Synaptic activity at the MNTB is disrupted in a mouse model with enhanced efferent olivocochlear system

Abstract

The auditory system in many mammals is immature at birth but precisely organized in adults. Spontaneous activity in the inner ear plays a critical role in guiding this process. This is shaped by an efferent pathway that descends from the brainstem and makes transient direct synaptic contacts with inner hair cells (IHCs). In this work, we used an β9 cholinergic receptor knock-in mouse model with enhanced medial efferent activity (Chrna9L9′T, L9′T) to understand the role of the olivocochlear system in the correct establishment of auditory circuits. Wave III amplitudes of auditory brainstem responses (which represent synchronized activity of synapses within the superior olivary complex) were smaller in L9′T mice, suggesting a central dysfunction. The mechanism underlying this functional alteration was analyzed in brain slices containing the medial nucleus of the trapezoid body (MNTB), where neurons are topographically organized along a medio-lateral axis. Electrophysiological recordings evidenced MNTB synaptic alterations. Spontaneous synaptic response (mEPSCs) displayed no changes in its amplitude among genotypes, while mEPSCs mean frequency displayed a significant increase in the L9’T lateral region (M: 2.52±0.56 Hz; L: 345 7.17±1.94 Hz; Mann-Whitney test, Z: -2.11, p=0.035). Moreover, evoked synaptic transmission was altered in the transgenic mice. While no significant differences in the unitary medial and lateral EPSC amplitudes were recorded in WT mice (M: 7.59±1.12 nA, n=9, 7 animals; L: 7.35±0.95 nA, n=10, 8 animals, ANOVA, F:0.027, p=0.87), evoked synaptic currents in the lateral side (5.07±0.87 nA, n=12, 11 animals) of L9’T mice were smaller compared to those of the medial side (8.05±1.37 nA, n=11, 11 animals; ANOVA, F:5.07, p=0.0357). These abnormalities were further supported by morphological alterations. Rhodamine-dextran labeling evidenced multiple innervation in L9’T MNTB principal cells suggesting an impairment during development. At the in-vivo level, multielectrode recordings showed that the overall level of MNTB activity was reduced in the L9’T. The average multi-unit activity in WT (11.49±3.58 Hz, n=6 animals) was larger than in L9’T mice (2.53-±0.43 Hz, n=8 animals; Mann-Whitney U Test, Z=2.19, 806 p=0.028). The present results suggest that the transient cochlear efferent innervation to IHCs during the critical period before the onset of hearing is involved in the refinement of topographic maps as well as in setting the correct synaptic transmission at central auditory nuclei.