Histochemical and electron microscopical demonstration of the sympathetic nerve fibers joining to the fourth and the sixth cranial nerves in rats.

The localization of sympathetic fibers on the floor of the cranium was studied in rats using amine fluorescence histochemistry, neuropeptide-Y (NPY) immunohistochemistry, and electron microscopy. The vast majority of amine fluorescent fibers joined the abducent nerve and were localized in the peripheral zone under the perineurium. After advancing along this nerve for some distance, the fibers diverged into many bundles that converged to form the cavernous plexus at a rostral end of the trigeminal ganglion. On the dorsal surface of the trigeminal ganglion, one or two medium-calibered fluorescent bundles ran inside or in close proximity to the trochlear nerve, while many small-calibered, brightly fluorescent bundles also extended longitudinally in the epidural connective tissue. In rats that had undergone nerve severance, NPY-immunoreactive fibers were detected at the cut ends of the abducent and trochlear nerve. The differing amounts of NPY accumulated at the rostral and the caudal stumps indicated the direction of the NPY-bearing fibers. Electron microscopy confirmed the presence of unmyelinated fibers in both the abducent and trochlear nerves.