Nitric oxide in glutamate-induced compound action potential threshold shifts

Protection against acoustic trauma by direct application of D-methionine to the inner ear

CONCLUSION

The findings from this study extend the use of the local application of D-methionine (D-met) to protect against acoustic trauma and demonstrate that D-met slowly diffuses from the perilymph.

OBJECTIVES

The objectives of the study were to determine the effect of D-met on auditory function and morphology after acoustic trauma and to measure the concentration of D-met in perilymph.

METHODS

Auditory thresholds were determine before, immediately after, and 24 h after acoustic trauma. Cochleae were analyzed using immunocytochemistry for c-Fos, TUJI, and cytochrome c. The concentration of D-met was determined from perilymph.

RESULTS

Protection against acoustic trauma (immediately and 24 h post trauma) on auditory brainstem thresholds was found at a time when the concentration of D-met in perilymph showed a fivefold increase above basal levels. The local application of D-met to the guinea pig cochlea results in elevated D-met concentrations that are maintained in the perilymph for at least 24 h.

New insights into glutamate ototoxicity in cochlear hair cells and spiral ganglion neurons

CONCLUSION

Excess glutamate (Glu) exposure (20 mM) in the cochlear perilymph affects the physiological function of outer hair cells (OHCs) within a 2 h period and induces apoptosis in the modiolus spiral ganglion neurons (SGNs) in an apoptosis-inducing factor (AIF)-dependent manner.

OBJECTIVES

To determine whether high-dose Glu affects the function of OHCs and whether it induces AIF- and caspase-3-dependent apoptosis in the cochlear SGNs.

METHODS

Perilymphatic perfusions of Glu (20 mM) and artificial perilymph (AP) solutions were performed in adult guinea pig cochleae. Both cochlear microphonics (CM) and electrical auditory brainstem response (eABR) were measured before and 2 h after perfusions. The hair cell morphologies were examined using transmission electron microscopy. The expression of two apoptotic indicators, AIF and caspase-3, was examined 8 h after perfusions.

RESULTS

In contrast to AP perfusions, the perfusion of 20 mM Glu caused significant reduction in the CM and eABR amplitudes. Inner hair cells (IHCs) after Glu perfusion were deformed and exhibited vacuolization in the postsynaptic region, whereas the OHC system appeared unaffected. AIF expression was detected in the nuclei of SGNs 8 h after Glu exposure, but the expression of caspase-3 was not shown in any cochlear tissues.