| Abstract: | Access to and clearance of ligands from binding sites on olfactorycilia are regulated by a complex interplay of molecular, physicaland cellular factors. Nasal/olfactory glands secrete mucus thatcontains many proteins, among them odorant-binding proteins(OBP) that may solubilize lipophilic odorants in the aqueousmucous phase and subsequently transport them to receptor sites.The rate of transport of the ligand–OBP complex or unboundodorant is a function of the diffusion coefficient that, underphysiological conditions, is determined largely by the molecularsize of the complex or unbound odorant, the viscosity of mucusand the tortuosity factor. The binding constants must favorassociation of the ligand with the binding protein, dissociationof the complex and possible reassociation of the ligand withthe odorant receptor. Neural regulation of secretion determinesthe properties of the olfactory mucus that affect ligand accessand clearance, including viscosity, water content and depth.Extrinsic autonomic (adrenergic, cholinergic) and peptidergic(substance P/CGRP, VIP) neurons innervate olfactory glands andregulate both secretory granule release and electrolyte/waterbalance. Extrinsic peptidergic (substance (P/CGRP, VIP) neuronsterminate near the epithelial surface in close apposition tosustentacular cells and olfactory receptor neurons. The substanceP/CGRP fibers, in addition to functioning as sensory fibers,appear to regulate secretion from sustentacular cells througha secretomotor reflex and to neuromodulate the sensitivity ofolfactory receptor neurons to odorant stimulation. The actionof regulatory factors in the olfactory mucosa is an emergingtopic of research focused on molecular, physical and cellularfactors that affect sensory transduction. |
