Corticosterone primes the neuroinflammatory response to Gulf War Illness‐relevant organophosphates independently of acetylcholinesterase inhibition

Abstract

Gulf War Illness (GWI) is a chronic multi‐symptom disorder affecting veterans of the 1991 Gulf War. Among the symptoms of GWI are those associated with sickness behavior, observations suggestive of underlying neuroinflammation. We have shown that exposure of mice to the stress hormone, corticosterone (CORT), and to diisopropyl fluorophosphate (DFP), as a nerve agent mimic, results in marked neuroinflammation, findings consistent with a stress/neuroimmune basis of GWI. Here, we examined the contribution of irreversible and reversible acetylcholinesterase (AChE) inhibitors to neuroinflammation in our mouse model of GWI. Male C57BL/6J mice received 4 days of CORT (400 mg/L) in the drinking water followed by a single dose of chlorpyrifos oxon (CPO; 8 mg/kg, i.p.), DFP (4 mg/kg, i.p.), pyridostigmine bromide (PB; 3 mg/kg, i.p.), or physostigmine (PHY; 0.5 mg/kg, i.p.). CPO and DFP alone caused cortical and hippocampal neuroinflammation assessed by qPCR of tumor necrosis factor‐alpha, IL‐6, C–C chemokine ligand 2, IL‐1β, leukemia inhibitory factor and oncostatin M; CORT pretreatment markedly augmented these effects. Additionally, CORT exposure prior to DFP or CPO enhanced activation of the neuroinflammation signal transducer, signal transducer and activator of transcription 3 (STAT3). In contrast, PHY or PB alone or with CORT pretreatment did not produce neuroinflammation or STAT3 activation. While all of the CNS‐acting AChE inhibitors (DFP, CPO, and PHY) decreased brain AChE activity, CORT pretreatment abrogated these effects for the irreversible inhibitors. Taken together, these findings suggest that irreversible AChE inhibitor‐induced neuroinflammation and particularly its exacerbation by CORT, result from non‐cholinergic effects of these compounds, pointing potentially to organophosphorylation of other neuroimmune targets.

Corticosterone potentiates DFP-induced neuroinflammation and affects high-order diffusion imaging in a rat model of Gulf War Illness

“…In addition to mimicking sarin, DFP shares chemical characteristics with other irreversible acetylcholinesterase, organophosphate compounds that veterans were exposed to during the Gulf War, like the insecticides chlorpyrifos and dichlorvos. As such, we have demonstrated the potential for both DFP, as a sarin surrogate, and chlorpyrifos to instigate similar neuroinflammatory responses following CORT pretreatment (Locker et al, 2017), supporting a role for these classes of compounds in the development of GWI.…”

“…In this study, we expanded our established GWI mouse model (O’Callaghan et al, 2015; Locker et al, 2017) by exposing adult male Sprague Dawley rats (Hilltop Lab Animals, Scottdale, PA, USA) to CORT in the drinking water (200 mg/L in 0.6% EtOH) for 4 days, followed by a single injection of DFP (1.5 mg/kg, i.p.). Rats were sacrificed 6 h post-DFP by either decapitation for the evaluation of cortex cytokine mRNA (n = 5 rats/group), or by a fatal dose of pentobarbital-based euthanasia solution (Fatal Plus; 300 mg/kg, i.p.)…”

“…followed by transcardial perfusion with 0.9% saline and fixation with 4% paraformaldehyde (n = 5 rats/group) for MRI. Total RNA from the frontal region of one cortical hemisphere was isolated as previously described (Locker et al, 2017). Real-time PCR analysis of the housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and of the proinflammatory mediators, TNFα, IL-6, CCL2, IL-1β, leukemia inhibitor factor (LIF), and oncostatin M (OSM) was performed in an ABI7500 Real-Time PCR System (Thermo Fisher Scientific, Waltham, MA, USA) in combination with TaqMan ® chemistry as previously described (Locker et al, 2017).…”

Modeling the Genetic Basis of Individual Differences in Susceptibility to Gulf War Illness

“…QPCR was used to analyze expression of mRNA for the proinflammatory cytokines, Il1β, Il6, and Tnfa in brain samples (medial prefrontal cortex). All procedures are described by O'Callaghan et al [10] and Locker et al [8]. Total RNA was isolated from medial prefrontal cortex at 6 h after DFP exposure.…”

“…These include depleted uranium, cholinesterase inhibitors used as insecticides or as prophylactics against nerve agents (e.g., chlorpyrifos and pyridostigmine bromide) and even small amounts of sarin. The evidence points to exposure to the irreversible cholinesterase inhibitors chlorpyrifos and the nerve gas, sarin [8]. The former is an insecticide applied to living quarters and the latter was inadvertently released from ammunition dumps during their destruction by allied troops.…”