Circadian regulation of cGMP-gated channels of vertebrate cone photoreceptors: role of cAMP and Ras.
Arylalkylamine N-acetyltransferase (AANAT) is the penultimate and key regulatory enzyme in the melatonin biosynthetic pathway. In chicken retina in vivo, AANAT is expressed in a circadian fashion, primarily in photoreceptor cells. AANAT activity is high at night in darkness, low during the daytime, and suppressed by light exposure at night. In the present study, we investigated the circadian and photic regulation of adenosine 3',5'-monophosphate (cAMP) in cultured retinal cells entrained to a daily light-dark (LD) cycle, as well as the role of Ca(2+) and cAMP in the regulation of AANAT activity. Similar to AANAT activity, cAMP levels fluctuate in a daily fashion, with high levels at night in darkness and low levels during the day in light. This daily fluctuation continued with reduced amplitude in constant (24 h/day) darkness (DD). These changes in cAMP appear to be causally related to control of AANAT activity. Adenylyl cyclase and protein kinase A inhibitors suppress the nocturnal increase of AANAT in DD, while 8Br-cAMP augments it. The nocturnal increase of AANAT activity also involves Ca(2+) influx, as it is inhibited by nitrendipine, an inhibitor of L-type voltage-gated channels, and augmented by Bay K 8644, a Ca(2+) channel agonist. The effect of Bay K 8644 was antagonized by the adenylyl cyclase inhibitor MDL 12330A, suggesting a link between Ca(2+) influx, cAMP formation, and AANAT activity in retinal cells. Light exposure at night, which rapidly suppresses AANAT activity, also suppressed cAMP levels. The effect of light on AANAT activity was reversed by Bay K 8644, 8Br-cAMP, and the proteasome inhibitor lactacystin. These results indicate a dynamic interplay of circadian oscillators and light in the regulation of cAMP levels and AANAT activity in photoreceptor cells.