Changes in prolactin release caused by GABA and endogenous GABA increase in rats

GABA and γ-acetylenic-GABA (GAG), a GABA-Transaminase inhibitor, were used in this study to determine the influence of GABA on the release of prolactin. In unrestrained male rats, intraventricular (IVT) injection of GABA in high doses (500 μg) markedly increased plasma prolactin levels. On the contrary, GABA at low doses (10 μg) and GAG (100 μg, IVT), were non effective. Systemic GABA administration (25,50,100 and 200 mg/kg IP) lacked action. Both IVT and systemic GAG (100 mg/kg) slightly decreased plasma prolactin levels. The stimulation of prolactin release by histamine was prevented, and the proestrus prolactin surge was blunted by GAG injected IP (100 mg/kg at 1300 hr) or IVT (100 μg). GABA (500 μg, IVT) exhibited a similar action on the histamine response. Results suggest that GABA in basal condition has a pharmacological ability to increase prolactin release. In contrast, the parallel inhibitory effects of GABA and GAG on the histamine effect, and on the proestrus prolactin surge give evidence in favor of a physiological inhibitory role of GABA in the control of prolactin. GABA might act in these conditions as a modulatory transmitter.