Abstract
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 (IAHP) 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 IAHP 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 IAHP in a dose-dependent, timolol-sensitive fashion. In addition, the α1-adrenergic receptor agonist methoxamine dose dependently inhibited the IAHP 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 IAHP, 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 IAHP. Thus, POA GABAergic neurons express an apamin-sensitive channel that mediates, at least in part, the IAHP, and tempers the excitability of these cells. Furthermore, these studies demonstrate that estrogen enhances the α1-adrenergic receptor-mediated inhibition of this current.
Footnotes
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The experiments described in this study were supported by Public Health Services Grants NS35944, NS38809, and DA05158.
- Abbreviations:
- BK
- large-conductance
- SK
- small-conductance
- AHP
- afterhyperpolarization
- E2
- 17β-estradiol
- EB
- estradiol benzoate
- GAD65
- glutamic acid decarboxylase, 65-kDa isomer
- GnRH
- gonadotropin-releasing hormone
- IAHP
- current underlying the AHP
- LH
- luteinizing hormone
- POA
- preoptic area
- TEA
- tetraethylammonium
- TTX
- tetrodotoxin
- ΔImax
- maximal inhibition of the peak current
- τinactivation
- time constant for inactivation
- SSC
- sodium chloride/sodium citrate buffer
- GABA
- γ-aminobutyric acid
- aCSF
- artificial cerebrospinal fluid
- Received April 3, 2001.
- Accepted June 6, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
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