PROTEIN INCORPORATION AND AXOPLASMIC FLOW IN MOTONEURON FIBRES FOLLOWING INTRA‐CORD INJECTION OF LABELLED LEUCINE *

SUBSTANCES synthesized in the cell bodies of neurons are believed to be carried by axoplasmic flow down inside the axons. The substances so carried maintain axonic excitability and most likely supply some part of the transmitter substances or their enzymes at the nerve terminals as well as trophic material to the postsynaptic cells (WEISS, 1961 ; GUTMANN, 1964; LUBI~SKA, 1964; OCHS, 1966). DROZ and LEBLOND (1962) injected the precursor [3H]leucine systemically and demonstrated with radioautography an intra-axonic location of the labelled material. However, their systemic injections of precursor gave rise to a large component of active material incorporated by the Schwann cells and the level of material incorporated inside the nerve fibres was relatively low. In our earlier axoplasmic flow studies, the precursor [32P]orthophosphate was injected directly into the spinal cord of cats: into the ventral horn region of the lumbar 7th and sacral 1st ventral segments (OCHS and BURGER, 1958; OCHS, DALRYMPLE and RICHARDS, 1962). As the ventral roots of these segments are relatively long, the distribution of the incorporated labelled material which has moved out in their fibres can more readily be measured. The proportion of activity found in the proximal as compared to the distal portions of the ventral roots changed with time after injection and this indicated an axoplasmic flow. The level of activity was high because the precursor was injected in the near vicinity of the motoneuron cell bodies in high activities. This technique of local injection was used in the present studies with the precursors [3H]leucine and [14C]leucine. It was inferred from an earlier study with [3H]leucine that the labelled material present in axons was due to [3H]leucine incorporated in the soluble protein and/or small particulate components (OCHS, 1965a, 1966). In those studies, the fibres were induced to 'bead' by stretching the roots and then subjecting them to freeze-substitution (OCHS, 1963, 19653). While the exact mechanism underlying beading is as yet unknown, practical use was made of the phenomenon in order to partition the axoplasmic contents. Electron microscopy of the fibres showed the neurofilaments and neurotubules to be present in relatively high concentrations in the constricted regions, while the fluid portion of the axoplasm appeared to be squeezed laterally into the expanded part of the beads. In radioautographs of these beaded fibres, the incorporated material was found in the expanded portions; more so than in association with the filamentous material in the constrictions. This would be the case if the labelled material were in the soluble protein and/or small particulates which could be free to move into the beaded portions. IN MOTONEURON FIBRES FOLLOWING INTRA-CORD