Adenosine stimulates cone photoreceptor myoid elongation via an adenosine A2-like receptor

In several parts of the nervous system, adenosine has been shown to functio n as an extracellular neuromodulator binding to surface receptors on target cells. This study examines the possible role of adenosine in mediating lig ht and circadian regulation of retinomotor movements in teleost cone photor eceptors. Teleost cones elongate in the dark and contract in the light. In continuous darkness, the cones continue to elongate and contract at subject ive dusk and dawn in response to circadian signals. We report here that exo genous adenosine triggers elongation (the dark/night movement) in isolated cone inner segment-cone outer segment preparations (CIS-COS) in vitro. Agon ist/antagonist potency profiles indicate that adenosine's effect on cone mo vement is mediated by an A2-like adenosine receptor, which like other A2 re ceptors enhances adenylate cyclase activity, Although closest to that expec ted for A2 receptors, the antagonist potency profile for CIS-COS does not c orrespond exactly to any known A2 receptor subtype, suggesting that the con e receptor may be a novel A2 subtype, Our findings are consistent with prev ious reports that retinal adenosine levels are higher in the dark, and furt her suggest that adenosine could act as a neuromodulatory "dark signal" inf luencing photoreceptor metabolism and function in the fish retina.