Induction of fertile oestrus in cats by administration of hCG and calcium-naloxone

The aim of this study was to evaluate the possibility of inducing fertile oestrus in queens by administering hCG in combination with Ca(2+)-naloxone. It is well established that an increase in endogenous opioids leads to a decrease in LH. The administration of naloxone, an opioid antagonist, inhibits endogenous opioidergic tone and induces the onset of pro-oestrus. The opioidergic block is related to the increase in binding of beta-endorphins to specific receptors, which determines calcium channel blockage. Pretreatment with hCG results in a rapid increase in the number of LH receptors and Ca(2+)-naloxone induces G protein activity. Twenty-one anoestrous queens were divided into four groups: (i) group 1, nine queens were treated with a single s.c. injection of hCG (1000 iu) and daily for 4 days with 0.1 ml kg-1 body weight i.m. of a solution containing 0.4 mg naloxone ml-1 dissolved in 20% calcium gluconate; (ii) group 2, four animals were treated with a single s.c. injection of hCG (1000 iu); (iii) group 3, four queens were treated with Ca(2+)-naloxone (0.1 ml body weight kg-1 i.m.) daily for 4 days; and (iv) group 4, four queens received no treatment (controls). Queens were monitored using vaginal cytology and blood progesterone concentrations, and pregnancy was detected using ultrasonography. In groups 2, 3 and 4 clinical signs of oestrus were not observed. In group 1, 88.8% of treated queens were mated (8 of 9) and ovulated on the basis of an increase in progesterone, and 75% (6 of 8) of these queens became pregnant. In conclusion, pretreatment with hCG increased the number of LH receptors and Ca(2+)-naloxone antagonized the hypothalamic GnRH opioid block thus inducing the pulsatility of LH leading to fertile oestrus in queens.