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 t han female rats. We have previously demonstrated that hypothalamic gamma-am inobutyric acid (GABA)ergic neuronal activity decreases following gonadecto my in male rats. To investigate whether these same hypothalamic GABA neuron s decrease their activity postcastration in female rats, and whether more r apid and or greater postcastration decreases occur in male rats, we determi ned the timing and magnitude of the postcastration decreases in GABA turnov er which are associated with the sexually dimorphic postcastration LH respo nse. Adult male and 4-day cycling female rats were castrated between 0800 a nd 1000 h (females ovariectomized on diestrus day 1). Serum LH levels incre ased significantly by 12 h postcastration in both males and females with th e magnitude of the increases being 6.2-fold in males and 2.8-fold in female s. GABA turnover was determined in 16 microdissected brain structures by th e 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 terminal is [DBB(ovlt)], the rate of GABA turnover decreased significantly already b y 6 h postcastration compared with the 0 h controls, and remained suppresse d through 6 days. This rapid down regulation of DBB(ovlt) GABAergic neurons also occurred in female rats, however, the duration of the decrease was no t as prolonged as in male rats. Similar changes occurred in the tuberoinfun dibular GABAergic (TIGA) neurons projecting to the median eminence in both males and females. Down regulation of these GABAergic neurons precedes or i s coincident with increased postcastration LH secretion in both sexes, and the duration of the decreases is consistent with the less robust postcastra tion LH response in female rats. In addition, the rate of GABA turnover dec reased after castration in the interstitial (bed) nucleus of the stria term inalis, ventral aspect (INSTv), the medial preoptic nucleus, dorsomedial as pect (MPNdm) and the ventromedial nucleus, ventrolateral aspect (VMNv1) in male rats, and in the INSTv and VMNv1 of female rats, while there was no ef fect of castration in other hypothalamic regions or control structures. The result in the female VMNv1 is consistent with reports that GABA facilitate s lordosis behavior in this hypothalamic structure. These findings are cons istent with the hypothesis that discrete hypothalamic populations of sex st eroid-sensitive GABAergic neurons mediate the postcastration LH responses i n both male and female rats, and may underlie other sexually dimorphic adul t phenotypes such as sex behavior. (C) 2000 Elsevier Science B.V. All right s reserved.