Brain hemodynamic changes mediated by dopamine receptors: Role of the cerebral microvasculature in dopamine-mediated neurovascular coupling

Increased dopamine release after working-memory updating training: Neurochemical correlates of transfer

Previous work demonstrates that working-memory (WM) updating training results in improved performance on a letter-memory criterion task, transfers to an untrained n-back task, and increases striatal dopamine (DA) activity during the criterion task. Here, we sought to replicate and extend these findings by also examining neurochemical correlates of transfer. Four positron emission tomography (PET) scans using the radioligand raclopride were performed. Two of these assessed DAD2 binding (letter memory; n-back) before 5 weeks of updating training, and the same two scans were performed post training. Key findings were (a) pronounced training-related behavioral gains in the letter-memory criterion task, (b) altered striatal DAD2 binding potential after training during letter-memory performance, suggesting training-induced increases in DA release, and (c) increased striatal DA activity also during the n-back transfer task after the intervention, but no concomitant behavioral transfer. The fact that the training-related DA alterations during the transfer task were not accompanied by behavioral transfer suggests that increased DA release may be a necessary, but not sufficient, condition for behavioral transfer to occur.

Dopaminergic activity and altered reward modulation in anorexia nervosa-insight from multimodal imaging

OBJECTIVE

Individuals with anorexia nervosa (AN) have anxious and inhibited temperaments with high concern for consequences. Studies using either positron emission tomography (PET) or functional magnetic resonance imaging (fMRI) suggest involvement of the middle and dorsal caudate (DC) in individuals recovered (REC) from AN. For example, dopamine (DA) D2/D3 receptor binding in the middle caudate and DC was associated with anxiety and harm avoidance, and blood-oxygen-level-dependent (BOLD) response in the DC was positively related to trait anxiety. It has not been shown yet whether BOLD response in individuals REC from AN was related to DA function.

METHODS

Post-hoc correlation analyses between the PET and fMRI studies by correlating D2/D3 binding in striatal regions and BOLD signal in the anteroventral striatum (AVS) and DC for wins and losses respectively in 12 individuals REC from AN.

RESULTS

Individuals REC from AN with the greatest BOLD response in the DC in a monetary choice task had higher middle caudate D2/D3 binding, and greater anxiety and/or harm avoidance.

DISCUSSION

Though preliminary, these findings suggest that increased dorsal striatal D2/D3 binding is associated with enhanced cognitive response to feedback, potentially related to anxious anticipation of consequences. © 2016 Wiley Periodicals, Inc.(Int J Eat Disord 2017; 50:593-596).

Differential effects of gaseous versus injectable anesthetics on changes in regional cerebral blood flow and metabolism induced by l-DOPA in a rat model of Parkinson's disease

Preclinical imaging of brain activity requires the use of anesthesia. In this study, we have compared the effects of two widely used anesthetics, inhaled isoflurane and ketamine/xylazine cocktail, on cerebral blood flow and metabolism in a rat model of Parkinson's disease and l-DOPA-induced dyskinesia. Specific tracers were used to estimate regional cerebral blood flow (rCBF - [¹⁴C]-iodoantipyrine) and regional cerebral metabolic rate (rCMR - [¹⁴C]-2-deoxyglucose) with a highly sensitive autoradiographic method. The two types of anesthetics had quite distinct effects on l-DOPA-induced changes in rCBF and rCMR. Isoflurane did not affect either the absolute rCBF values or the increases in rCBF in the basal ganglia after l-DOPA administration. On the contrary, rats anesthetized with ketamine/xylazine showed lower absolute rCBF values, and the rCBF increases induced by l-DOPA were masked. We developed a novel improved model to calculate rCMR, and found lower metabolic activities in rats anesthetized with isoflurane compared to animals anesthetized with ketamine/xylazine. Both anesthetics prevented changes in rCMR upon l-DOPA administration. Pharmacological challenges in isoflurane-anesthetized rats indicated that drugs mimicking the actions of ketamine/xylazine on adrenergic or glutamate receptors reproduced distinct effects of the injectable anesthetics on rCBF and rCMR. Our results highlight the importance of anesthesia in studies of cerebral flow and metabolism, and provide novel insights into mechanisms mediating abnormal neurovascular responses to l-DOPA in Parkinson's 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 beta-arrestin2 in shaping fMRI BOLD responses to dopaminergic stimulation

RATIONALE

The dopamine D₂ receptor (D₂R) couples to inhibitory G_i/o proteins and is targeted by antipsychotic and antiparkinsonian drugs. Beta-arrestin2 binds to the intracellular regions of the agonist-occupied D₂R to terminate G protein activation and promote internalization, but also to initiate downstream signaling cascades which have been implicated in psychosis. Functional magnetic resonance imaging (fMRI) has proven valuable for measuring dopamine receptor-mediated changes in neuronal activity, and might enable beta-arrestin2 function to be studied in vivo.

OBJECTIVES

The present study examined fMRI blood oxygenation level dependent (BOLD) signal changes elicited by a dopamine agonist in wild-type (WT) and beta-arrestin2 knockout (KO) mice, to investigate whether genetic deletion of beta-arrestin2 prolongs or otherwise modifies D₂R-dependent responses.

METHODS

fMRI BOLD data were acquired on a 9.4 T system. During scans, animals received 0.2 mg/kg apomorphine, i.v. In a subset of experiments, animals were pretreated with 2 mg/kg of the D₂R antagonist, eticlopride.

RESULTS

Following apomorphine administration, BOLD signal decreases were observed in caudate/putamen of WT and KO animals. The time course of response decay in caudate/putamen was significantly slower in KO vs. WT animals. In cingulate cortex, an initial BOLD signal decrease was followed by a positive response component in WT but not in KO animals. Eticlopride pretreatment significantly reduced apomorphine-induced BOLD signal changes.

CONCLUSIONS

The prolonged striatal response decay rates in KO animals might reflect impaired D₂R desensitization, consistent with the known function of beta-arrestin2. Furthermore, the apomorphine-induced positive response component in cingulate cortex may depend on beta-arrestin2 signaling downstream of D₂R.

Changes in functional organization and white matter integrity in the connectome in Parkinson's disease

Parkinson's disease (PD) leads to dysfunction in multiple cortico-striatal circuits. The neurodegeneration has also been associated with impaired white matter integrity. This structural and functional “disconnection” in PD needs further characterization.

We investigated the structural and functional organization of the PD whole brain connectome consisting of 200 nodes using diffusion tensor imaging and resting-state functional MRI, respectively. Data from 20 non-demented PD patients on dopaminergic medication and 20 matched controls were analyzed using graph theory-based methods. We focused on node strength, clustering coefficient, and local efficiency as measures of local network properties; and network modularity as a measure of information flow.

PD patients showed reduced white matter connectivity in frontoparietal-striatal nodes compared to controls, but no change in modular organization of the white matter tracts. PD group also showed reduction in functional local network metrics in many nodes distributed across the connectome. There was also decreased functional modularity in the core cognitive networks including the default mode and dorsal attention networks, and sensorimotor network, as well as a lack of modular distinction in the orbitofrontal and basal ganglia nodes in the PD group compared to controls.

Our results suggest that despite subtle white matter connectivity changes, the overall structural organization of the PD connectome remains robust at relatively early disease stages. However, there is a breakdown in the functional modular organization of the PD connectome.

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DTI and rs-fMRI investigation of the connectome in Parkinson's disease (PD)

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Local network properties and modularity were examined using graph theory.

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White matter connectivity was decreased in frontoparietal-striatal nodes in PD.

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Functional modularity of cognitive and motor networks was reduced in PD.

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.

The age-dependent effects of a single-dose methylphenidate challenge on cerebral perfusion in patients with attention-deficit/hyperactivity disorder

Methylphenidate (MPH) is a stimulant drug and an effective treatment for attention-deficit/hyperactivity disorder (ADHD) in both children and adults. Pre-clinical studies suggest that the response to stimulants is dependent on age, which may reflect the ontogeny of the dopamine (DA) system, which continues to develop throughout childhood and adolescence. Therefore, the aim of this study was to investigate the modulating effect of age on the cerebral blood flow (CBF) response to MPH in stimulant treatment-naive children and adults with ADHD. Ninety-eight stimulant treatment-naive male pediatric (10-12 years) and adult (23-40 years) patients with ADHD were included in this study. The CBF response to an acute challenge with MPH (0.5 mg/kg) was measured using arterial spin labeling (ASL) pharmacological magnetic resonance imaging, as a proxy for DA function. Region-of-interest (ROI) analyses were carried out for the striatum, thalamus and medial prefrontal cortex and in addition voxel-wise analyses were conducted. An acute challenge with MPH decreased CBF in both children and adults in cortical areas, although to a greater extent in adults. In contrast, ROI analyses showed that MPH decreased thalamic CBF only in children, but not adults. Our findings highlight the importance of taking the developmental perspective into account when studying the effects of stimulants in ADHD patients.

Modulation of Postnatal Neurogenesis by Perinatal Asphyxia: Effect of D1 and D2 Dopamine Receptor Agonists

Perinatal asphyxia (PA) is associated to delayed cell death, affecting neurocircuitries of basal ganglia and hippocampus, and long-term neuropsychiatric disabilities. Several compensatory mechanisms have been suggested to take place, including cell proliferation and neurogenesis. There is evidence that PA can increase postnatal neurogenesis in hippocampus and subventricular zone (SVZ), modulated by dopamine, by still unclear mechanisms. We have studied here the effect of selective dopamine receptor agonists on cell death, cell proliferation and neurogenesis in organotypic cultures from control and asphyxia-exposed rats. Hippocampus and SVZ sampled at 1-3 postnatal days were cultured for 20-21 days. At day in vitro (DIV) 19, cultures were treated either with SKF38393 (10 and 100 µM, a D₁ agonist), quinpirole (10 µM, a D₂ agonist) or sulpiride (10 μM, a D₂ antagonist) + quinpirole (10 μM) and BrdU (10 μM, a mitosis marker) for 24 h. At DIV 20-21, cultures were processed for immunocytochemistry for microtubule-associated protein-2 (MAP-2, a neuronal marker), and BrdU, evaluated by confocal microscopy. Some cultures were analysed for cell viability at DIV 20-21 (LIVE/DEAD kit). PA increased cell death, cell proliferation and neurogenesis in hippocampus and SVZ cultures. The increase in cell death, but not in cell proliferation, was inhibited by both SKF38393 and quinpirole treatment. Neurogenesis was increased by quinpirole, but only in hippocampus, in cultures from both asphyxia-exposed and control-animals, effect that was antagonised by sulpiride, leading to the conclusion that dopamine modulates neurogenesis in hippocampus, mainly via D₂ receptors.

Age-Dependent Effects of Methylphenidate on the Human Dopaminergic System in Young vs Adult Patients With Attention-Deficit/Hyperactivity Disorder: A Randomized Clinical Trial

IMPORTANCE

Although numerous children receive methylphenidate hydrochloride for the treatment of attention-deficit/hyperactivity disorder (ADHD), little is known about age-dependent and possibly lasting effects of methylphenidate on the human dopaminergic system.

OBJECTIVES

To determine whether the effects of methylphenidate on the dopaminergic system are modified by age and to test the hypothesis that methylphenidate treatment of young but not adult patients with ADHD induces lasting effects on the cerebral blood flow response to dopamine challenge, a noninvasive probe for dopamine function.

DESIGN, SETTING, AND PARTICIPANTS

A randomized, double-blind, placebo-controlled trial (Effects of Psychotropic Drugs on Developing Brain-Methylphenidate) among ADHD referral centers in the greater Amsterdam area in the Netherlands between June 1, 2011, and June 15, 2015. Additional inclusion criteria were male sex, age 10 to 12 years or 23 to 40 years, and stimulant treatment-naive status.

INTERVENTIONS

Treatment with either methylphenidate or a matched placebo for 16 weeks.

MAIN OUTCOMES AND MEASURES

Change in the cerebral blood flow response to an acute challenge with methylphenidate, noninvasively assessed using pharmacological magnetic resonance imaging, between baseline and 1 week after treatment. Data were analyzed using intent-to-treat analyses.

RESULTS

Among 131 individuals screened for eligibility, 99 patients met DSM-IV criteria for ADHD, and 50 participants were randomized to receive methylphenidate and 49 to placebo. Sixteen weeks of methylphenidate treatment increased the cerebral blood flow response to methylphenidate within the thalamus (mean difference, 6.5; 95% CI, 0.4-12.6; P = .04) of children aged 10 to 12 years old but not in adults or in the placebo group. In the striatum, the methylphenidate condition differed significantly from placebo in children but not in adults (mean difference, 7.7; 95% CI, 0.7-14.8; P = .03).

CONCLUSIONS AND RELEVANCE

We confirm preclinical data and demonstrate age-dependent effects of methylphenidate treatment on human extracellular dopamine striatal-thalamic circuitry. Given its societal relevance, these data warrant replication in larger groups with longer follow-up.

TRIAL REGISTRATION

identifier: NL34509.000.10 and trialregister.nl identifier: NTR3103.

Functional modulation of G-protein coupled receptors during Parkinson disease-like neurodegeneration

G-protein coupled dopamine and metabotropic glutamate receptors (mGlu) can modulate neurotransmission during Parkinson's disease (PD)-like neurodegeneration. PET imaging studies in a unilateral dopamine denervation model (6-OHDA) showed a significant inverse correlation of presynaptic mGlu4 and postsynaptic mGlu5 expression in the striatum and rapidly declining mGlu4 and enhanced mGlu5 expression in the hippocampus during progressive degeneration over time. Immunohistochemical studies verified the decreased mGlu4 expression in the hippocampus on the lesion side but did not show difference in mGlu5 expression between lesion and control side. Pharmacological MRI studies showed enhanced hemodynamic response in several brain areas on the lesion side compared to the control side after challenge with mGlu4 positive allosteric modulator or mGlu5 negative allosteric modulator. However, mGlu4 response was biphasic having short enhancement followed by negative response on both sides of brain. Studies in mGlu4 expressing cells demonstrated that glutamate induces cooperative increase in binding of mGlu4 ligands - especially at high glutamate levels consistent with in vivo concentration. This suggests that mGlu allosteric modulators as drug candidates will be highly sensitive to changes in glutamate concentration and hence metabolic state. These experiments demonstrate the importance of the longitudinal imaging studies to investigate temporal changes in receptor functions to obtain individual response for experimental drugs.

Neurofunctional Reward Processing Changes in Cocaine Dependence During Recovery

Although reward processing appears altered in addiction, few studies track neurofunctional changes following treatment or relate these to measures of reduced drug use. The current study examined neurofunctional alterations in reward processing in cocaine dependence (CD) pretreatment and posttreatment to determine whether these changes relate to clinically meaningful outcome indicators. Treatment-seeking CD outpatients (N=29) underwent functional magnetic resonance imaging while performing a monetary incentive delay task (MIDT) pretreatment and posttreatment. The MIDT parses anticipatory from outcome phases of reward/loss processing. Abstinence indicators (negative urines, days abstinent from cocaine during follow-up) were collected throughout treatment and up to 1 year later. Healthy control (HC) participants (N=28) were also scanned twice with the MIDT. Relative to pretreatment, at posttreatment CD participants demonstrated increased anticipatory reward activity in the midbrain, thalamus, and precuneus (pFWE<0.05). Increased midbrain activity correlated with cocaine abstinence during the 1-year follow-up. Ventral striatal (VS) activity during loss anticipation correlated negatively with negative urine screens. HC group test-retest results showed decreased ventromedial prefrontal cortex activity during winning outcomes. CD-HC group-by-time differences revealed increased left inferior frontal gyrus activity in the CD group during anticipatory phases at posttreatment. In CD participants, increased posttreatment activity in dopamine-innervated regions suggests lowered thresholds in anticipatory signaling for non-drug rewards. Midbrain and VS responses may represent biomarkers associated with CD abstinence. Abstinence-related neurobiological changes occur in similar regions implicated during active use and may possibly be used to track progress during short- and long-term recovery.

Dopamine D2 receptor availability is linked to hippocampal-caudate functional connectivity and episodic memory

D1 and D2 dopamine receptors (D1DRs and D2DRs) may contribute differently to various aspects of memory and cognition. The D1DR system has been linked to functions supported by the prefrontal cortex. By contrast, the role of the D2DR system is less clear, although it has been hypothesized that D2DRs make a specific contribution to hippocampus-based cognitive functions. Here we present results from 181 healthy adults between 64 and 68 y of age who underwent comprehensive assessment of episodic memory, working memory, and processing speed, along with MRI and D2DR assessment with [(11)C]raclopride and PET. Caudate D2DR availability was positively associated with episodic memory but not with working memory or speed. Whole-brain analyses further revealed a relation between hippocampal D2DR availability and episodic memory. Hippocampal and caudate D2DR availability were interrelated, and functional MRI-based resting-state functional connectivity between the ventral caudate and medial temporal cortex increased as a function of caudate D2DR availability. Collectively, these findings indicate that D2DRs make a specific contribution to hippocampus-based cognition by influencing striatal and hippocampal regions, and their interactions.

White-Matter Tract Connecting Anterior Insula to Nucleus Accumbens Correlates with Reduced Preference for Positively Skewed Gambles

Individuals sometimes show inconsistent risk preferences, including excessive attraction to gambles featuring small chances of winning large amounts (called "positively skewed" gambles). While functional neuroimaging research indicates that nucleus accumbens (NAcc) and anterior insula (AIns) activity inversely predict risky choice, structural connections between these regions have not been described in humans. By combining diffusion-weighted MRI with tractography, we identified the anatomical trajectory of white-matter tracts projecting from the AIns to the NAcc and statistically validated these tracts using Linear Fascicle Evaluation (LiFE) and virtual lesions. Coherence of the right AIns-NAcc tract correlated with reduced preferences for positively skewed gambles. Further, diminished NAcc activity during gamble presentation mediated the association between tract structure and choice. These results identify an unreported tract connecting the AIns to the NAcc in humans and support the notion that structural connections can alter behavior by influencing brain activity as individuals weigh uncertain gains against uncertain losses.

Imaging Agonist-Induced D2/D3 Receptor Desensitization and Internalization In Vivo with PET/fMRI

This study investigated the dynamics of dopamine receptor desensitization and internalization, thereby proposing a new technique for non-invasive, in vivo measurements of receptor adaptations. The D2/D3 agonist quinpirole, which induces receptor internalization in vitro, was administered at graded doses in non-human primates while imaging with simultaneous positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). A pronounced temporal divergence between receptor occupancy and fMRI signal was observed: occupancy remained elevated while fMRI responded transiently. Analogous experiments with an antagonist (prochlorperazine) and a lower-affinity agonist (ropinirole) exhibited reduced temporal dissociation between occupancy and function, consistent with a mechanism of desensitization and internalization that depends upon drug efficacy and affinity. We postulated a model that incorporates internalization into a neurovascular-coupling relationship. This model yielded in vivo desensitization/internalization rates (0.2/min for quinpirole) consistent with published in vitro measurements. Overall, these results suggest that simultaneous PET/fMRI enables characterization of dynamic neuroreceptor adaptations in vivo, and may offer a first non-invasive method for assessing receptor desensitization and internalization.

Dopamine D1 Binding Potential Predicts Fusiform BOLD Activity during Face-Recognition Performance

The importance of face memory in humans and primates is well established, but little is known about the neurotransmitter systems involved in face recognition. We tested the hypothesis that face recognition is linked to dopamine (DA) activity in fusiform gyrus (FFG). DA availability was assessed by measuring D1 binding potential (BP) during rest using PET. We further assessed blood-oxygen-level-dependent (BOLD) signal change while subjects performed a face-recognition task during fMRI scanning. There was a strong association between D1 BP and BOLD activity in FFG, whereas D1 BP in striatal and other extrastriatal regions were unrelated to neural activity in FFG. These results suggest that D1 BP locally modulates FFG function during face recognition. Observed relationships among D1 BP, BOLD activity, and face-recognition performance further suggest that D1 receptors place constraints on the responsiveness of FFG neurons.

SIGNIFICANCE STATEMENT

The importance of face memory in humans and primates is well established, but little is known about the neurotransmitter systems involved in face recognition. Our work shows a role for a specific neurotransmitter system in face memory.

Inverted optical intrinsic response accompanied by decreased cerebral blood flow are related to both neuronal inhibition and excitation

Negative hemodynamic response has been widely reported in blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging studies, however its origin is still controversial. Optical intrinsic signal (OIS) imaging can be used to study brain activity by simultaneously recording hemodynamic signals at different wavelengths with high spatial resolution. In this study, we found transcorneal electrical stimulation (TcES) could elicit both positive OIS response (POR) and negative OIS response (NOR) in cats’ visual cortex. We then investigated the property of this negative response to TcES and its relationship with cerebral blood flow (CBF) and neuronal activity. Results from laser speckle contrast imaging showed decreased CBF in the NOR region while increased CBF in the POR region. Both planar and laminar electrophysiological recordings in the middle (500–700 μm) cortical layers demonstrated that decreased and increased neuronal activities were coexisted in the NOR region. Furthermore, decreased neuronal activity was also detected in the deep cortical layers in the NOR region. This work provides evidence that the negative OIS together with the decreased CBF should be explained by mechanisms of both neuronal inhibition and excitation within middle cortical layers. Our results would be important for interpreting neurophysiological mechanisms underlying the negative BOLD signals.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Striatal and Pallidal Activation during Reward Modulated Movement Using a Translational Paradigm

Human neuroimaging studies of reward processing typically involve tasks that engage decision-making processes in the dorsal striatum or focus upon the ventral striatum's response to feedback expectancy. These studies are often compared to the animal literature; however, some animal studies include both feedback and nonfeedback events that activate the dorsal striatum during feedback expectancy. Differences in task parameters, movement complexity, and motoric effort to attain rewards may partly explain ventral and dorsal striatal response differences across species. We, therefore, used a target capture task during functional neuroimaging that was inspired by a study of single cell modulation in the internal globus pallidus during reward-cued, rotational arm movements in nonhuman primates. In this functional magnetic resonance imaging study, participants used a fiberoptic joystick to make a rotational response to an instruction stimulus that indicated both a target location for a capture movement and whether or not the trial would end with feedback indicating either a small financial gain or a neutral outcome. Portions of the dorsal striatum and pallidum demonstrated greater neural activation to visual cues predicting potential gains relative to cues with no associated outcome. Furthermore, both striatal and pallidal regions displayed a greater response to financial gains relative to neutral outcomes. This reward-dependent modulation of dorsal striatal and pallidal activation in a target-capture task is consistent with findings from reward studies in animals, supporting the use of motorically complex tasks as translational paradigms to investigate the neural substrates of reward expectancy and outcome in humans.

phMRI: methodological considerations for mitigating potential confounding factors

Pharmacological Magnetic Resonance Imaging (phMRI) is a variant of conventional MRI that adds pharmacological manipulations in order to study the effects of drugs, or uses pharmacological probes to investigate basic or applied (e.g., clinical) neuroscience questions. Issues that may confound the interpretation of results from various types of phMRI studies are briefly discussed, and a set of methodological strategies that can mitigate these problems are described. These include strategies that can be employed at every stage of investigation, from study design to interpretation of resulting data, and additional techniques suited for use with clinical populations are also featured. Pharmacological MRI is a challenging area of research that has both significant advantages and formidable difficulties, however with due consideration and use of these strategies many of the key obstacles can be overcome.

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.

Presynaptic Mechanisms of l-DOPA-Induced Dyskinesia: The Findings, the Debate, and the Therapeutic Implications

The dopamine (DA) precursor l-DOPA has been the most effective treatment for Parkinson’s disease (PD) for over 40 years. However, the response to this treatment changes with disease progression, and most patients develop dyskinesias (abnormal involuntary movements) and motor fluctuations within a few years of l-DOPA therapy. There is wide consensus that these motor complications depend on both pre- and post-synaptic disturbances of nigrostriatal DA transmission. Several presynaptic mechanisms converge to generate large DA swings in the brain concomitant with the peaks-and-troughs of plasma l-DOPA levels, while post-synaptic changes engender abnormal functional responses in dopaminoceptive neurons. While this general picture is well-accepted, the relative contribution of different factors remains a matter of debate. A particularly animated debate has been growing around putative players on the presynaptic side of the cascade. To what extent do presynaptic disturbances in DA transmission depend on deficiency/dysfunction of the DA transporter, aberrant release of DA from serotonin neurons, or gliovascular mechanisms? And does noradrenaline (which is synthetized from DA) play a role? This review article will summarize key findings, controversies, and pending questions regarding the presynaptic mechanisms of l-DOPA-induced dyskinesia. Intriguingly, the debate around these mechanisms has spurred research into previously unexplored facets of brain plasticity that have far-reaching implications to the treatment of neuropsychiatric disease.

Maximizing negative correlations in resting-state functional connectivity MRI by time-lag

This paper aims to better understand the physiological meaning of negative correlations in resting state functional connectivity MRI (r-fcMRI). The correlations between anatomy-based brain regions of 18 healthy humans were calculated and analyzed with and without a correction for global signal and with and without spatial smoothing. In addition, correlations between anatomy-based brain regions of 18 naïve anesthetized rats were calculated and compared to the human data. T-statistics were used to differentiate between positive and negative connections. The application of spatial smoothing and global signal correction increased the number of significant positive connections but their effect on negative connections was complex. Positive connections were mainly observed between cortical structures while most negative connections were observed between cortical and non-cortical structures with almost no negative connections between non-cortical structures. In both human and rats, negative connections were never observed between bilateral homologous regions. The main difference between positive and negative connections in both the human and rat data was that positive connections became less significant with time-lags, while negative connections became more significant with time-lag. This effect was evident in all four types of analyses (with and without global signal correction and spatial smoothing) but was most significant in the analysis with no correction for the global signal. We hypothesize that the valence of r-fcMRI connectivity reflects the relative contributions of cerebral blood volume (CBV) and flow (CBF) to the BOLD signal and that these relative contributions are location-specific. If cerebral circulation is primarily regulated by CBF in one region and by CBV in another, a functional connection between these regions can manifest as an r-fcMRI negative and time-delayed correlation. Similarly, negative correlations could result from spatially inhomogeneous responses of rCBV or rCBF alone. Consequently, neuronal regulation of brain circulation may be deduced from the valence of r-fcMRI connectivity.

Dopamine receptors and the persistent neurovascular dysregulation induced by methamphetamine self-administration in rats

Recently abstinent methamphetamine (Meth) abusers showed neurovascular dysregulation within the striatum. The factors that contribute to this dysregulation and the persistence of these effects are unclear. The current study addressed these knowledge gaps. First, we evaluated the brains of rats with a history of Meth self-administration following various periods of forced abstinence. Micro-computed tomography revealed a marked reduction in vessel diameter and vascular volume uniquely within the striatum between 1 and 28 days after Meth self-administration. Microvessels showed a greater impairment than larger vessels. Subsequently, we determined that dopamine (DA) D2 receptors regulated Meth-induced striatal vasoconstriction via acute noncontingent administration of Meth. These receptors likely regulated the response to striatal hypoxia, as hypoxia inducible factor 1α was elevated. Acute Meth exposure also increased striatal levels of endothelin receptor A and decreased neuronal nitric oxide synthase. Collectively, the data provide novel evidence that Meth-induced striatal neurovascular dysregulation involves DA receptor signaling that results in vasoconstriction via endothelin receptor A and nitric oxide signaling. As these effects can lead to hypoxia and trigger neuronal damage, these findings provide a mechanistic explanation for the selective striatal toxicity observed in the brains of Meth-abusing humans.

Imaging neurovascular function and functional recovery after stroke in the rat striatum using forepaw stimulation

Negative functional magnetic resonance imaging (fMRI) response in the striatum has been observed in several studies during peripheral sensory stimulation, but its relationship between local field potential (LFP) remains to be elucidated. We performed cerebral blood volume (CBV) fMRI and LFP recordings in normal rats during graded noxious forepaw stimulation at nine stimulus pulse widths. Albeit high LFP-CBV correlation was found in the ipsilateral and contralateral sensory cortices (r=0.89 and 0.95, respectively), the striatal CBV responses were neither positively, nor negatively correlated with LFP (r=0.04), demonstrating that the negative striatal CBV response is not originated from net regional inhibition. To further identify whether this negative CBV response can serve as a marker for striatal functional recovery, two groups of rats (n=5 each) underwent 20- and 45-minute middle cerebral artery occlusion (MCAO) were studied. No CBV response was found in the ipsilateral striatum in both groups immediately after stroke. Improved striatal CBV response was observed on day 28 in the 20-minute MCAO group compared with the 45-minute MCAO group (P<0.05). This study shows that fMRI signals could differ significantly from LFP and that the observed negative CBV response has potential to serve as a marker for striatal functional integrity in rats.

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'.

Striatal vessels receive phosphorylated tyrosine hydroxylase-rich innervation from midbrain dopaminergic neurons

Nowadays it is assumed that besides its roles in neuronal processing, dopamine (DA) is also involved in the regulation of cerebral blood flow. However, studies on the hemodynamic actions of DA have been mainly focused on the cerebral cortex, but the possibility that vessels in deeper brain structures receive dopaminergic axons and the origin of these axons have not been investigated. Bearing in mind the evidence of changes in the blood flow of basal ganglia in Parkinson's disease (PD), and the pivotal role of the dopaminergic mesostriatal pathway in the pathophysiology of this disease, here we studied whether striatal vessels receive inputs from midbrain dopaminergic neurons. The injection of an anterograde neuronal tracer in combination with immunohistochemistry for dopaminergic, vascular and astroglial markers, and dopaminergic lesions, revealed that midbrain dopaminergic axons are in close apposition to striatal vessels and perivascular astrocytes. These axons form dense perivascular plexuses restricted to striatal regions in rats and monkeys. Interestingly, they are intensely immunoreactive for tyrosine hydroxylase (TH) phosphorylated at Ser19 and Ser40 residues. The presence of phosphorylated TH in vessel terminals indicates they are probably the main source of basal TH activity in the striatum, and that after activation of midbrain dopaminergic neurons, DA release onto vessels precedes that onto neurons. Furthermore, the relative weight of this "vascular component" within the mesostriatal pathway suggests that it plays a relevant role in the pathophysiology of PD.

Contributions and complexities from the use of in vivo animal models to improve understanding of human neuroimaging signals

Many of the major advances in our understanding of how functional brain imaging signals relate to neuronal activity over the previous two decades have arisen from physiological research studies involving experimental animal models. This approach has been successful partly because it provides opportunities to measure both the hemodynamic changes that underpin many human functional brain imaging techniques and the neuronal activity about which we wish to make inferences. Although research into the coupling of neuronal and hemodynamic responses using animal models has provided a general validation of the correspondence of neuroimaging signals to specific types of neuronal activity, it is also highlighting the key complexities and uncertainties in estimating neural signals from hemodynamic markers. This review will detail how research in animal models is contributing to our rapidly evolving understanding of what human neuroimaging techniques tell us about neuronal activity. It will highlight emerging issues in the interpretation of neuroimaging data that arise from in vivo research studies, for example spatial and temporal constraints to neuroimaging signal interpretation, or the effects of disease and modulatory neurotransmitters upon neurovascular coupling. We will also give critical consideration to the limitations and possible complexities of translating data acquired in the typical animals models used in this area to the arena of human fMRI. These include the commonplace use of anesthesia in animal research studies and the fact that many neuropsychological questions that are being actively explored in humans have limited homologs within current animal models for neuroimaging research. Finally we will highlighting approaches, both in experimental animals models (e.g. imaging in conscious, behaving animals) and human studies (e.g. combined fMRI-EEG), that mitigate against these challenges.

Use of magnetic resonance imaging in pharmacogenomics

Because of the large variation in the response to psychoactive medication, many studies have attempted to uncover genetic factors that determine response. While considerable knowledge exists on the large effects of genetic polymorphisms on pharmacokinetics and plasma concentrations of drugs, effects of the concentration at the target site and pharmacodynamic effects on brain functions in disease are much less known. This article reviews the role of magnetic resonance imaging (MRI) to visualize response to medication in brain behaviour circuits in vivo in humans and assess the influence of pharmacogenetic factors. Two types of studies have been used to characterize effects of medication and genetic variation. In task-related activation studies the focus is on changes in the activity of a neural circuit associated with a specific psychological process. The second type of study investigates resting state perfusion. These studies provide an assessment of vascular changes associated with bioavailability of drugs in the brain, but may also assess changes in neural activity after binding of centrally active agents. Task-related pharmacogenetic studies of cognitive function have characterized the effects in the prefrontal cortex of genetic polymorphisms of dopamine receptors (DRD2), metabolic enzymes (COMT) and in the post-synaptic signalling cascade under the administration of dopamine agonists and antagonists. In contrast, pharmacogenetic imaging with resting state perfusion is still in its infancy. However, the quantitative nature of perfusion imaging, its non-invasive character and its repeatability might be crucial assets in visualizing the effects of medication in vivo in man during therapy.

Prefrontal hemodynamic changes during cigarette smoking in young adult smokers with and without ADHD

Individuals with attention-deficit/hyperactivity disorder (ADHD) have elevated smoking prevalence and reduced cessation rates compared to the general population. However, the effects of cigarette smoking on underlying brain activity in smokers with ADHD are not well characterized. Non-invasive near-infrared spectroscopy (NIRS) was used to characterize how cigarette smoking affects prefrontal brain hemodynamics in smokers with and without ADHD. Prefrontal changes of oxy- and deoxyhemoglobin (HbO2 and HHb) were measured in six male adult smokers with ADHD and six age- and gender-matched control smokers. NIRS measurements were separated into four sequential time intervals, i.e., before smoking, during smoking, after smoking, and during a breath hold. Prefrontal HbO2 was lower during smoking in smokers with ADHD compared to control smokers. More specifically, smokers with ADHD showed decreased prefrontal HbO2 during smoking compared to breath hold, before and after smoking periods. In contrast, control smokers showed increased prefrontal HbO2 from before smoking to breath hold. Decreased prefrontal HbO2 in smokers with ADHD may reflect a smoking-induced change in prefrontal brain activity and microvasculature, which is not found in smokers without ADHD. The lower prefrontal HbO2 may be a biomarker for increased susceptibility to tobacco smoke in smokers with ADHD. Smoking in individuals with ADHD may increase vasoconstriction of cerebral arteries in the prefrontal cortex, which may contribute to a reduction in HbO2. The findings highlight the importance of smoking cessation, in particular in those smokers who use nicotine to self-medicate ADHD symptoms.

Integrative physiological and computational approaches to understand autonomic control of cerebral autoregulation

The brain requires steady delivery of oxygen and glucose, without which neurodegeneration occurs within minutes. Thus, the ability of the cerebral vasculature to maintain relatively steady blood flow in the face of changing systemic pressure, i.e. cerebral autoregulation, is critical to neurophysiological health. Although the study of autoregulation dates to the early 20th century, only the recent availability of cerebral blood flow measures with high temporal resolution has allowed rapid, beat-by-beat measurements to explore the characteristics and mechanisms of autoregulation. These explorations have been further enhanced by the ability to apply sophisticated computational approaches that exploit the large amounts of data that can be acquired. These advances have led to unique insights. For example, recent studies have revealed characteristic time scales wherein cerebral autoregulation is most active, as well as specific regions wherein autonomic mechanisms are prepotent. However, given that effective cerebral autoregulation against pressure fluctuations results in relatively unchanging flow despite changing pressure, estimating the pressure-flow relationship can be limited by the error inherent in computational models of autoregulatory function. This review focuses on the autonomic neural control of the cerebral vasculature in health and disease from an integrative physiological perspective. It also provides a critical overview of the current analytical approaches to understand cerebral autoregulation.

Optical imaging of cortical networks via intracortical microstimulation

Understanding cortical organization is key to understanding brain function. Distinct neural networks underlie the functional organization of the cerebral cortex; however, little is known about how different nodes in the cortical network interact during perceptual processing and motor behavior. To study cortical network function we examined whether the optical imaging of intrinsic signals (OIS) reveals the functional patterns of activity evoked by electrical cortical microstimulation. We examined the effects of current amplitude, train duration, and depth of cortical stimulation on the hemodynamic response to electrical microstimulation (250-Hz train, 0.4-ms pulse duration) in anesthetized New World monkey somatosensory cortex. Electrical stimulation elicited a restricted cortical response that varied according to stimulation parameters and electrode depth. Higher currents of stimulation recruited more areas of cortex than smaller currents. The largest cortical responses were seen when stimulation was delivered around cortical layer 4. Distinct local patches of activation, highly suggestive of local projections, around the site of stimulation were observed at different depths of stimulation. Thus we find that specific electrical stimulation parameters can elicit activation of single cortical columns and their associated columnar networks, reminiscent of anatomically labeled networks. This novel functional tract tracing method will open new avenues for investigating relationships of local cortical organization.

Functional imaging of cerebral perfusion

The functional imaging of perfusion enables the study of its properties such as the vasoreactivity to circulating gases, the autoregulation and the neurovascular coupling. Downstream from arterial stenosis, this imaging can estimate the vascular reserve and the risk of ischemia in order to adapt the therapeutic strategy. This method reveals the hemodynamic disorders in patients suffering from Alzheimer's disease or with arteriovenous malformations revealed by epilepsy. Functional MRI of the vasoreactivity also helps to better interpret the functional MRI activation in practice and in clinical research.

Use of frontal lobe hemodynamics as reinforcement signals to an adaptive controller

Decision-making ability in the frontal lobe (among other brain structures) relies on the assignment of value to states of the animal and its environment. Then higher valued states can be pursued and lower (or negative) valued states avoided. The same principle forms the basis for computational reinforcement learning controllers, which have been fruitfully applied both as models of value estimation in the brain, and as artificial controllers in their own right. This work shows how state desirability signals decoded from frontal lobe hemodynamics, as measured with near-infrared spectroscopy (NIRS), can be applied as reinforcers to an adaptable artificial learning agent in order to guide its acquisition of skills. A set of experiments carried out on an alert macaque demonstrate that both oxy- and deoxyhemoglobin concentrations in the frontal lobe show differences in response to both primarily and secondarily desirable (versus undesirable) stimuli. This difference allows a NIRS signal classifier to serve successfully as a reinforcer for an adaptive controller performing a virtual tool-retrieval task. The agent's adaptability allows its performance to exceed the limits of the NIRS classifier decoding accuracy. We also show that decoding state desirabilities is more accurate when using relative concentrations of both oxyhemoglobin and deoxyhemoglobin, rather than either species alone.

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'.

Neurovascular abnormalities in brain disorders: highlights with angiogenesis and magnetic resonance imaging studies

The coupling between neuronal activity and vascular responses is controlled by the neurovascular unit (NVU), which comprises multiple cell types. Many different types of dysfunction in these cells may impair the proper control of vascular responses by the NVU. Magnetic resonance imaging, which is the most powerful tool available to investigate neurovascular structures or functions, will be discussed in the present article in relation to its applications and discoveries. Because aberrant angiogenesis and vascular remodeling have been increasingly reported as being implicated in brain pathogenesis, this review article will refer to this hallmark event when suitable.

Visual predictions in the orbitofrontal cortex rely on associative content

Predicting upcoming events from incomplete information is an essential brain function. The orbitofrontal cortex (OFC) plays a critical role in this process by facilitating recognition of sensory inputs via predictive feedback to sensory cortices. In the visual domain, the OFC is engaged by low spatial frequency (LSF) and magnocellular-biased inputs, but beyond this, we know little about the information content required to activate it. Is the OFC automatically engaged to analyze any LSF information for meaning? Or is it engaged only when LSF information matches preexisting memory associations? We tested these hypotheses and show that only LSF information that could be linked to memory associations engages the OFC. Specifically, LSF stimuli activated the OFC in 2 distinct medial and lateral regions only if they resembled known visual objects. More identifiable objects increased activity in the medial OFC, known for its function in affective responses. Furthermore, these objects also increased the connectivity of the lateral OFC with the ventral visual cortex, a crucial region for object identification. At the interface between sensory, memory, and affective processing, the OFC thus appears to be attuned to the associative content of visual information and to play a central role in visuo-affective prediction.

Neurovascular coupling to D2/D3 dopamine receptor occupancy using simultaneous PET/functional MRI

This study employed simultaneous neuroimaging with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to demonstrate the relationship between changes in receptor occupancy measured by PET and changes in brain activity inferred by fMRI. By administering the D2/D3 dopamine receptor antagonist [(11)C]raclopride at varying specific activities to anesthetized nonhuman primates, we mapped associations between changes in receptor occupancy and hemodynamics [cerebral blood volume (CBV)] in the domains of space, time, and dose. Mass doses of raclopride above tracer levels caused increases in CBV and reductions in binding potential that were localized to the dopamine-rich striatum. Moreover, similar temporal profiles were observed for specific binding estimates and changes in CBV. Injection of graded raclopride mass doses revealed a monotonic coupling between neurovascular responses and receptor occupancies. The distinct CBV magnitudes between putamen and caudate at matched occupancies approximately matched literature differences in basal dopamine levels, suggesting that the relative fMRI measurements reflect basal D2/D3 dopamine receptor occupancy. These results can provide a basis for models that relate dopaminergic occupancies to hemodynamic changes in the basal ganglia. Overall, these data demonstrate the utility of simultaneous PET/fMRI for investigations of neurovascular coupling that correlate neurochemistry with hemodynamic changes in vivo for any receptor system with an available PET tracer.

Perspective: Identification of genetic variants associated with dopaminergic compensatory mechanisms in early Parkinson's disease

Parkinson's disease (PD) is slowly progressive, and heterogeneity of its severity among individuals may be due to endogenous mechanisms that counterbalance the striatal dopamine loss. In this perspective paper, we introduce a neuroimaging-genetic approach to identify genetic variants, which may contribute to this compensation. First, we briefly review current known potential compensatory mechanisms for premotor and early disease PD, located in the striatum and other brain regions. Then, we claim that a mismatch between mild symptomatic disease, manifested by low motor score on the Unified PD Rating Scale (UPDRS), and extensive Nigro-Striatal (NS) degeneration, manifested by reduced uptake of [¹²³I]FP-CIT, is indicative of compensatory processes. If genetic variants are associated with the severity of motor symptoms, while the level of striatal terminals degeneration measured by ligand uptake is taken into account and controlled in the analysis, then these variants may be involved in functional compensatory mechanisms for striatal dopamine deficit. To demonstrate feasibility of this approach, we performed a small “proof of concept” study (candidate gene design) in a sample of 28 Jewish PD patients, and preliminary results are presented.

Prediction error in reinforcement learning: a meta-analysis of neuroimaging studies

Activation likelihood estimation (ALE) meta-analyses were used to examine the neural correlates of prediction error in reinforcement learning. The findings are interpreted in the light of current computational models of learning and action selection. In this context, particular consideration is given to the comparison of activation patterns from studies using instrumental and Pavlovian conditioning, and where reinforcement involved rewarding or punishing feedback. The striatum was the key brain area encoding for prediction error, with activity encompassing dorsal and ventral regions for instrumental and Pavlovian reinforcement alike, a finding which challenges the functional separation of the striatum into a dorsal 'actor' and a ventral 'critic'. Prediction error activity was further observed in diverse areas of predominantly anterior cerebral cortex including medial prefrontal cortex and anterior cingulate cortex. Distinct patterns of prediction error activity were found for studies using rewarding and aversive reinforcers; reward prediction errors were observed primarily in the striatum while aversive prediction errors were found more widely including insula and habenula.

Synchronized electrical stimulation of the rat medial forebrain bundle and perforant pathway generates an additive BOLD response in the nucleus accumbens and prefrontal cortex

To study how a synchronized activation of two independent pathways affects the fMRI response in a common targeted brain region, blood oxygen dependent (BOLD) signals were measured during electrical stimulation of the right medial forebrain bundle (MFB), the right perforant pathway (PP) and concurrent stimulation of the two fiber systems. Repetitive electrical stimulations of the MFB triggered significant positive BOLD responses in the nucleus accumbens (NAcc), septum, anterior cingulate cortex/medial prefrontal cortex (ACC/mPFC), ventral tegmental area/substantia nigra (VTA/SN), right entorhinal cortex (EC) and colliculus superior, which, in general, declined during later stimulation trains. At the same time, negative BOLD responses were observed in the striatum. Thus, the same stimulus caused region-specific hemodynamic responses. An identical electrical stimulation of the PP generated positive BOLD responses in the right dentate gyrus/hippocampus proper/subiculum (DG/HC), the right entorhinal cortex and the left entorhinal cortex, which remained almost stable during consecutive stimulation trains. Co-stimulation of the two fiber systems resulted in an additive activation pattern, i.e., the BOLD responses were stronger during the stimulation of the two pathways than during the stimulation of only one pathway. However, during the simultaneous stimulation of the two pathways, the development of the BOLD responses to consecutive trains changed. The BOLD responses in regions that were predominantly activated by MFB stimulation (i.e., NAcc, septum and ACC/mPFC) did not decline as fast as during pure MFB stimulation, thus an additive BOLD response was only observed during later trains. In contrast, in the brain regions that were predominantly activated by PP stimulation (i.e., right EC, DG/HC), co-stimulation of the MFB only resulted in an additive effect during early trains but not later trains. Consequently, the development of the BOLD responses during consecutive stimulations indicates the presence of an interaction between the two pathways in a target region, whereas the observed averaged BOLD responses do not.

Dorsolateral caudate nucleus differentiates cocaine from natural reward-associated contextual cues

Chronic drug administration induces neuroplastic changes within brain circuits regulating cognitive control and/or emotions. Following repeated pairings between drug intake and environmental cues, increased sensitivity to or salience of these contextual cues provoke conscious or unconscious craving and enhance susceptibility to relapse. To explore brain circuits participating in such experience-induced plasticity, we combined functional MRI with a preclinical drug vs. food self-administration (SA) withdrawal model. Specifically, two groups of rats were trained to associate odor cues with the availability of i.v. cocaine or oral sucrose, respectively. After 20 d of cocaine or sucrose SA followed by prolonged (30 d) forced abstinence, animals were presented with odor cues previously associated with or without (S+/S-) reinforcer (cocaine/sucrose) availability while undergoing functional MRI scans. ANOVA results demonstrate that a learning effect distinguishing S+ from S- was seen in the insula and nucleus accumbens, with the insula response reflecting the individual history of cocaine SA intake. A main effect of group, distinguishing cocaine from sucrose, was seen in the medial prefrontal cortex (infralimbic, prelimbic, and cingulate cortex) and dorsolateral striatum. Critically, only the dorsomedial striatum demonstrated a double dissociation between the two SA groups and learning (S+ vs. S-). These findings demonstrate altered cortico-limbic-striatal reward-related processing to learned, environment reward-associated contextual odor cues, which may serve as potential biomarkers for therapeutic interventions.

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.

Dopamine and training-related working-memory improvement

Converging evidence indicates that the neurotransmitter dopamine (DA) is implicated in working-memory (WM) functioning and that WM is trainable. We review recent work suggesting that DA is critically involved in the ability to benefit from WM interventions. Functional MRI studies reveal increased striatal BOLD activity following certain forms of WM interventions, such as updating training. Increased striatal BOLD activity has also been linked to transfer of learning to non-trained WM tasks, suggesting a neural signature of transfer. The striatal BOLD signal is partly determined by DA activity. Consistent with this assertion, PET research demonstrates increased striatal DA release during updating of information in WM after training. Genetic studies indicate larger increases in WM performance post training for those who carry advantageous alleles of DA-relevant genes. These patterns of results corroborate the role of DA in WM improvement. Future research avenues include: (a) neuromodulatory correlates of transfer; (b) the potential of WM training to enhance DA release in older adults; (c) comparisons among different WM processes (i.e., updating, switching, inhibition) regarding regional patterns of training-related DA release; and (d) gene-gene interactions in relation to training-related WM gains.