A conditioning lesion of the peripheral axons of dorsal root ganglion cells accelerates regeneration of only their peripheral axons.

Axotomy of the peripheral axon of dorsal root ganglion (DRG) cells is known to result in chromatolysis and changes in protein synthesis in DRG cells. We investigated whether a stimulus produced by peripheral branch axotomy would affect the regenerative properties of both the central and peripheral axon of the DRG cell equally. To examine this question, a conditioning crush lesion was made distally on the sciatic nerve 2 weeks prior to a testing lesion of either the dorsal root or peripheral branch axon near the DRG. Fast axonal transport of radioactive proteins was used to assess regeneration of DRG axons. In the adult rat, leading peripheral branch axons normally regenerate at a rate of 4.4 mm/day. If a conditioning lesion of the sciatic nerve is made 2 weeks before the test lesion, the rate of peripheral branch axonal regeneration increases by 25% to 5.5 mm/day. This effect is not limited to the fastest growing axons in the nerve since a population of more slowly growing axons also exhibits accelerated outgrowth in response to a prior peripheral axotomy. In contrast to this, the fastest growing central branch axons of DRG cells, which normally regenerate at a rate of 2.5 mm/day, are not significantly affected by a prior peripheral axotomy. A population of more slowly growing axons in the dorsal root also does not exhibit accelerated outgrowth in response to a peripheral conditioning lesion. The results of these experiments indicate that changes in the DRG neuron's metabolism induced by prior axotomy of its peripheral axon do not affect the regenerative properties of both axons equally. This raises the possibility that accelerated axonal outgrowth in only one axonal branch results from a differentially regulated supply of proteins to the two axons by the DRG cell body.