[P-32] ORTHOPHOSPHATE AND [S-35] METHIONINE LABEL SEPARATE POOLS OF NEUROFILAMENTS WITH MARKEDLY DIFFERENT AXONAL-TRANSPORT KINETICS IN MOUSE RETINAL GANGLION-CELLS IN-VIVO

Newly synthesized neurofilament proteins become highly phosphorylated within axons. Within 2 days after intravitreously injecting normal adu lt mice with [P-32]orthophosphate, we observed that neurofilaments alo ng the entire length of optic axons were radiolabeled by a soluble P-3 2-carrier that was axonally transported faster than neurofilaments. P- 32-incorporation into neurofilament proteins synthesized at the time o f injection was comparatively low and minimally influenced the labelin g pattern along axons. P-32-incorporation into axonal neurofilaments w as considerably higher in the middle region of the optic axons. This c haracteristic non-uniform distribution of radiolabel remained nearly u nchanged for at least 22 days. During this interval, less than 10% of the total P-32-labeled neurofilaments redistributed from the optic ner ve to the optic tract. By contrast, newly synthesized neurofilaments w ere selectively pulse-labeled in ganglion cell bodies by intravitreous injection of [S-35]methionine and about 60% of this pool translocated by slow axoplasmic transport to the optic tract during the same time interval. These findings indicate that the steady-state or resident po ol of neurofilaments in axons is not identical to the newly synthesize d neurofilament pool, the major portion of which moves at the slowest rate of axoplasmic transport. Taken together with earlier studies, the se results support the idea that, depending in part on their phosphory lation state, transported neurofilaments can interact for short or ver y long periods with a stationary but dynamic neurofilament lattice in axons.