Glutamate Excitotoxicity And The Effect Of Glutamate-glutamine Cycle In The Cochlea

The excitatory amino acid(EAA)--glutamate(Glu), is the primarycandidate for the neurotransmitter at the synapses between cochlear hair cells(HCs) and spiral ganglion cells. Excessive glutamate release has been identified as a basic mechanism of neurotoxicity through the process of excitotoxicity. All different subtypes of postsynaptic glutamate receptor can contribute independently to this excitotoxic cell damage, caused primarily by a receptor-gated massive sodium and hydroxide entry, intracellular calcium overflow. Glutamate receptor-linked neurotoxicity has been implicated in pathophysiological conditions such as noise trauma, ischemia, anoxia and so on. Glutamate-glutamine cycle is an efficient mechanism for glutamate uptake in the cochlear. Glutamate is released from the inner hair cells, and taken up by supporting cells through the glutamate transporter(GLAST). Glutamate is converted to glutamine(Gln) by glutamine synthetase(GS) and then transferred to inner hair cells. In the inner hair cells glutamine is converted to glutamate by phosphate-activated glutaminase(PAG). The GS is the key enzyme in the protection of noise-induced hearing loss(NIHL). To investigate glutamate excitotoxicity and the effect of glutamate-glutamine cycle in the cochlear, we did present research. The study was divided into the following three parts.Part one: To investigate the changes of the potentials and structure of the cochlear during whole cochlear perfusion with glutamate. CM, CAP, DPOAE, and ABR were measured to indicate the cochlear functional properties during wholecochlear perfusion. The morphology of the cochlear was monitored by transmission electron microscopy(TEM). After glutamate perfusion, CM I/O function maintained a nonlinear characteristic, but the amplitude of CM declined. The average CAP threshold was elevated. Glutamate induced the decline of cochlear function in a dose-dependent pattern. After glutamate perfusion, there were no significant DPOAE changes, while ABR latencies were delayed. The OHCs appeared normal, but IHCs and afferent dendrites showed cytoplasmic blebs after glutamate infusion. Our results suggest that glutamate may destroy the IHCs and spiral ganglion neurons, and the way it induced cochlear damage is dose-dependent furthermore. The method can also build an animal model of auditory neuropathy.Part two: To observe the effect of noise on the concentration of glutamate in perilymph, as well as on the physiological potentials and morphology of guinea pig cochlear. The concentration of glutamate in perilymph of both noise group and non-noise group was determined by high performance liquid chromatography. The CM and CAP were recorded before and 2 hours after noise exposure. The morphology of the cochlear was observed by transmission electron microscopy. The concentration of glutamate in perilymph was significantly elevated after noise exposure. The average concentration of glutamate in the noise group was 8.15 0.78 mol/L, while in the non-noise group, it was 4.27 0.40 mol/L. The amplitude of CM was declined, the nonlinear properties of CM I/O function became linearity after noise exposure. The average CAP threshold was elevated about 50dB SPL. There were vacuoles in OHCs, IHCs and radial auditory dendrites. The glutamate in the perilymph of the cochlear can be significantly elevated after noise exposure, and cause a series of changes of cochlear function and morphology. Our results suggest that glutamate might be over released fromthe inner hair cells after noise exposure, which caused its ototoxic effect in turn.Part three: To investigate the changes of the potentials and structure of the cochlear during whole cochlear perfusion with glutamine synthetase, and the effect of GS on noise-induced hearing loss. During whole cochlear infusion with GS alone, and the GS perfusion with noise exposure(white noise, lOOdB SPL), the functional properties of the cochlear were measured by CM and CAP, and the morphology of the cochlear was monitored by transmission electron microscopy.