GABA inhibition of immortalized gonadotropin-releasing hormone neuronal excitability involves GABA(A) receptors negatively coupled to cyclic adenosine monophosphate formation

gamma -Aminobutyric acid (GABA) has been implicated in the regulation of re production, particularly in the developmental modulation of gonadotropin-re leasing hormone (GnRH) secretion. GnRH neurons are innervated by GABA-conta ining processes, and the administration of GABA stimulates and inhibits GnR H secretion in vivo and in vitro. We have previously shown that GABA can ex ert both of these actions in sequence, by acting directly on immortalized G nRH neurons. While the stimulation is the result of a GABA(A) receptor-medi ated depolarization of the plasma membrane, the mechanism involved in the d elayed inhibition is the subject of the present investigation. GABA (1 nM-1 0 muM) decreased the intracellular concentration of cyclic adenosine monoph osphate (cAMP) in a dose- and time-dependent fashion. This effect was block ed by bicuculline and mimicked by muscimol but not by baclofen, To analyze the effect of GABA on cellular excitability, we used fura-2 loaded GT1-7 ce lls. Activation of voltage-sensitive calcium channels by high K+-induced de polarization (35 mM) increased [Ca2+](i), GABA (10 muM) and muscimol (10 mu M) reduced the amplitude of K+-induced [Ca2+](i) transients. This inhibitio n was blocked by forskolin (20 muM) or 8-Br-cAMP (1 mM), Altogether, these results show that GABA(A) receptors mediate a sustained inhibitory effect o f GABA on GnRH neurons, and suggest the involvement of the cAMP pathway dec reasing cellular excitability.