DOPAMINE STIMULATES GROWTH-HORMONE RELEASE FROM THE PITUITARY OF GOLDFISH, CARASSIUS-AURATUS, THROUGH THE DOPAMINE-D1 RECEPTORS

Previously, we have demonstrated that ip injection of apomorphine, a nonselective dopamine (DA) agonist, increases serum GH levels in the goldfish, suggesting a possible role of DA in GH regulation. In the present study, the effects of DA on GH release in the goldfish were further characterized using an in vitro perifusion system for pituitary fragments. DA increased GH release in a dose-dependent manner with an ED50 of 0.26 +/- 0.06-mu-M. SKF38393, a DA D1 agonist, mimicked the GH-releasing effect of DA with an ED50 of 0.41 +/- 0.12-mu-M. Stereoselectivity consistent with mammalian DA D1 systems was demonstrated for the GH response to SKF38393; only the (+)- but not (-)-enantiomer of SKF38393 induced a dose-dependent GH release. Two other D1 agonists, SKF77434 and SKF82958, were also found to have GH-releasing activity. In contrast, high doses (up to 1-mu-M) of the DA D2 agonists, bromocriptine and LY171555, did not affect basal GH levels. The receptor specificity for DA-stimulated GH release was further investigated by using D1 and D2 antagonists; the D1 antagonists SCH23390 and SKF83566 completely abolished the GH response to DA or the D1 agonist SKF38393, whereas the D2-specific antagonists domperidone and (-)-sulpiride were not effective in this respect. Taken together, the present study demonstrates that DA is stimulatory to GH release from the pituitary of goldfish, and its action is mediated through receptors resembling the mammalian DA D1 receptors. The apparent similarities of the DA D1 receptor pharmacology between the goldfish and the mammals also indicate that D1 receptor is highly conserved during vertebrate evolution.