Decoding smell
Neural code determines instinctual responses to attractive or aversive
odors

Date:
March 29, 2021
Source:
Stowers Institute for Medical Research
Summary:
How does the nose know? Scientists now detail how the inborn ability
to recognize certain odors is encoded in the nervous system of mice.



FULL STORY ========================================================================== Since the beginning of the pandemic, a loss of smell has emerged as one
of the telltale signs of COVID-19. Though most people regain their sense
of smell within a matter of weeks, others can find that familiar odors
become distorted.

Coffee smells like gasoline; roses smell like cigarettes; fresh bread
smells like rancid meat.


==========================================================================
This odd phenomenon is not just disconcerting. It also represents
the disruption of the ancient olfactory circuitry that has helped to
ensure the survival of our species and others by signaling when a reward (caffeine!) or a punishment (food poisoning!) is imminent.

Scientists have long known that animals possess an inborn ability
to recognize certain odors to avoid predators, seek food, and find
mates. Now, in two related studies, researchers from the Yu Lab at the
Stowers Institute for Medical Research show how that ability, known as
innate valence, is encoded.

The findings, published in the journals Current Biology and eLife,
indicate that our sense of smell is more complicated -- and malleable --
than previously thought.

Our current understanding of how the senses are encoded falls into two contradictory views -- the labeled-line theory and the pattern theory. The labeled-line theory suggests that sensory signals are communicated along a fixed, direct line connecting an input to a behavior. The pattern theory maintains that these signals are distributed across different pathways
and different neurons.

Some research has provided support for the labeled-line theory in simple species like insects. But evidence for or against that model has been
lacking in mammalian systems, says Ron Yu, PhD, an Investigator at the
Stowers Institute and corresponding author of the reports. According
to Yu, if the labeled-line model is true, then the information from one
odor should be insulated from the influence of other odors. Therefore,
his team mixed various odors and tested their impact on the predicted
innate responses of mice.

"It's a simple experiment," says Qiang Qiu, PhD, a research specialist
in the Yu Lab and first author of the studies. Qiu mixed up various combinations of odors that were innately attractive (such as the smell
of peanut butter or the urine of another mouse) or aversive (such as
the smell of rotting food or the urine of a predator). He then presented
those odor mixtures to the mice, using a device the lab specially designed
for the purpose. The device has a nose cone that can register how often
mice investigate an odor. If mice find a particular mixture attractive,
they poke their nose into the cone repeatedly. If they find the mixture aversive, they avoid the nose cone at all costs.



==========================================================================
To their surprise, the researchers discovered that mixing different
odors, even two attractive odors or two aversive odors, erased the
mice's innate behavioral responses. "That made us wonder whether it was
simply a case of one odor masking another, which the perfume industry
does all the time when they develop pleasant scents to mask foul ones,"
says Yu. However, when the team looked at the activity of the neurons
in the olfactory bulb that respond to aversive and attractive odors,
they found that was not the case.

Rather, the patterns of activity that represented the odor mixture were strikingly different from that for individual odors. Apparently, the
mouse brain perceived the mixture as a new odor identity, rather than
the combination of two odors. The finding supports the pattern theory,
whereby a sensory input activates not just one neuron but a population
of neurons, each to varying degrees, creating a pattern or population
code that is interpreted as a particular odor (coyote urine! run!). The
study was published online March 1, 2021, in Current Biology.

But is this complicated neural code hardwired from birth, or can it be influenced by new sensory experiences? Yu's team explored that question
by silencing sensory neurons early in life, when mice were only a week
old. They found that the manipulated mice lost their innate ability to recognize attractive or aversive odors, indicating that the olfactory
system is still malleable during this critical period of development.

Interestingly, the researchers found that when they exposed mice during
this critical period to a chemical component of bobcat urine called PEA,
the animals no longer avoided that odor later in life. "Because the mice encountered this odor while they were still with their mothers in a safe environment and found that it did not pose a danger, they learned to
not be afraid of it anymore," says Yu. This study was published online
March 26, 2021, in eLife.

Though the COVID-19 pandemic has warped the sense of smell in millions of people, Yu does not predict that it will have significant implications
for most adults who recover from the disease. However, he thinks this
altered sensory experience could have a major impact on affected infants
and children, especially considering the role that many odors play in
social connections and mental health.

"The sense of smell has a strong emotional component to it -- it's the
smell of home cooking that gives you a feeling of comfort and safety,"
says Yu. "Most people don't recognize how important it is until they
lose it." Other co-authors from Stowers include Yunming Wu, PhD Limei
Ma, PhD, Wenjing Xu, PhD, Max Hills, and Vivekanandan Ramalingam, PhD.

The work was funded by the Stowers Institute for Medical Research and the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health (award numbers R01DC008003, R01DC014701,
and R01DC016696). The content is solely the responsibility of the authors
and does not necessarily represent the official views of the NIH.

========================================================================== Story Source: Materials provided by
Stowers_Institute_for_Medical_Research. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Qiang Qiu, Yunming Wu, Limei Ma, C. Ron Yu. Encoding Innately
Recognized
Odors via a Generalized Population Code. Current Biology, 2021;
DOI: 10.1016/j.cub.2021.01.094 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210329153350.htm

--- up 4 days, 8 hours, 39 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)