Presynaptic alpha 2- and dopamine-receptor-mediated inhibitory mechanisms and dopamine nerve terminals in the rat hypothalamus.

Slices of rat hypothalamus were superfused and endogenous release of dopamine was measured. The potassium (20 mM)-evoked release in the presence of tetrodotoxin was Ca2+-dependent. The evoked release was reduced by adrenaline 10(-7) M, with a potent alpha 2-agonist action, and was reduced by a dopamine receptor agonist, apomorphine 10(-7) M, in the presence of yohimbine 10(-6) M. These inhibitory effects were antagonized by an alpha 2-antagonist, yohimbine 10(-6) M, and a dopamine receptor antagonist, (-)-sulpiride 10(-7) M, respectively. The electrically evoked release of DA was enhanced by (-)-sulpiride but not by the (+)-isomer. Taking into consideration that yohimbine at a relatively high concentration (10(-6) M) increases the electrically evoked DA release, it is suggested that DA release in the rat hypothalamus may be mediated not only via autoreceptors but also in part via presynaptic alpha 2-receptors on the DA nerve terminals.