An Investigation On The Effects Of The Pathway Of Glutamate-N-methyl-D-asparate-Nitric Oxide Synthase-Reactive Oxygen Species On Acoustic Trauma

The causes of noise induced hearing loss (NIHL) are multifactor and complicated. A variety of mechanisms have been proposed to account for hearing loss following intensity sound exposure. High-level noise exposures exceeding 140dB can cause direct mechanical trauma to the tectorial membrane-hair cells and even make the organ of Corti become detached from the basilar membrane. This type of damage-acoustic trauma occurs rapidly and results in an immediate, permanent hearing loss. Exposure to medium and low-level noise between 90 and 140dB damages the cochlea metabolically rather than mechanically, which induces the dysfunction of the enzyme system of the hair cells and supporting cells in the cochlea. This damage can also inhibit the energy metabolism and cause hearing loss.Glutamate (Glu) is known to be an excitatory neurotransmitter between inner hair cells (IHC) and the afferent neurons of the mammalian cochlea and to produce toxic effects upon the cochlea. Based on the theory of Glu excitotoxicity in the central nervous system. it was proposed the hypothesis that the pathway of glutamate (Glu)- N-methyl-D-aspartae (NMDA)-nitric oxide synthase (NOS)-reactive oxygen species (ROS) played an important role in the NIHL. The hypothesis assumes that acoustic overstimulation may result in excessive release of the Glu from the IHC and excess Glu release can cause excitotoxicity via increasing intracellular NOS activity evoked by calcium channels coupled to NMDA receptor activation. It states that increased NOS activity leads to large quantities of NO production and NO related ROS, which may be involved in the damage of hair cells and spiral ganglion cells and result in hearing loss.We investigated the changes of nitric oxide (NO) production and the serum total antioxidant capacity (TAC) of guinea pig exposed to noise, the effect of exogenous NO on the function of cochlea in guinea pig, the protective effect of NMDA receptor non-competitive antagonist - MK-801 against noise trauma, the protection of auditoryfunction against NIHL with NOS competitive antagonist- N~G-nitro-L-arginine methyl ester -(L-NAME) and the protection of free radicals scavenger a-lipoic acid (LA) against NIHL by using electrophysiological, morphological and immunohistochemistry methods. Main results and conclusions are as follows:1 .The noise exposure (4-kHz octave band, 115dB SPL. 5h) caused significant decrease of the TAC in serum and significant increase of the NO levels in cochlear tissue of guinea pig, which reached the peak level 3 days after noise expoure and was not different from baseline on day 7. This suggests that the mass formation of NO and related ROS after intense sound exposure may contribute to the NIHL.2. After NO donor -sodium nitroprusside (SNP) (20μl,100mM) applied on the round window membrane (RWM) , the NO levels in the cochlear tissue and the thresholds of auditory brainstem response (ABR) significantly elevated, and TAC in serum decreased significantly. Scanning eletron microscopy (SEM) showed an obvious missing of outer hair cells (OHC). Antioxidant LA can reduce the cochlear damage caused by SNP.These results suggest that an excessive production of NO in the inner ear could have an ototoxic effect on the cochlear function, and antioxidant can protect the cochlear damage from the excessive NO.3.1mmunoreactivity to iNOS was not identified in the cochlea tissue of the untreated animal, whereas after noise expoisure ( 4-kHz octave band ,115dB SPL 5h), immunoreacitvity to iNOS was seen in the organ of Corti , stria vascular and spiral ganlion cells.These results show that noise can upregulate the iNOS activity and iNOS-generated NO could be partly involved in the NIHL.4. The threshold shifts of the MK-801 (1mg/kg, i.p.)-treated animals 3 days after noise exposure (4-kHz octave band, 115dB SPL. 5h) were significantly less than those of the physiology saline-treated animals(p<0.05). The TAC in serum were significantly increased in the animals with treatment of MK-801.Noise-induced elevation in NO level and the expression of inducible nitric oxide synthase (iNOS) in the cochlear tissue were not significantly attenuated by MK-801. No vacuoles were found in the(IHC or in the-afferent dendrites of animals receiving MK-801. These results indicate that MK-801 could partially...