Regulatory factors in the vertebrate olfactory mucosa

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Abstract
Access to and clearance of ligands from binding sites on olfactory cilia are regulated by a complex interplay of molecular, physical and cellular factors. Nasal/olfactory glands secrete mucus that contains many proteins, among them odorant-binding proteins (OBP) that may solubilize lipophilic odorants in the aqueous mucous phase and subsequently transport them to receptor sites. The rate of transport of the ligand–OBP complex or unbound odorant is a function of the diffusion coefficient that, under physiological conditions, is determined largely by the molecular size of the complex or unbound odorant, the viscosity of mucus and the tortuosity factor. The binding constants must favor association of the ligand with the binding protein, dissociation of the complex and possible reassociation of the ligand with the odorant receptor. Neural regulation of secretion determines the properties of the olfactory mucus that affect ligand access and clearance, including viscosity, water content and depth. Extrinsic autonomic (adrenergic, cholinergic) and peptidergic (substance P/CGRP, VIP) neurons innervate olfactory glands and regulate both secretory granule release and electrolyte/water balance. Extrinsic peptidergic (substance (P/CGRP, VIP) neurons terminate near the epithelial surface in close apposition to sustentacular cells and olfactory receptor neurons. The substance P/CGRP fibers, in addition to functioning as sensory fibers, appear to regulate secretion from sustentacular cells through a secretomotor reflex and to neuromodulate the sensitivity of olfactory receptor neurons to odorant stimulation. The action of regulatory factors in the olfactory mucosa is an emerging topic of research focused on molecular, physical and cellular factors that affect sensory transduction.

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