Extended methamphetamine self-administration in rats results in a selective reduction of dopamine transporter levels in the prefrontal cortex and dorsal striatum not accompanied by marked monoaminergic depletion

Impacts of Self-Administered 3,4-Methylenedioxypyrovalerone (MDPV) Alone, and in Combination with Caffeine, on Recognition Memory and Striatal Monoamine Neurochemistry in Male Sprague Dawley Rats: Comparisons with Methamphetamine and Cocaine

Recent data suggest that 3,4-methylenedioxypyrovalerone (MDPV) has neurotoxic effects; however, the cognitive and neurochemical consequences of MDPV self-administration remain largely unexplored. Furthermore, despite the fact that drug preparations that contain MDPV often also contain caffeine, little is known regarding the toxic effects produced by the co-use of these two stimulants. The current study investigated the degree to which self-administered MDPV or a mixture of MDPV+caffeine can produce deficits in recognition memory and alter neurochemistry relative to prototypical stimulants. Male Sprague Dawley rats were provided 90 min or 12 h access to MDPV, MDPV+caffeine, methamphetamine, cocaine, or saline for 6 weeks. Novel object recognition (NOR) memory was evaluated prior to any drug self-administration history and 3 weeks after the final self-administration session. Rats that had 12 h access to methamphetamine and those that had 90 min or 12 h access to MDPV+caffeine exhibited significant deficits in NOR, whereas no significant deficits were observed in rats that self-administered cocaine or MDPV. Striatal monoamine levels were not systematically affected. These data demonstrate synergism between MDPV and caffeine with regard to producing recognition memory deficits, highlighting the importance of recapitulating the manner in which drugs are used (e.g., in mixtures containing multiple stimulants, binge-like patterns of intake).

Impacts of Self-Administered 3,4-Methylenedioxypyrovalerone (MDPV) Alone, and in Combination with Caffeine, on Recognition Memory and Striatal Monoamine Neurochemistry in Male Sprague-Dawley Rats: Comparisons with Methamphetamine and Cocaine

Recent data suggest that 3,4-methylenedioxypyrovalerone (MDPV) has neurotoxic effects; however, the cognitive and neurochemical consequences of MDPV self-administration remain largely unexplored. Furthermore, despite the fact that drug preparations that contain MDPV often also contain caffeine, little is known regarding the toxic effects produced by the co-use of these two stimulants. The current study investigated the degree to which self-administered MDPV, or a mixture of MDPV+caffeine can produce deficits in recognition memory and alter neurochemistry relative to prototypical stimulants. Male Sprague-Dawley rats were provided 90-min or 12-h access to MDPV, MDPV+caffeine, methamphetamine, cocaine, or saline for 6 weeks. Novel object recognition (NOR) memory was evaluated prior to any drug self-administration history and 3 weeks after the final self-administration session. Rats that had 12-h access to methamphetamine and those that had 90-min or 12-h access to MDPV+caffeine exhibited significant deficits in NOR, whereas no significant deficits were observed in rats that self-administered cocaine or MDPV. Striatal mono-amine levels were not systematically affected. These data demonstrate synergism between MDPV and caffeine with regard to producing recognition memory deficits and lethality, highlighting the importance of recapitulating the manner in which drugs are used (e.g., in mixtures containing multiple stimulants, binge-like patterns of intake).

Association of coffee consumption and striatal volume in patients with Parkinson's disease and healthy controls

BACKGROUND

Mounting studies have demonstrated that coffee consumption significantly reduces the risk of developing Parkinson's disease (PD). However, there have been few investigations about the role of chronic coffee consumption in nigrostriatal structural neurodegeneration in PD. We aimed to investigate whether chronic coffee consumption is associated with the change in striatal volume in PD.

METHODS

In this study, 130 de novo patients with PD and 69 healthy controls were enrolled from the Parkinson's Progression Markers Initiative cohort. Patients with PD and healthy controls were, respectively, divided into three subgroups, including current, ever, and never coffee consumers. Then, striatal volume was compared across the three subgroups. Correlation analyses were performed to assess the relationship between cups consumed per day and striatal volume. Furthermore, we included the factors that may have influenced nigrostriatal dopaminergic neurons in multiple linear regression analyses to identify significant contributing factors to striatal volume in each investigated striatal region.

RESULTS

Current coffee consumers had decreased striatal volume compared with ever consumers in controls but not patients with PD. Furthermore, the correlation analyses revealed that cups per day were negatively correlated with striatal volume in current consumers of patients with PD and controls. In addition, multiple linear regression analyses showed that current coffee consumption remained as an independent predictor of a decrease in striatal volume in controls.

CONCLUSIONS

Our study showed that chronic coffee consumption was negatively correlated with striatal volume. In addition, our study showed that chronic coffee consumption was associated with the change in striatal volume in current-rather than ever coffee consumers, which suggests that the chronic effects of caffeine on striatal morphology may fade and even compensate after quitting coffee. Our study provides evidence for the effect of chronic coffee consumption on striatal volume in human brain in vivo.

The Neurobiology of Methamphetamine Addiction and the Potential to Reduce Misuse Through Conjugate Vaccines Targeting Toll-Like Receptor 4

The methamphetamine epidemic continues to worsen each year and has contributed to more overdose deaths than opioids. Methamphetamine was listed in the top ten lethal drugs in 2021 in the United States. The drug has been shown to cause health problems such as addiction and neurological and behavioral changes. One possible solution to address this crisis is through vaccinations. Vaccinations consist of injecting a controlled substance with the goal of creating compound-specific antibodies. Although still early in development, vaccinations have been found to improve withdrawal symptoms and decrease drug-seeking behavior with minimal health side effects in rodent studies. This paper provides an overview of the clinical presentation and neurobiology of methamphetamine addiction and drug-seeking behaviors. The responses and adverse effects of conjugate vaccines IXTv-100 with adjuvant glucopyranosyl lipid A administered in oil-water stable emulsion and tetanus-toxoid conjugated to succinyl-methamphetamine adsorbed on aluminum hydroxide combined with adjuvant E6020 are examined.

Altered neuronal activity in the ventromedial prefrontal cortex drives nicotine intake escalation

Nicotine addiction develops after prolonged drug use and escalation of drug intake. However, because of difficulties in demonstrating escalation of nicotine use in rats, its underlying neuroadaptations still remain poorly understood. Here we report that access to unusually high doses of nicotine (i.e., from 30 µg to 240 µg/kg/injection) for self-administration precipitated a rapid and robust escalation of nicotine intake and increased the motivation for the drug in rats. This nicotine intake escalation also induced long-lasting changes in vmPFC neuronal activity both before and during nicotine self-administration. Specifically, after escalation of nicotine intake, basal vmPFC neuronal activity increased above pre-escalation and control activity levels, while ongoing nicotine self-administration restored these neuronal changes. Finally, simulation of the restoring effects of nicotine with in vivo optogenetic inhibition of vmPFC neurons caused a selective de-escalation of nicotine self-administration.

Negative impact of daily screen use on inhibitory control network in preadolescence: A two-year follow-up study

The COVID-19 pandemic has made an unprecedented shift in children's daily lives. Children are increasingly spending time with screens to learn and connect with others. As the online environment rapidly substitutes in-person experience, understanding children's neuropsychological trajectories associated with screen experiences is important. Previous findings suggest that excessive screen use can lead children to prefer more immediate rewards over delayed outcomes. We hypothesized that increased screen time delays a child's development of inhibitory control system in the brain (i.e., fronto-striatal circuitry). By analyzing neuropsychological data from 8324 children (9-11ys) from the ABCD Study, we found that children who had more screen time showed a higher reward orientation and weaker fronto-striatal connectivity. Importantly, we found that the daily screen exposure mediated the effect of reward sensitivity on the development of the inhibitory control system in the brain over a two year period. These findings suggest possible negative long-term impacts of increased daily screen time on children's neuropsychological development. The results further demonstrated that screen time influences dorsal striatum connectivity, which suggests that the effect of daily screen use is a habitual seeking behavior. The study provides neural and behavioral evidence for the negative impact of daily screen use on developing children.

Amphetamine-induced prolonged disturbances in tissue levels of dopamine and serotonin in the rat brain

BACKGROUND

A hallmark of psychostimulants is the persistence of neurobiological changes they produce. The difficulty in reversing long-time effects of psychostimulants use is why addiction therapy is so ineffective. This study aimed to look for such drug-induced changes that can be detected even after many weeks of abstinence.

METHODS

Rats were given 12 doses of amphetamine (Amph) at 1.5 mg/kg. The rewarding effect of Amph was assessed using ultrasonic vocalization. After 14 and 28 days of abstinence, tissue levels of dopamine (DA), serotonin (5-HT), and their metabolites were measured in the prefrontal cortex (PFC), nucleus accumbens (Acb), dorsomedial (CPuM), and dorsolateral (CPuL) striatum.

RESULTS

After 28 days of abstinence, DA levels were increased in the dorsal striatum while 5-HT levels were decreased in all brain regions studied. The opposite direction of changes in DA and 5-HT tissue levels observed in the dorsal striatum may be related to the changes in the emotional state during abstinence and may contribute to the incubation of craving and relapses. Tissue levels of 5-HT and DA showed intra- and inter-structural correlations, most pronounced after 14 days of abstinence. Most of them were absent in the control group (ctrl), which may indicate that their appearance was related to the changes induced by earlier Amph administration. We did not find any associations between reward sensitivity and the persistence of Amph-induced neurochemical disturbances.

CONCLUSIONS

Administration of 12 moderate doses of Amph causes prolonged changes in DA and 5-HT tissue levels. The direction and severity of the changes are dependent on the brain region and the neurotransmitter studied.

Biochemical Neuroadaptations in the Rat Striatal Dopaminergic System after Prolonged Exposure to Methamphetamine Self-Administration

Perturbations in striatal dopamine (DA) homeostasis might underlie the behavioral and pathobiological consequences of METH use disorder in humans. To identify potential consequences of long-term METH exposure, we modeled the adverse consequence DSM criterion of substance use disorders by giving footshocks to rats that had escalated their intake of METH during a drug self-administration procedure. Next, DA D1 receptor antagonist, SCH23390 was injected. Thereafter, rats were euthanized to measure several indices of the striatal dopaminergic system. Footshocks split the METH rats into two phenotypes: (i) shock-sensitive that decreased their METH-intake and (ii) shock-resistant that continued their METH intake. SCH23390 caused substantial dose-dependent reduction of METH taking in both groups. Stopping SCH23390 caused re-emergence of compulsive METH taking in shock-resistant rats. Compulsive METH takers also exhibited greater incubation of METH seeking than non-compulsive rats during withdrawal from METH SA. Analyses of DA metabolism revealed non-significant decreases (about 35%) in DA levels in resistant and sensitive rats. However, striatal contents of the deaminated metabolites, DOPAL and DOPAC, were significantly increased in sensitive rats. VMAT2 and DAT protein levels were decreased in both phenotypes. Moreover, protein expression levels of the D1-like DA receptor, D5R, and D2-like DA receptors, D3R and D4R, were significantly decreased in the compulsive METH takers. Our results parallel findings in post-mortem striatal tissues of human METH users who develop Parkinsonism after long-term METH intake and support the use of this model to investigate potential therapeutic interventions for METH use disorder.

Sex-Specific Alterations in Dopamine Metabolism in the Brain after Methamphetamine Self-Administration

Methamphetamine (METH) use disorder affects both sexes, with sex differences occurring in behavioral, structural, and biochemical consequences. The molecular mechanisms underlying these differences are unclear. Herein, we used a rat model to identify potential sex differences in the effects of METH on brain dopaminergic systems. Rats were trained to self-administer METH for 20 days, and a cue-induced drug-seeking test was performed on withdrawal days 3 and 30. Dopamine and its metabolites were measured in the prefrontal cortex (PFC), nucleus accumbens (NAc), dorsal striatum (dSTR), and hippocampus (HIP). Irrespective of conditions, in comparison to females, male rats showed increased 3,4-dihydroxyphenylalanine (DOPA) in the PFC, dSTR, and HIP; increased cys-dopamine in NAc; and increased 3,4-dihydroxyphenylethanol (DOPET) and 3,4-dihydroxyphenylacetic acid (DOPAC) in dSTR. Males also showed METH-associated decreases in DA levels in the HIP but increases in the NAc. Female rats showed METH-associated decreases in DA, DOPAL, and DOPAC levels in the PFC but increases in DOPET and DOPAC levels in the HIP. Both sexes showed METH-associated decreases in NAc DA metabolites. Together, these data document sex differences in METH SA-induced changes in DA metabolism. These observations provide further support for using sex as an essential variable when discussing therapeutic approaches against METH use disorder in humans.

The effect of self-administered methamphetamine on GABAergic interneuron populations and functional connectivity of the nucleus accumbens and prefrontal cortex

INTRODUCTION

Methamphetamine (METH, "ice") is a potent and addictive psychostimulant. Abuse of METH perturbs neurotransmitter systems and induces neurotoxicity; however, the neurobiological mechanisms which underlie addiction to METH are not fully understood, limiting the efficacy of available treatments. Here we investigate METH-induced changes to neuronal nitric oxide synthase (nNOS), parvalbumin and calretinin-expressing GABAergic interneuron populations within the nucleus accumbens (NAc), prefrontal cortex (PFC) and orbitofrontal cortex (OFC). We hypothesise that dysfunction or loss of these GABAergic interneuron populations may disrupt the excitatory/inhibitory balance within the brain.

METHODS

Male Long Evans rats (N = 32) were trained to lever press for intravenous METH or received yoked saline infusions. Following 14 days of behavioural extinction, animals were given a non-contingent injection of saline or METH (1 mg/kg, IP) to examine drug-primed reinstatement to METH-seeking behaviours. Ninety minutes post-IP injection, animals were culled and brain sections were analysed for Fos, nNOS, parvalbumin and calretinin immunoreactivity in eight distinct subregions of the NAc, PFC and OFC.

RESULTS

METH exposure differentially affected GABAergic populations, with METH self-administration increasing nNOS immunoreactivity at distinct locations in the prelimbic cortex and decreasing parvalbumin immunoreactivity in the NAc. METH self-administration triggered reduced calretinin immunoreactivity, whilst acute METH administration produced a significant increase in calretinin immunoreactivity. As expected, non-contingent METH-priming treatment increased Fos immunoreactivity in subregions of the NAc and PFC.

CONCLUSION

Here we report that METH exposure in this model may alter the function of GABAergic interneurons in more subtle ways, such as alterations in neuronal firing or synaptic connectivity.

Chemogenetic inhibition of corticostriatal circuits reduces cued reinstatement of methamphetamine seeking

Methamphetamine (meth) causes enduring changes within the medial prefrontal cortex (mPFC) and the nucleus accumbens (NA). Projections from the mPFC to the NA have a distinct dorsal-ventral distribution, with the prelimbic (PL) mPFC projecting to the NAcore, and the infralimbic (IL) mPFC projecting to the NAshell. Inhibition of these circuits has opposing effects on cocaine relapse. Inhibition of PL-NAcore reduces cued reinstatement of cocaine seeking and IL-NAshell inhibition reinstates cocaine seeking. Meth, however, exhibits a different profile, as pharmacological inhibition of either the PL or IL decrease cued reinstatement of meth-seeking. The potentially opposing roles of the PL-NAcore and IL-NAshell projections remain to be explored in the context of cued meth seeking. Here we used an intersectional viral vector approach that employs a retrograde delivery of Cre from the NA and Cre-dependent expression of DREADD in the mPFC, in both male and female rats to inhibit or activate these parallel pathways. Inhibition of the PL-NAcore circuit reduced cued reinstatement of meth seeking under short and long-access meth self-administration and after withdrawal with and without extinction. Inhibition of the IL-NAshell also decreased meth cued reinstatement. Activation of the parallel circuits was without an effect. These studies show that inhibition of the PL-NAcore or the IL-NAshell circuits can inhibit reinstated meth seeking. Thus, the neural circuitry mediating cued reinstatement of meth seeking is similar to cocaine in the dorsal, but not ventral, mPFC-NA circuit.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

To more effectively manage substance use disorders, it is imperative to understand the neural, genetic, and psychological underpinnings of addictive behavior. To contribute to this understanding, considerable efforts have been made to develop translational animal models that capture key behavioral characteristics of addiction on the basis of DSM5 criteria of substance use disorders. In this review, we summarize empirical evidence for the occurrence of addiction-like behavior in animals. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, exaggerated motivation for drugs, increased reinstatement of drug seeking after extinction, preference for drugs over nondrug rewards, and resistance to punishment. The occurrence of addiction-like behavior in laboratory animals has opened the opportunity to investigate the neural, genetic, and psychological background of key aspects of addiction, which may ultimately contribute to the prevention and treatment of substance use disorders.

Gut and brain profiles that resemble pre-motor and early-stage Parkinson's disease in methamphetamine self-administering rats

INTRODUCTION

Methamphetamine is a potent psychomotor stimulant, and methamphetamine abusers are up to three times more likely to develop Parkinson's disease (PD) later in life. Prodromal PD may involve gut inflammation and the accumulation of toxic proteins that are transported from the enteric nervous system to the central nervous system to mediate, in part, the degeneration of dopaminergic projections. We hypothesized that self-administration of methamphetamine in rats produces a gut and brain profile that mirrors pre-motor and early-stage PD.

METHODS

Rats self-administered methamphetamine in daily 3 h sessions for two weeks. Motor function was assessed before self-administration, during self-administration and throughout the 56 days of forced abstinence. Assays for pathogenic markers (tyrosine hydroxylase, glial fibrillary acidic protein (GFAP), α-synuclein) were conducted on brain and gut tissue collected at one or 56 days after cessation of methamphetamine self-administration.

RESULTS

Motor deficits emerged by day 14 of forced abstinence and progressively worsened up to 56 days of forced abstinence. In the pre-motor stage, we observed increased immunoreactivity for GFAP and α-synuclein within the ganglia of the myenteric plexus in the distal colon. Increased α-synuclein was also observed in the substantia nigra pars compacta. At 56 days, GFAP and α-synuclein normalized in the gut, but the accumulation of nigral α-synuclein persisted, and the dorsolateral striatum exhibited a significant loss of tyrosine hydroxylase.

CONCLUSION

The pre-motor profile is consistent with gut inflammation and gut/brain α-synuclein accumulation associated with prodromal PD and the eventual development of the neurological disease.

Alpha-pyrrolidinopentiophenone and mephedrone self-administration produce differential neurochemical changes following short- or long-access conditions in rats

Stimulant-induced neurochemical changes may occur at different times for different brain regions or neurotransmitter systems. This study sought to examine the behavioral and neurochemical effects of extended access to α-pyrrolidinopentiophenone (α-PVP) and 4-methylmethcathinone (4MMC). Male and female Sprague-Dawley rats were trained to self-administer α-PVP (0.1 mg/kg/infusion) or 4MMC (0.5 mg/kg/infusion) through autoshaping, and then self-administered for 21 days during 1 h (short access; ShA) or 6 h (long access; LgA) sessions. Separate rats were assigned to a naïve control group. Amygdala, hippocampus, hypothalamus, prefrontal cortex (PFC), striatum, and thalamus were extracted, and tissue was analyzed with electrochemical detection and liquid chromatography mass spectrometry. Rats acquired self-administration of α-PVP and 4MMC, and LgA rats showed more escalation of self-administration than ShA rats. Synthetic cathinone administration produced several effects on neurotransmitters. LgA self-administration of α-PVP increased 5-HIAA levels in all brain regions, compared to control. In contrast, both LgA and ShA 4MMC self-administration decreased 5-HT and 5-HIAA levels in most brain regions. LgA exposure to both synthetic cathinones increased DOPAC levels in hypothalamus and striatum, and increased HVA levels in striatum compared to control. LgA self-administration of either synthetic cathinone produced region-specific increases in NE levels, whereas ShA self-administration lowered NE levels in select locations compared to control. These alterations in neurotransmitter levels indicate that synthetic cathinone use may produce differential neurochemical changes during the transition from use to abuse, and that 21 days of self-administration only models the beginning stages of dysregulated drug intake.

Psychosocial Crowding Stress-Induced Changes in Synaptic Transmission and Glutamate Receptor Expression in the Rat Frontal Cortex

This study demonstrates how exposure to psychosocial crowding stress (CS) for 3, 7, and 14 days affects glutamate synapse functioning and signal transduction in the frontal cortex (FC) of rats. CS effects on synaptic activity were evaluated in FC slices of the primary motor cortex (M1) by measuring field potential (FP) amplitude, paired-pulse ratio (PPR), and long-term potentiation (LTP). Protein expression of GluA1, GluN2B mGluR1a/5, VGLUT1, and VGLUT2 was assessed in FC by western blot. The body’s response to CS was evaluated by measuring body weight and the plasma level of plasma corticosterone (CORT), adrenocorticotropic hormone (ACTH), and interleukin 1 beta (IL1B). CS 3 14d increased FP and attenuated LTP in M1, while PPR was augmented in CS 14d. The expression of GluA1, GluN2B, and mGluR1a/5 was up-regulated in CS 3d and downregulated in CS 14d. VGLUTs expression tended to increase in CS 7d. The failure to blunt the effects of chronic CS on FP and LTP in M1 suggests the impairment of habituation mechanisms by psychosocial stressors. PPR augmented by chronic CS with increased VGLUTs level in the CS 7d indicates that prolonged CS exposure changed presynaptic signaling within the FC. The CS bidirectional profile of changes in glutamate receptors’ expression seems to be a common mechanism evoked by stress in the FC.

Neural mechanisms underlying incubation of methamphetamine craving: A mini-review

Cue-induced drug craving and seeking progressively increases during abstinence. This "incubation of drug craving" phenomenon has been observed in both laboratory animals and humans. Preclinical studies identified several neural mechanisms underlying incubation of drug craving after forced abstinence, primarily focusing on cocaine. Recently, studies started focusing on another powerful psychostimulant, methamphetamine (Meth), and developed new incubation procedures (choice-induced and punishment-induced abstinence). Here, we review mechanistic studies at the circuit, synaptic and molecular levels on incubation of Meth craving. First, we provide an overview of neural adaptations associated with prolonged forced abstinence after extended-access Meth self-administration. Next, we review studies examining the causal roles of discrete brain regions and associated circuits, glutamate transmission, histone deacetylase 5 and oxytocin in incubation of Meth craving after forced abstinence. Lastly, we review causal and correlational studies examining the mechanisms underlying incubation of Meth craving after choice-induced voluntary abstinence and punishment-induced abstinence, respectively. We conclude by discussing the translational potential of these mechanistic studies in Meth relapse prevention in human drug users.

The emergence of cardiac changes following the self-administration of methamphetamine

BACKGROUND

Clinical observations suggest an association between methamphetamine (METH) use and cardiovascular disease, but preclinical studies are lacking. The purpose of the current study was to explore changes in left ventricular function as a potential precursor to cardiovascular disease in a rodent model of METH use.

METHODS

Male rats were allowed to self-administer either METH or saline for 9 d. On the day following the 4th and 9th self-administration sessions, an echocardiogram was performed to assess left-ventricular parameters under basal conditions and following a low-dose of METH (1 mg/kg).

RESULTS

A low challenge dose of METH resulted in subtle but statistically significant changes in cardiac function during the echocardiogram in both the METH and saline self-administering groups. Further, differences in left-ventricular parameters such as stroke volume and heart rate were observed between METH and saline groups following the 9th self-administration session. Finally, supervised machine learning correctly predicted the self-administration group assignment (saline or METH) using cardiac parameters following the 9th self-administration session.

CONCLUSIONS

The findings of the current study suggest the heart, specifically the left ventricle, is sensitive to METH. Overall, these findings and emerging clinical observations highlight the need for research to investigate the effects of METH use on the heart.

Neurochemical and behavioral comparisons of contingent and non-contingent methamphetamine exposure following binge or yoked long-access self-administration paradigms

RATIONALE

Abuse of the psychostimulant methamphetamine (METH) can cause long-lasting damage to brain monoaminergic systems and is associated with profound mental health problems for users, including lasting cognitive impairments. Animal models of METH exposure have been useful in dissecting the molecular effects of the drug on cognition, but many studies use acute, non-contingent "binge" administrations of METH which do not adequately approximate human METH use. Long-term METH exposure via long-access (LgA) self-administration paradigms has been proposed to more closely reflect human use and induce cognitive impairments.

OBJECTIVE

To better understand the role of contingency and patterns of exposure in METH-induced cognitive impairments, we analyzed behavioral and neurochemical outcomes in adult male rats, comparing non-contingent "binge" METH administration with contingent (LgA) METH self-administration and non-contingent yoked partners.

RESULTS

Binge METH (40 mg/kg, i.p., over 1 day) dramatically altered striatal and hippocampal dopamine, DOPAC, 5-HT, 5-HIAA, BDNF, and TrkB 75 days after drug exposure. In contrast, 6-h LgA METH self-administration (cumulative 24.8-48.9 mg METH, i.v., over 16 days) altered hippocampal BDNF in both contingent and yoked animals but reduced striatal 5-HIAA in only contingent animals. Neurochemical alterations following binge METH administration were not accompanied by cognitive deficits in Morris water maze, novel object recognition, or Y-maze tests. However, contingent LgA METH self-administration resulted in impaired spatial memory in the water maze.

CONCLUSIONS

Overall, substantial differences in neurochemical markers between METH exposure and self-administration paradigms did not consistently translate to deficits in cognitive tasks, highlighting the complexity of correlating METH-induced neurochemical changes with cognitive outcomes.

Long-term Changes in the Central Amygdala Proteome in Rats with a History of Chronic Cocaine Self-administration

The central nucleus of the amygdala (CeA) is a striatum-like structure that contains mainly inhibitory circuits controlling a repertoire of (mal)adaptive behaviors related to pain, anxiety, motivation, and addiction. Neural activity in the CeA is also necessary for the expression of persistent and robust drug seeking, also termed 'incubation of drug craving.' However, neuroadaptations within this brain region supporting incubated drug craving have not been characterized. Here, we conducted a comprehensive analysis of protein expression in the CeA of male rats after prolonged (45-day) abstinence from extended-access cocaine self-administration using a quantitative proteomic approach. The proteomic analysis identified 228 unique proteins altered in cocaine rats relative to animals that received saline. Out of the identified proteins, 160 were downregulated, while 68 upregulated. Upregulation of tyrosine hydroxylase and downregulation of neural cell-adhesion protein contactin-1 were validated by immunoblotting. Follow-up analysis by the Ingenuity Pathway Analysis tool revealed alterations in protein networks associated with several neurobehavioral disorders, cellular function and morphology, as well as axogenesis, long-term potentiation, and receptor signaling pathways. This study suggests that chronic cocaine self-administration, followed by a prolonged abstinence results in reorganization of specific protein signaling networks within the CeA that may underlie incubated cocaine craving and identifies potential novel 'druggable' targets for the treatment of cocaine use disorder (CUD).

In vivo reduction of striatal D1R by RNA interference alters expression of D1R signaling-related proteins and enhances methamphetamine addiction in male rats

This study sought to determine if reducing dopamine D1 receptor (D1R) expression in the dorsal striatum (DS) via RNA-interference alters methamphetamine self-administration. A lentiviral construct containing a short hairpin RNA (shRNA) was used to knock down D1R expression (D1RshRNA). D1RshRNA in male rats increased responding for methamphetamine (i.v.) under a fixed-ratio schedule in an extended access paradigm, compared to D1R-intact rats. D1RshRNA also produced a vertical shift in a dose-response paradigm and enhanced responding for methamphetamine in a progressive-ratio schedule, generating a drug-vulnerable phenotype. D1RshRNA did not alter responding for sucrose (oral) under a fixed-ratio schedule compared to D1R-intact rats. Western blotting confirmed reduced D1R expression in methamphetamine and sucrose D1RshRNA rats. D1RshRNA reduced the expression of PSD-95 and MAPK-1 and increased the expression of dopamine transporter (DAT) in the DS from methamphetamine, but not sucrose rats. Sucrose density gradient fractionation was performed in behavior-naïve controls, D1RshRNA- and D1R-intact rats to determine the subcellular localization of D1Rs, DAT and D1R signaling proteins. D1Rs, DAT, MAPK-1 and PSD-95 predominantly localized to heavy fractions, and the membrane/lipid raft protein caveolin-1 (Cav-1) and flotillin-1 were distributed equally between buoyant and heavy fractions in controls. Methamphetamine increased localization of PSD-95, Cav-1, and flotillin-1 in D1RshRNA and D1R-intact rats to buoyant fractions. Our studies indicate that reduced D1R expression in the DS increases vulnerability to methamphetamine addiction-like behavior, and this is accompanied by striatal alterations in the expression of DAT and D1R signaling proteins and is independent of the subcellular localization of these proteins.

Effects of a methamphetamine vaccine, IXT-v100, on methamphetamine-related behaviors

RATIONALE

Vaccines have been developed as a potential treatment for methamphetamine (meth) use disorder (MUD). Immunization with the meth vaccine IXT-v100 has previously been shown to elicit antibodies with high affinity for meth and thus may be an effective treatment for MUD.

OBJECTIVES

These studies were designed to determine the efficacy of IXT-v100 on meth-taking and meth-seeking behaviors in rats.

METHODS

In the acquisition and maintenance study, male and female rats were trained to self-administer meth (0.06 mg/kg/infusion) over an 8-week period following vaccination. In the last 4 weeks, the dose of meth was increased or decreased each week. To assess meth-seeking behavior, the meth-primed reactivity model was used. Rats were trained to self-administer meth for 5 weeks, followed by a 5-week or 11-week forced abstinence period during which the animals were vaccinated. Rats were then placed back into the self-administration chamber immediately after being injected with meth (1 mg/kg, i.p.) but did not receive meth during the session. Responses were recorded and used as a measure of meth seeking.

RESULTS

Results from the acquisition and maintenance study in Wistar rats show that vaccination with IXT-v100 adjuvanted with glucopyranosyl lipid A stable emulsion decreases the percentage of animals that will self-administer a moderate level of meth. In the meth-primed reactivity studies, results from males showed that vaccination significantly attenuates meth-seeking behavior.

CONCLUSION

Together, these results suggest vaccination with IXT-v100 may be effective at decreasing meth-taking and meth-seeking behaviors in humans suffering with MUD.

The effect of chronic oxytocin treatment during abstinence from methamphetamine self-administration on incubation of craving, reinstatement, and anxiety

Methamphetamine (METH) abuse is characterised by chronic relapse and anxiety, for which there are no effective pharmacotherapies. Acute treatment with the neuropeptide oxytocin has shown therapeutic potential for METH addiction and has social and anxiolytic effects in METH-naïve rats. However, the effects of chronic oxytocin treatment in METH-experienced rats are unknown. This study investigated the effects of repeated oxytocin treatment during abstinence from METH self-administration on incubation of cue-induced relapse, yohimbine- and METH-induced reinstatement, trait anxiety, and social interaction. Male and female Sprague-Dawley rats self-administered intravenous METH for 2 h/day (12 days) and then on short-access (2 h/day; ShA) or long-access (6 h/day; LgA) sessions (10 days). Rats underwent 30 days of drug abstinence, during which they received 15 days of intraperitoneal oxytocin (1 mg/kg) or saline (days 6-20) injections. Anxiety and social interaction were tested on days 25-28, and incubation was assessed by testing cue-induced relapse on days 2 and 30. Rats underwent extinction after the final cue-relapse test, followed by yohimbine- and METH-primed reinstatement. LgA, but not ShA rats exhibited incubation of METH-craving and enhanced METH-primed reinstatement in both sexes, and enhanced yohimbine-induced reinstatement in females. Importantly, chronic oxytocin attenuated incubation and METH-primed reinstatement in both sexes, and yohimbine-induced reinstatement in females, although only in LgA rats. LgA produced a heightened anxiety phenotype, which was partially rescued by chronic oxytocin treatment. Using a translatable addiction model, these findings demonstrate the therapeutic efficacy of chronic oxytocin after METH self-administration and supports the clinical utility of oxytocin for METH addiction in both sexes.

Neurobiological substrates of persistent working memory deficits and cocaine-seeking in the prelimbic cortex of rats with a history of extended access to cocaine self-administration

Cocaine use disorder (CUD) is associated with prefrontal cortex dysfunction and cognitive deficits that may contribute to persistent relapse susceptibility. As the relationship between cognitive deficits, cortical abnormalities and drug seeking is poorly understood, development of relevant animal models is of high clinical importance. Here, we used an animal model to characterize working memory and reversal learning in rats with a history of extended access cocaine self-administration and prolonged abstinence. We also investigated immediate and long-term functional changes within the prelimbic cortex (PrL) in relation to cognitive performance and drug-seeking. Adult male rats underwent 6 days of short-access (1 h/day) followed by 12 days of long-access (6 h/day) cocaine self-administration, or received passive saline infusions. Next, rats were tested in delayed match-to-sample (DMS) and (non)match-to-sample (NMS) tasks, and finally in a single context + cue relapse test on day 90 of abstinence. We found that a history of chronic cocaine self-administration impaired working memory, though sparing reversal learning, and that the components of these cognitive measures correlated with later drug-seeking. Further, we found that dysregulated metabolic activity and mGlu5 receptor signaling in the PrL of cocaine rats correlated with past working memory performance and/or drug-seeking, as indicated by the analysis of cytochrome oxidase reactivity, mGlu5 and Homer 1b/c protein expression, as well as Arc mRNA expression in mGlu5-positive cells. These findings advocate for a persistent post-cocaine PrL dysfunction, rooted in ineffective compensatory changes and manifested as impaired working memory performance and hyperreactivity to cocaine cues. Considering the possible interplay between the neural correlates underlying post-cocaine cognitive deficits and drug-seeking, cognitive function should be evaluated and considered when developing neurobiologically-based treatments of cocaine relapse.

Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques

Membrane transporters are expressed in various bodily tissues and play essential roles in the homeostasis of endogenous substances and the absortion, distribution and/or excretion of xenobiotics. For transporter assays, radioisotope-labeled compounds have been mainly used. However, commercially available radioisotope-labeled compounds are limited in number and relatively expensive. Chromatographic analyses such as high-performance liquid chromatography with ultraviolet absorptiometry and liquid chromatography with tandem mass spectrometry have also been applied for transport assays. To elucidate the transport properties of endogenous substrates, although there is no difficulty in performing assays using radioisotope-labeled probes, the endogenous background and the metabolism of the compound after its translocation across cell membranes must be considered when the intact compound is assayed. In this review, the current state of knowledge about the transport of endogenous substrates via membrane transporters as determined by chromatographic techniques is summarized. Chromatographic techniques have contributed to our understanding of the transport of endogenous substances including amino acids, catecholamines, bile acids, prostanoids and uremic toxins via membrane transporters.

Chronic methamphetamine self-administration dysregulates 5-HT2A and mGlu2 receptor expression in the rat prefrontal and perirhinal cortex: Comparison to chronic phencyclidine and MK-801

Chronic methamphetamine (meth) abuse often turns into a compulsive drug-taking disorder accompanied by persistent cognitive deficits and re-occurring psychosis. Possible common neurobiological substrates underlying meth-induced deficits and schizophrenia remain poorly understood. Serotonin 2A (5-HT2A) and metabotropic glutamate 2 (mGlu2) receptors co-regulate psychosis-like behaviors and cognitive function in animals. Therefore, in the present study we examined the effects of chronic exposure to three different drugs known to produce persistent deficits in sensorimotor gating and cognition [meth, phencyclidine (PCP) and MK-801] on the expression of 5-HT2A and mGlu2 within the rat medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC) and perirhinal cortex (PRh). Adult male rats underwent 14 days of: (a) meth self-administration (6 h/day), (b) phencyclidine (PCP; 5 mg/kg, twice/day) administration, or (c) MK-801 (0.3 mg/kg, twice/day) administration. Seven days after the discontinuation of drug administration, tissues of interest were collected for protein expression analysis. We found that despite different pharmacological mechanism of action, chronic meth, PCP, and MK-801 similarly dysregulated 5-HT2A and mGlu2, as indicated by an increase in the 5-HT2A/mGlu2 expression ratio in the mPFC (all three tested drugs), PRh (meth and PCP), and dHPC (MK-801 only). Complementary changes in G-protein expression (increase in Gαq and decrease in Gαi) were also observed in the mPFC of meth animals. Finally, we found that 5-HT2A/mGlu2 cooperation can be mediated in part by the formation of the receptor heteromer in some, but not all cortical regions. In summary, these data suggest that a shift towards increased availability (and G-protein coupling) of cortical 5-HT2A vs. mGlu2 receptors may represent a common neurobiological mechanism underlying the emergence of psychosis and cognitive deficits observed in subjects with meth use disorder and schizophrenia.

Transcriptome profiling of whisker follicles in methamphetamine self-administered rats

Methamphetamine (MA) is a highly addictive psychostimulant that disturbs the central nervous system; therefore, diagnosis of MA addiction is important in clinical and forensic toxicology. In this study, a MA self-administration rat model was used to illustrate the gene expression profiling of the rewarding effect caused by MA. RNA-sequencing was performed to examine changes in gene expression in rat whisker follicles collected before self-administration, after MA self-administration, and after withdrawal sessions. We identified six distinct groups of genes, with statistically significant expression patterns. By constructing the functional association network of these genes and performing the subsequent topological analysis, we identified 43 genes, which have the potential to regulate MA reward and addiction. The gene pathways were then analysed using the Reactome and Knowledgebase for Addiction-Related Gene database, and it was found that genes and pathways associated with Alzheimer's disease and the heparan sulfate biosynthesis were enriched in MA self-administration rats. The findings suggest that changes of the genes identified in rat whisker follicles may be useful indicators of the rewarding effect of MA. Further studies are needed to provide a comprehensive understanding of MA addiction.

Replacement treatment during extinction training with the atypical dopamine uptake inhibitor, JHW-007, reduces relapse to methamphetamine seeking

There are currently no approved medications to effectively counteract the effects of methamphetamine (METH), reduce its abuse and prolong abstinence from it. Data accumulated in recent years have shown that a range of N-substituted benztropine (BZT) analogues possesses psychopharmacological features consistent with those of a potential replacement or "substitute" treatment for stimulant addiction. On the other hand, the evidence that antidepressant therapy may effectively prevent relapse to stimulant seeking is controversial. Here, we compared in rats the ability of the BZT analogue and high affinity dopamine (DA) reuptake inhibitor, JHW-007, and the antidepressant, trazodone, administered during extinction sessions after chronic METH self-administration, to alter METH-primed reinstatement of drug seeking. The data showed that trazodone produced paradoxical effects on lever pressing during extinction of METH self-administration, decreasing active, but increasing inactive, lever pressing. JHW-007 did not have any observable effects on extinction training. Importantly, JHW-007 significantly attenuated METH-primed reinstatement, whereas trazodone enhanced it. These findings lend support to the candidacy of selective DA uptake blockers, such as JHW-007, as potential treatments for METH addiction, but not to the use of antidepressant medication as a single therapeutic approach for relapse prevention.

Inflammasome in drug abuse

Drug abuse disorders refer to a set of related negative health implications associated with compulsive drug seeking and use. Because almost all addictive drugs act on the brain, many of them cause neurological impairments after long-term abuse. Neuropathological studies have revealed a widespread impairment of the cellular elements. As the key components to limit the damage of neural cells, CNS immune system is also found affected by these drugs, directly or indirectly. It has been shown that drugs of abuse alter neuroimmune gene expression and signaling. Growing studies on neuroimmune factors further demonstrate their indispensable role in drugs-induced neurotoxicity. As an important proinflammatory intracellular receptor, inflammasome is activated in many neurodegenerative diseases in response to a broad range of damage-associated molecular patterns (DAMPs) signals. In the cases of drug abuse, especially in those with comorbid of HIV infection and sustained pain, inflammasome activation significantly promotes the neuroinflammation-associated toxicities. To understand inflammasome in drug-associated neurotoxic activity, we reviewed the role played by inflammasome in drug abuse-induced microglial neurotoxicity and evaluated the potential of imflammasone as a therapeutic target for drug abuse disorders based on recent development of various selective small-molecular inflammasome inhibitors.

MicroRNA expression signature of methamphetamine use and addiction in the rat nucleus accumbens

Methamphetamine (METH) is a highly addictive psycho-stimulant that induces behavioral changes due to high level of METH-induced dopamine in the brain. Nucleus accumbens (NAc) plays an important role in these changes, especially in drug addiction. However, little is known about the underlying molecular mechanisms of METH-induced addiction. The objective of this study was to establish a behavioral model of METH use and addiction using escalating doses of METH over 15 days and to determine the global miRNA expression profiling in NAc of METH-addicted rats. In the behavioral study, the experimental rats were divided into 3 groups of 9 each: a control group, a single dose METH (5 mg/kg) treatment group and a continuous 15 alternate days METH (0.25, 0.5, 1, 2, 3, 4, 5 mg/kg) treatment group. Following that, six rats in each group were randomly selected for global miRNA profiling. Addiction behavior in rats was established using Conditioned Place Preference task. The analysis of the miRNA profiling in the NAc was performed using Affymetric microarray GeneChip® System. The findings indicated that a continuous 15 alternate days METH treatment rats showed a preference for the drug-paired compartment of the CPP. However, a one-time acute treatment with 5 mg/kg METH did not show any significant difference in preference when compared with controls. Differential profiling of miRNAs indicated that 166 miRNAs were up-regulated and 4 down-regulated in the chronic METH-treatment group when compared to controls. In comparing the chronic treatment group with the acute treatment group, 52 miRNAs were shown to be up-regulated and 7 were down-regulated. MiRNAs including miR-496-3p, miR-194-5p, miR-200b-3p and miR-181a-5p, were found to be significantly associated with METH addiction. Canonical pathway analysis revealed that a high number of METH addiction-related miRNAs play important roles in the MAPK, CREB, G-Protein Couple Receptor and GnRH Signaling pathways. Our results suggest that dynamic changes occur in the expression of miRNAs following METH exposure and addiction.

Escalation of intravenous self-administration of methylone and mephedrone under extended access conditions

The recreational use of substituted cathinones continues to grow as a public health concern in the United States. Studies have shown that extended access to intravenous (i.v.) self-administration of stimulants, such as cocaine and methamphetamine, results in escalation of drug intake relative to shorter access; however, little is known about the impact of extended access on self-administration of entactogen class stimulants such as methylone and 4-methylmethcathinone (mephedrone). Male Wistar rats were randomly assigned to short-access (ShA, 2- h) and long-access (LgA, 6- h) groups and trained to self-administer methylone or mephedrone (0.5 mg/kg/infusion) using a fixed-ratio 1 response contingency. The methylone-trained groups were evaluated on a progressive-ratio (PR) procedure incorporating dose-substitution of methylone (0.125-2.5 mg/kg/infusion), mephedrone (0.125-2.5 mg/kg/infusion) or methamphetamine (MA; 0.01-0.5 mg/kg/infusion). Mephedrone-trained rats were similarly evaluated on a PR with mephedrone and MA. Rats trained with LgA to methylone and mephedrone earned more infusions during acquisition compared with ShA groups. Mephedrone-trained LgA rats reached significantly higher breakpoints than all other groups in mephedrone and MA PR tests. Methylone-trained LgA rats exhibited a rightward shift of the peak effective dose but no overall efficacy change compared with methylone-trained ShA rats. These findings show that the self-administration of mephedrone escalates under LgA conditions in a manner similar to traditional stimulants whereas escalation of 6 h intakes of methylone is not accompanied by differences in PR performance. Thus mephedrone represents the greater risk for dysregulated drug consumption.

Incubation of extinction responding and cue-induced reinstatement, but not context- or drug priming-induced reinstatement, after withdrawal from methamphetamine

In rats trained to self-administer methamphetamine, extinction responding in the presence of drug-associated contextual and discrete cues progressively increases after withdrawal (incubation of methamphetamine craving). The conditioning factors underlying this incubation are unknown. Here, we studied incubation of methamphetamine craving under different experimental conditions to identify factors contributing to this incubation. We also determined whether the rats' response to methamphetamine priming incubates after withdrawal. We trained rats to self-administer methamphetamine in a distinct context (context A) for 14 days (6 hours/day). Lever presses were paired with a discrete light cue. We then tested groups of rats in context A or a different non-drug context (context B) after 1 day, 1 week or 1 month for extinction responding with or without the discrete cue. Subsequently, we tested the rats for reinstatement of drug seeking induced by exposure to contextual, discrete cue, or drug priming (0, 0.25 and 0.5 mg/kg). Operant responding in the extinction sessions in contexts A or B was higher after 1 week and 1 month of withdrawal than after 1 day; this effect was context-independent. Independent of the withdrawal period, operant responding in the extinction sessions was higher when responding led to contingent delivery of the discrete cue. After extinction, discrete cue-induced reinstatement, but not context- or drug priming-induced reinstatement, progressively increased after withdrawal. Together, incubation of methamphetamine craving, as assessed in extinction tests, is primarily mediated by time-dependent increases in non-reinforced operant responding, and this effect is potentiated by exposure to discrete, but not contextual, cues.

Role of Mitochondria in Methamphetamine-Induced Dopaminergic Neurotoxicity: Involvement in Oxidative Stress, Neuroinflammation, and Pro-apoptosis-A Review

Methamphetamine (MA), an amphetamine-type psychostimulant, is associated with dopaminergic toxicity and has a high abuse potential. Numerous in vivo and in vitro studies have suggested that impaired mitochondria are critical in dopaminergic toxicity induced by MA. Mitochondria are important energy-producing organelles with dynamic nature. Evidence indicated that exposure to MA can disturb mitochondrial energetic metabolism by inhibiting the Krebs cycle and electron transport chain. Alterations in mitochondrial dynamic processes, including mitochondrial biogenesis, mitophagy, and fusion/fission, have recently been shown to contribute to dopaminergic toxicity induced by MA. Furthermore, it was demonstrated that MA-induced mitochondrial impairment enhances susceptibility to oxidative stress, pro-apoptosis, and neuroinflammation in a positive feedback loop. Protein kinase Cδ has emerged as a potential mediator between mitochondrial impairment and oxidative stress, pro-apoptosis, or neuroinflammation in MA neurotoxicity. Understanding the role and underlying mechanism of mitochondrial impairment could provide a molecular target to prevent or alleviate dopaminergic toxicity induced by MA.

Effects of chronic methamphetamine on psychomotor and cognitive functions and dopamine signaling in the brain

Methamphetamine (MA) studies in animals usually involve acute, binge, or short-term exposure to the drug. However, addicts take substantial amounts of MA for extended periods of time. Here we wished to study the effects of MA exposure on brain and behavior, using an animal model analogous to this pattern of MA intake. MA doses, 4 and 8mg/kg/day, were based on previously reported average daily freely available MA self-administration levels. We examined the effects of 16 week MA treatment on psychomotor and cognitive function in the rat using open field and novel object recognition tests and we studied the adaptations of the dopaminergic system, using in vitro and in vivo receptor imaging. We show that chronic MA treatment, at doses that correspond to the average daily freely available self-administration levels in the rat, disorganizes open field activity, impairs alert exploratory behavior and anxiety-like state, and downregulates dopamine transporter in the striatum. Under these treatment conditions, dopamine terminal functional integrity in the nucleus accumbens is also affected. In addition, lower dopamine D1 receptor binding density, and, to a smaller degree, lower dopamine D2 receptor binding density were observed. Potential mechanisms related to these alterations are discussed.

Chronic methamphetamine self-administration disrupts cortical control of cognition

Methamphetamine (meth) is one of the most abused substances worldwide. Chronic use has been associated with repeated relapse episodes that may be exacerbated by cognitive impairments during drug abstinence. Growing evidence demonstrates that meth compromises prefrontal cortex activity, resulting in persisting attentional and memory impairments. After summarizing recent studies of meth-induced cognitive dysfunction using a translationally relevant model of self-administered meth, this review emphasizes the cortical brain changes contributing to cognitive dysregulation during abstinence. Finally, we propose the use of cognitive enhancers during abstinence that may promote a drug-free state by reversing cortical dysfunction linked with prolonged meth abuse.

Methamphetamine addiction: involvement of CREB and neuroinflammatory signaling pathways

RATIONALE AND OBJECTIVES

Addiction to psychostimulant methamphetamine (METH) remains a major public health problem in the world. Animal models that use METH self-administration incorporate many features of human drug-taking behavior and are very helpful in elucidating mechanisms underlying METH addiction. These models are also helping to decipher the neurobiological substrates of associated neuropsychiatric complications. This review summarizes our work on the influence of METH self-administration on dopamine systems, transcription and immune responses in the brain.

METHODS

We used the rat model of METH self-administration with extended access (15 h/day for eight consecutive days) to investigate the effects of voluntary METH intake on the markers of dopamine system integrity and changes in gene expression observed in the brain at 2 h-1 month after cessation of drug exposure.

RESULTS

Extended access to METH self-administration caused changes in the rat brain that are consistent with clinical findings reported in neuroimaging and postmortem studies of human METH addicts. In addition, gene expression studies using striatal tissues from METH self-administering rats revealed increased expression of genes involved in cAMP response element binding protein (CREB) signaling pathway and in the activation of neuroinflammatory response in the brain.

CONCLUSION

These data show an association of METH exposure with activation of neuroplastic and neuroinflammatory cascades in the brain. The neuroplastic changes may be involved in promoting METH addiction. Neuroinflammatory processes in the striatum may underlie cognitive deficits, depression, and parkinsonism reported in METH addicts. Therapeutic approaches that include suppression of neuroinflammation may be beneficial to addicted patients.

The Effect of Chronic Methamphetamine Exposure on the Hippocampal and Olfactory Bulb Neuroproteomes of Rats

Nowadays, drug abuse and addiction are serious public health problems in the USA. Methamphetamine (METH) is one of the most abused drugs and is known to cause brain damage after repeated exposure. In this paper, we conducted a neuroproteomic study to evaluate METH-induced brain protein dynamics, following a two-week chronic regimen of an escalating dose of METH exposure. Proteins were extracted from rat brain hippocampal and olfactory bulb tissues and subjected to liquid chromatography-mass spectrometry (LC-MS/MS) analysis. Both shotgun and targeted proteomic analysis were performed. Protein quantification was initially based on comparing the spectral counts between METH exposed animals and their control counterparts. Quantitative differences were further confirmed through multiple reaction monitoring (MRM) LC-MS/MS experiments. According to the quantitative results, the expression of 18 proteins (11 in the hippocampus and 7 in the olfactory bulb) underwent a significant alteration as a result of exposing rats to METH. 13 of these proteins were up-regulated after METH exposure while 5 were down-regulated. The altered proteins belonging to different structural and functional families were involved in processes such as cell death, inflammation, oxidation, and apoptosis.

Phentermine induces conditioned rewarding effects via activation of the PI3K/Akt signaling pathway in the nucleus accumbens

RATIONALE

Phentermine is structurally similar to methamphetamine and is widely used as an anti-obesity drug in the USA and many other countries. The potential for reward of phentermine has been noted; however, the mechanisms of phentermine dependence have not been established.

OBJECTIVES

Here, we investigated the rewarding and dopaminergic behavioral responses to phentermine in mice and found that phentermine produced conditioned rewarding effects through the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway in the nucleus accumbens (NAc).

METHODS

The impact of phentermine was assessed using conditioned place preference (CPP) test, climbing behavior test, and western blot analysis.

RESULTS

Phentermine 1 and 3 mg/kg (i.p.) significantly increased CPP. Phentermine, a known dopamine releaser, boosted apomorphine-induced climbing behavior in mice, and methamphetamine (i.p.) also increased apomorphine-induced dopaminergic behavior. Phentermine and methamphetamine increased the level of expression of the dopamine transporter (DAT) and phospho-Akt proteins to a similar degree in the NAc of CPP mice. To determine whether the conditioned rewarding effects of phentermine were mediated through the PI3K/Akt pathway, we assessed the effects of the Akt inhibitor LY294002 on phentermine-induced place preference and climbing behavior. LY294002 (1 and 3 μg/site, i.c.v.) reduced phentermine-induced CPP and phentermine-increased climbing behavior. However, LY294002 did not change CPP and climbing behavior itself and also did not decrease apomorphine-induced climbing behavior in mice. Further, LY294002 decreased the phentermine-increased levels of DAT protein and phosphorylation of Akt in the NAc of CPP mice.

CONCLUSIONS

Thus, these findings suggest that phentermine induces conditioned rewarding effects via activation of the PI3K/Akt signaling pathway in the NAc.

Prefrontal glutamate correlates of methamphetamine sensitization and preference

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

Methamphetamine-, d-Amphetamine-, and p-Chloroamphetamine-Induced Neurotoxicity Differentially Effect Impulsive Responding on the Stop-Signal Task in Rats

Abused amphetamines, such as d-amphetamine (AMPH) and methamphetamine (METH), are highly addictive and destructive to health and productive lifestyles. The abuse of these drugs is associated with impulsive behavior, which is likely to contribute to addiction. The amphetamines also differentially damage dopamine (DA) and serotonin (5-HT) systems, which regulate impulsive behavior; therefore, exposure to these drugs may differentially alter impulsive behavior to effect the progression of addiction. We examined the impact of neurotoxicity induced by three amphetamines on impulsive action using a stop-signal task in rats. Animals were rewarded with a food pellet after lever pressing (i.e., a go trial), unless an auditory cue was presented and withholding lever press gained reward (i.e., a stop trial). Animals were trained on the task and then exposed to a neurotoxic regimen of either AMPH, p-chloroamphetamine (PCA), or METH. These regimens preferentially reduced DA transporter levels in striatum, 5-HT transporter levels in prefrontal cortex, or both, respectively. Assessment of performance on the stop-signal task beginning 1 week after the treatment revealed that AMPH produced a deficit in go-trial performance, whereas PCA did not alter performance on either trial type. In contrast, METH produced a deficit in stop-trial performance (i.e., impulsive action) but not go-trial performance. These findings suggest that the different neurotoxic consequences of substituted amphetamines are associated with different effects on inhibitory control over behavior. Thus, the course of addiction and maladaptive behavior resulting from exposure to these substances is likely to differ.

The potential utility of some legal highs in CNS disorders

Over the last decade there has been an explosion of new drugs of abuse, so called legal highs or novel psychoactive substances (NPS). Many of these abused drugs have unknown pharmacology, but their biological effects can be anticipated from their molecular structure and possibly also from online user reports. When considered with the findings that some prescription medications are increasingly abused and that some abused drugs have been tested clinically one could argue that there has been a blurring of the line between drugs of abuse and clinically used drugs. In this review we examine these legal highs/NPS and consider whether, based on their known or predicted pharmacology, some might have the potential to be clinically useful in CNS disorders.

Proteomics Analysis of Dorsal Striatum Reveals Changes in Synaptosomal Proteins following Methamphetamine Self-Administration in Rats

Methamphetamine is a widely abused, highly addictive drug. Regulation of synaptic proteins within the brain’s reward pathway modulates addiction behaviours, the progression of drug addiction and long-term changes in brain structure and function that result from drug use. Therefore, using large scale proteomics studies we aim to identify global protein expression changes within the dorsal striatum, a key brain region involved in the modulation of addiction. We performed LC-MS/MS analyses on rat striatal synaptosomes following 30 days of methamphetamine self-administration (2 hours/day) and 14 days abstinence. We identified a total of 84 differentially-expressed proteins with known roles in neuroprotection, neuroplasticity, cell cytoskeleton, energy regulation and synaptic vesicles. We identify significant expression changes in stress-induced phosphoprotein and tubulin polymerisation-promoting protein, which have not previously been associated with addiction. In addition, we confirm the role of amphiphysin and phosphatidylethanolamine binding protein in addiction. This approach has provided new insight into the effects of methamphetamine self-administration on synaptic protein expression in a key brain region associated with addiction, showing a large set of differentially-expressed proteins that persist into abstinence. The mass spectrometry proteomics data are available via ProteomeXchange with identifier PXD001443.

Extended access to methamphetamine self-administration up-regulates dopamine transporter levels 72 hours after withdrawal in rats

Previous studies have demonstrated that there are persistent changes in dopamine systems following withdrawal from methamphetamine (METH). This study examined changes in striatal dopamine transporter (DAT), tyrosine hydroxylase (TH) and dopamine receptor 2 (D2) 72 h after withdrawal from METH intravenous self- administration (IVSA). Rats were given limited (1h) or extended (6h) access to METH IVSA (0.05 mg/kg/0.1 ml infusion) for 22 days. Controls did not receive METH IVSA. The rats given extended access to IVSA displayed higher METH intake during the first hour of drug access compared to rats given limited access. Extended access to METH also produced a concomitant increase in striatal DAT levels relative to drug-naïve controls. There were no changes in TH or D2 levels across groups. Previous studies have reported a decrease in striatal DAT levels during protracted periods (>7 days) of withdrawal from METH IVSA. This study extends previous work by showing an increase in striatal DAT protein expression during an earlier time point of withdrawal from this drug. These results are an important step toward understanding the dynamic changes in dopamine systems that occur during different time points of withdrawal from METH IVSA.

Persistent palatable food preference in rats with a history of limited and extended access to methamphetamine self-administration

Recent studies have shown that when given a mutually exclusive choice between cocaine and palatable foods, most rats prefer the non-drug rewards over cocaine. Here, we used a discrete choice procedure to assess whether palatable food preference generalizes to rats with a history of limited (3 hours/day) or extended (6 or 9 hours/day) access to methamphetamine self-administration. On different daily sessions, we trained rats to lever-press for either methamphetamine (0.1-0.2 mg/kg/infusion) or palatable food (five pellets per reward delivery) for several weeks; regular food was freely available. We then assessed food-methamphetamine preference either during training, after priming methamphetamine injections (0.5-1.0 mg/kg), following a satiety manipulation (palatable food exposure in the home cage) or after 21 days of withdrawal from methamphetamine. We also assessed progressive ratio responding for palatable food and methamphetamine. We found that independent of the daily drug access conditions and the withdrawal period, the rats strongly preferred the palatable food over methamphetamine, even when they were given free access to the palatable food in the home cage. Intake of methamphetamine and progressive ratio responding for the drug, both of which increased or escalated over time, did not predict preference in the discrete choice test. Results demonstrate that most rats strongly prefer palatable food pellets over intravenous methamphetamine, confirming previous studies using discrete choice procedures with intravenous cocaine. Results also demonstrate that escalation of drug self-administration, a popular model of compulsive drug use, is not associated with a cardinal feature of human addiction of reduced behavioral responding for non-drug rewards.

Intravenous self-administration of entactogen-class stimulants in male rats

The intravenous self-administration (IVSA) of 3,4-methylenedioxymethamphetamine (MDMA) is inconsistent in rats, with up to half of subjects failing to acquire reliable drug intake. It is unknown if this changes under long-access conditions (6 h sessions) under which the IVSA of cocaine and methamphetamine escalates. The entactogen class cathinone stimulants which exhibit MDMA-like monoamine effects in the nucleus accumbens, mephedrone (4-methylmethcathinone) and methylone (3,4-methylenedioxymethcathinone), may support more reliable IVSA but results have been mixed. This study was designed to directly compare the IVSA of these three compounds. Groups of male Wistar rats were trained to self-administer mephedrone, methylone or MDMA (0.5 mg/kg/inf) under a Fixed-Ratio (FR) 1 schedule of reinforcement for 14 sessions. Following the acquisition interval, animals were evaluated in FR (0.0, 0.125, 0.25, 0.5, 1.0, 2.5 mg/kg/inf) and Progressive Ratio (PR; 0.125, 1.0 mg/kg/inf) dose-substitution procedures. Long access conditions escalated MDMA intake over the 6 h session but not in the first 2 h. In short access, drug intake was significantly higher in mephedrone-trained rats compared with either the methylone-trained or MDMA-trained groups during acquisition. Mephedrone resulted in the highest intakes during FR and PR dose-substitution in MDMA- and mephedrone-trained groups. Overall it was found that mephedrone is a more effective reinforcer than methylone or MDMA and represents a higher risk for compulsive use.

mRNA and microRNA analysis reveals modulation of biochemical pathways related to addiction in the ventral tegmental area of methamphetamine self-administering rats

BACKGROUND

Methamphetamine is a highly addictive central nervous system stimulant with increasing levels of abuse worldwide. Alterations to mRNA and miRNA expression within the mesolimbic system can affect addiction-like behaviors and thus play a role in the development of drug addiction. While many studies have investigated the effects of high-dose methamphetamine, and identified neurotoxic effects, few have looked at the role that persistent changes in gene regulation play following methamphetamine self-administration. Therefore, the aim of this study was to identify RNA changes in the ventral tegmental area following methamphetamine self-administration. We performed microarray analyses on RNA extracted from the ventral tegmental area of Sprague-Dawley rats following methamphetamine self-administration training (2 h/day) and 14 days of abstinence.

RESULTS

We identified 78 miRNA and 150 mRNA transcripts that were differentially expressed (fdr adjusted p < 0.05, absolute log2 fold change >0.5); these included genes not previously associated with addiction (miR-125a-5p, miR-145 and Foxa1), loci encoding receptors related to drug addiction behaviors and genes with previously recognized roles in addiction such as miR-124, miR-181a, DAT and Ret.

CONCLUSION

This study provides insight into the effects of methamphetamine on RNA expression in a key brain region associated with addiction, highlighting the possibility that persistent changes in the expression of genes with both known and previously unknown roles in addiction occur.

κ Opioid receptors in the nucleus accumbens shell mediate escalation of methamphetamine intake

Given that the κ opioid receptor (KOR) system has been implicated in psychostimulant abuse, we evaluated whether the selective KOR antagonist norbinaltorphimine dihydrochloride (nor-BNI) would attenuate the escalation of methamphetamine (METH) intake in an extended-access self-administration model. Systemic nor-BNI decreased the escalation of intake of long-access (LgA) but not short-access (ShA) self-administration. nor-BNI also decreased elevated progressive-ratio (PR) breakpoints in rats in the LgA condition and continued to decrease intake after 17 d of abstinence, demonstrating that the effects of a nor-BNI injection are long lasting. Rats with an ShA history showed an increase in prodynorphin immunoreactivity in both the nucleus accumbens (NAc) core and shell, but LgA animals showed a selective increase in the NAc shell. Other cohorts of rats received nor-BNI directly into the NAc shell or core and entered into ShA or LgA. nor-BNI infusion in the NAc shell, but not NAc core, attenuated escalation of intake and PR responding for METH in LgA rats. These data indicate that the development and/or expression of compulsive-like responding for METH under LgA conditions depends on activation of the KOR system in the NAc shell and suggest that the dynorphin-KOR system is a central component of the neuroplasticity associated with negative reinforcement systems that drive the dark side of addiction.

Histamine H3 receptor antagonist JNJ-39220675 modulates locomotor responses but not place conditioning by dopaminergic drugs

RATIONALE

Brain histaminergic system is involved in the regulation of the dopaminergic circuitry. The role of histamine H3 receptor (H3R) in behaviors linked to amphetamine addiction and other behaviors induced by dopaminergic compounds has remained unclear.

OBJECTIVE

Our aim was to study whether H3R antagonist JNJ-39220675 inhibits amphetamine-induced stimulation and reward. The effects of JNJ-39220675 on dopamine D2-like receptor (D2R-like) agonist quinpirole-induced behaviors were also investigated in order to clarify whether the possible effects of H3R antagonists are D2R-like dependent.

METHODS

The effects of JNJ-39220675 on amphetamine and quinpirole-induced behavioral responses in mice were studied assessing the locomotor activation after both acute and repeated administrations of amphetamine and quinpirole. The place conditioning paradigm was also used as a measure of reward or aversion.

RESULTS

JNJ-39220675 inhibited amphetamine-induced stimulation acutely but not after repeated administrations. Amphetamine (2 mg/kg) induced conditioned place preference that was not affected by either of the tested doses of JNJ-39220675 (1 and 10 mg/kg). Quinpirole (0.5 mg/kg) induced conditioned place aversion to which the pretreatment by JNJ-39220675 (10 mg/kg) had no effect. In repeated administration, JNJ-39220675 did, however, inhibit quinpirole-induced tolerance to hypokinesia.

CONCLUSIONS

Our results show that although H3R antagonists inhibit ethanol reward, they may not possess the same ability on psychostimulants, such as amphetamine. However, if H3R antagonists will become clinically available, it is of importance that these compounds potentiate neither the rewarding nor aversive effects of other drugs.