Concentration-dependent effects of muscimol to enhance pulsatile GnRH release from GT1-7 neurons in vitro

Immortalized GT1-7 neurons were used to characterize the effect of muscimol, a GABA(A) receptor agonist, to enhance pulsatile gonadotropin- releasing hormone (GnRH) release. GT1-7 neurons were grown on Cytodex-3 beads and placed in special superfusion microchambers. The cells were superfused at a rate of 6.2 ml · h^-1 with Media 199 (pH 7.35) using a commercially available perfusion system. After a pre-muscimol period of 120 min, the cells were exposed for 5 min to 0.35, 1, 5 or 10 μM muscimol or 5 μM muscimol + 20 μM of the GABA(A) receptor antagonist, bicuculline. Following removal of the muscimol (and bicuculline, in the case of the latter experiment), the superfusion was continued for another 115 min. Sample fractions were collected at 5 min intervals throughout the perfusion. Basal GnRH release from the GT1-7 neurons was pulsatile with an average interpulse interval of 45.4 ± 0.5 min and an average pulse amplitude of 191.5 ± 22.6 pg · min · ml^-1. Our results also demonstrated that the GABA(A) receptor agonist, muscimol, enhances pulsatile GnRH release from GT1-7 neurons in culture. The response to muscimol was saturable and concentration-dependent with an EC₅₀ of 0.47 μM. The effects of 5 μM muscimol to increase GnRH pulsatility were blocked by co-exposure to the GABA(A) receptor antagonist, bicuculline. The average GnRH interpulse intervals were 41.7 ± 1.8 min, 32.5 ± 2.9 min, 30.6 ± 0.7 min and 25.5 ± 0.4 min in the period following exposure to 0.35, 1, 5 and 10 μM of muscimol, respectively (post-muscimol period). GnRH pulse amplitude (mean-area for each pulse) was increased during exposure to muscimol but not during the pre- or post-muscimol periods. The GABA(A) receptor antagonist, bicuculline, itself had no effect on pulsatile GnRH release. These results are consistent with previously published reports suggesting that activation of the GABA(A) receptor stimulates hypothalamic GnRH release in embryonic and neonatal animals.