Norepinephrine innervation of the cochlear nuclei by locus coeruleus neurons in the rat

The cochlear nuclei (CN) contain a moderate concentration of norepinephrine (445 +/- 20 ng/g tissue) with dopamine levels (46 +/- 14 ng/g) that are low and within the precursor range expected for a norepinephrine (NE) terminal system. Lesion and horseradish peroxidase (HRP) experiments indicate that this innervation is bilateral and arises from fusiform and multipolar neurons in the locus coeruleus. Autoradiographic and fluorescence histochemical experiments demonstate that locus coeruleus fibers reach the ipsilateral ventral cochlear nuclei via a rostral pathway that projects from the rostral locus coeruleus laterally through the brain stem to the rostral tip of the ventral nuclei. This pathway is located dorsal to the motor and spinal trigeminal nuclei and ventral to the middle cerebellar peduncle. Descending coeruleo-cochlear fibers travel between the fourth ventricle and the vestibular nuclei to enter the acoustic striae. These fibers innervate both the dorsal and ventral nuclei. Contralateral locus fibers reach the CN by crossing in the pontine central gray at the rostral border of the fourth ventricle and by decussating with the fibers of the mesencephalic trigeminal nucleus ventral to the medial longitudinal fasciculus. The bilateral locus coeruleus innervation of the cochlear nuclei comprises a highly collateralized network of varicose axons which are not topographically organized. Unlike the cochlear nerve fibers in the CN which form specific projections, the locus coeruleus afferents to these sensory nuclei are diffuse and non-specific.