Vol. 299, Issue 1, 21-30, October 2001

The Noradrenergic Inhibition of an Apamin-Sensitive, Small-Conductance Ca²⁺-Activated K⁺ Channel in Hypothalamic -Aminobutyric Acid Neurons: Pharmacology, Estrogen Sensitivity, and Relevance to the Control of the Reproductive Axis

Edward J. Wagner, Oline K. Rønnekleiv and Martin J. Kelly

Department of Physiology and Pharmacology, Oregon Health Sciences University, Portland, Oregon

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 -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 -adrenergic receptor agonist isoproterenol inhibited the I_AHP in a dose-dependent, timolol-sensitive fashion. In addition, the ₁-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 17-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 ₁-adrenergic receptor-mediated inhibition of this current.