Developmental Profile of Ion Channel Specializations in the Avian Nucleus Magnocellularis

Glutamatergic CYLD deletion leads to aberrant excitatory activity in the basolateral amygdala: association with enhanced cued fear expression

JOURNAL/nrgr/04.03/01300535-202511000-00029/figure1/v/2024-12-20T164640Z/r/image-tiff Neuronal activity, synaptic transmission, and molecular changes in the basolateral amygdala play critical roles in fear memory. Cylindromatosis (CYLD) is a deubiquitinase that negatively regulates the nuclear factor kappa-B pathway. CYLD is well studied in non-neuronal cells, yet under-investigated in the brain, where it is highly expressed. Emerging studies have shown involvement of CYLD in the remodeling of glutamatergic synapses, neuroinflammation, fear memory, and anxiety- and autism-like behaviors. However, the precise role of CYLD in glutamatergic neurons is largely unknown. Here, we first proposed involvement of CYLD in cued fear expression. We next constructed transgenic model mice with specific deletion of Cyld from glutamatergic neurons. Our results show that glutamatergic CYLD deficiency exaggerated the expression of cued fear in only male mice. Further, loss of CYLD in glutamatergic neurons resulted in enhanced neuronal activation, impaired excitatory synaptic transmission, and altered levels of glutamate receptors accompanied by over-activation of microglia in the basolateral amygdala of male mice. Altogether, our study suggests a critical role of glutamatergic CYLD in maintaining normal neuronal, synaptic, and microglial activation. This may contribute, at least in part, to cued fear expression.

Electrophysiological correlates of divergent projections in the avian superior olivary nucleus

The physiological diversity of inhibitory neurons provides ample opportunity to influence a wide range of computational roles through their varied activity patterns, especially via feedback loops. In the avian auditory brain stem, inhibition originates primarily from the superior olivary nucleus (SON), and so it is critical to understand the intrinsic physiological properties and processing capabilities of these neurons. Neurons in the SON receive ascending input via the cochlear nuclei: directly from the intensity-coding cochlear nucleus angularis (NA) and indirectly via the interaural timing nucleus laminaris (NL), which itself receives input from cochlear nucleus magnocellularis (NM). Two distinct populations of SON neurons provide inhibitory feedback either to ipsilateral NA, NL, and the timing cochlear nucleus NM or to the contralateral SON. To determine whether these populations correspond to distinct response types, we investigated their electrophysiology in brain stem slices, using patch-clamp electrophysiology. We identified three phenotypes: single-spiking, chattering tonic, and regular tonic neurons. The two tonic phenotypes displayed distinct firing patterns and different membrane properties. Fluctuating "noisy" currents used to probe the capability of SON neurons to encode temporal features showed that each phenotype differed in sensitivity to temporally modulated input. By using cell fills and anatomical reconstructions, we could correlate the firing phenotypes with their axonal projection patterns. We found that SON axons exited via three fiber tracts, with each tract composed of specific phenotypes. These results provide a basis for understanding the role of specific inhibitory cell types in auditory function and elucidate the organization of the SON outputs.NEW & NOTEWORTHY Inhibitory inputs for the avian brain stem originate primarily from the superior olivary nucleus (SON). We describe three intrinsic phenotypes of SON neurons and show how they differ in their temporal processing and projection patterns. We propose that the two types of tonic firing neurons (including one novel type) and the single-spiking neurons in SON comprise separate feedback circuits that may differentially influence the auditory information flowing via the cochlear nuclei and nucleus laminaris.

BDNF Differentially Affects Low- and High-Frequency Neurons in a Primary Nucleus of the Chicken Auditory Brainstem

Neurotrophins are proteins that mediate neuronal development using spatiotemporal signaling gradients. The chicken nucleus magnocellularis (NM), an analogous structure to the mammalian anteroventral cochlear nucleus, provides a model system in which signaling between the brain-derived neurotrophic factor (BDNF) and tyrosine receptor kinase B (TrkB) is temporally regulated. In the NM, TrkB expression is high early in development (embryonic [E] day 9) and is downregulated until maturity (E18-21). It is currently unknown how BDNF-TrkB signaling affects neuronal properties throughout development and across a spatial (i.e., frequency) axis. To investigate this, we exogenously applied BDNF onto NM neurons ex vivo and studied intrinsic properties using whole-cell patch clamp electrophysiology. Early in development (E13), when TrkB expression is detectable with immunohistochemistry, BDNF application slowed the firing of high-frequency NM neurons, resembling an immature phenotype. Current measurements and biophysical modeling revealed that this was mediated by a decreased conductance of the voltage-dependent potassium channels. Interestingly, this effect was seen only in high-frequency neurons and not in low-frequency neurons. BDNF-TrkB signaling induced minimal changes in late-developing NM neurons (E20-21) of high and low frequencies. Our results indicate that normal developmental downregulation of BDNF-TrkB signaling promotes neuronal maturation tonotopically in the auditory brainstem, encouraging the appropriate development of neuronal properties.

Inhibition of Ionic Currents by Fluoxetine in Vestibular Calyces in Different Epithelial Loci

Previous studies have suggested a role for selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac®) in the treatment of dizziness and inner ear vestibular dysfunction. The potential mechanism of action within the vestibular system remains unclear; however, fluoxetine has been reported to block certain types of K+ channel in other systems. Here, we investigated the direct actions of fluoxetine on membrane currents in presynaptic hair cells and postsynaptic calyx afferents of the gerbil peripheral vestibular system using whole cell patch clamp recordings in crista slices. We explored differences in K+ currents in peripheral zone (PZ) and central zone (CZ) calyces of the crista and their response to fluoxetine application. Outward K+ currents in PZ calyces showed greater inactivation at depolarized membrane potentials compared to CZ calyces. The application of 100 μM fluoxetine notably reduced K+ currents in calyx terminals within both zones of the crista, and the remaining currents exhibited distinct traits. In PZ cells, fluoxetine inhibited a non-inactivating K+ current and revealed a rapidly activating and inactivating K+ current, which was sensitive to blocking by 4-aminopyridine. This was in contrast to CZ calyces, where low-voltage-activated and non-inactivating K+ currents persisted following application of 100 μM fluoxetine. Additionally, marked inhibition of transient inward Na+ currents by fluoxetine was observed in calyces from both crista zones. Different concentrations of fluoxetine were tested, and the EC50 values were found to be 40 µM and 32 µM for K+ and Na+ currents, respectively. In contrast, 100 μM fluoxetine had no impact on voltage-dependent K+ currents in mechanosensory type I and type II vestibular hair cells. In summary, micromolar concentrations of fluoxetine are expected to strongly reduce both Na+ and K+ conductance in afferent neurons of the peripheral vestibular system in vivo. This would lead to inhibition of action potential firing in vestibular sensory neurons and has therapeutic implications for disorders of balance.

Neuroanatomical Predictors of Transcranial Direct Current Stimulation (tDCS)-Induced Modifications in Neurocognitive Task Performance in Typically Developing Individuals

Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulation technique gaining more attention in neurodevelopmental disorders (NDDs). Due to the phenotypic heterogeneity of NDDs, tDCS is unlikely to be equally effective in all individuals. The present study aimed to establish neuroanatomical markers in typically developing (TD) individuals that may be used for the prediction of individual responses to tDCS. Fifty-seven male and female children received 2 mA anodal and sham tDCS, targeting the left dorsolateral prefrontal cortex (DLPFCleft), right inferior frontal gyrus, and bilateral temporoparietal junction. Response to tDCS was assessed based on task performance differences between anodal and sham tDCS in different neurocognitive tasks (N-back, flanker, Mooney faces detection, attentional emotional recognition task). Measures of cortical thickness (CT) and surface area (SA) were derived from 3 Tesla structural MRI scans. Associations between neuroanatomy and task performance were assessed using general linear models (GLM). Machine learning (ML) algorithms were employed to predict responses to tDCS. Vertex-wise estimates of SA were more closely linked to differences in task performance than measures of CT. Across ML algorithms, highest accuracies were observed for the prediction of N-back task performance differences following stimulation of the DLPFCleft, where 65% of behavioral variance was explained by variability in SA. Lower accuracies were observed for all other tasks and stimulated regions. This suggests that it may be possible to predict individual responses to tDCS for some behavioral measures and target regions. In the future, these models might be extended to predict treatment outcome in individuals with NDDs.

Revisiting the Chicken Auditory Brainstem Response: Frequency Specificity, Threshold Sensitivity, and Cross Species Comparison

The auditory brainstem response (ABR) is important for both clinical and basic auditory research. It is a non-invasive measure of hearing function with millisecond-level precision. The ABR can not only measure the synchrony, speed, and efficacy of auditory physiology but also detect different modalities of hearing pathology and hearing loss. ABRs are easily acquired in vertebrate animal models like reptiles, birds, and mammals, and complement existing molecular, developmental, and systems-level research. One such model system is the chicken; an excellent animal for studying auditory development, structure, and function. However, the ABR for chickens was last reported nearly 4 decades ago. The current study examines how decades of ABR characterization in other animal species support findings from the chicken ABR. We replicated and expanded on previous research using 43 chicken hatchlings 1- and 2-day post-hatch. We report that click-evoked chicken ABRs presented with a peak waveform morphology, amplitude, and latency like previous avian studies. Tone-evoked ABRs were found for frequencies from 250 to 4000 Hertz (Hz) and exhibited a range of best sensitivity between 750 and 2000 Hz. Objective click-evoked and tone-evoked ABR thresholds were comparable to subjective thresholds. With these revisited measurements, the chicken ABR still proves to be an excellent example of precocious avian development that complements decades of molecular, neuronal, and systems-level research in the same model organism.

Motor cortex analogue neurons in songbirds utilize Kv3 channels to generate ultranarrow spikes

Complex motor skills in vertebrates require specialized upper motor neurons with precise action potential (AP) firing. To examine how diverse populations of upper motor neurons subserve distinct functions and the specific repertoire of ion channels involved, we conducted a thorough study of the excitability of upper motor neurons controlling somatic motor function in the zebra finch. We found that robustus arcopallialis projection neurons (RAPNs), key command neurons for song production, exhibit ultranarrow spikes and higher firing rates compared to neurons controlling non-vocal somatic motor functions (dorsal intermediate arcopallium [AId] neurons). Pharmacological and molecular data indicate that this striking difference is associated with the higher expression in RAPNs of high threshold, fast-activating voltage-gated Kv3 channels, that likely contain Kv3.1 (KCNC1) subunits. The spike waveform and Kv3.1 expression in RAPNs mirror properties of Betz cells, specialized upper motor neurons involved in fine digit control in humans and other primates but absent in rodents. Our study thus provides evidence that songbirds and primates have convergently evolved the use of Kv3.1 to ensure precise, rapid AP firing in upper motor neurons controlling fast and complex motor skills.

Electrical signaling in cochlear efferents is driven by an intrinsic neuronal oscillator

Efferent neurons are believed to play essential roles in maintaining auditory function. The lateral olivocochlear (LOC) neurons-which project from the brainstem to the inner ear, where they release multiple transmitters including peptides, catecholamines, and acetylcholine-are the most numerous yet least understood elements of efferent control of the cochlea. Using in vitro calcium imaging and patch-clamp recordings, we found that LOC neurons in juvenile and young adult mice exhibited extremely slow waves of activity (∼0.1 Hz). These seconds-long bursts of Na⁺ spikes were driven by an intrinsic oscillator dependent on L-type Ca²⁺ channels and were not observed in prehearing mice, suggesting an age-dependent mechanism underlying the intrinsic oscillator. Using optogenetic approaches, we identified both ascending (T-stellate cells of the cochlear nucleus) and descending (auditory cortex) sources of synaptic excitation, as well as the synaptic receptors used for such excitation. Additionally, we identified potent inhibition originating in the glycinergic medial nucleus of trapezoid body (MNTB). Conductance-clamp experiments revealed an unusual mechanism of electrical signaling in LOC neurons, in which synaptic excitation and inhibition served to switch on and off the intrinsically generated spike burst mechanism, allowing for prolonged periods of activity or silence controlled by brief synaptic events. Protracted bursts of action potentials may be essential for effective exocytosis of the diverse transmitters released by LOC fibers in the cochlea.

Evaluation of Auditory Brainstem Response in Chicken Hatchlings

The auditory brainstem response (ABR) is an invaluable assay in clinical audiology, non-human animals, and human research. Despite the widespread use of ABRs in measuring auditory neural synchrony and estimating hearing sensitivity in other vertebrate model systems, methods for recording ABRs in the chicken have not been reported in nearly four decades. Chickens provide a robust animal research model because their auditory system is near functional maturation during late embryonic and early hatchling stages. We have demonstrated methods used to elicit one or two-channel ABR recordings using subdermal needle electrode arrays in chicken hatchlings. Regardless of electrode recording configuration (i.e., montage), ABR recordings included 3-4 positive-going peak waveforms within the first 6 ms of a suprathreshold click stimulus. Peak-to-trough waveform amplitudes ranged from 2-11 µV at high-intensity levels, with positive peaks exhibiting expected latency-intensity functions (i.e., increase in latency as a function of decreased intensity). Standardized earphone position was critical for optimal recordings as loose skin can occlude the ear canal, and animal movement can dislodge the stimulus transducer. Peak amplitudes were smaller, and latencies were longer as animal body temperature lowered, supporting the need for maintaining physiological body temperature. For young hatchlings (<3 h post-hatch day 1), thresholds were elevated by ~5 dB, peak latencies increased ~1-2 ms, and peak to trough amplitudes were decreased ~1 µV compared to older hatchlings. This suggests a potential conductive-related issue (i.e., fluid in the middle ear cavity) and should be considered for young hatchlings. Overall, the ABR methods outlined here permit accurate and reproducible recording of in-vivo auditory function in chicken hatchlings that could be applied to different stages of development. Such findings are easily compared to human and mammalian models of hearing loss, aging, or other auditory-related manipulations.

Modulation of Neuronal Excitability by Low- Intensity Ultrasound in Two Principal Neurons of Rat Anteroventral Cochlear Nucleus

Ultrasonic neuromodulation has proved to be a promising new approach for direct neuromodulation or potential noninvasive deep brain stimulation technology for treating various neurological disorders. Previous studies have demonstrated that ultrasonic waves can noninvasively diffuse through the intact skull and thus precisely target specific brain regions with high spatial resolution. However, its neuromodulatory effects over different cell types of target nuclei have not been fully elucidated. In the present study, we investigated the neuronal excitability resulted from ultrasound stimulation on the two major neurons of anteroventral cochlear nucleus (AVCN) in vitro. Our results demonstrated that bushy cells (BCs) were well maintaining one action potential (AP) in response to the pairing of a sequence of depolarizing current pulses and 60-s continuous low-intensity ultrasound (LIUS), and meanwhile, stellate cells (SCs) significantly increased the firing rate. The ultrasonic waves with an acoustic pressure of 0.13 MPa were elicited by an on-chip ultrasonic stimulation system compatible with patch-clamp recording. Furthermore, LIUS significantly improved the neuronal excitability in both BCs and SCs based on their intrinsic excitability. Modulation of membrane properties among cell types was due to the LIUS-induced increase in the total inward sodium currents ( I_Na ) and outward potassium currents ( I_Kv ). LIUS significantly, at a similar rate, increased the amplitude of total inward sodium currents in both cell types. Meanwhile, LIUS induces a higher rate of the outward potassium currents in the BCs compared with SCs. Therefore, this study could provide new evidence for safe use of ultrasonic neuromodulation and its potential therapy for many auditory diseases, such as the central auditory processing disorder.

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, and 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.NEW & NOTEWORTHY This study investigates the roles of Kv channels in effecting precision using electrophysiological and pharmacological methods in bushy cells. Different Kv channels have varying electrophysiological characteristics, which contribute to the interplay between changes in the membrane properties and regulation of neuronal excitability which then improve temporal coding. We conclude that the Kv channels are specialized to promote the precise and rapid coding of acoustic input by optimizing the generation of reliable APs.

Intrinsic properties of avian interaural level difference sound localizing neurons

Intrinsic properties of neurons are one major determinant for how neurons respond to their synaptic inputs and shape their outputs in neural circuits. Here, we studied the intrinsic properties of neurons in the chicken posterior portion of the dorsal nucleus of the lateral lemniscus (LLDp), the first interaural level difference (ILD) encoder of the avian auditory pathway. Using whole-cell recordings in brain slices, we revealed that the LLDp is composed of a heterogeneous neuron population based on their firing patterns. LLDp neurons were broadly classified as either phasic or tonic firing neurons, with further classification applied to tonic firing neurons, such as regular (most dominant, n = 82 out of 125 cells, 65.6%), pauser, or adaptive firing. Neurons with different firing patterns were distributed about evenly across the dorsoventral as well as mediolateral axis of LLDp. Phasic firing neurons were of faster membrane time constant, and lower excitability than tonic firing neurons. The action potentials (APs) elicited at the current thresholds displayed significant differences in first spike latency, AP peak amplitude, half-width, and maximal rising and falling rates. Interestingly, for APs elicited at suprathreshold currents (400 pA above thresholds), some of the differences diminished while a few others emerged. Remarkably, most parameters of the APs at thresholds were significantly different from those of APs at suprathresholds. Combined with our previous study (Curry and Lu, 2016), the results lend support to the two-cell type model for ILD coding in the avian system.

Effects of Neurotrophin-3 on Intrinsic Neuronal Properties at a Central Auditory Structure

Neurotrophins, a class of growth factor proteins that control neuronal proliferation, morphology, and apoptosis, are found ubiquitously throughout the nervous system. One particular neurotrophin (NT-3) and its cognate tyrosine receptor kinase (TrkC) have recently received attention as a possible therapeutic target for synaptopathic sensorineural hearing loss. Additionally, research shows that NT-3-TrkC signaling plays a role in establishing the sensory organization of frequency topology (ie, tonotopic order) in the cochlea of the peripheral inner ear. However, the neurotrophic effects of NT-3 on central auditory properties are unclear. In this study we examined whether NT-3-TrkC signaling affects the intrinsic electrophysiological properties at a first-order central auditory structure in chicken, known as nucleus magnocellularis (NM). Here, the expression pattern of specific neurotrophins is well known and tightly regulated. By using whole-cell patch-clamp electrophysiology, we show that NT-3 application to brainstem slices does not affect intrinsic properties of high-frequency neuronal regions but had robust effects for low-frequency neurons, altering voltage-dependent potassium functions, action potential repolarization kinetics, and passive membrane properties. We suggest that NT-3 may contribute to the precise establishment and organization of tonotopy in the central auditory pathway by playing a specialized role in regulating the development of intrinsic neuronal properties of low-frequency NM neurons.

Structural and Functional Refinement of the Axon Initial Segment in Avian Cochlear Nucleus during Development

The axon initial segment (AIS) is involved in action potential initiation. Structural and biophysical characteristics of the AIS differ among cell types and/or brain regions, but the underlying mechanisms remain elusive. Using immunofluorescence and electrophysiological methods, combined with super-resolution imaging, we show in the developing nucleus magnocellularis of the chicken in both sexes that the AIS is refined in a tonotopic region-dependent manner. This process of AIS refinement differs among cells tuned to different frequencies. At hearing onset, the AIS was ∼50 µm long with few voltage-gated sodium channels regardless of tonotopic region. However, after hatching, the AIS matured and displayed an ∼20-µm-long structure with a significant enrichment of sodium channels responsible for an increase in sodium current and a decrease in spike threshold. Moreover, the shortening was more pronounced, while the accumulation of channels was not, in neurons tuned to higher frequency, creating tonotopic differences in the AIS. We conclude that AIS shortening is mediated by disassembly of the cytoskeleton at the distal end of the AIS, despite intact periodicity of the submembranous cytoskeleton across the AIS. Importantly, deprivation of afferent input diminished the shortening in neurons tuned to a higher frequency to a larger extent in posthatch animals, with little effect on the accumulation of sodium channels. Thus, cytoskeletal reorganization and sodium channel enrichment at the AIS are differentially regulated depending on tonotopic region, but work synergistically to optimize neuronal output in the auditory nucleus.SIGNIFICANCE STATEMENT The axon initial segment (AIS) plays fundamental roles in determining neuronal output. The AIS varies structurally and molecularly across tonotopic regions in avian cochlear nucleus. However, the mechanism underlying these variations remains unclear. The AIS is immature around hearing onset, but becomes shorter and accumulates more sodium channels during maturation, with a pronounced shortening and a moderate channel accumulation at higher tonotopic regions. Afferent input adjusts sodium conductance at the AIS by augmenting AIS shortening (via disassembly of cytoskeletons at its distal end) specifically at higher-frequency regions. However, this had little effect on channel accumulation. Thus, cytoskeletal structure and sodium channel accumulation at the AIS are regulated differentially but work synergistically to optimize the neuronal output.

Maturation of pyramidal cells in anterior piriform cortex may be sufficient to explain the end of early olfactory learning in rats

Studies have shown that neonate rodents exhibit high ability to learn a preference for novel odors associated with thermo-tactile stimuli that mimics maternal care. Artificial odors paired with vigorous strokes in rat pups younger than 10 postnatal days (P), but not older, rapidly induce an orientation-approximation behavior toward the conditioned odor in a two-choice preference test. The olfactory bulb (OB) and the anterior olfactory cortex (aPC), both modulated by norepinephrine (NE), have been identified as part of a neural circuit supporting this transitory olfactory learning. One possible explanation at the neuronal level for why the odor-stroke pairing induces consistent orientation-approximation behavior in <P10 pups, but not in >P10, is the coincident activation of prior existent neurons in the aPC mediating this behavior. Specifically, odor-stroke conditioning in <P10 pups may activate more mother/nest odor's responsive aPC neurons than in >P10 pups, promoting orientation-approximation behavior in the former but not in the latter. In order to test this hypothesis, we performed in vitro patch-clamp recordings of the aPC pyramidal neurons from rat pups from two age groups (P5–P8 and P14–P17) and built computational models for the OB-aPC neural circuit based on this physiological data. We conditioned the P5–P8 OB-aPC artificial circuit to an odor associated with NE activation (representing the process of maternal odor learning during mother–infant interactions inside the nest) and then evaluated the response of the OB-aPC circuit to the presentation of the conditioned odor. The results show that the number of responsive aPC neurons to the presentation of the conditioned odor in the P14–P17 OB-aPC circuit was lower than in the P5–P8 circuit, suggesting that at P14–P17, the reduced number of responsive neurons to the conditioned (maternal) odor might not be coincident with the responsive neurons for a second conditioned odor.

Ultrasound Stimulation Modulates Voltage-Gated Potassium Currents Associated With Action Potential Shape in Hippocampal CA1 Pyramidal Neurons

Potassium channels (K⁺) play an important role in the regulation of cellular signaling. Dysfunction of potassium channels is associated with several severe ion channels diseases, such as long QT syndrome, episodic ataxia and epilepsy. Ultrasound stimulation has proven to be an effective non-invasive tool for the modulation of ion channels and neural activity. In this study, we demonstrate that ultrasound stimulation enables to modulate the potassium currents and has an impact on the shape modulation of action potentials (AP) in the hippocampal pyramidal neurons using whole-cell patch-clamp recordings in vitro. The results show that outward potassium currents in neurons increase significantly, approximately 13%, in response to 30 s ultrasound stimulation. Simultaneously, the increasing outward potassium currents directly decrease the resting membrane potential (RMP) from −64.67 ± 1.10 mV to −67.51 ± 1.35 mV. Moreover, the threshold current and AP fall rate increase while the reduction of AP half-width and after-hyperpolarization peak time is detected. During ultrasound stimulation, reduction of the membrane input resistance of pyramidal neurons can be found and shorter membrane time constant is achieved. Additionally, we verify that the regulation of potassium currents and shape of action potential is mainly due to the mechanical effects induced by ultrasound. Therefore, ultrasound stimulation may offer an alternative tool to treat some ion channels diseases related to potassium channels.

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.

Intrinsic physiological properties underlie auditory response diversity in the avian cochlear nucleus

Sensory systems exploit parallel processing of stimulus features to enable rapid, simultaneous extraction of information. Mechanisms that facilitate this differential extraction of stimulus features can be intrinsic or synaptic in origin. A subdivision of the avian cochlear nucleus, nucleus angularis (NA), extracts sound intensity information from the auditory nerve and contains neurons that exhibit diverse responses to sound and current injection. NA neurons project to multiple regions ascending the auditory brain stem including the superior olivary nucleus, lateral lemniscus, and avian inferior colliculus, with functional implications for inhibitory gain control and sound localization. Here we investigated whether the diversity of auditory response patterns in NA can be accounted for by variation in intrinsic physiological features. Modeled sound-evoked auditory nerve input was applied to NA neurons with dynamic clamp during in vitro whole cell recording at room temperature. Temporal responses to auditory nerve input depended on variation in intrinsic properties, and the low-threshold K⁺ current was implicated as a major contributor to temporal response diversity and neuronal input-output functions. An auditory nerve model of acoustic amplitude modulation produced synchrony coding of modulation frequency that depended on the intrinsic physiology of the individual neuron. In Primary-Like neurons, varying low-threshold K⁺ conductance with dynamic clamp altered temporal modulation tuning bidirectionally. Taken together, these data suggest that intrinsic physiological properties play a key role in shaping auditory response diversity to both simple and more naturalistic auditory stimuli in the avian cochlear nucleus. NEW & NOTEWORTHY This article addresses the question of how the nervous system extracts different information in sounds. Neurons in the cochlear nucleus show diverse responses to acoustic stimuli that may allow for parallel processing of acoustic features. The present studies suggest that diversity in intrinsic physiological features of individual neurons, including levels of a low voltage-activated K⁺ current, play a major role in regulating the diversity of auditory responses.

SYNGAP1 encephalopathy: A distinctive generalized developmental and epileptic encephalopathy

OBJECTIVE

To delineate the epileptology, a key part of the SYNGAP1 phenotypic spectrum, in a large patient cohort.

METHODS

Patients were recruited via investigators' practices or social media. We included patients with (likely) pathogenic SYNGAP1 variants or chromosome 6p21.32 microdeletions incorporating SYNGAP1. We analyzed patients' phenotypes using a standardized epilepsy questionnaire, medical records, EEG, MRI, and seizure videos.

RESULTS

We included 57 patients (53% male, median age 8 years) with SYNGAP1 mutations (n = 53) or microdeletions (n = 4). Of the 57 patients, 56 had epilepsy: generalized in 55, with focal seizures in 7 and infantile spasms in 1. Median seizure onset age was 2 years. A novel type of drop attack was identified comprising eyelid myoclonia evolving to a myoclonic-atonic (n = 5) or atonic (n = 8) seizure. Seizure types included eyelid myoclonia with absences (65%), myoclonic seizures (34%), atypical (20%) and typical (18%) absences, and atonic seizures (14%), triggered by eating in 25%. Developmental delay preceded seizure onset in 54 of 56 (96%) patients for whom early developmental history was available. Developmental plateauing or regression occurred with seizures in 56 in the context of a developmental and epileptic encephalopathy (DEE). Fifty-five of 57 patients had intellectual disability, which was moderate to severe in 50. Other common features included behavioral problems (73%); high pain threshold (72%); eating problems, including oral aversion (68%); hypotonia (67%); sleeping problems (62%); autism spectrum disorder (54%); and ataxia or gait abnormalities (51%).

CONCLUSIONS

SYNGAP1 mutations cause a generalized DEE with a distinctive syndrome combining epilepsy with eyelid myoclonia with absences and myoclonic-atonic seizures, as well as a predilection to seizures triggered by eating.

Need for Speed and Precision: Structural and Functional Specialization in the Cochlear Nucleus of the Avian Auditory System

Birds such as the barn owl and zebra finch are known for their remarkable hearing abilities that are critical for survival, communication, and vocal learning functions. A key to achieving these hearing abilities is the speed and precision required for the temporal coding of sound-a process heavily dependent on the structural, synaptic, and intrinsic specializations in the avian auditory brainstem. Here, we review recent work from us and others focusing on the specialization of neurons in the chicken cochlear nucleus magnocellularis (NM)-a first-order auditory brainstem structure analogous to bushy cells in the mammalian anteroventral cochlear nucleus. Similar to their mammalian counterpart, NM neurons are mostly adendritic and receive auditory nerve input through large axosomatic endbulb of Held synapses. Axonal projections from NM neurons to their downstream auditory targets are sophisticatedly programmed regarding their length, caliber, myelination, and conduction velocity. Specialized voltage-dependent potassium and sodium channel properties also play important and unique roles in shaping the functional phenotype of NM neurons. Working synergistically with potassium channels, an atypical current known as resurgent sodium current promotes rapid and precise action potential firing for NM neurons. Interestingly, these structural and functional specializations vary dramatically along the tonotopic axis and suggest a plethora of encoding strategies for sounds of different acoustic frequencies, mechanisms likely shared across species.

Experience-Dependent Intrinsic Plasticity During Auditory Learning

Song learning in zebra finches (Taeniopygia guttata) requires exposure to the song of a tutor, resulting in an auditory memory. This memory is the foundation for later sensorimotor learning, resulting in the production of a copy of the tutor's song. The cortical premotor nucleus HVC (proper name) is necessary for auditory and sensorimotor learning as well as the eventual production of adult song. We recently discovered that the intrinsic physiology of HVC neurons changes across stages of song learning, but are those changes the result of learning or are they experience-independent developmental changes? To test the role of auditory experience in driving intrinsic changes, patch-clamp experiments were performed comparing HVC neurons in juvenile birds with varying amounts of tutor exposure. The intrinsic physiology of HVC neurons changed as a function of tutor exposure. Counterintuitively, tutor deprivation resulted in juvenile HVC neurons showing an adult-like phenotype not present in tutor-exposed juveniles. Biophysical models were developed to predict which ion channels were modulated by experience. The models indicate that tutor exposure transiently suppressed the I _h and T-type Ca²⁺ currents in HVC neurons that target the basal ganglia, whereas tutor exposure increased the resting membrane potential and decreased the spike amplitude in HVC neurons that drive singing. Our findings suggest that intrinsic plasticity may be part of the mechanism for auditory learning in the HVC. More broadly, models of learning and memory should consider intrinsic plasticity as a possible mechanism by which the nervous system encodes the lasting effects of experience.SIGNIFICANCE STATEMENT It is well established that learning involves plasticity of the synapses between neurons. However, the activity of a neural circuit can also be dramatically altered by changes in the intrinsic properties (ion channels) of the component neurons. The present experiments show experience-dependent changes in the intrinsic physiology of neurons in the cortical premotor nucleus HVC (proper name) in juvenile zebra finches (Taeniopygia guttata) during auditory learning of a tutor's song. Tutor deprivation does not "arrest" development of intrinsic properties, but rather results in neurons with a premature adult-like physiological phenotype. It is possible that auditory learning involves a form of nonsynaptic plasticity and that experience-dependent suppression of specific ion channels may work in concert with synaptic plasticity to promote vocal learning.

Diverse Intrinsic Properties Shape Functional Phenotype of Low-Frequency Neurons in the Auditory Brainstem

In the auditory system, tonotopy is the spatial arrangement of where sounds of different frequencies are processed. Defined by the organization of neurons and their inputs, tonotopy emphasizes distinctions in neuronal structure and function across topographic gradients and is a common feature shared among vertebrates. In this study we characterized action potential firing patterns and ion channel properties from neurons located in the extremely low-frequency region of the chicken nucleus magnocellularis (NM), an auditory brainstem structure. We found that NM neurons responsible for encoding the lowest sound frequencies (termed NMc neurons) have enhanced excitability and fired bursts of action potentials to sinusoidal inputs ≤10 Hz; a distinct firing pattern compared to higher-frequency neurons. This response property was due to lower amounts of voltage dependent potassium (KV) conductances, unique combination of KV subunits and specialized sodium (NaV) channel properties. Particularly, NMc neurons had significantly lower KV1 and KV3 currents, but higher KV2 current. NMc neurons also showed larger and faster transient NaV current (INaT) with different voltage dependence of inactivation from higher-frequency neurons. In contrast, significantly smaller resurgent sodium current (INaR) was present in NMc with kinetics and voltage dependence that differed from higher-frequency neurons. Immunohistochemistry showed expression of NaV1.6 channel subtypes across the tonotopic axis. However, various immunoreactive patterns were observed between regions, likely underlying some tonotopic differences in INaT and INaR. Finally, using pharmacology and computational modeling, we concluded that KV3, KV2 channels and INaR work synergistically to regulate burst firing in NMc.

Auditory Input Shapes Tonotopic Differentiation of Kv1.1 Expression in Avian Cochlear Nucleus during Late Development

Tonotopic differentiation is fundamental for signal processing in the auditory system. However, when and how this differentiation arises remain elusive. We addressed this issue using electrophysiology and immunohistochemistry in nucleus magnocellularis of chickens of both sexes, which is known to differ in the expression of Kv1.1 channels depending on characteristic frequency (CF). Just after hearing onset (embryonic day 12-14), Kv1 current gradually increased to a slightly larger extent in neurons with higher CF, causing a tonotopic difference of Kv1 current before hatch. However, after hatch, a much larger increase of Kv1 current occurred, particularly in higher-CF neurons, due to an augmentation of Kv1.1 expression at the plasma membrane. This later change in expression led to the large tonotopic difference of Kv1 current characteristic of mature animals. Attenuation of auditory input by inducing conductive or sensorineural hearing loss around hatch suppressed the differentiation in a level-dependent manner. Moreover, elevation of auditory input during embryonic periods could not reproduce the differentiation, suggesting that the capacity of neurons to drive Kv1.1 expression via auditory input develops in a cell-specific manner, thus underlying the frequency-specific expression of the channel within the nucleus. The results indicated that the tonotopic differentiation of Kv1.1 in nucleus magnocellularis is partially determined before hatch, but largely driven by afferent input after hatch. Our results highlight the importance of neuronal capacity for sound to drive ion channel expression as well as the level of auditory experience in the frequency tuning of brainstem auditory circuits.SIGNIFICANCE STATEMENT Tuning-frequency-specific expression of ion channels is a prerequisite for auditory system function, but its underlying mechanisms remain unclear. Here, we revealed in avian cochlear nucleus that the expression of Kv1.1 became more dependent on auditory input at a late period of maturation in neurons tuned to higher-frequency sound, leading to frequency-specific Kv1.1 expression. Attenuation of auditory input during this period suppressed the differentiation in a level-dependent manner, whereas elevation of input in earlier periods could not reproduce the differentiation. Thus, the capacity of neurons to drive Kv1.1 expression via auditory input develops in a cell-specific manner and directs differentiation, highlighting the importance of neuronal character as well as the level of input in the frequency tuning of auditory circuits.

Resurgent sodium current promotes action potential firing in the avian auditory brainstem

Key points

Auditory brainstem neurons of all vertebrates fire phase‐locked action potentials (APs) at high rates with remarkable fidelity, a process controlled by specialized anatomical and biophysical properties.

This is especially true in the avian nucleus magnocellularis (NM) – the analogue of the mammalian anteroventral cochlear nucleus.

In addition to high voltage‐activated potassium (K_HVA) channels, we report, using whole cell physiology and modelling, that resurgent sodium current (I_NaR) of sodium channels (Na_V) is equally important and operates synergistically with K_HVA channels to enable rapid AP firing in NM.

Anatomically, we detected strong Na_V1.6 expression near hearing maturation, which was less distinct during hearing development despite functional evidence of I_NaR, suggesting that multiple Na_V channel subtypes may contribute to I_NaR.

We conclude that I_NaR plays an important role in regulating rapid AP firing for NM neurons, a property that may be evolutionarily conserved for functions related to similar avian and mammalian hearing.

Abstract

Auditory brainstem neurons are functionally primed to fire action potentials (APs) at markedly high rates in order to rapidly encode the acoustic information of sound. This specialization is critical for survival and the comprehension of behaviourally relevant communication functions, including sound localization and distinguishing speech from noise. Here, we investigated underlying ion channel mechanisms essential for high‐rate AP firing in neurons of the chicken nucleus magnocellularis (NM) – the avian analogue of bushy cells of the mammalian anteroventral cochlear nucleus. In addition to the established function of high voltage‐activated potassium channels, we found that resurgent sodium current (I_NaR) plays a role in regulating rapid firing activity of late‐developing (embryonic (E) days 19–21) NM neurons. I_NaR of late‐developing NM neurons showed similar properties to mammalian neurons in that its unique mechanism of an ‘open channel block state’ facilitated the recovery and increased the availability of sodium (Na_V) channels after depolarization. Using a computational model of NM neurons, we demonstrated that removal of I_NaR reduced high‐rate AP firing. We found weak I_NaR during a prehearing period (E11–12), which transformed to resemble late‐developing I_NaR properties around hearing onset (E14–16). Anatomically, we detected strong Na_V1.6 expression near maturation, which became increasingly less distinct at hearing onset and prehearing periods, suggesting that multiple Na_V channel subtypes may contribute to I_NaR during development. We conclude that I_NaR plays an important role in regulating rapid AP firing for NM neurons, a property that may be evolutionarily conserved for functions related to similar avian and mammalian hearing.

Auditory brainstem neurons of all vertebrates fire phase‐locked action potentials (APs) at high rates with remarkable fidelity, a process controlled by specialized anatomical and biophysical properties.

This is especially true in the avian nucleus magnocellularis (NM) – the analogue of the mammalian anteroventral cochlear nucleus.

In addition to high voltage‐activated potassium (K_HVA) channels, we report, using whole cell physiology and modelling, that resurgent sodium current (I_NaR) of sodium channels (Na_V) is equally important and operates synergistically with K_HVA channels to enable rapid AP firing in NM.

Anatomically, we detected strong Na_V1.6 expression near hearing maturation, which was less distinct during hearing development despite functional evidence of I_NaR, suggesting that multiple Na_V channel subtypes may contribute to I_NaR.

We conclude that I_NaR plays an important role in regulating rapid AP firing for NM neurons, a property that may be evolutionarily conserved for functions related to similar avian and mammalian hearing.

Neuronal Intrinsic Physiology Changes During Development of a Learned Behavior

Juvenile male zebra finches learn their songs over distinct auditory and sensorimotor stages, the former requiring exposure to an adult tutor song pattern. The cortical premotor nucleus HVC (acronym is name) plays a necessary role in both learning stages, as well as the production of adult song. Consistent with neural network models where synaptic plasticity mediates developmental forms of learning, exposure to tutor song drives changes in the turnover, density, and morphology of HVC synapses during vocal development. A network's output, however, is also influenced by the intrinsic properties (e.g., ion channels) of the component neurons, which could change over development. Here, we use patch clamp recordings to show cell-type-specific changes in the intrinsic physiology of HVC projection neurons as a function of vocal development. Developmental changes in HVC neurons that project to the basal ganglia include an increased voltage sag response to hyperpolarizing currents and an increased rebound depolarization following hyperpolarization. Developmental changes in HVC neurons that project to vocal-motor cortex include a decreased resting membrane potential and an increased spike amplitude. HVC interneurons, however, show a relatively stable range of intrinsic features across vocal development. We used mathematical models to deduce possible changes in ionic currents that underlie the physiological changes and to show that the magnitude of the observed changes could alter HVC circuit function. The results demonstrate developmental plasticity in the intrinsic physiology of HVC projection neurons and suggest that intrinsic plasticity may have a role in the process of song learning.

Intrinsic physiology of inhibitory neurons changes over auditory development

During auditory development, changes in membrane properties promote the ability of excitatory neurons in the brain stem to code aspects of sound, including the level and timing of a stimulus. Some of these changes coincide with hearing onset, suggesting that sound-driven neural activity produces developmental plasticity of ion channel expression. While it is known that the coding properties of excitatory neurons are modulated by inhibition in the mature system, it is unknown whether there are also developmental changes in the membrane properties of brain stem inhibitory neurons. We investigated the primary source of inhibition in the avian auditory brain stem, the superior olivary nucleus (SON). The present studies test the hypothesis that, as in excitatory neurons, the membrane properties of these inhibitory neurons change after hearing onset. We examined SON neurons at different stages of auditory development: embryonic days 14-16 (E14-E16), a time at which cochlear ganglion neurons are just beginning to respond to sound; later embryonic stages (E18-E19); and after hatching (P0-P2). We used in vitro whole cell patch electrophysiology to explore physiological changes in SON. Age-related changes were observed at the level of a single spike and in multispiking behavior. In particular, tonic behavior, measured as a neuron's ability to sustain tonic firing over a range of current steps, became more common later in development. Voltage-clamp recordings and biophysical models were employed to examine how age-related increases in ion currents enhance excitability in SON. Our findings suggest that concurrent increases in sodium and potassium currents underlie the emergence of tonic behavior. NEW & NOTEWORTHY This article is the first to examine heterogeneity of neuronal physiology in the inhibitory nucleus of the avian auditory system and demonstrate that tonic firing here emerges over development. By pairing computer simulations with physiological data, we show that increases in both sodium and potassium channels over development are necessary for the emergence of tonic firing.

Ion channel mechanisms underlying frequency-firing patterns of the avian nucleus magnocellularis: A computational model

We have previously shown that late-developing avian nucleus magnocellularis (NM) neurons (embryonic [E] days 19–21) fire action potentials (APs) that resembles a band-pass filter in response to sinusoidal current injections of varying frequencies. NM neurons located in the mid- to high-frequency regions of the nucleus fire preferentially at 75 Hz, but only fire a single onset AP to frequency inputs greater than 200 Hz. Surprisingly, NM neurons do not fire APs to sinusoidal inputs less than 20 Hz regardless of the strength of the current injection. In the present study we evaluated intrinsic mechanisms that prevent AP generation to low frequency inputs. We constructed a computational model to simulate the frequency-firing patterns of NM neurons based on experimental data at both room and near physiologic temperatures. The results from our model confirm that the interaction among low- and high-voltage activated potassium channels (K_LVA and K_HVA, respectively) and voltage dependent sodium channels (Na_V) give rise to the frequency-firing patterns observed in vitro. In particular, we evaluated the regulatory role of K_LVA during low frequency sinusoidal stimulation. The model shows that, in response to low frequency stimuli, activation of large K_LVA current counterbalances the slow-depolarizing current injection, likely permitting Na_V closed-state inactivation and preventing the generation of APs. When the K_LVA current density was reduced, the model neuron fired multiple APs per sinusoidal cycle, indicating that K_LVA channels regulate low frequency AP firing of NM neurons. This intrinsic property of NM neurons may assist in optimizing response to different rates of synaptic inputs.

Distinct Neural Properties in the Low-Frequency Region of the Chicken Cochlear Nucleus Magnocellularis

Topography in the avian cochlear nucleus magnocellularis (NM) is represented as gradually increasing characteristic frequency (CF) along the caudolateral-to-rostromedial axis. In this study, we characterized the organization and cell biophysics of the caudolateral NM (NMc) in chickens (Gallus gallus). Examination of cellular and dendritic architecture first revealed that NMc contains small neurons and extensive dendritic processes, in contrast to adendritic, large neurons located more rostromedially. Individual dye-filling study further demonstrated that NMc is divided into two subregions, with NMc2 neurons having larger and more complex dendritic fields than NMc1. Axonal tract tracing studies confirmed that NMc1 and NMc2 neurons receive afferent inputs from the auditory nerve and the superior olivary nucleus, similar to the adendritic NM. However, the auditory axons synapse with NMc neurons via small bouton-like terminals, unlike the large end bulb synapses on adendritic NM neurons. Immunocytochemistry demonstrated that most NMc2 neurons express cholecystokinin but not calretinin, distinct from NMc1 and adendritic NM neurons that are cholecystokinin negative and mostly calretinin positive. Finally, whole-cell current clamp recordings revealed that NMc neurons require significantly lower threshold current for action potential generation than adendritic NM neurons. Moreover, in contrast to adendritic NM neurons that generate a single-onset action potential, NMc neurons generate multiple action potentials to suprathreshold sustained depolarization. Taken together, our data indicate that NMc contains multiple neuron types that are structurally, connectively, molecularly, and physiologically different from traditionally defined NM neurons, emphasizing specialized neural properties for processing low-frequency sounds.