Role of Gamma-Aminobutyric Acid Neurons in the Release of Gonadotropin-Releasing Hormone in Cultured Rat Embryonic Olfactory Placodes

We recently established a primary cell culture system of gonadotropin-releasing hormone (GnRH) neurons originating from olfactory placodes of rat embryos at E13.5 and showed that cultured olfactory placodes released GnRH into the medium in a pulsatile fashion with an interpulse interval of about 30 min. Since the reported presence of γ-aminobutyric acid (GABA) neurons in the culture of rat olfactory placode raises questions as to the role played by these GABA neurons in the GnRH pulse generation, we immunostained GnRH neurons and GABA neurons in this culture system to examine the interrelationship between both types of neurons, and determined the effects of GABA and the GABA A receptor antagonist, bicuculline, on GnRH release. The immunohistochemical study showed that GnRH neurons received fiber terminals from GABA neurons. GnRH neurons in culture released GnRH into the medium at intervals of 30–40 min, confirming our previous study. Treatment with 20 µM GABA prolonged the interpulse interval and decreased the amplitude of GnRH pulses. Bicuculline administered at 20 µM did not affect either parameter, but 50 µM bicuculline elevated the mean GnRH level, although it did not affect either the interpulse interval or the amplitude of GnRH pulses. In addition, 50 µM bicuculline increased the mean trough levels of GnRH pulses, although 20 µM bicuculline did not. In light of the in vivo studies performed previously, we suggest that the GnRH pulse generator, which probably consists of a small population of GnRH neurons in the culture, does not involve GABA neurons to generate the pulsatile GnRH release, although it may be responsive to the inhibitory transmitter GABA. We also found that there may be another population of GnRH neurons in the culture whose activity is strongly suppressed by the tonic inhibition of GABA neurons. Although it is speculative, these latter GnRH neurons may be responsible for the surge of GnRH release.