Combined forced running stress and subclinical paraoxon exposure have little effect on pyridostigmine-induced acute toxicity in rats

A review of pre-clinical models for Gulf War Illness

Gulf War Illness (GWI) is a chronic multisymptomatic disorder that afflicts over 1/3rd of the 1991 GW veterans. It spans multiple bodily systems and presents itself as a syndrome exhibiting diverse symptoms including fatigue, depression, mood, and memory and concentration deficits, musculoskeletal pain and gastrointestinal distress in GW veterans. The etiology of GWI is complex and many factors, including chemical, physiological, and environmental stressors present in the GW arena, have been implicated for its development. It has been over 30 years since the end of the GW but, GWI has been persistent in suffering veterans who are also dealing with paucity of effective treatments. The multifactorial aspect of GWI along with genetic heterogeneity and lack of available data surrounding war-time exposures have proved to be challenging in developing pre-clinical models of GWI. Despite this, over a dozen GWI animal models exist in the literature. In this article, following a brief discussion of GW history, GWI definitions, and probable causes for its pathogenesis, we will expand upon various experimental models used in GWI laboratory research. These animal models will be discussed in the context of their attempts at mimicking GW-related exposures with regards to the variations in chemical combinations, doses, and frequency of exposures. We will discuss their advantages and limitations in modeling GWI followed by a discussion of behavioral and molecular findings in these models. The mechanistic data obtained from these preclinical studies have offered multiple molecular pathways including chronic inflammation, mitochondrial dysfunction, oxidative stress, lipid disturbances, calcium homeostatic alterations, changes in gut microbiota, and epigenetic modifications, amongst others for explaining GWI development and its persistence. Finally, these findings have also informed us on novel druggable targets in GWI. While, it has been difficult to conceive a single pre-clinical model that could express all the GWI signs and exhibit biological complexity reflective of the clinical presentation in GWI, animal models have been critical for identifying molecular underpinnings of GWI and evaluating treatment strategies for GWI.

Brain cholinergic alterations in rats subjected to repeated immobilization or forced swim stress on lambda-cyhalothrin exposure

Role of immobilization stress (IMS), a psychological stressor and forced swim stress (FSS), a physical stressor was investigated on the neurobehavioral toxicity of lambda-cyhalothrin (LCT), a new generation type-II synthetic pyrethroid. Pre-exposure of rats to IMS (15 min/day) or FSS (3 min/day) for 28 days on LCT (3.0 mg/kg body weight, p.o.) treatment for 3 days resulted to decrease spatial learning and memory and muscle strength associated with cholinergic-muscarinic receptors in frontal cortex and hippocampus as compared to those exposed to IMS or FSS or LCT alone. Decrease in acetylcholinesterase activity, protein expression of ChAT and PKC-β1 associated with decreased mRNA expression of CHRM2, AChE and ChAT in frontal cortex and hippocampus was also evident in rats pre-exposed to IMS or FSS on LCT treatment, compared to rats exposed to IMS or FSS or LCT alone. Interestingly, changes both in behavioral and neurochemical endpoints were marginal in rats subjected to IMS or FSS for 28 days or those exposed to LCT for 3 days alone, compared to controls. The results suggest that stress is an important contributor in LCT induced cholinergic deficits.

Carbamate nerve agent prophylatics exhibit distinct toxicological effects in the zebrafish embryo model

Pyridostigmine bromide (PB) is an FDA-approved drug for the treatment of myasthenia gravis and a prophylactic pre-treatment for organophosphate nerve agent poisoning. Current methods for evaluating nerve agent treatments include enzymatic studies and mammalian models. Rapid whole animal screening tools for assessing the effects of nerve agent pre-treatment and post-exposure drugs represent an underdeveloped area of research. We used zebrafish as a model for acute and chronic developmental exposure to PB and two related carbamate acetylcholinesterase (AChE) inhibitors, neostigmine bromide (NB) and physostigmine (PS). Lethal doses and gross morphological phenotypes resulting from exposure to sub-lethal doses of these compounds were determined. Quantitative analyses of motility impairment and AChE enzyme inhibition were used to determine optimal dosing conditions for evaluation of the effects of carbamate exposures on neuronal development; ~50% impairment of response to startle stimuli and >50% inhibition of AChE activity were observed at 80 mMPB, 20 mM NB and 0.1 mM PS. PB induced stunted somite length, but no other phenotypic effects were observed. In contrast, NB and PS induced more severe phenotypic morphological defects than PB as well as neurite outgrowth mislocalization. Additionally, NB induced mislocalization of nicotinic acetylcholine receptors, resulting in impaired synapse formation. Taken together, these data suggest that altered patterns of neuronal connectivity contribute to the developmental neurotoxicity of carbamates and demonstrate the utility of the zebrafish model for distinguishing subtle structure-based differential effects of AChE inhibitors, which include nerve agents, pesticides and drugs.

Repeated stress in combination with pyridostigmine Part I: long-term behavioural consequences

Since their return from the first Persian Gulf War, some veterans have complained of a variety of symptoms that were designated as "Gulf War Illness" (GWI). Among other factors, pyridostigmine, used as a prophylaxis treatment against intoxication by nerve agents, has been proposed by many authors as a cause of late social and/or cognitive dysfunction related to GWI. One of the hypotheses placed to explain these behavioural disorders is that operational stress has modified the side effects of pyridostigmine given to soldiers. In an attempt to establish an experimental model of GWI to evaluate the long-term behavioural effects of pyridostigmine administered in stressful conditions, we have developed a new model of repeated stress based on the pole-climbing avoidance technique. We used it to evaluate the effects of pyridostigmine treatment combined to repeated stress over the months following the end of the treatment. We observed that this stress induces impulsiveness and aggressiveness in adult male rat. Moreover, pyridostigmine treatment administered daily 30 min before each stressful session amplifies these behavioural disorders and induces long-term learning dysfunction and slight but significant decrease in phosphocholine level in hippocampus. This suggests that repeated administration of pyridostigmine combined to pole-climbing avoidance (PCA) stress conditions can induce adverse effects in rat central nervous system.

Repeated stress in combination with pyridostigmine Part II: changes in cerebral gene expression

Organophosphates (OP) represent a potential threat in terrorism or during military conflicts. Due to its faculty to protect cholinesterase (ChE) activity against irreversible inactivation by OP, pyridostigmine bromide (PB) was used as a prophylaxis treatment during the first Persian Gulf War. To explain dysfunctions reported by Gulf War Veterans (GWV), it was suggested a potentiation of the operational stress effects by PB given to soldiers. Our companion paper (see part 1 in the same journal issue) describes that PB treatment administered in repeated stress conditions results in long-term perturbations of learning and social behaviour. The present paper examines, in adult male Wistar rats, consequences of the association of repeated stress and PB treatment on gene expression in hypothalamus and hippocampus. PB treatment (1.5 mg/kg/day) was orally administered 30 min before each stress session to inhibit 40% of blood ChE as recommended by NATO. 10 days of stress alone induce a decrease in hypothalamic Il-1alpha expression. Treatment with PB alone increases mineralocorticoid receptor expression in hypothalamus which means that PB may thus modify stress perception by animals. Stressed-PB animals showed increase in hippocampal expression of BDNF, TrkB and CamKIIalpha, three genes implicated in memory development. As a supplement to previous studies showing behavioural and biochemical effects of the association of stress with PB, our data reveal that behavioural effects of this association may be linked with genomic changes in hippocampus. Mechanisms underlying these modifications and their link with memory disturbances reported by GWV remain to be further determined.

The effect of stress on the acute neurotoxicity of the organophosphate insecticide chlorpyrifos

A study was conducted to determine if multiple exposures to several stress paradigms might affect the anticholinesterase effect of subsequently administered organophosphate insecticide chlorpyrifos. Male Sprague-Dawley rats were subject to daily periods of restraint, swimming, a combination of the two, or neither of the two (controls) (n=8/group) for 5 days per week over a six-week period. The most profound stress, as measured by reduced body weight gain and elevated levels of plasma corticosterone, was swimming. On day 39 of the study, shortly after the daily stress episode, one half of the rats in each group was dosed with 60 mg/kg chlorpyrifos subcutaneously. This had no effect on subsequent levels of plasma corticosterone. There were no stress-related differences in the degree of chlorpyrifos-induced inhibition of brain acetylcholinesterase in animals sacrificed on day 43.

Effects of combined, multiple stressors on pyridostigmine-induced acute toxicity in rats

A number of studies have evaluated the possibility that stress-induced changes in blood-brain barrier permeability enhanced the central effects of the carbamate acetylcholinesterase inhibitor, pyridostigmine. We previously found relatively little evidence of stress-induced changes in the acute toxicity of pyridostigmine in rats using a variety of restraint, forced running and forced swimming stress conditions. In this study, we evaluated the effects of sequential pre-exposure to multiple stressors on the acute toxicity of pyridostigmine. Rats (n = 8 per treatment group) were either un-stressed or stressed by restraint (60 min), forced running (60 min, 15 m/min, 6 degrees incline) and forced swimming (15 min), and then given either vehicle (saline, 1 ml/kg, po) or pyridostigmine (30 mg/kg, po) immediately after the final stressor. Functional signs of cholinergic toxicity (involuntary movements, autonomic dysfunction) were recorded at 0.5, 1 and 2 h after dosing. Body temperature was measured both before stress and 2 h after dosing. Rats were sacrificed immediately after 2-h functional observations to collect tissues (whole blood, diaphragm, frontal cortex, hippocampus and cerebellum) for measurement of cholinesterase activity. Stressed rats treated with pyridostigmine exhibited higher lethality (2/8) compared to unstressed rats given pyridostigmine (0/8). Pyridostigmine elicited classical signs of cholinergic toxicity, but the rats that died did not show increased cholinergic signs and no significant differences in cholinergic signs were noted between treatment groups. Cholinesterase activity was significantly inhibited in blood (47-50%) and diaphragm (80%) following pyridostigmine exposure regardless of stress conditions. Slight but significant inhibition (11-15%) of cerebellar cholinesterase activity was observed following pyridostigmine exposure, but inhibition was not influenced by stress. We conclude that while acute lethality from pyridostigmine may be increased by combined, multiple stressors, increased lethality does not appear due to enhanced cholinergic toxicity or via increased cholinesterase inhibition in either central or peripheral tissues.