Quantitative in vivo receptor binding. I. Theory and application to the muscarinic cholinergic receptor

A novel approach to in vivo receptor binding experiments is presented which allows direct quantitation of binding site densities. The method is based on an equilibrium model of tracer uptake and is designed to produce a static distribution proportional to receptor density and to minimize possible confounding influences of regional blood flow, blood-brain barrier permeability, and nonspecific binding. This technique was applied to the measurement of regional muscarinic cholinergic receptor densities in rat brain using [3H]scopolamine. Specific in vivo binding of scopolamine demonstrated saturability, a pharmacologic profile, and regional densities which are consistent with interaction of the tracer with the muscarinic receptor. Estimates of receptor density obtained with the in vivo method and in vitro measurements in homogenates were highly correlated. Furthermore, reduction in striatal muscarinic receptors following ibotenic acid lesions resulted in a significant decrease in tracer uptake in vivo, indicating that the correlation between scopolamine distribution and receptor density may be used to demonstrate pathologic conditions. We propose that the general method presented here is directly applicable to investigation of high affinity binding sites for a variety of radioligands.     

Evaluation of novel PET ligands (+)N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) and its stereoisomer [11C](-)3-MPB for muscarinic cholinergic receptors in the conscious monkey brain: a PET study in comparison with

The novel muscarinic cholinergic ligands (+)N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) and its stereoisomer [11C](-)3-MPB were evaluated in comparison with [11C]4-MPB in the brains of conscious monkeys (Macaca mulatta) using high-resolution positron emission tomography (PET). The regional distribution patterns of [11C](+)3-MPB and [11C]4-MPB at 60-91 min postinjection were almost identical: highest in the striatum and occipital cortex; intermediate in the temporal and frontal cortices, cingulate gyrus, hippocampus, and thalamus; lower in the pons; and lowest in the cerebellum. The uptake of [11C](+)3-MPB in all regions was higher and the dynamic range of regional uptake differences of [11C](+)3-MPB was better than those of [11C]4-MPB. The levels of [11C](-)3-MPB were much lower in all regions of the brain than [11C](+)3-MPB and [11C]4-MPB. Administration of scopolamine, a muscarinic cholinergic antagonist, at a dose of 50 microg/kg reduced the radioactivity of [11C](+)3-MPB and [11C]4-MPB in all regions except the cerebellum. Time-activity curves of [11C](+)3-MPB peaked in all regions, while those of [11C]4-MPB showed gradual increases with time in all regions except the thalamus, pons, and cerebellum. Two graphical analyses (Logan plot and Patlak plot) with plasma radioactivity as an input function into the brain were applied to evaluate receptor binding in vivo. [11C](+)3-MPB showed linear regression curves on Logan plot analysis and nonlinear curves on Patlak plot in all regions, suggesting that [11C](+)3-MPB bound reversibly to the muscarinic receptors. The in vivo binding parameters as well as uptake at 60-91 min postinjection of [11C](+)3-MPB were consistent with muscarinic receptor density in the brain as reported in vitro.     

Age-related changes in muscarinic cholinergic receptors in the living brain: a PET study using N-[11C]methyl-4-piperidyl benzilate combined with cerebral blood flow measurement in conscious monkeys

The effects of changes in regional cerebral blood flow (rCBF) with aging on muscarinic cholinergic receptor binding were evaluated with [15O]H(2)O and N-[11C]methyl-4-piperidyl benzilate (4-MPB) in the living brains of young (5.9+/-1.8 years old) and aged (19.0+/-3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo with [11C]4-MPB, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and graphical Patlak plot analysis was applied. In addition, two-compartment model analysis using the radioactivity curve in the cerebellum as an input function (reference analysis) was also applied to determine the distribution volume (DV=K(1)/k(2)') for [11C]4-MPB. With metabolite-corrected arterial input, Patlak plot analysis of [11C]4-MPB indicated a regionally specific decrease in muscarinic cholinergic receptor binding in vivo in the frontal and temporal cortices as well as the striatum in aged compared with young animals, showing no correlation with the degree of reduced rCBF. In contrast, on the reference analysis with cerebellar input of [11C]4-MPB, all regions assayed except the pons showed a significant age-related decrease of DV, and the degree of reduction of DV was correlated with that of rCBF. These results demonstrated the usefulness of kinetic analysis of [11C]4-MPB with metabolite-corrected arterial input, not with reference region's input, as an indicator of the aging process of cortical muscarinic cholinergic receptors in vivo measured by PET with less blood flow dependency.     

Cholinergic neuronal modulation alters dopamine D2 receptor availability in vivo by regulating receptor affinity induced by facilitated synaptic dopamine turnover: positron emission tomography studies with microdialysis in the conscious monkey brain.

To evaluate the cholinergic and dopaminergic neuronal interaction in the striatum, the effects of scopolamine, a muscarinic cholinergic antagonist, on the striatal dopaminergic system were evaluated multi-parametrically in the conscious monkey brain using high-resolution positron emission tomography in combination with microdialysis. l-3,4-Dihydroxyphenylalanine (l-[beta-(11)C]DOPA) and 2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane ([beta-(11)C]CFT) were used to measure dopamine synthesis rate and dopamine transporter (DAT) availability, respectively. For assessment of dopamine D(2) receptor binding in vivo, [(11)C]raclopride was applied because this labeled compound, which has relatively low affinity to dopamine D(2) receptors, was hypothesized to be sensitive to the striatal synaptic dopamine concentration. Systemic administration of scopolamine at doses of 10 and 100 microg/kg dose-dependently increased both dopamine synthesis and DAT availability as measured by l-[beta-(11)C]DOPA and [beta-(11)C]CFT, respectively. Scopolamine decreased the binding of [(11)C]raclopride in a dose-dependent manner. Scopolamine induced no significant changes in dopamine concentration in the striatal extracellular fluid (ECF) as determined by microdialysis. However, scopolamine dose-dependently facilitated the striatal ECF dopamine induced by the DAT inhibitor GBR12909 at a dose of 0.5 mg/kg. Scatchard plot analysis in vivo of [(11)C]raclopride revealed that scopolamine reduced the apparent affinity of dopamine D(2) receptors. These results suggested that the inhibition of muscarinic cholinergic neuronal activity modulates dopamine turnover in the striatum by simultaneous enhancement of the dynamics of dopamine synthesis and DAT availability, resulting in no significant changes in apparent "static" ECF dopamine level but showing a decrease in [(11)C]raclopride binding in vivo attributable to the reduction of affinity of dopamine D(2) receptors.     

(R)-N-[11C]methyl-3-pyrrolidyl benzilate,a high-affinity reversible radioligand for PET studies of the muscarinic acetylcholine receptor

We recently reported the synthesis and binding affinity of ligands for the muscarinic acetylcholine receptor (mAChR) based on both the pyrrolidyl and piperidyl benzilate scaffold. One of these, (R)-3-pyrrolidyl benzilate, was successfully radiolabeled with [(11)C]methyl triflate and the resulting compound, (R)-N-[(11)C]methyl-3-pyrrolidyl benzilate (3-[(11)C]NMPYB), was evaluated as a reversible, acetylcholine-sensitive tracer for the mAChR (K(i) of unlabeled 3-NMPYB is 0.72 nM). This compound displayed high, receptor-mediated retention in regions of the mouse and rat brain known to have high concentrations of mAChRs. Moreover, bolus studies in a pigtail monkey showed that this compound had superior clearance from the brain when compared to muscarinic radiotracers previously employed in human PET studies. Infusion studies in the same monkey revealed that it was possible to achieve equilibrium of radiotracer distribution for 3-[(11)C]NMPYB in both the striatum and cortex. Sensitivity to endogenous acetylcholine levels was evaluated by injecting phenserine (5 mg/kg) into rats prior to administration of 3-[(11)C]NMPYB in an equilibrium infusion protocol. This pretreatment produced a modest, statistically significant decrease (9-11%) in the distribution volume ratios for muscarinic receptor rich regions of the rat brain as compared to controls.     

In vivo PET study of cerebral [11C] methyltetrahydroaminoacridine distribution and kinetics in healthy human subjects

It is unclear whether the palliative effects of tetrahydroaminoacridine (THA) (tacrine, Cognex) on the clinical symptoms of patients affected by Alzheimer's disease (AD) are the result of its inhibitory activity on acetylcholinesterase or on other complex sites of action. In order to investigate the cerebral distribution and kinetics of THA in the human brain in vivo, we performed positron emission tomography (PET) imaging with [11C]N-methyl-tetrahydro-aminoacridine (MTHA) in healthy human volunteers. After intravenous injection, [11C]MTHA crossed the blood-brain barrier and reached its maximum uptake between 10 and 40 minutes, depending on the brain regions. Uptake was higher in the grey matter structures, and lower in the white matter. After this peak, the radioactivity remained quasi- constant until 60 minutes in all regions with a half-life varying from 2.44 hours in the thalamus to 3.42 hours in the cerebral cortex. The ratios of regional to whole cerebral cortex brain radioactivity calculated between 50 and 70 minutes after the tracer injection were 1.14 +/- 0.04, 1.07 +/- 0. 03 and 1.06 +/- 0.04 in the putamen, cerebellum and thalamus, respectively. Overall, these results show that: (1) [11C]MTHA crosses the blood-brain barrier easily and is highly concentrated in the brain; (2) the regional brain distribution of [11C]MTHA does not parallel that of in vivo acetylcholinesterase (AChE) concentrations; and (3) the cerebral kinetics of [11C]MTHA are consistent with known plasmatic pharmacokinetics of THA in AD patients. We conclude that PET imaging with [11C]MTHA is a useful method for assessing the cerebral distribution and kinetics of THA in vivo.     

Differentiation of radioligand delivery and binding in the brain: validation of a two-compartment model for [11C]flumazenil

We recently developed a two-compartment, two-parameter tracer kinetic model to estimate the in vivo ligand transport rate (K1) and distribution volume (DV) for the benzodiazepine antagonist [11C]flumazenil (FMZ) as measured by positron emission tomography (PET). The aim of the present study was to validate that this simplified model provides a stable measure of regional benzodiazepine receptor availability even when ligand delivery is altered. Six young normal volunteers underwent two PET studies subsequent to intravenous injections of [11C]FMZ. Each FMZ study was immediately preceded by measurements of CBF following injection of [15O]water. One set of scans (water/FMZ) was acquired under resting conditions and the other set during audiovisual stimulation. Six additional volunteers underwent two FMZ studies under identical resting conditions. Parametric images were analyzed and a comparison of test-retest studies in the stimulation group revealed a significant increase of CBF and K1 of FMZ in the occipital cortex evoked by visual activation, whereas no regional changes were noted for the DV of FMZ. No significant changes were noted for either K1 or DV of FMZ when comparing studies in the rest-rest setting. The results indicate that the use of a simple two-compartment model for the tracer kinetic analysis of [11C]FMZ makes it possible to separate high-affinity binding from altered radio-ligand delivery to the human brain.     

Age-related changes in muscarinic cholinergic receptors in the living brain: a PET study using N-[C]methyl-4-piperidyl benzilate combined with cerebral blood flow measurement in conscious monkeys

The effects of changes in regional cerebral blood flow (rCBF) with aging on muscarinic cholinergic receptor binding were evaluated with [¹⁵O]H₂O and N-[¹¹C]methyl-4-piperidyl benzilate (4-MPB) in the living brains of young (5.9±1.8 years old) and aged (19.0±3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo with [¹¹C]4-MPB, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and graphical Patlak plot analysis was applied. In addition, two-compartment model analysis using the radioactivity curve in the cerebellum as an input function (reference analysis) was also applied to determine the distribution volume (DV=K₁/k₂′) for [¹¹C]4-MPB. With metabolite-corrected arterial input, Patlak plot analysis of [¹¹C]4-MPB indicated a regionally specific decrease in muscarinic cholinergic receptor binding in vivo in the frontal and temporal cortices as well as the striatum in aged compared with young animals, showing no correlation with the degree of reduced rCBF. In contrast, on the reference analysis with cerebellar input of [¹¹C]4-MPB, all regions assayed except the pons showed a significant age-related decrease of DV, and the degree of reduction of DV was correlated with that of rCBF. These results demonstrated the usefulness of kinetic analysis of [¹¹C]4-MPB with metabolite-corrected arterial input, not with reference region’s input, as an indicator of the aging process of cortical muscarinic cholinergic receptors in vivo measured by PET with less blood flow dependency.     

脑乙酰胆碱酯酶的PET显像剂[^11C]4-乙酰氧基-N-甲基哌啶在大鼠体内分布的研究

目的：研究脑乙酰胆碱酯酶（AChE）活性的正电子断层扫描（PET）诊断显像剂[^11C]4-乙酰氧基-N-甲基哌啶（[^11C]MP4A）在大鼠体内的摄取和分布。方法：正常雌性SD大鼠尾静脉注射[^11C]MP4A，按注射后5、15、25、35、45和60min时间点分组后断头处死动物，迅速取血，分离脏器和脑（大脑皮质、小脑和脑干），用自动γ放射性计数仪计算经衰变校正后不同时间点各脏器放射性摄取率，以％ID／g表示。结果：一次剂量的[^11C]MP4A静脉注射后，血中5min时％ID／g为（13．44±1．88），清除迅速，60min时接近于基线水平；各脏器组织中％ID／g 15-25min达到稳态；大脑皮质5min％ID／g为（2．26±0．29）。脑内各区域的摄取在注射后15～25min达到稳态。皮质的平均％ID/g〉小脑，两者比值范围为1．13～2．08；60min时小脑、脑干和皮质的％ID／g与其最高峰相比分别下降了77％、80％和40％；皮质放射活性清除最慢。结论：[^11C]MP4A血中清除迅速。脑各区域摄取在短时达到稳态，提示[^11C]MP4A有较好的脂溶性，易透过血脑屏障；皮质摄取比小脑高但清除慢，表明[^11C]MP4A能更好反映皮质AChE活性，适合作为脑AChE活性的PET诊断显像剂。     

Imaging muscarinic cholinergic receptors in human brain in vivo with Spect, [123I]4-iododexetimide, and [123I]4-iodolevetimide.

A method to image muscarinic acetylcholine receptors (muscarinic receptors) noninvasively in human brain in vivo was developed using [123I]4-iododexetimide ([123I]IDex), [123I]4-iodolevetimide ([123I]ILev), and single photon emission computed tomography (SPECT). [123I]IDex is a high-affinity muscarinic receptor antagonist. [123I]ILev is its pharmacologically inactive enantiomer and measures nonspecific binding of [123I]IDex in vitro. Regional brain activity after tracer injection was measured in four young normal volunteers for 24 h. Regional [123I]IDex and [123I]ILev activities were correlated early after injection, but not after 1.5 h. [123I]IDex activity increased over 7-12 h in neocortex, neostriatum, and thalamus, but decreased immediately after the injection peak in cerebellum. [123I]IDex activity was highest in neostriatum, followed in rank order by neocortex, thalamus, and cerebellum. [123I]IDex activity correlated with muscarinic receptor concentrations in matching brain regions. In contrast, [123I]ILev activity decreased immediately after the injection peak in all brain regions and did not correspond to muscarinic receptor concentrations. [123I]IDex activity in neocortex and neostriatum during equilibrium was six to seven times higher than [123I]ILev activity. The data demonstrate that [123I]IDex binds specifically to muscarinic receptors in vivo, whereas [123I]ILev represents the nonspecific part of [123I]IDex binding. Subtraction of [123I]ILev from [123I]IDex images on a pixel-by-pixel basis therefore reflects specific [123I]IDex binding to muscarinic receptors. Owing to its high specific binding, [123I]IDex has the potential to measure small changes in muscarinic receptor characteristics in vivo with SPECT. The use of stereoisomerism directly to measure nonspecific binding of [123I]IDex in vivo may reduce complexity in modeling approaches to muscarinic acetylcholine receptors in human brain.     

Mapping muscarinic receptors in human and baboon brain using [N-11C-methyl]-benztropine

The muscarinic cholinergic system has been mapped in vivo in human and baboon brain using [N-11C-methyl]-benztropine and high resolution positron emission tomography (PET). [N-11C-methyl]-benztropine uptake was observed in frontal, parietal, occipital, and temporal cortices as well as in subcortical structures including the corpus striatum and thalamus. Uptake continued to increase in baboon and human brain in all areas over an 80 minute experimental period with the exception of the cerebellum where the accumulation of radioactivity began to decrease by 25 minutes postinjection. The ratio of incorporation of [N-11C-methyl]-benztropine between corpus striatum/cerebellum was 1.53 and 1.46 in humans and baboons, respectively, at 60 minutes. Blocking studies in baboons using the muscarinic cholinergic antagonists scopolamine and benztropine and the muscarinic cholinergic agonist pilocarpine combined with blocking studies in humans using benztropine indicate that the binding of this compound is specific for the muscarinic cholinergic system. Pretreatment with the potent dopamine reuptake blocker nomifensine produced no effect on the incorporation of radioactivity in any baboon brain region examined. Analysis of labelled plasma metabolites indicates that in humans, the rate of metabolism of [N-11C-methyl]-benztropine is slow (83.0% unchanged at 30 minutes postinjection) differing quite dramatically from the rate of metabolism observed in baboons (43.4% unchanged at 30 minutes postinjection). These data combined with postmortem studies in humans and primates demonstrate that [N-11C-methyl]-benztropine is a suitable muscarinic cholinergic ligand for use in humans and baboons with PET.     

Positron emission tomography (PET) studies of dopaminergic/cholinergic interactions in the baboon brain

Interactions between the dopaminergic D2 receptor system and the muscarinic cholinergic system in the corpus striatum of adult female baboons (Papio anubis) were examined using positron emission tomography (PET) combined with [18F]N-methylspiroperidol [( 18F]NMSP) (to probe D2 receptor availability) and [N-11C-methyl]benztropine (to probe muscarinic cholinergic receptor availability). Pretreatment with benztropine, a long-lasting anticholinergic drug, bilaterally reduced the incorporation of radioactivity in the corpus striatum but did not alter that observed in the cerebellum or the rate of metabolism of [18F]NMSP in plasma. Pretreatment with unlabelled NMSP, a potent dopaminergic antagonist, reduced the incorporation of [N-11C-methyl]benztropine in all brain regions, with the greatest effect being in the corpus striatum greater than cortex greater than thalamus greater than cerebellum, but did not alter the rate of metabolism of the labelled benztropine in the plasma. These reductions in the incorporation of either [18F]NMSP or [N-11C-methyl]benztropine exceeded the normal variation in tracer incorporation in repeated studies in the same animal. This study demonstrates that PET can be used as a tool for investigating interactions between neurochemically different yet functionally linked neurotransmitters systems in vivo and provides insight into the consequences of multiple pharmacologic administration.     

A Novel Fluorine-18 β-Fluoroethoxy Organophosphate Positron Emission Tomography Imaging Tracer Targeted to Central Nervous System Acetylcholinesterase

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The synthesis and preclinical evaluation in rhesus monkey of [18F]MK‐6577 and [11C]CMPyPB glycine transporter 1 positron emission tomography radiotracers

Two positron emission tomography radiotracers for the glycine transporter 1 (GlyT1) are reported here. Each radiotracer is a propylsulfonamide‐containing benzamide and was labeled with either carbon‐11 or fluorine‐18. [¹¹C]CMPyPB was synthesized by the alkylation of a 3‐hydroxypyridine precursor using [¹¹C]MeI, and [¹⁸F]MK‐6577 was synthesized by a nucleophilic aromatic substitution reaction using a 2‐chloropyridine precursor. Each tracer shows good uptake into rhesus monkey brain with the expected distribution of highest uptake in the pons, thalamus, and cerebellum and lower uptake in the striatum and gray matter of the frontal cortex. In vivo blockade and chase studies of [¹⁸F]MK‐6577 showed a large specific signal and reversible binding. In vitro autoradiographic studies with [¹⁸F]MK‐6577 showed a large specific signal in both rhesus monkey and human brain slices and a distribution consistent with the in vivo results and those reported in the literature. In vivo metabolism studies in rhesus monkeys demonstrated that only more‐polar metabolites are formed for each tracer. Of these two tracers, [¹⁸F]MK‐6577 was more extensively characterized and is a promising clinical positron emission tomography tracer for imaging GlyT1 and for measuring GlyT1 occupancy of therapeutic compounds. Synapse, 2011. © 2010 Wiley‐Liss, Inc.     

A two-compartment description and kinetic procedure for measuring regional cerebral [11C]nomifensine uptake using positron emission tomography

S-[11C]Nomifensine (S-[11C]NMF) is a positron-emitting tracer suitable for positron emission tomography, which binds to both dopaminergic and noradrenergic reuptake sites in the striatum and the thalamus. Modelling of the cerebral distribution of this drug has been hampered by the rapid appearance of glucuronide metabolites in the plasma, which do not cross the blood--brain barrier. To date, [11C]NMF uptake has simply been expressed as regional versus nonspecific cerebellar activity ratios. We have calculated a "free" NMF input curve from red cell activity curves, using the fact that the free drug rapidly equilibrates between red cells and plasma, while glucuronides do not enter red cells. With this free [11C]NMF input function, all regional cerebral uptake curves could be fitted to a conventional two-compartment model, defining tracer distribution in terms of [11C]NMF regional volume of distribution. Assuming that the cerebellar volume of distribution of [11C]NMF represents the nonspecific volume of distribution of the tracer in striatum and thalamus, we have calculated an equilibrium partition coefficient for [11C]NMF between freely exchanging specific and nonspecific compartments in these regions, representing its "binding potential" to dopaminergic or noradrenergic uptake sites (or complexes). This partition coefficient was lower in the striatum when the racemate rather than the active S-enantiomer of [11C]NMF was administered. In the striatum of patients suffering from Parkinson's disease and multiple-system atrophy, the specific compartmentation of S-[11C]NMF was significantly decreased compared with that of age-matched volunteers.     

Inverse agonist histamine H3 receptor PET tracers labelled with carbon‐11 or fluorine‐18

Two histamine H3 receptor (H3R) inverse agonist PET tracers have been synthesized and characterized in preclinical studies. Each tracer has high affinity for the histamine H3 receptor, has suitable lipophilicity, and neither is a substrate for the P‐glycoprotein efflux pump. A common phenolic precursor was used to synthesize each tracer with high specific activity and radiochemical purity by an alkylation reaction using either [¹¹C]MeI or [¹⁸F]FCD₂Br. Autoradiographic studies in rhesus monkey and human brain slices showed that each tracer had a widespread distribution with high binding densities in frontal cortex, globus pallidus and striatum, and lower uptake in cerebellum. The specificity of this expression pattern was demonstrated by the blockade of the autoradiographic signal by either the H3R agonist R‐α‐methylhistamine or a histamine H3R inverse agonist. In vivo PET imaging studies in rhesus monkey showed rapid uptake of each tracer into the brain with the same distribution seen in the autoradiographic studies. Each tracer could be blocked by pretreatment with a histamine H3R inverse agonist giving a good specific signal. Comparison of the in vitro metabolism of each compound showed slower metabolism in human liver microsomes than in rhesus monkey liver microsomes, with each compound having a similar clearance rate in humans. The in vivo metabolism of 1b in rhesus monkey showed that at 60 min, ～35% of the circulating counts were due to the parent. These tracers are very promising candidates as clinical PET tracers to both study the histamine H3R system and measure receptor occupancy of H3R therapeutic compounds. Synapse 63:1122–1132, 2009. © 2009 Wiley‐Liss, Inc.     

Localization of serotonin 5-HT2 receptors in living human brain by positron emission tomography using N1-([11C]-methyl)-2-Br-LSD

N1-([11C]-Methyl)-2-Br-LSD ([11C]-MBL) has been developed as a positron emission tomography (PET) imaging agent for serotonin 5-HT2 receptors. In vitro receptor binding assays with nonradioactive MBL show high-affinity binding to serotonin 5-HT2 receptors (Ki = 0.5 nM), a secondary interaction of 8-fold lower affinity with dopamine D2 receptors, and low-affinity interactions with alpha 1-adrenergic as well as serotonin 5-HT1 receptors. Intravenous injection of [11C]-MBL in a baboon led to selective labeling of cortical regions that was markedly blocked by prior administration of ketanserin, a selective 5-HT2 receptor antagonist. Clinical trials with [11C]-MBL have been conducted in seven normal human volunteers, and the regional distribution of radioactivity in the brain was distinctly serotonergic. Labeling was highest in frontal, temporal, and parietal cortex with lower levels observed in caudate and putamen. The tracer rapidly washed from the cerebellum and the low levels of activity in this brain region were used to define nonspecific binding. The maximum specificity was reached between 30 and 60 minutes postinjection when frontal cortex to cerebellum ratios ranged from 1.7 for a 52-year-old male to 2.7 for a 30-year-old male. In agreement with previous studies, a trend towards lower ratios (lower serotonin 5-HT2 receptor levels) was observed in older volunteers. These studies indicate that [11C]-MBL is a selective radioligand that can be used to monitor serotonin 5-HT2 receptor densities in vivo in most regions of the human brain.     

Selectivity of pirenzepine in the central nervous system. III. Differential effects of multiple pirenzepine and scopolamine administrations on muscarinic receptors as measured autoradiographically

The effects of intrahippocampal injections of scopolamine and pirenzepine on muscarinic receptor binding were examined by quantitative autoradiographic techniques. Brain slices from animals which had received 7 injections of either scopolamine (n = 5) or pirenzepine (n = 5) over a 22-day injection schedule were compared with slices from 5 saline-injected controls for receptor binding to the whole slice and within selected regions of the brain as measured autoradiographically. The total number of receptors was determined from direct binding assays with 1-[3H]quinuclidinyl-benzilate ([3H]-1-QNB), while the binding of the selective ligands pirenzepine, carbamylcholine, and scopolamine was examined through inhibition studies. The data from the whole slices indicated that pirenzepine-treated animals contained more receptors for [3H]-1-QNB than either saline- or scopolamine-injected controls. Slices from the same animals also displayed a lower affinity for pirenzepine. Slices from scopolamine-injected animals revealed neither an increase in receptor number nor a decrease in antagonist affinity, although the binding of the agonist carbamylcholine was increased. Quantitative analysis of the autoradiograms generated from the slices indicated that the increase in receptor number for pirenzepine-injected animals was predominantly within the cerebral and cingulate cortices. The inhibition by pirenzepine was also lower in these areas in the same group of animals. Agonist inhibition was altered in the central layers of the cerebral cortex and in the pretectal area in scopolamine-treated animals. The results suggest separate mechanisms of drug action and adaptation for pirenzepine and scopolamine.     

In vivo studies of [125I]iodobenzamide and [11C]iodobenzamide: a ligand suitable for positron emission tomography and single photon emission tomography imaging of cerebral D2 dopamine receptors.

Iodobenzamide (IMB) labeled with either [11C] or [125I] was studied in mice and baboons. Pharmacological studies demonstrated an in vivo binding profile compatible with D2 dopamine receptors. Mouse biodistribution studies with both [11C]IMB and [125I]IMB showed a similar brain distribution of radioactivity. Mouse [125I]IMB studies with amphetamine and reserpine pretreatment suggested that IMB may be less susceptible to endogenous dopamine competition for D2 receptor binding in vivo as compared to raclopride. Preliminary baboon studies showed haloperidol competition for IMB binding sites.