The Role of Dopamine in Controlling Retinal Photoreceptor Function in Vertebrates

Abstract

Many and deep cyclical changes in measures of retinal functioning during the 24-h daily cycle are to a significant extent determined by the actions of two neuromodulators – melatonin and dopamine. Dopamine and melatonin form a reciprocal pair mutually inhibiting each other’s synthesis, and are released into the intercellular space of the retina essentially in contraphase. Dopamine is synthesized cyclically in a special population of dopaminergic amacrine cells and its content in the retina increases in the daytime and decreases at night. Like melatonin, dopamine acts on all the main cell types in the outer and inner layers of the retina. Excitation of D1- and D2-like receptors by dopamine regulates protein kinase A activity and the intracellular cAMP concentration, and can also trigger other regulatory pathways, including activation of phospholipase C. In photoreceptors, dopamine acting via D2-like dopamine receptors decreases the cAMP concentration, suppresses melatonin synthesis, and regulates the conductivity of gap junctions between rods and cones depending on the phase of the light cycle. Decreasing the cAMP concentration, dopamine may regulate the phototransduction cascade and other cellular functions in photoreceptors. One of the aims of this review is to address these possibilities.