ONTOGENY OF PINEAL MELATONIN RHYTHM IN RATS UNDER 12 12-HR AND 14/14-HR LIGHT-DARK CONDITIONS/

The aim of the study was to determine whether a discrepancy between th e genetically determined endogenous circadian period and an abnormally long Zeitgeber period disturbs the development of melatonin synthesis . Breeding pairs of rats were kept under 12:12- or 14:14-hr light:dark (LD) conditions. Pineal melatonin contents in the offspring were meas ured by radioimmunoassay. At 2 weeks of age high melatonin contents we re found from lights-off to lights-on in both conditions suggesting do minance of the photic regulation. At 3 weeks of age the signs of the c ircadian regulation in the melatonin profiles were evident: a lag peri od after the light offset in control conditions and a significant decl ine before the light onset in both conditions. However, in 14:14-hr LD conditions the melatonin content did not decrease to daytime levels u ntil the lights were on. This could suggest incomplete maturation of t he circadian system. The phase relationships between the melatonin pea k and LD cycle were different in the two conditions. A statistically s ignificant LD difference was first found at the age of 8-10 days in ma le pups and at 14 days in female pups under both lightings. The result s suggest that the abnormally long LD cycle did not cause any major di sorders in the development of photic or circadian regulation of the me latonin synthesis.