Immunohistological Localization of Mel1 a Melatonin Receptor in Pigeon Retina

Background: Melatonin (N-acetyl-5-methoxytryptamine), a significant indoleamine neuromodulator implicated in circadian rhythms and sleep patterns, regulates diverse rhythmic functions via activating its high-affinity G-protein-coupled receptors. However, the detailed cellular expression of the Mel1a receptor in the retina is still a research gap. Methods: The expression of the Mel1a receptor in pigeon retina was assessed using Western blot analysis and immunofluorescent staining. The cellular localization of the Mel1a receptor was studied using double immunofluorescent staining and laser-scanning confocal microscopy. Results: Our data suggested that the Mel1a receptor was extensively expressed in the outer segment of Rho4D2-labeled rod and L/M-opsin-labeled red/green cone and in the somata of the CB-labeled horizontal cell, TH-labeled dopaminergic amacrine cell, ChAT-labeled cholinergic amacrine cell, PV-labeled All amacrine cell, Brn3a-labeled conventional ganglion cell, melanopsin-containing ganglion cell and CRALBP-labeled Muller glial cell. In addition, the Mel1a receptor was diffusely distributed throughout the full thickness of the inner plexiform layer. However, the outer segment of S-opsin-labeled blue cone, the somata of ChX-10-labeled bipolar cell and outer plexiform layer seemed to lack immunoreactivity of the Mel1a receptor. Conclusion: The finding that multiple types of retinal cells express the Mel1a receptor provides a new neurobiological basis for the participation of melatonin in the regulation of retinal functions through activating the Mel1a receptor. Keywords: circadian rhythm, retinal Mel1a receptor, cellular localization, diurnal animal