Daily rhythms of analgesia in mice: effects of age and photoperiod.

Daily rhythms in response to aversive thermal stimulation and the analgesic effectiveness of morphine were assessed by the hot-plate method with young (1-2 months), mature (8-12 months) and old (20-30 months) mice exposed to various light-dark conditions (LD 12:12; 16:8; 8:16 h). The patterns of response after saline or morphine varied with the specific light-dark conditions examined, but routinely, there were increases in response latency from the early portions of the light phase to later time, and a further enhancement of the time to respond with onset of the dark phase. The dark-phase response declined abruptly with the start of the light-phase. Significant age-related declines were observed in the elevated, dark-phase morphine-analgesic and basal aversive responses, with no consistent effects of age evident in the light-phase.