Mediation of diurnal fluctuations in pain sensitivity in the rat by food intake patterns: reversal by naloxone.

Rats maintained on a 12-hour light-dark cycle were tested for pain sensitivity after being deprived of food during either the dark or the light phase of the cycle. Diurnal fluctuations in pain sensitivity were observed. The fluctuations followed food intake patterns rather than a natural circadian rhythm, with food deprivation producing a decrease in pain sensitivity. The analgesic response produced by this mild food deprivation was strongly attenuated by naloxone or feeding, suggesting that endogenous opioid systems may be related to patterns of food intake.