Axonal Transport of [35S]Methionine-Labeled Proteins in Two Intra-Brain Tracts of the Rat

[³⁵S]Methionine was stereotaxically injected into the dorsalateral geniculate body (DLGB) of adult male rats, and 1 h to 10 days post-injection the DLGB and projection site (striate cortex) were dissected out and solubilized in 1% sodium dodecyl sulfate. Samples were analyzed for acid-precipitable radioactivity, and radioactivity in different molecular weight classes was determined following discontinuous gel electrophoresis on both tube and slab gels. Acid-precipitable radioactivity in the DLGB peaked by 4 h and then declined over the time period studied. The molecular weight distribution pattern was complex and did not change appreciably with time. Radioactivity in the striate cortex arrived in at least three waves: rapidly transported proteins arrived between 2 and 4 h; a second wave of transport began to arrive at about 7 h post-injection and there was a slight rise in specific activity for 2 days; finally, at 3 days post-injection, there was a steep increase with the arrival of the bulk of the transported material. The electrophoretic distribution pattern of proteins arriving in the first wave included 40–50 identifiable bands ranging in molecular weight from 13,000 to 200,000. Of particular interest was a radioactive band of apparent molecular weight of 110,000, which was prominent at 4 h, but by 12 h showed very little labeling. The second wave of radioactivity contained primarily proteins of molecular weight classes already present, although there were quantitative differences. Several proteins in the molecular weight range of 43,000 to 78,000 were identifiable as characteristic of the third wave of transported material. Results from a study following injection of a hippocampus were similar: the electrophoretic distribution pattern of radioactive proteins extracted from the injected hippocampus resembled that of the DLGB, and also did not vary appreciably with time, while radioactive proteins in the contralateral hippocampus had an electrophoretic distribution pattern similar to that of the striate cortex and changed with time in a similar manner.