IMMUNOHISTOCHEMICAL LOCALIZATION OF L-DOPA AND AROMATIC L-AMINO-ACID DECARBOXYLASE IN THE RAT RETINA

Purpose. The purpose of this study was threefold: to determine if some catecholaminergic amacrine cells of the rat retina use L-DOPA as thei r neurotransmitter, especially the small (2CA) cells that are immunore active to tyrosine hydroxylase but not to dopamine; to understand bett er the possible existence of serotoninerge cells in the rat retina; an d to clarify the role of serotonin in the metabolism of melatonin. Met hods. Immunohistochemistry using antibodies against tyrosine hydroxyla se (TH), L-DOPA, aromatic L-amino acid decarboxylase (AADC), dopamine (DA), and tyramine in rat retinal wholemounts, serial sections, and va rious combinations of double labeling. Results. Paired wholemounts imm unoreacted with anti-TH/AADC antibodies did not show a significant dif ference in densities of TH+ and AADC+ amacrine cells. All the THC cell s exhibited AADC immunoreactivity. There were no AADC-immunoreactive c ells lacking TH. A few TH+ cells exhibited L-DOPA immunoreactivity; th ey also contained AADC. The inner segments of photoreceptor cells were labeled by the anti-AADC antibody. The antibody to tyramine did not l abel any cells in the rat retina. Conclusions. L-DOPA can be excluded as a candidate active substance for the small TH+ amacrine cells that do not exhibit DA-immunoreactivity, The L-DOPA-immunoreactivity restri cted to a small number of large TH+ amacrine cells probably does not r epresent an end product. Tyramine also does not appear to constitute a neurotransmitter in the rat retina. We confirm that there are no sero tonin-synthesizing amacrine cells in the rat retina. The localization of AADC-immunoreactivity in the photoreceptor cell inner segments is p ossibly related to the biosynthetic pathway of melatonin from 5-hydrox ytryptophan.