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 groups of unexposed control chicks between 2 and 15 days of age. One to nine animals were tested in each control or exposed group. The results in the control chicks revealed that the EP at 2 days of age (6.7 mV) was 36% of the mature value (15.2 mV) noted at 6 days of age. In the exposed animals, at 0-days recovery, the EP showed a 63% reduction relative 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 exposure exhibited a loss of only 7 and 3 percent relative to the age-matched controls. The recovery of EP was plotted against the recovery of evoked potential thresholds reported by others, and the time-course of the recovery functions were very similar. This relationship suggested that recovery of auditory function in the chick was highly correlated with the restoration of the EP. Damage to the tegmentum vasculosum and its capacity to secrete potassium, as well as the shunting of current through the damaged basilar papilla, are considered as mechanisms for the reduction of EP loss in the exposed ear.