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Convulsant

A convulsant is a pharmacological agent that induces convulsions, characterized by involuntary, rhythmic contractions of muscles due to abnormal, synchronized electrical activity in the central nervous system (CNS). These substances typically increase neuronal excitability by interfering with inhibitory neurotransmission, such as by antagonizing gamma-aminobutyric acid (GABA) or glycine receptors, thereby lowering the seizure threshold and provoking seizures at sufficiently high doses while minimally affecting mental function at lower levels.^[1]^[2] Convulsants encompass a diverse group of compounds, including synthetic drugs, natural toxins, and certain stimulants, with mechanisms that often target key inhibitory pathways in the brain and spinal cord. Prominent examples include pentylenetetrazol (PTZ), a non-competitive GABA_A receptor antagonist that blocks chloride ion influx, leading to hyperexcitability and clonic-tonic seizures; strychnine, a glycine receptor antagonist that disrupts inhibitory signaling in the spinal cord, resulting in severe muscle rigidity and spasms; bicuculline and picrotoxin, both GABA_A antagonists that produce focal or generalized seizures by enhancing excitatory neurotransmission; and excitatory agents like kainic acid, which activates glutamate receptors to model temporal lobe epilepsy. Other convulsants, such as organophosphates (e.g., sarin or soman), act as acetylcholinesterase inhibitors to cause cholinergic overstimulation and progressive tremors escalating to convulsions. These agents can induce a spectrum of seizure types, from myoclonic twitches and focal discharges to full tonic-clonic episodes, depending on the dose, route of administration (e.g., systemic intraperitoneal or focal intracerebral), and targeted brain region (e.g., hippocampus or amygdala).^[2]^[3] In pharmacology, convulsants hold significant value as experimental tools rather than therapeutic agents, primarily for creating animal models of epilepsy to investigate seizure mechanisms and screen potential anticonvulsant drugs. For instance, PTZ and pilocarpine (a muscarinic agonist inducing status epilepticus) are widely used in rodent studies to mimic human epileptiform activity, with EEG patterns revealing spikes, fast spiking, or spike-wave discharges that parallel clinical conditions. Due to their high toxicity—evidenced by low LD50 values, such as 8 µg/kg for VX in guinea pigs or 1-4 mg/kg for strychnine in rats—convulsants have no approved medical applications in humans and are instead studied for their role in toxicology, including as potential chemical warfare agents or environmental hazards from sources like certain essential oils (e.g., thujone in sage) or plants (e.g., Cicuta virosa). Ongoing research emphasizes their utility in understanding epileptogenesis, though ethical constraints limit their use to controlled laboratory settings.^[1]^[2]^[3]

Definition and Overview

Definition

A convulsant is a drug or substance that induces convulsions or epileptic seizures by disrupting the neural balance in the central nervous system, functioning as the pharmacological opposite of anticonvulsants.^[4] These agents typically increase reflex excitability and lead to abnormal electrical activity in the brain, resulting in seizure induction.^[1] The term "convulsant" originates from the Latin convulsio, meaning a violent pulling or cramping together, which underscores its association with sudden, involuntary disruptions in muscular control.^[5] This etymology highlights the focus on seizure-like events as the primary outcome of exposure. Convulsions refer to involuntary, rhythmic muscle contractions arising from excessive, hypersynchronous neuronal discharges in the brain, often manifesting as widespread bodily shaking or stiffening.^[6] Convulsants generally exhibit stimulant properties at lower doses but possess a narrow margin between effective and toxic levels, rapidly escalating to hyperexcitability and seizure activity at higher exposures.^[4]

Physiological Effects

Convulsants primarily induce synchronized, excessive neuronal firing in the brain, leading to seizures such as tonic-clonic convulsions characterized by sudden loss of consciousness, stiffening of muscles, and rhythmic jerking movements that can last from seconds to minutes.^[7] These episodes often manifest as myoclonus, involving brief, shock-like muscle contractions, or progress to status epilepticus, defined as a seizure lasting 5 minutes or more, or two or more seizures without full recovery of consciousness in between, which heightens the risk of life-threatening complications.^[8] This abnormal electrical activity arises from an imbalance favoring excitatory over inhibitory neurotransmission within cortical networks.^[9] Systemically, convulsant-induced seizures trigger autonomic responses including tachycardia and hypertension due to massive sympathetic activation, alongside potential cardiac arrhythmias from the intense physiological stress.^[10] Hyperthermia frequently develops during prolonged seizures as metabolic demands elevate body temperature, while respiratory patterns disrupt—initially halting during the tonic phase and becoming irregular or rapid afterward, sometimes culminating in arrest in severe cases.^[11] Hyperglycemia and metabolic acidosis also occur as the body mobilizes energy reserves amid the crisis.^[12] Neurologically, the post-seizure period, known as the postictal state, involves confusion, disorientation, and profound exhaustion lasting minutes to hours, reflecting cerebral recovery from the hyperexcitable state.^[13] In extended episodes, excitotoxicity—excessive glutamate-mediated calcium influx—can cause neuronal damage, particularly in vulnerable regions like the hippocampus, leading to potential long-term cognitive impairments or cell loss.^[14] The severity of these effects progresses dose-dependently: low doses may evoke mild tremors or heightened reflex excitability without full convulsions, whereas higher thresholds precipitate generalized seizures and systemic derangements.^[1]

Mechanisms of Action

Inhibitory Neurotransmission Blockade

Convulsants exert their effects by antagonizing GABA_A receptors, which are ligand-gated ion channels that mediate fast inhibitory synaptic transmission in the central nervous system. Upon GABA binding, these receptors open to allow chloride ion influx, hyperpolarizing the neuron and inhibiting action potential firing. Antagonism at GABA_A receptors prevents this chloride influx, thereby reducing hyperpolarization and promoting neuronal depolarization and hyperexcitability.^[15]^[16] Similarly, blockade of glycine receptors, which are also chloride-permeable ligand-gated channels, disrupts inhibitory signaling particularly in spinal cord and brainstem neurons. Glycine receptor activation normally facilitates chloride entry, stabilizing membrane potential and suppressing excitability in these regions. By inhibiting this process, convulsants diminish the inhibitory conductance, leading to unchecked neuronal firing and heightened network activity.^[17]^[18] In addition to receptor-level antagonism, some convulsants target GABA synthesis by inhibiting enzymes such as glutamic acid decarboxylase (GAD), which catalyzes the conversion of glutamate to GABA. This inhibition reduces overall GABA availability, lowering inhibitory tone across neural circuits and exacerbating disinhibition.^[19] Collectively, these mechanisms shift the excitation-inhibition balance toward excessive excitation, facilitating seizure initiation and propagation without directly enhancing excitatory neurotransmission. This imbalance arises from the failure of inhibitory pathways, promoting hypersynchronous activity in vulnerable brain regions.^[20]

Excitatory Neurotransmission Enhancement

Convulsants can enhance excitatory neurotransmission primarily through agonism at ionotropic glutamate receptors, including NMDA, AMPA, and kainate subtypes, which are ligand-gated ion channels mediating fast synaptic transmission in the central nervous system. Activation of these receptors by agonists facilitates the influx of sodium and calcium ions, leading to neuronal depolarization and heightened firing rates that propagate hyperexcitable states. This mechanism underlies the proconvulsant effects observed in experimental models, where excessive glutamatergic signaling disrupts the balance of neural activity.^[21]^[22] The pathophysiological cascade triggered by such enhancement involves sustained receptor activation, resulting in excitotoxic neuronal damage characterized by intracellular calcium overload. Elevated calcium levels activate proteases, lipases, and endonucleases, culminating in mitochondrial dysfunction, oxidative stress, and eventual cell death, particularly in vulnerable brain regions like the hippocampus. This excitotoxicity not only contributes to acute seizure propagation but also to long-term neuronal loss in chronic epilepsy models.^[23]^[24] Additionally, convulsants may amplify excitation via agonism at acetylcholine receptors, encompassing both nicotinic and muscarinic subtypes, which stimulate excessive cholinergic activity and thereby induce seizures. Nicotinic receptors, as ligand-gated cation channels, promote rapid depolarization, while muscarinic G-protein-coupled receptors modulate slower excitatory pathways, collectively lowering the seizure threshold through overactivation of neural circuits.^[25]^[26] The threshold for seizure induction via these excitatory enhancements varies based on the affinity of convulsants for specific receptor subtypes and the regional specificity of receptor distribution in the brain, with areas like the temporal lobe exhibiting heightened sensitivity due to dense glutamatergic and cholinergic innervation. This selectivity influences the onset and severity of convulsions, as higher-affinity interactions at NMDA receptors, for instance, more potently elicit widespread hyperexcitability compared to kainate-mediated effects.^[27]^[28]

Examples of Convulsants

GABAA Receptor Antagonists, Inverse Agonists, or Negative Allosteric Modulators

GABAA receptor antagonists, inverse agonists, and negative allosteric modulators are a class of convulsants that impair the function of GABAA receptors, the primary mediators of fast inhibitory synaptic transmission in the central nervous system, by blocking GABA binding, reducing channel conductance, or decreasing receptor efficacy. These agents counteract the hyperpolarizing chloride influx induced by GABA, thereby disinhibiting neuronal activity and promoting hyperexcitability that can culminate in seizures. Unlike direct agonists, antagonists such as bicuculline compete at the orthosteric GABA-binding site, while non-competitive blockers like picrotoxin occlude the ion channel pore, and negative allosteric modulators like fipronil bind at distinct sites to diminish GABA-evoked responses.^[29]^[30]^[31] Picrotoxin, a naturally occurring alkaloid derived from plants in the Cocculus genus, exemplifies non-competitive antagonism by binding within the transmembrane domain at or near the chloride channel pore of GABAA receptors, thereby stabilizing the closed state and preventing ion flow even in the presence of GABA. This pore-blocking mechanism reduces the duration and amplitude of GABA-activated currents without directly affecting GABA affinity for the receptor. In contrast, bicuculline, another plant-derived alkaloid from Dicentra species, functions as a competitive antagonist by binding to the orthosteric site on the β-α subunit interface, thereby inhibiting GABA binding and subsequent receptor activation with high specificity for GABAA over other chloride channels. Flumazenil, a synthetic imidazobenzodiazepine, acts primarily as an antagonist at the benzodiazepine-binding site on the α-γ subunit interface but exhibits inverse agonist properties by decreasing constitutive receptor activity in the absence of GABA, particularly at α1-, α2-, and α3-containing subtypes.^[30]^[29]^[32] The chemical diversity of these modulators spans natural alkaloids, synthetic pharmaceuticals, and environmental toxins, including insecticides like fipronil, a phenylpyrazole that serves as a negative allosteric modulator by binding at a site distinct from the GABA or benzodiazepine loci, likely in the channel vestibule, to allosterically reduce GABA potency and efficacy across various GABAA isoforms. Fipronil's sulfone metabolite enhances this blockade, contributing to its convulsant effects observed in mammalian systems. Such diversity allows for varied pharmacokinetic profiles, with lipophilic agents like picrotoxin exhibiting rapid brain penetration. Regarding potency and selectivity, most GABAA antagonists display relatively consistent IC50 values (typically in the micromolar range) across receptor subtypes, with bicuculline showing IC50 around 2 μM and picrotoxin around 1-5 μM, though subtle variations exist; for instance, certain α5-selective inverse agonists like those related to flumazenil derivatives demonstrate higher affinity for extrasynaptic receptors, influencing their convulsant potential.^[31]^[33]^[34] Pentylenetetrazol (PTZ), a synthetic hydrazide compound, is a well-known non-competitive GABAA receptor antagonist that inhibits chloride conductance, leading to neuronal hyperexcitability and clonic-tonic seizures; it is widely used in animal models to induce seizures for epilepsy research.^[35] Tetramethylenedisulfotetramine (TETS), a synthetic rodenticide, acts as a potent non-competitive GABAA receptor antagonist by binding within the chloride channel pore, similar to picrotoxin, resulting in severe convulsions and status epilepticus.^[36]

Glycine Receptor Antagonists

Glycine receptor antagonists are a class of convulsants that primarily disrupt inhibitory neurotransmission mediated by glycine receptors (GlyRs) in the central nervous system, particularly in the spinal cord, leading to motor control dysfunction and hyperexcitability.^[37] These agents block the receptor's chloride ion channel, preventing glycine from hyperpolarizing neurons and thereby removing inhibitory tone on motor pathways.^[38] The resulting disinhibition manifests as muscle rigidity and tetanic contractions, distinguishing these convulsants from those targeting GABAA receptors, which more commonly induce generalized seizures rather than spinal-specific spasms.^[37] The prototypical glycine receptor antagonist is strychnine, a naturally occurring alkaloid derived from the seeds of Strychnos nux-vomica. Strychnine acts as a competitive antagonist by binding to the strychnine-sensitive site on the GlyR, located within the chloride channel pore, thereby inhibiting glycine's ability to open the channel and allow chloride influx.^[39] This blockade predominantly affects spinal interneurons, where glycine serves as the primary inhibitory neurotransmitter, leading to unchecked excitatory input from motoneurons and the onset of violent, sustained muscle contractions known as tetany.^[40] In cases of strychnine poisoning, symptoms progress rapidly from stiffness to full-body convulsions, often culminating in respiratory failure due to diaphragmatic spasm, with a narrow therapeutic index that renders even small doses lethal.^[38] Glycine receptors are pentameric ligand-gated ion channels composed primarily of α and β subunits, with the α1 subunit being the key strychnine-binding component and conferring high sensitivity to the antagonist.^[41] The α1-containing GlyRs are widely distributed in the spinal cord, brainstem, and cranial nerve nuclei such as the trigeminal and vestibular nuclei, where they modulate motor reflexes and sensory processing.^[42] Interactions between α1 and β subunits stabilize the receptor's structure and influence its pharmacological properties, including strychnine's high-affinity binding (with dissociation constants in the nanomolar range), which underscores the alkaloid's potency as a convulsant.^[43] Other glycine receptor antagonists include brucine, a structurally related alkaloid also found in Strychnos species, which exhibits similar but weaker antagonistic effects on GlyRs due to its lower binding affinity compared to strychnine.^[44] Brucine poisoning presents with comparable toxicity profiles, including convulsions and muscle rigidity, though its reduced potency results in a slightly higher lethal dose threshold; clinical cases often involve accidental ingestion leading to rapid onset of symptoms treatable with supportive care and benzodiazepines to counter the hyperexcitability.^[44] Overall, the limited repertoire of clinically relevant glycine receptor antagonists highlights their niche role in toxicology and neuroscience research, with strychnine remaining the benchmark for studying inhibitory synapse disruption.^[45]

Ionotropic Glutamate Receptor Agonists

Ionotropic glutamate receptor agonists function as convulsants by excessively stimulating excitatory neurotransmission in the central nervous system, leading to hyperexcitability and seizure activity.^[22] These compounds bind to ionotropic glutamate receptors, including N-methyl-D-aspartate (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and kainate subtypes, thereby opening cation channels and causing neuronal depolarization.^[22] Unlike endogenous glutamate, which typically elicits transient activation followed by rapid desensitization, certain agonists promote sustained receptor activation, exacerbating excitotoxicity and convulsive behaviors.^[46] NMDA receptor agonists, such as quinolinic acid, an endogenous metabolite of the kynurenine pathway, induce convulsions through prolonged depolarization and calcium influx.^[47] Intracerebroventricular administration of quinolinic acid in rats triggers dose-dependent seizures and hippocampal neurodegeneration, mimicking aspects of excitotoxic damage seen in neurological disorders.^[48] This agonist selectively activates NMDA receptors containing NR2A and NR2B subunits, leading to massive intracellular calcium elevation that sustains neuronal firing and contributes to neurotoxic potential.^[49] AMPA and kainate receptor agonists, including exogenous compounds like kainic acid and domoic acid, similarly provoke seizures by enhancing non-NMDA receptor-mediated excitation.^[50] Kainic acid, a synthetic analogue derived from seaweed, binds with high affinity to kainate receptors, inducing limbic seizures and status epilepticus in rodent models, which has made it a standard tool for studying temporal lobe epilepsy.^[51] Domoic acid, a natural toxin produced by marine diatoms and accumulated in shellfish, acts as a potent kainate receptor agonist, causing amnesic shellfish poisoning characterized by gastrointestinal distress, memory loss, and convulsive seizures in humans and wildlife.^[52] Both compounds exemplify environmental and experimental convulsants that amplify excitatory signaling.^[53] The convulsant effects of these agonists stem from their binding properties, which often confer resistance to receptor desensitization, resulting in prolonged excitation.^[46] For instance, kainic acid evokes non-desensitizing responses in hippocampal neurons, maintaining continuous sodium influx and membrane depolarization that outlasts typical glutamatergic transmission.^[46] Similarly, domoic acid sustains kainate receptor activation, leading to persistent hyperexcitability and excitotoxic neuronal damage.^[52] Quinolinic acid, while endogenous, achieves comparable sustained depolarization via NMDA receptor stimulation, highlighting its role in pathological conditions involving kynurenine pathway dysregulation.^[47] Distinctions between endogenous and exogenous agonists underscore their origins and implications as convulsants. Endogenous agonists like quinolinic acid arise from tryptophan metabolism and contribute to neuroinflammation-linked seizures under physiological stress.^[49] In contrast, exogenous agents such as kainic acid (a research tool) and domoic acid (a marine toxin) represent environmental or iatrogenic risks, with the latter posing public health threats through contaminated seafood.^[50]^[52] This dichotomy illustrates how both natural and synthetic perturbations of glutamate signaling can precipitate convulsive states.^[22]

GABA Synthesis Inhibitors and Acetylcholine Receptor Agonists

GABA synthesis inhibitors are compounds that block the enzyme glutamic acid decarboxylase (GAD), which catalyzes the conversion of glutamate to gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the central nervous system.^[54] This blockade leads to a progressive depletion of GABA pools in neurons, reducing inhibitory neurotransmission and thereby promoting hyperexcitability and convulsions.^[55] Unlike direct receptor antagonists, these agents exert their effects over time as GABA levels diminish, often resulting in generalized seizures that correlate with the degree of GAD inhibition, typically reaching 40-60% reduction prior to seizure onset.^[56] Representative examples include 3-mercaptopropionic acid (3-MPA), a competitive GAD inhibitor that induces convulsions by rapidly lowering brain GABA concentrations, with effects evident within 30-60 minutes of administration.^[57] Another key agent is allylglycine, which irreversibly inhibits GAD through a mechanism involving its metabolic conversion to a pyridoxal phosphate-reactive intermediate, leading to epileptic seizures and neuronal damage in animal models.^[58] These inhibitors highlight how targeted disruption of GABA biosynthesis can mimic broader inhibitory blockade, underscoring the critical role of GAD in maintaining neural inhibition.^[59] Acetylcholine receptor agonists, particularly those targeting muscarinic or nicotinic subtypes, can induce convulsions by excessively stimulating cholinergic pathways, resulting in overstimulation of excitatory circuits in limbic and cortical regions.^[26] Pilocarpine, a muscarinic receptor agonist, primarily activates M1 subtypes to trigger cholinergic seizures, often used as a model for temporal lobe epilepsy due to its ability to cause status epilepticus through enhanced phosphoinositide signaling and calcium mobilization in hippocampal neurons.^[60] Similarly, high doses of nicotine, acting via nicotinic acetylcholine receptors (nAChRs) such as the α7 subtype, elicit tonic-clonic seizures by promoting excessive neuronal firing in the amygdala and other seizure-prone areas, with non-convulsive doses capable of kindling epileptogenic activity over repeated exposures.^[25] Organophosphate compounds, such as sarin and soman, act as acetylcholinesterase (AChE) inhibitors, preventing acetylcholine breakdown and causing indirect overstimulation of muscarinic and nicotinic receptors, which leads to cholinergic crisis with progressive tremors escalating to convulsions.^[61] This muscarinic and nicotinic overstimulation contrasts with GABA depletion by directly amplifying excitatory cholinergic tone, leading to limbic hyperexcitability and behavioral manifestations like running fits or wet-dog shakes in experimental settings.^[62]

Uses and Applications

Therapeutic and Clinical Uses

Convulsants have been employed in psychiatric treatment since the 1930s, primarily through chemical induction of seizures as an alternative to electroconvulsive therapy. Pentylenetetrazol (PTZ), also known as Metrazol, was introduced by Ladislas Meduna in 1934 for convulsive therapy in patients with schizophrenia and depression, based on the hypothesis that inducing grand mal seizures could alleviate psychotic symptoms. Clinical reports from the era indicated remission rates of 40% to 50% in schizophrenia cases treated with PTZ, leading to widespread adoption across psychiatric institutions worldwide by the late 1930s. Flurothyl, an inhalational convulsant, emerged in the 1950s as a less painful alternative to PTZ injections, administered via mask to provoke controlled seizures for treating severe mood disorders and schizophrenia; it was used until the 1960s when electroconvulsive therapy became preferred due to improved safety profiles. These therapies were largely phased out by the 1950s with the advent of antipsychotic medications, though flurothyl has been sporadically revisited in modern contexts for refractory depression. In clinical toxicology, certain convulsants serve as antagonists to reverse sedative overdoses. Bemegride, a central nervous system stimulant and GABAA receptor antagonist, was historically used in the mid-20th century to counteract barbiturate-induced coma by accelerating recovery from respiratory depression and unconsciousness, with controlled studies demonstrating its efficacy in shortening coma duration compared to supportive care alone. Flumazenil, a benzodiazepine receptor antagonist, is currently FDA-approved for reversing benzodiazepine overdose effects, including sedation and respiratory suppression, with intravenous administration achieving reversal in 80% of cases within three minutes; it is particularly valuable in emergency settings for isolated benzodiazepine intoxications without concurrent proconvulsant risks. These agents are administered cautiously to avoid precipitating withdrawal seizures in chronic users. Strychnine and tetramethylenedisulfotetramine (TETS) are utilized as rodenticides for pest control due to their potent convulsant properties, targeting glycine and GABAA receptors respectively to induce lethal seizures in rodents. Human exposures occur primarily through accidental ingestion or environmental contamination, particularly with illegally imported TETS products, leading to clinical management of acute poisoning cases that highlight the need for supportive care in emergency medicine. TETS, banned in many countries but still encountered in unregulated markets, poses a higher toxicity risk than strychnine, with potential for severe convulsions at doses far below those effective for pest control. Contemporary therapeutic applications of convulsants are limited, mainly to diagnostic provocation in specialized epilepsy evaluations. Pentylenetetrazol has been employed in rare cases to induce seizures during ictal single-photon emission computed tomography (SPECT) imaging for localizing epileptogenic foci in drug-resistant patients, providing enhanced diagnostic accuracy when spontaneous seizures are infrequent. Such uses are confined to controlled research or clinical settings due to ethical and safety considerations, with flumazenil occasionally referenced in differential diagnosis of seizure-like events.

Research and Experimental Applications

Convulsants play a crucial role in epilepsy research by enabling the creation of animal models that mimic human temporal lobe epilepsy (TLE). Kainic acid, administered systemically or intrahippocampally in rodents, induces status epilepticus followed by spontaneous recurrent seizures, providing insights into epileptogenesis and hippocampal sclerosis characteristic of TLE.^[63] Similarly, pilocarpine, a muscarinic acetylcholine receptor agonist, is used to trigger prolonged seizures in rats and mice, leading to chronic epilepsy with behavioral comorbidities, and recent refinements have improved survival rates in mouse models for studying pharmacoresistant epilepsy.^[64]^[65] In anticonvulsant drug development, pentylenetetrazol (PTZ) serves as a standard convulsant for screening potential therapies through the subcutaneous PTZ (scPTZ) test, which evaluates the ability of compounds to delay or prevent clonic seizures induced by GABA_A receptor antagonism.^[66] This model complements the maximal electroshock seizure (MES) test, where PTZ helps identify drugs effective against generalized seizures, contributing to the evaluation of antiepileptic drugs like benzodiazepines and phenytoin in preclinical phases.^[67] Domoic acid, a potent agonist of ionotropic glutamate receptors, is employed in neurotoxicity studies to investigate excitotoxic mechanisms underlying neuronal damage in conditions like TLE.^[68] It primarily activates AMPA and kainate receptors, leading to calcium influx and cell death, which has been used to model amnesic shellfish poisoning and hippocampal pathology. Recent investigations from 2020 to 2025 have utilized domoic acid to probe subtype-specific roles of glutamate receptors in developmental neurotoxicity and behavioral deficits, including effects on microglial morphology in zebrafish and chronic exposure risks in marine models.^[69]^[70] For in vitro electrophysiology, picrotoxin, a non-competitive GABA_A receptor antagonist, is widely applied in patch-clamp techniques to isolate and study inhibitory synaptic currents by blocking chloride conductance.^[71] This allows precise measurement of GABA-mediated inhibition in neuronal cultures or slices, revealing use-dependent blockade mechanisms and aiding research on synaptic transmission dynamics.^[72]

Advantages

Clinical Advantages

Historically, convulsants offered certain clinical advantages in therapeutic and diagnostic settings before their discontinuation due to safety concerns and the development of safer alternatives like electroconvulsive therapy (ECT). For instance, flurothyl, administered via inhalation, allowed for rapid induction of generalized seizures in psychiatric treatments for severe mood disorders, with onset within minutes due to its volatility. This method demonstrated efficacy comparable to ECT while potentially minimizing some cognitive side effects, such as memory disruption.^[73]^[74] In resource-limited settings, agents like pentylenetetrazol (PTZ) and flurothyl served as cost-effective alternatives to ECT, requiring only basic administration tools such as syringes or inhalation devices, unlike ECT's need for specialized electrical equipment.^[75] This accessibility facilitated broader application of convulsive therapy in early psychiatric practices. Additionally, PTZ was used historically in diagnostic procedures to provoke latent seizures and localize epileptic foci via techniques like single-photon emission computed tomography (SPET) in drug-resistant epilepsy cases, providing electrophysiological data for subtype classification.^[76] However, these applications were abandoned due to high risks of adverse effects and ethical concerns.

Research Advantages

Convulsants offer significant advantages in neuroscience and pharmacology research by enabling the creation of reproducible seizure models. Pentylenetetrazol (PTZ), a prototypical GABAA receptor antagonist, is widely employed in standardized protocols to induce seizures with consistent latency and severity, facilitating high-throughput screening of anticonvulsant compounds. The intravenous PTZ infusion test, for instance, determines individual seizure thresholds, allowing precise quantification of drug efficacy across large cohorts of rodents, which enhances statistical power and reduces inter-animal variability compared to less controlled methods.^[77] This reproducibility has made PTZ a cornerstone for evaluating novel therapeutics, as it mimics generalized tonic-clonic seizures and supports dose-response analyses in preclinical settings.^[78] Another key strength lies in the targeted probing of specific neural pathways using selective convulsants. Strychnine, a potent antagonist of glycine receptors (GlyRs), exemplifies this by selectively blocking inhibitory neurotransmission in the spinal cord, enabling researchers to isolate glycinergic circuits without broadly disrupting other systems. In spinal cord studies, strychnine application reveals the role of GlyRs in modulating motor neuron excitability and sensory processing, such as in nociceptive pathways, where its blockade induces hyperexcitability that mirrors pathological conditions like tactile allodynia.^[79] This specificity allows for detailed electrophysiological mapping of synaptic inhibition, contributing to insights into spinal reflex arcs and interneuronal networks.^[80] The versatility of convulsants extends across diverse experimental paradigms, from in vivo animal models to in vitro synaptic assays, supporting advanced investigations into epilepsy genetics. Chemoconvulsants like PTZ and bicuculline can induce seizures in genetically modified rodents, such as those harboring mutations in ion channel genes (e.g., SCN1A models of Dravet syndrome), to assess susceptibility and therapeutic responses in whole-brain contexts.^[81] In vitro, these agents applied to hippocampal slices or neuronal cultures from patient-derived induced pluripotent stem cells (iPSCs) replicate epileptiform activity, aiding the study of genetic variants' impact on network hyperexcitability from 2020 onward.^[82] This adaptability has accelerated research into polygenic epilepsy risks, integrating high-content screening with genetic manipulations to identify novel targets.^[83] Furthermore, convulsants serve as ethical alternatives to electrical stimulation in behavioral studies by providing non-invasive chemical induction of seizures. Unlike maximal electroshock (MES) protocols, which require electrode implantation and can cause tissue damage or stress, subcutaneous or intraperitoneal administration of chemoconvulsants like PTZ elicits reliable behavioral phenotypes—such as clonic convulsions—through systemic routes, minimizing procedural invasiveness and aligning with 3Rs principles (replacement, reduction, refinement).^[84] This approach reduces animal distress in acute models while still allowing observation of seizure progression, propagation, and behavioral correlates, thereby supporting more humane evaluations of anticonvulsant interventions.^[85]

Risks and Complications

Acute Risks

Exposure to convulsants can rapidly induce severe seizures, leading to immediate life-threatening hazards such as aspiration, physical trauma from falls, and progression to status epilepticus, which may cause brain hypoxia due to inadequate oxygenation during prolonged convulsive activity.^[8] During intense convulsions, individuals risk inhaling saliva, vomit, or food into the lungs, resulting in aspiration pneumonia, with studies reporting an incidence of up to 46% in cases of generalized convulsive status epilepticus.^[86] Additionally, uncontrolled muscle contractions can cause falls, leading to head trauma, fractures, or concussions, which may not manifest symptoms until hours or days later and require urgent medical intervention.^[87] Status epilepticus from convulsant-induced seizures exacerbates these dangers by sustaining high neuronal activity, depleting cerebral oxygen supplies and potentially causing irreversible brain damage, with mortality rates reaching 80% in severe hypoxic cases.^[8] Convulsants also impose acute cardiovascular strain through the physiological stress of convulsions, manifesting as arrhythmias, tachycardia, hypertension, or, in rare instances, bradycardia.^[88] The surge in catecholamines during seizures can trigger cardiac arrhythmias and even myocardial damage, while hypertension arises from intense muscle contractions and sympathetic activation.^[8] For glycine receptor antagonists like strychnine, these effects are particularly pronounced, often presenting with tachycardia and elevated blood pressure alongside muscle spasms.^[88] Respiratory failure represents another critical acute risk, frequently culminating in apnea or hypoventilation during severe convulsant exposure. In high-dose scenarios, sustained diaphragmatic and intercostal muscle spasms impair ventilation, leading to hypoxia, cyanosis, and rapid death, as seen in strychnine poisoning where respiratory arrest can occur within 15-30 minutes.^[89] Convulsants may also induce pulmonary edema or impaired airway protection, compounding respiratory compromise and increasing the likelihood of fatal outcomes.^[8] Overdose with certain convulsants, such as tetramethylenedisulfotetramine (TETS), progresses rapidly, with symptoms emerging in 1-5 minutes and escalating to refractory status epilepticus, coma, or death within hours, often without a specific antidote available.^[90] TETS poisoning specifically involves GABA receptor blockade, triggering uncontrollable seizures that lead to tachycardia, bradycardia, or myocarditis, alongside respiratory failure as a primary cause of mortality, necessitating aggressive supportive care like benzodiazepines.^[90]

Chronic and Long-Term Complications

Repeated or unresolved exposure to convulsants can result in excitotoxicity, a process where excessive glutamate receptor activation leads to calcium influx and neuronal death, particularly in vulnerable brain regions like the hippocampus. In recurrent kainic acid models, intrahippocampal injections of this glutamate agonist induce status epilepticus followed by spontaneous recurrent seizures, culminating in hippocampal sclerosis characterized by neuronal loss, gliosis, and mossy fiber sprouting, which closely recapitulates human temporal lobe epilepsy pathology.^[91]^[92] Cognitive deficits represent another key long-term outcome, with memory impairment frequently observed following convulsive therapies involving agents like flurothyl. Historical and experimental studies using flurothyl to induce generalized seizures in rodents have demonstrated persistent hippocampal-dependent memory deficits, such as impaired spatial learning and novel object recognition, persisting for days to weeks post-seizure, independent of gross pathology.^[93]^[94] These effects arise from disrupted synaptic plasticity and neurogenesis in the dentate gyrus, highlighting the vulnerability of developing or recovering neural circuits to convulsant-induced stress.

History

Early Discoveries

The toxic and medicinal effects of the seeds from the Strychnos nux-vomica tree, which contain strychnine, were recognized in ancient India and China, where they were employed as poisons and in traditional remedies for various ailments.^[95] These observations appear in early Ayurvedic texts, reflecting knowledge of the plant's ability to induce severe muscle spasms and convulsions dating back to ancient times.^[96] In the 19th century, key isolations advanced understanding of convulsants. French chemists Pierre Joseph Pelletier and Joseph Bienaimé Caventou first isolated pure strychnine from S. nux-vomica seeds in 1818, enabling detailed toxicological analysis of its convulsant properties.^[97] Concurrently, camphor's potential as a convulsant emerged through reports of toxicity; as early as 1785, British physician William Oliver documented seizures following oral camphor administration, with further 19th-century cases confirming its role in provoking epileptic-like fits via ingestion or exposure.^[98] By the mid-1800s, systematic observations, including a notable 1887 report of convulsions after solid camphor ingestion, highlighted its dangers in medical and household contexts.^[99] Initial medical trials of camphor as a convulsant occurred in the late 19th century, often involving subcutaneous injections to provoke seizures for studying epilepsy pathogenesis, though these were experimental and not intended for routine therapy.^[100] Prior to 1900, convulsants like strychnine featured prominently in toxicology studies without therapeutic aims; French physiologist Claude Bernard utilized strychnine in mid-19th-century experiments to dissect spinal reflex arcs and neuromuscular transmission, establishing foundational insights into convulsive physiology through animal models.^[101]

20th Century Developments

In the 1930s, Hungarian psychiatrist Ladislas von Meduna pioneered the use of pentylenetetrazol (also known as Metrazol or Cardiazol) in convulsive therapy for schizophrenia, hypothesizing a biological antagonism between epilepsy and the disorder based on histopathological observations of patient brain tissues.^[102] Initially experimenting with camphor oil injections in 1934 to induce seizures, Meduna shifted to the more reliable and soluble pentylenetetrazol, administering it intravenously to over 100 patients by 1937, reporting remission rates of up to 80% in catatonic cases.^[103] This approach marked a significant advancement in somatic psychiatry, spreading rapidly to Europe and the United States despite the therapy's intensity. The 1950s saw innovations aimed at mitigating the drawbacks of injectable convulsants, with flurothyl (Indoklon), a volatile ether derivative, introduced as an inhalational agent to induce seizures more controllably.^[104] First tested clinically in 1957, flurothyl demonstrated efficacy comparable to electroconvulsive therapy in treating depression and schizophrenia, with randomized trials showing similar response rates but reduced postictal confusion and memory impairment due to its rapid onset and offset via inhalation.^[74] Administered through a face mask in a closed-circuit system, it represented a pharmacological refinement, though adoption remained limited outside select centers. Parallel to therapeutic applications, the mid-20th century featured key research milestones in understanding convulsant mechanisms through neurotransmitter studies. In the late 1950s, picrotoxin emerged as a prototypical GABA antagonist, blocking inhibitory postsynaptic potentials at invertebrate and vertebrate synapses, which helped establish gamma-aminobutyric acid (GABA) as the primary inhibitory transmitter in the central nervous system.^[29] Building on this, bicuculline was characterized in the 1960s as a more selective competitive antagonist at GABA_A receptors, with foundational experiments in 1969-1970 demonstrating its ability to reverse GABA-mediated inhibition in cat spinal cord neurons, enabling precise dissection of GABAergic circuitry.^[105] These tools propelled neuropharmacological research, influencing models of epilepsy and anxiety. By the 1960s, chemical convulsant therapies waned in psychiatric practice, supplanted by electroconvulsive therapy owing to the former's risks, including unpredictable seizure thresholds, vertebral fractures, and severe patient distress from abrupt, explosive convulsions.^[106] The rise of antipsychotic medications further diminished their role in schizophrenia treatment, though select convulsants like picrotoxin and strychnine persisted in non-medical applications, such as rodenticides and piscicides, leveraging their potent neuroexcitatory effects.^[18]

Modern and Recent Developments

In the 2000s, toxicology research increasingly highlighted the risks of accidental human exposure to convulsants like tetramethylenedisulfotetramine (TETS) and fipronil, often stemming from their use in illegally imported or widely available pesticides. TETS, a potent GABA_A receptor antagonist, was implicated in multiple poisoning incidents in the United States and Asia, with a 2003 outbreak linked to contaminated Chinese rodenticides causing severe seizures and fatalities due to doses as low as 7-10 mg in adults.^[107] Similarly, fipronil, a phenylpyrazole insecticide blocking GABA- and glutamate-gated chloride channels, led to documented cases of accidental and intentional ingestion, characterized by agitation, vomiting, and tonic-clonic seizures, as reported in prospective studies from Sri Lanka and India during this period.^[108] These events underscored the need for enhanced surveillance and public health measures against pesticide-related convulsant exposures.^[109] From the 2010s onward, convulsants such as pentylenetetrazole (PTZ) have been integrated into advanced research models, including optogenetics, to dissect seizure initiation and propagation pathways. Optogenetic studies in chemical convulsant-induced seizure models in rodents have enabled precise manipulation of neural circuits, revealing how inhibitory interneuron activation can suppress hyperexcitability.^[110] CRISPR/Cas9 techniques have utilized kainic acid to induce seizures in genetically modified epilepsy models, validating therapeutic targets for network hyperexcitability.^[111] Regulatory frameworks have evolved significantly, with the European Union imposing a complete ban on strychnine as a rodenticide in 2006, leading to the recall of all supplies due to its high toxicity and environmental persistence.^[112] In the United States, aboveground uses of strychnine were suspended by the EPA in 1988, with further reregistration reviews in the 1990s and 2010s limiting applications to belowground uses and aligning with broader rodenticide reforms to curb secondary wildlife poisonings.^[113] The World Health Organization's 2024 brochure on preventing suicide by phasing out highly hazardous pesticides (HHPs) advocates for banning acutely toxic HHPs to reduce suicides and unintentional poisonings, particularly in low- and middle-income countries.^[114] Despite these advances, convulsants maintain limited direct therapeutic roles, primarily confined to experimental induction of seizures for drug screening, with ongoing concerns over their toxicity hindering clinical translation. However, their application in neuropharmacology is expanding, particularly in designing inverse agonists at GABA_A or other receptors to counteract constitutive activity in epilepsy and anxiety disorders, as evidenced by structure-activity studies optimizing selectivity to avoid pro-convulsant effects.^[115] Future research gaps include developing safer analogs for pathway modeling and addressing persistent illegal use in developing countries.^[90]

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Acute Human Self-Poisoning with the N-Phenylpyrazole Insecticide ...
Fipronil poisoning is characterized by vomiting, agitation, and seizures, and normally has a favorable outcome.Missing: convulsants | Show results with:convulsants
[114]
Acute human self-poisoning with the N-phenylpyrazole insecticide ...
Conclusions: Our experience with prospectively observed patients suggests that fipronil poisoning is characterized by vomiting, agitation, and seizures, and ...Missing: TETS convulsants 2000s
[115]
Novel Optogenetic Approaches in Epilepsy Research - Frontiers
Sep 5, 2019 · Lastly, behavioral alterations during stimulations start focally and evolve into generalized seizures, mimicking partial to complex seizure ...
[116]
CRISPR/Cas9-mediated neuronal deletion of 5-lipoxygenase ...
In the current study, we demonstrated the translational potential of neuronal Alox5 deletion in combating seizures, delaying epileptic progression and ...
[117]
Influence of glutamatergic and GABAergic neurotransmission on ...
Jul 12, 2023 · The concentration of extracellular glutamate is tightly regulated to prevent excitotoxicity – the phenomenon of increased glutamate-related ...
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Strychnine Poisoning: Gone but Not Forgotten - PubMed
Sep 1, 2010 · Strychnine was used as a pesticide until 1968 and a rodenticide until 2006 when its sale was banned throughout the EU and all supplies recalled ...Missing: US expansions
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[PDF] The Use of Strychnine in Wildlife Damage Management - usda aphis
Strychnine is a nonselective rodenticide first registered in the U.S. in 1947 (USEPA 1996b). Strychnine was registered to control rodents such as ground ...
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[PDF] Preventing suicide by phasing out highly hazardous pesticides
WHO therefore recommends banning acutely toxic HHPs to prevent suicides and other adverse health impacts, as articulated in the. Organization's LIVE LIFE ...
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Full article: Design, synthesis, and anticonvulsant effects evaluation ...
Jun 12, 2020 · Abstract. Histamine H3 receptors (H3R) antagonists/inverse agonists are becoming a promising therapeutic approach for epilepsy.