Dopaminergic innervation of the rainbow trout pituitary and stimulatory effect of dopamine on growth hormone secretion in vitro

To elucidate which factors regulate growth hormone (GH) secretion in rainbo w trout, dopaminergic innervation of the rainbow trout pituitary along with the action of dopamine in vitro, were studied. Brains with attached pituit aries were double-labeled for putative dopaminergic neuronal fibers and som atotropes, using fluorescence immunohistochemistry. A direct dopaminergic i nnervation to the proximal pars distalis (PPD) with dopaminergic fibers ter minating adjacent to somatotropes was demonstrated. Growth hormone secretio n from whole pituitaries was measured in perifusate using a homologous GH-R IA. Dopamine (DA; 10(-7)-2 x 10(-6) g ml(-1)) increased basal GH secretion, with the GH secretion normalizing again after the DA exposure was halted. When pituitaries were pre-treated with somatostatin-14 (SRIF-14; 10(-12)-10 (-9) g ml(-1)), before being exposed to different doses of DA, there was an inhibition of GH secretion which was not reversed after treatment of SRIF- 14 was halted, unless DA was added. It is concluded that dopamine can funct ion as a GH secretagogue in the rainbow trout pituitary gland. (C) 2000 Els evier Science Inc. All rights reserved.