Sex differences in GABA turnover and glutamic acid decarboxylase (GAD(65) and GAD(67)) mRNA in the rat hypothalamus

GABAergic neurons are estimated to make up more than half of the neuronal population of the hypothalamus and they likely account for some of the structural and functional sexual dimorphisms observed in the mammalian brain. We previously reported sex differences in the rate of GABA turnover in discrete hypothalamic structures of adult rats. In the present study, we extended our search for sex differences in GABA turnover to additional structures, and further determined whether these differences were associated with differences in GAD(65) and or GAD(67) mRNA levels. Utilizing the GABA transaminase inhibition method, we determined GABA turnover in 14 microdissected brain regions. The rate of GABA turnover was about 2-fold greater in male than in diestrous day one (D(1)) female rats in the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis [DBB(ovlt)], anteroventral periventricular nucleus (AVPv), median eminence (ME), and dorsomedial portion of the ventromedial nucleus (VMNdm). A sex difference also was noted in the DBB(ovlt) for GAD(65) mRNA determined by microlysate RNase protection assay. Here, GAD(65) levels were almost 2-fold greater in male rats, which suggests that differences in the activity of this GAD enzyme isoform contributes to the difference in turnover in this area. Additionally, in the dorsomedial nucleus (DMN), the GAD(65) mRNA level was significantly higher in female rats, and in the medial amygdaloid nucleus (Am), GAD(67) mRNA was higher in male rats. These data reveal striking sexual dimorphisms in the rate of GABA turnover and in GAD mRNA levels in specific populations of hypothalamic GABAergic neurons. The functional relationships between these GABAergic neurons and sexually dimorphic phenotypes associated with these structures, such as gonadotropin secretion, reproductive behaviors, seizure threshold and others, warrant further investigation.