Amphetamine-induced dopamine release in rat: Whole-brain spatiotemporal analysis with [11C]raclopride and positron emission tomography

Whole-brain mapping of drug effects are needed to understand the neural underpinnings of drug-related behaviors. Amphetamine administration is associated with robust increases in striatal dopamine (DA) release. Dopaminergic terminals are, however, present across several associative brain regions, which may contribute to behavioral effects of amphetamine. Yet the assessment of DA release has been restricted to a few brain regions of interest. The present work employed positron emission tomography (PET) with [11C]raclopride to investigate regional and temporal characteristics of amphetamine-induced DA release across twenty sessions in adult female Sprague Dawley rats. Amphetamine was injected intravenously (2 mg/kg) to cause displacement of [11C]raclopride binding from DA D2-like receptors, assessed using temporally sensitive pharmacokinetic PET model (lp-ntPET). We show amphetamine-induced [11C]raclopride displacement in the basal ganglia, and no changes following saline injections. Peak occupancy was highest in nucleus accumbens, followed by caudate-putamen and globus pallidus. Importantly, significant amphetamine-induced displacement was also observed in several extrastriatal regions, and specifically in thalamus, insula, orbitofrontal cortex, and secondary somatosensory area. For these, peak occupancy occurred later and was lower as compared to the striatum. Collectively, these findings demonstrate distinct amphetamine-induced DA responses across the brain, and that [11C]raclopride-PET can be employed to detect such spatiotemporal differences.

Effects of bariatric surgery and dietary interventions for obesity on brain neurotransmitter systems and metabolism: A systematic review of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) studies

This systematic review collates studies of dietary or bariatric surgery interventions for obesity using positron emission tomography and single-photon emission computed tomography. Of 604 publications identified, 22 met inclusion criteria. Twelve studies assessed bariatric surgery (seven gastric bypass, five gastric bypass/sleeve gastrectomy), and ten dietary interventions (six low-calorie diet, three very low-calorie diet, one prolonged fasting). Thirteen studies examined neurotransmitter systems (six used tracers for dopamine DRD2/3 receptors: two each for 11 C-raclopride, 18 F-fallypride, 123 I-IBZM; one for dopamine transporter, 123 I-FP-CIT; one used tracer for serotonin 5-HT2A receptor, 18 F-altanserin; two used tracers for serotonin transporter, 11 C-DASB or 123 I-FP-CIT; two used tracer for μ-opioid receptor, 11 C-carfentanil; one used tracer for noradrenaline transporter, 11 C-MRB); seven studies assessed glucose uptake using 18 F-fluorodeoxyglucose; four studies assessed regional cerebral blood flow using 15 O-H2 O (one study also used arterial spin labeling); and two studies measured fatty acid uptake using 18 F-FTHA and one using 11 C-palmitate. The review summarizes findings and correlations with clinical outcomes, eating behavior, and mechanistic mediators. The small number of studies using each tracer and intervention, lack of dietary intervention control groups in any surgical studies, heterogeneity in time since intervention and degree of weight loss, and small sample sizes hindered the drawing of robust conclusions across studies.

Striatal dopamine tone is positively associated with body mass index in humans as determined by PET using dual dopamine type-2 receptor antagonist tracers

The relationship between adiposity and dopamine type-2 receptor binding potential (D2BP) in the human brain has been repeatedly studied for >20 years with highly discrepant results, likely due to variable methodologies and differing study populations. We conducted a controlled inpatient feeding study to measure D2BP in the striatum using positron emission tomography with both [18F]fallypride and [11C]raclopride in pseudo-random order in 54 young adults with a wide range of body mass index (BMI 20-44 kg/m2). Within-subject D2BP measurements using the two tracers were moderately correlated (r=0.47, p<0.001). D2BP was negatively correlated with BMI as measured by [11C]raclopride (r= -0.51; p<0.0001) but not [18F]fallypride (r=-0.01; p=0.92) and these correlation coefficients were significantly different from each other (p<0.001). Given that [18F]fallypride has greater binding affinity to dopamine type-2 receptors than [11C]raclopride, which is more easily displaced by endogenous dopamine, our results suggest that adiposity is positively associated with increased striatal dopamine tone.

Polymorphism in the ZNF804A Gene and Variation in D1 and D2/D3 Dopamine Receptor Availability in the Healthy Human Brain: A Dual Positron Emission Tomography Study

BACKGROUND

The rs1344706 single nucleotide polymorphism in the ZNF804A gene has been associated with risk for psychosis in multiple genome-wide association studies, yet mechanisms underlying this association are not known. Given preclinical work suggesting an impact of ZNF804A on dopamine receptor gene transcription and clinical studies establishing dopaminergic dysfunction in patients with schizophrenia, we hypothesized that the ZNF804A risk single nucleotide polymorphism would be associated with variation in dopamine receptor availability in the human brain.

METHODS

In this study, 72 healthy individuals genotyped for rs1344706 completed both [18F]fallypride and [11C]NNC-112 positron emission tomography scans to measure D2/D3 and D1 receptor availability, respectively. Genetic effects on estimates of binding potential for each ligand were tested first with canonical subject-specific striatal regions of interest analyses, followed by exploratory whole-brain voxelwise analyses to test for more localized striatal signals and for extrastriatal effects.

RESULTS

Region of interest analyses revealed significantly less D2/D3 receptor availability in risk-allele homozygotes (TT) compared with non-risk allele carriers (G-allele carrier group: TG and GG) in the associative striatum and sensorimotor striatum, but no significant differences in striatal D1 receptor availability.

CONCLUSIONS

These data suggest that ZNF804A genotype may be meaningfully linked to dopaminergic function in the human brain. The results also may provide information to guide future studies of ZNF804A-related mechanisms of schizophrenia risk.

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.

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.

The effects of acute dopamine depletion on resting-state functional connectivity in healthy humans

Alpha-methyl-para-tyrosine (AMPT), a competitive inhibitor of tyrosine hydroxylase, can be used to deplete endogenous dopamine in humans. We examined how AMPT-induced dopamine depletion alters resting-state functional connectivity of the basal ganglia, and canonical resting-state networks, in healthy humans. Fourteen healthy participants (8 females; age [mean ± SD] = 27.93 ± 9.86) completed the study. Following dopamine depletion, the caudate showed reduced connectivity with the medial prefrontal cortex (mPFC) (Cohen's d = 1.89, p<.0001). Moreover, the caudate, putamen, globus pallidus, and midbrain all showed reduced connectivity with the occipital cortex (Cohen's d = 1.48-1.90; p<.0001-0.001). Notably, the dorsal caudate showed increased connectivity with the sensorimotor network (Cohen's d = 2.03, p=.002). AMPT significantly decreased self-reported motivation (t(13)=4.19, p=.001) and increased fatigue (t(13)=4.79, p=.0004). A greater increase in fatigue was associated with a greater reduction in connectivity between the substantia nigra and the mPFC (Cohen's d = 3.02, p<.00001), while decreased motivation was correlated with decreased connectivity between the VTA and left sensorimotor cortex (Cohen's d = 2.03, p=.00004). These findings help us to better understand the role of dopamine in basal ganglia function and may help us better understand neuropsychiatric diseases where abnormal dopamine levels are observed.

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).

Interaction of Ligands for PET with the Dopamine D3 Receptor: In Silico and In Vitro Methods

[¹⁸F]Fallypride and [¹⁸F]Fluortriopride (FTP) are two different PET radiotracers that bind with sub-nanomolar affinity to the dopamine D3 receptor (D₃R). In spite of their similar D₃ affinities, the two PET ligands display very different properties for labeling the D₃R in vivo: [¹⁸F]Fallypride is capable of binding to D₃R under "baseline" conditions, whereas [¹⁸F]FTP requires the depletion of synaptic dopamine in order to image the receptor in vivo. These data suggest that [¹⁸F]Fallypride is able to compete with synaptic dopamine for binding to the D₃R, whereas [¹⁸F]FTP is not. The goal of this study was to conduct a series of docking and molecular dynamic simulation studies to identify differences in the ability of each molecule to interact with the D₃R that could explain these differences with respect to competition with synaptic dopamine. Competition studies measuring the ability of each ligand to compete with dopamine in the β-arrestin assay were also conducted. The results of the in silico studies indicate that FTP has a weaker interaction with the orthosteric binding site of the D₃R versus that of Fallypride. The results of the in silico studies were also consistent with the IC50 values of each compound in the dopamine β-arrestin competition assays. The results of this study indicate that in silico methods may be able to predict the ability of a small molecule to compete with synaptic dopamine for binding to the D₃R.

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.

Imaging Cortical Dopamine Transmission in Cocaine Dependence: A [11C]FLB 457-Amphetamine Positron Emission Tomography Study

BACKGROUND

Positron emission tomography studies have demonstrated less dopamine D_2/3 receptor availability and blunted psychostimulant-induced dopamine release in cocaine-dependent subjects (CDSs). No studies in CDSs have reported the in vivo status of D_2/3 and dopamine release in the cortex. Basic and functional imaging studies suggest a role for prefrontal cortical dopaminergic abnormalities in impaired executive function and relapse in cocaine dependence. We used [¹¹C]FLB 457 positron emission tomography and amphetamine to measure cortical D_2/3 receptors and dopamine release in CDSs.

METHODS

[¹¹C]FLB 457 and positron emission tomography were used to measure D_2/3 receptor binding potential in cortical regions of interest in recently abstinent CDSs (n = 24) and healthy control subjects (n = 36) both before and after 0.5 mg kg^-1 of oral d-amphetamine. Binding potential relative to nondisplaceable uptake (BP_ND) and binding potential relative to total plasma concentration (BP_P) were derived using an arterial input function-based kinetic analysis. Cortical dopamine release in regions of interest was measured as the change in BP_ND and BP_P after amphetamine.

RESULTS

Baseline D_2/3 receptor availability (BP_P and BP_ND) and amphetamine-induced dopamine release (ΔBP_ND and ΔBP_P) were significantly lower in the cortical regions in CDSs compared with healthy control subjects. Fewer D_2/3 receptors and less dopamine release in CDSs were not associated with performance on working memory and attention tasks.

CONCLUSIONS

The results of this study suggest that deficits in dopamine D_2/3 transmission involve the cortex in cocaine dependence. Further studies to understand the clinical relevance of these findings are warranted.

Extra-striatal D2/3 receptor availability in youth at risk for addiction

The neurobiological traits that confer risk for addictions remain poorly understood. However, dopaminergic function throughout the prefrontal cortex, limbic system, and upper brainstem has been implicated in behavioral features that influence addiction vulnerability, including poor impulse control, and altered sensitivity to rewards and punishments (i.e., externalizing features). To test these associations in humans, we measured type-2/3 dopamine receptor (DA_2/3R) availability in youth at high vs. low risk for substance use disorders (SUDs). In this study, N = 58 youth (18.5 ± 0.6 years) were recruited from cohorts that have been followed since birth. Participants with either high (high EXT; N = 27; 16 F/11 M) or low pre-existing externalizing traits (low EXT; N = 31; 20 F/11 M) underwent a 90-min positron emission tomography [¹⁸F]fallypride scan, and completed the Barratt Impulsiveness Scale (BIS-11), Substance Use Risk Profile scale (SURPS), and Sensitivity to Punishment (SP) and Sensitivity to Reward (SR) questionnaire. We found that high vs. low EXT trait participants reported elevated substance use, BIS-11, SR, and SURPS impulsivity scores, had a greater prevalence of psychiatric disorders, and exhibited higher [¹⁸F]fallypride binding potential (BP_ND) values in prefrontal, limbic and paralimbic regions, even when controlling for substance use. Group differences were not evident in midbrain dopamine cell body regions, but, across all participants, low midbrain BP_ND values were associated with low SP scores. Together, the results suggest that altered DA_2/3R availability in terminal extra-striatal and dopamine cell body regions might constitute biological vulnerability traits, generating an EXT trajectory for addictions with and without co-occurring alterations in punishment sensitivity (i.e., an internalizing feature).

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.

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.

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'.

Selective D2 receptor PET in manganese-exposed workers

OBJECTIVE

To investigate the associations between manganese (Mn) exposure, D2 dopamine receptors (D2Rs), and parkinsonism using [11C](N-methyl)benperidol (NMB) PET.

METHODS

We used NMB PET to evaluate 50 workers with a range of Mn exposure: 22 Mn-exposed welders, 15 Mn-exposed workers, and 13 nonexposed workers. Cumulative Mn exposure was estimated from work histories, and movement disorder specialists examined all workers. We calculated NMB D2R nondisplaceable binding potential (BPND) for the striatum, globus pallidus, thalamus, and substantia nigra (SN). Multivariate analysis of covariance with post hoc descriptive discriminate analysis identified regional differences by exposure group. We used linear regression to examine the association among Mn exposure, Unified Parkinson's Disease Rating Scale motor subsection 3 (UPDRS3) score, and regional D2R BPND.

RESULTS

D2R BPND in the SN had the greatest discriminant power among exposure groups (p < 0.01). Age-adjusted SN D2R BPND was 0.073 (95% confidence interval [CI] 0.022-0.124) greater in Mn-exposed welders and 0.068 (95% CI 0.013-0.124) greater in Mn-exposed workers compared to nonexposed workers. After adjustment for age, SN D2R BPND was 0.0021 (95% CI 0.0005-0.0042) higher for each year of Mn exposure. Each 0.10 increase in SN D2R BPND was associated with a 2.65 (95% CI 0.56-4.75) increase in UPDRS3 score.

CONCLUSIONS AND RELEVANCE

Nigral D2R BPND increased with Mn exposure and clinical parkinsonism, indicating dose-dependent dopaminergic dysfunction of the SN in Mn neurotoxicity.

Failure to detect amphetamine-induced dopamine release in the cortex with [11 C]FLB 457 positron emission tomography (PET): Methodological considerations

Studies in nonhuman primates and humans have demonstrated that amphetamine-induced dopamine release in the cortex can be measured with [11 C]FLB 457 and PET imaging. This technique has been successfully used in recent clinical studies to show decreased dopamine transmission in the prefrontal cortex in schizophrenia and alcohol dependence. Here, we present data from a cohort of twelve healthy controls in whom an oral amphetamine challenge (0.5 mg kg-1 ) did not lead to a significant reduction in [11 C]FLB 457 BPND (i.e., binding potential relative to non-displaceable uptake). Two factors that likely contributed to the inability to displace [11 C]FLB 457 BPND in this cohort relative to successful cohorts are: (a) the acquisition of the baseline and post-amphetamine scans on different days as opposed to the same day and (b) the initiation of the post-amphetamine [11 C]FLB 457 scan at ∼5 hours as opposed to ∼3 hours following oral amphetamine. Furthermore, we show [11 C]FLB 457 reproducibility data from a legacy dataset to support greater variability in cortical BPND when the test and retest scans are acquired on different days as compared to the same day. These results highlight the methodological challenges that continue to plague the field with respect to imaging dopamine release in the cortex.

The role of striatal dopamine D2/3 receptors in cognitive performance in drug-free patients with schizophrenia

OBJECTIVE

A considerable body of research links cognitive function to dopaminergic transmission in the prefrontal cortex, but less is known about cognition in relation to striatal dopamine D_2/3 receptors in unmedicated patients with psychosis.

METHODS

We investigated this association by obtaining PET recordings with the high-affinity D_2/3 antagonist ligand [¹⁸F] fallypride in 15 medication-free patients with schizophrenia and 11 healthy controls. On the day of PET scanning, we undertook comprehensive neuropsychological testing and assessment of psychopathology using the Positive and Negative Syndrome Scale (PANSS).

RESULTS

The patients' performance in cognitive tests was significantly impaired in almost all domains. Irrespective of medication history, the mean [¹⁸F] fallypride binding potential (BP_ND) in the patient group tended to be globally 5-10% higher than that of the control group, but without reaching significance in any brain region. There were significant positive correlations between individual patient performance in the Trail Making Test (TMT(A) and TMT(B)) and Digit-Symbol-Substitution-Test with regional [¹⁸F] fallypride BP_ND, which remained significant after Bonferroni correction for the TMT(A) in caudate nucleus (CN) and for the TMT(B) in CN and putamen. No such correlations were evident in the control group.

DISCUSSION

The association between better cognitive performance and greater BP_ND in schizophrenia patients may imply that relatively lower receptor occupancy by endogenous dopamine favors better sparing of cognitive function. Absence of comparable correlations in healthy controls could indicate a greater involvement of signaling at dopamine D_2/3 receptors in certain cognitive functions in schizophrenia patients than in healthy controls.

[18F]fallypride-PET/CT Analysis of the Dopamine D₂/D₃ Receptor in the Hemiparkinsonian Rat Brain Following Intrastriatal Botulinum Neurotoxin A Injection

Intrastriatal injection of botulinum neurotoxin A (BoNT-A) results in improved motor behavior of hemiparkinsonian (hemi-PD) rats, an animal model for Parkinson’s disease. The caudate–putamen (CPu), as the main input nucleus of the basal ganglia loop, is fundamentally involved in motor function and directly interacts with the dopaminergic system. To determine receptor-mediated explanations for the BoNT-A effect, we analyzed the dopamine D₂/D₃ receptor (D₂/D₃R) in the CPu of 6-hydroxydopamine (6-OHDA)-induced hemi-PD rats by [¹⁸F]fallypride-PET/CT scans one, three, and six months post-BoNT-A or -sham-BoNT-A injection. Male Wistar rats were assigned to three different groups: controls, sham-injected hemi-PD rats, and BoNT-A-injected hemi-PD rats. Disease-specific motor impairment was verified by apomorphine and amphetamine rotation testing. Animal-specific magnetic resonance imaging was performed for co-registration and anatomical reference. PET quantification was achieved using PMOD software with the simplified reference tissue model 2. Hemi-PD rats exhibited a constant increase of 23% in D₂/D₃R availability in the CPu, which was almost normalized by intrastriatal application of BoNT-A. Importantly, the BoNT-A effect on striatal D₂/D₃R significantly correlated with behavioral results in the apomorphine rotation test. Our results suggest a therapeutic effect of BoNT-A on the impaired motor behavior of hemi-PD rats by reducing interhemispheric changes of striatal D₂/D₃R.

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.

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.

Elevated Dopamine D2/3 Receptor Availability in Obese Individuals: A PET Imaging Study with [11C](+)PHNO

Most prior work with positron emission tomography (PET) dopamine subtype 2/3 receptor (D_2/3R) non-selective antagonist tracers suggests that obese (OB) individuals exhibit lower D_2/3Rs when compared with normal weight (NW) individuals. A D₃-preferring D_2/3R agonist tracer, [¹¹C](+)PHNO, has demonstrated that body mass index (BMI) was positively associated with D_2/3R availability within striatal reward regions. To date, OB individuals have not been studied with [¹¹C](+)PHNO. We assessed D_2/3R availability in striatal and extrastriatal reward regions in 14 OB and 14 age- and gender-matched NW individuals with [¹¹C](+)PHNO PET utilizing a high-resolution research tomograph. Additionally, in regions where group D_2/3R differences were observed, secondary analyses of 42 individuals that constituted an overweight cohort was done to study the linear association between BMI and D_2/3R availability in those respective regions. A group-by-brain region interaction effect (F_{7, 182}=2.08, p=0.047) was observed. Post hoc analyses revealed that OB individuals exhibited higher tracer binding in D₃-rich regions: the substantia nigra/ventral tegmental area (SN/VTA) (+20%; p=0.02), ventral striatum (VST) (+14%; p<0.01), and pallidum (+11%; p=0.02). BMI was also positively associated with D_2/3R availability in the SN/VTA (r=0.34, p=0.03), VST (r=0.36, p=0.02), and pallidum (r=0.30, p=0.05) across all subjects. These data suggest that individuals who are obese have higher D_2/3R availability in brain reward regions densely populated with D₃Rs, potentially identifying a novel pharmacologic target for the treatment of obesity.

Sex Differences in Midbrain Dopamine D2-Type Receptor Availability and Association with Nicotine Dependence

Women differ from men in smoking-related behaviors, among them a greater difficulty in quitting smoking. Unlike female smokers, male smokers have lower striatal dopamine D2-type receptor availability (binding potential, BPND) than nonsmokers and exhibit greater smoking-induced striatal dopamine release. Because dopamine D2-type autoreceptors in the midbrain influence striatal dopamine release, a function that has been linked to addiction, we tested for sex differences in midbrain dopamine D2-type receptor BPND and in relationships between midbrain BPND, nicotine dependence and striatal dopamine D2-type receptor BPND. Positron emission tomography was used with [¹⁸F]fallypride to measure BPND in a midbrain region, encompassing the substantia nigra and ventral tegmental area, in 18 daily smokers (7 women, 11 men) and 19 nonsmokers (10 women, 9 men). A significant sex-by-group interaction reflected greater midbrain BPND in female but not male smokers than in corresponding nonsmokers (F_{1, 32}=5.089, p=0.03). Midbrain BPND was positively correlated with BPND in the caudate nucleus and putamen in nonsmokers and female smokers but not in male smokers and with nicotine dependence in female but not in male smokers. Striatal BPND was correlated negatively with nicotine dependence and smoking exposure. These findings extend observations on dopamine D2-type receptors in smokers and suggest a sex difference in how midbrain dopamine D2-type autoreceptors influence nicotine dependence.

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.

Cigarette Use and Striatal Dopamine D2/3 Receptors: Possible Role in the Link between Smoking and Nicotine Dependence

BACKGROUND

Cigarette smoking induces dopamine release in the striatum, and smoking- or nicotine-induced ventral striatal dopamine release is correlated with nicotine dependence. Smokers also exhibit lower dopamine D2/3 receptor availability in the dorsal striatum than nonsmokers. Negative correlations of striatal dopamine D2/3 receptor availability with smoking exposure and nicotine dependence, therefore, might be expected but have not been tested.

METHODS

Twenty smokers had positron emission tomography scans with [¹⁸F]fallypride to measure dopamine D2/3 receptor availability in ventral and dorsal regions of the striatum and provided self-report measures of recent and lifetime smoking and of nicotine dependence.

RESULTS

As reported before, lifetime smoking was correlated with nicotine dependence. New findings were that ventral striatal dopamine D2/3 receptor availability was negatively correlated with recent and lifetime smoking and also with nicotine dependence.

CONCLUSION

The results suggest an effect of smoking on ventral striatal D2/3 dopamine receptors that may contribute to nicotine dependence.

Consensus paper of the WFSBP Task Force on Biological Markers: Criteria for biomarkers and endophenotypes of schizophrenia part II: Cognition, neuroimaging and genetics

OBJECTIVES

Schizophrenia is a group of severe psychiatric disorders with high heritability but only low odds ratios of risk genes. Despite progress in the identification of pathophysiological processes, valid biomarkers of the disease are still lacking.

METHODS

This comprehensive review summarises recent efforts to identify genetic underpinnings, clinical and cognitive endophenotypes and symptom dimensions of schizophrenia and presents findings from neuroimaging studies with structural, functional and spectroscopy magnetic resonance imaging and positron emission tomography. The potential of findings to be biomarkers of schizophrenia is discussed.

RESULTS

Recent findings have not resulted in clear biomarkers for schizophrenia. However, we identified several biomarkers that are potential candidates for future research. Among them, copy number variations and links between genetic polymorphisms derived from genome-wide analysis studies, clinical or cognitive phenotypes, multimodal neuroimaging findings including positron emission tomography and magnetic resonance imaging, and the application of multivariate pattern analyses are promising.

CONCLUSIONS

Future studies should address the effects of treatment and stage of the disease more precisely and apply combinations of biomarker candidates. Although biomarkers for schizophrenia await validation, knowledge on candidate genomic and neuroimaging biomarkers is growing rapidly and research on this topic has the potential to identify psychiatric endophenotypes and in the future increase insight on individual treatment response in schizophrenia.

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.

Estimating the effect of endogenous dopamine on baseline [(11) C]-(+)-PHNO binding in the human brain

Endogenous dopamine (DA) levels at dopamine D2/3 receptors (D2/3 R) have been quantified in the living human brain using the agonist radiotracer [(11) C]-(+)-PHNO. As an agonist radiotracer, [(11) C]-(+)-PHNO is more sensitive to endogenous DA levels than antagonist radiotracers. We sought to determine the proportion of the variance in baseline [(11) C]-(+)-PHNO binding to D2/3 Rs which can be accounted for by variation in endogenous DA levels. This was done by computing the Pearson's coefficient for the correlation between baseline binding potential (BPND ) and the change in BPND after acute DA depletion, using previously published data. All correlations were inverse, and the proportion of the variance in baseline [(11) C]-(+)-PHNO BPND that can be accounted for by variation in endogenous DA levels across the striatal subregions ranged from 42-59%. These results indicate that lower baseline values of [(11) C]-(+)-PHNO BPND reflect greater stimulation by endogenous DA. To further validate this interpretation, we sought to examine whether these data could be used to estimate the dissociation constant (Kd) of DA at D2/3 R. In line with previous in vitro work, we estimated the in vivo Kd of DA to be around 20 nM. In summary, the agonist radiotracer [(11) C]-(+)-PHNO can detect the impact of endogenous DA levels at D2/3 R in the living human brain from a single baseline scan, and may be more sensitive to this impact than other commonly employed radiotracers.

Amphetamine alters neural response to sucrose in healthy women

Amphetamine, likely via action on the brain's dopaminergic systems, induces anorectic eating behavior and blunts dopaminergic midbrain activation to rewards. Past work has hypothesized that this blunted reward responsivity is a result of increasing tonic over phasic DA activity. We sought to extend past findings to sweet taste during fMRI following single-blind administration of dextroamphetamine and placebo in 11 healthy women. We hypothesized that neural response in both limbic and cognitive sweet taste circuits would mirror past work with monetary rewards by effectively blunting sweet taste reward, and 'equalizing' it's rewarding taste with receipt of water. Behavioral results showed that amphetamine reduced self-reported hunger (supporting the existence of amphetamine anorexia) and increased self-report euphoria. In addition, region of Interest analysis revealed significant treatment by taste interactions in the middle insula and dorsal anterior cingulate confirming the 'equalizing' hypothesis in the cingulate, but unlike monetary reinforcers, the insula actually evinced enhanced separation between tastes on the amphetamine day. These results suggest a divergence from prior research using monetary reinforcers when extended to primary reinforcers, and may hint that altering dopaminergic signaling in the insula and anterior cingulate may be a target for pharmacological manipulation of appetite, and the treatment of obesity.

A regularized full reference tissue model for PET neuroreceptor mapping

The full reference tissue model (FRTM) is a PET analysis framework that includes both free and specifically bound compartments within tissues, together with rate constants defining association and dissociation from the specifically bound compartment. The simplified reference tissue model (SRTM) assumes instantaneous exchange between tissue compartments, and this "1-tissue" approximation reduces the number of parameters and enables more robust mapping of non-displaceable binding potentials. Simulations based upon FRTM have shown that SRTM exhibits biases that are spatially dependent, because biases depend upon binding potentials. In this work, we describe a regularized model (rFRTM) that employs a global estimate of the dissociation rate constant from the specifically bound compartment (k₄). The model provides an internal calibration for optimizing k₄ through the reference-region outflow rate k₂', a model parameter that should be a global constant but varies regionally in SRTM. Estimates of k₄ by rFRTM are presented for four PET radioligands. We show that SRTM introduces bias in parameter estimates by assuming an infinite value for k₄, and that rFRTM ameliorates bias with an appropriate choice of k₄. Theoretical considerations and simulations demonstrate that rFRTM reduces bias in non-displaceable binding potentials. A two-parameter reduction of the model (rFRTM2) provides robust mapping at a voxel-wise level. With a structure similar to SRTM, the model is easily implemented and can be applied as a PET reference region analysis that reduces parameter bias without substantially altering parameter variance.

Cortical Dopamine Transmission as Measured with the [11C]FLB 457 - Amphetamine PET Imaging Paradigm Is Not Influenced by COMT Genotype

Basic investigations link a Val158Met polymorphism (rs4680) in the catechol-O-methyltransferase (COMT) gene to not only its enzymatic activity, but also to its dopaminergic tone in the prefrontal cortex. Previous PET studies have documented the relationship between COMT Val158Met polymorphism and D₁ and D_2/3 receptor binding potential (BP), and interpreted them in terms of dopaminergic tone. The use of baseline dopamine D₁ and D_2/3 receptor binding potential (BP_ND) as a proxy for dopaminergic tone is problematic because they reflect both endogenous dopamine levels (a change in radiotracer's apparent affinity) and receptor density. In this analysis of 31 healthy controls genotyped for the Val158Met polymorphism (Val/Val, Val/Met, and Met/Met), we used amphetamine-induced displacement of [¹¹C]FLB 457 as a direct measure of dopamine release. Our analysis failed to show a relationship between COMT genotype status and prefrontal cortical dopamine release. COMT genotype was also not predictive of baseline dopamine D_2/3 receptor BP_ND.

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.

Variability in paralimbic dopamine signaling correlates with subjective responses to d-amphetamine

Subjective responses to psychostimulants vary, the basis of which is poorly understood, especially in relation to possible cortical contributions. Here, we tested for relationships between participants' positive subjective responses to oral d-amphetamine (dAMPH) versus placebo and variability in striatal and extrastriatal dopamine (DA) receptor availability and release, measured via positron emission tomography (PET) with the radiotracer (18)F-fallypride. Analyses focused on 35 healthy adult participants showing positive subjective effects to dAMPH measured via the Drug Effects Questionnaire (DEQ) Feel, Like, High, and Want More subscales (Responders), and were repeated after inclusion of 11 subjects who lacked subjective responses. Associations between peak DEQ subscale ratings and both baseline (18)F-fallypride binding potential (BPnd; an index of D2/D3 receptor availability) and the percentage change in BPnd post dAMPH (%ΔBPnd; a measure of DA release) were assessed. Baseline BPnd in ventromedial prefrontal cortex (vmPFC) predicted the peak level of High reported following dAMPH. Furthermore, %ΔBPnd in vmPFC positively correlated with DEQ Want More ratings. DEQ Want More was also positively correlated with %ΔBPnd in right ventral striatum and left insula. This work indicates that characteristics of DA functioning in vmPFC, a cortical area implicated in subjective valuation, are associated with both subjective high and incentive (wanting) responses. The observation that insula %ΔBPnd was associated with drug wanting converges with evidence suggesting its role in drug craving. These findings highlight the importance of variability in DA signaling in specific paralimbic cortical regions in dAMPH's subjective response, which may confer risk for abusing psychostimulants.

Effect of Exercise Training on Striatal Dopamine D2/D3 Receptors in Methamphetamine Users during Behavioral Treatment

Methamphetamine use disorder is associated with striatal dopaminergic deficits that have been linked to poor treatment outcomes, identifying these deficits as an important therapeutic target. Exercise attenuates methamphetamine-induced neurochemical damage in the rat brain, and a preliminary observation suggests that exercise increases striatal D2/D3 receptor availability (measured as nondisplaceable binding potential (BPND)) in patients with Parkinson's disease. The goal of this study was to evaluate whether adding an exercise training program to an inpatient behavioral intervention for methamphetamine use disorder reverses deficits in striatal D2/D3 receptors. Participants were adult men and women who met DSM-IV criteria for methamphetamine dependence and were enrolled in a residential facility, where they maintained abstinence from illicit drugs of abuse and received behavioral therapy for their addiction. They were randomized to a group that received 1 h supervised exercise training (n=10) or one that received equal-time health education training (n=9), 3 days/week for 8 weeks. They came to an academic research center for positron emission tomography (PET) using [(18)F]fallypride to determine the effects of the 8-week interventions on striatal D2/D3 receptor BPND. At baseline, striatal D2/D3 BPND did not differ between groups. However, after 8 weeks, participants in the exercise group displayed a significant increase in striatal D2/D3 BPND, whereas those in the education group did not. There were no changes in D2/D3 BPND in extrastriatal regions in either group. These findings suggest that structured exercise training can ameliorate striatal D2/D3 receptor deficits in methamphetamine users, and warrants further evaluation as an adjunctive treatment for stimulant dependence.

Convergent evidence from alcohol-dependent humans and rats for a hyperdopaminergic state in protracted abstinence

A major hypothesis in addiction research is that alcohol induces neuroadaptations in the mesolimbic dopamine (DA) system and that these neuroadaptations represent a key neurochemical event in compulsive drug use and relapse. Whether these neuroadaptations lead to a hypo- or hyperdopaminergic state during abstinence is a long-standing, unresolved debate among addiction researchers. The answer is of critical importance for understanding the neurobiological mechanism of addictive behavior. Here we set out to study systematically the neuroadaptive changes in the DA system during the addiction cycle in alcohol-dependent patients and rats. In postmortem brain samples from human alcoholics we found a strong down-regulation of the D1 receptor- and DA transporter (DAT)-binding sites, but D2-like receptor binding was unaffected. To gain insight into the time course of these neuroadaptations, we compared the human data with that from alcohol-dependent rats at several time points during abstinence. We found a dynamic regulation of D1 and DAT during 3 wk of abstinence. After the third week the rat data mirrored our human data. This time point was characterized by elevated extracellular DA levels, lack of synaptic response to D1 stimulation, and augmented motor activity. Further functional evidence is given by a genetic rat model for hyperdopaminergia that resembles a phenocopy of alcohol-dependent rats during protracted abstinence. In summary, we provide a new dynamic model of abstinence-related changes in the striatal DA system; in this model a hyperdopaminergic state during protracted abstinence is associated with vulnerability for relapse.

Safety of Oral Amphetamine Administered during Positron Emission Tomography Scans in Medically Screened Humans

Changes in endogenous dopamine levels can be detected in humans using positron emission tomography scans by measuring the amount by which a specific D_2/3 radioligand is displaced. In some cases, a challenge drug such as amphetamine is introduced to increase the amount of dopamine released into the synaptic cleft. Although intravenous amphetamine is often utilized, oral amphetamine has been shown to be just as effective in increasing endogenous dopamine levels. Based on our own use of oral amphetamine as a challenge drug, we have retroactively reviewed our study charts to determine the cardiovascular safety of 0.5 mg kg^-1 oral d-amphetamine. Of 172 amphetamine administrations in 144 individuals, only 2.8% of subjects experienced any transient adverse effects. In addition, we found no clinically relevant differences in increases of vital signs between healthy controls and patients. We therefore reaffirm the safety of 0.5 mg kg^-1 oral amphetamine in subjects previously screened for cardiovascular risk factors.

Relationship of Alexithymia Ratings to Dopamine D2-type Receptors in Anterior Cingulate and Insula of Healthy Control Subjects but Not Methamphetamine-Dependent Individuals

BACKGROUND

Individuals with substance-use disorders exhibit emotional problems, including deficits in emotion recognition and processing, and this class of disorders also has been linked to deficits in dopaminergic markers in the brain. Because associations between these phenomena have not been explored, we compared a group of recently abstinent methamphetamine-dependent individuals (n=23) with a healthy-control group (n=17) on dopamine D2-type receptor availability, measured using positron emission tomography with [(18)F]fallypride.

METHODS

The anterior cingulate and anterior insular cortices were selected as the brain regions of interest, because they receive dopaminergic innervation and are thought to be involved in emotion awareness and processing. The Toronto Alexithymia Scale, which includes items that assess difficulty in identifying and describing feelings as well as externally oriented thinking, was administered, and the scores were tested for association with D2-type receptor availability.

RESULTS

Relative to controls, methamphetamine-dependent individuals showed higher alexithymia scores, reporting difficulty in identifying feelings. The groups did not differ in D2-type receptor availability in the anterior cingulate or anterior insular cortices, but a significant interaction between group and D2-type receptor availability in both regions, on self-report score, reflected significant positive correlations in the control group (higher receptor availability linked to higher alexithymia) but nonsignificant, negative correlations (lower receptor availability linked to higher alexithymia) in methamphetamine-dependent subjects.

CONCLUSIONS

The results suggest that neurotransmission through D2-type receptors in the anterior cingulate and anterior insular cortices influences capacity of emotion processing in healthy people but that this association is absent in individuals with methamphetamine dependence.

Dopamine in the Brain: Hypothesizing Surfeit or Deficit Links to Reward and Addiction

Recently there has been debate concerning the role of brain dopamine in reward and addiction. David Nutt and associates eloquently proposed that dopamine (DA) may be central to psycho stimulant dependence and some what important for alcohol, but not important for opiates, nicotine or even cannabis. Others have also argued that surfeit theories can explain for example cocaine seeking behavior as well as non-substance-related addictive behaviors. It seems prudent to distinguish between what constitutes “surfeit” compared to” deficit” in terms of short-term (acute) and long-term (chronic) brain reward circuitry responsivity. In an attempt to resolve controversy regarding the contributions of mesolimbic DA systems to reward, we review the three main competing explanatory categories: “liking”, “learning”, and “wanting”. They are (a) the hedonic impact -liking reward, (b) the ability to predict rewarding effects-learning and (c) the incentive salience of reward-related stimuli -wanting. In terms of acute effects, most of the evidence seems to favor the “surfeit theory”. Due to preferential dopamine release at mesolimbic-VTA-caudate-accumbens loci most drugs of abuse and Reward Deficiency Syndrome (RDS) behaviors have been linked to heightened feelings of well-being and hyperdopaminergic states.The “dopamine hypotheses” originally thought to be simple, is now believed to be quite complex and involves encoding the set point of hedonic tone, encoding attention, reward expectancy, and incentive motivation. Importantly, Willuhn et al. shows that in a self-administration paradigm, (chronic) excessive use of cocaine is caused by decreased phasic dopamine signaling in the striatum. In terms of chronic addictions, others have shown a blunted responsivity at brain reward sites with food, nicotine, and even gambling behavior. Finally, we are cognizant of the differences in dopaminergic function as addiction progresses and argue that relapse may be tied to dopamine deficiency. Vulnerability to addiction and relapse may be the result of the cumulative effects of dopaminergic and other neurotransmitter genetic variants and elevated stress levels. We therefore propose that dopamine homeostasis may be a preferred goal to combat relapse.

Striatal and extrastriatal dopamine release in the common marmoset brain measured by positron emission tomography and [(18)F]fallypride

Previous studies have demonstrated that patients with schizophrenia show greater sensitivity to psychostimulants than healthy subjects. Sensitization to psychostimulants and resultant alteration of dopaminergic neurotransmission in rodents has been suggested as a useful model of schizophrenia. This study sought to examine the use of methylphenidate as a psychostimulant to induce dopamine release and that of [(18)F]fallypride as a radioligand to quantify the release in a primate model of schizophrenia. Four common marmosets were scanned by positron emission tomography twice, before and after methylphenidate challenge, to evaluate dopamine release. Four other marmosets were sensitized by repeated methamphetamine (MAP) administration. Then, they were scanned twice, before and after methylphenidate challenge, to evaluate whether MAP-sensitization induced greater sensitivity to methylphenidate. We revealed a main effect of the methylphenidate challenge but not the MAP pretreatment on the striatal binding potential. These results suggest that methylphenidate-induced striatal dopamine release in the common marmoset could be evaluated by [(18)F]fallypride.

Insulin, Central Dopamine D2 Receptors, and Monetary Reward Discounting in Obesity

Animal research finds that insulin regulates dopamine signaling and reward behavior, but similar research in humans is lacking. We investigated whether individual differences in body mass index, percent body fat, pancreatic β-cell function, and dopamine D2 receptor binding were related to reward discounting in obese and non-obese adult men and women. Obese (n = 27; body mass index>30) and non-obese (n = 20; body mass index<30) adults were assessed for percent body fat with dual-energy X-ray absorptiometry and for β-cell function using disposition index. Choice of larger, but delayed or less certain, monetary rewards relative to immediate, certain smaller monetary rewards was measured using delayed and probabilistic reward discounting tasks. Positron emission tomography using a non-displaceable D2-specific radioligand, [¹¹C](N-methyl)benperidol quantified striatal D2 receptor binding. Groups differed in body mass index, percent body fat, and disposition index, but not in striatal D2 receptor specific binding or reward discounting. Higher percent body fat in non-obese women related to preference for a smaller, certain reward over a larger, less likely one (greater probabilistic discounting). Lower β-cell function in the total sample and lower insulin sensitivity in obese related to stronger preference for an immediate and smaller monetary reward over delayed receipt of a larger one (greater delay discounting). In obese adults, higher striatal D2 receptor binding related to greater delay discounting. Interestingly, striatal D2 receptor binding was not significantly related to body mass index, percent body fat, or β-cell function in either group. Our findings indicate that individual differences in percent body fat, β-cell function, and striatal D2 receptor binding may each contribute to altered reward discounting behavior in non-obese and obese individuals. These results raise interesting questions about whether and how striatal D2 receptor binding and metabolic factors, including β-cell function, interact to affect reward discounting in humans.

Emotional Eating Phenotype is Associated with Central Dopamine D2 Receptor Binding Independent of Body Mass Index

PET studies have provided mixed evidence regarding central D2/D3 dopamine receptor binding and its relationship with obesity as measured by body mass index (BMI). Other aspects of obesity may be more tightly coupled to the dopaminergic system. We characterized obesity-associated behaviors and determined if these related to central D2 receptor (D2R) specific binding independent of BMI. Twenty-two obese and 17 normal-weight participants completed eating- and reward-related questionnaires and underwent PET scans using the D2R-selective and nondisplaceable radioligand (N-[¹¹C]methyl)benperidol. Questionnaires were grouped by domain (eating related to emotion, eating related to reward, non-eating behavior motivated by reward or sensitivity to punishment). Normalized, summed scores for each domain were compared between obese and normal-weight groups and correlated with striatal and midbrain D2R binding. Compared to normal-weight individuals, the obese group self-reported higher rates of eating related to both emotion and reward (p < 0.001), greater sensitivity to punishment (p = 0.06), and lower non-food reward behavior (p < 0.01). Across normal-weight and obese participants, self-reported emotional eating and non-food reward behavior positively correlated with striatal (p < 0.05) and midbrain (p < 0.05) D2R binding, respectively. In conclusion, an emotional eating phenotype may reflect altered central D2R function better than other commonly used obesity-related measures such as BMI.

No evidence for attenuated stress-induced extrastriatal dopamine signaling in psychotic disorder

Stress is an important risk factor in the etiology of psychotic disorder. Preclinical work has shown that stress primarily increases dopamine (DA) transmission in the frontal cortex. Given that DA-mediated hypofrontality is hypothesized to be a cardinal feature of psychotic disorder, stress-related extrastriatal DA release may be altered in psychotic disorder. Here we quantified for the first time stress-induced extrastriatal DA release and the spatial extent of extrastriatal DA release in individuals with non-affective psychotic disorder (NAPD). Twelve healthy volunteers (HV) and 12 matched drug-free NAPD patients underwent a single infusion [(18)F]fallypride positron emission tomography scan during which they completed the control and stress condition of the Montreal Imaging Stress Task. HV and NAPD did not differ in stress-induced [(18)F]fallypride displacement and the spatial extent of stress-induced [(18)F]fallypride displacement in medial prefrontal cortex (mPFC) and temporal cortex (TC). In the whole sample, the spatial extent of stress-induced radioligand displacement in right ventro-mPFC, but not dorso-mPFC or TC, was positively associated with task-induced subjective stress. Psychotic symptoms during the scan or negative, positive and general subscales of the Positive and Negative Syndrome Scale were not associated with stress-induced [(18)F]fallypride displacement nor the spatial extent of stress-induced [(18)F]fallypride displacement in NAPD. Our results do not offer evidence for altered stress-induced extrastriatal DA signaling in NAPD, nor altered functional relevance. The implications of these findings for the role of the DA system in NAPD and stress processing are discussed.

Low Dopamine D2/D3 Receptor Availability is Associated with Steep Discounting of Delayed Rewards in Methamphetamine Dependence

BACKGROUND

Individuals with substance use disorders typically exhibit a predilection toward instant gratification with apparent disregard for the future consequences of their actions. Indirect evidence suggests that low dopamine D2-type receptor availability in the striatum contributes to the propensity of these individuals to sacrifice long-term goals for short-term gain; however, this possibility has not been tested directly. We investigated whether striatal D2/D3 receptor availability is negatively correlated with the preference for smaller, more immediate rewards over larger, delayed alternatives among research participants who met DSM-IV criteria for methamphetamine (MA) dependence.

METHODS

Fifty-four adults (n = 27 each: MA-dependent, non-user controls) completed the Kirby Monetary Choice Questionnaire, and underwent positron emission tomography scanning with [(18)F]fallypride.

RESULTS

MA users displayed steeper temporal discounting (p = 0.030) and lower striatal D2/D3 receptor availability (p < 0.0005) than controls. Discount rate was negatively correlated with striatal D2/D3 receptor availability, with the relationship reaching statistical significance in the combined sample (r = -0.291, p = 0.016) and among MA users alone (r = -0.342, p = 0.041), but not among controls alone (r = -0.179, p = 0.185); the slopes did not differ significantly between MA users and controls (p = 0.5).

CONCLUSIONS

These results provide the first direct evidence of a link between deficient D2/D3 receptor availability and steep temporal discounting. This finding fits with reports that low striatal D2/D3 receptor availability is associated with a higher risk of relapse among stimulant users, and may help to explain why some individuals choose to continue using drugs despite knowledge of their eventual negative consequences. Future research directions and therapeutic implications are discussed.

Application of cross-species PET imaging to assess neurotransmitter release in brain

RATIONALE

This review attempts to summarize the current status in relation to the use of positron emission tomography (PET) imaging in the assessment of synaptic concentrations of endogenous mediators in the living brain.

OBJECTIVES

Although PET radioligands are now available for more than 40 CNS targets, at the initiation of the Innovative Medicines Initiative (IMI) "Novel Methods leading to New Medications in Depression and Schizophrenia" (NEWMEDS) in 2009, PET radioligands sensitive to an endogenous neurotransmitter were only validated for dopamine. NEWMEDS work-package 5, "Cross-species and neurochemical imaging (PET) methods for drug discovery", commenced with a focus on developing methods enabling assessment of changes in extracellular concentrations of serotonin and noradrenaline in the brain.

RESULTS

Sharing the workload across institutions, we utilized in vitro techniques with cells and tissues, in vivo receptor binding and microdialysis techniques in rodents, and in vivo PET imaging in non-human primates and humans. Here, we discuss these efforts and review other recently published reports on the use of radioligands to assess changes in endogenous levels of dopamine, serotonin, noradrenaline, γ-aminobutyric acid, glutamate, acetylcholine, and opioid peptides. The emphasis is on assessment of the availability of appropriate translational tools (PET radioligands, pharmacological challenge agents) and on studies in non-human primates and human subjects, as well as current challenges and future directions.

CONCLUSIONS

PET imaging directed at investigating changes in endogenous neurochemicals, including the work done in NEWMEDS, have highlighted an opportunity to further extend the capability and application of this technology in drug development.

Efficiency gains in tracer identification for nuclear imaging: can in vivo LC-MS/MS evaluation of small molecules screen for successful PET tracers?

Positron emission tomography (PET) imaging has become a useful noninvasive technique to explore molecular biology within living systems; however, the utility of this method is limited by the availability of suitable radiotracers to probe specific targets and disease biology. Methods to identify potential areas of improvement in the ability to predict small molecule performance as tracers prior to radiolabeling would speed the discovery of novel tracers. In this retrospective analysis, we characterized the brain penetration or peak SUV (standardized uptake value), binding potential (BP), and brain exposure kinetics across a series of known, nonradiolabeled PET ligands using in vivo LC-MS/MS (liquid chromatography coupled to mass spectrometry) and correlated these parameters with the reported PET ligand performance in nonhuman primates and humans available in the literature. The PET tracers studied included those reported to label G protein-coupled receptors (GPCRs), intracellular enzymes, and transporters. Additionally, data for each tracer was obtained from a mouse brain uptake assay (MBUA), previously published, where blood-brain barrier (BBB) penetration and clearance parameters were assessed and compared against similar data collected on a broad compound set of central nervous system (CNS) therapeutic compounds. The BP and SUV identified via nonradiolabeled LC-MS/MS, while different from the published values observed in the literature PET tracer data, allowed for an identification of initial criteria values we sought to facilitate increased potential for success from our early discovery screening paradigm. Our analysis showed that successful, as well as novel, clinical PET tracers exhibited BP of greater than 1.5 and peak SUVs greater than approximately 150% at 5 min post dose in rodents. The brain kinetics appeared similar between both techniques despite differences in tracer dose, suggesting linearity across these dose ranges. The assessment of tracers in a CNS exposure model, the mouse brain uptake assessment (MBUA), showed that those compound with initial brain-to-plasma ratios >2 and unbound fraction in brain homogenate >0.01 were more likely to be clinically successful PET ligands. Taken together, early incorporation of a LC/MS/MS cold tracer discovery assay and a parallel MBUA can be an useful screening paradigm to prioritize and rank order potential novel PET radioligands during early tracer discovery efforts. Compounds considered for continued in vivo PET assessments can be identified quickly by leveraging in vitro affinity and selectivity measures, coupled with data from a MBUA, primarily the 5 min brain-to-plasma ratio and unbound fraction data. Coupled utilization of these data creates a strategy to efficiently screen for the identification of appropriate chemical space to invest in for radiotracer discovery.

Towards a therapeutic window of D2/3 occupancy for treatment of psychosis in Alzheimer's disease, with [18F]fallypride positron emission tomography

OBJECTIVE

Dopamine D2/3 receptor positron emission tomography tracers have guided antipsychotic prescribing in young people with schizophrenia by establishing a 'therapeutic window' of striatal D2/3 receptor occupancy. Older people, particularly those with dementia, are highly susceptible to motor side effects and may benefit from the appropriate application of imaging techniques. The study aimed to adapt [18F]fallypride imaging for use in occupancy studies in Alzheimer's disease (AD) and to investigate whether data acquisition could be made more tolerable by piloting the protocol in a small sample.

METHODS

Six participants with AD (three men; 85.0 ± 5.6 years old; MMSE = 16.0 ± 2.4) were recruited prior to commencing amisulpride for the treatment of psychosis and associated agitation. [18F]fallypride binding potential (BPND ) was determined using an interrupted scanning protocol at baseline (n = 6) and after 27.0 ± 6.1 days of amisulpride (25-50 mg) treatment (n = 4). D2/3 occupancy was calculated by percentage reduction in BPND between scanning sessions. Image data were re-analysed after reducing individual sampling times to 20 min.

RESULTS

The protocol was tolerated well, apart from the final (40 min) session of the post-treatment scan in one participant. Higher occupancies were achieved in the striatum (caudate 47-70%, putamen 31-58%) and thalamus (54-76%) than in the inferior temporal gyrus (27-43%). There was high agreement between occupancy values derived using longer and shorter sampling times (mean absolute difference 6.1% in the inferior temporal gyrus; <2% all other regions).

CONCLUSIONS

The protocol is feasible for use in AD and represents the first step towards establishing dose-occupancy relationships across older clinical populations.

The simplified reference tissue model with 18F-fallypride positron emission tomography: choice of reference region

The development of high-affinity radiotracers for positron emission tomography (PET) has allowed for quantification of dopamine receptors in extrastriatal and striatal regions of the brain. As these new radiotracers have distinctly different kinetic properties than their predecessors, it is important to examine the suitability of kinetic models to represent their uptake, distribution, and in vivo washout. Using the simplified reference tissue model, we investigated the influence of reference region choice on the striatal binding potential of 18F-fallypride, a high-affinity dopamine D2/D3 receptor ligand. We compared the use of the visual cortex and a white matter region (superior longitudinal fasciculus) to the cerebellum, a commonly used reference tissue, in a PET-fallypride study of healthy and methamphetamine-dependent subjects. Compared to the cerebellum, use of the visual cortex produced significantly greater sample variance in binding potential relative to nondisplaceable uptake (BP(ND)). Use of the white matter region was associated with BP(ND) values and sample variance similar to those obtained with the cerebellum and a larger effect size for the group differences in striatal BP(ND) between healthy and methamphetamine-dependent subjects. Our results do not support the use of the visual cortex as a reference region in 18F-fallypride studies and suggest that white matter may be a reasonable alternative to the cerebellum as it displays similar statistical and kinetic properties.

Alterations in cortical and extrastriatal subcortical dopamine function in schizophrenia: systematic review and meta-analysis of imaging studies

BACKGROUND

The hypothesis that cortical dopaminergic alterations underlie aspects of schizophrenia has been highly influential.

AIMS

To bring together and evaluate the imaging evidence for dopaminergic alterations in cortical and other extrastriatal regions in schizophrenia.

METHOD

Electronic databases were searched for in vivo molecular studies of extrastriatal dopaminergic function in schizophrenia. Twenty-three studies (278 patients and 265 controls) were identified. Clinicodemographic and imaging variables were extracted and effect sizes determined for the dopaminergic measures. There were sufficient data to permit meta-analyses for the temporal cortex, thalamus and substantia nigra but not for other regions.

RESULTS

The meta-analysis of dopamine D2/D3 receptor availability found summary effect sizes of d = -0.32 (95% CI -0.68 to 0.03) for the thalamus, d = -0.23 (95% CI -0.54 to 0.07) for the temporal cortex and d = 0.04 (95% CI -0.92 to 0.99) for the substantia nigra. Confidence intervals were wide and all included no difference between groups. Evidence for other measures/regions is limited because of the small number of studies and in some instances inconsistent findings, although significant differences were reported for D2/D3 receptors in the cingulate and uncus, for D1 receptors in the prefrontal cortex and for dopamine transporter availability in the thalamus.

CONCLUSIONS

There is a relative paucity of direct evidence for cortical dopaminergic alterations in schizophrenia, and findings are inconclusive. This is surprising given the wide influence of the hypothesis. Large, well-controlled studies in drug-naive patients are warranted to definitively test this hypothesis.