Olfaction, Memory, and Neurodegenerative Diseases

The olfactory system is a sensory system that helps humans smell. This process occurs when volatile molecules, or odorants, enter the nasal cavity and reach the olfactory epithelium. Odorants bind to odorant receptors on the olfactory epithelium which leads to a signal transduction in the olfactory sensory neurons. Recently, there have been studies that suggest olfaction can predict behavior and even diseases such as Parkinson’s Disease (PD), and Alzheimer’s Disease (AD). 

According to an article in Scientific American called “Smell Test May Sniff Out Oncoming Parkinson’s and Alzheimer’s,” Davangere Devanand, a researcher from Columbia University, says that studies that relate to impaired olfaction serve as better predictors of cognitive decline than memory studies. This can be important because according to the Alzheimer’s Association, about 15% to 20% of people older than 65 have mild cognitive impairment. If olfaction tests can help identify the onset of AD, this could help doctors intervene earlier with treatment. For PD, loss of smell often occurs before the traditional motor deficits that come with PD. While olfactory dysfunction can be a common early symptom of AD and PD, this does not mean everyone with olfactory dysfunction is at a high risk of developing these neurodegenerative diseases (Noonan, 2017).

Similarly, Dr. Laura Shanahan reviewed articles that related to olfaction and memory consolidation during sleep. Rasch et al. found that reactivation of odor neurons associated with the context of learned object locations during slow-wave sleep improves memory consolidation. Using a memory task and odor delivery during learning and sleep, researchers found that slow-wave sleep with an odor delivery had significantly greater memory recall (Rasch et al., 2007). Dr. Shanahan used this information to study if sleep born odors could influence memory and what is happening in the brain during sleep to improve memory. Using a similar approach as Rasch et al., participants did a memory task in an MRI where participants associated faces, buildings, tools, and animals to a specific odor. They found different reactivation of the prefrontal cortex during sleep promotes memory consolidation (Shanahan et al., 2018).

Both of these findings are important because they could potentially be used together to find out more about AD and PD. Future studies could perhaps focus on taking a similar approach to study memory consolidation with regard to olfaction. Researchers could potentially use a similar model as Dr. Shanahan to study what mechanisms go wrong in memory consolidation in patients who show olfactory dysfunction. Since odor-evoked reactivation of the prefrontal cortex during sleep can promote memory consolidation, researchers might be able to use deep brain stimulation techniques in mice AD and PD models to see if this could potentially help reduce cognitive decline in mice. 

References

Rasch B, Buchel C, Gais S, Born J. 2007. Odor cues during slow-wave sleep prompt declarative memory consolidation. Science 315:1426–1429. DOI: https://doi.org/10.1126/science.1138581

Shanahan LK, Gjorgieva E, Paller KA, Kahnt T, Gottfried JA. 2018. Odor-evoked category reactivation in human ventromedial prefrontal cortex during sleep promotes memory consolidation. eLIFE. DOI: https://doi.org/10.7554/eLife.39681

Shanahan LK, Gottfried JA. 2017. Scents and Reminiscence: Olfactory Influences on Memory Consolidation in the Sleep Human Brain. Springer. DOI: 10.1007/978-3-319-45066-7_20