The mGlu5 receptor regulates extinction of cocaine-driven behaviours

Background: There is extensive evidence implicating the metabotropic glutamate 5 (mGlu5) receptor in aspects of addiction-related behaviours. Methods: Here, we used a well-characterized line of mGlu5-deficient mice to further examine the role of this receptor in cocaine-driven behaviours. We confirmed the previously reported deficit in hippocampal long-term potentiation and associated spatial learning impairment. Results: Despite a spatial learning deficit, mGlu5-deficient mice developed and maintained a conditioned place preference to cocaine, suggesting cocaine reward and Pavlovian conditioning are intact in these animals. Notably, however, mGlu5-deficient mice exhibited a marked deficit in the extinction of a cocaine-conditioned place preference compared to wild type littermates. Moreover, in a fixed ratio operant intravenous self-administration paradigm, both genotypes showed similar responding for cocaine over two different doses, while mGlu5-deficient mice displayed enhanced responding on a progressive ratio schedule. In addition, cue-induced drug-seeking after abstinence was exaggerated in mGlu5-deficient mice. Conclusion: Collectively, these findings suggest that while the mGlu5 receptor may be involved in mediating the rewarding effects of cocaine, it appears necessary for the extinction of cocaine-driven behaviours.