GABA(A) receptor mediated elevation of Ca2+ and modulation of gonadotropin-releasing hormone action in αT3-1 gonadotropes

B. Williams, M. Bence, H. Everest, W. Forrest-Owen, S. L. Lightman, Craig A. McArdle

Research output: Contribution to journalArticlepeer-review

13 Scopus citations

Abstract

γ-amino butyric acid (GABA) is the major inhibitory neurotransmitter in the CNS, mediating fast inhibitory synaptic transmission, by activating GABA, receptors. However, these GABA-gated CI- channels can also be excitatory, causing depolarization, and increasing Ca2+ entry via voltage-operated Ca2+ channels (VOCCs). Evidence exists for excitatory ionotropic GABA receptors in anterior pituitary cells, including gonadotropes, but these have not been directly characterized and their pharmacology remains controversial. Here we have measured the cytosolic Ca2+ concentration ([Ca2+](i)) in αT3-1 gonadotropes, to test for expression of excitatory GABA receptors. The GABA(A) agonists, GABA and muscimol, both caused rapid, robust and dose-dependent increases in [Ca2+](i) (EC50 values 2.7 and 1 μM), whereas the GABA(B) agonist, baclofen, did not. The GABA(A) antagonist, bicuculline, inhibited muscimol's effect, whereas the GABA(B) antagonist, baclofen, did not. The neuroactive steroid 5α-pregnan-3α-ol-11,20-dione (an allosteric activator of GABA(A) receptors) increased [Ca2+](i) and this effect, like that of muscimol, was inhibited by picrotoxin. The muscimol effect on [Ca2+](i) was blocked by the VOCC antagonist, nifedipine, or by Ca2+-free medium. When cells were pretreated with muscimol this increased the spike phase of the [Ca2+](i) response to subsequent stimulation with gonadotropin-releasing hormone (GnRH). Similar amplification was seen in muscimol-pretreated cells stimulated with GnRH in Ca2+-free medium, but not when cells were pretreated with muscimol in Ca2+-free medium. The amplification was not, however, GnRH receptor-specific, because the spike response to ionomycin was also increased by muscimol pretreatment. These data provide the first direct evidence for expression of excitatory GABA(A) receptors, and the first demonstration of acute steroid effects, on GnRH-responsive pituitary cells. They also reveal a novel mechanism by which GABA(A) activation modulates GnRH action, raising the possibility that this may also influence gonadotrophin secretion from non-immortalized gonadotropes.

Original languageEnglish
Pages (from-to)159-166
Number of pages8
JournalJournal of Neuroendocrinology
Volume12
Issue number2
DOIs
StatePublished - 2000
Externally publishedYes

Keywords

  • Ca
  • GABA
  • GnRH
  • Gonadotrope
  • αT3-1

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