Effects of conductive hearing loss on auditory nerve activity in gerbil.

The goal of this research was to define the effects of conductive hearing loss (CHL) produced by malleus removal (MR) on auditory nerve activity in the absence of acoustic stimulation. Subjects were gerbils in two age groups: 21 days (P21) and 42 days (P42). Ensemble background activity (EBA) of the auditory nerve was measured by spectral analysis of a signal recorded from the round window. The EBA measure was studied following MR in an acute setting in P42 animals, and in chronic experiments either 5 days (P21 and P42 animals) or 3 weeks (P21 animals) after MR. Findings were: (1) an EBA can be reliably measured in the gerbil, and this measure is highly stable over time; (2) the EBA is eliminated by application of tetrodotoxin, confirming the auditory nerve as the source of this measure; (3) MR results in a statistically significant decrease in the magnitude of the EBA in adult animals; and (4) the EBA measure is age-dependent. The EBA was significantly lower in P21 than in P42 animals, and response to MR varied as a function of age. We conclude that CHL decreases 'spontaneous' activity of the auditory nerve in young adult gerbils.