Stereotaxic summation analysis of human cerebral benzodiazepine binding maps

Serotonin transporter density in isolated rapid eye movement sleep behavioral disorder

Background/objective

The serotoninergic nervous system is known to play a role in the maintenance of rapid eye movement (REM) sleep. Serotoninergic projections are known to be vulnerable in synucleinopathies. To date, positron emission tomography (PET) studies using serotonin-specific tracers have not been reported in isolated REM sleep behavior disorder (iRBD).

Methods

We conducted a cross-sectional imaging study using serotonin transporter (SERT) 11C-3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitrile (DASB) PET to identify differences in serotonin system integrity between 11 participants with iRBD and 16 older healthy controls.

Results

Participants with iRBD showed lower DASB distribution volume ratios (DVRs) in the total neocortical mantle [1.13 (SD: 0.07) vs. 1.19 (SD: 0.06); t = 2.33, p = 0.028)], putamen [2.07 (SD: 0.19) vs. 2.25 (SD: 0.18); t = 2.55, p = 0.017], and insula [1.26 (SD: 0.11) vs. 1.39 (SD: 0.09); t = 3.58, p = 0.001]. Paradoxical increases relative to controls were seen in cerebellar hemispheres [0.98 (SD: 0.04) vs. 0.95 (SD: 0.02); t = 2.93, p = 0.007)]. No intergroup differences were seen in caudate, substantia nigra, or other brainstem regions with the exception of the dorsal mesencephalic raphe [3.08 (SD: 0.53) vs. 3.47 (SD: 0.48); t = 2.00, p = 0.056] that showed a non-significant trend toward lower values in iRBD.

Conclusions

Insular, neocortical, and striatal serotoninergic terminal loss may be common in prodromal synucleinopathies before the onset of parkinsonism or dementia. Given our small sample size, these results should be interpreted as hypothesis-generating/exploratory in nature.

Serotonin Transporter Imaging in Multiple System Atrophy and Parkinson's Disease

BACKGROUND

Both Parkinson's disease (PD) and multiple system atrophy (MSA) exhibit degeneration of brainstem serotoninergic nuclei, affecting multiple subcortical and cortical serotoninergic projections. In MSA, medullary serotoninergic neuron pathology is well documented, but serotonin system changes throughout the rest of the brain are less well characterized.

OBJECTIVES

To use serotonin transporter [¹¹ C]3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitrile positron emission tomography (PET) to compare serotoninergic innervation in patients with MSA and PD.

METHODS

We performed serotonin transporter PET imaging in 18 patients with MSA, 23 patients with PD, and 16 healthy controls to explore differences in brainstem, subcortical, and cortical regions of interest.

RESULTS

Patients with MSA showed lower serotonin transporter distribution volume ratios compared with patients with PD in the medulla, raphe pontis, ventral striatum, limbic cortex, and thalamic regions, but no differences in the dorsal striatal, ventral anterior cingulate, or total cortical regions. Controls showed greater cortical serotonin transporter binding compared with PD or MSA groups but lower serotonin transporter binding in the striatum and other relevant basal ganglia regions. There were no regional differences in binding between patients with MSA-parkinsonian subtype (n = 8) and patients with MSA-cerebellar subtype (n = 10). Serotonin transporter distribution volume ratios in multiple different regions of interest showed an inverse correlation with the severity of Movement Disorders Society Unified Parkinson's Disease Rating Scale motor score in patients with MSA but not patients with PD.

CONCLUSIONS

Brainstem and some forebrain subcortical region serotoninergic deficits are more severe in MSA compared with PD and show an MSA-specific correlation with the severity of motor impairments. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

CNS Assessment Using Functional Neuro-PET Imaging

Disorders of the brain manifest in a variety of manners ranging from feeling or thought abnormalities to total paralysis. Until recently, most imaging methods of the brain have been limited to anatomic considerations, with little information about actual function of the brain except that deduced from clinical examination and physical and cognitive assessment testing. With the advent of positron emission tomography (PET) and enhanced computer and scintigraphic image detection systems, there is keen interest in applying this imaging technique to better understand brain physiology and pathophysiology. Potential applications of PET in CNS assessment is expanding avenues for improved diagnosis and staging of disease, as well as monitoring therapeutic interventions. A general review of the radiopharmaceuticals used for neuro-PET imaging, as well as their application in situations such as cerebrovascular disease, brain activation studies, various movement disorders and dementias, depression, epilepsy, obsessive-compulsive disorder, schizophrenia, and neuropharmacology (including cerebral receptor studies) will be presented.

Differentiating Alzheimer's disease from dementia with Lewy bodies and Parkinson's disease with (+)-[11C]dihydrotetrabenazine positron emission tomography

BACKGROUND

Several progressive neurologic disorders begin with cognitive decline or parkinsonism, notably Alzheimer's disease (AD), Parkinson's disease (PD), and dementia with Lewy bodies (DLB). We used positron emission tomography (PET) in attempts to differentiate these disorders.

METHODS

We performed PET with (+)-[11C]dihydrotetrabenazine ([11C]DTBZ) to examine blood-to-brain ligand transport (K(1)) and striatal monoaminergic presynaptic binding (distribution volume [DV]) in 25 DLB, 30 PD, and 25 AD patients and 57 elderly controls (NC).

RESULTS

[11C]DTBZ DV was decreased significantly in caudate nucleus, anterior putamen, and posterior putamen in DLB and PD compared with AD and NC. DLB and PD groups showed an anterior-to-posterior gradient of binding loss relative to NC, least in caudate nucleus and largest in posterior putamen. The gradient was significantly steeper in PD than DLB. Both PD and DLB showed significantly greater interhemispheric striatal binding asymmetry than NC, and PD had greater asymmetry than DLB. Cerebral cortical [11C]DTBZ K(1) was decreased diffusely by 4% to 8% in PD. Larger K(1) deficits occurred in AD and DLB temporoparietal and prefrontal association cortices and posterior cingulate cortex. Greater reduction of K(1) occurred in occipital cortex in DLB than AD. Receiver operating characteristic curve analyses distinguished DLB from AD more effectively on the basis of striatal DV than occipital K(1) and distinguished DLB from PD more effectively on the basis of cerebral cortical K(1) than striatal DV patterns. Overall, 90% of cases were properly classified by combining these measures.

CONCLUSIONS

PET with [11C]DTBZ can differentiate DLB from both PD and AD in a single neuroimaging study.

Receptor binding techniques

This overview first discusses issues relating to the selection of radioligand for receptor binding assays, including the isotopic label and considerations pertaining to the pharmacological and chemical profile of the ligand. This is followed by a section on characterization of ligand-binding assays, starting with tissue preparation methods, followed by detection of specific binding, determination of incubation and washing conditions and a discussion of saturation and competition assay formats. Quantification of the assay results can be accomplished by autoradiography or film densitometry. Finally, methods and considerations for analysis of the resulting data are presented.

Dose effects of triazolam on brain activity during episodic memory encoding: a PET study

RATIONALE

Although it is well established that acute benzodiazepine administration impairs episodic memory encoding, little is known about the neuroanatomical substrates of this effect.

OBJECTIVE

The objective was to examine the acute dose effects of the benzodiazepine hypnotic triazolam on brain activity during episodic memory encoding.

METHODS

After oral capsule administration (placebo, 0.1, 0.2, and 0.4 mg/70 kg triazolam), regional cerebral blood flow (rCBF) was measured using positron emission tomography (PET) with 15O-H2O during performance of semantic categorization and orthographic categorization tasks in a double-blind, within-subject design in 12 healthy volunteers. The rCBF associated with episodic memory encoding was measured by subtracting the rCBF during orthographic categorization from that during semantic categorization and by examining correlations between brain activity during encoding and subsequent recognition memory performance.

RESULTS

Results in the placebo condition replicated those of nonpharmacological encoding studies, including activation in left ventrolateral prefrontal cortex. Correlations between brain activity and subsequent memory performance additionally showed medial temporal activation. Triazolam produced dose-related impairment in memory performance and dose-related deactivation in encoding-associated areas including right prefrontal cortex, left parahippocampal gyrus, and left anterior cingulate cortex.

CONCLUSIONS

Results are consistent with behavioral evidence that benzodiazepines impair prefrontal control processes as well as contextual memory and episodic binding processes thought to be controlled by the medial temporal lobe. In addition to elucidating the brain mechanisms underlying these benzodiazepine-induced behavioral deficits, results of this study also help validate hypotheses generated in nonpharmacological neuroimaging studies regarding the processes controlled by these brain regions.

Positron emission tomography of monoaminergic vesicular binding in aging and Parkinson disease

The type-2 vesicular monoamine transporter (VMAT2) might serve as an objective biomarker of Parkinson disease (PD) severity. Thirty-one subjects with early-stage PD and 75 normal subjects underwent continuous intravenous infusion of (+)-[(11)C]dihydrotetrabenazine (DTBZ) and positron emission tomography (PET) imaging to estimate the striatal VMAT2 binding site density with equilibrium tracer modeling. Parkinson disease patients were evaluated clinically in the practically defined 'off' state with the Unified Parkinson Disease Rating Scale (UPDRS), the Hoehn and Yahr Scale (HY), and the Schwab and England Activities of Daily Living Scale (SE). In normal subjects there was age-related decline in striatal DTBZ binding, approximating 0.5% per year. In PD subjects, specific DTBZ binding was reduced in the caudate nucleus (CD; -44%), anterior putamen (-68%), and posterior putamen (PP; -77%). The PP-to-CD ratio of binding was reduced significantly in PD subjects. Dihydrotetrabenazine binding was also reduced by approximately 50% in the PD substantia nigra. Striatal binding reductions correlated significantly with PD duration and SE scores, but not with HY stage or with UPDRS motor subscale (UPDRS(III)) scores. Striatal and midbrain DTBZ binding was asymmetric in PD subjects, with greatest reductions contralateral to the most clinically affected limbs. There was significant correlation between asymmetry of DTBZ binding and clinical asymmetry measured with the UPDRS(III). In HY stage 1 and 1.5 subjects (n=16), PP DTBZ binding contralateral to the clinically unaffected body side was reduced by 73%, indicating substantial preclinical nigrostriatal pathology in PD. We conclude that (+)-[(11)C]DTBZ-PET imaging displays many properties necessary of a PD biomarker.

GABAA receptor abnormalities in Prader–Willi syndrome assessed with positron emission tomography and [11C]flumazenil

Prader-Willi syndrome (PWS) is a multi-system disorder characterized clinically by abnormal mental and physical development. PWS patients have a deletion in an imprinted region on paternal chromosome 15 (15q11-13), maternal disomy for this segment, or rarely, a chromosomal imprinting center deletion that gives rise to suppression of the equivalent paternal genes. Within the affected segment of chromosome 15 are genes encoding the alpha(5), beta(3) and gamma(3) subunits of the gamma-aminobutyric acid type-A (GABA(A)) receptor. Therefore, altered neurobehavioral function could arise in PWS due directly to altered GABA(A) receptor composition and expression, or alternatively, from brain developmental and maturational effects of these or other genes in the imprinted region. The aim of the present study was to assess cerebral GABA(A) receptors in PWS with the use of positron emission tomography of the benzodiazepine binding site employing [11C]flumazenil ([11C]FMZ). A reduction in [11C]FMZ binding was found predominantly in the cingulate, frontal and temporal neocortices and insula in six adult PWS patients compared to nine normal subjects. A possible role for the deleted beta(3) subunit gene in PWS is supported in part by the wide cortical distribution of its mRNA expression and the effects of experimental knockouts on benzodiazepine binding described in prior studies. Altered GABA(A) receptor composition or number in these cortical regions may account for neurobehavioral abnormalities in PWS including mild mental retardation, poor impulse control, and impaired responses to somatic pain.

Vesicular monoamine transporter concentrations in bipolar disorder type I, schizophrenia, and healthy subjects

BACKGROUND

Previous analyses of vesicular monoamine transporter (VMAT2) binding in euthymic bipolar disorder type I (BDI) patients have shown increases of this presynaptic marker in the thalamus and ventral midbrain. To assess the diagnostic specificity of those findings, we compared VMAT2 concentrations between euthymic BDI patients, patients diagnosed with schizophrenia (SCH), and age-matched healthy volunteers.

METHODS

Binding sites for VMAT2 were quantified with (+)-alpha-[11C]DTBZ (dihydrotetrabenazine) and positron emission tomography. Fifteen euthymic BDI and 12 SCH patients and 15 group-matched healthy controls were studied. [11C]DTBZ tracer transport and binding potentials were examined in the thalamus and ventral midbrain with factorial analyses of variance and post hoc Tukey's honestly significantly different tests.

RESULTS

Analysis of variance detected diagnosis effects in binding potentials in both brain regions. Binding of VMAT2 in the thalamus was higher in BDI patients than in control subjects and SCH patients. Conversely, ventral brainstem binding was nearly identical between BDI and SCH patients and were higher than in the control group.

CONCLUSIONS

The patterns of regional VMAT2 expression, and by extension, the concentration of monoaminergic synaptic terminals, differ between BDI, SCH, and a control group. These findings may relate to both similarities and differences in the presentation or clinical course of these syndromes and require further examination.

Extramotor involvement in ALS: PET studies with the GABA(A) ligand [(11)C]flumazenil

We used the benzodiazepine GABA(A) marker [(11)C] flumazenil to study cerebral dysfunction in amyotrophic lateral sclerosis (ALS) with PET. Seventeen non-demented patients with clinically definite or probable ALS were scanned and statistical parametric maps were derived to localize changes in regional flumazenil volumes of distribution (FMZVD), which correlate closely with receptor density (B(max)), and the results were compared with those of 17 controls. The ALS group showed statistically significant decreases in relative FMZVD in the prefrontal cortex (areas 9 and 10 bilaterally), parietal cortex (area 7 bilaterally), visual association cortex (area 18 bilaterally) and left motor/premotor cortex (including area 4) (P < 0.001). Relative reductions in FMZVD were also seen in the left ventrolateral and dorsolateral prefrontal cortex (areas 45, 46 and 47), Broca's area and the right temporal (area 21) and right visual association cortex (area 19). These observations suggest that cerebral dysfunction in ALS involves motor/premotor and extramotor areas, particularly the prefrontal regions.

Effects of vigabatrin on the GABAergic system as determined by [123I]iomazenil SPECT and GABA MRS

PURPOSE

To evaluate effects of vigabatrin (VGB) by using [123I]iomazenil single-photon emission computed tomography (SPECT) to estimate central gamma-aminobutyric acid (GABA(A))/benzodiazepine receptors (BZRs), and magnetic resonance spectroscopy (MRS) to assess tissue GABA levels.

METHODS

Six patients with partial seizures had both SPECT and MRS before and 25-84 days after starting VGB (3 g p.o., q.d.). SPECT was acquired by using the constant-infusion method and, after nonuniform attenuation correction, coregistered with T1-weighted MR Imaging (MRI) A volume of interest (VOI) of 3 x 2 x 2 cc over the occipital cortex, used for MRS acquisition, was positioned on both MRI and coregistered SPECT. Occipital activity was divided by either total plasma activity or plasma [123I]iomazenil concentration to estimate BZR distribution volume (V(T)-p and V'(T), respectively). Wilcoxon's test was used for VOI differences in GABA levels, BZR V(T)-p or V'(T). SPM96 (either no global normalization or proportional scaling) was used to compare BZR V(T)-p changes in the patients with and without VGB with test-retest data in eight healthy age-matched controls.

RESULTS

Occipital GABA levels were increased threefold (without VGB, 1.1+/-0.1 micromol/g; with VGB, 2.9+/-0.5 micromol/g; p = 0.027). BZR distribution volumes showed no change, when estimated by either V(T)-p (without VGB, 6.00+/-0.91 ml/g; with VGB, 5.86+/-0.44 ml/g; p = 0.92) or V(T) (without VGB, 41.1+/-11.2 ml/g; with VGB, 41.2+/-9.9 ml/g; p = 0.75). No significant changes were detected by SPM96.

CONCLUSIONS

A clinically effective dose of VGB caused a threefold increase in tissue GABA levels but was not associated with a substantial BZR downregulation.

Preserved benzodiazepine receptors in Alzheimer's disease measured with C-11 flumazenil PET and I-123 iomazenil SPECT in comparison with CBF

This study evaluates the regional cerebral blood flow (CBF) with H2(15)O-PET and the distribution of central benzodiazepine receptor (BZR) with C-11 flumazenil (FMZ) by PET and I-123 iomazenil (IMZ) by SPECT in Alzheimer's disease (AD). In AD, whereas the CBF was diminished in the frontal, temporal, parietal, and occipital cortex, the distribution volume of FMZ and delayed activity of IMZ were relatively preserved in these cortices, suggesting that the BZR reduction, reflecting neuronal loss, is less prominent than the CBF suppression. The mini-mental state examination score (MMS) was weakly correlated with the CBF in the parietal cortex but not with BZR. It is speculated that the neuronal density reflected by BZR is less impaired than the neuronal function assessed with blood flow in the association cortex of AD. High correlation was found between the uptake of FMZ and the delayed activity of IMZ. The delayed image of IMZ-SPECT is clinically useful to evaluate the preservation of neuronal density in the affected temoporoparietal association cortex in AD.

Assessment of spatial normalization of PET ligand images using ligand-specific templates

Recent advances allow robust computation of parametric maps of ligand-receptor binding from PET data sets. Parametric maps may be statistically analyzed at the voxel level, given suitable techniques for both the spatial normalization of image data into a standard space and the application of appropriate statistical tests. The purpose of this study was to spatially normalize parametric maps of [carbonyl-11C]WAY-100635 and [11C]raclopride binding using SPM 96 and ligand-specific templates. Ligand-specific templates were created from integral images taken from healthy subjects. For this, a MRI-based spatial normalization was used: T1-weighted MRI scans were coregistered to the PET integral images, and the spatial normalization of the MRI to the SPM 96 T1 MRI template was applied to the integral images. These integral images were meaned and smoothed to form [carbonyl-11C]WAY-100635 and [11C]raclopride templates. Reliability of spatial normalization using the ligand template method and the previous MRI-based spatial normalization was investigated by using a second set of integral images taken from a different cohort: Landmark coordinates were defined on all spatially normalized integral images. Mean coordinates were found in order to produce an overall (average) landmark for each location. For each image, at each location, the distance from the landmark coordinates to the overall landmark were found. A multivariate analysis of variance was used to examine the effects of observer variance, landmark location, and the method used. Visually acceptable templates were created. While observer variance was not significant, the landmark x method interaction was significant. The ligand template method had significantly smaller distances: Among the landmark locations with this method, the mean distances between individual image landmarks and overall image landmarks ranged from 1. 1 to 4.9 mm. The ligand template method provides a reliable approach for spatial normalization of PET ligand images.

Mapping of central D2 dopamine receptors in man using [11C]raclopride: PET with anatomic standardization technique

D2 dopamine receptors are of interest in the pathophysiology of schizophrenia. For group comparisons of neuroreceptor distribution measured by PET on a pixel-by-pixel basis, an anatomic standardization technique is required. The aim of the present study is to build a database of normal D2 dopamine receptor distribution using [11C]raclopride and an anatomic standardization technique. In each subject, two PET measurements were performed with rapid bolus injection and with continuous infusion of [11C]raclopride. The radioactivity of the PET images were integrated in the time interval. Integrated images were normalized by the radioactivity of the cerebellum, providing a measure of the binding potential (BP) in each pixel. Each PET image was transformed into a standard brain anatomy using a Computerized Brain Atlas system. From the standardized PET images, the sample mean and the SD of the BP were calculated in each pixel. On the anatomically standardized average images for the both rapid bolus injection and continuous infusion, high BP was observed in the putamen and the caudate nucleus, whereas low BP was observed in the cerebral cortices. The BP for the thalamus and the substantia nigra were slightly higher than those for the cerebral cortices. This regional distribution is in good agreement with the distribution of D2 dopamine receptors known from in vitro studies. The anatomic standardization technique permits to build a database of the normal D2 dopamine receptor distribution in the living human brain. This technique can be applied for group comparisons on a pixel-by-pixel basis.

11C-diprenorphine binding in Huntington's disease: a comparison of region of interest analysis with statistical parametric mapping

We compare region of interest (ROI) analytical approaches with statistical parametric mapping (SPM) of 11C-diprenorphine positron emission tomography findings in five patients with Huntington's disease (HD) and nine age-matched controls. The ROI were placed on caudate, putamen, and an occipital reference area. Ratios of striatal-occipital uptake from averaged static images centered at 60 minutes showed a mean 20% reduction in caudate (P = 0.034) and 15% reduction in putamen (P = 0.095) receptor binding in the HD patients. Dynamic data from caudate and putamen ROI, together with a plasma tracer input function, were analyzed using spectral analysis to give regional impulse response functions. Regional data at 60 minutes after impulse showed a mean 29% decrease in caudate (P = 0.006) and 23% decrease in putamen (P = 0.029) opioid binding in the HD cohort. Parametric images of tracer binding also were produced with spectral analysis on a voxel basis. The images of the unit impulse response function at 60 minutes showed a mean 31% decrease in caudate (P = 0.005) and a 26% decrease in putamen binding (P = 0.011) in HD. The voxel-based parametric images were transformed into standard stereotactic space, and a between-group comparison (patient versus controls) was performed with SPM. This approach revealed symmetrical decreases in caudate (peak 40% decrease, z score = 4.38) and putamen opioid binding (peak 24% decrease, z score = 4.686) with additional nonhypothesized changes in cingulate, prefrontal, and thalamic areas. The significance and precision of changes measured with spectral analysis applied to dynamic data sets were superior to ROI-based ratio analysis on static images. The SPM replicated the striatal reductions in opioid binding in HD and detected additional nonpredicted changes. This study suggests that SPM is a valid alternative to conventional ROI analytical approaches for determining binding changes with positron emission tomography and may have advantages over region-based analyses in exploratory studies.