Effects of dopamine, norepinephrine and serotonin on plasma concentrations of luteinizing hormone and prolactin in ovariectomized and anestrous ewes

Abstract

Two experiments were conducted in ovariectomized and anestrous ewes to examine for effects of dopamine (DA), norepinephrine (NE) and serotonin (5-HT) on basal concentrations of luteinizing hormone (LH) and prolactin (Prl) and the gonadotropin-releasing hormone (GnRH)-induced release of LH. In Experiment 1, ovariectomized ewes, during the breeding and anestrous seasons, were infused for 6 h intravenously with DA (0.66, 6.6 or 66 μg/kg per min), NE or 5-HT (each at doses of 0.06, 0.66 or 6.6μg/kg per min). Effects of the biogenic amines on tonic concentrations and GnRH-induced release of LH were not different between seasons. Prior to GnRH, mean concentrations of LH in plasma were affected by DA, NE and 5-HT in a dose-dependent manner. The high dose of DA and NE decreased mean concentrations of LH, while the high dose of 5-HT increased mean concentrations of LH during the first 2 h of infusion. Mean concentrations of LH in the 4 h following administration of GnRH were related to mean concentrations of LH prior to GnRH. After adjustment by analysis of covariance, there was a significant effect of the interaction of biogenic amine x dose on mean concentrations of LH following administration of GnRH. Dose-response patterns of LH to administration of GnRH following infusion of biogenic amines were similar to those observed prior to treatment with GnRH. While 5-HT did not affect concentrations of Prl, DA and NE reduced concentrations of Prl. Dopamine was more effective in inhibiting secretion of Prl in fall than in spring. Changes in Prl and LH were not correlated. In Experiment 2, anestrous ewes were infused intravenously with DA (0.66 and 66 μg/kg per min), NE or 5-HT (each at doses of 0.06 and 6.6 μg/kg per min). Concentrations of LH were uniformly low prior to administration of GnRH. Following treatment with GnRH, NE inhibited and 5-HT potentiated the release of LH (P<0.01), while DA had no effect. Concentrations of Prl in plasma were inhibited by DA and NE in a dose-related manner (P<0.01). Intravenous infusion of 5-HT did not affect Prl. Again, there was no apparent relationship between changes in LH and Prl.