Distribution and projections of nitric oxide synthase neurons in the rodent superior olivary complex

The superior olivary complex (SOC), a group of interrelated brainstem nuclei, sends efferents to a variety of neuronal structures including the cochlea and the inferior colliculus. The present review describes data obtained from rodents providing evidence that the gaseous, short-living neuroactive substance nitric oxide (NO) is produced in the SOC. The NO-synthesizing enzyme neuronal NO-synthase (nNOS) has been localized by means of several methods including histochemistry and immunohistochemistry. Perikarya containing nNOS were found in several nuclei of the SOC. Their largest numbers and percentages of total cells were observed in the medial nucleus of the trapezoid body. Stained terminals were observed mainly in the lateral superior olivary nucleus and in the superior paraolivary nucleus. While retrograde neuronal tracing identified a considerable number of nNOS-immunoreactive neurons as to be part of the olivo-cochlear pathway, the projection patterns of other nNOS-immunoreactive SOC cell groups remain to be investigated. We also review other putative sources of cochlear NO, and discuss the possible role of NO in the lower auditory brainstem and organ of Corti with regard to physiological and pathophysiological mechanisms.