Effects of hypothalamic dopamine on growth hormone‐releasing hormone‐induced growth hormone secretion and thyrotropin‐releasing hormone‐induced prolactin secretion in goats

The aim of the present study was to clarify the effects of hypothalamic dopamine ( DA ) on the secretion of growth hormone ( GH ) in goats. The GH ‐releasing response to an intravenous (i.v.) injection of GH ‐releasing hormone ( GHRH , 0.25 μg/kg body weight (BW)) was examined after treatments to augment central DA using carbidopa (carbi, 1 mg/kg BW) and L ‐dopa (1 mg/kg BW) in male and female goats under a 16‐h photoperiod (16 h light, 8 h dark) condition. GHRH significantly and rapidly stimulated the release of GH after its i.v. administration to goats ( P  < 0.05). The carbi and L ‐dopa treatments completely suppressed GH ‐releasing responses to GHRH in both male and female goats ( P  < 0.05). The prolactin ( PRL )‐releasing response to an i.v. injection of thyrotropin‐releasing hormone ( TRH , 1 μg/kg BW) was additionally examined in male goats in this study to confirm modifications to central DA concentrations. The treatments with carbi and L ‐dopa significantly reduced TRH ‐induced PRL release in goats ( P  < 0.05). These results demonstrated that hypothalamic DA was involved in the regulatory mechanisms of GH , as well as PRL secretion in goats.