Influence of light and neural circuitry on tyrosine hydroxylase phosphorylation in the rat retina

Light has been shown to increase dopamine synthesis and release in vertebrate retinas, but the retinal circuits mediating the light signal are unknown. We utilized three antibodies which recognize phosphorylated forms of tyrosine hydroxylase (TH) at serines 19, 31, and 40 to study the effects of light and neuroactive drugs on TH phosphorylation in the rat retina. Phosphorylated TH and total TH immunoreactivities were co-localized exclusively in retinal neurons whose shape and location are characteristic of dopaminergic interplexiform cells. Phosphorylated TH was weak to absent in darkness, but light strongly stimulated phosphorylation in all the three serine residues. Light-induced phosphorylation of TH induction by light was uniformly blocked by a combination of NMDA and AMPA glutamate receptor antagonists. In darkness, the combination of NMDA+AMPA induced phosphosphorylation of TH at serines 19 and 40 but it was weak at serine 31. A GABA(A) antagonist had the same effect. An agonist of depolarizing (ON) bipolar cells, L-(+)-2-amino-4-phosphonobutyric acid, did not prevent light-induced phosphorylated TH formation. Carbachol, a non-specific cholinergic agonist, selectively induced phosphorylation of TH at serine 31 in darkness, an effect which was blocked by the nicotinic antagonist, d-tubocurarine. These results show that retinal circuits involving glutamatergic, GABAergic and cholinergic synapses influence phospho-TH formation at different serine residues in this enzyme. Gamma amino butyric acid (GABA) and glutamate influence TH phosphorylation at serines 19 and 40, whereas cholinergic inputs affect its phosphorylation at serine 31.