Sex differences in glutamic acid decarboxylase mRNA in neonatal rat brain: implications for sexual differentiation.

Sexual differentiation of rodent brain is dependent upon hormonal exposure during a "critical period" beginning in late gestation and ending in early neonatal life. Steroid hormone action at this time results in anatomical and physiological sexual dimorphisms in adult brain, but the mechanism mediating these changes is essentially unknown. The inhibitory neurotransmitter, GABA, is involved in regulation of sexually dimorphic patterns of behavior and gonadotropin secretion in the adult. Recent evidence suggests that during development GABA is excitatory and provides critical neurotrophic and neuromodulatory influences. We hypothesized that steroid-induced changes in GABAergic neurotransmission during this critical period are important mediators of sexual differentiation in brain. Therefore, we quantified levels of mRNA for GAD, the rate-limiting enzyme in GABA synthesis. On Postnatal Day 1, males had significantly higher levels of GAD mRNA in the dorsomedial nucleus, arcuate nucleus, and CA1 region of hippocampus. On Postnatal Day 15, after the critical period for sexual differentiation has ended, these differences were no longer present. We examined the role of gonadal steroids in regulating GAD by removing testes of males and administering testosterone to females at birth. Exposure to testosterone was correlated with increased GAD mRNA in the dorsomedial nucleus. A sex difference in GAD mRNA was also observed in the medial preoptic area, but the influence of testosterone was inconclusive. We conclude that sex differences in the GABAergic system during development are partially hormonally mediated, and that these differences may contribute to the development of sexually dimorphic characteristics in adult brain.