MT(1) 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 MT(1) melatonin receptor in human retina was examined. METHODS Postmortem nonpathologic human eyes from nine donors were investigated, three by reverse transcription-polymerase chain reaction (RT-PCR) for MT(1) and MT(2) transcripts and six by immunocytochemistry, using a peptide-specific anti-MT(1) receptor antibody. RESULTS RT-PCR suggested that both MT(1) and MT(2) transcripts had similar levels of expression. Vertical slices of human retina demonstrated MT(1) 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-MT(1) and tyrosine hydroxylase revealed that 69% of CA1 and 63% of CA2 dopaminergic neurons exhibited MT(1) immunoreaction. Double immunolabeling with anti-parvalbumin, a horizontal cell marker, showed that MT(1)-positive cells along the outer INL border were exclusively horizontal cells, and that 18% of horizontal cells in central retina expressed MT(1). Double staining with MT(1) and markers for both rod and cone photoreceptors suggest that IS staining is present on rod cells. CONCLUSIONS The MT(1) 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.