In response to evidence of an association between the rs1799971 SNP and opioid dependence, studies attempted to investigate the potential functional effects of the rs1799971 variant on cellular activity and binding affinity. Such functions were tested in cells expressing the most common μ receptor and the rs1799971 variant [73]. Bond et al [73] found that the rs1799971 receptor variant showed similar binding affinities to small endogenous opioid peptide agonists and exogenous agonists, suggesting that the rs1799971 SNP does not significantly alter the binding properties of the μ receptor [73]. However, the reaction between the rs1799971 receptor and human β -endorphin displayed a high binding affinity, which was represented by an association constant ratio of 3.46 ± 0.31 [73]. Bond et al [73] confirmed this association by measuring the potency of β-endorphin as an rs1799971 agonist and found that it was able to induce a greater response at a lower concentration when compared to the common μ receptor. Using different methods, a study by Kroslak et al [71] found no changes in β-endorphin signaling at the rs1799971 μ receptor, and also
