Metabolic presbycusis: differential changes in auditory brainstem and otoacoustic emission responses with chronic furosemide application in the gerbil.

Auditory characteristics of metabolic or strial presbycusis were investigated using an animal model in which young adult Mongolian gerbils ( Meriones unguiculates) were implanted with an osmotic pump supplying furosemide continuously to the round window. This model causes chronic lowering of the endocochlear potential (EP) and results in auditory responses very similar to those seen in quiet-aged gerbils (Schmiedt et al., J. Neurosci. 22:9643-9650, 2002). Auditory function was examined up to one week post-implant by measurement of auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). Emission "threshold" was defined as the stimulus level required to reach a criterion emission amplitude. Comparing all responses on a "threshold-shift diagram," where emission threshold increases were plotted versus ABR threshold increases, the following results were obtained: (1) On average, the increase of the emission threshold was about 55% of the increase in ABR threshold, with comparatively little scatter. (2) The main dysfunction in metabolic presbycusis appears to be a decrease in the gain of the cochlear amplifier, combined with an additional, smaller increase in neural threshold, both effects caused by a chronically low EP. (3) For ABR threshold increases over 20 dB, the points for the chronic low-EP condition were largely separate from those previously found for permanent acoustic damage. The threshold-shift diagram therefore provides a method for noninvasive differential diagnosis of two common hearing dysfunctions.