Abstract
We show that the pineal melatonin synthesis of rats can be uncoupled from the circadian regulation by exposing the animals to abnormally long light periods. Male rats were kept 7 days under 22.5 1.5- h light/dark conditions and then exposed to darkness at different times of the day. After a 60-min dark exposure, the melatonin synthesis increased independently of the time of the day (Expt. 1). During 22 h in darkness, the mean melatonin content did not return to the low daytime level (Expt. 2). A dark-induced, time-independent increase of melatonin was also found after the rats had been 3–7 days under constant light (Expts. 3 and 4).
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