Noradrenergic and serotonergic projections to the superior olive: potential for modulation of olivocochlear neurons.

The distribution and density of noradrenergic (NA) and serotonergic (5-HT) varicosities in the superior olive (SO) and periolivary region (PO) and their relationship to olivocochlear neurons was studied. Antibodies against 5-HT and the NA precursor enzyme dopamine beta-hydroxylase were utilized to examine the density of innervation of SO and PO. To determine the relationship of these varicosities to efferent neurons projecting to the cochlea, olivocochlear neurons were retrogradely labeled with biotinylated dextranamine (BDA). NA and 5-HT varicosities were found adjacent to labeled olivocochlear neuron cell bodies and dendrites. More than 50% of labeled medial olivocochlear (MOC) neurons showed likely contact with NA varicosities and more than 90% of labeled MOC neurons with 5-HT varicosities. There was no apparent difference in the number of lateral olivocochlear (LOC) neurons in close proximity to NA and 5-HT varicosities versus MOCs in close proximity to NA and 5-HT varicosities. Our results suggest that the NA and 5-HT systems are in a position to modulate auditory brainstem processing. The specific relationship of NA and 5-HT varicosities to olivocochlear neurons suggests that one possible level of modulation is prior to signal transduction. Copyright 1999 Elsevier Science B.V.