Adrenergic fibers in the spinal cord of the monkey: light and electron microscopic study

The adrenergic innervation of the monkey (Macaca fascicularis) thoracic spinal cord was examined by means of peroxidase-antiperoxidase immunohistochemical method using antisera directed phenylethanolamine N-methyl transferase (PNMT). At light microscopic level the PNMT-positive profiles are seen as brown granules, presumably axon terminals, or varicose fibers. They are localized in the intermediolateral nucleus, central gray and the intermediate gray which connects the two. Occasional fibers are seen in ventral and dorsal horns. The descending adrenergic fiber tract is found in the lateral margin of the lateral funiculus. At electron microscopic level, the PNMT-positive presynaptic profiles exhibit densely packed small clear vesicles, a few large dense core vesicles and numerous mitochondria. They make synaptic contact with dendritic profiles (97%) and somatic profiles (3%) and demonstrate either symmetric or asymmetric synaptic specialization. The descending adrenergic fiber tract consists mainly of unmyelinated fibers and is located in the ventral half of the lateral funiculus.