Age-related differences in forced walking stress-induced analgesia in mice

We investigated the effects of aging on forced walking stress-induced analgesia using a formalin-induced paw licking test in male mice. Exposure to forced walking stress for 6 h showed forced walking stress-induced analgesia in all mice aged 4, 24 and 48 weeks in the second phase (10-30 min), but not in the first phase (0-10 min). In the second phase, the degree of stress-induced analgesia was age-dependent (4 > 24 > 48 weeks). LY-235959, a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, blocked forced walking stress-induced analgesia in mice aged 4 and 24 weeks, but not in those aged 48 weeks. Naloxone did not antagonize forced walking stress-induced analgesia in mice in any of the age groups. The present study suggests that the degree of forced walking stress-induced analgesia depends on the age of mice and confirms previous findings that forced walking stress-induced analgesia is involved in the nonopioid system via NMDA receptors.