Proliferation of primary sensory neurons in adult rat dorsal root ganglion and the kinetics of retrograde cell loss after sciatic nerve section

This study was aimed at measuring the kinetics of retrograde death among primary sensory neurons axotomized by transection of the ipsilateral sciatic nerve in adult rats. Using electrophysiological and retrograde transport methods, we first determined that most sciatic afferents enter the spinal cord along the L₄ and L₅ dorsal roots (DRs), and that about 54% of the cells in the L₄ and L₅ dorsal root ganglia (DRGs) project an axon into the sciatic nerve. Knowing this value, we could then calculate the rate of loss of axotomized neurons from the overall rate of neuron loss in the DRGs at different times after the lesion. Following unilateral sciatic neurectomy, we found a steady falloff in the ratio of DRG neurons on the operated versus the intact control sides in cresyl-violet-stained serial paraffin sections. We were surprised to note, however, that on the control side there was a steady increase in the cell count with age. Counts done on a series of unoperated rats of various ages confirmed this natural increase. Overall, new neurons accrete at an average rate of 18.1 cells per day to the combined L₄ and L₅ DRGs, nearly doubling their numbers during the adult life of the animal. The new cells add mostly to the small-diameter neuronal compartment. Evidence from neonatally operated rats indicates that the decline in the ratio of neurons in operated versus control DRGs following sciatic nerve section in the adult results more from a halt in the accretion of new neurons to the sciatic compartment than from frank cell death. From our data, we calculate that the loss of axotomized neurons occurs at a rate of only about 8% per 100 postoperative days.