Unique genetic factors influence sensitivity to the rewarding and aversive effects of methamphetamine versus cocaine

Impact of the Aversive Effects of Drugs on Their Use and Abuse

Drug use and abuse are complex issues in that the basis of each may involve different determinants and consequences, and the transition from one to the other may be equally multifaceted. A recent model of the addiction cycle (as proposed by Koob and his colleagues) illustrates how drug-taking patterns transition from impulsive (acute use) to compulsive (chronic use) as a function of various neuroadaptations leading to the downregulation of DA systems, upregulation of stress systems, and the dysregulation of the prefrontal/orbitofrontal cortex. Although the nature of reinforcement in the initiation and mediation of these effects may differ (positive vs. negative), the role of reinforcement in drug intake (acute and chronic) is well characterized. However, drugs of abuse have other stimulus properties that may be important in their use and abuse. One such property is their aversive effects that limit drug intake instead of initiating and maintaining it. Evidence of such effects comes from both clinical and preclinical populations. In support of this position, the present review describes the aversive effects of drugs (assessed primarily in conditioned taste aversion learning), the fact that they occur concurrently with reward as assessed in combined taste aversion/place preference designs, the role of aversive effects in drug-taking (in balance with their rewarding effects), the dissociation of these affective properties in that they can be affected in different ways by the same manipulations, and the impact of various parametric, experiential, and subject factors on the aversive effects of drugs and the consequent impact of these factors on their use and abuse potential.

Confirmation of a Causal Taar1 Allelic Variant in Addiction-Relevant Methamphetamine Behaviors

Sensitivity to rewarding and reinforcing drug effects has a critical role in initial use, but the role of initial aversive drug effects has received less attention. Methamphetamine effects on dopamine re-uptake and efflux are associated with its addiction potential. However, methamphetamine also serves as a substrate for the trace amine-associated receptor 1 (TAAR1). Growing evidence in animal models indicates that increasing TAAR1 function reduces drug self-administration and intake. We previously determined that a non-synonymous single nucleotide polymorphism (SNP) in Taar1 predicts a conformational change in the receptor that has functional consequences. A Taar1 m1J mutant allele existing in DBA/2J mice expresses a non-functional receptor. In comparison to mice that possess one or more copies of the reference Taar1 allele (Taar1 +/+ or Taar1 +/m1J ), mice with the Taar1 m1J/m1J genotype readily consume methamphetamine, express low sensitivity to aversive effects of methamphetamine, and lack sensitivity to acute methamphetamine-induced hypothermia. We used three sets of knock-in and control mice in which one Taar1 allele was exchanged with the alternative allele to determine if other methamphetamine-related traits and an opioid trait are impacted by the same Taar1 SNP proven to affect MA consumption and hypothermia. First, we measured sensitivity to conditioned rewarding and aversive effects of methamphetamine to determine if an impact of the Taar1 SNP on these traits could be proven. Next, we used multiple genetic backgrounds to study the consistency of Taar1 allelic effects on methamphetamine intake and hypothermia. Finally, we studied morphine-induced hypothermia to confirm prior data suggesting that a gene in linkage disequilibrium with Taar1, rather than Taar1, accounts for prior observed differences in sensitivity. We found that a single SNP exchange reduced sensitivity to methamphetamine conditioned reward and increased sensitivity to conditioned aversion. Profound differences in methamphetamine intake and hypothermia consistently corresponded with genotype at the SNP location, with only slight variation in magnitude across genetic backgrounds. Morphine-induced hypothermia was not dependent on Taar1 genotype. Thus, Taar1 genotype and TAAR1 function impact multiple methamphetamine-related effects that likely predict the potential for methamphetamine use. These data support further investigation of their potential roles in risk for methamphetamine addiction and therapeutic development.

A breeding strategy to identify modifiers of high genetic risk for methamphetamine intake

Trace amine-associated receptor 1 (Taar1) impacts methamphetamine (MA) intake. A mutant allele (Taar1m1J ) derived from the DBA/2J mouse strain codes for a non-functional receptor, and Taar1m1J/m1J mice consume more MA than mice possessing the reference Taar1+ allele. To study the impact of this mutation in a genetically diverse population, heterogeneous stock-collaborative cross (HS-CC) mice, the product of an eight-way cross of standard and wild-derived strains, were tested for MA intake. HS-CC had low MA intake, so an HS-CC by DBA/2J strain F2 intercross was created to transfer the mutant allele onto the diverse background, and used for selective breeding. To study residual variation in MA intake existing in Taar1m1J/m1J mice, selective breeding for higher (MAH) vs lower (MAL) MA intake was initiated from Taar1m1J/m1J F2 individuals; a control line of Taar1+/+ individuals (MAC) was retained. The lines were also examined for MA-induced locomotor and thermal responses, and fluid and tastant consumption. Taar1m1J/m1J F2 mice consumed significantly more MA than Taar1+/+ F2 mice. Response to selection was significant by generation 2 and there were corresponding differences in fluid consumed. Fluid consumption was not different in non-MA drinking studies. Taar1m1J/m1J genotype (MAL or MAH vs MAC mice) was associated with heighted MA locomotor and reduced hypothermic responses. MAL mice exhibited greater sensitization than MAH mice, but the selected lines did not consistently differ for thermal or tastant phenotypes. Residual variation among high-risk Taar1m1J/m1J mice appears to involve mechanisms associated with neuroadaptation to MA, but not sensitivity to hypothermic effects of MA.

Increased alcohol preference and intake in nicotine-preferring rats

BACKGROUND

Alcohol and tobacco are among the leading substances that are misused together and shared genetic vulnerability is likely. Increased susceptibility to nicotine self-administration has been shown in alcohol-preferring rat-lines. However, a nicotine-preferring (nP) rat-line has not been studied for alcohol preference.

OBJECTIVES

To evaluate alcohol preference and intake in male and female nP rats. We hypothesized that nP rats and females would drink more ethanol than control rats and males, respectively.

METHODS

nP rats are being selectively outbred for high oral nicotine intake at Ege University. Seventeen nP (18^th generation) and 20 naïve female and male SD rats, not previously exposed to alcohol or nicotine, were used. Twelve-week-old rats were given intermittent access to 20% ethanol in a 2-bottle-choice-procedure for six weeks. After one week withdrawal, six weeks of oral nicotine self-administration was applied.

RESULTS

nP rats drank significantly more ethanol than controls and their preference for ethanol over water was higher. Female rats' ethanol intake was higher than males'. The nP rats' nicotine preference and intake were higher than controls, and they gained less weight.

CONCLUSION

We have shown for the first time that nP rats also have high alcohol intake. Our results support the hypothesis that shared genetic factors may underlie concurrent addiction to nicotine and alcohol and have translational value in understanding their misuse. Considering the increased vulnerability for alcohol use disorder in smokers and sex differences observed, early preventive measures in families with a history of tobacco addiction, specifically targeting female members, could have public health benefits.

Differential genetic risk for methamphetamine intake confers differential sensitivity to the temperature-altering effects of other addictive drugs

Mice selectively bred for high methamphetamine (MA) drinking (MAHDR), compared with mice bred for low MA drinking (MALDR), exhibit greater sensitivity to MA reward and insensitivity to aversive and hypothermic effects of MA. Previous work identified the trace amine-associated receptor 1 gene (Taar1) as a quantitative trait gene for MA intake that also impacts thermal response to MA. All MAHDR mice are homozygous for the mutant Taar1 m1J allele, whereas all MALDR mice possess at least one copy of the reference Taar1 + allele. To determine if their differential sensitivity to MA-induced hypothermia extends to drugs of similar and different classes, we examined sensitivity to the hypothermic effect of the stimulant cocaine, the amphetamine-like substance 3,4-methylenedioxymethamphetamine (MDMA), and the opioid morphine in these lines. The lines did not differ in thermal response to cocaine, only MALDR mice exhibited a hypothermic response to MDMA, and MAHDR mice were more sensitive to the hypothermic effect of morphine than MALDR mice. We speculated that the μ-opioid receptor gene (Oprm1) impacts morphine response, and genotyped the mice tested for morphine-induced hypothermia. We report genetic linkage between Taar1 and Oprm1; MAHDR mice more often inherit the Oprm1 D2 allele and MALDR mice more often inherit the Oprm1 B6 allele. Data from a family of recombinant inbred mouse strains support the influence of Oprm1 genotype, but not Taar1 genotype, on thermal response to morphine. These results nominate Oprm1 as a genetic risk factor for morphine-induced hypothermia, and provide additional evidence for a connection between drug preference and drug thermal response.

Verification of a genetic locus for methamphetamine intake and the impact of morphine

A quantitative trait locus (QTL) on proximal chromosome (Chr) 10 accounts for > 50% of the genetic variance in methamphetamine (MA) intake in mice selectively bred for high (MAHDR) and low (MALDR) voluntary MA drinking. The µ-opioid receptor (MOP-r) gene, Oprm1, resides at the proximal end of Chr 10, and buprenorphine reduces MA intake in MAHDR mice. However, this drug has only partial agonist effects at MOP-r. We investigated the impact of a full MOP-r agonist, morphine, on MA intake and saccharin intake, measured MOP-r density and affinity in several brain regions of the MA drinking lines and their C57BL/6J (B6) and DBA/2J (D2) progenitor strains, and measured MA intake in two congenic strains of mice to verify the QTL and reduce the QTL interval. Morphine reduced MA intake in the MAHDR line, but also reduced saccharin and total fluid intake. MOP-r density was lower in the medial prefrontal cortex of MAHDR, compared to MALDR, mice, but not in the nucleus accumbens or ventral midbrain; there were no MOP-r affinity differences. No significant differences in MOP-r density or affinity were found between the progenitor strains. Finally, Chr 10 congenic results were consistent with previous data suggesting that Oprm1 is not a quantitative trait gene, but is impacted by the gene network underlying MA intake. Stimulation of opioid pathways by a full agonist can reduce MA intake, but may also non-specifically affect consummatory behavior; thus, a partial agonist may be a better pharmacotherapeutic.

Sex as a biological variable: Drug use and abuse

The study of sex as a biological variable is a necessary emphasis across a wide array of endpoints, including basic neuroscience, medicine, mental health, physiology and behavior. The present review summarizes work from clinical and preclinical populations on sex differences in drug use and abuse, ranging from initiation to escalation/dysregulation and from drug cessation/abstinence to relapse. These differences are analyzed in the context of the addiction cycle conceptualization of Koob and his colleagues and address patterns of drug use (binge/intoxication), motivation underlying its use (withdrawal/negative affect) and likelihood and causes of craving and relapse of drug taking (preoccupation/anticipation). Following this overview, an assessment of the basis for the reported sex differences is discussed in the context of the affective (rewarding and aversive) properties of drugs of abuse and how such properties and their balance vary with sex and contribute to drug intake. Finally, the interaction of sex with several experiential (drug history) and subject (age) factors and how these interactions affect reward and aversion are discussed to highlight the importance of understanding such interactions in predicting drug use and abuse. We note that sex as a biological variable remains one of critical evaluation and that such investigations of sex differences in drug use and abuse continue and be expanded to assess all facets of their mediation, including these affective properties, how their balance may be impacted by the multiple conditions under which drugs are taken and how this overall balance affects drug use and addiction vulnerability.

Identification of Treatment Targets in a Genetic Mouse Model of Voluntary Methamphetamine Drinking

Methamphetamine has powerful stimulant and euphoric effects that are experienced as rewarding and encourage use. Methamphetamine addiction is associated with debilitating illnesses, destroyed relationships, child neglect, violence, and crime; but after many years of research, broadly effective medications have not been identified. Individual differences that may impact not only risk for developing a methamphetamine use disorder but also affect treatment response have not been fully considered. Human studies have identified candidate genes that may be relevant, but lack of control over drug history, the common use or coabuse of multiple addictive drugs, and restrictions on the types of data that can be collected in humans are barriers to progress. To overcome some of these issues, a genetic animal model comprised of lines of mice selectively bred for high and low voluntary methamphetamine intake was developed to identify risk and protective alleles for methamphetamine consumption, and identify therapeutic targets. The mu opioid receptor gene was supported as a target for genes within a top-ranked transcription factor network associated with level of methamphetamine intake. In addition, mice that consume high levels of methamphetamine were found to possess a nonfunctional form of the trace amine-associated receptor 1 (TAAR1). The Taar1 gene is within a mouse chromosome 10 quantitative trait locus for methamphetamine consumption, and TAAR1 function determines sensitivity to aversive effects of methamphetamine that may curb intake. The genes, gene interaction partners, and protein products identified in this genetic mouse model represent treatment target candidates for methamphetamine addiction.

Prefrontal glutamate correlates of methamphetamine sensitization and preference

Methamphetamine (MA) is a widely misused, highly addictive psychostimulant that elicits pronounced deficits in neurocognitive function related to hypo-functioning of the prefrontal cortex (PFC). Our understanding of how repeated MA impacts excitatory glutamatergic transmission within the PFC is limited, as is information about the relationship between PFC glutamate and addiction vulnerability/resiliency. In vivo microdialysis and immunoblotting studies characterized the effects of MA (ten injections of 2 mg/kg, i.p.) upon extracellular glutamate in C57BL/6J mice and upon glutamate receptor and transporter expression, within the medial PFC. Glutamatergic correlates of both genetic and idiopathic variance in MA preference/intake were determined through studies of high vs. low MA-drinking selectively bred mouse lines (MAHDR vs. MALDR, respectively) and inbred C57BL/6J mice exhibiting spontaneously divergent place-conditioning phenotypes. Repeated MA sensitized drug-induced glutamate release and lowered indices of N-methyl-d-aspartate receptor expression in C57BL/6J mice, but did not alter basal extracellular glutamate content or total protein expression of Homer proteins, or metabotropic or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors. Elevated basal glutamate, blunted MA-induced glutamate release and ERK activation, as well as reduced protein expression of mGlu2/3 and Homer2a/b were all correlated biochemical traits of selection for high vs. low MA drinking, and Homer2a/b levels were inversely correlated with the motivational valence of MA in C57BL/6J mice. These data provide novel evidence that repeated, low-dose MA is sufficient to perturb pre- and post-synaptic aspects of glutamate transmission within the medial PFC and that glutamate anomalies within this region may contribute to both genetic and idiopathic variance in MA addiction vulnerability/resiliency.

An animal model of differential genetic risk for methamphetamine intake

The question of whether genetic factors contribute to risk for methamphetamine (MA) use and dependence has not been intensively investigated. Compared to human populations, genetic animal models offer the advantages of control over genetic family history and drug exposure. Using selective breeding, we created lines of mice that differ in genetic risk for voluntary MA intake and identified the chromosomal addresses of contributory genes. A quantitative trait locus was identified on chromosome 10 that accounts for more than 50% of the genetic variance in MA intake in the selected mouse lines. In addition, behavioral and physiological screening identified differences corresponding with risk for MA intake that have generated hypotheses that are testable in humans. Heightened sensitivity to aversive and certain physiological effects of MA, such as MA-induced reduction in body temperature, are hallmarks of mice bred for low MA intake. Furthermore, unlike MA-avoiding mice, MA-preferring mice are sensitive to rewarding and reinforcing MA effects, and to MA-induced increases in brain extracellular dopamine levels. Gene expression analyses implicate the importance of a network enriched in transcription factor genes, some of which regulate the mu opioid receptor gene, Oprm1, in risk for MA use. Neuroimmune factors appear to play a role in differential response to MA between the mice bred for high and low intake. In addition, chromosome 10 candidate gene studies provide strong support for a trace amine-associated receptor 1 gene, Taar1, polymorphism in risk for MA intake. MA is a trace amine-associated receptor 1 (TAAR1) agonist, and a non-functional Taar1 allele segregates with high MA consumption. Thus, reduced TAAR1 function has the potential to increase risk for MA use. Overall, existing findings support the MA drinking lines as a powerful model for identifying genetic factors involved in determining risk for harmful MA use. Future directions include the development of a binge model of MA intake, examining the effect of withdrawal from chronic MA on MA intake, and studying potential Taar1 gene × gene and gene × environment interactions. These and other studies are intended to improve our genetic model with regard to its translational value to human addiction.

Methamphetamine drinking microstructure in mice bred to drink high or low amounts of methamphetamine

Genetic factors likely influence individual sensitivity to positive and negative effects of methamphetamine (MA) and risk for MA dependence. Genetic influence on MA consumption has been confirmed by selectively breeding mouse lines to consume high (MAHDR) or low (MALDR) amounts of MA, using a two-bottle choice MA drinking (MADR) procedure. Here, we employed a lickometer system to characterize the microstructure of MA (20, 40, and 80mg/l) and water intake in MAHDR and MALDR mice in 4-h limited access sessions, during the initial 4hours of the dark phase of their 12:12h light:dark cycle. Licks at one-minute intervals and total volume consumed were recorded, and bout analysis was performed. MAHDR and MALDR mice consumed similar amounts of MA in mg/kg on the first day of access, but MAHDR mice consumed significantly more MA than MALDR mice during all subsequent sessions. The higher MA intake of MAHDR mice was associated with a larger number of MA bouts, longer bout duration, shorter interbout interval, and shorter latency to the first bout. In a separate 4-h limited access MA drinking study, MALDR and MAHDR mice had similar blood MA levels on the first day MA was offered, but MAHDR mice had higher blood MA levels on all subsequent days, which corresponded with MA intake. These data provide insight into the microstructure of MA intake in an animal model of differential genetic risk for MA consumption, which may be pertinent to MA use patterns relevant to genetic risk for MA dependence.