Combined effect of sulpiride and light treatment on the onset of cyclicity in anestrous mares

摘要

Studies on the role of dopamine antagonists on the onset of cyclicity in various laboratories have demonstrated that inhibition of dopamine activity in late anestrous mares shortens the interval to first ovulation [1]. However, the mechanism by which ovarian activity is stimulated remains unclear. Several studies have failed to demonstrate an effect of dopamine antagonists on gonadotropin secretion. In contrast to other species, sulpiride (a dopamine antagonist) does not increase pulse frequency of FSH and LH in anestrous mares. Throughout past studies, it has become evident that environmental factors such as photoperiod, temperature, nutrition and body condition may influence the impact of dopamine antagonists on the time of first ovulation. For example, we have observed that anestrous mares maintained inside at temperatures above freezing had a shorter treatment-to-ovulation interval than mares subjected to the same treatment but maintained outside at sub-freezing temperatures. Observations in some long-day breeders indicate that dopamine may not exert its regulating role through the inhibition of gonadotropin secretion (LH and FSH) but rather acts via the regulation of prolactin which stimulates the expression of ovarian gonadotropin receptors. Extrapolation of these observations to anestrous mares leads us to propose that in anestrous mares the dopamine antagonist increases circulating levels of prolactin which in turn stimulate the expression of ovarian gonadotropin receptors. The result is that with similar levels of gonadotropin secretion, mares treated with dopamine antagonist are more apt to respond to gonadotropin stimulation than untreated mares with lower expression of gonadotropin receptors.