Orexins, composed of orexin A and orexin B, are identified as endogenous ligands of two orphan G-protein-coupled receptors: orexin 1 and orexin 2 receptors (OX1R and OX2R). Orexins are implicated in regulating wake/sleep states, feeding behaviors, etc. Using reverse transcription-polymerase chain reactive (RT-PCR) analysis and immunofluorescence double labeling, we investigated the distributions of orexin A, orexin B, OX1R and OX2R in rat retina. RT-PCR analysis revealed the presence of mRNAs of prepro-orexin, OX1R and OX2R in rat retina. Immunostaining for orexin A and orexin B was observed in many cells in the inner nuclear layer and the ganglion cell layer. In the outer retina, horizontal cells, labeled by calbindin, and bipolar cells, labeled by homeobox protein Chx10, were orexin A- and orexin B-positive. In the inner retina, two orexins were both found in GABAergic amacrine cells (ACs), including dopaminergic and cholinergic ones, stained by tyrosine hydroxylase and choline acetyltransferase respectively. Glycinergic ACs, including AII ACs, also expressed orexins. Weak to moderate labeling for orexin A and orexin B was diffusely distributed in the inner plexiform layer. Additionally, orexins were expressed in almost all ganglion cells (GCs) retrogradely labeled by cholera toxin B subunit. Specifically, double-labeling experiments demonstrated that melanopsin-positive GCs (intrinsically photosensitive retinal GCs, ipRGCs) were labeled by two orexins. Morever, OX1R immunoreactivity was observed in most of GCs and all dopaminergic ACs, as well as in both outer and inner plexiform layers. In contrast, no obvious OX2R immunostaining was detectable in the rat retina. These results suggest that orexins may modulate the function of neurons, especially in the inner retina. We further hypothesize that the orexin signaling via ipRGCs may be involved in setting the suprachiasmatic nucleus (SCN) circadian clock.
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