Differential contributions of voltage-gated potassium channel subunits in enhancing temporal coding in the bushy cells of the ventral cochlear nucleus

Temporal coding precision of bushy cells in the ventral cochlear nucleus (VCN), critical for sound localization and communication, depends on the generation of rapid and temporally precise action potentials (APs). Voltage-gated potassium (Kv) channels are critically involved in this. The bushy cells in rat VCN express Kv1.1, 1.2, 1.3, 1.6, 3.1, 4.2 and 4.3 subunits. The Kv1.1 subunit contributes to the generation of a temporally precise single AP. However, the understanding of the functions of other Kv subunits expressed in the bushy cells is limited. Here, we investigated the functional diversity of Kv subunits concerning their contributions to temporal coding. We characterized the electrophysiological properties of the Kv channels with different subunits using whole-cell patch-clamp recording and pharmacological methods. The neuronal firing pattern changed from single to multiple APs only when the Kv1.1 subunit was blocked. The Kv subunits, including the Kv1.1, 1.2, 1.6 or 3.1, were involved in enhancing temporal coding by lowering membrane excitability, shortening AP latencies, reducing jitter and regulating AP kinetics. Meanwhile, all the Kv subunits contributed to rapid repolarization and sharpening peaks by narrowing half-width and accelerating fall rate, while the Kv1.1 subunit also affected the depolarization of AP. The Kv1.1, 1.2 and 1.6 subunits endowed bushy cells with a rapid time constant and a low input resistance of membrane for enhancing spike timing precision. The present results indicate that the Kv channels differentially affect intrinsic membrane properties to optimize the generation of rapid and reliable APs for temporal coding.

Audiotactile interactions in the mouse cochlear nucleus

Multisensory integration of auditory and tactile information occurs already at the level of the cochlear nucleus. Rodents use their whiskers for tactile perception to guide them in their exploration of the world. As nocturnal animals with relatively poor vision, audiotactile interactions are of great importance for this species. Here, the influence of whisker deflections on sound-evoked spiking in the cochlear nucleus was investigated in vivo in anesthetized mice. Multichannel, silicon-probe electrophysiological recordings were obtained from both the dorsal and ventral cochlear nucleus. Whisker deflections evoked an increased spiking activity in fusiform cells of the dorsal cochlear nucleus and t-stellate cells in ventral cochlear nucleus, whereas bushy cells in the ventral cochlear nucleus showed a more variable response. The response to broadband noise stimulation increased in fusiform cells and primary-like bushy cells when the sound stimulation was preceded (~ 20 ms) by whisker stimulation. Multi-sensory integration of auditory and whisker input can thus occur already in this early brainstem nucleus, emphasizing the importance of early integration of auditory and somatosensory information.

RIM-Binding Protein 2 organizes Ca2+ channel topography and regulates release probability and vesicle replenishment at a fast central synapse

RIM-Binding Protein 2 (RIM-BP2) is a multi-domain protein of the presynaptic active zone (AZ). By binding to Rab-interacting protein (RIM), bassoon and voltage-gated Ca2+ channels (CaV), it is considered to be a central organizer of the topography of CaV and release sites of synaptic vesicles (SVs) at the AZ. Here, we investigated the role of RIM-BP2 at the endbulb of Held synapse of auditory nerve fibers with bushy cells of the cochlear nucleus, a fast relay of the auditory pathway with high release probability. Disruption of RIM-BP2 lowered release probability altering short-term plasticity and reduced evoked excitatory postsynaptic currents (EPSCs). Analysis of SV pool dynamics during high frequency train stimulation indicated a reduction of SVs with high release probability but an overall normal size of the readily releasable SV pool (RRP). The Ca2+-dependent fast component of SV replenishment after RRP depletion was slowed. Augmenting Ca2+ influx by adding extracellular Ca2+ restored release probability but not EPSC amplitude, and uncovered an impairment of SV replenishment during train stimulation. Ultrastructural analysis by super-resolution light and electron microscopy revealed an impaired topography of presynaptic CaV and a reduction of docked and membrane-proximal SVs at the AZ. We conclude that RIM-BP2 organizes the topography of CaV, and promotes SV tethering and docking. This way RIM-BP2 is critical for establishing a high initial release probability as required to reliably signal sound onset information that we found to be degraded in bushy cells of RIM-BP2-deficient mice in vivo.

Small dendritic synapses enhance temporal coding in a model of cochlear nucleus bushy cells

Spherical bushy cells (SBCs) in the the anteroventral cochlear nucleus receive a single or very few powerful axosomatic inputs from the auditory nerve. However, SBCs are also contacted by small regular bouton synapses of the auditory nerve, located in their dendritic tree. The function of these small inputs is unknown. It was speculated that the interaction of axosomatic inputs with small dendritic inputs improved temporal precision, but direct evidence for this is missing. In a compartment model of spherical bushy cells with a stylized or realistic 3D-representation of the bushy dendrite we thus explored this proposal. Phase-locked dendritic inputs caused both tonic depolarization and a modulation of the model SBC membrane potential at the frequency of the stimulus. For plausible model parameters dendritic inputs were subthreshold. Instead, the tonic depolarization increased the excitability of the SBC model and the modulation of the membrane potential caused a phase-dependent increase in the efficacy of the main axosomatic input. This improved rate, entrainment and temporal precision of output action potentials. However, these effects showed differential dependency on the stimulus frequency. A careful exploration of morphological and biophysical parameters of the bushy dendrite suggested a functional explanation for the peculiar shape of the bushy dendrite. Our model for the first time directly implied a role for the small excitatory dendritic inputs in auditory processing: they modulate the efficacy of the main input and are thus a plausible mechanism for the improvement of temporal precision and fidelity in these central auditory neurons.

Ventral cochlear nucleus bushy cells encode hyperacusis in guinea pigs

Psychophysical studies characterize hyperacusis as increased loudness growth over a wide-frequency range, decreased tolerance to loud sounds and reduced behavioral reaction time latencies to high-intensity sounds. While commonly associated with hearing loss, hyperacusis can also occur without hearing loss, implicating the central nervous system in the generation of hyperacusis. Previous studies suggest that ventral cochlear nucleus bushy cells may be putative neural contributors to hyperacusis. Compared to other ventral cochlear nucleus output neurons, bushy cells show high firing rates as well as lower and less variable first-spike latencies at suprathreshold intensities. Following cochlear damage, bushy cells show increased spontaneous firing rates across a wide-frequency range, suggesting that they might also show increased sound-evoked responses and reduced latencies to higher-intensity sounds. However, no studies have examined bushy cells in relationship to hyperacusis. Herein, we test the hypothesis that bushy cells may contribute to the neural basis of hyperacusis by employing noise-overexposure and single-unit electrophysiology. We find that bushy cells exhibit hyperacusis-like neural firing patterns, which are comprised of enhanced sound-driven firing rates, reduced first-spike latencies and wideband increases in excitability.

Small dendritic synapses enhance temporal coding in a model of cochlear nucleus bushy cells

Spherical bushy cells (SBC) in the the anteroventral cochlear nucleus can improve the temporal precision of the auditory nerve spiking activity despite receiving sometimes only a single suprathreshold axosomatic input. The interaction with small dendritic inputs could provide a possible explanation for this phenomenon. In a compartment model of spherical bushy cells with a stylized or realistic three-dimensional representation of the bushy dendrite we explored this proposal. Phase-locked dendritic inputs caused both a tonic depolarization and a modulation of the SBC membrane potential at the frequency of the stimulus but for plausible model parameters do not cause output action potentials (AP). The tonic depolarization increased the excitability of the SBC model. The modulation of the membrane potential caused a phase-dependent increase in the efficacy of the main axosomatic input to cause output AP. These effects increased the rate and the temporal precision of output AP. Rate was mainly increased for stimulus frequencies at and below the characteristic frequency of the main input. Precision mostly increased for higher frequencies above about 1 kHz. Dendritic morphological parameters, biophysical parameters of the dendrite and the synaptic inputs and tonotopic parameters of the inputs all affected the impact of dendritic synapses. This suggested the possibility of fine tuning of the effect the dendritic inputs have for different coding demands or input frequency ranges. Excitatory dendritic inputs modulate the processing of the main input and are thus a plausible mechanism for the improvement of temporal precision in spherical bushy cells.