Comparison between the early changes in isoniazid intoxication and the chromatolytic response to nerve ligation in spinal ganglion cells of the rat.

The early effects of acute isoniazid intoxication and the early responses of the neuron to axotomy have been compared in lumbar spinal ganglion cells in the rat. The cells were studied by light and electron microscopy during the 4 days after a single oral dose (2g/kg) of isoniazid and for four days after application of a tight ligature to the sciatic nerve at the level of the sciatic notch. After axotomy a greater proportion of cells showed changes visible by light and electron microscopy than after INH. Small dark neurons were affected more rapidly and more severely by both insults than the large light cells as evidenced principally by changes in their ribosomal organization. Large light cells showed little response to INH. The first response to axotomy was visible about 12 h after injury. The response to INH began 24 h after the single oral dose. The early cell responses to INH therefore apparently precede axon degeneration which is first seen 72 h later and are thus probably a direct toxic effect.