DOPAMINE AGONISTIC EFFECTS AS OPPOSED TO PROLACTIN CONCENTRATIONS IN PLASMA AS THE INFLUENCING FACTOR ON THE DURATION OF ANESTRUS IN BITCHES

The duration of the interoestrous interval and plasma concentrations o f prolactin were determined in seven bitches treated with 0.1 mg meter goline kg(-1) body mass orally, twice a day during anoestrus to determ ine whether shortening of the interoestrous interval in dogs is due to suppression of prolactin secretion. The results were compared with th ose of: a previous experiment in which four bitches were treated with 20 mu g bromocriptine kg(-1) body mass orally during the luteal period and anoestrus. The interoestrous interval of the dogs treated with me tergoline, 240+/-67 days, was similar to the interoestrous interval be fore treatment, 225+/-54 days. However in the dogs treated with bromoc riptine, the interoestrous interval was shortened to 123+/-46 days. Th e average of the means of the plasma concentrations of prolactin durin g metergoline treatment, 1.5+/-0.5 mu g l(-1), was lower (P = 0.03) th an the average of the means of the plasma concentrations of prolactin during the 2 weeks before treatment, 3.6+/-2.3 mu g l(-1), but did not differ from the average of the means of the plasma concentrations of prolactin, 2.1+/-0.5 mu g l(-1), of the dogs treated with bromocriptin e during anoestrus. As the plasma concentrations of prolactin were sim ilar during metergoline and bromocriptine treatment, and the anoestrou s period was shortened during bromocriptine treatment, but not during metergoline treatment, these results suggest that the induction of oes trus by dopamine agonists is not initiated by suppression of prolactin secretion, but by other direct or indirect dopaminergic effects.