ROLE OF DOPAMINE AS A REGULATOR OF VERTEBRATE PHOTORECEPTORS
Numerous and profound cyclical changes in retina functioning during a 24-hour daily cycle are largely determined by the influence of two neuromodulators - melatonin and dopamine. Dopamine and melatonin form a reciprocal pair by mutually inhibiting the synthesis of each other, and they release into extracellular space of the retina approximately in antiphase. Dopamine is cyclically synthesized by special populations of dopaminergic amacrine cells in the retina and its content increases during the day and decreases at night. Like melatonin, dopamine affects all the major cell types of the outer and inner retinal layers. Activation of dopamine D1- and D2-type receptors regulate the activity of protein kinase A and the intracellular concentration of cAMP, and may trigger other regulatory pathways, including activation of phospholipase C. In photoreceptors, dopamine acting on D2-type receptors reduces cAMP concentration, suppresses melatonin synthesis and regulates the conduction of gap junctions between rods and cones, depending on the phase of the light cycle. By decreasing the concentration of cAMP, dopamine could also be aregulator of the phototransduction cascade and other cellular functions of the photoreceptor. Here we review some of these possibilities.