The effect of small doses of naloxone on the initiation and duration of the first estrus after weaning in sows

The present research was conducted with the objective of studying the pharmacological effect of small doses of naloxone on the initiation and duration of the first estrus after weaning in the sow. For this purpose, 32 multiparous sows were used. Sows were divided at random into two groups. Group 1 (n=16) was treated by i.m. injection with 2mg naloxone at 12h intervals from 3 days before until 3 days after weaning. Group 2 (n=16) served as the control group and received saline solution at the same times as treatments for group 1. First estrus after weaning occurred at 85+/-5.2 and 108.3+/-5h (P<0.05) in naloxone- and saline-treated sows. Duration of estrus was 89.6+/-3.9 and 49.6+/-3.9h (P<0.05) in naloxone-treated and control animals, respectively. It was concluded that naloxone treatment advanced the time of appearance and duration of the first estrus after weaning in sows giving further support that endogenous opioids (EOP) are modulators of sexual behavior in female pigs.