GRM8 and heroin dependence: Although there are, at present, very few studies have investigated potential roles of these type III metabotropic receptors in animal models of addiction, our data are consistent with those of Nielsen et al. (2008) who were able to provide evidence that metabotropic receptors, mGluR6 and mGluR8, were correlated with the risk of developing heroin addiction in a genome-wide association study of 110 heroin addicted individuals.