Disruption of lateral olivocochlear neurons with a dopaminergic neurotoxin depresses spontaneous auditory nerve activity

Abstract

Neurons of the lateral olivocochlear (LOC) system project from the auditory brainstem to the cochlea, where they synapse on radial dendrites of auditory nerve fibers. Selective LOC disruption depresses sound-evoked auditory nerve activity in the guinea pig, but enhances it in the mouse. Here, LOC disruption depressed spontaneous auditory nerve activity in the guinea pig. Recordings from single auditory nerve fibers revealed a significantly reduced proportion of fibers with the highest spontaneous firing rates (SRs) and an increased proportion of neurons with lower SRs. Ensemble activity, estimated using round window noise, also decreased after LOC disruption. Decreased spontaneous activity after LOC disruption may be a consequence of reduced tonic release of excitatory transmitters from the LOC terminals in guinea pigs.

Keywords: Auditory nerve; Cochlea; Guinea pig; MPTP; Olivocochlear efferents.

Figure 1

MPTP decreases RW noise. Spectral waveforms were obtained 30-min after 6-μl of artificial perilymph, 30- and 60-min after 6-μl of 50-mM MPTP and post-mortem. The waveform obtained 30-min after AP (solid line) shows the characteristic broad peak in the 0.5-1.0 kHz range (arrow). In the waveforms obtained 30- and 60-min post-MPTP, RWN waveform amplitude is diminished, with RWN waveform eliminated post-mortem.

Figure 2

Spontaneous activity recorded from single auditory nerve fibers in (2A) normal animals and (2B) MPTP lesioned animals. Due to unequal sample sizes, the same data are normalized as percentages (of the number of fibers within each group) in 2C and 2D. In 2E, spontaneous rate distributions for Control and MPTP-lesioned animals are shown. The data from panels 2C and 2D have been binned into the SR groups as indicated on the ordinate; the distributions are significantly different.