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 proces sing. It acts in the brain largely through activation of soluble guanylyl c yclase (sGC), a heterodimer comprised of alpha and beta subunits. The autho rs used immunohistochemistry to study the NO/guanosine 3',5'-cyclic monopho sphate (cGMP) pathway in the cochlear nucleus of Sprague-Dawley rats. Centr al fibers of the cochlear nerve were stained for neuronal nitric oxide synt hase (NOS-I) but not for sGC beta. Within the ventral cochlear nucleus, a l arge fraction of principal cells were immunopositive for both NOS-I and sGC beta; these cells could be seen at times receiving contacts from NOS-I-pos itive fibers. sGC staining of somatic cytoplasm extended into the distal de ndritic tree. At variance with this pattern, NOS-I was concentrated mainly in somata. Double-labeling experiments showed that most of the principal ne urons expressed both antigens. By contrast, in the granule cell domain, sma ll cells that were immunopositive for NOS-I rarely corresponded to those th at were immunopositive for sGC. To assess whether NOS-I and sGC immunoreact ivities colocalize with their respective catalytic activities, the authors performed multiple labeling with L-citrulline (a by-product of the formatio n of NO from L-arginine) and cGMP, respectively. L-citrulline was restricte d to NOS-I-positive elements, and the large majority of NOS-expressing neur ons were positive for citrulline. Multiple labeling revealed that almost al l sGC-positive neurons also accumulated cGMP both in the ventral cochlear n ucleus and in the granule cell domain. These data suggest that NO is a sign aling molecule in the cochlear nucleus, perhaps functioning in both a parac rine manner and an autocrine manner. J. Comp, Neurol. 431:1-10, 2001. (C) 2 001 Wiley-Liss. Inc.