Effects of bromocriptine-induced hypoprolactinaemia on gonadotrophin secretion and testicular function in rams (Ovis aries) during two seasons.

The influence of low circulating concentrations of prolactin on gonadotrophin and testosterone secretion, sperm production and testicular growth was investigated in rams during two different seasons. Treatment of Dorset rams (n = 23) with bromocriptine (4 mg day-1) during the spring (n = 11) and autumn (n = 12) caused a significant decrease in basal, mean and total serum prolactin concentrations (P < 0.01). In spring, serum prolactin concentrations returned to pretreatment values, one week after the termination of treatment. Basal, mean and total serum concentrations of LH were significantly higher in treated rams than in controls during the treatment period in autumn (P < 0.05). Secretion of LH was not affected by bromocriptine treatment during spring. There were no differences in the secretion of FSH between treated and control rams in either season. Serum concentrations of testosterone were significantly lower in treated rams than in control rams during the treatment period in autumn (P < 0.05) but not during spring. Semen volume from treated rams was significantly lower during the period after treatment in autumn (P < 0.05). Scrotal circumference decreased during both seasons in treated animals, but this change in size was significant only during spring (P < 0.05). Conversely, there was an increase in scrotal circumference in control rams during both seasons. It is concluded that prolactin may (i) affect LH secretion and, (ii) influence testicular function in rams, by directly affecting testosterone and semen production during autumn, and retarding testicular growth in spring.