The μ opiate receptor as a candidate gene for pain:: Polymorphisms, variations in expression, nociception, and opiate responses

There are differences between human individuals and between mouse strains in levels of mu opiate receptor (mu OR) expression, responses to painful stimuli, and responses to opiate drugs. One of the best candidates for contributing to these differences is variation at the mu OR gene locus. Support for this idea comes from analyses of the human and murine mu OR genes. Assessments of individual differences in human mu OR expression add further support. Studies with mice, including knockout-transgenic, quantitative trait locus, and, strain-comparison studies, also strongly support the possibility that mu OR gene alleles would be strong candidates for contributing to individual differences in human nociception and opiate drug responses. This paper reviews current analyses of the murine and human mu OR genes, their important variants, and correlations between these variants and opiate influences on pain.