Positron emission tomographic evaluation of the putative dopamine-D3 receptor ligand, [11C]RGH-1756 in the monkey brain

The dopamine-D3 receptor is of special interest due to its postulated role in the pathophysiology and treatment of schizophrenia and Parkinson's Disease. Increasing evidences support the assumption that the D3 receptors are occupied to a high degree by dopamine at physiological conditions. Research on the functional role of the D3 receptors in brain has however been hampered by the lack of D3 selective ligands. In the present Positron Emission Tomography (PET) study the binding of the novel, putative dopamine-D3 receptor ligand, [11C]RGH-1756 was characterized in the cynomolgus monkey brain. [11C]RGH-1756 was rather homogenously distributed in brain and the regional binding potential (BP) values ranged between 0.17 and 0.48. Pretreatment with unlabelled RGH-1756 decreased radioligand binding to the level of the cerebellum in most brain areas. The regional BP values were lower after intravenous injection of a higher mass of RGH-1756, indicating saturable binding of [11C]RGH-1756. The D2/D3 antagonist raclopride partly inhibited the binding of [11C]RGH-1756 in several brain areas, including the striatum, mesencephalon and neocortex, whereas the 5HT(1A) antagonist WAY-100635 had no evident effect on [11C]RGH-1756 binding. Despite the promising binding characteristics of RGH-1756 in vitro the present PET-study indicates that [11C]RGH-1756 provides a low signal for specific binding to the D3 receptor in vivo. One explanation is that the favorable binding characteristics of RGH-1756 in vitro are not manifested in vivo. Alternatively, the results may support the hypothesis that the dopamine-D3 receptors are indeed occupied to a high extent by dopamine in vivo and thus not available for radioligand binding.     

Neuroimaging of syntax and syntactic processing

Recent results challenge and refine the prevailing view of the way language is represented in the human brain. Syntactic knowledge and processing mechanisms that implement syntax in use are mapped onto neural tissue in experiments that harness both syntactic concepts and imaging technologies to the study of brain mechanisms in healthy and impaired populations. In the emerging picture, syntax is neurologically segregated, and its component parts are housed in several distinct cerebral loci that extend beyond the traditional ones - Broca's and Wernicke's regions in the left hemisphere. In particular, the new brain map for syntax implicates portions of the right cerebral hemisphere.     

Positron emission tomography: quantitative measurement of brain acetylcholinesterase activity using radiolabeled substrates

A new method for quantitative measurement of brain acetylcholinesterase (AChE) activity in living human brain using positron emission tomography (PET) is described. We tested several radiolabeled lipophilic acetylcholine analogs, e.g., N-methylpiperidyl esters, which readily entered the brain via the blood-brain barrier, were hydrolyzed selectively by AChE, and were then trapped in the brain. Among them, and tested and N-[11C]methylpiperidin-4-yl acetate ([11C]MP4A) was chosen as the tracer for PET. Quantitative measurement of cortical AChE was accomplished by fitting the time course of cerebral radioactivity concentration measured by PET and the metabolite-corrected arterial plasma input function using a nonlinear least-squares fitting method. Normal control studies of subjects with a wide range in age (24-89 years) showed no decrease in AChE activity in the cerebral cortex with age. Studies on patients with Alzheimer's disease demonstrated a widespread reduction of AChE activity in the cerebral cortex (more profound in early-onset than in late-onset Alzheimer's disease). Parkinson's disease and progressive supranuclear palsy, clinically similar disorders, could be differentiated with [11C]MP4A/PET studies. Simple methods without using an arterial input function are also proposed. The method provides a quantitative measure of the cholinergic aspect of brain function and proved to be useful in diagnosis of neurodegenerative disorders including Alzheimer's disease.     

脑内阿片受体PET成像及其在痛与镇痛研究中的应用

脑内阿片受体在痛与镇痛中的作用机制一直是神经科学领域研究热点之一。正电子发射体层扫描(positron emission tomography,PET)是目前在体定量检测脑内相关分子参与神经信号转导的唯一途径。本文在简要回顾阿片受体和内源性阿片肽的发现、生理功能及其脑内分布的基础上,对已应用或有望应用于人体的阿片受体选择性和非选择性示踪剂及其在PET成像中的应用进行介绍,并对阿片成像结果所反映的神经机制进行解读。鉴于脑内阿片受体在介导痛与镇痛中的重要作用,文中着重就近年来有关痛与镇痛的脑内阿片受体PET成像研究进展予以综述。     

Positron emission tomography imaging of prostate cancer

Prostate cancer (PCa) is the second leading cause of cancer death among men in the United States. Positron emission tomography (PET), a non-invasive, sensitive, and quantitative imaging technique, can facilitate personalized management of PCa patients. There are two critical needs for PET imaging of PCa, early detection of primary lesions and accurate imaging of PCa bone metastasis, the predominant cause of death in PCa. Because the most widely used PET tracer in the clinic, ¹⁸F-fluoro-2-deoxy-2-d-glucose (¹⁸F-FDG), does not meet these needs, a wide variety of PET tracers have been developed for PCa imaging that span an enormous size range from small molecules to intact antibodies. In this review, we will first summarize small-molecule-based PET tracers for PCa imaging, which measure certain biological events, such as cell membrane metabolism, fatty acid synthesis, and receptor expression. Next, we will discuss radiolabeled amino acid derivatives (e.g. methionine, leucine, tryptophan, and cysteine analogs), which are primarily based on the increased amino acid transport of PCa cells. Peptide-based tracers for PET imaging of PCa, mostly based on the bombesin peptide and its derivatives which bind to the gastrin-releasing peptide receptor, will then be presented in detail. We will also cover radiolabeled antibodies and antibody fragments (e.g. diabodies and minibodies) for PET imaging of PCa, targeting integrin α_vβ₃, EphA2, the epidermal growth factor receptor, or the prostate stem cell antigen. Lastly, we will identify future directions for the development of novel PET tracers for PCa imaging, which may eventually lead to personalized management of PCa patients.     

The syntactic and semantic processing of mass and count nouns: an ERP study

The present study addressed the question of whether count and mass nouns are differentially processed in the brain. In two different ERP (Event-Related Potentials) tasks we explored the semantic and syntactic levels of such distinction. Mass and count nouns typically differ in concreteness, hence the effect of this important variable was factorially examined in each task. Thus the stimuli presented were: count concrete, count abstract, mass concrete or mass abstract. The first experiment (concrete/abstract semantic judgment task) involved the interaction between the N400 concreteness effect and the Mass/Count condition, revealing a substantial effect between mass and count nouns at the semantic level. The second experiment (sentence syntactic violation task) showed a Mass/Count distinction on left anterior negativity (LAN) and on P600 components, confirming the difference at the syntactic level. This study suggests that the brain differentiates between count and mass nouns not only at the syntactic level but also at the semantic level. Implications for our understanding of the brain mechanisms underlying the Mass/Count distinction are discussed.     

Cerebral white matter blood flow is constant during human non-rapid eye movement sleep: a positron emission tomographic study.

This study aimed to identify brain regions with the least decreased cerebral blood flow (CBF) and their relationship to physiological parameters during human non-rapid eye movement (NREM) sleep. Using [(15)O]H(2)O positron emission tomography, CBF was measured for nine normal young adults during nighttime. As NREM sleep progressed, mean arterial blood pressure and whole brain mean CBF decreased significantly; arterial partial pressure of CO(2) and, selectively, relative CBF of the cerebral white matter increased significantly. Absolute CBF remained constant in the cerebral white matter, registering 25.9 +/- 3.8 during wakefulness, 25.8 +/- 3.3 during light NREM sleep, and 26.9 +/- 3.0 (ml.100 g(-1).min(-1)) during deep NREM sleep (P = 0.592), and in the occipital cortex (P = 0.611). The regression slope of the absolute CBF significantly differed with respect to arterial partial pressure of CO(2) between the cerebral white matter (slope 0.054, R = - 0.04) and frontoparietal association cortex (slope - 0.776, R = - 0.31) (P = 0.005) or thalamus (slope - 1.933, R = - 0.47) (P = 0.004) and between the occipital cortex (slope 0.084, R = 0.06) and frontoparietal association cortex (P = 0.021) or thalamus (P < 0.001), and, with respect to mean arterial blood pressure, between the cerebral white matter (slope - 0.067, R = - 0.10) and thalamus (slope 0.637, R = 0.31) (P = 0.044). The cerebral white matter CBF keeps constant during NREM sleep as well as the occipital cortical CBF, and may be specifically regulated by both CO(2) vasoreactivity and pressure autoregulation.     

Positron emission tomography in venous ulceration and liposclerosis: study of regional tissue function.

Oxygen-15 administered by continuous inhalation and emission computed tomography were used to study regional tissue oxygen utilisation and blood flow in the limbs of 11 patients with venous ulceration and five patients with liposclerosis due to venous insufficiency. The results showed increased blood flow and appreciably reduced fractional oxygen extraction in the diseased tissues. These findings indicated a local, functional shunting of blood through the abnormal microcirculation of the skin and subcutaneous tissues. This may be an important factor in the aetiology of the skin changes seen in patients with calf muscle pump failure.     

Asymmetrical activation in the human brain during processing of fearful faces

Traditional split-field studies and patient research indicate a privileged role for the right hemisphere in emotional processing [1-7], but there has been little direct fMRI evidence for this, despite many studies on emotional-face processing [8-10](see Supplemental Background). With fMRI, we addressed differential hemispheric processing of fearful versus neutral faces by presenting subjects with faces bilaterally [11-13]and orthogonally manipulating whether each hemifield showed a fearful or neutral expression prior to presentation of a checkerboard target. Target discrimination in the left visual field was more accurate after a fearful face was presented there. Event-related fMRI showed right-lateralized brain activations for fearful minus neutral left-hemifield faces in right visual areas, as well as more activity in the right than in the left amygdala. These activations occurred regardless of the type of right-hemifield face shown concurrently, concordant with the behavioral effect. No analogous behavioral or fMRI effects were observed for fearful faces in the right visual field (left hemisphere). The amygdala showed enhanced functional coupling with right-middle and anterior-fusiform areas in the context of a left-hemifield fearful face. These data provide behavioral and fMRI evidence for right-lateralized emotional processing during bilateral stimulation involving enhanced coupling of the amygdala and right-hemispheric extrastriate cortex.     

Positron emission tomography: measurement of the activity of second messenger systems

Phosphoinositide turnover is closely connected to modulation of synaptic function and is part of an important second messenger-producing system. New radioligands for imaging second messenger systems by positron emission tomography have been developed: carbon-11-labeled 1,2-diacylglycerols. The theoretical background of second messenger imaging is described in detail and the relation between the biologically active compounds and potential tracers for imaging second messenger systems is discussed. We report informative findings on postsynaptic biological responses in the living human brain of healthy normal subjects and with various diseases.     

Positron emission tomography: the conceptual idea using a multidisciplinary approach

Positron emission tomography (PET) is a method for quantitatively measuring biochemical and physiological processes in vivo by using radiopharmaceuticals labeled with positron-emitting radionuclides such as 11C, 13N, 15O, and 18F and by measuring the annihilation radiation using a coincidence technique. This technique is also used for measurement of the pharmacokinetics of labeled drugs and measurement of the effects of drugs on metabolism. Deviations from normal metabolism can be measured and insight into biological processes responsible for diseases can be obtained.