Effects of soman poisoning on mitochondrial respiratory enzyme activity in the mouse hippocampus and cerebral cortex.

Mitochondrial dysfunctions have been highlighted as a contributing factor in epileptic seizures and subsequent neuronal cell death. Soman is an irreversible inhibitor of cholinesterase, triggering epileptic seizures leading to massive neuronal cell death in brain areas, such as the hippocampus and cerebral cortex. Mitochondrial respiratory chain enzymatic assays were performed in hippocampus and cerebral cortex homogenates from mouse brains collected 3 hours, 24 hours, 3 days, and 7 days after soman poisoning. Our results suggest that mitochondrial enzymatic alterations stem more likely from secondary effects of the poisoning, rather than from any fallout effect from neuronal cell death.

Long-term consequences of soman poisoning in mice Part 1. Neuropathology and neuronal regeneration in the amygdala.

To date, studies on soman-induced neuropathology mainly focused on the hippocampus, since this brain region is a well-delimited area with easily detectable pyramidal neurons. Moreover, the hippocampus is severely damaged after soman exposure leading to a substantial alteration of behavioral mnemonic processes. The neuropathology described in the hippocampus, however, and its behavioral consequences cannot be extrapolated to all other limbic damaged brain areas such as the amygdala. Accordingly, in this inaugural paper, using hemalun-phloxin staining and NeuN immunohistochemistry, the number of damaged and residual healthy neurons was quantified in the amygdala in mice over a 90-day period after soman injection (1.2LD(50) of soman). On post-soman day 1, a moderate neuronal cell death (about 23% of the whole neurons) was evidenced. In parallel, a large quantity of degenerating neurons (about 36% of the whole neurons) occurred in this brain region and survived from post-soman day 1 to day 15. The death of these damaged neurons was initiated on post-soman day 30, and ended on post-soman day 90. Concomitantly, as quantified by NeuN immunohistochemistry, a clear neuronal regeneration was demonstrated in the amygdala of soman-poisoned mice between 60 and 90 days after neurotoxicant exposure. In the companion paper (see part 2), the possible effects of both long-term neuropathology and delayed neuronal regeneration were evaluated on amygdala-driven emotional processes.

Soman poisoning induces delayed astrogliotic scar and angiogenesis in damaged mouse brain areas.

Gliotic scar formation and angiogenesis are two biological events involved in the tissue reparative process generally occurring in the brain after mechanically induced injury, ischemia or cerebral tumor development. For the first time, in this study, neo-vascularization and glial scar formation were investigated in the brain of soman-poisoned mice over a 3-month period after nerve agent exposure (1.2 LD50 of soman). Using anti-claudin-5 and anti-vascular endothelial growth factor (VEGF) immunostaining techniques on brain sections, blood vessels were quantified and VEGF expression was verified to appraise the level of neo-angiogenesis induced in damaged brain areas. Furthermore, glial scar formation and neuropathology were estimated over time in the same injured brain regions by anti-glial fibrillary acidic protein (GFAP) immunohistochemistry and hemalun-phloxin (H&P) dye staining, respectively. VEGF over-expression was noticed on post-soman day 3 in lesioned areas such as the hippocampal CA1 field and amygdala. This was followed by an increase in the quantity of mature blood vessels, 3 months after soman poisoning, in the same brain areas. On the other hand, massive astroglial cell activation was demonstrated on post-soman day 8. Reactive astroglial cells were located only in damaged cerebral regions where H&P-stained eosinophilic neurons were found. For longer experimental times, astroglial response slowly decreased overtime but remained detectable on post-soman day 90 in some discrete brain regions (i.e. CA1 field and amygdala) evidencing the formation of a glial scar. In this study, we discuss the key role of VEGF in the angiogenic process and in the glial or neuronal response induced by soman poisoning.

Long-term behavioral consequences of soman poisoning in mice.

We investigated the long-term (up to 90 days) consequences of soman intoxication in mice on weight, motor performances (grip strength, rotarod) and mnemonic cognitive processes (T-maze, Morris water maze test). First, a relative weight loss of 20%, measured 3 days after intoxication, was evidenced as a threshold beyond which neuropathological damage was observed in the hippocampus. Animals were then distributed into either low weight loss (LWL) or high weight loss (HWL) groups according to the relative 20% weight loss threshold. Compared to controls, both groups of poisoned mice quickly exhibited a decrease in their motor performance subsequent to an acute soman toxicity phase. Then, total motor recovery occurred for the LWL group. Comparatively, HWL mice showed only transient recovery prior to a second decrease phase due to soman-induced delayed toxicity. One month after intoxication, mnemonic cognitive performances of the LWL group were similar to controls while the HWL group did not exhibit any learning skill. Three months after poisoning, compared to controls, the LWL group showed similar mnemonic performances in the maze test but a mild deficit in the Morris water maze task. At the same time, learning skills slightly recovered in the HWL group. Mnemonic cognitive data are discussed in relation to the neuropathology, neurogenesis and sprouting occurring in the hippocampus of soman-intoxicated animals.

Effects of aspirin and mefenamic acid on soman poisoning-induced neuropathology in mice.

The efficacy of aspirin and mefenamic acid to counteract soman-induced brain damage was investigated in mice. Neuronal damage was evaluated in the hippocampus and amygdala by performing omega3 receptor density measurements and hemalun-phloxin staining. The effect of both drugs on the proliferation of neural progenitors after soman exposure was also assessed. Mefenamic acid aggravated the soman-induced hippocampal neuropathology. On the other hand, aspirin recorded a weak neuroprotective effect in the amygdala. However, this drug also diminished the proliferation of neural precursor cells. The possible neurochemical mechanisms underlying such differences in the efficacy of the two drugs are also reviewed.

Early reduction of NeuN antigenicity induced by soman poisoning in mice can be used to predict delayed neuronal degeneration in the hippocampus.

The neuronal nuclei (NeuN) antigen is increasingly being used as a specific marker to identify neuronal cell loss under various pathological conditions. However, recent studies pointed out that a decrease in NeuN labeling could also be due to the reduction of protein expression level or loss of antigenicity and this was not necessarily related to neuronal cell disappearance. We also investigated the presence of damaged neurons, the loss of NeuN immunoreactivity and the level of NeuN protein in the brain hippocampus of mice subjected to soman poisoning (1.2 LD50 of soman). Damaged neurons were detected using hemalun-phloxin (H&P) and Fluoro-Jade B (FJB) staining on brain sections. NeuN immunohistochemistry was also performed on adjacent brain sections and NeuN protein level quantified by Western blot analysis. One and eight days after soman exposure, about 49% of hippocampal neurons were damaged, as assessed by H&P or FJB staining. NeuN immunohistochemistry indicated that all these damaged neurons were deprived of NeuN immunoreactivity. Using Western blot analysis, we proved that loss of NeuN immunoreactivity in degenerating neurons was due to reduced NeuN antigenicity rather than a fall in protein expression level. In this study, we discuss the potential use of NeuN immunohistochemistry as a good biomarker to predict delayed neuronal degeneration in the rodent hippocampus after various brain injuries.

Neuronal regeneration partially compensates the delayed neuronal cell death observed in the hippocampal CA1 field of soman-poisoned mice.

Soman poisoning induces long-term neuropathology characterized by the presence of damaged neurons up to 2 months after exposure in various central brain areas, especially the hippocampal CA1 layer. Rapid depletion of this layer could therefore be expected. Surprisingly, the CA1 layer remained consistently visible, suggesting delayed death of these damaged neurons, potentially accompanied by neuronal regeneration. To address this issue, mice were exposed to a convulsive dose of soman (110 microg/kg followed by 5.0mg/kg of atropine methyl nitrate (MNA) 1 min later) and brains were collected from day 1 to day 90 post-exposure. Damaged and residual healthy neurons were quantified on brain sections using hemalun-phloxin and fluorojade staining or neuronal nuclei antigen (NeuN) immunohistochemistry. On post-soman day 1, a moderate neuronal cell death was noticed in the hippocampal CA1 layer. In this area, an important and steady quantity of damaged neurons (about 48% of the whole pyramidal neurons) was detected from post-soman day 1 to day 30. Thus, throughout this period, damaged neurons seemed to survive, as confirmed by the unmodified depth of the hippocampal CA1 layer. The dramatic disappearance of the damaged neurons occurred only later during the experiment and was almost complete at day 90 after soman exposure. Interestingly, between day 30 and day 90 following poisoning, an increase in the number of residual healthy pyramidal neurons was observed. These different kinetic patterns related to the density of total, damaged and residual healthy neurons after soman poisoning demonstrate that neuronal regeneration is delayed in the hippocampal CA1 layer and is concomitant to the death of damaged neurons.

Effect of cytokine treatment on the neurogenesis process in the brain of soman-poisoned mice.

We previously described that enhanced proliferation of neural progenitors occurred in the subgranular zone (SGZ) of the dentate gyrus and in the subventricular zone (SVZ) of the mouse brain following soman poisoning. Then, a discrete number of these cells seemed to migrate and engraft into the main damaged brain regions (hippocampus; septum and amygdala) and subsequently differentiate into neurons. In the present study, the effect of a cytokine treatment on the neurogenesis process was evaluated. For this purpose, subcutaneous injection of a cocktail of 40 microg/kg epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) was administered daily to soman-poisoned mice (110 microg/kg soman and 5.0 mg/kg methyl nitrate atropine), from post-soman days 1 to 8. To label replicating neural progenitors, 200 mg/kg bromodeoxyuridine (BrdU) was injected twice a day between post-soman days 6 and 8. Mice were sacrificed on post-soman day 9 or 34. On post-soman day 9, the cytokine treatment had no effect on the proliferation of neural progenitors in the SVZ and SGZ, as assessed by BrdU immunochemistry. However, this treatment seemed to promote the migration of neural precursor cells from the proliferative areas towards damaged brain regions. Indeed, in the CA1 hippocampal layer of soman-poisoned mice, on post-soman day 34, the cytokine treatment increased the number of healthy pyramidal neurons stained by hemalun-eosin dye. The cytokine treatment also augmented the number of BrdU-labeled cells in the CA1 hippocampal layer and amygdala. Interestingly, the administration of cytokines resulted in the differentiation of BrdU-positive cells into new neurons in the CA1 hippocampal layer, whereas astrocytic differentiation was preferentially observed in the amygdala.

Soman poisoning increases neural progenitor proliferation and induces long-term glial activation in mouse brain.

To date, only short-term glial reaction has been extensively studied following soman or other warfare neurotoxicant poisoning. In a context of cell therapy by neural progenitor engraftment to repair brain damage, the long-term effect of soman on glial reaction and neural progenitor division was analyzed in the present study. The effect of soman poisoning was estimated in mouse brains at various times ranging from 1 to 90 days post-poisoning. Using immunochemistry and dye staining techniques (hemalun-eosin staining), the number of degenerating neurons, the number of dividing neural progenitors, and microglial, astroglial or oligodendroglial cell activation were studied. Soman poisoning led to rapid and massive (post-soman day 1) death of mature neurons as assessed by hemalun-eosin staining. Following this acute poisoning phase, a weak toxicity effect on mature neurons was still observed for a period of 1 month after poisoning. A massive short-termed microgliosis peaked on day 3 post-poisoning. Delayed astrogliosis was observed from 3 to 90 days after soman poisoning, contributing to glial scar formation. On the other hand, oligodendroglial cells or their precursors were practically unaffected by soman poisoning. Interestingly, neural progenitors located in the subgranular zone of the dentate gyrus (SGZ) or in the subventricular zone (SVZ) of the brain survived soman poisoning. Furthermore, soman poisoning significantly increased neural progenitor proliferation in both SGZ and SVZ brain areas on post-soman day 3 or day 8, respectively. This increased proliferation rate was detected up to 1 month after poisoning.

Effect of soman poisoning on populations of bone marrow and peripheral blood cells in mice.

According to recent reports, brain lesions resulting from ischemia, mechanical injury or neurodegenerative diseases can be partially treated using bone marrow-derived stromal cell (BMSC) engraftment approaches. Nevertheless, for brain lesions resulting from organophosphate poisoning, nerve agents such as soman (pinacolyl methylphosphono-fluoridate) could affect blood and bone marrow (BM) micro-environments, thus preventing efficient BMSC migration and engraftment. It is therefore necessary to verify the hematologic response to soman exposure. To assess this issue, the survival of BM cells, in particular hematopoietic progenitor and precursor cells (HPC), as well as distribution of the different populations of peripheral blood cells, were investigated in soman-intoxicated mice. Nine-week-old adult male B6D2F1 mice were treated with 110 microg/kg soman and 5.0 mg/kg methyl nitrate atropine. BM and peripheral blood (PB) samples were collected 1, 4, 8 and 22 days after poisoning. Various parameters were determined such as PB cell counting or, for BM samples, myelogram, in vitro colony-forming cells and phenotypic flow cytometry analysis. On post-soman day 1, a significant decrease in numbers of white blood cells and an increase in erythrocyte and platelet counts were noted. On post-soman day 4, the number of HPC decreased significantly, probably due to reduction of the replication rate of these immature cells. However, the number of more immature cells (Sca1+/Lin- phenotype) remained unchanged. On post-soman day 8 and day 22, the number of monocytes and granulocytes in the blood had considerably increased, probably due to a strong inflammatory reaction in response to soman poisoning. In conclusion, PB cell and BM-derived HPC populations are affected by acute soman poisoning, suggesting particular care, mainly for graft kinetic aspects, during future development of autologous BM stem cell therapy strategy to treat nerve agent-induced brain damage.

Early changes in MAP2 protein in the rat hippocampus following soman intoxication.

We investigated the time course of both MAP2 (microtubule-associated protein 2) levels and its phosphorylation degree in the rat hippocampus during the first 90 min following the onset of soman-induced seizures. The quantitative immunoblot analysis of hippocampal extracts revealed that MAP2 increased significantly in response to a sustained epileptic activity (from 60 min of seizure duration). In addition, intense MAP2 dephosphorylation was also observed 60 to 90 min after the onset of seizures. The possible neuropathological consequences of these two early MAP2 changes are discussed in relation to the both excessive stimulation of glutamate receptors and subsequent dendritic spine alterations occurring in hippocampal neurons soon after soman intoxication.

Review of the value of huperzine as pretreatment of organophosphate poisoning.

Today, organophosphate (OP) nerve agents are still considered as potential threats in both military or terrorism situations. OP agents are potent irreversible inhibitors of central and peripheral acetylcholinesterases. Pretreatment of OP poisoning relies on the subchronic administration of the reversible acetylcholinesterase (AChE) inhibitor pyridostigmine (PYR). Since PYR does not penetrate into the brain, it does not afford protection against seizures and subsequent neuropathology induced by an OP agent such as soman. Comparatively, huperzine (HUP) is a reversible AChE inhibitor that crosses the blood-brain barrier. HUP is presently approved for human use or is in course of clinical trials for the treatment of Alzheimer's disease or myasthenia gravis. HUP is also used as supplementary drug in the USA for correction of memory impairment. Besides, HUP has also been successfully tested for pretreatment of OP poisoning. This review summarizes the therapeutical value of HUP in this field. Moreover, the modes of action of HUP underlying its efficacy against OP agents are described. Efficacy appears mainly related to both the selectivity of HUP for red cell AChE which preserves scavenger capacity of plasma butyrylcholinesterases for OP agents and to the protection conferred by HUP on cerebral AChE. Finally, recent data, showing that HUP seems to be devoid of deleterious effects in healthy subjects, are also presented. Globally, this review reinforces the therapeutical value of HUP for the optimal pretreatment of OP poisoning.