Evidence for a Short Feedback of Prolactin on the Tubero-Infundibular Endings: Differential Effect on the Release of [³ H]Gamma-Aminobutyric Acid and [³ H]Dopamine from Superfused Median Eminence

Abstract Release experiments were undertaken on the tubero-infundibular terminals of the hypothalamic median eminence. Isolated median eminences were preloaded with [(3) H]y-aminobutyric acid (GABA) or [(3) H]dopamine in such conditions that non-specific uptake was pharmacologically impaired. Each median eminence was continuously superfused with Krebs bicarbonate medium and tritium releases were evoked with high potassium solutions. The evoked release was estimated by measuring the total radioactivity of which actual dopamine and GABA represented more than 60% and 80%, respectively. The experiments were done by applying two successive 5-min pulses of potassium, the second in the presence of prolactin. These stimulations were submaximal: for each, a KCl concentration was chosen that gave about half the maximal response (45 mM for GABA and 30 mM for dopamine); the responses were suppressed by calcium-free medium. Rat prolactin induced a significant increase of dopamine-evoked release in a dose-dependent manner with a maximum (30%) at 25 mug prolactin/ml of medium. Over the same range of concentration a significant but still weak (10%) increase of evoked GABA was observed only with the highest prolactin concentration (25 mug prolactin/ml). At the same concentrations, ovine prolactin induced a markedly reduced effect on dopamine release and had no effect on GABA release. Spontaneous releases of dopamine and GABA were not affected by prolactin. These results show that prolactin enhances acutely and directly, but differently, the evoked release of GABA and dopamine by the median eminence. This differential effect may reflect either a differential regulation or a greater heterogeneity of GABAergic endings. Our data give support for a short feedback exerted by prolactin directly on the tubero-infundibular neurons.