Ageing, opioid analgesia and the pineal gland.

The effects of ageing on day-night rhythms of analgesia was examined with young (1-2 months), mature (8-12 months) and old (20-30 months) mice. Significant age-related declines were observed both in the absolute levels and diel rhythms of morphine analgesia, with the most pronounced changes occurring at night. Administration of the pineal hormone, melatonin, augmented day-time levels of analgesia in all age classes and reversed the age-related decline in nocturnal morphine analgesia in old mice. Inhibition of pineal function in young mice by either exposure to light pulses or treatment with benserazide mimicked the effects of ageing on nocturnal morphine analgesia. These findings suggest that the pineal gland and melatonin are involved in modulating diel rhythms of analgesia and have an influential role on age-related changes in opioid responses.