The noradrenergic inhibition of an apamin-sensitive, small-conductance Ca2+-activated K+ channel in hypothalamic gamma-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 gamma-aminobutyric acid (GABA)ergic neurons, exhibited an AHP and spike frequency adaptation. The corresponding I(AHP) was sensitive to antagonism by CdCl2 (200 microM), apamin (0.3-1 microM), and dequalinium (3 microM). The beta-adrenergic receptor agonist isoproterenol inhibited the I(AHP) in a dose-dependent, timolol-sensitive fashion. In addition, the alpha1-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 microg, s.c.) markedly potentiated the inhibitory effect of methoxamine on the I(AHP), whereas that for isoproterenol was unaffected. Similarly, bath application of 17beta-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 alpha1-adrenergic receptor-mediated inhibition of this current.
Wagner
E J
EJ
Department of Physiology and Pharmacology, Oregon Health Sciences University, Portland, Oregon 97201, USA. wagnere@ohsu.edu
Rønnekleiv
O K
OK
Kelly
M J
MJ
D000818:Animals; D001030:Apamin / Q000494:pharmacology*; D004967:Estrogens / Q000494:pharmacology*; D005260:Female; D005456:Fluorescent Dyes; D005968:Glutamate Decarboxylase / Q000378:metabolism; D006168:Guinea Pigs; D007031:Hypothalamus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009638:Norepinephrine / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects*; D024681:Potassium Channels, Calcium-Activated; D011301:Preoptic Area / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects / Q000502:physiology; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000502:physiology; D012098:Reproduction / Q000187:drug effects*; D051640:Small-Conductance Calcium-Activated Potassium Channels; D005680:gamma-Aminobutyric Acid / Q000502:physiology
D004967:Estrogens; D005456:Fluorescent Dyes; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D018340:Receptors, Adrenergic, alpha-1; D011943:Receptors, Adrenergic, beta; D051640:Small-Conductance Calcium-Activated Potassium Channels; D001030:Apamin; D005680:gamma-Aminobutyric Acid; D005968:Glutamate Decarboxylase; D009638:Norepinephrine