Neuroanatomical characterization of perineuronal net components in the human cochlear nucleus and superior olivary complex.

The human auditory brainstem, especially the cochlear nucleus (CN) and the superior olivary complex (SOC) are characterized by a high density of neurons associated with perineuronal nets (PNs). PNs build a specific form of extracellular matrix surrounding the neuronal somata, proximal dendrites and axon initial segments. They restrict synaptic plasticity and control high-frequency synaptic activity, a prominent characteristic of neurons of the auditory brainstem. The distribution of PNs within the auditory brainstem has been investigated in a number of mammalian species. However, much less is known regarding PNs in the human auditory brainstem. The present study aimed at the immunohistochemical identification of PNs in the cochlear nucleus (CN) and superior olivary complex (SOC) in the human brainstem. We focused on the complex nature and molecular variability of PNs in the CN and SOC by using specific antibodies against the main PN components (aggrecan, brevican, neurocan and hyaluronan and proteoglycan link protein 1). Virtually all subnuclei within the ventral CN and SOC were found to be associated with PNs. Direct comparison between gerbil and human yielded similar fine structure of PNs and confirmed the typical tight interdigitation of PNs with synaptic terminals in both species. Noticeably, an elaborate combination of immunohistochemical labelings clearly supports the still debated existence of the medial nucleus of trapezoid body (MNTB) in the human brain. In conclusion, the present study demonstrates that PNs form a prominent extracellular structure on CN and SOC neurons in the human brain, potentially stabilizing synaptic contacts, which is in agreement with many other mammalian species.

In vivo Whole-Cell Recordings Combined with Electron Microscopy Reveal Unexpected Morphological and Physiological Properties in the Lateral Nucleus of the Trapezoid Body in the Auditory Brainstem.

The lateral nucleus of the trapezoid body (LNTB) is a prominent nucleus in the superior olivary complex in mammals including humans. Its physiology in vivo is poorly understood due to a paucity of recordings. It is thought to provide a glycinergic projection to the medial superior olive (MSO) with an important role in binaural processing and sound localization. We combined in vivo patch clamp recordings with labeling of individual neurons in the Mongolian gerbil. Labeling of the recorded neurons allowed us to relate physiological properties to anatomy at the light and electron microscopic level. We identified a population of quite dorsally located neurons with surprisingly large dendritic trees on which most of the synaptic input impinges. In most neurons, one or more of these dendrites run through and are then medial to the MSO. These neurons were often binaural and could even show sensitivity to interaural time differences (ITDs) of stimulus fine structure or envelope. Moreover, a subpopulation showed enhanced phase-locking to tones delivered in the tuning curve tail. We propose that these neurons constitute the gerbil main LNTB (mLNTB). In contrast, a smaller sample of neurons was identified that was located more ventrally and that we designate to be in posteroventral LNTB (pvLNTB). These cells receive large somatic excitatory terminals from globular bushy cells. We also identified previously undescribed synaptic inputs from the lateral superior olive. pvLNTB neurons are usually monaural, display a primary-like-with-notch response to ipsilateral short tones at CF and can phase-lock to low frequency tones. We conclude that mLNTB contains a population of neurons with extended dendritic trees where most of the synaptic input is found, that can show enhanced phase-locking and sensitivity to ITD. pvLNTB cells, presumed to provide glycinergic input to the MSO, get large somatic globular bushy synaptic inputs and are typically monaural with short tone responses similar to their primary input from the cochlear nucleus.

Inhibitory projections from the ventral nucleus of the trapezoid body to the medial nucleus of the trapezoid body in the mouse.

Neurons in the medial nucleus of the trapezoid body (MNTB) receive prominent excitatory input through the calyx of Held, a giant synapse that produces large and fast excitatory currents. MNTB neurons also receive inhibitory glycinergic inputs that are also large and fast, and match the calyceal excitation in terms of synaptic strength. GABAergic inputs provide additional inhibition to MNTB neurons. Inhibitory inputs to MNTB modify spiking of MNTB neurons both in-vitro and in-vivo, underscoring their importance. Surprisingly, the origin of the inhibitory inputs to MNTB has not been shown conclusively. We performed retrograde tracing, anterograde tracing, immunohistochemical experiments, and electrophysiological recordings to address this question. The results support the ventral nucleus of the trapezoid body (VNTB) as at least one major source of glycinergic input to MNTB. VNTB fibers enter the ipsilateral MNTB, travel along MNTB principal neurons and produce several bouton-like presynaptic terminals. Further, the contribution of GABA to the total inhibition declines during development, resulting in only a very minor fraction of GABAergic inhibition in adulthood, which is matched in time by a reduction in expression of a GABA synthetic enzyme in VNTB principal neurons.