The opioid receptor kappa 1 (OPRK1) gene, located on chromosome 8q11.2 [78], codes for the κ opioid receptor and has been implicated in the regulation of stress responses, as well as counteracting the euphoric sensations produced by μ opioid receptor agonists [59]. Activation of the κ receptor by endogenous ligand dynorphin A has been found to decrease the dopamine release in the striatum [79], however there is inconclusive evidence to support an association of OPRK1 SNPs with opioid dependence and the results varied greatly with different ethnic populations.
