NADPH-diaphorase histochemistry reveals an autonomic-like innervation in the postnatal hamster cochlea

Previous studies used nicotinamide adenine diphosphate (NADPH)-diaphorase h istochemistry as an indicator of nitric oxide synthase (NOS) expression in the adult mammalian cochlea. In this study, we investigated the early postn atal expression of diaphorase activity in the hamster cochlea. Two types of extrinsic fibers were intensely labeled as early as postnatal day 3 (P3) i n the portion of the cochlear nerve that innervates the base of the modiolu s. By P10, these fibers had reached the spiral ganglion and were projecting toward the organ of Corti. The perivascular type of fiber did not project into the organ of Corti; however, the nonperivascular type could be traced among the supporting cells below the outer hair cells. Spiral ganglion cell somata were also labeled as early as P3. The onset of diaphorase expressio n in the spiral ganglion cells corresponds to a critical period of synaptog enesis for these sensorineural cells. If NADPH-diaphorase activity is an in dicator of NOS, then our results suggest that NO may play a role during pos tnatal cochlear development. (C) 1999 Wiley-Liss, Inc.