[Effects of the pineal body and melatonin on sensitivity to pain in mice]

Sensitivity to painful stimuli was measured by hot plate, writhing tests and electric caudal stimulation in mice. The mice were kept under light-dark 12/12 cycle with light out at 18:00 for at least 2 wk. Both basal analgesia and meperidine (pethidine)-induced analgesic effect exhibited parallel circadian rhythms, with the marked peak and through occurring at mid-dark and mid-light phases, respectively. The day-night differences in pain threshold 10 d after pinealectomy were not evident, especially in the loss of dark time augmentation of analgesic responses, but persisted in sham operated mice. Melatonin (MT) 50-200 mg/kg ip during light phase produced analgesic activity. MT 250 mg/kg ip resulted in a loss of the righting reflex. In pinealectomized mice, the pre-treatment of MT 5 mg/kg potentiated levels of analgesia induced by meperidine 10 mg/kg or morphine 5 mg/kg. It is possible that MT is one of the endogenous nocifensor inhibitors in CNS.