Estradiol Enhances Excitatory Gammabutyric Acid-Mediated Calcium Signaling in Neonatal Hypothalamic Neurons.

Contrary to the situation in adulthood, gammabutyric acid (GABA)A receptor activation during early brain development depolarizes neurons sufficiently to open L-type voltage-gated Ca channels. Because GABA is excitatory during the sensitive period of steroid-mediated brain sexual differentiation, we investigated whether estradiol modulates excitatory GABA during this period, by examining two parameters: 1) magnitude of GABA-induced calcium transients; and 2) developmental duration of excitatory GABA. Dissociated hypothalamic neurons from embryonic-day-15 rat embryos were loaded with the Ca indicator, fura-2, and transient rises in [Ca]i (Ca transient) were measured after application of 10 mM muscimol, a GABAA receptor agonist. Cells were treated with 10 210 M estradiol or vehicle from 0–3 days in vitro (DIV) and imaged on 4 DIV, whereas others were treated from 3–6 DIV and imaged on 7 DIV. The mean amplitude of Ca transients after muscimol administration were 68% and 61% higher in estradiol-treated neurons on 4 DIV and 7 DIV, respectively, relative to controls. Consistent with GABA becoming inhibitory in mature neurons, 50% fewer control neurons responded on DIV 7, relative to DIV 4. However, estradiol treatment maintained excitatory GABA on DIV 7 (72% in estradiol-treated vs. 35% in control). This is the first report of hormonal modulation of excitatory GABA, and it suggests that estradiol may mediate sexual differentiation by enhancing GABA-induced increases in intracellular Ca. (Endocrinology 142: 2238–2243, 2001) S ARE IMPORTANT regulators of gene expression in the brain during development and in the adult. In the rat, there is a developmental window beginning the last week of gestation and terminating a few days after birth, during which high levels of estradiol (aromatized from testicularly derived testosterone) masculinize the male brain (1). Steroid-mediated effects during this period include alterations in dendritic spines, neuronal branching, myelination, and cell death and are the basis of adult sex differences in brain structure and function (2). Estradiol also displays trophic properties in dissociated cell culture systems, enhancing the growth and arborization of axons and dendrites in organotypic cultures from various brain regions (3). Despite the considerable in vivo and in vitro evidence for estradiol’s trophic actions, the underlying cellular mechanisms remain