| Opioid drugs play important roles in the clinical management of pain, as well as in the development and treatment of drug abuse. The mu opioid receptor (MOR) is the primary site of action for the most commonly used opioids, including morphine, heroin, fentanyl, and methadone. Individuals vary in their responses to drug treatments and in their susceptibility to the development of drug dependence. It is possible that functional variation in the MOR contributes to the underlying cause of these inter-individual differences. A total of 173 DNA samples from former heroin addicts in methadone maintenance treatment and control individuals with no history of drug or alcohol abuse were examined for sequence polymorphism in the coding region of the human mu opioid receptor gene. Five single-nucleotide variations were identified, three of which were predicted to result in an amino acid change in the receptor protein. The most prevalent polymorphic receptors, C17T and A118G, had allele frequencies of 6.6 and 10.5% respectively. Significant differences in allele frequency distribution were observed among the ethnic groups studied. The C17T variant was present in a higher overall proportion of opioid-dependent persons in the study group (p = 0.05). The variants resulting in amino acid changes in the receptor protein, C17T, A118G, and G779A, were recreated by site-directed mutagenesis of the cloned wild-type MOR. The mutant receptor cDNA was expressed in a mammalian cell line for pharmacological characterization and analysis of functional coupling to adenylyl cyclase. In addition, the A118G MOR was expressed in Xenopus oocytes to examine functional coupling of the receptor to an inwardly rectifying potassium channel (GIRK). Each of the receptor variants exhibited subtle alterations in ligand binding compared with the wild-type receptor. In addition, differences were observed between variant and wild-type receptors in their abilities to inhibit adenylyl cyclase activity and stimulate activation of GIRK. These results demonstrate that single nucleotide changes in the MOR can alter binding and signal transduction in the resulting receptor and may have implications for normal physiology and pain therapeutics, as well as for the development of opioid dependence. |
