The impact of deafness: maturation of synaptic and biophysical properties in an auditory brainstem nucleus

The ventral nucleus of the lateral lemniscus (VNLL) is a monaural brainstem nucleus of the ascending auditory pathway that may play a critical role in the analysis of temporal sound patterns. Its underlying intrinsic and synaptic properties have not been systematically studied in a ventrodorsal orientation in mice. Using in vitro electrophysiological recordings from acute brain slices of mice and immunohistochemistry, we aimed to discover the margins of the VNLL and the neuronal and synaptic characteristics of the neurons lying within. We demonstrate that the biophysical membrane properties and excitatory input characteristics differ between dorsal and ventral VNLL neurons. Neurons in the ventral VNLL displayed an onset-type firing pattern and little hyperpolarization-activated current (Ih), and, in the ventrolateral VNLL (vVNLL), received calyx-like inputs that were similar to calyx of Held synapses in the medial nucleus of the trapezoid body (MNTB). Neurons of the dorsal part received several and weak input fibers that were slower than in the vVNLL, while displaying either onset- or sustained-type firing patterns with large Ih. Using a mouse model that expresses channelrhodopsin under the promotor of the vesicular GABA transporter (VGAT) suggests that dorsal and ventral neurons are inhibitory, since they were all depolarized by light stimulation. The diverse membrane and input properties in dorsal and ventral VNLL neurons suggest differential roles of these neurons in sound processing. Furthermore, we were interested in discovering when the excitatory calyx-like inputs in the vVNLL develop. Before hearing onset, neurons of the vVNLL received generally more than one excitatory input, which refined shortly after hearing onset. Additionally, NMDA receptor-mediated currents were almost completely reduced after hearing onset, resulting in fast and large postsynaptic current responses mediated by AMPA receptors. Next, we wanted to elucidate whether sound-evoked activity is essential for the observed synaptic refinement. Therefore, we used a congenitally deaf mouse that lacks otoferlin and compared it with young and mature hearing mice. Otoferlin is found in inner hair cells, among other places, and mediates exocytosis of neurotransmitters. We demonstrate that KO mice have an impaired firing behavior. While neurons displayed either an onset-type or sustained-type firing in KO and young WT animals, mature WT mice had only onset-firing neurons. Additionally, synaptic refinement was also altered and showed great similarities to young WT mice. Neurons of mature WT mice received on average one large calyx-like synapse, whereas neurons of KO and young WT mice received up to five excitatory inputs. Fiber stimulations of the inputs in KO mice evoked smaller and slower postsynaptic currents than in mature WT mice. However, establishment of the readily releasable pool was unaffected by deafness. Our results suggest that sensory experience is necessary for a typical maturation of biophysical properties and the calyx-like input to the vVNLL. Consequently, our results encourage the use of cochlear implants in patients with an otoferlin-related deafness, since it may be possible that the synaptic and biophysical impairment can be reduced by providing sound-evoked activity to the auditory brainstem