The effect of acetylcholinesterase-inhibition on depolarization-induced GABA release from rat striatal slices

Quantitative Proteomic Changes after Organophosphorous Nerve Agent Exposure in the Rat Hippocampus

The widespread use of organophosphorous (OP) compounds and recent misuse of nerve agents on civilians requires an urgent need to decode their complex biological response to develop effective drugs. Proteomic profiling of biological target tissues helps in identification of molecular toxicity mechanisms. Quantitative proteomics profiling of the rat hippocampus was studied in this study. Liquid chromatography mass spectrometry (LC-MS) analysis of tandem mass tag (TMT)-labeled lysates identified 6356 proteins. A total of 69, 61, and 77 proteins were upregulated, and 66, 35, and 70 proteins were downregulated at 30 min, 1 day, and 7 days after soman exposure. This is the first report on the soman-induced proteomic changes to the best of our knowledge. Bioinformatics analysis revealed soman-induced broad-range proteomic changes in key pathways related to glutamate, acetylcholine, GABA, 5-hydroxytryptamine, and adrenergic receptors, G-protein signaling, chemokine and cytokine-mediated inflammation, cytoskeleton, neurodegeneration (Parkinson's and Alzheimer's), Wnt signaling, synaptic vesicle trafficking, MAP kinases, proteosome degradation, metabolism, and cell death. Selected protein changes were verified by immunoblotting, and neuropathological findings indicated significant brain damage. Results demonstrate that persistent proteomic changes in the brain can cause multiple neurological effects through cholinergic and non-cholinergic pathways, and these mechanistic insights are useful in the development of novel drugs.

α-Linolenic Acid, A Nutraceutical with Pleiotropic Properties That Targets Endogenous Neuroprotective Pathways to Protect against Organophosphate Nerve Agent-Induced Neuropathology

α-Linolenic acid (ALA) is a nutraceutical found in vegetable products such as flax and walnuts. The pleiotropic properties of ALA target endogenous neuroprotective and neurorestorative pathways in brain and involve the transcription factor nuclear factor kappa B (NF-κB), brain-derived neurotrophic factor (BDNF), a major neuroprotective protein in brain, and downstream signaling pathways likely mediated via activation of TrkB, the cognate receptor of BDNF. In this review, we discuss possible mechanisms of ALA efficacy against the highly toxic OP nerve agent soman. Organophosphate (OP) nerve agents are highly toxic chemical warfare agents and a threat to military and civilian populations. Once considered only for battlefield use, these agents are now used by terrorists to inflict mass casualties. OP nerve agents inhibit the critical enzyme acetylcholinesterase (AChE) that rapidly leads to a cholinergic crisis involving multiple organs. Status epilepticus results from the excessive accumulation of synaptic acetylcholine which in turn leads to the overactivation of muscarinic receptors; prolonged seizures cause the neuropathology and long-term consequences in survivors. Current countermeasures mitigate symptoms and signs as well as reduce brain damage, but must be given within minutes after exposure to OP nerve agents supporting interest in newer and more effective therapies. The pleiotropic properties of ALA result in a coordinated molecular and cellular program to restore neuronal networks and improve cognitive function in soman-exposed animals. Collectively, ALA should be brought to the clinic to treat the long-term consequences of nerve agents in survivors. ALA may be an effective therapy for other acute and chronic neurodegenerative disorders.

Alpha-linolenic acid is a potent neuroprotective agent against soman-induced neuropathology

Nerve agents are deadly threats to military and civilian populations around the world. Nerve agents cause toxicity to peripheral and central sites through the irreversible inhibition of acetylcholinesterase, the enzyme that metabolizes acetylcholine. Excessive acetylcholine accumulation in synapses results in status epilepticus in the central nervous system. Prolonged status epilepticus leads to brain damage, neurological dysfunction and poor outcome. Anticonvulsants are effective but must be given rapidly following exposure. Because these agents cause mass casualties, effective neuroprotective agents are needed to reduce brain damage and improve cognitive outcome. α-Linolenic acid is an omega-3 fatty acid that is found in vegetable products and has no known side effects. α-Linolenic acid is neuroprotective against kainic acid-induced brain damage in vivo, but its neuroprotective efficacy against nerve agents is unknown. α-Linolenic acid also exerts anti-depressant and anti-inflammatory activities and enhances synaptic plasticity in vivo. These properties make this polyunsaturated fatty acid (PUFA) a potential candidate against nerve agent-induced neuropathology. Here we show that α-linolenic acid is neuroprotective against soman-induced neuropathology in either a pretreatment or post-treatment paradigm. We also show that subcutaneous injection of α-linolenic acid shows greater neuroprotective efficacy compared with intravenous injection in a brain region-specific manner.

Organophosphate-induced brain damage: mechanisms, neuropsychiatric and neurological consequences, and potential therapeutic strategies

Organophosphate (OP)-induced brain damage is defined as progressive damage to the brain, resulting from the cholinergic neuronal excitotoxicity and dysfunction induced by OP-induced irreversible AChE inhibition. This delayed secondary neuronal damage that occurs mainly in the cholinergic regions of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, might be largely responsible for persistent profound neuropsychiatric and neurological impairments (memory, cognitive, mental, emotional, motor and sensory deficits) in the victims of OP poisoning. Neuroprotective strategies for attenuating OP-induced brain damage should target different development stages of OP-induced brain damage, and may include but not limited to: (1) Antidote therapies with atropine and related efficient anticholinergic drugs; (2) Anti-excitotoxic therapies targeting attenuation of cerebral edema and inflammatory reaction, blockage of calcium influx, inhibition of apoptosis program, and the control of seizures; (3) Neuroprotective strategies using cytokines, antioxidants and NMDAR antagonists (a single drug or a combination of drugs) to slow down the process of secondary neuronal damage; and (4) Therapies targeting individual symptoms or clusters of chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may help limit or prevent secondary neuronal damage at the early stage of OP poisoning and attenuate the subsequent neuropsychiatric and neurological impairments, thus reducing the long-term disability caused by exposure to OPs.

Primary brain targets of nerve agents: the role of the amygdala in comparison to the hippocampus

Exposure to nerve agents and other organophosphorus acetylcholinesterases used in industry and agriculture can cause death, or brain damage, producing long-term cognitive and behavioral deficits. Brain damage is primarily caused by the intense seizure activity induced by these agents. Identifying the brain regions that respond most intensely to nerve agents, in terms of generating and spreading seizure activity, along with knowledge of the physiology and biochemistry of these regions, can facilitate the development of pharmacological treatments that will effectively control seizures even if administered when seizures are well underway. Here, we contrast the pathological (neuronal damage) and pathophysiological (neuronal activity) findings of responses to nerve agents in the amygdala and the hippocampus, the two brain structures that play a central role in the generation and spread of seizures. The evidence so far suggests that exposure to nerve agents causes significantly more damage in the amygdala than in the hippocampus. Furthermore, in in vitro brain slices, the amygdala generates prolonged, seizure-like neuronal discharges in response to the nerve agent soman, at a time when the hippocampus generates only interictal-like activity. In vivo experiments are now required to confirm the primary role that the amygdala seems to play in nerve agent-induced seizure generation.

Soman induces ictogenesis in the amygdala and interictal activity in the hippocampus that are blocked by a GluR5 kainate receptor antagonist in vitro

Exposure to organophosphorus nerve agents induces brain seizures, which can cause profound brain damage resulting in death or long-term cognitive deficits. The amygdala and the hippocampus are two of the most seizure-prone brain structures, but their relative contribution to the generation of seizures after nerve agent exposure is unclear. Here, we report that application of 1 muM soman for 30 min, in rat coronal brain slices containing both the hippocampus and the amygdala, produces prolonged synchronous neuronal discharges (10-40 s duration, 1.5-5 min interval of occurrence) resembling ictal activity in the basolateral nucleus of the amygdala (BLA), but only interictal-like activity ("spikes" of 100-250 ms duration; 2-5 s interval) in the pyramidal cell layer of the CA1 hippocampal area. BLA ictal- and CA1 interictal-like activity were synaptically driven, as they were blocked by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. As the expression of the GluR5 subunit of kainate receptors is high in the amygdala, and kainate receptors containing this subunit (GluR5KRs) play an important role in the regulation of neuronal excitability in both the amygdala and the hippocampus, we tested the efficacy of a GluR5KR antagonist against the epileptiform activity induced by soman. The GluR5KR antagonist UBP302 reduced the amplitude of the hippocampal interictal-like spikes, and eliminated the seizure-like discharges in the BLA, or reduced their duration and frequency, with no significant effect on the evoked field potentials. This is the first study reporting in vitro ictal-like activity in response to a nerve agent. Our findings, along with previous literature, suggest that the amygdala may play a more important role than the hippocampus in the generation of seizures following soman exposure, and provide the first evidence that GluR5KR antagonists may be an effective treatment against nerve agent-induced seizures.

Ketamine reduces cholinergic modulated GABA release from rat striatal slices

Striatal GABA release has been shown to be enhanced under pathological conditions of cholinergic overstimulation, e.g. inhibition of acetylcholine esterase. This increase in striatal GABA release during cholinergic overstimulation is mediated by M-cholinoceptors and is associated with clinical symptoms, e.g. the occurrence of seizures. Little is known about the effects of drugs on cholinergic modulated GABA release in the striatum. To investigate the effects of the N-methyl-D-aspartate (NMDA) antagonist MK-801 and the intravenous anaesthetic ketamine on cholinergic modulated depolarisation-induced GABA release, both drugs were coadministered with the M-cholinoceptor agonist pilocarpine in a superfusion model of rat striatal slices. The concentration of GABA was determined in the superfusate by use of high performance liquid chromatography. Evoked GABA release was increased by pilocarpine with a half-effective concentration of 53.8 microM. This increase could be attenuated by the M1-cholinoceptor antagonist pirenzepine (10 microM). MK-801 and ketamine reduced evoked GABA release enhanced by pilocarpine dose dependently with half-effective concentrations of 6.7 microM (MK-801) and 6.9 microM (ketamine), a concentration that is clinically relevant for ketamine anaesthesia. This reduction of striatal GABA release may therefore contribute to the beneficial effect of both drugs in pathological situations of cholinergic overstimulation, e.g. during intoxication with acetylcholine esterase inhibitors.