Tryptophan hydroxylase mRNA levels are regulated by the circadian clock, temperature, and cAMP in chick pineal cells

Chick pineal cells contain a circadian oscillator that drives rhythmic synthesis and secretion of melatonin even in dispersed cell culture. Here, we demonstrate that the mRNA encoding tryptophan hydroxylase (TPH), the first enzyme in the melatonin synthetic pathway, is expressed rhythmically under the control of the circadian clock. TPH message levels doubled between early day and early night, under both cyclic lighting and constant lighting conditions. The amplitude of the TPH mRNA rhythm was increased to 4-fold by culturing the cells at 43.3°C for 48 h instead of 36.7°C. Addition of forskolin to the cultures in early day produced a modest increase (50%) in TPH message levels but had no effect at other times. Because TPH mRNA levels are regulated by the endogenous pineal circadian clock, this provides a valuable system in which to study the molecular mechanism of clock control of gene expression.