Abstract
Principal neurons in the medial nucleus of the trapezoid body (MNTB) receive strong and temporally precise excitatory input from globular bushy cells in the cochlear nucleus through the calyx of Held. The extremely large synaptic currents produced by the calyx have sometimes led to the view of the MNTB as a simple relay synapse which converts incoming excitation to outgoing inhibition. However, electrophysiological and anatomical studies have shown the additional presence of inhibitory glycinergic currents that are large enough to suppress action potentials in MNTB neurons at least in some cases. The source(s) of glycinergic inhibition to MNTB are not fully understood. One major extrinsic source of glycinergic inhibitory input to MNTB is the ventral nucleus of the trapezoid body. However, it has been suggested that MNTB neurons receive additional inhibitory inputs via intrinsic connections (collaterals of glycinergic projections of MNTB neurons). While several authors have postulated their presence, these collaterals have never been examined in detail. Here we test the hypothesis that collaterals of MNTB principal cells provide glycinergic inhibition to the MNTB. We injected dye into single principal neurons in the MNTB, traced their projections, and immunohistochemically identified their synapses. We found that collaterals terminate within the MNTB and provide an additional source of inhibition to other principal cells, creating an inhibitory microcircuit within the MNTB. Only about a quarter to a third of MNTB neurons receive such collateral inputs. This microcircuit could produce side band inhibition and enhance frequency tuning of MNTB neurons, consistent with physiological observations.
Highlights
The medial nucleus of the trapezoid body (MNTB) is an auditory brainstem nucleus involved in the sound source localization pathway, as well as in a number of other auditory circuits[1,2,3,4]
The goal of our study was to test the hypothesis that axons from MNTB neurons form collateral inhibitory inputs onto neighboring MNTB neurons
We tested this hypothesis by labeling axons of single MNTB neurons, identifying potential recurrent collaterals, characterizing their terminals, and estimating the number of neurons that produce such collaterals
Summary
The medial nucleus of the trapezoid body (MNTB) is an auditory brainstem nucleus involved in the sound source localization pathway, as well as in a number of other auditory circuits[1,2,3,4]. It receives excitatory input from globular bushy cells (GBCs) located in the contralateral anterior ventral cochlear nucleus (aVCN) [5,6,7,8,9,10]. Golgi staining and electron microscopy (EM) studies have characterized three types of neurons in the MNTB: stellate, elongate and principal cells ([5], cat) with the latter representing the majority (82%) of cells ([16], rat)
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