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

doi:10.1016/j.neuroscience.2008.11.053

Neuroscience

Volume 159, Issue 1, 3 March 2009, Pages 380-389

https://doi.org/10.1016/j.neuroscience.2008.11.053 Get rights and content

Abstract

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 μM 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.

Section snippets

Experimental procedures

Coronal slices containing both the amygdala and the hippocampus were prepared from male Sprague–Dawley rats, weighing 445–570 g (498.1±8.5, mean±S.E.; n=25; age range: 4–5.5 months). The rats were deeply anesthetized with isoflurane and decapitated. The brain was rapidly removed and placed, for 1–2 min, in ice-cold artificial cerebrospinal fluid (ACSF) consisting of (in mM) 125 NaCl, 3 KCl, 2.0 CaCl₂, 2 MgCl₂, 25 NaHCO₃, 1.25 NaH₂PO₄, and 10 glucose, and bubbled with 95% O₂ and 5% CO₂ to

Results

In all of the experiments described below, the recording slice medium consisted of (in mM) 125 NaCl, 5 KCl, 2.0 CaCl₂, 1 MgCl₂, 25 NaHCO₃, 1.25 NaH₂PO₄, and 10 glucose. All extracellularly recorded signals were collected in the gap-free mode to capture spontaneous events. Control recordings were obtained for 20–40 min before application of soman. Single stimulus pulses were delivered every 30 s to the external capsule and the Schaffer collaterals to evoke field potentials in the BLA and the CA1

Discussion

Previous studies in guinea-pig hippocampal slices have shown that in the CA1 region, paraoxon (Endres et al 1989, Harrison et al 2004) and soman (Harrison et al 2004, Harrison et al 2005, Apland 2001) induce interictal activity. There are no previous reports of the effects of nerve agents on amygdala slices. The in vitro generation of seizure-like prolonged neuronal discharges by the amygdala in response to a nerve agent, when the hippocampus, under the same conditions, generates only

Acknowledgments

This work was supported by the Uniformed Services University of the Health Sciences grant H070SG and by the National Institutes of Health CounterACT Program through the National Institute of Neurological Disorders and Stroke (award # U01 NS058162-01). The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. This work was also supported by the

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Temporal lobe epileptic seizures are one of the most common and well-characterized types of epilepsies. The current knowledge on the pathology of temporal lobe epilepsy relies strongly on studies of epileptogenesis caused by experimentally induced status epilepticus (SE). Although several temporal lobe structures have been implicated in the epileptogenic process, the hippocampal formation is the temporal lobe structure studied in the greatest amount and detail. However, studies in human patients and animal models of temporal lobe epilepsy indicate that the amygdaloid complex can be also an important seizure generator, and several pathological processes have been shown in the amygdala during epileptogenesis.
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Nerve agents are organophosphorus acetylcholinesterase inhibitors. Acute exposure to nerve agents can cause rapid death. In this review, we summarize the history of nerve agent development and use in warfare, the mechanisms by which these agents cause death or long-term brain damage, and the treatments for preventing death or long-term morbidity. The G-series nerve agents, tabun, sarin, soman, ethyl sarin, and cyclosarin, were developed by the Nazis. VX, the best-known of the V-series agents, was synthesized in the 1950's by a British scientist. Little is known about the development of the novichoks (the “A-series”) by the former Soviet Union. Nerve agents were used for the first time in the battlefield by the Iraqi government in the Iran–Iraq War, in the 1980s. The Chemical Weapons Convention, in 1993, banned all chemical weapons production and use, yet, sarin was subsequently used in terrorist attacks in Japan and, recently, in the war in Syria. Pyridostigmine has been used as a prophylactic treatment, and bioscavengers are presently investigated as a better alternative. Atropine, along with an oxime, can prevent rapid death from the nerve agent-induced peripheral cholinergic crisis. Treatment with diazepam or midazolam for the cessation of nerve agent-induced status epilepticus cannot protect against brain damage, and, therefore, these benzodiazepines should be replaced by novel anticonvulsants and neuroprotectants. The AMPA/GluK1 receptor antagonist LY293558 (tezampanel) has shown superior antiseizure and neuroprotective efficacy against soman, particularly when administered in combination with caramiphen, an antagonist of muscarinic and NMDA receptors.
This article is part of the special issue entitled ‘Acetylcholinesterase Inhibitors: From Bench to Bedside to Battlefield’.
Targeting the glutamatergic system to counteract organophosphate poisoning: A novel therapeutic strategy
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One of the devastating effects of acute exposure to organophosphates, like nerve agents, is the induction of severe and prolonged status epilepticus (SE), which can cause death, or brain damage if death is prevented. Seizures after exposure are initiated by muscarinic receptor hyperstimulation—after inhibition of acetylcholinesterase by the organophosphorus agent and subsequent elevation of acetylcholine—but they are reinforced and sustained by glutamatergic hyperexcitation, which is the primary cause of brain damage. Diazepam is the FDA-approved anticonvulsant for the treatment of nerve agent-induced SE, and its replacement by midazolam is currently under consideration. However, clinical data derived from the treatment of SE of any etiology, as well as studies on the control of nerve agent-induced SE in animal models, have indicated that diazepam and midazolam control seizures only temporarily, their antiseizure efficacy is reduced as the latency of treatment from the onset of SE increases, and their neuroprotective efficacy is limited or absent. Here, we review data on the discovery of a novel anticonvulsant and neuroprotectant, LY293558, an AMPA/GluK1 receptor antagonist. Treatment of soman-exposed immature, young-adult, and aged rats with LY293558, terminates SE with limited recurrence of seizures, significantly protects from brain damage, and prevents long-term behavioral deficits, even when LY293558 is administered 1 h post-exposure. More beneficial effects and complete neuroprotection is obtained when LY293558 administration is combined with caramiphen, which antagonizes NMDA receptors. Further efficacy studies may bring the LY293558 + caramiphen combination therapy on the pathway to approval for human use.
Retrograde activation of CB1R by muscarinic receptors protects against central organophosphorus toxicity
2019, Neuropharmacology
Citation Excerpt :
The soman concentration was chosen during pilot studies, which revealed no significant differences in ictogenic responses between 1 and 10 μM soman, whereas perfusion with 0.1 μM soman did not have an apparent neurophysiological effect within 20 min (not shown). Use of 10 μM soman is consistent with previous hippocampal soman perfusion studies (Apland et al., 2009; Wang et al., 2011) and corresponds to the estimated maximum plasma concentration (Cmax) following subcutaneous administration of 1 LD50 soman to mice. Soman elicited a 36.0 ± 7.2% reduction in fEPSP amplitudes, with a median time-to-maximal depression (t50) of 3.42 ± 1.67 min (n = 12; Fig. 1A).
The acute toxicity of organophosphorus-based compounds is primarily a result of acetylcholinesterase inhibition in the central and peripheral nervous systems. The resulting cholinergic crisis manifests as seizure, paralysis, respiratory failure and neurotoxicity. Though overstimulation of muscarinic receptors is the mechanistic basis of central organophosphorus (OP) toxicities, short-term changes in synapse physiology that precede OP-induced seizures have not been investigated in detail. To study acute effects of OP exposure on synaptic function, field excitatory postsynaptic potentials (fEPSPs) were recorded from Schaffer collateral synapses in the mouse hippocampus CA1 stratum radiatum during perfusion with various OP compounds. Administration of the OPs paraoxon, soman or VX rapidly and stably depressed fEPSPs via a presynaptic mechanism, while the non-OP proconvulsant tetramethylenedisulfotetramine had no effect on fEPSP amplitudes. OP-induced presynaptic long-term depression manifested prior to interictal spiking, occurred independent of recurrent firing, and did not require NMDA receptor currents, suggesting that it was not mediated by activity-dependent calcium uptake. Pharmacological dissection revealed that the presynaptic endocannabinoid type 1 receptor (CB1R) as well as postsynaptic M₁ and M₃ muscarinic acetylcholine receptors were necessary for OP-LTD. Administration of CB1R antagonists significantly reduced survival in mice after a soman challenge, revealing an acute protective role for endogenous CB1R signaling during OP exposure. Collectively these data demonstrate that the endocannabinoid system alters glutamatergic synaptic function during the acute response to OP acetylcholinesterase inhibitors.
A rat model of nerve agent exposure applicable to the pediatric population: The anticonvulsant efficacies of atropine and GluK1 antagonists
2015, Toxicology and Applied Pharmacology
Inhibition of acetylcholinesterase (AChE) after nerve agent exposure induces status epilepticus (SE), which causes brain damage or death. The development of countermeasures appropriate for the pediatric population requires testing of anticonvulsant treatments in immature animals. In the present study, exposure of 21-day-old (P21) rats to different doses of soman, followed by probit analysis, produced an LD₅₀ of 62 μg/kg. The onset of behaviorally-observed SE was accompanied by a dramatic decrease in brain AChE activity; rats who did not develop SE had significantly less reduction of AChE activity in the basolateral amygdala than rats who developed SE. Atropine sulfate (ATS) at 2 mg/kg, administered 20 min after soman exposure (1.2 × LD₅₀), terminated seizures. ATS at 0.5 mg/kg, given along with an oxime within 1 min after exposure, allowed testing of anticonvulsants at delayed time-points. The AMPA/GluK1 receptor antagonist LY293558, or the specific GluK1 antagonist UBP302, administered 1 h post-exposure, terminated SE. There were no degenerating neurons in soman-exposed P21 rats, but both the amygdala and the hippocampus were smaller than in control rats at 30 and 90 days post-exposure; this pathology was not present in rats treated with LY293558. Behavioral deficits present at 30 days post-exposure, were also prevented by LY293558 treatment. Thus, in immature animals, a single injection of atropine is sufficient to halt nerve agent-induced seizures, if administered timely. Testing anticonvulsants at delayed time-points requires early administration of ATS at a low dose, sufficient to counteract only peripheral toxicity. LY293558 administered 1 h post-exposure, prevents brain pathology and behavioral deficits.
LY293558 prevents soman-induced pathophysiological alterations in the basolateral amygdala and the development of anxiety
2015, Neuropharmacology
Citation Excerpt :
First, it is a key brain area for the initiation and propagation of seizures after exposure; seizures are not induced unless AChE decreases significantly in the BLA (Prager et al., 2013), and microinjection of nerve agents into different brain regions can elicit convulsions only when the nerve agent is injected into the BLA (McDonough et al., 1987). Second, it is one of the most severely damaged brain region after nerve agent exposure (Apland et al., 2010; Aroniadou-Anderjaska et al., 2009; Shih et al., 2003), suggesting more intense seizure activity in this area (Apland et al., 2009), and/or high susceptibility to seizure-induced damage. Finally, the most prevalent behavioral deficit resulting from nerve agent exposure is anxiety (Coubard et al., 2008; Filliat et al., 2007; Hoffman et al., 2007; Langston et al., 2012; Ohtani et al., 2004; Prager et al., 2014), and a characteristic feature of anxiety is a derangement in the excitability of the BLA (Koenigs and Grafman, 2009; Mahan and Ressler, 2012; Rauch et al., 2000; Sajdyk and Shekhar, 1997; Zhou et al., 2010).
Exposure to nerve agents can cause brain damage due to prolonged seizure activity, producing long-term behavioral deficits. We have previously shown that LY293558, a GluK1/AMPA receptor antagonist, is a very effective anticonvulsant and neuroprotectant against nerve agent exposure. In the present study, we examined whether the protection against nerve agent-induced seizures and neuropathology conferred by LY293558 translates into protection against pathophysiological alterations in the basolateral amygdala (BLA) and the development of anxiety, which is the most prevalent behavioral deficit resulting from exposure. LY293558 (15 mg/kg) was administered to rats, along with atropine and HI-6, at 20 min after exposure to soman (1.2 × LD50). At 24 h, 7 days, and 30 days after exposure, soman-exposed rats who did not receive LY293558 had reduced but prolonged evoked field potentials in the BLA, as well as increased paired-pulse ratio, suggesting neuronal damage and impaired synaptic inhibition; rats who received LY293558 did not differ from controls in these parameters. Long-term potentiation of synaptic transmission was impaired at 7 days after exposure in the soman-exposed rats who did not receive anticonvulsant treatment, but not in the LY293558-treated rats. Anxiety-like behavior assessed by the open field and acoustic startle response tests was increased in the soman-exposed rats at 30 and 90 days after exposure, while rats treated with LY293558 did not differ from controls. Along with our previous findings, the present data demonstrate the remarkable efficacy of LY293558 in counteracting nerve agent-induced seizures, neuropathology, pathophysiological alterations in the BLA, and anxiety-related behavioral deficits.

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¹: These authors have contributed equally to the study.

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

Abstract

Section snippets

Experimental procedures

Results

Discussion

Acknowledgments

Epilepsy Res

Toxicology

Crit Care Clin

Neuroscience

Neuron

Neurotoxicology

Toxicol Sci

Trends Pharmacol Sci

Toxicology

Epilepsy Res

Eur J Pharmacol

Neuroscience

Neurol Sci

Prog Neurobiol

Neuropharmacology

Brain Res

Neuroscience

Brain Res

Neurosci Biobehav Rev

Neurosci Biobehav Rev

Epilepsy Res

Neuropharmacology

Neuropharmacology

Epilepsy Res

Toxicol Applied Pharmacol

Prog Brain Res

Organophosphorus ester-induced delayed neurotoxicity

Annu Rev Pharmacol Toxicol

Effects of anticonvulsant drugs on soman-induced epileptiform activity in guinea pig hippocampal-entorhinal slicesUSAMRICD-TR-01-02

Input-specific LTP and depotentiation in the basolateral amygdala

Neuroreport

Mechanisms regulating GABAergic inhibitory transmission in the basolateral amygdala: Implications for epilepsy and anxiety disorders

Amino Acids

Complex view on poisoning with nerve agents and organophosphates

Acta Medica (Hradec Kralove)

Biochemical and behavioral effects of soman vapors in low concentrations

Inhal Toxicol

Soman-induced morphological changes: an overview in the nonhuman primate

J Appl Toxicol

Assessing the extent of RNA editing in the TMII regions of GluR5 and GluR6 kainate receptors during rat brain development

J Neurochem

Bidirectional modulation of GABA release by presynaptic glutamate receptor 5 kainate receptors in the basolateral amygdala

J Neurosci

The physiological role of kainate receptors in the amygdala

Mol Neurobiol

Review of health consequences from high, intermediate, and low-level exposure to organophosphorus nerve agents

J Appl Toxicol

Dimensions and ion selectivity of recombinant AMPA and kainate receptor channels and their dependence on Q/R site residues

J Physiol (Lond)

A hippocampal GluR5 kainate receptor regulating inhibitory synaptic transmission

Nature

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