Physiological and Anatomical Development of Glycinergic Inhibition in the Mouse Superior Paraolivary Nucleus Following Hearing Onset.

Neural circuits require balanced synaptic excitation and inhibition to ensure accurate neural computation. Our knowledge about the development and maturation of inhibitory synaptic inputs is less well developed than that concerning excitation. Here we describe the maturation of an inhibitory circuit within the mammalian auditory brainstem where counter-intuitively, inhibition drives action potential firing of principal neurons. Using combined anatomical tracing and electrophysiological recordings from mice, neurons of the superior paraolivary nucleus (SPN) are shown to receive converging glycinergic input from at least four neurons of the medial nucleus of the trapezoid body (MNTB). These four axons formed 30.71 ±2.72 (mean ±s.e.m.) synaptic boutons onto each SPN neuronal soma, generating a total inhibitory conductance of 80nS. Such strong inhibition drives the underlying post-inhibitory rebound firing mechanism, which is a hallmark of SPN physiology. In contrast to inhibitory projections to the medial and lateral superior olives, the inhibitory projection to the SPN does not exhibit experience-dependent synaptic refinement following the onset of hearing. These findings emphasize that the development and function of neural circuits cannot be inferred from one synaptic target to another, even if both originate from the same neuron.