EFFECTS OF LOOP DIURETICS ON THE SUPPRESSION TUNING OF DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS IN RABBITS

The suppression tuning of distortion-product otoacoustic emissions (DP OAEs) is commonly assumed to measure frequency selectivity, because th e dominant features of suppression-tuning curves (STCs) are similar to the principal properties of the neural-tuning curves (NTCs) of single auditory-nerve fibers. In the present study, several common loop diur etics were used to affect the DPOAE-generation process to determine if reversible ototoxicity could adversely modify the characteristics of STCs, in a manner similar to that shown previously for NTCs. Contour p lots of DPOAE level in the presence of a series of variable-level supp ressor tones were obtained before and after administering diuretic dru gs that reversibly reduced or eliminated DPOAEs. Primary-tone pairs we re centered at 2.8 or 4 kHz, with L-1 = L-2, or L-2 < L-1. From the re sulting plots, STC parameters including tip frequency, threshold at th e tip frequency, and Q(10) (dB) measures of tuning were extracted for four suppression criteria of 3, 6, 9, and 12 dB. In the pre-drug nonot otoxic state, suppression tuning depended on both primary-tone level ( L-1, L-2), and the relative levels of the primaries (L-1 - L-2), with tuning being sharper for lower- than for higher-level equilevel primar ies, and sharpest for offset-level primary tones. Following drug injec tion, the expected decrease in sharpness of tuning evidenced by change s in Q(10) (dB) as well as the dramatically elevated tip thresholds no rmally seen for NTCs under similar conditions, were not observed. Over all, Q(10) (dB) increased or decreased more or less randomly, with a s light tendency for STCs to become sharper than prior to drug dosing, f or the two highest suppression criteria. The STC-tip frequencies demon strated significant decreases following diuretic administration that w ere weakly correlated with the associated decreases in DPOAE amplitude . The most consistent changes in response to the drug-induced reductio n in DPOAE level were increases in the STC-tip thresholds. However, th ese changes were relatively small and rarely exceeded 10 dB. In the ab sence of notable changes in overall STC shape, a major finding was a c hange in the effectiveness of suppression following ototoxic insult. H owever, when the amount of suppression was expressed as a percentage o f the DPOAE remaining, the effects of diuretic dosing were often almos t completely obscured. Overall, the results demonstrated that when the generation of DPOAEs was interfered with by the introduction of a sup pressor tone to produce STCs that resemble NTCs, STCs behaved quite di fferently following reversible cochlear insult than their previously d ocumented neural counterparts. These findings imply that STCs do not a ssess the frequency-selective aspects of the cochlear amplification pr ocess in a manner similar to NTCs. (C) 1998 Acoustical Society of Amer ica. [S0001-4966(98)06808-8].