Involvement of the pineal gland and melatonin in murine analgesia

The data presented herein suggest that an intact pineal gland is required for the expression of the increased nocturnal sensitivity to morphine observed in mice. We report that the day/night rhythm of morphine analgesia was not evident in pinealectomized mice. Further, mice treated with melatonin exhibited a dose-related analgesic response. The decrease in sensitivity to pain was not observed in animals in which melatonin administration was followed by the opiate antagonist, naloxone. These data suggest that information derived from environmental lighting regulates sensitivity to pain via the pineal gland hormone melatonin, which is released and acts upon other areas of the CNS.