Brain norepinephrine reductions in soman-intoxicated rats: association with convulsions and AChE inhibition, time course, and relation to other monoamines

Sex-specific responses to juvenile stress on the dopaminergic system in an animal model of attention-deficit hyperactivity disorder

The etiology of attention-deficit hyperactivity disorder (ADHD) strongly suggests a genetic component as the main cause; however, environmental factors such as early adverse experiences in childhood may play an interactive role with the genetic susceptibility. Spontaneously hypertensive rats (SHRs), a genetic ADHD model, and control Wistar Kyoto rats (WKYs) were subjected to chronic unpredictable mild stress during the juvenile period. The behavioral characteristics were monitored, and dopamine-related factors in the core regions of dopaminergic pathways were measured. Higher ADHD symptom-related behaviors were observed in response to juvenile stress in male SHRs than control WKYs. For the SHRs subjected to juvenile stress, hyperactivity in males, recognition in females, and depressant potential in both sexes were markedly observed. In the expression of 17 dopamine-related genes and proteins, greater changes were detected in male SHRs subjected to juvenile stress, especially in dopamine metabolic factors. Dopamine clearance factors involved in dopamine degradation and transport, especially catechol-O-methyltransferase (COMT) and dopamine transporter (DAT), showed sex-specific differences induced by juvenile stress in dopamine metabolite assays. Moreover, stressed male SHRs treated with methylphenidate showed better improvement in behavior than the females, resulting in different levels of COMT and DAT amelioration. These results suggest that juvenile stress potentially increased the incidence of ADHD in a genetic rat model, which showed sex-specific differences based on the expression of COMT and DAT. Therefore, our results could help develop gender-specific diagnostics and healthcare options for juvenile stress in patients with ADHD.

Elevated Anxiety and Impaired Attention in Super-Smeller, Kv1.3 Knockout Mice

It has long been recognized that olfaction and emotion are linked. While chemosensory research using both human and rodent models have indicated a change in emotion can contribute to olfactory dysfunction, there are few studies addressing the contribution of olfaction to a modulation in emotion. In mice, olfactory deficits have been linked with heightened anxiety levels, suggesting that there could be an inverse relationship between olfaction and anxiety. Furthermore, increased anxiety is often co-morbid with psychiatric conditions such as attention disorders. Our study aimed to investigate the roles of olfaction in modulating anxiety. Voltage-gated potassium ion channel Kv1.3 knockout mice (Kv1.3−/−), which have heightened olfaction, and wild-type (WT) mice were examined for anxiety-like behaviors using marble burying (MB), light-dark box (LDB) and elevated-plus maze (EPM) tests. Because Kv1.3−/− mice have increased locomotor activity, inattentive and hyperactive behaviors were quantified for both genotypes. Kv1.3−/− mice showed increased anxiety levels compared to their WT counterparts and administration of methylphenidate (MPH) via oral gavage alleviated their increased anxiety. Object-based attention testing indicated young and older Kv1.3−/− mice had attention deficits and treatment with MPH also ameliorated this condition. Locomotor testing through use of a metabolic chamber indicated that Kv1.3−/− mice were not significantly hyperactive and MPH treatment failed to modify this activity. Our data suggest that heightened olfaction does not necessarily lead to decreased anxiety levels, and that Kv1.3−/− mice may have behaviors associated with inattentiveness.

Dexmedetomidine stops benzodiazepine-refractory nerve agent-induced status epilepticus

Nerve agents are highly toxic chemicals that pose an imminent threat to soldiers and civilians alike. Nerve agent exposure leads to an increase in acetylcholine within the central nervous system, resulting in development of protracted seizures known as status epilepticus (SE). Currently, benzodiazepines are the standard of care for nerve agent-induced SE, but their efficacy quickly wanes as the time to treatment increases. Here, we examine the role of the α2-adrenoceptor in termination of nerve agent-induced SE using the highly specific agonist dexmedetomidine (DEX). Adult male rats were exposed to soman and entered SE as confirmed by electroencephalograph (EEG). We observed that administration of DEX in combination with the benzodiazepine midazolam (MDZ) 20 or 40 min after the onset of SE stopped seizures and returned processed EEG measurements to baseline levels. The protective effect of DEX was blocked by the α2-adrenoceptor antagonist atipamezole (ATI), but ATI failed to restore seizure activity after it was already halted by DEX in most cases, suggesting that α2-adrenoceptors may be involved in initiating SE cessation rather than merely suppressing seizure activity. Histologically, treatment with DEX + MDZ significantly reduced the number of dying neurons as measured by FluoroJade B in the amygdala, thalamus, and piriform cortex, but did not protect the hippocampus or parietal cortex even when SE was successfully halted. We conclude that DEX serves not just as a valuable potential addition to the anticonvulsant regimen for nerve agent exposure, but also as a tool for dissecting the neural circuitry that drives SE.

Subsecond Regulation of Synaptically Released Dopamine by COMT in the Olfactory Bulb

The efficacy of neurotransmission depends on multiple factors, including presynaptic vesicular release of transmitter, postsynaptic receptor populations and clearance/inactivation of the transmitter. In the olfactory bulb (OB), short axon cells (SACs) form an interglomerular circuit that uses GABA and dopamine (DA) as cotransmitters. Selective optical activation of SACs causes GABA and DA co-release, resulting in a fast, postsynaptic GABA inhibitory response and a slower G-protein-coupled DA rebound excitation. In most systems, vesicular release of DA is cleared by the dopamine transporter (DAT). However, in the OB, high levels of specific DA metabolites suggest that enzymatic catalysis by catechol-O-methyl-transferase (COMT) predominates over DAT re-uptake. To assess this possibility we measured the amount of the DA breakdown enzyme, COMT, present in the OB. Compared with the striatum, the brain structure richest in DA terminals, the OB contains 50% more COMT per unit of tissue. Furthermore, the OB has dramatically less DAT compared with striatum, supporting the idea that COMT enzymatic breakdown, rather than DAT recycling, is the predominant mechanism for DA clearance. To functionally assess COMT inactivation of vesicular release of DA we used fast-scan cyclic voltammetry and pharmacological blockade of COMT. In mice expressing ChR2 in tyrosine hydroxylase-containing neurons, optical activation of SACs evoked robust DA release in the glomerular layer. The COMT inhibitor, tolcapone, increased the DA signal ∼2-fold, whereas the DAT inhibitor GBR12909 had no effect. Together, these data indicate that the OB preferentially employs COMT enzymatic inactivation of vesicular release of DA.

SIGNIFICANCE STATEMENT

In the olfactory bulb (OB), odors are encoded by glomerular activation patterns. Dopaminergic short axon neurons (SACs) form an extensive network of lateral connections that mediate cross talk among glomeruli, releasing GABA and DA onto sensory nerve terminals and postsynaptic neurons. DA neurons are ∼10-fold more numerous in OB than in ventral tegmental areas that innervate the striatum. We show that OB has abundant expression of the DA catalytic enzyme catechol-O-methyl-transferase (COMT), but negligible expression of the dopamine transporter. Using optogenetics and fast-scan cyclic voltammetry, we show that inhibition of COMT increases DA signals ∼2-fold. Thus, in contrast to the striatum, which has the brain's highest proportion of DAergic synapses, the DA catalytic pathway involving COMT predominates over re-uptake in OB.

Mechanism for the acute effects of organophosphate pesticides on the adult 5-HT system

The neurotransmitter serotonin (5-HT) is involved in mood disorder aetiology and it has been reported that (organophosphate) OP exposure affects 5-HT turnover. The aim of this study was to elucidate the mechanism underlying OP effects on the adult 5-HT system. First, acute in vivo administration of the OP diazinon (0, 1.3, 13 or 39 mg/kg i.p.) to male Hooded Lister rats inhibited the activity of the cholinergic enzyme acetylcholinesterase in blood and in the hippocampus, dorsal raphe nucleus (DRN), striatum and prefrontal cortex. Diazinon-induced cholinesterase inhibition was greatest in the DRN, the brain's major source of 5-HT neurones. Second, acute in vivo diazinon exposure (0 or 39 mg/kg i.p.) increased the basal firing rate of DRN neurones measured ex vivo in brain slices. The excitatory responses of DRN neurones to α₁-adrenoceptor or AMPA/kainate receptor activation were not affected by in vivo diazinon exposure but the inhibitory response to 5-HT was attenuated, indicating 5-HT_1A autoreceptor down-regulation. Finally, direct application of the diazinon metabolite diazinon oxon to naive rat brain slices increased the firing rate of DRN 5-HT neurones, as did chlorpyrifos-oxon, indicating the effect was not unique to diazinon. The oxon-induced augmentation of firing was blocked by the nicotinic acetylcholine receptor antagonist mecamylamine and the AMPA/kainate glutamate receptor antagonist DNQX. Together these data indicate that 1) acute OP exposure inhibits DRN cholinesterase, leading to acetylcholine accumulation, 2) the acetylcholine activates nicotinic receptors on 5-HT neurones and also on glutamatergic neurones, thus releasing glutamate and activating 5-HT neuronal AMPA/kainate receptors 3) the increase in 5-HT neuronal activity, and resulting 5-HT release, may lead to 5-HT_1A autoreceptor down-regulation. This mechanism may be involved in the reported increase in risk of developing anxiety and depression following occupational OP exposure.

•

Organophosphate exposure inhibits dorsal raphe nucleus cholinesterase activity.

•

Organophosphate oxon exposure activates 5-HT neurones in the dorsal raphe nucleus.

•

Nicotinic and AMPA receptors mediate the oxon-induced activation of 5-HT neurones.

•

Organophosphate exposure attenuates the response to 5-HT1A autoreceptor activation.

Dramatic Increase in Cerebral Blood Flow following Soman Intoxication If Signs of Symptoms Can Be Seen

Organophosphate poisoning is associated with adverse effects on the central nervous system such as seizure/convulsive activity and long term changes in neuronal networks. This study report an investigation designed to assess the consequences of Soman, a highly toxic organophosphorus compound, exposure on regional blood flow in the rat brain and peripheral organs. We performed repeated blood flow measurements in the same animal, using the microspheres technique, to characterize changes in regional blood flow at different times after Soman intoxication. In addition, the cardiopulmonary effects of Soman were followed during the intoxication. Administration of Soman (1 LD50; 90 µg/kg, s.c.) to anaesthetized rats produced a decrease in blood acetylcholinesterase activity in all animals tested. Although, only six out of ten rats showed signs of poisoning like a decrease in respiratory rate, the results show that only animals with significant signs of poisoning demonstrated an increase in cerebral blood flow. We conclude that it is of great importance to treat all data individually. An overall mean can easily be misinterpreted and conceal important effects. We also conclude that the increase in cerebral blood flow has an important role in the effect on respiration and that this effect is independent of the blood acetylcholinesterase activity.

Low-level repeated exposure to diazinon and chlorpyrifos decrease anxiety-like behaviour in adult male rats as assessed by marble burying behaviour

Occupational exposure to organophosphate (OPs) pesticides is reported to increase in the risk of developing anxiety and depression. Preclinical studies using OP levels, which inhibit acetylcholinesterase activity, support the clinical observations, but little is known of the effects of exposure below this threshold. We examined the effects of low level OP exposure on behaviours and neurochemistry associated with affective disorders. Adult rats were administered either diazinon (1 mg/kg i.p.) which is present in sheep dip and flea collars, chlorpyrifos (1 mg/kg i.p.) which is present in crop sprays, or vehicle for 5 days. OP exposure did not affect acetylcholinesterase activity (blood, cerebellum, caudate putamen, hippocampus, prefrontal cortex), anhedonia-like behaviour (sucrose preference), working memory (novel object recognition), locomotor activity or anxiety-like behaviour in the open field arena. In contrast OP exposure attenuated marble burying behaviour, an ethological measure of anxiety. The diazinon-induced reduction in marble burying persisted after exposure cessation. In comparison to vehicle, dopamine levels were lowered by chlorpyrifos, but not diazinon. 5-HT levels and turnover were unaffected by OP exposure. However, 5-HT transporter expression was reduced by diazinon suggesting subtle changes in 5-HT transmission. These data indicate exposure to occupational and domestic OPs, below the threshold to inhibit acetylcholinesterase, can subtly alter behaviour and neurochemistry.

Brain regional heterogeneity and toxicological mechanisms of organophosphates and carbamates

The brain is a well-organized, yet highly complex, organ in the mammalian system. Most investigators use the whole brain, instead of a selected brain region(s), for biochemical analytes as toxicological endpoints. As a result, the obtained data is often of limited value, since their significance is compromised due to a reduced effect, and the investigators often arrive at an erroneous conclusion(s). By now, a plethora of knowledge reveals the brain regional variability for various biochemical/neurochemical determinants. This review describes the importance of brain regional heterogeneity in relation to cholinergic and noncholinergic determinants with particular reference to organophosphate (OP) and carbamate pesticides and OP nerve agents.

Suicide and potential occupational exposure to pesticides, Colorado 1990-1999

A number of occupational studies have reported high rates of suicide among selected occupations, including farmers. Limited work has focused on occupational exposures that may increase the risk of suicide. The purpose of this study is to describe suicide among individuals potentially exposed to pesticides through their occupation. Data from Colorado death certificate files for the period 1990-1999 were obtained. Eligible records were those individuals who were Colorado residents at the time of death who had an occupation listed on their death certificates. Cases had suicide listed as the primary cause of death on the death certificates. The comparison group included Colorado residents who died from any cause during the same period other than cancer, mental disorders and injuries. A total of 4,991 suicide deaths were included and a total of 107,692 other deaths served as the comparison group. Occupations considered pesticide exposed included: veterinarians; pest control occupations; farmers and farm workers; farm managers and supervisors; marine life cultivators; nursery workers; groundskeepers and gardeners; animal caretakers; graders, sorters and inspectors of agricultural products; and forestry workers, supervisors and loggers. All other occupational categories were coded as unexposed. Logistic regression was used to compare the groups, separately for males and females. After controlling for age, race, Hispanic ethnicity, years of education, and marital status, males who were in pesticide exposed occupations had higher odds of suicide (odds ratio 1.14; 95% confidence interval 0.97, 1.34) and females in pesticide exposed occupations also had higher odds of suicide (odds ratio 1.98; 95% confidence interval 1.01, 3.88).

Anticonvulsant effects of damage to structures involved in seizure induction in rats exposed to soman

In nerve agent research, it is assumed that the regions from which seizure activity is triggered may offer clues for the designing of effective anticonvulsive therapy. In the present study, selective brain lesions were made to identify critical cholinergic pathways and seizure controlling areas involved in the induction of epileptiform activity in rats challenged with soman. The results showed that rats with bilateral aspiration lesion of the seizure controlling substrate, area tempestas (AT) in the piriform cortex, displayed marked anticonvulsant effects, whereas such effects were not seen when substantia nigra was destroyed. Aspiration lesion of the medial septal area (MS) including the vertical limb of the diagonal band nucleus (DBN) caused increased latency to the onset of convulsions, whereas damage to the nucleus basalis magnocellularis (NBM), nucleus accumbens, or both MS and NBM did not cause anticonvulsant effects. Saporin lesion of MS, DBN (horizontal limb), or MS+DBN had no anticonvulsant effects, suggesting that aspiration lesion of MS disrupted pathways beyond cholinergic ones. Severe aphagia/adipsia and reduced body weight occurred in rats with lesions in the septal area. In separate sham operated rats, a strong positive correlation was found between body weight and latency to onset of convulsions in response to soman. Thus, weight loss and a relatively high dose of soman (1.6 x LD(50)) in this context may have masked potential anticonvulsant effects among some lesioned animals. It is inferred that MS and AT/piriform cortex occur as prime target areas for induction of seizures by soman.

Morphometric studies of cardiac myocytes of rats chronically treated with an organophosphate

Organophosphate intoxication induces an acute cholinergic syndrome, but the long-term effects of these compounds in the cardiocirculatory system are not known. The objective of the present work is to investigate if experimental chronic exposition to repetitive sublethal doses of organophosphate methamidophos can induce morphological changes in rat's hearts. Wistar albino adult male rats received a weekly enteral sublethal dose of the organophosphate methamidophos for 12 consecutive weeks. After that we have performed histological and morphometric studies of their hearts. We have observed hypertrophy of cardiac myocites in treated animals, which was confirmed by morphometric studies (measure of smaller diameter of cardiac myocites). One of the possible explanations for the cardiac hypertrophy would be persistent systemic arterial hypertension in treated animals. However, another possible explanation would be direct sympathetic stimulation.

Morphometric studies of specific brain regions of rats chronically intoxicated with the organophosphate methamidophos

Subtle neurological disturbances have been described in organophosphorus intoxication. Experimental studies have reported neuronal necrosis, particularly in animals experiencing seizures. The objective of the present work was to investigate if in rats (without seizures) exposed to an organophosphate agent, morphological changes occur in specific regions of the brain. The animals received 2.5 or 5.0 mg/kg methamidophos once a week for 2 months and were decapitated after 2 months 7 days of drug administration. We observed atrophy of the molecular layer of the parietal cortex without neuronal loss in specific cerebral regions. This would be due to atrophy or loss of neuronal ramifications but without neuronal loss.

Suicide and exposure to organophosphate insecticides: cause or effect?

BACKGROUND

Suicide using pesticides as agent is recognized as a major cause of pesticide poisoning.

METHODS

A literature review of mortality and morbidity studies related to suicide among pesticide-exposed populations, and of human and animal studies of central nervous system toxicity related to organophosphate (OP) pesticides was performed.

RESULTS

Suicide rates are high in farming populations. Animal studies link OP exposure to serotonin disturbances in the central nervous system, which are implicated in depression and suicide in humans. Epidemiological studies conclude that acute and chronic OP exposure is associated with affective disorders. Case series and ecological studies also support a causal association between OP use and suicide.

CONCLUSIONS

OPs are not only agents for suicide. They may be part of the causal pathway. Emphasizing OPs solely as agents for suicide shifts responsibility for prevention to the individual, reducing corporate responsibility and limiting policy options available for control.

Methyl parathion increases neuronal activities in the rat locus coeruleus

Exposure to organophosphate insecticides induces undesirable behavioral changes in humans, including anxiety and irritability, depression, cognitive disturbances and sleep disorders. Little information currently exists concerning the neural mechanisms underlying such behavioral changes. The brain stem locus coeruleus (LC) could be a mediator of organophosphate insecticide-induced behavioral toxicities since it contains high levels of acetylcholinesterase and is involved in the regulation of the sleep-wake cycle, attention, arousal, memory, and pathological processes, including anxiety and depression. In the present study, using a multi-wire recording technique, we examined the effects of methyl parathion, a commonly used organophosphate insecticide, on the firing patterns of LC neurons in rats. Systemic administration of a single dose of methyl parathion (1 mg/kg, i.v.) increased the spontaneous firing rates of LC neurons by 240% but did not change the temporal relationships among the activities of multiple LC neurons. This dose of methyl parathion induced a 50% decrease in blood acetylcholinesterase activity and a 48% decrease in LC acetylcholinesterase activity. The methyl parathion-induced excitation of LC neurons was reversed by administration of atropine sulfate, a muscarinic receptor antagonist, indicating an involvement of muscarinic receptors. The methyl parathion-induced increase in LC neuronal activity returned to normal within 30 min while the blood acetylcholinesterase activity remained inhibited for over 1 h. These data indicate that methyl parathion treatment can elicit excitation of LC neurons. Such excitation could contribute to the neuronal basis of organophosphate insecticide-induced behavioral changes in human.

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.

Striatal dopaminergic pathways as a target for the insecticides permethrin and chlorpyrifos

Because insecticide exposure has been linked to both Parkinsons disease and Gulf War illness, the neurotoxic actions of pyrethroid and organophosphate insecticides on behavior and striatal dopaminergic pathways were investigated in C57BL/6 mice treated with permethrin (three i.p. doses at 0.2-200 mg/kg) or chlorpyrifos (three s.c. doses at 25-100 mg/kg) over a 2-week period. Permethrin altered maximal [3H]dopamine uptake in striatal synaptosomes from treated mice, with changes in Vmax displaying a bell-shaped curve. Uptake was increased to 134% of control at a dose of 1.5 mg/kg. At higher doses of PM (25 mg/kg), dopamine uptake declined to a level significantly below that of control (50% of control at 200 mg/kg, P < 0.01). We also observed a small, but statistically significant decrease in [3H]dopamine uptake by chlorpyrifos, when given at a dose of 100 mg/kg. There was no significant effect on the Km for dopamine transport. Evidence of cell stress was observed in measures of mitochondrialfunction, which were reduced in mice given high-end doses of chlorpyrifos and permethrin. Although cytotoxicity was not reflected in decreased levels of striatal dopamine in either 200 mg/kg PM or 100 mg/kg CPF treatment groups, an increase in dopamine turnover at 100 mg/kg CPF was indicated by a significant increase in titers of the dopamine metabolite, 3,4-dihydroxyphenylacetic acid. Both permethrin and chlorpyrifos caused a decrease in open field behavior at the highest doses tested. Although frank Parkinsonism was not observed, these findings confirm that dopaminergic neurotransmission is affected by exposure to pyrethroid and organophosphorus insecticides, and may contribute to the overall spectrum of neurotoxicity caused by these compounds.

Evidence for cholinergic regulation of basal norepinephrine release in the rat olfactory bulb

The effects of locally infused cholinergic agonists on extracellular levels of norepinephrine in the olfactory bulb of anesthetized rats were determined using in vivo microdialysis coupled with high-performance liquid chromatography and electrochemical detection. Using chronically implanted microdialysis probes, the basal norepinephrine level in the olfactory bulb was 0.55 pg/10 microl dialysate. Local infusion of K+ (30 mM) or the norepinephrine re-uptake inhibitor desipramine (1 microM) through the dialysis probe significantly increased basal norepinephrine levels. Focal activation of noradrenergic locus coeruleus neurons, the sole source of norepinephrine innervation of the olfactory bulb, increased norepinephrine levels by 247% of control. Local infusion of the acetylcholinesterase inhibitor soman (0.4 mM) into the olfactory bulb increased basal norepinephrine levels by 134% of control, suggesting that endogenously released acetylcholine modulates norepinephrine release. Intrabulbar infusion of acetylcholine (40 mM) or nicotine (40 mM) increased norepinephrine levels (317% and 178% of control, respectively), while infusion of the muscarinic receptor agonist pilocarpine (40 mM) reduced norepinephrine levels (54% of control). These results demonstrate that basal norepinephrine release in the olfactory bulb is potently modulated by stimulation of local cholinergic receptors. Nicotinic receptors stimulate, and muscarinic receptors inhibit, norepinephrine release from locus coeruleus terminals.

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.

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.

Soman-induced seizures rapidly activate astrocytes and microglia in discrete brain regions

Neurons in the piriform cortex and the pontine nucleus locus coeruleus express elevated levels of the immediate early gene protein product, Fos, within 30-45 minutes of a seizurogenic dose of the anticholinesterase, soman (Zimmer et al., [1997] J. Comp. Neurol. 378:468-481). By 24 hours following soman injection, there is marked neuropathology in the piriform cortex. These findings suggest selective, regional vulnerability in response to the seizurogenic actions of soman. In the present study, we determined that soman-induced seizures also cause selective, rapid activation of astrocytes and microglia in the piriform cortex and other brain regions. Animals were killed at different intervals between 1 hour and 24 hours after a convulsive dose of soman. Brain sections were processed for immunocytochemical detection of astrocytes with antibodies against glial fibrillary acidic protein, and microglia and macrophages with antibodies against the complement receptor 3 protein, OX-42. The results demonstrate that following soman administration: (1) there is a rapid increase in glial fibrillary acidic protein staining in astrocytes of the piriform cortex (1 hour); (ii) reactive astrocytes are specifically restricted to layer II and the superficial boundaries of layer III of the piriform cortex. These are the same layers in which neurons express Fos within 30-45 minutes following soman administration; (3) between 1 and 4 hours, resting (ramified) microglia in the piriform cortex and the hippocampus alter their morphology to resemble active microglia. From 4-8 hours, active microglia undergo morphological changes characteristic of reactive microglia that resemble macrophages. Taken together, these observations indicate that astrocytes and microglia in brain regions susceptible to soman become rapidly "reactive" in response to seizures. The highly specific anatomical codistribution of reactive glia and Fos-expressing neurons suggests that intensely active neurons provide local signals that trigger reactive changes in neighboring glia.

Tonic activation of locus coeruleus neurons by systemic or intracoerulear microinjection of an irreversible acetylcholinesterase inhibitor: increased discharge rate and induction of C-fos

Recent studies in this laboratory have demonstrated that intramuscular injection of the irreversible acetylcholinesterase (AChE) inhibitor, soman (pinacolylmethylphosphonofluoridate), produces a rapid (1-2 h) and profound depletion (70% of control) of norepinephrine (NE) in the olfactory bulb and forebrain. NE is decreased only in convulsing animals. As NE-containing locus coeruleus (LC) neurons provide the only NE input to the olfactory bulb and the major NE innervation of the forebrain, the reduction of NE suggests that soman may cause tonic activation of LC neurons leading to rapid depletion of NE. Activation of LC may result from: (i) facilitation of cholinergic transmission in LC; (ii) soman-induced activation of excitatory inputs to LC; or (iii) generalized activation of LC neurons due to seizures. The present experiments were designed to assess these alternatives. We examined whether LC neuronal activity, c-fos expression, and AChE staining are altered after peripheral (systemic) or direct intracoerulear injection of soman in anesthetized rats. Both modes of soman administration rapidly and potently increase the spontaneous discharge rate of LC neurons. This activation was associated with a desynchronization of the electroencephalogram, but not with seizures. The discharge of LC neurons remained elevated at all postsoman intervals examined (up to 2 h) and was rapidly and completely reversed by systemic injection of the muscarinic receptor antagonist scopolamine hydrochloride, but not by the nicotinic receptor antagonist mecamylamine. Both systemic and intracoerulear soman administration completely inhibited AChE staining in LC and rapidly induced the expression of c-fos in LC neurons. These results demonstrate that soman potently and tonically activates LC neurons. This effect appears to be mediated by direct inhibition of AChE in LC leading to a rapid accumulation of ACh. Unhydrolyzed ACh tonically activates LC neurons via muscarinic receptors. Soman-induced activation of LC neurons does not require seizures. We conclude that depletion of forebrain and olfactory bulb NE after systemic administration of soman results from tonic hypercholinergic stimulation of LC.