Persistent cognitive and dopamine transporter deficits in abstinent methamphetamine users

Nucleus accumbens invulnerability to methamphetamine neurotoxicity

Methamphetamine (Meth) is a neurotoxic drug of abuse that damages neurons and nerve endings throughout the central nervous system. Emerging studies of human Meth addicts using both postmortem analyses of brain tissue and noninvasive imaging studies of intact brains have confirmed that Meth causes persistent structural abnormalities. Animal and human studies have also defined a number of significant functional problems and comorbid psychiatric disorders associated with long-term Meth abuse. This review summarizes the salient features of Meth-induced neurotoxicity with a focus on the dopamine (DA) neuronal system. DA nerve endings in the caudate-putamen (CPu) are damaged by Meth in a highly delimited manner. Even within the CPu, damage is remarkably heterogeneous, with ventral and lateral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared the damage that accompanies binge Meth intoxication, but relatively subtle changes in the disposition of DA in its nerve endings can lead to dramatic increases in Meth-induced toxicity in the CPu and overcome the normal resistance of the NAc to damage. In contrast to the CPu, where DA neuronal deficiencies are persistent, alterations in the NAc show a partial recovery. Animal models have been indispensable in studies of the causes and consequences of Meth neurotoxicity and in the development of new therapies. This research has shown that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of Meth to include brain structures not normally targeted for damage. The resistance of the NAc to Meth-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of Meth neurotoxicity by alterations in DA homeostasis is significant in light of the numerous important roles played by this brain structure.

Fragment C Domain of Tetanus Toxin Mitigates Methamphetamine Neurotoxicity and Its Motor Consequences in Mice

BACKGROUND

The C-terminal domain of the heavy chain of tetanus toxin (Hc-TeTx) is a nontoxic peptide with demonstrated in vitro and in vivo neuroprotective effects against striatal dopaminergic damage induced by 1-methyl-4-phenylpyridinium and 6-hydoxydopamine, suggesting its possible therapeutic potential in Parkinson's disease. Methamphetamine, a widely abused psychostimulant, has selective dopaminergic neurotoxicity in rodents, monkeys, and humans. This study was undertaken to determine whether Hc-TeTx might also protect against methamphetamine-induced dopaminergic neurotoxicity and the consequent motor impairment.

METHODS

For this purpose, we treated mice with a toxic regimen of methamphetamine (4mg/kg, 3 consecutive i.p. injections, 3 hours apart) followed by 3 injections of 40 ug/kg of Hc-TeTx into grastrocnemius muscle at 1, 24, and 48 hours post methamphetamine treatment.

RESULTS

We found that Hc-TeTx significantly reduced the loss of dopaminergic markers tyrosine hydroxylase and dopamine transporter and the increases in silver staining (a well stablished degeneration marker) induced by methamphetamine in the striatum. Moreover, Hc-TeTx prevented the increase of neuronal nitric oxide synthase but did not affect microglia activation induced by methamphetamine. Stereological neuronal count in the substantia nigra indicated loss of tyrosine hydroxylase-positive neurons after methamphetamine that was partially prevented by Hc-TeTx. Importantly, impairment in motor behaviors post methamphetamine treatment were significantly reduced by Hc-TeTx.

CONCLUSIONS

Here we demonstrate that Hc-TeTx can provide significant protection against acute methamphetamine-induced neurotoxicity and motor impairment, suggesting its therapeutic potential in methamphetamine abusers.

Impulsivity, Stimulant Abuse, and Dopamine Receptor Signaling

The nonmedical use of amphetamine-type stimulants is a worldwide problem, with substantial medical and social consequences. Nonetheless, the identification of a pharmacological treatment for amphetamine use disorder remains elusive. Stimulant users exhibit neurochemical evidence of dopamine-system dysfunction as well as impulsive behaviors that may interfere with the success of treatments for their addiction. This review focuses on the potential role of dopaminergic neurotransmission in impulsivity, both in healthy individuals and chronic stimulant users who meet criteria for methamphetamine dependence. Presented are findings related to the potential contributions of signaling through dopamine D1- and D2-type receptors to self-control impulsivity in methamphetamine- dependent users. The information available points to signaling through striatal D2-type dopamine receptors as a potential therapeutic target for stimulant use disorders, but medications that target D2-type dopamine receptors have not been successful in treating stimulant-use disorders, possibly because D2-type receptors are downregulated. Other means to augment D2-type receptor signaling are therefore under consideration, and one promising approach is the addition of exercise training as an adjunct to behavioral treatment for addiction.

Rapid Recovery of Vesicular Dopamine Levels in Methamphetamine Users in Early Abstinence

We previously reported very low levels of dopamine in post-mortem striatum of chronic methamphetamine users, raising the possibility that restoration of normal dopamine levels could help in this addiction and perhaps prevent early relapse. To establish relevance of this finding to the living brain, we tested whether striatal [(11)C]-(+)-dihydrotetrabenazine binding, a vesicular monoamine transporter probe sensitive to changes in (stored) vesicular dopamine, is elevated in methamphetamine users. Chronic methamphetamine users underwent [(11)C]-(+)-dihydrotetrabenazine positron emission tomography scans during early (mean 2.6 days) and later (~10 days) abstinence. Striatal [(11)C]-(+)-dihydrotetrabenazine binding was elevated (suggesting low stored dopamine) in methamphetamine users (n=28; 2.6 days after last use) relative to controls (n=22) (+28%, p<0.0001) and correlated with severity and recency of drug use and with cognitive impairment and withdrawal symptoms. Mean [(11)C]-(+)-dihydrotetrabenazine binding levels in the subgroup of methamphetamine users who could remain abstinent ~10 days following last use (n=17) were normal at the follow-up scan. Our imaging data support post-mortem findings and suggest that chronic methamphetamine users have low brain levels of stored dopamine during very early abstinence from MA, which could contribute to behavioral and cognitive deficits. Findings also suggest a rapid recovery of stored dopamine in some methamphetamine users who become abstinent and who therefore might not benefit from dopamine replacement medication (eg, levodopa). Further study is necessary to establish whether those users who could not maintain abstinence for the second scan might have a more severe and persistent dopamine deficiency and who could benefit from this medication.

Prefrontal glutamate correlates of methamphetamine sensitization and preference

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

An acute, epitope-specific modification in the dopamine transporter associated with methamphetamine-induced neurotoxicity

Preclinical studies demonstrate that repeated, high-dose methamphetamine administrations rapidly decrease plasmalemmal dopamine uptake, which may contribute to aberrant dopamine accumulation, reactive species generation, and long-term dopaminergic deficits. The present study extends these findings by demonstrating a heretofore unreported, epitope-specific modification in the dopamine transporter caused by a methamphetamine regimen that induces these deficits. Specifically, repeated, high-dose methamphetamine injections (4 × 10 mg/kg/injection, 2-h intervals) rapidly decreased immunohistochemical detection of striatal dopamine transporter as assessed 1 h after the final methamphetamine exposure. In contrast, neither a single high dose (1 × 10 mg/kg) nor repeated injections of a lower dose (4 × 2 mg/kg/injection) induced this change. The high-dose regimen-induced alteration was only detected using antibodies directed against the N-terminus. Immunohistochemical staining using antibodies directed against the C-terminus did not reveal any changes. The high-dose regimen also did not alter dopamine transporter expression as assessed using [(125) I]RTI-55 autoradiography. These data suggest that the repeated, high-dose methamphetamine regimen alters the N-terminus of the dopamine transporter. Further, these data may be predictive of persistent dopamine deficits caused by the stimulant. Future studies of the signaling cascades involved should provide novel insight into potential mechanisms underlying the physiological and pathophysiological regulation of the dopamine transporter.

Striatal Dopamine D2/D3 Receptor Availability Is Associated with Executive Function in Healthy Controls but Not Methamphetamine Users

Background

Dopamine D₂/D₃ receptor availability in the striatum has been linked with executive function in healthy individuals, and is below control levels among drug addicts, possibly contributing to diminished executive function in the latter group. This study tested for an association of striatal D₂/D₃ receptor availability with a measure of executive function among research participants who met DSM-IV criteria for methamphetamine dependence.

Methods

Methamphetamine users and non-user controls (n = 18 per group) completed the Wisconsin Card Sorting Test and positron emission tomography with [¹⁸F]fallypride.

Results

The methamphetamine users displayed significantly lower striatal D₂/D₃ receptor availability on average than controls after controlling for age and education (p = 0.008), but they did not register greater proportions of either perseverative or non-perseverative errors when controlling for education (both ps ≥ 0.622). The proportion of non-perseverative, but not perseverative, errors was negatively correlated with striatal D₂/D₃ receptor availability among controls (r = -0.588, p = 0.010), but not methamphetamine users (r = 0.281, p = 0.258), and the group-wise interaction was significant (p = 0.030).

Conclusions

These results suggest that cognitive flexibility, as measured by perseverative errors on the Wisconsin Card Sorting Test, is not determined by signaling through striatal D₂/D₃ receptors in healthy controls, and that in stimulant abusers, who have lower D₂/D₃ receptor availability, compensation can effectively maintain other executive functions, which are associated with D₂/D₃ receptor signaling in controls.

Failure to Recognize Novelty after Extended Methamphetamine Self-Administration Results from Loss of Long-Term Depression in the Perirhinal Cortex

Exposure to methamphetamine (meth) can produce lasting memory impairments in humans and rodents. We recently demonstrated that extended access meth self-administration results in novel object recognition (NOR) memory deficits in rats. Recognition of novelty depends upon intact perirhinal (pRh) cortex function, which is compromised by meth-induced downregulation of GluN2B-containing N-methyl-D-aspartate (NMDA) receptors. NMDA receptors containing this subunit have a critical role in pRh long-term depression (LTD), one of the primary physiological processes thought to underlie object recognition memory. We hypothesized that meth-induced downregulation of GluN2B receptors would compromise pRh LTD, leading to loss of NOR memory. We found that meth self-administration resulted in an inability to induce pRh LTD following 1 Hz stimulation, an effect that was reversed with bath application of the NMDA receptor partial agonist D-cycloserine (DCS). In addition, pRh microinfusion of DCS restored meth-induced memory deficits. Furthermore, blockade of GluN2B-containing NMDA receptors with Ro 25-6981 prevented DCS restoration of pRh LTD in meth subjects. Thus, targeting pRh LTD may be a promising strategy to treat meth-induced cognitive impairment.

Chronic Nicotine Exposure Attenuates Methamphetamine-Induced Dopaminergic Deficits

Repeated methamphetamine (METH) administrations cause persistent dopaminergic deficits resembling aspects of Parkinson's disease. Many METH abusers smoke cigarettes and thus self-administer nicotine; yet few studies have investigated the effects of nicotine on METH-induced dopaminergic deficits. This interaction is of interest because preclinical studies demonstrate that nicotine can be neuroprotective, perhaps owing to effects involving α4β2 and α6β2 nicotinic acetylcholine receptors (nAChRs). This study revealed that oral nicotine exposure beginning in adolescence [postnatal day (PND) 40] through adulthood [PND 96] attenuated METH-induced striatal dopaminergic deficits when METH was administered at PND 89. This protection did not appear to be due to nicotine-induced alterations in METH pharmacokinetics. Short-term (i.e., 21-day) high-dose nicotine exposure also protected when administered from PND 40 to PND 61 (with METH at PND 54), but this protective effect did not persist. Short-term (i.e., 21-day) high-dose nicotine exposure did not protect when administered postadolescence (i.e., beginning at PND 61, with METH at PND 75). However, protection was engendered if the duration of nicotine exposure was extended to 39 days (with METH at PND 93). Autoradiographic analysis revealed that nicotine increased striatal α4β2 expression, as assessed using [(125)I]epibatidine. Both METH and nicotine decreased striatal α6β2 expression, as assessed using [(125)I]α-conotoxin MII. These findings indicate that nicotine protects against METH-induced striatal dopaminergic deficits, perhaps by affecting α4β2 and/or α6β2 expression, and that both age of onset and duration of nicotine exposure affect this protection.

Memory and brain-derived neurotrophic factor after subchronic or chronic amphetamine treatment in an animal model of mania

Progression of bipolar disorder (BD) has been associated with cognitive impairment and changes in neuroplasticity, including a decrease in serum brain-derived neurotrophic factor (BDNF). However, no study could examine BDNF levels directly in different brain regions after repeated mood episodes to date. The proposed animal model was designed to mimic several manic episodes and evaluate whether the performance in memory tasks and BDNF levels in hippocampus, prefrontal cortex, and amygdala would change after repeated amphetamine (AMPH) exposure. Adult male Wistar rats were divided into subchronic (AMPH for 7 days) and chronic groups (35 days), mimicking manic episodes at early and late stages of BD, respectively. After open field habituation or inhibitory avoidance test, rats were killed, brain regions were isolated, and BDNF mRNA and protein levels were measured by quantitative real-time PCR and ELISA, respectively. AMPH impaired habituation memory in both subchronic and chronic groups, and the impairment was worse in the chronic group. This was accompanied by increased Bdnf mRNA levels in the prefrontal cortex and amygdala region, as well as reduced BDNF protein in the hippocampus. In the inhibitory avoidance, AMPH significantly decreased the change from training to test when compared to saline. No difference was observed between subchronic and chronic groups, although chronically AMPH-treated rats presented increased Bdnf mRNA levels and decreased protein levels in hippocampus when compared to the subchronic group. Our results suggest that the cognitive impairment related to BD neuroprogression may be associated with BDNF alterations in hippocampus, prefrontal cortex, and amygdala.

Essential Oils from the Medicinal Herbs Upregulate Dopamine Transporter in Rat Pheochromocytoma Cells

The dopamine transporter (DAT) protein, a component of the dopamine system, undergoes adaptive neurobiological changes from drug abuse. Prevention of relapse and reduction of withdrawal symptoms are still the major limitations in the current pharmacological treatments of drug addiction. The present study aimed to investigate the effects of essential oils extracted from Elsholtzia ciliata, Shinchim, Angelicae gigantis Radix, and Eugenia caryophyllata, well-known traditional Korean medicines for addiction, on the modulation of dopamine system in amphetamine-treated cells and to explore the possible mechanism underlying its therapeutic effect. The potential cytotoxic effect of essential oils was evaluated in PC12 rat pheochromocytoma cells using cell viability assays. Quantification of DAT, p-CREB, p-MAPK, and p-Akt was done by immunoblotting. DAT was significantly reduced in cells treated with 50 μM of amphetamine in a time-dependent manner. No significant toxicity of essential oils from Elsholtzia ciliata and Shinchim was observed at doses of 10, 25, and 50 μg/mL. However, essential oils from A. gigantis Radix at a dose of 100 μg/mL and E. caryophyllata at doses of 50 and 100 μg/mL showed cytotoxicity. Treatment with GBR 12909, a highly selective DAT inhibitor, significantly increased DAT expression compared with that of amphetamine only by enhancing phosphorylation of mitogen-activated protein kinases (MAPK) and Akt. In addition, essential oils effectively induced hyperphosphorylation of cyclic-AMP response element-binding protein (CREB), MAPK, and Akt, which resulted in DAT upregulation. Our study implies that the essential oils may rehabilitate brain dopamine function through increased DAT availability in abstinent former drug users.

Nicotine Administration Attenuates Methamphetamine-Induced Novel Object Recognition Deficits

BACKGROUND

Previous studies have demonstrated that methamphetamine abuse leads to memory deficits and these are associated with relapse. Furthermore, extensive evidence indicates that nicotine prevents and/or improves memory deficits in different models of cognitive dysfunction and these nicotinic effects might be mediated by hippocampal or cortical nicotinic acetylcholine receptors. The present study investigated whether nicotine attenuates methamphetamine-induced novel object recognition deficits in rats and explored potential underlying mechanisms.

METHODS

Adolescent or adult male Sprague-Dawley rats received either nicotine water (10-75 μg/mL) or tap water for several weeks. Methamphetamine (4 × 7.5mg/kg/injection) or saline was administered either before or after chronic nicotine exposure. Novel object recognition was evaluated 6 days after methamphetamine or saline. Serotonin transporter function and density and α4β2 nicotinic acetylcholine receptor density were assessed on the following day.

RESULTS

Chronic nicotine intake via drinking water beginning during either adolescence or adulthood attenuated the novel object recognition deficits caused by a high-dose methamphetamine administration. Similarly, nicotine attenuated methamphetamine-induced deficits in novel object recognition when administered after methamphetamine treatment. However, nicotine did not attenuate the serotonergic deficits caused by methamphetamine in adults. Conversely, nicotine attenuated methamphetamine-induced deficits in α4β2 nicotinic acetylcholine receptor density in the hippocampal CA1 region. Furthermore, nicotine increased α4β2 nicotinic acetylcholine receptor density in the hippocampal CA3, dentate gyrus and perirhinal cortex in both saline- and methamphetamine-treated rats.

CONCLUSIONS

Overall, these findings suggest that nicotine-induced increases in α4β2 nicotinic acetylcholine receptors in the hippocampus and perirhinal cortex might be one mechanism by which novel object recognition deficits are attenuated by nicotine in methamphetamine-treated rats.

Neurologic Complications of Psychomotor Stimulant Abuse

Psychomotor stimulants are drugs that act on the central nervous system (CNS) to increase alertness, elevate mood, and produce a sense of well-being. These drugs also decrease appetite and the need for sleep. Stimulants can enhance stamina and improve performance in tasks that have been impaired by fatigue or boredom. Approved therapeutic applications of stimulants include attention deficit hyperactivity disorder (ADHD), narcolepsy, and obesity. These agents also possess potent reinforcing properties that can result in excessive self-administration and abuse. Chronic use is associated with adverse effects including psychosis, seizures, and cerebrovascular accidents, though these complications usually occur in individuals with preexisting risk factors. This chapter reviews the adverse neurologic consequences of chronic psychomotor stimulant use and abuse, with a focus on two prototypical stimulants methamphetamine and cocaine.

Monoamine reuptake inhibitors in Parkinson's disease

The motor manifestations of Parkinson's disease (PD) are secondary to a dopamine deficiency in the striatum. However, the degenerative process in PD is not limited to the dopaminergic system and also affects serotonergic and noradrenergic neurons. Because they can increase monoamine levels throughout the brain, monoamine reuptake inhibitors (MAUIs) represent potential therapeutic agents in PD. However, they are seldom used in clinical practice other than as antidepressants and wake-promoting agents. This review article summarises all of the available literature on use of 50 MAUIs in PD. The compounds are divided according to their relative potency for each of the monoamine transporters. Despite wide discrepancy in the methodology of the studies reviewed, the following conclusions can be drawn: (1) selective serotonin transporter (SERT), selective noradrenaline transporter (NET), and dual SERT/NET inhibitors are effective against PD depression; (2) selective dopamine transporter (DAT) and dual DAT/NET inhibitors exert an anti-Parkinsonian effect when administered as monotherapy but do not enhance the anti-Parkinsonian actions of L-3,4-dihydroxyphenylalanine (L-DOPA); (3) dual DAT/SERT inhibitors might enhance the anti-Parkinsonian actions of L-DOPA without worsening dyskinesia; (4) triple DAT/NET/SERT inhibitors might exert an anti-Parkinsonian action as monotherapy and might enhance the anti-Parkinsonian effects of L-DOPA, though at the expense of worsening dyskinesia.

Methamphetamine and cannabis abuse in adolescence: a quasi-experimental study on specific and long-term neurocognitive effects

OBJECTIVES

Methamphetamine abuse affects brain structure and function. Although methamphetamine and cannabis are commonly abused together, few studies have investigated the differential neurocognitive consequences of methamphetamine abuse with or without cannabis. Furthermore, the effects of drug use on the developing adolescent brain remain poorly understood. We compared neurocognitive function between adolescents with 'pure' methamphetamine abuse, those with comorbid methamphetamine and cannabis abuse, and healthy controls at baseline and follow-up.

METHODS

Individuals residing in the greater Cape Town region, between the ages of 13 and 18 years, were recruited into either Methamphetamine only group (Meth-only; n=10), Methamphetamine and cannabis group (Meth-cann; n=10) or healthy control (n=20) groups using a quasi-experimental design. All participants underwent a comprehensive neurocognitive assessment. Substance-use variables and psychiatric symptom counts were also recorded. A portion of the Meth-only and control participants completed 12-month follow-up assessments.

RESULTS

While the Meth-cann group demonstrated widespread neurocognitive deficits at baseline, these deficits were restricted to the self-monitoring domain in the Meth-only group at baseline and at follow-up.

CONCLUSIONS

Methamphetamine abuse with cannabis abuse is associated with significantly more neurocognitive impairment than methamphetamine abuse alone, and such deficits may be enduring.

Structural, functional and spectroscopic MRI studies of methamphetamine addiction

This chapter reviews selected neuroimaging findings related to long-term amphetamine and methamphetamine (MA) use. An overview of structural and functional (fMRI) MR studies, Diffusion Tensor Imaging (DTI), Magnetic Resonance Spectroscopy (MRS) and Positron Emission Tomography (PET) studies conducted in long-term MA abusers is presented. The focus of this chapter is to present the relevant studies as tools to understand brain changes following drug abstinence and recovery from addiction. The behavioral relevance of these neuroimaging studies is discussed as they relate to clinical symptoms and treatment. Within each imaging section this chapter includes a discussion of the relevant imaging studies as they relate to patterns of drug use (i.e., duration of MA use, cumulative lifetime dose and time MA abstinent) as well as an overview of studies that link the imaging findings to cognitive measures. In our conclusion we discuss some of the future directions of neuroimaging as it relates to the pathophysiology of addiction.

Amphetamine- and methamphetamine-induced hyperthermia: Implications of the effects produced in brain vasculature and peripheral organs to forebrain neurotoxicity

The adverse effects of amphetamine- (AMPH) and methamphetamine- (METH) induced hyperthermia on vasculature, peripheral organs and peripheral immune system are discussed. Hyperthermia alone does not produce amphetamine-like neurotoxicity but AMPH and METH exposures that do not produce hyperthermia (≥40°C) are minimally neurotoxic. Hyperthermia likely enhances AMPH and METH neurotoxicity directly through disruption of protein function, ion channels and enhanced ROS production. Forebrain neurotoxicity can also be indirectly influenced through the effects of AMPH- and METH- induced hyperthermia on vasculature. The hyperthermia and the hypertension produced by high doses amphetamines are a primary cause of transient breakdowns in the blood-brain barrier (BBB) resulting in concomitant regional neurodegeneration and neuroinflammation in laboratory animals. This BBB breakdown can occur in the amygdala, thalamus, striatum, sensory and motor cortex and hippocampus. Under these conditions, repetitive seizures greatly enhance neurodegeneration in hippocampus, thalamus and amygdala. Even when the BBB is less disrupted, AMPH- or METH- induced hyperthermia effects on brain vasculature may play a role in neurotoxicity. In this case, striatal and cortical vascular function are adversely affected, and even greater ROS, immune and damage responses are seen in the meninges and cortical surface vasculature. Finally, muscle and liver damage and elevated cytokines in blood can result when amphetamines produce hyperthermia. Proteins, from damaged muscle may activate the peripheral immune system and exacerbate liver damage. Liver damage can further increase cytokine levels, immune system activation and increase ammonia levels. These effects could potentially enhance vascular damage and neurotoxicity.

Chronic methamphetamine abuse and corticostriatal deficits revealed by neuroimaging

Despite aggressive efforts to contain it, methamphetamine use disorder continues to be major public health problem; and with generic behavioral therapies still the mainstay of treatment for methamphetamine abuse, rates of attrition and relapse remain high. This review summarizes the findings of structural, molecular, and functional neuroimaging studies of methamphetamine abusers, focusing on cortical and striatal abnormalities and their potential contributions to cognitive and behavioral phenotypes that can serve to promote compulsive drug use. These studies indicate that individuals with a history of chronic methamphetamine abuse often display several signs of corticostriatal dysfunction, including abnormal gray- and white-matter integrity, monoamine neurotransmitter system deficiencies, neuroinflammation, poor neuronal integrity, and aberrant patterns of brain connectivity and function, both when engaged in cognitive tasks and at rest. More importantly, many of these neural abnormalities were found to be linked with certain addiction-related phenotypes that may influence treatment response (e.g., poor self-control, cognitive inflexibility, maladaptive decision-making), raising the possibility that they may represent novel therapeutic targets.

Role of adenosine receptor subtypes in methamphetamine reward and reinforcement

The neurobiology of methamphetamine (MA) remains largely unknown despite its high abuse liability. The present series of studies explored the role of adenosine receptors on MA reward and reinforcement and identified alterations in the expression of adenosine receptors in dopamine terminal areas following MA administration in rats. We tested whether stimulating adenosine A1 or A2A receptor subtypes would influence MA-induced place preference or MA self-administration on fixed and progressive ratio schedules in male Sprague-Dawley rats. Stimulation of either adenosine A1 or A2A receptors significantly reduced the development of MA-induced place preference. Stimulating adenosine A1, but not A2A, receptors reduced MA self-administration responding. We next tested whether repeated experimenter-delivered MA administration would alter the expression of adenosine receptors in the striatal areas using immunoblotting. We observed no change in the expression of adenosine receptors. Lastly, rats were trained to self-administer MA or saline for 14 days and we detected changes in adenosine A1 and A2A receptor expression using immunoblotting. MA self-administration significantly increased adenosine A1 in the nucleus accumbens shell, caudate-putamen and prefrontal cortex. MA self-administration significantly decreased adenosine A2A receptor expression in the nucleus accumbens shell, but increased A2A receptor expression in the amygdala. These findings demonstrate that MA self-administration produces selective alterations in adenosine receptor expression in the nucleus accumbens shell and that stimulation of adenosine receptors reduces several behavioral indices of MA addiction. Together, these studies shed light onto the neurobiological alterations incurred through chronic MA use that may aid in the development of treatments for MA addiction.

Long-lasting alterations in 5-HT2A receptor after a binge regimen of methamphetamine in mice

The repeated administration of methamphetamine (MA) to animals in a single-day 'binge' dosing regimen produces damage to dopamine and serotonin terminals and psychosis-like behaviours similar to those observed in MA abusers. The present study aimed to examine the effects of MA binge exposure on 5-HT2A receptors, the subtype of serotonin receptors putatively involved in psychosis. ICR male mice were treated with MA (4 × 5 mg/kg) or saline at 2 h intervals. Recognition memory and social behaviours were sequentially evaluated by a novel location recognition test, a novel object recognition test, a social interaction and a nest-building test to confirm the persistent cognitive and behavioural impairments after this dosing regimen. Subsequently, a hallucinogenic 5-HT2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI)-induced head-twitch, molecular and electrophysiological responses were monitored. Finally, the levels of 5-HT2C, 5-HT1A, 5-HT2A and mGlu2 receptors in the medial prefrontal cortex were determined. MA binge exposure produced recognition memory impairment, reduced social behaviours, and increased DOI-induced head-twitch response, c-Fos and Egr-2 expression and field potentials in the medial prefrontal cortex. Furthermore, MA binge exposure increased 5-HT2A and decreased mGlu2 receptor expression in the medial frontal cortex, whereas 5-HT2C and 5-HT1A receptors were unaffected. These data reveal that the increased behavioural, molecular and electrophysiological responses to DOI might be associated with an up-regulation of 5-HT2A receptors in the medial prefrontal cortex after MA binge exposure. Identifying the biochemical alterations that parallel the behavioural changes in a mouse model of MA binge exposure may facilitate targeting therapies for treatment of MA-related psychiatric disorders.

New perspectives on using brain imaging to study CNS stimulants

While the recent application of brain imaging to study CNS stimulants has offered new insights into the fundamental factors that contribute to their use and abuse, many gaps remain. Brain circuits that mediate pleasure, dependence, craving and relapse are anatomically, neurophysiologically and neurochemically distinct from one another, which has guided the search for correlates of stimulant-seeking and taking behavior. However, unlike other drugs of abuse, metrics for tolerance and physical dependence on stimulants are not obvious. The dopamine theory of stimulant abuse does not sufficiently explain this disorder as serotonergic, GABAergic and glutamagergic circuits are clearly involved in stimulant pharmacology and so tracking the source of the "addictive" processes must adopt a more multimodal, multidisciplinary approach. To this end, both anatomical and functional magnetic resonance imaging (MRI), MR spectroscopy (MRS) and positron emission tomography (PET) are complementary and have equally contributed to our understanding of how stimulants affect the brain and behavior. New vistas in this area include nanotechnology approaches to deliver small molecules to receptors and use MRI to resolve receptor dynamics. Anatomical and blood flow imaging has yielded data showing that cognitive enhancers might be useful adjuncts in treating CNS stimulant dependence, while MRS has opened opportunities to examine the brain's readiness to accept treatment as GABA tone normalizes after detoxification. A desired outcome of the above approaches is being able to offer evidence-based rationales for treatment approaches that can be implemented in a more broad geographic area, where access to brain imaging facilities may be limited. This article is part of the Special Issue entitled 'CNS Stimulants'.

Methamphetamine self-administration results in persistent dopaminergic pathology: implications for Parkinson's disease risk and reward-seeking

Methamphetamine (Meth) abuse may be a risk factor for Parkinson's disease (PD); a problematic event as approximately 33 million people abuse Meth worldwide. The current study determined if a mild form of PD-like nigrostriatal pathology occurred following forced abstinence in Meth self-administering rats. The average daily intake of self-administered Meth was 3.6 ± 0.2 mg/kg/3 h over 14 sessions. Subsequently, animals were killed and the brains harvested at 1, 7, 28 or 56 days of abstinence. Post mortem, tyrosine hydroxylase (TH) immunostaining in the dorsal striatum progressively decreased throughout abstinence, reaching a 50% loss at 56 days. In the substantia nigra, there was marked reduction of TH+ cells, and Fluorogold (retrograde tracer) transport from the striatum to the nigra, at 28 and 56 days after Meth. Thus, Meth-induced progressive nigrostriatal damage occurred retrogradely, similar to PD pathology. The mesolimbic dopamine pathway [i.e. ventral tegmental area (VTA) and nucleus accumbens (NAc)], critical for Meth-induced reward, was also evaluated. TH immunostaining was decreased in the NAc-core at 28 and 56 days of forced abstinence, while staining in the dorsomedial NAc-shell was preserved. Accordingly, TH+ cell loss was evident in the lateral VTA, the origin of projections to the NAc-core, but not the medial VTA where NAc-shell projections originate. Thus, after Meth-taking ceased, a time-dependent, progressive degeneration occurred within nigrostriatal projections that eventually engulfed lateral mesolimbic projections. This pathological pattern is consistent with a trajectory for developing PD; therefore, these findings provide preclinical support for Meth abuse to increase vulnerability to developing PD.

Repeated Acquisition in the Morris Swim Task: Effects of MDMA, Methamphetamine and Methylphenidate

Acute effects of methylenedioxymethamphetamine (MDMA), methamphetamine (MA) and methylphenidate (MPD) were studied using a within-subject, repeated acquisition/performance procedure adapted to the Morris Swim Task. To investigate place learning, the acquisition component consisted of a hidden platform that varied in location across experimental sessions. As a control for drug effects not specific to acquisition, a performance component was included in which the hidden platform was in the same pool location in every experimental session. All three drugs increased escape latencies and swim distances in dose-dependent fashion. However, impairment in the acquisition component was generally observed only at doses that also produced impairment in the performance component, suggesting that effects were not selective to place learning. None of the drugs produced enhancement of learning or performance at any dose. Taken together, the results suggest that acute exposure to these psychomotor stimulants produce global impairment of performance in the Morris task, rather than specific deficits in place learning.

Striatum and insula dysfunction during reinforcement learning differentiates abstinent and relapsed methamphetamine-dependent individuals

BACKGROUND AND AIMS

Individuals with methamphetamine dependence (MD) exhibit dysfunction in brain regions involved in goal maintenance and reward processing when compared with healthy individuals. We examined whether these characteristics also reflect relapse vulnerability within a sample of MD patients.

DESIGN

Longitudinal, with functional magnetic resonance imaging (fMRI) and clinical interview data collected at baseline and relapse status collected at 1-year follow-up interview.

SETTING

Keck Imaging Center, University of California San Diego, USA.

PARTICIPANTS

MD patients (n = 60) enrolled into an in-patient drug treatment program at baseline. MD participants remaining abstinent at 1-year follow-up (abstinent MD group; n = 42) were compared with MD participants who relapsed within this period (relapsed MD group; n = 18).

MEASUREMENTS

Behavioral and neural responses to a reinforcement learning (paper-scissors-rock) paradigm recorded during an fMRI session at time of treatment.

FINDINGS

The relapsed MD group exhibited greater bilateral inferior frontal gyrus (IFG) and right striatal activation than the abstinent MD group during the learning of reward contingencies (Cohen's d range: 0.60-0.83). In contrast, the relapsed MD group displayed lower bilateral striatum, bilateral insula, left IFG and left anterior cingulate activation than the abstinent MD group (Cohen's d range: 0.90-1.23) in response to winning, tying and losing feedback.

CONCLUSIONS

Methamphetamine-dependent individuals who achieve abstinence and then relapse show greater inferior frontal gyrus activation during learning, and relatively attenuated striatal, insular and frontal activation in response to feedback, compared with methamphetamine-dependent people who remain abstinent.

Dysregulation of dopamine and glutamate release in the prefrontal cortex and nucleus accumbens following methamphetamine self-administration and during reinstatement in rats

Methamphetamine (meth) addicts often exhibit enduring cognitive and neural deficits that likely contribute to persistent drug seeking and the high rates of relapse. These deficits may be related to changes in the prefrontal cortex (PFC) and its glutamatergic projections to the nucleus accumbens (NAc). Here, we performed in vivo microdialysis in the PFC and NAc in rats following either meth self-administration or yoked-saline control histories to assess baseline glutamate (GLU) levels, or reinstatement-evoked GLU and dopamine (DA) efflux in both regions simultaneously under cue-induced, meth-primed, or combined cues+meth reinstatement conditions. Our results show that meth self-administration (1) reduced basal GLU levels in both the dmPFC and NAc, (2) concurrently increased dmPFC and NAc GLU efflux during reinstatement, and (3) increased DA efflux in the dmPFC, but not in the NAc, under all reinstatement conditions when compared with yoked-saline controls. These data demonstrate for the first time that a history of psychostimulant self-administration alters GLU homeostasis not only in the NAc, but also in the dmPFC, its primary GLU projection source. Furthermore, combined cues+meth-primed reinstatement conditions produced the most pronounced increases in mPFC and NAc extracellular GLU, suggesting that the cue and meth prime conditions are additive in promoting reinstatement. Finally, increased efflux of DA in the dmPFC, but not in the NAc, across reinstatement conditions suggests that DA release in the dmPFC may be an important mediator of drug seeking initiated by multiple relapse triggers.

Detrimental impact of remote methamphetamine dependence on neurocognitive and everyday functioning in older but not younger HIV+ adults: evidence for a legacy effect?

Prior studies examining the combined adverse effects of HIV and methamphetamine (MA) on the central nervous system (CNS) have focused on younger to middle-aged adults with recent MA use diagnoses. Aging, HIV, and MA all converge on prefrontal and temporolimbic neural systems and confer independent risk for neurocognitive and functional decline. Thus, this study sought to determine the residual impact of a remote history of MA dependence on neurocognitive and real-world outcomes in older people living with HIV (PLWH). Participants included 116 older (≥50 years) and 94 younger (<40 years) adults classified into one of six study groups based on HIV serostatus (HIV+/HIV-) and lifetime histories of MA dependence (MA+/MA-): older HIV-MA- (n = 36), older HIV+MA- (n = 49), older HIV+MA+ (n = 31), younger HIV-MA- (n = 27), younger HIV+MA- (n = 33), and younger HIV+MA+ (n = 34). No participant-met criteria for current MA use disorders and histories of MA dependence were remote in both the older (average of nearly 9 years prior to evaluation) and younger (average of over 2 years prior to evaluation) HIV+MA+ groups. Findings revealed that a remote history of MA dependence exerts a significant detrimental impact on specific aspects of neurocognitive performance (e.g., memory) and a broad range of real-world functioning outcomes (e.g., employment) among older, but not younger PLWH. These results suggest that MA-associated neurotoxicity may have significant "legacy" effects on both neurocognitive and functional outcomes to which older PLWH are particularly vulnerable.

Dopamine transporter dysfunction in Han Chinese people with chronic methamphetamine dependence after a short-term abstinence

Single-photon emission-computed tomography (SPECT) after the administration of (99m)Tc-TRODAT-1 was performed on healthy subjects and subjects with methamphetamine (METH)dependence at time 1 (T1) after 24-48 h of abstinence, time 2 (T2) after 2 weeks of abstinence, and time 3 (T3) after 4 weeks of abstinence. In contrast to values in controls, the values of the striatal DAT specific uptake ratios (SURs) in subjects with METH dependence were significantly lower at T1 (n=25), T2 (n=9), and T3 (n=8); a mild increase in SURs was observed at T2 and T3, but values were still significantly lower than those in controls. In subjects with METH dependence, there was a trend for a negative correlation of striatal DAT SURs and craving for METH at T1. METH craving, anxiety and depression scores significantly decreased from T1 to T2 to T3. We conclude that Han Chinese people with METH dependence experience significant striatal DAT dysfunction, and that these changes may be mildly reversible after 4 weeks of abstinence, but that DAT levels still remain significantly lower than those in healthy subjects. The mild recovery of striatal DAT may parallel improvements in craving, anxiety and depression.

Monoamine transporter inhibitors and substrates as treatments for stimulant abuse

The acute and chronic effects of abused psychostimulants on monoamine transporters and associated neurobiology have encouraged development of candidate medications that target these transporters. Monoamine transporters, in general, and dopamine transporters, in particular, are critical molecular targets that mediate abuse-related effects of psychostimulants such as cocaine and amphetamine. Moreover, chronic administration of psychostimulants can cause enduring changes in neurobiology reflected in dysregulation of monoamine neurochemistry and behavior. The current review will evaluate evidence for the efficacy of monoamine transporter inhibitors and substrates to reduce abuse-related effects of stimulants in preclinical assays of stimulant self-administration, drug discrimination, and reinstatement. In considering deployment of monoamine transport inhibitors and substrates as agonist-type medications to treat stimulant abuse, the safety and abuse liability of the medications are an obvious concern, and this will also be addressed. Future directions in drug discovery should identify novel medications that retain efficacy to decrease stimulant use but possess lower abuse liability and evaluate the degree to which efficacious medications can attenuate or reverse neurobiological effects of chronic stimulant use.

Exercise for methamphetamine dependence: rationale, design, and methodology

BACKGROUND

Effective pharmacotherapies to treat methamphetamine (MA) dependence have not been identified, and behavioral therapies are marginally effective. Based on behavioral studies demonstrating the potential efficacy of aerobic exercise for improving depressive symptoms, anxiety, cognitive deficits, and substance use outcomes, the study described here is examining exercise as a potential treatment for MA-dependent individuals.

METHODS

This study is randomizing 150 participants with MA dependence at a residential treatment facility for addictive disorders to receive either a thrice-weekly structured aerobic and resistance exercise intervention or a health education condition. Recruitment commenced in March, 2010. Enrollment and follow-up phases are ongoing, and recruitment is exceeding targeted enrollment rates.

CONCLUSIONS

Seeking evidence for a possibly effective adjunct to traditional behavioral approaches for treatment of MA dependence, this study is assessing the ability of an 8-week aerobic and resistance exercise protocol to reduce relapse to MA use during a 12-week follow-up period after discharge from residential-based treatment. The study also is evaluating improvements in health and functional outcomes during and after the protocol. This paper describes the design and methods of the study.

Pharmacotherapy for stimulant-related disorders

Stimulant-related disorders (SRD) continue to be an important public health problem for which there are presently no approved pharmacotherapies. Although behavioral interventions provide some benefit response varies. The development of novel and effective pharmacotherapies continues to be a research priority. Understanding neural mechanisms critical to the action of stimulants has helped reveal several potential pharmacotherapies that have already shown promise in controlled clinical trials. Common to some of these medications is the ability to reverse neural deficits in individuals with SRD. Results from thoroughly conducted clinical trials continue to broaden our knowledge increasing the possibility of soon developing effective pharmacotherapies for SRD.

Dopamine transporter availability in heroin-dependent subjects and controls: longitudinal changes during abstinence and the effects of Jitai tablets treatment

RATIONALE

Previous imaging studies have indicated that the levels of the dopamine transporter (DAT) are reduced in the brains of heroin users. However, whether these changes can be reversed by abstinence and/or treatment remains unclear.

OBJECTIVES

This study aims to investigate DAT availability in heroin users and changes in DAT availability after abstinence and treatment with the Jitai tablets, a traditional Chinese medicinal product that is approved for the treatment of opioid addiction.

METHODS

Single-photon emission computed tomography (SPECT) with [(99m)Tc] TRODAT-1 was performed on heroin-dependent patients (n = 64) and healthy controls (n = 15). The patients were randomly assigned to treatment with either placebo or the Jitai. All patients underwent SPECT imaging both at baseline and after 6 months of treatment. DAT availability was assessed in the caudate and putamen. Depression and anxiety were evaluated at baseline.

RESULTS

DAT availability remained at low levels during a 6-month period in the placebo-treated group but was increased (14-17 %) in the Jitai-treated group. The ratio of DAT availability at month 6 to that at baseline in the Jitai-treated group was significantly higher than that in the placebo-treated group in both the bilateral caudate and putamen. DAT uptake in the striatum was significantly correlated with daily heroin dose, years of heroin use, and depression.

CONCLUSIONS

These findings suggest that chronic heroin use induces long-lasting striatal DAT reductions. DAT availability remained unchanged during a 6-month period of abstinence. Treatment with Jitai appears to be effective at increasing striatal DAT availability.

Substance use and mental health characteristics associated with cognitive functioning among adults who use methamphetamine

This study describes cognitive functioning and its relation to psychiatric and substance use severity among adults with long duration methamphetamine use. Study participants (N = 405) completed a battery of tests from the Automated Neuropsychological Assessment Metrics that examined cognitive accuracy, processing speed, and efficiency. Multivariate analyses indicate that lower accuracy but faster speed on learning, spatial memory and delayed memory were correlated with more days of past-month methamphetamine use. Lifetime months of methamphetamine use was not related to cognitive functioning. Poorer cognitive efficiency was related to other problems, including crack/cocaine use, symptoms of depression, and poorer emotional state.

Pharmacotherapeutics for substance-use disorders: a focus on dopaminergic medications

INTRODUCTION

Illicit substance-use is a substantial public health concern, contributing over $150 billion in costs annually to Americans. A complex disease, a substance-use disorder affects neural circuits involved in reinforcement, motivation, learning and memory, and inhibitory control.

AREAS COVERED

The modulatory influence of dopamine in mesocorticolimbic circuits contributes to encoding the primary reinforcing effects of substances and numerous studies suggest that aberrant signaling within these circuits contributes to the development of a substance-use disorder in some individuals. Decades of research focused on the clinical development of medications that directly target dopamine receptors has led to recent studies of agonist-like dopaminergic treatments for stimulant-use disorders and, more recently, cannabis-use disorder. Human studies evaluating the efficacy of dopaminergic agonist-like medications to reduce reinforcing effects and substance-use provide some insight into the design of future pharmacotherapy trials. A search of PubMed using specific brain regions, medications, and/or the terms 'dopamine', 'cognition', 'reinforcement', 'cocaine', 'methamphetamine', 'amphetamine', 'cannabis', 'treatment/pharmacotherapy', 'addiction/abuse/dependence' identified articles relevant to this review.

EXPERT OPINION

Conceptualization of substance-use disorders and their treatment continues to evolve. Current efforts increasingly focus on a strategy fostering combination pharmacotherapies that target multiple neurotransmitter systems.

A role for D1 dopamine receptors in striatal methamphetamine-induced neurotoxicity

Methamphetamine (METH) exposure results in long-term damage to the dopamine system in both human METH abusers and animal models. One factor that has been heavily implicated in this METH-induced damage to the dopaminergic system is the activation of D1 dopamine (DA) receptors. However, a significant caveat to the studies investigating the role of the receptor in such toxicity is that genetic and pharmacological manipulations of the D1 DA receptor also mitigate METH-induced hyperthermia. Importantly, METH-induced hyperthermia is tightly associated with the neurotoxicity, such that simply cooling animals during METH exposure protects against the neurotoxicity. Therefore, it is difficult to determine whether D1 DA receptors per se play an important role in METH-induced neurotoxicity or whether the protection observed simply resulted from a mitigation of METH-induced hyperthermia. To answer this important question, the current study infused a D1 DA receptor antagonist into striatum during METH exposure while controlling for METH-induced hyperthermia. Here we found that even when METH-induced hyperthermia is maintained, the coadministration of a D1 DA receptor antagonist protects against METH-induced neurotoxicity, strongly suggesting that D1 DA receptors play an important role in METH-induced neurotoxicity apart from the mitigation of METH-induced hyperthermia.

Is there a role for nitric oxide in methamphetamine-induced dopamine terminal degeneration?

Methamphetamine (METH) abuse results in long-term damage to the dopaminergic system, manifesting as decreases in dopamine (DA) tissue content, DA transporter binding, as well as tyrosine hydroxylase and vesicular monoamine transporter immunostaining. However, the exact cascade of events that ultimately result in this damage has not been clearly elucidated. One factor that has been heavily implicated in METH-induced DA terminal degeneration is the production of nitric oxide (NO). Unfortunately, many of the studies attempting to clarify the role of NO in METH-induced neurotoxicity have been confounded by issues such as the disruption of METH-induced hyperthermia, preventing the formation of strong conclusions. As a result, there is a body of work suggesting that NO is sufficient for METH-induced neurotoxicity, while other studies suggest that NO does not play a role in METH-induced degeneration of DA nerve terminals. This review summarizes the existing studies investigating the role of NO in METH-induced neurotoxicity, and argues that while NO may be necessary for METH-induced neurotoxicity, it is not sufficient. Finally, important areas of future investigation are highlighted and discussed.

Imaging addiction: D2 receptors and dopamine signaling in the striatum as biomarkers for impulsivity

Dependence to drugs of abuse is closely associated with impulsivity, or the propensity to choose a lower, but immediate, reward over a delayed, but more valuable outcome. Here, we review clinical and preclinical studies showing that striatal dopamine signaling and D2 receptor levels - which have been shown to be decreased in addiction - directly impact impulsivity, which is itself predictive of drug self-administration. Based on these studies, we propose that the alterations in D2 receptor binding and dopamine release seen in imaging studies of addiction constitute neurobiological markers of impulsivity. Recent studies in animals also show that higher striatal dopamine signaling at the D2 receptor is associated with a greater willingness to expend effort to reach goals, and we propose that this same relationship applies to humans, particularly with respect to recovery from addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Pharmacological imaging as a tool to visualise dopaminergic neurotoxicity

Dopamine abnormalities underlie a wide variety of psychopathologies, including ADHD and schizophrenia. A new imaging technique, pharmacological magnetic resonance imaging (phMRI), is a promising non-invasive technique to visualize the dopaminergic system in the brain. In this review we explore the clinical potential of phMRI in detecting dopamine dysfunction or neurotoxicity, assess its strengths and weaknesses and identify directions for future research. Preclinically, phMRI is able to detect severe dopaminergic abnormalities quite similar to conventional techniques such as PET and SPECT. phMRI benefits from its high spatial resolution and the possibility to visualize both local and downstream effects of dopaminergic neurotransmission. In addition, it allows for repeated measurements and assessments in vulnerable populations. The major challenge is the complex interpretation of phMRI results. Future studies in patients with dopaminergic abnormalities need to confirm the currently reviewed preclinical findings to validate the technique in a clinical setting. Eventually, based on the current review we expect that phMRI can be of use in a clinical setting involving vulnerable populations (such as children and adolescents) for diagnosis and monitoring treatment efficacy. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Neurologic manifestations of chronic methamphetamine abuse

The drug with perhaps the greatest impact on the practice of Psychiatry is Methamphetamine. By increasing the extracellular concentrations of dopamine while slowly damaging the dopaminergic neurotransmission, Meth is a powerfully addictive drug whose chronic use preferentially causes psychiatric complications. Chronic Meth users have deficits in memory and executive functioning as well as higher rates of anxiety, depression, and most notably psychosis. It is because of addiction and chronic psychosis from Meth abuse that the Meth user is most likely to come to the attention of the practicing Psychiatrist/Psychologist. Understanding the chronic neurologic manifestations of Meth abuse will better arm practitioners with the diagnostic and therapeutic tools needed to make the Meth epidemic one of historical interest only.

Methamphetamine use: a comprehensive review of molecular, preclinical and clinical findings

Methamphetamine (MA) is a highly addictive psychostimulant drug that principally affects the monoamine neurotransmitter systems of the brain and results in feelings of alertness, increased energy and euphoria. The drug is particularly popular with young adults, due to its wide availability, relatively low cost, and long duration of psychoactive effects. Extended use of MA is associated with many health problems that are not limited to the central nervous system, and contribute to increased morbidity and mortality in drug users. Numerous studies, using complementary techniques, have provided evidence that chronic MA use is associated with substantial neurotoxicity and cognitive impairment. These pathological effects of the drug, combined with the addictive properties of MA, contribute to a spectrum of psychosocial issues that include medical and legal problems, at-risk behaviors and high societal costs, such as public health consequences, loss of family support and housing instability. Treatment options include pharmacological, psychological or combination therapies. The present review summarizes the key findings in the literature spanning from molecular through to clinical effects.

Methamphetamine-induced neurotoxicity disrupts naturally occurring phasic dopamine signaling

Methamphetamine (METH) is a highly addictive drug that is also neurotoxic to central dopamine (DA) systems. Although striatal DA depletions induced by METH are associated with behavioral and cognitive impairments, the link between these phenomena remains poorly understood. Previous work in both METH-pretreated animals and the 6-hydroxydopamine model of Parkinson's disease suggests that a disruption of phasic DA signaling, which is important for learning and goal-directed behavior, may be such a link. However, previous studies used electrical stimulation to elicit phasic-like DA responses and were also performed under anesthesia, which alters DA neuron activity and presynaptic function. Here we investigated the consequences of METH-induced DA terminal loss on both electrically evoked phasic-like DA signals and so-called 'spontaneous' phasic DA transients measured by voltammetry in awake rats. Not ostensibly attributable to discrete stimuli, these subsecond DA changes may play a role in enhancing reward-cue associations. METH pretreatment reduced tissue DA content in the dorsomedial striatum and nucleus accumbens by ~55%. Analysis of phasic-like DA responses elicited by reinforcing stimulation revealed that METH pretreatment decreased their amplitude and underlying mechanisms for release and uptake to a similar degree as DA content in both striatal subregions. Most importantly, characteristics of DA transients were altered by METH-induced DA terminal loss, with amplitude and frequency decreased and duration increased. These results demonstrate for the first time that denervation of DA neurons alters naturally occurring DA transients and are consistent with diminished phasic DA signaling as a plausible mechanism linking METH-induced striatal DA depletions and cognitive deficits.

Chronic methamphetamine exposure produces a delayed, long-lasting memory deficit

Methamphetamine (METH) is a highly addictive and neurotoxic psychostimulant. Its use in humans is often associated with neurocognitive impairment. Whether this is due to long-term deficits in short-term memory and/or hippocampal plasticity remains unclear. Recently, we reported that METH increases baseline synaptic transmission and reduces LTP in an ex vivo preparation of the hippocampal CA1 region from young mice. In the current study, we tested the hypothesis that a repeated neurotoxic regimen of METH exposure in adolescent mice decreases hippocampal synaptic plasticity and produces a deficit in short-term memory. Contrary to our prediction, there was no change in the hippocampal plasticity or short-term memory when measured after 14 days of METH exposure. However, we found that at 7, 14, and 21 days of drug abstinence, METH-exposed mice exhibited a deficit in spatial memory, which was accompanied by a decrease in hippocampal plasticity. Our results support the interpretation that the deleterious cognitive consequences of neurotoxic levels of METH exposure may manifest and persist after drug abstinence. Therefore, therapeutic strategies should consider short-term as well as long-term consequences of methamphetamine exposure.

Methamphetamine and Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder predominantly affecting the elderly. The aetiology of the disease is not known, but age and environmental factors play an important role. Although more than a dozen gene mutations associated with familial forms of Parkinson's disease have been described, fewer than 10% of all cases can be explained by genetic abnormalities. The molecular basis of Parkinson's disease is the loss of dopamine in the basal ganglia (caudate/putamen) due to the degeneration of dopaminergic neurons in the substantia nigra, which leads to the motor impairment characteristic of the disease. Methamphetamine is the second most widely used illicit drug in the world. In rodents, methamphetamine exposure damages dopaminergic neurons in the substantia nigra, resulting in a significant loss of dopamine in the striatum. Biochemical and neuroimaging studies in human methamphetamine users have shown decreased levels of dopamine and dopamine transporter as well as prominent microglial activation in the striatum and other areas of the brain, changes similar to those observed in PD patients. Consistent with these similarities, recent epidemiological studies have shown that methamphetamine users are almost twice as likely as non-users to develop PD, despite the fact that methamphetamine abuse and PD have distinct symptomatic profiles.

Drugs of abuse and increased risk of psychosis development

OBJECTIVE

There is considerable evidence to suggest that the abuse of illicit drugs, particularly cannabis and methamphetamine, has aetiological roles in the pathogenesis of psychosis and schizophrenia. Factors that may increase susceptibility to the propsychotic effects of these drugs include the age at which the abuse starts as well as family history of genetic polymorphisms relevant to the pathophysiology of this disorder. However, the neurobiological mechanisms involved in drug abuse-associated psychosis remain largely unclear.

METHODS AND RESULTS

This paper presents an overview of the available evidence, including clinical, animal model, and molecular studies, with a focus on brain regions and neurotransmitters systems, such as dopamine and glutamate, previously implicated in psychosis.

CONCLUSION

It is clear that further studies are urgently needed to provide a greater insight into the mechanisms that mediate the long-term and neurodevelopmental effects of cannabis and methamphetamine. A dialogue between basic science and clinical research may help to identify at-risk individuals and novel pathways for treatment and prevention.

Amphetamines, the pregnant woman and her children: a review

The objective of this study is to review and summarize available evidence regarding the impact of amphetamines on pregnancy, the newborn infant and the child. Amphetamines are neurostimulants and neurotoxins that are some of the most widely abused illicit drugs in the world. Users are at high risk of psychiatric co-morbidities, and evidence suggests that perinatal amphetamine exposure is associated with poor pregnancy outcomes, but data is confounded by other adverse factors associated with drug-dependency. Data sources are Government data, published articles, conference abstracts and book chapters. The global incidence of perinatal amphetamine exposure is most likely severely underestimated but acknowledged to be increasing rapidly, whereas exposure to other drugs, for example, heroin, is decreasing. Mothers known to be using amphetamines are at high risk of psychiatric co-morbidity and poorer obstetric outcomes, but their infants may escape detection, because the signs of withdrawal are usually less pronounced than opiate-exposed infants. There is little evidence of amphetamine-induced neurotoxicity and long-term neurodevelopmental impact, as data is scarce and difficult to extricate from the influence of other factors associated with children living in households where one or more parent uses drugs in terms of poverty and neglect. Perinatal amphetamine-exposure is an increasing worldwide concern, but robust research, especially for childhood outcomes, remains scarce. We suggest that exposed children may be at risk of ongoing developmental and behavioral impediment, and recommend that efforts be made to improve early detection of perinatal exposure and to increase provision of early-intervention services for affected children and their families.

Acute modafinil exposure reduces daytime sleepiness in abstinent methamphetamine-dependent volunteers

The purpose of this study was to evaluate the effects of acute, oral modafinil (200 mg) exposure on daytime sleepiness in methamphetamine (Meth)-dependent individuals. Eighteen Meth-dependent subjects were enrolled in a 7-d inpatient study and were administered placebo or modafinil on day 6 and the counter-condition on day 7 (randomized) of the protocol. Subjects completed several subjective daily assessments (such as the Epworth Sleepiness Scale, Pittsburgh Sleep Quality Index, Beck Depression Inventory and visual analogue scale) throughout the protocol as well as objective assessments on days 5-7, when the Multiple Sleep Latency Test was performed. The results of the current study suggest that short-term abstinence from Meth is associated with increased daytime sleepiness and that a single dose of 200 mg modafinil reduces daytime somnolence in this population. In addition, a positive correlation was found between subjective reporting of the likelihood of taking a nap and craving and desire for Meth, as well as the likelihood of using Meth and whether Meth would make the participant feel better. The results of this study should be considered when investigating candidate medications for Meth-dependence, especially in those individuals who attribute their Meth use to overcoming deficits resulting from sleep abnormalities.

Decreased dopamine activity predicts relapse in methamphetamine abusers

Studies in methamphetamine (METH) abusers showed that the decreases in brain dopamine (DA) function might recover with protracted detoxification. However, the extent to which striatal DA function in METH predicts recovery has not been evaluated. Here we assessed whether striatal DA activity in METH abusers is associated with clinical outcomes. Brain DA D2 receptor (D2R) availability was measured with positron emission tomography and [(11)C]raclopride in 16 METH abusers, both after placebo and after challenge with 60 mg oral methylphenidate (MPH) (to measure DA release) to assess whether it predicted clinical outcomes. For this purpose, METH abusers were tested within 6 months of last METH use and then followed up for 9 months of abstinence. In parallel, 15 healthy controls were tested. METH abusers had lower D2R availability in caudate than in controls. Both METH abusers and controls showed decreased striatal D2R availability after MPH and these decreases were smaller in METH than in controls in left putamen. The six METH abusers who relapsed during the follow-up period had lower D2R availability in dorsal striatum than in controls, and had no D2R changes after MPH challenge. The 10 METH abusers who completed detoxification did not differ from controls neither in striatal D2R availability nor in MPH-induced striatal DA changes. These results provide preliminary evidence that low striatal DA function in METH abusers is associated with a greater likelihood of relapse during treatment. Detection of the extent of DA dysfunction may be helpful in predicting therapeutic outcomes.

Dose-dependent changes in the locomotor responses to methamphetamine in BALB/c mice: low doses induce hypolocomotion

The overall goal of the present study was to determine the effects of different doses of (+)-methamphetamine (meth) on locomotor activity of Balb/C mice. Four experiments were designed to test a wide range of meth doses in BALB/c female mice. In Experiment 1, we examined locomotor activity induced by an acute administration of low doses of meth (0.01 and 0.03mg/kg) in a 90-min session. Experiment 2 was conducted to test higher meth doses (0.3-10mg/kg). In Experiment 3, separate sets of mice were pre-treated with various meth doses once or twice (one injection/week) prior to a locomotor challenge with a low meth dose. Finally, in Experiment 4, we tested whether locomotor activation would be affected by pretreatment with a low or moderate dose of meth one month prior to the low meth dose challenge. Results show that low doses of meth induce hypolocomotion whereas moderate to high doses induce hyperlocomotion. Prior exposure to either one moderate or high dose of meth or to two, low doses of meth attenuated the hypolocomotor effect of a low meth dose one week later. This effect was also attenuated in mice tested one month after administration of a moderate meth dose. These results show that low and high doses of meth can have opposing effects on locomotor activity. Further, prior exposure to the drug leads to tolerance, rather than sensitization, of the hypolocomotor response to low meth doses.