[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.
Ying
S W
SW
Department of Biomedical Sciences, McMaster University, Ontario, Canada.
Huang
Z Q
ZQ
D000818:Animals; D002940:Circadian Rhythm; D005260:Female; D008550:Melatonin / Q000494:pharmacology*; D008614:Meperidine / Q000494:pharmacology; D051379:Mice; D009020:Morphine / Q000494:pharmacology; D010146:Pain / Q000503:physiopathology*; D010870:Pineal Gland / Q000502:physiology*; D012684:Sensory Thresholds / Q000187:drug effects
D009020:Morphine; D008614:Meperidine; D008550:Melatonin