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-FIT) on basal concentrations of luteinizing hormone (LH) and prolactin (PrI) 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 pg/kg per mm), NE or 5-HT (each at doses of 0.06, 0.66 or 6.6 pg/kg per mm). 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-FIT increased mean concentrations of LH during the first 2 h of infusion. Mean concentrations of LH in the 4 h following administration of CnRH 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 CnRH. Dose-response patterns of LH to administration of GnRFI following infusion of biogenic arnines were similar to those observed prior to treatment with GnRH. While 5-FIT did not affect concentrations of PrI, DA and NE reduced concentrations of PrI. Dopamine was more effective in inhibiting secretion of PrI in fall than in spring. Changes in PrI and LH were not correlated. In Experiment 2, anestrous ewes were infused intravenously with DA (0.66 and 66 pg/kg per mm), NE or 5-HT (each at doses of 0.06 and 6.6 pg/kg per mm). Concentrations of LH were uniformly low prior to administration of CnRH. Following treatment with GnRH, NE inhibited and 5-HT potentiated the release of LH (P<0.01), while DA had no effect. Concentrations of PrI 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 PrI.