for rapid degradation.^[25] NO diffuses freely across cell membranes, with its signaling terminated by reactions with superoxide or hemoglobin, leading to a half-life of seconds in biological tissues.^[25] Glial cells, particularly astrocytes, play a crucial role in clearing excitatory amino acids like glutamate to prevent excitotoxicity. Astrocytes express excitatory amino acid transporters (EAATs), such as EAAT1 and EAAT2, which mediate sodium- and potassium-dependent uptake of glutamate from the synaptic cleft into glial cells for conversion to glutamine.^[26] This process helps regulate extracellular glutamate levels and supports neuronal glutamine supply for resynthesis.^[27] Recycling of cleared components enables efficient neurotransmitter replenishment. For instance, choline produced from AChE-mediated hydrolysis is reuptaken by the high-affinity choline transporter (CHT1) into presynaptic terminals, where it is reused in ACh synthesis via choline acetyltransferase.^[28] Similarly, reuptaken monoamines like dopamine are repackaged into vesicles by the vesicular monoamine transporter (VMAT2).^[20] The timescales of clearance vary by neurotransmitter type and mechanism. Small-molecule neurotransmitters, such as ACh and glutamate, are typically cleared within milliseconds through enzymatic degradation or reuptake, ensuring precise temporal control of signaling.^[20] In contrast, neuropeptides rely mainly on diffusion and extracellular proteolysis, resulting in slower clearance over seconds to minutes, which contributes to their prolonged modulatory effects.^[29]

Molecular Mechanisms of Action

Receptor Interactions

Neurotransmitters exert their effects primarily through binding to specific receptors on the postsynaptic membrane or other target sites, initiating rapid or modulated responses in neural signaling. Receptors are broadly classified into two main types: ionotropic and metabotropic. Ionotropic receptors function as ligand-gated ion channels, allowing direct ion flow upon neurotransmitter binding, which leads to fast synaptic transmission. For instance, the N-methyl-D-aspartate (NMDA) receptor, activated by glutamate, permits influx of sodium and calcium ions, generating fast excitatory postsynaptic potentials (EPSPs) essential for synaptic excitation.^[30] In contrast, metabotropic receptors are G-protein-coupled receptors (GPCRs) that indirectly influence ion channels or cellular processes via intermediary signaling molecules, resulting in slower, modulatory effects. The D2 dopamine receptor exemplifies this class, coupling to Gi/o proteins to inhibit adenylyl cyclase and modulate neuronal excitability over longer timescales.^[31] Binding specificity between neurotransmitters and their receptors is governed by precise molecular interactions, characterized by affinity constants that quantify the strength of association. The dissociation constant (Kd) typically ranges from nanomolar to micromolar levels, reflecting high selectivity; for example, acetylcholine binds to nicotinic receptors with a Kd of approximately 160 μM.^[32] This specificity ensures that only cognate neurotransmitters or structurally similar ligands effectively bind, minimizing off-target effects in diverse neural circuits. Agonist binding often stabilizes the receptor in an active conformation, whereas antagonists lock it in an inactive state, directly influencing synaptic efficacy.^[33] Receptors are strategically localized to optimize signal transmission and regulation. Postsynaptic densities (PSDs) are specialized protein scaffolds in excitatory synapses where ionotropic receptors, such as AMPA and NMDA types for glutamate, cluster to enhance sensitivity to released neurotransmitters and facilitate rapid postsynaptic responses.^[34] Conversely, presynaptic autoreceptors, often metabotropic, reside on the presynaptic terminal and detect spillover neurotransmitter to provide negative feedback, thereby inhibiting further release and maintaining synaptic homeostasis; for example, presynaptic D2 autoreceptors on dopaminergic terminals reduce dopamine secretion in response to elevated extracellular levels.^[35] Allosteric modulation fine-tunes receptor function by binding at sites distinct from the orthosteric neurotransmitter site, altering binding affinity or efficacy. Positive allosteric modulators enhance the receptor's response to the endogenous ligand, while negative modulators diminish it. A prominent example is the action of benzodiazepines on GABA_A receptors, where they bind at an allosteric site on the γ2 subunit interface, increasing GABA affinity and potentiating chloride influx for enhanced inhibition.^[36] Such modulation allows for nuanced control of synaptic strength without directly competing with the neurotransmitter. From an evolutionary perspective, neurotransmitter receptor families exhibit remarkable conservation across species, reflecting ancient origins in early metazoans. Ionotropic glutamate receptors and GPCR families share core structural motifs preserved across diverse phyla, underscoring their fundamental role in neural function.

Signal Transduction Pathways

Upon binding to their receptors, neurotransmitters initiate intracellular signal transduction pathways that convert extracellular signals into cellular responses, primarily through two major classes: ionotropic and metabotropic receptors.^[37] Ionotropic receptors function as ligand-gated ion channels, enabling direct and rapid ion flux across the postsynaptic membrane to alter neuronal excitability.^[37] In ionotropic pathways, neurotransmitter binding directly opens the ion channel pore, allowing specific ions to flow and generate immediate postsynaptic potentials. For excitatory transmission, such as with glutamate binding to AMPA receptors, sodium (Na⁺) and potassium (K⁺) influx causes membrane depolarization, facilitating action potential initiation.^[38] In contrast, inhibitory neurotransmitters like GABA or glycine activate receptors that permit chloride (Cl⁻) influx, leading to hyperpolarization and reduced neuronal firing.^[37] These pathways operate on a millisecond timescale, enabling fast synaptic transmission essential for precise neural communication.^[38] Metabotropic pathways, mediated by G-protein-coupled receptors (GPCRs), involve indirect signaling through heterotrimeric G proteins that dissociate upon receptor activation to modulate intracellular effectors.^[37] Different G protein subtypes direct distinct cascades: Gs proteins stimulate adenylyl cyclase to increase cyclic AMP (cAMP) levels, activating protein kinase A (PKA); Gq proteins activate phospholipase C, producing inositol trisphosphate (IP₃) and diacylglycerol (DAG), which release intracellular calcium (Ca²⁺) and activate protein kinase C (PKC); and Gi proteins inhibit adenylyl cyclase or open potassium (K⁺) channels, promoting hyperpolarization.^[37] Examples include norepinephrine acting via β-adrenergic receptors (Gs-coupled) to enhance excitability or serotonin via 5-HT1 receptors (Gi-coupled) to suppress it.^[39] These slower processes, lasting seconds to minutes, allow for neuromodulation and broader cellular adjustments.^[37] Signal amplification in these pathways occurs through enzymatic cascades, particularly in metabotropic routes, where second messengers like cAMP activate kinases such as PKA, which phosphorylate ion channels, pumps, or transcription factors to propagate and intensify the signal.^[37] For instance, PKA can phosphorylate voltage-gated calcium channels to modulate neurotransmitter release probability.^[40] This multistep amplification enables a single receptor activation to elicit widespread effects within the neuron.^[38] Crosstalk between neurotransmitter pathways and other signaling systems, such as those involving neurotrophins like brain-derived neurotrophic factor (BDNF), integrates synaptic activity with trophic support, where neurotrophin receptors (e.g., TrkB) can enhance G-protein cascades to fine-tune neuronal survival and excitability.^[41] Overall, ionotropic pathways drive rapid, localized responses, while metabotropic ones provide sustained modulation, together enabling the nervous system's dynamic information processing.^[37]

Synaptic Plasticity Effects

Neurotransmitters play a central role in Hebbian plasticity, a form of synaptic strengthening or weakening based on correlated pre- and postsynaptic activity, often summarized by the principle "neurons that fire together wire together." Long-term potentiation (LTP) and long-term depression (LTD) represent key manifestations of this process, where high-frequency stimulation induces persistent increases or decreases in synaptic efficacy, respectively.^[42] In LTP, glutamate release activates NMDA receptors, allowing calcium influx upon coincident presynaptic and postsynaptic depolarization, which triggers intracellular cascades leading to enhanced synaptic transmission. This calcium-dependent signaling promotes the insertion of AMPA receptors into the postsynaptic membrane via trafficking mechanisms involving exocytosis and stabilization at the synapse.^[43] Conversely, in LTD, moderate calcium levels through NMDA receptors facilitate AMPA receptor endocytosis, reducing synaptic strength.^[43] Glutamate is pivotal in hippocampal LTP, where its binding to NMDA receptors during theta-burst or high-frequency stimulation initiates the calcium influx essential for AMPA receptor trafficking and long-lasting synaptic enhancement. Dopamine, acting as a neuromodulator, contributes to reward-based reinforcement by gating plasticity in circuits like the nucleus accumbens and prefrontal cortex, where phasic dopamine release during unexpected rewards strengthens synapses via D1/D5 receptor activation and enhancement of LTP-like processes.^[44] This dopaminergic modulation integrates motivational signals with Hebbian rules, enabling associative learning.^[45] Homeostatic scaling provides a complementary mechanism to Hebbian plasticity, maintaining overall network stability by globally adjusting synaptic strengths in response to chronic activity changes, often through balanced regulation of excitatory glutamate and inhibitory GABA neurotransmission. Reduced network activity triggers multiplicative upregulation of AMPA receptor surface expression at glutamatergic synapses and increased GABA receptor clustering, compensating to restore firing rates. Conversely, heightened activity downscales these synapses via receptor removal, preventing runaway excitation or silencing. This form of plasticity operates on slower timescales than Hebbian mechanisms, ensuring long-term circuit homeostasis.^[46]^[47] Excessive glutamate release can lead to pathological plasticity through excitotoxicity, where prolonged NMDA receptor activation causes overwhelming calcium influx, activating proteases, lipases, and endonucleases that damage cellular components and induce neuronal death.^[48] This process underlies synaptic weakening and circuit dysfunction in conditions of unchecked excitatory neurotransmission.^[49] Experimental evidence for these effects derives from in vitro hippocampal slice preparations, where frequency-dependent protocols reveal LTP induction with high-frequency stimulation (e.g., 100 Hz tetani) and LTD with low-frequency stimulation (1-5 Hz), as measured by field excitatory postsynaptic potentials. Blocking NMDA receptors with antagonists like APV abolishes LTP in these slices, confirming the calcium influx pathway, while manipulations of AMPA trafficking mimic or occlude plasticity changes.^[42] Such studies demonstrate how neurotransmitter dynamics directly sculpt adaptive synaptic modifications.^[50]

Historical and Methodological Foundations

Key Discoveries

In the early 19th century, the debate between electrical and chemical modes of neural transmission began to favor the latter through experiments with curare, a paralytic poison. French physiologist Claude Bernard demonstrated in the 1850s that curare blocks neuromuscular transmission specifically at the junction between nerve and muscle, without affecting nerve conduction itself, providing early evidence for a chemical intermediary in synaptic signaling.^[51]^[52] A pivotal breakthrough occurred in 1921 when Otto Loewi conducted his famous frog heart experiments, proving chemical neurotransmission. By stimulating the vagus nerve of one frog heart and transferring the perfusate to a second heart, Loewi observed slowed beating in the recipient, attributing the effect to a released substance he termed "vagusstoff," later identified as acetylcholine.^[53]^[54] This work, shared with Henry Dale in the 1936 Nobel Prize, established the chemical basis of synaptic transmission. The mid-20th century saw the identification of additional key neurotransmitters. In 1946, Ulf von Euler isolated norepinephrine from sympathetic nerves and confirmed its role as the primary transmitter in the sympathetic nervous system, earning him a share of the 1970 Nobel Prize for elucidating its storage and release mechanisms.^[55]64948-3/pdf) Concurrently, in 1948, Maurice Rapport and colleagues at the Cleveland Clinic isolated a vasoconstrictive factor from blood serum, naming it serotonin (5-hydroxytryptamine), which was soon recognized as a neurotransmitter modulating mood and other functions.^[56] The 1950s and 1960s expanded the roster to include amino acid transmitters. Japanese pharmacologist Takashi Hayashi reported in 1954 that sodium glutamate excites the central nervous system, injecting it into the motor cortex of dogs to induce rhythmic movements, proposing glutamate as an excitatory transmitter.^[57] Independently, Austrian-born physiologist Ernst Florey identified an inhibitory factor in crustacean nervous systems during the 1950s, which collaborative work in 1957 chemically confirmed as gamma-aminobutyric acid (GABA), the brain's chief inhibitory neurotransmitter.^[58]^[59] Later decades revealed more diverse transmitters, including neuropeptides and gases. In the early 1970s, Susan Leeman's team purified and sequenced substance P, an 11-amino-acid peptide first described in the 1930s, establishing it as the archetypal neuropeptide involved in pain transmission and inflammation. By the 1990s, nitric oxide (NO) emerged as a unconventional gaseous transmitter; Robert Furchgott, Louis Ignarro, and Ferid Murad's discoveries of NO's role in vascular relaxation and signaling earned the 1998 Nobel Prize, with Ignarro specifically highlighting its neuronal functions.^[60]^[61]

Techniques for Identification

Classical techniques for identifying neurotransmitters primarily involved bioassays that assessed physiological responses to candidate substances extracted from neural tissue. For instance, organ bath preparations, such as frog rectus abdominis muscle assays, measured contractions induced by acetylcholine to confirm its transmitter role through dose-response similarities.^[62] Pharmacological mimics further validated identifications by replicating or blocking effects; eserine (physostigmine), an acetylcholinesterase inhibitor, prolonged acetylcholine-mediated responses in bioassays, distinguishing it from other candidates like adrenaline.^[63] Biochemical approaches provide direct quantification and structural confirmation of neurotransmitters in tissues or fluids. High-performance liquid chromatography (HPLC), often coupled with electrochemical detection (HPLC-ECD), separates and detects small-molecule transmitters like dopamine and serotonin with nanomolar sensitivity, enabling analysis of microdialysate samples from brain regions.^[62] Mass spectrometry (MS), typically integrated with liquid chromatography (LC-MS/MS), offers high specificity for simultaneous identification of multiple transmitters, including amino acids like glutamate, achieving picomolar limits of detection without derivatization.^[64] For neuropeptides, radioimmunoassays (RIA) employ specific antibodies to quantify low-abundance transmitters like substance P in neural extracts, providing quantitative measures of content and release.^[63] Electrophysiological methods record synaptic events to infer neurotransmitter involvement through receptor-mediated currents. Patch-clamp techniques, in whole-cell or voltage-clamp configurations, measure postsynaptic potentials or currents evoked by synaptic stimulation, identifying transmitters via pharmacology (e.g., bicuculline-sensitive GABA_A currents for GABA).^[65] Optogenetics enables selective activation of transmitter-specific neurons using channelrhodopsins, allowing paired electrophysiological recordings to confirm release dynamics, such as dopamine-induced currents in target cells.^[66] Imaging techniques visualize neurotransmitter dynamics in living tissue with spatiotemporal precision. Fluorescent sensors, including genetically encoded indicators like GRAB (GPCR-activation-based) for dopamine, bind transmitters and undergo conformational changes that increase fluorescence, enabling real-time monitoring of release in behaving animals with sub-second resolution.^[67] Calcium indicators such as GCaMP, while primarily tracking presynaptic calcium influx, indirectly reveal transmitter release probability during synaptic events.^[68] Genetic approaches use engineered models to verify neurotransmitter roles by disrupting synthesis, storage, or release. Knockout mice lacking vesicular monoamine transporter 2 (VMAT2) exhibit depleted monoamine storage, confirming its essentiality for dopamine and serotonin vesicular loading through reduced evoked release and behavioral deficits.^[69] Such models, combined with rescue experiments, distinguish transmitter-specific functions from compensatory mechanisms.^[70]

Classification and Diversity

Major Classes

Neurotransmitters are classified into several major categories based on their chemical structure and mode of action, including small-molecule neurotransmitters (such as amino acids, biogenic amines, acetylcholine, and purines), neuropeptides, and gaseous molecules. Small-molecule neurotransmitters encompass biogenic amines and amino acids, which mediate fast synaptic transmission at the majority of central nervous system (CNS) synapses. Biogenic amines, such as dopamine, serotonin, and histamine, are involved in modulating mood, arousal, and reward pathways and are estimated to operate at 5-10% of synapses. Amino acids include the excitatory neurotransmitter glutamate, which serves as the primary excitatory agent at approximately 80% of CNS synapses, and the inhibitory neurotransmitters GABA and glycine, which together account for 20-30% of synapses, with GABA predominant in the brain and glycine more common in the spinal cord. Acetylcholine functions as a key small-molecule neurotransmitter primarily at neuromuscular junctions in the somatic nervous system, where it triggers muscle contraction, and in autonomic ganglia, facilitating transmission in both sympathetic and parasympathetic pathways.^[21] Neuropeptides represent a diverse class of over 100 identified types, often acting as neuromodulators with slower onset and longer duration compared to small molecules; examples include endorphins, which alleviate pain by binding to opioid receptors, and orexins, which promote wakefulness and arousal.^[71]^[72] Gaseous neurotransmitters, such as nitric oxide (NO) and carbon monoxide (CO), differ from classical transmitters by being freely diffusible, non-vesicular messengers involved in retrograde signaling to modulate synaptic plasticity and vasodilation.^[73]^[74] Purines, notably adenosine triphosphate (ATP), often serve as co-transmitters alongside other small molecules, exerting excitatory effects in sensory neurons, autonomic ganglia, and certain CNS circuits.^[6]

Structural and Functional Criteria

Neurotransmitters are defined by a set of established structural and functional criteria that ensure their role as specific signaling molecules in synaptic transmission. These criteria, originally outlined by John C. Eccles in his seminal 1964 work on synaptic physiology, require that a candidate substance be synthesized and stored in the presynaptic neuron, released in a calcium-dependent manner upon presynaptic depolarization, bind to specific receptors on the postsynaptic cell to elicit a physiological response, mimic the natural synaptic effect when applied exogenously, and possess a mechanism for rapid inactivation or clearance from the synapse.^[2] These standards, refined in subsequent neuroscientific literature, emphasize the substance's localization within presynaptic terminals and its Ca²⁺-evoked release, typically validated through techniques like immunocytochemistry for presence and microdialysis for release dynamics.^[75] Functionally, neurotransmitters exhibit diverse modes of action that influence synaptic communication speed and scope. Ionotropic receptors mediate fast, point-to-point transmission by directly gating ion channels, resulting in rapid excitatory or inhibitory postsynaptic potentials within milliseconds, as seen with glutamate acting on AMPA receptors.^[5] In contrast, metabotropic receptors trigger slower, diffuse modulation through G-protein-coupled signaling cascades, often leading to prolonged changes in cellular excitability via second messengers like cAMP, exemplified by norepinephrine's effects on β-adrenergic receptors.^[5] This dichotomy allows neurotransmitters to support both precise, localized signaling and broader network modulation, with functional efficacy typically requiring synaptic concentrations exceeding 1 μM to activate postsynaptic responses effectively. Structurally, neurotransmitters vary in polarity, which dictates their solubility, transport mechanisms, and diffusion properties. Polar molecules, such as amino acid derivatives like glutamate and GABA, are hydrophilic and rely on specific transmembrane transporters (e.g., EAATs for glutamate) for uptake and reuptake due to limited passive diffusion across lipid bilayers.^[76] Non-polar or lipophilic substances, including gaseous transmitters like nitric oxide, exhibit greater membrane permeability and diffuse freely without dedicated transporters, enabling volume transmission over larger distances.^[76] These structural features ensure targeted delivery and prevent nonspecific spillover, with polar neurotransmitters often stored in vesicles for regulated release. Controversies arise with substances like endocannabinoids (e.g., 2-arachidonoylglycerol), which function as retrograde messengers synthesized postsynaptically and diffusing backward to inhibit presynaptic release via CB1 receptors, thus deviating from classical anterograde criteria.^[77] Unlike traditional neurotransmitters, they lack vesicular storage and Ca²⁺-dependent exocytosis from presynaptic terminals, challenging strict classification while highlighting expanded definitions of synaptic signaling.^[78]

Neurotransmitter Systems in the Nervous System

Central Nervous System Roles

In the central nervous system (CNS), neurotransmitters play essential roles in modulating neural circuits within the brain and spinal cord, facilitating processes such as cognition, motor control, and emotional regulation. Glutamate serves as the primary excitatory neurotransmitter, predominantly released by pyramidal neurons in the cerebral cortex and hippocampus. These neurons utilize ionotropic receptors like NMDA and AMPA to mediate synaptic transmission, enabling long-term potentiation (LTP), a key mechanism underlying learning and memory formation.^[79] For instance, NMDA receptor activation in hippocampal CA1 pyramidal neurons allows calcium influx that triggers signaling cascades for synaptic strengthening, as demonstrated in studies of spatial memory tasks.^[79] Complementing glutamatergic excitation, gamma-aminobutyric acid (GABA) acts as the main inhibitory neurotransmitter, primarily through GABAergic interneurons that constitute about 10-15% of hippocampal neurons. These interneurons target principal cells at somatic, dendritic, or axonal sites, providing fast phasic inhibition via GABA_A receptors and slower tonic inhibition to maintain excitation-inhibition balance and prevent hyperexcitability.^[80] Subtypes such as parvalbumin-expressing basket cells deliver precise perisomatic inhibition to synchronize network oscillations, while somatostatin-expressing cells modulate distal dendritic inputs, both critical for averting seizure propagation in cortical and hippocampal circuits.^[80] Dopamine, a key modulatory neurotransmitter, operates through distinct pathways originating in the midbrain. The mesolimbic pathway, arising from ventral tegmental area neurons, projects to the nucleus accumbens and prefrontal cortex, influencing reward processing, motivation, and reinforcement learning by modulating synaptic plasticity in limbic regions.^[81] In contrast, the nigrostriatal pathway from substantia nigra pars compacta neurons innervates the dorsal striatum, regulating voluntary motor control and habit formation within basal ganglia circuits.^[81] Serotonin, released by neurons in the raphe nuclei, exerts widespread influence on mood-regulating areas through projections across the forebrain. Dorsal raphe serotonergic neurons promote antidepressant-like effects by reducing passive coping behaviors in stress models, while median raphe neurons contribute to anxiety modulation via inputs to the hippocampus and amygdala.^[82] This serotonergic modulation fine-tunes emotional responses, with balanced activity between dorsal and median raphe subpopulations essential for adaptive behavior.^[82] Neurotransmitter systems integrate in structures like the basal ganglia and cerebellum to ensure coordinated motor function. In the basal ganglia, dopaminergic inputs from the substantia nigra interact with glutamatergic and GABAergic signals to balance direct and indirect pathways, facilitating smooth movement initiation and vigor.^[83] Cerebellar circuits, receiving glutamatergic inputs via thalamic relays, refine these signals for precise coordination and error correction, with climbing fiber activity encoding adjustments that prevent ataxia.^[83] This interplay maintains overall CNS stability, underscoring the reciprocal modulation between excitatory, inhibitory, and modulatory transmitters.^[83]

Peripheral and Non-Neuronal Roles

Neurotransmitters play crucial roles beyond the central nervous system, influencing peripheral organs, sensory pathways, and even non-neuronal tissues through autonomic, enteric, and other signaling mechanisms. In the autonomic nervous system, acetylcholine (ACh) serves as the primary neurotransmitter in the parasympathetic branch, where it binds to muscarinic receptors—particularly M2 subtypes in the heart—to slow heart rate and reduce contractility, promoting rest and digestion.^[84] Conversely, norepinephrine acts as the main postganglionic neurotransmitter in the sympathetic nervous system, facilitating the "fight-or-flight" response by increasing heart rate, vasoconstriction in certain vascular beds, and mobilization of energy reserves through adrenergic receptors.^[85] In the enteric nervous system (ENS), which governs gastrointestinal function independently of central control, serotonin (5-HT) is a key modulator, with approximately 95% of the body's serotonin synthesized and stored in enterochromaffin cells of the gut. This neurotransmitter regulates gut motility by activating 5-HT receptors on enteric neurons and smooth muscle, enhancing peristalsis and secretion to facilitate digestion and propulsion of contents.^[86] Dysregulation of enteric serotonin signaling can alter motility patterns, underscoring its essential role in maintaining gastrointestinal homeostasis.^[87] Non-neuronal cells also utilize neurotransmitters for intercellular communication. Glutamate, traditionally known as an excitatory neurotransmitter, functions as an immunomodulator in immune cells such as T lymphocytes, where it binds to glutamate receptors to influence activation, proliferation, and cytokine release, thereby shaping adaptive immune responses.^[88] Similarly, nitric oxide (NO), a gaseous neurotransmitter produced by endothelial cells in blood vessels, induces vasodilation by activating guanylate cyclase in smooth muscle cells, relaxing vessels to regulate blood flow and prevent thrombosis.^[89] These actions highlight neurotransmitters' broader paracrine effects in non-neural contexts. In peripheral sensory systems, substance P, a neuropeptide neurotransmitter, is released from nociceptors—primary afferent neurons detecting pain—and transmits nociceptive signals to the spinal cord via neurokinin-1 receptors, contributing to the perception of inflammatory and neuropathic pain.^[90] This release also promotes neurogenic inflammation by inducing plasma extravasation and immune cell recruitment at peripheral sites. Inter-tissue communication further extends these roles, as seen in the gut-brain axis where serotonin from the ENS signals via vagal afferents to influence central autonomic regulation, modulating mood, appetite, and stress responses through bidirectional neural pathways.^[91]

Pharmacological and Therapeutic Implications

Drug Modulation Strategies

Drug modulation strategies aim to therapeutically alter neurotransmitter levels or activity by targeting various stages of their lifecycle, from synthesis and storage to release and reuptake, without directly interacting with receptors. These approaches enhance or diminish synaptic transmission to restore balance in neurotransmitter systems, often addressing deficiencies or excesses associated with neurological conditions. By intervening at presynaptic mechanisms, such strategies can increase extracellular neurotransmitter availability, thereby influencing downstream signaling pathways. One primary method involves reuptake inhibitors, which block the presynaptic transporters responsible for clearing neurotransmitters from the synaptic cleft, thereby elevating their concentrations. For instance, selective serotonin reuptake inhibitors (SSRIs) target the serotonin transporter (SERT) on the presynaptic axon terminal, preventing serotonin reuptake and allowing greater accumulation in the synapse. This inhibition prolongs serotonergic signaling, a mechanism central to their therapeutic effects. Similarly, inhibitors of other monoamine transporters, such as norepinephrine or dopamine, operate on analogous principles to sustain elevated levels of these neurotransmitters. Enzyme inhibitors represent another key strategy, focusing on preventing the degradation of neurotransmitters to maintain higher cytosolic pools available for release. Monoamine oxidase inhibitors (MAOIs) irreversibly bind to the enzyme monoamine oxidase (MAO), which catalyzes the oxidative deamination of monoamines like serotonin, norepinephrine, and dopamine. By blocking this breakdown, MAOIs increase the intracellular concentrations of these neurotransmitters, enhancing their vesicular packaging and subsequent synaptic release. This approach is particularly relevant for monoaminergic systems, where enzymatic catabolism limits transmitter availability. Vesicular modulators target the storage of neurotransmitters within synaptic vesicles, altering the reserves available for exocytosis. Reserpine, a classic example, inhibits the vesicular monoamine transporter (VMAT), which normally sequesters monoamines into vesicles using a proton gradient. By binding to VMAT and blocking this uptake, reserpine causes depletion of vesicular stores, leading to reduced neurotransmitter release upon depolarization and eventual cytosolic leakage, which can be degraded or reverse-transported. This mechanism disrupts monoamine transmission, historically used to model depletion states. Release enhancers promote the expulsion of neurotransmitters into the synapse by manipulating transporter dynamics or vesicular trafficking. Amphetamines act as substrates for plasma membrane transporters like the dopamine transporter (DAT), inducing a reversal of their normal uptake function through conformational changes and phosphorylation events. This reverse transport effluxes dopamine (and other monoamines) from the cytosol to the extracellular space, independent of vesicular release, thereby acutely boosting synaptic levels. Such actions amplify dopaminergic and noradrenergic signaling. Clinical strategies often combine these tactics to balance neurotransmitter systems, particularly by augmenting precursor availability to replenish depleted pools. Levodopa serves as a dopamine precursor that crosses the blood-brain barrier via large neutral amino acid transporters and is decarboxylated by aromatic L-amino acid decarboxylase into dopamine within dopaminergic neurons. This loading approach increases cytosolic dopamine for vesicular storage and release, compensating for synthetic deficits in affected pathways. Integration of these strategies allows for tailored modulation, optimizing therapeutic outcomes while minimizing off-target effects.

Agonists and Antagonists

Agonists are substances that bind to neurotransmitter receptors and activate them, mimicking the effects of endogenous neurotransmitters to elicit a biological response. These compounds can be classified based on their degree of activation relative to the natural ligand. Full agonists produce the maximum possible response upon receptor binding, equivalent to that of the endogenous neurotransmitter. For instance, nicotine acts as a full agonist at nicotinic acetylcholine receptors (nAChRs), binding to the α4β2 subtype and opening the ion channel to allow cation influx, thereby depolarizing the postsynaptic membrane.^[92] Partial agonists, in contrast, bind to the same site but elicit a submaximal response even at full receptor occupancy, often due to a lower capacity for conformational change in the receptor. Buprenorphine exemplifies this at mu-opioid receptors, where it activates the receptor to a lesser extent than full agonists like morphine, resulting in ceiling effects on analgesia and respiratory depression.^[93] Inverse agonists bind to the same receptor but stabilize an inactive conformation, reducing basal activity in constitutively active receptors; pimavanserin functions as an inverse agonist at 5-HT2A serotonin receptors, decreasing receptor signaling even in the absence of agonist stimulation.^[94] Antagonists inhibit neurotransmitter effects by binding to receptors without activating them, thereby preventing or reducing the action of agonists. Competitive antagonists bind reversibly to the orthosteric site, competing directly with the neurotransmitter or agonist for binding; their effects can be overcome by increasing agonist concentration. Naloxone serves as a classic competitive antagonist at mu-opioid receptors, rapidly displacing opioids like heroin to reverse overdose effects by blocking G-protein coupling and downstream inhibition of adenylyl cyclase.^[95] Non-competitive antagonists, however, bind to an allosteric site or irreversibly to the orthosteric site, altering receptor function without direct competition; their inhibition persists regardless of agonist concentration. Ketamine exemplifies non-competitive antagonism at NMDA glutamate receptors, entering the ion channel pore to block calcium influx in an open-channel state, thereby disrupting excitatory transmission.^[96] Selectivity in agonists and antagonists refers to the preference for specific receptor subtypes, minimizing off-target effects. Many compounds target subtypes within neurotransmitter families, such as adrenergic receptors. Propranolol, a non-selective beta-blocker, antagonizes both β1- and β2-adrenergic receptors by binding to the catecholamine site, inhibiting norepinephrine-mediated increases in heart rate and bronchodilation.^[97] In contrast, selective agents like metoprolol primarily target β1-receptors in cardiac tissue, reducing cardiovascular effects while sparing β2-mediated pulmonary functions.^[98] Receptor binding kinetics distinguish between affinity, which measures the strength of ligand-receptor association (quantified by the dissociation constant K_d), and efficacy, which reflects the ligand's ability to stabilize the active receptor state and produce a response (often denoted as \epsilon). High-affinity ligands bind tightly but may have low efficacy if they fail to induce conformational changes, as seen in partial agonists.^[99] These properties determine potency (EC_{50}, the concentration for half-maximal effect) and therapeutic windows, with slow dissociation kinetics prolonging blockade in antagonists.^[100] Chronic exposure to agonists can lead to receptor desensitization, a side effect where prolonged activation reduces receptor responsiveness through mechanisms like phosphorylation by kinases (e.g., GRKs) and subsequent arrestin binding, uncoupling the receptor from G-proteins. This tachyphylaxis is prominent in β2-adrenergic receptors with long-acting agonists, contributing to diminished bronchodilation over time.^[101] Desensitization may also involve receptor internalization and downregulation, exacerbating tolerance in systems like opioid signaling.^[102]

Pathophysiological Associations

Imbalances and Disorders

Imbalances in neurotransmitter systems arise from disruptions in synthesis, release, reuptake, or receptor function, leading to either excess or deficiency that contributes to neurological and psychiatric disorders. Excess neurotransmitter activity, such as glutamate-mediated excitotoxicity, occurs when prolonged activation of glutamate receptors overwhelms cellular calcium homeostasis, triggering neuronal death; this mechanism is prominent in ischemic stroke, where rapid glutamate release during energy failure exacerbates brain injury. Conversely, deficiencies, exemplified by dopamine loss in Parkinson's disease, result from degeneration of dopaminergic neurons in the substantia nigra, reducing striatal dopamine levels and impairing motor control. These imbalances highlight how neurotransmitter dysregulation can directly impair neural signaling and circuit integrity.^[103]^[104] Many disorders involve multifactorial interactions across neurotransmitter systems rather than isolated deficits. For instance, in schizophrenia, dysregulated serotonin modulation of dopamine release in mesolimbic pathways can amplify dopaminergic hyperactivity, contributing to psychotic symptoms through altered prefrontal and striatal signaling. Such interactions underscore the interconnected nature of neurotransmitter networks, where compensatory changes in one system may exacerbate vulnerabilities in another, complicating disease etiology.^[105] Diagnosis of neurotransmitter imbalances often relies on direct measurement techniques. Cerebrospinal fluid (CSF) analysis quantifies neurotransmitter metabolites like homovanillic acid (for dopamine) or 5-hydroxyindoleacetic acid (for serotonin), providing insights into central nervous system turnover; this method is particularly valuable for identifying primary neurotransmitter disorders. Positron emission tomography (PET) imaging of transporters, using radioligands such as [11C]raclopride for dopamine, visualizes binding potential and endogenous release dynamics in vivo, enabling non-invasive assessment of system integrity across brain regions.^[106]^[107] Epidemiologically, neurotransmitter imbalances are linked to high-prevalence conditions; for example, dysregulation of serotonin systems is implicated in major depressive disorder, which has a lifetime prevalence of approximately 12-16% in the general population, with reduced serotonin signaling observed in a significant subset of cases through CSF metabolite studies. These associations emphasize the broad public health impact of neurotransmitter-related pathologies.^[108]^[109] Recent post-2020 research has revealed the gut microbiome's role in modulating neurotransmitter production, particularly serotonin, where commensal bacteria influence host tryptophan metabolism and enterochromaffin cell activity to produce up to 90% of peripheral serotonin, potentially affecting central levels via the gut-brain axis and contributing to mood disorders. This insight highlights emerging environmental factors in neurotransmitter homeostasis.^[110]

Specific Neurotransmitter Dysfunctions

Dysfunctions in dopamine neurotransmission are central to several neurological and psychiatric disorders. In Parkinson's disease, the progressive loss of dopaminergic neurons in the substantia nigra pars compacta leads to dopamine depletion in the striatum, resulting in motor symptoms such as bradykinesia, rigidity, and tremor.^[111] This nigral degeneration disrupts the balance between direct and indirect pathways in the basal ganglia, causing hypoactivity in motor-facilitating circuits and contributing to slowed movements.^[112] In contrast, schizophrenia is associated with hyperactivity in the mesolimbic dopamine pathway, particularly from the ventral tegmental area to the nucleus accumbens, which underlies positive symptoms like hallucinations and delusions.^[113] This hyperdopaminergic state is thought to arise from dysregulated dopamine release and receptor sensitivity, amplifying aberrant salience attribution to internal stimuli.^[114] Serotonin dysregulation contributes to mood and anxiety disorders through altered firing and autoregulatory mechanisms in the raphe nuclei. In major depressive disorder, reduced serotonergic neuron firing in the dorsal raphe nucleus diminishes serotonin release across projection areas like the prefrontal cortex and hippocampus, impairing mood regulation and leading to symptoms such as persistent sadness and anhedonia.^[115] This hypoactivity may stem from enhanced negative feedback via 5-HT1A autoreceptors, limiting serotonin synthesis and transmission.^[82] For anxiety disorders, impaired serotonin autoregulation, particularly involving 5-HT1A somatodendritic autoreceptors, results in excessive serotonergic inhibition of raphe neurons, disrupting adaptive responses to stress and manifesting as heightened worry, panic, and autonomic arousal.^[116] Such dysregulation can lead to unbalanced excitation in limbic regions like the amygdala.^[117] Glutamate, the primary excitatory neurotransmitter, is implicated in neurodegenerative and seizure disorders via excitotoxic mechanisms. In epilepsy, excessive glutamate release and impaired uptake cause hyperexcitability in neuronal networks, particularly through overactivation of NMDA and AMPA receptors, leading to synchronized firing, seizures, and potential neuronal damage.^[118] This imbalance overwhelms inhibitory GABAergic control, propagating ictal activity across brain regions like the hippocampus and cortex.^[119] In Alzheimer's disease, amyloid-beta peptides induce changes in glutamate receptor function, including enhanced NMDA receptor activity and reduced AMPA receptor trafficking, which promote synaptic dysfunction, calcium overload, and progressive cognitive decline such as memory loss.^[120] These alterations exacerbate tau pathology and neuronal loss in affected areas.^[121] Acetylcholine (ACh) deficits underlie neuromuscular and cognitive impairments in autoimmune and degenerative conditions. Myasthenia gravis is primarily caused by autoantibodies against postsynaptic nicotinic ACh receptors at the neuromuscular junction, leading to receptor blockade, internalization, and complement-mediated destruction, which impairs muscle contraction and results in fluctuating weakness, fatigue, and ptosis.^[122] This autoimmune attack reduces the number of functional receptors by up to 70-80%, disrupting endplate potentials.^[123] In Alzheimer's disease, selective loss of cholinergic neurons in the basal forebrain, particularly the nucleus basalis of Meynert, causes ACh depletion in cortical and hippocampal targets, contributing to attentional deficits, memory impairment, and overall cognitive decline.^[124] This cholinergic hypofunction correlates with amyloid and tau pathology, amplifying synaptic failure.^[125] GABAergic dysfunction disrupts inhibitory control, contributing to sleep and substance use disorders. In insomnia, reduced GABA levels or impaired GABA_A receptor function in sleep-regulating circuits, such as the ventrolateral preoptic nucleus, fail to suppress wake-promoting neurons in the arousal centers, leading to prolonged sleep latency, fragmented sleep, and daytime fatigue.^[126] This inhibitory shortfall heightens cortical excitability during intended rest periods.^[127] In addiction, chronic exposure to substances like alcohol or benzodiazepines induces tolerance through downregulation of GABA_A receptors, particularly α1-containing subtypes, in reward pathways like the ventral tegmental area, necessitating higher doses for effect and increasing vulnerability to dependence and withdrawal symptoms such as anxiety and seizures.^[128] This adaptive decrease in receptor density and sensitivity underlies the diminished inhibitory tone.^[129]

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[72]
Identification of neurotransmitters and co-localization of transmitters ...
A more recent method for identifying the major transmitter is through the use of in situ hybridization (ISH) for definitive markers of glutamatergic ...
[73]
Overview of neuropeptides: awakening the senses? - PMC
Classical small molecule transmitters, such as glutamate, are stored in clear synaptic vesicles, while peptides are stored in dense core vesicles. The ...
[74]
Protective Role and Functional Engineering of Neuropeptides in ...
Feb 16, 2023 · There are two types of orexin, orexin-A ... There are three types of endorphins, with beta-endorphins being the most studied and prevalent.
[75]
Gases as biological messengers: nitric oxide and carbon monoxide ...
NO and CO have been recognized as putative neurotransmitters. These two novel messenger molecules have greatly expanded the criteria for candidacy of a ...
[76]
Carbon monoxide and nitric oxide as coneurotransmitters in the ...
Nitric oxide (NO) and carbon monoxide (CO) seem to be neurotransmitters in the brain. The colocalization of their respective biosynthetic enzymes, neuronal NO ...
[77]
Box A, Criteria That Define a Neurotransmitter - Neuroscience - NCBI Bookshelf
### Criteria for a Substance to Be Considered a Neurotransmitter
[78]
Neurotransmitter - an overview | ScienceDirect Topics
Neurotransmitters may be biogenic amines (e.g., acetylcholine, dopamine, norepinephrine, epinephrine, serotonin, histamine), amino acids (e.g., glutamate, GABA) ...
[79]
Endocannabinoid System: Chemical Characteristics and Biological ...
Endocannabinoids, unlike classical neurotransmitters, are considered atypical messengers because of the modulation of information from postsynaptic terminals to ...
[80]
Endocannabinoid signaling and synaptic function - PMC
Retrograde signaling is the principal mode by which endocannabinoids mediate short- and long-term forms of plasticity at both excitatory and inhibitory synapses ...
[81]
Overview of Glutamatergic Neurotransmission in the Nervous System
The authors of the following articles will review the preclinical and clinical evidence for aberrant glutamatergic neurotransmission in neuropsychiatric ...
[82]
Hippocampal GABAergic Inhibitory Interneurons
Sep 27, 2017 · In the hippocampus GABAergic local circuit inhibitory interneurons represent only ~10–15% of the total neuronal population.
[83]
Dopaminergic Neurons and Brain Reward Pathways
Dopamine released from the nigrostriatal pathway modulates corticostriatal transmission in medium spiny neurons expressing dopamine D1 or D2 receptors, which ...
[84]
Activity of raphé serotonergic neurons controls emotional behaviors
Thus we find a critical role of serotonergic neuronal activity in emotional regulation and uncover opposing roles of median and dorsal raphe function.
[85]
Cerebellar Contributions to the Basal Ganglia Influence Motor ...
Nov 9, 2022 · Both the cerebellum and basal ganglia are implicated in diverse functions, such as motor, cognitive, reward, and emotional processing.
[86]
Autonomic and endocrine control of cardiovascular function - PMC
In relation to the cardiovascular system the parasympathetic nervous system has two different kinds of muscarinic receptors: the M2 and M3 receptors (Table 1).
[87]
Physiology, Noradrenergic Synapse - StatPearls - NCBI Bookshelf
One major role it is involved in is the body's “fight or flight” response. During states of stress or anxiety, norepinephrine and epinephrine are released and ...
[88]
Novel aspects of enteric serotonergic signaling in health and brain ...
Approximately 95% of the body's serotonin ... Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks.
[89]
Serotonergic Mechanisms Regulating the GI Tract - PubMed Central
5-HT has been found to modulate enteric nervous system (ENS) development and neurogenesis, motility, secretion, inflammation, sensation, and epithelial ...
[90]
Role of glutamate on T-cell mediated immunity - PubMed
Recent studies have shown that glutamate not only has a role as neurotransmitter, but also as an important immunomodulator. In this regard, several glutamate ...
[91]
Vascular nitric oxide: formation and function - PMC - PubMed Central
NO plays a versatile role in the vasculature, maintaining vascular tone, inhibiting or reversing platelet activity, and preventing endothelial dysfunction.Missing: neurotransmitter | Show results with:neurotransmitter
[92]
Substance P and pain chronicity - PMC - PubMed Central
SP is an 11-amino acid neuropeptide that preferentially activates the neurokinin-1 receptor (NK1R). It transmits nociceptive signals via primary afferent fibers ...
[93]
Interaction of the Vagus Nerve and Serotonin in the Gut–Brain Axis
This review synthesizes the current knowledge on serotonin signaling, vagal nerve pathways, and central autonomic regulation.
[94]
Nicotinic Acetylcholine Receptors and Nicotine Addiction: A Brief ...
Nicotine binds to neuronal nicotinic acetylcholine receptors (nAChRs), which come in a diverse collection of subtypes. The nAChRs that contain the α4 and β2 ...
[95]
CPNP 2019 Annual Meeting Poster Abstracts - PMC - PubMed Central
May 13, 2019 · When administered concomitantly with full opioid agonists, buprenorphine will antagonize the full agonist, resulting in suboptimal analgesia.
[96]
The behavioral pharmacology and therapeutic potential of lorcaserin ...
Although 5-HT2A receptor antagonists (e.g., ketanserin, MDL100907), and inverse agonists (e.g., pimavanserin) appear to be more effective at reducing the ...Missing: types | Show results with:types
[97]
In vivo pharmacological resultant analysis reveals noncompetitive ...
In vivo, naltrexone, naloxone, CTAP and CTOP appear to be selective μ-opioid receptor antagonists in non-dependent subjects (Gulya et al., 1988; Kramer et al., ...Results · Antagonists Alone · Discussion
[98]
Ketamine Toxicity - StatPearls - NCBI Bookshelf
Ketamine's main site action is a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) glutamate receptor, though it exhibits effects on a myriad of ...
[99]
Beta Blockers - StatPearls - NCBI Bookshelf
Beta-blockers vary in their specificity towards different receptors, and accordingly, the effects produced depend on the type of receptor(s) blocked as well as ...
[100]
Selective Beta-1 Blockers - StatPearls - NCBI Bookshelf
The cardio-selective beta-1-blockers include atenolol, betaxolol, bisoprolol, esmolol, acebutolol, metoprolol, and nebivolol.Continuing Education Activity · Indications · Mechanism of Action · Adverse Effects
[101]
Molecular determinants of drug–receptor binding kinetics
molecular size, conformational fluctuations, ...
[102]
Agonist efficacy and receptor desensitization: from partial truths to a ...
One risk of long-term exposure of a G protein-coupled receptor (GPCR) agonist is desensitization of the response, most commonly via receptor phosphorylation and ...Missing: side | Show results with:side
[103]
Agonist-Directed Desensitization of the β2-Adrenergic Receptor - PMC
Apr 26, 2011 · However, prolonged or repeated use of current β2-agonist drugs leads to loss of their effects, a pervasive phenomenon termed tachyphylaxis, ...Missing: side | Show results with:side
[104]
Glutamate excitotoxicity: Potential therapeutic target for ischemic ...
May 24, 2022 · In this review, we highlight recent findings on glutamate release, reuptake mechanisms, NMDAR and its downstream cellular signaling pathways in post-ischemic ...
[105]
Dopamine and Parkinson's Disease - NCBI - NIH
Neurons of the substantia nigra communicate with neurons of the basal ganglia by liberating the neurotransmitter dopamine (DA).
[106]
Serotonin-dopamine interaction and its relevance to schizophrenia
Conclusions: Serotonergic modulation of dopaminergic function provides a viable mechanism for enhancing therapeutics in schizophrenia, but much remains unclear.
[107]
Clinical Use of CSF Neurotransmitters - PubMed
Cerebrospinal fluid pterins and monoamine metabolites are diagnostic in a range of primary neurotransmitter disorders, including disorders of biogenic amine ...
[108]
PET neurochemical imaging modes - PMC - PubMed Central - NIH
The most important principle for measuring endogenous neurotransmitter level changes with PET is radioligand sensitivity to competition with the endogenous ...
[109]
Major Depressive Disorder - StatPearls - NCBI Bookshelf
It has a lifetime prevalence of about 5 to 17 percent, with the average being 12 percent. The prevalence rate is almost double in women than in men.[5] This ...
[110]
What has serotonin to do with depression? - PMC - PubMed Central
Jun 4, 2015 · The best evidence that serotonin plays a role in the pathophysiology of depression comes from studies of “tryptophan depletion”.Missing: prevalence | Show results with:prevalence
[111]
Microbiota–gut–brain axis and its therapeutic applications ... - Nature
Feb 16, 2024 · Studies have shown that gut microbiota is capable of producing or stimulating the production of neurotransmitters, including serotonin, dopamine ...
[112]
Parkinson's Disease: Etiology, Neuropathology, and Pathogenesis
The movement disorder arises due to the loss of dopaminergic neurons of the substantia nigra pars compacta, with the pathological hallmark being intracellular ...Parkinson's Disease... · Etiology · Lewy Body Pathology
[113]
Determinants of dopaminergic neuron loss in Parkinson's disease
The cardinal motor symptoms of Parkinson's disease (PD) are caused by the death of dopaminergic neurons in the substantia nigra pars compacta (SNc).
[114]
Schizophrenia, Dopamine and the Striatum - PubMed Central - NIH
Sep 28, 2018 · The mesolimbic hypothesis has been a central dogma of schizophrenia for decades, positing that aberrant functioning of midbrain dopamine ...
[115]
The Dopamine Hypothesis of Schizophrenia: Version III—The Final ...
Mar 26, 2009 · The hypothesis that dopamine and dopaminergic mechanisms are central to schizophrenia, and particularly psychosis, has been one of the most ...Missing: hyperactivity | Show results with:hyperactivity
[116]
Revisiting the role of raphe and serotonin in neuropsychiatric ... - NIH
The monoaminergic hypothesis of depression posits that this illness results from a deficit in serotonin (5-HT), noradrenaline, and dopamine signaling in the ...
[117]
Increased Serotonin-1A (5-HT1A) Autoreceptor Expression ... - NIH
The serotonin-1A (5-HT1A) somatodendritic autoreceptor inhibits the firing of raphe serotonin neurons to negatively regulate the serotonin system (4).
[118]
Regulation of dorsal raphe nucleus function by serotonin ...
Serotonin 1B Autoreceptors Originating in the Caudal Dorsal Raphe Nucleus Reduce Expression of Fear and Depression-Like Behavior. Biol Psychiatry. 2011;69 ...5 Influence Of 5-Ht... · 6. Role Of 5-Ht... · 6.3 Stress And 5-Ht...
[119]
Glutamatergic Mechanisms Associated with Seizures and Epilepsy
Seizures elevate extracellular glutamate—the main excitatory neurotransmitter of the brain—which leads to aberrant neuronal signaling and connectivity.
[120]
The Role of Glutamate Receptors in Epilepsy - PMC - PubMed Central
Glutamate plays a role in both pre- and post-synaptic excitatory neurotransmission resulting in cellular and network hyperactivity, and underlies the formation ...
[121]
Amyloid β, Glutamate, Excitotoxicity in Alzheimer's Disease
A rise in glutamate would activate synaptic NMDA receptors, subsequently inducing a desensitization of the receptors with ultimately synaptic depression. A ...Glutamate And Synaptic... · Synaptic Plasticity In... · Glutamate Hypothesis And...
[122]
Glutamatergic receptor expression changes in the Alzheimer's ...
This study aimed to characterize the expression of specific glutamate receptor subunits in post‐mortem human brain tissue using immunohistochemistry and ...
[123]
Autoimmune myasthenia gravis: emerging clinical and biological ...
In most cases, it is caused by pathogenic autoantibodies directed towards the skeletal muscle acetylcholine receptor (AChR).
[124]
Mechanisms of acetylcholine receptor loss in myasthenia gravis
The fundamental abnormality affecting the neuromuscular junctions of myasthenic patients is a reduction of available AChRs, due to an autoimmune attack ...
[125]
The cholinergic system in the pathophysiology and treatment of ...
The cholinergic hypothesis of Alzheimer's disease centres on the progressive loss of limbic and neocortical cholinergic innervation. Neurofibrillary ...
[126]
Cholinergic system during the progression of Alzheimer's disease
Alzheimer's disease (AD) is characterized by a progressive phenotypic downregulation of markers within cholinergic basal forebrain (CBF) neurons.
[127]
The Role of GABA in Primary Insomnia - PMC - PubMed Central
GABA, the primary inhibitory neurotransmitter in the central nervous system (CNS), plays a role in the etiology of chronic insomnia.
[128]
GABA mechanisms and sleep - PubMed
It has been shown that GABA(B) receptor antagonists increase brain-activated behavioral states (waking and paradoxical sleep: dreaming stage). Recently, a ...
[129]
The Role of GABAA Receptors in the Development of Alcoholism
GABA A receptors undergo allosteric modulation by ethanol, anesthetics, benzodiazepines and neurosteroids and have been implicated in the acute as well as the ...
[130]
The role of GABAA receptors in the acute and chronic effects of ...
The effects of chronic ethanol administration are influenced by adaptations in GABA A receptor function, expression, trafficking, and subcellular localization.<|control11|><|separator|>