Why do we sneeze?

Sneezing is an important human reflex that expels particles, irritants or even pathogens. We must not hold back, especially to prevent certain diseases such as Alzheimer’s. The underlying mechanism that causes sneezing remains poorly understood. To pierce it up to date, American researchers from the Faculty of Medicine at Washington University in Saint-Louis have studied this phenomenon. They published a study on June 15 in the journal Cell in which they describe the biological process that leads to sneezing and highlight the key role played by a specific molecule.

Neuromedin B, the key molecule in sneezing

Sneezing is the strongest and most common way to spread infectious droplets from respiratory infections. “Better understanding what makes us sneeze – particularly how neurons behave in response to allergens and viruses – may point to treatments that can slow the spread of infectious respiratory diseases via sneezing”, estimates Qin Liu, professor of anesthesiology and principal author of the study. The region of the central nervous system that causes sneezing has already been identified, but how the sneeze reflex works at the cellular and molecular level has remained a mystery.

Researchers conducted experiments on mice to identify the process that leads to sneezing. To do this, they exposed them to aerosol droplets containing either histamine or capsaicin, a pungent compound made from chili peppers. Unsurprisingly, both products caused sneezing in rodents. By examining nerve cells already known to respond to capsaicin, scientists discovered a class of small neurons linked to sneezing. They then searched for molecules, called neuropeptides, capable of transmitting sneeze signals to these nerve cells. This allowed them to realize the key role played by the neuromedin B molecule (NMB). Moreover, by eliminating neurons sensitive to NMB, they found that this blocked the sneeze reflex.

A connection with the cells of respiration

“Interestingly, none of these sneeze-inducing neurons are housed in any of the brainstem regions known to be related to respiration.Qin Liu wondered. Although we discovered that the cells causing sneezing are in a different region of the brain than that which controls breathing, we also found that the cells in these two regions were directly connected via their axons, the nerve cell wiring..”

During their experiments, the researchers also found that they were able to stimulate the sneeze reflex by exposing part of the mouse brain to the NMB peptide. The mice sneezed despite not being exposed to capsaicin, histamine or other allergens.

Control sneezing to limit the spread of pathogens

For the researchers, this discovery is not only symbolic and can have resonances in terms of health prevention. Many viruses and other pathogens, including SARS-CoV-2, are spread in part by aerosolized droplets that can be expelled with a sneeze. By targeting the NMB or its receptor, it would then be possible to limit the spread among infected people.

“A sneeze can create 20,000 virus-containing droplets that can stay in the air for up to 10 minutesassures the researcher. In contrast, a cough produces more than 3,000 droplets, roughly the same number produced by speaking for a few minutes. 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 it. By identifying neurons that mediate the sneeze reflex, as well as the neuropeptides that activate these neurons, we have discovered targets that could lead to treatments for pathological sneezing or strategies to limit the spread of infections..”