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.
John
T M
TM
Clarke Institute of Psychiatry, Toronto, Ontario, Canada.
Brown
M C
MC
Wideman
L
L
Brown
G M
GM
D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D008027:Light; D008297:Male; D008550:Melatonin / Q000502:physiology*; D009619:Nociceptors / Q000502:physiology*; D017288:Pain Threshold / Q000502:physiology; D010870:Pineal Gland / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D011930:Reaction Time / Q000502:physiology
D008550:Melatonin