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

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.