Kinetic analysis of 3-quinuclidinyl 4-[125I]iodobenzilate transport and specific binding to muscarinic acetylcholine receptor in rat brain in vivo: implications for human studies

Radioiodinated R- and S-Quinuclidinyl derivatives of RS-benzilate (R- and S-125IQNB) have been synthesized for quantitative evaluation of muscarinic acetylcholine receptor binding in vivo. Two sets of experiments were performed in rats. The first involved determining the metabolite-corrected blood concentration and tissue distribution of tracer R-IQNB (active enantiomer) and S-IQNB (inactive enantiomer) in brain 1 min to 26 h after intravenous injection. The second involved the measurement of brain tissue washout over a 2-min period after loading the brain by an intracarotid artery injection of the ligands. Various pharmacokinetic models were tested, which included transport across the blood-brain barrier (BBB), nonspecific binding, low-affinity binding, and high-affinity binding. Our analysis demonstrated that the assumptions of rapid equilibrium across the BBB and rapid nonspecific binding are incorrect and result in erroneous estimates of the forward rate constant for binding at the high-affinity receptor sites (k3). The estimated values for influx across the BBB (K1), the steady-state accumulation rate in cerebrum (K), and the dissociation rate constant at the high-affinity site (k4) of R-IQNB were independent of the specific compartmental model used to analyze these data (K1 approximately 0.23 ml/min/g, K approximately 0.13 ml/min/g, and k4 approximately 0.0019 min-1 for caudate). In contrast, the estimated values of k3 and the efflux rate constant (k2) varied over a 10-fold range between different compartmental models (k3 approximately 2.3-22 min-1 and k2 approximately 1.6-16 min-1 in caudate), but their ratios were constant (k3/k2 approximately 1.4). Our analysis demonstrates that the estimates of k3 (and derived values such as the binding potential) are model dependent, that the rate of R-IQNB accumulation in cerebrum depends on transport across the BBB as well as the rate of binding, and that uptake in cerebrum is essentially irreversible during the first 360 min after intravenous administration. Graphical analysis was consistent with compartmental analysis of the data and indicated that steady-state uptake of R-IQNB in cerebrum is established within 1-5 min after intravenous injection. We propose a new approach to the analysis of R-IQNB time-activity data that yields reliable quantitative estimates of k3, k4, and the nonspecific binding equilibrium constant (Keq) by either compartmental or graphical analysis. The approach is based on determining the free unbound fraction of radiolabeled ligand in blood and an estimate of K1.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of vascular activity in the determination of rate constants for the uptake of 18F-labeled 2-fluoro-2-deoxy-D-glucose: error analysis and normal values in older subjects

Regional cerebral blood volume (CBV) can be calculated using data obtained during the kinetic analysis of 18F-labeled 2-fluoro-2-deoxy-D-glucose (FDG) uptake measured by positron emission tomography (PET). As a result the influence of vascular activity upon the determination of FDG rate constants can be minimized. The method is investigated by simulation experiments and by analysis of PET studies on seven older, healthy human volunteers aged 52-70 years. The accuracy of measured FDG rate constants k1, k2, and k3, obtained either by omitting the early portion of the uptake curve or by explicit inclusion of CBV as a fit parameter, is compared. The root mean square error in measured rate constant for the latter method is equivalent to that obtained by omitting the first 2.5-3 min of tissue data and neglecting the CBV term. Hence, added information about the physiological state of the tissue is obtained without compromising the accuracy of the (FDG) rate constant measurement. In hyperemic tissue the explicit determination of the vascular fraction results in more accurate estimates of the FDG rate constants. The ratio of CBV determined by this method to CBV obtained using C15O in six subjects with CBV in the normal range was 0.92 +/- 0.32. A comparison of the CBV image obtained by this method with that obtained using C15O in an arteriovenous malformation case demonstrates the accuracy of the approach over a wide range of CBV values. The mean value for CBV fraction in gray matter obtained by this method in the older control group was 0.040 +/- 0.014. Average gray matter rate constants obtained were k1 = 0.084 +/- 0.012, k2 = 0.150 +/- 0.071, and k3 = 0.099 +/- 0.045 min-1.     

Effect of aging on lazabemide binding,monoamine oxidase activity and monoamine metabolites in human frontal cortex

Summary Age-related modifications of monoamine oxidase-A and -B (MAO-A and MAO-B) and amine metabolite concentrations were studied in human frontal cortex taken postmortem from 22 subjects of various ages (21–75 years). Qualitative and quantitative analysis for MAO-B was provided by kinetic studies with a specific radioligand, [³H]lazabemide. The data demonstrated a significant (P < 0.05) positive correlation between the density of [³H]lazabemide binding sites (B_max) and age of the subject, without showing an apparent modification in the dissociation constant (K_D) of the radioligand. In parallel experiments, MAO-B but not MAO-A activity was shown to correlate with age (P < 0.05). The concentrations of the amine metabolites 4-hydroxy-3-methoxyphenylacetic acid (HVA), 5-hydroxyindole-3-acetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), 4-hydroxy-3-methoxyphenylglycol (MHPG) and 3,4-dihydroxyphenylglycol (DHPG) were all devoid of a correlation with age. Neither did the concentrations of these metabolites relate to the respective subject's MAO-B enzymatic activity nor to [³H]lazabemide B_max. A correlation, though rather weak, was obtained between MAO-A activity and MHPG concentration (P=0.045). The MAO-A and -B enzyme characteristics in subjects who had committed suicide (n=9) did not differ from those of subjects deceased for other causes (n=13). Among the measured monoamine metabolites the concentrations of DOPAC and HVA were higher in the suicide versus control group (P < 0.05). The present data confirm in a direct manner that the increase in MAO-B activity in aging brain is due to an enhancement of the number of active sites of the enzyme and not through modifications of its kinetic characteristics. Furthermore, that neither the characteristics nor the activity of the enzyme are changed in the frontal cortex of suicide victims compared to control subjects.     

Dissociation between I2-imidazoline receptors and MAO-B activity in platelets of patients with Alzheimer's type dementia.

The I2-imidazoline receptor is expressed in brain and platelets and could represent a new binding domain on MAO-B enzyme. Brain I2-imidazoline receptors and MAO-B sites have been found to be increased in Alzheimer's disease. The study sought to evaluate I2-imidazoline receptors and MAO-B activity in platelets from patients with Alzheimer's type dementia (ATD) and matched controls. Preliminary saturation experiments of [3H]idazoxan binding to platelet purified mitochondrial membranes were performed to determine the maximal number of binding sites (Bmax) and the apparent dissociation constant (Kd). Afterwards, the I2-imidazoline receptor density ([3H]idazoxan at 8 and 20 nM in the presence of 2 x 10(-6) M efaroxan) was evaluated in 20 patients with ATD and 17 controls. MAO-B activity was quantified by [14C]PEA oxidation. All subjects were screened for cognitive evaluation by the Mini-Mental State Examination. The density of I2-imidazoline receptors was similar in ATD patients (8.4 and 14.3 fmol/mg protein) and controls (8.3 and 14.0 fmol/mg protein). MAO-B activity was 22% higher in ATD subjects. Significant correlations between I2-imidazoline receptors and MAO-B activity were observed. No relationships between I2-imidazoline receptors or MAO-B activity and the cognitive score were observed. In conclusion, platelet I2-imidazoline receptors do not show the increase of I2-imidazoline receptors previously observed in brain of subjects with ATD. The dissociation between I2-imidazoline receptors and MAO-B in platelets suggests that the enzyme contributes to but not exclusively represents the I2-imidazoline receptor.     

The retention of [99mTc]-d,l-HM-PAO in the human brain after intracarotid bolus injection: a kinetic analysis

[99mTc]-d,l-HM-PAO (HM-PAO) was injected rapidly into the internal carotid artery and its retention in the brain was recorded by external scintillation cameras in eight human subjects. A model is described based on three compartments: the lipophilic tracer in the blood pool of the brain, the lipophilic tracer inside the brain, and the hydrophilic form retained in the brain. The retention curve initially drops abruptly, corresponding to the nonextracted fraction of the injectate leaving the brain; it then falls exponentially towards the asymptotic level of the fractional steady-state retention R. Cerebral blood flow (F) was measured using the xenon-133 intracarotid injection method. The first-pass extraction E of HM-PAO was calculated from F using an empiric regression equation. The residue curves for the whole brain after intracarotid HM-PAO injection were analyzed to yield a retention fraction (R') and the brain clearance backflux constant of lipophilic HM-PAO (k). From the kinetic model and the measured values of R', k, and F, the following parameter values could be calculated: the average retained fraction of all tracer supplied to the brain, R = 0.38 +/- 0.05 (mean +/- SD), the conversion rate constant (lipophilic to hydrophilic tracer) in the brain k3 = 0.80 +/- 0.12 min-1, the efflux rate constant (brain to blood) k2 = 0.69 +/- 0.11 min-1, the conversion/clearance ratio alpha = k3/k2 = 1.18 +/- 0.25, the influx (blood clearance) constant K1 = 0.45 +/- 0.11 ml/g/min, and the brain/blood partition ratio lambda = K1/k2 = 0.67 +/- 0.23 ml/g. Using the kinetic model and assuming constancy of alpha, an algorithm was developed that corrects for the blood flow dependent backflux of HM-PAO and results in a more linear relation between regional cerebral blood flow (rCBF) and HM-PAO distribution.     

PET quantification of muscarinic cholinergic receptors with [N-11C-methyl]-benztropine and application to studies of propofol-induced unconsciousness in healthy human volunteers

This work evaluated kinetic analysis methods for estimation of the receptor availability of the muscarinic receptor using dynamic positron emission tomography (PET) studies with [N-(11)C-methyl]-benztropine. The study also investigated the effect of propofol on central muscarinic receptor availability during general anesthesia. Six volunteers were scanned three times, once for baseline while awake, once during unconsciousness, and once after recovery to conscious level. An irreversible two-tissue compartment model was used to estimate the [N-(11)C-methyl]-benztropine specific binding rate constant k(3), a measure of muscarinic receptor availability. Two different estimation methods were used: 1) optimization with positivity constraints on all the parameters; 2) optimization with additional constraints determined from a one-tissue compartment fit to the cerebellum. In regions with low to middle muscarinic receptor density, the k(3) values from method (2) had lower standard errors than that for method (1) and gave a higher correlation with the density of muscarinic receptors measured in human tissue by in vitro studies (r(2) of 0.98 for Method 2 and r(2) of 0.72 for Method 1). But the k(3) values determined by Method 2 had higher errors for regions with high muscarinic receptor density compared to Method 1. For both methods the mean k(3) values during unconsciousness were generally lower than those during awake for most regions evaluated. Therefore, the method with additional constraints derived from the cerebellum (Method 2) was deemed superior for regions with low to middle muscarinic receptor density, while the method with positivity constraint is the better choice in the regions with high muscarinic receptor density. Our results also suggest the existence of propofol-related reductions in muscarinic receptor availability.     

Quantification of neuroreceptors in the living human brain. I. Irreversible binding of ligands

A first step in the quantification of receptor density in the living human brain is the measurement of the binding of a labeled ligand to the receptor in question. In the present study, we determined the rate of binding of 11C-labeled N-methylspiperone (NMSP) to the D2 dopamine receptor in 11 normal volunteers, using a three-compartment model to relate the time integral of the measured plasma concentration to the distribution of the tracer in the caudate nucleus. The plasma concentrations of NMSP were separated from the contaminating metabolites by the ratio of radioactivities in cerebellum and blood plasma. Plasma concentrations calculated in this way agreed with plasma concentrations determined by HPLC. The rate of binding of labeled NMSP to its receptors (k3) was defined as the product of the bimolecular association rate (kon) and the quantity of available receptors (B'max) and calculated as the ratio between the steady-state rate of accumulation and the volume of distribution of labeled NMSP in the caudate nucleus. The average value of k3 in the 11 normal volunteers was 0.065 min-1. The fractional clearance of labeled NMSP from the caudate nucleus (k2) was 0.070 min-1 and thus close to the value of k3. We also examined several indexes of binding based on ratios between different regions in brain. The indexes required that binding be negligible compared to the efflux of labeled NMSP (i.e., k2 much greater than k3) and therefore yielded incorrectly low values of k3. Thus, the only accurate approach used measured plasma concentrations to estimate transfer constants at steady state and yielded the absolute rate of binding k3. The approach is applicable to other irreversibly bound ligands.     

Kinetic analysis of [(11)C]MP4A using a high-radioactivity brain region that represents an integrated input function for measurement of cerebral acetylcholinesterase activity without arterial blood sampling.

N -[(11)C]methylpiperidin-4-yl acetate ([(11)C]MP4A) is an acetylcholine analog. It has been used successfully for the quantitative measurement of acetylcholinesterase (AChE) activity in the human brain with positron emission tomography (PET). [(11)C]MP4A is specifically hydrolyzed by AChE in the brain to a hydrophilic metabolite, which is irreversibly trapped locally in the brain. The authors propose a new method of kinetic analysis of brain AChE activity by PET without arterial blood sampling, that is, reference tissue-based linear least squares (RLS) analysis. In this method, cerebellum or striatum is used as a reference tissue. These regions, because of their high AChE activity, act as a biologic integrator of plasma input function during PET scanning, when regional metabolic rates of [(11)C]MP4A through AChE (k(3); an AChE index) are calculated by using Blomqvist's linear least squares analysis. Computer simulation studies showed that RLS analysis yielded k(3) with almost the same accuracy as the standard nonlinear least squares (NLS) analysis in brain regions with low (such as neocortex and hippocampus) and moderately high (thalamus) k(3) values. The authors then applied these methods to [(11) C]MP4A PET data in 12 healthy subjects and 26 patients with Alzheimer disease (AD) using the cerebellum as the reference region. There was a highly significant linear correlation in regional k(3) estimates between RLS and NLS analyses (456 cerebral regions, [RLS k(3) ] = 0.98 x [NLS k(3) ], r = 0.92, P < 0.001). Significant reductions were observed in k(3) estimates of frontal, temporal, parietal, occipital, and sensorimotor cerebral neocortices (P < 0.001, single-tailed t-test), and hippocampus (P = 0.012) in patients with AD as compared with controls when using RLS analysis. Mean reductions (19.6%) in these 6 regions by RLS were almost the same as those by NLS analysis (20.5%). The sensitivity of RLS analysis for detecting cortical regions with abnormally low k 3 in the 26 patients with AD (138 of 312 regions, 44%) was somewhat less than NLS analysis (52%), but was greater than shape analysis (33%), another method of [(11)C]MP4A kinetic analysis without blood sampling. The authors conclude that RLS analysis is practical and useful for routine analysis of clinical [(11)C]MP4A studies.     

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

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

Monoamine oxidases of the brains and livers of macaque and cercopithecus monkeys.

We assessed the two forms of monoamine oxidase (MAO), MAO-A and MAO-B, in discrete regions of the brain and in cerebral micro- and macrovessels, choroid plexus, and liver of three species of monkeys: African Green, rhesus, and cynomolgus. MAO was determined by specific [3H]pargyline binding which is stoichiometric and irreversible and by measuring the rate of oxidation of several substrates. Cerebral micro- and macrovessels had low MAO content. Regional brain MAO did not vary by more than one-fold in the brains of each of the three species of monkeys and was higher in the basal ganglia than in the cerebral cortex or cerebellum. MAO in the choroid plexus was low, while the liver had higher MAO activity than any of the brain samples. The vast majority of MAO in all the tissues that we examined was of the MAO-B type, and specific [3H]pargyline binding correlated well with the oxidation rate of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. These results show marked similarities in brain MAO distribution between monkey and man. Or the three monkey species, the African Green monkey had the lowest MAO activity in its cerebral microvessels, which constitute the blood-brain barrier, although the small number of observations in each group did not allow statistical analyses of the differences.     

Chronic haloperidol affects striatal D2-dopamine receptor reappearance after irreversible receptor blockade

The time course of recovery of [3H]spiperone binding in the rat striatum after administration of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was studied in chronically haloperidol-treated rats (0.5 mg/kg, i.p., twice a day for two weeks). Chronic neuroleptic treatment significantly enhanced the [3H]spiperone Bmax value. EEDQ (6.0 mg/kg, i.p.) produced a similar profound decrease of [3H]spiperone binding site density in both saline- and haloperidol-treated rats. However, the receptor degradation rate constant in the haloperidol-treated animals (k = 0.0051 h-1) and the receptor production rate (r = 1.6 fmol/mg prot/h) were lower than in the saline-treated rats (k = 0.0074 h-1; r = 1.8 fmol/mg prot/h). These results are different from what is found in 6-OH-dopamine lesioned rats. D2-receptor recovery after EEDQ administration is enhanced in chronically (4-5 weeks) denervated striatum (Brain Research, 329 (1985) 225-231) while the degradation rate constant is unchanged. Thus, the present results indicate that chronic haloperidol treatment reduces both the degradation and production rates of striatal D2-receptors.     

5-Hydroxy-L-[beta-11C]tryptophan versus alpha-[11C]methyl-L-tryptophan for positron emission tomography imaging of serotonin synthesis capacity in the rhesus monkey brain.

The purpose of this study was to compare two positron emission tomography (PET) tracers that were developed to follow serotonin (5HT) synthesis by performing sequential PET scanning of the same rhesus monkey (n=4) on the same day. alpha-[11C]Methyl-L-tryptophan ([11C]AMT) and 5-Hydroxy-L-[beta-11C]tryptophan ([11C]HTP) are substrates in the first and second enzymatic steps, respectively, in the biosynthesis of 5HT. Regional net accumulation rate constants were derived from kinetic (two-tissue compartment model with irreversible tracer trapping) and graphic (Patlak) analyses, using the arterial plasma concentrations as input. The kinetic data analysis showed that the rate constant for the transfer of [11C]HTP into the brain (K1) was higher than that for [11C]AMT in the striatum and thalamus but was similar in other brain regions. The rate constant for tracer trapping (k3) was also higher for [11C]HTP than for [11C]AMT in the striatum (0.046+/-0.024 versus 0.019+/-0.006 min(-1)) and thalamus (0.039+/-0.013 versus 0.016+/-0.007 min(-1)). In agreement with previously reported regional HTP accumulation rates, the net accumulation rate constant (K(acc)) for [11C]HTP was also higher in these regions than in other brain regions; this is in contrast to the uniform distribution of [11C]AMT K(acc) values. This suggests that the regional net accumulation rates obtained with these two PET tracers will be of different magnitude, which might be related to the activity of each targeted enzyme.     

Kinetic modeling of N-[11C]methylpiperidin-4-yl propionate: alternatives for analysis of an irreversible positron emission tomography trace for measurement of acetylcholinesterase activity in human brain.

N-[11C]Methylpiperidin-4-yl propionate ([11C]PMP) is a substrate for hydrolysis by acetylcholinesterase (AChE). This work evaluates kinetic analysis alternatives for estimation of relative AChE activity using dynamic positron emission tomography (PET) studies of [11C]PMP. The PET studies were performed on three groups of subjects: (1) 12 normal volunteer subjects, aged 20 to 45 years, who received a single intravenous injection of 16 to 32 mCi of [11C]PMP; (2) six subjects, aged 21 to 44 years, who received two 16-mCi injections of [11C]PMP (baseline and visual stimulation, respectively); and (3) five subjects, aged 24 to 40 years, who received two 16-mCi injections separated by 200 minutes (baseline and after a 1-hour constant infusion of 1.5 mg of physostigmine, respectively). Dynamic acquisition consisted of a 17-frame sequence over 80 minutes. All analysis methods were based on a first-order kinetic model consisting of two tissue compartments with the parameter k3, representing PMP hydrolysis, being the index of AChE activity. Four different schemes were used to estimate k3: (1) an unconstrained non-linear least-squares fit estimating blood-brain barrier transport parameters, K1 and k2, in addition to the hydrolysis rate constant k3; (2) and (3), two methods of constraining the fit by fixing the volume of distribution of free tracer (DVfree); and (4), a direct estimation of k3 without use of an arterial input function based on the shape of the tissue time-activity curve alone. Results showed that k3 values from the unconstrained fitting and no input methods were estimated with similar accuracy, whereas the two methods using DVfree constraints yielded similar results. The authors conclude that the optimal analysis method for [11C]PMP differs as a function of AChE activity. All four methods gave precise measures of k3 in regions with low AChE activity (approximately 10% coefficient of variation in cortex), but surprisingly, with unconstrained methods yielding estimates with lower variability than constrained methods. In regions with moderate to high AChE activity, constrained methods were required to yield meaningful estimates and were superior to the unconstrained methods.     

Kinetic analysis of central [76Br]bromolisuride binding to dopamine D2 receptors studied by PET.

The in vivo kinetic analysis of dopamine D2 receptors was obtained in baboon brain using positron emission tomography (PET) and [76Br]bromolisuride [( 76Br]BLIS) as radioligand. An injection of a trace amount of [76Br]BLIS was followed 3 h later by an injection of a mixture of [76Br]BLIS and BLIS in the same syringe (coinjection experiment). A third injection performed at 6 h was either an excess of unlabeled ligand (displacement experiment) or a second coinjection. This protocol allowed us to evaluate in the striatum of each animal and after a single experiment the quantity of available receptors (B'max) and the kinetic parameters including the association and dissociation rate constants (k + 1VR and k-1, respectively, where VR is the volume of reaction). The cerebellum data were fitted using a model without specific binding. All the parameters were estimated using nonlinear mathematical models of the ligand-receptor interactions including or not including nonspecific binding. The plasma time-concentration curve was used as an input function after correction for the metabolites. An estimate of standard errors was obtained for each PET study and for each identified parameter using the covariance matrix. The average values of B'max and KdVR were 73 +/- 11 pmol/ml tissue and 1.9 +/- 0.9 pmol/ml, respectively. The nonspecific binding was identifiable in the experiment where the last injection corresponded to a second coinjection. We found that approximately 6% of the striatal binding was nonspecific after a tracer injection of [76Br]BLIS. The nonspecific binding appeared to be reversible in the striatum but irreversible in the cerebellum.     

Monoamine oxidase B expression is selectively regulated by dexamethasone in cultured rat astrocytes

The influence of dexamethasone on monoamine oxidase (MAO) A and B expression and activity was investigated in primary cultures of rat type 1 astrocytes cultured under serum free, defined conditions. Dexamethasone treatment resulted in a dose- and time-dependent induction of MAO-B, but not of MAO-A, activity. The selective MAO-B increase was substantially reduced by the antagonist RU 486, thus suggesting a glucocorticoid receptor-mediated action of the hormone. Kinetic analysis showed an increase inV_max of MAO-B with no change in apparentK_m. The dexamethasone-induced selective rise in MAO-B activity appeared to be due to enhanced enzyme synthesis, since MAO-B mRNA was markedly increased by dexamethasone treatment and the recovery of MAO-B activity after its irreversible inhibition by deprenyl was more pronounced in the presence than in the absence of the hormone. Furthermore, the dexamethasone effect was abolished by the protein synthesis inhibitors actinomycin D or cycloheximide. The present study demonstrates that dexamethasone is able to selectively induce MAO-B in type 1 astrocytes and leads to speculation of a possible role for glucocorticoids in the increase in brain MAO-B associated with neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases.     

The effect of antipsychotic drugs on dopamine-receptors on caudate calf]

Using radioligand receptor binding assay (RRA), dopamine receptor in the calf caudate were identified with 3H-Spiperone as radioactive ligand. The total number of maximal binding sites (Bmax), the equilibrium dissociation affinity (Kd), the concentrations of drug required to inhibited 50% specific binding (Ic50) and competitive inhibition constant (Ki) of antipsychotics in calf caudate were determined by sturative binding experiment and inhibitive binding experiment. The result showed the mean Bmax was 0.659 pmol/mg protein membrane and Kd was 0.31 nmol/L. Scatchard analysis denotes that it was hyperbole and there were two binding positions in calf caudte. Antipsychotics can inhibit 3H-Spiperone binding specifically the order of affinity constants (1/Ki) was Haloperdol and sulpiride, Chlorpromazine and Clozapin, Tranquis and Taractan. This abilities of drugs to compete with 3H-Spi binding will be used as basis of RRA for measuring blood levels of antipsychotics.     

The quantitative analysis of D2-dopamine receptors in baboon striatum in vivo with 3-N-[2'-18F]fluoroethylspiperone using positron emission tomography

We used the ligand 3-N-[2'-18F]fluoroethylspiperone (FESP), which binds to D2-dopamine receptors in the striatum, and positron emission tomography (PET) to quantify striatal D2-dopamine densities (Bmax) and binding kinetics in baboon brain in vivo. Sequential PET scans were obtained for 4 h post injection. Various similar models based on a nonlinear kinetic four-compartment model that takes into account the effect of ligand specific activity were used. We investigated the effect of exact model configuration on the reliability of Bmax and other kinetic transfer coefficients. We found that with the ligand FESP and dynamic PET studies, the estimated values of Bmax and other model parameters are sensitive to the choice of model configuration, ligand specific activity, and data analysis technique. The limitations of the reliability of parameter estimates in a complex kinetic model for receptor ligands were studied in simulation calculations. Results showed that the accuracy of estimated values of Bmax is affected by both the ligand binding properties and the injected dose of ligand. The estimated average value of kinetic model parameters was as follows: ligand-receptor dissociation constant k4 = 0.0080 min-1; the product of ligand-receptor association constant and fraction of ligand available to bind to specific receptors f2ka = 0.0052 (min nM)-1; and D2-dopamine receptor density Bmax = 37.5 pmol g-1.     

Evaluation of simplified kinetic analyses for measurement of brain acetylcholinesterase activity using N-[11C]Methylpiperidin-4-yl propionate and positron emission tomography.

The applicability of two reference tissue-based analyses without arterial blood sampling for the measurement of brain regional acetylcholinesterase (AChE) activity using N-[11C]methylpiperidin-4-yl propionate ([11C]MP4P) was evaluated in 12 healthy subjects. One was a linear least squares analysis derived from Blomqvist's equation, and the other was the analysis of the ratio of target-tissue radioactivity relative to reference-tissue radioactivity proposed by Herholz and coworkers. The standard compartment analysis using arterial input function provided reliable quantification of k3 (an index of AChE activity) estimates in regions with low (neocortex and hippocampus), moderate (thalamus), and high (cerebellum) AChE activity with a coefficient of variation (COV) of 12% to 19%. However, the precise k3 value in the striatum, where AChE activity is the highest, was not obtained. The striatum was used as a reference because its time-radioactivity curve was proportional to the time integral of the arterial input function. Reliable k3 estimates were also obtained in regions with low-to-moderate AChE activity with a COV of less than 21% by striatal reference analyses, though not obtained in the cerebellum. Shape analysis, the previous method of direct k3 estimation from the shape of time-radioactivity data, gave k3 estimates in the cortex and thalamus with a somewhat larger COV. In comparison with the standard analysis, a moderate overestimation of k3 by 9% to 18% in the linear analysis and a moderate underestimation by 2% to 13% in the Herholz method were observed, which were appropriately explained by the results of computer simulation. In conclusion, simplified kinetic analyses are practical and useful for the routine analysis of clinical [11C]MP4P studies and are nearly as effective as the standard analysis for detecting regions with abnormal AChE activity.     

Multicompartmental analysis of [11C]-carfentanil binding to opiate receptors in humans measured by positron emission tomography

[11C]-Carfentanil is a high affinity opiate agonist that can be used to localize mu opiate receptors in humans by positron emission tomography (PET). A four-compartment model was used to obtain quantitative estimates of rate constants for receptor association and dissociation. PET studies were performed in five normal subjects in the absence and presence of 1 mg/kg naloxone. Arterial plasma concentration of [11C]-carfentanil and its labeled metabolites were determined during each PET study. The value of k3/k4 = Bmax/kD was determined for each subject in the presence and absence of naloxone. There was a significant reduction in the value of k3/k4 from 3.4 +/- 0.92 to 0.26 +/- 0.13 in the thalamus (p less than 0.01) and from 1.8 +/- 0.33 to 0.16 +/- 0.065 in the frontal cortex (p less than 0.001). Mean values of frontal cortex/occipital cortex and thalamus/occipital cortex ratios were determined for the interval 35-70 min after injection when receptor binding is high relative to nonspecific binding. The relationship between the measured region/occipital cortex values and the corresponding values of k3/k4 in the presence and absence of naloxone was: regions/occipital cortex = 0.95 + 0.74 (k3/k4) with r = 0.98 (n = 20). Simulation studies also demonstrated a linear relationship between the thalamus/occipital cortex or frontal cortex/occipital cortex ratio and k3/k4 for less than twofold increases or decreases in k3/k4. Simulation studies in which thalamic blood flow was varied demonstrated no significant effect on the region/occipital cortex ratio at 35-70 min for a twofold increase or fourfold decrease in blood flow. Therefore, the region/occipital cortex ratio can be used to quantitate changes in k3/k4 when tracer kinetic modeling is not feasible.     

Positron emission tomographic measurement of brain acetylcholinesterase activity using N-[(11)C]methylpiperidin-4-yl acetate without arterial blood sampling: methodology of shape analysis and its diagnostic power for Alzheimer's disease.

N-[11C]methylpiperidin-4-yl acetate ([11C]MP4A) is a radiotracer that has been used successfully for the quantitative measurement of acetylcholinesterase (AChE) activity in the human brain with positron emission tomography (PET) using a standard compartment model analysis and a metabolite-corrected arterial input function. In the current study, the authors evaluated the applicability of a simple kinetic analysis without blood sampling, namely shape analysis. First, the authors used computer simulations to analyze factors that affect the precision and bias of shape analysis, then optimized the shape analysis procedure for [11C]MP4A. Before shape analysis execution, the later part of dynamic PET data except for the initial 3 minutes were smoothed by fitting to a bi-exponential function followed by linear interpolation of 8 data points between each of adjacent scan frames. Simulations showed that shape analysis yielded estimates of regional metabolic rates of [11C]MP4A by AChE (k3) with acceptable precision and bias in brain regions with low k3 values such as neocortex. Estimates in regions with higher k3 values became progressively more inaccurate. The authors then applied the method to [11C]MP4A PET data in 10 healthy subjects and 20 patients with Alzheimer's disease (AD). There was a highly significant linear correlation in regional k3 estimates between shape and compartment analyses (300 neocortical regions, [shape k3] = 0.93 x [NLS k3], r = 0.89, P < 0.001). Significant reductions in k3 estimates of frontal, temporal, parietal, occipital, and sensorimotor cerebral cortices in patients with AD as compared with controls were observed when using shape analysis (P < 0.013, two-tailed t-test), although these reductions (17% to 20%) were somewhat less than those obtained by compartment analysis (22% to 27%). The sensitivity of shape analysis for detecting neocortical regions with abnormally low k3 in the 20 patients with AD (92 out of 200 regions, 46%) also was somewhat less than compartment analysis (136 out of 200 regions, 68%). However, taking its simplicity and noninvasiveness into account, the authors conclude that quantitative measurement of neocortical AChE activity with shape analysis and [11C]MP4A PET is practical and useful for clinical diagnosis of AD.