The involvement of nitric oxide in a rodent model of post-traumatic stress disorder
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Post-traumatic stress disorder (PTSD), an anxiety disorder, may develop after experiencing or witnessing a severe traumatic event. Characteristic symptoms include hyper arousal and amnesic symptoms, while volume reductions in the hippocampus of these patients appear correlated with illness severity and the degree of cognitive deficit. Stress-induced increases in plasma cortisol have been implicated in this apparent atrophy of the hippocampus, although, clinical studies have described a marked suppression of plasma cortisol in PTSD. Given this hypocortisolemia, the basis for hippocampal neuro degeneration and cognitive decline remains unclear. While stress-related hippocampal structural changes have been linked to the neurotoxic effects of glucocorticoids and glutamate. NMDA-NO pathways have been found to play a causal role in anxiety-related behaviours. Prior exposure to trauma is an important risk factor for PTSD. In most instances the disorder becomes progressively worse over time, possibly with a delayed onset, suggesting a role for sensitization. In this study a time-dependent sensitization (TDS) model was used to induce PTSD-like sequelae in male Spraque-Dawley rats. The TDS-model is based on exposure to acute stressors, with a reminder of the trauma, in the form of re-exposure to one of the acute stressor, seven days later. NOS-activity, NMDA receptor parameters (Bmax and Kd) and GABA levels in the hippocampus of rats, as well as plasma corticosterone levels were determined 21 days after exposure to the TDS-model. Increased levels of corticosterone were measured after exposure to acute stress, but these levels were found to decrease below basal levels 21 days after the re-exposure, thus mimicking glucocorticoid levels in patients with PTSD. These findings may also imply that the increase in glucocorticoid levels after stress exposure is only the initial step in a cascade of events leading to neuronal damage in the hippocampus. This study also found that stress-restress evoked a long-lasting increase in hippocampal NOS activity that was accompanied by a reactive down-regulation of hippocampal NMDA receptors and dysregulation of inhibitory GABA pathways. Subsequently, animals were chronically treated with certain pharmacological agents prior to exposure to the TDS-model to determine possible approaches for inhibiting the induction of PTSD. Pre-treatment with fluoxetine, currently indicated in the treatment of PTSD. and the nNOS inhibitor, 7-nitroindazole, had no effect on the increased NOS activity measured 21 days afler exposure to the TDS-model. Pre-treatment with the iNOS inhibitor, aminoguanidine, however, resulted in inhibition of the observed increase in hippocampal NOS-activity, implicating a possible role for the iNOS isoform in the etiology of PTSD. Treatment with ketoconazole, an inhibitor of glucoccfticoid synthesis, resulted in inhibition of the increase in NOS-activity observed after exposure to TDS-stress, thus indicating a possible link between stress glucocorticoid-release and NO synthesis. These perturbations may have importance in explaining the increasing evidence for stress-related hippocampal degenerative pathology and cognitive deficits seen in patients with PTSD. Uncovering and understanding the role of NO in PTSD will hopefully lead to the development of selective therapeutic agents in disorders like PTSD. as well as providing a better understanding of basic processes underlying normal and pathological neuronal functions in PTSD.
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