GABA is a predominant neurotransmittter in the hypothalamus and an important regulator of hypothalamic function, To elucidate the cellular basis for GABAergic action in this region, we used intracellular recordings from identified hypothalamic neurons, Ninety-three percent of the mediobasal hypothalamic neurons responded to GABA(B) receptor stimulation, and the presence of bicuculline-sensitive synaptic potentials indicated a tonic, GABA(A) receptor-mediated input. Stimulation of GABA(B) receptors hyperpolarized these cells by activating an inwardly rectifying potassium conductance. We characterized GABA(B) responses by generating concentration-response curves to the GABA(B) agonist baclofen. There was heterogeneity in the responses to baclofen, with one third of the cells having low baclofen potency (EC(50) = 5.0 mu M). Two thirds of the neurons had a 4-fold higher potency (EC(50) = 1.2 mu M), larger somas and a more lateral distribution. Previous work has shown that hypothalamic GABA(B) and mu-opioid receptors open the same K+ channels and that the response to mu-opioid agonists is rapidly attenuated by 17 beta-estradiol (E(2)). In order to test the hypothesis that the coupling of GABA(B) receptors to K+ channels is also altered, baclofen concentration-response curves were generated before and after an E(2) challenge (100 nM, 20 min). Consistent with our hypothesis, the potency of baclofen was decreased nearly 4-fold in a subset of the cells that had a high potency response to baclofen. Furthermore, decreased baclofen potency only occurred in those cells in which E(2) also altered the mu-opioid responses. Therefore, our findings suggest that a discrete subpopulation of hypothalamic neurons is sensitive to estrogen actions to alter inhibitory transmission. We propose that the alteration of GABA(B) and mu-opioid input is consistent with estrogen's rapid inhibition of the reproductive axis.
