Sex and day-night differences in opiate-induced responses of insular wild deer mice, Peromyscus maniculatus...

We examined the effects of mu and kappa opiate agonists on the day- and night-time nociceptive, locomotory and ingestive behaviors of an island population of wild male and female deer mice, Peromyscus maniculatus triangularis. The prototypical mu opiate agonist, morphine, had significant analgesic and locomotory effects, which were blocked by naloxone, and the specific delta opiate antagonist, ICI 154,129, respectively. The specific kappa opiate agonist, U-50,488, had significant analgesic actions and inhibitory effects on locomotor activity, as well as stimulating feeding. Significant day-night variations occurred in the analgesic and activity responses, with the mu and kappa opiate agonists having significantly greater effects at night. There were also prominent sex differences in responses; male deer mice displaying significantly greater levels of mu and kappa opiate-induced analgesia and alterations in activity than female animals. These sex differences in opiate-induced effects were most pronounced at night, female deer mice displaying reduced day-night rhythms of responsiveness. These results demonstrate the existence of significant day-night rhythms and sex differences in the mu and kappa opiate behavioral responses of a wild population of rodents.