Soman-induced convulsions: significance of changes in levels of blood electrolytes, gases, glucose, and insulin

Toxic influence of organophosphate, carbamate, and organochlorine pesticides on cellular metabolism of lipids, proteins, and carbohydrates: a systematic review

Pesticides, including organophosphate (OP), organochlorine (OC), and carbamate (CB) compounds, are widely used in agricultural and indoor purposes. OP and CB act as acetyl cholinesterase (AChE) inhibitors that affect lots of organs such as peripheral and central nervous systems, muscles, liver, pancreas, and brain, whereas OC are neurotoxic involved in alteration of ion channels. There are several reports about metabolic disorders, hyperglycemia, and also oxidative stress in acute and chronic exposures to pesticides that are linked with diabetes and other metabolic disorders. In this respect, there are several in vitro and in vivo but few clinical studies about mechanism underlying these effects. Bibliographic databases were searched for the years 1963-2010 and resulted in 1652 articles. After elimination of duplicates or irrelevant papers, 204 papers were included and reviewed. Results indicated that OP and CB impair the enzymatic pathways involved in metabolism of carbohydrates, fats and protein within cytoplasm, mitochondria, and proxisomes. It is believed that OP and CB show this effect through inhibition of AChE or affecting target organs directly. OC mostly affect lipid metabolism in the adipose tissues and change glucose pathway in other cells. As a shared mechanism, all OP, CB and OC induce cellular oxidative stress via affecting mitochondrial function and therefore disrupt neuronal and hormonal status of the body. Establishing proper epidemiological studies to explore exact relationships between exposure levels to these pesticides and rate of resulted metabolic disorders in human will be helpful.

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.

Early histopathologic and ultrastructural changes in the heart of Sprague-Dawley rats following administration of soman

Male Sprague-Dawley rats were given atropine methylnitrate (20 mg/kg) and HI-6 (125 mg/kg) ip 10 min before a single injection of 130 micrograms soman/kg sc, and the heart was examined by light and electron microscopy 10, 25, 45, 90, and 180 min after the onset of seizures. Seizures appeared within 6-11 min after treatment. Control rats were given saline sc in place of soman. Early myocardial lesions consisting of hypercontraction and hyperextension of sarcomeres, focal myocytolysis, and contraction bands were detected in individual or groups of myocardial fibers. Hypercontraction was characterized by shortening of the sarcomere length, disappearance of the I and H bands, and thickening of the Z line. In contrast, hyperextended sarcomeres had thickened I and H bands. Myocytolysis was characterized by a progressively severe focal dissolution of myofilaments and edema of the affected sarcoplasmic area. Contraction bands appeared to result from the breakdown of markedly hypercontracted myofibril bundles. Due to the presence of a number of surviving myofilaments and the preservation of the sarcolemmal tube, distortion of the overall myocytic structure was minimal. Changes in the mitochondria and other intracellular organelles were also minimal and nonspecific. The close resemblance of morphologic findings to those induced by catecholamines supports the view that soman-induced myocardial damage is secondary to a treatment-related release of unphysiologic amounts of endogenous catecholamines.

A study of the N-methyl-D-aspartate antagonistic properties of anticholinergic drugs

Drugs that act at the N-methyl-D-aspartate (NMDA) receptor complex have the ability to terminate nerve agent-induced seizures and modulate the neuropathologic consequences of agent exposure. Drugs with mixed anticholinergic and anti-NMDA properties potentially provide an ideal class of compounds for development as anticonvulsant treatments for nerve agent casualties. The present experiment evaluated the potential NMDA antagonist activity of 11 anticholinergic drugs by determining whether pretreatment with the compound was capable of protecting mice from the lethal effects of NMDA. The following anticholinergic drugs antagonized NMDA lethality and are ranked according to their potency: mecamylamine > procyclidine = benactyzine > biperiden > trihexyphenidyl. The anticholinergics atropine, aprophen, azaprophen, benztropine, 3-quinuclidinyl benzilate (QNB), and scopolamine failed to show NMDA antagonist properties. In addition, and unexpectedly, diazepam, ethanol, and pentobarbital were also shown to be capable of antagonizing NMDA lethality over a certain range of doses. The advantages and limitations of using antagonism of NMDA lethality in mice as a bioassay for determining the NMDA antagonist properties of drugs are also discussed.

Assessment of primary neuronal culture as a model for soman-induced neurotoxicity and effectiveness of memantine as a neuroprotective drug

An in vitro mammalian model neuronal system to evaluate the intrinsic toxicity of soman and other neurotoxicants as well as the efficacy of potential countermeasures was investigated. The link between soman toxicity, glutamate hyperactivity and neuronal death in the central nervous system was investigated in primary dissociated cell cultures from rat hippocampus and cerebral neocortex. Exposure of cortical or hippocampal neurons to glutamate for 30 min produced neuronal death in almost 80% of the cells examined at 24 h. Hippocampal neurons exposed to soman for 15-120 min at 0.1 microM concentration caused almost complete inhibition (> or = 90%) of acetylcholinesterase but failed to show any evidence of effects on cell viability, indicating a lack of direct cytotoxicity by this agent. Acetylcholine (ACh, 0.1 mM), alone or in combination with soman, did not potentiate glutamate toxicity in hippocampal neurons. Memantine, a drug used for the therapy of Parkinson's disease, spasticity and other brain disorders, significantly protected hippocampal and cortical neurons in culture against glutamate and N-methyl-D-aspartate (NMDA) excitotoxicity. In rats a single dose of memantine (18 mg/kg) administered 1 h prior to a s.c. injection of a 0.9 LD50 dose of soman reduced the severity of convulsions and increased survival. Survival, however, was accompanied by neuronal loss in the frontal cortex, piriform cortex and hippocampus.

Pharmacological modulation of soman-induced seizures

Anticholinergics, benzodiazepines and N-methyl-D-aspartate (NMDA) antagonists have been shown to modulate the expression of nerve agent-induced seizures. This study examined whether the anticonvulsant actions of these drugs varied depending on the duration of prior seizure activity. Rats implanted with electrodes to record electroencephalographic (EEG) activity were pretreated with the oxime HI-6 (125 mg/kg, IP) to prolong survival, and then challenged with a convulsant dose of the nerve agent soman (180 micrograms/kg, SC); treatment compounds (scopolamine, diazepam, MK-801, atropine, benactyzine, and trihexyphenidyl) were delivered IV at specific times after seizure onset. Both diazepam and MK-801 displayed a similar profile of activity: At both short or long times after seizure initiation the anticonvulsant efficacy of each drug remained the same. Diazepam, and especially MK-801, enhanced the lethal actions of soman by potentiating the respiratory depressant effects of the agent; scopolamine given prior to diazepam or MK-801 protected against the respiratory depression. Scopolamine and atropine showed a dose- and time-dependent effectiveness; the longer the seizure progressed the higher the dose of drug required to terminate the seizure, with eventual loss of anticonvulsant activity if the seizure had progressed for 40 min. In contrast, benactyzine and trihexyphenidyl showed a third profile of activity: There was a smaller increase in drug dosage required for anticonvulsant activity as seizure duration increased, and both drugs could terminate seizures that had progressed for 40 min. The early anticonvulsant action of anticholinergics is interpreted as a specific effect that blocks the primary cholinergic excitatory drive that initiates, and first maintains, nerve agent seizures. If allowed to progress, the seizure activity itself recruits excitatory neurotransmitter systems (i.e., NMDA) that eventually maintain the seizure independent of the initial cholinergic drive. This is indicated by the eventual ineffectiveness of scopolamine and atropine as the duration of the seizure progresses. Diazepam and MK-801 appear to act to moderate nerve agent seizures by enhancing inhibitory activity (diazepam) or dampening the secondarily activated noncholinergic excitatory system (MK-801). Benactyzine and trihexyphenidyl represent compounds that possibly have both anticholinergic and NMDA antagonistic properties.

Vx enhances neuronal excitability and alters membrane properties of Rana catesbeiana sympathetic ganglion neurons

1. The effects of VX (10 microM) were examined on sympathetic ganglion neurons from the bullfrog using intracellular recording techniques. 2. VX significantly increased the amplitude of the residual EPSP from 4.8 +/- 0.86 mV (n = 4) to 13.7 +/- 1.23 mV (n = 4). 3. VX significantly decreased the membrane potential 5.2 +/- 0.75 mV (n = 6). The input resistance and the duration of the spike afterhyperpolarization (AHP) were also reduced 69.8% and 69.6% of control, respectively. 4. VX increased neuronal excitability greater than 200% (n = 5) of control. 5. The VX-induced neuronal excitability may result from a reduction in the duration of the AHP and contribute to the CNS toxicity.

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.

Prevention of physostigmine-, DFP-, and diazinon-induced acute toxicity by monoethylcholine and N-aminodeanol

1. Choline, and the choline analogues monoethylcholine (MEC) and N-aminodeanol (NAD) were examined for prophylactic activity in acute acetylcholinesterase inhibitor toxicity in mice. The rank order of potency of the compounds was MEC greater than NAD greater than choline. 2. Simultaneous administration of MEC (60 mg kg-1) or NAD (200 mg kg-1) with physostigmine reduced lethality to 17 and 13% respectively. MEC (60 mg kg-1) completely protected against disopropylfluorophosphate (DFP) and diazinon toxicity, and NAD reduced lethality to 17% for both agents. Choline (200 mg kg-1) exhibited only negligible antidotal activity against the inhibitors. 3. In vitro concentrations of choline, MEC, and NAD, similar to the estimated concentration obtained in vivo in the acute toxicity study, produced mixed inhibition of mouse brain acetylcholinesterase. The inhibition was dose-related and was additive to the inhibition produced by the cholinesterase inhibitors. 4. All three analogues reduced ligand binding at the nicotinic, M1, and M2 receptors. The rank order of potencies for the analogues at each receptor was nicotinic: (choline greater than MEC greater than NAD), M1: (MEC greater than choline greater than NAD), and M2: (MEC greater than choline greater than NAD). 5. It is proposed that the analogues prevent acetylcholinesterase inhibitor toxicity peripherally by interacting with acetylcholinesterase, and/or by competing with acetylcholine for binding to cholinoceptors.

Review: cholinergic mechanisms and epileptogenesis. The seizures induced by pilocarpine: a novel experimental model of intractable epilepsy

High-dose treatment with pilocarpine hydrochloride, a cholinergic muscarinic agonist, induces seizures in rodents following systemic or intracerebral administration. Pilocarpine seizures are characterized by a sequential development of behavioral patterns and electrographic activity. Hypoactivity, tremor, scratching, head bobbing, and myoclonic movements of the limbs progress to recurrent myoclonic convulsions with rearing, salivation, and falling, and status epilepticus. The sustained convulsions induced by pilocarpine are followed by widespread damage to the forebrain. The amygdala, thalamus, olfactory cortex, hippocampus, neocortex, and substantia nigra are the most sensitive regions to epilepsy-related damage following convulsions produced by pilocarpine. Spontaneous seizures are observed in the long-term period following the administration of convulsant doses of pilocarpine. Developmental studies show age-dependent differences in the response of rats to pilocarpine. Seizures are first noted in 7-12 day-old rats, and the adult pattern of behavioral and electroencephalographic sequelae of pilocarpine is seen in 15-21-day-old rats. During the third week of life the rats show an increased susceptibility to the convulsant action of pilocarpine relative to older and younger animals. The developmental progress of the convulsive response to pilocarpine does not correlate with evolution of the brain damage. The adult pattern of the damage is seen after a delay of 1-2 weeks in comparison with the evolution of seizures and status epilepticus. The susceptibility to seizures induced by pilocarpine increases in rats aged over 4 months. The basal ganglia curtail the generation and spread of seizures induced by pilocarpine. The caudate putamen, the substantia nigra, and the entopeduncular nucleus govern the propagation of pilocarpine-induced seizures. The antiepileptic drugs diazepam, clonazepam, phenobarbital, valproate, and trimethadione protect against pilocarpine-induced convulsions, while diphenylhydantoin and carbamazepine are ineffective. Ethosuximide and acetazolamide increase the susceptibility to convulsant action of pilocarpine. Lithium, morphine, and aminophylline also increase the susceptibility of rats to pilocarpine seizures. The pilocarpine seizure model may be of value in designing new therapeutic approaches to epilepsy.

Direct microinjection of soman or VX into the amygdala produces repetitive limbic convulsions and neuropathology

Rats were injected in the amygdala and other forebrain sites with nmolar amounts of the highly toxic organophosphate 'nerve agent' compounds soman or VX (O-ethyl-S-(2-diisopropylaminoethyl)-methylphosphonothioate) in an attempt to determine the mechanism(s) responsible for the permanent brain pathology that has been observed following systemic intoxication with these agents. Injections were performed using a stereotaxically guided microsyringe in animals maintained under halothane/oxygen anesthesia or using chronically implanted cannulae in conscious animals. Bilateral microsyringe injections of up to 11.0 nmol soman into the amygdala failed to evoke abnormal behavior or brain pathology. When rats were pretreated with lithium chloride, or when carbachol was coadministered, soman injections evoked repetitive clonic convulsions and neuropathology. Unilateral injections of 3.4 nmol of VX into the amygdala elicited convulsions and brain damage in 67% of the animals tested. Atropine pretreatment (15.0 mg/kg, i.p.) prevented the development of convulsions and brain damage. Neuropathology was observed only in animals that developed repetitive convulsions; the piriform and entorhinal cortex, amygdala, hippocampus and thalamus were the brain structures most consistently damaged. With unilateral injections, the damage was more severe on the side ipsilateral to the injection. The behavioral topography of the convulsions and the neuroanatomical distribution and nature of the subsequent pathology closely resemble that observed with systemic administration of these compounds. The results indicate that the nerve agents are not directly neurotoxic, that peripherally induced hypoxia or anoxia are unlikely mechanisms of the neuropathology, and that the brain damage produced by these compounds is primarily seizure-mediated.

Cholinergic systems influence local cerebral glucose use in specific anatomical areas: diisopropyl phosphorofluoridate versus soman

The organophosphates, diisopropyl phosphorofluoridate and soman have a common mechanism of action (inhibition of acetylcholinesterase), but result in very different behavioral responses in the rat. Soman rapidly produced persistent tonic convulsions whereas diisopropyl phosphorofluoridate only infrequently produced transient convulsive-like activity. Soman increased local cerebral glucose use in most of the cortex, striato-pallido-nigral pathway, limbic system and in specific thalamic nuclei whereas diisopropyl phosphorofluoridate increased glucose use in a limited fashion, primarily in the dorsal striato-pallido-nigral pathway. When diazepam blocked soman-induced convulsions, the pattern of glucose use was strikingly similar to that caused by diisopropyl phosphorofluoridate. Soman or diisopropyl phosphorofluoridate depressed local cerebral glucose use in rats pretreated with the antidotal mixture of trimedoxime, atropine and benactyzine (muscarinic antagonists). Also, this antidotal mixture blocked the increased glucose use in the dorsal striato-pallido-nigral system produced by either acetylcholinesterase inhibitor, indicating that muscarinic receptors mediate the excitation of this pathway. Both diisopropyl phosphorofluoridate and soman activate the striato-pallido-nigral pathway but soman also causes spread of activity producing overt motor convulsions. Possible explanations for this difference in response to the organophosphates are differential responses in cholinergic actions within specific brain regions or some non-cholinergic action of soman.

Effect of HI-6, applied into the cerebral ventricles, on the inhibition of brain acetylcholinesterase by soman in rats

When applied to rats (intraperitoneally) immediately after subcutaneous injection of soman (120 micrograms/kg) HI-6 (100 mg/kg) protected about 40% of the activity of acetylcholinesterase (AChE) in the motor end plate region of the diaphragm but did not protect AChE in the brain. However, a partial protection of AChE in brain against inhibition by soman was obtained in anaesthetized, atropinized rats by the oxime injected into the cerebral ventricle 5 min before parenteral exposure to soman. The AChE activity in brain of rats pretreated with HI-6, analyzed 60 min after the injection of soman was between 10 and 19%, while that in non-protected animals did not exceed 1% of the control. The degree of protection of AChE in brain was dose-dependent. Large doses of HI-6 (greater than or equal to 100 micrograms) were tolerated by animals because of the pentobarbital anaesthesia which counteracted the lethal action of HI-6. The rate of "aging" of AChE in brain inhibited by soman was analyzed by intracerebroventricular injection of 200 micrograms of HI-6 at different time intervals after the subcutaneous injection of soman. A statistically-significant reactivation of inhibited AChE activity in brain was demonstrated when HI-6 was applied up to 20 min after soman. The protection and reactivation by HI-6 of both AChE in brain and AChE in muscle end plates in poisoning with soman appear to be quite similar.

Effects of antidotes on soman-induced brain changes

Rats were pretreated with either diazepam, atropine or benactyzine 10 min prior to soman injection. Local cerebral glucose use (LCGU) was determined during the seizure phase (15 min post soman) or pathology phase (72 h post soman). Diazepam and benactyzine pretreatment prevented convulsive activity, whereas atropine pretreatment only reduced the duration of convulsive activity after soman exposure. Each pretreatment agent had a unique impact on LCGU pattern during the seizure phase. During the pathology phase, the marked reduction in LCGU and the conspicuous brain damage associated with soman-induced seizures was minimized by all three pretreatments.

Time course effects of soman on acetylcholine and choline levels in six discrete areas of the rat brain

The time course of changes in rat brain levels of acetylcholine (ACh) and choline (Ch) was investigated following a single SC injection of soman (0.9 LD50, 120 micrograms/kg) to understand the relationship between central neurotransmitter alteration and soman toxicity. Of the animals exposed to the dose of soman, 46% died within 24 h, with maximum mortality occurring during the first 40 min following soman administration. In a second group, surviving rats were killed at various times after treatment by a beam of focused microwave radiation to the head, and ACh and Ch levels were determined by gas chromatography-mass spectrometry. Soman produced a maximal ACh elevation in the brain stem at 20 min (34.4%), in cerebellum at 40 min (51.9%), in cortex and striatum at 2 h (320.3% and 35.2%, respectively), and in hippocampus and midbrain at 3 h (94.5% and 56.8%, respectively). ACh levels remained above normal approximately 30 min in the brain stem; 2 h in the midbrain, cerebellum, and striatum; 8 h in the cortex; and 16 h in the hippocampus. Ch levels were elevated in all areas except the striatum. Ch maxima occurred at 10-40 min and returned to control levels approximately 3 h after injection. Results suggest that perturbation of ACh levels due to soman was not uniform throughout the brain and that soman toxicity may reflect ACh changes in multiple areas, rather than changes in any given area. These data further suggest a possible relationship between elevated Ch levels and soman toxicity.