Aromatic L-amino acid decarboxylase activity of retina is modulated via aminergic receptors.

The catecholamine neurotransmitters, dopamine (DA), norepinephrine (NE) and epinephrine (EPI), are found in the retina in separate structures (Haggendal and Malmfors,1965; Hadjiconstantinou et al., 1984; Dowling et al., 1976). DA is associated with a subpopulation of amacrine cells and with interplexiform cells (Dowling et al., 1976). It is the most abundant catecholamine in the rat retina representing about 95 percent of the total catecholamine. NE is found primarily in the sympathetic nerves that originate in the superior cervical ganglion and innervate the retina blood vessels (Hadjiconstantinou et al., 1983). EPI is located in a subtype of amacrine cell that is distinct from the DA amacrines (Hadjiconstantinou et al., 1984). The EPI cells undoubtedly contain small quantities of precursor DA and NE.Tyrosine hydroxylase (EC 1.14.16.2; TH) is the first and ratelimiting enzyme for the synthesis of catecholamines (Levitt et al., 1965). The presence of TH is characteristic of catecholamine-containing cells. TH activity is modulated by neuronal activity. In the retina, this is easily observed because dopaminergic neurons become activated when animals are placed in a lighted environment from the dark. The consequences of exposure to light is activation of TH (Iuvone et al., 1978b), increased release of DA (Kramer, 1971) and increased formation of DA metabolites (Cohen et al., 1983). Activation of the enzyme may result from phosphorylation of the enzyme.