GABAERGIC NEURONAL-ACTIVITY AND MESSENGER-RNA LEVELS FOR BOTH FORMS OF GLUTAMIC-ACID DECARBOXYLASE (GAD(65) AND GAD(67)) ARE REDUCED IN THEDIAGONAL BAND OF BROCA DURING THE AFTERNOON OF PROESTRUS

There is considerable evidence that GABAergic neurons play an importan t role in the regulation of gonadotropin-releasing hormone (GnRH) secr etion, and that these neurons may mediate the feedback actions of gona dal steroids on GnRH neurons. The aim of the present study was to inve stigate whether endogenous changes in ovarian steroid secretion during the estrous cycle influenced GABAergic neuronal activity in the preop tic region of the hypothalamus, and in other steroid-sensitive brain r egions, Intact, adult female rats were sacrificed at various times dur ing the days of metestrus or proestrus, GABAergic neuronal activity wa s estimated by measuring the rate of accumulation of GABA in microdiss ected brain regions after pharmacological inhibition of GABA degradati on. Concentrations of mRNA for both forms of glutamic acid decarboxyla se (GAD(65) and GAD(67)) were quantified in microdissected brain regio ns by a microlysate ribonuclease protection assay. In the diagonal ban d of Broca at the level of the organum vasculosum of the lamina termin alis (DBB(ovlt)), GABAergic neuronal activity was significantly reduce d during the afternoon of proestrus compared with tile morning of eith er proestrus or metestrus. in the lateral septal nucleus, GABAergic ne uronal activity was significantly increased in the afternoon of proest rus compared with the morning. There were no significant effects of ti me of day or day of estrous cycle in the medial preoptic nucleus, medi an eminence, ventromedial nucleus, suprachiasmatic nucleus, medial sep tal nucleus, hippocampus (CA1 region), or cingulate cortex. in the DBB (ovlt), mRNA levels for both GAD(65) and GAD(67) were significantly re duced in the afternoon of proestrus compared with the afternoon of met estrus. By contrast, there was no change in GAD(65) and GAD(67), mRNA levels in the cingulate cortex at any of the times examined. These res ults demonstrate that GABAergic neuronal activity, and mRNA levels for both GAD(65) and GAD(67), are reduced in the DBB(ovlt) during the aft ernoon of proestrus. These results support the hypothesis that decreas ed GABAergic neuronal activity in this region plays a major permissive role in the generation and maintenance of the estrogen-induced LH sur ge.