Termination of dioestrus and induction of oestrus in dioestrous nonpregnant bitches by the prolactin antagonist cabergoline.

The effects of treatment with a dopamine agonist from day 30 after the LH surge of a nonpregnant oestrous cycle were analysed in five bitches previously demonstrated to be fertile. Five untreated nonpregnant bitches were studied as controls. Cabergoline was given (5 micrograms kg-1 body weight) daily from day 30 after the LH peak until +/- 2 days after the first sign of pro-oestrus. Cabergoline decreased progesterone concentrations to < 1 ng ml-1 within 15-20 days. Four of the five treated bitches came into pro-oestrus after a mean of 29.75 +/- 5 days of treatment, at 66.5 +/- 7.1 days after the previous LH surges. The previous and following interoestrous intervals in the treated and responding animals were 216 +/- 6 and 66.5 +/- 7.1 days, respectively, and were significantly different (P < 0.05). In untreated animals, interoestrous intervals were longer (P < 0.05) than in treated animals, and not different from previous interoestrous intervals. The duration of pro-oestrus and oestrus was normal in the induced oestrous cycle compared with previous or untreated cycles in both groups. Variations in progesterone in the induced cycles were not different from those in control cycles. Prolactin concentrations were lower (P < 0.05) in treated animals compared with untreated animals. All bitches mated. However, none of the treated bitches became pregnant at the induced oestrous cycle. Lack of fertility was presumably due to the inability of the uterus to receive the embryos and was related to insufficient uterine involution and regeneration. Nevertheless, this study demonstrates that it is possible to induce oestrus not only in anoestrus but also in dioestrous nonpregnant bitches using a D2 dopamine agonist.