CHARACTERIZATION OF D1-RECEPTORS MEDIATING DOPAMINE-STIMULATED GROWTH-HORMONE RELEASE FROM PITUITARY-CELLS OF THE GOLDFISH, CARASSIUS-AURATUS

Previously, we demonstrated that dopamine (DA) stimulates GH release f rom the pituitary of goldfish, and this action is mediated by Dl-like receptors. In the current study, we have provided evidence for the pre sence of D1-specific binding sites in the pituitary cells of goldfish. These D1-binding sites were found to be saturable, stereospecific, an d selective for D1 ligands. The rank order of binding affinity of thes e D1-binding sites is (+)SCH23390 > SKF83566 > > (-)SCH23390 > domperi done > LY171555 > > serotonin. The association of these D1-binding sit es with [H-3]SCH23390, a D1-specific radioligand, was rapid, reversibl e, and exhibited a high binding affinity in the nanomolar range. The K (d) values were estimated to be 33.7 +/- 8.5 nm for mixed populations of pituitary cells and 10.9 +/- 2.5 nm for pituitary cell preparations enriched with somatotrophs. Autoradiographic studies revealed that sp ecific binding of [H-3]SCH23390 was predominantly localized in the par s distalis, not in the neurointermediate lobe of the goldfish pituitar y. Furthermore, these D1-binding sites in the goldfish pituitary cells could be functionally correlated with the GH-releasing actions of DA. Since these D1-binding sites exhibited the expected pharmacological p roperties of mammalian D1 receptors, we conclude that DA DI receptors are present in the goldfish pituitary and are responsible for the medi ation of DA Dl-stimulated GH release. The apparent similarities of the D1 receptor pharmacology between goldfish and mammals also suggests t hat DA D1 receptors are highly conserved during vertebrate evolution.