Transient impairment of the gabaergic function during initiation of soman-induced seizures

Synthesis and preliminary biological evaluation of gabactyzine, a benactyzine-GABA mutual prodrug, as an organophosphate antidote

Organophosphates (OPs) are inhibitors of acetylcholinesterase and have deleterious effects on the central nervous system. Clinical manifestations of OP poisoning include convulsions, which represent an underlying toxic neuro-pathological process, leading to permanent neuronal damage. This neurotoxicity is mediated through the cholinergic, GABAergic and glutamatergic (NMDA) systems. Pharmacological interventions in OP poisoning are designed to mitigate these specific neuro-pathological pathways, using anticholinergic drugs and GABAergic agents. Benactyzine is a combined anticholinergic, anti-NMDA compound. Based on previous development of novel GABA derivatives (such as prodrugs based on perphenazine for the treatment of schizophrenia and nortriptyline against neuropathic pain), we describe the synthesis and preliminary testing of a mutual prodrug ester of benactyzine and GABA. It is assumed that once the ester crosses the blood-brain-barrier it will undergo hydrolysis, releasing benactyzine and GABA, which are expected to act synergistically. The combined release of both compounds in the brain offers several advantages over the current OP poisoning treatment protocol: improved efficacy and safety profile (where the inhibitory properties of GABA are expected to counteract the anticholinergic cognitive adverse effects of benactyzine) and enhanced chemical stability compared to benactyzine alone. We present here preliminary results of animal studies, showing promising results with early gabactyzine administration.

Enaminone Modulators of Extrasynaptic α4β3δ γ-Aminobutyric AcidA Receptors Reverse Electrographic Status Epilepticus in the Rat After Acute Organophosphorus Poisoning

Seizures induced by organophosphorus nerve agent exposure become refractory to treatment with benzodiazepines because these drugs engage synaptic γ-aminobutyric acid-A receptors (GABA_ARs) that rapidly internalize during status epilepticus (SE). Extrasynaptic GABA_ARs, such as those containing α₄β₃δ subunits, are a putative pharmacological target to comprehensively manage nerve agent-induced seizures since they do not internalize during SE and are continuously available for activation. Neurosteroids related to allopregnanolone have been tested as a possible replacement for benzodiazepines because they target both synaptic and extrasynaptic GABA_ARs receptors. A longer effective treatment window, extended treatment efficacy, and enhanced neuroprotection represent significant advantages of neurosteroids over benzodiazepines. However, neurosteroid use is limited by poor physicochemical properties arising from the intrinsic requirement of the pregnane steroid core structure for efficacy rendering drug formulation problematic. We tested a non-steroidal enaminone GABA_AR modulator that interacts with both synaptic and extrasynaptic GABA_ARs on a binding site distinct from neurosteroids or benzodiazepines for efficacy to control electrographic SE induced by diisopropyl fluorophosphate or soman intoxication in rats. Animals were treated with standard antidotes, and experimental therapeutic treatment was given following 1 h (diisopropyl fluorophosphate model) or 20 min (soman model) after SE onset. We found that the enaminone 2-261 had an extended duration of seizure termination (>10 h) in the diisopropyl fluorophosphate intoxication model in the presence or absence of midazolam (MDZ). 2-261 also moderately potentiated MDZ in the soman-induced seizure model but had limited efficacy as a stand-alone anticonvulsant treatment due to slow onset of action. 2-261 significantly reduced neuronal death in brain areas associated with either diisopropyl fluorophosphate- or soman-induced SE. 2-261 represents an alternate chemical template from neurosteroids for enhancing extrasynaptic α₄β₃δ GABA_AR activity to reverse SE from organophosphorous intoxication.

Pharmacokinetics and pharmacodynamics of some oximes and associated therapeutic consequences: a critical review

Undoubtedly, the use of oximes represents real progress in counteracting intoxications with organophosphates (OP), through potentiating antidotal effects of atropine. The penetration extent of these compounds through the blood-brain barrier (BBB) to significantly reactivate phosphorylated or phosphonylated acetylcholinesterase (AChE) in the brain still remains a debatable issue. Penetration of biological barriers by oximes was investigated mainly through determination of several quantitative parameters characterizing digestive absorption and BBB penetration. A weak penetration of biological barriers could be concluded from the available experimental data. The functional parameters/therapeutic effects following the penetration of oximes through BBB, more precisely the antagonism of OP-induced seizures and hypothermia, prevention of brain damage and respiratory center protection, leading to the final end-point, the survival of intoxicated organisms, are of high interest. It seems obvious that oximes are weakly penetrating the BBB, with minimal brain AChE reactivation (<5%) in important functional areas, such as the ponto-medullar. The cerebral protection achieved through administration of oximes is only partial, without major impact on the antagonism of OP-induced seizures, hypothermia and respiratory center inhibition. The antidotal effects probably result from synergic effects of other PD properties, different from the brain AChE reactivation process. Oxime structures especially designed for enhanced BBB penetration, through potentiating the hydrophobic characteristics, more often produce neurotoxic effects. Certainly, obtaining oximes with broad action spectrum (active against all OP types) would make a sense, but certainly, such a target is not achievable only through the increase in their penetrability in the brain.

Epilepsy, regulation of brain energy metabolism and neurotransmission

Seizures are the result of a sudden and temporary synchronization of neuronal activity, the reason for which is not clearly understood. Astrocytes participate in the control of neurotransmitter storage and neurotransmission efficacy. They provide fuel to neurons, which need a high level of energy to sustain normal and pathological neuronal activities, such as during epilepsy. Various genetic or induced animal models have been developed and used to study epileptogenic mechanisms. Methionine sulfoximine induces both seizures and the accumulation of brain glycogen, which might be considered as a putative energy store to neurons in various animals. Animals subjected to methionine sulfoximine develop seizures similar to the most striking form of human epilepsy, with a long pre-convulsive period of several hours, a long convulsive period during up to 48 hours and a post convulsive period during which they recover normal behavior. The accumulation of brain glycogen has been demonstrated in both the cortex and cerebellum as early as the pre-convulsive period, indicating that this accumulation is not a consequence of seizures. The accumulation results from an activation of gluconeogenesis specifically localized to astrocytes, both in vivo and in vitro. Both seizures and brain glycogen accumulation vary when using different inbred strains of mice. C57BL/6J is the most "resistant" strain to methionine sulfoximine, while CBA/J is the most "sensitive" one. The present review describes the data obtained on methionine sulfoximine dependent seizures and brain glycogen in the light of neurotransmission, highlighting the relevance of brain glycogen content in epilepsies.

Soman-induced convulsions: the neuropathology revisited

The organophosphorus compound soman, an irreversible inhibitor of cholinesterases, produces seizure activity and related brain damage. Studies using various biochemical markers of programmed cell death (PCD) suggested that soman-induced cell damage in the brain was apoptotic rather than necrotic. However, it has recently become clear that not all PCD is apoptotic, and the unequivocal demonstration of apoptosis requires ultrastructural examination. Therefore, the present study was undertaken to reinvestigate the damage produced in the brains of mice sacrificed at various times within the first 24 h or at 7 days after a convulsive dose of soman. Classical histology and ultrastructural examination were performed. The immunohistochemical expression of proteins (p53, Bax) involved in PCD, DNA fragmentation (TUNEL method at light and electron microscopy levels) and the glial reaction were also explored. Our study confirms that the severity of lesions depended on the duration of convulsions and shows that cerebral changes were still occurring as late as 7 days after the onset of long-lasting convulsions. Our observations also establish that there was a large variety of ultrastructurally distinct types of cell damage, including hybrid forms between apoptosis and necrosis, but that pure apoptosis was very rare. A prominent expression of p53 and Bax proteins was detected indicating that PCD mechanisms were certainly involved in the morphologically diverse forms of cell death. Since purely apoptotic cells were very rare, these protein expressions were presumably involved either in nonapoptotic cell death mechanisms or in apoptotic mechanisms occurring in parallel with nonapoptotic ones. Moreover, evidence for DNA fragmentation by the TUNEL method was found in apoptotic but also in numerous other morphotypes of cell damage. Therefore, TUNEL-positivity and the expression of PCD-related proteins, in the absence of ultrastructural confirmation, were here shown not to provide proof of apoptosis. In soman poisoning as well as in other cerebral pathologies, premature conclusions on this question can potentially be misleading and might even lead to detrimental therapies.

Disruption of mice sleep stages induced by low doses of organophosphorus compound soman

We have explored during 7 days, EEG spectral response and sleep pattern of mice after a mild intoxication with soman. Using an automatic staging method, we have quantified the sleep stage of the mice to identify disruptions of the sleep pattern. The 50 microg/kg dose of soman produced several effects during several time windows after intoxication. A first decrease followed by an increase of theta energy, a disturbance of slow wave sleep during 5 days and an increase of the REM sleep during the first and second day after intoxication. During the first 6h, we have observed some effects which were not consistent with a muscarinic activation and might have involved GABA-ergic system. After this early period, the observed effects were in accordance with a muscarinic activation. We observed an increase of energy in the EEG theta band during 3 days after soman injection and an increase of slow wave sleep during the second to the fifth day after soman injection.

Nerve agents: a comprehensive review

Nerve agents are perhaps the most feared of potential agents of chemical attack. The authors review the history, physical characteristics, pharmacology, clinical effects, and treatment of these agents.

Acute exposure to a low or mild dose of soman: biochemical, behavioral and histopathological effects

Effects of low to mild doses of soman on central and blood cholinesterase (ChE) activities and anxiety behavior were studied in mice 30 min, 24 h and 7 days after poisoning. At these two latter time points, histopathological consequences of soman intoxication were also studied. The 30-microg/kg dose of soman produced 30 min after intoxication, about 35% of central ChE inhibition, and an anxiolytic effect without toxic signs or histopathological changes. The 50-microg/kg dose of soman produced at the same time, about 56% of central ChE inhibition, slight clinical signs of poisoning without convulsions, an anxiogenic effect with a slight hypolocomotion but no brain damage. A mild dose of soman (90 microg/kg) produced at this same time point about 80% of central ChE inhibition, and led to ataxia and tremors in every mouse and to convulsions in some of them. Thirty minutes and 24 h after poisoning, the behavioral tests revealed neither anxiolytic nor anxiogenic responses despite a clear hypolocomotion. Only mice that experienced long-lasting convulsions developed neuropathological changes. The functional implication of our results, as well as the biological relevance of blood vs. brain ChE levels, as an index of intoxication severity are discussed.

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.

Effects of soman-induced convulsions on phosphoinositide metabolism

Turnover of [3H]phosphoinositides (PI) was examined in brain slices from the hippocampus of rats undergoing soman-induced seizure activity. Hydrolysis of PI was determined by measuring the accumulation of [3H]inositol-1-phosphate (IP1). Incubation of hippocampal slices in the presence of carbachol or norepinephrine (NE) increased PI hydrolysis. Stimulated hydrolysis by NE, but not carbachol was significantly reduced in slices from soman-challenged rats undergoing convulsive activity. NE-stimulated PI hydrolysis was not reduced in slices from animals exposed to soman that did not exhibit convulsive activity. In rats surviving for 24 h, the response to NE was not different from control rats. In control slices, NE-stimulated hydrolysis of PI was potentiated by GABA. No potentiation by GABA was seen in slices from animals undergoing seizures. Uptake and incorporation of myo-[2-3H]inositol into phospholipids was reduced in slices from rats undergoing convulsions. Reduced IP1 production appeared to be owing, in part, to decreased synthesis of inositol lipids. These observations suggest that during soman-induced seizure activity, there is an apparent decrease in the response of the PI second messenger system to NE stimulation, and that this may contribute to the severity and duration of convulsions and brain damage resulting from exposure to soman and other anticholinesterase compounds.

Brain microdialysis of GABA and glutamate: what does it signify?

Microdialysis has become a frequently used method to study extracellular levels of GABA and glutamate in the central nervous system. However, the fact that the major part of GABA and glutamate as measured by microdialysis does not fulfill the classical criteria for exocytotic release questions the vesicular origin of the amino acids in dialysates. Glial metabolism or reversal of the (re)uptake sites has been suggested to be responsible for the pool of nonexocytotically released amino-acid transmitters that seem to predominate over the neuronal exocytotic pool. The origin of extracellular GABA and glutamate levels and, as a consequence, the implications of changes in these levels upon manipulations are therefore obscure. This review critically analyzes what microdialysis data signify, i.e., whether amino-acid neurotransmitters sampled by microdialysis represent synaptic release, carrier-mediated release, or glial metabolism. The basal levels of GABA and glutamate are virtually tetrodotoxin- and calcium-independent. Given the fact that evidence for nonexocytotic release mediated by reversal of the uptake sites as a release mechanism relevant for normal neurotransmission is so far limited to conditions of "excessive stimulation," basal levels most likely reflect a nonneuronal pool of amino acids. Extracellular GABA and glutamate concentrations can be enhanced by a wide variety of pharmacological and physiological manipulations. However, it is presently impossible to ascertain that the stimulated GABA and glutamate in dialysates are of neuronal origin. On the other hand, under certain stimulatory conditions, increases in amino-acid transmitters can be obtained in the presence of tetrodotoxin, again suggesting that aspecific factors not directly related to neurotransmission underlie these changes in extracellular levels. It is concluded that synaptic transmission of GABA and glutamate is strictly compartmentalized and as a result, these amino acids can hardly leak out of the synaptic cleft and reach the extracellular space where the dialysis probe samples.

Efficacy of huperzine in preventing soman-induced seizures, neuropathological changes and lethality

Huperzine A (HUP) is a potent reversible inhibitor of acetylcholinesterase (AChE) that crosses the blood-brain barrier. Its ability to prevent seizures and subsequent hippocampal neuropathological changes induced by the organophosphate soman was studied in guinea pigs. Results were compared to guinea pigs treated with pyridostigmine (PYR, 0.2 mg/kg, subcutaneously). HUP pretreatment at 0.5 mg/kg, intraperitoneally, totally prevented seizures and ensured the survival of all animals for 24 h after intoxication. Hippocampal tissue was then free of any neuronal damage. Comparatively, all animals pretreated with PYR exhibited epileptic activity after soman poisoning and five of six animals died. Examination of the hippocampus of the only surviving guinea pig pretreated with PYR showed extensive neuropathological changes. Although HUP or PYR induced similar inhibitions of blood AChE activity, only HUP pretreatment led to a decrease in central AChE activity. In binding studies on guinea-pig brain homogenates, HUP had no affinity for muscarinic, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and gamma-aminobutyric acid (GABA)A receptors and only a very low one for N-methyl-D-aspartate (NMDA) receptors. In conclusion, HUP, unlike PYR, protects against soman-induced convulsions and neuropathological changes in the hippocampus. This efficacy seems to be related to a protection by HUP of both peripheral and central stores of AChE.

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.

Comparison of seizure related amino acid release in human epileptic hippocampus versus a chronic, kainate rat model of hippocampal epilepsy

Recent microdialysis studies of excitatory and inhibitory amino acid release associated with paroxysmal hippocampal activity have found significant increases in the hippocampus of epileptic patients, but minimal or variable increases in animal models. One possible reason for the difference is that the animal models employed in these studies have not adequately reflected the pathophysiology of human epilepsy. The present study sought to verify the amino acid release reported in human epileptic hippocampus and then employs animal studies using a chronic rat model of epilepsy, in which rats exhibit spontaneous seizure activity 3 to 4 months after injection of kainic acid into the hippocampus. In agreement with earlier reports, we found increases in glutamate, aspartate and GABA during seizures in human hippocampus. In addition we found increases in taurine which have not previously been reported. The chronic rat model shows increases in the same amino acids as in the human epileptic hippocampus, both during spontaneous seizures and stimulation evoked after-discharges (ADs). In contrast, minimal increases are elicited by hippocampal stimulation in control (non-kainate injected) animals. These results correlate with the degree of mossy fiber reorganization found in the dentate gyrus of kainate rats or epileptic humans.

Histomorphogenesis of soman-induced encephalocardiomyopathy in Sprague-Dawley rats

Although myocardial damage caused by soman has been previously reported, its relation to brain damage is unclear. In order to clarify this relationship, we examined the histomorphogenesis of central nervous system (CNS) and myocardial lesions in Sprague-Dawley rats, given atropine methylnitrate (20 mg/kg) and HI-6 (125 mg/kg) ip 10 min before a single injection of 0 or 130 micrograms soman/kg (sc) and sacrificed 45 min and 1.5 hr, 3 hr, 24 hr, and 72 hr later. Bilaterally symmetrical CNS damage began with vacuolation of the neuropil and was followed by astrocytic degeneration and neuronal necrosis culminating in liquefaction necrosis and focal hemorrhage. The cerebral cortex, limbic system, thalamus, and substantia nigra were common target sites. Repair in affected sites was characterized by capillary endothelial cell proliferation, microgliosis, and reversal of microvacuolation. Myocardial damage began with myocytolysis and contraction bands and evolved into coagulative myocytolysis and replacement fibrosis with a transient recruitment of acute inflammatory cells. The left ventricle, especially its free wall and papillary muscles, was consistently affected. There was good correlation among seizures, CNS damage, and myocardial lesions at all times following treatment. The results support the view that CNS lesions are associated with protracted seizure activity and provide evidence that myocardial damage is neurogenic.