Nitric oxide synthase is an active enzyme in the spiral ganglion cells of the rat cochlea.

Nitric oxide (NO) mediates the effects of the excitatory amino acids in the central nervous system. Excitatory amino acids, in particular L-glutamate, are thought to be the neurotransmitter(s) present at the cochlear hair cell-afferent nerve synapse. To our knowledge, no studies to date have documented the presence of NO in the cochlea nor attempted to elucidate the role of NO in hearing. Rat cochlea frozen sections were examined for the presence of nitric oxide synthase (NOS) by NADPH diaphorase histochemistry. Vibratome sections of rat cochlea were examined by immunocytochemistry with an antibody to citrulline, an indication of NOS activity. Spiral ganglion cells in the rat cochlea were positive by NADPH diaphorase histochemistry and by anti-citrulline immunocytochemistry. These results indicate that NOS is present and that the enzyme actively produces nitric oxide in the spiral ganglion cells of the rat cochlea. Given our current understanding of neurotransmission in the cochlea, it is reasonable to postulate that the actions of NO in cochlear neuronal tissue are similar to the actions of NO in the CNS and that NO acts as a neurotransmitter/neuromodulator in the cochlea. In addition, because NO has been implicated as a mediator of excitotoxicity in the CNS, NO may play a role in neurotoxicity in the cochlea.