New Biosensors Show Opioids Acting on Receptors in Living Brains

Scientists still understand remarkably little about precisely how opioids work, despite the enormous impact these substances have had around the world, for good and ill.

A new study led by researchers at the University of California, Davis, sets out a way to observe the nervous system's response as opioid receptors are activated, in real time.

By introducing fluorescent receptors into mouse neurons, the researchers demonstrated that opioid activity can be visualised within a living brain.

The body's opioid receptor system

For many people, the term opioid immediately suggests substances such as heroin and fentanyl. Yet the body produces its own opioid system, a key component of the brain circuits involved in reward and aversion.

This system is centred on three receptors: kappa (κOR), delta (δOR) and mu (µOR). They are triggered by various short-chain amino acids released by neurons in response to pleasure, pain and stress. The most familiar opioid neuropeptides are likely to be endorphins - a blend of the words “endogenous morphine” - although many others exist.

Opioid medicines also attach strongly to and activate opioid receptors. Some opioids are non-psychoactive, including the anti-diarrhoeal medicine loperamide. Most, however, reduce pain and create intense euphoria while causing the brain to adjust to a new opioid baseline. That makes them useful painkillers, but also highly addictive.

Much of this is established knowledge. Even so, the apparently simple process contains numerous complexities. As the paper explains, working out how neuropeptides engage with the opioid system is challenging in itself: "[The receptors] can be activated by at least 20 endogenous opioid peptides with differential affinity and selectivity".

Why opioid activity is difficult to measure

The reference to "differential affinity and selectivity" matters because opioids bind to each receptor with varying strength. Rather than functioning as a straightforward on-or-off switch, receptor activation therefore resembles a light controlled by a dimmer switch.

The extremely small amounts of neuropeptides used by the opioid system have created another obstacle. According to the paper, "the released concentration may also be at orders of magnitude lower than classical neurotransmitters … As a result, it has been exceedingly difficult to study the processes that regulate opioid neuropeptide release."

There are also many more synthetic opioids than natural neuropeptides, with each affecting the brain in subtly distinct ways.

Fluorescent biosensors reveal opioid receptors in mice

The researchers' paper outlines a technique for examining exactly what occurs when opioid receptors encounter a binding substance, whether an opioid drug or an endogenous neuropeptide.

It uses three precisely calibrated biosensor molecules, each based on one of the κOR, δOR and µOR receptors.

These molecules fluoresce when a substance activates their respective receptor, before the glow fades as that receptor becomes inactive again. The fluorescence depends on the dose and is reduced by opioid blockers such as naloxone, which bind tightly to receptors without activating them.

When used in mice fitted with fluorescent biosensors in the hippocampus, the approach enabled the team to observe how opioid receptors reacted to different drugs and neuropeptides.

The results could improve understanding of the opioid system and aid the search for possible treatments for anxiety and depression. They may also support the longstanding effort to develop a medicine that delivers the pain relief of current opioids without their associated risk of addiction.

The paper was published in Nature Neuroscience.