Castration rapidly decreases hypothalamic gamma-aminobutyric acidergic neuronal activity in both male and female rats

The postcastration LH response is greater and somewhat more rapid in male than female rats. We have previously demonstrated that hypothalamic gamma-aminobutyric acid (GABA)ergic neuronal activity decreases following gonadectomy in male rats. To investigate whether these same hypothalamic GABA neurons decrease their activity postcastration in female rats, and whether more rapid and or greater postcastration decreases occur in male rats, we determined the timing and magnitude of the postcastration decreases in GABA turnover which are associated with the sexually dimorphic postcastration LH response. Adult male and 4-day cycling female rats were castrated between 0800 and 1000 h (females ovariectomized on diestrus day 1). Serum LH levels increased significantly by 12 h postcastration in both males and females with the magnitude of the increases being 6.2-fold in males and 2.8-fold in females. GABA turnover was determined in 16 microdissected brain structures by the GABA transaminase inhibition method at 0 h (sham-operated controls), 6 h, 12 h and 1, 2, 4 and 6 days postcastration. In male rats, in the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis [DBB(ovlt)], the rate of GABA turnover decreased significantly already by 6 h postcastration compared with the 0 h controls, and remained suppressed through 6 days. This rapid down regulation of DBB(ovlt) GABAergic neurons also occurred in female rats, however, the duration of the decrease was not as prolonged as in male rats. Similar changes occurred in the tuberoinfundibular GABAergic (TIGA) neurons projecting to the median eminence in both males and females. Down regulation of these GABAergic neurons precedes or is coincident with increased postcastration LH secretion in both sexes, and the duration of the decreases is consistent with the less robust postcastration LH response in female rats. In addition, the rate of GABA turnover decreased after castration in the interstitial (bed) nucleus of the stria terminalis, ventral aspect (INSTv), the medial preoptic nucleus, dorsomedial aspect (MPNdm) and the ventromedial nucleus, ventrolateral aspect (VMNvl) in male rats, and in the INSTv and VMNvl of female rats, while there was no effect of castration in other hypothalamic regions or control structures. The result in the female VMNvl is consistent with reports that GABA facilitates lordosis behavior in this hypothalamic structure. These findings are consistent with the hypothesis that discrete hypothalamic populations of sex steroid-sensitive GABAergic neurons mediate the postcastration LH responses in both male and female rats, and may underlie other sexually dimorphic adult phenotypes such as sex behavior.