Activation of A-type γ-aminobutyric acid receptors excites gonadotropin-releasing hormone neurons

γ,-Aminobutyric acid (GABA), acting through GABA_A receptors (GABA_AR), is hypothesized to suppress reproduction by inhibiting GnRH secretion, but GABA actions directly on GnRH neurons are not well established. In green fluorescent protein-identified adult mouse GnRH neurons in brain slices, gramicidin-perforated-patch-clamp experiments revealed the reversal potential (E_GABA) for current through GABA_ARs was depolarized relative to the resting potential. Furthermore, rapid GABA application elicited action potentials in GnRH neurons but not controls. The consequence of GABA_AR activation depends on intracellular chloride levels, which are maintained by homeostatic mechanisms. Membrane proteins that typically extrude chloride (KCC-2 cotransporter, CLC-2 channel) were absent from the GT1-7 immortalized GnRH cell line and GnRH neurons in situ or were not localized to the proper cell compartment for function. In contrast, GT1-7 cells and some GnRH neurons expressed the chloride-accumulating cotransporter, NKCC-1. Patch-clamp experiments showed that blockade of NKCC hyperpolarized E_GABA by lowering intracellular chloride. Regardless of reproductive state, rapid GABA application excited GnRH neurons. In contrast, bath application of the GABA_AR agonist muscimol transiently increased then suppressed firing; suppression persisted 4-15 min. Rapid activation of GABA_AR thus excites GnRH neurons whereas prolonged activation reduces excitability, suggesting the physiological consequence of synaptic activation of GABA_AR in GnRH neurons is excitation.