Striatal and extrastriatal dopamine release measured with PET and [(18)F] fallypride

Correlation of striatal dopamine D2/3 receptor availability with GABA level in the anterior cingulate cortex in healthy controls but not in alcohol-dependent subjects and individuals at high risk: A multimodal magnetic resonance spectroscopy and positron emission tomography study

BACKGROUND

The association of impaired dopaminergic neurotransmission with the development and maintenance of alcohol use disorder is well known. More specifically, reduced dopamine D2/3 receptors in the striatum of subjects with alcohol dependence (AD) compared to healthy controls have been found in previous studies. Furthermore, alterations of gamma-aminobutyric acid (GABA) and glutamate (Glu) levels in the anterior cingulate cortex (ACC) of AD subjects have been documented in several studies. However, the interaction between cortical Glu levels and striatal dopamine D2/3 receptors has not been investigated in AD thus far.

METHODS

This study investigated dopamine D2/3 receptor availability via 18F-fallypride positron emission tomography (PET) and GABA as well as Glu levels via magnetic resonance spectroscopy (MRS) in 19 detoxified AD subjects, 18 healthy controls (low risk, LR) controls and 19 individuals at high risk (HR) for developing AD, carefully matched for sex, age and smoking status.

RESULTS

We found a significant negative correlation between GABA levels in the ACC and dopamine D2/3 receptor availability in the associative striatum of LR but not in AD or HR individuals. Contrary to our expectations, we did not observe a correlation between Glu concentrations in the ACC and striatal D2/3 receptor availability.

CONCLUSIONS

The results may reflect potential regulatory cortical mechanisms on mesolimbic dopamine receptors and their disruption in AD and individuals at high risk, mirroring complex neurotransmitter interactions associated with the pathogenesis of addiction. This is the first study combining 18F-fallypride PET and MRS in AD subjects and individuals at high risk.

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Imaging the Vesicular Acetylcholine Transporter in Schizophrenia: A Positron Emission Tomography Study Using [18F]-VAT

BACKGROUND

Despite longstanding interest in the central cholinergic system in schizophrenia (SCZ), cholinergic imaging studies with patients have been limited to receptors. Here, we conducted a proof-of-concept positron emission tomography study using [18F]-VAT, a new radiotracer that targets the vesicular acetylcholine transporter as a proxy measure of acetylcholine transmission capacity, in patients with SCZ and explored relationships of vesicular acetylcholine transporter with clinical symptoms and cognition.

METHODS

A total of 18 adult patients with SCZ or schizoaffective disorder (the SCZ group) and 14 healthy control participants underwent a positron emission tomography scan with [18F]-VAT. Distribution volume (VT) for [18F]-VAT was derived for each region of interest, and group differences in VT were assessed with 2-sample t tests. Functional significance was explored through correlations between VT and scores on the Positive and Negative Syndrome Scale and a computerized neurocognitive battery (PennCNB).

RESULTS

No group differences in [18F]-VAT VT were observed. However, within the SCZ group, psychosis symptom severity was positively associated with VT in multiple regions of interest, with the strongest effects in the hippocampus, thalamus, midbrain, cerebellum, and cortex. In addition, in the SCZ group, working memory performance was negatively associated with VT in the substantia innominata and several cortical regions of interest including the dorsolateral prefrontal cortex.

CONCLUSIONS

In this initial study, the severity of 2 important features of SCZ-psychosis and working memory deficit-was strongly associated with [18F]-VAT VT in several cortical and subcortical regions. These correlations provide preliminary evidence of cholinergic activity involvement in SCZ and, if replicated in larger samples, could lead to a more complete mechanistic understanding of psychosis and cognitive deficits in SCZ and the development of therapeutic targets.

Insulin effects on core neurotransmitter pathways involved in schizophrenia neurobiology: a meta-analysis of preclinical studies. Implications for the treatment

Impairment of insulin action and metabolic dysregulation have traditionally been associated with schizophrenia, although the molecular basis of such association remains still elusive. The present meta-analysis aims to assess the impact of insulin action manipulations (i.e., hyperinsulinemia, hypoinsulinemia, systemic or brain insulin resistance) on glutamatergic, dopaminergic, γ-aminobutyric acid (GABA)ergic, and serotonergic pathways in the central nervous system. More than one hundred outcomes, including transcript or protein levels, kinetic parameters, and other components of the neurotransmitter pathways, were collected from cultured cells, animals, or humans, and meta-analyzed by applying a random-effects model and adopting Hedges'g to compare means. Two hundred fifteen studies met the inclusion criteria, of which 180 entered the quantitative synthesis. Significant impairments in key regulators of synaptic plasticity processes were detected as the result of insulin handlings. Specifically, protein levels of N-methyl-D-aspartate receptor (NMDAR) subunits including type 2A (NR2A) (Hedges' g = -0.95, 95%C.I. = -1.50, -0.39; p = 0.001; I2 = 47.46%) and 2B (NR2B) (Hedges'g = -0.69, 95%C.I. = -1.35, -0.02; p = 0.043; I2 = 62.09%), and Postsynaptic density protein 95 (PSD-95) (Hedges'g = -0.91, 95%C.I. = -1.51, -0.32; p = 0.003; I2 = 77.81%) were found reduced in insulin-resistant animal models. Moreover, insulin-resistant animals showed significantly impaired dopamine transporter activity, whereas the dopamine D2 receptor mRNA expression (Hedges'g = 3.259; 95%C.I. = 0.497, 6.020; p = 0.021; I2 = 90.61%) increased under insulin deficiency conditions. Insulin action modulated glutamate and GABA release, as well as several enzymes involved in GABA and serotonin synthesis. These results suggest that brain neurotransmitter systems are susceptible to insulin signaling abnormalities, resembling the discrete psychotic disorders' neurobiology and possibly contributing to the development of neurobiological hallmarks of treatment-resistant schizophrenia.

Antipsychotics-Induced Changes in Synaptic Architecture and Functional Connectivity: Translational Implications for Treatment Response and Resistance

Schizophrenia is a severe mental illness characterized by alterations in processes that regulate both synaptic plasticity and functional connectivity between brain regions. Antipsychotics are the cornerstone of schizophrenia pharmacological treatment and, beyond occupying dopamine D2 receptors, can affect multiple molecular targets, pre- and postsynaptic sites, as well as intracellular effectors. Multiple lines of evidence point to the involvement of antipsychotics in sculpting synaptic architecture and remodeling the neuronal functional unit. Furthermore, there is an increasing awareness that antipsychotics with different receptor profiles could yield different interregional patterns of co-activation. In the present systematic review, we explored the fundamental changes that occur under antipsychotics' administration, the molecular underpinning, and the consequences in both acute and chronic paradigms. In addition, we investigated the relationship between synaptic plasticity and functional connectivity and systematized evidence on different topographical patterns of activation induced by typical and atypical antipsychotics.

Amphetamine-induced dopamine release and impulsivity in Parkinson's disease

Impulsive-compulsive behaviours manifest in a substantial proportion of subjects with Parkinson's disease. Reduced ventral striatum dopamine receptor availability, and increased dopamine release is noted in patients with these symptoms. Prior studies of impulsivity suggest that midbrain D2 autoreceptors regulate striatal dopamine release in a feedback inhibitory manner, and in healthy populations, greater impulsivity is linked to poor proficiency of this inhibition. This has not been assessed in a Parkinson's disease population. Here, we applied 18F-fallypride PET studies to assess striatal and extrastriatal D2-like receptor uptake in a placebo-controlled oral dextroamphetamine sequence. We hypothesized that Parkinson's disease patients with impulsive-compulsive behaviours would have greater ventral striatal dopaminergic response to dextroamphetamine, and that an inability to attenuate ventral striatal dopamine release via midbrain D2 autoreceptors would underlie this response. Twenty patients with Parkinson's disease (mean age = 64.1 ± 5.8 years) both with (n = 10) and without (n = 10) impulsive-compulsive behaviours, participated in a single-blind dextroamphetamine challenge (oral; 0.43 mg/kg) in an OFF dopamine state. All completed PET imaging with 18F-fallypride, a high-affinity D2-like receptor ligand, in the placebo and dextroamphetamine state. Both voxelwise and region of interest analyses revealed dextroamphetamine-induced endogenous dopamine release localized to the ventral striatum, and the caudal-medial orbitofrontal cortex. The endogenous dopamine release observed in the ventral striatum correlated positively with patient-reported participation in reward-based behaviours, as quantified by the self-reported Questionnaire for Impulsivity in Parkinson's disease Rating Scale. In participants without impulsive-compulsive behaviours, baseline midbrain D2 receptor availability negatively correlated with ventral striatal dopamine release; however, this relationship was absent in those with impulsive-compulsive behaviours. These findings emphasize that reward-based behaviours in Parkinson's disease are regulated by ventral striatal dopamine release, and suggest that loss of inhibitory feedback from midbrain autoreceptors may underlie the manifestation of impulsive-compulsive behaviours.

Rational and Translational Implications of D-Amino Acids for Treatment-Resistant Schizophrenia: From Neurobiology to the Clinics

Schizophrenia has been conceptualized as a neurodevelopmental disorder with synaptic alterations and aberrant cortical–subcortical connections. Antipsychotics are the mainstay of schizophrenia treatment and nearly all share the common feature of dopamine D2 receptor occupancy, whereas glutamatergic abnormalities are not targeted by the presently available therapies. D-amino acids, acting as N-methyl-D-aspartate receptor (NMDAR) modulators, have emerged in the last few years as a potential augmentation strategy in those cases of schizophrenia that do not respond well to antipsychotics, a condition defined as treatment-resistant schizophrenia (TRS), affecting almost 30–40% of patients, and characterized by serious cognitive deficits and functional impairment. In the present systematic review, we address with a direct and reverse translational perspective the efficacy of D-amino acids, including D-serine, D-aspartate, and D-alanine, in poor responders. The impact of these molecules on the synaptic architecture is also considered in the light of dendritic spine changes reported in schizophrenia and antipsychotics’ effect on postsynaptic density proteins. Moreover, we describe compounds targeting D-amino acid oxidase and D-aspartate oxidase enzymes. Finally, other drugs acting at NMDAR and proxy of D-amino acids function, such as D-cycloserine, sarcosine, and glycine, are considered in the light of the clinical burden of TRS, together with other emerging molecules.

Cannabinoid receptor availability modulates the magnitude of dopamine release in vivo in the human reward system: A preliminary multitracer positron emission tomography study

The established role of dopamine (DA) in the mediation of reward and positive reinforcement, reward processing is strongly influenced by the type 1 cannabinoid receptors (CB₁ Rs). Although considerable preclinical evidence has demonstrated several functional CB₁ R-DA interactions, the relation between human CB₁ R availability, DA release capacity and drug-reinforcing effects has been never investigated so far. Here, we perform a multitracer [¹⁸ F]MK-9470 and [¹⁸ F]fallypride positron emission tomography (PET) study in 10 healthy male subjects using a placebo-controlled and single-blinded amphetamine (AMPH) (30 mg) administration paradigm to (1) investigate possible functional interactions between CB₁ R expression levels and DA release capacity in a normo-DAergic state, relating in vivo AMPH-induced DA release to CB₁ R availability, and (2) to test the hypothesis that the influence of striatal DAergic signalling on the positive reinforcing effects of AMPH may be regulated by prefrontal CB₁ R levels. Compared with placebo, AMPH significantly reduced [¹⁸ F]fallypride binding potential (hence increase DA release; ΔBP_ND ranging from -6.1% to -9.6%) in both striatal (p < 0.005, corrected for multiple comparisons) and limbic extrastriatal regions (p ≤ 0.04, uncorrected). Subjects who reported a greater dopaminergic response in the putamen also showed higher CB₁ R availability in the medial and dorsolateral prefrontal cortex (r = 0.72; p = 0.02), which are regions involved in salience attribution, motivation and decision making. On the other hand, the magnitude of DA release was greater in those subjects with lower CB₁ R availability in the anterior cingulate cortex (ACC) (r = -0.66; p = 0.03). Also, the correlation between the DA release in the nucleus accumbens with the subjective AMPH effect liking was mediated through the CB₁ R availability in the ACC (c' = -0.76; p = 0.01). Our small preliminary study reports for the first time that the human prefrontal CB₁ R availability is a determinant of DA release within both the ventral and dorsal reward corticostriatal circuit, contributing to a number of studies supporting the existence of an interaction between CB₁ R and DA receptors at the molecular and behavioural level. These preliminary findings warrant further investigation in pathological conditions characterized by hypo/hyper excitability to DA release such as addiction and schizophrenia.

Pharmacological and behavioral divergence of ketamine enantiomers: implications for abuse liability

Ketamine, a racemic mixture of (S)-ketamine and (R)-ketamine enantiomers, has been used as an anesthetic, analgesic and more recently, as an antidepressant. However, ketamine has known abuse liability (the tendency of a drug to be used in non-medical situations due to its psychoactive effects), which raises concerns for its therapeutic use. (S)-ketamine was recently approved by the United States' FDA for treatment-resistant depression. Recent studies showed that (R)-ketamine has greater efficacy than (S)-ketamine in preclinical models of depression, but its clinical antidepressant efficacy has not been established. The behavioral effects of racemic ketamine have been studied extensively in preclinical models predictive of abuse liability in humans (self-administration and conditioned place preference [CPP]). In contrast, the behavioral effects of each enantiomer in these models are unknown. We show here that in the intravenous drug self-administration model, the gold standard procedure to assess potential abuse liability of drugs in humans, rats self-administered (S)-ketamine but not (R)-ketamine. Subanesthetic, antidepressant-like doses of (S)-ketamine, but not of (R)-ketamine, induced locomotor activity (in an opioid receptor-dependent manner), induced psychomotor sensitization, induced CPP in mice, and selectively increased metabolic activity and dopamine tone in medial prefrontal cortex (mPFC) of rats. Pharmacological screening across thousands of human proteins and at biological targets known to interact with ketamine yielded divergent binding and functional enantiomer profiles, including selective mu and kappa opioid receptor activation by (S)-ketamine in mPFC. Our results demonstrate divergence in the pharmacological, functional, and behavioral effects of ketamine enantiomers, and suggest that racemic ketamine's abuse liability in humans is primarily due to the pharmacological effects of its (S)-enantiomer.

18F-fallypride and 11C-methionine positron emission tomography/computed tomography for the study of prolactinomas and nonfunctioning pituitary adenomas: A case series

Some studies have assessed the expression of dopaminergic dopamine 2 (D2)/3 receptors in prolactinomas and nonfunctioning pituitary adenomas (NFPA) by positron emission tomography/computed tomography (PET/CT) with ¹¹C-raclopride, proving that this modality can be useful to predict the response to treatment with dopamine agonists. However, the use of ¹¹C-labeled radiotracers is limited, as it requires a cyclotron in the PET center. ¹⁸F-fallypride is a radiotracer that has proven useful in assessing the expression of D2/3 receptors. As it is labeled with ¹⁸F, it can be produced and transported to distant PET centers. There are no studies on the usefulness of ¹⁸F-fallypride for the evaluation of patients with prolactinomas and NFPA. The aim of this study was to describe the first case series of patients with prolactinomas and NFPA studied with ¹⁸F-fallypride and ¹¹C-methionine PET/CT to reveal D2/3 expression and amino acid (AA) metabolism. ¹⁸F-fallypride and ¹¹C-methionine uptake were assessed in a case series of six patients, five with prolactinomas and one with a NFPA, and compared with clinical presentation and follow-up at 6–18 months. All patients presented with macroadenomas, with a wide range of AA metabolism, as revealed by ¹¹C-methionine PET/CT. ¹⁸F-fallypride PET/CT identified low to moderate/high D2/3 expression in the tumors. The patient that presented low expression of D2/3 in the tumor and high AA metabolism showed a poor response to DA therapy. ¹⁸F-fallypride was able to reveal D2/3 receptor expression in prolactinomas and NFPA, with the advantage of been a more accessible radiotracer in comparison with previous ¹¹C labeled analogs.

The Role of Positron Emission Tomography in Bariatric Surgery Research: a Review

Bariatric surgery, initially understood as restricting or bypassing the amount of food that reaches the stomach to reduce food intake and/or increase malabsorption of food to promote weight loss, is now recognized to also affect incretin signaling in the gut and promote improvements in system-wide metabolism. Positron emission tomography (PET) is an imaging technique whereby patients are injected with picomolar concentrations of radioactive molecules, below the threshold of having physiological effects, to measure spatial distributions of blood flow, metabolism, receptor, and enzyme pharmacology. Recent advances in both whole-body PET imaging and radioligand development will allow for novel research that may help clarify the roles of peripheral and central receptor/enzyme systems in treating obesity with bariatric surgery.

Dopamine D1 and D2 receptors are distinctly associated with rest-activity rhythms and drug reward

BACKGROUND

Certain components of rest-activity rhythms such as greater eveningness (delayed phase), physical inactivity (blunted amplitude) and shift work (irregularity) are associated with increased risk for drug use. Dopaminergic (DA) signaling has been hypothesized to mediate the associations, though clinical evidence is lacking.

METHODS

We examined associations between rhythm components and striatal D1 (D1R) and D2/3 receptor (D2/3R) availability in 32 healthy adults (12 female, age: 42.40±12.22) and its relationship to drug reward. Rest-activity rhythms were assessed by one-week actigraphy combined with self-reports. [11C]NNC112 and [11C]raclopride Positron Emission Tomography (PET) scans were conducted to measure D1R and D2/3R availability, respectively. Additionally, self-reported drug-rewarding effects of 60 mg oral methylphenidate were assessed.

RESULTS

We found that delayed rhythm was associated with higher D1R availability in caudate, which was not attributable to sleep loss or 'social jet lag', whereas physical inactivity was associated with higher D2/3R availability in nucleus accumbens (NAc). Delayed rest-activity rhythm, higher caudate D1R and NAc D2/3R availability were associated with greater sensitivity to the rewarding effects of methylphenidate.

CONCLUSION

These findings reveal specific components of rest-activity rhythms associated with striatal D1R, D2/3R availability and drug-rewarding effects. Personalized interventions that target rest-activity rhythms may help prevent and treat substance use disorders.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT03190954FUNDING. This work was accomplished with support from the National Institute on Alcohol Abuse and Alcoholism (ZIAAA000550).

Translational PET applications for brain circuit mapping with transgenic neuromodulation tools

Transgenic neuromodulation tools have transformed the field of neuroscience over the past two decades by enabling targeted manipulation of neuronal populations and circuits with unprecedented specificity. Chemogenetic and optogenetic neuromodulation systems are among the most widely used and allow targeted control of neuronal activity through the administration of a selective compound or light, respectively. Innovative genetic targeting strategies are utilized to transduce specific cells to express transgenic receptors and opsins capable of manipulating neuronal activity. These allow mapping of neuroanatomical projection sites and link cellular manipulations with brain circuit functions and behavior. As these tools continue to expand knowledge of the nervous system in preclinical models, developing translational applications for human therapies is becoming increasingly possible. However, new strategies for implementing and monitoring transgenic tools are needed for safe and effective use in translational research and potential clinical applications. A major challenge for such applications is the need to track the location and function of chemogenetic receptors and opsins in vivo, and new developments in positron emission tomography (PET) imaging techniques offer promising solutions. The goal of this review is to summarize current research combining transgenic tools with PET for in vivo mapping and manipulation of brain circuits and to propose future directions for translational applications.

Dopamine receptor density and white mater integrity: 18F-fallypride positron emission tomography and diffusion tensor imaging study in healthy and schizophrenia subjects

Dopaminergic dysfunction and changes in white matter integrity are among the most replicated findings in schizophrenia. A modulating role of dopamine in myelin formation has been proposed in animal models and healthy human brain, but has not yet been systematically explored in schizophrenia. We used diffusion tensor imaging and ¹⁸F-fallypride positron emission tomography in 19 healthy and 25 schizophrenia subjects to assess the relationship between gray matter dopamine D₂/D₃ receptor density and white matter fractional anisotropy in each diagnostic group. AFNI regions of interest were acquired for 42 cortical Brodmann areas and subcortical gray matter structures as well as stereotaxically placed in representative white matter areas implicated in schizophrenia neuroimaging literature. Welch's t-test with permutation-based p value adjustment was used to compare means of z-transformed correlations between fractional anisotropy and ¹⁸F-fallypride binding potentials in hypothesis-driven regions of interest in the diagnostic groups. Healthy subjects displayed an extensive pattern of predominantly negative correlations between ¹⁸F-fallypride binding across a range of cortical and subcortical gray matter regions and fractional anisotropy in rostral white matter regions (internal capsule, frontal lobe, anterior corpus callosum). These patterns were disrupted in subjects with schizophrenia, who displayed significantly weaker overall correlations as well as comparatively scant numbers of significant correlations with the internal capsule and frontal (but not temporal) white matter, especially for dopamine receptor density in thalamic nuclei. Dopamine D₂/D₃ receptor density and white matter integrity appear to be interrelated, and their decreases in schizophrenia may stem from hyperdopaminergia with dysregulation of dopaminergic impact on axonal myelination.

Dopamine D2/3 receptor availability in alcohol use disorder and individuals at high risk: Towards a dimensional approach

Alcohol use disorder (AUD) is the most common substance use disorder worldwide. Although dopamine-related findings were often observed in AUD, associated neurobiological mechanisms are still poorly understood. Therefore, in the present study, we investigate D2/3 receptor availability in healthy participants, participants at high risk (HR) to develop addiction (not diagnosed with AUD), and AUD patients in a detoxified stage, applying ¹⁸ F-fallypride positron emission tomography (¹⁸ F-PET). Specifically, D2/3 receptor availability was investigated in (1) 19 low-risk (LR) controls, (2) 19 HR participants, and (3) 20 AUD patients after alcohol detoxification. Quality and severity of addiction were assessed with clinical questionnaires and (neuro)psychological tests. PET data were corrected for age of participants and smoking status. In the dorsal striatum, we observed significant reductions of D2/3 receptor availability in AUD patients compared with LR participants. Further, receptor availability in HR participants was observed to be intermediate between LR and AUD groups (linearly decreasing). Still, in direct comparison, no group difference was observed between LR and HR groups or between HR and AUD groups. Further, the score of the Alcohol Dependence Scale (ADS) was inversely correlated with D2/3 receptor availability in the combined sample. Thus, in line with a dimensional approach, striatal D2/3 receptor availability showed a linear decrease from LR participants to HR participants to AUD patients, which was paralleled by clinical measures. Our study shows that a core neurobiological feature in AUD seems to be detectable in an early, subclinical state, allowing more individualized alcohol prevention programs in the future.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

PET Imaging of Dopamine Neurotransmission During EEG Neurofeedback

Neurofeedback (NFB) is a brain-based training method that enables users to control their own cortical oscillations using real-time feedback from the electroencephalogram (EEG). Importantly, no investigations to date have directly explored the potential impact of NFB on the brain's key neuromodulatory systems. Our study's objective was to assess the capacity of NFB to induce dopamine release as revealed by positron emission tomography (PET). Thirty-two healthy volunteers were randomized to either EEG-neurofeedback (NFB) or EEG-electromyography (EMG), and scanned while performing self-regulation during a single session of dynamic PET brain imaging using the high affinity D2/3 receptor radiotracer, [18F]Fallypride. NFB and EMG groups down-regulated cortical alpha power and facial muscle tone, respectively. Task-induced effects on endogenous dopamine release were estimated in the frontal cortex, anterior cingulate cortex, and thalamus, using the linearized simplified reference region model (LSRRM), which accounts for time-dependent changes in radiotracer binding following task initiation. Contrary to our hypothesis of a differential effect for NFB vs. EMG training, significant dopamine release was observed in both training groups in the frontal and anterior cingulate cortex, but not in thalamus. Interestingly, a significant negative correlation was observed between dopamine release in frontal cortex and pre-to-post NFB change in spontaneous alpha power, suggesting that intra-individual changes in brain state (i.e., alpha power) could partly result from changes in neuromodulatory tone. Overall, our findings constitute the first direct investigation of neurofeedback's effect on the endogenous release of a key neuromodulator, demonstrating its feasibility and paving the way for future studies using this methodology.

High long-term test-retest reliability for extrastriatal 11C-raclopride binding in healthy older adults

In vivo dopamine D2-receptor availability is frequently assessed with ¹¹C-raclopride and positron emission tomography. Due to low signal-to-noise ratios for ¹¹C-raclopride in areas with low D2 receptor densities, the ligand has been considered unreliable for measurements outside the dopamine-dense striatum. Intriguingly, recent studies show that extrastriatal ¹¹C-raclopride binding potential (BP_ND) values are (i) reliably higher than in the cerebellum (where D2-receptor levels are negligible), (ii) correlate with behavior in the expected direction, and (iii) showed good test-retest reliability in a sample of younger adults. The present work demonstrates high seven-month test-retest reliability of striatal and extrastriatal ¹¹C-raclopride BP_ND values in healthy, older adults (n = 27, age: 64-78 years). Mean ¹¹C-raclopride BP_ND values were stable between test sessions in subcortical nuclei, and in frontal and temporal cortices (p > 0.05). Across all structures analyzed, intraclass correlation coefficients were high (0.85-0.96), absolute variability was low (mean: 4-8%), and coefficients of variance ranged between 9 and 25%. Furthermore, regional ¹¹C-raclopride BP_ND values correlated with previously determined ¹⁸F-fallypride BP_ND values (ρ = 0.97 and 0.92 in correlations with and without striatal values, respectively, p < 0.01) and postmortem determined D2-receptor densities (including striatum: ρ = 0.92; p < 0.001; excluding striatum: ρ = 0.75; p = 0.067). These observations suggest that extrastriatal ¹¹C-raclopride measurements represent a true D2 signal.

Dopamine transporter neuroimaging accurately assesses the maturation of dopamine neurons in a preclinical model of Parkinson's disease

BACKGROUND

Significant developments in stem cell therapy for Parkinson's disease (PD) have already been achieved; however, methods for reliable assessment of dopamine neuron maturation in vivo are lacking. Establishing the efficacy of new cellular therapies using non-invasive methodologies will be critical for future regulatory approval and application. The current study examines the utility of neuroimaging to characterise the in vivo maturation, innervation and functional dopamine release of transplanted human embryonic stem cell-derived midbrain dopaminergic neurons (hESC-mDAs) in a preclinical model of PD.

METHODS

Female NIH RNu rats received a unilateral stereotaxic injection of 6-OHDA into the left medial forebrain bundle to create the PD lesion. hESC-mDA cell and sham transplantations were carried out 1 month post-lesion, with treated animals receiving approximately 4 × 10⁵ cells per transplantation. Behavioural analysis, [¹⁸F]FBCTT and [¹⁸F]fallypride microPET/CT, was conducted at 1, 3 and 6 months post-transplantation and compared with histological characterisation at 6 months.

RESULTS

PET imaging revealed transplant survival and maturation into functional dopaminergic neurons. [¹⁸F]FBCTT-PET/CT dopamine transporter (DAT) imaging demonstrated pre-synaptic restoration and [¹⁸F]fallypride-PET/CT indicated functional dopamine release, whilst amphetamine-induced rotation showed significant behavioural recovery. Moreover, histology revealed that the grafted cells matured differently in vivo producing high- and low-tyrosine hydroxylase (TH) expressing cohorts, and only [¹⁸F]FBCTT uptake was well correlated with differentiation.

CONCLUSIONS

This study provides further evidence for the value of in vivo functional imaging for the assessment of cell therapies and highlights the utility of DAT imaging for the determination of early post-transplant cell maturation and differentiation of hESC-mDAs.

Dopaminergic Plasticity in the Bilateral Hippocampus Following Threat Reversal in Humans

When a cue no longer predicts a threat, a diminished ability to extinguish or reverse this association is thought to increase risk for stress-related disorders. Despite the clear clinical relevance, the mediating neurochemical mechanisms of threat reversal have received relatively little study. One neurotransmitter implicated in rodent research of changing associations with threat is dopamine. To study whether dopamine is involved in threat reversal in humans, we used high-resolution positron emission tomography (PET) coupled with 18F-fallypride. Twelve healthy volunteers (6 F/6 M) underwent three PET scans: (i) at baseline, (ii) following threat conditioning (the response to a cue associated with electric wrist shock), and (iii) following threat reversal (the response to the same cue now associated with safety). We observed moderate evidence of reduced dopamine D2/3 receptor availability, consistent with greater dopamine release, in the bilateral anterior hippocampus following threat reversal, in response to a safety cue that was previously associated with threat, as compared to both baseline and during exposure to the same cue prior to threat reversal. These findings offer the first preliminary evidence that the response to a previously threatening cue that has since become associated with safety involves dopaminergic neurotransmission within the hippocampus in healthy humans.

A Comparative Study of in vitro Assays for Predicting the Nonspecific Binding of PET Imaging Agents in vivo

Nonspecific binding (NSB) is a key parameter in optimizing PET imaging tracers. We compared the ability to predict NSB of three available methods: LIMBA, rat f_u,brain , and CHI(IAM). Even though NSB is often associated with lipophilicity, we observed that logD does not correlate with any of these assays, clearly indicating that lipophilicity, while influencing NSB, is insufficient to predict it. A cross-comparison of the methods showed that all three correlate and are useful predictors of NSB. The three assays, however, rank the molecules slightly differently, illustrating the challenge of comparing molecules within a narrow chemical space. We also noted that CHI(IAM) values more effectively predict V_NS , a measure of in vivo NSB in the human brain. CHI(IAM) measurements might be a closer model of the actual physicochemical interaction between PET tracer candidates and cell membranes, and seems to be the method of choice for the optimization of in vivo NSB.

Higher striatal D2-receptor availability in aerobically fit older adults but non-selective intervention effects after aerobic versus resistance training

There is much evidence that dopamine is vital for cognitive functioning in aging. Here we tested the hypothesis that aerobic exercise and fitness influence dopaminergic neurotransmission in the striatum, and in turn performance on offline working-memory updating tasks. Dopaminergic neurotransmission was measured by positron emission tomography (PET) and the non-displacable binding potential (BP_ND) of [¹¹C]raclopride, i.e. dopamine (DA) D2-receptor (D2R) availability. Fifty-four sedentary older adults underwent a six-months exercise intervention, performing either aerobic exercise or stretching, toning, and resistance active control training. At baseline, higher aerobic fitness levels (VO_2peak) were associated with higher BP_ND in the striatum, providing evidence of a link between an objective measure of aerobic fitness and D2R in older adults. BP_ND decreased substantially over the intervention in both groups but the intervention effects were non-selective with respect to exercise group. The decrease was several times larger than any previously estimated annual decline in D2R, potentially due to increased endogenous DA. Working-memory was unrelated to D2R both at baseline and following the intervention. To conclude, we provide partial evidence for a link between physical exercise and DA. Utilizing a PET protocol able to disentangle both D2R and DA levels could shed further light on whether, and how, aerobic exercise impacts the dopaminergic system in older adults.

Investigating the vestibular system using modern imaging techniques-A review on the available stimulation and imaging methods

The vestibular organs, located in the inner ear, sense linear and rotational acceleration of the head and its position relative to the gravitational field of the earth. These signals are essential for many fundamental skills such as the coordination of eye and head movements in the three-dimensional space or the bipedal locomotion of humans. Furthermore, the vestibular signals have been shown to contribute to higher cognitive functions such as navigation. As the main aim of the vestibular system is the sensation of motion it is a challenging system to be studied in combination with modern imaging methods. Over the last years various different methods were used for stimulating the vestibular system. These methods range from artificial approaches like galvanic or caloric vestibular stimulation to passive full body accelerations using hexapod motion platforms, or rotatory chairs. In the first section of this review we provide an overview over all methods used in vestibular stimulation in combination with imaging methods (fMRI, PET, E/MEG, fNIRS). The advantages and disadvantages of every method are discussed, and we summarize typical settings and parameters used in previous studies. In the second section the role of the four imaging techniques are discussed in the context of vestibular research and their potential strengths and interactions with the presented stimulation methods are outlined.

Antipsychotics: Mechanisms underlying clinical response and side-effects and novel treatment approaches based on pathophysiology

Antipsychotic drugs are central to the treatment of schizophrenia and other psychotic disorders but are ineffective for some patients and associated with side-effects and nonadherence in others. We review the in vitro, pre-clinical, clinical and molecular imaging evidence on the mode of action of antipsychotics and their side-effects. This identifies the key role of striatal dopamine D2 receptor blockade for clinical response, but also for endocrine and motor side-effects, indicating a therapeutic window for D2 blockade. We consider how partial D2/3 receptor agonists fit within this framework, and the role of off-target effects of antipsychotics, particularly at serotonergic, histaminergic, cholinergic, and adrenergic receptors for efficacy and side-effects such as weight gain, sedation and dysphoria. We review the neurobiology of schizophrenia relevant to the mode of action of antipsychotics, and for the identification of new treatment targets. This shows elevated striatal dopamine synthesis and release capacity in dorsal regions of the striatum underlies the positive symptoms of psychosis and suggests reduced dopamine release in cortical regions contributes to cognitive and negative symptoms. Current drugs act downstream of the major dopamine abnormalities in schizophrenia, and potentially worsen cortical dopamine function. We consider new approaches including targeting dopamine synthesis and storage, autoreceptors, and trace amine receptors, and the cannabinoid, muscarinic, GABAergic and glutamatergic regulation of dopamine neurons, as well as post-synaptic modulation through phosphodiesterase inhibitors. Finally, we consider treatments for cognitive and negative symptoms such dopamine agonists, nicotinic agents and AMPA modulators before discussing immunological approaches which may be disease modifying. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

What proportion of striatal D2 receptors are occupied by endogenous dopamine at baseline? A meta-analysis with implications for understanding antipsychotic occupancy

Using molecular imaging techniques - positron emission tomography (PET) and single-photon emission computed tomography (SPECT) - in conjunction with an acute dopamine depletion challenge (alpha-methyl-para-tyrosine) it is possible to estimate endogenous dopamine levels occupying striatal dopamine D₂ receptors (D₂R) in humans in vivo. However, it is unclear what proportion of striatal D₂R are occupied by endogenous dopamine under normal conditions. This is important since it has been suggested that in schizophrenia there may be a substantial proportion of striatal D₂R which are occupied by endogenous dopamine and not accessible by therapeutic doses of antipsychotics. In order to clarify these issues, we conducted a meta-analysis of dopamine depletion studies using substituted benzamide radiotracers in healthy persons. This meta-analysis suggests that anywhere from 8 to 21% (weighted average 11%) of striatal D₂R may be occupied by endogenous dopamine at baseline. Using these estimates, we propose an updated occupancy model and tentatively suggest that antipsychotics inhibit a smaller proportion of the total pool of striatal D₂R in vivo than previously acknowledged. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

A role for foregut tyrosine metabolism in glucose tolerance

Objective

We hypothesized that DA and L-DOPA derived from nutritional tyrosine and the resultant observed postprandial plasma excursions of L-DOPA and DA might affect glucose tolerance via their ability to be taken-up by beta cells and inhibit glucose-stimulated β-cell insulin secretion.

Methods

To investigate a possible circuit between meal-stimulated 3,4-dihydroxy-L-phenylalanine (L-DOPA) and dopamine (DA) production in the GI tract and pancreatic β-cells, we: 1) mapped GI mucosal expression of tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase (AADC); 2) measured L-DOPA and DA content of GI mucosal tissues following meal challenges with different L-tyrosine (TYR) content, 3) determined whether meal TYR content impacts plasma insulin and glucose excursions; and 4) characterized postprandial plasma excursions of L-DOPA and DA in response to meal tyrosine content in rodents and a population of bariatric surgery patients. Next, we characterized: 1) the metabolic transformation of TYR and L-DOPA into DA in vitro using purified islet tissue; 2) the metabolic transformation of orally administrated stable isotope labeled TYR into pancreatic DA, and 3) using a nuclear medicine technique, we studied endocrine beta cells in situ release and binding of DA in response to a glucose challenge.

Results

We demonstrate in rodents that intestinal content and circulatory concentrations L-DOPA and DA, plasma glucose and insulin are responsive to the tyrosine (TYR) content of a test meal. Intestinal expression of two enzymes, Tyrosine hydroxylase (TH) and Aromatic Amino acid Decarboxylase (AADC), essential to the transformation of TYR to DA was mapped and the metabolism of metabolism of TYR to DA was traced in human islets and a rodent beta cell line in vitro and from gut to the pancreas in vivo. Lastly, we show that β cells secrete and bind DA in situ in response to glucose stimulation.

Conclusions

We provide proof-of-principle evidence for the existence of a novel postprandial circuit of glucose homeostasis dependent on nutritional tyrosine. DA and L-DOPA derived from nutritional tyrosine may serve to defend against hypoglycemia via inhibition of glucose-stimulated β-cell insulin secretion as proposed by the anti-incretin hypothesis.

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Nutritional tyrosine is metabolized to L DOPA and DA in the foregut.

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Postprandial L-DOPA and DA plasma concentrations rise in response to tyrosine.

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Oral stable isotope labeled tyrosine is found postprandially in the pancreas as DA.

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L-DOPA and DA are inhibitors of beta cell glucose-stimulated insulin secretion.

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Postprandial L-DOPA and DA excursions are muted in certain bariatric surgery patients.

Time-dependent assessment of stimulus-evoked regional dopamine release

To date, the spatiotemporal release of specific neurotransmitters at physiological levels in the human brain cannot be detected. Here, we present a method that relates minute-by-minute fluctuations of the positron emission tomography (PET) radioligand [11C]raclopride directly to subsecond dopamine release events. We show theoretically that synaptic dopamine release induces low frequency temporal variations of extrasynaptic extracellular dopamine levels, at time scales of one minute, that can evoke detectable temporal variations in the [11C]raclopride signal. Hence, dopaminergic activity can be monitored via temporal fluctuations in the [11C]raclopride PET signal. We validate this theory using fast-scan cyclic voltammetry and [11C]raclopride PET in mice during chemogenetic activation of dopaminergic neurons. We then apply the method to data from human subjects given a palatable milkshake and discover immediate and-for the first time-delayed food-induced dopamine release. This method enables time-dependent regional monitoring of stimulus-evoked dopamine release at physiological levels.

Effects of flow changes on radiotracer binding: Simultaneous measurement of neuroreceptor binding and cerebral blood flow modulation

The potential effects of changes in blood flow on the delivery and washout of radiotracers has been an ongoing question in PET bolus injection studies. This study provides practical insight into this topic by experimentally measuring cerebral blood flow (CBF) and neuroreceptor binding using simultaneous PET/MRI. Hypercapnic challenges (7% CO₂) were administered to non-human primates in order to induce controlled increases in CBF, measured with pseudo-continuous arterial spin labeling. Simultaneously, dopamine D₂/D₃ receptor binding of [¹¹C]raclopride or [¹⁸F]fallypride was monitored with dynamic PET. Experiments showed that neither time activity curves nor quantification of binding through binding potentials ( BP_ND) were measurably affected by CBF increases, which were larger than two-fold. Simulations of experimental procedures showed that even large changes in CBF should have little effect on the time activity curves of radiotracers, given a set of realistic assumptions. The proposed method can be applied to experimentally assess the flow sensitivity of other radiotracers. Results demonstrate that CBF changes, which often occur due to behavioral tasks or pharmacological challenges, do not affect PET [¹¹C]raclopride or [¹⁸F]fallypride binding studies and their quantification. The results from this study suggest flow effects may have limited impact on many PET neuroreceptor tracers with similar properties.

Application of molecular imaging technology in neurotoxicology research

Molecular imaging has been widely applied in preclinical research. Among these new molecular imaging modalities, microPET imaging can be utilized as a very powerful tool that can obtain the measurements of multiple biological processes in various organs repeatedly in a same subject. This review discusses how this new approach provides noninvasive biomarker for neurotoxicology research and summarizes microPET findings with multiple radiotracers on the variety of neurotoxicity induced by toxic agents in both the rodent and the nonhuman primate brain.

The relationship between excitement symptom severity and extrastriatal dopamine D2/3 receptor availability in patients with schizophrenia: a high-resolution PET study with [18F]fallypride

The purpose of this study was to investigate the relationship between specific symptom severity and D_2/3 receptor availability in extrastriatal regions in outpatients with schizophrenia to shed light on the role of extrastriatal dopaminergic neurotransmission in the pathophysiology of symptoms of schizophrenia. Sixteen schizophrenia patients receiving relatively low-dose maintenance atypical antipsychotics and seventeen healthy controls underwent 3-Tesla magnetic resonance imaging and high-resolution positron emission tomography with [¹⁸F]fallypride. For D_2/3 receptor availability, the binding potential with respect to non-displaceable compartment (BP_ND) was derived using the simplified reference tissue model. The BP_ND values were lower in patients on antipsychotic treatment than in controls across all regions with large effect sizes (1.03-1.42). The regions with the largest effect size were the substantia nigra, amygdala, and insula. Symptoms of schizophrenia were assessed using a five-factor model of the Positive and Negative Syndrome Scale (PANSS). The region of interest-based analysis showed that PANSS excitement factor score had a significant positive correlation with the [¹⁸F]fallypride BP_ND in the insula. The equivalent dose of antipsychotics was not significantly correlated with PANSS factor scores or regional BP_ND values. The voxel-based analysis also revealed a significant positive association between the PANSS excitement factor and the [¹⁸F]fallypride BP_ND in the insula. The present study revealed a significant association between excitement symptom severity and D_2/3 receptor availability in the insula in schizophrenia, suggesting a possible important role of D_2/3 receptor-mediated neurotransmission in the insula and related limbic system in the pathophysiology of this specific symptom cluster.

Striatal dopamine release and impaired reinforcement learning in adults with 22q11.2 deletion syndrome

22q11.2 deletion syndrome (22q11DS) is a genetic disorder caused by a microdeletion on chromosome 22q11.2 and associated with an increased risk for developing psychosis. The catechol-O-methyltransferase (COMT) gene is located in the deleted region and involved in dopamine (DA) breakdown. Impaired reinforcement learning (RL) is a recurrent feature in psychosis and thought to be related to abnormal striatal DA function. This study aims to examine RL and the potential association with striatal DA-ergic neuromodulation in 22q11DS. Twelve non-psychotic adults with 22q11DS and 16 healthy controls (HC) were included. A dopamine D_2/3 receptor [¹⁸F]fallypride positron emission tomography (PET) scan was acquired while participants performed a modified version of the probabilistic stimulus selection task. RL-task performance was significantly worse in 22q11DS compared to HC. There were no group difference in striatal nondisplaceable binding potential (BP_ND) and task-induced DA release. In HC, striatal task-induced DA release was positively associated with task performance, but no such relation was found in 22q11DS subjects. Moreover, higher caudate nucleus task-induced DA release was found in COMT Met hemizygotes relative to Val hemizygotes. This study is the first to show impairments in RL in 22q11DS. It suggests that potentially motivational impairments are not only present in psychosis, but also in this genetic high risk group. These deficits may be underlain by abnormal striatal task-induced DA release, perhaps as a consequence of COMT haplo-insufficiency.

PET imaging of dopamine-D2 receptor internalization in schizophrenia

Recent genetic, molecular and post-mortem studies suggest impaired dopamine (DA)-D2 receptor (D2R) trafficking in patients with schizophrenia (SZ). Imaging and preclinical studies have shown agonist-induced D2R internalization can be imaged with positron emission tomography (PET) using D2R radiotracers combined with psychostimulant challenge. This is feasible if radiotracer binding is measured when postchallenge DA levels have returned to baseline, following the initial competition phase between DA and radiotracer for binding to D2R. Here we used 'late-phase' imaging after challenge to test the hypothesis that impaired D2R internalization in SZ leads to blunted late-phase displacement, or a faster return to baseline, in patients compared with healthy controls (HCs). We imaged 10 patients with SZ and 9 HCs with PET and [¹¹C]raclopride at baseline and two times (3-5 and 6-10 h) following 0.5 mg kg^-1 dextroamphetamine. We measured binding potential relative to non-displaceable compartment (BP_ND) and derived percent reduction from baseline (ΔBP_ND) for each postamphetamine scan. To test the hypothesis that time course of return of striatal BP_ND to baseline differed between SZ and HCs, we implemented a linear model with ΔBP_ND as dependent variable, time after amphetamine as repeated measure and time after amphetamine and diagnostic group as fixed effects. Neither diagnostic group nor interaction of diagnostic group-by-time after amphetamine significantly affected striatal ΔBP_ND (F=1.38, P=0.26; F=0.51, P=0.61). These results show similar pattern of return of BP_ND to baseline as a function of time in patients with SZ and HC, suggesting that striatal D2R internalization as measured by our imaging paradigm is normal in patients with SZ.

Cerebral dopaminergic and glutamatergic transmission relate to different subjective responses of acute alcohol intake: an in vivo multimodal imaging study

Converging preclinical evidence links extrastriatal dopamine release and glutamatergic transmission via the metabotropic glutamate receptor 5 (mGluR5) to the rewarding properties of alcohol. To date, human evidence is lacking on how and where in the brain these processes occur. Mesocorticolimbic dopamine release upon intravenous alcohol administration and mGluR5 availability were measured in 11 moderate social drinkers by single-session [¹⁸ F]fallypride and [¹⁸ F]FPEB positron emission tomography, respectively. Additionally, baseline and postalcohol glutamate and glutamine levels in the anterior cingulate cortex (ACC) were measured by using proton-magnetic resonance spectroscopy. To investigate differences in reward domains linked to both neurotransmitters, regional imaging data were related to subjective alcohol responses. Alcohol induced significant [¹⁸ F]fallypride displacement in the prefrontal cortex (PFC), temporal and parietal cortices and thalamus (P < 0.05, corrected for multiple comparisons). Dopamine release in the ACC and orbitofrontal and ventromedial PFCs were correlated with subjective 'liking' and 'wanting' effects (P < 0.05). In contrast, baseline mGluR5 availability was positively correlated with the 'high' effect of alcohol in dorsolateral, ventrolateral and ventromedial PFCs and in the medial temporal lobe, thalamus and caudate nucleus (P < 0.05). Although neither proton-magnetic resonance spectroscopy glutamate nor glutamine levels were affected by alcohol, baseline ACC glutamate levels were negatively associated with the alcohol 'liking' effect (P < 0.003). These data reveal new mechanistic understanding and differential neurobiological underpinnings of the effects of acute alcohol consumption on human behavior. Specifically, prefrontal dopamine release may encode alcohol 'liking' and 'wanting' effects in specific areas underlying value processing and motivation, whereas mGluR5 availability in distinct prefrontal-temporal-subcortical regions is more related to the alcohol 'high' effect.

Neuroimaging correlates of narcolepsy with cataplexy: A systematic review

Recent developments in neuroimaging techniques have advanced our understanding of biological mechanisms underpinning narcolepsy. We used MEDLINE to retrieve neuroimaging studies to compare patients with narcolepsy and healthy controls. Thirty-seven studies were identified and demonstrated several replicated abnormalities: (1) gray matter reductions in superior frontal, superior and inferior temporal, and middle occipital gyri, hypothalamus, amygdala, insula, hippocampus, cingulate cortex, thalamus, and nucleus accumbens, (2) decreased fractional anisotropy in white matter of fronto-orbital and cingulate area, (3) reduced brain metabolism or cerebral blood flow in middle and superior frontal, and cingulate cortex (4) increased activity in inferior frontal gyri, insula, amygdala, and nucleus accumbens, and (5) N-acetylaspartate/creatine-phosphocreatine level reduction in hypothalamus. In conclusion, all the replicated findings are still controversial due to the limitations such as heterogeneity or size of the samples and lack of multimodal imaging or follow-up. Thus, future neuroimaging studies should employ multimodal imaging methods in a large sample size of patients with narcolepsy and consider age, duration of disease, age at onset, severity, human leukocyte antigen type, cerebrospinal fluid hypocretin levels, and medication intake in order to elucidate possible neuroimaging characteristic of narcolepsy and identify therapeutic targets.

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.

D2/D3 dopamine receptor binding with [F-18]fallypride correlates of executive function in medication-naïve patients with schizophrenia

Converging evidence indicates that the prefrontal cortex is critically involved in executive control and that executive dysfunction is implicated in schizophrenia. Reduced dopamine D2/D3 receptor binding potential has been reported in schizophrenia, and the correlations with neuropsychological test scores have been positive and negative for different tasks. The aim of this study was to examine the relation between dopamine D2/D3 receptor levels with frontal and temporal neurocognitive performance in schizophrenia. Resting-state ¹⁸F-fallypride positron emission tomography was performed on 20 medication-naïve and 5 previously medicated for brief earlier periods patients with schizophrenia and 19 age- and sex-matched healthy volunteers. Striatal and extra-striatal dopamine D2/D3 receptor levels were quantified as binding potential using fallypride imaging. Magnetic resonance images in standard Talairach position and segmented into gray and white matter were co-registered to the fallypride images, and the AFNI stereotaxic atlas was applied. Two neuropsychological tasks known to activate frontal and temporal lobe function were chosen, specifically the Wisconsin Card Sorting Test (WCST) and the California Verbal Learning Test (CVLT). Images of the correlation coefficient between fallypride binding and WCST and CVLT performance showed a negative correlation in contrast to positive correlations in healthy volunteers. The results of this study demonstrate that lower fallypride binding potential in patients with schizophrenia may be associated with better performance. Our findings are consistent with previous studies that failed to find cognitive improvements with typical dopamine-blocking medications.

Predicting the dopamine D2 receptor occupancy of ropinirole in rats using positron emission tomography and pharmacokinetic-pharmacodynamic modeling

1. The purpose of this study was to measure dopamine D_2/3 receptor occupancy (RO) as a marker of the clinical efficacy of ropinirole in rats via positron emission tomography (PET) using ¹⁸F-fallypride as the radiotracer and to explore the relationship between dopamine RO and the plasma concentration of ropinirole via pharmacokinetic-pharmacodynamic modeling. 2. Plasma was collected from 16 rats treated with one of four doses of ropinirole. For the time-dependent study, the data of 16 rats in the 15 mg/kg dose group at four time points were averaged, and another 24 rats were divided into three dose groups (5 mg/kg, 30 mg/kg and 60 mg/kg) for the dose-dependent study; the animals were assessed via ¹⁸F-fallypride PET scans. The correlation between dopamine RO and the ropinirole plasma concentration was investigated, and a pharmacokinetic-pharmacodynamic (PK-PD) model was established with WinNonlin 6.3 software. Both the plasma concentration and the binding potential changed in a time- and dose-dependent manner, and the plasma concentration that induces 50% RO (EC₅₀) as calculated by the PK-PD model was 1391 ng/mL. 3. ¹⁸F-fallypride appeared to be a suitable radiotracer for ropinirole imaging, and its binding to the dopamine D₂ receptor has time- and concentration-dependent characteristics. A theory-based PK-PD model was developed to describe the relationship between the plasma ropinirole concentration and RO, providing a methodological foundation for noninvasive and in vivo clinical evaluations of ropinirole treatment.

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.

Acute effect of intravenously applied alcohol in the human striatal and extrastriatal D2 /D3 dopamine system

Investigations on the acute effects of alcohol in the human mesolimbic dopamine D₂ /D₃ receptor system have yielded conflicting results. With respect to the effects of alcohol on extrastriatal D₂ /D₃ dopamine receptors no investigations have been reported yet. Therefore we applied PET imaging using the postsynaptic dopamine D₂ /D₃ receptor ligand [¹⁸ F]fallypride addressing the question, whether intravenously applied alcohol stimulates the extrastriatal and striatal dopamine system. We measured subjective effects of alcohol and made correlation analyses with the striatal and extrastriatal D₂ /D₃ binding potential. Twenty-four healthy male μ-opioid receptor (OPRM1)118G allele carriers underwent a standardized intravenous and placebo alcohol administration. The subjective effects of alcohol were measured with a visual analogue scale. For the evaluation of the dopamine response we calculated the binding potential (BP_ND ) by using the simplified reference tissue model (SRTM). In addition, we calculated distribution volumes (target and reference regions) in 10 subjects for which metabolite corrected arterial samples were available. In the alcohol condition no significant dopamine response in terms of a reduction of BP_ND was observed in striatal and extrastriatal brain regions. We found a positive correlation for 'liking' alcohol and the BP_ND in extrastriatal brain regions (Inferior frontal cortex (IFC) (r = 0.533, p = 0.007), orbitofrontal cortex (OFC) (r = 0.416, p = 0.043) and prefrontal cortex (PFC) (r = 0.625, p = 0.001)). The acute alcohol effects on the D₂ /D₃ dopamine receptor binding potential of the striatal and extrastriatal system in our experiment were insignificant. A positive correlation of the subjective effect of 'liking' alcohol with cortical D₂ /D₃ receptors may hint at an addiction relevant trait.

The impact of common dopamine D2 receptor gene polymorphisms on D2/3 receptor availability: C957T as a key determinant in putamen and ventral striatum

Dopamine function is broadly implicated in multiple neuropsychiatric conditions believed to have a genetic basis. Although a few positron emission tomography (PET) studies have investigated the impact of single-nucleotide polymorphisms (SNPs) in the dopamine D2 receptor gene (DRD2) on D2/3 receptor availability (binding potential, BP_ND), these studies have often been limited by small sample size. Furthermore, the most commonly studied SNP in D2/3 BP_ND (Taq1A) is not located in the DRD2 gene itself, suggesting that its linkage with other DRD2 SNPs may explain previous PET findings. Here, in the largest PET genetic study to date (n=84), we tested for effects of the C957T and -141C Ins/Del SNPs (located within DRD2) as well as Taq1A on BP_ND of the high-affinity D2 receptor tracer ¹⁸F-Fallypride. In a whole-brain voxelwise analysis, we found a positive linear effect of C957T T allele status on striatal BP_ND bilaterally. The multilocus genetic scores containing C957T and one or both of the other SNPs produced qualitatively similar striatal results to C957T alone. The number of C957T T alleles predicted BP_ND in anatomically defined putamen and ventral striatum (but not caudate) regions of interest, suggesting some regional specificity of effects in the striatum. By contrast, no significant effects arose in cortical regions. Taken together, our data support the critical role of C957T in striatal D2/3 receptor availability. This work has implications for a number of psychiatric conditions in which dopamine signaling and variation in C957T status have been implicated, including schizophrenia and substance use disorders.

Pathway-Specific Dopamine Abnormalities in Schizophrenia

In light of the clinical evidence implicating dopamine in schizophrenia and the prominent hypotheses put forth regarding alterations in dopaminergic transmission in this disease, molecular imaging has been used to examine multiple aspects of the dopaminergic system. We review the imaging methods used and compare the findings across the different molecular targets. Findings have converged to suggest early dysregulation in the striatum, especially in the rostral caudate, manifesting as excess synthesis and release. Recent data showed deficit extending to most cortical regions and even to other extrastriatal subcortical regions not previously considered to be "hypodopaminergic" in schizophrenia. These findings yield a new topography for the dopaminergic dysregulation in schizophrenia. We discuss the dopaminergic innervation within the individual projection fields to provide a topographical map of this dual dysregulation and explore potential cellular and circuit-based mechanisms for brain region-dependent alterations in dopaminergic parameters. This refined knowledge is essential to better guide translational studies and efforts in early drug development.

Recent Developments in Molecular Brain Imaging of Neuropsychiatric Disorders

Molecular imaging with PET or SPECT has been an important research tool in psychiatry for as long as these modalities have been available. Here, we discuss two areas of neuroimaging relevant to current psychiatry research. The first is the use of imaging to study neurotransmission. We discuss the use of pharmacologic probes to induce changes in levels of neurotransmitters that can be inferred through their effects on outcome measures of imaging experiments, from their historical origins focusing on dopamine transmission through recent developments involving serotonin, GABA, and glutamate. Next, we examine imaging of neuroinflammation in the context of psychiatry. Imaging markers of neuroinflammation have been studied extensively in other areas of brain research, but they have more recently attracted interest in psychiatry research, based on accumulating evidence that there may be an inflammatory component to some psychiatric conditions. Furthermore, new probes are under development that would allow unprecedented insights into cellular processes. In summary, molecular imaging would continue to offer great potential as a unique tool to further our understanding of brain function in health and disease.

Blunted Dopamine Transmission in Addiction: Potential Mechanisms and Implications for Behavior

Positron emission tomography (PET) imaging consistently shows blunted striatal dopamine release and decreased dopamine D2 receptor availability in addiction. Here, we review the preclinical and clinical studies indicating that this neurobiological phenotype is likely to be both a consequence of chronic drug consumption and a vulnerability factor in the development of addiction. We propose that, behaviorally, blunted striatal dopamine transmission could reflect the increased impulsivity and altered cost/benefit computations that are associated with addiction. The factors that influence blunted striatal dopamine transmission in addiction are unknown. Herein, we give an overview of various factors, genetic, environmental, and social, that are known to affect dopamine transmission and that have been associated with the vulnerability to develop addiction. Altogether, these data suggest that blunted dopamine transmission and decreased D2 receptor availability are biomarkers both for the development of addiction and resistance to treatment. These findings support the view that blunted dopamine reflects impulsive behavior and deficits in motivation, which lead to the escalation of drug use.

Automation of the Radiosynthesis of Six Different 18F-labeled radiotracers on the AllinOne

Background

Fast implementation of positron emission tomography (PET) into clinical and preclinical studies highly demands automated synthesis for the preparation of PET radiopharmaceuticals in a safe and reproducible manner. The aim of this study was to develop automated synthesis methods for these six ¹⁸F-labeled radiopharmaceuticals produced on a routine basis at the University of Pennsylvania using the AllinOne synthesis module.

Results

The development of automated syntheses with varying complexity was accomplished including HPLC purification, SPE procedures and final formulation with sterile filtration. The six radiopharmaceuticals were obtained in high yield and high specific activity with full automation on the AllinOne synthesis module under current good manufacturing practice (cGMP) guidelines.

Conclusion

The study demonstrates the versatility of this synthesis module for the preparation of a wide variety of ¹⁸F-labeled radiopharmaceuticals for PET imaging studies.

Cocaine cue-induced dopamine release in the human prefrontal cortex

BACKGROUND

Accumulating evidence indicates that drug-related cues can induce dopamine (DA) release in the striatum of substance abusers. Whether these same cues provoke DA release in the human prefrontal cortex remains unknown.

METHODS

We used high-resolution positron emission tomography with [18F]fallypride to measure cortical and striatal DA D2/3 receptor availability in the presence versus absence of drug-related cues in volunteers with current cocaine dependence.

RESULTS

Twelve individuals participated in our study. Among participants reporting a craving response (9 of 12), exposure to the cocaine cues significantly decreased [18F]fallypride binding potential (BPND) values in the medial orbitofrontal cortex and striatum. In all 12 participants, individual differences in the magnitude of craving correlated with BPND changes in the medial orbitofrontal cortex, dorsolateral prefrontal cortex, anterior cingulate, and striatum. Consistent with the presence of autoreceptors on mesostriatal but not mesocortical DA cell bodies, midbrain BPND values were significantly correlated with changes in BPND within the striatum but not the cortex. The lower the midbrain D2 receptor levels, the greater the striatal change in BPND and self-reported craving.

LIMITATIONS

Limitations of this study include its modest sample size, with only 2 female participants. Newer tracers might have greater sensitivity to cortical DA release.

CONCLUSION

In people with cocaine use disorders, the presentation of drug-related cues induces DA release within cortical and striatal regions. Both effects are associated with craving, but only the latter is regulated by midbrain autoreceptors. Together, the results suggest that cortical and subcortical DA responses might both influence drug-focused incentive motivational states, but with separate regulatory mechanisms.

Occupancy of pramipexole (Sifrol) at cerebral dopamine D2/3 receptors in Parkinson's disease patients

Whereas positron emission tomography (PET) with the antagonist ligand [¹⁸F]fallypride reveals the composite of dopamine D2 and D3 receptors in brain, treatment of Parkinson's disease (PD) patients with the D3-prefering agonist pramipexole should result in preferential occupancy in the nucleus accumbens, where the D3-subtype is most abundant. To test this prediction we obtained pairs of [¹⁸F]fallypride PET recordings in a group of nine PD patients, first in a condition of treatment as usual with pramipexole (ON-Sifrol; 3 × 0.7 mg p.d.), and again at a later date, after withholding pramipexole 48–72 h (OFF-Sifrol); in that condition the serum pramipexole concentration had declined by 90% and prolactin levels had increased four-fold, in conjunction with a small but significant worsening of PD motor symptoms. Exploratory comparison with historical control material showed 14% higher dopamine D2/3 availability in the more-affected putamen of patients OFF medication. On-Sifrol there was significant (p ˂ 0.01) occupancy at [¹⁸F]fallypride binding sites in globus pallidus (8%) thalamus (9%) and substantia nigra (19%), as well as marginally significant occupancy in frontal and temporal cortex of patients. Contrary to expectation, comparison of ON- and OFF-Sifrol results did not reveal any discernible occupancy in nucleus accumbens, or elsewhere in the extended striatum; present methods should be sensitive to a 10% change in dopamine D2/3 receptor availability in striatum; the significant findings elsewhere in the basal ganglia and in cerebral cortex are consistent with a predominance of D3 receptors in those structures, especially in substantia nigra, and imply that therapeutic effects of pramipexole may be obtained at sites outside the extended striatum.

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Fallypride PET recordings in nine PD patients, scanned on- and off medication with pramipexole

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No occupancy in the striatum, despite improved motor symptoms

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Substantial occupancy in substantia nigra, thalamus and globus pallidus

Associations between dopamine D2 receptor availability and BMI depend on age

OBJECTIVE

The dopamine D2/3 receptor subtypes (DRD2/3) are the most widely studied neurotransmitter biomarker in research on obesity, but results to date have been inconsistent, have typically involved small samples, and have rarely accounted for subjects' ages despite the large impact of age on DRD2/3 levels. We aimed to clarify the relation between DRD2/3 availability and BMI by examining this association in a large sample of subjects with BMI spanning the continuum from underweight to extremely obese.

SUBJECTS

130 healthy subjects between 18 and 81years old underwent PET with [18F]fallypride, a high affinity DRD2/3 ligand.

RESULTS

As expected, DRD2/3 availability declined with age. Critically, age significantly interacted with DRD2/3 availability in predicting BMI in the midbrain and striatal regions (caudate, putamen, and ventral striatum). Among subjects under 30years old, BMI was not associated with DRD2/3 availability. By contrast, among subjects over 30years old, BMI was positively associated with DRD2/3 availability in the midbrain, putamen, and ventral striatum.

CONCLUSION

The present results are incompatible with the prominent dopaminergic hypofunction hypothesis that proposes that a reduction in DRD2/3 availability is associated with increased BMI, and highlights the importance of age in assessing correlates of DRD2/3 function.