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MOHAMED A. VIRMANI, STANKO S. STOJILKOVIĆ, KEVIN J. CATT, Stimulation of Luteinizing Hormone Release by γ-Aminobutyric Acid (GABA) Agonists: Mediation by GABAA-Type Receptors and Activation of Chloride and Voltage-Sensitive Calcium Channels, Endocrinology, Volume 126, Issue 5, 1 May 1990, Pages 2499–2505, https://doi.org/10.1210/endo-126-5-2499
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The mechanism by which →-aminobutyric acid (GABA) stimulates the release of LH was analyzed in cultured female rat pituitary cells. In 3-h incubations, GABA (1-100 μM) caused a dose-dependent increase in LH release, with the maximal response about 16% of that evoked by 10 nM GnRH. GABA action was independent of the GnRH receptor, since 1 /iM GnRH antagonist ([N-acetyl-D-p-Cl-Phe11-2,D-Trp3,D-Lys6,D-Ala10] GnRH), which completely inhibits GnRH action, did not affect the response to GABA. In studies on the effects of GABA receptor agonists and antagonists, 4,5,6,7-tetrahydoisoxazolo- [5,4-c]pyridin-3(2H)-one (THIP) and muscimol (GABAA agonists) gave similar response patterns, with the same maximal stimulation as GABA but much higher potencies. In contrast, the GABAB receptor agonist baclofen did not stimulate LH release. The GABAA receptor antagonist SR95531 caused dosedependent inhibition of the LH-releasing effects of GABA and muscimol (10 μM), with complete blockade at 10 μM SR95531. T-Butylbicyclophosphorothionate, an inhibitor of the GABAA receptor-associated chloride channel, also dose-dependently reduced the releasing effect of 100 μM GABA. These results indicate that GABA action is mediated by the chloride channelassociated GABAA receptor. However, the other GABAA receptor antagonists, including bicuculline, picrotoxin, and strychnine, did not attenuate the LH-releasing effect of 100 μM GABA in concentrations up to 100 μM, suggesting that GABA action is mediated by nonclassical GABAA receptors. Incubation in the presence of nifedipine (1 μM) or in calcium-free medium inhibited the LH-releasing action of GABA, indicating that calcium influx through voltage-sensitive calcium channels (VSCC) is required for GABA-induced LH release. Such entry of Ca2+ would result from activation of VSCC by depolarization due to the increased Cl- conductance caused by GABAA receptor activation. In cell perfusion studies, the actions of GABA and muscimol were attenuated or abolished after repetitive stimulation, consistent with desensitization of the GABA receptors. These findings have demonstrated that the stimulation of LH release by GABA is independent of GnRH action, occurs via binding to nonclassical GABAA receptors, which rapidly desensitize, and is mediated by the activation of VSCC. (Endocrinology126: 2499–2505,1990)
