Journal of biological rhythmsApr 9, 1999

Day length controls key enzyme genes in the rat pineal gland and affects melatonin production

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 2, 2026

Abstract

Evening increases in AA-NAT and HIOMT gene expression coincide with rising melatonin levels as scotophase shortens.

AA-NAT and HIOMT gene expression may be influenced by changes in photoperiod.
Evening increases in AA-NAT and HIOMT mRNA were observed alongside circulating melatonin levels.
Morning declines in AA-NAT, HIOMT, and melatonin occurred at different rates but were linked to light onset.
The duration of the nocturnal peak in HIOMT mRNA showed a positive correlation with HIOMT activity.
Photoperiodic modulation of melatonin rhythm primarily involves changes in AA-NAT gene expression.

Simplified

Photoperiodic changes of pineal melatonin (MEL) profile are accompanied by parallel changes of arylalkylamine-N-acetyltransferase (AA-NAT) activity. In the present study, the authors investigated, for the first time, whether two other important variables of pineal metabolism, AA-NAT and hydroxyindole-O-methyltransferase (HIOMT) gene expression, also may be affected by the photoperiod. Evening rises in AA-NAT and HIOMT mRNA and in circulating MEL occurred concomitantly with an increased delay from dark onset as scotophase shortened. On the opposite, the morning declines of all three variables occurred with different kinetics but were locked to light onset. These observations demonstrate that the daily rhythms in AA-NAT and HIOMT gene expression are modulated by the photoperiod and bring further evidence in favor of nor adrenaline as the possible link between the endogenous clock and MEL. Interestingly, the duration of the nocturnal peak in HIOMT mRNA was positively correlated with HIOMT activity. In conclusion, this study adds two important links to the chain of mechanisms involved in the photoperiodic control of pineal metabolism. First, photoperiodic modulation of the MEL rhythm primarily results from changes in the AA-NAT gene expression. Second, the photoperiodic regulation of HIOMT activity occurs at the transcriptional level.

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Day length controls key enzyme genes in the rat pineal gland and affects melatonin production

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