Age and day-night changes in clonidine-induced analgesia in mice.

Administrations of clonidine hydrochloride (0.1, 1.0 mg/kg) to young, mature, and old mice influenced their response time on a thermally aversive surface (50 degrees C) in an age-dependent manner. This analgesic effect was of short duration. During daytime measurement periods, young and mature mice showed significantly greater analgesic responses than did the old animals. Although all animals were able to perform the appropriate paw-licking response, the old mice displayed tremors and locomotor disturbances after receiving clonidine. These effects were not seen in the younger groups. Both the young and mature animals showed a substantial enhancement of their analgesic responses after receiving clonidine at night, whereas a significant but much reduced nighttime increase in antinociceptive effect was seen in old animals. Yohimbine, but not prazosin, inhibited clonidine-induced analgesia in young animals. Old mice given combinations of clonidine and these adrenergic antagonists showed elevations in response times, accompanied by severe behavioural changes.