MT1 melatonin receptor in the human retina:: Expression and localization

PURPOSE. Melatonin's function in human vision is far from understood, in part because of the lack of information on its cellular targets. Therefore, expression and localization of the MT1 melatonin receptor in human retina was examined. METHODS. Postmortem nonpathologic human eyes from nine donors were investigated, three by reverse transcriptionpolymerase chain reaction (RT-PCR) for MT1 and MT2 transcripts and six by immunocytochemistry, using a peptide-specific anti-MT1 receptor antibody. RESULTS. RT-PCR suggested that both MT1 and MT2 transcripts had similar levels of expression. Vertical slices of human retina demonstrated MT1 immunoreactivity in cell bodies along the outer border of the inner nuclear layer (INL), along the inner border of the INL, in cell bodies within the ganglion cell layer (GCL), and in the inner segments (IS) of photoreceptors. Double immunolabeling using anti-MT1 and tyrosine hydroxylase revealed that 69% of CA1 and 63% of CA2 dopaminergic neurons exhibited MT1 immunoreaction. Double immunolabeling with anti-parvalbumin, a horizontal cell marker, showed that MT1-positive cells along the outer INL border were exclusively horizontal cells, and that 18% of horizontal cells in central retina expressed MT1. Double staining with MT1 and markers for both rod and cone photoreceptors suggest that IS staining is present on rod cells. CONCLUSIONS. The MT1 receptor is expressed in diverse neuronal cell types in the human retina, providing evidence of a significant role for melatonin and this receptor subtype in human vision.