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How Memories of Smell Are Planted in the Brain

Smells get etched into the brain’s permanent wiring by avoiding neural pruning.

Key points

  • Smells have the ability to trigger vivid memories from the past.
  • New research explored how memories of smell are encoded and stored in the brain.
  • A protein called BMPR-2 may strengthen neural connections linked to specific smells encountered during early development and prune others.
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Specific smells that are encountered during early postnatal development can become hardwired in the mammalian brain if their neural connections to the olfactory bulb aren't pruned, a new study in mice reports.
Source: Design_Cells/Shutterstock

Over a century ago, Marcel Proust (1871-1922) described the French protagonist in his novel À la recherche du temps perdu (first translated into English as Remembrance of Things Past, but now commonly referred to as In Search of Lost Time) dipping a Madeleine in a cup of tea and having aroma-triggered flashbacks to his youth.

Experiencing a "Proustian moment" refers to the phenomenon of having a particular scent conjure up vivid memories of a specific time or place from one's past.

As Cretian van Campen explains in the abstract for his non-fiction book, The Proust Effect: The Senses as Doorways to Lost Memories:

"[This effect] refers to the vivid reliving of events from the past through sensory stimuli. Many of us are familiar with those special moments when you are taken by surprise by a tiny sensory stimulus (e.g., the scent of your mother's soap) that evokes an intense and emotional memory of an episode from your childhood."

How do these vivid "Proustian" memories from early stages of childhood development get permanently etched into our brains? A new olfaction-based study (Aihara et al., 2021) in mice by researchers from Kyushu University offers some fresh clues. These open-access findings were recently published in the journal Cell Reports.

The Kyushu University researchers investigated mouse olfactory neurons and identified how the protein BMPR-2 regulates the selective stabilization of neuron branching by strengthening neural connections associated with a specific smell during early development.

The complex way that BMPR-2 "gates activity-dependent stabilization of primary dendrites during mitral cell remodeling" is a multi-faceted process that involves a protein called LIMK and glutamic acid neurotransmitters. For details on how this all works, check out the paper's open-access Results.

 myboys[dot]me/Shutterstock
Much like a young, untrimmed bonsai tree with an abundance of excessive branches, the olfactory bulb starts with countless mitral cells that are waiting to carry olfactory information from nose-to-brain via neuronal branches (dendrites).
Source: myboys[dot]me/Shutterstock

Neuroplasticity in Action: Neural Pruning Trims Unused Olfactory Dendrites Like Bonsai Tree Branches

Using a layperson-friendly metaphor, the Japanese neuroscientists describe the mechanisms involved in selectively pruning or strengthening neural connections involved in the brain's sense of smell during early development as a "biomolecular bonsai" tree.

In a "use it or lose it" way, only the dendrites that get activated by a specific smell form strong, lasting connections, while unused dendrites are pruned or snipped away like excess branches on a manicured bonsai tree.

"At the start of neuronal development, neurons form excessive amounts of connections that are gradually eliminated as others are strengthened," the authors explain in a July 15 news release. "How neuronal circuits remodel themselves over time, especially during early development, is an open question in neurobiology."

"At an early stage of mouse development, the mitral cells connect to multiple glomeruli. As development progresses, excess branches are pruned away, and eventually each mitral cell establishes a single branch to only one glomerulus innervating for a single odor," the authors note.

"A main reason we use olfactory neurons is because they are easy to access and study, and mitral cells develop only a single branch," first author Shuhei Aihara said in the news release. "When an olfactory neuron detects a specific molecule that we smell, it sends the signal to a specific 'way station' in the brain's olfactory bulb called a glomerulus. That signal is then relayed to the brain through mitral cells. One mitral cell receives signals for one specific smell."

"Hopefully these new insights into neural development can lead to further understanding of the fundamental mechanisms behind critical brain functions and possible treatments into pathologies underlined by synaptic dysfunction," senior author Takeshi Imai added.

"Our next step is to find the factors that promote dendrite pruning, and we also want to see if this mechanism in the olfactory bulb is fundamental throughout the neocortex," he concluded.

References

Shuhei Aihara, Satoshi Fujimoto, Richi Sakaguchi, Takeshi Imai. "BMPR-2 Gates Activity-Dependent Stabilization of Primary Dendrites During Mitral Cell Remodeling." Cell Reports (First published: June 22, 2021) DOI: 10.1016/j.celrep.2021.109276

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