New molecular probes for opioid receptors

New molecular probes for opioid receptors

Strong painkillers are very important in the management of patients with cancer and heart attack or requiring surgery. They extert their effect by binding to so-called opioid receptors in the body.

These painkillers have excellent efficacy, but also severe side effects. On the one hand, there is the danger of dependency, on the other hand, patients may become tolerant - i.e. the effectiveness of the drugs decreases with repeated use. This means that the dose must be increased over time to achieve the same effect. Painkillers with less drastic undesirable effects and equally good efficacy would therefore be highly desirable.

The team dealt with a question that has been highly debated in the field so far. "It is still unclear whether the analgesic effect of opioids is mediated by individual receptors or whether it is necessary for the receptors to aggregate into pairs or larger molecular complexes," said the senior author. Evidence has already been found for all these possibilities.

"Our results help reconciling some of the previously contradictory observations," says another author. "We found that most opioid receptors exist as individual entities in the cell membrane. However, a small proportion forms pairs of two. Although the lifespan of these pairs is short, they might contribute to the function of this important family of receptors."

This finding might be very important: "There is evidence that the receptor pairs have different pharmacological effects than individual receptors," said the senior author. Therefore, it might be possible to develop new painkillers with a more favourable effect profile on the basis of this knowledge.

Due to the importance of these new findings, the journal "Angewandte Chemie" has classified the publication of the JMU researchers as "highly important". It is freely available on the Web. In addition, the work was selected for one of the journal covers.

The research team came to its conclusion because it had previously developed highly selective fluorescent ligands for a subtype of the receptors, the so-called mu opioid receptor (MOR). This is the most important of the three receptor subtypes and is responsible for the desired pain-relieving but also for the addictive effect. The new ligands can be used as molecular probes to label the receptor highly specifically and to observe its behaviour in living cells using single-molecule fluorescence microscopy.

The new fluorescent ligands are based on the μ‐OR antagonist Ep‐nitrocinnamoylamino‐dihydrocodeinone (CACO), that display high affinity, long residence time and pronounced selectivity. Using these ligands, the authors achieved single‐molecule imaging of μ‐ORs on the surface of living cells at physiological expression levels.

The results reveal a high heterogeneity in the diffusion of μ‐ORs, with a relevant immobile fraction. Using a pair of fluorescent ligands of different color,they provide evidence that μ‐ORs interact with each other to form short‐lived homodimers on the plasma membrane. This approach provides a new strategy to investigate μ‐OR pharmacology at single‐molecule level.