Modulation of Retinal Aromatic l‐Amino Acid Decarboxylase via α2 Adrenoceptors

Abstract

Aromatic L-amino acid decarboxylase (AAAD) activity of the rat retina increases when animals are placed in a lighted environment from the dark. The rise of activity can be inhibited by administering .alpha.2 adrenoceptor agonists. In the dark, the enzyme activity can be made to increase by administering .alpha.2 adrenoceptor agonist drugs. Kinetic analysis indicates that the maximum velocity of the enzyme increases with little change of the Km for the substrate L-3,4-dihydroxyphenylalanine or the cofactor pyridoxal-5''-phosphate. The rise of activity in the light and in the dark after .alpha.2 antagonists can be blocked by administering cycloheximide, suggesting that protein synthesis is needed for the response. We speculate that epinephrine released in the dark from a subpopulation of retinal amacrine cells onto .alpha.2 receptors suppresses AAAD activity that is associated with dopaminergic amacrines.