Forskolin, an activator of adenylate cyclase activity, promotes large increases in N-acetyl transferase activity and melatonin production in the Syrian hamster pineal gland only during the late dark period.

The exposure of organ cultured pineal glands of Syrian hamsters to forskolin, an adenylate cyclase activator, caused marked increases in serotonin N-acetyltransferase activity and melatonin content in a dose-related manner (1-100 microM) when glands were collected in the second half of the dark period. However, addition of forskolin to glands collected anytime during the light period or at the beginning of the dark period failed or only modestly stimulated either pineal N-acetyltransferase activity or melatonin levels. Similar results were obtained with isoproterenol. The results suggest that intrapinealocyte regulatory mechanisms may determine the nocturnal rise in the Syrian hamster pineal gland.