Seasonal variation in endogenous serum melatonin profiles in goats: A difference between spring and fall?

The pineal hormone melatonin serves as a signal of day length in the regula tion of annual rhythms of physiological functions and behavior. The duratio n of high melatonin levels in body fluids is proportional to the duration o f the dark period of the day. Due to the direct suppression of melatonin by light, the overt melatonin rhythm may differ from the endogenous rhythm dr iven by the hypothalamic circadian clock. The aim of this study was to find out possible differences between the overt and endogenous melatonin rhythm s in goats during the course of a year. Seven Finnish landrace goats (nonla ctating females) were kept under artificial lighting that approximately sim ulated the annual changes of day length at 60 degreesN. Blood samples for m elatonin measurements by radioimmunoassay were collected at 2-h intervals d uring six seasons: winter (light:dark 6:18 h), early spring (10:14), late s pring (14:10), summer (18:6), early fall (14:10), and late fall (10:14). Me latonin profiles were determined for 2 consecutive days, first in light-dar k (LD) conditions and then in continuous darkness (DID). In LD conditions, the profiles matched the dark period with one exception: In winter, the mea n peak duration was significantly shorter than the scotoperiod. In DID cond itions, two types of endogenous melatonin patterns were found: a "winter pa ttern" (peak duration 13-15 h) in winter, early spring, early fall, and lat e fall, and a "summer pattern" (duration about 11 h) in late spring and sum mer. Thus, in equal habitual LID conditions in late spring and early fall ( LD 14:10), the endogenous melatonin rhythms were not quite similar: The pat tern in late spring resembled that in summer, and the pattern in early fall that in winter. These results suggest that, in addition to the light-adjus ted overt melatonin rhythm, the endogenous rhythm of melatonin secretion va ries during the course of a year.