Expression of mt 1 melatonin receptor in rat retina: evidence for multiple cell targets for melatonin

Abstract

Melatonin is synthesized in the retina at night and acts as a local modulator within this tissue by mediating the effects of darkness. We investigated the expression and localization of the mt 1 (Mel 1a) melatonin receptor in rat retina in order to disclose the cellular and molecular bases of melatonin's action in the mammalian retina. Western blotting of the mt 1 receptor in rat retina exhibited a single immunoreactive band of approximately 37,000 mol. wt, which corresponds to the predicted molecular size of the receptor. The mt 1 receptor was immunocytochemically localized to both the inner and outer plexiform layers. During postnatal development, retina from two-week-old rats showed the highest mt 1 immunoreactivity; the outer plexiform layer and horizontal cell bodies were strongly immunolabeled, with weaker labeling in the inner plexiform layer. Expression of mt 1 receptor messenger RNA in the rat retina was demonstrated by reverse transcription–polymerase chain reaction and in situ hybridization. mt 1 receptor transcripts were localized to ganglion cells, amacrine cells and horizontal cells. These results suggest that melatonin influences retinal physiology by acting on multiple retinal cell types, including ganglion, amacrine and horizontal cells, via the mt 1 receptor expressed in their processes.