Glycerol injection to the rat trigeminal nerve: histological and immunohistochemical studies

The effect of topical glycerol injection into the rat trigeminal nerves was investigated histologically and immunohistochemically. Anhydrous glycerol was injected into the preganglionic portion of the trigeminal nerves via a ventral approach. Extensive myelin swelling and axonolysis were observed in the rats killed 1 and 2 weeks after glycerol injection. Numerous inflammatory cells were seen especially in the animals sacrificed 1 week after surgery. Myelin disintegration continued up to 4 weeks after glycerol injection. In normal and saline injected sham operated nerves, calcitonin gene-related peptide (CGRP)- and substance P(SP)-like immunoreactivities were densely localized in the nerve fibers. A marked decrease in both CGRP- and SP-like immunofluorescence was seen in the nerves after glycerol injection. The remaining nerve fibers often had blunt endings with increased fluorescence. Swollen and winding structures were also found. These immunohistochemical changes were observed in the rats killed 1 and 2 weeks following surgery. A similar change but of lesser degree was seen in the 4-week-animal. The present study suggests that topical glycerol injection into the trigeminal nerve induces degeneration of the nerves immunoreactive to CGRP and SP. These changes emphasize the putative functional implications of the peptides in relieving the pain of trigeminal neuralgia after topical glycerol injection.