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Mu-opioid receptors (MOPRs) are the target of heroin and other prescription opioids, which are currently responsible for massive addiction morbidity in the US. The gene coding for the human MOPR (OPRM1) has an important functional single nucleotide polymorphism (SNP), A118G. The OPRM1 A118G genotype results in substantially increased risk of heroin addiction in humans; however, the neurobiological mechanism for this increased risk is not fully understood. This study examined heroin self-administration (SA) behavior in A112G (G/G) mice, harboring a functionally equivalent SNP in Oprm1 with a similar amino acid substitution, in extended (4 h) SA sessions. Adult male and female G/G mice and 'wild-type' litter mates (A/A) were allowed to self-administer heroin (0.25 mg/kg/unit dose, FR1 with a nose poke response) for 4 h/day, for 10 consecutive days. Half of the mice then continued in a heroin dose-response study, while extinction from heroin SA was studied in the other half. In vivo microdialysis was used to measure acute heroin-induced increases of striatal dopamine in the GG vs AA genotypes. Male and female G/G mice responded for heroin significantly more (and thus had greater intake) than A/A mice, in the initial 10 days of heroin SA, and in the subsequent dose-response study. There were no significant differences in extinction of SA between the A/A and G/G mice. Heroin-induced increases in striatal dopamine levels are higher in the GG mice than in the AA mice. Both male and female G/G mice self-administered more heroin than did A/A mice over a 10-day period, possibly because of the greater increases of heroin-induced striatal dopamine in the GG mice. Furthermore, G/G male mice escalated the amount of heroin self-administration across 10 extended-access sessions more than A/A male mice did. These are the first studies to examine the acquisition of heroin SA in this mouse model. These studies may lead to a better understanding of the neurobiological and behavioral mechanisms that underlie greater risk of heroin addiction in carriers of the A118G SNP. [ABSTRACT FROM AUTHOR]

Dynorphin peptides and the κ-opioid receptor are important in the rewarding properties of cocaine, heroin, and alcohol. We tested polymorphisms of the prodynorphin gene (PDYN) for association with cocaine dependence and cocaine/alcohol codependence. We genotyped six single nucleotide polymorphisms (SNPs), located in the promoter region, exon 4 coding, and 3′ untranslated region, in 106 Caucasians and 204 African Americans who were cocaine dependent, cocaine/alcohol codependent, or controls. In Caucasians, we found point-wise significant associations of 3′UTR SNPs (rs910080, rs910079, and rs2235749) with cocaine dependence and cocaine/alcohol codependence. These SNPs are in high linkage disequilibrium, comprising a haplotype block. The haplotype CCT was significantly experiment-wise associated with cocaine dependence and with combined cocaine dependence and cocaine/alcohol codependence (false discovery rate, q=0.04 and 0.03, respectively). We investigated allele-specific gene expression of PDYN, using SNP rs910079 as a reporter, in postmortem human brains from eight heterozygous subjects, using SNaPshot assay. There was significantly lower expression for C allele (rs910079), with ratios ranging from 0.48 to 0.78, indicating lower expression of the CCT haplotype of PDYN in both the caudate and nucleus accumbens. Analysis of total PDYN expression in 43 postmortem brains also showed significantly lower levels of preprodynorphin mRNA in subjects having the risk CCT haplotype. This study provides evidence that a 3′UTR PDYN haplotype, implicated in vulnerability to develop cocaine addiction and/or cocaine/alcohol codependence, is related to lower mRNA expression of the PDYN gene in human dorsal and ventral striatum.Neuropsychopharmacology (2009) 34, 1185–1197; doi:10.1038/npp.2008.187; published online 15 October 2008 [ABSTRACT FROM AUTHOR]

Nonmedical use of the prescription opioid analgesic oxycodone is a major problem in the United States, particularly among adolescents and young adults. This study characterized self-administration of oxycodone by adolescent and adult mice, and how this affects striatal dopamine levels. Male C57BL/6J mice (4 or 10 weeks old) were allowed to acquire oxycodone self-administration (0.25 mg/kg per infusion) for 9 days, and then tested with varying doses of oxycodone (0, 0.125, 0.25, 0.5, and 0.75 mg/kg per infusion). On completion of the self-administration study, a guide cannula was implanted into the striatum of these mice. Six days later, microdialysis was conducted on the freely moving mouse. After collection of baseline samples, oxycodone was administered i.p. (1.25, 2.5, and 5.0 mg/kg) and samples were collected for 1 h after each dose. Adult mice self-administered significantly more oxycodone across the doses tested. After 1 week, basal striatal dopamine levels were lower in mice of both ages that had self-administered oxycodone than in yoked saline controls. Oxycodone challenge increased striatal dopamine levels in a dose-dependent manner in both age groups. Of interest, the lowest dose of oxycodone led to increased striatal dopamine levels in the mice that had self-administered oxycodone during adolescence but not those that self-administered it as adults. The lower number of infusions of oxycodone self-administered by adolescent mice, and their later increased striatal dopamine in response to the lowest dose of oxycodone (not found in adults), suggest differential sensitivity to the reinforcing and neurobiological effects of oxycodone in the younger mice.Neuropsychopharmacology (2009) 34, 912–922; doi:10.1038/npp.2008.134; published online 10 September 2008 [ABSTRACT FROM AUTHOR]

The μ-opioid receptor is the site of action of opiates and opioids. We examined whether there are differences in cytosine:guanine (CpG) dinucleotide methylation in the OPRM1 promoter between former heroin addicts and controls. We analyzed methylation at 16 CpG dinucleotides in DNA obtained from lymphocytes of 194 Caucasian former severe heroin addicts stabilized in methadone maintenance treatment and 135 Caucasian control subjects. Direct sequencing of bisulfite-treated DNA showed that the percent methylation at two CpG sites was significantly associated with heroin addiction. The level of methylation at the −18 CpG site was 25.4% in the stabilized methadone-maintained former heroin addicts and 21.4% in controls (p=0.0035, generalized estimating equations (GEE); p=0.0077, t-test; false discovery rate (FDR)=0.048), and the level of methylation at the +84 CpG dinucleotide site was 7.4% in cases and 5.6% in controls (p=0.0095, GEE; p=0.0067, t-test; FDR=0.080). Both the −18 and the +84 CpG sites are located in potential Sp1 transcription factor-binding sites. Methylation of these CpG sites may lead to reduced OPRM1 expression in the lymphocytes of these former heroin addicts.Neuropsychopharmacology (2009) 34, 867–873; doi:10.1038/npp.2008.108; published online 23 July 2008 [ABSTRACT FROM AUTHOR]