Immunohistochemical localization of nitric oxide synthase and soluble guanylyl cyclase in the ventral cochlear nucleus of the rat.

The diffusible messenger nitric oxide (NO) is implicated in auditory processing. It acts in the brain largely through activation of soluble guanylyl cyclase (sGC), a heterodimer comprised of alpha and beta subunits. The authors used immunohistochemistry to study the NO/guanosine 3',5'-cyclic monophosphate (cGMP) pathway in the cochlear nucleus of Sprague-Dawley rats. Central fibers of the cochlear nerve were stained for neuronal nitric oxide synthase (NOS-I) but not for sGCbeta. Within the ventral cochlear nucleus, a large fraction of principal cells were immunopositive for both NOS-I and sGCbeta; these cells could be seen at times receiving contacts from NOS-I-positive fibers. sGC staining of somatic cytoplasm extended into the distal dendritic tree. At variance with this pattern, NOS-I was concentrated mainly in somata. Double-labeling experiments showed that most of the principal neurons expressed both antigens. By contrast, in the granule cell domain, small cells that were immunopositive for NOS-I rarely corresponded to those that were immunopositive for sGC. To assess whether NOS-I and sGC immunoreactivities colocalize with their respective catalytic activities, the authors performed multiple labeling with L-citrulline (a by-product of the formation of NO from L-arginine) and cGMP, respectively. L-citrulline was restricted to NOS-I-positive elements, and the large majority of NOS-expressing neurons were positive for citrulline. Multiple labeling revealed that almost all sGC-positive neurons also accumulated cGMP both in the ventral cochlear nucleus and in the granule cell domain. These data suggest that NO is a signaling molecule in the cochlear nucleus, perhaps functioning in both a paracrine manner and an autocrine manner.