Neural basis of opioid-induced respiratory depression and its rescue

Neural basis of opioid-induced respiratory depression and its rescue


The scientists show how blocking opioid receptors in specific neurons can restore breathing during an overdose

It’s long been known that opioid overdose deaths are caused by disrupted breathing, but the actual mechanism by which these drugs suppress respiration was not understood. Now, a new study has identified a group of neurons in the brainstem that plays a key role in this process.

The findings, published in the journal Proceedings of the National Academy of Sciences, show how triggering specific receptors in these neurons causes opioid-induced respiratory depression, or OIRD, the disrupted breathing that causes overdose deaths. It also shows how blocking these receptors can cause OIRD to be reversed.

“The underlying mechanism of why opiates slow down and depress the breathing rhythm has not been fully characterized,” says senior investigator. “This knowledge can provide a stepping stone to better treatment options for OIRD.”

Opioids work by binding to proteins on nerve cells (neurons) called opioid receptors and subsequently inhibiting their activity. Currently, naloxone is the only medication known to block the effects of opioids and reverse an overdose. But naloxone has limitations, including a short duration that requires it to be administered multiple times. It also works systemically, blocking opioid receptors throughout the entire body, including those that control pain.

To develop strategies to rescue OIRD with more specificity, the team set out to search for the breathing neurons in the brain that also carry opioid receptors. In the new study, the researchers identified a group of neurons that express a certain type of opioid receptor (the mu opoid receptor) and are located in the brainstem breathing modulation center; they then characterized these neurons’ role in OIRD. 

They found that mice that were genetically engineered to lack opioid receptors in these neurons didn’t have their breathing disrupted when exposed to morphine, as mice in the control group did. The researchers also found that, without introducing opioids, stimulating these receptors in control mice caused symptoms of OIRD.

he team then looked at ways to reverse the process by treating the overdosed mice with chemical compounds targeted to other receptors on the same neurons, which play an opposite role as the opioid receptor (activating rather than inhibiting them).

“We discovered four different chemical compounds that successfully activated these neurons and brought back the breathing rate during OIRD,” said the first author. Recovery in overdosed mice was close to 100 percent, which surprised the team.

The researchers next plan to look at whether other cell groups also play a role in OIRD. Further study would also examine the connection between breathing regulation and pain perception in the brain, potentially opening the door to developing more targeted treatments for OIRD.

“We hope to explain the pain-breathing segregation at the molecular or microcircuit level,” says the senior author. “By doing that, we can try to restore breathing without touching analgesic effects of opioids.”

https://www.pnas.org/content/118/23/e2022134118

http://sciencemission.com/site/index.php?page=news&type=view&id=publications%2Fparadoxical-hyperexcitab&filter=22

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