Memory of "delicious smells" facilitated in the brain by the action of dopamine

How is the memory of delicious smells formed? The joint research team of Assistant Professor Kei M. Igarashi of the University of California, Irvine School of Medicine and Assistant Professor Koshi Murata of the University of Fukui School of Medical Sciences sought to answer this question and has revealed for the first time in vertebrates that the secretion of dopamine into nerve cells in the entorhinal cortex is associated with establishment of memories related to odor and experience of delicious foods. Associate Professor Igarashi said, "Many researchers have focused on the hippocampus and ignored the entorhinal cortex; however, it has turned out that this is the key. This time, we focused on two factors, dopamine secretion and the entorhinal cortex. However, I would like to consider other substances and anatomical locations in the future." These findings were published in Nature.

An example of this is Fukui's specialty, "heshiko" (pickled mackerel), as it is often shunned because of its strong odor; however, once you eat it, the taste is addictive. In order to be able to think the smell of heshiko is delicious, it is necessary to memorize the "smell" and "experience of deliciousness" in combination.

The researchers used optogenetic techniques to control the activity of the entorhinal cortex while mice were performing associative memory tasks related to odor and sugar water. Electrodes were attached to the approximately 200 fan cells in the second layer of the entorhinal cortex and electrophysiological recordings were taken. When the mouse experienced odors associated with sugar water, it was discovered that the fan cells of the entorhinal cortex formed associative memories. When sugar water was immediately received after smelling a new odor, dopamine was released in the entorhinal cortex, and the fan cells acquired responsiveness to the odor through this release of dopamine, establishing an associative memory. This was clarified using calcium imaging.

This finding reveals a previously completely unknown dopamine-mediated associative memory function in the entorhinal cortex. In Alzheimer's disease, the sense of odor is impaired early, so it is hoped that the current study will lead to future therapies for memory disorders.