Cholinergic actions of diazepam and atropine sulfate in soman poisoning

Seizure suppression and neuroprotection in soman-exposed rats following delayed intramuscular treatment of adenosine A1 receptor agonist as an adjunct to standard medical treatment

Soman produces excitotoxic effects by inhibiting acetylcholinesterase in the cholinergic synapses and neuromuscular junctions, resulting in soman-induced sustained status epilepticus (SSE). Our previous work showed delayed intramuscular (i.m.) treatment with A1 adenosine receptor agonist N-bicyclo-[2.2.1]-hept-2-yl-5'-chloro-5'-deoxyadenosine (ENBA) alone suppressed soman-induced SSE and prevented neuropathology. Using this same rat soman seizure model, we tested if delayed therapy with ENBA (60 mg/kg, i.m.) would terminate seizure, protect neuropathology, and aid in survival when given in conjunction with current standard medical countermeasures (MCMs): atropine sulfate, 2-PAM, and midazolam (MDZ). Either 15- or 30-min following soman-induced SSE onset, male rats received atropine and 2-PAM plus either MDZ or MDZ + ENBA. Electroencephalographic (EEG) activity, physiologic parameters, and motor function were recorded. Either 2- or 14-days following exposure surviving rats were euthanized and perfused for histology. All animals treated with MDZ + ENBA at both time points had 100% EEG seizure termination and reduced total neuropathology compared to animals treated with MDZ (2-day, p = 0.015 for 15-min, p = 0.002 for 30-min; 14-day, p < 0.001 for 15-min, p = 0.006 for 30-min), showing ENBA enhanced MDZ's anticonvulsant and neuroprotectant efficacy. However, combined MDZ + ENBA treatment, when compared to MDZ treatment groups, had a reduction in the 14-day survival rate regardless of treatment time, indicating possible enhancement of MDZ's neuronal inhibitory effects by ENBA. Based on our findings, ENBA shows promise as an anticonvulsant and neuroprotectant in a combined treatment regimen following soman exposure; when given as an adjunct to standard MCMs, the dose of ENBA needs to be adjusted.

Acute and chronic effects of diazepam on the polychaete Hediste diversicolor: Antioxidant, metabolic, pharmacologic, neurotoxic and behavioural mechanistic traits

Pharmaceutical drugs are widespread environmental contaminants, but data about their adverse effects are still limited to a few compounds. This study analyzed the acute (96 h) and chronic (28 days) impacts of environmentally realistic levels of diazepam (acute exposure: 0.001, 0.01, 0.1, 1, 10 μg/L; chronic exposure: 0.1, 1, 10, 100, 1000 ng/L), in the polychaete Hediste diversicolor, by measuring behavioral and biochemical (catalase [CAT], glutathione-S-transferases [GSTs], cholinesterases [ChEs], glutathione peroxidase [GPx], lipid peroxidation [TBARS]) parameters. Acute exposure to diazepam altered behavioral traits, decreasing burrowing times and causing hyperactivity, whilst burrowing time increased and hypoactivity resulted after chronic exposure. All biomarkers were affected after the chronic exposure, with the exception of lipid peroxidation. Our data demonstrate that realistic levels of diazepam may impair behavioral and biochemical traits in polychaetes, suggesting that diazepam exposure presents a significant challenge to the environment that supports these organisms.

The combination of donepezil and procyclidine protects against soman-induced seizures in rats

Current treatment of nerve agent poisoning consists of prophylactic administration of pyridostigmine and therapy using atropine, an oxime and a benzodiazepine. Pyridostigmine does however not readily penetrate the blood-brain barrier giving ineffective protection of the brain against centrally mediated seizure activity. In this study, we have evaluated donepezil hydrochloride, a partial reversible inhibitor of acetylcholinesterase (AChE) clinically used for treating Alzheimer's disease, in combination with procyclidine, used in treatment of Parkinson's disease and schizophrenia, as prophylaxis against intoxication by the nerve agent soman. The results demonstrated significant protective efficacy of donepezil (2.5 mg/kg) combined with procyclidine (3 or 6 mg/kg) when given prophylactically against a lethal dose of soman (1.6 x LD(50)) in Wistar rats. No neuropathological changes were found in rats treated with this combination 48 h after soman intoxication. Six hours after soman exposure cerebral AChE activity and acetylcholine (ACh) concentration was 5% and 188% of control, respectively. The ACh concentration had returned to basal levels 24 h after soman intoxication, while AChE activity had recovered to 20% of control. Loss of functioning muscarinic ACh receptors (17%) but not nicotinic receptors was evident at this time point. The recovery in brain AChE activity seen in our study may be due to the reversible binding of donepezil to the enzyme. Donepezil is well tolerated in humans, and a combination of donepezil and procyclidine may prove useful as an alternative to the currently used prophylaxis against nerve agent intoxication.

Unequal efficacy of pyridinium oximes in acute organophosphate poisoning

The use of organophosphorus pesticides results in toxicity risk to non-target organisms. Organophosphorus compounds share a common mode of action, exerting their toxic effects primarily via acetylcholinesterase (AChE) inhibition. Consequently, acetylcholine accumulates in the synaptic clefts of muscles and nerves, leading to overstimulation of cholinergic receptors. Acute cholinergic crisis immediately follows exposure to organophosphate and includes signs and symptoms resulting from hyperstimulation of central and peripheral muscarinic and nicotinic receptors. The current view of the treatment of organophosphate poisoning includes three strategies, i.e. the use of an anticholinergic drug (e.g., atropine), cholinesterase-reactivating agents (e.g., oximes) and anticonvulsant drugs (e.g., benzodiazepines). Oximes, as a part of antidotal therapy, ensure the recovery of phosphylated enzymes via a process denoted as reactivation of inhibited AChE. However, both experimental results and clinical findings have demonstrated that different oximes are not equally effective against poisonings caused by structurally different organophosphorus compounds. Therefore, antidotal characteristics of conventionally used oximes can be evaluated regarding how close the certain substance is to the theoretical concept of the universal oxime. Pralidoxime (PAM-2), trimedoxime (TMB-4), obidoxime (LüH-6), HI-6 and HLö-7 have all been demonstrated to be very effective in experimental poisonings with sarin and VX. TMB-4 and LüH-6 may reactivate tabun-inhibited AChE, whereas HI-6 possesses the ability to reactivate the soman-inhibited enzyme. An oxime HLö-7 seems to be an efficient reactivator of AChE inhibited by any of the four organophosphorus warfare agents. According to the available literature, the oximes LüH-6 and TMB-4, although relatively toxic, are the most potent to induce reactivation of AChE inhibited by the majority of organophosphorus pesticides. Since there are no reports of controlled clinical trials on the use of TMB-4 in human organophosphate pesticide poisoning, LüH-6 may be a better option.

Anticonvulsant effects of phencynonate hydrochloride and other anticholinergic drugs in soman poisoning: neurochemical mechanisms

Previous studies have paid little attention to the anticonvulsant effect of anticholinergic drugs that act on both muscarinic (M) and nicotinic (N) receptors during soman-induced seizures. Therefore, with the establishment of a soman-induced seizures model in rats, this study evaluated the efficacy in preventing soman-induced convulsions of two antagonists of both the M and N receptors, phencynonate hydrochloride (PCH) and penehyclidine hydrochloride (8018), which were synthesized by our institute, and of other anticholinergic drugs, and investigated the mechanisms of their antiseizures responses. Male rats, previously prepared with electrodes to record electroencephalographic (EEG) activity, were pretreated with the oxime HI-6 (125 mg kg-1, i.p.) 30 min before they were administered soman (180 microg kg-1, s.c.). All animals developed seizures subsequent to this treatment. Different drugs were given at different times (5, 20 and 40 min after seizures onset) and their anticonvulsant effects were monitored and compared using the two variables, i.e. the dose that could totally control the ongoing seizures, as well as the speed of seizures control. The anticonvulsant effects of atropine, scopolamine and 8018 decreased with the progression of the seizures, and they eventually lost their anticonvulsant activity when the seizures had progressed for 40 min. In contrast, PCH showed good anticonvulsant effectiveness at 5 and 20 min, and especially at 40 min after seizures onset. Of the anticholinergic drugs tested, atropine, scopolamine, and 8018 showed no obvious protection against pentylenetetrazol (PTZ)-induced convulsions or N-methyl-D-aspartate (NMDA)-induced lethality in mice. However, PCH antagonized the PTZ-induced convulsions in a dose-dependant manner with an ED50 of 10.8 mg kg-1, i.p. (range of 7.1-15.2 mg kg-1) and partly blocked the lethal effects of NMDA in mice. PCH also dose-dependently inhibited NMDA-induced injury in rat primary hippocampal neuronal cultures, suggesting a possible neuroprotective action in vivo. In conclusion, our study suggests that the mechanisms of PCH action against soman-induced seizures might differ from those of the M receptor antagonists atropine and scopolamine, and that of the antagonist of both the M and N receptors, 8018. The pharmacological profile of PCH might include anticholinergic and anti-NMDA properties. Compared with the currently recommended anticonvulsant drug diazepam, with known NMDA receptor antagonists such as MK-801 and with conventional anticholinergics such as scopolamine and atropine, the potent anticonvulsant effects of PCH during the entire initial 40 min period of soman poisoning, and its fewer adverse effects, all suggest that PCH might serve as a new type of anticonvulsant for the treatment of seizures induced by soman.

Inhibition effects of benactyzine and drofenine on human serum butyrylcholinesterase

Benactyzine and drofenine are widely used anticholinergic drugs. Benactyzine is used to treat organophosphate poisoning and drofenine acts on smooth muscle to stop muscle spasms. Both of these drugs are esters. After they enter the bloodstream, they will interact with butyrylcholinesterase (BChE; acylcholine acyl hydrolase: EC 3.1.1.8), which has an ability to hydrolyze a wide variety of esters. Therefore, the kinetic analysis of their inhibitory effects on human serum BChE was examined using butyrylthiocholine as substrate. Both drugs were competitive inhibitors of BChE and the Ki values of benactyzine and drofenine were calculated to be 0.010 +/- 0.001 and 0.003 +/- 0.000 mM, respectively, using the Systat (version 5.03, 1991) nonlinear regression analysis software package. According to these parameters, drofenine is a more potent competitive inhibitor of BChE than benactyzine.

Magnetic resonance imaging predicts neuropathology from soman-mediated seizures in the rodent

Intoxication by the organophosphate compound soman causes prolonged seizures that lead to neuropathology in the brain. This MRI-based study describes the temporal and spatial evolution of brain pathology that follows soman-induced convulsions. We observed significant decreases in apparent diffusion coefficients (ADC; 23% below control) of the hippocampus and thalamus by 12 h after soman treatment. The ADC then returned to near normal values in all regions at 24 h but declined again during the next 7 days. These data suggest that the initial cellular degradation may be resolved but is ultimately followed by regional cellular remodeling. T2 relaxation values declined significantly at 12 h (37% decrease) returning to near normal values by 24 h. These data lend detail to the model suggesting that injured tissues experience an edematous influx that is resolved by 24 h. The imaging data was fully supported by histopathological comparisons where moderate cell loss and swelling within the hippocampus and piriform cortex was observed. This is the first report providing excellenttemporal and spatial resolution of emerging soman-mediated, seizure-induced neuropathology using MRI with histological correlation.

Effects of Huperzine used as pre-treatment against soman-induced seizures

Huperzine A (HUP), an alkaloid isolated from the Chinese club moss, Huperzia serrata is a reversible inhibitor of cholinesterases which crosses the blood-brain barrier and shows high specificity for acetylcholinesterase (AChE) and a prolonged biological half-life. We tested, in vivo, its efficiency in protecting cortical AChE from soman inhibition and preventing subsequent seizures. The release of acetylcholine (ACh) was also followed in the cortex of freely moving rats using microdialysis techniques. We previously found that soman-induced seizures occurred in rodents only when the cortical AChE inhibition was over 65% and when the increase of ACh level was over 200 times the baseline level. This was verified in the present study in control animals intoxicated by 1 LD50 of soman (90 microg/kg). Using the same dose of soman in rats pre-treated with 500 microg/kg of HUP, we observed that 93% of the animals survived and none of them had seizures. This dose of HUP reduced AChE inhibition to 54% and increase of ACh level to 230 times baseline value. HUP thus appears as a promising compound to protect subjects against organophosphorus intoxication.

Neuropharmacological mechanisms of nerve agent-induced seizure and neuropathology

This paper proposes a three phase "model" of the neuropharmacological processes responsible for the seizures and neuropathology produced by nerve agent intoxication. Initiation and early expression of the seizures are cholinergic phenomenon; anticholinergics readily terminate seizures at this stage and no neuropathology is evident. However, if not checked, a transition phase occurs during which the neuronal excitation of the seizure per se perturbs other neurotransmitter systems: excitatory amino acid (EAA) levels increase reinforcing the seizure activity; control with anticholinergics becomes less effective; mild neuropathology is occasionally observed. With prolonged epileptiform activity the seizure enters a predominantly non-cholinergic phase: it becomes refractory to some anticholinergics; benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists remain effective as anticonvulsants, but require anticholinergic co-administration; mild neuropathology is evident in multiple brain regions. Excessive influx of calcium due to repeated seizure-induced depolarization and prolonged stimulation of NMDA receptors is proposed as the ultimate cause of neuropathology. The model and data indicate that rapid and aggressive management of seizures is essential to prevent neuropathology from nerve agent exposure.

The stoichiometry of protection against soman and VX toxicity in monkeys pretreated with human butyrylcholinesterase

Bioscavengers of organophophates (OP) have been examined as potential substitutes for the currently approved drug treatment against OP toxicity. The present work was designed to assess the ability of butyrylcholinesterase, purified from human serum (HuBChE), to prevent the toxicity induced by soman and VX in rhesus monkeys. The consistency of the data across species was then evaluated as the basis for the extrapolation of the data to humans. The average mean residence time of the enzyme in the circulation of monkeys following an intravenous loading was 34 hr. High bioavailability of HuBChE in blood (>80%) was demonstrated after intramuscular injection. A molar ratio of HuBChE:OP approximately 1.2 protected against an i.v. bolus injection of 2.1 x LD50 VX, while a ratio of 0.62 was sufficient to protect monkeys against an i.v. dose of 3.3 x LD50 of soman, with no additional postexposure therapy. A remarkable protection was also seen against soman-induced behavioral deficits detected in the performance of a spatial discrimination task. The consistency of the results across several species offers a reliable prediction of both the stoichiometry of the scavenging and the extent of prophylaxis with HuBChE against nerve agent toxicity in humans.

Prevention of soman-induced cognitive deficits by pretreatment with human butyrylcholinesterase in rats

This study examined the ability of pretreatment with human serum butyrylcholinesterase (HuBChE) to prevent soman-induced cognitive impairments. Behavioral testing was carried out using the Morris water maze task evaluating learning, memory, and reversal learning processes. Pretreatment with HuBChE significantly prevented the memory and reversal learning impairments induced by soman. A small deficiency in performance was observed only during part of the learning period in HuBChE-treated rats after administration of soman. Results support the contention that pretreatment alone with HuBChE is sufficient to increase survival and to prevent impairment in cognitive functioning following exposure to soman.

Human butyrylcholinesterase as a general prophylactic antidote for nerve agent toxicity. In vitro and in vivo quantitative characterization

Butyrylcholinesterase purified from human plasma (HuBChE) was evaluated both in vitro and in vivo in mice and rats as a single prophylactic antidote against the lethal effects of highly toxic organophosphates (OP). The variation among the bimolecular rate constants for the inhibition of HuBChE by tabun, VX, sarin, and soman was 10-fold (0.47 to 5.12 x 10(7) M-1 min-1; pH 8.0, 26 degrees). The half-life of HuBChE in blood after its i.v. administration in mice and rats was 21 and 46 hr, respectively. The peak blood-enzyme level was obtained in both species approximately 9-13 hr following i.m. injection of HuBChE, and the fraction of the enzyme activity absorbed into the blood was 0.9 and 0.54 for rats and mice, respectively. The stoichiometry of the in vivo sequestration of the anti-cholinesterase toxicants was consistent with the HuBChE/OP ratio of the molar concentration required to inhibit 100% enzyme activity in vitro. Linear correlation was demonstrated between the blood level of HuBChE and the extent of protection conferred against the toxicity of nerve agents. Pretreatment with HuBChE alone was sufficient not only to increase survivability following exposure to multiple median lethal doses of a wide range of potent OPs, but also to alleviate manifestation of toxic symptoms in mice and rats without the need for additional post-exposure therapy. It appeared that in order to confer protection against lethality nerve agents had to be scavenged to a level below their median lethal dose LD50 within less than one blood circulation time. Since the high rate of sequestration of nerve agents by HuBChE is expected to underlie the activity of the scavenger in other species as well, a reliable extrapolation of its efficacy from experimental animals to humans can be made.

Effects of anticholinergic-antiparkinsonian drugs on striatal neurotransmitter levels of rats intoxicated with soman

Antimuscarinic drugs possessing antiparkinson activity that were effective in preventing convulsions induced by the organophosphorus cholinesterase (ChE) inhibitor soman were studied for their effects on spinal cord ChE activity and striatal levels of acetylcholine (ACh) and catecholamines in soman-intoxicated rats. Either biperiden (BPR) or trihexyphenidyl (THP) was administered to rats at an anticonvulsant dose (0.125 mg/kg, IM) in the presence or absence of soman (100 micrograms/kg, SC). The time course (up to 2 h) for ChE activity and levels of ACh and catecholamines were measured after soman, BPR, THP, soman and BPR, or soman and THP treatment. Soman rapidly inhibited ChE activity (65-75%; 15-120 min) and increased ACh levels (35%; at 30 min). It did not affect norepinephrine or dopamine (DA), but elevated at later time points (60-120 min) levels of the DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), thus indicating increased DA turnover. BPR and THP alone reduced striatal ACh level from control, but did not affect any other neurochemical parameters studied. THP and BPR each reversed the effects of soman on DOPAC and HVA levels, but neither affected ChE activity nor ACh level induced by soman. Thus, our findings suggest that the anticonvulsant effects of BPR and THP in soman poisoning may be attributed to their earlier reported muscarinic receptor blocking properties.

Influence of medial septal cholinoceptive cells on c-Fos-like proteins induced by soman

The effects of intraseptal application of atropine on c-fos proto-oncogene expression related to soman treatment were studied by immunohistochemistry for c-Fos-like proteins. In control rats, 2 h after the onset of convulsion, c-Fos-like immunoreactivity was intense in the piriform and entorhinal cortices, but also in the cingulate, frontoparietal and retrosplenial cortices. In addition, the staining was moderate in the hypothalamus, amygdala and fascia dentata. The intraseptal application of atropine, which prevented soman-induced convulsions, reduced or even blocked c-Fos-like protein production related to soman treatment. This inhibition of Fos induction was significant in most of the limbic structures but also in non-limbic areas. The data in this study strongly suggest that the cholinergic cells of the medial septal area play a key role in soman-induced seizures, and confirm that c-Fos-like protein induction is closely related to neuronal hyperactivity.

Effects of paraldehyde on the convulsions induced by administration of soman in rats

The ability of paraldehyde, a potent central nervous system depressant, to prevent the convulsions induced by the organophosphate soman, an irreversible inhibitor of acetylcholinesterase, was studied in rats. Paraldehyde (0.1-500 mg/kg, im) administered 10 min before soman (100 micrograms/kg, sc) did not protect against seizures. Co-administered with atropine sulfate (10 mg/kg, im), paraldehyde produced a clear dose-dependent anticonvulsant response. Although this pre-treatment could delay the occurrence of death, it did not produce any change in the soman-induced 24 h mortality rate. Thus, co-administration of paraldehyde and atropine sulfate might constitute a valuable tool to be used against the convulsant consequences of soman poisoning. However, supplementary pre-medication, in addition to paraldehyde and atropine sulfate, remains necessary to improve the antilethal capacity of the pre-treatment.

Cerebral blood flow and metabolism in soman-induced convulsions

Regional cerebral blood flow (CBF) and regional cerebral glucose utilization (CGU) were studied by quantitative autoradiographic techniques in rats. Animals were treated either with a toxic dose of soman, an irreversible organophosphorus cholinesterase inhibitor, that produced convulsions or with saline as controls. An increased arterial blood pressure (mean increase = 41% of control) always preceded onset of convulsions. Convulsive activity was associated with an increase of plasma glucose concentration and marked increases over controls of CGU [average of all regions: control = 75 +/- 5 mumol.100 g-1.min-1, n = regions/animals (304/8); seizures = 451 +/- 20 mumol.100 g-1.min-1, n = 190/5] and CBF [average of all regions: control = 135 +/- 6 ml.100 g-1.min-1, n = 190/5; seizures = 619 +/- 29 ml.100 g-1.min-1, n = 190/5). Regional distribution of these effects revealed a greater proportional increase of CBF over CGU in cingulate, motor, and occipital cortex and caudate-putamen. In contrast, a lower proportional increase of CBF over CGU in CA3 region of hippocampus, dentate gyrus, medial thalamus, and substantia nigra was observed, implying the existence of a relative ischemia in these brain areas. These findings may be relevant to the pathogenesis of brain lesions associated with soman-induced convulsions.

Prophylactic and therapeutic efficacy of memantine against seizures produced by soman in the rat

Male Sprague-Dawley rats injected sc with a single sublethal dose of the organophosphate nerve agent, soman (100 micrograms/kg), had motor limbic seizures within 5-15 min. Pretreatment with a single dose of memantine HCl (MEM, 18 mg/kg, sc), alone or in combination with atropine sulfate (ATS, 16 mg/kg, sc), before soman prevented seizures without sedation or ataxia. Rats appeared normal or demonstrated increased exploratory activity. Excessive salivation, a peripheral manifestation of soman intoxication, was decreased by ATS, but pretreatment with ATS alone did not prevent seizures. After seizure onset, MEM +/- ATS, but not ATS, abolished seizures. Acetylcholinesterase (AChE) activity in several brain regions (cortex, stem, striatum, and hippocampus) was markedly reduced by soman, but not by MEM, ATS, or MEM + ATS. Preadministration of MEM + ATS in vivo significantly protected AChE from inhibition by soman. Memantine reduced inhibition of AChE activity in crude brain homogenates by soman, but not by edrophonium (anionic site inhibitor) or decamethonium (peripheral site inhibitor). Thus, MEM may bind to a different modulatory site, not yet characterized, to protect AChE. When given after onset of soman-induced seizures, treatment with MEM +/- ATS did not reactivate AChE although seizures were controlled, suggesting additional anticonvulsant mechanisms of action. At concentrations (10(-4) to 5 x 10(-4) M) which did not significantly alter the spontaneous firing of action potentials (APs), MEM limited sustained high frequency repetitive firing (SRF) induced by depolarization of spinal cord (mouse and rat) and neocortical (mouse) neurons in monolayer-dissociated cell culture. In the same range of concentrations, ATS both limited SRF and suppressed spontaneous activity, suggesting toxicity. In addition, MEM and ATS reversibly produced use-dependent block of depolarizing responses to acetylcholine (ACh) applied by pressure ejection to spinal cord neurons. Thus, the anticonvulsant efficacy of MEM, with or without ATS, may have resulted from a combination of actions, including protection of AChE from inhibition by soman, limitation of high frequency firing of APs, and blockade of excitatory postsynaptic responses to ACh.

Anticonvulsants for poisoning by the organophosphorus compound soman: pharmacological mechanisms

Exposure to high doses of organophosphorus nerve agents such as soman, even with carbamate pretreatment, produces a variety of toxic cholinergic signs, including secretions, convulsions and death. Evidence suggests that soman-induced convulsions may be associated with postexposure brain neuropathology. The purpose of this study was to investigate the pharmacologic mechanism of action of soman-induced convulsions and of anticonvulsant drugs. Various classes of compounds were evaluated for their efficacy in preventing soman-induced convulsions in rats pretreated with the oxime HI-6 to increase survival time, along with various doses of the test compounds (IM) either in the absence or presence of atropine sulfate (16 mg/kg, IM) 30 minutes prior to a soman challenge dose (180 micrograms/kg, SC; equivalent to 1.6 x LD50) that produced 100% convulsions. Without atropine sulfate, only tertiary anticholinergics (scopolamine, trihexyphenidyl, biperiden, benactyzine, benztropine, azaprophen and aprophen), caramiphen, carbetapentane and MK-801 were effective anticonvulsants. In the presence of atropine sulfate, the benzodiazepines (diazepam, midazolam, clonazepam, loprazolam and alprazolam), mecamylamine, flunarizine, diphenylhydantoin, clonidine, CGS 19755 and Organon 6370 studied were effective. We have examined the possibility that diazepam may exert some of its anticonvulsant effects through cholinergic mechanisms and found that a reduced release of ACh into synapses after diazepam and atropine treatment may account for diazepam's anticonvulsant activity against soman. We also found that at anticonvulsant doses biperiden and trihexyphenidyl each significantly reversed the effects of soman on striatal levels of DOPAC and HVA, the metabolites of dopamine, and have concluded that in addition to actions on muscarinic receptors, the anticonvulsant effects of these anticholinergics in soman poisoning may be partially related to their actions on the striatal dopaminergic system. These findings allow us to postulate that central muscarinic cholinergic mechanisms are primarily involved in eliciting the convulsions following exposure to soman and that subsequent recruitment of other excitatory neurotransmitter systems and loss of inhibitory control may be responsible for sustaining the convulsions and for producing the subsequent brain damage. Future studies to confirm these neuropharmacological mechanisms are proposed.