Determination of Regional Rates of Cerebral Protein Synthesis in Vivo with L-[1-14C]Leucine as the Tracer Amino Acid

Abstract

The quantitative autoradiographic L-[1-14C]leucine method for the determination of regional rates of cerebral protein synthesis in vivo takes into account recycling of unlabeled leucine derived from protein degradation into the precursor pool for protein synthesis. We have evaluated the degree of recycling in whole brain by measuring the ratio of the apparent steady state leucine specific activity in the precursor pool (tRNA-bound leucine) to that in arterial plasma. In normal, conscious, adult rats this ratio (λWB) equals 0.58 indicating that 42% of leucine in the precursor amino acid pool is derived from protein breakdown. Evaluation of λi in local brain regions indicates that the degree of recycling does vary regionally. Local rates of leucine incorporation into protein determined with the quantitative autoradiographic technique and regional values of λi ranged from 11.0 in hypoglossal nucleus to 3.8 nmol/g/min in white matter. The average rate in the brain as a whole was found to be 6.1 nmol/g/min. Results of our studies of regeneration in the hypoglossal nucleus and plasticity in the developing monkey visual system suggest that chronic changes in functional activity in a pathway are more likely than acute changes to result in effects on rates of protein synthesis in structures of the pathway.