RESEARCH ARTICLE SUMMARY
◥NEUROSCIENCE
Widespread receptor-driven modulation
in peripheral olfactory coding
Lu Xu, Wenze Li, Venkatakaushik Voleti, Dong-Jing Zou, Elizabeth M. C. Hillman†, Stuart Firestein†
INTRODUCTION:The mammalian nose is ar-
guably the best chemical sensor on the planet,
able to detect and discriminate among a large
and diverse repertoire of mostly small, organic
molecules. It accomplishes this, at least in part,
through a large family of G protein–coupled
receptors (GPCRs) expressed in specialized
olfactory sensory neurons (OSNs) arrayed over
an epithelium deep within the nasal cavity.
Each neuron expresses only one of the ~1000
receptor genes. It is thought that the specific
activation of subsets of these receptors by a
particular odor translates into a code that can
be read by higher brain centers to create a per-
ception. However, we rarely encounter pure
odors. Our daily life is a stream of encounters
with rich blends of odors, from garbage to co-
logne. Even a simple cup of coffee has >800
volatile components. To study how the olfac-
tory system encodes this much more complex
information, we explored how neurons with-
in the peripheral olfactory epithelium of a
mouse’s nose responded to a series of odor
mixtures. Our analysis was enabled by a new
high-speed three-dimensional (3D) imaging
method called SCAPE (Swept Confocally Aligned
Planar Excitation) microscopy, which allows
the responses of thousands of single neurons
within the intact olfactory epithelium to be
monitored in parallel during delivery of re-
peated odor combinations.RATIONALE:Previous studies using single, mono-
molecular odors suggested that the diverse
expression of receptors in OSNs could provide
unambiguous representations of individual
odors. However, it is unclear how the brain
mightbeabletodecodesignalswhenmultiple
odor components within a blend generate over-
lapping patterns. Moreover, when smelling a
mixture of odors, it is common to perceive one
odor dominating another. Psychophysical tests
have revealed both suppressive and enhancing
effects of particular odors within a blend.
However, it has long been assumed that such
odor-coding interactions occur at higher pro-
cessing levels within the brain. Here, we studied
whether combinatorial effects of odors affectneural representations at the peripheral sen-
sory level.RESULTS:Using SCAPE microscopy to image
calcium-sensitive fluorescent proteins in OSNs,
we were able to simultaneously monitor the
activity of cells within a large volume of the
intact mouse olfactory epithelium with single-
cell resolution. Analyzing the responses of
thousands of single neurons to blends of up
to three odors, we discovered a series of sur-
prising interactions that distorted the repre-
sentation of the odor mixture compared with
a simple combinatorial
sum of responses to in-
dividual odors. Among the
eight chemically distinct
odors tested, we observed
that the presence of one
odor could either enhance
or suppress the response of a neuron to another
odor, even if the modulating odor by itself did
not elicit a response from the neuron. This
means that an odorless molecule could alter the
perception of another odor, and that a neuron’s
response to an odor blend can be much larger
or smaller than its response to components of
the blend. Overall, we observed clear evidence
of agonism, antagonism, partial agonism, and
enhancement occurring at the receptor level,
suggesting a richer repertoire of receptor mod-
ulation mechanisms than previously thought.
Finally, we note that enhancement of responses
may be evidence of an allosteric modulatory
site, a rare finding in class A GPCRs that bind
small molecules.CONCLUSION:Although inhibition and enhance-
ment are well established in sensory systems,
they are only a feature of higher circuit pro-
cessing. Here, we observed complex receptor
modulation at the level of peripheral olfac-
tory sensory receptors. We propose that these
peripheral modulatory interactions are cru-
cial for discriminating complex blends of
odors with overlapping activation patterns
because they prevent the saturation of re-
ceptors and allow each new component to
alter the overall activation pattern, rendering
it distinctive. This result suggests that higher
brain regions may rely on pattern recognition
rather than on reading an additive combina-
torial code to build a perception. This work
also demonstrates an exciting and versatile
new paradigm for high-throughput charac-
terization of single-cell responses in intact
systems.
▪RESEARCH
154 10 APRIL 2020•VOL 368 ISSUE 6487 sciencemag.org SCIENCE
The list of author affiliations is available in the full article online.
*These authors contributed equally to this work.
†Corresponding author. Email: sjf24@columbi a.edu (S.F.);
[email protected] (E.M.C.H.)
Cite this article as L. Xuet al.,Science 368 , eaaz5390
(2020). DOI: 10.1126/science.aaz5390SCAPEOOO
O11,936 neurons200μmLarge-scale single-cell recording of OSNs reveals receptor-driven modulation effects.Volumetric
imaging of GCaMP in intact olfactory epithelium using high-speed SCAPE microscopy enables analysis of
responses to mixtures of different odor molecules. Single-neuron response time courses show that odor
mixtures can enhance (red) or suppress (green) responses compared with individual odors. Heatmap shows
assessment of >10,000 individual neurons across five mice.
ON OUR WEBSITE◥Read the full article
at http://dx.doi.
org/10.1126/
science.aaz5390
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