INVOLVEMENT OF COCHLEAR EFFERENT PATHWAYS IN PROTECTIVE EFFECTS ELICITED WITH BINAURAL LOUD SOUND EXPOSURE IN CATS

1. Studies in guinea pigs have shown that the crossed olivocochlear efferent [crossed olivocochlear bundle (OCB)] pathways can reduce the cochlear neural desensitization caused by loud sounds. In one experimental paradigm, binaural loud sound exposure produced less damage than did monaural exposure, and various tests confirmed that this could be attributed to the OCB. In contrast, a study in cats has shown no such protection from the OCB for binaural exposures. There are some methodological differences between the cat and guinea pig studies that could account for the difference. The present study was carried out to determine whether two factors, namely anesthetic or exposure frequency, could account for the difference. Experiments were carried out in cats using barbiturate anesthesia, as in the guinea pig experiments. 2. Using a unilateral middle ear muscle (MEM) tenotomy, it was confirmed that under Nembutal anesthesia the MEM did not affect the threshold losses to monaural or binaural exposure. However, comparing results for monaural versus binaural 11-kHz exposures, there were significantly less threshold losses to the binaural exposure, independent of the presence of the MEM. No such difference between monaural and binaural exposures was seen for 3-kHz exposures. 3. By employing unilateral surgical strategies such as MEM tenotomy and/or various brain stem incisions, it was confirmed that the protection with binaural compared with monaural 11-kHz exposure was due specifically to the OCB. In unilaterally deefferented animals, binaural 11-kHz exposure always produced lower threshold losses on the OCB-intact side than on the OCB-cut side, regardless of the status of the MEM. Brain stem cuts that affected other rostrally or laterally located structures but not the OCB produced similar threshold losses bilaterally after binaural exposure, and the losses were comparable with the protected levels seen in other cases with intact OCB and binaural exposure. These data confirmed that when using an exposure frequency very similar to that used in the guinea pig experiments, a protective OCB effect could be demonstrated in cats, as previously seen in guinea pigs. The MEMs appeared to be inactive in barbiturate-anesthetized animals and were not activated by 3-, 7-, or 11-kHz exposures at 100 dB SPL for 10 min. The companion manuscript demonstrates this same effect across a wider range of exposures. 4. Finally, across-group comparisons showed that binaural exposures at 11 kHz activated OCB-mediated protection, whereas monaural exposures did not appear to do so, and that, in this study in which the selection of animals for experimentation was tightly controlled, there was very good similarity in the threshold losses to the same exposure between groups in which the net theoretical outcome was the same.