CIRCADIAN EXPRESSION OF TRYPTOPHAN-HYDROXYLASE MESSENGER-RNA IN THE CHICKEN RETINA

Many aspects of retinal physiology are controlled by a circadian clock located within the eye. This clock controls the rhythmic synthesis of melatonin, which results in elevated levels during the night and low levels during the day. The rate-limiting enzyme in melatonin biosynthe sis in retina appears to be tryptophan hydroxylase (TPH)[G.M. Cahill a nd J.C. Besharse, Circadian regulation of melatonin in the retina of X enopus laevis: Limitation by serotonin availability, J. Neurochem. 54 (1990) 716-719]. In this report, we found that TPH mRNA is strongly ex pressed in the photoreceptor layer and the vitread portion of the inne r nuclear layer; the message is also expressed, but to a lesser extent , in the ganglion cell layer. The abundance of retinal TPH mRNA exhibi ts a circadian rhythm which persists in constant light or constant dar kness. The phase of the rhythm can be reversed by reversing the light: dark cycle. In parallel experiments we found a similar pattern of expr ession in the chicken pineal gland. However, whereas a pulse of light at midnight suppressed retinal TPH mRNA by 25%, it did not alter pinea l TPH mRNA, suggesting that there are tissue-specific differences in p hotic regulation of TPH mRNA. In retinas treated with kainic acid to d estroy serotonin-containing amacrine and bipolar cells, a high amplitu de rhythm of TPH mRNA was observed indicating that melatonin-synthesiz ing photoreceptors are the primary source of the rhythmic message. The se observations provide the first evidence that chick retinal TPH mRNA is under control of a circadian clock. (C) 1998 Elsevier Science B.V. All rights reserved.