Cyclic AMP increases hydroxyindole-O-methyltransferase mRNA levels in the chicken pineal gland, but is not required for circadian rhythmicity of this transcript.

Hydroxyindole-O-methyltransferase (HIOMT) plays an important role as the final enzyme in the synthesis of melatonin. In the chicken pineal gland, HIOMT mRNA concentration exhibits a circadian rhythm with a threefold peak at midday. The present study sought to evaluate the possible role of cyclic AMP in this transcriptional rhythm. In cultured pineal glands from 4-day-old chicks, cyclic AMP analogs and the adenylate cyclase activator, forskolin, increased HIOMT mRNA levels twofold to threefold in a dose-dependent manner. Vasoactive intestinal polypeptide increased HIOMT mRNA levels by 50%. Actinomycin-D chase experiments indicated that cyclic AMP did not affect the stability of HIOMT mRNA, thus providing indirect evidence that the effect of cyclic AMP was exerted at transcriptional level. In cultured pineal glands from 11 days embryos, HIOMT mRNA levels failed to respond to cyclic AMP. However, a daily rhythm of HIOMT mRNA, with an endogenous component in constant darkness was clearly observed at this developmental stage. This observation indicates that cyclic AMP is not required for circadian rhythmicity of HIOMT gene transcription.