Same axonal regeneration rate after different endoneurial response to intraneural glycerol and phenol injection

Glycerol (an atoxic alcohol) and phenol (a toxic monohydroxybenzene) are currently used as neurolytic blocking agents to relieve pain or spasticity. In the present study we compared the endoneurial response of anhydrous glycerol and 7% phenol-aqua after intraneural injection into rat sciatic nerve, using electron microscopy and immunohistochemical stainings. Despite the wide use of these drugs, a systematic morphological study of their action has not been done. Electron microscope studies showed different patterns of nerve damage for glycerol and phenol. Glycerol injection resulted in gross sciatic nerve injury, with myelin fragments widely dispersed in the endoneurium 1-2 weeks after the injury. Phenol-aqua injection resulted in gross sciatic nerve injury with focal haemorrhagic necrosis; nerve fibres were segmentally dissolved 1-2 weeks after the injury. In both groups the first axonal sprouts appeared in the area of the lesion 2 weeks after the injury and the sprouts became myelinated in both groups by 4 weeks. Immunohistochemical staining showed that in the glycerol-treated nerves macrophages were widely scattered in the endoneurium by day 3; the number of macrophages proximal to the lesion site and at the lesion site was significantly higher in the glycerol-treated nerves than in the phenol-treated nerves both at days 3 and 7. In the phenol-treated nerves, macrophages appeared after 1 week and they exceeded the number of macrophages in the glycerol-treated nerves at 2 weeks. The number of Schwann cells remained low until 4 weeks in both groups. The results show that glycerol-induced nerve fibre damage with breaching of myelin fragments is followed by invasion of macrophages into the endoneurium after 3 days. The delayed invasion of macrophages after phenol injection may be due to occluded vessels or may be related to the denaturing effect of phenol on the proteins needed for macrophage attraction. Despite the rapid invasion of macrophages after glycerol injection axonal regeneration was delayed when compared to that seen after traumatic axotomy, but the axonal regeneration occurred at the same time in both experimental groups. Thus, the results suggest that after chemical axonotmesis the axonal regeneration rate is not dependent on the macrophage invasion rate alone and that other endoneurial changes also play a role.