The axon reaction of lamprey spinal interneurons.

Axotomy and partial denervation of giant interneurons (GIs) and lateral cells (LCs) were produced by complete spinal transection in the larval lamprey spinal cord. Both cell types demonstrated a reduction in cytoplasmic basophilia, increase in cell size, nuclear eccentricity, and formation of a chromophilic nuclear cap. This was quantified in the case of cell diameter. During the first 8 weeks of recovery, the GIs with the largest diameters were found progressively further from the scar and this peak change moved at approximately 0.5 mm/day. The increase in size of GIs remained up to 20 weeks post-transection, long after the time required for their axons to regenerate across the scar and form functioning synapses. GIs injected intracellularly with horseradish peroxidase (HRP) also showed this increase in diameter as well as a simplification of their dendritic trees. Intracellular recordings from GIs revealed changes in the frequency and amplitude of spontaneous synaptic input. In the first two weeks after transection, spontaneous excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) were less frequent than in control cells. After 6 weeks of recovery they became more frequent than in control cells. EPSPs predominated in axotomized GIs, while in control cells they constituted only 36% of the total of spontaneous potentials. A reversible increase in the amplitude of these EPSPs occurred at 3-4 weeks of recovery time. The resting membrane potential was significantly reduced by the 6th week after transection and returned to normal after the 22nd week.