Dopamine and [D-ALA2, D-Leu5]enkephalin Inhibit the Electrically Stimulated Neurohypophyseal Release of Vasopressin in Vitro: Evidence for Calcium-dependent Opiate Action

The effects of both dopamine and enkephalin agonists were assessed on electrically stimulated vasopressin secretion from the isolated neural lobe. Both dopamine (10(-6) M) and the dopamine agonist ADTN (2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene; 10(-6) M) significantly reduced the stimulated release of vasopressin. This reduction in vasopressin secretion was prevented by the dopamine antagonist sulpiride (10(-6) M). [D-Ala2, D-Leu5]Enkephalin (DADLE) also inhibited vasopressin release and its effect was additive to that of ADTN. The effect of DADLE was prevented by the opiate antagonist naloxone and was also sensitive to the calcium content of the medium. Low calcium concentrations markedly increased the inhibitory effect of DADLE, while high concentrations abolished its effect. These findings provide evidence of a direct inhibitory action of both dopamine and enkephalins at the level of the neural lobe and suggest that the effect of enkephalins may be intimately related to calcium-sensitive neurosecretory events.