Monoamine oxidase-containing nerve fibers in the major cerebral arteries of rats.

The localization of monoamine oxidase (MAO) in nerve fibers associated with the major cerebral arteries in rats was studied using a new coupled peroxidation method modified by adding nickel ammonium sulfate at the electron microscopic level. MAO was, localized in some unmyelinated axons, both in the adventitia and in the periadventitial nerve bundles. Schwann cell cytoplasm encircling myelinated axons in the periadventitial nerve bundles also contained a MAO-reactive substance. The incidences of MAO-containing axons in the adventitial layer of the anterior cerebral, middle cerebral, internal carotid and basilar arteries were 32.3%, 29.5%, 29.6% and 21.1%, respectively. In the periadventitial nerve bundles, MAO activity was also demonstrated in 10.8% among unmyelinated axons. Preincubation with clorgyline, a specific inhibitor of MAOA, suppressed staining in the axons, both in adventitia and in periadventitial nerve bundles, but not in the Schwann cell cytoplasm. Conversely, preincubation with deprenyl, a specific inhibitor of MAOB, suppressed staining in the Schwann cell cytoplasm, but not in the axons. Therefore, MAO in the axons is regarded as MAOA and MAO in the Schwann cell cytoplasm as MAOB. In immunosympathectomized rats (anti-NGF-treated rats), MAO reactivity was suppressed in axons associated with cerebral arteries, but was retained in some Schwann cell cytoplasm. The results indicate that the MAO-containing unmyelinated axons coincide with the postganglionic noradrenergic ones. Thus, histochemical MAO staining may be utilized to study postganglionic sympathetic nerve fibers presumably innervating the major cerebral arteries at the electron microscopic level.