Increased rates of brain protein synthesis during [N1,N2] sleep: L-[1-11C]leucine PET studies in human subjects.

During sleep, reduced brain energy demands provide an opportunity for biosynthetic processes like protein synthesis. Sleep is required for some forms of memory consolidation which requires de novo protein synthesis. We measured regional cerebral protein synthesis rates (rCPS) in human subjects to ascertain how rCPS is affected during sleep. Subjects underwent three consecutive L-[1-11C]leucine PET scans with simultaneous polysomnography: 1. rested awake, 2. sleep-deprived awake, 3. sleep. Measured rCPS were similar across the three conditions. Variations in sleep stage times during sleep scans were used to estimate rCPS in sleep stages under the assumption that measured rCPS is the weighted sum of rCPS in each stage, with weights reflecting time and availability of [11C]leucine in that stage. During sleep scans, subjects spent most of the time in N2, N3, and awake and very little time in N1 and REM; rCPS in N1 and REM could not be reliably estimated. When stages N1 and N2 were combined [N1,N2], estimates of rCPS were more robust. In selective regions, estimated rCPS were statistically significantly higher (30-39%) in [N1,N2] compared with N3; estimated rCPS in N3 were similar to values measured in sleep-deprived awake scans. Results indicate increased rates of protein synthesis linked to [N1,N2] sleep.

Decreased rates of cerebral protein synthesis in conscious young adults with fragile X syndrome demonstrated by L-[1-11C]leucine PET.

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. Fragile X mental retardation protein, a putative translation suppressor, is significantly reduced in FXS. The prevailing hypothesis is that rates of cerebral protein synthesis (rCPS) are increased by the absence of this regulatory protein. We have previously reported increased rCPS in the Fmr1 knockout mouse model of FXS. To address the hypothesis in human subjects, we measured rCPS in young men with FXS with L-[1-11C]leucine PET. In previous studies we had used sedation during imaging, and we did not find increases in rCPS as had been seen in the mouse model. Since mouse measurements were conducted in awake animals, we considered the possibility that sedation may have confounded our results. In the present study we used a modified and validated PET protocol that made it easier for participants with FXS to undergo the study awake. We compared rCPS in 10 fragile X participants and 16 healthy controls all studied while awake. Contrary to the prevailing hypothesis and findings in Fmr1 knockout mice, results indicate that rCPS in awake participants with FXS are decreased in whole brain and most brain regions by 13-21% compared to healthy controls.

Regional rates of brain protein synthesis are unaltered in dexmedetomidine sedated young men with fragile X syndrome: A L-[1-11C]leucine PET study.

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. Fragile X mental retardation protein (FMRP), a putative translation suppressor, is absent or significantly reduced in FXS. One prevailing hypothesis is that rates of protein synthesis are increased by the absence of this regulatory protein. In accord with this hypothesis, we have previously reported increased rates of cerebral protein synthesis (rCPS) in the Fmr1 knockout mouse model of FXS and others have reported similar effects in hippocampal slices. To address the hypothesis in human subjects, we applied the L[1-11C]leucine PET method to measure rCPS in adults with FXS and healthy controls. All subjects were males between the ages of 18 and 24 years and free of psychotropic medication. As most fragile X participants were not able to undergo the PET study awake, we used dexmedetomidine for sedation during the imaging studies. We found no differences between rCPS measured during dexmedetomidine-sedation and the awake state in ten healthy controls. In the comparison of rCPS in dexmedetomidine-sedated fragile X participants (n = 9) and healthy controls (n = 14) we found no statistically significant differences. Our results from in vivo measurements in human brain do not support the hypothesis that rCPS are elevated due to the absence of FMRP. This hypothesis is based on findings in animal models and in vitro measurements in human peripheral cells. The absence of a translation suppressor may produce a more complex response in pathways regulating translation than previously thought. We may need to revise our working hypotheses regarding FXS and our thinking about potential therapeutics.

Substitution of venous for arterial blood sampling in the determination of regional rates of cerebral protein synthesis with L-[1-11C]leucine PET: A validation study.

We developed and validated a method to estimate input functions for determination of regional rates of cerebral protein synthesis (rCPS) with L-[1-11C]leucine PET without arterial sampling. The method is based on a population-derived input function (PDIF) approach, with venous samples for calibration. Population input functions were constructed from arterial blood data measured in 25 healthy 18-24-year-old males who underwent L-[1-11C]leucine PET scans while awake. To validate the approach, three additional groups of 18-27-year-old males underwent L-[1-11C]leucine PET scans with both arterial and venous blood sampling: 13 awake healthy volunteers, 10 sedated healthy volunteers, and 5 sedated subjects with fragile X syndrome. Rate constants of the L-[1-11C]leucine kinetic model were estimated voxel-wise with measured arterial input functions and with venous-calibrated PDIFs. Venous plasma leucine measurements were used with venous-calibrated PDIFs for rCPS computation. rCPS determined with PDIFs calibrated with 30-60 min venous samples had small errors (RMSE: 4-9%), and no statistically significant differences were found in any group when compared to rCPS determined with arterial input functions. We conclude that in young adult males, PDIFs calibrated with 30-60 min venous samples can be used in place of arterial input functions for determination of rCPS with L-[1-11C]leucine PET.

Rates of cerebral protein synthesis in primary visual cortex during sleep-dependent memory consolidation, a study in human subjects.

If protein synthesis during sleep is required for sleep-dependent memory consolidation, we might expect rates of cerebral protein synthesis (rCPS) to increase during sleep in the local brain circuits that support performance on a particular task following training on that task. To measure circuit-specific brain protein synthesis during a daytime nap opportunity, we used the L-[1-(11)C]leucine positron emission tomography (PET) method with simultaneous polysomnography. We trained subjects on the visual texture discrimination task (TDT). This was followed by a nap opportunity during the PET scan, and we retested them later in the day after the scan. The TDT is considered retinotopically specific, so we hypothesized that higher rCPS in primary visual cortex would be observed in the trained hemisphere compared to the untrained hemisphere in subjects who were randomized to a sleep condition. Our results indicate that the changes in rCPS in primary visual cortex depended on whether subjects were in the wakefulness or sleep condition but were independent of the side of the visual field trained. That is, only in the subjects randomized to sleep, rCPS in the right primary visual cortex was higher than the left regardless of side trained. Other brain regions examined were not so affected. In the subjects who slept, performance on the TDT improved similarly regardless of the side trained. Results indicate a regionally selective and sleep-dependent effect that occurs with improved performance on the TDT.

Effects of shortened scanning intervals on calculated regional rates of cerebral protein synthesis determined with the L-[1-11C]leucine PET method.

To examine effects of scan duration on estimates of regional rates of cerebral protein synthesis (rCPS), we reanalyzed data from thirty-nine previously reported L-[1-11C]leucine PET studies. Subjects consisted of 12 healthy volunteers studied twice, awake and under propofol sedation, and 15 subjects with fragile X syndrome (FXS) studied once under propofol sedation. All scans were acquired on a high resolution scanner. We used a basis function method for voxelwise estimation of parameters of the kinetic model of L-[1-11C]leucine and rCPS over the interval beginning at the time of tracer injection and ending 30, 45, 60, 75 or 90 min later. For each study and scan interval, regional estimates in nine regions and whole brain were obtained by averaging voxelwise estimates over all voxels in the region. In all three groups rCPS was only slightly affected by scan interval length and was very stable between 60 and 90 min. Furthermore, statistical comparisons of rCPS between awake and sedated healthy volunteers provided almost identical results when they were based on 60 min scan data as when they were based on data from the full 90 min interval. Statistical comparisons between sedated healthy volunteers and sedated subjects with FXS also yielded almost identical results when based on 60 and 90 min scan intervals. We conclude that, under the conditions of our studies, scan duration can be shortened to 60 min without loss of precision.

Impact of tissue kinetic heterogeneity on PET quantification: case study with the L-[1-11C]leucine PET method for cerebral protein synthesis rates.

Functional quantification with PET is generally based on modeling that assumes tissue regions are kinetically homogeneous. Even in regions sufficiently small to approach homogeneity, spillover due to resolution limitations of PET scanners may introduce heterogeneous kinetics into measured data. Herein we consider effects of kinetic heterogeneity at the smallest volume accessible, the single image voxel. We used L-[1-11C]leucine PET and compared rates of cerebral protein synthesis (rCPS) estimated voxelwise with methods that do (Spectral Analysis Iterative Filter, SAIF) and do not (Basis Function Method, BFM) allow for kinetic heterogeneity. In high resolution PET data with good counting statistics BFM produced estimates of rCPS comparable to SAIF, but at lower computational cost; thus the simpler, less costly method can be applied. With poorer counting statistics (lower injected radiotracer doses), BFM estimates were more biased. In data smoothed to simulate lower resolution PET, BFM produced estimates of rCPS 9-14% higher than SAIF, overestimation consistent with applying a homogeneous tissue model to kinetically heterogeneous data. Hence with lower resolution data it is necessary to account for kinetic heterogeneity in the analysis. Kinetic heterogeneity may impact analyses of other tracers and scanning protocols differently; assessments should be made on a case by case basis.

Altered cerebral protein synthesis in fragile X syndrome: studies in human subjects and knockout mice.

Dysregulated protein synthesis is thought to be a core phenotype of fragile X syndrome (FXS). In a mouse model (Fmr1 knockout (KO)) of FXS, rates of cerebral protein synthesis (rCPS) are increased in selective brain regions. We hypothesized that rCPS are also increased in FXS subjects. We measured rCPS with the L-[1-(11)C]leucine positron emission tomography (PET) method in whole brain and 10 regions in 15 FXS subjects who, because of their impairments, were studied under deep sedation with propofol. We compared results with those of 12 age-matched controls studied both awake and sedated. In controls, we found no differences in rCPS between awake and propofol sedation. Contrary to our hypothesis, FXS subjects under propofol sedation had reduced rCPS in whole brain, cerebellum, and cortex compared with sedated controls. To investigate whether propofol could have a disparate effect in FXS subjects masking usually elevated rCPS, we measured rCPS in C57Bl/6 wild-type (WT) and KO mice awake or under propofol sedation. Propofol decreased rCPS substantially in most regions examined in KO mice, but in WT mice caused few discrete changes. Propofol acts by decreasing neuronal activity either directly or by increasing inhibitory synaptic activity. Our results suggest that changes in synaptic signaling can correct increased rCPS in FXS.

Use of spectral analysis with iterative filter for voxelwise determination of regional rates of cerebral protein synthesis with L-[1-11C]leucine PET.

A spectral analysis approach was used to estimate kinetic parameters of the L-[1-(11)C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS) on a voxel-by-voxel basis. Spectral analysis applies to both heterogeneous and homogeneous tissues; it does not require prior assumptions concerning number of tissue compartments. Parameters estimated with spectral analysis can be strongly affected by noise, but numerical filters improve estimation performance. Spectral analysis with iterative filter (SAIF) was originally developed to improve estimation of leucine kinetic parameters and rCPS in region-of-interest (ROI) data analyses. In the present study, we optimized SAIF for application at the voxel level. In measured L-[1-(11)C]leucine PET data, voxel-level SAIF parameter estimates averaged over all voxels within a ROI (mean voxel-SAIF) generally agreed well with corresponding estimates derived by applying the originally developed SAIF to ROI time-activity curves (ROI-SAIF). Region-of-interest-SAIF and mean voxel-SAIF estimates of rCPS were highly correlated. Simulations showed that mean voxel-SAIF rCPS estimates were less biased and less variable than ROI-SAIF estimates in the whole brain and cortex; biases were similar in white matter. We conclude that estimation of rCPS with SAIF is improved when the method is applied at voxel level than in ROI analysis.

A spectral analysis approach for determination of regional rates of cerebral protein synthesis with the L-[1-(11)C]leucine PET method.

A spectral analysis approach was used to estimate kinetic model parameters of the L-[1-(11)C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS) in predefined regions of interest (ROIs). Unlike analyses based on the assumption that tissue ROIs are kinetically homogeneous, spectral analysis allows for heterogeneity within a region. To improve estimation performance, a new approach was developed-spectral analysis with iterative filter (SAIF). In simulation SAIF produced low bias, low variance estimates of the influx rate constant for leucine (K(1)), blood volume fraction (V(b)), fraction of unlabeled leucine in the tissue precursor pool for protein synthesis derived from arterial plasma (lambda), and rCPS. Simulation of normal count rate studies showed that SAIF applied to ROI time-activity curves (TACs) performed comparably to the basis function method (BFM) applied to voxel TACs when voxelwise estimates were averaged over all voxels in the ROI. At low count rates, however, SAIF performed better. In measured L-[1-(11)C]leucine PET data, there was good agreement between ROI-based SAIF estimates and average voxelwise BFM estimates of K(1), V(b), lambda, and rCPS. We conclude that SAIF sufficiently addresses the problem of tissue heterogeneity in ROI data and provides a valid tool for estimation of rCPS, even in low count rate studies.

Voxel-based estimation of kinetic model parameters of the L-[1-(11)C]leucine PET method for determination of regional rates of cerebral protein synthesis: validation and comparison with region-of-interest-based methods.

We adapted and validated a basis function method (BFM) to estimate at the voxel level parameters of the kinetic model of the L-[1-(11)C]leucine positron emission tomography (PET) method and regional rates of cerebral protein synthesis (rCPS). In simulation at noise levels typical of voxel data, BFM yielded low-bias estimates of rCPS; in measured data, BFM and nonlinear least-squares parameter estimates were in good agreement. We also examined whether there are advantages to using voxel-level estimates averaged over regions of interest (ROIs) in place of estimates obtained by directly fitting ROI time-activity curves (TACs). In both simulated and measured data, fits of ROI TACs were poor, likely because of tissue heterogeneity not taken into account in the kinetic model. In simulation, rCPS determined from fitting ROI TACs was substantially overestimated and BFM-estimated rCPS averaged over all voxels in an ROI was slightly underestimated. In measured data, rCPS determined by regional averaging of voxel estimates was lower than rCPS determined from ROI TACs, consistent with simulation. In both simulated and measured data, intersubject variability of BFM-estimated rCPS averaged over all voxels in a ROI was low. We conclude that voxelwise estimation is preferable to fitting ROI TACs using a homogeneous tissue model.

Propofol anesthesia does not alter regional rates of cerebral protein synthesis measured with L-[1-(11)C]leucine and PET in healthy male subjects.

We report regional rates of cerebral protein synthesis (rCPS) in 10 healthy young males, each studied under two conditions: awake and anesthetized with propofol. We used the quantitative L-[1-(11)C]leucine positron emission tomography (PET) method to measure rCPS. The method accounts for the fraction (lambda) of unlabeled leucine in the precursor pool for protein synthesis that is derived from arterial plasma; the remainder comes from proteolysis of tissue proteins. Across 18 regions and whole brain, mean differences in rCPS between studies ranged from -5% to 5% and were within the variability of rCPS in awake studies (coefficient of variation range: 7% to 14%). Similarly, differences in lambda (range: 1% to 4%) were typically within the variability of lambda (coefficient of variation range: 3% to 6%). Intersubject variances and patterns of regional variation were also similar under both conditions. In propofol-anesthetized subjects, rCPS varied regionally from 0.98+/-0.12 to 2.39+/-0.23 nmol g(-1) min(-1) in the corona radiata and in the cerebellum, respectively. Our data indicate that the values, variances, and patterns of regional variation in rCPS and lambda measured by the L-[1-(11)C]leucine PET method are not significantly altered by anesthesia with propofol.

Regional rates of cerebral protein synthesis measured with L-[1-11C]leucine and PET in conscious, young adult men: normal values, variability, and reproducibility.

We report regional rates of cerebral protein synthesis (rCPS) measured with the fully quantitative L-[1-(11)C]leucine positron emission tomography (PET) method. The method accounts for the fraction (lambda) of unlabeled amino acids in the precursor pool for protein synthesis derived from arterial plasma; the remainder (1-lambda) comes from tissue proteolysis. We determined rCPS and lambda in 18 regions and whole brain in 10 healthy men (21 to 24 years). Subjects underwent two 90-min dynamic PET studies with arterial blood sampling at least 2 weeks apart. Rates of cerebral protein synthesis varied regionally and ranged from 0.97+/-0.70 to 2.25+/-0.20 nmol/g per min. Values of rCPS were in good agreement between the two PET studies. Mean differences in rCPS between studies ranged from 9% in cortical regions to 15% in white matter. The lambda value was comparatively more uniform across regions, ranging from 0.63+/-0.03 to 0.79+/-0.02. Mean differences in lambda between studies were 2% to 8%. Intersubject variability in rCPS was on average 6% in cortical areas, 9% in subcortical regions, and 12% in white matter; intersubject variability in lambda was 2% to 8%. Our data indicate that in human subjects low variance and highly reproducible measures of rCPS can be made with the L-[1-(11)C]leucine PET method.

Use of acute hyperphenylalaninemia in rhesus monkeys to examine sensitivity and stability of the L-[1-11C]leucine method for measurement of regional rates of cerebral protein synthesis with PET.

We have previously shown by direct comparison with autoradiographic and biochemical measurements that the L-[1-(11)C]leucine positron emission tomography method provides accurate determinations of regional rates of cerebral protein synthesis (rCPS) and the fraction (lambda) of unlabeled leucine in the precursor pool for protein synthesis derived from arterial plasma. In this study, we examine sensitivity of the method to detect changes in lambda and stability of the method to measure rCPS in the face of these changes. We studied four isoflurane-anesthetized monkeys dynamically scanned with the high resolution research tomograph under control and mild hyperphenylalaninemic conditions. Hyperphenylalaninemia was produced by an infusion of phenylalanine that increased plasma phenylalanine concentrations three- to five-fold. In phenylalanine-infused monkeys, plasma leucine concentrations remained relatively constant, but values of lambda were statistically significantly decreased by 11% to 15%; rCPS was unaffected. Effects on lambda are consistent with competitive inhibition of leucine transport by increased plasma phenylalanine. The effect on lambda shows that competition for the transporter results in a reduction in the fraction of leucine in the precursor pool for protein synthesis coming from plasma. Even under these hyperphenylalaninemic conditions, rCPS remains unchanged due to the compensating increased contribution of leucine from protein degradation to the precursor pool.

Astrocyte activation in vivo during graded photic stimulation.

Astrocytes have important roles in control of extracellular environment, de novo synthesis of neurotransmitters, and regulation of neurotransmission and blood flow. All of these functions require energy, suggesting that astrocytic metabolism should rise and fall with changes in neuronal activity and that brain imaging can be used to visualize and quantify astrocytic activation in vivo. A unilateral photic stimulation paradigm was used to test the hypothesis that graded sensory stimuli cause progressive increases in the uptake coefficient of [2-(14)C]acetate, a substrate preferentially oxidized by astrocytes. The acetate uptake coefficient fell in deafferented visual structures and it rose in intact tissue during photic stimulation of conscious rats; the increase was highest in structures with monosynaptic input from the eye and was much smaller in magnitude than the change in glucose utilization (CMR(glc)) by all cells. The acetate uptake coefficient was not proportional to stimulus rate and did not correlate with CMR(glc) in resting or activated structures. Simulation studies support the conclusions that acetate uptake coefficients represent mainly metabolism and respond to changes in metabolism rate, with a lower response at high rates. A model portraying regulation of acetate oxidation illustrates complex relationships among functional activation, cation levels, and astrocytic metabolism.

Operational lumped constant for FDG in normal adult male rats.

UNLABELLED: We determined an operational value for the lumped constant to be used in measurements of the local rate of cerebral glucose use (lCMR(glc)) with FDG in normal adult male rats. METHODS: The standard quantitative autoradiographic method was used with 2-deoxy-d-(14)C-glucose ((14)C-DG) and with (14)C-FDG in awake normal adult male rats. Timed arterial blood samples were drawn for 45 min after the bolus and assayed for plasma glucose and (14)C concentrations. At the end of the 45-min experimental period, the rats were killed, and their brains were removed and divided in half sagittally. One hemisphere was immediately frozen and assayed for local (14)C concentrations by quantitative autoradiography; the other was weighed, homogenized in t-octylphenoxypolyethoxyethanol solution, and assayed for (14)C concentrations in the whole brain by liquid scintillation counting. Paired rats (3 pairs), one in each pair receiving (14)C-DG and the other receiving (14)C-FDG, were studied in parallel on the same day. Additional unpaired animals (n = 8) were studied with either (14)C-DG or (14)C-FDG but not in parallel on the same day. To calculate the lCMR(glc) in rats studied with (14)C-FDG, the rate constants for (14)C-FDG were estimated from the (14)C-DG values determined for rats and the (14)C-FDG/(14)C-DG ratios determined for humans. In all of the rats studied with either (14)C-DG or (14)C-FDG, the lCMR(glc) was first calculated in 12 representative brain structures with the lumped constant of 0.48 previously determined for (14)C-DG in rats. The ratio of the lCMR(glc) thus determined with (14)C-FDG to that determined with (14)C-DG for each structure was then multiplied by the lumped constant for (14)C-DG to estimate the lumped constant for (14)C-FDG. The lCMR(glc) and the lumped constant for FDG in the brain as a whole were similarly estimated from the tracer concentrations in the brain homogenates. RESULTS: The mean values for the lumped constant for FDG were found to be 0.71 and 0.70 in the autoradiographic assays and the assays with brain homogenates, respectively. CONCLUSION: The appropriate value for the lumped constant to be used in determinations of the lCMR(glc) in normal adult male rat studies with (18)F-FDG and small-animal PET scanners is 0.71.

Distribution of the 5-HT(1A) receptor antagonist [ (18)F]FPWAY in blood and brain of the rat with and without isoflurane anesthesia.

PURPOSE: To determine whether brain and plasma equilibrium of a proposed PET tracer for 5-HT(1A), [(18)F]FPWAY, can be achieved in a sufficiently short time for practical use of the brain to plasma equilibrium distribution ratio (DR) to monitor receptor availability with and without isoflurane anesthesia. METHODS: Awake (n=4) and isoflurane-anesthetized (n=4) rats were administered a continuous 60 min intravenous infusion of [(18)F]FPWAY with timed arterial blood sampling. Brains of the isoflurane-anesthetized rats were scanned with the ATLAS small animal PET scanner; awake rats were not. All rats were killed at 60 min and scanned postmortem for 15 min, followed by brain slicing for autoradiography. Several regions of interest (ROIs) were defined in the PET images as well as in the autoradiographic images. Regional DRs were calculated as total activity in the brain ROI divided by plasma [(18)F]FPWAY activity. RESULTS: DRs in the anesthetized animals were constant between 30 and 60 min, indicating that near equilibrium between brain and plasma had been achieved by approximately 30 min. DRs determined from postmortem PET data were higher in the isoflurane-anesthetized rats by 24% (not significant) and 33% (p=0.065) in whole brain and hippocampus, respectively. DRs determined from autoradiographic data were greater in isoflurane-anesthetized rats in medial hippocampus, lateral hippocampus, and cerebellum by 33% (p=0.054), 63% (p<0.01), and 32% (p<0.05), respectively. CONCLUSION: [(18)F]FPWAY could be an appropriate ligand for monitoring changes in receptor availability in the serotonergic system using a bolus/infusion paradigm. One possible explanation for higher DRs in anesthetized rats may be a reduction in endogenous 5-HT secretion under isoflurane anesthesia.

Resolution, sensitivity and precision with autoradiography and small animal positron emission tomography: implications for functional brain imaging in animal research.

Quantitative autoradiographic methods for in vivo measurement of regional rates of cerebral blood flow, glucose metabolism, and protein synthesis contribute significantly to our understanding of phsysiological and biochemical responses of the brain to changes in the environment. A disadvantage of these autoradiographic methods is that experimental animals can be studied only once. With the advent of small animal positron emission tomography (PET) and with increases in the sensitivity and spatial resolution of scanners it is now possible to use adaptations of these methods in experimental animals with PET. These developments allow repeated studies of the same animal, including studies of the same animal under different conditions, and longitudinal studies. In this review we summarize the tradeoffs between the use of autoradiography and small animal PET for functional brain imaging studies in animal research.

Measurement of regional rates of cerebral protein synthesis with L-[1-11C]leucine and PET with correction for recycling of tissue amino acids: I. Kinetic modeling approach.

Measurements of regional rates of cerebral protein synthesis (rCPS) require correction for the effect of recycling of tissue amino acids back into the precursor pool for protein synthesis. The fraction of the precursor pool derived from arterial plasma, lambda, can be evaluated as the steady-state ratio of the specific activity of leucine in the tissue tRNA-bound fraction to that in arterial plasma. While lambda can be directly measured in terminal experiments in animals, an alternative method is required for use with PET. We report a method to estimate lambda based on a kinetic model of labeled and unlabeled leucine and labeled CO2 in the tissue. The kinetic model is also used to estimate the amount of labeled protein and rCPS. We measured time courses of [14C]leucine, [14C]protein, and 14CO2 in the blood and brain of anesthetized rats and estimated parameters of the kinetic model from these data. Simulation studies based on the kinetic parameters were then performed to examine the feasibility of this approach for use with L-[1-11C]leucine and PET. Lambda and rCPS were estimated with low bias, which suggests that PET can be used for quantitative measurement of rCPS with L-[1-11C]leucine and a kinetic modeling approach for correction for recycling of tissue amino acids.