DOPAMINERGIC REGULATION OF GONADOTROPIN-SECRETION IN SEASONALLY ANESTROUS MARES

We have previously demonstrated that daily administration of the dopam ine D2 antagonist, sulpiride, during seasonal anoestrus, effectively a dvances the mean time of onset of the breeding season in mares. The pu rpose of this study was to examine the effect of sulpiride administrat ion on pulsatile FSH and LH secretion in seasonally anoestrous mares, follicular development, time of first ovulation and the fertility at t he first ovulation. Fourteen anoestrous mares were selected based on p rogesterone concentrations < 1 ng ml(-1) for 3 weeks and largest folli cle diameter < 20 mm. Starting 30 January, eight seasonally anoestrous mares were treated daily with sulpiride until the first ovulation of the year, and six untreated control mares were maintained under the sa me environmental conditions. Ovarian activity was monitored and plasma samples were collected every other day. On days 1, 11 and 21 of treat ment, plasma samples were collected every 15 min for 11 h in six treat ed and six control mares. Mares were bred during the first oestrus. Me an time of first ovulation was significantly advanced in sulpiride-tre ated mares compared with control mares. Pregnancy rate 18 days after o vulation was similar between groups. Mean FSH pulse frequency on the f irst day of treatment and mean plasma FSH concentrations on day 11 of treatment were significantly higher in sulpiride-treated mares compare d with control mares. No significant difference was observed between g roups for parameters of LH pulsatile secretion. The results of this st udy suggest that dopamine inhibits FSH pulsatile secretion in seasonal ly anoestrous mares.