Scientists uncover how your nose knows to sneeze

ST. LOUIS, Mo. — Is sneezing really something doctors can cure? In a new paper, scientists at the University of Washington School of Medicine in St. Louis have uncovered what makes us go, “AH-CHOO.” This discovery represents progress towards what the team believes is a “treatment” for sneezing.

Controlling the sneeze reflex can help prevent infections

Irritants in dust, pollen, and even spicy food can make us sneeze. A sneeze is our body’s natural response to rid itself of something dangerous or unpleasant. Irritants can also cause inflammation or damage the mucous membrane of our nose that normally shields us from pathogens like bacteria and viruses.

Although our reflexive sneeze protects us from infection, a sneeze can spread infectious droplets through the air to infect others. Indeed, these droplets can contain microbes and pathogens such as the virus responsible for COVID-19 — SARS-COV2.

“A sneeze can create 40,000 virus-containing droplets that can stay in the air for up to 10 minutes,” explains Dr. Qin Liu, senior study author and an associate professor of anesthesiology, in a university release.

One way to slow the spread of infection is to control the sneezing reflex. However, scientists do not fully understand how the reflex occurs.

“Better understanding what causes us to sneeze – specifically how neurons behave in response to allergens and viruses – may point to treatments capable of slowing the spread of infectious respiratory diseases via sneezes,” Liu adds.

The brain controls sneezing

Liu’s team studies the neural mechanism behind sneezing, meaning how the brain integrates information to initiate a sneeze.

“Our goal is to understand how neurons behave in response to allergies and viral infections, including how they contribute to itchy eyes, sneezing and other symptoms,” the study author explains.

The brain has separate regions involved in respiration and in sneezing. However, the neurons involved in sneezing are connected to the respiratory neurons. The respiratory neurons receive information that something is wrong in the nose, then send a signal to the respiratory muscles. The muscles contract during a sneeze and expel the irritant from the nose. Humans share this process with other animals, including mice which Liu’s team used in the study.

Mouse neurons are sensitive to chili pepper compound

In their study, Liu and her team exposed the mice to aerosolized droplets with compounds that cause a sneeze. One of the compounds was capsaicin, which is found in chili peppers.

First, the team searched for the neurons responsible for the sneezing response to capsaicin. They started their search with sensory nerve cells already known to react to capsaicin, and honed in on the neurons which react to capsaicin by releasing  neuropeptides. The team hypothesized that a neuropeptide was involved in the sneeze response by transmitting sneeze signals across the respiration and sneezing centers of the brain.

Neuropeptide required for sneeze response

As it turns out, Liu’s team was correct. Researchers discovered that the neuropeptide, neuromedin B (NMB), is required for sneezing. Mice sneezed less frequently if they lacked NMB. Additionally, mice sneezed less if neurons that talk to the respiratory center of the brain lack the ability to respond to NMB. Both experiments suggest that the sneeze-evoking center of the brain and respiratory center are connected through NMB.

In a key experiment, the researchers initiated the sneeze response in the absence of capsaicin. Results show directly stimulating the brain with NMB is sufficient enough to initiate a sneeze. In this experiment, the mice began to sneeze even though they had not been exposed to capsaicin.

The study demonstrates that in response to capsaicin, the body produces NMB in neurons that sense irritants, and releases NMB to bind to the NMB receptor. In response to binding NMB, neurons that talk to the respiratory center evoke a sneeze.

How does this lead to a future sneezing treatment?

The current study from Liu’s team contributes to their ongoing efforts to uncover links between nerve cells and other body systems to cause sneezing. This research is important because it can inform future treatments for sneezing or for fighting respiratory infections.

“To prevent future viral outbreaks and help treat pathological sneezing caused by allergens, it will be important to understand the pathways that cause sneezing in order to block them,” Liu concludes. “By identifying neurons that mediate the sneeze reflex, as well as neuropeptides that activate these neurons, we have discovered targets that could lead to treatments for pathological sneezing or strategies for limiting the spread of infections.”

The study appears in the journal Cell.