Catecholaminergic innervation in the vestibular labyrinth and vestibular nucleus of guinea pigs.

The vestibular sympathetic fibers of 20 guinea pigs were examined by immunohistochemical demonstration using tyrosine hydroxylase and dopamine B-hydroxylase. The vestibular sympathetics originated in the ipsilateral superior cervical ganglion and entered the internal auditory meatus along the labyrinthine artery. At the Schwann-glial border, some of the sympathetic fibers left the artery and went into the superior and inferior divisions of the vestibular nerve and made a loose meshwork among the Scarpa's ganglion cells, while other fibers kept following the labyrinthine artery. Both groups of fibers entered the cristae ampullaris and saccular and utricular maculas after several bifurcations in the cribrose areas and terminated either near the capillaries beneath the sensory epithelia, or among the vestibular nerve fibers. These fibers travelled freely in the vestibular labyrinth without being restricted to following blood vessels or vestibular nerve fibers. Some sympathetic fibers made direct contacts with the vestibular efferent fibers or the vestibular afferent fibers at the node of Ranvier. Sympathetic fibers were not observed in the sensory epithelia or semicircular canals, and were rarely found in the vicinity of the dark cells. The vestibular nucleus was also innervated by other catecholaminergic nerve fibers originating from the locus ceruleus, and the synaptic contacts were observed between the catecholaminergic nerve terminals and other neurons. These two peripheral and central catecholaminergic nervous systems were quite independent at the level of Schwann-glial border.