The noradrenergic inhibition of an apamin-sensitive, small-conductance Ca²⁺-activated K⁺ channel in hypothalamic γ-aminobutyric acid neurons: Pharmacology, estrogen sensitivity, and relevance to the control of the reproductive axis

The present study sought to determine whether small-conductance, Ca²⁺-activated K⁺ currents underlie the afterhyperpolarization (AHP) in neurons of the preoptic area (POA), a brain region important in controlling reproduction. We used an ovariectomized, female guinea pig model to test two hypotheses: 1) the current associated with the AHP (I_AHP) regulates the firing rate of POA neurons and 2) amine neurotransmitters modulate it in a gonadal steroid-sensitive manner. Intracellular recordings followed by combined histofluorescence/in situ hybridization for glutamic acid decarboxylase, 65-kDa isomer, revealed that POA neurons, including γ-aminobutyric acid (GABA)ergic neurons, exhibited an AHP and spike frequency adaptation. The corresponding I_AHP was sensitive to antagonism by CdCl₂ (200 μM), apamin (0.3-1 μM), and dequalinium (3 μM). The β-adrenergic receptor agonist isoproterenol inhibited the I_AHP in a dose-dependent, timolol-sensitive fashion. In addition, the α₁-adrenergic receptor agonist methoxamine dose dependently inhibited the I_AHP in a prazosin-sensitive manner and increased neuronal firing rate. Twenty-four-hour pretreatment with estradiol benzoate (EB; 25 μg, s.c.) markedly potentiated the inhibitory effect of methoxamine on the I_AHP, whereas that for isoproterenol was unaffected. Similarly, bath application of 17β-estradiol (100 nM; 15-20 min) mimicked the effect of EB on the methoxamine-induced inhibition of the I_AHP. Thus, POA GABAergic neurons express an apamin-sensitive channel that mediates, at least in part, the I_AHP, and tempers the excitability of these cells. Furthermore, these studies demonstrate that estrogen enhances the α₁-adrenergic receptor-mediated inhibition of this current.