Region-specific differences in MOPR levels between A118 and G118 have also been demonstrated in humans. In a recent positron emission tomography study using [11C]carfentanil as the MOPR ligand, Ray et al. (2011) reported that smokers heterozygous for the G118 allele had lower levels of MOPR-binding potential in the amygdala, thalamus, and anterior cingulate cortex than those homozygous for the A118 allele, but similar MOPR levels were observed in the ventral striatum/NAc and caudate.
