Glutamate receptor mediation of round window noise in the gerbil

Voltage recordings from the round window (RW) have been described to include information on auditory nerve activity. The spectrum of the "noise" recorded from the RW has a 900 Hz peak. The aim of this study was to determine the role of the kainate glutamate receptor in mediating this electrical noise recorded from the RW. Using mongolian gerbils, with a silver ball-tipped electrode placed in the RW niche and a Ag-AgCl electrode placed in soft tissue of the neck, recordings were made of the RW noise in several conditions. DNQX is a competitive blocker of the kainate glutamate receptor. Recordings of the RW noise in intentionally unstimulated (i.e., in quiet) and in intentionally stimulated (narrow band acoustic noise) ears of mongolian gerbils, revealed that the RW noise was susceptible to intracochlear administration of this drug (3 uL, 200uM, infused into the basal turn scala tympani). The DNQX-induced suppression of 0.8-1.0 kHz RW noise, in the quiet condition, peaked at approximately 21% baseline, approximately 15 minutes after injection, and RW noise levels recovered to 90% pre-DNQX levels after approximately 90 minutes. Administration of TTX (3 uL, 200uM, infused into the basal turn scala tympani) reduced RW noise levels further than DNQX, in both the quiet and noise-stimulated state. These results suggest that RW noise consists of kainate-receptor-mediated component, as well as a component that is not mediated by the kainate receptor. This marginal, non-kainate-receptor-mediated, TTX-suppressible noise may result from spontaneous auditory nerve activity. The possible origins of the kainate-receptor mediated RW noise is discussed.