Which myelinated sensory axons sprout into an end-to-side coapted peripheral nerve in the rat?

The high-threshold sensory afferents, which express trkA, are predominantly involved in terminal collateral sprouting in the skin of adult mammals. We explored which sensory axons are capable of sprouting into the end-to-side coapted nerve in the rat.

The distal stump of the transected peroneal nerve was sutured to the side of the uninjured sural nerve. After 36 weeks, sprouting of sensory axons into the end-to-side coapted nerve was assessed by the electrophysiologic measurements of compound action potential and by counting the myelinated axons. The neurons in the dorsal root ganglia (DRG) L5 whose axons sprouted into the end-to-side coapted nerve were retrogradely labelled by the fluorescent dye Fluorogold. The expression of trkA in sprouting DRG neurons was investigated by immunohistochemistry.

Predominantly thin myelinated axons were found in the end-to-side coapted peroneal nerve. Their mean conduction velocity (CV) was between the average CVs of the Adelta and Abeta fibres in the normal sural nerves. About 90% of the sprouting DRG neurons were small and medium sized, and about 10% were large. About 85% of sprouting DRG neurons was immunoreactive to trkA, but the rest were not.

Mostly the high-threshold sensory afferents sprouted into the end-to-side coapted nerve, which resembles the collateral sprouting of sensory axons in the skin. However, our results suggest that also some low-threshold mechanoreceptors can sprout after the end-to-side nerve repair.