Dopamine receptors influence vasoactive intestinal peptide release from turkey hypothalamic explants

Vasoactive intestinal peptide (VIP) is a significant prolactin-releasing factor (PRF) in avian species, and dopamine (DA) exhibits both a stimulatory and inhibitory influence upon this prolactin (PRL) secretion. The stimulatory effect of DA upon PRL release appears to be mediated by VIP. This study investigated DAergic actions upon VIP release using turkey hypothalamic explants perifused with DA and its agonists or antagonists. VIP release was stimulated by DA in a dose-dependent manner (10 nmol DA/min, from 67.2 ± 3.9 to 164.3 ± 3.1 pg/5 min; 100 nmol DA/min, from 70.1 ± 3.2 to 291.0 ± 7.5 pg/5 min; 1,000 nmol DA/min, from 72.0 ± 4.8 to 501.0 ± 24.7 pg/5 min). The D₁ DA receptor antagonist (R+)-SCH-23390 HC1 completely negated the stimulatory effect of DA (100 nmol/min) upon VIP release. Perifusion with the D₂ DA receptor antagonist S(-)-eticlopride HC1 by itself stimulated VIP release from the hypothalamic explants, increasing VIP from 38.1 ± 5.3 to 161.9 ± 9.7 pg/5 min, where release stabilized until perifusion was terminated. The D₁ DA agonist (+)-SKF-38393 HC1 increased VIP release from 52.7 ± 4.6 to 192.6 ± 16.9 pg/5 min, and this stimulated release was partially inhibited by the D₂ DA receptor agonist R(-)-NPA HC1 (from 192.6 ± 16.9 to 139.7 ± 13.8 pg/5 min). These results suggest that VIP secretion is in part regulated by possible opposite actions between stimulatory D₁ and inhibitory D₂ DA receptors in the turkey hypothalamus.