(1) We have validated a method for quantitative determination of regional rates of cerebral protein synthesis (rCPS) with C-11 leucine and PET (see project MH000889). The method uses a kinetic modeling approach to estimate lamda, the fraction of the precursor pool for protein synthesis derived from arterial plasma, in order to correct for recycling of tissue amino acids (Schmidt et al., JCBFM 2005; 25:617-28). Because estimates of kinetic model rate constants decline with time in heterogeneous tissues until all precursor pools equilibrate with arterial plasma (Schmidt et al., JCBFM 1991; 11:10-24), estimated nlamda and rCPS may also change with time until equilibrium has been achieved. We are currently examining data from conscious human subjects dynamically scanned for 90 min after injection of C-11 leucine in order to determine the optimal scanning interval measurement of rCPS. Kinetic model rate constants and lamda have been estimated over varying time intervals for whole brain and several regions of interest and rCPS computed. Preliminary results suggest that an interval between 60 and 75 min may be optimal; these data were presented at the 23rd International Symposium on Cerebral Blood Flow & Metabolism and 8th International Conference on Quantification of Brain Function with PET (BrainPET07 Abstract BO2-6U). We are also examining data from anesthetized monkeys dynamically scanned for 120 min following injection of C-11 leucine in order to determine the optimal interval for measurement of rCPS in rhesus monkeys. ? ? (2) We have applied the C-11 leucine PET method to healthy human subjects and determined normal values and variability of rCPS as well as reproducibility of the method (Bishu et al., JCBFM 2008, 28:1502-1513; see project MH000889). Analyses were performed on a region-of-interest (ROI) basis; this provides quantitative measures of rCPS for preselected brain regions that have been outlined on each subjects MRI. We are currently extending the analyses to the voxel level, i.e., each volume element in the image will be analyzed to produce parametric images of rCPS and of the individual parameters of the kinetic model. This allows not only new visual representations of the data, but also the ability to see heterogeneity of rates of protein synthesis within individual ROIs. Furthermore, this analysis method may reduce effects of tissue heterogeneity on kinetic model estimates. Due to the high number of voxels within each image (106V107), however, it is not feasible to use conventional nonlinear least squares (NLLS) algorithms because of their high computational cost. We have adapted a basis function method (BFM) to the estimation problem that reduces computation time to 4V6 hours/image, and have validated this approach by comparison of its results with those obtained by use of NLLS methods. Preliminary results indicate that BFM and NLLS estimates are in good agreement. Voxelwise estimates of rCPS were found to be somewhat lower on average than those obtained by ROI-based analyses. This may be due to larger tissue heterogeneity effects with ROI-based than voxel-based methods at a measurement time before all precursor pools in the tissue have equilibrated with arterial plasma. A manuscript detailing these findings is in preparation.? ? (3) 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. 2-14Cacetate is a substrate preferentially oxidized by astrocytes and is a potential tracer for brain imaging with quantitative autoradiography to visualize and quantify astrocytic activation in vivo. In a unilateral photic stimulation paradigm that was used to test the hypothesis that graded sensory stimuli cause progressive increases in the uptake coefficient of 2-14Cacetate, the acetate uptake coefficient fell in deafferented visual structures and it rose in intact tissue during photic stimulation of conscious rats. The increase in the acetate uptake coefficient, however, was not proportional to the increase in stimulus rate and was not proportional to increases in the rate of glucose utilization by all cells. Simulation studies were undertaken to investigate whether acetate uptake was limited by delivery of tracer to the tissue. Simulation results support the conclusion that acetate uptake coefficients represent mainly metabolism and respond to changes in metabolic rate, but with progressively lower responses at progressively higher metabolic rates. Therefore, acetate metabolism may actually increase to a much greater extent than is reflected in autoradiographic images. Results have been published in the Journal of Neurochemistry (Dienel, Schmidt and Cruz, J Neurochem, 2007).
Showing the most recent 10 out of 11 publications
