Perireceptor events and peripheral modulation of olfactory signals in the olfactory epithelium of vertebrates

Abstract The olfactory epithelium (OE) and its associated perireceptor space, i.e., the mucus layer (ML) covering the epithelium, are the most peripheral parts of the vertebrate olfactory system. The olfactory receptor neurons (ORNs), one of the cell types of the OE, are the odorant detectors of the olfactory system. These bipolar neurons extend their apical appendages, which express odorant receptors, into the ML. The binding of odorants to odorant receptors is the initial step of odor processing. The vast majority of research on the peripheral olfactory system has focused on the ORNs and the molecular components of the olfactory transduction cascades. Less attention has been directed to the other cell types of the OE and their physiological functions. For a long time, it was assumed that the olfactory signals detected in the OE are transmitted to the olfactory bulb without preprocessing, but this view turned out to be over-simplistic. It has been shown that the olfactory signals are critically modulated already in the OE. Despite compelling evidence, many descriptions of the olfactory system still ignore the existence of these peripheral modulatory mechanisms. The importance of peripheral modulation of the olfactory signals, the physiological functions of the other epithelial cell types, the extrinsic innervation of the olfactory mucosa, and the perireceptor space are only slowly coming into focus in the olfactory research. Furthermore, several intraepithelial signaling pathways that signal epithelial damage and initiate regenerative processes have recently been discovered. This review provides a concise overview of the current knowledge of peripheral events in the olfactory mucosa and the perireceptor space.