Immunohistochemical localization of L-dopa and aromatic L-amino acid-decarboxylase in the rat retina.
OBJECTIVE
The purpose of this study was threefold: to determine if some catecholaminergic amacrine cells of the rat retina use L-DOPA as their neurotransmitter, especially the small (2CA) cells that are immunoreactive to tyrosine hydroxylase but not to dopamine; to understand better the possible existence of serotoninergic cells in the rat retina; and to clarify the role of serotonin in the metabolism of melatonin.


METHODS
Immunohistochemistry using antibodies against tyrosine hydroxylase (TH), L-DOPA, aromatic L-amino acid decarboxylase (AADC), dopamine (DA), and tyramine in rat retinal wholemounts, serial sections, and various combinations of double labeling.


RESULTS
Paired wholemounts immunoreacted with anti-TH/AADC antibodies did not show a significant difference in densities of TH+ and AADC+ amacrine cells. All the TH+ cells exhibited AADC immunoreactivity. There were no AADC-immunoreactive cells lacking TH. A few TH+ cells exhibited L-DOPA immunoreactivity; they also contained AADC. The inner segments of photoreceptor cells were labeled by the anti-AADC antibody. The antibody to tyramine did not label 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 restricted to a small number of large TH+ amacrine cells probably does not represent an end product. Tyramine also does not appear to constitute a neurotransmitter in the rat retina. We confirm that there are no serotonin-synthesizing amacrine cells in the rat retina. The localization of AADC-immunoreactivity in the photoreceptor cell inner segments is possibly related to the biosynthetic pathway of melatonin from 5-hydroxytryptophan.
Nguyen-Legros
J
J
Laboratoire de Neurocytologie Oculaire, INSERM U-86, Paris, France.
Krieger
M
M
Simon
A
A
D000818:Animals; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000378:metabolism*; D004298:Dopamine / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D007124:Immunoenzyme Techniques; D007980:Levodopa / Q000378:metabolism*; D008297:Male; D008550:Melatonin / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012160:Retina / Q000378:metabolism*; D012701:Serotonin / Q000502:physiology; D014439:Tyramine / Q000378:metabolism
D012701:Serotonin; D007980:Levodopa; D001142:Aromatic-L-Amino-Acid Decarboxylases; D008550:Melatonin; D004298:Dopamine; D014439:Tyramine