REGULATION OF TRYPTOPHAN-HYDROXYLASE EXPRESSION BY A RETINAL CIRCADIAN OSCILLATOR IN-VITRO

Many aspects of retinal physiology are controlled by a circadian clock including at least two steps in the melatonin synthetic pathway: the activity of the enzyme, N-acetyltransferase (NAT), and mRNA levels of the rate-limiting enzyme tryptophan hydroxylase (TPH). Light and dopam ine (through D2-like dopamine receptors) can phase shift the clock, an d can also acutely inhibit NAT activity, resulting in supressed melato nin synthesis. In this paper, we show that eyecups cultured in constan t darkness maintain a clock-controlled rhythm in TPH mRNA, with low le vels in early day, rising to a peak in early night. Both eyecups and i solated retinas, cultured in light during the day, also exhibit a simi lar increase in TPH mRNA levels, indicating that this expression is no t acutely inhibited by light. Treatment with light or quinpirole (D2 d opamine receptor agonist) in early night, at a time and dose that acut ely inhibits NAT activity, does not change levels of TPH mRNA. Additio n of eticlopride (D2 dopamine receptor antagonist) during the day, als o has no effect on the normal daytime increase in TPH message levels. Therefore, TPH mRNA level is controlled by a circadian clock located w ithin the eye, but acute effects of light or dopamine are not detected .