MR-PET image coregistration for quantitation of striatal dopamine D2 receptors

Striatal dopamine D2/3 receptors in medication-naïve schizophrenia: an [123I] IBZM SPECT study

BACKGROUND

The hyper-function of the striatal dopamine system has been suggested to underlie key pathophysiological mechanisms in schizophrenia. Moreover, patients have been observed to present a significant elevation of dopamine receptor availability compared to healthy controls. Although it is difficult to measure dopamine levels directly in humans, neurochemical imaging techniques such as single-photon emission computed tomography (SPECT) provide indirect indices of in vivo dopamine synthesis and release, and putative synaptic levels.

METHODS

We focused on the role of dopamine postsynaptic regulation using [¹²³I] iodobenzamide (IBZM) SPECT. We compared D_2/3 receptor availability between 53 healthy controls and 21 medication-naive patients with recent-onset schizophrenia.

RESULT

The mean specific striatal binding showed no significant difference between patients and controls (estimated difference = 0.001; 95% CI -0.11 to 0.11; F = 0.00, df = 1, 69; p = 0.99). There was a highly significant effect of age whereby IBZM binding declined with advancing age [estimated change per decade of age = -0.01(binding ratio); 95% CI -0.01 to -0.004; F = 11.5, df = 1, 69; p = 0.001]. No significant correlations were found between the mean specific striatal binding and psychopathological or cognitive rating scores.

CONCLUSIONS

Medication-naïve patients with recent-onset schizophrenia have similar D_2/3 receptor availability to healthy controls. We suggest that, rather than focusing exclusively on postsynaptic receptors, future treatments should target the presynaptic control of dopamine synthesis and release.

Decreased dopamine brain reactivity in marijuana abusers is associated with negative emotionality and addiction severity

Moves to legalize marijuana highlight the urgency to investigate effects of chronic marijuana in the human brain. Here, we challenged 48 participants (24 controls and 24 marijuana abusers) with methylphenidate (MP), a drug that elevates extracellular dopamine (DA) as a surrogate for probing the reactivity of the brain to DA stimulation. We compared the subjective, cardiovascular, and brain DA responses (measured with PET and [(11)C]raclopride) to MP between controls and marijuana abusers. Although baseline (placebo) measures of striatal DA D2 receptor availability did not differ between groups, the marijuana abusers showed markedly blunted responses when challenged with MP. Specifically, compared with controls, marijuana abusers had significantly attenuated behavioral ("self-reports" for high, drug effects, anxiety, and restlessness), cardiovascular (pulse rate and diastolic blood pressure), and brain DA [reduced decreases in distribution volumes (DVs) of [(11)C]raclopride, although normal reductions in striatal nondisplaceable binding potential (BPND)] responses to MP. In ventral striatum (key brain reward region), MP-induced reductions in DVs and BPND (reflecting DA increases) were inversely correlated with scores of negative emotionality, which were significantly higher for marijuana abusers than controls. In marijuana abusers, DA responses in ventral striatum were also inversely correlated with addiction severity and craving. The attenuated responses to MP, including reduced decreases in striatal DVs, are consistent with decreased brain reactivity to the DA stimulation in marijuana abusers that might contribute to their negative emotionality (increased stress reactivity and irritability) and addictive behaviors.

Striatal dopamine transporter availability in drug-naive patients with schizophrenia: a case-control SPECT study with [(99m)Tc]-TRODAT-1 and a meta-analysis

Central dopaminergic hyperactivity has been one of the main hypotheses of the pathophysiology of schizophrenia since the 1970s. Excess dopamine (DA) neurotransmission in the striatum is hypothesized to alter the processing of information and result in psychotic symptoms in schizophrenia. Single photon emission computerized tomography (SPECT) provides in vivo indices of DA neurotransmission. Our study aimed to compare dopamine transporter (DAT) availability between drug-naive patients with schizophrenia and controls using SPECT. DAT availability through [(99m)Tc]-TRODAT-1 SPECT was compared between 47 drug-naive patients with recent-onset schizophrenia and 112 healthy controls. We also conducted a random-effects meta-analysis of the available literature synthesizing the results of 6 comparable published articles as well as our current data. The mean specific striatal binding showed a statistical trend for a reduction among the patients compared with controls (estimated difference = 0.071; 95% CI -0.01, 0.15; P = .08). There was an effect of gender, whereby females had a higher ratio of specific striatal binding than males. Age was negatively correlated with the ratio of specific striatal binding, both in patients and controls. The meta-analysis provided a pooled standardized effect size (Cohen's d) of -0.07 (95% CI -0.31, 0.18; P = .60) for the patient vs control comparison in TRODAT binding, with no evidence of heterogeneity between studies or publication bias. Our findings suggest that striatal DAT levels are not altered in the early stages of schizophrenia before medication is introduced. We identified gender differences and aging effects that could have significance for future studies.

Methylphenidate-elicited dopamine increases in ventral striatum are associated with long-term symptom improvement in adults with attention deficit hyperactivity disorder

Stimulant medications, such as methylphenidate, which are effective treatments for attention deficit hyperactivity disorder (ADHD), enhance brain dopamine signaling. However, the relationship between regional brain dopamine enhancement and treatment response has not been evaluated. Here, we assessed whether the dopamine increases elicited by methylphenidate are associated with long-term clinical response. We used a prospective design to study 20 treatment-naive adults with ADHD who were evaluated before treatment initiation and after 12 months of clinical treatment with a titrated regimen of oral methylphenidate. Methylphenidate-induced dopamine changes were evaluated with positron emission tomography and [(11)C]raclopride (D(2)/D(3) receptor radioligand sensitive to competition with endogenous dopamine). Clinical responses were assessed using the Conners' Adult ADHD Rating Scale and revealed a significant reduction in symptoms of inattention and hyperactivity with long-term methylphenidate treatment. A challenge dose of 0.5 mg/kg intravenous methylphenidate significantly increased dopamine in striatum (assessed as decreases in D(2)/D(3) receptor availability). In the ventral striatum, these dopamine increases were associated with the reductions in ratings of symptoms of inattention with clinical treatment. Statistical parametric mapping additionally showed dopamine increases in prefrontal and temporal cortices with intravenous methylphenidate that were also associated with decreases in symptoms of inattention. Our findings indicate that dopamine enhancement in ventral striatum (the brain region involved with reward and motivation) was associated with therapeutic response to methylphenidate, further corroborating the relevance of the dopamine reward/motivation circuitry in ADHD. It also provides preliminary evidence that methylphenidate-elicited dopamine increases in prefrontal and temporal cortices may also contribute to the clinical response.

Dopaminergic activity in depressed smokers: a positron emission tomography study

Tobacco dependence is highly prevalent in depressed patients. We assessed changes in [(11)C]-raclopride binding potential (BP) using positron emission tomography (PET) before and after the oral administration of d-amphetamine in healthy controls and unmedicated patients with current depression with and without current tobacco dependence. Over a single study day 2 [(11)C]-raclopride positron emission tomography scans were taken in 38 subjects: at baseline and 2 h following oral d-amphetamine 30 mg. Twenty controls (9 smokers, 11 nonsmokers) and 18 subjects with current major depressive episode (8 smokers, 10 non-smokers). Striatal [(11)C]-raclopride binding potential was measured before and after d-amphetamine administration. Depressed smokers had a lower baseline [(11)C]-raclopride binding potential compared with both control non-smokers (P < 0.007) and depressed non-smokers (P < 0.001). There was a main effect of smoking status on amphetamine-induced change in [(11)C]-raclopride binding potential (P < 0.02), but no main effect of depression. This may be due to a floor effect because of the low BP at baseline. Depressed subjects reported significant increase of positive mood after d-amphetamine administration compared with controls (depressed smokers vs. control smokers: P < 0.05; depressed non-smokers vs. controls: P < 0.055). Tobacco dependence appears to decrease d-amphetamine-induced changes in [(11)C]-raclopride binding potential as measured by positron emission tomography. Comorbid major depression and tobacco dependence exacerbates this effect, suggesting an altered dopamine system in comorbid patients.

Striatal dopamine predicts outcome-specific reversal learning and its sensitivity to dopaminergic drug administration

Individual variability in reward-based learning has been ascribed to quantitative variation in baseline levels of striatal dopamine. However, direct evidence for this pervasive hypothesis has hitherto been unavailable. We demonstrate that individual differences in reward-based reversal learning reflect variation in baseline striatal dopamine synthesis capacity, as measured with neurochemical positron emission tomography. Subjects with high baseline dopamine synthesis in the striatum showed relatively better reversal learning from unexpected rewards than from unexpected punishments, whereas subjects with low baseline dopamine synthesis in the striatum showed the reverse pattern. In addition, baseline dopamine synthesis predicted the direction of dopaminergic drug effects. The D(2) receptor agonist bromocriptine improved reward-based relative to punishment-based reversal learning in subjects with low baseline dopamine synthesis capacity, while impairing it in subjects with high baseline dopamine synthesis capacity in the striatum. Finally, this pattern of drug effects was outcome-specific, and driven primarily by drug effects on punishment-, but not reward-based reversal learning. These data demonstrate that the effects of D(2) receptor stimulation on reversal learning in humans depend on task demands and baseline striatal dopamine synthesis capacity.

Striatal dopamine and working memory

Recent studies have emphasized the importance of dopamine projections to the prefrontal cortex (PFC) for working memory (WM) function, although this system has rarely been studied in humans in vivo. However, dopamine and PFC activity can be directly measured with positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), respectively. In this study, we examined WM capacity, dopamine, and PFC function in healthy older participants in order to test the hypothesis that there is a relationship between these 3 factors. We used the PET tracer 6-[18F]fluoro-L-m-tyrosine to measure dopamine synthesis capacity in the striatum (caudate, putamen), and event-related fMRI to measure brain activation during different epochs (cue, delay, probe) of a WM task. Caudate (but not putamen) dopamine correlated positively with WM capacity, whereas putamen (but not caudate) dopamine correlated positively with motor speed. In addition, delay-related fMRI activation in a left inferior prefrontal region was related to both caudate dopamine and task accuracy, suggesting that this may be a critical site for the integration of WM maintenance processes. These results provide new evidence that striatal dopaminergic function is related to PFC-dependent functions, particularly brain activation and behavioral performance during WM tasks.

Working memory capacity predicts dopamine synthesis capacity in the human striatum

Evidence from psychopharmacological research has revealed that dopamine receptor agents have opposite effects on cognitive function depending on baseline levels of working memory capacity. These contrasting effects have been interpreted to reflect differential baseline levels of dopamine. Here we demonstrate for the first time that working memory capacity as measured by listening span predicts dopamine synthesis capacity in the striatum, indicating that subjects with low working memory capacity have low DA synthesis capacity in the striatum, whereas subjects with high working memory capacity have high DA synthesis capacity in the striatum.

Striatal amphetamine-induced dopamine release in patients with schizotypal personality disorder studied with single photon emission computed tomography and [123I]iodobenzamide

BACKGROUND

Previous imaging studies demonstrated that schizophrenia is associated with increased amphetamine-induced dopamine (DA) release in the striatum, most pronounced during episodes of illness exacerbation. Schizotypal personality disorder (SPD) is a schizophrenia spectrum disorder, genetically related to schizophrenia. The goal of this study was to investigate striatal DA function in patients with SPD.

METHODS

In our study, 13 SPD patients and 13 matched healthy control subjects underwent single photon emission computed tomography (SPECT) scan during bolus plus constant infusion of the D2/3 radiotracer [123I]iodobenzamide (IBZM). Striatal specific to nonspecific equilibrium partition coefficient (V(3)") was measured at baseline and following amphetamine administration (.3 mg/kg).

RESULTS

No significant differences were observed in baseline V(3)" between groups. Amphetamine induced a larger decrease in [123I]IBZM V(3)" in SPD patients (-12 +/- 5%) compared with control subjects (-7 +/- 5%, p =.03).

CONCLUSIONS

The reduction in [123I]IBZM V(3)" induced by amphetamine in SPD was similar to that observed in remitted schizophrenia patients (-10 +/- 9%, n = 17), but significantly lower than that observed during illness exacerbation (-24 +/- 13%, n = 17). This suggests that DA dysregulation in schizophrenia spectrum disorders might have a trait component, present in remitted patients with schizophrenia and in SPD, and a state component, associated with psychotic exacerbations but not SPD.

Expectation of caffeine induces dopaminergic responses in humans

Recent neuroimaging studies indicate that placebo treatments can induce clinically relevant neurobiological responses in patients with Parkinson's disease, depression and pain. The present study aimed to investigate neurotransmitter function in psychostimulant expectation, with the focus on dopaminergic effects of placebo caffeine in healthy human subjects. Eight habitual coffee drinkers were examined twice with [11C]raclopride positron emission tomography after no treatment and after oral placebo tablets in a counter-balanced setting. During the placebo condition the subjects were instructed that they had a 50% chance of receiving caffeine, but all received placebo. As compared with no treatment, placebo induced a significant bilateral dopamine release in the thalamus, as reflected by a 15% reduction in thalamic [11C]raclopride binding (P < 0.001). The level of arousal after placebo correlated positively with the tracer binding in the putamen (r = -0.91, P = 0.004). The results indicate that caffeine expectation induces dopaminergic placebo effects, and that these effects are similar to previous findings with oral caffeine. The results therefore suggest that caffeine and placebo caffeine may share some dopaminergic mechanisms of action.

Dopaminergic effects of caffeine in the human striatum and thalamus

Epidemiological studies have provided evidence that caffeine, an adenosine receptor antagonist, reduces the risk for Parkinson's disease. There are indications of specific interactions between striatal adenosine A(2A) and dopamine D(2) receptors, but the in vivo effects of caffeine on human dopamine system have not been investigated. In the present study, the dopaminergic effects of caffeine were examined with [(11)C]raclopride positron emission tomography (PET) in eight healthy habitual coffee drinkers after 24 h caffeine abstinence. Compared to oral placebo, 200 mg oral caffeine induced a 12% decrease in midline thalamic binding potential (p < 0.001). A trend-level increase in ventral striatal [(11)C]raclopride binding potential was seen with a correlation between caffeine-related arousal and putaminal dopamine D(2) receptor binding (r = -0.81, p = 0.03). The findings indicate that caffeine has effects on dopaminergic neurotransmission in the human brain, which may be differential in the striatum and the thalamus.

OralD-amphetamine causes prolonged displacement of [11C]raclopride as measured by PET

Parenterally administered D-amphetamine has been used as a challenge drug to release dopamine, which in turns inhibits [11C]raclopride binding to dopaminergic D2 receptors as measured using positron emission tomography (PET) techniques. The primary objective of this study was to determine whether orally administered D-amphetamine would inhibit [11C]raclopride binding in a manner similar to that produced by intravenously administered D-amphetamine. The secondary objective was to assess the timeline of these effects. Twelve healthy human volunteers participated in this study. Subjects were scanned at baseline and 2 h after D-amphetamine administration (n = 5); at baseline, 2 and 6 h postdrug (n = 4); or at baseline, 2 and 24 h postdrug (n = 3). Orally administered D-amphetamine caused a significant decrease in [11C]raclopride binding at 2 h (13% +/- 5%). Receptor availability was still decreased at 6 h (18% +/- 6%), even though physiological effects had completely returned to baseline. [11C]Raclopride binding returned to baseline at 24 h. The percentage of [11C]raclopride displacement was not correlated with plasma D-amphetamine concentrations. In conclusion, orally administered D-amphetamine caused a reliable and prolonged [11C]raclopride displacement, the magnitude of which is similar to that observed after intravenous administration. Possible mechanisms for the observed prolonged displacement may include persistence of intrasynaptic dopamine and/or receptor internalization.

Dopamine release in human ventral striatum and expectation of reward

Using the ability of [11C]raclopride to compete with dopamine for D(2)/D(3) receptors, we investigated by positron emission tomography the effect of placebo (saline) injection on dopamine release in the ventral striatum of patients with Parkinson's disease. We found evidence for placebo-induced dopamine release of similar magnitude to that reported in healthy volunteers after amphetamine administration. However, in contrast to the dorsal striatum, there were no differences in [11C]raclopride binding potential changes between patients who experienced the reward (those who reported placebo-induced clinical benefit) and those who did not. We conclude that the release of dopamine in the ventral striatum (nucleus accumbens) is related to the expectation of reward and not to the reward itself. These observations have potential implications for the treatment of drug addiction.

Measurement of striatal and extrastriatal dopamine D1 receptor binding potential with [11C]NNC 112 in humans: validation and reproducibility

To evaluate the postulated role of extrastriatal D1 receptors in human cognition and psychopathology requires an accurate and reliable method for quantification of these receptors in the living human brain. [11C]NNC 112 is a promising novel radiotracer for positron emission tomography imaging of the D1 receptor. The goal of this study was to develop and evaluate methods to derive D1 receptor parameters in striatal and extrastriatal regions of the human brain with [11C]NNC 112. Six healthy volunteers were studied twice. Two methods of analysis (kinetic and graphical) were applied to 12 regions (neocortical, limbic, and subcortical regions) to derive four outcome measures: total distribution volume, distribution volume ratio, binding potential (BP), and specific-to-nonspecific equilibrium partition coefficient (k3/k4). Both kinetic and graphic analyses provided BP and k3/k4 values in good agreement with the known distribution of D1 receptors (striatum > limbic regions = neocortical regions > thalamus). The identifiability of outcome measures derived by kinetic analysis was excellent. Time-stability analysis indicated that 90 minutes of data collection generated stable outcome measures. Derivation of BP and k3/k4 by kinetic analysis was highly reliable, with intraclass correlation coefficients (ICCs) of 0.90+/-0.06 (mean +/- SD of 12 regions) and 0.84+/-0.11, respectively. The reliability of these parameters derived by graphical analysis was lower, with ICCs of 0.72+/-0.17 and 0.58+/-0.21, respectively. Noise analysis revealed a noise-dependent bias in the graphical but not the kinetic analysis. In conclusion, kinetic analysis of [11C]NNC 112 uptake provides an appropriate method with which to derive D1 receptor parameters in regions with both high (striatal) and low (extrastriatal) D1 receptor density.

Intimate combination of low- and high-resolution image data: I. Real-space PET and (1)H(2)O MRI, PETAMRI

Two different types of (co-registered) images of the same slice of tissue will generally have different spatial resolutions. The judicious pixel-by-pixel combination of their data can be accomplished to yield a single image exhibiting properties of both. Here, axial (18)FDG PET and (1)H(2)O MR images of the human brain are used as the low- and high-resolution members of the pair. A color scale is necessary in order to provide for separate intensity parameters from the two image types. However, not all color scales can accommodate this separability. The HSV color model allows one to choose a color scale in which the intensity of the low-resolution image type is coded as hue, while that of the high-resolution type is coded as value, a reasonably independent parameter. Furthermore, the high-resolution image must have high contrast and be quantitative in the same sense as the low-resolution image almost always is. Here, relaxographic MR images (naturally segmented quantitative (1)H(2)O spin-density components) are used. Their essentially complete contrast serves to effect an apparent editing function when encoded as the value of the color scale. Thus, the combination of (18)FDG PET images with gray-matter (GM) relaxographic (1)H(2)O images produces visually "GM-edited" (18)FDG PETAMR (positron emission tomography and magnetic resonance) images. These exhibit the high sensitivity to tracer amounts characteristic of PET along with the high spatial resolution of (1)H(2)O MRI. At the same time, however, they retain the complete quantitative measures of each of their basis images. Magn Reson Med 42:345-360, 1999. Published 1999 Wiley-Liss, Inc.

In vivo imaging of the spinal cord cholinergic system with PET

PURPOSE

Our goal was to demonstrate the feasibility of an in vivo noninvasive method for imaging spinal cord cholinergic terminals using (+)-4-[18F]fluorobenzyltrozamicol ([18F]FBT) and PET.

METHOD

In vitro and in vivo experiments in rats were conducted to demonstrate the specific binding characteristics, localization, and time course of [3H]FBT binding in the spinal cord. PET imaging was then performed on seven rhesus monkeys.

RESULTS

The rat studies demonstrate high specific binding in the spinal cord with a distribution coinciding with the known distribution of cholinergic terminals. In vivo tracer concentrations in the spinal cord and basal ganglia were of the same magnitude. With use of [18F]FBT and PET in the rhesus monkey, the spinal cord was clearly visualized, with tracer concentration in the spinal cord being approximately one-fourth of that seen in the basal ganglia.

CONCLUSION

This work demonstrates the feasibility of imaging cholinergic terminals in vivo in the spinal cord using [18F]FBT and PET.

A survey of medical image registration

The purpose of this paper is to present a survey of recent (published in 1993 or later) publications concerning medical image registration techniques. These publications will be classified according to a model based on nine salient criteria, the main dichotomy of which is extrinsic versus intrinsic methods. The statistics of the classification show definite trends in the evolving registration techniques, which will be discussed. At this moment, the bulk of interesting intrinsic methods is based on either segmented points or surfaces, or on techniques endeavouring to use the full information content of the images involved.