Effects of Light on Dopamine Metabolism in the Chick Retina

Abstract

The effect of prolonged exposure to light on the activity of dopaminergic neurons and dopamine (DA) metabolism of chick retinae was investigated. .alpha.-Fluoromethyldopa, a potent and specific irreversible inactivator of aromatic amino acid decarboxylase, was used to assess DA turnover after inhibition of synthesis and also to assess in vivo tyrosine hydroxylase activity by dihydroxyphenylalanine accumulation. After 48 h of light exposure, retinal DA in 12-day-old chicks was .apprx. 30% higher (P < 0.005); dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were elevated 2-3 times (P < 0.005) the level of controls kept in the dark for the same period. DA turnover was .apprx. 2-fold faster in the light (t1/2 [half time] = 31 min) than in the dark (t1/2 = 65 min). Tyrosine hydroxylase, assayed in vitro with saturating levels of cofactor and substrate, increased by .apprx. 50% after light exposure. The apparent tyrosine hydroxylase activity in vivo was .apprx. 6-fold higher in the light than the dark. These results are interpreted and discussed in terms of the regulation of DA synthesis and the use of DOPAC and HVA as indices of DA function in the retina.