Prior collateral sprouting of sensory axons delays recovery of pain sensitivity after subsequent nerve crush

Regeneration of motor axons is enhanced if they have sprouted prior to nerve injury. We examined whether sensory axon regeneration and recovery of pain response was affected by previous collateral sprouting. In the experimental group of rats, the right saphenous, tibial, and sural nerves were transected and ligated. The peroneal nerve was left to sprout into the adjacent denervated skin, Two months later, the axons of the peroneal nerve were crushed in the sciatic nerve. In the control group, the right sciatic nerve was crushed at the same time that the saphenous, tibial, and sural nerves were transected, Recovery of pain response in the foot was determined by the skin pinch test. Sensory axon elongation rate was measured by the nerve pinch test. The number of myelinated axons was determined in nerve cross sections stained by Azur blue. Recovery of pain sensitivity in the animals of the experimental group was delayed for 2-3 weeks in comparison to the control group. Moreover, the spatial pattern of pain response in the experimental group was irregular, displaying residual regions of insensitive skin which were not present in controls. The elongation rate of regenerating sensory axons in the experimental group was not decreased, and the number of myelinated axons in the peroneal nerves was even about 10% higher than in the control group, Therefore, we assume that the terminal arborization of the neurilemmal tubes pertaining to the former axon sprouts delayed regrowth of sensory axon terminals in the skin. (C) 1996 Academic Press, Inc.