Stimulation Of Retinal Dopamine Biosynthesis Invivo By Exogenous Tetrahydrobiopterin - Relationship To Tyrosine-Hydroxylase Activation

Intravitreal injection of tetrahydrobiopterin (BH 4 ), the cofactor for tyrosine hydroxylase (TH), increases 3,4-dihydroxyphenylalanine (DOPA) accumulation in retinas of dark-adapted rats, as does exposure to light. In contrast, BH 4 had no significant effect on DOPA accumulation in retinas of light-exposed rats. The levels of endogenous retinal BH 4 and the uptake of injected BH 4 into the retinal tissue were not affected by light exposure. These data indicate that TH is not saturated with endogenous BH 4 in the retinas of dark-adapted rats. In addition, the observations support the interpretation that the decrease in apparent K m of TH for the cofactor in response to light exposure is of sufficient magnitude to allow near saturation of TH by endogenous BH 4 and, thus, is causally related to the increase of dopamine biosynthesis in response to short-term photic stimulation.