Dopamine mediates circadian rhythms of rod-cone dominance in the Japanese quail retina

A circadian clock modulates the functional organization of the Japanese qua il retina. Under conditions of constant darkness, rods dominate electroreti nogram (ERG) b-wave responses at night, and cones dominate them during the day, yielding a circadian rhythm in retinal sensitivity and rod-cone domina nce. The activity of tyrosine hydroxylase, the rate-limiting enzyme in dopa mine synthesis, also exhibits a circadian rhythm in the retina with approxi mately threefold higher levels during the day than at night. The rhythm of tyrosine hydroxylase activity is opposite in phase to the circadian activit y of tryptophan hydroxylase, the first enzyme in the melatonin biosynthetic pathway. We tested whether dopamine may be related to the physiological rh ythms of the retina by examining the actions of pharmacological agents that effect dopamine receptors. We found that blocking dopamine D2 receptors in the retina during the day mimics the nighttime state by increasing the amp litude of the b-wave and shifting the retina to rod dominance. Conversely, activating D2 receptors at night mimics the daytime state by decreasing the amplitude of the b-wave and shifting the retina to cone dominance. A selec tive antagonist for D1 dopamine receptors has no effect on retinal sensitiv ity or rod-cone dominance. Reducing retinal dopamine partially abolishes rh ythms in sensitivity and yields a rod-dominated retina regardless of the ti me of day. These results suggest that dopamine, under the control of a circ adian oscillator, has a key role in modulating sensitivity and rod-cone dom inance in the Japanese quail retina.