Dept. of Behavioral Neuroscience
Oregon Health & Science University
Methamphetamine (MA) is a powerful psychostimulant and its excessive use is linked to neurotoxicity and neuropsychiatric disorders. However, not all initial users develop drug use disorders and it is possible that genetic differences render some individuals more susceptible to the addictive properties of MA compared to others. Genetic differences in avidity for MA have been studied using two replicate sets of selectively bred MA drinking (MADR) mouse lines that voluntarily consume either high (MAHDR) or low (MALDR) amounts of MA. Selective breeding alters allele frequencies; thus, alleles that increase MA drinking (MADR) have aggregated in the MAHDR line mice, whereas alleles that reduce MA intake have aggregated in the MALDR line mice. A gene mapping study identified a major effect genetic locus on mouse chromosome (Chr) 10 that accounts for more than 50% of the genetic variance associated with this differential MA intake. Oprm1 lies within the mapped region and previous gene expression analysis added support for Oprm1 as a candidate gene on Chr 10 that influences MA drinking. Based on published basic and human clinical data and preliminary data obtained within our laboratory, I hypothesized that Oprm1 genetic variation and mu-opioid receptor (MOP-r)-regulated systems are important in influencing MA intake. The first goal of this project was to examine potential differences in sensitivity to MOP-r-mediated effects in MADR mice. It was hypothesized that MOP-rs may be involved in the differences in MA intake between the MADR lines, and this might be reflected in a difference in MOP-r sensitivity and avidity. Sensitivity to the locomotor stimulant effects of the MOP-r agonist drugs morphine (MOR) and fentanyl (FENT) was measured and avidity for MOR was evaluated in a two-bottle choice MA drinking procedure. Sensitivity to the analgesic effects of MOP-r drugs was assessed using hot plate, tail flick, and the magnesium-sulfate-induced writhing test. In addition, MOP-r density and affinity were assessed between the MADR lines and also between C57BL/6J (B6) and DBA/2J (D2) strain mice, which were the founding strains for the selected lines. Opioid pharmacokinetics were also evaluated. No differences between the lines were detected for sensitivity to the analgesic effects of MOP-r drugs, but MALDR mice had greater sensitivity to the locomotor stimulant effects of MOP-r agonist drugs, and consumed more MOR, compared to MAHDR mice. These data suggested that a negative genetic correlation exists between sensitivity to MOP-r agonist drugs and MA intake and also between MOP-r agonist intake and MA intake. In addition, MALDR mice had greater MOP-r density in the medial prefrontal cortex (mPFC), but not nucleus accumbens or ventral midbrain, compared to MAHDR mice. These data are consistent with the difference in Oprm1 gene expression previously identified in the mPFC, but not the other two brain regions, and support my hypothesis that MOP-r-regulated effects may be involved in MA intake in MADR mice. Based on differences in response to MOP-r drugs, the second goal of this proposal sought to examine the efficacy of MOP-r drugs to alter MA intake and drinking patterns. These studies administered either MOP-r agonist or antagonist drugs in a limited access two-bottle choice MA drinking procedure. It was hypothesized that MAHDR mice, which in comparison to MALDR mice, had lower expression of Oprm1 in the mPFC, less MOP-r agonist-induced acute locomotor stimulation, and consumed less MOR, would more closely resemble MALDR mice for MA intake, when given a MOP-r agonist prior to MA drinking sessions. Some doses of the partial MOP-r agonist drug, buprenorphine, and the full agonist MOP-r drugs, MOR and FENT, reduced MA intake and altered drinking patterns in MAHDR mice. However MOR and FENT also reduced total volume consumed, suggesting that MOP-r agonist drugs may have induced a behavioral response that impeded drinking behavior. The MOP-r antagonist drug naltrexone did not alter MA intake. These data partially supported my hypothesis that MOP-r agonist drugs could alter MA intake and drinking patterns. The final goals of this project were to verify the existence of the Chr 10 QTL for MA consumption using a more isogenic background and to gain better mapping resolution of the Chr 10 QTL. This aim was addressed using congenic strains of mice, which were created from B6 and D2 inbred mouse strains, the progenitor strains of the selected lines. One congenic strain had a B6 segment that spanned Chr 10 0-7.72 Mb (the region between 7.58 and 7.72 Mb is of unknown genotype due to marker spacing and thus may or may not be of B6 origin), whereas the other had a B6 segment that spanned Chr 10 0-20.4 Mb (the region between 18.8 and 20.4 Mb is of unknown genotype due to marker spacing and thus may or may not be of B6 origin), both of which carried the B6 allele for Oprm1, located at Chr 10 6.75 Mb. Genotyping data used to detect the QTL for MA intake on Chr 10 had demonstrated that D2 alleles were associated with higher MA intake. I predicted that MA intake would be reduced in both congenic strains, compared to the D2 background strain. Contrary to my hypothesis, only one congenic strain (Chr 10 0-20.4 Mb) had decreased MA intake, compared to the D2 strain. These data indicate that genes proximal to the 7.58 Mb location on Chr 10 may be eliminated from consideration as quantitative trait genes influencing MA intake on Chr 10 and that a gene(s) that resides in the non-overlapping, up to 12.86 Mb segment (Chr 10 7.58-20.4 Mb) likely contributes to the genetic variation in MA intake between the MADR lines of mice. Though these data exclude Oprm1 from consideration as a QTG, overall, the data support a genetic correlation between MOP-r density in the mPFC and MA consumption in the MADR lines of mice.
School of Medicine
Eastwood, Emily C., "Opioid pathways in methamphetamine intake" (2015). Scholar Archive. 3656.