THE EFFECTS OF EXPOSURE TO INTENSE SOUND ON THE DC ENDOCOCHLEAR POTENTIAL IN THE CHICK

Chicks were exposed to an intense pure tone (0.9 kHz, 120 dB SPL) for 48 h. The DC endocochlear potential was measured with a microelectrode inserted into scala media in six separate groups of animals between 0 and 12 days post exposure. Similar data were obtained from seven grou ps of unexposed control chicks between 2 and 15 days of age. One to ni ne animals were tested in each control or exposed group. The results i n the control chicks revealed that the EP at 2 days of age (6.7 mV) wa s 36% of the mature value (15.2 mV) noted at 6 days of age. In the exp osed animals, at 0-days recovery, the EP showed a 63% reduction relati ve to that measured in age matched control chicks. The level of EP in the exposed animals quickly recovered, and by 3 and 6 days post exposu re exhibited a loss of only 7 and 3 percent relative to the age-matche d controls. The recovery of EP was plotted against the recovery of evo ked potential thresholds reported by others, and the time-course of th e recovery functions were very similar. This relationship suggested th at recovery of auditory function in the chick was highly correlated wi th the restoration of the EP. Damage to the tegmentum vasculosum and i ts capacity to secrete potassium, as well as the shunting of current t hrough the damaged basilar papilla, are considered as mechanisms for t he reduction of EP loss in the exposed ear.