Dopaminergic and GABAergic amacrine cells are direct targets of melatonin:: Immunocytochemical study of mt1 melatonin receptor in guinea pig retina

Distribution of the mt(1) melatonin receptor in the guinea pig retina was immunocytochemically investigated using peptide-specific anfi-mt(1) receptor antibody. Western blots of the guinea pig retina showed a single band at approximately 37 kilodalton (kD) immunoreactive to the anti-mt(1) antibody. The most intense immunoreactivity for the mt(1) receptor was detected in the cell bodies of ganglion cells. Their dendrites and axons were also immunolabeled. Subpopulations of amacrine cells, the inner plexiform layer, and the outer plexiform layer also exhibited moderate to weak immunolabeling. The mt(1)-positive amacrine cells were located either at the vitreal border of the inner nuclear layer or displaced in the ganglion cell layer. Double immunolabeling using antibodies to the mt(1) receptor and tyrosine hydroxylase revealed that the majority of dopaminergic amacrine cells showed mt(1) immunoreactivity. Almost all the ICA type dopaminergic cells were mt(1) positive while the 2CA type cells less frequently exhibited mt(1) immunoreaction. By double immunolabeling for the mt(1) receptor and GABA, more than 50% of the mt(1)-immunoreactive amacrine cells were shown to be GABAergic neurons. Approximately one-third of the GABAergic amacrine cells were immunolabeled for the mrl receptor. The present results demonstrate expression of the mt(1) receptor in diverse neuronal cell types in the guinea pig retina and provide the first evidence for the direct effect of melatonin on dopaminergic and GABAergic amacrine cells via the mt(1) receptor.