Although many complex environmental and social factors contribute to the development of heroin addiction, increasing evidence suggests that there is also genetic component. The mu opioid receptor has been implicated because of its ability to bind addictive opioids. This lab has identified two single nucleotide polymorphisms (SNPs) in this receptor that correlate with heroin addiction and are common in the population. An A to G transition at base pair 118 (A118G) may confer a protective effects against addiction in one of the ethnic groups studied, while a C to T transition at base pair 17 (C17T) may predispose patients to the development of heroin addiction. The A118G SNP has been shown to alter binding affinity to the endogenous ligand beta-endorphin, and reduce G-protein coupled Inwardly Rectifying K+ (GIRK) channel activation. This project will determine if these SNPs have altered binding affinity to clinically relevant opioids and other endogenous peptide agonists. Electrophysiological studies will focus on the functional conseque4nces of these SNPs on channel function: effects on GIRK channel activation will be performed in Xenopus oocytes, while effects on N-type Ca2+ channel inhibition will be studied in the NG108-15 cell line. Changes in channel function or activation may indicate alterations in cellular response to opioids that could affect a patient's susceptibility to addiction. Dose response curves for each agonist will be generated for each polymorphism, and these will be compared to those of the prototype receptor. Cyclic AMP levels will be measured in transfected NG108-15 cells to determine the effects of each SNP on tolerance and the response to sudden morphine withdrawal.
