Developmental changes in opiate-induced analgesia in deer mice: sex and population differences

We examined developmental changes in nociception and μ (morphine) and κ (U-50, 488) opiate-induced analgesia in male and female deer mice of two different populations; Peromyscus maniculatus artemisiae from a mainland region and P.m. angustus from a small island. Both populations displayed significant developmental changes in nociception and morphine (10 mg/kg) and U-50, 488 (10 mg/kg)-induced analgesia. Basal thermal response latencies (nociceptive responses) and the levels of μ and κ opiate-induced analgesia increased over 14–35 days of age, with maximum analgesic responses in adults (35+ days of age). In both of the populations, young (neonatal-weaning) male mice displayed significantly higher thermal response latencies and greater levels of naloxone (1.0 mg/kg) antagonized opiate-induced analgesia than young females. There were also population differences in the levels of analgesia, the insular mice displaying greater μ and lower κ opiate-induced analgesic responses than the mainland animals. The population differences in μ and κ opiate-induced analgesia were evident in young and adult mice of both sexes. These results show that there are significant sex and population differences in nociception and opiate-induced analgesia in young (neonatal-weaning) and adult deer mice.