Circadian expression of XXXX hydroxylase mRNA in the @entity1573 retina.

@entity1238 @entity2493 @entity2340 @entity1573 @entity4176 @entity750
Many aspects of retinal physiology are controlled by a circadian clock located within the eye. This clock controls the rhythmic synthesis of @entity2493 , which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in @entity2493 biosynthesis in retina appears to be @entity2340 hydroxylase (TPH)[G.M. Cahill and J.C. Besharse, Circadian regulation of @entity2493 in the retina of @entity1238 : Limitation by @entity750 availability, J. Neurochem. 54 (1990) 716-719]. In this report, we found that TPH mRNA is strongly expressed in the photoreceptor layer and the vitread portion of the inner nuclear layer; the message is also expressed, but to a lesser extent, in the ganglion cell layer. The abundance of retinal TPH mRNA exhibits a circadian rhythm which persists in constant light or constant darkness. The phase of the rhythm can be reversed by reversing the light:dark cycle. In parallel experiments we found a similar pattern of expression in the @entity1573 pineal gland. However, whereas a pulse of light at midnight suppressed retinal TPH mRNA by 25%, it did not alter pineal TPH mRNA, suggesting that there are tissue-specific differences in photic regulation of TPH mRNA. In retinas treated with @entity4176 to destroy @entity750 -containing amacrine and bipolar cells, a high amplitude rhythm of TPH mRNA was observed indicating that @entity2493 -synthesizing photoreceptors are the primary source of the rhythmic message. These observations provide the first evidence that @entity1573 retinal TPH mRNA is under control of a circadian clock.
