Effects of prior exposure to prolonged continuous light on the pattern of melatonin secretion in sheep held under continuous darkness.

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.