Axoplasmic transport in the toad Bufo marinus

Abstract

The rate and course of axoplasmic transport from the eighth dorsal root ganglion cell bodies into the sciatic nerve of the toad Bufo marinus were studied. Concentrated tritiated proline was hydraulically injected into a surgically exposed dorsal root ganglion of animals maintained at 19 ± 0.5 °C. At postinjection intervals of 1, 6, and 10 h, the animals were sacrificed and the dorsal root, ganglion, and sciatic nerve were removed bilaterally. The dorsal roots and peripheral nerves were cut into 3 mm segments measured from the ganglion. In some experiments all tissues were prepared for liquid scintillation counting techniques. In others the ganglion and every fifth 3 mm nerve segment were fixed in Bouin's fixative for radioautography, and the remaining tissue segments were prepared for liquid scintillation counting methods. Scintillation counts (counts/min) of consecutive segments along the labeled nerve were plotted against distance (mm) for each animal. Examination of these profiles showed a peak of radioactivity in the injected ganglion for each animal that was followed distally by an abrupt in the adjacent segments of the nerve. Radioactivity remained relatively stable in subsequent segments forming a plateau and then dropped to baseline levels forming a wavefront in the distal portion of the peripheral nerves of the 6 and 10 h toads. Movement of this wavefront during the 6 to 10 h time interval provided evidence for an axoplasmic flow rate of about 120 mm/day. Radioautographs of the ganglion and representative segments along the sciatic nerve were examined with both bright- and dark-field microscopy. Accumulations of silver grains were observed overlying the injected ganglion cell bodies and labeled axons of the nerve. Values for the number of silver grains/unit area were obtained from dark-field radioautographs of the nerve segments through the use of a computer-microscope system. The relative amount of radioactivity present in the axons was thereby demonstrated. Radioautographic data confirmed the axoplasmic flow rate of about 120 mm/day determined by liquid scintillation counting methods for the bulk of radioactive materials transported at this fast rate. In addition, the evaluation of radioautographic data suggests that an even faster flow rate of 185-–215 mm/day may exist for a small portion of the labeled materials transported in the axon.