Inhibitory properties underlying non-monotonic input-output relationship in low-frequency spherical bushy neurons of the gerbil

Enhancement of phase-locking in rodents. II. An axonal recording study in chinchilla

The trapezoid body (TB) contains axons of neurons residing in the anteroventral cochlear nucleus (AVCN) that provide excitatory and inhibitory inputs to the main monaural and binaural nuclei in the superior olivary complex (SOC). To understand the monaural and binaural response properties of neurons in the medial and lateral superior olive (MSO and LSO), it is important to characterize the temporal firing properties of these inputs. Because of its exceptional low-frequency hearing, the chinchilla (Chinchilla lanigera) is one of the widely used small animal models for studies of hearing. However, the characterization of the output of its ventral cochlear nucleus to the nuclei of the SOC is fragmentary. We obtained responses of TB axons to stimuli typically used in binaural studies and compared these responses to those of auditory nerve (AN) fibers, with a focus on temporal coding. We found enhancement of phase-locking and entrainment, i.e., the ability of a neuron to fire action potentials at a certain stimulus phase for nearly every stimulus period, in TB axons relative to AN fibers. Enhancement in phase-locking and entrainment are quantitatively more modest than in the cat but greater than in the gerbil. As in these species, these phenomena occur not only in low-frequency neurons stimulated at their characteristic frequency but also in neurons tuned to higher frequencies when stimulated with low-frequency tones, to which complex phase-locking behavior with multiple modes of firing per stimulus cycle is frequently observed.NEW & NOTEWORTHY The sensitivity of neurons to small time differences in sustained sounds to both ears is important for binaural hearing, and this sensitivity is critically dependent on phase-locking in the monaural pathways. Although studies in cat showed a marked improvement in phase-locking from the peripheral to the central auditory nervous system, the evidence in rodents is mixed. Here, we recorded from AN and TB of chinchilla and found temporal enhancement, though more limited than in cat.

Dendritic Degeneration and Altered Synaptic Innervation of a Central Auditory Neuron During Age-related Hearing Loss

Cellular morphology and synaptic configuration are key determinants of neuronal function and are often modified under pathological conditions. In the first nucleus of the central auditory system, the cochlear nucleus (CN), principal bushy neurons specialize in processing temporal information of sound critical for hearing. These neurons alter their physiological properties during aging that contribute to age-related hearing loss (ARHL). The structural basis of such changes remains unclear, especially age-related modifications in their dendritic morphology and the innervating auditory nerve (AN) synapses. Using young (2-5 months) and aged (28-33 months) CBA/CaJ mice of either sex, we filled individual bushy neurons with fluorescent dye in acute brain slices to characterize their dendritic morphology, followed by immunostaining against vesicular glutamate transporter 1 (VGluT1) and calretinin (CR) to identify innervating AN synapses. We found that dendritic morphology of aged bushy neurons had significantly reduced complexity, suggesting age-dependent dendritic degeneration, especially in neurons with predominantly non-CR-expressing synapses on the soma. These dendrites were innervated by AN bouton synapses, which were predominantly non-CR-expressing in young mice but had increased proportion of CR-expressing synapses in old mice. While somatic AN synapses degenerated substantially with age, as quantified by VGluT1-labeled puncta volume, no significant difference was observed in the total volume of dendritic synapses between young and old mice. Consequently, synaptic density on dendrites was significantly higher in old mice. The findings suggest that dendritic degeneration and altered synaptic innervation in bushy neurons during aging may underlie their changed physiological activity and contribute to the development of ARHL.

Auditory Brainstem Models: Adapting Cochlear Nuclei Improve Spatial Encoding by the Medial Superior Olive in Reverberation

Listeners typically perceive a sound as originating from the direction of its source, even as direct sound is followed milliseconds later by reflected sound from multiple different directions. Early-arriving sound is emphasised in the ascending auditory pathway, including the medial superior olive (MSO) where binaural neurons encode the interaural-time-difference (ITD) cue for spatial location. Perceptually, weighting of ITD conveyed during rising sound energy is stronger at 600 Hz than at 200 Hz, consistent with the minimum stimulus rate for binaural adaptation, and with the longer reverberation times at 600 Hz, compared with 200 Hz, in many natural outdoor environments. Here, we computationally explore the combined efficacy of adaptation prior to the binaural encoding of ITD cues, and excitatory binaural coincidence detection within MSO neurons, in emphasising ITDs conveyed in early-arriving sound. With excitatory inputs from adapting, nonlinear model spherical bushy cells (SBCs) of the bilateral cochlear nuclei, a nonlinear model MSO neuron with low-threshold potassium channels reproduces the rate-dependent emphasis of rising vs. peak sound energy in ITD encoding; adaptation is equally effective in the model MSO. Maintaining adaptation in model SBCs, and adjusting membrane speed in model MSO neurons, 'left' and 'right' populations of computationally efficient, linear model SBCs and MSO neurons reproduce this stronger weighting of ITD conveyed during rising sound energy at 600 Hz compared to 200 Hz. This hemispheric population model demonstrates a link between strong weighting of spatial information during rising sound energy, and correct unambiguous lateralisation of a speech source in reverberation.

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

Spherical bushy cells (SBCs) in 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 three-dimensional (3-D) representation of the bushy dendrite, we explored this hypothesis. 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 response rate and entrainment for low-input frequencies and temporal precision of output at and above the characteristic 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.NEW & NOTEWORTHY We modeled dendritic inputs from the auditory nerve that spherical bushy cells of the cochlear nucleus receive. Dendritic inputs caused both tonic depolarization and modulation of the membrane potential at the input frequency. This improved the rate, entrainment, and temporal precision of output action potentials. Our simulations suggest a role for small dendritic inputs in auditory processing: they modulate the efficacy of the main input supporting temporal precision and fidelity in these central auditory neurons.

Purinergic Modulation of Activity in the Developing Auditory Pathway

Purinergic P2 receptors, activated by endogenous ATP, are prominently expressed on neuronal and non-neuronal cells during development of the auditory periphery and central auditory neurons. In the mature cochlea, extracellular ATP contributes to ion homeostasis, and has a protective function against noise exposure. Here, we focus on the modulation of activity by extracellular ATP during early postnatal development of the lower auditory pathway. In mammals, spontaneous patterned activity is conveyed along afferent auditory pathways before the onset of acoustically evoked signal processing. During this critical developmental period, inner hair cells fire bursts of action potentials that are believed to provide a developmental code for synaptic maturation and refinement of auditory circuits, thereby establishing a precise tonotopic organization. Endogenous ATP-release triggers such patterned activity by raising the extracellular K⁺ concentration and contributes to firing by increasing the excitability of auditory nerve fibers, spiral ganglion neurons, and specific neuron types within the auditory brainstem, through the activation of diverse P2 receptors. We review recent studies that provide new models on the contribution of purinergic signaling to early development of the afferent auditory pathway. Further, we discuss potential future directions of purinergic research in the auditory system.

Neuronal population model of globular bushy cells covering unit-to-unit variability

Computations of acoustic information along the central auditory pathways start in the cochlear nucleus. Bushy cells in the anteroventral cochlear nucleus, which innervate monaural and binaural stations in the superior olivary complex, process and transfer temporal cues relevant for sound localization. These cells are categorized into two groups: spherical and globular bushy cells (SBCs/GBCs). Spontaneous rates of GBCs innervated by multiple auditory nerve (AN) fibers are generally lower than those of SBCs that receive a small number of large AN synapses. In response to low-frequency tonal stimulation, both types of bushy cells show improved phase-locking and entrainment compared to AN fibers. When driven by high-frequency tones, GBCs show primary-like-with-notch or onset-L peristimulus time histograms and relatively irregular spiking. However, previous in vivo physiological studies of bushy cells also found considerable unit-to-unit variability in these response patterns. Here we present a population of models that can simulate the observed variation in GBCs. We used a simple coincidence detection model with an adaptive threshold and systematically varied its six parameters. Out of 567000 parameter combinations tested, 7520 primary-like-with-notch models and 4094 onset-L models were selected that satisfied a set of physiological criteria for a GBC unit. Analyses of the model parameters and output measures revealed that the parameters of the accepted model population are weakly correlated with each other to retain major GBC properties, and that the output spiking patterns of the model are affected by a combination of multiple parameters. Simulations of frequency-dependent temporal properties of the model GBCs showed a reasonable fit to empirical data, supporting the validity of our population modeling. The computational simplicity and efficiency of the model structure makes our approach suitable for future large-scale simulations of binaural information processing that may involve thousands of GBC units.

In the auditory system, specialized neuronal circuits process various types of acoustic information. A group of neurons, called globular bushy cells (GBCs), faithfully transfer timing information of acoustic signals to their downstream neurons responsible for the perception of sound location. Previous physiological studies found representative activity patterns of GBCs, but with substantial individual variations among them. In this study, we present a population of models, instead of creating one best model, to account for the observed variations of GBCs. We varied all six parameters of a simple auditory neuron model and selected the combinations of parameters that led to acceptable activity patterns of GBCs. In total, we tested more than half a million combinations and accepted ~11600 GBC models. Temporal spiking patterns of real GBCs depend on the sound frequency, and our model population was able to replicate this trend. The model used here is computationally efficient and can thus serve as a building block for future large-scale simulations of auditory information processing.

Principal Neurons in the Anteroventral Cochlear Nucleus Express Cell-Type Specific Glycine Receptor α Subunits

Signal processing in the principal neurons of the anteroventral cochlear nucleus (AVCN) is modulated by glycinergic inhibition. The kinetics of IPSCs are specific to the target neurons. It remains unclear what glycine receptor subunits are involved in generating such target-specific IPSC kinetics in AVCN principal neurons. We investigated the expression patterns of glycine receptor α (GlyRα) subunits in AVCN using immunohistochemical labeling of four isoforms of GlyRα subunits (GlyRα₁-α₄), and found that AVCN neurons express GlyRα₁ and GlyRα₄, but not GlyRα₂ and GlyRα₃ subunits. To further identify the cell type-specific expression patterns of GlyRα subunits, we combined whole-cell patch clamp recording with immunohistochemistry by recording from all three types of AVCN principal neurons, characterizing the synaptic properties of their glycinergic inhibition, dye-filling the neurons, and processing the slice for immunostaining of different GlyRα subunits. We found that AVCN bushy neurons express both GlyRα₁ and GlyRα₄ subunits that underlie their slow IPSC kinetics, whereas both T-stellate and D-stellate neurons express only GlyRα₁ subunit that underlies their fast IPSC kinetics. In conclusion, AVCN principal neurons express cell-type specific GlyRα subunits that underlie their distinct IPSC kinetics, which enables glycinergic inhibition from the same source to exert target cell-specific modulation of activity to support the unique physiological function of these neurons.

Functional Development of Principal Neurons in the Anteroventral Cochlear Nucleus Extends Beyond Hearing Onset

Sound information is transduced into graded receptor potential by cochlear hair cells and encoded as discrete action potentials of auditory nerve fibers. In the cochlear nucleus, auditory nerve fibers convey this information through morphologically distinct synaptic terminals onto bushy cells (BCs) and stellate cells (SCs) for processing of different sound features. With expanding use of transgenic mouse models, it is increasingly important to understand the in vivo functional development of these neurons in mice. We characterized the maturation of spontaneous and acoustically evoked activity in BCs and SCs by acquiring single-unit juxtacellular recordings between hearing onset (P12) and young adulthood (P30) of anesthetized CBA/J mice. In both cell types, hearing sensitivity and characteristic frequency (CF) range are mostly adult-like by P14, consistent with rapid maturation of the auditory periphery. In BCs, however, some physiological features like maximal firing rate, dynamic range, temporal response properties, recovery from post-stimulus depression, first spike latency (FSL) and encoding of sinusoid amplitude modulation undergo further maturation up to P18. In SCs, the development of excitatory responses is even more prolonged, indicated by a gradual increase in spontaneous and maximum firing rates up to P30. In the same cell type, broadly tuned acoustically evoked inhibition is immediately effective at hearing onset, covering the low- and high-frequency flanks of the excitatory response area. Together, these data suggest that maturation of auditory processing in the parallel ascending BC and SC streams engages distinct mechanisms at the first central synapses that may differently depend on the early auditory experience.

Developmental Shift of Inhibitory Transmitter Content at a Central Auditory Synapse

Synaptic inhibition in the CNS is mostly mediated by GABA or glycine. Generally, the use of the two transmitters is spatially segregated, but there are central synapses employing both, which allows for spatial and temporal variability of inhibitory mechanisms. Spherical bushy cells (SBCs) in the mammalian cochlear nucleus receive primary excitatory inputs through auditory nerve fibers arising from the organ of Corti and non-primary inhibition mediated by a dual glycine-GABA transmission. Slow kinetics IPSCs enable activity dependent tonic-like conductance build up, functioning as a gain control by filtering out small or temporally imprecise EPSPs. However, it remained elusive whether GABA and glycine are released as content of the same vesicle or from distinct presynaptic terminals. The developmental profile of quantal release was investigated with whole cell recordings of miniature inhibitory postsynaptic currents (mIPSCs) from P1–P25 SBCs of Mongolian gerbils. GABA is the initial transmitter eliciting slow-rising and -decaying events of relatively small amplitudes, occurring only during early postnatal life. Around and after hearing onset, the inhibitory quanta are predominantly containing glycine that—with maturity—triggers progressively larger and longer mIPSC. In addition, GABA corelease with glycine evokes mIPSCs of particularly large amplitudes consistently occurring across all ages, but with low probability. Together, these results suggest that GABA, as the primary transmitter released from immature inhibitory terminals, initially plays a developmental role. In maturity, GABA is contained in synaptic vesicles only in addition to glycine to increase the inhibitory potency, thereby fulfilling solely a modulatory function.

Tonotopic Optimization for Temporal Processing in the Cochlear Nucleus

In the auditory system, sounds are processed in parallel frequency-tuned circuits, beginning in the cochlea. Auditory nerve fibers reflect this tonotopy and encode temporal properties of acoustic stimuli by "locking" discharges to a particular stimulus phase. However, physiological constraints on phase-locking depend on stimulus frequency. Interestingly, low characteristic frequency (LCF) neurons in the cochlear nucleus improve phase-locking precision relative to their auditory nerve inputs. This is proposed to arise through synaptic integration, but the postsynaptic membrane's selectivity for varying levels of synaptic convergence is poorly understood. The chick cochlear nucleus, nucleus magnocellularis (NM), exhibits tonotopic distribution of both input and membrane properties. LCF neurons receive many small inputs and have low input thresholds, whereas high characteristic frequency (HCF) neurons receive few, large synapses and require larger currents to spike. NM therefore presents an opportunity to study how small membrane variations interact with a systematic topographic gradient of synaptic inputs. We investigated membrane input selectivity and observed that HCF neurons preferentially select faster input than their LCF counterparts, and that this preference is tolerant of changes to membrane voltage. We then used computational models to probe which properties are crucial to phase-locking. The model predicted that the optimal arrangement of synaptic and membrane properties for phase-locking is specific to stimulus frequency and that the tonotopic distribution of input number and membrane excitability in NM closely tracks a stimulus-defined optimum. These findings were then confirmed physiologically with dynamic-clamp simulations of inputs to NM neurons.

SIGNIFICANCE STATEMENT

One way that neurons represent temporal information is by phase-locking, which is discharging in response to a particular phase of the stimulus waveform. In the auditory system, central neurons are optimized to retain or improve phase-locking precision compared with input from the auditory nerve. However, the difficulty of this computation varies systematically with stimulus frequency. We examined properties that contribute to temporal processing both physiologically and in a computational model. Neurons processing low-frequency input benefit from integration of many weak inputs, whereas those processing higher frequencies progressively lose precision by integration of multiple inputs. Here, we reveal general features of input-output optimization that apply to all neurons that process time varying input.

High Entrainment Constrains Synaptic Depression Levels of an In vivo Globular Bushy Cell Model

Globular bushy cells (GBCs) located in the ventral cochlear nucleus are an essential part of the sound localization pathway in the mammalian auditory system. They receive inputs directly from the auditory nerve and are particularly sensitive to temporal cues due to their synaptic and membrane specializations. GBCs act as coincidence detectors for incoming spikes through large synapses-endbulbs of Held-which connect to their soma. Since endbulbs of Held are an integral part of the auditory information conveying and processing pathway, they were extensively studied. Virtually all in vitro studies showed large synaptic depression, but on the other hand a few in vivo studies showed relatively small depression. It is also still not well understood how synaptic properties functionally influence firing properties of GBCs. Here we show how different levels of synaptic depression shape firing properties of GBCs in in vivo-like conditions using computer simulations. We analyzed how an interplay of synaptic depression (0-70%) and the number of auditory nerve fiber inputs (10-70) contributes to the variability of the experimental data from previous studies. We predict that the majority of synapses of GBCs with high characteristic frequencies (CF > 500 Hz) have a rate dependent depression of less than 20%. GBCs with lower CF (<500 Hz) work also with strong depressing synapses (up to 50% or more). We also showed that synapses explicitly fitted to in vitro experiments with paired-pulse stimuli did not operate properly in in vivo-like conditions and required further extension to capture the differences between in vitro and in vivo experimental conditions. Overall, this study helps to understand how synaptic properties shape temporal processing in the auditory system. It also integrates, compares, and reconciles results of various experimental studies.

Inhibition in the auditory brainstem enhances signal representation and regulates gain in complex acoustic environments

Inhibition plays a crucial role in neural signal processing, shaping and limiting responses. In the auditory system, inhibition already modulates second order neurons in the cochlear nucleus, e.g. spherical bushy cells (SBCs). While the physiological basis of inhibition and excitation is well described, their functional interaction in signal processing remains elusive. Using a combination of in vivo loose-patch recordings, iontophoretic drug application, and detailed signal analysis in the Mongolian Gerbil, we demonstrate that inhibition is widely co-tuned with excitation, and leads only to minor sharpening of the spectral response properties. Combinations of complex stimuli and neuronal input-output analysis based on spectrotemporal receptive fields revealed inhibition to render the neuronal output temporally sparser and more reproducible than the input. Overall, inhibition plays a central role in improving the temporal response fidelity of SBCs across a wide range of input intensities and thereby provides the basis for high-fidelity signal processing.

Slow Cholinergic Modulation of Spike Probability in Ultra-Fast Time-Coding Sensory Neurons

Sensory processing in the lower auditory pathway is generally considered to be rigid and thus less subject to modulation than central processing. However, in addition to the powerful bottom-up excitation by auditory nerve fibers, the ventral cochlear nucleus also receives efferent cholinergic innervation from both auditory and nonauditory top-down sources. We thus tested the influence of cholinergic modulation on highly precise time-coding neurons in the cochlear nucleus of the Mongolian gerbil. By combining electrophysiological recordings with pharmacological application in vitro and in vivo, we found 55-72% of spherical bushy cells (SBCs) to be depolarized by carbachol on two time scales, ranging from hundreds of milliseconds to minutes. These effects were mediated by nicotinic and muscarinic acetylcholine receptors, respectively. Pharmacological block of muscarinic receptors hyperpolarized the resting membrane potential, suggesting a novel mechanism of setting the resting membrane potential for SBC. The cholinergic depolarization led to an increase of spike probability in SBCs without compromising the temporal precision of the SBC output in vitro. In vivo, iontophoretic application of carbachol resulted in an increase in spontaneous SBC activity. The inclusion of cholinergic modulation in an SBC model predicted an expansion of the dynamic range of sound responses and increased temporal acuity. Our results thus suggest of a top-down modulatory system mediated by acetylcholine which influences temporally precise information processing in the lower auditory pathway.

Inhibition shapes acoustic responsiveness in spherical bushy cells

Signal processing in the auditory brainstem is based on an interaction of neuronal excitation and inhibition. To date, we have incomplete knowledge of how the dynamic interplay of both contributes to the processing power and temporal characteristics of signal coding. The spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN) receive their primary excitatory input through auditory nerve fibers via large, axosomatic synaptic terminals called the endbulbs of Held and by additional, acoustically driven inhibitory inputs. SBCs provide the input to downstream nuclei of the brainstem sound source localization circuitry, such as the medial and lateral superior olive, which rely on temporal precise inputs. In this study, we used juxtacellular recordings in anesthetized Mongolian gerbils to assess the effect of acoustically evoked inhibition on the SBCs input-output function and on temporal precision of SBC spiking. Acoustically evoked inhibition proved to be strong enough to suppress action potentials (APs) of SBCs in a stimulus-dependent manner. Inhibition shows slow onset and offset dynamics and increasing strength at higher sound intensities. In addition, inhibition decreases the rising slope of the EPSP and prolongs the EPSP-to-AP transition time. Both effects can be mimicked by iontophoretic application of glycine. Inhibition also improves phase locking of SBC APs to low-frequency tones by acting as a gain control to suppress poorly timed EPSPs from generating postsynaptic APs to maintain precise SBC spiking across sound intensities. The present data suggest that inhibition substantially contributes to the processing power of second-order neurons in the ascending auditory system.