Chemical stimuli in the nasal cavity can be detected via olfactory, vomeronasal, and trigeminal receptors. Often, however, trigeminal chemoreception is overlooked by investigators in the chemical senses. Hence, behaviors and other effects of odors attributed primarily to olfactory or vomeronasal stimulation may in fact have a significant trigeminal component. Although trigeminal nerve endings in the nasal cavity respond to chemical stimuli, especially irritants, the nature of this response as well as the morphology of the receptors are not well characterized. Few data are available regarding the kinds and concentrations of compounds which stimulate trigeminal chemoreceptors. In addition, little is known about the location and ultrastructure of trigeminal chemoreceptors in the nasal cavity. The proposed research will examine the physiology and morphology of nasal trigeminal chemoreceptors. Single and multiunit recordings will be obtained from the ethmoid branch of the trigeminal nerve in the rat. Stimulus-response functions, threshold ranges, and latencies will be derived from a structurally diverse range of odorants, some commonly used in olfactory research, others obvious pollutants. Analysis also will focus on whether adaptation differs for different odorants. Cross adaptation experiments will examine the specificity of trigeminal chemoreceptors, i.e., can trigeminal chemoreceptors discriminate different odorants? Capsaicin desensitization, which depletes substance P and other neuropeptides, eliminates trigeminal responses to odorants. Therefore, light and high voltage electron microscopic immunocytochemical techniques will be used to identify the capsaicin-sensitive neuropeptide-containing trigeminal fibers and endings in the nasal epithelium. The distribution and ultrastructure of these neuropeptide-containing fibers will be determined.
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