Endocrine Abstracts

Prolactin is associated with seasonal transitions into and out of anestrus in the mare. The earliest anestrous transitional changes are declining progesterone secretion and a concomitant prolactin decline. Ovarian changes include slowed follicular development and prolonged luteal activity. Dopamine antagonist administration during the autumnal transition prevented a seasonal progesterone decline, suggesting a cause-and-effect between prolactin and progesterone production. The present study sought to mimic decreased autumn prolactin secretion during summer cyclicity to investigate the prolactin–progesterone secretion relationship, possible prolactin influence over ovarian events and sexual behavior.

In total, 20 mares were divided into treatment (T) or control (C) groups (May 15–Aug 10) and assessed regularly for sexual behavior (teasing), endocrine function (plasma collection and RIA) and ovarian function (ultrasonography). Treatment mares received cabergoline, a long-acting dopamine agonist (5 mg orally every third day). Comparisons between groups (repeated measures ANOVA) included plasma prolactin and luteal progesterone concentrations, length of estrous cycle and its luteal and follicular phases, average daily follicular growth rates and numbers of small, medium and large-sized follicles. Sexual behaviors were scored as positive and negative and aggregate daily scores were compared between groups.

Plasma prolactin declined (P<0.05) within 24 h post-cabergoline treatment and remained lower than controls during the study. Luteal progesterone was not statistically different between groups, but there were some indications of progesterone decline in response to cabergoline. Although estrous cycle intervals were not different between T and C groups, late summer luteal phases tended to be shortened in T mares. Late diestrus (day 15) follicular growth wave suppression was demonstrated in T mares. Sexual behavior was different (P<0.05) between groups insofar as T mares demonstrated greater sexual ambivalence to the stallion, often displaying both positive and negative behaviors simultaneously.

Results suggest that prolactin/dopamine does influence some aspects of reproduction in the mare.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however, funding details unavailable.