Localization of orexins and their receptors in the rat olfactory system: possible modulation of olfactory perception by a neuropeptide synthetized centrally or locally.

Orexin-A and -B, also known as hypocretins, are two neuropeptides acting on feeding and sleep. They are specific ligands for two different receptors belonging to the G-protein coupled receptors family. Orexin fibers and orexin receptor neurons have been previously described in the forebrain olfactory system. Using immunocytochemistry, we showed that both orexin-A and -B as well as their receptors were present at different levels of the olfactory system, from the nasal mucosa to nuclei of the amygdala. A punctuated staining for orexins and their receptors was detected at the apical part of the olfactory epithelium; in the lamina propria of the mucosa, the staining was localized around olfactory nerves. At the ultrastructural level, olfactory neurons and supporting cells were found immunoreactive for orexins and their receptors. The labeling was localized in dendritic knobs and cilia of neurons, in the apical part and microvilli of supporting cells. The finding of immunolabeled cisternae of reticulum strongly suggests a local synthesis of both peptides and receptors, confirmed by RT-PCR experiments. In forebrain and amygdala regions, we detected numerous orexin fibers. Orexin receptors were present in mitral-tufted cells of the bulb and in many neuronal perikarya in the anterior olfactory nuclei, piriform cortex and amygdala nuclei. Altogether, these results show that orexins and their receptors are present at all levels of the olfactory system, from cilia where odors bind to their receptors to central regions where integration of olfactory signals occurs. They suggest a possible modulation of olfactory perception by these neuropeptides.