Why does this familiar smell take us straight back to the past with the associated feelings? Smell is the only sense directly linked to the limbic brain, the seat of memory and emotions. Besides, loss of smell would be a precursor symptom of several diseases linked to the central nervous system such as Parkinson’s disease and Alzheimer’s disease.
In order to help elderly people suffering from dementia, non-drug solutions such as aromatherapy – which can engage olfaction – can be considered in addition to traditional treatments. The fine molecules of essential oils act quickly on the patient’s limbic system once inhaled.
Several studies have highlighted the effects of odorous substances on the brain and on the modulation of the immune system.
Spanish researchers were interested in this subject. Their work on mice was published last year in the journal Frontiers in Immunology. The smell that interests us here emanates from menthol, one of the main components of peppermint essential oil.
“Our researchers have shown in animal models of Alzheimer’s disease that inhaling menthol improves cognitive abilities. With this study, they discovered that repeated short exposures to this substance can modulate the immune system and prevent the cognitive deterioration typical of this neurodegenerative disease,” reports the study press release.
“Surprisingly, we observed that short exposures to this substance for six months prevented cognitive decline in mice with Alzheimer’s. It also improved the cognitive ability of healthy young mice,” added Dr. Juan José Lasarte, director of the immunology and immunotherapy program at Cima and lead author of the study.
The mechanism of action of menthol
By analyzing its mechanism of action, researchers found that smelling this aroma had an action on the level of interleukin-1-beta (IL-1β). This critical protein (in the origin of dementia) and influential on the inflammatory response was reduced in the brain until it reached a safe level.
Furthermore, by inhibiting this protein using a drug approved for the treatment of certain autoimmune diseases, they were able to improve the cognitive abilities of mice affected by Alzheimer’s.
Dr. Ana García-Osta, co-author of the study, adds that “blocking the activity of regulatory T cells, a type of immune cell with immunosuppressive activity, also improved the cognitive ability of mice affected by the Alzheimer’s disease and resulted in a clear benefit in the cognitive ability of healthy young mice.”
Brain, sense of smell and immune system
Research paves the way for the development of new “odorous” therapies to prevent or alleviate the effects of Alzheimer’s disease and other central nervous system diseases. “It constitutes an important step in understanding the link between the immune system, the central nervous system and the sense of smell, because the results suggest that odors and immune modulators could play an important role in this type of diseases”, concludes the Dr Noelia Casares, researcher in the immunology and immunotherapy program at Cima and co-author of the study.
But there is still much to do, both in mice and humans, and further research will have to confirm the results found.
