Dopaminergic response to graded dopamine concentration elicited by four amphetamine doses

Treatment with dextroamphetamine decreases the reacquisition of cocaine self-administration: Consistency across social contexts

RATIONALE

A return to cocaine use following abstinence frequently occurs in a social context, and the presence of other individuals using cocaine may contribute to the likelihood of use. Previous studies have reported that chronic d-amphetamine treatment decreases cocaine self-administration in laboratory animals and reduces a return to cocaine use following abstinence in humans.

OBJECTIVE

The purpose of this study was to examine the effects of chronic d-amphetamine treatment on the reacquisition of cocaine use in rats self-administering cocaine in different social contexts.

METHODS

Male and female rats were implanted with intravenous catheters and trained to self-administer cocaine during daily 6-hr sessions. After 14 days, cocaine self-administration was extinguished by substituting saline for the cocaine stimulus. At this time, rats were randomized to receive chronic treatment with either d-amphetamine or saline. After 9 days of extinction, cocaine was again made available during daily 6-hr sessions. At this time, rats were further randomized into three social conditions: (1) rats continued self-administering cocaine in isolation, (2) rats self-administered cocaine in the presence of a same-sex partner that also self-administered cocaine, or (3) rats self-administered cocaine in the presence of a same-sex partner that did not have access to cocaine. Daily treatment with d-amphetamine or saline continued for the duration of reacquisition testing.

RESULTS

Chronic treatment with d-amphetamine decreased cocaine intake during reacquisition, but these effects were not influenced by the social context. No sex differences were observed.

CONCLUSION

These data support previous studies reporting that d-amphetamine decreases cocaine intake and demonstrate its efficacy across social contexts.

Personalization of pharmacological treatments for ADHD: Why it is advisable and possible options to achieve it

Abstract:

Attention-deficit hyperactivity disorder is a neurodevelopmental disorder diagnosed primarily in children, although it is also present in adults. Patients present inattention, impulsivity, and hyperactivity symptoms that create difficulties in their daily lives. Pharmacological treatment with stimulants or non-stimulants is used most commonly to reduce ADHD symptoms. Although generally effective and safe, pharmacological treatments have different effects among patients, including lack of response and adverse reactions. The reasons for these differences are not fully understood, but they may derive from the highly diverse etiology of ADHD. Strategies to guide optimal pharmacological treatment selection on the basis of individual patients’ physiological markers are being developed. In this review, we describe the main pharmacological ADHD treatments used and their main drawbacks. We present alternatives under study that would allow the customization of pharmacological treatments to overcome these drawbacks and achieve more reliable improvement of ADHD symptoms.

Persistent CO2 reactivity deficits are associated with neurological dysfunction up to one year after repetitive mild closed head injury in adolescent mice

Cerebrovascular reactivity (CVR) deficits in adolescents with concussion may persist after resolution of neurological symptoms. Whether or not CVR deficits predict long term neurological function is unknown. We used adolescent mice closed head injury (CHI) models (54 g, 107 cm or 117 cm drop height), followed by blood oxygenation level dependent (BOLD)-functional MRI with CO₂ challenge to assess CVR and brain connectivity. At one week, 3HD 107 cm mice showed delayed BOLD responses (p = 0.0074), normal striatal connectivity_, and an impaired respiratory rate response to CO₂ challenge (p = 0.0061 in ΔRmax). The 107 cm group developed rotarod deficits at 6 months (p = 0.02) and altered post-CO₂ brain connectivity (3-fold increase in striatum to motor cortex correlation coefficient) by one year, but resolved their CVR and respiratory rate impairments, and did not develop cognitive or circadian activity deficits. In contrast, the 117 cm group had persistent CVR (delay time: p = 0.016; washout time: p = 0.039) and circadian activity deficits (free-running period: 23.7 hr in sham vs 23.9 hr in 3HD; amplitude: 0.15 in sham vs 0.2 in 3HD; peak activity: 18 in sham vs 21 in 3HD) at one year. Persistent CVR deficits after concussion may portend long-term neurological dysfunction. Further studies are warranted to determine the utility of CVR to predict chronic neurological outcome after mild traumatic brain injury.

Simultaneous fMRI and fast-scan cyclic voltammetry bridges evoked oxygen and neurotransmitter dynamics across spatiotemporal scales

The vascular contributions of neurotransmitters to the hemodynamic response are gaining more attention in neuroimaging studies, as many neurotransmitters are vasomodulatory. To date, well-established electrochemical techniques that detect neurotransmission in high magnetic field environments are limited. Here, we propose an experimental setting enabling simultaneous fast-scan cyclic voltammetry (FSCV) and blood oxygenation level-dependent functional magnetic imaging (BOLD fMRI) to measure both local tissue oxygen and dopamine responses, and global BOLD changes, respectively. By using MR-compatible materials and the proposed data acquisition schemes, FSCV detected physiological analyte concentrations with high temporal resolution and spatial specificity inside of a 9.4 T MRI bore. We found that tissue oxygen and BOLD correlate strongly, and brain regions that encode dopamine amplitude differences can be identified via modeling simultaneously acquired dopamine FSCV and BOLD fMRI time-courses. This technique provides complementary neurochemical and hemodynamic information and expands the scope of studying the influence of local neurotransmitter release over the entire brain.

Imaging of the dopamine system with focus on pharmacological MRI and neuromelanin imaging

The dopamine system in the brain is involved in a variety of neurologic and psychiatric disorders, such as Parkinson's disease, attention-deficit/hyperactivity disorder and psychosis. Different aspects of the dopamine system can be visualized and measured with positron emission tomography (PET) and single photon emission computed tomography (SPECT), including dopamine receptors, dopamine transporters, and dopamine release. New developments in MR imaging also provide proxy measures of the dopamine system in the brain, offering alternatives with the advantages MR imaging, i.e. no radiation, lower costs, usually less invasive and time consuming. This review will give an overview of these developments with a focus on the most developed techniques: pharmacological MRI (phMRI) and neuromelanin sensitive MRI (NM-MRI). PhMRI is a collective term for functional MRI techniques that administer a pharmacological challenge to assess its effects on brain hemodynamics. By doing so, it indirectly assesses brain neurotransmitter function such as dopamine function. NM-MRI is an upcoming MRI technique that enables in vivo visualization and semi-quantification of neuromelanin in the substantia nigra. Neuromelanin is located in the cell bodies of dopaminergic neurons of the nigrostriatal pathway and can be used as a proxy measure for long term dopamine function or degeneration of dopaminergic neurons. Both techniques are still primarily used in clinical research, but there is promise for clinical application, in particular for NM-MRI in dopaminergic neurodegenerative diseases like Parkinson's disease.

The Vasomotor Response to Dopamine Is Altered in the Rat Model of l-dopa-Induced Dyskinesia

BACKGROUND

Levodopa (l-dopa) is the frontline treatment for motor symptoms of Parkinson's disease. However, prolonged use of l-dopa results in a motor complication known as levodopa-induced dyskinesia (LID) in ~50% of patients over 5 years.

OBJECTIVES

We investigated neurovascular abnormalities in a rat model of LID by examining changes in angiogenesis and dopamine-dependent vessel diameter changes.

METHODS

Differences in striatal and nigral angiogenesis in a parkinsonian rat model (6-OHDA lesion) treated with 2 doses of l-dopa (saline, 2, and 10 mg/kg/day subcutaneous l-dopa treatment for 22 days) by 5-bromo-2'-deoxyuridine (BrdU)-RECA1 co-immunofluorescence. Difference in the vasomotor response to dopamine was examined with 2-photon laser scanning microscopy and Dodt gradient imaging.

RESULTS

We found that the 10 mg/kg l-dopa dosing regimen induced LID in all animals (n = 5) and induced significant angiogenesis in the striatum and substantia nigra. In contrast, the 2 mg/kg treatment induced LID in 6 out of 12 rats and led to linearly increasing LID severity over the 22-day treatment period, making this a promising model for studying LID progression longitudinally. However, no significantly different level of angiogenesis was observed between LID versus non-LID animals. Dopamine-induced vasodilatory responses were exaggerated only in rats that show LID-like signs compared to the rest of groups. Additionally, in juvenile rats, we showed that DA-induced vasodilation is preceded by increased Ca2+ release in the adjacent astrocytes.

CONCLUSION

This finding supports that astrocytic dopamine signaling controls striatal blood flow bidirectionally, and the balance is altered in LID. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

No effect of a dopaminergic modulation fMRI task by amisulpride and L-DOPA on reward anticipation in healthy volunteers

RATIONALE

Dysregulation of dopaminergic neurotransmission, specifically altered reward processing assessed via the reward anticipation in the MID task, plays a central role in the etiopathogenesis of neuropsychiatric disorders.

OBJECTIVES

We hypothesized to find a difference in the activity level of the reward system (measured by the proxy reward anticipation) under drug administration versus placebo, in that amisulpride reduces, and L-DOPA enhances, its activity.

METHODS

We studied the influence of dopamine agonist L-DOPA and the antagonist amisulpride on the reward system using functional magnetic resonance imaging (fMRI) during a monetary incentive delay (MID) task in n = 45 healthy volunteers in a randomized, blinded, cross-over study.

RESULTS

The MID paradigm elicits strong activation in reward-dependent structures (such as ventral striatum, putamen, caudate, anterior insula) during reward anticipation. The placebo effect demonstrated the expected significant blood oxygen level-dependent activity in reward-dependent brain regions. Neither amisulpride nor L-DOPA led to significant changes in comparison with the placebo condition. This was true for whole-brain analysis as well as analysis of a pre-defined nucleus accumbens region-of-interest mask.

CONCLUSION

The present results cast doubt on the sensitivity of reward anticipation contrast in the MID task for assessing dopamine-specific changes in healthy volunteers by pharmaco-fMRI. While our task was not well-suited for detailed analysis of the outcome phase, we provide reasonable arguments that the lack of effect in the anticipation phase is not due to an inefficient task but points to unexpected behavior of the reward system during pharmacological challenge. Group differences of reward anticipation should therefore not be seen as simple representatives of dopaminergic states.

Evaluation of electrochemical methods for tonic dopamine detection in vivo

Dysfunction in dopaminergic neuronal systems underlie a number of neurologic and psychiatric disorders such as Parkinson's disease, drug addiction, and schizophrenia. Dopamine systems communicate via two mechanisms, a fast "phasic" release (sub-second to second) that is related to salient stimuli and a slower "tonic" release (minutes to hours) that regulates receptor tone. Alterations in tonic levels are thought to be more critically important in enabling normal motor, cognitive, and motivational functions, and dysregulation in tonic dopamine levels are associated with neuropsychiatric disorders. Therefore, development of neurochemical recording techniques that enable rapid, selective, and quantitative measurements of changes in tonic extracellular levels are essential in determining the role of dopamine in both normal and disease states. Here, we review state-of-the-art advanced analytical techniques for in vivo detection of tonic levels, with special focus on electrochemical techniques for detection in humans.

The selective GSK3 inhibitor, SAR502250, displays neuroprotective activity and attenuates behavioral impairments in models of neuropsychiatric symptoms of Alzheimer's disease in rodents

Glycogen synthase kinase 3 (GSK3) has been identified as a promising target for the treatment of Alzheimer’s disease (AD), where abnormal activation of this enzyme has been associated with hyperphosphorylation of tau proteins. This study describes the effects of the selective GSK3 inhibitor, SAR502250, in models of neuroprotection and neuropsychiatric symptoms (NPS) associated with AD. In P301L human tau transgenic mice, SAR502250 attenuated tau hyperphosphorylation in the cortex and spinal cord. SAR502250 prevented the increase in neuronal cell death in rat embryonic hippocampal neurons following application of the neurotoxic peptide, Aβ_25–35. In behavioral studies, SAR502250 improved the cognitive deficit in aged transgenic APP(SW)/Tau(VLW) mice or in adult mice after infusion of Aβ_25–35. It attenuated aggression in the mouse defense test battery and improved depressive-like state of mice in the chronic mild stress procedure after 4 weeks of treatment. Moreover, SAR502250 decreased hyperactivity produced by psychostimulants. In contrast, the drug failed to modify anxiety-related behaviors or sensorimotor gating deficit. This profile confirms the neuroprotective effects of GSK3 inhibitors and suggests an additional potential in the treatment of some NPS associated with AD.

Effects of Exercise on Cognitive Performance in Children and Adolescents with ADHD: Potential Mechanisms and Evidence-based Recommendations

Attention Deficit Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder with a complex symptomatology, and core symptoms as well as functional impairment often persist into adulthood. Recent investigations estimate the worldwide prevalence of ADHD in children and adolescents to be ~7%, which is a substantial increase compared to a decade ago. Conventional treatment most often includes pharmacotherapy with central nervous stimulants, but the number of non-responders and adverse effects call for treatment alternatives. Exercise has been suggested as a safe and low-cost adjunctive therapy for ADHD and is reported to be accompanied by positive effects on several aspects of cognitive functions in the general child population. Here we review existing evidence that exercise affects cognitive functions in children with and without ADHD and present likely neurophysiological mechanisms of action. We find well-described associations between physical activity and ADHD, as well as causal evidence in the form of small to moderate beneficial effects following acute aerobic exercise on executive functions in children with ADHD. Despite large heterogeneity, meta-analyses find small positive effects of exercise in population-based control (PBC) children, and our extracted effect sizes from long-term interventions suggest consistent positive effects in children and adolescents with ADHD. Paucity of studies probing the effect of different exercise parameters impedes finite conclusions in this regard. Large-scale clinical trials with appropriately timed exercise are needed. In summary, the existing preliminary evidence suggests that exercise can improve cognitive performance intimately linked to ADHD presentations in children with and without an ADHD diagnosis. Based on the findings from both PBC and ADHD children, we cautiously provide recommendations for parameters of exercise.

Dopamine is a double-edged sword: dopaminergic modulation enhances memory retrieval performance but impairs metacognition

While memory encoding and consolidation processes have been linked with dopaminergic signaling for a long time, the role of dopamine in episodic memory retrieval remained mostly unexplored. Based on previous observations of striatal activity during memory retrieval, we used pharmacological functional magnetic resonance imaging to investigate the effects of dopamine on retrieval performance and metacognitive memory confidence in healthy humans. Dopaminergic modulation by the D2 antagonist haloperidol administered acutely during the retrieval phase improved recognition accuracy of previously learned pictures significantly and was associated with increased activity in the substantia nigra/ventral tegmental area, locus coeruleus, hippocampus, and amygdala during retrieval. In contrast, confidence for new decisions was impaired by unsystematically increased activity of the striatum across confidence levels and restricted range of responsiveness in frontostriatal networks under haloperidol. These findings offer new insights into the mechanisms underlying memory retrieval and metacognition and provide a broader perspective on the presence of memory problems in dopamine-related diseases and the treatment of memory disorders.

The Relationship Between Dopamine Neurotransmitter Dynamics and the Blood-Oxygen-Level-Dependent (BOLD) Signal: A Review of Pharmacological Functional Magnetic Resonance Imaging

Functional magnetic resonance imaging (fMRI) is widely used in investigations of normal cognition and brain disease and in various clinical applications. Pharmacological fMRI (pharma-fMRI) is a relatively new application, which is being used to elucidate the effects and mechanisms of pharmacological modulation of brain activity. Characterizing the effects of neuropharmacological agents on regional brain activity using fMRI is challenging because drugs modulate neuronal function in a wide variety of ways, including through receptor agonist, antagonist, and neurotransmitter reuptake blocker events. Here we review current knowledge on neurotransmitter-mediated blood-oxygen-level dependent (BOLD) fMRI mechanisms as well as recently updated methodologies aimed at more fully describing the effects of neuropharmacologic agents on the BOLD signal. We limit our discussion to dopaminergic signaling as a useful lens through which to analyze and interpret neurochemical-mediated changes in the hemodynamic BOLD response. We also discuss the need for future studies that use multi-modal approaches to expand the understanding and application of pharma-fMRI.

The pharmacology of amphetamine and methylphenidate: Relevance to the neurobiology of attention-deficit/hyperactivity disorder and other psychiatric comorbidities

Psychostimulants, including amphetamines and methylphenidate, are first-line pharmacotherapies for individuals with attention-deficit/hyperactivity disorder (ADHD). This review aims to educate physicians regarding differences in pharmacology and mechanisms of action between amphetamine and methylphenidate, thus enhancing physician understanding of psychostimulants and their use in managing individuals with ADHD who may have comorbid psychiatric conditions. A systematic literature review of PubMed was conducted in April 2017, focusing on cellular- and brain system-level effects of amphetamine and methylphenidate. The primary pharmacologic effect of both amphetamine and methylphenidate is to increase central dopamine and norepinephrine activity, which impacts executive and attentional function. Amphetamine actions include dopamine and norepinephrine transporter inhibition, vesicular monoamine transporter 2 (VMAT-2) inhibition, and monoamine oxidase activity inhibition. Methylphenidate actions include dopamine and norepinephrine transporter inhibition, agonist activity at the serotonin type 1A receptor, and redistribution of the VMAT-2. There is also evidence for interactions with glutamate and opioid systems. Clinical implications of these actions in individuals with ADHD with comorbid depression, anxiety, substance use disorder, and sleep disturbances are discussed.

News and views on in-vivo imaging of neurotransmission using PET and MRI

Molecular neuroimaging with PET is an integrated tool in psychiatry research and drug-development for as long as this modality has been available, in particular for studying neurotransmission and endogenous neurotransmitter release. Pharmacologic, behavioral and other types of challenges are currently applied to induce changes in neurochemical levels that can be inferred through their effects on changes in receptor binding and related outcome measures. Based on the availability of tracers that are sensitive for measuring neurotransmitter release these experiments have focused on the brain's dopamine system, while recent developments have extended those studies to other targets such as the serotonin or choline system. With the introduction of hybrid, truly simultaneous PET/MRI systems, in-vivo imaging of the dynamics of neuroreceptor signal transmission in the brain using PET and functional MRI (fMRI) has become possible. fMRI has the ability to provide information about the effects of receptor function that are complementary to the PET measurement. Dynamic acquisition of both PET and fMRI signals enables not only an in-vivo real-time assessment of neurotransmitter or drug binding to receptors but also dynamic receptor adaptations and receptor-specific neurotransmission. While fMRI temporal resolution is comparatively fast in relation to PET, the timescale of observable biological processes is highly dependent on the kinetics of radiotracers and study design. Overall, the combination of the specificity of PET radiotracers to neuroreceptors, fMRI signal as a functional readout and integrated study design promises to expand our understanding of the location, propagation and connections of brain activity in health and disease.

Repeated dexamphetamine treatment alters the dopaminergic system and increases the phMRI response to methylphenidate

Dexamphetamine (AMPH) is a psychostimulant drug that is used both recreationally and as medication for attention deficit hyperactivity disorder. Preclinical studies have demonstrated that repeated exposure to AMPH can induce damage to nerve terminals of dopamine (DA) neurons. We here assessed the underlying neurobiological changes in the DA system following repeated AMPH exposure and pre-treated rats with AMPH or saline (4 times 5 mg/kg s.c., 2 hours apart), followed by a 1-week washout period. We then used pharmacological MRI (phMRI) with a methylphenidate (MPH) challenge, as a sensitive and non-invasive in-vivo measure of DAergic function. We subsequently validated the DA-ergic changes post-mortem, using a.o. high-performance liquid chromatography (HPLC) and autoradiography. In the AMPH pre-treated group, we observed a significantly larger BOLD response to the MPH challenge, particularly in DA-ergic brain areas and their downstream projections. Subsequent autoradiography studies showed that AMPH pre-treatment significantly reduced DA transporter (DAT) density in the caudate-putamen (CPu) and nucleus accumbens, whereas HPLC analysis revealed increases in the DA metabolite homovanillic acid in the CPu. Our results suggest that AMPH pre-treatment alters DAergic responsivity, a change that can be detected with phMRI in rats. These phMRI changes likely reflect increased DA release together with reduced DAT binding. The ability to assess subtle synaptic changes using phMRI is promising for both preclinical studies of drug discovery, and for clinical studies where phMRI can be a useful tool to non-invasively investigate DA abnormalities, e.g. in neuropsychiatric disorders.

The role of the mesolimbic dopamine system in the formation of blood-oxygen-level dependent responses in the medial prefrontal/anterior cingulate cortex during high-frequency stimulation of the rat perforant pathway

Several human functional magnetic resonance imaging studies point to an activation of the mesolimbic dopamine system during reward, addiction and learning. We previously found activation of the mesolimbic system in response to continuous but not to discontinuous perforant pathway stimulation in an experimental model that we now used to investigate the role of dopamine release for the formation of functional magnetic resonance imaging responses. The two stimulation protocols elicited blood-oxygen-level dependent responses in the medial prefrontal/anterior cingulate cortex and nucleus accumbens. Inhibition of dopamine D_1/5 receptors abolished the formation of functional magnetic resonance imaging responses in the medial prefrontal/anterior cingulate cortex during continuous but not during discontinuous pulse stimulations, i.e. only when the mesolimbic system was activated. Direct electrical or optogenetic stimulation of the ventral tegmental area caused strong dopamine release but only electrical stimulation triggered significant blood-oxygen level-dependent responses in the medial prefrontal/anterior cingulate cortex and nucleus accumbens. These functional magnetic resonance imaging responses were not affected by the D_1/5 receptor antagonist SCH23390 but reduced by the N-methyl-D-aspartate receptor antagonist MK801. Therefore, glutamatergic ventral tegmental area neurons are already sufficient to trigger blood-oxygen-level dependent responses in the medial prefrontal/anterior cingulate cortex and nucleus accumbens. Although dopamine release alone does not affect blood-oxygen-level dependent responses it can act as a switch, permitting the formation of blood-oxygen-level dependent responses.

Striatal phosphodiesterase 10A availability is altered secondary to chronic changes in dopamine neurotransmission

Background

Phosphodiesterase 10A (PDE10A) is an important regulator of nigrostriatal dopamine (DA) neurotransmission. However, little is known on the effect of alterations in DA neurotransmission on PDE10A availability. Here, we used [¹⁸F]JNJ42259152 PET to measure changes in PDE10A availability, secondary to pharmacological alterations in DA release and to investigate whether these are D₁- or D₂-receptor driven.

Results

Acute treatment of rats using D-amphetamine (5 mg, s.c. and 1 mg/kg i.v.) did not result in a significant change in PDE10A BP_ND compared to baseline conditions. 5-day D-amphetamine treatment (5 mg/kg, s.c.) increased striatal PDE10A BP_ND compared to the baseline (+24 %, p = 0.03). Treatment with the selective D2 antagonist SCH23390 (1 mg/kg) and D-amphetamine decreased PDE10A binding (-22 %, p = 0.03). Treatment with only SCH23390 further decreased PDE10A binding (-26 %, p = 0.03). No significant alterations in PDE10A mRNA levels were observed.

Conclusions

Repeated D-amphetamine treatment significantly increased PDE10A binding, which is not observed upon selective D₁ receptor blocking. This study suggests a potential pharmacological interaction between PDE10A enzymes and drugs modifying DA neurotransmission. Therefore, PDE10A binding in patients with neuropsychiatric disorders might be modulated by chronic DA-related treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s41181-016-0005-5) contains supplementary material, which is available to authorized users.

Inhibition of parvalbumin-expressing interneurons results in complex behavioral changes

Reduced expression of the Gad1 gene-encoded 67-kDa protein isoform of glutamic acid decarboxylase (GAD67) is a hallmark of schizophrenia. GAD67 downregulation occurs in multiple interneuronal sub-populations, including the parvalbumin-positive (PVALB+) cells. To investigate the role of the PV-positive GABAergic interneurons in behavioral and molecular processes, we knocked down the Gad1 transcript using a microRNA engineered to target specifically Gad1 mRNA under the control of Pvalb bacterial artificial chromosome. Verification of construct expression was performed by immunohistochemistry. Follow-up electrophysiological studies revealed a significant reduction in γ-aminobutyric acid (GABA) release probability without alterations in postsynaptic membrane properties or changes in glutamatergic release probability in the prefrontal cortex pyramidal neurons. Behavioral characterization of our transgenic (Tg) mice uncovered that the Pvalb/Gad1 Tg mice have pronounced sensorimotor gating deficits, increased novelty-seeking and reduced fear extinction. Furthermore, NMDA (N-methyl-d-aspartate) receptor antagonism by ketamine had an opposing dose-dependent effect, suggesting that the differential dosage of ketamine might have divergent effects on behavioral processes. All behavioral studies were validated using a second cohort of animals. Our results suggest that reduction of GABAergic transmission from PVALB+ interneurons primarily impacts behavioral domains related to fear and novelty seeking and that these alterations might be related to the behavioral phenotype observed in schizophrenia.

Investigation of the mechanisms mediating MDMA "Ecstasy"-induced increases in cerebro-cortical perfusion determined by btASL MRI

RATIONALE

Acute administration of the recreational drug of abuse 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy) has previously been shown to increase cerebro-cortical perfusion as determined by bolus-tracking arterial spin labelling (btASL) MRI.

OBJECTIVES

The purpose of the current study was to assess the mechanisms mediating these changes following systemic administration of MDMA to rats.

METHODS

Pharmacological manipulation of serotonergic, dopaminergic and nitrergic transmission was carried out to determine the mechanism of action of MDMA-induced increases in cortical perfusion using btASL MRI.

RESULTS

Fenfluramine (10 mg/kg), like MDMA (20 mg/kg), increased cortical perfusion. Increased cortical perfusion was not obtained with the 5-HT2 receptor agonist 2,5-dimethoxy-4-iodophenyl-aminopropane hydrochloride (DOI) (1 mg/kg). Depletion of central 5-HT following systemic administration of the tryptophan hydroxylase inhibitor para-chlorophenylalanine (pCPA) produced effects similar to those observed with MDMA. Pre-treatment with the 5-HT receptor antagonist metergoline (4 mg/kg) or with the 5-HT reuptake inhibitor citalopram (30 mg/kg), however, failed to produce any effect alone or influence the response to MDMA. Pre-treatment with the dopamine D1 receptor antagonist SCH 23390 (1 mg/kg) failed to influence the changes in cortical perfusion obtained with MDMA. Treatment with the neuronal nitric oxide (NO) synthase inhibitor 7-nitroindazole (7-NI) (25 mg/kg) provoked no change in cerebral perfusion alone yet attenuated the MDMA-related increase in cortical perfusion.

CONCLUSIONS

Cortical 5-HT depletion is associated with increases in perfusion although this mechanism alone does not account for MDMA-related changes. A role for NO, a key regulator of cerebrovascular perfusion, is implicated in MDMA-induced increases in cortical perfusion.

Pharmacological MRI response to a selective dopamine transporter inhibitor, GBR12909, in awake and anesthetized rats

Pharmacological magnetic resonance imaging (phMRI) is a powerful tool for imaging the effects of drugs on brain activity. In preclinical phMRI studies, general anesthesia used for minimizing head movements is thought to influence the phMRI responses to drugs. In this study we investigated the phMRI responses to a selective dopamine transporter (DAT) inhibitor, GBR12909, and a dopamine (DA) releaser, d-amphetamine (AMPH), in the isoflurane anesthetized and awake rats using a relative cerebral blood volume (rCBV) method. AMPH (1 mg/kg i.p.) caused an increase in rCBV in the dopaminergic circuitry in the both anesthetized and awake rats. The striatal rCBV change was correlated with the change of the striatal DA concentration induced by AMPH in the both anesthetized and awake rats. GBR12909 (10 mg/kg i.p.) caused a positive rCBV response and showed a similar regional pattern of rCBV response to AMPH in the awake rats, and the correlation between the change of the striatal rCBV and the striatal DA concentration was observed. However, in the anesthetized rats, GBR12909 induced a widespread negative rCBV response, whereas an increase in striatal DA concentration was observed. These findings indicate that phMRI responses to activation of DA neurotransmission by GBR12909 or AMPH are overall identical in the awake state, while the phMRI response to a DAT inhibitor, GBR12909 but not to AMPH was changed by isoflurane anesthesia. For the evaluation of neuroactive drugs using phMRI, isoflurane anesthesia might be complicated the interpretation of pharmacodynamic effects of drugs in preclinical studies.

Data collection and analysis strategies for phMRI

Although functional MRI traditionally has been applied mainly to study changes in task-induced brain function, evolving acquisition methodologies and improved knowledge of signal mechanisms have increased the utility of this method for studying responses to pharmacological stimuli, a technique often dubbed "phMRI". The proliferation of higher magnetic field strengths and the use of exogenous contrast agent have boosted detection power, a critical factor for successful phMRI due to the restricted ability to average multiple stimuli within subjects. Receptor-based models of neurovascular coupling, including explicit pharmacological models incorporating receptor densities and affinities and data-driven models that incorporate weak biophysical constraints, have demonstrated compelling descriptions of phMRI signal induced by dopaminergic stimuli. This report describes phMRI acquisition and analysis methodologies, with an emphasis on data-driven analyses. As an example application, statistically efficient data-driven regressors were used to describe the biphasic response to the mu-opioid agonist remifentanil, and antagonism using dopaminergic and GABAergic ligands revealed modulation of the mesolimbic pathway. Results illustrate the power of phMRI as well as our incomplete understanding of mechanisms underlying the signal. Future directions are discussed for phMRI acquisitions in human studies, for evolving analysis methodologies, and for interpretative studies using the new generation of simultaneous PET/MRI scanners. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Dysfunctional dopaminergic neurotransmission in asocial BTBR mice

Autism spectrum disorders (ASD) are neurodevelopmental conditions characterized by pronounced social and communication deficits and stereotyped behaviours. Recent psychosocial and neuroimaging studies have highlighted reward-processing deficits and reduced dopamine (DA) mesolimbic circuit reactivity in ASD patients. However, the neurobiological and molecular determinants of these deficits remain undetermined. Mouse models recapitulating ASD-like phenotypes could help generate hypotheses about the origin and neurophysiological underpinnings of clinically relevant traits. Here we used functional magnetic resonance imaging (fMRI), behavioural and molecular readouts to probe dopamine neurotransmission responsivity in BTBR T(+) Itpr3(tf)/J mice (BTBR), an inbred mouse line widely used to model ASD-like symptoms owing to its robust social and communication deficits, and high level of repetitive stereotyped behaviours. C57BL/6J (B6) mice were used as normosocial reference comparators. DA reuptake inhibition with GBR 12909 produced significant striatal DA release in both strains, but failed to elicit fMRI activation in widespread forebrain areas of BTBR mice, including mesolimbic reward and striatal terminals. In addition, BTBR mice exhibited no appreciable motor responses to GBR 12909. DA D1 receptor-dependent behavioural and signalling responses were found to be unaltered in BTBR mice, whereas dramatic reductions in pre- and postsynaptic DA D2 and adenosine A2A receptor function was observed in these animals. Overall these results document profoundly compromised DA D2-mediated neurotransmission in BTBR mice, a finding that is likely to have a role in the distinctive social and behavioural deficits exhibited by these mice. Our results call for a deeper investigation of the role of dopaminergic dysfunction in mouse lines exhibiting ASD-like phenotypes, and possibly in ASD patient populations.

Long-term oral methylphenidate treatment in adolescent and adult rats: differential effects on brain morphology and function

Methylphenidate is a widely prescribed psychostimulant for treatment of attention deficit hyperactivity disorder (ADHD) in children and adolescents, which raises questions regarding its potential interference with the developing brain. In the present study, we investigated effects of 3 weeks oral methylphenidate (5 mg/kg) vs vehicle treatment on brain structure and function in adolescent (post-natal day [P]25) and adult (P65) rats. Following a 1-week washout period, we used multimodal magnetic resonance imaging (MRI) to assess effects of age and treatment on independent component analysis-based functional connectivity (resting-state functional MRI), D-amphetamine-induced neural activation responses (pharmacological MRI), gray and white matter tissue volumes and cortical thickness (postmortem structural MRI), and white matter structural integrity (postmortem diffusion tensor imaging (DTI)). Many age-related differences were found, including cortical thinning, white matter development, larger dopamine-mediated activation responses and increased striatal functional connectivity. Methylphenidate reduced anterior cingulate cortical network strength in both adolescents and adults. In contrast to clinical observations from ADHD patient studies, methylphenidate did not increase white matter tissue volume or cortical thickness in rat. Nevertheless, DTI-based fractional anisotropy was higher in the anterior part of the corpus callosum following adolescent treatment. Furthermore, methylphenidate differentially affected adolescents and adults as evidenced by reduced striatal volume and myelination upon adolescent treatment, although we did not observe adverse treatment effects on striatal functional activity. Our findings of small but significant age-dependent effects of psychostimulant treatment in the striatum of healthy rats highlights the importance of further research in children and adolescents that are exposed to methylphenidate.

Cerebral blood volume MRI with intravascular superparamagnetic iron oxide nanoparticles

The cerebral blood volume (CBV) is a crucial physiological indicator of tissue viability and vascular reactivity. Thus, noninvasive CBV mapping has been of great interest. For this, ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles, including monocrystalline iron oxide nanoparticles, can be used as long-half-life, intravascular susceptibility agents of CBV MRI measurements. Moreover, CBV-weighted functional MRI (fMRI) with USPIO nanoparticles provides enhanced sensitivity, reduced large vessel contribution and improved spatial specificity relative to conventional blood oxygenation level-dependent fMRI, and measures a single physiological parameter that is easily interpretable. We review the physiochemical and magnetic properties, and pharmacokinetics, of USPIO nanoparticles in brief. We then extensively discuss quantifications of baseline CBV, vessel size index and functional CBV change. We also provide reviews of dose-dependent sensitivity, vascular filter function, specificity, characteristics and impulse response function of CBV fMRI. Examples of CBV fMRI specificity at the laminar and columnar resolution are provided. Finally, we briefly review the application of CBV measurements to functional and pharmacological studies in animals. Overall, the use of USPIO nanoparticles can determine baseline CBV and its changes induced by functional activity and pharmacological interventions.

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'.

A receptor-based model for dopamine-induced fMRI signal

This report describes a multi-receptor physiological model of the fMRI temporal response and signal magnitude evoked by drugs that elevate synaptic dopamine in basal ganglia. The model is formulated as a summation of dopamine's effects at D1-like and D2-like receptor families, which produce functional excitation and inhibition, respectively, as measured by molecular indicators like adenylate cyclase or neuroimaging techniques like fMRI. Functional effects within the model are described in terms of relative changes in receptor occupancies scaled by receptor densities and neuro-vascular coupling constants. Using literature parameters, the model reconciles many discrepant observations and interpretations of pre-clinical data. Additionally, we present data showing that amphetamine stimulation produces fMRI inhibition at low doses and a biphasic response at higher doses in the basal ganglia of non-human primates (NHP), in agreement with model predictions based upon the respective levels of evoked dopamine. Because information about dopamine release is required to inform the fMRI model, we simultaneously acquired PET (11)C-raclopride data in several studies to evaluate the relationship between raclopride displacement and assumptions about dopamine release. At high levels of dopamine release, results suggest that refinements of the model will be required to consistently describe the PET and fMRI data. Overall, the remarkable success of the model in describing a wide range of preclinical fMRI data indicate that this approach will be useful for guiding the design and analysis of basic science and clinical investigations and for interpreting the functional consequences of dopaminergic stimulation in normal subjects and in populations with dopaminergic neuroadaptations.

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.

Cortical responses to amphetamine exposure studied by pCASL MRI and pharmacokinetic/pharmacodynamic dose modeling

INTRODUCTION

Perfusion measurement by arterial spin labeling (ASL) techniques is well suited for pharmaceutical magnetic resonance imaging (phMRI) studies to investigate how drugs change the cerebral perfusion status and further, neuronal activity.

MATERIALS AND METHOD

Twelve healthy normal male volunteers participated in the study which was based on a double blinded design. Six subjects were randomly selected to receive a single oral dose of 20mg d-amphetamine and six were given placebo. Perfusion measurements by pseudo-continuous ASL (pCASL) technique were repeatedly performed at 10 different time points with a 3T clinical MRI scanner during a 10 hour period after dose together with physiologic data and blood sample collections. The dynamic changes in cerebral perfusion in response to the plasma concentration variations of d-amphetamine were analyzed at voxel-level and for regions of interest.

RESULTS

Compared to the placebo group a 20% reduction in cerebral blood flow (CBF) was observed in gray matter for the subjects that received d-amphetamine. The most significant reduction of regional CBF (rCBF) was detected in the basal ganglia, frontal region and insular cortex using voxel based analysis. A relation between d-amphetamine exposure and CBF response was found using PK/PD modeling, which predicted on average a 15% decrease of the CBF in gray matter at a plasma concentration of 30 ng/ml.

CONCLUSION

In this study we have demonstrated that repeated perfusion measurements by pCASL technique was sufficiently robust to differentiate the neurological response between the groups that received d-amphetamine and placebo. Quantitative and repetitive CBF measurements can be used for PK/PD modeling of CNS drug responses in humans.

Inhibition of GSK3 attenuates amphetamine-induced hyperactivity and sensitization in the mouse

Glycogen synthase kinase 3 (GSK3) is implicated in mediating dopamine-dependent behaviors. Previous studies have demonstrated the ability of amphetamine, which increases extracellular dopamine levels and influences behavior, to regulate the activity of GSK3. This study used valproic acid and the selective GSK3 inhibitor, SB 216763, to examine the role of GSK3 in amphetamine-induced hyperactivity and the development of sensitized stereotypic behavior. Pretreatment with valproic acid (50-300 mg/kg, i.p.) or SB 216763 (2.5-5 mg/kg, i.p.) prior to amphetamine (2 mg/kg, i.p.) significantly reduced amphetamineinduced ambulation and stereotypy. To assess the development of sensitization to the stereotypic effects of amphetamine, mice were pretreated daily with valproic acid (300 mg/kg) or SB 216763 (5 mg/kg) prior to amphetamine (2 mg/kg) for 5 days. Upon amphetamine challenge (1 mg/kg) 7 days later, mice pretreated with valproate or SB 216763 showed a significant attenuation of amphetamine-induced sensitization of stereotypy. To determine whether regulation of GSK3 activity was associated with attenuation of acute amphetamine-induced hyperactivity by valproic acid, valproate (300 mg/kg) or vehicle was injected prior to amphetamine (2 mg/kg) or saline and brain tissue obtained. Analysis of the levels of phospho-GSK3α and β by immunoblot indicated that valproate increased phosphorylation of ser²¹-GSK3α in the frontal cortex, as well as ser⁹-GSK3β in the frontal cortex and caudate putamen of amphetamine-injected mice. These data support a role for GSK3 in acute amphetamine-induced hyperactivity and the development of sensitization to amphetamine-induced stereotypy.

Restoring cerebral dopamine homeostasis by electrical forepaw stimulation: an FMRI study

Deviation of dopamine homeostasis is known to be associated with disorders like drug addiction and Parkinson's disease. As dopamine function is tightly regulated within the basal ganglia circuitry, cortical perturbation would lead to modulation of dopaminergic activity in the striatum. We proposed and tested if somatosensory activity such as forepaw stimulation could modulate dopaminergic function. Specifically, we tested in rats if electrical forepaw stimulation (EFS) could attenuate dopamine release in the brain if dopamine is excessive, and boost dopamine release if dopamine is deficient. We had previously demonstrated that EFS effectively attenuated excessive DA concentration in the striatum. We now show in this manuscript with fMRI that EFS boosted DA release on two DA deficient conditions: (1) with quinpirole challenge, and (2) partial Parkinsonism model (PD). Quinpirole alone decreased dopamine release and thus the cerebral blood volume (CBV) that was restored by EFS. EFS also succeeded in increasing CBV in the basal-ganglia circuitry of the PD rats, but not in the controls. Context-dependent connectivity analysis showed increased connectivity during the basal state in the PD rats, compared with the controls. This "enhanced" yet abnormal connectivity of PD rats was reduced post-EFS. Our results suggest that EFS resets the deficient DA system by partially increasing DA release, in the meanwhile lessening the need for recruiting extra functional network in the basal ganglia circuitry. This study shows not only the capacity of peripheral stimulation to perturb neurotransmitter function, but also the potential of peripheral stimulation to restore neurotransmitter homeostasis.

Pharmacologic magnetic resonance imaging (phMRI): imaging drug action in the brain

The technique of functional magnetic resonance (fMRI), using various cognitive, motor and sensory stimuli has led to a revolution in the ability to map brain function. Drugs can also be used as stimuli to elicit an hemodynamic change. Stimulation with a pharmaceutical has a number of very different consequences compared to user controllable stimuli, most importantly in the time course of stimulus and response that is not, in general, controllable by the experimenter. Therefore, this type of experiment has been termed pharmacologic MRI (phMRI). The use of a drug stimulus leads to a number of interesting possibilities compared to conventional fMRI. Using receptor specific ligands one can characterize brain circuitry specific to neurotransmitter systems. The possibility exists to measure parameters reflecting neurotransmitter release and binding associated with the pharmacokinetics and/or the pharmacodynamics of drugs. There is also the ability to measure up- and down-regulation of receptors in specific disease states. phMRI can be characterized as a molecular imaging technique using the natural hemodynamic transduction related to neuro-receptor stimulus. This provides a coupling mechanism with very high sensitivity that can rival positron emission tomography (PET) in some circumstances. The large numbers of molecules available, that do not require a radio-label, means that phMRI becomes a very useful tool for performing drug discovery. Data and arguments will be presented to show that phMRI can provide information on neuro-receptor signaling and function that complements the static picture generated by PET studies of receptor numbers and occupancies.

Postprandial plasma PYY concentrations are associated with increased regional gray matter volume and rCBF declines in caudate nuclei--a combined MRI and H2(15)O PET study

The anorexigenic gastrointestinal hormone Peptide YY plays an important role in the communication between the gastrointestinal tract and the central nervous system. PYY has been shown to modulate brain activity in regions implicated in reward and food related behavior. Its effects on brain structure however, remain unknown. Voxel-based morphometry was used to investigate the relationship between fasting and postprandial plasma PYY concentrations and regional gray matter volume (GMV). For this analysis twenty adult, non diabetic Caucasians were included (18F/2M, age 31±9 y, percentage of body fat [PFAT] 32±8%) who had volumetric brain magnetic resonance images and underwent H(2)(15)O positron emission tomographic (PET) measurements of regional cerebral blood flow (rCBF), a marker of local neuronal activity, and measurements of plasma total PYY, prior to (fasting) and following a satiating liquid meal. Voxel-wise analysis revealed a regional positive association between postprandial PYY and gray matter volume bilaterally in the caudate nuclei. These associations remained significant (p<0.05) after small volume correction for multiple comparisons. Based on these findings we investigated whether postprandial PYY is associated with PET measured rCBF of the caudate nucleus. We found a significant negative association between average postprandial caudate rCBF and postprandial plasma PYY concentrations (r=-0.60, p<0.02, age, sex and PFAT adjusted). Average postprandial caudate rCBF was also negatively associated with rCBF in the right medial orbitofrontal cortex and the right hippocampal formation (p<0.05, corrected for multiple comparisons). Total PYY is positively associated with gray matter but negatively with postprandial activity in the caudate nuclei while caudate activity is negatively associated with rCBF in prefrontal and paralimbic regions implicated in reward behavior. Thus, PYY may act centrally to modulate eating behavior via striatal networks.

Neuroimaging evidence of altered fronto-cortical and striatal function after prolonged cocaine self-administration in the rat

Cocaine addiction is often modeled in experimental paradigms where rodents learn to self-administer (SA) the drug. However, the extent to which these models replicate the functional alterations observed in clinical neuroimaging studies of cocaine addiction remains unknown. We used magnetic resonance imaging (MRI) to assess basal and evoked brain function in rats subjected to a prolonged, extended-access cocaine SA scheme. Specifically, we measured basal cerebral blood volume (bCBV), an established correlate of basal metabolism, and assessed the reactivity of the dopaminergic system by mapping the pharmacological MRI (phMRI) response evoked by the dopamine-releaser amphetamine. Cocaine-exposed subjects exhibited reduced bCBV in fronto-cortical areas, nucleus accumbens, ventral hippocampus, and thalamus. The cocaine group also showed an attenuated functional response to amphetamine in ventrostriatal areas, an effect that was significantly correlated with total cocaine intake. An inverse relationship between bCBV in the reticular thalamus and the frontal response elicited by amphetamine was found in control subjects but not in the cocaine group, suggesting that the inhibitory interplay within this attentional circuit may be compromised by the drug. Importantly, histopathological analysis did not reveal significant alterations of the microvascular bed in the brain of cocaine-exposed subjects, suggesting that the imaging findings cannot be merely ascribed to cocaine-induced vascular damage. These results document that chronic, extended-access cocaine SA in the rat produces focal fronto-cortical and striatal alterations that serve as plausible neurobiological substrate for the behavioral expression of compulsive drug intake in laboratory animals.

Cocaine self-administration leads to alterations in temporal responses to cocaine challenge in limbic and motor circuitry

Chronic use of cocaine is associated with lasting alterations in brain metabolism, circuitry, and receptor properties. We used neuroimaging with pharmacological magnetic resonance imaging to assess alterations in response to cocaine (0.5 mg/kg) in animals trained to self-administer cocaine on a fixed-ratio 5 schedule of reinforcement, as well as saline-yoked controls, after 28 days of cocaine abstinence. We fitted the cerebral blood volume (CBV) curves for full-width half-maximum (FWHM) as well as peak CBV response. There were significant increases in the FWHM of the response curves in the cocaine self-administering (SA) animals as compared with saline-yoked controls in the medial prefrontal cortex (mPFC) and the caudate/putamen (CPu), and increases in peak CBV in the M1 motor cortex, CPu, and pedunculopontine tegmental nucleus. Functional connectivity analysis showed increased correlations in the cocaine SA rats upon acute cocaine challenge, especially in the S1, mPFC, and thalamus. As D3 receptor expression is postulated to increase following chronic cocaine administration, we also examined the response to 0.2 mg/kg of the D3-preferring agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OHDPAT). Cocaine SA animals showed a decreased overall CBV response to this drug, except in the globus pallidus. The hypothalamus showed a negative CBV change in response to cocaine challenge, similar to that noted with the D3 agonist, and showed a smaller response in the cocaine SA animals than in the controls. Given the good coupling of cerebral hemodynamics with dopamine dynamics previously observed with pharmacological magnetic resonance imaging, these data suggest that increased persistence of dopamine in the prefrontal cortex may be responsible for some of the behavioral alterations observed subsequent to chronic cocaine use.

MDMA ("ecstasy") abuse as an example of dopamine neuroplasticity

A number of reviews have focused on the short- and long-term effects of MDMA and, in particular, on the persistent deficits in serotonin neurotransmission that accompany some exposure regimens. The mechanisms underlying the serotonin deficits and their relevance to various behavioral and cognitive consequences of MDMA use are still being debated. It has become clear, however, that some individuals develop compulsive and uncontrolled drug-taking that is consistent with abuse. For other drugs of abuse, this transition has been attributed to neuroadaptations in central dopamine mechanisms that occur as a function of repeated drug exposure. A question remains as to whether similar neuroadaptations occur as a function of exposure to MDMA and the impact of serotonin neurotoxicity in the transition from use to abuse. This review focuses specifically on this issue by first providing an overview of human studies and then reviewing the animal literature with specific emphasis on paradigms that measure subjective effects of drugs and self-administration as indices of abuse liability. It is suggested that serotonin deficits resulting from repeated exposure to MDMA self-administration lead to a sensitized dopaminergic response to the drug and that this sensitized response renders MDMA comparable to other drugs of abuse.

Pharmacologic neuroimaging of the ontogeny of dopamine receptor function

Characterization of the ontogeny of the cerebral dopaminergic system is crucial for gaining a greater understanding of normal brain development and its alterations in response to drugs of abuse or conditions such as attention-deficit hyperactivity disorder. Pharmacological MRI (phMRI) was used to determine the response to dopamine transporter (DAT) blockers cocaine and methylphenidate (MPH), the dopamine releaser D-amphetamine (AMPH), the selective D1 agonist dihydrexidine, and the D2/D3 agonist quinpirole in young (<30 days old) and adult (>60 days old) rats. In adult rats, cocaine (0.5 mg/kg i.v.) or MPH (2 mg/kg) induced primarily positive cerebral blood volume (rCBV) changes in the dopaminergic circuitry, but negative rCBV changes in the young animals. Microdialysis measurements in the striatum showed that young rats have a smaller increase in extracellular dopamine in response to cocaine than adults. The young rats showed little rCBV response to the selective D1 agonist dihydrexidine in contrast to robust rCBV increases observed in the adults, whereas there was a similar negative rCBV response in the young and adult rats to the D2 agonist quinpirole. We also performed a meta-analysis of literature data on the development of D1 and D2 receptors and the DAT. These data suggest a predominance of D2-like over D1-like function between 20 and 30 days of age. These combined results suggested that the dopamine D1 receptor is functionally inhibited at young age.