Effects of the dopamine agonist cabergoline on the pulsatile and TRH-induced secretion of prolactin, LH, and testosterone in male beagle dogs

In the present study, the pulsatile serum profiles of prolactin, LH and testosterone were investigated in eight clinically healthy fertile male beagles of one to six years of age. Serum hormone concentrations were determined in blood samples collected at 15 min intervals over a period of 6 h before (control) and six days before the end of a four weeks treatment with the dopamine agonist cabergoline (5 μg kg −1 bodyweight/day). In addition, the effect of cabergoline administration was investigated on thyrotropin-releasing hormone (TRH)-induced changes in the serum concentrations of these hormones. In all eight dogs, the serum prolactin concentrations (mean 3.0 ± 0.3 ng ml −1) were on a relatively constant level not showing any pulsatility, while the secretion patterns of LH and testosterone were characterised by several hormone pulses. Cabergoline administration caused a minor but significant reduction of the mean prolactin concentration (2.9 ± 0.2 ng ml −1, p < 0.05) and did not affect the secretion of LH (mean 4.6 ± 1.3 ng ml −1 versus 4.4 ± 1.7 ng ml −1) or testosterone (2.5 ± 0.9 ng ml −1 versus 2.4 ± 1.2 ng ml −1). Under control conditions, a significant prolactin release was induced by intravenous TRH administration (before TRH: 3.8 ± 0.9 ng ml −1, 20 min after TRH: 9.1 ± 5.9 ng ml −1) demonstrating the role of TRH as potent prolactin releasing factor. This prolactin increase was almost completely suppressed under cabergoline medication (before TRH: 3.0 ± 0.2 ng ml −1, 20 min after TRH: 3.3 ± 0.5 ng ml −1). The concentrations of LH and testosterone were not affected by TRH administration. The results of these studies suggest that dopamine agonists mainly affect suprabasal secretion of prolactin in the dog.