Imaging techniques reveal opioid receptor localization throughout the brain

Review by Emily Henderson, Bachelor of ScienceNovember 12, 2020

The maze-like twisting and twisting brain is made up of a network of elaborate pathways through which information flows at high speeds, rapidly producing thoughts, emotions, and physical reactions.

Much of this information is relayed by chemicals or neurotransmitters such as dopamine and serotonin.

Although fine-tuned and evolved for complex processing, the brain and its neurotransmitters are vulnerable to hijacking by opioids such as oxycodone, psychostimulants such as cocaine, and chemicals such as alcohol.

Chronic use of any of these substances increases the activity of a molecule known as the kappa opioid receptor (KOR), which is active in the brain’s reward circuit. Activation of KOR makes you unable to feel discomfort and pleasure. Its enhanced activity after the use of chronic drugs or alcohol plays an important role in substance abuse.

KOR is known to be present in specific brain regions, especially those involved in pain treatment, rewards, and stress responses, but a new study from Temple University’s Lewis Katz School of Medicine (LKSOM) found that these receptors were in fact. It has been shown to be widely distributed in. brain.

Temple researchers made this discovery after illuminating the mouse brain using a technique called CLARITY, followed by three-dimensional (3D) fluorescence imaging.

This study is the first to apply imaging techniques to better understand the localization of opioid receptors throughout the brain in 3D images.

Normally, we look at the brain in sections, so two-dimensional (2D) images are generated. In this case, you cannot actually see the whole picture of the protein distribution. However, CLARITY can be used to generate 3D images of the entire brain and organs. This allowed us to expose the entire range of the KOR distribution... “

Lee-Yuan Liu-Chen, PhD, Professor, Substance Abuse Research Center and Faculty of Pharmacy, Temple University, Lewiskats School of Medicine

Liu-Chen Senior Researcher for new research.

The study was published online in the journal eNeuro..

CLARITY technology makes brain tissue transparent and allows researchers to visualize fluorescent probes linked to the protein of interest (KOR in this case).

The fluorescence emitted by the probe is then detected by confocal imaging to produce a very detailed 3D image of the distribution of a particular protein throughout the brain.

Dr. Liu-Chen and colleagues aim to better understand the localization of KOR in the brain by applying CLARITY to the conserved brain of mice designed to express a fluorescent tag known as tdTomato on the KOR protein. I applied it.

Imaging revealed a very specific area of ​​the brain of KOR-tdTomato mice lit with a bright red tint, revealing a 3D distribution of KOR throughout the brain. Next, the researchers examined the 2D section of brain tissue to obtain detailed information about the spatial localization of KOR at the cellular level.

3D analysis and observation from brain sections allowed researchers to plan specific locations for KOR expression.

Based on their existing knowledge of the association of KOR with these pathways, they identified a wide range of pain and reward-related areas and discovered many previously unknown neural areas that express KOR. Did.

“When I looked at KOR in 3D space, I noticed that the receptors were expressed in areas of the brain beyond what was previously explained,” said Dr. Liu-Chen. “The function of KOR in these additional neural circuits is unknown.” The team’s current main goal is to understand what KOR does in these newly identified circuits.

The success of the team’s approach itself is important and may open the door to the study of other neurotransmitter receptors in the brain. KOR and other opioid receptors are a type of G protein-coupled receptor (GPCR).

“No one has ever done a 3D study of GPCR distributions in the brain,” said Dr. Liu-Chen. “The approach we used is a very useful tool and can be applied to study different types of GPCRs and other proteins throughout the neural tract.”