REDISTRIBUTION OF SYNAPTIC INPUTS IN THE NEONATAL RODENT AUDITORY BRAINSTEM OCCURS IN THE ABSENCE OF ACOUSTICALLY DRIVEN ACTIVITY

Abstract

Principal neurons in the lateral superior olive (LSO) receive excitatory input from the anteroventral cochlear nucleus (AVCN) and inhibitory inputs from the medial nucleus of the trapezoid body (MNTB). These two phenotypically different inputs arriving at individual neurons in mature LSO are precisely matched for stimulus frequency. However, in the immature LSO, excitatory and inhibitory inputs arriving at individual neurons are crudely matched for stimulus frequency. During postnatal development, both excitatory and inhibitory pathways undergo substantially physiological measurable refinement before hearing onset, presumably under the guidance of spontaneous activity generated in the immature cochlea. In the neighbouring nucleus, MSO, inhibitory synapses are redistributed toward the soma, arguably through an experience-dependent manner. Here, in LSO, I investigate how excitatory and inhibitory synapses are redistributed before hearing onset. In order to do so, I implemented hardware to label single cells in acute living slices, Matlab scripts to segment synapses in 3D, and Matlab scripts for spike sorting and firing state reconstruction (not reported here). Using the tools that I developed, I investigated the developmental distribution of excitatory and inhibitory synapses in prehearing LSO. At birth, excitatory synapses surprisingly outnumbered inhibitory synapses at the soma and the proximal dendrite. In the ensuing week, before hearing onset, while excitatory synapses are redistributed away from the soma, inhibitory synapses are redistributed toward the soma, such that by hearing onset inhibitory synapses outnumber excitatory synapses at the soma and the proximal dendrite.