IN-VIVO INVESTIGATION OF LUTEAL FUNCTION IN DOGS - EFFECTS OF CABERGOLINE, A DOPAMINE AGONIST, AND PROLACTIN ON PROGESTERONE SECRETION DURING MID-PREGNANCY AND MID-DIESTRUS

The role of prolactin on luteal function in dogs was investigated in v ivo. The function of prolactin in mid-luteal phase was compared in pre gnant and nonpregnant dogs. A dopamine agonist, cabergoline, known for its prolactin secretion inhibitory effects, was injected subcutaneous ly at a dose of 5 mu g/kg body weight in five pregnant and five nonpre gnant Beagle bitches. Mean plasma prolactin and progesterone were dram atically suppressed for 4 to 5 days after injection in both groups whe n compared with control pregnant and non-pregnant animals, whereas no effect on luteinizing hormone (LH) secretion was observed. The decline in plasma progesterone occurred after that in prolactin, suggesting p lasma progesterone was impaired by inhibition of prolactin secretion. These results confirm the luteotropic importance of prolactin in pregn ant bitches, and also demonstrate its importance in luteal phase of th e nonpregnant dog. Second, to demonstrate that the effects of cabergol ine were mediated by prolactin inhibition and not by a direct action o n the corpus luteum, concomitant administration on Day 30 of cabergoli ne and prolactin (375 mu g IV twice daily on Days 30 and 31) or caberg oline and LH (750 mu g IV twice daily on Days 30 and 31) was affected in two groups of five pregnant animals each. Results showed that only prolactin was able to reverse the negative effects of cabergoline on c irculating progesterone. This confirms the indirect mode of action of the dopamine agonist, cabergoline on corpus luteum function. Third, fu rther investigation on the precise luteotropic role of prolactin was m ade by IV injection of 375 mu g pure canine prolactin twice daily in f ive pregnant bitches on Days 30 and 31, and in five pregnant bitches o n Days 40 and 41. No direct stimulatory effect of prolactin on plasma progesterone secretion occurred. Nor was there a noticeable effect on plasma LH secretion. These results suggest that prolactin is unable to directly stimulate progesterone secretion by the corpus luteum of pre gnancy. The results of this study suggest that prolactin is an essenti al luteotropin in the dog from mid-luteal phase in both pregnant and n onpregnant animals. However, it appears to act by sustaining corpus lu teum lifespan and function rather than by direct stimulatory effects o n progesterone secretion. (C) Elsevier Science Inc. 1997.