Abstract
Two experiments were conducted in ovariectomized, pituitary stalk-transected ewes to determine if dopamine (DA), norepinephrine (NE) or serotonin (5-HT) alter secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL). In experiment 1, ewes were infused ( iv) with saline (control), DA (66 μg/kg/min), NE (6.6 μg/kg/min) or 5-HT (6.6 μg/kg/min). Treatments did not alter pulse frequency, but 5-HT increased (P<.05) amplitude of pulses of LH and mean concentrations of LH. DA and NE were without effect on basal secretion of LH. DA but not NE or 5-HT decreased (P<.05) the release of LH in response to gonadotropin hormone-releasing hormone (GnRH, 25 μg, im). Concentrations of FSH were not affected by treatments. Secretion of PRL was reduced (P<.05) by treatment with DA and NE but not 5-HT. Each amine reduced (P<.05) the release of PRL in response to thyrotropin-releasing hormone (TRH; 3 μg, im). In experiment 2, ewes were given DA at doses of 0, 0.66, 6.6 or 66.0 μg/kg/min, iv. No dose altered basal LH, but each dose reduced (P<.05) basal and TRH-induced release of PRL. Key findings from these studies include direct pituitary action for: (1) 5-HT enhanced basal secretion of LH, (2) suppression of GnRH-induced secretion of LH by DA, (3) DA and NE inhibition of PRL secretion, and (4) DA, NE and 5-HT inhibition of release of PRL in response to TRH.
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