Autoradiographic localization of 5-HT1A receptors in the post-mortem human brain using [H]WAY-100635 and [C]WAY-100635

The distribution of 5-HT_1A receptors was examined in the post-mortem human brain using whole hemisphere autoradiography and the selective 5-HT_1A receptor antagonist [³H]WAY-100635 ([O-methyl-³H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride). The autoradiograms showed very dense binding to hippocampus, raphe nuclei and neocortex. The labeling in neocortex was slightly lower than in the hippocampus and was mainly at superficial layers, although a faintly labeled band could be seen in deeper neocortical layers. Other regions, such as the amygdala, septum and claustrum, showed low densities of [³H]WAY-100635 binding, reflecting low densities of 5-HT_1A receptors. The labeling was very low in basal ganglia, such as nucleus caudatus and putamen, in cerebellum or in structures of the brain stem except in the raphe nuclei. The labeling of human 5-HT_1A receptors with [³H]WAY-100635 was antagonized by the addition of the 5-HT_1A receptor ligands, 5-HT, buspirone, pindolol or 8-OH-DPAT (10 μM), leaving a very low background of non-specific binding. Saturation analysis of semiquantitative data from several human regions indicated that [³H]WAY-100635 has a K_d of approximately 2.5 nM. The selective labeling of 5-HT_1A receptors with [³H]WAY-100635 clearly show that this compound is useful for further studies of the human 5-HT_1A receptor subtype in vitro. [¹¹C]WAY-100635 is used for the characterization of 5-HT_1A receptors with positron emission tomography (PET). WAY-100635 was also radiolabeled with the short-lived positron-emitting radionuclide carbon-11 (t_1/2=20 min) and used for in vitro autoradiography on human whole hemisphere cryosections. [¹¹C]WAY-100635 gave images qualitatively similar to those of [³H]WAY-100635, although with a lower resolution. Thus, the hippocampal formation was densely labeled, with lower density in the neocortex. Buspirone, pindolol or 8-OH-DPAT (10 μM), blocked all binding of [¹¹C]WAY-100635. The in vitro autoradiography of the distribution of 5-HT_1A receptors obtained with radiolabeled WAY-100635 provide detailed qualitative and quantitative information on the distribution of 5-HT_1A-receptors in the human brain. Moreover, the studies give reference information for the interpretation of previous initial results at much lower resolution in humans with PET and [¹¹C]WAY-100635. These data provide a strong basis for expecting [¹¹C]WAY-100635 to behave as a highly selective radioligand in vivo.     

Validation and quantification of [18F]altanserin binding in the rat brain using blood input and reference tissue modeling

The 5-hydroxytryptamine type 2a (5-HT_2A) selective radiotracer [¹⁸F]altanserin has been subjected to a quantitative micro-positron emission tomography study in Lister Hooded rats. Metabolite-corrected plasma input modeling was compared with reference tissue modeling using the cerebellum as reference tissue. [¹⁸F]altanserin showed sufficient brain uptake in a distribution pattern consistent with the known distribution of 5-HT_2A receptors. Full binding saturation and displacement was documented, and no significant uptake of radioactive metabolites was detected in the brain. Blood input as well as reference tissue models were equally appropriate to describe the radiotracer kinetics. [¹⁸F]altanserin is suitable for quantification of 5-HT_2A receptor availability in rats.     

Age-related reduction of [C]MDL100,907 binding to central 5-HT2A receptors:: PET study in the conscious monkey brain

Age-related alterations of serotonin (5-hydroxytryptamine; 5-HT) type 2A receptors (5-HT_2A) in the living brains of young (6.0±1.3 years old) and aged (19.2±3.0 years old) monkeys (Macaca mulatta) were evaluated with [¹¹C]MDL100,907 in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of 5-HT_2A binding in vivo, PET scan of [¹¹C]MDL100,907 was performed with arterial blood sampling in each animal, and the metabolic-corrected arterial input function was used for Logan graphical analysis. Higher cerebral binding of [¹¹C]MDL100,907 was observed in the hippocampus, cingulate gyrus, frontal, temporal and occipital cortices, regions known to contain high densities of 5-HT_2A, by in vitro assay. Binding was intermediate in the striatum and thalamus, and lower in the pons and cerebellum in both young and aged monkeys. The age-related decrease in [¹¹C]MDL100,907 binding to 5-HT_2A receptors was prominent in the hippocampus, cingulate gyrus, frontal, temporal and occipital cortices, but not in the striatum, thalamus and pons. These observation demonstrated the usefulness of [¹¹C]MDL100,907 as an labeled compound for assessment of the aging process of the cortical 5-HT_2A measured by PET.     

Imaging cortical dopamine D1 receptors using [11C]NNC112 and ketanserin blockade of the 5-HT2A receptors

[¹¹C]NNC112 (8-chloro-7-hydroxy-3-methyl-5-(7-benzofuranyl)-2,3,4,5-tetrahydro-IH-3-benzazepine), a selective positron-emission tomography (PET) ligand for the D₁ receptor (R) over the 5-HT_2A R in vitro, has shown lower selectivity in vivo, hampering measurement of D₁ R in the cortex. [¹¹C]NNC112 PET and intravenous (i.v) ketanserin challenge were used to (1) confirm the previous findings of [¹¹C]NNC112 in vivo D₁ R selectivity, and (2) develop a feasible methodology for imaging cortical D₁ R without contamination by 5-HT_2A R. Seven healthy volunteers underwent [¹¹C]NNC112 PET scans at baseline and after a 5-HT_2A R-blocking dose of ketanserin (0.15 mg/kg, i.v.). Percent BP_ND change between the post-ketanserin and baseline scans was calculated. Irrespective of the quantification method used, ketanserin pretreatment led to significant decrease of BP_ND in the cortical (∼30%) and limbic regions (∼20%) but not in the striatum, which contains a much lower amount of 5-HT_2A R. Therefore, ketanserin allows D₁ R signal to be detected by [¹¹C]NNC112 PET without significant 5-HT_2A R contamination. These data confirm the presence of a significant 5-HT_2A R contribution to cortical [¹¹C]NNC112 signal, and call for caution in the interpretation of published [¹¹C]NNC112 PET findings on cortical D₁ R in humans. In the absence of more selective ligands, [¹¹C]NNC112 PET with ketanserin can be used for cortical D₁ R imaging in vivo.     

Kinetic modeling of the serotonin 5-HT1B receptor radioligand [11C]P943 in humans

[¹¹C]P943 is a new radioligand recently developed to image and quantify serotonin 5-Hydroxytryptamine (5-HT_1B) receptors with positron emission tomography (PET). The purpose of this study was to evaluate [¹¹C]P943 for this application in humans, and to determine the most suitable quantification method. Positron emission tomography data and arterial input function measurements were acquired in a cohort of 32 human subjects. Using arterial input functions, compartmental modeling, the Logan graphical analysis, and the multilinear method MA1 were tested. Both the two tissue-compartment model and MA1 provided good fits of the PET data and reliable distribution volume estimates. Using the cerebellum as a reference region, BP_ND binding potential estimates were computed. [¹¹C]P943 BP_ND estimates were significantly correlated with in vitro measurements of the density of 5-HT_1B receptors, with highest values in the occipital cortex and pallidum. To evaluate noninvasive methods, two- and three-parameter graphical analyses, Simplified Reference Tissue Models (SRTM and SRTM2), and Multilinear Reference Tissue Models (MRTM and MRTM2) were tested. The MRTM2 model provided the best correlation with MA1 binding-potential estimates. Parametric images of the volume of distribution or binding potential of [¹¹C]P943 could be computed using both MA1 and MRTM2. The results show that [¹¹C]P943 provides quantitative measurements of 5-HT_1B binding potential.     

Atypical in vitro and in vivo binding of [3H]S-14506 to brain 5-HT1A receptors

Summary The tritiated derivative of the potent 5-HT_1A receptor agonist S-14506 {1[2-(4-fluorobenzoylamino)ethyl]-4-(7-methoxynaphtyl)piperazine} was tested for its capacity to selectively label the serotonin 5-HT_1A receptors both in vitro in the rat and the mouse brain, and in vivo in the mouse. In vitro studies showed that the pharmacological profile and the distribution of [³H]S-14506 specific binding sites (Kd=0.15 nM) in different brain regions matched perfectly those of the prototypical 5-HT_1A receptor ligand [³H]8-OH-DPAT. However, in the three regions examined (hippocampus, septum, cerebral cortex), the density of [³H]S-14506 specific binding sites was significantly higher (+ 66–90%) than that found with [³H]8-OH-DPAT. Whereas the specific binding of [³H]8-OH-DPAT was markedly reduced by GTP and Gpp(NH)p and increased by Mn²⁺, that of [³H]S-14506 was essentially unaffected by these compounds. In addition, the alkylating agent N-ethylmaleimide was much less potent to inhibit the specific binding of [³H]S-14506 than that of [³H]8-OH-DPAT. Measurement of in vivo accumulation of tritium one hour after i.v. injection of [³H]S-14506 to mice revealed marked regional differences, with about 2.5 times more radioactivity in the hippocampus than in the cerebellum. Pretreatment with 5-HT_1A receptor ligands prevented tritium accumulation in the hippocampus but not in the cerebellum. Autoradiograms from brain sections of injected mice confirmed the specific in vivo labeling of 5-HT_1A receptors by [³H]S-14506, therefore suggesting further developments with derivatives of this molecule for positron emission tomography in vivo in man.     

Noninvasive blood-free full quantification of positron emission tomography radioligand binding

Full quantification of a positron emission tomography (PET) radioligand binding to its target is preferred because it requires the fewest assumptions, but generally involves measuring the concentration of free radioligand in the arterial plasma by collecting blood samples from the subject''s radial artery during the scan, and performing metabolite analysis. This invasive, costly procedure deters subjects'' participation, and requires specialized staff and equipment. Simultaneous estimation (SIME) can fully quantify binding using only PET data from multiple brain regions and one individual anchor value, which is based on a single arterial blood sample. Drawing this sample can still be challenging in clinical settings, particularly when using simultaneous PET/magnetic resonance scanners. Here we propose a methodology for full quantification of binding that does not require any blood samples. The methodology substitutes the SIME blood-based anchor with a value predicted using multiple linear regression of noninvasive, easy-to-collect variables related to the radioligand blood concentration, and individual metabolism, such as injected dose, body mass index, or body surface area. As a study case, we show here the methodology in comparison to analysis with full arterial-line blood sampling in a cohort of 23 available scans with [¹¹C]CUMI-101, a partial agonist of the serotonin 5-HT_1A receptors.     

Test–retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

The role of serotonin in CNS function and in many neuropsychiatric diseases (e.g., schizophrenia, affective disorders, degenerative dementias) support the development of a reliable measure of serotonin receptor binding in vivo in human subjects. To this end, the regional distribution and intrasubject test–retest variability of the binding of [¹⁸F]altanserin were measured as important steps in the further development of [¹⁸F]altanserin as a radiotracer for positron emission tomography (PET) studies of the serotonin 5-HT_2A receptor. Two high specific activity [¹⁸F]altanserin PET studies were performed in normal control subjects (n = 8) on two separate days (2–16 days apart). Regional specific binding was assessed by distribution volume (DV), estimates that were derived using a conventional four compartment (4C) model, and the Logan graphical analysis method. For both analysis methods, levels of [¹⁸F]altanserin binding were highest in cortical areas, lower in the striatum and thalamus, and lowest in the cerebellum. Similar average differences of 13% or less were observed for the 4C model DV determined in regions with high receptor concentrations with greater variability in regions with low concentrations (16–20%). For all regions, the absolute value of the test–retest differences in the Logan DV values averaged 12% or less. The test–retest differences in the DV ratios (regional DV values normalized to the cerebellar DV) determined by both data analysis methods averaged less than 10%. The regional [¹⁸F]altanserin DV values using both of these methods were significantly correlated with literature-based values of the regional concentrations of 5-HT_2A receptors determined by postmortem autoradiographic studies (r² = 0.95, P < 0.001 for the 4C model and r² = 0.96, P < 0.001 for the Logan method). Brain uptake studies in rats demonstrated that two different radiolabeled metabolites of [¹⁸F]altanserin (present at levels of 3–25% of the total radioactivity in human plasma 10–120 min postinjection) were able to penetrate the blood–brain barrier. However, neither of these radiolabeled metabolites bound specifically to the 5-HT_2A receptor and did not interfere with the interpretation of regional [¹⁸F]altanserin-specific binding parameters obtained using either a conventional 4C model or the Logan graphical analysis method. In summary, these results demonstrate that the test–retest variability of [¹⁸F]altanserin-specific binding is comparable to that of other PET radiotracers and that the regional specific binding of [¹⁸F]altanserin in human brain was correlated with the known regional distribution of 5-HT_2A receptors. These findings support the usefulness of [¹⁸F]altanserin as a radioligand for PET studies of 5-HT_2A receptors. Synapse 30:380–392, 1998. © 1998 Wiley-Liss, Inc.     

Serotonin 1B receptor density mapping of the human brainstem using positron emission tomography and autoradiography

The serotonin 1B (5-HT_1B) receptor has lately received considerable interest in relation to psychiatric and neurological diseases, partly due to findings based on quantification using Positron Emission Tomography (PET). Although the brainstem is an important structure in this regard, PET radioligand binding quantification in brainstem areas often shows poor reliability. This study aims to improve PET quantification of 5-HT_1B receptor binding in the brainstem.Volumes of interest (VOIs) were selected based on a 3D [³H]AZ10419369 Autoradiography brainstem model, which visualized 5-HT_1B receptor distribution in high resolution. Two previously developed VOI delineation methods were tested and compared to a conventional manual method. For a method based on template data, a [¹¹C]AZ10419369 PET template was created by averaging parametric binding potential (BP_ND) images of 52 healthy subjects. VOIs were generated based on a predefined volume and BP_ND thresholding and subsequently applied to test-retest [¹¹C]AZ10419369 parametric BP_ND images of 8 healthy subjects. For a method based on individual subject data, VOIs were generated directly on each individual parametric image.Both methods showed improved reliability compared to a conventional manual VOI. The VOIs created with [¹¹C]AZ10419369 template data can be automatically applied to future PET studies measuring 5-HT_1B receptor binding in the brainstem.     

In vivo binding of [123I]4-(2′-methoxy phenyl)-1-[2′-(N-2″-pyridinyl)-P-iodobenzamido-]ethyl-piperazine,p-MPPI,to 5-HT1A receptors in rat brain

The in vivo regional distribution and pharmacological profile of a novel iodinated phenylpiperazine derivative, [¹²³I]p-MPPI (4-(2′-methoxy-)phenyl-l-[2′-(N-2″pyridinyl)-p-iodobenzamido-]ethyl-piperazine), in the rat brain were evaluated for use as a potential in vivo imaging agent for 5-HT_1A receptors. The new ligand penetrated the blood-brain barrier quickly and efficiently, with 1.2% of the injected dose found in the whole brain at 2 min post i.v. injection. The rate of radioactivity washout was slowest from the hippocampal region, followed by the hypothalamus, cortex, striatum, and cerebellum. The maximum ratio of hippocampus/cerebellum was 3.3 at 30 min postinjection. The specific binding of the radioligand in the hippocampal region, an area rich in 5-HT_1A receptor density, was blocked by pretreatment with a dose of (±) 8-OH-DPAT (2 mg/kg, i.v.) or WAY 100635 (1 mg/kg, i.v.), whereas the regional distribution of [¹²³I]p-MPPI was unaffected by pretreatment with non-5-HT_1A agents such as ketanserin or haloperidol. Ex vivo autoradiographic studies further confirmed that the specific binding of [¹²³I]p- MPPI is associated with 5-HT_1A receptor sites. These results indicate that [¹²³I]p-MPPI may be a useful candidate for noninvasive single photon emission computed tomography (SPECT) imaging of 5-HT_1A receptor sites in the living human brain. © 1994 Wiley-Liss, Inc.     

Automated reference region extraction and population-based input function for brain [11C]TMSX PET image analyses

[¹¹C]TMSX ([7-N-methyl-¹¹C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine) is a selective adenosine A2A receptor (A_2AR) radioligand. In the central nervous system (CNS), A_2AR are linked to dopamine D₂ receptor function in striatum, but they are also important modulators of inflammation. The golden standard for kinetic modeling of brain [¹¹C]TMSX positron emission tomography (PET) is to obtain arterial input function via arterial blood sampling. However, this method is laborious, prone to errors and unpleasant for study subjects. The aim of this work was to evaluate alternative input function acquisition methods for brain [¹¹C]TMSX PET imaging. First, a noninvasive, automated method for the extraction of gray matter reference region using supervised clustering (SCgm) was developed. Second, a method for obtaining a population-based arterial input function (PBIF) was implemented. These methods were created using data from 28 study subjects (7 healthy controls, 12 multiple sclerosis patients, and 9 patients with Parkinson''s disease). The results with PBIF correlated well with original plasma input, and the SCgm yielded similar results compared with cerebellum as a reference region. The clustering method for extracting reference region and the population-based approach for acquiring input for dynamic [¹¹C]TMSX brain PET image analyses appear to be feasible and robust methods, that can be applied in patients with CNS pathology.     

Autoradiographic evidence for the heterogeneity of 5-HT1 sites in the rat brain

The distribution of the binding sites of a new, potent agonist of serotonin (5-HT), 8-OH-N,N-dipropoyl-2-aminotetralin (PAT), was studied in the rat brain with the quantitative autoradiographic technique utilizing tritium-sensitive LKB film. The localization of [³H]PAT binding sites was very similar to that of [³H]5-HT binding sites, except in some discrete regions (choroid plexus, striatum, area preoptica lateralis, subiculum, and substantia nigra), which exhibited very low levels of labeling with [³H]PAT and high levels with [³H]5-HT. These results indicate that 5-HT₁ receptors are heterogeneous, and that [³H]PAT recognizes only a 5-HT₅ subclass (called 5-HT_1A).     

Two distinct serotonin receptors: regional variations in receptor binding in mammalian brain

Two distinct serotonin receptors in mammalian brain are labeled respectively with [³H]serotonin (5-HT₁) and [³H]spiperone (5-HT₂). In general, agonists display highest affinities for 5-HT₁ while antagonists prefer 5-HT₂ sites. To conduct regional studies of 5-HT receptors, we estimated 5-HT₂ sites with [³H]spiperone, using the 5-HT₂ specific antagonist cinanserin to displace binding to 5-HT₂ but not dopamine receptors and sulpiride to displace [³H]spiperone from dopamine but not 5-HT receptors. About 15% of cerebral cortical [³H]spiperone binding appears to involve dopamine sites and the remainder involves 5-HT₂ receptors. In the corpus striatum about 80% of [³H]spiperone binding labels dopamine receptors and the rest involves 5-HT₂ sites. [³H]mianserin binds about equally to 5-HT₂ sites are studies selectively by displacing histamine H₁-receptor binding with the H₁-antihistamine triprolidine. [³H]LSD labels both 5-HT₁ and 5-HT₂ receptors. Its binding to 5-HT₁ sites is displaced selectively with 5-HT, while its binding to 5-HT₂ receptors is displaced with cinanserin. [³H]5-HT labels only 5-HT₁ receptors. The regional distribution of the two 5-HT receptors is similar in rat, guinea pig and bovine brain. However, the regional patterns of 5-HT₁ and 5-HT₂ receptors differ considerably in all 3 species. The hippocampus is quite high in 5-HT₁ receptors but low in 5-HT₂ sites. The cerebellum contains the lowest levels of both 5-HT₁ and 5-HT₂ receptors. In bovine brain, most areas contain similar numbers of 5-HT₁ and 5-HT₂ receptors. However, the substantia nigra, the richest 5-HT₁ area inbovine brain, possesses 10 times more 5-HT₁ than 5-HT₂ sites.     

Autoradiographic mapping of 5-HT1B and 5-HT1D receptors in the post mortem human brain using [H]GR 125743

The distribution of 5-HT_1B and 5-HT_1D receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [³H]GR 125743. [³H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [³H]GR 125743 binding was observed, reflecting low densities of 5-HT_1B/1D receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT_1D receptors and the second using SB 224289 to inhibit 5-HT_1B receptor binding. The autoradiograms indicated that in the human brain, the 5-HT_1B receptor is much more abundant than the 5-HT_1D receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [³H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT_1B receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT_1D receptor in the human brain.     

[I]4-Aminobenzyl-5′-N-methylcarboxamidoadenosine ([I]AB-MECA) labels multiple adenosine receptor subtypes in rat brain

Adenosine modulates neuronal activity and neurotransmitter release through interaction with cell surface receptors. Four adenosine receptor subtypes, A₁, A_2A, A_2B, and A₃ receptors, have been cloned and characterized. The agonist ligand, [¹²⁵I]AB-MECA ([¹²⁵I]4-aminobenzyl-5′-N-methylcarboxamidoadenosine) has high affinity for recombinant A₁ and A₃ receptors [Olah et al., Mol. Pharmacol., 45 (1994) 978–982]. Rodent A₃ receptors are relatively insensitive to xanthines; inhibition of A₁ receptors with xanthines allows selective detection of A₃ receptors despite the lack of selectivity of the ligand. We studied whether [¹²⁵I]AB-MECA is useful for localization and characterization of A₃ receptors in rat brain. The autoradiographic distribution of total [¹²⁵I]AB-MECA (400 pM) binding closely resembled the pattern of A₁ receptor binding, with highest levels in cerebellum, hippocampus, and thalamus, and moderate levels in cortex and striatum. Drug competition studies confirmed that almost all [¹²⁵I]AB-MECA binding could be attributed to labeling of A₁ receptors. Xanthine amine congener (1 μM) reduced specific [¹²⁵I]AB-MECA binding by >95%, indicating that xanthine-resistant A₃ receptors represent a quantitatively minor subtype. Despite the use of a radioligand with high affinity and high specific activity, the low density of A₃ receptors in rat brain appears insufficient to allow localization, or even consistent detection, of this receptor subtype. In the presence of DPCPX (50 nM, to block A₁ receptors), residual [¹²⁵I]AB-MECA binding to A_2A receptors was observed in the striatum. Thus, [¹²⁵I]AB-MECA labels primarily A₁ and A_2A adenosine receptors in rat brain.     

In Vitro Autoradiographic Localization of 5-HT1A Receptor-Activated G-Proteins in the Rat Brain

Serotonin 5-HT_1A receptors belong to the superfamily of G-protein-coupled receptors. Receptor activation of G-proteins can be determined by agonist-stimulated [³⁵S]GTPγS binding in the presence of excess GDP, and in vitro autoradiographic adaptation of this technique allows visualization of receptor-activated G-proteins in tissue sections. The present study was performed to examine 5-HT_1A receptor activation of G-proteins using 8-OH-DPAT-stimulated [³⁵S]GTPγS binding in membranes and brain sections. In hippocampal membranes, 8-OH-DPAT stimulated [³⁵S]GTPγS binding by twofold, with an ED₅₀ value of 25 nM. 5-HT₁ antagonists, but not 5-HT₂ antagonists, increased the ED₅₀ of 8-OH-DPAT in a manner consistent with competitive antagonists. Scatchard analysis of [³⁵S]GTPγS binding showed that 8-OH-DPAT induced the formation of high affinity [³⁵S]GTPγS binding sites with a K_D for GTPγS of 3.2 nM. [³⁵S]GTPγS autoradiography, performed in brain sections with the 5-HT_1A agonist 8-OH-DPAT, revealed high levels of 5-HT_1A-stimulated [³⁵S]GTPγS binding in the hippocampus, lateral septum, prelimbic cortex, entorhinal cortex, and dorsal raphe nucleus. 5-HT_1A-stimulated [³⁵S]GTPγS binding in sections was blocked by the addition of the 5-HT₁ antagonist methiothepin. These results show that the use of agonist-stimulated [³⁵S]GTPγS autoradiography for the 5-HT_1A receptor system should provide new information regarding signal transduction in specific brain regions.     

Decreased density of serotonin 2A receptors in the superior temporal gyrus in schizophrenia-a postmortem study

The superior temporal gyrus (STG) is strongly implicated in the pathophysiology of schizophrenia, particularly with regards to auditory hallucinations. In this study, using in situ quantitative autoradiography in postmortem tissue, we investigated the binding of the [³H]ketanserin to 5-HT_2A receptors and [³H]mesulergine to 5-HT_2C receptors in the left STG of 8 male schizophrenic patients compared to 8 control subjects. A strong [³H]ketanserin binding was observed in the STG, however there was a very weak [³H]mesulergine binding in the STG. A significant decrease in binding of [³H]ketanserin was clearly observed in schizophrenia patients in comparison with control subjects. There were no significant correlations between 5-HT_2A binding density and age, postmortem intervals, or brain pH. These results suggest that the alterations of the 5-HT_2A receptors contribute to the pathophysiology of the STG in schizophrenia. Furthermore, there is a clear tendency for a positive correlation between 5-HT_2A and muscarinic M1 receptor bindings, and for negative correlations between 5-HT_2A and GABA_A receptor bindings and between muscarinic M1 and GABA_A receptor bindings. This provides a possible mechanism of auditory hallucinations through interactions between 5-HT_2A, acetylcholine muscarinic and GABA transmissions in the STG in schizophrenia.     

Evaluation of [4‐O‐methyl‐11C]KW‐6002 as a potential PET ligand for mapping central adenosine A2A receptors in rats

KW‐6002, a xanthine‐based adenosine A_2A antagonist, was labelled with the positron emitter carbon‐11 by O‐methylation of its precursor, KF23325, using [¹¹C]iodomethane and was evaluated in rats as a putative in vivo radioligand for positron emission tomography (PET). Following intravenous injection of [¹¹C]KW‐6002, radioactivity was measured in blood, plasma, peripheral tissues, and in discrete brain tissues over a 2‐h time period commensurate with PET scanning. In brain, [¹¹C]KW‐6002 showed highest retention in striata, with evidence of saturable binding, and lowest retention in frontal cortex (a tissue low in adenosine A_2A receptors). PET scanning with [¹¹C]KW‐6002 demonstrated a specific signal in the striata which could be described using compartmental modelling. Specific binding was, however, also detected in extrastriatal regions, including brain areas reported to have low adenosine A_2A receptor density. Blocking studies with the A₁ selective antagonist KF15372 and the non xanthine‐type A_2A antagonist ZM 241385 failed to elucidate the nature of this binding. Thus, although [¹¹C]KW‐6002 shows some potential for development as a PET ligand for quantifying striatal adenosine A_2A receptor function, its in vivo selectivity requires further investigation. Synapse 42:164–176, 2001. © 2001 Wiley‐Liss, Inc.     

Quantitative analysis of [11C]AZ10419369 binding to 5-HT1B receptors in human brain

A novel radioligand for positron emission tomography (PET) imaging of serotonin 5-HT_1B receptors, [¹¹C]AZ10419369, has been recently described. In this study, the potential for quantitative analysis of [¹¹C]AZ10419369 binding to central 5-HT_1B receptors was evaluated in human subjects. PET measurements were performed after injection of [¹¹C]AZ10419369 in 10 subjects. Data were analyzed with kinetic modeling and linear graphical analysis using the arterial plasma as input function, and with reference tissue models using cerebellar cortex as the reference region. Binding of [¹¹C]AZ10419369 was highest in pallidum, ventral striatum, and occipital cortex and lowest in cerebellum. The percentage of unchanged radioligand in plasma was 97% to 99%, indicating that no significant amounts of radioactive metabolites were formed during the time of analysis. Time–activity curves of [¹¹C]AZ10419369 could be described with both one-tissue compartment (1-TC) and two-tissue compartment (2-TC) models in the majority of subjects. The 2-TC model failed to deliver reasonable estimates of the kinetic parameters. However, stable estimates of binding potential (BP_ND) were obtained by constraining K₁/k₂ to the distribution volume obtained with the 1-TC model in the cerebellar cortex. BP_ND values estimated with reference tissue models were correlated with the corresponding values obtained with kinetic modeling. The findings support the use of reference tissue models in applied clinical studies with [¹¹C]AZ10419369.     

Characterization of in vivo pharmacokinetic properties of the dopamine D1 receptor agonist DAR-0100A in nonhuman primates using PET with [11C] NNC112 and [11C] raclopride

DAR-0100A, the active enantiomer of dihydrexidine, is a potent dopamine D1 agonist under investigation for treatment of cognitive impairment and negative symptoms of schizophrenia. We measured the dose–occupancy relationship for DAR-0100A at D1 receptors using positron emission tomography (PET) imaging in baboons with [¹¹C] NNC112 and its binding to D2 with [¹¹C] raclopride. Two baboons were scanned with [¹¹C] NNC112 at baseline and after three different doses of DAR-0100A. Two baboons were scanned with [¹¹C] raclopride at baseline and after one dose of DAR-0100A. Occupancy (ΔBP_ND) was computed in the striatum and cortex. A clear relationship was observed between plasma concentration of DAR-0100A and ΔBP_ND. ΔBP_ND was larger in the striatum than in the cortex, consistent with reports showing that 25% of [¹¹C] NNC112 BP_ND in the cortex is attributed to 5-HT_2A. Plasma EC₅₀ estimates ranged from 150 to 550 ng/mL according to the constraints on the model. There was no detectable effect of DAR-0100A on [¹¹C] raclopride BP_ND. These data suggest that at doses likely to be administered to patients, occupancy will not be detectable with [¹¹C] NNC112 PET and binding of DAR-0100A to D2 will be negligible. This is the first demonstration with PET of a significant occupancy by a full D1 agonist in vivo.