Melatonin replacement nullifies the effect of light-induced functional pinealectomy on nociceptive rhythm in the rat.

Rats maintained on a 12 h daily photoperiod (12:12 LD cycle), exhibited a diurnal variation in sensitivity to both heat-elicited and pressure-elicited pain, with low sensitivity at 2 h before the end of the scotophase and higher at 4 h after the onset of photophase. Functional pinealectomy induced by a single LL day effaced the baseline diurnal rhythm of sensitivity to pressure-elicited pain, and reversed that to heat-elicited pain. Oral administration of physiological doses of melatonin into functionally pinealectomized rats, nullified the effect of functional pinealectomy, restoring the normal baseline rhythms of both pressure-elicited and heat-elicited nociceptive responses. The role of melatonin in modulating nociception is discussed in light of an indoleaminergic-opioid system.