D. C. Klein coauthored a 1970 study connecting a circadian rhythm to the activity of serotonin N-acetyltransferase in the rat pineal gland. With J. L. Weller, D. C. Klein found activity more than fifteen times higher at night than during the day.[ref]
The rhythm persisted in complete darkness and in blinded animals, while constant illumination suppressed it. These comparisons distinguished an internally maintained rhythm from a response requiring the eye to detect each transition between day and night.[ref]
The study also tested regulation in isolated preparations: norepinephrine, rather than serotonin, regulated the measured activity under those conditions. Combining the time course with an intervention on the biochemical system connected timing to a particular step associated with melatonin synthesis.[ref]
Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase by D. C. Klein and J. L. Weller. Abstract: in-vitro regulation and relation to melatonin rhythm
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