New Biosensors Show Opioid Receptors Working in the Living Brain

Given opioids’ profound effect on the world – for better or worse – scientists still understand surprisingly little about precisely how these drugs function.

A new study, led by researchers at the University of California, Davis, outlines a way for scientists to observe in real time how the nervous system reacts when its opioid receptors are activated.

By introducing fluorescent receptors into the neurons of mice, the researchers demonstrated that opioid activity can be visualised as it unfolds within a living brain.

The body’s opioid system

For many people, the term ‘opioid’ suggests drugs such as heroin and fentanyl. Yet the human body has its own opioid system, an essential component of the brain pathways involved in reward and aversion.

This system centres on three receptors: kappa (κOR), delta (δOR) and mu (µOR). They are triggered by various short-chain amino acids that neurons release in response to pleasure, pain and stress. The most familiar opioid neuropeptides are likely endorphins – a blend of ‘endogenous morphine’ – though there are many others.

Opioid drugs likewise bind tightly to and activate opioid receptors. Although some opioids are non-psychoactive – including the anti-diarrhoeal medicine loperamide – most relieve pain and create intense euphoria while compelling the brain to adjust to a new opioid baseline. That makes them useful painkillers, but also highly addictive.

Why opioid receptors are difficult to study

That much is established. Yet a process that appears relatively simple on the surface contains numerous subtleties and complications. As the study explains, even determining how neuropeptides engage with the opioid system is challenging: "[The receptors] can be activated by at least 20 endogenous opioid peptides with differential affinity and selectivity".

The phrase “differential affinity and selectivity” matters because individual opioids attach more or less strongly to each receptor. Consequently, the level of receptor activation resembles a light bulb controlled by a dimmer rather than a basic on/off switch.

The minute amounts of neuropeptides involved in the opioid system have created another obstacle. The paper states that "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."

Naturally, synthetic opioids are far more numerous than neuropeptides, and each influences the brain in subtly distinct ways.

Fluorescent biosensors reveal opioid activity

The researchers describe a new approach for examining exactly what occurs at opioid receptors when they meet a substance that binds to them, whether an endogenous neuropeptide or an opioid drug.

The method uses three precisely calibrated molecules, known as biosensors, with one derived from each of the κOR, δOR and µOR receptors.

These molecules fluoresce when a substance activates the relevant receptor, before the glow fades as the receptor becomes inactive again. Fluorescence depends on the dose, and it is also reduced by opioid blockers such as naloxone, which bind strongly to receptors without activating them.

Using mice fitted with fluorescent biosensors in the hippocampus region of their brains, the team could observe opioid receptors responding to different drugs and neuropeptides.

The results could both deepen understanding of the opioid system and aid the hunt for possible treatments for anxiety and depression. They may also support the seemingly endless search for a medicine that offers the pain relief of current opioids without their associated risk of addiction.

The paper has been published in Nature Neuroscience.