Cerebellar nucleocortical projection with a survey of factors affecting the transport of radioactive tracers

Abstract

The nucleocortical projection has been studied with the method of anterograde transport of tritiated amino acids and autoradiography. It was observed that this projection is made up of a small number of fibers. Counts of silver grain aggregates in projection sites of the granular layer were compared with counts from material on the cuneocerebellar projection. As it had been concluded in previous studies that the projection is quantitatively important, several modifications of the experimental conditions were tested. Radioactive leucine of high or low specific activity as well as a mixture of amino acids were used. It was observed that labeling is always more intense with a high specific activity tracer. Variations of the aspect and density of the projection were observed with survival periods lasting from a few hours to several days. With survival times between 9 and 15 hours, round aggregates of silver grains measuring from 10 to 20 microns were observed in the granular layer of labeled regions. At longer survival periods, the terminal structures in the granular layer appeared as coarse linear aggregates. It is suggested that the latter represent preterminal labeled fragments while the round accumulations correspond to terminal rosettes. Maximum density of the labeling in the granular layer occurred between 9 and 15 hours. At 2 days, the density was lowest, and at 5 or 6 days it was somewhat higher. Even under optimal conditions, the density of the nucleocortical terminals was low.