Researchers light-up mouse brain, revealing previously hidden areas susceptible to opioids

Date:
November 11, 2020
Source:
Temple University Health System
Summary:
New work shows that kappa opioid receptors actually are distributed
widely throughout the brain. The researchers made this discovery
after lighting up the brains of mice using a technique called
CLARITY followed by three-dimensional (3D) fluorescent imaging. The
study is the first to apply the imaging technique to better
understand opioid receptor localization across the whole brain in
3D images.



FULL STORY ========================================================================== Winding and twisting like a labyrinth, the brain consists of an elaborate network of passages through which information flows at high speeds,
rapidly generating thoughts, emotions, and physical responses. Much of
this information is relayed by chemical messengers, or neurotransmitters
-- like dopamine and serotonin.


========================================================================== Although fine-tuned and evolved for complex processing, the brain and its neurotransmitters are vulnerable to hijacking by chemical substances,
including opioid drugs such as oxycodone, psychostimulants such as
cocaine, and alcohol.

Chronic use of any of these substances enhances the activity of a molecule known as the kappa opioid receptor (KOR), which is active in the brain's
reward circuitry. KOR activation produces dysphoria and an inability to
feel pleasure.

Its enhanced activity following chronic drug or alcohol use plays a
crucial role in substance abuse.

KORs have been known to exist in certain brain regions, particularly
those involved in pain processing, reward, and stress responses, but new
work at the Lewis Katz School of Medicine at Temple University (LKSOM)
shows that these receptors actually are distributed widely throughout
the brain. The Temple researchers made this discovery after lighting
up the brains of mice using a technique called CLARITY followed by three-dimensional (3D) fluorescent imaging. The study is the first
to apply the imaging technique to better understand opioid receptor localization across the whole brain in 3D images.

"Typically, we look at the brain in sections, thus yielding
two-dimensional (2D) images, in which case we are not really able to
see get a big picture view of protein distribution," explained Lee-Yuan Liu-Chen, PhD, Professor in the Center for Substance Abuse Research and Department of Pharmacology at LKSOM and senior investigator on the new
study. "But with CLARITY we are able to produce 3D images of the entire
brain, as a whole organ, and this allowed us to expose the full extent of
KOR distribution." The study was published online in the journal eNeuro.

The CLARITY technique renders brain tissue transparent, enabling
researchers to visualize fluorescent probes linked to a protein of
interest, in this case KOR.

Fluorescence emitted from the probes is then detected via confocal
imaging methods to yield highly detailed 3D images of the specific
protein's distribution in the whole brain.

Seeking to gain a deeper understanding of KOR localization in the
brain, Dr.

Liu-Chen and colleagues applied CLARITY to preserved brains from mice
that had been engineered to express a fluorescent tag known as tdTomato
on KOR proteins.

Upon imaging, very specific regions of the KOR-tdTomato mouse brain lit
up a bright shade of red, revealing the 3D distribution of KOR throughout
the brain.

The researchers then examined 2D sections of brain tissue to gain detailed information on the spatial localization of KOR at the cellular level.

The 3D analyses and observations from brain sectioning enabled the
researchers to map out the specific places of KOR expression. They
identified extensive tracts related to pain and reward, building on
existing knowledge of KOR's relevance to these pathways, and they
discovered many neural tracts not previously known to express KOR.

"Seeing KOR in 3D space led to the realization that the receptor is
expressed in areas of the brain beyond those that had been described
before," Dr. Liu- Chen said. "The function of KOR in these additional
neural circuits is unknown." A major goal for the team now is to figure
out what KOR does in these newly identified circuits.

The success of the team's approach in itself is significant and could
open doors to the study of other neurotransmitter receptors in the
brain. KOR and other opioid receptors are types of G-protein coupled
receptors (GPCRs). "No one has done a 3D study of GPCR distribution in
the brain before," Dr. Liu-Chen said. "The approach we used is a very
useful tool and could be applied to study many different types of GPCRs
and other proteins across neural tracts."

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


========================================================================== Journal Reference:
1. Chongguang Chen, Alex H. Willhouse, Peng Huang, Nora Ko, Yujun
Wang, Bin
Xu, Lan Hsuan Melody Huang, Brigitte Kieffer, Mary F. Barbe,
Lee-Yuan Liu-Chen. Characterization of a Knock-In Mouse Line
Expressing a Fusion Protein of k Opioid Receptor Conjugated with
tdTomato: 3-Dimensional Brain Imaging via CLARITY. eneuro, 2020;
7 (4): ENEURO.0028-20.2020 DOI: 10.1523/ENEURO.0028-20.2020 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/11/201111122838.htm

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