SYNCHRONOUS NYCTOHEMERAL RHYTHMS IN HUMAN BLOOD MELATONIN AND IN HUMAN POST‐MORTEM PINEAL ENZYME

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Abstract
Serum melatonin measured by radioimmunoassay at 4-hourly intervals for 24 h in 5 normal men exhibited nyctohemeral variations. The highest concentrations of serum melatonin occurred at 02.00 h, and the lowest concentrations at 14.00 h. The synthetic enzymes for melatonin, serotonin-N-acetyl transferase (SNAT) and hydroxyindole-O-methyltransferase (HIOMT) were measured in post-mortem pineal organs from 36 men and women, and the activity of both enzymes was directly related to the time of death of the subject, resulting in a similar nyctohemeral curve as that observed with serum melatonin. SNAT and HIOMT enzymes were stable when measured in pineal organs held under the usual post-mortem conditions. These observations indicate a nyctohemeral rhythm in human pineal function and suggests that any clinical interpretation of pineal dysfunction in disease states should take into consideration this normal rhythm.