Blocking Intracellular Glutamate May Halt Neuropathic Pain
Interview with Karen O'Malley, PhD
Understanding how the body processes pain is essential to understanding how to better treat it. Aiding in that understanding has been intense research on one of the key players that transmit nociceptive information through the body. G-protein-couple receptors (GPCRs) play a critical role in various bodily functions, including as a mediator of neuroplasticity underlying chronic pain. One specific receptor, metabotropic glutamate 5 receptor (mGluR5), is heavily abundant in the dorsal horn (DH),1,2 prime real estate for the relay of pain information throughout the central nervous system (CNS).3
While researchers already know that mGluR5 influences hypersensitivity following a nerve injury, they haven’t understood whether these effects are due to signaling of the receptor on the surface of spinal cells or inside of them. In a new study published online in Nature Communications,4 it appears that not only does mGluR5 proliferate in the DH following a nerve injury, the receptor is far more influential inside the cell, specifically on the membranes of the nucleus.
“To our knowledge, these are the first experiments demonstrating a role for an intracellular GPCR in an in vivo behavioral model,” wrote the researchers. While many GPCR’s can be found on the nuclear membrane, deducing their significance has been tricky. Now, the researcher team from Washington University School of Medicine in St. Louis, Missouri, and McGill University in Montreal, Canada, believe nuclear mGluR5 could become a measurable treatment target for analgesia, something that would influence future therapies to come.
Block the Receptor, Curb the Pain
The investigators wanted to analyze how mGluR5 responded when rats received a spared-nerve injury (SNI), which is a typical neuropathic pain model that helps to simulate human neuropathic pain, like spontaneous pain, allodynia, and hyperalgesia. The researchers found nuclear mGluR5 inside DH neurons increased following the trauma. The mGluR5 binding sites also appeared to increase, suggesting a pathophysiological role. Effector molecules, known to impact neuropathic plasticity, increased as well. Based on this reaction, mGluR5 appeared to be a key component in a systemic chain signaling nociception, something that potentially may be suppressed using selective drugs, the authors noted.
For instance, when researchers blocked intracellular mGluR5 using a permeable antagonist called fenobam (IC50 80 nM), they found marked, dose-dependent reductions in the rats’ pain behaviors for about one hour after the injection. By contrast, inhibiting only the mGluR5 found on the cell surface using an impermeable antagonist had no effect, which supported the notion that mGluR5 found inside the cell is far more influential in pain transmission.
Interestingly, when rats without nerve injury were treated with fenobam, their pain sensitivities didn’t change at all. For the animals with nerve injury, not only were allodynia and spontaneous pain significantly reduced when treated with the intracellular mGluR5 antagonist, downstream signalers of the pain also decreased.
Developing a Selective Inhibitor
Given the measurable analgesic effects achieved in the study, mGluR5 inhibition may be a viable target for future pain pharmacotherapy techniques. "If we can find ways to specifically block pain receptors inside of cells rather than on the cell surface, we may make a big dent in chronic pain with fewer drug-induced side effects," said Karen O'Malley, PhD, a professor of neuroscience at Washington University and a coauthor of the study.
Interestingly, before this research was published there were already drugs optimized for mGluR5 selectivity, affinity, and pharmacokinetic parameters.5-7 These drugs overcome the off-target effects and short-half-lives that marred past attempts at developing an agent. However, up to this point, researchers have not looked at which specific receptor pool ligands act in neuropathic pain models.
Fenobam, a negative allosteric modulator, was far more successful at inhibiting intracellular mGluR5 than the impermeable antagonist, LY393053, which had little effect on cell surface mGluR5. “The fenobam-induced analgesic [conditioned place preference (CPP) paradigm] in neuropathic, but not naive, rats suggests that agents acting at intracellular mGluR5 may produce analgesia with low potential for abuse,” noted the researchers.
While fenobam was used in the study because of its strong specificity for mGluR5 antagonists, its short half-life wouldn’t make it a strong candidate for pharmacological development, Dr. O’Malley said. However, there are other negative allosteric modulators available now that may hold promise, such as basimglurant, an mGluR5 negative allosteric modulator currently under development by Hoffmann-La Roche. “Currently, there is no targeted way to get a negative allosteric modulator inside the cell without affecting the cell surface receptor—although people are working on alternative modes of delivery.”
There are still many questions about mGluR5 that are still left to be answered. “Although our study emphasizes the physiological significance of the two pools of mGluR5, the mechanism by which nuclear mGluR5 is increased in neuropathic rats remains unknown,” the authors noted. The next step is to investigate the significance of various effector proteins that are associated with nuclear mGluR5’s effects on nociceptive processing.
“We are now trying to look at whether there are unique proteins scaffolded to mGluR5 on intracellular membranes versus the cell surface. Unfortunately, the tools for these studies are not great so it's taking longer than we would have hoped for,” Dr. O’Malley said.
The study was supported through grants from the Canadian Institutes of Health Research, the Louise and Alan Edwards Foundation, the National Institutes of Health, as well as studentships from Natural Sciences and Engineering Research Council of Canada and a fellowship from the Canadian Institutes of Health Strategic Training Program in Pain: Molecules to Community. The authors of the study declared no competing conflicts of interest.