Since pain is a complex bio-psychosocial phenomenon, the task of searching for and finding single gene variations to explain individual differences could be seen as futile. However, in this study, the focus is on three genes, all of which encode proteins related to the pain-relieving effects of opioids, and on the endogenous opioid peptide β-endorphin. The search for candidate genes containing SNPs possibly associated with pain and opioid sensitivity resulted in the choice of the μ-opioid peptide receptor (OPRM1) gene [10-12], the ATP-binding cassette B1 (ABCB1) gene [13,14] and the calcium channel complex subunit (CACNA2D2) gene [8]. In the present study, the occurrence of SNPs in these genes was correlated with levels of circulating β-endorphin and clinical data from a well-characterized patient group with chronic low-back pain and differential sensitivity to the opioid remifentanil. The patients were classified as high responders, normal responders or non-responders to remifentanil by an intravenous opioid testing procedure (for details see Methods).
