Light onset stimulates tyrosine hydroxylase activity in isolated teleost retinas

Abstract

The action of tyrosine hydroxylase is the rate-limiting step in the synthesis of dopamine, the most abundant catecholamine in vertebrate retinas. I have examined the activation and regulation of this enzyme in isolated retinas of green sunfish, Lepomis cyanellus. Exposing previously dark-adapted retinas to constant illumination for a period of 10 min increased enzymatic activity 2.2-fold over that present in retinas incubated in darkness. Thus, light onset activates tyrosine hydroxylase in teleost retinas. Stimulation of the activity of tyrosine hydroxylase under these conditions was associated with a decrease in the apparent K m of the enzyme for its pteridine cofactor without a change in the apparent V max of the reaction. This result suggests that short-term exposure to light increases dopamine synthesis by enhancing the affinity of the enzyme for its naturally occurring cofactor. These findings are consistent with the idea that light activates dopaminergic neurons in teleost retinas.