Anticonvulsants for poisoning by the organophosphorus compound soman: pharmacological mechanisms

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.

SCH 23390 affords protection against soman-evoked seizures in the freely moving guinea-pig: a concomitant neurochemical, electrophysiological and behavioural study

We studied the role of striatal dopamine (DA) release in seizure activity evoked by the subcutaneous administration of the cholinesterase inhibitor pinacolyl methylphosphonofluoridate (soman), in the guinea-pig. The involvement of the dopamine receptor subtypes was studied by systemic administration of the D(1)-like receptor antagonist SCH 23390 (0.5 mg kg(-1)) or the D(2)-like receptor antagonist sulpiride (30 mg kg(-1)). Microdialysis and HPLC with electrochemical detection were used to monitor changes in extracellular levels of striatal DA and its metabolites, acetylcholine and choline. These data were correlated with changes in the striatal and cortical electroencephalogram and observation of predefined clinical signs. We found that the blockade of the D(1) receptor with SCH 23390 can inhibit seizure activity, while blockade of the D(2) receptor with sulpiride can augment the evoked seizure activity. These results clarify the involvement of the dopaminergic system in soman-evoked seizures.

Novel method of monitoring electroencephalography at the site of microdialysis during chemically evoked seizures in a freely moving animal

This paper covers the design, development and operation of a novel piece of equipment, based around the CMA/12 guide probe (Carnegie Medicin, Sweden), which offers a low cost alternative for monitoring EEG at the site of microdialysis in a freely moving animal. This equipment is entirely based on commercially available parts, and thus can be easily replicated. Moreover, it is less intrusive than earlier models, offering advantages for experiments in which behavioural testing or chronic monitoring is required. We illustrate its use in a study of changes in electrical seizure activity, in both cortex and basal nuclei, evoked by the administration of the chemoconvulsant soman. The inference from the many experimental paradigms looking at the mechanisms of chemoconvulsants is that paroxysmal discharges are a better correlate of seizure activity than behavioural signs. The correlation of the EEG with extracellular neurotransmitter data, over a period of hours post-injection of chemoconvulsant, allows the determination of whether extracellular neurotransmitter changes are a cause or consequence of the evoked electrical activity.

Anticonvulsant treatment of nerve agent seizures: anticholinergics versus diazepam in soman-intoxicated guinea pigs

A total of eight anticholinergic drugs (aprophen, atropine, azaprophen, benactyzine, biperiden, procyclidine, scopolamine, trihexyphenidyl) were tested in parallel with diazepam for the ability to terminate seizure activity induced by the nerve agent soman. Guinea pigs, implanted with electrodes to record cortical electroencephalographic (EEG) activity, were pretreated with pyridostigmine Br (0.026 mg/kg, i.m.) and 30 min later challenged with 2 x LD50 soman (56 microg/kg, s.c.) followed 1 min later by treatment with atropine SO4 (2 mg/kg, i.m.) and pralidoxime chloride (2-PAM Cl; 25 mg/kg, i.m.). All guinea pigs developed sustained seizure activity following this treatment. Dose-effect curves were determined for the ability of each drug to terminate seizure activity when anticonvulsant treatment was given either 5 or 40 min after seizure onset. Body weight gain and recovery of behavioral performance of a previously trained one-way avoidance task were measured after exposure. With the exception of atropine, all anticholinergic drugs were effective at lower doses than diazepam in terminating seizures when given 5 min after seizure onset; benactyzine, procyclidine and aprophen terminated seizures most rapidly while scopolamine, trihexyphenidyl, biperiden, and diazepam were significantly slower. When given 40 min after seizure onset, diazepam was the most potent compound tested, followed by scopolamine, benactyzine and biperiden; atropine was not effective when tested 40 min after seizure onset. For diazepam, the time to terminate the seizure was the same whether it was given at the 5- or 40-min delay. In contrast, most anticholinergics were significantly slower in terminating seizure activity when

Nitrone spin trapping compound N-tert-butyl-alpha-phenylnitrone prevents seizures induced by anticholinesterases

The neuroprotection afforded by spin trapping agents such as N-tert-butyl-alpha-phenylnitrone (PBN) has lent support to the hypothesis that increased production of reactive oxygen species (ROS) is a major contributing factor to excitotoxicity, aging and cognitive decline. Little is known, however, about the pharmacological properties of PBN. We have compared the acute effects of PBN on the development of seizures induced by the irreversible acetylcholinesterase (AChE) inhibitor diisopropylphosphorofluoridate (DFP), the reversible AChE inhibitor physostigmine (PHY), the muscarinic cholinergic receptor agonist pilocarpine (PIL) and the glutamatergic receptor agonist kainic acid (KA). Rats were sacrificed 90 min after the injection of seizure-inducing agents. In situ hybridization was used to detect the induction of immediate early gene (IEG) c-fos and c-jun mRNA's and the levels of AChE mRNA. The activity of AChE was visualized by AChE staining and quantified using an in vitro AChE assay. The seizures correlated with the induction of IEG mRNA's with all agents used. The pre-treatment with 150 mg/kg of PBN prevented DFP- and PHY-induced seizures and the related expression of IEG mRNA's, but had no effect on PIL- or KA-induced seizures and associated IEG mRNA's changes. PBN prevented seizures and significantly protected AChE activity against DFP inhibition when given before, but not when given after DFP. This study shows that PBN specifically protects against anticholinesterase-induced seizures by reversible protection of AChE activity and not by the blockade of muscarinic or glutamate receptors, reactivation of AChE or scavenging of ROS. The anticholinesterase properties should be considered when using PBN in studies of cholinergic dysfunction.

Organophosphorus nerve agents-induced seizures and efficacy of atropine sulfate as anticonvulsant treatment

The ability of five organophosphorus nerve agents (tabun, sarin, soman, GF, and VX) to produce brain seizures and the effectiveness of atropine as an anticonvulsant treatment against these nerve agents were studied in two different animal models--the rat and guinea pig. All animals were implanted with cortical electrodes for EEG recordings. Five minutes after the start of nerve agent-induced EEG seizures, animals were treated intramuscularly (IM) with different doses of atropine sulfate and observed for seizure termination. The anticonvulsant ED50 of atropine sulfate for termination of seizures induced by each nerve agent was calculated and compared. In the rat model, selected oximes were administered either before, concurrent with, or following challenge with a 1.6 x LD50 dose of a given nerve agent to maximize seizure development with certain agent/oxime combinations. The choice and the timing of oxime administration significantly effected the incidence of seizure development by different nerve agents. When oxime administration did not effect seizure development (tabun, soman) the anticonvulsant ED50 for atropine sulfate was the same, regardless of the nerve agent used to elicit the seizure. When oxime administration reduced the incidence of seizure occurrence (sarin, GF, VX), the anticonvulsant ED50 dose of atropine sulfate for a nerve agent was lower. In the guinea pig model, animals were pretreated with pyridostigmine prior to challenge with 2 x LD50 of a given agent, and treated 1 min later with atropine sulfate (2 mg/kg) and 2-PAM (25 mg/kg). Under these conditions, the incidence, latency of seizure development, and anticonvulsant ED50s of atropine for soman-, tabun-, and GF-elicited seizures were virtually identical. With sarin, although the latency of seizure development was the same as with soman, tabun, and GF, seizures occurred with a lower incidence, and the anticonvulsant ED50 of atropine was lower. With VX, the latency of seizure development was notably longer, while the incidence of seizure development and anticonvulsant ED50 of atropine were significantly lower than with soman, tabun, or GF. In both models, a lower incidence of seizure development predicted a lower anticonvulsant dose of atropine. In the rat, the incidence of seizure development and the anticonvulsant effectiveness of atropine was highly dependent on the oxime used. In the guinea pig, higher doses of atropine sulfate were required to control soman-, tabun-, or GF-induced seizures, perhaps reflecting the lower cholinesterase reactivating ability of 2-PAM against these agents.

Soman-induced hypertension in conscious rats is mediated by prolonged central muscarinic stimulation

The acetylcholinesterase inhibitor, soman, induces marked and sustained hypertension and tachycardia associated with a convulsive syndrome in rats. The aims of the present study were to distinguish between the cardiovascular and convulsant effects of soman and to determine whether the maintenance of the soman-induced hypertension and tachycardia depends solely on a central muscarinic effect. To this end, using a computerised analysis of blood pressure (BP) in conscious freely moving rats, we examined the consequences on the increase in mean BP (MBP) and heart rate (HR) induced by soman (60 micrograms/kg, i.v.) of 1) a pre-treatment with the anticonvulsant drug diazepam (3 mg/kg, i.v.) and 2) atropine sulphate (10 mg/kg, i.v.) administered 10 or 60 min after the intoxication. Pretreatment with diazepam prevented the convulsions, assessed by electroencephalogram (EEG) recording, but modified neither the magnitude nor the kinetics of the pressor and tachycardic effects of soman (delta MBP = 74 +/- 2 and 73 +/- 5 mmHg, delta HR = 69 +/- 10 and 79 +/- 7 bpm, maximum MBP = 186 +/- 3 and 182 +/- 6 mmHg, maximum HR = 545 +/- 9 and 522 +/- 16 bpm in solvent- (n = 8) and diazepam- (n = 8) pre-treated rats, respectively). Whatever its time of administration, atropine sulphate fully and immediately reversed the rise in BP induced by soman. The soman-induced tachycardia was also suppressed by atropine administered 10 min after soman whereas it persisted when atropine was injected 60 min after the intoxication. These results show that the cardiovascular effects of soman can occur independently of the convulsive syndrome and that the maintenance of the soman-induced hypertension depends entirely on a permanent central muscarinic stimulation.

Sex differences in dizocilpine (MK-801) neurotoxicity in rats

The sex differences in the clinical signs and the distribution of astrocytic glial fibrillary acidic protein (GFAP) induced by an N-methyl-d-aspartate antagonist, dizocilpine (MK-801), were examined. A single intraperitoneal injection of MK-801 (5 mg/kg body weight) caused a prolonged recumbency (35-40 h), leading to a severe loss of body weight in female rats, in contrast to a light effect in males, independent of age. Early salivation or lacrimation was also severe in females and delayed bloody lacrimation was observed in females only. The pretreatment with 17β-estradiol (0.1 or 1.0 mg/kg body weight) made early signs worse in both sexes, but a remarkable mortality (20-40%) was observed in females only. The treatment with MK-801 greatly enhanced GFAP expression in retrospenial cortex of both sexes with a higher enhancement in females. The MK-801-induced expression of GFAP was further increased by the pretreatment with 17β-estradiol (1 mg/kg body weight) in females. Overall, the expression of GFAP in the retrospenial cortex of rats treated with MK-801 appeared to be higher in females than males, somewhat in parallel with more severe clinical signs in females. The results indicate the higher sensitivity of female rats to MK-801 neurotixicity, and the possible involvement of 17β-estradiol in the sex differences of the sensitivity.

Organophosphate-induced brain injuries: delayed apoptosis mediated by nitric oxide

The features of organophosphate-induced brain injuries were investigated. Rats were poisoned intraperitoneally with 9 mg/kg (1.8 LD(50)) of diisopropylfluorophosphate. Pyridostigmine bromide (0.1 mg/kg) and atropine methylnitrate (20 mg/kg), which are centrally inactive, were pre-treated intramuscularly to reduce the mortality and eliminate peripheral signs. Diisopropylfluorophosphate induced severe limbic seizures, and early necrotic and delayed apoptotic brain injuries. The necrotic brain injury was observed to be maximal as early as 1 h after diisopropylfluorophosphate treatment predominently in hippocampus and piriform/entorhinal cortices, showing a spongiform change (malacia) of neuropils in severe cases. In contrast, typical apoptotic (TUNEL-positive) cells started to appear at 12 h in thalamus, and a mixed type in amygdala. Separately, nitrite/nitrate content in cerebrospinal fluid was found to significantly increase after 2 h, reaching a maximal level at 6 h. Pre-treatment with l-N(G)-nitroarginine, an inhibitor of nitric oxide synthase, reduced nitrite/nitrate content and, noteworthy, attenuated only apoptotic brain injury in all four brain regions without affecting seizure intensity and necrotic injury. Taken together, the delayed apoptotic injury of brain induced by diisopropylfluorophosphate poisoning in rats might be mediated in part through nitric oxide production.

Anticonvulsants for soman-induced seizure activity

This report describes studies of anticonvulsants for the organophosphorus (OP) nerve agent soman: a basic research effort to understand how different pharmacological classes of compounds influence the expression of seizure produced by soman in rats, and a drug screening effort to determine whether clinically useful antiepileptics can modulate soman-induced seizures in rats. Electroencephalographic (EEG) recordings were used in these studies. Basic studies were conducted in rats pretreated with HI-6 and challenged with 1.6 x LD50 soman. Antimuscarinic compounds were extremely effective in blocking (pretreatment) or terminating soman seizures when given 5 min after seizure onset. However, significantly higher doses were required when treatment was delayed for more than 10 min, and some antimuscarinic compounds lost anticonvulsant efficacy when treatment was delayed for more than 40 min. Diazepam blocked seizure onset, yet seizures could recur after an initial period of anticonvulsant effect at doses </=2.5 mg/kg. Diazepam could terminate ongoing seizures when given 5 min after seizure onset, but doses up to 20 mg/kg were ineffective when treatment was delayed for 40 min. The GABA uptake inhibitor, tiagabine, was ineffective in blocking or terminating soman motor convulsions or seizures. The glutamate receptor antagonists, NBQX, GYKI 52466, and memantine, had weak or minimal antiseizure activity, even at doses that virtually eliminated signs of motor convulsions. The antinicotinic, mecamylamine, was ineffective in blocking or stopping seizure activity. Pretreatment with a narrow range of doses of alpha2-adrenergic agonist, clonidine, produced variable protection (40-60%) against seizure onset; treatment after seizure onset with clonidine was not effective. Screening studies in rats, using HI-6 pretreatment, showed that benzodiazepines (diazepam, midazolam and lorazepam) were quite effective when given 5 min after seizure onset, but lost their efficacy when given 40 min after onset. The barbiturate, pentobarbital, was modestly effective in terminating seizures when given 5 or 40 min after seizure onset, while other clinically effective antiepileptic drugs, trimethadione and valproic acid, were only slightly effective when given 5 min after onset. In contrast, phenytoin, carbamazepine, ethosuximide, magnesium sulfate, lamotrigine, primidone, felbamate, acetazolamide, and ketamine were ineffective.

Medical management of organophosphate-induced seizures

Recent studies concerning management of soman-induced seizures are reviewed. While drugs classically used against epilepsy in hospital appear ineffective against soman, muscarinic receptor blockers are shown to be able to prevent or stop seizures within the first 5 min after their onset. Benzodiazepine could also be considered as an emergency treatment useful during the first 10 min of seizure. Comparatively NMDA antagonists appear to be able to terminate soman-induced seizures even if the treatment is delayed after 40 min of epileptic activity. Drugs with both antimuscarinic and anti-NMDA properties may represent the most adequate pharmacological treatment to treat soman intoxication. However, the results obtained until now with these drugs must be completed in relation with their possible efficacy after i.m. administration. Propositions for future studies are reviewed.

Electrocorticographic changes during generalized convulsive status epilepticus in soman intoxicated rats

Generalized convulsive status epilepticus (GCSE) is the most common and potentially most damaging form of status epilepticus (SE). It has been previously reported, in both human GCSE and animal models of GCSE, that the electroencephalographs (EEGs) and electrocorticographs (ECoGs) recorded during GCSE contain an ordered sequence of five identifiable patterns: discrete seizures (phase 1), waxing and waning ictal discharges (phase 2), continuous ictal discharges (phase 3), continuous activity with flat periods (phase 4), and periodic epileptiform discharge on a flat background (phase 5). In this paper, we report the same pattern of ECoG changes in 15 rats exposed to soman, an acetylcholinesterase (AChE) inhibitor. Phase 1 was observed in 12 of 15 animals, but phases 2-5 were recorded in all the animals. Taken together, these findings suggest that the sequence of EEG changes is independent of the initiating cause, represent a common electrical response to GCSE, and reflect a common underlying neurochemical mechanism.

Effectiveness of procyclidine in combination with carbamate prophylactics against diisopropylfluorophosphate poisoning

The protective effect of cholinolytics such as procyclidine and atropine, in combination with carbamate prophylactics, against diisopropylfluorophosphate poisoning was examined in mice. Doses of carbamates were optimized, based on the maximum sign-free dose, the time course of cholinesterase inhibition and the protective potential against diisopropylfluorophosphate poisoning. Centrally-active physostigmine was more toxic than centrally-inactive pyridostigmine and the toxic signs of carbamates appeared to be closely related to the level of inhibition of brain cholinesterase activity. In combination with atropine, physostigmine was more effective than pyridostigmine in protecting mice intoxicated with diisopropylfluorophosphate. Moreover, centrally-active atropine sulfate was a more effective co-antidote to carbamates than centrally-inactive atropine methylnitrate. The most prominent protection was achieved with the combination of carbamates and procyclidine, a centrally-active cholinolytic showing anticonvulsion, which was also observed to prevent diisopropylfluorophosphate-induced convulsions (Kim et al., 1997). Taken together, it is suggested that procyclidine could be a possible substitute for atropine as an antidote to diisopropylfluorophosphate poisoning.

Evaluation of dextromethorphan and dextrorphan as a preventive treatment of soman toxicity in mice

Phencyclidine-like drugs are effective against convulsions and brain lesions related to soman intoxication but induce severe side effects. The well tolerated antitussive dextromethorphan (DM) and its metabolite dextrorphan (DX) have antiepileptic and neuroprotective properties that we evaluated in mice against 2 LD50 of soman in a three-drug pretreatment (atropine sulfate and oxime HI-6 plus DM: 20-50 mg/kg or DX: 10-40 mg/kg i.p). Neuroprotection was evaluated by measurement of hippocampal omega 3 binding site density. DM and DX have weak anticonvulsant and neuroprotective activities which are counterbalanced at high doses by an increased mortality due to respiratory distress for DM and by ataxia for DX. Thus DM and DX do not appear to be appropriate for the pretreatment of soman 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.

Antagonism of soman-induced convulsions by midazolam, diazepam and scopolamine

The effects of midazolam (MDZ), diazepam (DZ) and scopolamine (SCP) therapies on soman-induced electrocorticogram (ECoG) and biceps femoris electromyogram (EMG) activities and brain lesions were assessed in male rats. Animals received pyridostigmine (26 micrograms/kg, im) 30 min before soman (87.1 micrograms/kg, im) followed by therapy consisting of atropine (1.5 mg/kg) admixed with 2-PAM (25 mg/kg, im) 1 min later; MDZ (0.5 mg/kg), DZ (1.77 mg/kg) or SCP (0.43 mg/kg) was administered im at 1 min after the onset of convulsions (CVs). Typically, within 5 min after soman the ECoG profile changed to a full-blown, spike-and-dome epileptiform (SDE) pattern followed by CVs and increased amplitude of EMG activity. Treatment with SCP restored ECoG and EMG profiles by 30 min. At 2 hr after exposure only 1 animal demonstrated a slight abnormality in ECoG activity which was normal at 24 hr. Similarly, DZ and MDZ restored EcoG and EMG profiles by 30 min; however, in contrast to SCP, 83% of the animals demonstrated reappearance of SDE 2 hrs after soman. SCP therapy also enabled rats to move about in their cages by 30 min post treatment. In contrast, DZ- and MDZ-treated rats remained incapacitated as late as 2 hr post-exposure. Animals were euthanized at 24 hr, and the extent of soman-induced brain lesions was determined by light microscopic analysis. When present, brain lesions were minimal in SCP-treated rats. The mean brain lesion scores across all experimental conditions ranked as follows: soman control > MDZ > DZ > or = SCP = saline control. These observations suggest that SCP may be highly effective in severe soman intoxication.

Pharmacokinetics and effects of HI 6 in blood and brain of soman-intoxicated rats: a microdialysis study

The bispyridinium oxime HI 6 (1-(((4-amino-carbonyl)pyridino)methoxy)methyl)-2-(hydroxyimino )methyl)-pyridinium dichloride monohydrate), combined with atropine, is effective for treating poisoning with organophosphate nerve agents. The protective action of HI 6 in soman poisoning has been attributed mainly to its peripheral reactivation of inhibited acetylcholinesterase. In the present study we investigated whether high intramuscular doses of HI 6 can reach the brain in a sufficient amount to reactivate inhibited brain acetylcholinesterase. Microdialysis probes were implanted in the jugular vein and striatum and dialysis samples were collected simultaneously from the two sites in awake, freely moving rats. Pharmacokinetic parameters of unbound HI 6 in blood and brain were calculated after administration of HI 6 (50, 75 or 100 mg/kg i.m.) in control rats and rats injected with soman (90 microg/kg s.c., 0.9 LD50) 1 min before HI 6 treatment. We found that signs of soman poisoning correlated positively to acetylcholinesterase inhibition and negatively to the concentration of unbound HI 6 in the brain and that soman intoxication significantly decreased uptake of HI 6 into the brain.

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.

A role of nitric oxide in organophosphate-induced convulsions

The effects of nitric oxide-regulating compounds on convulsions and mortality of rats administered i.p. with diisopropylfluorophosphate was investigated. l-N(G)-nitroarginine methyl ester, a nitric oxide synthase inhibitor possessing an anticholinergic action, markedly attenuated the intensity of convulsions and significantly reduced the mortality rate. A similar result was obtained with anticholinergic procyclidine, an N-methyl-d-aspartate receptor antagonist. Noteworthy, l-N(G)-nitroarginine, another inhibitor of nitric oxide synthase, significantly attenuated the seizure intensity when administered in combination with atropine sulfate (5 mg/kg), though either l-N(G)-nitroarginine or atropine sulfate was inactive alone. It is suggested that nitric oxide may be a proconvulsant or a convulsion-promoting factor in anticholinesterase poisoning, and both the reduction of nitric oxide level and blockade of cholinergic systems may be required for more effective protection of seizures.

Thyrotropin-releasing hormone (TRH) is markedly increased in the rat brain following soman-induced convulsions

Soman is an organophosphorus (OP) compound which irreversibly inhibits acetylcholinesterase (AChE), the primary synaptic inactivator of acetylcholine. Resultant excessive cholinergic activity elicits generalized convulsions and brain lesions. Recent evidence suggests that other neurotransmitter/neuromodulator systems may be affected by the OP compounds as well. Since we have shown that both electrically and chemically induced seizures cause significant and prolonged increases in the neuropeptide thyrotropin-releasing hormone (TRH) in epileptogenic sites, we examined soman-induced convulsion effects on CNS TRH. Rats were injected with either soman (100 microg/kg SC; equivalent to 0.9 LD50) or saline and observed for convulsive activity. Forty-eight hours post injection, dramatic increases of TRH over control levels were seen in frontal cortex (30-fold), pooled cortex (24-fold), hippocampus (16-fold), piriform cortex (14-fold), entorhinal cortex (11-fold), and amygdala (2-fold). No change was observed in either hypothalamus or pituitary. Our results demonstrate, for the first time, a substantial effect of an OP on a specific neuropeptide system in vivo. The neurochemical and behavioral consequences of the soman-induced increases in TRH, especially in the frontal cortex, are presently unknown. Clearly, much more work is required to discern the exact role TRH has following soman exposure.

Mapping of cortical metabolic activation in soman-induced convulsions in rats

The metabolic activation of the cerebral cortex during convulsions induced by the organophosphorus cholinesterase inhibitor soman was studied in detail. Soman was given at a dose equivalent to 0.9 LD50 (100 microgram/kg SC after pretreatment with 26 microgram/kg pyridostigmine, IM, to decrease lethality) to examine separately the metabolic effects of severe acetylcholinesterase inhibition, present always with this dose, and convulsions, present only in some of the animals. Cerebral glucose utilization (CGU) values of cortex divided by CGU of brain stem (nCGU) were calculated for 96 locations in nine coronal slices. Animals injected with pyridostigmine-soman and that developed convulsions (n = 7) showed statistically significant increases of nCGU with regard to animals injected with saline (n = 5) in 33 locations, 27 of which were in a single cluster, with the piriform cortex at its center. Perirhinal cortex, and insular cortex also showed significantly higher nCGU in convulsing rats. Other foci of elevated nCGU were found in frontal and parietal locations. In animals injected with pyridostigmine-soman and that did not develop convulsions (n = 5) in spite of severe cholinesterase inhibition, a single location (piriform cortex) showed significantly higher nCGU than controls. Neuropathology evaluation showed a significant decrease in viable cells only in animals that developed convulsions. This effect correlated with enhanced nCGU. It is concluded that the presence of convulsions, and not exposure to pyridostigmine-soman, determined the pattern of nCGU cortical activation, which correlated closely with the structural changes.

Measurement of the oxime HI-6 after peripheral administration in tandem with neurotransmitter levels in striatal dialysates: effects of soman intoxication

In the present study, the technique of microdialysis combined with tandem high-performance liquid chromatography was used to determine the striatal levels of HI-6 and neurotransmitters following peripheral administration of HI-6 (50 mg/kg i.m.) in conscious, freely moving rats. The results were compared with those obtained in animals given soman (135 micrograms/kg i.p.) 1 min before HI-6 (50 mg/kg i.m.). Principal component analysis was used to study the effects of the different treatments on neurotransmitters and signs of poisoning. In all animals given HI-6, maximum levels of HI-6 appeared in the second 20-min fraction after administration of HI-6, then gradually declined, reaching the lower limits of detection after 3 hr. There was a correlation between severity of poisoning and neurochemical changes observed; dopamine and GABA levels increased as the severity of signs of poisoning increased. These results clearly demonstrate that HI-6 can penetrate into the brain of control and soman-intoxicated animals. Tandem measurement of dopamine electrochemically and HI-6 by UV detection provides a simple method for obtaining data on HI-6 penetration into the brain in neurochemical studies of soman poisoning and its treatment.

Effects of antimuscarinic antiparkinsonian drugs on brightness discrimination performance in rats

Biperiden (BPR) and trihexyphenidyl (THP), the current antimuscarinic drugs of choice in the management of parkinsonism, have been shown to exert anticonvulsant effects induced by poisoning by the organophosphorus compound soman. The present study was undertaken to evaluate the effects of these drugs on performance of a simple light-intensity discrimination task in rats under a tandem schedule of fixed-ratio (FR) reward/ differential-reinforcement-of-low-rate (DRL) nonreward contingencies, for water reinforcement in 2-h experimental sessions. Both BPR (0.125-2.0 mg/kg, SC) and THP (0.25-8.0 mg/kg, SC) in general decreased overall reinforcement rates in a similar dose dependent and parallel manner, concurrent with increased overall nonreinforced responses in an inverted U-shaped dose-response relationship. Lower doses of BPR (0.125-0.5 mg/kg) and and THP (0.25-2.0 mg/kg) produced a moderate reduction in reinforcement (> or = 50% of baseline controls), which was correlated well with increases in nonreinforced responses emitted, whereas, higher doses of BPR (> 0.5 mg/kg) and TPH (> or = 2.0 mg/kg) markedly decreased reinforcements, which mainly resulted from the pausing of responding in the presence of stereotyped behavior. The behavioral disruption induced by BPR was much more rapid than that induced by THP. The ED50 values (0.6 mg/kg vs. 1.3 mg/kg, respectively) and parallel dose-effect curves suggest that these drugs have similar efficacy, and that BPR is about twice as potent as THP, a ranking that corresponds with their binding affinity at M-1 muscarinic acetylcholine receptors in rat cerebral cortex. Based on the similarity between the anticonvulsant doses of these drugs and the maximal doses that in this study did not disrupt operant responses (0.125 mg/kg vs. 0.25 mg/kg, respectively), it is suggested that both drugs may be useful in protection against seizures produced by the cholinesterase inhibitor soman. Overall, these results suggest that this multiple schedule operant contingency may have promise as a behavioral model to identify the therapeutic or toxic potentials of centrally acting antimuscarinic antiparkinsonian drugs based on their congnitive side effects.

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.

Anticonvulsant activity of caramiphen analogs

Caramiphen potently blocks maximal electroshock (MES)-induced seizures in mice and rats. The anticonvulsant mechanism has been hypothesized to be due to high-affinity binding to sigma recognition sites in brain. To study the structure-activity relationship for anticonvulsant activity of caramiphen we evaluated 8 analogs in MES-induced seizures in rats and also determined whether a correlation exists between anticonvulsant potency and sigma binding affinity. Some of the analogs potently inhibited sigma binding but were devoid of anticonvulsant activity. Aminocaramiphen 2 (ED50 = 3.4 mg/kg) and N-methyl-4-piperidinyl 1-phenylcyclopentanecarboxylate 9 (ED50 = 4.8 mg/kg) showed anticonvulsant activity comparable to caramiphen (ED50 = 3.1 mg/kg), although in sigma binding assays the affinities were 3-and 30-fold less than caramiphen, respectively. In the presence of 250 microM of phenytoin, caramiphen and p-aminocaramiphen showed 3- to 5-fold increases in affinity for [3H](+)pentazocine binding, whereas piodocaramiphen, which was inactive as an anticonvulsant, showed no change in affinity for sigma binding. These results indicate that anticonvulsant activity of the caramiphen analogs is not due to interaction with sigma binding sites.

Efficacy of injectable anticholinergic drugs against soman-induced convulsive/subconvulsive activity

Six FDA approved, injectable compounds [benztropine (BZT); biperiden (BIP); dicyclomine (DCL); l-hyoscyamine (HYO); orphenadrine (ORP); scopolamine (SCP)] were each compared to diazepam (DZ, the standard) in male guinea pigs against ongoing soman-induced convulsive or sub-CV (CV/sub-CV) activity. Three trained graders concurrently assigned CV/sub-CV scores to each animal based on signs of intoxication at various times post-soman. Animals received (im) pyridostigmine (26 micrograms/kg) 30 min before soman (56 micrograms/kg; 2 x LD50), atropine (2 mg/kg) admixed with 2-PAM (25 mg/kg) at one min after soman, and the candidate drug preparation at 5.67 min post soman, a time when CV activity was assured. BIP and SCP were effective over dosage ranges between 10 and 0.3, and 1.0 and 0.13 mg/kg, respectively, while the other preparations were less effective at their respective maximum dosages. At the most effective dosages of SCP (1.0 mg/kg) and BIP (10 mg/kg), the CV/sub-CV scores were significantly lower (p < 0.05) than those of DZ. Only 33% survival was observed at each of two doses of ORP and one dose of HYO; therefore, no further testing was done with these compounds. Using freshly prepared solutions, DCL (up to 40 mg/kg) and BZT (up to 96 mg/kg) were tested with mixed results; DCL lowered lethality while BZT increased lethality. CV/sub-CV scores for the most effective dose of DCL and BZT were, however, lower than those of DZ. SCP is an antimuscarinic drug devoid of antinicotinic activity, while BIP possesses antimuscarinic, antinicotinic, antispasmodic and anti-N-methyl-D-aspartate activity. Recent evidence suggests that, in late stages of intoxication by nerve agents, noncholinergic, excitatory amino acid receptors may become involved and necessitate the use of a multi-action drug like BIP. The findings herein suggest that SCP and BIP are superior to DZ, but further studies are needed to determine which drug or drug class should be pursued in more advanced testing.

Efficacy comparison of scopolamine (SCP) and diazepam (DZ) against soman-induced lethality in guinea pigs

Diazepam (DZ) and scopolamine (SCP) are known to be beneficial when each is used in combination with atropine (AT) + oxime therapy against intoxication by soman, but the efficacy of each might be expected to vary with the dosage of AT. Thus the therapeutic efficacy of SCP (5 doses; 0-0.86 mg/kg) versus DZ (5 doses; 0-5 mg/kg), when used in conjunction with AT (3 doses; 0.5-8 mg/kg) + 2-PAM (25 mg/kg) therapy, was tested in groups of pyridostigmine pretreated guinea pigs exposed to 1.6, 2.0, 2.5 or 3.2 LD50s of soman. Response surface methodology was employed to describe the relationship between lethality and the AT/DZ or AT/SCP dosages. Results show that within the indicated dose ranges used, the efficacy of SCP is not dependent on the presence of AT, whereas AT is needed for DZ to maintain the lowest probability of death. These findings suggest that in guinea pigs SCP could supplement AT or replace DZ as therapy against nerve agent intoxication.

Neuroprotective activity of glutamate receptor antagonists against soman-induced hippocampal damage: quantification with an omega 3 site ligand

Previous investigations have indicated that the measurement of omega 3 (peripheral-type benzodiazepine) binding site densities could be of widespread applicability in the localization and quantification of neural tissue damage in the central nervous system. In the first step of the present study, the suitability of this approach for the assessment of soman-induced brain damage was validated. Autoradiographic study revealed marked increases of omega 3 site densities in several brain areas of convulsing rats 2 days after soman challenge. These increases were well-correlated with the pattern and the amplitude of neuropathological alterations due to soman and closely related to both glial reaction and macrophage invasion of the lesioned tissues. We then used this marker to assess, in mouse hippocampus, the neuroprotective activity against soman-induced brain damage of NBQX and TCP which are respective antagonists of non-NMDA and NMDA glutamatergic receptors. Injection of NBQX at 20 or 40 mg/kg 5 min prior to soman totally prevented the neuronal damage. Comparatively, TCP had neuroprotective efficacy when administered at 1 mg/kg 5 min prior to soman followed by a reinjection 1 h after. These results demonstrate that both NBQX and TCP afford a satisfactory neuroprotection against soman-induced brain damage. Since it is known that the neuropathology due to soman is closely seizure-related, the neuroprotective activities of NBQX and TCP are discussed in relation with the respective roles of non-NMDA and NMDA receptors in the onset and maintenance of soman-induced seizures.

Prolonged effects of cholinesterase inhibition with eptastigmine on the cerebral blood flow-metabolism ratio of normal rats

The cerebrovascular and metabolic effects of the novel cholinesterase inhibitor eptastigmine were tested in conscious rats. The drug was administered by single intravenous injection, and blood flow or glucose utilization were assessed in 38 brain regions by quantitative autoradiographic techniques. A dose-dependent increase in regional cerebral blood flow (rCBF) was obtained for i.v. doses ranging from 0.5 to 3 mg kg-1. Forty minutes after the dose of 1.5 mg kg-1, average rCBF of the 38 regions studied was (mean +/- SD) 2.62 +/- 0.62 ml g-1 min-1, a value significantly higher than that of saline-injected controls (1.46 +/- 0.26; p < 0.005). In contrast, a similar dose of eptastigmine did not significantly alter regional cerebral glucose utilization (rCGU) (0.90 +/- 0.21 mumol g-1 min-1) when compared with saline-injected controls (0.99 +/- 0.08 mumol g-1 min-1). A linear correlation between rCBF and rCGU was observed both in saline (r = 0.871) and eptastigmine (r = 0.873)-injected animals but the slope of the regression line of rCBF on rCGU was significantly higher (p < 0.01) in the eptastigmine group (2.863 +/- 0.266) than in the controls that received saline (1.00 +/- 0.094). The cerebral vasodilatation induced by eptastigmine peaked at 40 min after drug administration. No toxic signs were observed at the doses used. Mean arterial blood pressure decreased after 0.5 mg kg-1 (control = 109.3 +/- 10.56 mm Hg; eptastigmine = 96.6 +/- 8.10 mm Hg) but did not differ from control at the higher doses. It is concluded that eptastigmine induces a long-lasting increase in rCBF and a significant enhancement of the rCBF:rCGU ratio in most regions. The results suggest an important role of endogenous acetylcholine in the control of cerebral perfusion.

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.

Effects of the nerve agents soman and tabun on the uptake and release of GABA and glutamate in synaptosomes of guinea pig cerebral cortex

1. Crude and purified synaptosomes were prepared from the cerebral cortex of the rat or the guinea pig and used to study the uptake and release of [3H]GABA and [3H]glutamate. 2. Baclofen at 10(-5) M inhibited stimulated release of [3H]GABA from crude rat and guinea pig synaptosomes, but not from purified rat synaptosomes. 3. 1-2 mM tabun decreased the uptake of [3H]GABA and increased the uptake of [3H]glutamate by purified guinea pig synaptosomes. 4. Soman and tabun at 10(-6) M and 10(-5) M inhibited basal release of [3H]GABA and [3H]glutamate from crude guinea pig synaptosomes. Tabun at 10(-5) M decreased stimulated release of [3H]GABA while soman had no effect. 5. The results do not sustain the possibility that nerve agents cause convulsions by affecting the uptake or release of GABA or glutamate. However indirect evidence was obtained that soman and tabun inhibit catabolism of GABA and glutamate.

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.

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.