Nitric Oxide/Cyclic Guanosine Monophosphate Pathway in the Peripheral and Central Auditory System of the Rat JAMES D. FESSENDEN, 1 RICHARD A. ALTSCHULER, 1 AUDREY F. SEASHOLTZ, 2 AND JOCHEN SCHACHT 1 * 1 Kresge Hearing Research Institute, University of Michigan, Ann Arbor, Michigan 48109–0506 2 Mental Health Research Institute, University of Michigan, Ann Arbor, Michigan 48109–0720 ABSTRACT The neuronal isoform of nitric oxide synthase (nNOS) and soluble guanylate cyclase (sGC) were localized in the cochlea, the cochlear nucleus (CN), and the superior olivary complex (SOC) of Fisher 344 rats. In the cochlea, nNOS was identified in spiral ganglion cells by using nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and in situ hybridization. NADPH-diaphorase staining also was detected in blood vessels of the modiolus. By using immunohistochemistry against cyclic guanosine monophosphate, cochlear sGC activity was localized to pericytes in the spiral ligament as well as nerve fibers innervating outer hair cells. In the lower auditory brainstem, nNOS was localized to principal cells of the medial nucleus of the trapezoid body (MNTB) with NADPH-diaphorase histochem- istry and in situ hybridization. NADPH-diaphorase activity also was observed in the lateral and medial superior olive (LSO and MSO, respectively), the superior periolivary nucleus (SPN), the ventral and lateral nuclei of the trapezoid body (VNTB and LNTB, respectively), and the ventral cochlear nucleus (VCN). Transcripts of the -subunit of sGC were localized in rat brainstem by using in situ hybridization. mRNA for sGC was expressed in neurons within the SPN, LSO, MSO, LNTB, MNTB, VNTB, and VCN. Highest levels of sGC expression were seen in the SPN. These results suggest that the NO/cGMP pathway is involved in both the ascending and descending pathways of the auditory brainstem. J. Comp. Neurol. 404:52–63, 1999.  1999 Wiley-Liss, Inc. Indexing terms: auditory brainstem; cochlea; nicotinamide adenine dinucleotide phosphate- diaphorase histochemistry; nitric oxide synthase; soluble guanylate cyclase Many neurotransmitters are amino acids or amino acid derivatives stored within neuronal synaptic vesicles. On depolarization of the neuron and subsequent calcium influx, these bioactive molecules are released into the synaptic cleft, where they bind to receptors that usually are present in the postsynaptic cell membrane. Nitric oxide (NO) is a recently discovered neurotransmitter that also is derived from an amino acid (L-arginine) but with a different mode of action. NO is a gas; therefore, it can diffuse across cell membranes. NO does not act upon plasma membrane receptors; instead, it stimulates a cyto- solic protein, soluble guanylate cyclase (sGC), to produce cyclic guanosine monophosphate (cGMP). Thus, the NO/ cGMP pathway is a unique neurotransmitter/neuromodu- lator system with a role in the nervous system that is only beginning to be understood. In the central nervous system (CNS), the NO/cGMP pathway has been best characterized in the cerebellum (Garthwaite et al., 1988; Bredt and Snyder, 1989), where it has been implicated in the induction of long-term depres- Grant sponsor: National Institutes for Deafness and Other Communica- tion Disorders, National Institutes of Health; Grant number: DC-02982. *Correspondence to: Jochen Schacht, Ph.D., Kresge Hearing Research Institute, 1301 E. Ann Street, Ann Arbor, MI 48109–0506. E-mail: schacht@umich.edu Received 30 October 1997; Revised 20 May 1998; Accepted 4 June 1998 THE JOURNAL OF COMPARATIVE NEUROLOGY 404:52–63 (1999)  1999 WILEY-LISS, INC.