An endogenous ligand modulates dopamine-containing neurons of retina via alpha-2 adrenoceptors.

Environmental light induces the activation of dopamine (DA)-containing neurons of rat retina and as a consequence DA turnover increases. The state of DA metabolism is directly related to the content of 3,4-dihydroxyphenylacetic acid in retina. Alpha-2 adrenoceptors are present in the retina and their activation diminishes the retinal content of 3,4-dihydroxyphenylacetic acid of rats placed in the light, but not of rats placed in the dark. When alpha-2 antagonists are administered, they increase retinal DA metabolism of rats in the light as well as of rats in the dark. These results are consistent with the notion that an endogenous agonist fully occupies the alpha-2 receptor in the dark and only partially occupies the receptors in the light. The most likely endogenous agonist for these receptors is epinephrine released from a newly identified population of epinephrine-containing amacrine cells.