Specialized Nasal Sensory Cells Detect Irritants in the Environment

PRESS RELEASE

Newswise — Many chemicals, including most odorants in highly concentrated form, are irritating to the nose and can cause burning, stinging, warmth, itching or pain. The sensations occur when high levels of the chemical odors activate the trigeminal sensory system in the nasal cavity. The sensations are a signal of poor air quality and potential heath risks.

It is not yet known how the initial process occurs but a small, important step towards understanding it has been made. A team of researchers have discovered that specialized receptor cells of the upper nasal cavity respond to a broad array of chemical irritants including odorants at high concentrations. These cells allow an animal to quickly monitor and avoid potentially irritating and toxic compounds in the environments.

The StudyThe authors of the study, "TRPM5-expressing Solitary Chemosensory Cells Respond to Odorous Irritants" are Weihong Lin and Tatsuya Ogura, Department of Biological Sciences, University of Maryland, Baltimore County, MD; Robert F. Margolskee, Department of Neuroscience, Mount Sinai School of Medicine, New York, NY; and Thomas E. Finger and Diego Restrepo, Department of Cell and Developmental Biology, The Neuroscience Program, and Rocky Mountain Taste and Smell Center, University of Colorado Denver School of Medicine, Aurora, CO. Their findings appear in the March 2008 edition of the Journal of Neurophysiology, (doi:10.1152/jn.01195.2007), a publication of the American Physiological Society (APS; http://www.the-aps.org/).

BackgroundSolitary chemosensory cells (also called "solitary chemoreceptor cells" ) affecting the trigeminal nerve were first identified in the nasal cavity of rodents with an antibody against α-gustducin (a protein found in taste receptor cells). A broader class of chemosensory cells expressing the transient receptor potential ion channel M5 (TRPM5) within the airway and gastrointestinal tract has also been identified.

Summary of Methodology and FindingsThis research was designed to investigate nasal TRPM5-expressing cells and their chemical responsiveness to volatile irritants. Histological analysis of TRPM5-GFP (green fluorescent protein) transgenic mice showed the presence of over 5,000 sensory cells on each side of the nasal cavity demonstrating the significance of this sensory modality.

Functional studies of the TRPM5-expressing solitary chemosensory cells with a technique to image intracellular Ca2+ levels showed that these cells respond to many different odorous chemicals at a high concentration. Previously it was thought that only olfactory sensory neurons (for the sense of smell) detect odorous molecules. Yet olfactory responses would not account for the irritating quality of odorants at high concentrations. Thus the current study provides a mechanism for our sensitivity to a broad array of chemical irritants.

ConclusionsThis is the first direct demonstration that TRPM5-expressing solitary chemosensory cells in the nasal cavity respond to odorous chemical irritants. This is an example of the Law of Specific Nerve Energies (Johannes Peter Müller: 1826) who said that the quality of a perception is not dictated by the physical nature of the stimulus, but rather by the perceptual apparatus or sensory nerve conveying the information. The classic example is that if you sit in a dark place, close your eyes to adapt to the dark, and then push gently on the eyeball, you would perceive a blob of light even though the stimulus is mechanical pressure. Similarly, here in this system, an odorant is not perceived as being, e.g. flowery, but rather is perceived as an irritant at high concentrations because the information is transmitted via the trigeminal nerve. According to Dr. Lin, the first author of the study, "We have discovered a new cell function that we did not know existed. These findings bring us a step closer to understanding the way in which certain cells inside the nose protect us from potential harm."

Physiology is the study of how molecules, cells, tissues and organs function to create health or disease. The American Physiological Society (APS; http://www.the-aps.org/press has been an integral part of the scientific discovery process since it was established in 1887.