γ-Aminobutyric acid (GABA) is one of the most important neurotransmitters that regulate the excitability of GnRH neurons. Numerous studies have shown that GABA activates Cl currents in GnRH neurons, and these effects are antagonized by GABAA receptor antagonists. The GABAB receptor is a heterodimer composed of GABAB R1 and R2, and although both subunits have been localized in GnRH neurons, nothing is known about the cellular signaling of this Gαi,o-coupled receptor in GnRH neurons. Using whole-cell recordings from mouse enhanced green fluorescent protein-GnRH neurons, we found that the GABAB receptor agonist baclofen hyperpolarized GnRH neurons through activation of an inwardly rectifying K+ current in a concentration-dependent manner. The effects of baclofen were antagonized by the selective GABAB receptor antagonist CGP 52432 with a Ki (inhibitory constant) of 85 nm. Furthermore, in the presence of the GABAA receptor antagonist picrotoxin, GABA hyperpolarized GnRH neurons in a similar manner. Treatment with 17β-estradiol as compared with oil vehicle did not significantly alter either the EC50 for the baclofen-induced response (0.8 ± 0.1 vs. 1.0 ± 0.1 μm, respectively) or the maximal outward current (10.8 ± 1.7 pA vs. 11.4 ± 0.6 pA, respectively) in GnRH neurons. However, the outward current (and membrane hyperpolarization) was abrogated by submaximal concentrations of the G protein-coupled receptor 54 (GPR54) agonist kisspeptin-10 in both groups, indicating that Gαq-coupled (GPR54) can desensitize the GABAB receptor-mediated response. Therefore, the activation of GABAB receptors in GnRH neurons may provide increased inhibitory tone during estrogen-negative feedback states that is attenuated by kisspeptin during positive feedback.

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