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 α₂ adrenoceptor agonists. In the dark, the enzyme activity can be made to increase by administering α₂ adrenoceptor antagonist drugs. Kinetic analysis indicates that the maximum velocity of the enzyme increases with little change of the K_m 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 α₂ 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 α₂ receptors suppresses AAAD activity that is associated with dopaminergic amacrines.