Influence of the dopamine antagonist domperidone on the vernal transition in seasonally anoestrous mares.

The aim of the present study was to determine the effect of the prolonged administration of the dopamine antagonist domperidone on follicular development, ovulation and endocrine profiles in anoestrous mares. Anoestrous mares (n = 16) were maintained under natural photoperiod and ambient temperature. Eight of the mares were treated with domperidone each day from 15 January until the first ovulation of the year. The mean number and size of follicles ≥ 20 mm in diameter were significantly greater in domperidone-treated mares than in control mares by day 14 of treatment. The day of first ovulation was significantly earlier in domperidone-treated mares than in control mares (mean ± SEM: 51 ± 8.2 and 129 ± 13.6 days, respectively; P < 0.01). The eight domperidonetreated mares all ovulated during treatment and the mean interval from the start of treatment to the day of first ovulation was 27 days (range 15-55 days). Six of the eight domperidone-treated mares underwent normal cycles after first ovulation, whereas there was a prolonged interval (mean = 67 days) before the second ovulation in the other two mares. Domperidone administration resulted in significantly higher plasma prolactin concentrations measured at 0,4,8,12, and 24 h on day 7 of treatment compared with untreated controls (25.5 ± 15.8 versus 2.5 ± 3.0 ng ml -1 ). The concentrations of LH and oestrogen conjugates were significantly higher in domperidone-treated mares compared with control mares by day 28 of treatment. There were no differences in FSH concentrations in domperidone-treated and control mares. The significant increases in follicular development and concentrations of oestrogen conjugates that were observed to occur without concurrent increases in FSH concentration indicate that either domperidone or prolactin may have a direct effect on the ovary.