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 i n the central nervous system. Excitatory amino acids, in particular L- glutamate, are thought to be the neurotransmitter(s) present at the co chlear hair cell-afferent nerve synapse. To our knowledge, no studies to date have documented the presence of NO in the cochlea nor attempte d 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 exa mined by immunocytochemistry with an antibody to citrulline, an indica tion of NOS activity. Spiral ganglion cells in the rat cochlea were po sitive by NADPH diaphorase histochemistry and by anti-citrulline immun ocytochemistry. These results indicate that NOS is present and that th e enzyme actively produces nitric oxide in the spiral ganglion cells o f the rat cochlea. Given our current understanding of neurotransmissio n 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 C NS and that NO acts as a neurotransmitter/neuromodulator in the cochle a. In addition, because NO has been implicated as a mediator of excito toxicity in the CNS, NO may play a role in neurotoxicity in the cochle a.