Excitotoxic effect of kainic acid on chicken otoacoustic emissions and cochlear potentials.

Kainic acid (KA) is a potent glutamate analog that can temporarily or permanently damage glutamatergic neurons. The purpose of the present study was to determine the short- and long-term effects of KA on chicken otoacoustic emissions and cochlear potentials. A chronic electrode was used to record the compound action potential (CAP), cochlear microphonic (CM), and the slow, positive neural potential (SPNP), a predominantly dc response. The CM, CAP, SPNP, and distortion product otoacoustic emissions (DPOAEs) were recorded before and after infusing 10 microl of a low dose (KA-L, 0.3 mM) or high dose (KA-H, 5 mM) of KA into scala tympani. KA caused a rapid and large reduction in CAP and SPNP amplitude in both the KA-H and KA-L groups; however, the CM and DPOAEs were largely unchanged. The amplitude of the CAP and SPNP in the KA-L group began to recover around 1 week post-KA, but was approximately 50% below normal at 4 weeks post-KA. In contrast, the CAP and SPNP showed no signs of recovery in the KA-H group. The results suggest that KA has no effect on the CM and DPOAEs generated by the hair cells, but selectively damages the CAP generated by the cochlear ganglion neurons. The reduction in the avian SPNP suggests that the response originates in the cochlear afferent neurons, unlike the summating potential (SP) in mammals that is generated in hair cells.