Effect of Repeated Predator Scent Exposure on Excitability of Serotonin Neurons and Stress Markers in Rats

Exposure to predator scent (PS) has been used aa a model of stress associated with danger to life and body integrity. We tested the hypothesis that repeated PS exposure alters the excitability of 5-HT neurons of the dorsal raphe nucleus. To study the mechanisms involved, we approached serum and adrenal corticosterone and aldosterone concentrations, as well as cortical brain-derived neurotrophic factor (BDNF) expression. Adult male Sprague-Dawleys rats were exposed to PS for ten minutes daily for ten consecutive days. Two weeks after the last exposure, the selected electrophysiological and biochemical assessments were performed. Measurements by in vivo electrophysiology showed increased spontaneous firing activity of 5-HT neurons in rats exposed to repeated PS. Repeated PS exposure resulted in reduced serum corticosterone and aldosterone concentrations. Concentrations of both corticosteroids in the adrenal glands, as well as the relative weight of the adrenals, were unaffected. The gene expression of hippocampal BDNF of rats exposed to PS remained unaltered. In conclusion, repeated exposure of rats to PS leads to enhanced firing activity of 5HT neurons accompanied by reduced serum, but not adrenal aldosterone and corticosterone concentrations. Reduced corticosteroid concentrations in the blood appear to be the result of increased metabolism and/or tissue uptake. The decrease in circulating corticosterone in rats experienced repeated PS may represent part of the mechanisms leading to increased excitability of 5-HT neurons. The increase in 5-HT neuronal firing activity might be an important compensatory mechanism designated to diminish the harmful effects of the repeated PS exposure on the brain.