Mechanistic Insights into Light-Driven Allosteric Control of GPCR Biological Activity

G protein-coupled receptors (GPCR), including the metabotrobic glutamate 5 receptor (mGlu₅), are important therapeutic targets and the development of allosteric ligands for targeting GPCRs has become a desirable approach toward modulating receptor activity. Traditional pharmacological approaches toward modulating GPCR activity are still limited since precise spatiotemporal control of a ligand is lost as soon as it is administered. Photopharmacology proposes the use of photoswitchable ligands to overcome this limitation, since their activity can be reversibly controlled by light with high precision. As this is still a growing field, our understanding of the molecular mechanisms underlying the light-induced changes of different photoswitchable ligand pharmacology is suboptimal. For this reason, we have studied the mechanisms of action of alloswitch-1 and MCS0331; two freely diffusible, mGlu₅ phenylazopyridine photoswitchable negative allosteric modulators. We combined photochemical, cell-based, and in vivo photopharmacological approaches to investigate the effects of trans-cis azobenzene photoisomerization on the functional activity and binding ability of these ligands to the mGlu₅ allosteric pocket. From these results, we conclude that photoisomerization can take place inside and outside the ligand binding pocket, and this leads to a reversible loss in affinity, in part, due to changes in dissociation rates from the receptor. Ligand activity for both photoswitchable ligands deviates from high-affinity mGlu₅ negative allosteric modulation (in the trans configuration) to reduced affinity for the mGlu₅ in their cis configuration. Importantly, this mechanism translates to dynamic and reversible control over pain following local injection and illumination of negative allosteric modulators into a brain region implicated in pain control.