Dopamine agonistic effects as opposed to prolactin concentrations in plasma as the influencing factor on the duration of anoestrus in bitches.

The duration of the interoestrous interval and plasma concentrations of prolactin were determined in seven bitches treated with 0.1 mg metergoline kg-1 body mass orally, twice a day during anoestrus to determine whether shortening of the interoestrous interval in dogs is due to suppression of prolactin secretion. The results were compared with those of a previous experiment in which four bitches were treated with 20 micrograms bromocriptine kg-1 body mass orally during the luteal period and anoestrus. The interoestrous interval of the dogs treated with metergoline, 240 +/- 67 days, was similar to the interoestrous interval before treatment, 225 +/- 54 days. However in the dogs treated with bromocriptine, the interoestrous interval was shortened to 123 +/- 46 days. The average of the means of the plasma concentrations of prolactin during metergoline treatment, 1.5 +/- 0.5 micrograms l-1, was lower (P = 0.03) than the average of the means of the plasma concentrations of prolactin during the 2 weeks before treatment, 3.6 +/- 2.3 micrograms l-1, but did not differ from the average of the means of the plasma concentrations of prolactin, 2.1 +/- 0.5 micrograms l-1, of the dogs treated with bromocriptine during anoestrus. As the plasma concentrations of prolactin were similar during metergoline and bromocriptine treatment, and the anoestrous period was shortened during bromocriptine treatment, but not during metergoline treatment, these results suggest that the induction of oestrus by dopamine agonists is not initiated by suppression of prolactin secretion, but by other direct or indirect dopaminergic effects.