Organization of inhibitory feed-forward synapses from the dorsal to the ventral cochlear nucleus in the cat: a quantitative analysis of endings by vesicle morphology

A biophysical modelling platform of the cochlear nucleus and other auditory circuits: From channels to networks

Models of the auditory brainstem have been an invaluable tool for testing hypotheses about auditory information processing and for highlighting the most important gaps in the experimental literature. Due to the complexity of the auditory brainstem, and indeed most brain circuits, the dynamic behavior of the system may be difficult to predict without a detailed, biologically realistic computational model. Despite the sensitivity of models to their exact construction and parameters, most prior models of the cochlear nucleus have incorporated only a small subset of the known biological properties. This confounds the interpretation of modelling results and also limits the potential future uses of these models, which require a large effort to develop. To address these issues, we have developed a general purpose, biophysically detailed model of the cochlear nucleus for use both in testing hypotheses about cochlear nucleus function and also as an input to models of downstream auditory nuclei. The model implements conductance-based Hodgkin-Huxley representations of cells using a Python-based interface to the NEURON simulator. Our model incorporates most of the quantitatively characterized intrinsic cell properties, synaptic properties, and connectivity available in the literature, and also aims to reproduce the known response properties of the canonical cochlear nucleus cell types. Although we currently lack the empirical data to completely constrain this model, our intent is for the model to continue to incorporate new experimental results as they become available.

Synaptic plasticity in the medial superior olive of hearing, deaf, and cochlear-implanted cats

The medial superior olive (MSO) is a key auditory brainstem structure that receives binaural inputs and is implicated in processing interaural time disparities used for sound localization. The deaf white cat, a proven model of congenital deafness, was used to examine how deafness and cochlear implantation affected the synaptic organization at this binaural center in the ascending auditory pathway. The patterns of axosomatic and axodendritic organization were determined for principal neurons from the MSO of hearing, deaf, and deaf cats with cochlear implants. The nature of the synapses was evaluated through electron microscopy, ultrastructure analysis of the synaptic vesicles, and immunohistochemistry. The results show that the proportion of inhibitory axosomatic terminals was significantly smaller in deaf animals when compared with hearing animals. However, after a period of electrical stimulation via cochlear implants the proportion of inhibitory inputs resembled that of hearing animals. Additionally, the excitatory axodendritic boutons of hearing cats were found to be significantly larger than those of deaf cats. Boutons of stimulated cats were significantly larger than the boutons in deaf cats, although not as large as in the hearing cats, indicating a partial recovery of excitatory inputs to MSO dendrites after stimulation. These results exemplify dynamic plasticity in the auditory brainstem and reveal that electrical stimulation through cochlear implants has a restorative effect on synaptic organization in the MSO.

Degeneration in the ventral cochlear nucleus after severe noise damage in mice

To study the mechanisms of noise-induced hearing loss and the phantom noise, or tinnitus, often associated with it, we used a mouse model of noise damage designed for reproducible and quantitative structural analyses. We selected the posteroventral cochlear nucleus, which has shown considerable plasticity in past studies, and correlated its changes with the distribution of neurotrophin 3 (NT3). We used volume change, optical density analysis, and microscopic cluster analysis to measure the degeneration after noise exposure. There was a fluctuation pattern in the reorganization of nerve terminals. The data suggest that the source and size of the nerve terminals affect their capacity for regeneration. We hypothesize that the deafferentation of ventral cochlear nucleus is the structural basis of noise-induced tinnitus. In addition, the immunofluorescent data show a possible connection between NT3 and astrocytes. There appears to be a compensatory process in the supporting glial cells during this degeneration. Glia may play a role in the mechanisms of noise-induced hearing loss.

Electrical stimulation of the dorsal cochlear nucleus induces hearing in rats

Auditory brainstem implants (ABIs) restore hearing by electrical stimulation of the cochlear nucleus (CN). Depending on the physiological condition, duration of the pre-existing deafness, extent of damage to the CN, and the number of channels accessible to the tonotopic frequency gradients of the CN, ABIs improve speech understanding to varying degrees. Although the ventral cochlear nucleus, a mainstream auditory structure, has been considered a logic target for ABI stimulation, it is not yet clear how the dorsal cochlear nucleus (DCN) contributes to patients' hearing during ABI stimulation. To better understand the mechanisms underlying ABIs, we tested if electrical stimulation of the rat DCN induces hearing using a novel electrical prepulse inhibition (ePPI) of startle reflex behavior model. Our results showed that bipolar electrical stimulation of all channels in the DCN induced behavioral manifestation of hearing and that electrical stimulation of certain channels in the DCN induced robust neural activity in auditory cortex channels that responded to acoustic stimulation and demonstrated well-defined frequency tuning curves. This suggests that the DCN plays an important role in electrical hearing and should be further pursued in designing new ABIs. The novel ePPI behavioral paradigm may potentially be developed into an efficient method for testing hearing in animals with an implantable prosthesis.

Review: cytological characteristics of commissural and tuberculo-ventral neurons in the rat dorsal cochlear nucleus

The goal of the present review is to summarize the main ultrastructural and immunocytochemical characteristics for glycine and GABA in commissural (COM) and tuberculo-ventral neurons (TV) of the DCN. These neurons are localized in similar areas of the DCN multipolar but are connected to different targets. About 2/3rd of COM-neurons are large to bipolar neurons, mainly glycinergic, often GABA-ergic, with scarce ergastoplasm and axo-somatic boutons. About 1/3rd of COM-neurons are glycine and GABA-negative, and show little ergastoplasm and synaptic coverage. Occasional giant COM-neurons are glycine-positive and GABA-negative, and are covered with synaptic boutons. Other infrequent large neurons, rich in dense core vesicles, glycine- and GABA-negative, are most covered with boutons. TV-neurons are most glycinergic but 9% are glycine-negative. They have little ergastoplasm and a developed Golgi apparatus. Axo-somatic terminals are scarce and mainly contain flat and pleomorphic vesicles, glycine and sometimes GABA (inhibitory). TV-neurons receive a lower number of boutons than COM, which contain mainly flat-pleomorphic terminals. Putative COM-inhibitory boutons contact excitatory pyramidal and giant neurons (monosynaptic inhibition). Some putative inhibitory COM-terminals contact inhibitory cartwheel and tuberculo-ventral neurons. This indicates direct disinhibition and therefore excitation in the DCN (di-three-synaptic). Putative COM-mossy fibers reach the granule areas of the DCN, including unipolar brush cell dendrites, another possible excitatory commissural pathway.

Presynaptic ryanodine receptors are required for normal quantal size at the Caenorhabditis elegans neuromuscular junction

Analyses of the effect of ryanodine in vertebrate brain slices have led to the conclusion that presynaptic ryanodine receptors (RYRs) may have several functions in synaptic release, including causing large-amplitude miniature postsynaptic currents (mPSCs) by promoting concerted multivesicular release. However, the role of RYRs in synaptic release is controversial. To better understand the role of RYRs in synaptic release, we analyzed the effect of RYR mutation on mPSCs and evoked postsynaptic currents (ePSCs) at the Caenorhabditis elegans neuromuscular junction (NMJ). Amplitudes of mPSCs varied greatly at the C. elegans NMJ. Loss-of-function mutations of the RYR gene unc-68 (uncoordinated 68) essentially abolished large-amplitude mPSCs. The amplitude of ePSCs was also greatly suppressed. These defects were completely rescued by expressing wild-type UNC-68 specifically in neurons but not in muscle cells, suggesting that RYRs acted presynaptically. A combination of removing extracellular Ca2+ and UNC-68 function eliminated mPSCs, suggesting that influx and RYR-mediated release are likely the exclusive sources of Ca2+ for synaptic release. Large-amplitude mPSCs did not appear to be caused by multivesicular release, as has been suggested to occur at vertebrate central synapses, because the rise time of mPSCs was constant regardless of the amplitude but distinctive from that of ePSCs, and because large-amplitude mPSCs persisted under conditions that inhibit synchronized synaptic release, including elimination of extracellular Ca2+, and mutations of syntaxin and SNAP25 (soluble N-ethylmaleimide-sensitive factor attachment protein 25). These observations suggest that RYRs are essential to normal quantal size and are potential regulators of quantal size.