Castration-induced decrease in the activity of medial preoptic and tuberoinfundibular GABAergic neurons is prevented by testosterone.

We recently determined that castration specifically decreased GABA turnover in discrete rostral and mediobasal hypothalamic structures. This study aimed to investigate whether testosterone could stimulate GABAergic neuronal activity in these hypothalamic GABAergic neurons in the castrate rat, and to compare the effects of episodic testosterone replacement with the constant levels provided by subcutaneous testosterone implants. Animals were divided into 4 experimental groups: intact, 48 h castrate, 48 h castrate+testosterone capsules (2 x 30 mm Silastic implants, 1.57 mm ID, 3.18 mm OD) and 48 h castrate+testosterone injections (100 micrograms/injection s.c., every 8 h). GABA concentrations were measured in 4 microdissected brain regions either before or 60 min after inhibition of the GABA degrading enzyme, GABA transaminase, by injection of aminooxyacetic acid (AOAA, 100 mg/kg i.p.). The rate of GABA accumulation in the tissue following injection of AOAA was used as an index of GABAergic neuronal activity. Castration resulted in a 10-fold increase in serum LH concentrations compared with intact rats. Either mode of testosterone administration completely prevented this castration-induced LH rise. In the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis, the medial preoptic nucleus and in the median eminence, GABA turnover was significantly reduced by castration to approximately 50% that of intact rats. Either testosterone implants or testosterone injections prevented this castration-induced decrease in GABA turnover, such that the turnover rates were not significantly different from intact rats. There was no effect of castration with or without testosterone replacement in the cingulate cortex.(ABSTRACT TRUNCATED AT 250 WORDS)