Methamphetamine Addiction Vulnerability: The Glutamate, the Bad, and the Ugly

The acute neurotoxicity of inorganic mercury in Mactra chinensis philippi

Inorganic mercury (IHg) is hazardous to marine organisms especially resulting in neurotoxicity, bivalves are sensitive to pollutants as "ocean sentinel", but data on the neurotoxicity of IHg in bivalves are sparse. So we chosed M. chinensis philippi with typical neural structures in bivalves to investigate the neurotoxicity of IHg, which could be helpful to understand the specificity of neural regulation and the response characteristics of bivalves. After acute exposed to IHg (HgCl2) for 24 h, the metabolites of ganglion tissues in M. chinensis philippi were evaluated using 1H-nuclear magnetic resonance based metabolomics; Ca2+, neurotransmitters (nitric oxide, glutamate, acetylcholine) and related enzymes (calcineurin, nitric oxide synthase and acetylcholinesterase) were measured using biochemical detection. Compared to the control group, the levels of the nitric oxide (81.04 ± 12.84 μmol/g prot) and acetylcholine (30.93 ± 12.57 μg/mg prot) in M. chinensis philippi of IHg-treated were decreased, while glutamate (2.11 ± 0.61 mmol/L) increased significantly; the activity of nitric oxide synthase (679.34 ± 135.33 U/mg prot) was increased, while acetylcholinesterase (1.39 ± 0.44 U/mg prot) decreased significantly, and the activity of calcineurin (0.52 ± 0.02 U/mg prot) had a statistically insignificant increasing tendency. The concentration of Ca2+ (0.92 ± 0.46 mmol/g prot) in the IHg-treated group was significantly higher than that in the control group. OPLS-DA was performed to reveal the difference in metabolites between the control and IHg-challenged groups, the metabolites of glucose, glutamine, inosine, succinate, glutamate, homarine, and alanine were sensitive to IHg, subsequently metabolic pathways that were affected including glucose metabolism, glutamine metabolism, nucleotide metabolism, Krebs cycle, amino acid metabolism and osmotic regulation. In our study, IHg interfered with metabolites in M. chinensis philippi, thus the corresponding metabolic pathways were changed, which influenced the neurotransmitters subsequently. Furthermore, Ca2+overload affected the synthesis or degradation of the neurotransmitters, and then the altered neurotransmitters involved in changes in metabolic pathways again. Overall, we hypothesized that the neurotoxic effects of IHg on bivalve were in close contact with metabolism, neurotransmitters, related enzymes and Ca2+, which could be effective neurotoxic biomarkers for marine environmental quality assessment, and also provide effective data for the study of the regulatory mechanism of the nervous system in response to IHg in bivalves.

Effects of systemic pretreatment with the NAALADase inhibitor 2-PMPA on oral methamphetamine reinforcement in C57BL/6J mice

Introduction

Repeated exposure to methamphetamine (MA) in laboratory rodents induces a sensitization of glutamate release within the corticoaccumbens pathway that drives both the rewarding and reinforcing properties of this highly addictive drug. Such findings argue the potential for pharmaceutical agents inhibiting glutamate release or its postsynaptic actions at glutamate receptors as treatment strategies for MA use disorder. One compound that may accomplish both of these pharmacological actions is the N-acetylated-alpha-linked-acidic dipeptidase (NAALADase) inhibitor 2-(phosphonomethyl)pentanedioic acid (2-PMPA). 2-PMPA elevates brain levels of the endogenous agonist of glutamate mGluR3 autoreceptors, N-acetyl-aspartatylglutamate (NAAG), while potentially acting as an NMDA glutamate receptor antagonist. Of relevance to treating psychomotor stimulant use disorders, 2-PMPA is reported to reduce indices of both cocaine and synthetic cathinone reward, as well as cocaine reinforcement in preclinical rodent studies.

Method

Herein, we conducted three experiments to pilot the effects of systemic pretreatment with 2-PMPA (0-100 mg/kg, IP) on oral MA self-administration in C57BL/6J mice. The first experiment employed female mice with a prolonged history of MA exposure, while the mice in the second (females) and third (males and females) experiment were MA-naïve prior to study. In all experiments, mice were trained daily to nose-poke for delivery of unadulterated MA solutions until responding stabilized. Then, mice were pretreated with 2-PMPA prior to operant-conditioning sessions in which nose-poking behavior was reinforced by delivery of 120 mg/L or 200 mg/L MA (respectively, in Experiments 1 and 2/3).

Results

Contrary to our expectations, 30 mg/kg 2-PMPA pretreatment altered neither appetitive nor consummatory measures related to MA self-administration. In Experiment 3, 100 mg/kg 2-PMPA reduced responding in the MA-reinforced hole, as well as the number of reinforcers earned, but did not significantly lower drug intake.

Discussion

These results provide mixed evidenced related to the efficacy of this NAALADase inhibitor for reducing oral MA reinforcement in female mice.

Neuropharmacological Evidence Implicating Drug-Induced Glutamate Receptor Dysfunction in Affective and Cognitive Sequelae of Subchronic Methamphetamine Self-Administration in Mice

Methamphetamine (MA) is a highly addictive drug, and MA use disorder is often comorbid with anxiety and cognitive impairment. These comorbid conditions are theorized to reflect glutamate-related neurotoxicity within the frontal cortical regions. However, our prior studies of MA-sensitized mice indicate that subchronic, behaviorally non-contingent MA treatment is sufficient to dysregulate glutamate transmission in mouse brain. Here, we extend this prior work to a mouse model of high-dose oral MA self-administration (0.8, 1.6, or 3.2 g/L; 1 h sessions × 7 days) and show that while female C57BL/6J mice consumed more MA than males, MA-experienced mice of both sexes exhibited some signs of anxiety-like behavior in a behavioral test battery, although not all effects were concentration-dependent. No MA effects were detected for our measures of visually cued spatial navigation, spatial learning, or memory in the Morris water maze; however, females with a history of 3.2 g/L MA exhibited reversal-learning deficits in this task, and mice with a history of 1.6 g/L MA committed more working-memory incorrect errors and relied upon a non-spatial navigation strategy during the radial-arm maze testing. Relative to naïve controls, MA-experienced mice exhibited several changes in the expression of certain glutamate receptor-related proteins and their downstream effectors within the ventral and dorsal areas of the prefrontal cortex, the hippocampus, and the amygdala, many of which were sex-selective. Systemic pretreatment with the mGlu1-negative allosteric modulator JNJ 162596858 reversed the anxiety-like behavior expressed by MA-experienced mice in the marble-burying test, while systemic pretreatment with NMDA or the NMDA antagonist MK-801 bi-directionally affected the MA-induced reversal-learning deficit. Taken together, these data indicate that a relatively brief history of oral MA is sufficient to induce some signs of anxiety-like behavior and cognitive dysfunction during early withdrawal that reflect, at least in part, MA-induced changes in the corticolimbic expression of certain glutamate receptor subtypes of potential relevance to treating symptoms of MA use disorder.

Inefficacy of N-acetylcysteine in mitigating cue-induced amphetamine-seeking

Glutamatergic imbalances are characteristic of SUDs. Astrocytic and neuronal transporters help regulate glutamate homeostasis and disruptions in this homeostasis engender SUD. The cysteine-glutamate exchanger (xCT) is primarily localized on astrocytes and maintains glutamate concentrations. This process is disrupted by cocaine use, and the therapeutic N-acetylcysteine (NAC) lowers cue-induced relapse to cocaine by restoring xCT function. However, little research has shown how these effects extend to other psychostimulants, such as amphetamine (AMP). Here, we assessed xCT expression following relapse to AMP cues, and if NAC can attenuate relapse via changes to astrocyte and xCT expression. We administered NAC (100 mg/kg ip) daily during a 14-day abstinence period following AMP (0.1 mg/kg/infusion; 2 h sessions) self-administration. Relapse was tested following one (WD 1) or 14 days (WD 14) of withdrawal. The overall number of astrocytes was also quantified within the medial prefrontal cortex (mPFC) and nucleus accumbens (ACb). NAC failed to lower cue-induced AMP craving via cue-induced relapse and reinstatement testing. Cue-induced craving did not increase from WD 1 to WD 14. AMP-exposed rats had greater astrocyte counts in the mPFC and ACb when compared AMP-naïve rats. Repeated injection with NAC decreased xCT expression within the mPFC and ACb. Overall, these results suggest that NAC may be an ineffective treatment option for lowering cue-induced relapse to AMP. Further, the results suggest that stimulating xCT via NAC may not be an effective therapeutic approach for decreasing cue-seeking for AMP.

Modeling methamphetamine use disorder and relapse in animals: short- and long-term epigenetic, transcriptional., and biochemical consequences in the rat brain

Methamphetamine use disorder (MUD) is a neuropsychiatric disorder characterized by binge drug taking episodes, intervals of abstinence, and relapses to drug use even during treatment. MUD has been modeled in rodents and investigators are attempting to identify its molecular bases. Preclinical experiments have shown that different schedules of methamphetamine self-administration can cause diverse transcriptional changes in the dorsal striatum of Sprague-Dawley rats. In the present review, we present data on differentially expressed genes (DEGs) identified in the rat striatum following methamphetamine intake. These include genes involved in transcription regulation, potassium channel function, and neuroinflammation. We then use the striatal data to discuss the potential significance of the molecular changes induced by methamphetamine by reviewing concordant or discordant data from the literature. This review identified potential molecular targets for pharmacological interventions. Nevertheless, there is a need for more research on methamphetamine-induced transcriptional consequences in various brain regions. These data should provide a more detailed neuroanatomical map of methamphetamine-induced changes and should better inform therapeutic interventions against MUD.

Rats have low motivation to self-administer oral methamphetamine across increasing response requirements

Methamphetamine (METH) is a psychostimulant drug that has become increasingly popular in recent years, with overdose deaths more than doubling during the second half of the 2010s. As methamphetamine use disorder rates continue to increase, finding effective treatment strategies to decrease METH dependence is important. Animal studies are well-suited for studying the neurobiological mechanisms underlying addiction-like behaviors. Although individuals can ingest METH orally, few studies have examined oral METH self-administration in animals. Mice show decreased responding for oral METH as the response requirement increases across sessions. The purpose of the current study was to determine if rats show a similar decrease in motivation to earn oral METH across increasing response requirements. Sixteen Sprague Dawley rats were trained to emit a response in an aperture to receive a 0.1-ml METH solution (40 mg/l) according to an FR 1 schedule. The FR requirement increased across sessions to a terminal FR 10. Responses for METH decreased significantly when an FR 10 schedule was used. These results suggest that rats, similarly to mice, have low motivation to self-administer oral METH.

Deletion of Cryab increases the vulnerability of mice to the addiction-like effects of the cannabinoid JWH-018 via upregulation of striatal NF-κB expression

Synthetic cannabinoids have exhibited unpredictable abuse liabilities, especially self-administration (SA) responses in normal rodent models, despite seemingly inducing addiction-like effects in humans. Thus, an efficient pre-clinical model must be developed to determine cannabinoid abuse potential in animals and describe the mechanism that may mediate cannabinoid sensitivity. The Cryab knockout (KO) mice were recently discovered to be potentially sensitive to the addictive effects of psychoactive drugs. Herein, we examined the responses of Cryab KO mice to JWH-018 using SA, conditioned place preference, and electroencephalography. Additionally, the effects of repeated JWH-018 exposure on endocannabinoid- and dopamine-related genes in various addiction-associated brain regions were examined, along with protein expressions involving neuroinflammation and synaptic plasticity. Cryab KO mice exhibited greater cannabinoid-induced SA responses and place preference, along with divergent gamma wave alterations, compared to wild-type (WT) mice, implying their higher sensitivity to cannabinoids. Endocannabinoid- or dopamine-related mRNA expressions and accumbal dopamine concentrations after repeated JWH-018 exposure were not significantly different between the WT and Cryab KO mice. Further analyses revealed that repeated JWH-018 administration led to possibly greater neuroinflammation in Cryab KO mice, which may arise from upregulated NF-κB, accompanied by higher expressions of synaptic plasticity markers, which might have contributed to the development of cannabinoid addiction-related behavior in Cryab KO mice. These findings signify that increased neuroinflammation via NF-κB may mediate the enhanced addiction-like responses of Cryab KO mice to cannabinoids. Altogether, Cryab KO mice may be a potential model for cannabinoid abuse susceptibility.

An unconventional cancer-promoting function of methamphetamine in hepatocellular carcinoma

For the past decade, the prevalence and mortality of methamphetamine (METH) use have doubled, suggesting that METH use could be the next substance use crisis worldwide. Ingested METH is transformed into other products in the liver, a major metabolic organ. Studies have revealed that METH causes deleterious inflammatory response, oxidative stress, and extensive DNA damage. These pathological damages are driving factors of hepatocellular carcinoma (HCC). Nonetheless, the potential role of METH in HCC and the underlying mechanisms remain unknown. Herein, we found a higher HCC incidence in METH abusers. METH promoted cellular proliferation, migration, and invasion in two human-derived HCC cells. Consistently, METH uptake promoted HCC progression in a xenograft mouse model. Mechanistically, METH exposure induced ROS production, which activated the Ras/MEK/ERK signaling pathway. Clearance of ROS by NAC suppressed METH-induced activation of Ras/ERK1/2 pathways, leading to arrest of HCC xenograft formation in nude mice. To the best of our knowledge, this is the first study to substantiate that METH promotes HCC progression and inhibition of ROS may reverse this process.

The role of metabotropic glutamate receptors in neurobehavioral effects associated with methamphetamine use

Metabotropic glutamate (mGlu) receptors are expressed throughout the central nervous system and act as important regulators of drug-induced neuroplasticity and behavior. Preclinical research suggests that mGlu receptors play a critical role in a spectrum of neural and behavioral consequences arising from methamphetamine (meth) exposure. However, an overview of mGlu-dependent mechanisms linked to neurochemical, synaptic, and behavioral changes produced by meth has been lacking. This chapter provides a comprehensive review of the role of mGlu receptor subtypes (mGlu1-8) in meth-induced neural effects, such as neurotoxicity, as well as meth-associated behaviors, such as psychomotor activation, reward, reinforcement, and meth-seeking. Additionally, evidence linking altered mGlu receptor function to post-meth learning and cognitive deficits is critically evaluated. The chapter also considers the role of receptor-receptor interactions involving mGlu receptors and other neurotransmitter receptors in meth-induced neural and behavioral changes. Taken together, the literature indicates that mGlu5 regulates the neurotoxic effects of meth by attenuating hyperthermia and possibly through altering meth-induced phosphorylation of the dopamine transporter. A cohesive body of work also shows that mGlu5 antagonism (and mGlu2/3 agonism) reduce meth-seeking, though some mGlu5-blocking drugs also attenuate food-seeking. Further, evidence suggests that mGlu5 plays an important role in extinction of meth-seeking behavior. In the context of a history of meth intake, mGlu5 also co-regulates aspects of episodic memory, with mGlu5 stimulation restoring impaired memory. Based on these findings, we propose several avenues for the development of novel pharmacotherapies for Methamphetamine Use Disorder based on the selective modulation mGlu receptor subtype activity.

Endocannabinoids and addiction memory: Relevance to methamphetamine/morphine abuse

AIM

This review aims to summarise the role of endocannabinoid system (ECS), incluing cannabinoid receptors and their endogenous lipid ligands in the modulation of methamphetamine (METH)/morphine-induced memory impairments.

METHODS

Here, we utilized the results from researches which have investigated regulatory role of ECS (including cannabinoid receptor agonists and antagonists) on METH/morphine-induced memory impairments.

RESULTS

Among the neurotransmitters, glutamate and dopamine seem to play a critical role in association with the ECS to heal the drug-induced memory damages. Also, the amygdala, hippocampus, and prefrontal cortex are three important brain regions that participate in both drug addiction and memory task processes, and endocannabinoid neurotransmission have been investigated.

CONCLUSION

ECS can be regarded as a treatment for the side effects of METH and morphine, and their memory-impairing effects.

Selective Inhibition of PDE4B Reduces Methamphetamine Reinforcement in Two C57BL/6 Substrains

Methamphetamine (MA) is a highly addictive psychostimulant drug, and the number of MA-related overdose deaths has reached epidemic proportions. Repeated MA exposure induces a robust and persistent neuroinflammatory response, and the evidence supports the potential utility of targeting neuroimmune function using non-selective phosphodiesterase 4 (PDE4) inhibitors as a therapeutic strategy for attenuating addiction-related behavior. Off-target, emetic effects associated with non-selective PDE4 blockade led to the development of isozyme-selective inhibitors, of which the PDE4B-selective inhibitor A33 was demonstrated recently to reduce binge drinking in two genetically related C57BL/6 (B6) substrains (C57BL/6NJ (B6NJ) and C57BL/6J (B6J)) that differ in their innate neuroimmune response. Herein, we determined the efficacy of A33 for reducing MA self-administration and MA-seeking behavior in these two B6 substrains. Female and male mice of both substrains were first trained to nose poke for a 100 mg/L MA solution followed by a characterization of the dose-response function for oral MA reinforcement (20 mg/L-3.2 g/L), the demand-response function for 400 mg/L MA, and cue-elicited MA seeking following a period of forced abstinence. During this substrain comparison of MA self-administration, we also determined the dose-response function for A33 pretreatment (0-1 mg/kg) on the maintenance of MA self-administration and cue-elicited MA seeking. Relative to B6NJ mice, B6J mice earned fewer reinforcers, consumed less MA, and took longer to reach acquisition criterion with males of both substrains exhibiting some signs of lower MA reinforcement than their female counterparts during the acquisition phase of the study. A33 pretreatment reduced MA reinforcement at all doses tested. These findings provide the first evidence that pretreatment with a selective PDE4B inhibitor effectively reduces MA self-administration in both male and female mice of two genetically distinct substrains but does not impact cue-elicited MA seeking following abstinence. If relevant to humans, these results posit the potential clinical utility of A33 or other selective PDE4B inhibitors for curbing active drug-taking in MA use disorder.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

Electroacupuncture alleviates spatial memory deficits in METH withdrawal mice by enhancing astrocyte-mediated glutamate clearance in the dCA1

Methamphetamine (METH) elicits endogenous glutamate (Glu) in the brain, which could partially explain METH-induced memory deficits. Here, we investigated the therapeutic effects of electroacupuncture (EA) on spatial memory deficits in METH withdrawal mice and its potential synaptic mechanisms. We found that EA at acupoints 'Baihui' and 'Yintang' ameliorated the impaired spatial memory in METH withdrawal mice. In parallel, EA attenuated the Glu levels in vivo and suppressed the neuronal activities within dCA1 of METH withdrawal mice, as indicated by the decreasing c-Fos levels and the amplitude of mEPSP. In the dCA1, EA decreased A1-like astrocytes but increased astrocytic glutamatergic transporting molecules including glutamate transporter 1 and glutamine synthase. However, EA seemed to have no effects on presynaptic Glu transmission from the dCA3, as evidenced by the similiar levels of c-Fos in the dCA3 neurons, synaptic vesicular markers of dCA3 neural terminals and values of paired-pulse ratio in the dCA1 neurons between EA-treated and sham EA-treated METH withdrawal mice. These findings suggest that EA might normalize the dCA1 Glu levels at least in part through enhancing astrocyte-mediated Glu clearance. Taken together, astrocytes might be a novel target for developing therapeutic interventions against the impaired memory behaviours in METH users, and EA represents a promising non-invasive therapeutic strategy for the management of drug-caused memory deficits.

Methamphetamine, Neurotransmitters and Neurodevelopment

Methamphetamine (MA), as massively abused psychoactive stimulant, has been associated with many neurological diseases. It has various potent and neurotoxic properties. There are many mechanisms of action that contribute to its neurotoxic and degenerative effects, including excessive neurotransmitter (NEU) release, blockage of NEU uptake transporters, degeneration of NEU receptors, process of oxidative stress etc. MA intoxication is caused by blood-brain barrier disruption resulted from MA-induced oxidation stress. In our laboratory we constantly work on animal research of MA. Our current interest is to investigate processes of MA-induced alteration in neurotransmission, especially during development of laboratory rat. This review will describe current understanding in role of NEUs, which are affected by MA-induced neurotoxicity caused by altering the action of NEUs in the central nervous system (CNS). It also briefly brings information about NEUs development in critical periods of development.

Nature of glutamate alterations in substance dependence: A systematic review and meta-analysis of proton magnetic resonance spectroscopy studies

Animal studies have reported the brain glutamatergic dysfunction in substance dependence. However, proton magnetic resonance spectroscopy (¹H-MRS) studies of glutamate in substance-dependent patients published contradicting results. In order to investigate the characteristics of brain glutamatergic alterations in substance-dependent patients, we conducted systematic reviews and meta-analyses of ¹H-MRS studies that have investigated the glutamate, glutamine, and Glx (glutamate + glutamine) concentration in substance-dependent patients. Multiple databases were searched until Sep 10, 2020. Twenty-nine studies comprising 982 patients and 787 controls were included. There was significantly decreased glutamate level in dorsolateral prefrontal cortex in patients compared with controls. Higher glutamate levels in medial prefrontal cortex and basal ganglia region were also demonstrated in patients compared with controls. Subgroup analyses based on the substance type and abstinence period (short vs medium-term abstinence period) were performed. The results revealed Glx and glutamate concentrations in all investigated brain regions were not different in patients with any types of substance dependence compared with controls. The abstinence period had no effect on the glutamate levels. In summary, substance dependence is associated with glutamatergic dysfunction of prefrontal cortex and basal ganglia. Present findings partially support the hypothesis that addiction is associated with abnormal brain glutamatergic neurotransmission.

Combined and sequential effects of alcohol and methamphetamine in animal models

Comorbid drug use, often alcohol with other drugs, poses significant health and societal concerns. Methamphetamine is among the illicit drugs most often co-used with alcohol. The current review examines the animal literature for impacts of comorbid alcohol and methamphetamine exposure. We found evidence for additive or synergistic effects of combined or sequential exposure on behavior and physiology. Dopaminergic, serotonergic, and glutamatergic systems are all impacted by combined exposure to alcohol and methamphetamine and cyclooxygenase-2 activity plays an important role in their combined neurotoxic effects. Adverse consequences of comorbid exposure include altered brain development with prenatal exposure, impaired learning and memory, motor deficits, gastrotoxicity, hepatotoxicity, and augmented intake under some conditions. Given high susceptibility to drug experimentation in adolescence, studies of co-exposure during the adolescent period and of how adolescent exposure to one drug impacts later use or sensitivity to the other drug should be a priority. Further, to gain traction on prevention and treatment, additional research to identify motivational and neurobiological drivers and consequences of comorbid use is needed.

The effects of minocycline in improving of methamphetamine withdrawal syndrome in male mice

Methamphetamine (METH) is a potent psychostimulant drug with an increasing rate of abuse over recent years. Depressive-like behaviors are one of the major symptoms patients in the METH withdrawal period experience. There is limited evidence regarding the METH withdrawal treatment, and conventional managements are not completely effective. Furthermore, extensive promising literature supports minocycline, a well-known antibiotic with anti-oxidant, anti-inflammatory properties, to treat depressive-like behaviors. Therefore, we hypothesized that administration of minocycline might mitigate the methamphetamine (METH) induced depression in male mice. Administration of METH (2 mg/kg) to mice two times a day for 14 constitutive days was done to induce the METH-induced withdrawal syndrome model. Animals were divided into 10 groups (n = 10 in each group), and three doses of minocycline (2.5, 5 and 10 mg/kg) were daily administered to male albino mice for 10 days. Following the behavioral test, the animals were scarified, their hippocampus were dissected to measure oxidative stress parameters. Our data revealed that chronic administration of minocycline provoked antidepressant effects in behavioral tests, such as forced swim test (FST), tail suspension test (TST) and splash test. Additionally, minocycline was able to improve oxidative stresses and neuronal damage in the hippocampus and restore the body's antioxidant system by increasing glutathione (GSH) and the cellular energy (ATP) and reducing the malondialdehyde (MDA) level. According to our promising results of minocycline on targeting mitochondria and its performance, we suggest minocycline as a new therapeutic option in clinical trials of depression treatment.

Astrocyte-selective STAT3 knockdown rescues methamphetamine withdrawal-disrupted spatial memory in mice via restoring the astrocytic capacity of glutamate clearance in dCA1

Methamphetamine (METH) is a common abused drug. METH-triggered glutamate (Glu) levels in dorsal CA1 (dCA1) could partially explain the etiology of METH-caused abnormal memory, but the synaptic mechanism remains unclear. Here, we found that METH withdrawal disrupted spatial memory in mice, accompanied by the increases in Glu levels and postsynaptic neuronal activities at dCA1 synapses. METH withdrawal weakened the capacity of Glu clearance in astrocytes, as indicated by increasing the A1-like astrocytes and phosphorylated signal transducer and activator of transcription 3 (p-STAT3), decreasing the Glu transporter 1(GLT-1, also known as EAAT2 or SLC1A2), Glu-aspartate-transporter (GLAST also known as EAAT1 or SLC1A3) and astrocytic glutamine synthase (GS), but failed to affect the presynaptic Glu release from dCA3 within dCA1. Moreover, we identified that in vitro A1-like astrocytes exhibited an increased STAT3 activation and the impaired capacity of Glu clearance. Most importantly, selective knockdown of astrocytic STAT3 in vivo in dCA1 restored the astrocytic capacity of Glu clearance, normalized Glu levels at dCA1 synapses, and finally rescued METH withdrawal-disrupted spatial memory in mice. Thus, astrocytic Glu clearance system, especially STAT3, serves as a novel target for future therapies against METH neurotoxicity.

Targeting mGlu5 for Methamphetamine Use Disorder

Methamphetamine abuse leads to devastating consequences, including addiction, crime, and death. Despite decades of research, no medication has been approved by the U.S. Food and Drug Administration for the treatment of Methamphetamine Use Disorder. Thus, there is a need for new therapeutic approaches. Animal studies demonstrate that methamphetamine exposure dysregulates forebrain function involving the Group-I metabotropic glutamate receptor subtype 5 (mGlu₅), which is predominantly localized to postsynaptic sites. Allosteric modulators of mGlu₅ offer a unique opportunity to modulate glutamatergic neurotransmission selectively, thereby potentially ameliorating methamphetamine-induced disruptions. Negative allosteric modulators of mGlu₅ attenuate the effects of methamphetamine, including rewarding/reinforcing properties of the drug across animal models, and have shown promising effects in clinical trials for Anxiety Disorder and Major Depressive Disorder. Preclinical studies have also sparked great interest in mGlu₅ positive allosteric modulators, which exhibit antipsychotic and anxiolytic properties, and facilitate extinction learning when access to methamphetamine is removed, possibly via the amelioration of methamphetamine-induced cognitive deficits. Clinical research is now needed to elucidate the mechanisms underlying the mGlu₅ receptor-related effects of methamphetamine and the contributions of these effects to addictive behaviors. The growing array of mGlu₅ allosteric modulators provides excellent tools for this purpose and may offer the prospect of developing tailored and effective medications for Methamphetamine Use Disorder.

Glutamate homeostasis and dopamine signaling: Implications for psychostimulant addiction behavior

Cocaine, amphetamine, and methamphetamine abuse disorders are serious worldwide health problems. To date, there are no FDA-approved medications for the treatment of these disorders. Elucidation of the biochemical underpinnings contributing to psychostimulant addiction is critical for the development of effective therapies. Excitatory signaling and glutamate homeostasis are well known pathophysiological substrates underlying addiction-related behaviors spanning multiple types of psychostimulants. To alleviate relapse behavior to psychostimulants, considerable interest has focused on GLT-1, the major glutamate transporter in the brain. While many brain regions are implicated in addiction behavior, this review focuses on two regions well known for their role in mediating the effects of cocaine and amphetamines, namely the nucleus accumbens (NAc) and the ventral tegmental area (VTA). In addition, because many investigators have utilized Cre-driver lines to selectively control gene expression in defined cell populations relevant for psychostimulant addiction, we discuss potential off-target effects of Cre-recombinase that should be considered in the design and interpretation of such experiments.

Hnrnph1 is a novel regulator of alcohol reward

BACKGROUND

Hnrnph1 is a validated quantitative trait gene for methamphetamine behavioral sensitivity that encodes for heterogeneous nuclear ribonucleoprotein H1 (hnRNP H1). This RNA-binding protein is involved in all stages of RNA metabolism that impacts mesocorticolimbic dopamine neurotransmission to influence addiction-related behavior.

METHODS

We characterized the alcohol behavioral phenotypes of mice heterozygous for a deletion in the first coding exon of Hnrnph1 (Hnrnph1+/-). We examined alcohol intake under both continuous- and limited-access procedures, as well as alcohol-induced place-conditioning. Follow-up studies examined genotypic differences in the psychomotor-activating and sedative-hypnotic effects of acute and repeated alcohol, and a behavioral test battery was employed to determine the effects of Hnrnph1 deletion on the manifestation of negative affect during alcohol withdrawal.

RESULTS

Relative to wild-type (WT) controls, Hnrnph1+/- males exhibited blunted intake of high alcohol concentrations under both drinking procedures. Hnrnph1 deletion did not impact the conditioned rewarding properties of low-dose alcohol, but reversed the conditioned place-aversion elicited by higher alcohol doses (2 and 4 g/kg), with more robust effects in male versus female mice. No genotypic differences were observed for alcohol-induced locomotor activity. Hnrnph1+/- mice exhibited a modest increase in sensitivity to alcohol's sedative-hypnotic effects, but did not differ from WT mice with regard to tolerance to alcohol's sedative-hypnotic effects or alcohol metabolism, Inconsistent effects of Hnrnph1 deletion were observed in models for withdrawal-induced negative affect.

CONCLUSIONS

These data identify Hnrnph1 as a novel, male-selective, driver of alcohol consumption and high-dose alcohol aversion that is potentially relevant to the neurobiology of alcohol abuse and alcoholism.

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.

mGlu5 function in the nucleus accumbens core during the incubation of methamphetamine craving

Many studies have demonstrated that negative allosteric modulators (NAM) of metabotropic glutamate receptor 5 (mGlu5) reduce cocaine and methamphetamine seeking in extinction-reinstatement animal models of addiction. Less is known about effects of mGlu5 NAMs in abstinence models, particularly for methamphetamine. We used the incubation of drug craving model, in which cue-induced craving progressively intensifies after withdrawal from drug self-administration, to conduct the first studies of the following aspects of mGlu5 function in the rat nucleus accumbens (NAc) core during abstinence from methamphetamine self-administration: 1) functionality of the major form of synaptic depression in NAc medium spiny neurons, which is induced postsynaptically via mGlu5 and expressed presynaptically via cannabinoid type 1 receptors (CB1Rs), 2) mGlu5 surface expression and physical associations between mGlu5, Homer proteins, and diacylglycerol lipase-α, and 3) the effect of systemic and intra-NAc core administration of the mGlu5 NAM 3-((2-methyl-4-)ethynyl)pyridine (MTEP) on expression of incubated methamphetamine craving. We found that mGlu5/CB1R-dependent synaptic depression was lost during the rising phase of methamphetamine incubation but then recovered, in contrast to its persistent impairment during the plateau phase of incubation of cocaine craving. Furthermore, whereas the cocaine-induced impairment was accompanied by reduced mGlu5 levels and mGlu5-Homer associations, this was not the case for methamphetamine. Systemic MTEP reduced incubated methamphetamine seeking, but also reduced inactive hole nose-pokes and locomotion, while intra-NAc core MTEP had no significant effects. These findings provide the first insight into the role of mGlu5 in the incubation of methamphetamine craving and reveal differences from incubation of cocaine craving.

Metabotropic glutamate type 5 receptor binding availability during dextroamphetamine sensitization in mice and humans

BACKGROUND

Glutamate transmission is implicated in drug-induced behavioural sensitization and the associated long-lasting increases in mesolimbic output. Metabotropic glutamate type 5 (mGlu5) receptors might be particularly important, but most details are poorly understood.

METHODS

We first assessed in mice (n = 51, all male) the effects of repeated dextroamphetamine administration (2.0 mg/kg, i.p.) on locomotor activity and binding of the mGlu5 ligand [3H]ABP688. In a parallel study, in 19 stimulant-drug-naïve healthy human volunteers (14 female) we administered 3 doses of dextroamphetamine (0.3 mg/kg, p.o.) or placebo, followed by a fourth dose 2 weeks later. We measured [11C]ABP688 binding using positron emission tomography before and after the induction phase. We assessed psychomotor and behavioural sensitization using speech rate, eye blink rate and self-report. We measured the localization of mGlu5 relative to synaptic markers in mouse striatum using immunofluorescence.

RESULTS

We observed amphetamine-induced psychomotor sensitization in mice and humans. We did not see group differences in mGlu5 availability following 3 pre-challenge amphetamine doses, but group differences did develop in mice administered 5 doses. In mice and humans, individual differences in mGlu5 binding after repeated amphetamine administration were negatively correlated with the extent of behavioural sensitization. In drug-naïve mice, mGlu5 was expressed at 67% of excitatory synapses on dendrites of striatal medium spiny neur.

LIMITATIONS

Correlational results should be interpreted as suggestive because of the limited sample size. We did not assess sex differences.

CONCLUSION

Together, these results suggest that changes in mGlu5 availability are not part of the earliest neural adaptations in stimulant-induced behavioural sensitization, but low mGlu5 binding might identify a higher propensity for sensitization.

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.

Enduring dysregulation of nucleus accumbens catecholamine and glutamate transmission by developmental exposure to phenylpropanolamine

For over 50 years, the sympathomimetic phenylpropanolamine (PPA; ±-norephedrine) was a primary active ingredient in over-the-counter nasal decongestants for both children and adults and continues to be prevalent in the vast majority of countries today. Previously, we reported that juvenile PPA exposure alters the developmental trajectory of catecholamine and amino acid neurotransmitter systems in the nucleus accumbens (NAC), impacting the motivational valence of cocaine in later life. The present study employed a combination of in vivo microdialysis and immunoblotting approaches to better understand how juvenile PPA exposure impacts catecholamine and glutamate function within the NAC. For this, C57BL/6J mice were pretreated repeatedly with PPA (0 or 40 mg/kg) during postnatal days 21-33. Starting at 70 days of age, the function and expression of receptors and transporters regulating extracellular dopamine and glutamate were determined. Juvenile PPA pretreatment completely abolished the capacity of selective dopamine and epinephrine reuptake inhibitors to increase NAC levels of both catecholamines, without impacting D2 or α2 receptor regulation of catecholamine release. Juvenile PPA pretreatment facilitated the rise in NAC glutamate elicited by dopamine, norepinephrine and glutamate transporter inhibitors and blunted mGlu2/3 inhibition of glutamate release in this region. These data confirm that juvenile exposure to PPA produces protracted perturbations in the regulation of extracellular catecholamine and glutamate levels within the NAC and further the hypothesis that early exposure to sympathomimetic drugs found in cough, cold and allergy medicines, have long-lasting effects upon neurotransmission within brain regions gating motivation.

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.

mGlu5 receptor availability in youth at risk for addictions: effects of vulnerability traits and cannabis use

The excitatory neurotransmitter glutamate has been implicated in experience-dependent neuroplasticity and drug-seeking behaviors. Type 5 metabotropic glutamate (mGlu5) receptors might be particularly important. They are critically involved in synaptic plasticity and their availability has been reported to be lower in people with alcohol, tobacco, and cocaine use disorders. Since these reductions could reflect effects of drug use or pre-existing traits, we used positron emission tomography to measure mGlu5 receptor availability in young adults at elevated risk for addictions. Fifty-nine participants (age 18.5 ± 0.6) were recruited from a longitudinal study that has followed them since birth. Based on externalizing traits that predict future substance use problems, half were at low risk, half were at high risk. Cannabis use histories varied markedly and participants were divided into three subgroups: zero, low, and high use. Compared to low risk volunteers, those at elevated risk had lower [11C]ABP688 binding potential (BPND) values in the striatum, amygdala, insula, and orbitofrontal cortex (OFC). Cannabis use by risk group interactions were observed in the striatum and OFC. In these regions, low [11C]ABP688 BPND values were only seen in the high risk group that used high quantities of cannabis. When these high risk, high cannabis use individuals were compared to all other participants, [11C]ABP688 BPND values were lower in the striatum, OFC, and insula. Together, these results provide evidence that mGlu5 receptor availability is low in youth at elevated risk for addictions, particularly those who frequently use cannabis.

Neurotransmitter system aberrations in patients with drug addiction

Drug dependence may affect the neurotransmitter system levels in the human body. This study recruited 113 healthy control subjects, 118 heroin-dependent patients and 118 methamphetamine-dependent patients and examined the serum 5-HT, dopamine, glutamate and norepinephrine levels in the 349 volunteers. ELISA assays demonstrated that the serum 5-HT levels were significantly reduced in the drug-dependent patients, whereas the serum dopamine and glutamate levels were both significantly increased in the drug-dependent patients when compared with control subjects. In contrast, the norepinephrine levels did not exhibit a significant difference between the drug-dependent and control subjects. We also used qRT-PCR to analyze the transcriptional expression levels of 5-HT_1A, 5-HT_1B, dopmaine-D1 and dopamine-D2 receptors in the blood of drug-dependent patients and controls, and the results show that only 5-HT_1B receptor levels were dysfunctional in the heroin abusers. In addition, our results suggest that serum 5-HT, dopamine, and glutamate levels had the potential to differ between drug abusers and controls, and combining those three potential biomarkers provided an accurate means to differentiate between the drug-dependent and control subjects. Taken together, our study reveals a differential profile of neurotransmitters in the heroin-dependent patients and methamphetamine-dependent patients, and this revelation may contribute to understanding the pathophysiology of drug addiction.

Effects of nicotine exposure on oral methamphetamine self-administration, extinction, and drug-primed reinstatement in adolescent male and female rats

BACKGROUND

Adolescent nicotine exposure increases methamphetamine (MA) intake in adult male rats; however, little is known about how nicotine affects MA self-administration during the adolescent period. Therefore, we assessed whether exposing rats to nicotine during early or late adolescence affects oral MA self-administration.

METHODS

146 male and female rats were treated with saline or nicotine (0.16 or 0.64 mg/kg) from postnatal day (PD) 25-PD 34 (the early exposure phase) and/or PD 35-PD 55 (the late exposure phase). Rats began an oral MA self-administration procedure on PD 35.

RESULTS

Only the sex variable, but not nicotine, affected sucrose and MA acquisition, as female rats had more nose pokes than males during training. On the test sessions, female rats exposed to nicotine (0.64 mg/kg) in the early exposure phase had more active nose pokes than saline-treated female rats or nicotine-treated male rats. Rats exposed to nicotine (0.16 mg/kg) in the late exposure phase had fewer active nose pokes during testing than rats exposed to saline. Nose poke responding during extinction was not altered by nicotine exposure, but administering nicotine (0.16 or 0.64 mg/kg) to male rats in the early exposure phase did decrease nose pokes during the drug-primed reinstatement session.

CONCLUSIONS

Our results show that adolescent female rats are more sensitive to the reinforcing effects of oral sucrose and MA than adolescent males, and that preadolescent nicotine exposure enhances oral MA self-administration in female rats. These findings suggest that preteen nicotine use may increase vulnerability to later MA abuse in teenage girls.

Genome-Wide DNA Methylation Analysis in Male Methamphetamine Users With Different Addiction Qualities

This paper aimed to explore the genome-wide DNA methylation status of methamphetamine (MA) abusers with different qualities to addiction and to identify differentially methylated candidate genes. A total of 207 male MA abusers with an MA abuse frequency of ≥10 times and an MA abuse duration of ≥1 year were assigned to the high MA addiction quality group (HMAQ group; 168 subjects who met the diagnostic criteria for MA dependence according to the DSM-IV) or to the low MA addictive quality group (LMAQ group; 39 subjects who did not meet the criteria for MA dependence). In addition 105 healthy controls were recruited. Eight HMAQ subjects, eight LMAQ subjects, and eight healthy controls underwent genome-wide DNA methylation scans with an Infinium Human Methylation 450 array (Illumina). The differentially methylated region (DMR) data were entered into pathway analysis, and the differentially methylated position (DMP) data were screened for candidate genes and verified by MethyLight qPCR with all samples. Seven specific pathways with an abnormal methylation status were identified, including the circadian entrainment, cholinergic synapse, glutamatergic synapse, retrograde endocannabinoid signaling, GABAergic synapse, morphine addiction and PI3K-Akt signaling pathways. SLC1A6, BHLHB9, LYNX1, CAV2, and PCSK9 showed differences in their methylation levels in the three groups. Only the number of methylated copies of CAV2 was significantly higher in the LMAQ group than in the HMAQ group. Our findings suggest that the circadian entrainment pathway and the caveolin-2 gene may play key roles in MA addiction quality. Further studies on their functions and mechanisms will help us to better understand the pathogenesis of MA addiction and to explore new targets for drug intervention.

Transgenic Analyses of Homer2 Function Within Nucleus Accumbens Subregions in the Regulation of Methamphetamine Reward and Reinforcement in Mice

Problems associated with the abuse of amphetamine-type stimulants, including methamphetamine (MA), pose serious health and socioeconomic issues world-wide. While it is well-established that MA's psychopharmacological effects involve interactions with monoamine neurotransmission, accumulating evidence from animal models implicates dysregulated glutamate in MA addiction vulnerability and use disorder. Recently, we discovered an association between genetic vulnerability to MA-taking and increased expression of the glutamate receptor scaffolding protein Homer2 within both the shell and core subregions of the nucleus accumbens (NAC) and demonstrated a necessary role for Homer2 within the shell subregion in MA reward and reinforcement in mice. This report extends our earlier work by interrogating the functional relevance of Homer2 within the NAC core for the conditioned rewarding and reinforcing properties of MA. C57BL/6J mice with a virus-mediated knockdown of Homer2b expression within the NAC core were first tested for the development and expression of a MA-induced conditioned place-preference/CPP (four pairings of 2 mg/kg MA) and then were trained to self-administer oral MA under operant-conditioning procedures (5-80 mg/L). Homer2b knockdown in the NAC core augmented a MA-CPP and shifted the dose-response function for MA-reinforced responding, above control levels. To determine whether Homer2b within NAC subregions played an active role in regulating MA reward and reinforcement, we characterized the MA phenotype of constitutive Homer2 knockout (KO) mice and then assayed the effects of virus-mediated overexpression of Homer2b within the NAC shell and core of wild-type and KO mice. In line with the results of NAC core knockdown, Homer2 deletion potentiated MA-induced CPP, MA-reinforced responding and intake, as well as both cue- and MA-primed reinstatement of MA-seeking following extinction. However, there was no effect of Homer2b overexpression within the NAC core or the shell on the KO phenotype. These data provide new evidence indicating a globally suppressive role for Homer2 in MA-seeking and MA-taking but argue against specific NAC subregions as the neural loci through which Homer2 actively regulates MA addiction-related behaviors.

Individual Differences in Amphetamine Locomotor Sensitization are Accompanied with Changes in Dopamine Release and Firing Pattern in the Dorsolateral Striatum of Rats

Not all the people that consume drugs of abuse develop addiction. In this sense, just a percentage of rats express locomotor sensitization after repeated psychostimulant exposure. Neurochemical evidence has shown that locomotor sensitization is associated with changes in dorsolateral striatum (DLS) activity. However, it is unknown if individual differences observed in locomotor sensitization are related to differential neuro-adaptations in DLS activity. In this study, we measured basal dopamine (DA) levels and single unit activity in the DLS of anesthetized rats, after repeated amphetamine (AMPH) administration. Rats were treated with AMPH 1.0 mg/kg ip or saline ip for 5 days. Following 5 days of withdrawal, a challenge dose of AMPH 1.0 mg/kg ip was injected. In-vivo microdialysis experiments and single unit recording were carried out twenty-four hours after the last AMPH injection. Sensitized rats showed increased basal DA levels and baseline firing rate of medium spiny neurons (MSNs) compared to non-sensitized rats. The local variation index (Lv) was used to measure the firing pattern of MSNs. In saline rats, a bursty firing pattern was observed in MSNs. A decrease in MSNs baseline Lv accompanies the expression of AMPH locomotor sensitization. Moreover, a decrease in Lv after an acute AMPH 1.0 mg/kg injection was only observed in saline and sensitized rats. Our results show individual differences in DLS basal DA levels and firing pattern after repeated AMPH administration, suggesting that an hyperfunction of nigrostriatal pathway, accompanied by a decrease in DLS MSNs firing irregularity underlies the expression of AMPH locomotor sensitization.

A Mutation in Hnrnph1 That Decreases Methamphetamine-Induced Reinforcement, Reward, and Dopamine Release and Increases Synaptosomal hnRNP H and Mitochondrial Proteins

Individual variation in the addiction liability of amphetamines has a heritable genetic component. We previously identified Hnrnph1 (heterogeneous nuclear ribonucleoprotein H1) as a quantitative trait gene underlying decreased methamphetamine-induced locomotor activity in mice. Here, we showed that mice (both females and males) with a heterozygous mutation in the first coding exon of Hnrnph1 (H1^+/-) showed reduced methamphetamine reinforcement and intake and dose-dependent changes in methamphetamine reward as measured via conditioned place preference. Furthermore, H1^+/- mice showed a robust decrease in methamphetamine-induced dopamine release in the NAc with no change in baseline extracellular dopamine, striatal whole-tissue dopamine, dopamine transporter protein, dopamine uptake, or striatal methamphetamine and amphetamine metabolite levels. Immunohistochemical and immunoblot staining of midbrain dopaminergic neurons and their forebrain projections for TH did not reveal any major changes in staining intensity, cell number, or forebrain puncta counts. Surprisingly, there was a twofold increase in hnRNP H protein in the striatal synaptosome of H1^+/- mice with no change in whole-tissue levels. To gain insight into the mechanisms linking increased synaptic hnRNP H with decreased methamphetamine-induced dopamine release and behaviors, synaptosomal proteomic analysis identified an increased baseline abundance of several mitochondrial complex I and V proteins that rapidly decreased at 30 min after methamphetamine administration in H1^+/- mice. In contrast, the much lower level of basal synaptosomal mitochondrial proteins in WT mice showed a rapid increase. We conclude that H1^+/- decreases methamphetamine-induced dopamine release, reward, and reinforcement and induces dynamic changes in basal and methamphetamine-induced synaptic mitochondrial function.SIGNIFICANCE STATEMENT Methamphetamine dependence is a significant public health concern with no FDA-approved treatment. We discovered a role for the RNA binding protein hnRNP H in methamphetamine reward and reinforcement. Hnrnph1 mutation also blunted methamphetamine-induced dopamine release in the NAc, a key neurochemical event contributing to methamphetamine addiction liability. Finally, Hnrnph1 mutants showed a marked increase in basal level of synaptosomal hnRNP H and mitochondrial proteins that decreased in response to methamphetamine, whereas WT mice showed a methamphetamine-induced increase in synaptosomal mitochondrial proteins. Thus, we identified a potential role for hnRNP H in basal and dynamic mitochondrial function that informs methamphetamine-induced cellular adaptations associated with reduced addiction liability.

PI3K activation within ventromedial prefrontal cortex regulates the expression of drug-seeking in two rodent species

Phosphatidylinositide 3-kinases (PI3Ks) are intracellular signal transducer enzymes that recruit protein kinase B (aka Akt) to the cell membrane, the subsequent activation of which regulates many cellular functions. PI3K/Akt activity is up-regulated within mesocorticolimbic structures in animal models of alcoholism, but less is known regarding PI3K/Akt activity in animal models of cocaine addiction. Given that prefrontal cortex (PFC) is grossly dysregulated in addiction, we studied how cocaine affects protein indices of PFC PI3K/Akt activity in rat and mouse models and examined the relevance of PI3K activity for cocaine-related learning. Immunoblotting of mouse medial PFC at 3 weeks withdrawal from a cocaine-sensitization regimen (seven injections of 30 mg/kg, intraperitoneal [IP]) revealed increased kinase activity, as did immunoblotting of tissue from the ventral PFC of rats with a history of long-access intravenous cocaine self-administration (0.25 mg/0.1 mL infusion; 10 days of 6 h/d cocaine access). Interestingly, increased Akt phosphorylation was observed in rat ventromedial PFC at both 3- and 30-day withdrawal only in animals re-exposed to cocaine-associated cues. A conditioned place-preference paradigm in mice and a cue-elicited drug-seeking test in rats were conducted to determine the functional relevance for elevated PI3K activity for addiction-related behavior. In both cases, an intra-PFC infusion of the PI3K inhibitor wortmannin (50μM) reduced drug-seeking behavior. Taken together, this cross-species, interdisciplinary, study provides convincing evidence that cocaine history produces an enduring increase in PI3K/Akt-dependent signaling within the more ventral aspect of the PFC that is relevant to behavioral reactivity to drug-associated cues/contexts. As such, PI3K inhibitors may well serve as an effective strategy for reducing drug cue reactivity and craving in cocaine addiction.

Who is HOT and who is LOT? Detailed characterization of prescription opioid-induced changes in behavior between 129P3/J and 129S1/SvlmJ mouse substrains

Genetic factors are theorized to contribute to the substantial inter-individual variability in opioid abuse/addiction. To advance the behavioral genetics of prescription opioid abuse, our prior work identified the 129S1/SvlmJ (S1) and related 129P3/J (P3) mouse substrains, respectively, as low and high opioid-taking. Herein, we related our prior results to measures of sucrose reward/reinforcement, basal anxiety, opioid-induced place-conditioning, locomotor activity and Straub tail reaction, as well as behavioral and physiological signs of withdrawal. Substrains were also re-examined for higher-dose oxycodone and fentanyl intake under limited-access drinking procedures. S1 mice failed to acquire sucrose self-administration under various operant-conditioning procedures and exhibited lower sucrose intake in the home-cage. However, sucrose intake under limited-access procedures escalated in both substrains with repeated sucrose experience. S1 mice exhibited less spontaneous locomotor activity, as well as less opioid-induced locomotor activity and Straub tail reaction, than P3 mice and failed to exhibit an oxycodone-induced place-preference. The lack of conditioned behavior by S1 mice was unrelated to behavioral signs of withdrawal-induced negative affect or dependence severity, but might reflect high levels of basal anxiety-like behavior. Intriguingly, S1 and P3 mice initially exhibited equivalent oxycodone and fentanyl consumption in the home-cage; however opioid intake escalated only in P3 mice with repeated opioid experience. No sex differences were observed for any of our measures. These data provide additional evidence for robust differences in opioid addiction-related behaviors between P3 and S1 substrains and suggest that anxiety, learning, and/or motivational impairments might confound interpretation of operant- and place-conditioning studies employing the S1 substrain.

Taar1 gene variants have a causal role in methamphetamine intake and response and interact with Oprm1

We identified a locus on mouse chromosome 10 that accounts for 60% of the genetic variance in methamphetamine intake in mice selectively bred for high versus low methamphetamine consumption. We nominated the trace amine-associated receptor 1 gene, Taar1, as the strongest candidate and identified regulation of the mu-opioid receptor 1 gene, Oprm1, as another contributor. This study exploited CRISPR-Cas9 to test the causal role of Taar1 in methamphetamine intake and a genetically-associated thermal response to methamphetamine. The methamphetamine-related traits were rescued, converting them to levels found in methamphetamine-avoiding animals. We used a family of recombinant inbred mouse strains for interval mapping and to examine independent and epistatic effects of Taar1 and Oprm1. Both methamphetamine intake and the thermal response mapped to Taar1 and the independent effect of Taar1 was dependent on genotype at Oprm1. Our findings encourage investigation of the contribution of Taar1 and Oprm1 variants to human methamphetamine addiction.

Regional Analysis of the Brain Transcriptome in Mice Bred for High and Low Methamphetamine Consumption

Transcriptome profiling can broadly characterize drug effects and risk for addiction in the absence of drug exposure. Modern large-scale molecular methods, including RNA-sequencing (RNA-Seq), have been extensively applied to alcohol-related disease traits, but rarely to risk for methamphetamine (MA) addiction. We used RNA-Seq data from selectively bred mice with high or low risk for voluntary MA intake to construct coexpression and cosplicing networks for differential risk. Three brain reward circuitry regions were explored, the nucleus accumbens (NAc), prefrontal cortex (PFC), and ventral midbrain (VMB). With respect to differential gene expression and wiring, the VMB was more strongly affected than either the PFC or NAc. Coexpression network connectivity was higher in the low MA drinking line than in the high MA drinking line in the VMB, oppositely affected in the NAc, and little impacted in the PFC. Gene modules protected from the effects of selection may help to eliminate certain mechanisms from significant involvement in risk for MA intake. One such module was enriched in genes with dopamine-associated annotations. Overall, the data suggest that mitochondrial function and glutamate-mediated synaptic plasticity have key roles in the outcomes of selective breeding for high versus low levels of MA intake.

Effects of Methamphetamine Self-Administration and Extinction on Astrocyte Structure and Function in the Nucleus Accumbens Core

Astrocytes provide support for neurons, regulate metabolic processes, and influence neuronal communication in a variety of ways, including through the homeostatic regulation of glutamate. Following 2-h cocaine or methamphetamine self-administration (SA) and extinction, rodents display decreased levels of basal glutamate in the nucleus accumbens core (NAcore), which transitions to elevated glutamate levels during drug seeking. We hypothesized that, like cocaine, this glutamate 'overflow' during methamphetamine seeking arises via decreased expression of the astroglial glutamate transporter GLT-1, and withdrawal of perisynaptic astroglial processes (PAPs) from synapses. As expected, methamphetamine self-administration and extinction decreased the level of contact made by PAPs in the NAcore, yet did not impact glutamate uptake, GLT-1 expression, or the general structural characteristics of astrocytes. Interestingly, systemic administration of N-acetylcysteine (NAC), a drug that both upregulates GLT-1 and promotes glial-glutamate release, reduced cued methamphetamine seeking. In order to test the impact of astrocyte activation and the induction of glial glutamate release within the NAcore, we employed astrocyte-specific expression of designer receptors exclusively activated by designer drugs (DREADDs). We show here that acute activation of Gq-coupled DREADDs in this region inhibited cued methamphetamine seeking. Taken together, these data indicate that cued methamphetamine seeking following two-hour SA is not mediated by deficient glutamate clearance in the NAcore, yet can be inhibited by engaging NAcore astrocytes.

Dynamic interactions of ceftriaxone and environmental variables suppress amphetamine seeking

Extrasynaptic glutamate within the nucleus accumbens (NAc) is a driver of relapse. Cocaine, ethanol, and methamphetamine reduce the expression of cystine-glutamate antiporter (xCT) and primary glial glutamate transporter 1 (GLT1) leading to increased extrasynaptic glutamate. Ceftriaxone (CTX) restores xCT and GLT1 expression and effectively suppresses cocaine and ethanol reinstatement, however, the effects of CTX on amphetamine (AMP) reinstatement are not determined. Rodents were reared in an enriched condition (EC), isolated (IC), or standard condition (SC) and trained in AMP self-administration (0.1 mg/kg/infusion). EC, IC, and SC rats received injections of SAL or CTX (200 mg/kg) after daily extinction sessions. Then rats were tested in cue- and AMP-induced reinstatement tests. We hypothesized that EC rearing would reduce reinstatement by altering GLT1 or xCT expression in the NAc and medial prefrontal cortex (mPFC). In Experiment 2, pair-housed rats received once-daily AMP (1.0 mg/kg i.p.) or SAL for eight days followed by once-daily CTX (200 mg/kg i.p.) or SAL injections for 10 days. CTX treatment reduced cue-induced drug seeking in EC rats but not IC or SC rats. In an AMP-induced reinstatement test, CTX reduced AMP-induced drug seeking in EC and SC rats, but not IC rats. Western blot analyses revealed that AMP self-administration and non-contingent repeated AMP exposure did not downregulate GLT1 or xCT in the NAc or mPFC. Therefore, the ability for EC housing to reduce amphetamine seeking may work through other mechanisms.

Association studies of up to 1.2 million individuals yield new insights into the genetic etiology of tobacco and alcohol use

Tobacco and alcohol use are leading causes of mortality that influence risk for many complex diseases and disorders1. They are heritable2,3 and etiologically related4,5 behaviors that have been resistant to gene discovery efforts6-11. In sample sizes up to 1.2 million individuals, we discovered 566 genetic variants in 406 loci associated with multiple stages of tobacco use (initiation, cessation, and heaviness) as well as alcohol use, with 150 loci evidencing pleiotropic association. Smoking phenotypes were positively genetically correlated with many health conditions, whereas alcohol use was negatively correlated with these conditions, such that increased genetic risk for alcohol use is associated with lower disease risk. We report evidence for the involvement of many systems in tobacco and alcohol use, including genes involved in nicotinic, dopaminergic, and glutamatergic neurotransmission. The results provide a solid starting point to evaluate the effects of these loci in model organisms and more precise substance use measures.

Depression-like symptoms of withdrawal in a genetic mouse model of binge methamphetamine intake

Binge methamphetamine (MA) users have higher MA consumption, relapse rates and depression-like symptoms during early periods of withdrawal, compared with non-binge users. The impact of varying durations of MA abstinence on depression-like symptoms and on subsequent MA intake was examined in mice genetically prone to binge-level MA consumption. Binge-level MA intake was induced using a multiple-bottle choice procedure in which mice were offered one water drinking tube and three tubes containing increasing concentrations of MA in water, or four water tubes (control group). In two studies, depression-like symptoms were measured using a tail-suspension test and a subsequent forced-swim test, after forced abstinence of 6 and 30 hours from a 28-day course of chronic MA intake. An additional study measured the same depression-like symptoms, as well as MA intake, after prolonged abstinence of 1 and 2 weeks. MA high drinking mice and one of their progenitor strains DBA/2J escalated their MA intake with increasing MA concentration; however, MA high drinking mice consumed almost twice as much MA as DBA/2J mice. Depression-like symptoms were significantly higher early after MA access was withdrawn, compared to levels in drug-naïve controls, with more robust effects of MA withdrawal observed in MA high drinking than DBA/2J mice. When depression-like symptoms were examined after 1 or 2 weeks of forced abstinence in MA high drinking mice, depression-like symptoms dissipated, and subsequent MA intake was high. The MA high drinking genetic mouse model has strong face validity for human binge MA use and behavioral sequelae associated with abstinence.

Prior binge-drinking history promotes the positive affective valence of methamphetamine in mice

An alcohol use disorder is a major predisposing factor for methamphetamine (MA) abuse. Further, MA-alcohol co-abuse is a risk factor for treatment discontinuation and non-compliance in MA-dependent individuals. No effective treatment exists for MA addiction, let alone treatments directed at those suffering from MA-alcohol addiction co-morbidity. Thus, it is imperative that we develop high-throughput animal models to study the biobehavioral interactions between MA and alcohol of relevance to the etiology and treatment of co-abuse. To this end, we reported that a history of binge alcohol-drinking [5,10, 20 and 40% (v/v); 2 h/day for 10-14 days] reduces MA reinforcement and intake, but it augments MA-preference and intake when drug availability is behaviorally non-contingent. To reconcile this apparent discrepancy in findings, we employed a comparable 2-week binge-drinking paradigm as that employed in our previous studies followed by place-conditioning procedures (4 pairings of 0.25, 0.5, 1, 2 or 4 mg/kg MA, i.p.). This was meant to determine how a prior binge-drinking history impacts the affective valence of MA and sensitivity to MA-induced psychomotor-activation/sensitization. Prior binge-drinking history blunted spontaneous locomotor activity and shifted the MA dose-place-preference function upwards of water drinking controls. The potentiation of MA-conditioned reward by prior binge-drinking history was independent of any alcohol effects upon the locomotor-activating or -sensitizing effects of MA. Based on these results we propose that the reduced MA reinforcement reported previously by our group likely reflects a compensatory response to an increased sensitivity to MA's positive subjective effects rather than increased sensitivity to the drug's psychomotor-activating effects.

A Spontaneous Mutation in Taar1 Impacts Methamphetamine-Related Traits Exclusively in DBA/2 Mice from a Single Vendor

Major gene effects on traits associated with substance use disorders are rare. Previous findings in methamphetamine drinking (MADR) lines of mice, bred for high or low voluntary MA intake, and in null mutants demonstrate a major impact of the trace amine-associated receptor 1 (Taar1) gene on a triad of MA-related traits: MA consumption, MA-induced conditioned taste aversion and MA-induced hypothermia. While inbred strains are fundamentally genetically stable, rare spontaneous mutations can become fixed and result in new or aberrant phenotypes. A single nucleotide polymorphism in Taar1 that encodes a missense proline to threonine mutation in the second transmembrane domain (Taar1^m1J) has been identified in the DBA/2J strain. MA is an agonist at this receptor, but the receptor produced by Taar1^m1J does not respond to MA or endogenous ligands. In the present study, we used progeny of the C57BL/6J × DBA/2J F2 cross, the MADR lines, C57BL/6J × DBA/2J recombinant inbred strains, and DBA/2 mice sourced from four vendors to further examine Taar1-MA phenotype relations and to define the chronology of the fixation of the Taar1^m1J mutation. Mice homozygous for Taar1^m1J were found at high frequency early in selection for high MA intake in multiple replicates of the high MADR line, whereas Taar1^m1J homozygotes were absent in the low MADR line. The homozygous Taar1^m1J genotype is causally linked to increased MA intake, reduced MA-induced conditioned taste aversion, and reduced MA-induced hypothermia across models. Genotype-phenotype correlations range from 0.68 to 0.96. This Taar1 polymorphism exists in DBA/2J mice sourced directly from The Jackson Laboratory, but not DBA/2 mice sourced from Charles River (DBA/2NCrl), Envigo (formerly Harlan Sprague Dawley; DBA/2NHsd) or Taconic (DBA/2NTac). By genotyping archived samples from The Jackson Laboratory, we have determined that this mutation arose in 2001–2003. Our data strengthen the conclusion that the mutant Taar1^m1J allele, which codes for a non-functional receptor protein, increases risk for multiple MA-related traits, including MA intake, in homozygous Taar1^m1J individuals.

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.

Behavioral and Neurochemical Phenotyping of Mice Incapable of Homer1a Induction

Immediate early and constitutively expressed products of the Homer1 gene regulate the functional assembly of post-synaptic density proteins at glutamatergic synapses to influence excitatory neurotransmission and synaptic plasticity. Earlier studies of Homer1 gene knock-out (KO) mice indicated active, but distinct, roles for IEG and constitutively expressed Homer1 gene products in regulating cognitive, emotional, motivational and sensorimotor processing, as well as behavioral and neurochemical sensitivity to cocaine. More recent characterization of transgenic mice engineered to prevent generation of the IEG form (a.k.a Homer1a KO) pose a critical role for Homer1a in cocaine-induced behavioral and neurochemical sensitization of relevance to drug addiction and related neuropsychiatric disorders. Here, we extend our characterization of the Homer1a KO mouse and report a modest pro-depressant phenotype, but no deleterious effects of the KO upon spatial learning/memory, prepulse inhibition, or cocaine-induced place-conditioning. As we reported previously, Homer1a KO mice did not develop cocaine-induced behavioral or neurochemical sensitization within the nucleus accumbens; however, virus-mediated Homer1a over-expression within the nucleus accumbens reversed the sensitization phenotype of KO mice. We also report several neurochemical abnormalities within the nucleus accumbens of Homer1a KO mice that include: elevated basal dopamine and reduced basal glutamate content, Group1 mGluR agonist-induced glutamate release and high K⁺-stimulated release of dopamine and glutamate within this region. Many of the neurochemical anomalies exhibited by Homer1a KO mice are recapitulated upon deletion of the entire Homer1 gene; however, Homer1 deletion did not affect NAC dopamine or alter K⁺-stimulated neurotransmitter release within this region. These data show that the selective deletion of Homer1a produces a behavioral and neurochemical phenotype that is distinguishable from that produced by deletion of the entire Homer1 gene. Moreover, the data indicate a specific role for Homer1a in regulating cocaine-induced behavioral and neurochemical sensitization of potential relevance to the psychotogenic properties of this drug.

Compulsive methamphetamine taking in the presence of punishment is associated with increased oxytocin expression in the nucleus accumbens of rats

Methamphetamine addiction is mimicked in rats that self-administer the drug. However, these self-administration (SA) models do not include adverse consequences that are necessary to reach a diagnosis of addiction in humans. Herein, we measured genome-wide transcriptional consequences of methamphetamine SA and footshocks in the rat brain. We trained rats to self-administer methamphetamine for 20 days. Thereafter, lever-presses for methamphetamine were punished by mild footshocks for 5 days. Response-contingent punishment significantly reduced methamphetamine taking in some rats (shock-sensitive, SS) but not in others (shock-resistant, SR). Rats also underwent extinction test at one day and 30 days after the last shock session. Rats were euthanized one day after the second extinction test and the nucleus accumbens (NAc) and dorsal striatum were collected to measure gene expression with microarray analysis. In the NAc, there were changes in the expression of 13 genes in the SRvsControl and 9 genes in the SRvsSS comparison. In the striatum, there were 9 (6 up, 3 down) affected genes in the SRvsSS comparison. Among the upregulated genes was oxytocin in the NAc and CARTpt in the striatum of SR rats. These observations support a regional role of neuropeptides in the brain after a long withdrawal interval when animals show incubation of methamphetamine craving.