Circadian oscillators play an important role in the regulation of visual system function, and may also play a role in retinal dysfunction. Very little is known about circadian control of any individual components of the phototransduction cascades of rods or cones, although modulation of many of these components has been studied within the context of background and bleaching adaptation. We have recently demonstrated that cGMP-gated cationic channels of chick cones are under circadian control, and are more sensitive to cGMP during the subjective night than during the subjective day. Circadian control of these channels is a posttranslational effect driven by two different protein kinases. Thus, the MAP kinase Erk is more active during the night whereas the serine-threonine kinase CaMKII is more active during the day. The Erk rhythm is essential for the rhythm in CaMKII activity and thus appears to be """"""""closer"""""""" to the oscillator. Additional preliminary data suggest a role for cAMP in driving the circadian rhythm in channel gating and Erk activation. Because cGMP-gated channels represent the final step in vertebrate phototransduction, circadian control of these channels could have profound effects on the overall dynamics of phototransduction. This research will focus on three main questions. First, what is the mechanism underlying circadian control of Erk and CaMKII? We will examine if the Erk activity rhythms are driven by free-running rhythms in cAMP cascades that act by driving Ras activation (which is also rhythmic in these cells). Second, do Erk, CaMKII, or Ca2+-calmodulin cause direct effects on cGMP-gated channels or closely associated membrane proteins? Finally, we will examine the mechanism of dopamine modulation of the cGMP-gated channels. We have shown that this amacrine cell neurohormone causes a decrease in the affinity of the channels for cGMP during the nighttime, but has no effect during the daytime. This effect is mediated by D2 family receptors. We will test the hypothesis that dopamine acts through inhibition of cAMP and Erk signaling, and/or increases in CaMKII activity, and we will carry out in vivo experiments to determine the role of dopaminergic modulation of these channels in vivo.
