IN-VITRO AND IN-VIVO EVIDENCE THAT DOPAMINE EXERTS GROWTH HORMONE-RELEASING ACTIVITY IN GOLDFISH

We have previously demonstrated that dopamine (DA) and the DA D1 agoni st SKF 38393 stimulate growth hormone (GH) release from perifused pitu itary fragments of the goldfish, suggesting an involvement of DA D1 re ceptors in GH regulation. In the present study, the role of DA on GH r elease and body growth of the goldfish was further investigated both i n vivo and in vitro. DA consistently stimulated GH release in a dose-d ependent manner from perifused goldfish pituitary fragments. The GH-re leasing action of DA was seasonal, being the highest in sexually regre ssed fish, intermediate in recrudescent fish, and the lowest in sexual ly mature (prespawning) fish. Somatostatin, a known GH-release inhibit or in the goldfish, suppressed basal GH release and abolished the GH r esponse to DA in perifused pituitary fragments as well as pituitary ce lls under static incubation. Intraperitoneal administration of the non selective DA agonist apomorphine and the D1 agonist SKF 82958 increase d the plasma GH levels in the goldfish. These GH responses were blocke d by simultaneous treatment with the D1 antagonist Sch 23390 but not t he D2 antagonist pimozide. Apomorphine administered orally also induce d a similar elevation in plasma GH levels. Long-term feeding with apom orphine was found to be stimulatory to the body growth of goldfish. Th ese results provide evidence that the neurotransmitter DA, by acting t hrough DA D1 receptors in the pituitary, also functions as a GH-releas ing factor in the goldfish.