ANALYSIS OF PROJECTIONS FROM THE COCHLEAR NUCLEUS TO THE LATERAL PARAGIGANTOCELLULAR RETICULAR NUCLEUS IN THE RAT

Golgi-staining, retrograde and anterograde tract-tracing, and a two-co lor immunoperoxidase technique have been employed, at the light- and e lectron-miicroscopic levels, to analyze the auditory projections from the cochlear nucleus (CN) to the lateral paragigantocellular reticular nucleus (LPGi) in the rat. We have found that the auditory input orig inates predominantly in the posteroventral and cochlear root nuclei. T he auditory axons terminate in the cell-poor, ventral portion of the L PGi, which is strongly invaded by beaded dendritic profiles, originati ng from parent cell bodies located in the dorsal half of the LPGi. Ult rastructural analysis has revealed that the anterogradely labeled audi tory axons form functional synapses preferentially with dendritic shaf ts. These axo-dendritic contacts are apparently excitatory in nature. By means of a sequential two-color immunoperoxidase staining method, w e have further characterized potential postsynaptic neurons in the LPG i. Black-stained auditory fibers intermingle with brown-stained seroto nergic or adrenergic neurons. Varicose auditory axons are often closel y apposed to immunoreactive dendritic profiles of serotonergic and adr energic neurons, indicating the presence of possible synaptic contacts of auditory terminal fibers with these transmitter-classified cells. The monosynaptic auditory input from the CN may modulate the activity of B3 serotonergic and Cl adrenergic cells in the LPGi and may thus in duce adaptive changes in response to acoustic stimuli.