The relationship between brain γ-aminobutyric acid and plasma corticosterone levels in mice exposed to high ambient temperature or dexamethasone

The influence of GABAergic system on plasma corticosterone levels was studied in stress-exposed and dexamethasone-treated mice. When mice were exposed to high ambient temperature (38°C), plasma corticosterone content increased at 30 and 60 min. At 120 min corticosterone was similar to the control concentration. Dexamethasone (1 mg/kg, i.p.) decreased plasma corticosterone significantly and induced sustained increase in the GABA content in the diencephalon. Pre-treatment with bicuculline (2 mg/kg, i.p.) blocked the dexamethasone induced decrease in plasma corticosterone. The treatment with bicuculline after dexamethasone-treatment did not increase the plasma corticosterone level, but diminished the feedback effect of dexamethasone on the plasma corticosterone level. GABA contents in the diencephalon of the mice exposed to heat-stress significantly decreased at the time point when plasma corticosterone content increased. When GABA contents returned to the control levels, the plasma concentration of corticosterone was similar to the control levels. Bicuculline (2 mg/kg, i.p.) potentiated the stress-induced increase of plasma corticosterone. From these results, it appears that GABAergic system in the diencephalon may play a regulatory role of ACTH release in mice exposed to high ambient temperature.