6-[18F]fluoro-L-DOPA-PET studies show partial reversibility of long-term effects of chronic amphetamine in monkeys

Sex differences in neurobehavioral consequences of methamphetamine exposure in adult mice

RATIONALE

Recreational and medical use of stimulants is increasing, and their use may increase susceptibility to aging and promote neurobehavioral impairments. The long-term consequences of these psychostimulants and how they interact with age have not been fully studied.

OBJECTIVES

Our study investigated whether chronic exposure to the prototypical psychostimulant, methamphetamine (METH), at doses designed to emulate human therapeutic dosing, would confer a pro-oxidizing redox shift promoting long-lasting neurobehavioral impairments.

METHODS

Groups of 4-month-old male and female C57BL/6 J mice were administered non-contingent intraperitoneal injections of either saline or METH (1.4 mg/kg) twice a day for 4 weeks. Mice were randomly assigned to one experimental group: (i) short-term cognitive assessments (at 5 months), (ii) long-term cognitive assessments (at 9.5 months), and (ii) longitudinal motor assessments (at 5, 7, and 9 months). Brain regions were assessed for oxidative stress and markers of neurotoxicity after behavior testing.

RESULTS

Chronic METH exposure induced short-term effects on associative memory, gait speed, dopamine (DA) signaling, astrogliosis in females, and spatial learning and memory, balance, DA signaling, and excitotoxicity in males. There were no long-term effects of chronic METH on cognition; however, it decreased markers of excitotoxicity in the striatum and exacerbated age-associated motor impairments in males.

CONCLUSION

In conclusion, cognitive and motor functions were differentially and sex-dependently affected by METH exposure, and oxidative stress did not seem to play a role in the observed behavioral outcomes. Future studies are necessary to continue exploring the long-term neurobehavioral consequences of drug use in both sexes and the relationship between aging and drugs.

Changes in ALFF and ReHo values in methamphetamine abstinent individuals based on the Harvard-Oxford atlas: A longitudinal resting-state fMRI study

Methamphetamine (MA) abuse has become a global public health problem due to damage to various systems throughout the body, especially the central nervous system. However, the differences in resting‐state brain function between short‐term and long‐term abstinence, the pros and cons of treatments, and the relationship between resting‐state brain function and behavioral tests are unknown. Sixty‐three MA abstinent individuals were followed up for nearly 1 year and treated with three different methods. The amplitude of low‐frequency fluctuation (ALFF) and regional homogeneity (ReHo) based on the Harvard‐Oxford atlas (HOA) were measured by resting‐state functional magnetic resonance imaging (fMRI). Impulsivity was evaluated by the Barratt Impulsivity Scale‐11 (BIS‐11). Brain regions with significant increases in ALFF and ReHo values in the long‐term abstinent group compared to the short‐term abstinent group were around the right frontal pole (McKetin et al., 2012, https://doi.org/10.1111/j.1360-0443.2012.03933.x) and right middle frontal gyrus (Wang et al., 2015, https://doi.org/10.1371/journal.pone.0133431). There were no significant differences among the three groups that experienced long‐term abstinence. The changes in ALFF and ReHo in the right middle frontal gyrus were significantly associated with BIS total scores, BIS attention scores, and BIS nonplanning scores. The right middle frontal gyrus is a critical region in MA long‐term abstinent individuals exposed to therapeutic intervention, and this region may be useful, when combined with BIS‐11, as a potential biomarker to identify the effect of abstinence with therapeutic intervention in MA individuals.

Adults with a history of illicit amphetamine use exhibit abnormal substantia nigra morphology and parkinsonism

INTRODUCTION

The sonographic appearance of the substantia nigra is abnormally bright and enlarged (hyperechogenic) in young adults with a history of illicit stimulant use. The abnormality is a risk factor for Parkinson's disease. The aim of the current study was to identify the type of illicit stimulant drug associated with substantia nigra hyperechogenicity and to determine if individuals with a history of illicit stimulant use exhibit clinical signs of parkinsonism. We hypothesised that use of amphetamines (primarily methamphetamine) is associated with substantia nigra hyperechogenicity and clinical signs of parkinsonism.

METHODS

The area of echogenic signal in the substantia nigra was measured in abstinent human amphetamine users (n = 27; 33 ± 8 years) and in three control groups comprising a) 'ecstasy' users (n = 19; 23 ± 3 years), b) cannabis users (n = 30; 26 ± 8 years), and c) non-drug users (n = 37; 25 ± 7 years). A subset of subjects (n = 55) also underwent a neurological examination comprising the third and fifth part of the Unified Parkinson's Disease Rating Scale.

RESULTS

Area of substantia nigra echogenicity was significantly larger in the amphetamine group (0.276 ± 0.080 cm(2)) than in the control groups (0.200 ± 0.075, 0.190 ± 0.049, 0.191 ± 0.055 cm(2), respectively; P < 0.002). The score on the clinical rating scale was also significantly higher in the amphetamine group (8.4 ± 8.1) than in pooled controls (3.3 ± 2.8; P = 0.002).

CONCLUSION

Illicit use of amphetamines is associated with abnormal substantia nigra morphology and subtle clinical signs of parkinsonism. The results support epidemiological findings linking use of amphetamines, particularly methamphetamine, with increased risk of developing Parkinson's disease later in life.

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.

Cognitive judgment bias in the psychostimulant-induced model of mania in rats

RATIONALE

Animal models of mania lack genuine cognitive parameters. The present gold standard of mania models, amphetamine-induced hyperlocomotion, is rather unspecific and does not necessarily target its cardinal symptoms. Therefore, alternative behavioral markers that are sensitive to stimulants are required.

OBJECTIVES

In the present study, by combining the psychostimulant-induced model of mania in rodents with the recently developed ambiguous-cue interpretation (ACI) tests, we investigated the effects of chronic administration of D-amphetamine and cocaine on the cognitive judgment bias of rats.

METHODS

To accomplish this goal, in two separate experiments, previously trained animals received chronic, daily injections of either D-amphetamine (2 mg/kg) or cocaine (10 mg/kg) for 2 weeks and were subsequently tested with the ACI procedure.

RESULTS

Chronic treatment with both psychostimulants did not make rats more "optimistic."

CONCLUSIONS

The results are discussed in terms of behavioral and pharmacological actions of the tested compounds and their implications for modeling mania in animals.

Monoaminergic dysfunction in recreational users of dexamphetamine

Preclinical studies suggest that dexamphetamine (dAMPH) can lead to monoaminergic neurotoxicity. This exploratory study aimed to investigate effects of recreational dAMPH use on the dopamine (DA) and noradrenaline (NA) systems in humans. To that purpose, eight male abstinent dAMPH (26.0 ± 4.0 years) users and 10 age- and IQ-matched male healthy control subjects (23.0 ± 3.8) underwent neuropsychological testing sensitive to DAergic function and single photon emission computed tomography (SPECT) scanning with [(123)I]FP-CIT to determine striatal DA transporter (DAT) binding. In addition, changes in cerebral blood flow (CBF) induced by the DA/NA reuptake inhibitor methylphenidate (MPH) were measured using pharmacological magnetic resonance imaging (phMRI). Performance of dAMPH users was significantly worse on executive function and verbal memory tasks. Striatal DAT binding ratios were on average lower in dAMPH users (near-significant, p=0.05). In addition, CBF in control subjects decreased significantly in response to MPH in gray matter and basal ganglia, among which the striatum, thalamus and hippocampus by 10% to 29%. However, in dAMPH users the CBF response was blunted in most brain areas studied, only decreasing in the hippocampus and orbitofrontal cortex. When comparing groups, CBF response was found to be significantly different in the thalamus with a decrease for healthy controls and a blunted response in dAMPH users. Collectively, our findings of a blunted hemodynamic response in monoaminergic regions, in combination with indications for lower striatal DAT binding and poorer behavioral measures are likely to represent DAergic dysfunction in dAMPH users, although NAergic dysfunction may also play a role.

Presynaptic striatal dopamine dysfunction in people at ultra-high risk for psychosis: findings in a second cohort

BACKGROUND

Using positron emission tomography (PET), we previously observed increases in 3,4-dihydroxy-6-[(18)F]fluoro-L-phenylalanine ((18)F-DOPA) uptake in the striatum of subjects at ultra-high risk (UHR) for psychosis, indicating elevated presynaptic dopamine synthesis capacity. The purpose of this study was to test if this finding would be replicated in a second UHR cohort.

METHODS

(18)F-DOPA PET was used to estimate dopamine synthesis capacity in the striatum of an entirely new cohort of 26 individuals at UHR for psychosis (14 males, mean±SD age = 22.7±4.7 years) and 20 healthy volunteers matched for age and gender (11 males, mean±SD age = 24.5±4.5 years).

RESULTS

Dopamine synthesis capacity was elevated in the whole [t(44) = 2.6; p = .01, effect size = .81] and associative striatum [t(44) = 2.6; p = .01, effect size = .73] of UHR compared with control subjects. When the two samples were combined to give a final sample of 32 control and 50 UHR subjects, the higher levels of dopamine synthesis capacity in the UHR group reached significance across the whole [F(1,81) = 11.0; p = .001], associative [F(1,81) = 12.7; p = .001], and sensorimotor [F(1,81) = 4.7; p = .03], but not the limbic [F(1,81) = 2.1; p = .2], striatum.

CONCLUSIONS

The findings indicate that elevated dopamine synthesis capacity in the dorsal striatum is a robust feature of individuals at UHR for psychosis and provide further evidence that dopaminergic abnormalities precede the onset of psychosis.

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'.

Dopaminergic dysfunction in abstinent dexamphetamine users: results from a pharmacological fMRI study using a reward anticipation task and a methylphenidate challenge

BACKGROUND

Dopamine (DA) is involved in systems governing motor actions, motivational processes and cognitive functions. Preclinical studies have shown that even relatively low doses of d-amphetamine (dAMPH) (equivalent to doses used in clinical Practice) can lead to DA neurotoxicity in rodents and non-human primates (Ricaurte et al., 2005).

METHODS

Therefore, we investigated the DAergic function in eight male recreational users of dAMPH and eight male healthy controls using functional magnetic resonance imaging (fMRI). We compared brain activation between both groups during a monetary incentive delay task (Knutson et al., 2001) with and without an oral methylphenidate (MPH) challenge. All subjects were abstinent for at least 2 weeks during the baseline scan. The second scan was performed on the same day 1.5 h after receiving an oral dose of 35 mg MPH (approximately 0.5 mg/kg) when peak MPH binding was assumed.

RESULTS

When anticipating reward, dAMPH users showed lower striatal activation in comparison to control subjects. In addition, MPH induced a reduction in the striatal activation during reward anticipation in healthy controls, whereas no such effect was observed in dAMPH users.

CONCLUSION

The combination of these findings provides further evidence for frontostriatal DAergic dysfunction in recreational dAMPH users and is consistent with preclinical data suggesting neurotoxic effects of chronic dAMPH use. The findings of this explorative study could have important implications for humans in need for treatment with dAMPH, such as patients suffering from ADHD and therefore this study needs replication in a larger sample.

Illicit stimulant use is associated with abnormal substantia nigra morphology in humans

Use of illicit stimulants such as methamphetamine, cocaine, and ecstasy is an increasing health problem. Chronic use can cause neurotoxicity in animals and humans but the long-term consequences are not well understood. The aim of the current study was to investigate the long-term effect of stimulant use on the morphology of the human substantia nigra. We hypothesised that history of illicit stimulant use is associated with an abnormally bright and enlarged substantia nigra (termed 'hyperechogenicity') when viewed with transcranial sonography. Substantia nigra morphology was assessed in abstinent stimulant users (n = 36; 31±9 yrs) and in two groups of control subjects: non-drug users (n = 29; 24±5 yrs) and cannabis users (n = 12; 25±7 yrs). Substantia nigra morphology was viewed with transcranial sonography and the area of echogenicity at the anatomical site of the substantia nigra was measured at its greatest extent. The area of substantia nigra echogenicity was significantly larger in the stimulant group (0.273±0.078 cm(2)) than in the control (0.201±0.054 cm(2); P<0.001) and cannabis (0.202±0.045 cm(2); P<0.007) groups. 53% of stimulant users exhibited echogenicity that exceeded the 90(th) percentile for the control group. The results of the current study suggest that individuals with a history of illicit stimulant use exhibit abnormal substantia nigra morphology. Substantia nigra hyperechogenicity is a strong risk factor for developing Parkinson's disease later in life and further research is required to determine if the observed abnormality in stimulant users is associated with a functional deficit of the nigro-striatal system.

Mapping the hemodynamic response in human subjects to a dopaminergic challenge with dextroamphetamine using ASL-based pharmacological MRI

Pharmacological magnetic resonance imaging (phMRI) maps the neurovascular response to a pharmacological challenge and is increasingly used to assess neurotransmitter systems. Here we investigated the hemodynamic response to a dopaminergic (DAergic) challenge with dextroamphetamine (dAMPH) in humans using arterial spin labeling (ASL) based phMRI. Twelve healthy male subjects aged 21.0years (±1.5) were included. We used a pseudo-continuous ASL sequence (40min) to quantify cerebral blood flow (CBF) and started dAMPH infusion (0.3mg/kg) after 10min. On another day, we measured baseline dopamine D2/3 receptor availability with [(123)I]IBZM single photon emission computed tomography (SPECT). Baseline measures on mood and impulsivity and subjective behavioral responses to dAMPH were obtained. CBF response was corrected for cardiovascular effects using an occipital cortex mask for internal reference. Corrected CBF (sCBF) was analyzed using ROI-based and voxel-based analysis, in addition to independent component analysis (ICA). CBF data was correlated to D2/3 receptor availability and behavioral measures. Subjects reported experiencing euphoria following dAMPH administration. In the striatum sCBF significantly increased, as demonstrated by all three analysis methods. Voxel-based analysis and ICA also showed increased sCBF in the thalamus, anterior cingulate and cerebellum. Decreased sCBF was observed in several cortical areas, the posterior cingulated and paracingulate cortex. Apart from one ICA component, no correlations were found with sCBF changes and D2/3 receptor availability and behavioral measures. Our observations are in line with literature and provide further evidence that ASL-based phMRI with dAMPH is a promising technique to assess DAergic function in human subjects.

Extended findings of brain metabolite normalization in MA-dependent subjects across sustained abstinence: a proton MRS study

OBJECTIVE

The goal of the present study was to extend our previous findings on long-term methamphetamine (MA) use and drug abstinence on brain metabolite levels in an expanded group of MA-dependent individuals.

METHODS

Seventeen MA abusers with sustained drug abstinence (1-5 years), 30 MA abusers with short-term drug abstinence (1-6 months) and 24 non-substance using controls were studied using MR spectroscopy (MRS). MRS measures of NAA/Cr, Cho/Cr and Cho/NAA were obtained in the anterior cingulate cortex (ACC) and in the primary visual cortex (PVC).

RESULTS

ACC-Cho/NAA values were abnormally high in the short-term abstinent group compared to controls [F(1,52) = 18.76, p < 0.0001]. No differences were observed between controls and the long-term abstinent group [F(1,39) = 0.97, p = 0.97]. New evidence of lower ACC-NAA/Cr levels were observed in the short-term abstinent MA abusers compared to controls [F(1,52) = 23.05, p < 0.0001] and long-term abstinent MA abusers [F(1,45) = 7.06, p = 0.01]. No differences were observed between long-term abstinent MA abusers and controls [F(1,39) = 0.48, p = 0.49].

CONCLUSIONS

The new findings of relative NAA/Cr normalization across periods of abstinence suggest that adaptive changes following cessation of MA abuse may be broader than initially thought. These changes may contribute to some degree of normalization of neuronal function in the ACC.

Delusional infestation: neural correlates and antipsychotic therapy investigated by multimodal neuroimaging

INTRODUCTION

In delusional infestation (DI), as with other non-schizophrenic psychotic disorders, little is known about the neural basis and the mechanisms of antipsychotic treatment. We aimed at investigating the brain circuitry involved in DI and the role of postsynaptic D2 receptors in mediating the effects of antipsychotics by means of multimodal neuroimaging.

METHODS

In Case 1, a patient with DI (initially drug-induced), cerebral glucose metabolism and dopaminergic neurotransmission were studied in the untreated state (FDG-PET, FDOPA-PET, 123I-FP-CIT-SPECT, and IBZM-SPECT) and after effective aripiprazole treatment (FDG-PET and IBZM-SPECT), with negative drug screenings at both imaging sessions. In Case 2 (DI secondary to mild vascular encephalopathy) cerebral perfusion and gray matter volume changes were investigated in the untreated state and compared to N=8 [corrected] age-matched healthy controls (MRI-based CASL and VBM).

RESULTS

In Case 1, before treatment, glucose metabolism was left-dominant in the thalamus and the putamen. Pre- and postsynaptic dopaminergic neurotransmissions were altered in the striatum, again mainly the left putamen. Full remission to aripiprazole was associated with 63 to 78% striatal D2 receptor occupancy and glucose metabolism changes in the bilateral thalamus. In Case 2, significant perfusion and GMV changes were observed in the bilateral putamen, frontal and parietal somatosensory cortices as compared to controls. Symptoms partially remitted to ziprasidone therapy.

DISCUSSION/CONCLUSION

Six imaging techniques were first used to study the neural basis of DI and mechanisms of antipsychotic therapy. The study provides first low-level evidence in vivo evidence of fronto-striato-thalamo-parietal network to mediate core symptoms of DI, i.e. a priori brain regions involved in judgment (frontal cortex), sensory gating (thalamus) and body perception (dorsal striatum, thalamus and somatic cortices). This is also the first report of effective treatment with aripiprazole in drug-induced DI and with ziprasidone in organic DI, adding to existing limited evidence that SGAs are helpful in various forms of DI. Effective antipsychotic treatment seems to depend on blocking striatal D2 receptors with similar occupancy rates as in schizophrenia. Larger samples are needed to confirm our preliminary findings and further evaluate their relevance for the different forms of DI.

Neurochemistry of drug action: insights from proton magnetic resonance spectroscopic imaging and their relevance to addiction

Proton magnetic resonance spectroscopy ((1)H MRS) is a noninvasive imaging technique that permits measurement of particular compounds or metabolites within the tissue of interest. In the brain, (1)H MRS provides a snapshot of the neurochemical environment within a defined volume of interest. A search of the literature demonstrates the widespread utility of this technique for characterizing tumors, tracking the progress of neurodegenerative disease, and for understanding the neurobiological basis of psychiatric disorders. As of relatively recently, (1)H MRS has found its way into substance abuse research, and it is beginning to become recognized as a valuable complement in the brain imaging toolbox that also contains positron emission tomography, single-photon-emission computed tomography, and functional magnetic resonance imaging. Drug abuse studies using (1)H MRS have identified several biochemical changes in the brain. The most consistent alterations across drug class were reductions in N-acetylaspartate and elevations in myo-inositol, whereas changes in choline, creatine, and amino acid transmitters also were abundant. Together, the studies discussed herein provide evidence that drugs of abuse may have a profound effect on neuronal health, energy metabolism and maintenance, inflammatory processes, cell membrane turnover, and neurotransmission, and these biochemical changes may underlie the neuropathology within brain tissue that subsequently gives rise to the cognitive and behavioral impairments associated with drug addiction.

Cyclosporine, a P-glycoprotein modulator, increases [18F]MPPF uptake in rat brain and peripheral tissues: microPET and ex vivo studies

PURPOSE

Pretreatment with cyclosporine, a P-glycoprotein (P-gp) modulator increases brain uptake of 4-(2'-methoxyphenyl)-1-[2'-(N-2"-pyridinyl)-p-[(18)F]fluorobenzamido]ethylpiperazine ([(18)F]MPPF) for binding to hydroxytryptamine(1A) (5-HT(1A)) receptors. Those increases were quantified in rat brain with in vivo microPET and ex vivo tissue studies.

MATERIALS AND METHODS

Each Sprague-Dawley rat (n = 4) received a baseline [(18)F]MPPF microPET scan followed by second scan 2-3 weeks later that included cyclosporine pretreatment (50 mg/kg, i.p.). Maximum a posteriori reconstructed images and volumetric ROIs were used to generate dynamic radioactivity concentration measurements for hippocampus, striatum, and cerebellum, with simplified reference tissue method (SRTM) analysis. Western blots were used to semiquantify P-gp regional distribution in brain.

RESULTS

MicroPET studies showed that hippocampus uptake of [(18)F]MPPF was increased after cyclosporine; ex vivo studies showed similar increases in hippocampus and frontal cortex at 30 min, and for heart and kidney at 2.5 and 5 min, without concomitant increases in [(18)F]MPPF plasma concentration. P-gp content in cerebellum was twofold higher than in hippocampus or frontal cortex.

CONCLUSIONS

These studies confirm and extend prior ex vivo results (J. Passchier, et al., Eur J Pharmacol, 2000) that showed [(18)F]MPPF as a substrate for P-gp. Our microPET results showed that P-gp modulation of [(18)F]MPPF binding to 5-HT(1A) receptors can be imaged in rat hippocampus. The heterogeneous brain distribution of P-gp appeared to invalidate the use of cerebellum as a nonspecific reference region for SRTM modeling. Regional quantitation of P-gp may be necessary for accurate PET assessment of 5-HT(1A) receptor density when based on tracer uptake sensitive to P-gp modulation.

Update on amphetamine neurotoxicity and its relevance to the treatment of ADHD

OBJECTIVE

A review of amphetamine treatment for attention-deficit/hyperactivity disorder (ADHD) was conducted, to obtain information on the long-term neurological consequences of this therapy.

METHOD

Several databases were accessed for research articles on the effects of amphetamine in the brain of laboratory animals and ADHD diagnosed individuals.

RESULTS

In early studies, high doses of amphetamine, comparable to amounts used by addicts, were shown to damage dopaminergic pathways. More recent studies, using therapeutic regimens, appear contradictory. One paradigm shows significant decreases in striatal dopamine and transporter density after oral administration of "therapeutic" doses in primates. Another shows morphological evidence of "trophic" dendritic growth in the brains of adult and juvenile rats given systemic injections mimicking "therapeutic" treatment. Imaging studies of ADHD-diagnosed individuals show an increase in striatal dopamine transporter availability that may be reduced by methylphenidate treatment.

CONCLUSION

Clarification of the neurological consequences of chronic AMPH treatment for ADHD is needed.

Implications of chronic methamphetamine use: a literature review

Methamphetamine (MA) abuse is increasing to epidemic proportions, both nationally and globally. Chronic MA use has been linked to significant impairments in different arenas of neuropsychological function. To better understand this issue, a computerized literature search (PubMed, 1964-2004) was used to collect research studies examining the neurobiological and neuropsychiatric consequences of chronic MA use. Availability of MA has markedly increased in the United States due to recent technological improvements in both mass production and clandestine synthesis, leading to significant public health, legal, and environmental problems. MA intoxication has been associated with significant psychiatric and medical comorbidity. Research in animal models and human subjects reveals complicated mechanisms of neurotoxicity by which chronic MA use affects catecholamine neurotransmission. This pathology may underlie the characteristic cognitive deficits that plague chronic MA users, who experience impairments in memory and learning, psychomotor speed, and information processing. These impairments have the potential to compromise, in turn, the ability of MA abusers to engage in, and benefit from, psychosocially based chemical-dependency treatment. Development of pharmacological interventions to improve these cognitive impairments in this population may significantly improve the degree to which they may be able to participate in treatment. Atypical antipsychotics may have some promise in this regard.

Amphetamine induces apoptosis of medium spiny striatal projection neurons via the mitochondria‐dependent pathway

Amphetamine (AMPH) is a psychostimulant whose chronic abuse may cause impairments in attention and memory in humans. These cognitive deficits might be related to neurotoxic effects of the drug. One such toxic effect is the well-described destruction of striatal dopaminergic terminals in mammals. In the present study, we investigated the possibility that AMPH might also cause neuronal apoptosis in the rodent striatum. Administration of a dose of the drug (10 mg/kg, 4 times, every 2 h) that is toxic to dopaminergic terminals resulted in the appearance of striatal cells that were positive for cleaved caspase-3 and for terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick-end labeling (TUNEL), observations that are indicative of an ongoing apoptotic process. Dual immunofluorescence staining revealed that cleaved caspase-3-positive cells express calbindin and DARPP-32, but not somatostatin, parvalbumin, or cholinergic markers. In addition, AMPH also caused increased expression of p53 and Bax at both transcript and protein levels; in contrast, Bcl-2 levels were decreased after the AMPH injections. Moreover, Bax knockout mice showed resistance to AMPH-induced apoptotic cell death but not to AMPH-induced destruction of dopaminergic terminals. When taken together, these observations indicate that injections of doses of AMPH that are known to destroy striatal dopamine terminals can also cause apoptotic death of postsynaptic medium spiny projection neurons via mitochondria-dependent mechanisms.

Designer drugs: how dangerous are they?

Of the designer drugs, the amphetamine analogues are the most popular and extensively studied, ecstasy (3,4-methylenedioxymethamphetamine; MDMA) in particular. They are used recreationally with increasing popularity despite animal studies showing neurotoxic effects to serotonin (5-HT) and/or dopamine (DA) neurones. However, few detailed assessments of risks of these drugs exist in humans. Previously, there were no methods available for directly evaluating the neurotoxic effects of amphetamine analogues in the living human brain. However, development of in vivo neuroimaging tools have begun to provide insights into the effects of MDMA in human brain. In this review, contributions of brain imaging studies on the potential 5-HT and/or DA neurotoxic effects of amphetamine analogues will be highlighted in order to delineate the risks these drugs engender in humans, focusing on MDMA. An overview will be given of PET, SPECT and MR Spectroscopy studies employed in human users of these drugs. Most of these studies provide suggestive evidence that MDMA is neurotoxic to 5-HT neurones, and (meth)amphetamine to DA neurones in humans. These effects seem to be dose-related, leading to functional impairments such as memory loss, and are reversible in several brain regions. However most studies have had a retrospective design, in which evidence is indirect and differs in the degree to which any causative links can be implied between drug use and neurotoxicity. Therefore, at this moment, it cannot be ascertained that humans are susceptible to MDMA-induced 5-HT injury or (meth)amphetamine-induced DA injury. Finally, although little is known about other amphetamine analogues there are important questions as to the safety of these designer drugs as well, in view of the fact that they are chemically closely related to MDMA and some have been shown to be 5-HT neurotoxins in animals.

Is methamphetamine abuse a risk factor in parkinsonism?

Parkinsons disease (PD) is a neurodegenerative disorder with increased incidence in individuals beyond 50 years of age. The etiology of PD is currently not known, but it appears that environmental factors may play an important role. The molecular basis of PD is the nearly complete loss of the neurotransmitter dopamine (DA) in the basal ganglia (caudate/putamen). The decrease in dopamine levels is the result of degeneration of dopamine-containing neurons in the substantia nigra. This biochemical deficit in the nigrostriatal pathway leads to the emergence of motor impairments typical of PD. Methamphetamine (METH) is a psychostimulant drug with increasing use in certain segments of the population in the United States and worldwide. In experimental animal models and human studies, METH administration has been shown to decrease markers of dopaminergic neuron terminal integrity in the basal ganglia. A long-standing question has been whether the reductions in dopaminergic markers induced by METH constitute degenerative changes or reflect drug-induced modulation. Resolving this question is important because the irreversible loss of dopaminergic function may increase the likelihood of Parkinsonism with advancing age.

FDOPA metabolism in the adult porcine brain: influence of tracer circulation time and VOI selection on estimates of striatal DOPA decarboxylation

Different methodologies for PET data analysis influence the magnitude of estimates of blood-brain transfer coefficients and rate constants for the metabolism of FDOPA in living striatum. We now test the effects on several kinetic parameters of automatic procedures for volume of interest (VOI) selection. We also tested the sensitivity of the estimates to dynamic frame sequence duration, and produced a standard method for minimizing the variations in physiological estimates for FDOPA kinetics in minipig brain. We used minipigs because our previous work has shown them to provide an appropriate animal model for study normal and pathological cerebral DOPA metabolism using PET. Time-activity curves in striatum of adult minipigs were acquired in VOIs defined manually on MR-images, or alternatively on the basis of the radioactivity concentration based on the most radioactive voxel in the last scan frame. For all frame sequences, the relative decarboxylase activity (k(3)(D)) declined significantly (P < 0.006) as the VOI threshold declined from 95 to 70% of the most radioactive voxel. Irrespective of VOI size, the magnitude of k(3)(D) declined significantly (P < 0.001) from 0.074+/-0.008 to 0.045+/-0.005 per min (mean+/-S.E.M.) as total sequence length increased from 60 to 120 min circulation. The method of VOI selection had no significant effect on the striatum decarboxylation index of FDOPA calculated relative to the radioactivity in cerebellum (k(3)(S)).

Methamphetamine neurotoxicity: necrotic and apoptotic mechanisms and relevance to human abuse and treatment

Research into methamphetamine-induced neurotoxicity has experienced a resurgence in recent years. This is due to (1) greater understanding of the mechanisms underlying methamphetamine neurotoxicity, (2) its usefulness as a model for Parkinson's disease and (3) an increased abuse of the substance, especially in the American Mid-West and Japan. It is suggested that the commonly used experimental one-day methamphetamine dosing regimen better models the acute overdose pathologies seen in humans, whereas chronic models are needed to accurately model human long-term abuse. Further, we suggest that these two dosing regimens will result in quite different neurochemical, neuropathological and behavioral outcomes. The relative importance of the dopamine transporter and vesicular monoamine transporter knockout is discussed and insights into oxidative mechanisms are described from observations of nNOS knockout and SOD overexpression. This review not only describes the neuropathologies associated with methamphetamine in rodents, non-human primates and human abusers, but also focuses on the more recent literature associated with reactive oxygen and nitrogen species and their contribution to neuronal death via necrosis and/or apoptosis. The effect of methamphetamine on the mitochondrial membrane potential and electron transport chain and subsequent apoptotic cascades are also emphasized. Finally, we describe potential treatments for methamphetamine abusers with reference to the time after withdrawal. We suggest that potential treatments can be divided into three categories; (1) the prevention of neurotoxicity if recidivism occurs, (2) amelioration of apoptotic cascades that may occur even in the withdrawal period and (3) treatment of the atypical depression associated with withdrawal.

Recovery from methamphetamine induced long-term nigrostriatal dopaminergic deficits without substantia nigra cell loss

After administration of methamphetamine (METH) (2x2 mg/kg, 6 h apart) to vervet monkeys, long term but reversible dopaminergic deficits were observed in both in vivo and post-mortem studies. Longitudinal studies using positron emission tomography (PET) with the dopamine transporter (DAT)-binding ligand, [11C]WIN 35,428 (WIN), were used to show decreases in striatal WIN binding of 80% at 1 week and only 10% at 1.5 years. A post-mortem characterization of other METH subjects at 1 month showed extensive decreases in immunoreactivity (IR) profiles of tyrosine hydroxylase (TH), DAT and vesicular monoamine transporter-2 (VMAT) in the striatum, medial forebrain bundle and the ventral midbrain dopamine (VMD) cell region. These IR deficits were not associated with a loss of VMD cell number when assessed at 1.5 years by stereological methods. Further, at 1.5 years, IR profiles of METH subjects throughout the nigrostriatal dopamine system appeared similar to controls although some regional deficits persisted. Collectively, the magnitude and extent of the dopaminergic deficits, and the subsequent recovery were not suggestive of extensive axonal degeneration followed by regeneration. Alternatively, this apparent reversibility of the METH-induced neuroadaptations may be related primarily to long-term decreases in expression of VMD-related proteins that recover over time.

Compartmental analysis of dopa decarboxylation in living brain from dynamic positron emission tomograms

The trapping of decarboxylation products of radiolabelled dopa analogs in living human brain occurs as a function of the activity of dopa decarboxylase. This enzyme is now understood to regulate, with tyrosine hydroxylase, cerebral dopamine synthesis. Influx into brain of dopa decarboxylase substrates such as 6-[18F]fluorodopa and beta-[11C]dopa measured by positron emission tomography can be analyzed by solution of linear differential equations, assuming irreversible trapping of the decarboxylated products in brain. The isolation of specific physiological steps in the pathway for catecholamine synthesis requires compartmental modelling of the observed dynamic time-activity curves in plasma and in brain. The several approaches to the compartmental modelling of the kinetics of labelled substrates of dopa decarboxylase are now systematically and critically reviewed. Labelled catechols are extensively metabolized by hepatic catechol-O-methyltransferase yielding brain-penetrating metabolites. The assumption of a fixed blood-brain permeability ratio for O-methyl-6-[18F]fluorodopa or O-methyl-beta-[11C]dopa to the parent compounds eliminates several parameters from compartmental models. However, catechol-O-methyltransferase activity within brain remains a possible factor in underestimation of cerebral dopa decarboxylase activity. The O-methylation of labelled catechols is blocked with specific enzyme inhibitors, but dopa decarboxylase substrates derived from m-tyrosine may supplant the catechol tracers. The elimination from brain of decarboxylated tracer metabolites can be neglected without great prejudice to the estimation of dopa decarboxylase activity when tracer circulation is less than 60 minutes. However, elimination of dopamine metabolites from brain occurs at a rate close to that observed previously for metabolites of glucose labelled in the 6-position. This phenomenon can cause systematic underestimation of the rate of dopa decarboxylation in brain. The spillover of radioactivity due to the limited spatial resolution of tomographs also results in underestimation of dopa decarboxylase activity, but correction for partial volume effects is now possible. Estimates of dopa decarboxylase activity in human brain are increased several-fold by this correction. Abnormally low influx of dopa decarboxylase tracers in the basal ganglia is characteristic of Parkinson's disease and other movement disorders. Consistent with postmortem results, the impaired retention of labelled dopa is more pronounced in the putamen than in the caudate nucleus of patients with Parkinson's disease; this heterogeneity persists after correction for spillover. Current in vivo assays of dopa decarboxylase activity fail to discriminate clinically distinct stages in the progression of Parkinson's disease and are, by themselves, insufficient for differential diagnosis of Parkinson's disease and other subcortical movement disorders. However, potential new avenues for therapeutics can be tested by quantifying the rate of metabolism of exogenous dopa in living human brain.

Brain dopamine neurotoxicity in baboons treated with doses of methamphetamine comparable to those recreationally abused by humans: evidence from [11C]WIN-35,428 positron emission tomography studies and direct in vitro determinations

The present study sought to determine whether doses of methamphetamine in the range of those used recreationally by humans produce brain dopamine (DA) neurotoxicity in baboons and to ascertain whether positron emission tomography (PET) imaging with the DA transporter (DAT) ligand [11C]WIN-35,428 ([11C]2beta-carbomethoxy-3beta-(4-fluorophenyl)-tropane) could be used to detect methamphetamine-induced DAT loss in living primates. Baboons were treated with saline (n = 3) or one of three doses of methamphetamine [0.5 mg/kg (n = 2); 1 mg/kg (n = 2); and 2 mg/kg (n = 3)], each of which was given intramuscularly four times at 2 hr intervals. PET studies were performed before and 2-3 weeks after methamphetamine treatment. After the final PET studies, animals were killed for direct neurochemical determination of brain DA axonal markers. PET-derived binding potential values, used to index striatal DAT density, were significantly decreased after methamphetamine, with larger decreases occurring after higher methamphetamine doses. Reductions in striatal DAT documented by PET were associated with decreases in DA, dihydroxyphenylacetic acid, and specific [3H]WIN-35,428 and [3H]DTBZ binding determined in vitro. Decreases in DAT detected with PET were highly correlated with decreases in specific [3H]WIN-35,428 binding determined in vitro in the caudate of the same animal (r = 0.77; p = 0.042). These results indicate that methamphetamine, at doses used by some humans, produces long-term reductions in brain DA axonal markers in baboons, and that it is possible to detect methamphetamine-induced DAT loss in living nonhuman primates by means of PET.

Recovery of striatal dopamine function after acute amphetamine- and methamphetamine-induced neurotoxicity in the vervet monkey

In six vervet monkeys, presynaptic striatal dopamine function was assessed longitudinally by [18F]fluoro-L-DOPA (FDOPA)-positron emission tomography (PET) after administration (2 x 2 mg/kg, i.m., 4 h apart) of either amphetamine (Amp), n = 3, or methamphetamine (MeAmp), n = 3. At 1-2 weeks postdrug, both Amp and MeAmp exposure effected similar decreases (60-70%) in the FDOPA influx rate constant (FDOPA Ki), an index of striatal dopamine synthesis capacity. Subsequent studies in these subjects showed that FDOPA Ki values were decreased by 45-67% at 3-6 weeks, by 25% at 10-12 weeks and by 16% in one Amp-treated subject at 32 weeks. Biochemical analysis showed that striatal dopamine concentrations were decreased by 75% at 3-4 weeks and by 55% at 10-12 weeks. These results indicate that in vervet monkey striatum, an acute Amp or MeAmp drug dosage produces extensive striatal dopamine system neurotoxicity. However, these effects were reversible; observed time-dependent recovery in both FDOPA Ki and dopamine concentrations indicates that neurochemical plasticity remains active in the adult primate striatum. At 3-4 and 10-12 weeks postdrug, the concurrent characterization of the striatal FDOPA Ki and dopamine concentrations for individual subjects showed that Ki decreases between 24 and 67% corresponded to dopamine depletions of 55-85%. These relatively larger postdrug decrements in steady-state striatal dopamine concentrations suggest that compensatory increases in dopamine synthesis capacity develop in the partially lesioned striatum. In contrast to the dopamine depletion in striatum, substantia nigra concentrations remained unchanged from referent values at both 3-4 and 10-12 weeks postdrug. Thus, the integrity of the substantia nigra could not be inferred from decreases in the striatal FDOPA Ki parameter. This disparity between striatum and substantia nigra reactivity to systemic administration of amphetamines suggests that each has unique dopamine system regulatory mechanisms.

Ethological and 6-[18F]fluoro-L-DOPA-PET profiles of long-term vulnerability to chronic amphetamine

A chronic 10-day amphetamine (Amp) protocol was used to induce significant long-term decrements of the striatal [18F]fluoro-L-DOPA influx rate constant (FDOPA Ki) in the vervet monkey. Longitudinal FDOPA-positron emission tomography (PET) assessment in Amp-treated subjects subsequently revealed a gradual recovery of striatal dopamine function: FDOPA Ki values were decreased by approximately 70% at 1 month, approximately 45% at 6 months, approximately 20% at 12 months and were similar to pre-Amp values at 24 months. Motoric and social behavioral measures were obtained on all subjects within a species-typical group setting. Behavioral observations were conducted during both basal and stressor-challenge conditions, the latter being created by placing a potential intruder-animal in an individual cage adjacent to the subject's group enclosure. During basal conditions, post-Amp stereotypies were present at 2 weeks and locomotor behaviors were increased throughout 1 month; both alterations occurred while FDOPA Ki values were significantly decreased. Social behaviors were also significantly affected; affiliative behavior was decreased up to 6 months while aggressive behavior was increased for 12 months. However, a different pattern of behavioral changes emerged under stressor-challenge conditions. Motoric and social changes were of greater magnitude and persisted longer than in basal settings while aggressive behavior remained elevated at 24 months. These results indicate that chronic Amp-induced decreases in FDOPA Ki values and behavioral alterations are reversible. Changes in striatal dopamine function as indexed with FDOPA-PET are not correlated with post-Amp alterations in behaviors and moreover, expression of those behaviors is context-dependent.