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

Trace amine-associated receptor 1 and drug abuse

Trace amine-associated receptor 1 (TAAR1) is the best characterized receptor selectively activated by trace amines. It is broadly expressed in the monoaminergic system in the brain including ventral tegmental area (VTA), nucleus accumbens (NAc), dorsal raphe (DR) and substantial nigra (SN). Extensive studies have suggested that TAAR1 plays an important role in the modulation of monoaminergic system, especially dopamine (DA) transmission which may underlie the mechanisms by which TAAR1 interventions affect drug abuse-like behaviors. TAAR1 activation inhibits the rewarding and reinforcing effects of drugs from different classes including psychostimulants, opioid and alcohol as well as drug-induced increase in DA accumulation. The mechanisms of TAAR1's function in mediating drug abuse-like behaviors are not clear. However, it is hypothesized that TAAR1 interaction with DA transporter (DAT) and dopamine D2 receptor (D2) and the subsequent modulation of cellular cascades may contribute to the effects of TAAR1 in regulating drug abuse. Further studies are needed to investigate the role of TAAR1 in other drugs of abuse-related behaviors and its safety and efficacy for prolonged medications. Together, TAAR1 inhibits drug-induced DA transmission and drug abuse-related behaviors. Therefore, TAAR1 may be a promising therapeutic target for the treatment of drug addiction.

Potential of Ligands for Trace Amine-Associated Receptor 1 (TAAR1) in the Management of Substance Use Disorders

Trace amines, including β-phenylethylamine (β-PEA), p-tyramine (TYR), tryptamine (TRP), and p-octopamine (OCT), represent a group of amines expressed at low levels in the mammalian brain. Given the close structural similarities to traditional monoamines, links between trace amines and the monoaminergic system have long been suspected. Trace amine-associated receptor 1 (TAAR1), the most well characterized receptor in the TAAR family, has been shown to be potently activated by trace amines such as TYR and PEA. Further, catecholamine metabolites and amphetamine analogs are also potent agonists of TAAR1, implicating the receptor in mediating the monoaminergic system and in substance use disorders. In the central nervous system, TAAR1 is expressed in brain regions involved in dopaminergic, serotonergic, and glutamatergic transmission, and genetic animal models and electrophysiological studies have revealed that TAAR1 is a potent modulator of the monoaminergic system. Selective and potent engineered TAAR1 ligands, including full (RO5166017 and RO5256390) and partial (RO5203648, RO5263397 and RO5073012) agonists and the antagonist EPPTB (N-(3-ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl) benzamide, RO5212773), serve as invaluable tools for the investigation of TAAR1 functions and display significant potential for the development of TAAR1-based pharmacotherapies for the treatment of substance use disorders. Despite a number of advances that have been made, more clinical studies are warranted in order to test the potential and efficacy of TAAR1 ligands in the treatment of psychiatric disorders, including substance use disorders.

Trace amine-associated receptor 1 (TAAR1): Potential application in mood disorders: A systematic review

There is a need for innovation with respect to therapeutics in psychiatry. Available evidence indicates that the trace amine-associated receptor 1 (TAAR1) agonist SEP-363856 is promising, as it improves measures of cognitive and reward function in schizophrenia. Hedonic and cognitive impairments are transdiagnostic and constitute major burdens in mood disorders. Herein, we systematically review the behavioural and genetic literature documenting the role of TAAR1 in reward and cognitive function, and propose a mechanistic model of TAAR1's functions in the brain. Notably, TAAR1 activity confers antidepressant-like effects, enhances attention and response inhibition, and reduces compulsive reward seeking without impairing normal function. Further characterization of the responsible mechanisms suggests ion-homeostatic, metabolic, neurotrophic, and anti-inflammatory enhancements in the limbic system. Multiple lines of evidence establish the viability of TAAR1 as a biological target for the treatment of mood disorders. Furthermore, the evidence suggests a role for TAAR1 in reward and cognitive function, which is attributed to a cascade of events that are relevant to the cellular integrity and function of the central nervous system.

ERK-Directed Phosphorylation of mGlu5 Gates Methamphetamine Reward and Reinforcement in Mouse

Methamphetamine (MA) is a highly addictive psychomotor stimulant drug. In recent years, MA use has increased exponentially on a global scale, with the number of MA-involved deaths reaching epidemic proportions. There is no approved pharmacotherapy for treating MA use disorder, and we know relatively little regarding the neurobiological determinants of vulnerability to this disease. Extracellular signal-regulated kinase (ERK) is an important signaling molecule implicated in the long-lasting neuroadaptations purported to underlie the development of substance use disorders, but the role for this kinase in the propensity to develop addiction, particularly MA use disorder, is uncharacterized. In a previous MA-induced place-conditioning study of C57BL/6J mice, we characterized mice as MA-preferring, -neutral, or -avoiding and collected tissue from the medial prefrontal cortex (mPFC). Using immunoblotting, we determined that elevated phosphorylated ERK expression within the medial prefrontal cortex (mPFC) is a biochemical correlate of the affective valence of MA in a population of C57BL/6J mice. We confirmed the functional relevance for mPFC ERK activation for MA-induced place-preference via site-directed infusion of the MEK inhibitor U0126. By contrast, ERK inhibition did not have any effect upon MA-induced locomotion or its sensitization upon repeated MA treatment. Through studies of transgenic mice with alanine point mutations on T1123/S1126 of mGlu5 that disrupt ERK-dependent phosphorylation of the receptor, we discovered that ERK-dependent mGlu5 phosphorylation normally suppresses MA-induced conditioned place-preference (MA-CPP), but is necessary for this drug’s reinforcing properties. If relevant to humans, the present results implicate individual differences in the capacity of MA-associated cues/contexts to hyper-activate ERK signaling within mPFC in MA Use Disorder vulnerability and pose mGlu5 as one ERK-directed target contributing to the propensity to seek out and take MA.

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.

The motivational valence of methamphetamine relates inversely to subsequent methamphetamine self-administration in female C57BL/6J mice

Understanding the mechanisms underpinning individual variance in addiction vulnerability requires the development of validated, high-throughput screens. In a prior study of a large sample of male isogenic C57BL/6J mice, the direction and magnitude of methamphetamine (MA)-induced place-conditioning predicts the propensity to acquire oral MA self-administration, as well as the efficacy of MA to serve as a reinforcer. The present study examined whether or not such a predictive relationship also exists in females. Adult C57BL/6J females underwent a 4-day MA place-conditioning paradigm (once daily injections of 2 mg/kg) and were then trained to nose-poke for delivery of a 20 mg/L MA solution under increasing schedules of reinforcement, followed by dose-response testing (5-400 mg/L MA). Akin to males, 53 % of the females exhibited a conditioned place-preference, while 32 % of the mice were MA-neutral and 15 % exhibited a conditioned place-aversion. However, unlike males, the place-conditioning phenotype did not transfer to MA-reinforced nose-poking behavior under operant-conditioning procedures, with 400 mg/L MA intake being inversely correlated place-conditioning. While only one MA-conditioning dose has been assayed to date, these data indicate that sex does not significantly shift the proportion of C57BL/6J mice that perceive MA's interoceptive effects as positive, neutral or aversive. However, a sex difference appears to exist regarding the predictive relationship between the motivational valence of MA and subsequent drug-taking behavior; females exhibit MA-taking behavior and reinforcement, despite their initial perception of the stimulant interoceptive effects as positive, neutral or negative.