Genetic Effects on Dopamine Response to an Opiate

Objectives Mesolimbic dopamine (DA) release is a key signal for drug reward, and endogenous opioids are thought to exert their effects in part by modulating the activity of this system. A functional µ-opioid receptor (OPRM1) A118G single nucleotide polymorphism (SNP) has been associated with increased risk for heroin addiction in some studies. This polymorphism has been shown to confer differential pain sensitivity and to alter the release of DA following an alcohol challenge. The objective of this study is to examine the role of the A118G OPRM1 polymorphism for responses to a challenge of an opiate (morphine) with regard to psycho-physiological variables measured in the laboratory and for brain dopamine release measured by \[11C\]raclopride PET. Study Population Healthy male participants who have had experience with oral prescription analgesics (e.g., Oxycontin, Vicodin, Percocet, oxycodone) will be recruited for the study. These volunteers will be screened to obtain samples of two groups of subjects: 1) persons homozygous for the major 118A allele (118AA genotype); 2) persons carrying one or two copies of the variant 118G allele (118AG or 118GG genotype, hereafter called 118GX). We will recruit up to 120 participants to obtain 40 completers per genotype for the study. Design We will compare the response of these groups to a challenge with morphine given intravenously. Participants will receive a standardized IV challenge of morphine (10.0 mg/70 kg over 1 minute; morphine concentration 2 mg/ml). Pre and post injection measures will be made in two areas: 1) subjective response as measured by standardized questionnaires, and 2) measures of physiological response, including pupil response to light, respiratory rate, oxygen saturation and a pain rating from putting a hand in cold water and blood chemistries. In addition, during this visit, participants will wear the AutoSense mobile physiological monitor; parameters measured by AutoSense include respiration rate, heart rate, heart-rate variability, skin conductance, and activity level. The injection will be repeated in all participants in the PET scanner, once with morphine and once with normal saline. Dopamine release will be assessed by determining the difference between the binding potential for \[11C\]raclopride, a positron emitter labeled ligand which binds preferentially to D2 receptors during saline administration and its binding potential during morphine administration. Outcome measures We hypothesize that 118GX subjects will have significantly different subjective response to the challenge than 118AA subjects as observed in participants receiving alcohol in a similar study (Ramchandani et al. 2011). However, the genotype effect on the response may be opposite from the effect of genotype on the alcohol response. We also hypothesize that the PET studies with \[11C\]raclopride will show that 118GX subjects have less dopamine release during morphine administration than 118 AA subjects.