Seasonal Assessment of Duration of Prolactin Suppression Following Cabergoline Treatment in Mares: Unstimulated Versus Sulpiride and Thyrotropin-Releasing Hormone-Stimulated Responses

Experiments 1 through 4 were performed to test the hypothesis that season affects the duration of prolactin suppression by a single injection of the dopaminergic agonist, cabergoline. In each, six mares received cabergoline intramuscularly and four received the vehicle. The protocol was repeated around the vernal equinox, summer solstice, autumnal equinox, and winter solstice. In all experiments, cabergoline suppressed (P < .01) prolactin for at least 5 days; concentrations in fall and winter were naturally low; thus, treatment effects were harder to detect. Experiments 5 and 6 tested whether basal (unstimulated) prolactin concentrations recovered from cabergoline suppression faster (earlier) than secretagogue-induced prolactin secretion. In July, five mares each were treated with either compounded cabergoline or cabergoline in oil; four others received oil. Cabergoline suppressed (P < .0001) prolactin in both treated groups for approximately 6 days. Prolactin in daily samples returned to control levels thereafter, whereas low-dose sulpiride-stimulated secretion remained suppressed by >50% at 11 days after treatment. In the last experiment, treatment with cabergoline or oil (n = 6 mares each) in October followed by thyrotropin-releasing hormone (TRH) challenges resulted in similar responses to those in experiment 5; TRH-induced prolactin responses were still suppressed (P < .05) on day 11. In conclusion, little seasonal variation in duration of prolactin suppression in response to cabergoline was detected. In general, secretagogue-induced secretion is suppressed much longer than basal secretion. This dichotomy may indicate at least two subpopulations of lactotropes regulated differentially by dopamine input.