Retinal tyrosine hydroxylase: comparison of short-term and long-term stimulation by light

Dopamine (DA) is a putative neurotransmitter within some retinal amacrine neurons. Exposure of rats to light for either 15 min or 96 hr increases retinal tyrosine hydroxylase activity. Concomitant with the increase of enzyme activity is a 4-fold increase of DA formation. The molecular mechanism for the increased enzyme activity for the two exposures to light is apparently different. Short-term exposure to light decreases the K(m) but not the V(max) of the enzyme for the pteridine cofactor, while 96 hr of exposure to light increases the V(max) for tyrosine but has no significant effect on the K(m) for tyrosine or cofactor. Enzyme activity of rats exposed to 15 min of light decreases to the level found in 96 hr dark-adapted rats within 30 min of darkness, while more than 6 hr of darkness are required after 96 hr of light exposure. These studies are consistent with the hypothesis that retinal DA formation is modulated by different molecular mechanisms depending on the duration of exposure of light. Short-term exposure to light activates tyrosine hydroxylase while long-term exposure to light results in the formation of more active molecules of enzyme.