Measurement of cortical dopamine d1 receptor binding with 11C[SCH23390]: a test-retest analysis

Dopamine D1 Receptor Agonist PET Tracer Development: Assessment in Nonhuman Primates

Non-catechol-based high-affinity selective dopamine D1 receptor (D1R) agonists were recently described, and candidate PET ligands were selected on the basis of favorable properties. The objective of this study was to characterize in vivo in nonhuman primates 2 novel D1R agonist PET radiotracers, racemic 18F-MNI-800 and its more active atropisomeric (-)-enantiomer, 18F-MNI-968. Methods: Ten brain PET experiments were conducted with 18F-MNI-800 on 2 adult rhesus macaques and 2 adult cynomolgus macaques, and 8 brain PET experiments were conducted with 18F-MNI-968 on 2 adult rhesus macaques and 2 adult cynomolgus macaques. PET data were analyzed with both plasma-input-based methods and reference-region-based methods. Whole-body PET images were acquired with 18F-MNI-800 from 2 adult rhesus macaques for radiation dosimetry estimates. Results:18F-MNI-800 and 18F-MNI-968 exhibited regional uptake consistent with D1R distribution. Specificity and selectivity were demonstrated by dose-dependent blocking with the D1 antagonist SCH-23390. 18F-MNI-968 showed a 30% higher specific signal than 18F-MNI-800, with a nondisplaceable binding potential of approximately 0.3 in the cortex and approximately 1.1 in the striatum. Dosimetry radiation exposure was favorable, with an effective dose of about 0.023 mSv/MBq. Conclusion:18F-MNI-968 has significant potential as a D1R agonist PET radiotracer, and further characterization in human subjects is warranted.

In vivo imaging of dopamine D1 receptor and activated microglia in attention-deficit/hyperactivity disorder: a positron emission tomography study

Alterations in the cortical dopamine system and microglial activation have been implicated in the pathophysiology of attention-deficit/hyperactivity disorder (ADHD), one of neurodevelopmental disorders that can be conventionally treated with a dopamine enhancer (methylphenidate) albeit unsatisfactorily. Here, we investigated the contributions of the dopamine D1 receptor (D1R) and activated microglia and their interactions to the clinical severities in ADHD individuals using positron emission tomography (PET). Twenty-four psychotropic-naïve ADHD individuals and 24 age- and sex-matched typically developing (TD) subjects underwent PET measurements with [¹¹C]SCH23390 for the D1R and [¹¹C](R)PK11195 for activated microglia as well as assessments of clinical symptoms and cognitive functions. The ADHD individuals showed decreased D1R in the anterior cingulate cortex (ACC) and increased activated microglia in the dorsolateral prefrontal cortex (DLPFC) and orbitofrontal cortex (OFC) compared with the TD subjects. The decreased D1R in the ACC was associated with severe hyperactivity in the participants with ADHD. Microglial activation in the DLPFC were associated with deficits in processing speed and attentional ability, and that in the OFC was correlated with lower processing speed in the ADHD individuals. Furthermore, positive correlations between the D1R and activated microglia in both the DLPFC and the OFC were found to be significantly specific to the ADHD group and not to the TD group. The current findings suggest that microglial activation and the D1R reduction as well as their aberrant interactions underpin the neurophysiological mechanism of ADHD and indicate these biomolecular changes as a novel therapeutic target.

Kinetic Modelling and Test-Retest Reproducibility for the Dopamine D1R Radioligand [11C]SCH23390 in Healthy and Diseased Mice

Purpose

Our aim in this study was to compare different non-invasive pharmacokinetic models and assess test–retest reproducibility of the radioligand [¹¹C]SCH23390 for the quantification of dopamine D₁-like receptor (D₁R) in both wild-type (WT) mice and heterozygous (HET) Q175DN mice as Huntington’s disease (HD) model.

Procedures

Adult WT (n = 9) and HET (n = 14) mice underwent a 90-min [¹¹C]SCH23390 positron emission tomography (PET) scan followed by computed tomography (CT) to evaluate the pharmacokinetic modelling in healthy and diseased conditions. Additionally, 5 WT mice and 7 HET animals received a second [¹¹C]SCH23390 PET scan for test–retest reproducibility. Parallel assessment of the simplified reference tissue model (SRTM), the multilinear reference tissue model (MRTM) and the Logan reference tissue model (Logan Ref) using the striatum as a receptor-rich region and the cerebellum as a receptor-free (reference) region was performed to define the most suitable method for regional- and voxel-based quantification of the binding potential (BP_ND). Finally, standardised uptake value ratio (SUVR-1) was assessed as a potential simplified measurement.

Results

For all models, we measured a significant decline in dopamine D₁R density (e.g. SRTM = − 38.5 ± 5.0 %, p < 0.0001) in HET mice compared to WT littermates. Shortening the 90-min scan duration resulted in large underestimation of striatal BP_ND in both WT mice (SRTM 60 min: − 17.7 ± 2.8 %, p = 0.0078) and diseased HET (SRTM 60 min: − 13.1 ± 4.1 %, p = 0.0001). Striatal BP_ND measurements were very reproducible with an average test–retest variability below 5 % when using both MRTM and SRTM. Parametric BP_ND maps generated with SRTM were highly reliable, showing nearly perfect agreement to the regional analysis (r² = 0.99, p < 0.0001). Finally, SRTM provided the most accurate estimate for relative tracer delivery R₁ with both regional- and voxel-based analyses. SUVR-1 at different time intervals were not sufficiently reliable when compared to BP_ND (r² < 0.66).

Conclusions

Ninety-minute acquisition and the use of SRTM for pharmacokinetic modelling is recommended. [¹¹C]SCH23390 PET imaging demonstrates optimal characteristics for the study of dopamine D₁R density in models of psychiatric and neurological disorders as exemplified in the Q175DN mouse model of HD.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11307-020-01561-1.

[11C]SCH23390 binding to the D1-dopamine receptor in the human brain-a comparison of manual and automated methods for image analysis

Background

The D₁-dopamine receptor radioligand [¹¹C]SCH23390 has been frequently used in PET studies. In drug-naïve patients with schizophrenia, the findings have been inconsistent, with decreases, increases, and no change in the frontal cortex D₁-dopamine receptors. While these discrepancies are likely primarily due to a lack of statistical power in these studies, we speculated that an additional explanation may be the differences due to methods of image analysis between studies, affecting reliability as well as bias between groups.

Methods

Fifteen healthy subjects underwent two PET measurements with [¹¹C]SCH23390 on the same day. The binding potential (BP_ND) was compared using a 95% confidence interval following manual and automated delineation of a region of interest (ROI) as well as with and without frame-by-frame realignment.

Results

Automated target region delineation produced lower BP_ND values, while automated delineation of the reference region yielded higher BP_ND values. However, no significant differences were observed for repeatability using automated and manual delineation methods. Frame-by-frame realignment generated higher BP_ND values and improved repeatability.

Conclusions

The results suggest that the choice of ROI delineation method is not an important factor for reliability, whereas the improved results following movement correction confirm its importance in PET image analysis. Realignment is therefore especially important for measurements in patient populations such as schizophrenia or Parkinson’s disease, where motion artifacts may be more prevalent.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0416-2) contains supplementary material, which is available to authorized users.

PET radioligands for the dopamine D1-receptor: Application in psychiatric disorders

The dopamine (DA) system is considered to be centrally involved in the pathophysiology of several major psychiatric disorders. Using positron emission tomography (PET), aberrations in dopamine D2/D3-receptors (D2-R) levels and uptake of the DA precursor FDOPA have been shown for schizophrenia, substance abuse and depression. Radioligands for the dopamine D1-receptor (D1-R) have been available for more than three decades, however this receptor subtype has received much less attention in psychiatry research. Here, studies investigating D1-R in psychiatric patients in comparison to healthy control subjects are summarized. Although small sample sizes, medication effects and heterogeneous methods of quantification limit the conclusions that can be drawn, the data is suggestive of higher levels of cortical D1-R in drug naïve patients with psychosis, and lower D1-R in patients with affective disorders. Data sharing and reanalysis using harmonized methodology are important next steps towards clarifying the role of D1-R in these disorders.

Posterior dopamine D2/3 receptors and brain network functional connectivity

Recent studies suggest that dopaminergic tone influences resting state activity in multiple brain networks. Although dopamine receptors and transporters have been identified in the posteromedial and parietal cortices, which are linked to functional networks such as the default mode network (DMN), the relationship between dopamine receptor distribution in these posterior regions and resting-state connectivity has yet to be explored. Here, we used a multi-modal neuroimaging strategy, combining resting-state functional magnetic resonance imaging (rsfMRI) and [18 F]-fallypride high-resolution positron emission tomography (PET), to examine the association between within-network functional connectivity and the dopamine D2/3 receptor distribution in the posterior portion of the brain in 13 healthy adults. Our results indicate that the posterior distribution of D2/3 receptors coincides primarily with the posterior portion of the DMN. Furthermore, in the posterior portion of the brain, the level of [18 F]-fallypride binding in the posteromedial cortex correlated positively with the functional connectivity strength of the DMN and sensorimotor network, and negatively with the functional connectivity strength of the dorsal attention network, the salience network, and a network that included the anterior part of the temporo-parietal junction. On the basis of these findings, we propose that posterior brain dopamine influences the configuration of the posterior DMN and several other functional brain networks. The posterior distribution of D2/3 receptors binding (hot colour spectrum) coincides with the functional connectivity of the posterior portion of the default mode network (green colour spectrum). The mean BP_ND in a posteromedial cortex and the mean ICA-Z score in the precuneus showed significant positive correlation.

Test-retest measurements of dopamine D1-type receptors using simultaneous PET/MRI imaging

PURPOSE

The role of dopamine D₁-type receptor (D₁R)-expressing neurons in the regulation of motivated behavior and reward prediction has not yet been fully established. As a prerequisite for future research assessing D₁-mediated neuronal network regulation using simultaneous PET/MRI and D₁R-selective [¹¹C]SCH23390, this study investigated the stability of central D₁R measurements between two independent PET/MRI sessions under baseline conditions.

METHODS

Thirteen healthy volunteers (7 female, age 33 ± 13 yrs) underwent 90-min emission scans, each after 90-s bolus injection of 486 ± 16 MBq [¹¹C]SCH23390, on two separate days within 2-4 weeks using a PET/MRI system. Parametric images of D₁R distribution volume ratio (DVR) and binding potential (BP_ND) were generated by a multi-linear reference tissue model with two parameters and the cerebellar cortex as receptor-free reference region. Volume-of-interest (VOI) analysis was performed with manual VOIs drawn on consecutive transverse MRI slices for brain regions with high and low D₁R density.

RESULTS

The DVR varied from 2.5 ± 0.3 to 2.9 ± 0.5 in regions with high D₁R density (e.g. the head of the caudate) and from 1.2 ± 0.1 to 1.6 ± 0.2 in regions with low D₁R density (e.g. the prefrontal cortex). The absolute variability of the DVR ranged from 2.4% ± 1.3% to 5.1% ± 5.3%, while Bland-Altman analyses revealed very low differences in mean DVR (e.g. 0.013 ± 0.17 for the nucleus accumbens). Intraclass correlation (one-way, random) indicated very high agreement (0.93 in average) for both DVR and BP_ND values. Accordingly, the absolute variability of BP_ND ranged from 7.0% ± 4.7% to 12.5% ± 10.6%; however, there were regions with very low D₁R content, such as the occipital cortex, with higher mean variability.

CONCLUSION

The test-retest reliability of D₁R measurements in this study was very high. This was the case not only for D₁R-rich brain areas, but also for regions with low D₁R density. These results will provide a solid base for future joint PET/MRI data analyses in stimulation-dependent mapping of D₁R-containing neurons and their effects on projections in neuronal circuits that determine behavior.

Brain dopaminergic system changes in drug addiction: a review of positron emission tomography findings

Dopamine (DA) is considered crucial for the rewarding effects of drugs of abuse, but its role in addiction remains unclear. Positron emission tomography (PET) is the first technology used for in vivo measurement of components of the dopaminergic system in the human brain. In this article, we review the major findings from PET imaging studies on the involvement of DA in drug addiction, including presynaptic DA synthesis, vesicular monoamine transporter 2, the DA transporter, and postsynaptic DA receptors. These results have corroborated the role of DA in addiction and increased the understanding of its underlying mechanisms.

18F-FPEB, a PET radiopharmaceutical for quantifying metabotropic glutamate 5 receptors: a first-in-human study of radiochemical safety, biokinetics, and radiation dosimetry

Identification of safe and valid PET radioligands for metabotropic glutamate receptor, type 5 (mGluR5), is essential to measure changes in brain mGluR5 in neuropsychiatric disorders, to confirm central mGluR5 occupancy of drug candidates, and to guide dose selection for obtaining an optimum therapeutic window. Here we present the results of a first-in-human study assessing the safety and effectiveness of a novel PET radiopharmaceutical, (18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB), for quantifying regional brain concentrations of mGluR5.

METHODS

Quantification of whole-body biokinetics was conducted in 6 healthy adults (3 men and 3 women). The radiation safety profile was estimated with OLINDA/EXM software. Subsequently, pairs of dynamic brain scans were obtained for 11 healthy men to identify optimal methods for derivation of regional distribution volume and binding potential and to determine the repeatability of measurement.

RESULTS

The whole-body effective radiation dose was approximately 17 μSv/MBq (62 mrem/mCi), with the gallbladder receiving the highest dose of 190 μSv/MBq. In brain studies, time-activity curves showed high accumulation in the insula/caudate nucleus, moderate uptake in the thalamus, and the lowest concentration in the cerebellum/pons. The plasma reference graphical analysis method appeared optimal for (18)F-FPEB; it showed acceptable test-retest variability of nondisplaceable binding potential (<10%) and identified the highest nondisplaceable binding potential values (from ∼0.5 in the globus pallidus to ∼3.5 in the insula) for target regions. Safety assessments revealed no clinically meaningful changes in vital signs, electrocardiogram, or laboratory values.

CONCLUSION

(18)F-FPEB is safe and well tolerated, and its regional cerebral distribution is consistent with previous reports in the literature for metabotropic glutamate receptors. The repeatability of measurement suggests that (18)F-FPEB is suitable for quantifying mGluR5 in humans.

Positron emission tomography molecular imaging of dopaminergic system in drug addiction

Dopamine (DA) is involved in drug reinforcement, but its role in drug addiction remains unclear. Positron emission tomography (PET) is the first technology used for the direct measurement of components of the dopaminergic system in the living human brain. In this article, we reviewed the major findings of PET imaging studies on the involvement of DA in drug addiction, especially in heroin addiction. Furthermore, we summarized PET radiotracers that have been used to study the role of DA in drug addiction. To investigate presynaptic function in drug addiction, PET tracers have been developed to measure DA synthesis and transport. For the investigation of postsynaptic function, several radioligands targeting dopamine one (D1) receptor and dopamine two (D2) receptor are extensively used in PET imaging studies. Moreover, we also summarized the PET imaging findings of heroin addiction studies, including heroin-induced DA increases and the reinforcement, role of DA in the long-term effects of heroin abuse, DA and vulnerability to heroin abuse and the treatment implications. PET imaging studies have corroborated the role of DA in drug addiction and increase our understanding the mechanism of drug addiction.

Positron emission tomography imaging of dopamine D₂/₃ receptors in the human cortex with [¹¹C]FLB 457: reproducibility studies

In a recent PET study, we demonstrated the ability to measure amphetamine-induced DA release in the human cortex with the relatively high affinity dopamine D₂/₃ radioligand [¹¹C]FLB 457 (Narendran et al., [2009] Synapse 63:447-461). The aim of this study was to evaluate the reproducibility and reliability of [¹¹C]FLB 457 in the same imaging paradigm we used to measure amphetamine-induced DA transmission. Six healthy human subjects (three males/three females)were studied twice with [¹¹C]FLB 457, once at baseline and again 3 h following the end of the baseline scan. D₂/₃ receptor binding parameters were estimated using a two-tissue compartment kinetic analysis in the cortical regions of interest and cerebellum (reference region). The test-retest variability and intraclass correlation coefficient were assessed for distribution volume (VT), binding potential relative to plasma concentration (BP(P)), and binding potential relative to non-displaceable uptake (BP(ND)) of [¹¹C]FLB 457. The test-retest variability of [¹¹C]FLB 457 VT, BPP, and BP(ND) were ≤15%, consistent with the published test-retest variability for this ligand in other brain regions (Sudo et al., [2001] Nucl Med Commun 22:1215-1221; Vilkman et al., [2000] Eur J Nucl Med 27:1666-1673). In addition, no significant decrease in [¹¹C]FLB457 BP(ND) was observed in the second scan compared to the first one. This suggests that the contribution of carryover mass of [¹¹C]FLB 457 to the measured reduction in[¹¹C]FLB 457 BP(ND) following amphetamine was relatively low. These data support the further validation of [¹¹C]FLB 457 as a tool to measure amphetamine-induced dopamine release in the human cortex.

Dopamine D₁ receptors and nonlinear probability weighting in risky choice

Misestimating risk could lead to disadvantaged choices such as initiation of drug use (or gambling) and transition to regular drug use (or gambling). Although the normative theory in decision-making under risks assumes that people typically take the probability-weighted expectation over possible utilities, experimental studies of choices among risks suggest that outcome probabilities are transformed nonlinearly into subjective decision weights by a nonlinear weighting function that overweights low probabilities and underweights high probabilities. Recent studies have revealed the neurocognitive mechanism of decision-making under risk. However, the role of modulatory neurotransmission in this process remains unclear. Using positron emission tomography, we directly investigated whether dopamine D₁ and D₂ receptors in the brain are associated with transformation of probabilities into decision weights in healthy volunteers. The binding of striatal D₁ receptors is negatively correlated with the degree of nonlinearity of weighting function. Individuals with lower striatal D₁ receptor density showed more pronounced overestimation of low probabilities and underestimation of high probabilities. This finding should contribute to a better understanding of the molecular mechanism of risky choice, and extreme or impaired decision-making observed in drug and gambling addiction.

Dopamine D1 Receptor Imaging in the Rodent and Primate Brain Using the Isoquinoline (+)-[11C]A-69024 and Positron Emission Tomography

In vivo pharmacokinetic and brain binding characteristics of (+)-[(11)C]A-69024, a high-affinity-D1-selective dopamine receptor antagonist, were assessed with micro-PET and beta-microprobes in the rat and PET in the baboon. The biodistribution of (+)-[(11)C]A-69024 in rats and baboons showed a rapid brain uptake (reaching a maximal value at 5 and 15 min postinjection in rats and baboons, respectively), followed by a slow wash out. The region/cerebellum concentration ratio was characterized by a fourfold higher uptake in striatum and a twofold higher uptake in cortical regions, consistent with in vivo specific binding of the radiotracer in these cerebral regions. Furthermore, this specific (+)-[(11)C]A-69024 binding significantly correlated with the reported in vitro distribution of dopamine D1-receptors. Finally, the specific uptake of the tracer in the striatum and cortical regions was completely prevented by either a pretreatment with large doses of nonradioactive (+/-)A-69024 or of the D1-selective antagonist SCH23390, resulting in a similar uptake in the reference region (cerebellum) and in other brain regions. Thus, (+)-[(11)C]A-69024 appears to be a specific and enantioselective radioligand to visualize and quantify brain dopamine D1 receptors in vivo using positron emission tomography.

Balancing bias, reliability, noise properties and the need for parametric maps in quantitative ligand PET: [(11)C]diprenorphine test-retest data

[(11)C]diprenorphine (DPN) is a non-subtype selective opioid receptor PET ligand with slow kinetics and no region devoid of specific binding. Parametric maps are desirable but have to overcome high noise at the voxel level. We obtained parameter values, parametric map image quality, test-retest reproducibility and reliability (using intraclass correlation coefficients (ICCs)) for conventional spectral analysis and a derived method (rank shaping), compared them with values obtained through sampling of volumes of interest (VOIs) on the dynamic data sets and tested whether smaller amounts of radioactivity injected maintained reliability. Ten subjects were injected twice with either approximately 185 MBq or approximately 135 MBq of [(11)C]DPN, followed by dynamic PET for 90 min. Data were movement corrected with a frame-to-frame co-registration method. Arterial plasma input functions corrected for radiolabelled metabolites were created. There was no overall effect of movement correction except for one subject with substantial movement whose test-retest differences decreased by approximately 50%. Actual parametric values depended heavily on the cutoff for slow frequencies (between 0.0008 s(-1) and 0.00063 s(-1)). Image quality was satisfactory for restricted base ranges when using conventional spectral analysis. The rank shaping method allowed maximising of this range but had similar bias. VOI-based methods had the widest dynamic range between regions. Average percentage test-retest differences were smallest for the parametric maps with restricted base ranges; similarly ICCs were highest for these (up to 0.86) but unacceptably low for VOI-derived VD estimates at the low doses of injected radioactivity (0.24/0.04). Our data can inform the choice of methodology for a given biological problem.

Dopamine receptor modulation of hypoxic-ischemic neuronal injury in striatum of newborn piglets

Dopamine receptors regulate glutamatergic neurotransmission and Na(+),K(+)-ATPase via protein kinase A (PKA) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32)-dependent signaling. Consequently, dopamine receptor activation may modulate neonatal hypoxic-ischemic (H-I) neuronal damage in the selectively vulnerable putamen enriched with dopaminergic receptors. Piglets subjected to two durations of hypoxia followed by asphyxic cardiac arrest were treated with a D1-like (SCH23390) or D2-like (sulpiride) receptor antagonist. At 4 days of recovery from less severe H-I, the remaining viable neurons in putamen were 60% of control, but nearly completely salvaged by pretreatment with SCH23390 or sulpiride. After more severe H-I in which only 18% of neurons were viable, partial neuroprotection was seen with SCH23390 pretreatment (50%) and posttreatment (39%) and with sulpiride pretreatment (35%), but not with sulpiride posttreatment (24%). Dopamine was significantly elevated in microdialysis samples from putamen during asphyxia and the first 15 mins of reoxygenation. Pretreatment with SCH23390 or sulpiride largely attenuated the increased nitrotyrosine and the decreased Na(+),K(+)-ATPase activity that occurred at 3 h after severe H-I. Pretreatment with SCH23390, but not sulpiride, also attenuated H-I-induced increases in PKA-dependent phosphorylation of Thr34 on DARPP-32, Ser943 on the alpha subunit of Na(+),K(+)-ATPase, and Ser897 of the N-methyl-D-aspartate (NMDA) receptor NR1 subunit. These findings indicate that D1 and D2 dopamine receptor activation contribute to neuronal death in newborn putamen after H-I in association with increased protein nitration and decreased Na(+),K(+)-ATPase activity. Furthermore, mechanisms of D1 receptor toxicity may involve DARPP-32-dependent phosphorylation of NMDA receptor NR1 and Na(+),K(+)-ATPase.

In vivo DA D(1) receptor selectivity of NNC 112 and SCH 23390

PURPOSE

[(11)C]NNC 112 and [(11)C]SCH 23390 are selective positron emission tomography (PET) tracers for visualizing dopamine D(1) receptors. It is known that both have some affinity for serotonin 2A receptors, but previous studies have suggested this is negligible compared to D(1) affinity. We sought to verify this property in vivo.

PROCEDURES

Two baboons were scanned to measure the selectivity of both tracers with a displacement paradigm. Four baboons were scanned to directly assess [(11)C] NNC 112 affinity for both receptors.

RESULTS

In vivo, D(1) to 5-HT(2A) selectivity is six to fourteenfold, not 100-fold as previously reported by other investigators.

CONCLUSION

We conclude that about 1/4 of the cortical signal of both [(11)C]NNC 112 and [(11)C]SCH 23390 is due to binding to 5-HT(2A) receptors. If confirmed in humans, this suggests caution should be exercised when drawing conclusions from studies using either tracer. These results also indicate the need for more selective tracers for the D(1) receptor.

Role of ventral striatal dopamine D1 receptor in cigarette craving

BACKGROUND

Several theories of cigarette craving suggest that dopaminergic function in the ventral striatum plays an important role. The objective of this study was to determine correlations between craving-related brain activation and dopamine D1 receptor (D1R) binding in smokers.

METHODS

Twelve smokers and 12 nonsmoking controls underwent [(15)O]H(2)O-positron emission tomography activation study and D1R-binding study using [(11)C]SCH 23390, and the correlations between receptor binding and cue-induced regional cerebral blood flow (rCBF) changes were assessed. Consecutive D1R-binding changes were examined during a period of 6 months of postsmoking abstinence in five smokers.

RESULTS

Cue-induced activation was observed in the left ventral striatum including the nucleus accumbens in smokers. D1R binding in the ventral striatum showed a negative relationship with cue-induced craving and rCBF changes. D1R binding was significantly low in smokers, and there was a trend of increase after smoking abstinence.

CONCLUSIONS

D1R binding and cue-induced rCBF changes in the ventral striatum suggest the important role of D1R in this region in cigarette craving.

Blood-brain barrier penetration of zolmitriptan--modelling of positron emission tomography data

Positron emission tomography (PET) with the drug radiolabelled allows a direct measurement of brain or other organ kinetics, information which can be essential in drug development. Usually, however, a PET-tracer is administered intravenously (i.v.), whereas the therapeutic drug is mostly given orally or by a different route to the PET-tracer. In such cases, a recalculation is needed to make the PET data representative for the alternative administration route. To investigate the blood-brain barrier penetration of a drug (zolmitriptan) using dynamic PET and by PK modelling quantify the brain concentration of the drug after the nasal administration of a therapeutic dose. [11C]Zolmitriptan at tracer dose was administered as a short i.v. infusion and the brain tissue and venous blood kinetics of [11C]zolmitriptan was measured by PET in 7 healthy volunteers. One PET study was performed before and one 30 min after the administration of 5 mg zolmitriptan as nasal spray. At each of the instances, the brain radioactivity concentration after subtraction of the vascular component was determined up to 90 min after administration and compared to venous plasma radioactivity concentration after correction for radiolabelled metabolites. Convolution methods were used to describe the relationship between arterial and venous tracer concentrations, respectively between brain and arterial tracer concentration. Finally, the impulse response functions derived from the PET studies were applied on plasma PK data to estimate the brain zolmitriptan concentration after a nasal administration of a therapeutic dose. The studies shows that the PET data on brain kinetics could well be described as the convolution of venous tracer kinetics with an impulse response including terms for arterial-to-venous plasma and arterial-to-brain impulse responses. Application of the PET derived impulse responses on the plasma PK from nasal administration demonstrated that brain PK of zolmitriptan increased with time, achieving about 0.5 mg/ml at 30 min and close to a maximum of 1.5 mg/ml after 2 hr. A significant brain concentration was observed already after 5 min. The data support the notation of a rapid brain availability of zolmitriptan after nasal administration.

Striatal dopamine D1 and D2 receptor balance in twins at increased genetic risk for schizophrenia

The dopamine hypothesis of schizophrenia postulates that a dysfunctional dopaminergic system is a major pathophysiological mechanism in the disease. Most studies have focused on striatal dopamine D2 receptors, but a disturbed link between dopamine D1 and D2 receptors has also been proposed. Schizophrenia is highly heritable, and recent evidence suggests that alterations in the dopaminergic system confer susceptibility for schizophrenia instead of being solely related to the to overt expression of the disease. To explore the impact of genetic vulnerability for schizophrenia on the balance of striatal dopamine D1 and D2 receptors, we studied monozygotic (MZ) and dizygotic (DZ) unaffected co-twins from twin pairs discordant for schizophrenia as well as healthy control twins using positron emission tomography (PET). Both [(11)C]SCH 23390 and [(11)C]raclopride were used to quantitate D1 and D2 receptor binding, respectively, in the same individuals during the same day. The association between D1 and D2 receptor binding was analyzed using conventional region of interests as well as voxel-wise D1/D2 ratio maps. All levels of analyses failed to show any differences in D1/D2 ratio between the unaffected MZ or DZ co-twins and control twins. We noted rostrocaudally declining and dorsoventrally increasing gradients in D1/D2 ratio in the striatum, with no differences between groups in these gradients. In this sample, we did not find evidence for an association between increased genetic risk for schizophrenia and altered D1/D2 receptor balance in the striatum.

Measurement of serotonin transporter binding with PET and [11C]MADAM: A test–retest reproducibility study

[(11)C]MADAM, or [(11)C]N,N-dimethyl-2-(2-amino-4-methylphenyl thio)benzylamine, is a radioligand suitable for positron emission tomography (PET) studies of the serotonin transporter (5-HTT) in man. The purpose of this study was to examine the test-retest reproducibility using a design tailored for future applied studies. Nine healthy male subjects were examined with PET and [(11)C]MADAM under baseline conditions at two occasions 4-8 weeks apart. The subjects participated in a Phase 1 trial to which the present study was an addendum. Eight regions of interest were studied, including frontal cortex, hippocampal complex, and the raphe nuclei. All regions, but the raphe nuclei, were defined on MR-images to which the PET-images were coregistered using SPM2. Binding potentials were calculated using the simplified reference tissue model, with cerebellum as reference region. Test-retest data were calculated from the binding potentials, and included binding potential (BP) quotient, BP difference, and the intraclass correlation coefficient. The quotient was about one in all regions, and the mean difference varied between 0 and 11%. The intraclass correlation coefficient varied between 0.96 and 0.51 in the raphe nuclei and averaged bilateral regions. [(11)C]MADAM was shown to have good to excellent reliability in measurements of 5-HTT binding in brain regions of interest in research on psychiatric disorders.

An application of a new planar positron imaging system (PPIS) in a small animal: MPTP-induced Parkinsonism in mouse

OBJECTIVE

Recent animal PET research has led to the development of PET scanners for small animals. A planar positron imaging system (PPIS) was newly developed to study physiological function in small animals and plants in recent years. To examine the usefulness of PPIS for functional study in small animals, we examined dopaminergic images of mouse striata in MPTP-induced parkinsonism.

METHODS

Male C57BL/6NCrj mice were treated with MPTP 7 days before the PPIS study. Scans were performed to measure dopamine D1 receptor binding and dopamine transporter availability with [11C]SCH23390 (about 2 MBq) and [11C]beta-CFT (about 2 MBq), respectively. After the PPIS study, dopamine content in the striatum was measured by HPLC.

RESULTS

The MPTP treatment significantly reduced dopamine content in the striatum 7 days after treatment. In the MPTP-treated group, [11C]beta-CFT binding in the striatum was significantly decreased compared with the control group, while striatal [11C]SCH23390 binding was not affected. Dopamine content in the striatum was significantly correlated with the striatal binding of [11C]beta-CFT.

CONCLUSION

The present results suggest that PPIS is able to determine brain function in a small animal. Using PPIS, high throughput imaging of small animal brain functions could be achieved.

Neuroreceptor imaging of stress and mood disorders

Techniques such as positron emission tomography and single photon emission computed tomography allow for the imaging of neurotransmitter receptors and transporters in the brain. These tools have been used to investigate serotonergic, dopaminergic, and opioidergic function in healthy subjects as well as in patients with major depressive disorder, bipolar disorder, and other mood disorders. Pharmacologic challenges, such as amphetamine challenge, and physiologic stressors, such as pain challenge, have been used to further examine the function of these neurotransmitter systems. Neuroimaging of patient populations before and after medication treatment may be useful to understand changes in neurotransmission that accompany disease remission. As new radiotracers with higher selectivity for the various receptors and transporters are developed, imaging techniques may provide new insights into the pathophysiology of mood disorders, leading to improved diagnosis and treatment.

Dopamine depletion and in vivo binding of PET D1 receptor radioligands: implications for imaging studies in schizophrenia

Recent positron emission tomography (PET) studies have assessed the level of dopamine (DA) D1 receptors in the prefrontal cortex (PFC) in patients with schizophrenia and have generated contradictory findings. In the PFC of patients with schizophrenia, the binding potential (BP) of [11C]NNC 112 has been reported as increased, while the BP of [11C]SCH 23390 was reported as decreased or unchanged. In this study, the effect of acute and subchronic DA depletion on the in vivo binding of [11C]NNC 112 and [3H]SCH 23390 was evaluated in rats. Acute DA depletion did not affect [11C]NNC 112 in vivo binding, but paradoxically decreased [3H]SCH 23390 in vivo binding. Subchronic DA depletion was associated with increased [11C]NNC 112 in vivo binding and decreased [3H]SCH 23390 in vivo binding. Together, these data demonstrate that the in vivo binding of these radiotracers is differentially affected by changes in endogenous DA tone, and suggest that alterations in the binding of these tracers in the PFC of patients with schizophrenia might reflect changes in D1 receptors secondary to sustained deficit in prefrontal DA function.