Protection against soman-induced seizures in rats: relationship among doses of prophylactics, soman, and adjuncts

The combined effects of physostigmine and procyclidine (antagonizing muscarinic, nicotinic, and NMDA receptors) were tested against various doses of soman. Physostigmine (0.1 mg/kg) in combination with procyclidine doses of 1, 3, or 6 mg/kg effectively prevented the development of convulsions and hippocampally monitored seizures when the doses of soman were 1.3, 1.6, or 2 × LD50, respectively. Results from [³H]MK-801-binding experiments showed that procyclidine inhibits the phencyclidine site at the NMDA receptor in a concentration-dependent manner. Physostigmine (0.1 mg/kg) and procyclidine in a dose of 1 mg/kg did not prevent convulsions or seizures when the soman dose was 1.6 × LD50. Subsequent treatment with scopolamine in doses of 0.5 or 1 mg/kg immediately after (3 min) seizure onset showed that only the highest dose produced a reliable termination. When scopolamine (1 mg/kg) was given later (10 min) after onset of seizures, no effect was obtained. The sustained seizures were subsequently treated with diazepam (10 mg/kg) and pentobarbital (30 mg/kg) and finally terminated 25 min after onset. In rats given inadequate prophylaxis, both modified convulsions and seizures were seen. It is suggested that moderate doses of prophylactics should be preferred to avoid adverse effects on cognitive functions because insufficient prophylaxis can be compensated for by adjunct treatment.     

Two medical therapies very effective shortly after high levels of soman poisoning in rats,but only one with universal utility

A treatment regimen consisting of HI-6, scopolamine, and physostigmine (termed the physostigmine regimen) has been based on the serendipitous discovery that it exerts powerful antidotal effects against high levels of soman poisoning if it is administered 1 min after exposure. A medical therapy with corresponding efficacy, but without the time limitation of the latter regimen, has been developed through studies of microinfusions of anticonvulsants into seizure controlling sites in the forebrain of rats. From these studies procyclidine emerged as the most potent anticonvulsant, and its potency was further enhanced when being combined with the antiepileptic levetiracetam during systemic administration. In the present study, the capacity of HI-6, levetiracetam, and procyclidine (termed the procyclidine regimen) was tested against that of the physostigmine regimen. The results showed that both regimens were very effective against supralethal doses of soman (3, 4, 5 × LD₅₀) when given 1 and 5 min after intoxication. When the treatments were administered 10 and 14 or 20 and 24 min after soman exposure, only the procyclidine regimen was able to terminate seizures and preserve lives. When used as prophylactic therapies, both regimens protected equally well against seizures, but only the procyclidine regimen provided neuroprotection. The procyclidine regimen has apparently capacities to serve as a universal therapy against soman intoxication in rats.     

Assessment of a combination of physostigmine and scopolamine as pretreatment against the behavioural effects of organophosphates in the common marmoset (<Emphasis Type="Italic">Callithrix jacchus</Emphasis>)

Rationale. There is a requirement to ensure that UK armed forces are provided with the best possible medical countermeasures to prevent or mitigate the effects of exposure to nerve agents. When pretreatments are under consideration, it is of particular importance to ensure that they do not in themselves give rise to adverse effects and do not exacerbate the effects of agent exposure. Objectives. The present study was designed to address these considerations for a combination of physostigmine and scopolamine as a potential pretreatment regimen. Methods. Common marmosets were trained to perform a two-choice discrimination serial reversal task, and baseline data were collected. Subjects received a dose of either soman or sarin after 2 weeks of pretreatment with either saline or physostigmine and scopolamine via miniosmotic pump. Results. No effects of physostigmine and scopolamine were seen on task accuracy or response rates. Neither accuracy of reversal performance nor number of responses made were significantly changed by administration of either soman or sarin subsequent to pretreatment with physostigmine/scopolamine. In the groups pretreated with saline, performance of the behavioural task, in terms of responses made, was virtually abolished on the day the OP was administered, but a significant increase in accuracy of performance was seen over the 2- to 14-day period following administration. Conclusions. A combination of physostigmine and scopolamine, which is known to protect against nerve-agent lethality, offers protection against the effects of soman and sarin on behavioural performance, as measured by a discrimination reversal task. The improved performance observed following nerve agent requires further investigation. Electronic Publication     

Protection of guinea pigs against soman poisoning with ferrocene carbamate

The protective effect of ferrocene carbamate pretreatment against soman poisoning was studied in guinea pigs. At doses corresponding to 1/20 x and 1/10 × LD₅₀ of this carbamate a 20% and 45% decrease of the acetylcholinesterase in blood and brain, respectively, was obtained. In combination with additional pretreatment, diazepam, and therapy, HI-6 and atropine, the protective ratios (LD₅₀ of soman in treated animals/LD₅₀ of soman in untreated animals) were around 20 and 40, respectively. Animals pretreated with the high dose of the ferrocene carbamate that survived 10 x and 15xLD_50s of soman showed no remaining signs of poisoning after 24 h. Thus, the ferrocene carbamate afforded a better protection against soman than physostigmine. The explanation for this could be due to the properties of the ferrocene carbamate, not correlated to its cholinesterase inhibiting activity. This hypothesis is discussed.     

Minimum effective drug concentrations of a transdermal patch system containing procyclidine and physostigmine for prophylaxis against soman poisoning in rhesus monkeys

A transdermal patch system containing procyclidine, an N-methyl-d-aspartate receptor antagonist possessing anticholinergic action, and physostigmine, a reversible cholinesterase inhibitor, was developed, and its prophylactic efficacy against soman intoxication was investigated. Male rhesus monkeys were shaved on the dorsal area, attached with a matrix-type patch with various sizes (2 × 2 to 7 × 7 cm) for 24 or 72 h, and challenged with 2 × LD₅₀ doses (13 μg/kg) of soman. The smallest patch size for the protection against lethality induced by soman intoxication was 3 × 3 cm, resulting in blood procyclidine concentration of 10.8 ng/ml, blood physostigmine concentration of 0.54 ng/ml, which are much lower concentrations than maximum sign-free doses, and blood cholinesterase inhibition of 42%. The drug concentrations and enzyme inhibition rate corresponding to a diverging point of survivability were presumably estimated to be around 7 ng/ml for procyclidine, 0.35 ng/ml for physostigmine, and 37% of enzyme inhibition. Separately, in combination with the patch treatment, the post treatment consisting of atropine (0.5 mg/kg) plus 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 50 mg/kg) exerted protection against 5 × LD₅₀ challenge of soman, which means the posttreatment remarkably augmented the efficacy of the patch. Additionally, it was found that brain injuries induced by soman toxicity were effectively prevented by the patch treatment according to histopathological examinations. These results suggest that the patch system could be an effective alternative for diazepam, an anticonvulsant, and the current pyridostigmine pretreatment, and especially in combination with atropine plus HI-6, could be a choice for quality survival from nerve-agent poisoning.     

Potentiation effects of (+/-)huprine X, a new acetylcholinesterase inhibitor, on nicotinic receptors in rat cortical synaptosomes

The present experiments were developed to analyze the direct and/or potentiation effect of (+/-)-12-amino-3-chloro-9-ethyl-6,7,10,11-tetrahydro-7,11-methanocycloocta[b]quinoline hydrochloride ((+/-)huprine X) on nicotinic receptors using a synaptosomal superfusion method. (+/-)Huprine X (1 microM, 10 microM) increased [(3)H]-ACh release only at 10 microM (46%; P < 0.001) in basal, but not in stimulated, conditions. This effect was completely reverted by mecamylamine (100 microM; MEC). Potentiation of evoked-[3H]-ACh release induced by ACh (1 microM) and by galantamine (GAL) 0.4 microM and physostigmine (PHY) 10 microM (55% and 50%, respectively; P < 0.001), two well-known allosteric compounds, corroborate that the present experimental approach is a suitable method to study potentiation effects on nicotinic receptors in the central nervous system nerve terminals. (+/-)Huprine X potentiated the evoked-[3H]-ACh release induced by ACh (1 microM) by 166% and 90% (P < 0.001) at 10 microM and 30 microM, respectively, and this effect was completely blocked by MEC (100 microM). In the presence of different ACh concentrations, (+/-)huprine X 10 microM potentiated evoked-[3H]-ACh release at low ACh concentrations, while a decrease in neurotransmitter release was observed at high ACh concentrations. The highest potentiation effect was obtained at the ACh/(+/-)huprine X concentration ratio of 1:10, and this potentiation was observed at as low a (+/-)huprine X concentration as 0.1 microM (P < 0.05). While the results suggest that huprine may enhance the potency or effectiveness of ACh by an effect involving nicotinic receptors we cannot completely discard that the results could be explained by acetylcholine esterase inhibition.     

Effectiveness of 1,2,3,4-Tetrahydro-9-aminoacridine (THA) as a pretreatment drug for protection of mice from acute diisopropylfluorophosphate (DFP) intoxication

The protective action of 1,2,3,4-tetrahydro-9-aminoacridine (THA) against acute diisopropylfluorophosphate (DFP) intoxication was evaluated in mice by measuring the effects of the pretreatment of the animals with various doses of the drug on the DFP LD₅₀. In the same experiments, the action of physostigmine and pyridostigmine were compared. THA at the doses 2.5, 5 and 7.5 mg/kg injected subcutaneously 15 min before DFP caused a dose-dependent increase in the DFP LD₅₀, resulting in protection ratios equal to 3, 3.1 and 4.4, respectively, in the absence of atropine and 4.5, 8.6 and 14.5, respectively, in the presence of atropine sulfate (17.4 mg/kg) therapy. Under the same experimental conditions, the protective ratios of 0.1 mg/kg physostigmine and pyridostigmine were 2.2 and 1.3, respectively, without atropine and 11.0 and 12.2, respectively, with atropine. The effectiveness of THA antidotal effect was inversely correlated to the time between pretreatment and DFP administration, being maximal when THA was injected 15 min before poisoning. In separate experiments, the time-course of acetylcholinesterase (AChE; EC 3.1.1.7) activity recovery was evaluated in the whole brain and diaphragm tissues of mice pretreated with THA (5 mg/kg) and physostigmine (0.1 mg/kg) 15 min before poisoning with DFP (8 mg/kg). At 10 min after DFP administration residual AChE activity in the brain averaged 4, 25 and 15% of that in controls in the animals pretreated with atropine alone, atropine plus THA or atropine plus physostigmine, respectively. At 24 h after poisoning, brain AChE activity averaged 34 and 47% of that in controls in the mice protected by THA and physostigmine, respectively. As for the diaphragm, AChE activity in THA-pretreated animals was 29% of controls 10 min after poisoning versus 8 and 23% in unprotected and physostigmine-pretreated animals, respectively. At 24 h after poisoning, diaphragm AChE activities averaged 56 and 53% of controls in THA- and physostigminetreated animals, respectively. From these experiments it is concluded that THA exerts a considerable protective action against acute DFP-poisoning in the mouse, presumably by protecting neuronal AChE from irreversible inactivation by this organophosphorus agent.     

Evaluation of oxime efficacy in nerve agent poisoning: development of a kinetic-based dynamic model

The widespread use of organophosphorus compounds (OP) as pesticides and the repeated misuse of highly toxic OP as chemical warfare agents (nerve agents) emphasize the necessity for the development of effective medical countermeasures. Standard treatment with atropine and the established acetylcholinesterase (AChE) reactivators, obidoxime and pralidoxime, is considered to be ineffective with certain nerve agents due to low oxime effectiveness. From obvious ethical reasons only animal experiments can be used to evaluate new oximes as nerve agent antidotes. However, the extrapolation of data from animal to humans is hampered by marked species differences. Since reactivation of OP-inhibited AChE is considered to be the main mechanism of action of oximes, human erythrocyte AChE can be exploited to test the efficacy of new oximes. By combining enzyme kinetics (inhibition, reactivation, aging) with OP toxicokinetics and oxime pharmacokinetics a dynamic in vitro model was developed which allows the calculation of AChE activities at different scenarios. This model was validated with data from pesticide-poisoned patients and simulations were performed for intravenous and percutaneous nerve agent exposure and intramuscular oxime treatment using published data. The model presented may serve as a tool for defining effective oxime concentrations and for optimizing oxime treatment. In addition, this model can be useful for the development of meaningful therapeutic animal models.     

Subchronic physostigmine pretreatment in marmosets: absence of side effects and effectiveness against soman poisoning with negligible postintoxication incapacitation.

Subchronic pretreatment with physostigmine (PHY) (0.0125 mg/kg/h) leading to a blood acetylcholinesterase inhibition of about 30% caused no side effects when applied to marmoset monkeys. This was evident on behavioral parameters and on EEG and cortical visual evoked response. Furthermore, this treatment regime, followed by atropine as postintoxication therapy, protected the marmosets against lethality after a 2 x LD50 dose of soman with negligible postintoxication incapacitation. These findings suggest that a symptom-free pretreatment with subchronic PHY could protect man sufficiently against severe soman intoxication.     

Stimulation of central A1 adenosine receptors suppresses seizure and neuropathology in a soman nerve agent seizure rat model

The current regimen for treating nerve agent poisoning does not sufficiently suppress the excitotoxic activity that causes severe brain damage, especially in cases where treatment is delayed and nerve agent-induced status epilepticus develops. New therapeutic targets are required to improve survivability and minimize neuropathology after irreversible acetylcholinesterase inactivation. Earlier studies have shown that systemic delivery of adenosine agonists decreases nerve agent lethality; however, the mechanism of protection remains to be understood. The primary aim of this study was to investigate the role of central adenosine receptor (AR) stimulation in neuroprotection by directly injecting (6)-cyclopentyladenosine (CPA), an adenosine agonist specific to the A1 receptor subtype (A1R), into the brain intracerebroventricularly (ICV) in a soman seizure rat model. In addition to general A1R stimulation, we hypothesized that bilateral micro-injection of CPA into the cholinergic basal forebrain (BF) could also suppress excitotoxic activity. The results from these studies demonstrated that centrally administered adenosine agonists are anti-seizure and neuroprotective. CPA-delivered ICV prevented seizure and convulsion in 100% of the animals. Moreover, neuropathological evaluation indicated that adenosine treatments reduced brain damage from severe to minimal. Inhibition of the BF via CPA had varied results. Some animals were protected by treatment; however, others displayed similar pathology to the control. Overall, these data suggest that stimulating central ARs could be an effective target for the next generation countermeasures for nerve agent intoxication.     

Acute poisoning with a glyphosate-surfactant herbicide ('Roundup'): a review of 93 cases.

Between 1 January 1980, and 30 September 1989, 93 cases of exposure to herbicides containing glyphosphate and surfactant ('Roundup') were treated at Changhua Christian Hospital. The average amount of the 41% solution of glyphosate herbicide ingested by non-survivors was 184 +/- 70 ml (range 85-200 ml), but much larger amounts (500 ml) were reported to have been ingested by some patients and only resulted in mild to moderate symptomatology. Accidental exposure was asymptomatic after dermal contact with spray (six cases), while mild oral discomfort occurred after accidental ingestion (13 cases). Intentional ingestion (80 cases) resulted in erosion of the gastrointestinal tract (66%), seen as sore throat (43%), dysphagia (31%), and gastrointestinal haemorrhage (8%). Other organs were affected less often (non-specific leucocytosis 65%, lung 23%, liver 19%, cardiovascular 18%, kidney 14%, and CNS 12%). There were seven deaths, all of which occurred within hours of ingestion, two before the patient arrived at the hospital. Deaths following ingestion of 'Roundup' alone were due to a syndrome that involved hypotension, unresponsive to intravenous fluids or vasopressor drugs, and sometimes pulmonary oedema, in the presence of normal central venous pressure.     

Anticholinesterase (DFP) toxicity antagonism by chronic donepezil: a potential nerve agent treatment

Animal studies exploring the antagonism of irreversible cholinesterase inhibitors (i.e. nerve agents) such as soman and sarin have shown that pretreatment with the reversible centrally acting cholinesterase inhibitor, physostigmine, alone or in conjunction with the centrally acting anticholinergic drug, scopolamine, antagonizes the lethality and toxicity of these agents. This study evaluated the effects of pretreatment with the oral cholinesterase inhibitor and anti-Alzheimer's agent, donepezil (Aricept) on the hypokinetic, hypothermic and diarrhea-inducing effects of the irreversible long-acting cholinesterase inhibitor, diisopropylfluorophosphate (DFP) in adult Sprague-Dawley rats. Donepezil (2 mg/kg), given acutely (30 min pretreatment) or chronically (10 daily treatments), significantly antagonized the hypothermia, hypoactivity and diarrhea induced by DFP (1.25 mg/kg) administration. The effects were most prominent 4 and 6 h after the injection of DFP and some protection was observed even when the last treatment of the chronic donepezil protocol was given 24 h before the DFP injection. Although these phenomena are not the same as lethality, they may be parallel phenomena, and our results may have therapeutic implications for the treatment of nerve agent toxicity.     

Pharmacogenetics of paraoxonases: a brief review

The human paraoxonase (PON) gene family consists of three members, PON1, PON2, and PON3, aligned next to each other on chromosome 7. By far the most-studied member of the family is the serum paraoxonase 1 (PON1), a high-density lipoprotein-associated esterase/lactonase. Early research focused on its capability to hydrolyze toxic organophosphates, and its name derives from one of its most commonly used in vitro substrates, paraoxon. Studies in the last 2 decades have demonstrated PON1's ability to protect against atherosclerosis by hydrolyzing specific derivatives of oxidized cholesterol and/or phospholipids in oxidized low-density lipoprotein and in atherosclerotic lesions. Levels and genetic variability of PON1 influence sensitivity to specific insecticides and nerve agents, as well as the risk of cardiovascular disease. More recently, the other two members of the PON family, PON2 and PON3, have also been shown to have antioxidant properties. A major goal in present research on the paraoxonases is to identify their natural substrates and to elucidate the mechanism(s) of their catalytic activities.     

Cardiorespiratory function in nerve agent poisoned and oxime+atropine treated guinea-pigs: effect of pyridostigmine pretreatment

The effect of pyridostigmine pretreatment on the efficacy of oxime+atropine treatment in sarin and VX poisoning was investigated in a guinea-pig model with on-line monitoring of respiratory and circulatory parameters. The carotid artery, jugular vein and trachea were cannulated in female urethane-anesthetized Pirbright-white guinea-pigs. After baseline measurements the animals received pyridostigmine (PYR, 0.05 μmol/kg, i.v.), 30 min later sarin (100 or 200 μg/kg=5 or 10xLD₅₀, i.v.) or VX (45 or 90 μg/kg=10 or 20xLD₅₀, i.v.), followed 2 min later by atropine (10 mg/kg, i.v.) plus HI 6 or HLö 7 (30 μmol/kg each, i.v.). Sixty-minute survival time and rate and respiratory and circulatory parameters were recorded. Diaphragm acetylcholinesterase (AChE) activity was determined spectrophotometrically. Identical groups without PYR were included for comparison. With regard to survival time and rate, PYR pretreatment slightly improved the efficacy of HI 6 plus atropine in sarin 5xLD₅₀ poisoned animals, reduced the efficacy of oxime+atropine treatment in the other sarin groups and had no effect in VX poisoning. Compared to nonpretreated oxime+atropine groups, PYR slightly improved respiratory function in sarin and in VX poisoning (only HI 6). PYR did not affect circulatory function in VX poisoning but reduced circulatory parameters in sarin poisoning to varying extent’s. The oxime efficacy in reactivating diaphragm AChE decreased in the order sarin > VX without significant differences between pretreated and non-pretreated groups. The data suggest that pyridostigmine pretreatment does not enhance the efficacy of oxime+atropine in sarin or VX poisoning.     

Polydrug use (the use of drugs in combination): A brief review

This brief review provides an overview of this topic; it is not a critical review. Polydrug use is a poorly defined concept; it is taken here to mean the simultaneous use of more than one drug. Data on polydrug use is seldom systematically collected, and the use of alcohol and tobacco in combination with illegal drugs is frequently overlooked. If alcohol and tobacco are included, most users are polydrug users, since these drugs are used by most users of illegal drugs, and many people with alcohol problems also use illegal drugs. Polydrug use has increased; drug availability, cultural context and the 'normalization' of drug use are important factors. Gender and racial differences are observed in the prevalence of polydrug use. Implications for drug prevention include the importance of targeting multiple substance use rather than single substances. Aiming prevention campaigns at younger people and focusing on 'gateway' drugs might be more effective. But polydrug users are diverse, and a variety of strategies (aimed at different types of polydrug users) are necessary. Implications for treatment include the evidence of the greater difficulty that polydrug users have in quitting; they may therefore require additional help. A fuller, systematic, review is required; it is expected that this would recommend the commissioning of additional research on this poorly understood phenomenon.     

Polydrug use (the use of drugs in combination): A brief review

This brief review provides an overview of this topic; it is not a critical review. Polydrug use is a poorly defined concept; it is taken here to mean the simultaneous use of more than one drug. Data on polydrug use is seldom systematically collected, and the use of alcohol and tobacco in combination with illegal drugs is frequently overlooked. If alcohol and tobacco are included, most users are polydrug users, since these drugs are used by most users of illegal drugs, and many people with alcohol problems also use illegal drugs. Polydrug use has increased; drug availability, cultural context and the ‘normalization’ of drug use are important factors. Gender and racial differences are observed in the prevalence of polydrug use. Implications for drug prevention include the importance of targeting multiple substance use rather than single substances. Aiming prevention campaigns at younger people and focusing on ‘gateway’ drugs might be more effective. But polydrug users are diverse, and a variety of strategies (aimed at different types of polydrug users) are necessary. Implications for treatment include the evidence of the greater difficulty that polydrug users have in quitting; they may therefore require additional help. A fuller, systematic, review is required; it is expected that this would recommend the commissioning of additional research on this poorly understood phenomenon.     

Acute soman poisoning in primates neither pretreated nor receiving immediate therapy: value of gacyclidine (GK-11) in delayed medical support

Organophosphorus (OP) nerve agents are still used as warfare and terrorism compounds. Classical delayed treatment of victims of organophosphate poisoning includes combined i.v. administration of a cholinesterase reactivator (an oxime), a muscarinic cholinergic receptor antagonist (atropine) and a benzodiazepine anticonvulsant (diazepam). The objective of this study was to evaluate, in a realistic setting, the therapeutic benefit of administration of GK-11 (gacyclidine), an antiglutamatergic compound, as a complement to the above therapy against organophosphate poisoning. Gacyclidine was injected (i.v.) in combination with atropine/diazepam/pralidoxime at man-equivalent doses after a 45- or 30-min latency period to intoxicated primates (2 LD₅₀). The effects of gacyclidine on the animals' survival, electroencephalographic (EEG) activity, signs of toxicity, recovery after challenge and central nervous system histology were examined. The present data demonstrated that atropine/diazepam/pralidoxime alone or combined with gacyclidine did not prevent signs of soman toxicity when treatment was delayed 45 min after poisoning. Atropine/diazepam/pralidoxime also did not control seizures or prevent neuropathology in primates exhibiting severe signs of poisoning when treatment was commenced 30 min after intoxication. However, in this latter case, EEG recordings revealed that additional treatment with gacyclidine was able to stop soman-induced seizures and restore normal EEG activity. This drug also totally prevented the neuropathology observed 5 weeks after soman exposure in animals treated with atropine/diazepam/pralidoxime alone. Overall, in the case of severe OP-poisoning, gacyclidine represents a promising adjuvant therapy to the currently available polymedication to ensure optimal management of organophosphate poisoning in man. This drug is presently being evaluated in a human clinical trial for a different neuroprotective indication. However, it should always be kept in mind that, in the case of severe OP-poisoning, medical intervention must be conducted as early as possible. Received: 30 November 1998 / Accepted: 12 January 1999     

Changes of protein oxidation, calpain and cytoskeletal proteins (alpha tubulin and pNF-H) levels in rat brain after nerve agent poisoning

Highly toxic organophosphorus (OP) nerve agents, sarin and soman act by inhibiting acetylcholinesterase (AChE) function at neuronal synapses and cause many toxic effects including death within minutes. The effect of nerve agents on protein oxidation, calpain, and cytoskeletal protein levels was not well known. In the present study we investigated these parameters after subcutaneous injection of sarin (120 μg/kg) and soman (80 μg/kg) in the rat brain. Results indicate that several rat brain proteins were intensely oxidized after nerve agent poisoning. Immunoreactivity levels of μ-calpain were significantly elevated in cerebral cortex and cerebellum regions of rat brain from 2.5 h to 30 days. Alpha tubulin levels reduced from 1 to 7 days in the supernatant and 1 to 3 days in the pellet fractions of cerebellum and cerebral cortex, where as phosphorylation of high molecular weight neurofilament (pNF-H) was increased significantly in nerve agent intoxicated rat brains as compared to control rats. AChE activity was inhibited up to 3 days after nerve agent exposure in plasma and brain. Results suggest that altered protein oxidation, calpain and cytoskeletal protein levels are due to multiple mechanisms of nerve agents actions and these changes might be involved in nerve agent induced complex neurotoxicity.     

The 'ABC' of GABA receptors: a brief review

1. In the mammalian central nervous system, GABA is the main inhibitory neurotransmitter. GABA is a highly flexible molecule and, thus, can exist in many low-energy conformations. Conformationally restricted analogues of GABA have been used to help identify three major GABA receptors, termed GABAA, GABAB and GABAC receptors. 2. GABAA and GABAC receptors are members of a super-family of transmitter-gated ion channels that include nicotinic acetylcholine, strychnine-sensitive glycine and 5HT3 receptors. GABAA receptors are hetero-oligomeric Cl- channels that are selectively blocked by the alkaloid bicuculline and modulated by steroids, barbiturates and benzodiazepines. To date, 16 human GABAA receptor cDNA have been cloned. 3. GABAB receptors are seven transmembrane receptors that are coupled to G-proteins and activate second messenger systems and Ca2+ and K+ ion channels. To date, three GABAB receptor proteins have been cloned and these resemble metabotropic glutamate receptors. GABAB receptors are hetero-oligomeric receptors made up of a mixture of a combination of the subunits. These receptors are selectively activated by (-)-baclofen and CCGP27492 and are blocked by phaclofen, the phosphonic acid analogue of baclofen. 4. In contrast, GABAC receptors represent a relatively simple form of transmitter-gated Cl- channel made up of a single type of protein subunit. Two human GABAC receptor cDNA have been cloned. These receptors are not blocked by bicuculline nor are they modulated by steroids, barbiturates or benzodiazepines. Instead, GABAC receptors are selectively activated by the conformationally restricted analogues of GABA in the folded conformation cis-4-aminocrotonic acid and (1s,2R)-2-(aminomethyl)-1-carboxycyclopropane. (1,2,5,6-Tetrahydropyridine-4-yl)methylphosphinic acid, a methylphosphinic acid analogue of GABA in a partially folded conformation, is a selective antagonist at GABAC receptors.