Induction of fertile oestrus in anoestrous dogs by constant infusion of GnRH agonist

Abstract

The continuous, subcutaneous administration of the GnRH superagonist lutrelin for 14 days was evaluated as a means to induce oestrus in 24 anoestrous beagle bitches, beginning 90-146 days after the previous oestrus. The recent occurrence of pregnancy and lactation (N = 12) vs a non-pregnant cycle (N = 12) did not appear to affect the incidence of pro-oestrus (10/12 vs 11/12 bitches), or oestrus (8/12 vs 11/12 bitches), the timing of pro-oestrus (5 +/- 0.4 days) or oestrus (13 +/- 0.3 days) or the incidence of ovulation (8/12 vs 10/12 bitches), but did reduce the number of ovulating bitches that gave birth to litters after mating the induced oestrus (3/8 vs 6/10). The mean magnitude of the endogenous preovulatory or preovulatory-like LH surges at Day 8-15 varied (P less than 0.5) among the 9 bitches that became pregnant (4.7 +/- 0.9 ng/ml), the 9 bitches that ovulated but did not become pregnant (2.7 +/- 0.6 ng/ml), and the 6 bitches that failed to ovulate in response to treatment (1.9 +/- 0.3 ng/ml). Overall, mean LH surges (3.3 +/- 0.5 ng/ml) were lower than those in 12 contemporary spontaneous control cycles (16 +/- 3 ng/ml) and may have been compromised by pituitary down-regulation caused by continued agonist administration. The latter was suggested in ovariectomized bitches treated similarly with agonist by the suppression, within 10 days of start of treatment, of LH from 35 +/- 6 to 3 +/- 0.5 ng/ml. The 50% success rate obtained in early anoestrous bitches after non-pregnant cycles suggests that constant infusion of GnRH agonist might be useful for the induction or synchronization of fertile oestrus in dogs, particularly if initiated later in anoestrus.