Modulation of Retinal Aromatic l-Amino Acid Decarboxylase via α2 Adrenoceptors |
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Authors: | Zvani Rossetti Dimitrij Krajnc Norton H Neff Maria Hadjiconstantinou |
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Institution: | Department of Pharmacology, Ohio State University College of Medicine, Columbus 43210-1239. |
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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 antagonist 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. |
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Keywords: | Aromatic l-amino acid decarboxylase Retina Dopamine Epinephrine α2 adrenoceptors Catecholamines |
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