Effects of Axotomy on Protein Synthesis in the Rat Hypoglossal Nucleus: Examination of the Influence of Local Recycling of Leucine Derived from Protein Degradation into the Precursor Pool

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Abstract
The quantitative autoradiographic l-[1-14C] leucine method for the determination of regional rates of cerebral protein synthesis (lCPSleu) requires knowledge of the degree of recycling of leucine derived from protein degradation into the precursor pool for protein synthesis, which can be evaluated by measuring λi; the steady-state ratio of the leucine-specific activity in the precursor amino acid pool (tRNA-bound leucine) to that of the arterial plasma. To define the changes in lCPSleu during regeneration of the hypoglossal nerve, we examined the effects of axotomy on the value of λi. Because the concentration of tRNA-bound leucine in the hypoglossal nucleus is too low to measure, we measured the equivalent ratio for the total acid-soluble pool (ψi) and applied the linear relationship between λ and ψ found in the whole brain to calculate a value of λi in the ipsilateral and contralateral hypoglossal nuclei of 22 adult female rats 2, 18, 35, and 60 days after unilateral hypoglossal axotomy. Statistically significant but quantitatively inconsequential effects of axotomy on values of ψi and λi were found. Therefore, the mean value for λi (0.64) of the left and right hypoglossal nuclei in all 22 axotomized rats was used to calculate lCPSleu. In a separate group of 15 unilaterally axotomized rats, lCPSleu was determined by the autoradiographic technique; lCPSleu was increased on the axotomized side by 23% on day 2, 30% on day 18, and 13% on day 35. By postaxotomy day 60, lCPSleu had returned to normal.