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Molecular Pharmacology, Vol 14, 1212-1219, Copyright © 1978 by the American Society for Pharmacology and Experimental Therapeutics

Retinal Tyrosine Hydroxylase: Comparison of Short-term and Long-term Stimulation by Light

P. MICHAEL IUVONE 1, C. L. GALLI 1, and N. H. NEFF 1

1 Laboratory of Preclinical Pharmacology, National Institute of Mental Health, Saint Elizabeth’s Hospital, Washington, D. C. 20032

Dopamime (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 Km but not the Vmax of the enzyme for the pteridine cofactor, while 96 hr of exposure to light increases the Vmax for tyrosine but has no significant effect on the Km 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. Our studies are consistant 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.

Submitted on March 27, 1978
Accepted on July 19, 1978




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