mRNA transcription determines the lag period for the induction of pineal melatonin synthesis in the Syrian hamster pineal gland

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Abstract
The nocturnal pattern of Syrian hamster pineal melatonin synthesis is characterized by a 6–8 h lag period, followed by a late‐night, short‐duration peak in both N‐acetyltransferase (NAT) activity and melatonin content. Administration of cycloheximide (20 mg/kg body weight) given either at the time of lights out or 4 h into the dark phase to Syrian hamsters blocked the nocturnal increase in both pineal NAT activity and melatonin content. Actinomycin D (5 mg/kg body weight) prevented the nocturnal increase in both constituents only when it was administered at darkness onset, being significantly less effective when injected after 4 h of dark exposure. Reinduction of melatonin production by isoproterenol (2 mg/kg body weight) administration to acutely light‐exposed animals during late darkness was prevented by cycloheximide, but not by actinomycin D administration. The results suggest that whereas Syrian hamster pineal melatonin production requires protein synthesis both early and late in the dark phase, the transcription of a putative NAT‐related mRNA, which occurs only during the early night, seems to determine the lag period in melatonin synthesis and pineal responsiveness to β‐adrenergic receptor agonist stimulation.

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