Dopamine inhibition of gonadotropin and α-melanocyte-stimulating hormone release in vitro from the pituitary of the goldfish ( Carassius auratus)

Abstract

The purpose of the present investigation was to examine the receptor specificity of dopamine inhibition of gonadotropin (GtH) and alpha-melanocyte-stimulating hormone (alpha-MSH) release from the goldfish (Carassius auratus) pituitary in vitro. Pars distalis (PD) and neurointermediate lobe (NIL) fragments of the goldfish pituitary were superfused in vitro under various experimental paradigms; eluate from PD and NIL fragments was analyzed for (GtH) and (alpha-MSH), respectively. Spontaneous GtH release from PD fragments was relatively constant over 6 hr; continuous superfusion with dopamine reversibly inhibited spontaneous GtH release with an estimated ED50 of 10(-4.4) M. Domperidone, a specific D-2 receptor antagonist, reversed the inhibitory action of dopamine and increased spontaneous GtH release. Acute treatment of PD fragments with salmon GnRH (sGnRH) stimulated GtH release; dopamine inhibited GtH release from similarly treated fragments with an ED50 of 10(-7.5) M. The spontaneous release of alpha-MSH from NIL fragments was relatively constant over 6 hr; continuous superfusion with dopamine reversibly inhibited this release with an ED50 of 10(-7.2) M. Acute treatment of NIL fragments with thyrotropin-releasing hormone (TRH) caused acute dose-related increases in alpha-MSH release with an ED50 of 10(-8.2) M; dopamine reversibly inhibited alpha-MSH release from similarly treated fragments with an ED50 of 10(-7.7) M. Both stereoisomers of apomorphine, a dopamine agonist, inhibited GtH release from PD fragments treated with sGnRH; in contrast, alpha-MSH release from NIL fragments treated with TRH was stereospecifically inhibited by (-)-apomorphine, but not by (+)-apomorphine. Domperidone reversed (ED50 = 10(-6.6) M) dopamine (10(-6.3) M) inhibition of GtH release from PD fragments treated with sGnRH. In NIL fragments, the inhibitory action of dopamine (10(-6.3) M) was reversed by domperidone (ED50 = 10(-5.5) M), which restored the acute alpha-MSH release response to TRH. These results suggest the involvement of a low-affinity dopamine/neuroleptic receptor in dopamine inhibition of GtH and alpha-MSH release from the pituitary of the goldfish.