Intrahippocampal cholinesterase inhibition induces epileptogenesis in mice without evidence of neurodegenerative events

Electro-behavioral phenotype and cell injury following exposure to paraoxon-ethyl in mice: Effect of the genetic background

Organophosphorus compounds (OP) are irreversible inhibitors of both central and peripheral cholinesterases (ChE). They still represent a major health issue in some countries as well as a terrorist and military threat. In order to design appropriate medical counter-measures, a better understanding of the pathophysiology of the poisoning is needed. Little to nothing is known regarding the impact of the genetic background on OP-induced seizures and seizure-related cell injury. Using two different mouse strains, Swiss and C57BL/6J, exposed to a convulsing dose of the OP pesticide paraoxon-ethyl (POX), our study focused on seizure susceptibility, especially the occurrence of SE and related mortality. We also evaluated the initial neuropathological response and SE-induced cell injury. Following the administration of 2.4 mg/kg POX, more Swiss mice experienced SE than C57BL/6J (55.6% versus 17.2%) but the duration of their SE, based on EEG recordings, was shorter (64.3 ± 19.5 min versus 180.8 ± 36.8 min). No significant difference was observed between strains regarding mortality (33% versus 14%). In both strains limited cell injury was observed in the medial temporal cortex, the dentate gyrus and the CA3 field without inter-strain differences (Fluorojade C-positive cells/mm²). Conversely, only C57BL/6J mice showed cell injury in the CA1 field. There was no obvious correlation between the number of Fluorojade C-positive cells and the duration of the EEG discharges. Our work suggests some differences between Swiss and C57BL/6J mice and lay ground to further studies on the impact of strains in the development of central nervous system toxicity of OP.

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.

Muscarinic excitation of parvalbumin-positive interneurons contributes to the severity of pilocarpine-induced seizures

OBJECTIVE

A common rodent model in epilepsy research employs the muscarinic acetylcholine receptor (mAChR) agonist pilocarpine, yet the mechanisms underlying the induction of pilocarpine-induced seizures (PISs) remain unclear. Global M1 mAChR (M1 R) knockout mice are resistant to PISs, implying that M1 R activation disrupts excitation/inhibition balance. Parvalbumin-positive (PV) inhibitory neurons express M1 Rs, participate in cholinergically induced oscillations, and can enter a state of depolarization block (DB) during epileptiform activity. Here, we test the hypothesis that pilocarpine activation of M1 Rs expressed on PV cells contributes to PISs.

METHODS

CA1 PV cells in PV-CRE mice were visualized with a floxed YFP or hM3Dq-mCherry adeno-associated virus, or by crossing PV-CRE mice with the RosaYFP reporter line. To eliminate M1 Rs from PV cells, we generated PV-M1 knockout (KO) mice by crossing PV-CRE and floxed M1 mice. Action potential (AP) frequency was monitored during application of pilocarpine (200 μm). In behavioral experiments, locomotion and seizure symptoms were recorded in wild-type (WT) or PV-M1 KO mice during PISs.

RESULTS

Pilocarpine significantly increased AP frequency in CA1 PV cells into the gamma range. In the continued presence of pilocarpine, a subset (5/7) of PV cells progressed to DB, which was mimicked by hM3Dq activation of Gq-receptor signaling. Pilocarpine-induced depolarization, AP firing at gamma frequency, and progression to DB were prevented in CA1 PV cells of PV-M1 KO mice. Finally, compared to WT mice, PV-M1 KO mice were associated with reduced severity of PISs.

SIGNIFICANCE

Pilocarpine can directly depolarize PV+ cells via M1 R activation, but a subset of these cells progress to DB. Our electrophysiologic and behavioral results suggest that this mechanism is active during PISs, contributing to a collapse of PV-mediated γ-aminobutyric acid (GABA)ergic inhibition, dysregulation of excitation/inhibition balance, and increased susceptibility to PISs.

Acetylcholinesterase activity in the cerebrospinal fluid of dogs with seizures

Recent studies in animal models have focused on the role of cholinergic elements, mainly acetylcholinesterase (AChE) and the 'readthrough' acetylcholinesterase isoform (AChE-R), in seizures. A prospective double-masked study was conducted to assess the activity of AChE and AChE-R in cerebrospinal fluid (CSF) of 26 dogs post-seizure, 28 dogs with intervertebral disc disease (IVDD) and 16 healthy dogs. AChE was also measured in the serum in the post-seizure and IVDD groups. The results showed no significant differences in CSF AChE among the three groups. AChE-R was not detected in any dog and AChE in the serum was similar between groups. This preliminary study provides new information on AChE and AChE-R in the CSF and sera of dogs following naturally-occurring seizures.

Treatment of status epilepticus with ketamine, are we there yet?

Status epilepticus (SE), a neurological emergency both in adults and in children, could lead to brain damage and even death if untreated. Generalized convulsive SE (GCSE) is the most common and severe form, an example of which is that induced by organophosphorus nerve agents. First- and second-line pharmacotherapies are relatively consensual, but if seizures are still not controlled, there is currently no definitive data to guide the optimal choice of therapy. The medical community seems largely reluctant to use ketamine, a noncompetitive antagonist of the N-methyl-d-aspartate glutamate receptor. However, a review of the literature clearly shows that ketamine possesses, in preclinical studies, antiepileptic properties and provides neuroprotection. Clinical evidences are scarcer and more difficult to analyze, owing to a use in situations of polytherapy. In absence of existing or planned randomized clinical trials, the medical community should make up its mind from well-conducted preclinical studies performed on appropriate models. Although potentially active, ketamine has no real place for the treatment of isolated seizures, better accepted drugs being used. Its best usage should be during GCSE, but not waiting for SE to become totally refractory. Concerns about possible developmental neurotoxicity might limit its pediatric use for refractory SE.

Exposure to nerve agents: from status epilepticus to neuroinflammation, brain damage, neurogenesis and epilepsy

Epilepsy is a common neurological disorder characterized by an initial injury due to stroke, traumatic brain injury, brain infection, or febrile seizures causing status epilepticus (SE). This phenomenon precedes recurrent (secondary) seizures, the latent period (period without seizures) and downstream appearance of spontaneous recurrent seizures (SRS). Epilepsy inducers include the organophosphorous (OP) compounds modified as chemical warfare nerve agents, such as soman. SE induced by soman is a result of cholinergic system hyperactivity caused by the irreversible inhibition of acetylcholinesterase, and the subsequent increase in the amount of the neurotransmitter acetylcholine at central and peripheral sites. SE leads to profound, permanent, complex and widespread brain damage and associated cognitive and behavioral deficits, accompanied by impaired neurogenesis. Several anticonvulsant and neuroprotective strategies have been studied in order to avoid the epileptogenesis which occurs after SE caused by soman exposure. In recent studies, we showed that SRS occur post-soman exposure and neuropathology can be reduced with diazepam (DZP) and valproic acid (VPA) when administered in combination treatment. These effects are accompanied by neurogenesis seen 15 days post-exposure in the hippocampal dentate gyrus (DG). This review discusses several findings about epilepsy induced by soman exposure such as behavioral changes, EEG anomalies, neuropathology, neuroinflammation, neurogenesis, possible circuitry changes and current strategies for treatment. The soman seizure model is an important model of temporal lobe epilepsy (TLE) and comparable in certain respects with well studied models in the literature such as pilocarpine and kainic acid. All these models together allow for a greater understanding of the different mechanisms of seizure induction, propagation and options for treatment. These studies are very necessary for current military and civilian treatment regimens, against OP nerve agent exposure, which fail to prevent SE resulting in severe neuropathology and epilepsy.

Acetylcholinesterase loosens the brain's cholinergic anti-inflammatory response and promotes epileptogenesis

Recent studies show a key role of brain inflammation in epilepsy. However, the mechanisms controlling brain immune response are only partly understood. In the periphery, acetylcholine (ACh) release by the vagus nerve restrains inflammation by inhibiting the activation of leukocytes. Recent reports suggested a similar anti-inflammatory effect for ACh in the brain. Since brain cholinergic dysfunctions are documented in epileptic animals, we explored changes in brain cholinergic gene expression and associated immune response during pilocarpine-induced epileptogenesis. Levels of acetylcholinesterase (AChE) and inflammatory markers were measured using real-time RT-PCR, in-situ hybridization and immunostaining in wild type (WT) and transgenic mice over-expressing the "synaptic" splice variant AChE-S (TgS). One month following pilocarpine, mice were video-monitored for spontaneous seizures. To test directly the effect of ACh on the brain's innate immune response, cytokines expression levels were measured in acute brain slices treated with cholinergic agents. We report a robust up-regulation of AChE as early as 48 h following pilocarpine-induced status epilepticus (SE). AChE was expressed in hippocampal neurons, microglia, and endothelial cells but rarely in astrocytes. TgS mice overexpressing AChE showed constitutive increased microglial activation, elevated levels of pro-inflammatory cytokines 48 h after SE and accelerated epileptogenesis compared to their WT counterparts. Finally we show a direct, muscarine-receptor dependant, nicotine-receptor independent anti-inflammatory effect of ACh in brain slices maintained ex vivo. Our work demonstrates for the first time, that ACh directly suppresses brain innate immune response and that AChE up-regulation after SE is associated with enhanced immune response, facilitating the epileptogenic process. Our results highlight the cholinergic system as a potential new target for the prevention of seizures and epilepsy.

Detection of adduct on tyrosine 411 of albumin in humans poisoned by dichlorvos

Studies in mice and guinea pigs have shown that albumin is a new biomarker of organophosphorus toxicant (OP) and nerve agent exposure. Our goal was to determine whether OP-labeled albumin could be detected in the blood of humans exposed to OP. Blood from four OP-exposed patients was prepared for mass spectrometry analysis by digesting 0.010 ml of serum with pepsin and purifying the labeled albumin peptide by offline high performance liquid chromatography. Dimethoxyphosphate-labeled tyrosine 411 was identified in albumin peptides VRY(411)TKKVPQVSTPTL and LVRY(411)TKKVPQVSTPTL from two patients who had attempted suicide with dichlorvos. The butyrylcholinesterase activity in these serum samples was inhibited 80%. A third patient whose serum BChE activity was inhibited 8% by accidental inhalation of dichlorvos had undetectable levels of adduct on albumin. A fourth patient whose BChE activity was inhibited 60% by exposure to chlorpyrifos had no detectable adduct on albumin. This is the first report to demonstrate the presence of OP-labeled albumin in human patients. It is concluded that tyrosine 411 of human albumin is covalently modified in the serum of humans poisoned by dichlorvos and that the modification is detectable by mass spectrometry. The special reactivity of tyrosine 411 with OP suggests that other proteins may also be modified on tyrosine. Identification of other OP-modified proteins may lead to an understanding of neurotoxic symptoms that appear long after the initial OP exposure.

In vitro effects of antiepileptic drugs on acetylcholinesterase and ectonucleotidase activities in zebrafish (Danio rerio) brain

Carbamazepine (CBZ), phenytoin (PHT), and gabapentine (GBP) are classical antiepileptic drugs (AEDs) that act through a variety of mechanisms. We have tested the in vitro effects of CBZ, PHT, and GBP at different concentrations on ectonucleotidase and acetylcholinesterase activities in zebrafish brain. CBZ inhibited ATP hydrolysis at 1000 microM (32%) whereas acetylcholine hydrolysis decreased at 500 microM (25.2%) and 1000 microM (38.7%). PHT increased AMP hydrolysis both at 500 microM (65%) and 1000 microM (64.8%). GBP did not promote any significant changes on ectonucleotidase and acetylcholinesterase activities. These results have shown that CBZ can reduce NTPDase (nucleoside triphosphate diphosphohydrolase) and PHT enhance ecto 5'-nucleotidase activities. Therefore, it is possible to suggest that the AEDs induced-effects on ectonucleotidases are related to enzyme anchorage form. Our findings have also shown that high CBZ concentrations inhibit acetylcholinesterase activity, which can induce an increase of acetylcholine levels. Taken together, these results showed a complex interaction among AEDs, purinergic, and cholinergic systems, providing a better understanding of the AEDs pharmacodynamics.