Targeting Intracellular Calcium Stores Alleviates Neurological Morbidities in a DFP-Based Rat Model of Gulf War Illness

Abstract

Gulf War Illness (GWI) is a chronic multi-symptom disorder afflicting the veterans of the First Gulf War, and includes neurological symptoms characterized by depression and memory deficits. Chronic exposure to organophosphates (OPs) is considered a leading cause for GWI, yet its pathobiology is not fully understood. We recently observed chronic elevations in neuronal Ca 2+ levels ([Ca 2+ ] i ) in an OP-diisopropyl fluorophosphate (DFP)-based rat model for GWI. This study was aimed at identifying mechanisms underlying elevated [Ca 2+ ] i in this DFP model and investigating whether their therapeutic targeting could improve GWI-like neurological morbidities. Male Sprague-Dawley rats (9 weeks) were exposed to DFP (0.5 mg/kg, s.c . , 1×-daily for 5 days) and at 3 months postDFP exposure, behavior was assessed and rats were euthanized for protein estimations and ratiometric Fura-2 [Ca 2+ ] i estimations in acutely dissociated hippocampal neurons. In DFP rats, a sustained elevation in intracellular Ca 2+ levels occurred, and pharmacological blockade of Ca 2+ -induced Ca 2+ -release mechanisms significantly lowered elevated [Ca 2+ ] i in DFP neurons. Significant reductions in the protein levels of the ryanodine receptor (RyR) stabilizing protein Calstabin2 were also noted. Such a posttranslational modification would render RyR “leaky” resulting in sustained DFP [Ca 2+ ] i elevations. Antagonism of RyR with levetiracetam significantly lower elevated [Ca 2+ ] i in DFP neurons and improved GWI-like behavioral symptoms. Since Ca 2+ is a major second messenger molecule, such chronic increases in its levels could underlie pathological synaptic plasticity that expresses itself as GWI morbidities. Our studies show that treatment with drugs targeted at blocking intracellular Ca 2+ release could be effective therapies for GWI neurological morbidities.

Assessment of Ketamine and its Enantiomers in an Organophosphate-Based Rat Model for Features of Gulf War Illness

“…We therefore used a combination of behavioral assays that measure certain aspects of signs of depression that are reported in humans. In agreement with our previous studies [32][33][34], DFP-exposed rats exhibited significantly higher immobility time in the FST, did not show higher preference towards sweetened water in the SPT, and exhibited lowered exploration in the open-arm of the EPM. Taken together, these behaviors suggest that repeated low-dose DFP exposure produces chronic neuropsychiatric signs in rodents.…”

“…The activity of eEF2 kinase, also known as CamKIII, is modulated by calcium and calmodulin. We recently demonstrated that hippocampal neurons from GWI rats exhibited chronic calcium elevations [33,34]. It will be interesting to investigate the role of KET on GWI calcium homeostatic mechanisms and its subsequent effect on modulating the rapid behavioral response in this model.…”

“…DFP exposures were carried out as described previously [32][33][34]. Briefly, rats were injected with DFP (0.5 mg/kg) once daily for 5 days, while control rats received DFP vehicle injections (PBS) for the same period.…”

Dorsoventral-Specific Effects of Nerve Agent Surrogate Diisopropylfluorophosphate on Synaptic Transmission in the Mouse Hippocampus

“…Accordingly, DFP is one of the most widely studied OP alkylphosphates in the laboratory (Prendergast et al, 1997;Prendergast et al, 1998;Pope et al, 2005;Grigoryan et al, 2008;Nallapaneni et al, 2008;Wright et al, 2010;Terry Jr et al, 2011;Terry Jr et al, 2012;Ferchmin et al, 2015;. Moreover, DFP is currently employed as an initiator of status epilepticus in seizure models (Deshpande et al, 2010;Todorovic et al, 2012;Pouliot et al, 2016) and a nerve agent surrogate in models of Gulf War Illness (O'Callaghan et al, 2015;Phillips et al, 2019). Nonetheless, there is a lack of information characterizing the cholinergic and noncholinergic mechanisms of OPs like DFP on hippocampal synaptic transmission.…”

“…JPET # 263053 NMDAR in initiating Ca 2+ -induced Ca 2+ release mechanisms after OP exposure (Deshpande et al, 2010;Phillips et al, 2019). Interestingly, DFP has been shown to directly interact with the glutamate binding pocket of the NMDAR, irreversibly inhibiting the receptor (Johnson and Michaelis, 1992).…”