title: Effects of prior exposure to prolonged continuous light on the pattern of melatonin secretion in sheep held under continuous darkness.
abstract: To gain insight into the nature of the neural centre regulating melatonin production by the pineal gland we have measured hourly changes in plasma melatonin in 16 ewes held for 42-56 h under continuous darkness following a 28-day period of exposure to continuous light. Plasma melatonin was undetectable while animals were in constant light but increased to normal night time levels (120-2,200 pmol/litre) within 10 min of the onset of darkness in 14 out of the 16 ewes regardless of whether this occurred at 1,200 h (group 1) or 2,400 h (group 2). In four ewes melatonin secretion was maintained for approximately 36 h, indicating that the pineal gland remained responsive to a static stimulatory signal presumably emanating from the supra chiasmatic nucleus. In all ewes, melatonin secretion ceased at approximately 12 and or 36 h after the onset of darkness and did not require a separate zeitgeber. Although the increase and the decline of plasma melatonin levels were synchronized within groups 1 and 2, there were sufficient differences in the pattern of melatonin secretion between the groups to suggest that the regulatory mechanism had an intrinsic component. A hypothesis involving a diurnal change in sensitivity to circulating melatonin is advanced to explain the observed differences in the synchrony of melatonin secretion and the prolonged episodes of melatonin secretion.
authors.affiliation: Department of Obstetrics and Gynecology, Medical School, University of Adelaide, South Australia.
authors.forename: P F
authors.initials: PF
authors.lastname: Shaw
authors.forename: D J
authors.initials: DJ
authors.lastname: Kennaway
authors.forename: R F
authors.initials: RF
authors.lastname: Seamark
mesh_terms: D000818:Animals; D002940:Circadian Rhythm; D003624:Darkness; D005260:Female; D008027:Light; D008550:Melatonin / Q000378:metabolism*; D010507:Periodicity; D012756:Sheep
chemical_list: D008550:Melatonin