Brett Smith for redOrbit.com – Your Universe Online
New research published in the American Journal of Respiratory Cell and Molecular Biology has revealed that odor receptors aren’t just found in the nose – they also line the lungs as well. However, instead of being used to detect the aroma of blueberry cobbler or a succulent roasting chicken, the odor receptors in the lungs trigger a response designed to protect the airways.
“We forget,” said study author Yehuda Ben-Shahar, “that our body plan is a tube within a tube, so our lungs and our gut are open to the external environment. Although they’re inside us, they’re actually part of our external layer.”
“So they constantly suffer environmental insults, and it makes sense that we evolved mechanisms to protect ourselves,” added Ben-Shahar, and assistant professor of biology and medicine at Washington University in St. Louis.
While odor receptors in the nose are embedded in the cell membranes of nerve cells, receptors in the lungs are found in the membranes of neuroendocrine cells. When the lung receptors detect a harsh smell, they set off a hormonal response that constricts the airways, protecting them from harm.
Since airway diseases are marked by hypersensitivity to volatile molecules, the study team theorized that the lungs must have a way of sensing inhaled chemicals. A study published in the journal Science in 2009 found a group of ciliated cells in the respiratory system that have bitter-taste receptors. When bitter odor molecules were detected, the cilia worked harder to sweep them out of the airway.
In the new study, the research team decided to investigate this phenomenon further. By exposing cultured cells to volatile chemicals, the researchers found that respiratory tissues also express odor receptors. These receptors were not found on ciliated cells, but on neuroendocrine cells that release serotonin and other neuropeptides when they are activated.
Ben-Shahar said his team’s finding should have been expected.
“When people with airway disease have pathological responses to odors, they’re usually pretty fast and violent,” said Ben-Shahar. “Patients suddenly shut down and can’t breathe, and these cells may explain why.”
The researchers noted that the cells in the airways are secretory, not neuronal – meaning they are broadly tuned with multiple receptors. Also, instead of sending signals to the brain, the lung receptors inundate local nerves and muscles with hormones.
“They are possibly designed to elicit a rapid, physiological response if you inhale something that is bad for you,” Ben-Shahar explained.
The study team suspected that these pulmonary neurosecretory cells play a role in the hypersensitivity of patients with chronic obstructive pulmonary disease (COPD) to airborne irritants. COPD is a class of diseases that are marked by coughing, wheezing, shortness of breath and chest tightness.
When the researchers examined tissues from patients with COPD, they found more neurosecretory cells than in airway tissues from healthy donors. Ben-Shahar says that future research should focus on exploring the genetic mechanisms behind these neurosecretory cells.
“Clearly, primates have evolved distinct cell lineages and signaling systems for respiratory-specific functions,” he added.