Novelty-induced opioid analgesia in deer mice (Peromyscus maniculatus): sex and population differences.

Exposure to a new environment elicited significant, naloxone (1.0 mg/kg) reversible analgesic responses in three different populations of deer mice; Peromyscus maniculatus artemisiae from the mainland, and P. m. angustus and P. m. triangularis from small marine islands. In all cases male deer mice displayed significantly greater levels of analgesia than females. In addition, the levels of analgesia were significantly greater in the insular than in the mainland populations. These results indicate that there are substantial sex and population differences in the novelty-induced analgesia displayed by natural and laboratory populations of deer mice.