Structural tuning and plasticity of the axon initial segment in auditory neurons

Motor learning changes the axon initial segment of the spinal motoneuron

We are studying the mechanisms of H-reflex operant conditioning, a simple form of learning. Modelling studies in the literature and our previous data suggested that changes in the axon initial segment (AIS) might contribute. To explore this, we used blinded quantitative histological and immunohistochemical methods to study in adult rats the impact of H-reflex conditioning on the AIS of the spinal motoneuron that produces the reflex. Successful, but not unsuccessful, H-reflex up-conditioning was associated with greater AIS length and distance from soma; greater length correlated with greater H-reflex increase. Modelling studies in the literature suggest that these increases may increase motoneuron excitability, supporting the hypothesis that they may contribute to H-reflex increase. Up-conditioning did not affect AIS ankyrin G (AnkG) immunoreactivity (IR), p-p38 protein kinase IR, or GABAergic terminals. Successful, but not unsuccessful, H-reflex down-conditioning was associated with more GABAergic terminals on the AIS, weaker AnkG-IR, and stronger p-p38-IR. More GABAergic terminals and weaker AnkG-IR correlated with greater H-reflex decrease. These changes might potentially contribute to the positive shift in motoneuron firing threshold underlying H-reflex decrease; they are consistent with modelling suggesting that sodium channel change may be responsible. H-reflex down-conditioning did not affect AIS dimensions. This evidence that AIS plasticity is associated with and might contribute to H-reflex conditioning adds to evidence that motor learning involves both spinal and brain plasticity, and both neuronal and synaptic plasticity. AIS properties of spinal motoneurons are likely to reflect the combined influence of all the motor skills that share these motoneurons. KEY POINTS: Neuronal action potentials normally begin in the axon initial segment (AIS). AIS plasticity affects neuronal excitability in development and disease. Whether it does so in learning is unknown. Operant conditioning of a spinal reflex, a simple learning model, changes the rat spinal motoneuron AIS. Successful, but not unsuccessful, H-reflex up-conditioning is associated with greater AIS length and distance from soma. Successful, but not unsuccessful, down-conditioning is associated with more AIS GABAergic terminals, less ankyrin G, and more p-p38 protein kinase. The associations between AIS plasticity and successful H-reflex conditioning are consistent with those between AIS plasticity and functional changes in development and disease, and with those predicted by modelling studies in the literature. Motor learning changes neurons and synapses in spinal cord and brain. Because spinal motoneurons are the final common pathway for behaviour, their AIS properties probably reflect the combined impact of all the behaviours that use these motoneurons.

A class-specific effect of dysmyelination on the excitability of hippocampal interneurons

The role of myelination for axonal conduction is well-established in projection neurons but little is known about its significance in GABAergic interneurons. Myelination is discontinuous along interneuron axons and the mechanisms controlling myelin patterning and segregation of ion channels at the nodes of Ranvier have not been elucidated. Protein 4.1B is implicated in the organization of the nodes of Ranvier as a linker between paranodal and juxtaparanodal membrane proteins to the spectrin cytoskeleton. In the present study, 4.1B KO mice are used as a genetic model to analyze the functional role of myelin in Lhx6-positive parvalbumin (PV) and somatostatin (SST) neurons, two major classes of GABAergic neurons in the hippocampus. We show that 4.1B-deficiency induces disruption of juxtaparanodal K+ channel clustering and mislocalization of nodal or heminodal Na+ channels. Strikingly, 4.1B-deficiency causes loss of myelin in GABAergic axons in the hippocampus. In particular, stratum oriens SST cells display severe axonal dysmyelination and a reduced excitability. This reduced excitability is associated with a decrease in occurrence probability of small amplitude synaptic inhibitory events on pyramidal cells. In contrast, stratum pyramidale fast-spiking PV cells do not appear affected. In conclusion, our results indicate a class-specific effect of dysmyelination on the excitability of hippocampal interneurons associated with a functional alteration of inhibitory drive.

An adaptive behavioral control motif mediated by cortical axo-axonic inhibition

Genetically defined subgroups of inhibitory interneurons are thought to play distinct roles in learning, but heterogeneity within these subgroups has limited our understanding of the scope and nature of their specific contributions. Here we reveal that the chandelier cell (ChC), an interneuron type that specializes in inhibiting the axon-initial segment (AIS) of pyramidal neurons, establishes cortical microcircuits for organizing neural coding through selective axo-axonic synaptic plasticity. We found that organized motor control is mediated by enhanced population coding of direction-tuned premotor neurons, with tuning refined through suppression of irrelevant neuronal activity. ChCs contribute to learning-dependent refinements by providing selective inhibitory control over individual pyramidal neurons rather than global suppression. Quantitative analysis of structural plasticity across axo-axonic synapses revealed that ChCs redistributed inhibitory weights to individual pyramidal neurons during learning. These results demonstrate an adaptive logic of the inhibitory circuit motif responsible for organizing distributed neural representations. Thus, ChCs permit efficient cortical computation in a targeted cell-specific manner.

High-resolution volumetric imaging constrains compartmental models to explore synaptic integration and temporal processing by cochlear nucleus globular bushy cells

Globular bushy cells (GBCs) of the cochlear nucleus play central roles in the temporal processing of sound. Despite investigation over many decades, fundamental questions remain about their dendrite structure, afferent innervation, and integration of synaptic inputs. Here, we use volume electron microscopy (EM) of the mouse cochlear nucleus to construct synaptic maps that precisely specify convergence ratios and synaptic weights for auditory nerve innervation and accurate surface areas of all postsynaptic compartments. Detailed biophysically based compartmental models can help develop hypotheses regarding how GBCs integrate inputs to yield their recorded responses to sound. We established a pipeline to export a precise reconstruction of auditory nerve axons and their endbulb terminals together with high-resolution dendrite, soma, and axon reconstructions into biophysically detailed compartmental models that could be activated by a standard cochlear transduction model. With these constraints, the models predict auditory nerve input profiles whereby all endbulbs onto a GBC are subthreshold (coincidence detection mode), or one or two inputs are suprathreshold (mixed mode). The models also predict the relative importance of dendrite geometry, soma size, and axon initial segment length in setting action potential threshold and generating heterogeneity in sound-evoked responses, and thereby propose mechanisms by which GBCs may homeostatically adjust their excitability. Volume EM also reveals new dendritic structures and dendrites that lack innervation. This framework defines a pathway from subcellular morphology to synaptic connectivity, and facilitates investigation into the roles of specific cellular features in sound encoding. We also clarify the need for new experimental measurements to provide missing cellular parameters, and predict responses to sound for further in vivo studies, thereby serving as a template for investigation of other neuron classes.

Structure and dynamics that specialize neurons for high-frequency coincidence detection in the barn owl nucleus laminaris

A principal cue for sound source localization is the difference in arrival times of sounds at an animal's two ears (interaural time difference, ITD). Neurons that process ITDs are specialized to compare the timing of inputs with submillisecond precision. In the barn owl, ITD processing begins in the nucleus laminaris (NL) region of the auditory brain stem. Remarkably, NL neurons are sensitive to ITDs in high-frequency sounds (kilohertz-range). This contrasts with ITD-based sound localization in analogous regions in mammals where ITD sensitivity is typically restricted to lower-frequency sounds. Guided by previous experiments and modeling studies of tone-evoked responses of NL neurons, we propose NL neurons achieve high-frequency ITD sensitivity if they respond selectively to the small-amplitude, high-frequency oscillations in their inputs, and remain relatively non-responsive to mean input level. We use a biophysically based model to study the effects of soma-axon coupling on dynamics and function in NL neurons. First, we show that electrical separation of the soma from the axon region in the neuron enhances high-frequency ITD sensitivity. This soma-axon coupling configuration promotes linear subthreshold dynamics and rapid spike initiation, making the model more responsive to input oscillations, rather than mean input level. Second, we provide new evidence for the essential role of phasic dynamics for high-frequency neural coincidence detection. Transforming our model to the phasic firing mode further tunes the model to respond selectively to the oscillating inputs that carry ITD information. Similar structural and dynamical mechanisms specialize mammalian auditory brain stem neurons for ITD sensitivity, and thus, our work identifies common principles of ITD processing and neural coincidence detection across species and for sounds at widely different frequencies.

An adaptive behavioral control motif mediated by cortical axo-axonic inhibition

Neural circuits are reorganized with specificity during learning. Genetically-defined subgroups of inhibitory interneurons are thought to play distinct roles in learning, but heterogeneity within these subgroups has limited our understanding of the scope and nature of their specific contributions to learning. Here we reveal that the chandelier cell (ChC), an interneuron type that specializes in inhibiting the axon-initial segment (AIS) of pyramidal neurons, establishes cortical microcircuits for organizing neural coding through selective axo-axonic synaptic plasticity. We find that organized motor control is mediated by enhanced population coding of direction-tuned premotor neurons, whose tuning is refined through suppression of irrelevant neuronal activity. ChCs are required for learning-dependent refinements via providing selective inhibitory control over pyramidal neurons rather than global suppression. Quantitative analysis on structural plasticity of axo-axonic synapses revealed that ChCs redistributed inhibitory weights to individual pyramidal neurons during learning. These results demonstrate an adaptive logic of the inhibitory circuit motif responsible for organizing distributed neural representations. Thus, ChCs permit efficient cortical computation in a target cell specific manner, which highlights the significance of interneuron diversity.

The Amyloid Precursor Protein Modulates the Position and Length of the Axon Initial Segment

The amyloid precursor protein (APP) is linked to the genetics and pathogenesis of Alzheimer's disease (AD). It is the parent protein of the β-amyloid (Aβ) peptide, the main constituent of the amyloid plaques found in an AD brain. The pathways from APP to Aβ are intensively studied, yet the normal functions of APP itself have generated less interest. We report here that glutamate stimulation of neuronal activity leads to a rapid increase in App gene expression. In mouse and human neurons, elevated APP protein changes the structure of the axon initial segment (AIS) where action potentials are initiated. The AIS is shortened in length and shifts away from the cell body. The GCaMP8f Ca2+ reporter confirms the predicted decrease in neuronal activity. NMDA antagonists or knockdown of App block the glutamate effects. The actions of APP on the AIS are cell-autonomous; exogenous Aβ, either fibrillar or oligomeric, has no effect. In culture, APPSwe (a familial AD mutation) induces larger AIS changes than wild type APP. Ankyrin G and βIV-spectrin, scaffolding proteins of the AIS, both physically associate with APP, more so in AD brains. Finally, in humans with sporadic AD or in the R1.40 AD mouse model, both females and males, neurons have elevated levels of APP protein that invade the AIS. In vivo as in vitro, this increased APP is associated with a significant shortening of the AIS. The findings outline a new role for the APP and encourage a reconsideration of its relationship to AD.SIGNIFICANCE STATEMENT While the amyloid precursor protein (APP) has long been associated with Alzheimer's disease (AD), the normal functions of the full-length Type I membrane protein have been largely unexplored. We report here that the levels of APP protein increase with neuronal activity. In vivo and in vitro, modest amounts of excess APP alter the properties of the axon initial segment. The β-amyloid peptide derived from APP is without effect. Consistent with the observed changes in the axon initial segment which would be expected to decrease action potential firing, we show that APP expression depresses neuronal activity. In mouse AD models and human sporadic AD, APP physically associates with the scaffolding proteins of the axon initial segment, suggesting a relationship with AD dementia.

CDK5/p35-Dependent Microtubule Reorganization Contributes to Homeostatic Shortening of the Axon Initial Segment

The structural plasticity of the axon initial segment (AIS) contributes to the homeostatic control of activity and optimizes the function of neural circuits; however, the underlying mechanisms are not fully understood. In this study, we prepared a slice culture containing nucleus magnocellularis from chickens of both sexes that reproduces most features of AIS plasticity in vivo, regarding its effects on characteristics of AIS and cell-type specificity, and revealed that microtubule reorganization via activation of CDK5 underlies plasticity. Treating the culture with a high-K⁺ medium shortened the AIS and reduced sodium current and membrane excitability, specifically in neurons tuned to high-frequency sound, creating a tonotopic difference in AIS length in the nucleus. Pharmacological analyses revealed that this AIS shortening was driven by multiple Ca²⁺ pathways and subsequent signaling molecules that converge on CDK5 via the activation of ERK1/2. AIS shortening was suppressed by overexpression of dominant-negative CDK5, whereas it was facilitated by the overexpression of p35, an activator of CDK5. Notably, p35(T138A), a phosphorylation-inactive mutant of p35, did not shorten the AIS. Moreover, microtubule stabilizers occluded AIS shortening during the p35 overexpression, indicating that CDK5/p35 mediated AIS shortening by promoting disassembly of microtubules at distal AIS. This study highlights the importance of microtubule reorganization and regulation of CDK5 activity in structural AIS plasticity and the tuning of AIS characteristics in neurons.SIGNIFICANCE STATEMENT The structural plasticity of AIS has a strong impact on the output of neurons and plays a fundamental role in the physiology and pathology of the brain. However, the mechanisms linking neuronal activity to structural changes in AIS are not well understood. In this study, we prepared an organotypic culture of avian auditory brainstem, reproducing most AIS plasticity features in vivo, and we revealed that activity-dependent AIS shortening occurs through the disassembly of microtubules at distal AIS via activation of CDK5/p35 signals. This study emphasizes the importance of microtubule reorganization and regulation of CDK5 activity in structural AIS plasticity and tonotopic differentiation of AIS structures in the brainstem auditory circuit.

Cortical control of chandelier cells in neural codes

Various cortical functions arise from the dynamic interplay of excitation and inhibition. GABAergic interneurons that mediate synaptic inhibition display significant diversity in cell morphology, electrophysiology, plasticity rule, and connectivity. These heterogeneous features are thought to underlie their functional diversity. Emerging attention on specific properties of the various interneuron types has emphasized the crucial role of cell-type specific inhibition in cortical neural processing. However, knowledge is still limited on how each interneuron type forms distinct neural circuits and regulates network activity in health and disease. To dissect interneuron heterogeneity at single cell-type precision, we focus on the chandelier cell (ChC), one of the most distinctive GABAergic interneuron types that exclusively innervate the axon initial segments (AIS) of excitatory pyramidal neurons. Here we review the current understanding of the structural and functional properties of ChCs and their implications in behavioral functions, network activity, and psychiatric disorders. These findings provide insights into the distinctive roles of various single-type interneurons in cortical neural coding and the pathophysiology of cortical dysfunction.

Cellular Strategies for Frequency-Dependent Computation of Interaural Time Difference

Binaural coincidence detection is the initial step in encoding interaural time differences (ITDs) for sound-source localization. In birds, neurons in the nucleus laminaris (NL) play a central role in this process. These neurons receive excitatory synaptic inputs on dendrites from both sides of the cochlear nucleus and compare their coincidences at the soma. The NL is tonotopically organized, and individual neurons receive a pattern of synaptic inputs that are specific to their tuning frequency. NL neurons differ in their dendritic morphology along the tonotopic axis; their length increases with lower tuning frequency. In addition, our series of studies have revealed several frequency-dependent refinements in the morphological and biophysical characteristics of NL neurons, such as the amount and subcellular distribution of ion channels and excitatory and inhibitory synapses, which enable the neurons to process the frequency-specific pattern of inputs appropriately and encode ITDs at each frequency band. In this review, we will summarize these refinements of NL neurons and their implications for the ITD coding. We will also discuss the similarities and differences between avian and mammalian coincidence detectors.

Altered action potential waveform and shorter axonal initial segment in hiPSC-derived motor neurons with mutations in VRK1

We recently described new pathogenic variants in VRK1, in patients affected with distal Hereditary Motor Neuropathy associated with upper motor neurons signs. Specifically, we provided evidences that hiPSC-derived Motor Neurons (hiPSC-MN) from these patients display Cajal Bodies (CBs) disassembly and defects in neurite outgrowth and branching. We here focused on the Axonal Initial Segment (AIS) and the related firing properties of hiPSC-MNs from these patients. We found that the patient's Action Potential (AP) was smaller in amplitude, larger in duration, and displayed a more depolarized threshold while the firing patterns were not altered. These alterations were accompanied by a decrease in the AIS length measured in patients' hiPSC-MNs. These data indicate that mutations in VRK1 impact the AP waveform and the AIS organization in MNs and may ultimately lead to the related motor neuron disease.

A general principle of dendritic constancy: A neuron's size- and shape-invariant excitability

Reducing neuronal size results in less membrane and therefore lower input conductance. Smaller neurons are thus more excitable, as seen in their responses to somatic current injections. However, the impact of a neuron's size and shape on its voltage responses to dendritic synaptic activation is much less understood. Here we use analytical cable theory to predict voltage responses to distributed synaptic inputs in unbranched cables, showing that these are entirely independent of dendritic length. For a given synaptic density, neuronal responses depend only on the average dendritic diameter and intrinsic conductivity. This remains valid for a wide range of morphologies irrespective of their arborization complexity. Spiking models indicate that morphology-invariant numbers of spikes approximate the percentage of active synapses. In contrast to spike rate, spike times do depend on dendrite morphology. In summary, neuronal excitability in response to distributed synaptic inputs is largely unaffected by dendrite length or complexity.

Physiological Diversity Influences Detection of Stimulus Envelope and Fine Structure in Neurons of the Medial Superior Olive

The neurons of the medial superior olive (MSO) of mammals extract azimuthal information from the delays between sounds reaching the two ears [interaural time differences (ITDs)]. Traditionally, all models of sound localization have assumed that MSO neurons represent a single population of cells with specialized and homogeneous intrinsic and synaptic properties that enable the detection of synaptic coincidence on a timescale of tens to hundreds of microseconds. Here, using patch-clamp recordings from large populations of anatomically labeled neurons in brainstem slices from male and female Mongolian gerbils (Meriones unguiculatus), we show that MSO neurons are far more physiologically diverse than previously appreciated, with properties that depend regionally on cell position along the topographic map of frequency. Despite exhibiting a similar morphology, neurons in the MSO exhibit subthreshold oscillations of differing magnitudes that drive action potentials at rates between 100 and 800 Hz. These oscillations are driven primarily by voltage-gated sodium channels and are distinct from resonant properties derived from other active membrane properties. We show that graded differences in these and other physiological properties across the MSO neuron population enable the MSO to duplex the encoding of ITD information in both fast, submillisecond time-varying signals as well as in slower envelopes.SIGNIFICANCE STATEMENT Neurons in the medial superior olive (MSO) encode sound localization cues by detecting microsecond differences in the arrival times of inputs from the left and right ears, and it has been assumed that this computation is made possible by highly stereotyped structural and physiological specializations. Here we report using a large (>400) sample size in which MSO neurons show a strikingly large continuum of functional properties despite exhibiting similar morphologies. We demonstrate that subthreshold oscillations mediated by voltage-gated Na+ channels play a key role in conferring graded differences in firing properties. This functional diversity likely confers capabilities of processing both fast, submillisecond-scale synaptic activity (acoustic "fine structure"), and slow-rising envelope information that is found in amplitude-modulated sounds and speech patterns.

Electrical match between initial segment and somatodendritic compartment for action potential backpropagation in retinal ganglion cells

The action potential of most vertebrate neurons initiates in the axon initial segment (AIS) and is then transmitted to the soma where it is regenerated by somatodendritic sodium channels. For successful transmission, the AIS must produce a strong axial current, so as to depolarize the soma to the threshold for somatic regeneration. Theoretically, this axial current depends on AIS geometry and Na⁺ conductance density. We measured the axial current of mouse retinal ganglion cells using whole cell recordings with post hoc AIS labeling. We found that this current is large, implying high Na⁺ conductance density, and carries a charge that covaries with capacitance so as to depolarize the soma by ∼30 mV. Additionally, we observed that the axial current attenuates strongly with depolarization, consistent with sodium channel inactivation, but temporally broadens so as to preserve the transmitted charge. Thus, the AIS appears to be organized so as to reliably backpropagate the axonal action potential.NEW & NOTEWORTHY We measured the axial current produced at spike initiation by the axon initial segment of mouse retinal ganglion cells. We found that it is a large current, requiring high sodium channel conductance density, which covaries with cell capacitance so as to ensure a ∼30 mV depolarization. During sustained depolarization the current attenuated, but it broadened to preserve somatic depolarization. Thus, properties of the initial segment are adjusted to ensure backpropagation of the axonal action potential.

Sensory input drives rapid homeostatic scaling of the axon initial segment in mouse barrel cortex

The axon initial segment (AIS) is a critical microdomain for action potential initiation and implicated in the regulation of neuronal excitability during activity-dependent plasticity. While structural AIS plasticity has been suggested to fine-tune neuronal activity when network states change, whether it acts in vivo as a homeostatic regulatory mechanism in behaviorally relevant contexts remains poorly understood. Using the mouse whisker-to-barrel pathway as a model system in combination with immunofluorescence, confocal analysis and electrophysiological recordings, we observed bidirectional AIS plasticity in cortical pyramidal neurons. Furthermore, we find that structural and functional AIS remodeling occurs in distinct temporal domains: Long-term sensory deprivation elicits an AIS length increase, accompanied with an increase in neuronal excitability, while sensory enrichment results in a rapid AIS shortening, accompanied by a decrease in action potential generation. Our findings highlight a central role of the AIS in the homeostatic regulation of neuronal input-output relations.

Neurofascin and Kv7.3 are delivered to somatic and axon terminal surface membranes en route to the axon initial segment

Ion channel complexes promote action potential initiation at the mammalian axon initial segment (AIS), and modulation of AIS size by recruitment or loss of proteins can influence neuron excitability. Although endocytosis contributes to AIS turnover, how membrane proteins traffic to this proximal axonal domain is incompletely understood. Neurofascin186 (Nfasc186) has an essential role in stabilising the AIS complex to the proximal axon, and the AIS channel protein Kv7.3 regulates neuron excitability. Therefore, we have studied how these proteins reach the AIS. Vesicles transport Nfasc186 to the soma and axon terminal where they fuse with the neuronal plasma membrane. Nfasc186 is highly mobile after insertion in the axonal membrane and diffuses bidirectionally until immobilised at the AIS through its interaction with AnkyrinG. Kv7.3 is similarly recruited to the AIS. This study reveals how key proteins are delivered to the AIS and thereby how they may contribute to its functional plasticity.

Input-Output Relationship of CA1 Pyramidal Neurons Reveals Intact Homeostatic Mechanisms in a Mouse Model of Fragile X Syndrome

Cellular hyperexcitability is a salient feature of fragile X syndrome animal models. The cellular basis of hyperexcitability and how it responds to changing activity states is not fully understood. Here, we show increased axon initial segment length in CA1 of the Fmr1^-/y mouse hippocampus, with increased cellular excitability. This change in length does not result from reduced AIS plasticity, as prolonged depolarization induces changes in AIS length independent of genotype. However, depolarization does reduce cellular excitability, the magnitude of which is greater in Fmr1^-/y neurons. Finally, we observe reduced functional inputs from the entorhinal cortex, with no genotypic difference in the firing rates of CA1 pyramidal neurons. This suggests that AIS-dependent hyperexcitability in Fmr1^-/y mice may result from adaptive or homeostatic regulation induced by reduced functional synaptic connectivity. Thus, while AIS length and intrinsic excitability contribute to cellular hyperexcitability, they may reflect a homeostatic mechanism for reduced synaptic input onto CA1 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.

Diversity of Axonal and Dendritic Contributions to Neuronal Output

Our general understanding of neuronal function is that dendrites receive information that is transmitted to the axon, where action potentials (APs) are initiated and propagated to eventually trigger neurotransmitter release at synaptic terminals. Even though this canonical division of labor is true for a number of neuronal types in the mammalian brain (including neocortical and hippocampal pyramidal neurons or cerebellar Purkinje neurons), many neuronal types do not comply with this classical polarity scheme. In fact, dendrites can be the site of AP initiation and propagation, and even neurotransmitter release. In several interneuron types, all functions are carried out by dendrites as these neurons are devoid of a canonical axon. In this article, we present a few examples of "misbehaving" neurons (with a non-canonical polarity scheme) to highlight the diversity of solutions that are used by mammalian neurons to transmit information. Moreover, we discuss how the contribution of dendrites and axons to neuronal excitability may impose constraints on the morphology of these compartments in specific functional contexts.

Impact of Auditory Experience on the Structural Plasticity of the AIS in the Mouse Brainstem Throughout the Lifespan

Sound input critically influences the development and maintenance of neuronal circuits in the mammalian brain throughout life. We investigate the structural and functional plasticity of auditory neurons in response to various auditory experiences during development, adulthood, and aging. Using electrophysiology, computer simulation, and immunohistochemistry, we study the structural plasticity of the axon initial segment (AIS) in the medial nucleus of the trapezoid body (MNTB) from the auditory brainstem of the mice (either sex), in different ages and auditory environments. The structure and spatial location of the AIS of MNTB neurons depend on their functional topographic location along the tonotopic axis, aligning high- to low-frequency sound-responding neurons (HF or LF neurons). HF neurons dramatically undergo structural remodeling of the AIS throughout life. The AIS progressively shortens during development, is stabilized in adulthood, and becomes longer in aging. Sound inputs are critically associated with setting and maintaining AIS plasticity and tonotopy at various ages. Sound stimulation increases the excitability of auditory neurons. Computer simulation shows that modification of the AIS length, location, and diameter can affect firing properties of MNTB neurons in the developing brainstem. The adaptive capability of axonal structure in response to various auditory experiences at different ages suggests that sound input is important for the development and maintenance of the structural and functional properties of the auditory brain throughout life.

Robustness to Axon Initial Segment Variation Is Explained by Somatodendritic Excitability in Rat Substantia Nigra Dopaminergic Neurons

In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. Interestingly, the axon of rat SNc dopaminergic (DA) neurons displays a highly variable location and most often arises from an axon-bearing dendrite (ABD). We combined current-clamp somatic and dendritic recordings, outside-out recordings of dendritic sodium and potassium currents, morphological reconstructions and multicompartment modeling on male and female rat SNc DA neurons to determine cell-to-cell variations in AIS and ABD geometry, and their influence on neuronal output (spontaneous pacemaking frequency, action potential [AP] shape). Both AIS and ABD geometries were found to be highly variable from neuron to neuron. Surprisingly, we found that AP shape and pacemaking frequency were independent of AIS geometry. Modeling realistic morphological and biophysical variations helped us clarify this result: in SNc DA neurons, the complexity of the ABD combined with its excitability predominantly define pacemaking frequency and AP shape, such that large variations in AIS geometry negligibly affect neuronal output and are tolerated.SIGNIFICANCE STATEMENT In many neuronal types, axon initial segment (AIS) geometry critically influences neuronal excitability. In the current study, we describe large cell-to-cell variations in AIS length or distance from the soma in rat substantia nigra pars compacta dopaminergic neurons. Using neuronal reconstruction and electrophysiological recordings, we show that this morphological variability does not seem to affect their electrophysiological output, as neither action potential properties nor pacemaking frequency is correlated with AIS morphology. Realistic multicompartment modeling suggests that this robustness to AIS variation is mainly explained by the complexity and excitability of the somatodendritic compartment.

Axon initial segment plasticity accompanies enhanced excitation of visual cortical neurons in aged rats

Recent studies have indicated that the structure of the axon initial segment (AIS) of neurons is highly plastic in response to changes in neuronal activity. Whether an age-related enhancement of neuronal responses in the visual cortex is coupled with plasticity of AISs is unknown. Here, we compare the AIS length and the distribution of Nav1.6, a key Na⁺ ion channel in action potential (AP) initiation, along the AIS of layer II/III neurons in the primary visual cortex (V1) of young adult and aged rats, which were examined previously in a single-unit recording study. In that study, we found that V1 neurons in aged rats showed a significantly higher spontaneous activity and stronger visually evoked responses than did neurons in young rats. Our present study shows that the mean AIS length of layer II/III neurons in the V1 area of aged rats was significantly shorter than that of young adult rats. Further, the proportion of AIS with the Nav1.6 distribution was also reduced significantly in aged rats relative to young rats, as indicated by a decrease in the mean Nav1.6 immunofluorescence optical density within AISs and a specific decrease in Nav1.6 immunofluorescence optical density near the proximal region of the AIS. Our results indicate that aging results in both shortening of AISs and reduction of Nav1.6 Na⁺ ion channel distribution along AISs, which accompanies enhanced neuronal activity. This age-related morphological plasticity may lower the AP amplitude by reducing Na⁺ ion entry during AP initiation, spare ATPs consumed by Na⁺ ion pumps during membrane potential restoration, and thus balance the energy expenditure caused by an increased firing rate of cortical neurons during the aging process.

Maturation Dynamics of the Axon Initial Segment (AIS) of Newborn Dentate Granule Cells in Young Adult C57BL/6J Mice

Newborn dentate granule cells (DGCs) are generated in the hippocampal dentate gyrus (DG) of rodents through a process called adult hippocampal neurogenesis, which is subjected to tight intrinsic and extrinsic regulation. The use of retroviruses encoding fluorescent proteins has allowed the characterization of the maturation dynamics of newborn DGCs, including their morphological development and the establishment and maturation of their afferent and efferent synaptic connections. However, the study of a crucial cellular compartment of these cells, namely, the axon initial segment (AIS), has remained unexplored to date. The AIS is not only the site of action potential initiation, but it also has a unique molecular identity that makes it one of the master regulators of neural plasticity and excitability. Here we examined the dynamics of AIS formation in newborn DGCs of young female adult C57BL/6J mice in vivo Our data reveal notable changes in AIS length and thickness throughout cell maturation under physiological conditions and show that the most remarkable structural changes coincide with periods of intense morphological and functional remodeling. Moreover, we demonstrate that AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuli.SIGNIFICANCE STATEMENT The hippocampal dentate gyrus (DG) of rodents generates newborn dentate granule cells (DGCs) throughout life. This process, named adult hippocampal neurogenesis, confers a unique degree of plasticity to the hippocampal circuit, and it is crucial for learning and memory. Here we studied, for the first time, the formation of a key cellular compartment of newborn DGCs, namely, the axon initial segment (AIS) in vivo Our data reveal remarkable AIS structural remodeling throughout the maturation of these cells under physiological conditions. Moreover, AIS development can be modulated extrinsically by both neuroprotective (environmental enrichment) and detrimental (lipopolysaccharide from Escherichia coli) stimuli.

Molecular Disorganization of Axons Adjacent to Human Cortical Microinfarcts

Cortical microinfarcts (CMIs) are microscopically identified wedge-shaped ischemic lesions that occur at or near the cortical surface and result from occlusion of penetrating arterioles. These microscopic lesions can be observed with high-resolution magnetic resonance imaging in aging brains and in patients with cerebrovascular disease. Recent studies have suggested that strategically located microinfarcts strongly correlate with cognitive deficits, which can contribute to Alzheimer’s disease as well as other forms of dementia. We have recently shown that the molecular organization of axons into functional microdomains is altered in areas adjacent to white matter lacunar and microinfarcts, creating a peri-infarct penumbral injury in surviving axons. Whether similar changes in nodal, adjacent paranodal, and proximal axon initial segment molecular organization occur in the cortex adjacent to human CMIs is not known. Paraffin-embedded sections of autopsy brain tissue from five patients with CMIs were immunofluorescently labeled for nodal and paranodal markers including beta-IV spectrin, ankyrin-G, and contactin-associated protein. High magnification images from the peri-infarct cortical tissue were generated using confocal microscopy. In surviving cortical tissue adjacent to microinfarcts, we observed a dramatic loss of axon initial segments, suggesting that neuronal firing capacity in adjacent cortical tissue is likely compromised. The number of identifiable nodal/paranodal complexes in surviving cortical tissue is reduced adjacent to microinfarcts, while the average paranodal length is increased indicating a breakdown of axoglial contact. This axonal microdomain disorganization occurs in the relative absence of changes in the structural integrity of myelinated axons as measured by myelin basic protein and neurofilament staining. These findings indicate that the molecular organization of surviving axons adjacent to human CMIs is abnormal, reflecting lost axoglial contact and the functional elements necessary for neural transmission. This study provides support for the concept of a microinfarct penumbral injury that may account for the cumulative cognitive effect of these tiny strokes.

Mild Traumatic Brain Injury Evokes Pyramidal Neuron Axon Initial Segment Plasticity and Diffuse Presynaptic Inhibitory Terminal Loss

The axon initial segment (AIS) is the site of action potential (AP) initiation, thus a crucial regulator of neuronal activity. In excitatory pyramidal neurons, the high density of voltage-gated sodium channels (NaV1.6) at the distal AIS regulates AP initiation. A surrogate AIS marker, ankyrin-G (ankG) is a structural protein regulating neuronal functional via clustering voltage-gated ion channels. In neuronal circuits, changes in presynaptic input can alter postsynaptic output via AIS structural-functional plasticity. Recently, we showed experimental mild traumatic brain injury (mTBI) evokes neocortical circuit disruption via diffuse axonal injury (DAI) of excitatory and inhibitory neuronal systems. A key finding was that mTBI-induced neocortical electrophysiological changes involved non-DAI/ intact excitatory pyramidal neurons consistent with AIS-specific alterations. In the current study we employed Thy1-yellow fluorescent protein (YFP)-H mice to test if mTBI induces AIS structural and/or functional plasticity within intact pyramidal neurons 2 days after mTBI. We used confocal microscopy to assess intact YFP+ pyramidal neurons in layer 5 of primary somatosensory barrel field (S1BF), whose axons were continuous from the soma of origin to the subcortical white matter (SCWM). YFP+ axonal traces were superimposed on ankG and NaV1.6 immunofluorescent profiles to determine AIS position and length. We found that while mTBI had no effect on ankG start position, the length significantly decreased from the distal end, consistent with the site of AP initiation at the AIS. However, NaV1.6 structure did not change after mTBI, suggesting uncoupling from ankG. Parallel quantitative analysis of presynaptic inhibitory terminals along the postsynaptic perisomatic domain of these same intact YFP+ excitatory pyramidal neurons revealed a significant decrease in GABAergic bouton density. Also within this non-DAI population, patch-clamp recordings of intact YFP+ pyramidal neurons showed AP acceleration decreased 2 days post-mTBI, consistent with AIS functional plasticity. Simulations of realistic pyramidal neuron computational models using experimentally determined AIS lengths showed a subtle decrease is NaV1.6 density is sufficient to attenuate AP acceleration. Collectively, these findings highlight the complexity of mTBI-induced neocortical circuit disruption, involving changes in extrinsic/presynaptic inhibitory perisomatic input interfaced with intrinsic/postsynaptic intact excitatory neuron AIS output.

The basis of sharp spike onset in standard biophysical models

In most vertebrate neurons, spikes initiate in the axonal initial segment (AIS). When recorded in the soma, they have a surprisingly sharp onset, as if sodium (Na) channels opened abruptly. The main view stipulates that spikes initiate in a conventional manner at the distal end of the AIS, then progressively sharpen as they backpropagate to the soma. We examined the biophysical models used to substantiate this view, and we found that spikes do not initiate through a local axonal current loop that propagates along the axon, but through a global current loop encompassing the AIS and soma, which forms an electrical dipole. Therefore, the phenomenon is not adequately modeled as the backpropagation of an electrical wave along the axon, since the wavelength would be as large as the entire system. Instead, in these models, we found that spike initiation rather follows the critical resistive coupling model proposed recently, where the Na current entering the AIS is matched by the axial resistive current flowing to the soma. Besides demonstrating it by examining the balance of currents at spike initiation, we show that the observed increase in spike sharpness along the axon is artifactual and disappears when an appropriate measure of rapidness is used; instead, somatic onset rapidness can be predicted from spike shape at initiation site. Finally, we reproduce the phenomenon in a two-compartment model, showing that it does not rely on propagation. In these models, the sharp onset of somatic spikes is therefore not an artifact of observing spikes at the incorrect location, but rather the signature that spikes are initiated through a global soma-AIS current loop forming an electrical dipole.

MTCL1 plays an essential role in maintaining Purkinje neuron axon initial segment

The axon initial segment (AIS) is a specialized domain essential for neuronal function, the formation of which begins with localization of an ankyrin-G (AnkG) scaffold. However, the mechanism directing and maintaining AnkG localization is largely unknown. In this study, we demonstrate that in vivo knockdown of microtubule cross-linking factor 1 (MTCL1) in cerebellar Purkinje cells causes loss of axonal polarity coupled with AnkG mislocalization. MTCL1 lacking MT-stabilizing activity failed to restore these defects, and stable MT bundles spanning the AIS were disorganized in knockdown cells. Interestingly, during early postnatal development, colocalization of MTCL1 with these stable MT bundles was observed prominently in the axon hillock and proximal axon. These results indicate that MTCL1-mediated formation of stable MT bundles is crucial for maintenance of AnkG localization. We also demonstrate that Mtcl1 gene disruption results in abnormal motor coordination with Purkinje cell degeneration, and provide evidence suggesting possible involvement of MTCL1 dysfunction in the pathogenesis of spinocerebellar ataxia.

Structural and Functional Plasticity at the Axon Initial Segment

The axon initial segment (AIS) is positioned between the axonal and somato-dendritic compartments and plays a pivotal role in triggering action potentials (APs) and determining neuronal output. It is now widely accepted that structural properties of the AIS, such as length and/or location relative to the soma, change in an activity-dependent manner. This structural plasticity of the AIS is known to be crucial for homeostatic control of neuronal excitability. However, it is obvious that the impact of the AIS on neuronal excitability is critically dependent on the biophysical properties of the AIS, which are primarily determined by the composition and characteristics of ion channels in this domain. Moreover, these properties can be altered via phosphorylation and/or redistribution of the channels. Recently, studies in auditory neurons showed that alterations in the composition of voltage-gated K⁺ (Kv) channels at the AIS coincide with elongation of the AIS, thereby enhancing the neuronal excitability, suggesting that the interaction between structural and functional plasticities of the AIS is important in the control of neuronal excitability. In this review, we will summarize the current knowledge regarding structural and functional alterations of the AIS and discuss how they interact and contribute to regulating the neuronal output.

Transmission of auditory sensory information decreases in rate and temporal precision at the endbulb of Held synapse during age-related hearing loss

Age-related hearing loss (ARHL) is largely attributed to structural changes and functional declines in the peripheral auditory system, which include synaptopathy at the inner hair cell/spiral ganglion cell (SGC) connection and the loss of SGCs. However, functional changes at the central terminals of SGCs, namely the auditory nerve synapses in the cochlear nucleus, are not yet fully understood during ARHL. With the use of young (1-3 mo) and old (25-30 mo) CBA/CaJ mice, this study evaluated the intrinsic properties of the bushy neurons postsynaptic to the endbulb of Held synapses, and the firing properties of these neurons to direct current injections as well as to synaptic inputs from the auditory nerve. Results showed that bushy neurons in old mice are more excitable and are able to fire spikes at similar rate and timing to direct current injections as those in young mice. In response to synaptic inputs, however, bushy neurons from old mice fired spikes with significantly decreased rate and reduced temporal precision to stimulus trains at 100 and 400 Hz, with the drop in firing probability more profound at 400 Hz. It suggests that transmission of auditory information at the endbulb is declined in both rate and timing during aging, which signifies the loss of sensory inputs to the central auditory system under ARHL. The study proposes that, in addition to damages at the peripheral terminals of SGCs as well as the loss of SGCs, functional decline at the central terminals of surviving SGCs is also an essential component of ARHL.

Neuron Morphology Influences Axon Initial Segment Plasticity

In most vertebrate neurons, action potentials are initiated in the axon initial segment (AIS), a specialized region of the axon containing a high density of voltage-gated sodium and potassium channels.

In most vertebrate neurons, action potentials are initiated in the axon initial segment (AIS), a specialized region of the axon containing a high density of voltage-gated sodium and potassium channels. It has recently been proposed that neurons use plasticity of AIS length and/or location to regulate their intrinsic excitability. Here we quantify the impact of neuron morphology on AIS plasticity using computational models of simplified and realistic somatodendritic morphologies. In small neurons (e.g., dentate granule neurons), excitability was highest when the AIS was of intermediate length and located adjacent to the soma. Conversely, neurons having larger dendritic trees (e.g., pyramidal neurons) were most excitable when the AIS was longer and/or located away from the soma. For any given somatodendritic morphology, increasing dendritic membrane capacitance and/or conductance favored a longer and more distally located AIS. Overall, changes to AIS length, with corresponding changes in total sodium conductance, were far more effective in regulating neuron excitability than were changes in AIS location, while dendritic capacitance had a larger impact on AIS performance than did dendritic conductance. The somatodendritic influence on AIS performance reflects modest soma-to-AIS voltage attenuation combined with neuron size-dependent changes in AIS input resistance, effective membrane time constant, and isolation from somatodendritic capacitance. We conclude that the impact of AIS plasticity on neuron excitability will depend largely on somatodendritic morphology, and that, in some neurons, a shorter or more distally located AIS may promote, rather than limit, action potential generation.

Redistribution of Kv1 and Kv7 enhances neuronal excitability during structural axon initial segment plasticity

Structural plasticity of the axon initial segment (AIS), the trigger zone of neurons, is a powerful means for regulating neuronal activity. Here, we show that AIS plasticity is not limited to structural changes; it also occurs as changes in ion-channel expression, which substantially augments the efficacy of regulation. In the avian cochlear nucleus, depriving afferent inputs by removing cochlea elongated the AIS, and simultaneously switched the dominant Kv channels at the AIS from Kv1.1 to Kv7.2. Due to the slow activation kinetics of Kv7.2, the redistribution of the Kv channels reduced the shunting conductance at the elongated AIS during the initiation of action potentials and effectively enhanced the excitability of the deprived neurons. The results indicate that the functional plasticity of the AIS works cooperatively with the structural plasticity and compensates for the loss of afferent inputs to maintain the homeostasis of auditory circuits after hearing loss by cochlea removal.

Sensory deprivation in the avian brain can lead to structural changes in the axon initial segment. Here, the authors build on their previous work by showing that such homeostatic AIS plasticity also involves changes in Kv channel expression, which contributes to enhanced neuronal excitability.

Imaging with organic indicators and high-speed charge-coupled device cameras in neurons: some applications where these classic techniques have advantages

Dynamic calcium and voltage imaging is a major tool in modern cellular neuroscience. Since the beginning of their use over 40 years ago, there have been major improvements in indicators, microscopes, imaging systems, and computers. While cutting edge research has trended toward the use of genetically encoded calcium or voltage indicators, two-photon microscopes, and in vivo preparations, it is worth noting that some questions still may be best approached using more classical methodologies and preparations. In this review, we highlight a few examples in neurons where the combination of charge-coupled device (CCD) imaging and classical organic indicators has revealed information that has so far been more informative than results using the more modern systems. These experiments take advantage of the high frame rates, sensitivity, and spatial integration of the best CCD cameras. These cameras can respond to the faster kinetics of organic voltage and calcium indicators, which closely reflect the fast dynamics of the underlying cellular events.

Strategically positioned inhibitory synapses of axo-axonic cells potently control principal neuron spiking in the basolateral amygdala

Axo-axonic cells (AACs) in cortical regions selectively innervate the axon initial segments (AISs) of principal cells (PCs), where the action potentials are generated. These GABAergic interneurons can alter the activity of PCs, but how the efficacy of spike control correlates with the number of output synapses remains unclear. Moreover, the relationship between the spatial distribution of GABAergic synapses and the action potential initiation site along the AISs is not well defined. Using paired recordings obtained in the mouse basolateral amygdala, we found that AACs powerfully inhibited or delayed the timing of PC spiking by 30 ms, if AAC output preceded PC spiking with no more than 80 ms. By correlating the number of synapses and the probability of spiking, we revealed that larger numbers of presynaptic AAC boutons giving rise to larger postsynaptic responses provided more effective inhibition of PC spiking. At least 10-12 AAC synapses, which could originate from 2-3 AACs on average, were necessary to veto the PC firing under our recording conditions. Furthermore, we determined that the threshold for the action potential generation along PC axons is the lowest between 20 and 40 μm from soma, which axonal segment received the highest density of GABAergic inputs. Single AACs preferentially innervated this narrow portion of the AIS where action potentials were generated with the highest likelihood, regardless of the number of synapses forming a given connection. Our results uncovered a fine organization of AAC innervation maximizing their inhibitory efficacy by strategically positioning synapses along the AISs.

Structural homeostasis in the nervous system: a balancing act for wiring plasticity and stability

Experience-dependent modifications of neural circuits provide the cellular basis for functional adaptation and learning, while presenting significant challenges to the stability of neural networks. The nervous system copes with these perturbations through a variety of compensatory mechanisms with distinct spatial and temporal profiles. Mounting evidence suggests that structural plasticity, through modifications of the number and structure of synapses, or changes in local and long-range connectivity, might contribute to the stabilization of network activity and serve as an important component of the homeostatic regulation of the nervous system. Conceptually similar to the homeostatic regulation of synaptic strength and efficacy, homeostatic structural plasticity has a profound and lasting impact on the intrinsic excitability of the neuron and circuit properties, yet remains largely unexplored. In this review, we examine recent reports describing structural modifications associated with functional compensation in both developing and adult nervous systems, and discuss the potential role for structural homeostasis in maintaining network stability and its implications in physiological and pathological conditions of the nervous systems.

The emerging functions of oligodendrocytes in regulating neuronal network behaviour

Myelin is required for efficient nerve conduction, but not all axons are myelinated to the same extent. Here we review recent studies that have revealed distinct myelination patterns of different axonal paths, suggesting that myelination is not an all or none phenomenon and that its presence is finely regulated in central nervous system networks. Whereas powerful reductionist biology has led to important knowledge of how oligodendrocytes function by themselves, little is known about their role in neuronal networks. We still do not understand how oligodendrocytes integrate information from neurons to adapt their function to the need of the system. An intricate cross talk between neurons and glia is likely to exist and to determine how neuronal circuits operate as a whole. Dissecting these mechanisms by using integrative systems biology approaches is one of the major challenges ahead.

Plasticity of the Axonal Trigger Zone

The axon initial segment (AIS) is a specialized axonal compartment that is involved in conversion of synaptic potentials into action potentials. Recent studies revealed that structural properties of the AIS, such as length and position relative to the soma, are differentiated in a cell-specific manner and shape signal processing of individual neurons. Moreover, these structural properties are not fixed but vary in response to prolonged changes of neuronal activity, which readjusts action potential threshold and compensates for the changes of activity, indicating that this structural plasticity of the AIS works as a homeostatic mechanism and contributes to maintain neuronal activity. Neuronal activity plays a crucial role in formation, maintenance, and refinement of neural circuits as well as in pathogenesis and/or pathophysiology of diseases. Thus, this plasticity should be a key to understand physiology and pathology of the brain.

Activity-dependent and activity-independent development of the axon initial segment

The axon initial segment (AIS) is the site of spike initiation in neurons. Previous studies revealed that spatial distribution of the AIS varies greatly among neurons to meet their specific needs. However, when and how this differentiation arises is unknown. Neurons in the avian nucleus laminaris (NL) are binaural coincidence detectors for sound localization and show differentiation in the distribution of the AIS, with shorter length and a more distal position from the soma with an increase in tuning frequency. We studied these characteristics of the AIS in NL neurons of the chicken during development and found that the AIS differentiates in its distribution after initial formation, and this is driven by activity-dependent and activity-independent mechanisms that differentially regulate distal and proximal boundaries of the AIS. Before hearing onset, the ankyrinG-positive AIS existed at a wide stretch of proximal axon regardless of tuning frequency, but Na+ channels were only partially distributed within the AIS. Shortly after hearing onset, Na+ channels accumulated along the entire AIS, which started shortening and relocating distally to a larger extent in neurons with higher tuning frequencies. Ablation of inner ears abolished the shortening of the AIS without affecting the position of its proximal boundary, indicating that both distal and proximal AIS boundaries are disassembled during development, and the former is dependent on afferent activity. Thus, interaction of these activity-dependent and activity-independent mechanisms determines the cell-specific distribution of the AIS in NL neurons and plays a critical role in establishing the function of sound localization circuit.

Neural ECM molecules in axonal and synaptic homeostatic plasticity

Neural circuits can express different forms of plasticity. So far, Hebbian synaptic plasticity was considered the most important plastic phenomenon, but over the last decade, homeostatic mechanisms gained more interest because they can explain how a neuronal network maintains stable baseline function despite multiple plastic challenges, like developmental plasticity, learning, or lesion. Such destabilizing influences can be counterbalanced by the mechanisms of homeostatic plasticity, which restore the stability of neuronal circuits. Synaptic scaling is a mechanism in which neurons can detect changes in their own firing rates through a set of molecular sensors that then regulate receptor trafficking to scale the accumulation of glutamate receptors at synaptic sites. Additional homeostatic mechanisms allow local changes in synaptic activation to generate local synaptic adaptations and network-wide changes in activity, which lead to adjustments in the balance between excitation and inhibition. The molecular pathways underlying these forms of homeostatic plasticity are currently under intense investigation, and it becomes clear that the extracellular matrix (ECM) of the brain, which surrounds individual neurons and integrates them into the tissue, is an important element in these processes. As a highly dynamic structure, which can be remodeled and degraded in an activity-dependent manner and in concerted action of neurons and glial cells, it can on one hand promote structural and functional plasticity and on the other hand stabilize neural microcircuits. This chapter highlights the composition of brain ECM with particular emphasis on perisynaptic and axonal matrix formations and its involvement in plastic and adaptive processes of the central nervous system.

Mild traumatic brain injury in the mouse induces axotomy primarily within the axon initial segment

Traumatic axonal injury (TAI) is a consistent component of traumatic brain injury (TBI), and is associated with much of its morbidity. Increasingly, it has also been recognized as a major pathology of mild TBI (mTBI). In terms of its pathogenesis, numerous studies have investigated the susceptibility of the nodes of Ranvier, the paranode and internodal regions to TAI. The nodes of Ranvier, with their unique composition and concentration of ion channels, have been suggested as the primary site of injury, initiating a cascade of abnormalities in the related paranodal and internodal domains that lead to local axonal swellings and detachment. No investigation, however, has determined the effect of TAI upon the axon initial segment (AIS), a segment critical to regulating polarity and excitability. The current study sought to identify the susceptibility of these different axon domains to TAI within the neocortex, where each axonal domain could be simultaneously assessed. Utilizing a mouse model of mTBI, a temporal and spatial heterogeneity of axonal injury was found within the neocortical gray matter. Although axonal swellings were found in all domains along myelinated neocortical axons, the majority of TAI occurred within the AIS, which progressed without overt structural disruption of the AIS itself. The finding of primary AIS involvement has important implications regarding neuronal polarity and the fate of axotomized processes, while also raising therapeutic implications, as the mechanisms underlying such axonal injury in the AIS may be distinct from those described for nodal/paranodal injury.

Development of on-off spiking in superior paraolivary nucleus neurons of the mouse

The superior paraolivary nucleus (SPON) is a prominent cell group in the auditory brain stem that has been increasingly implicated in representing temporal sound structure. Although SPON neurons selectively respond to acoustic signals important for sound periodicity, the underlying physiological specializations enabling these responses are poorly understood. We used in vitro and in vivo recordings to investigate how SPON neurons develop intrinsic cellular properties that make them well suited for encoding temporal sound features. In addition to their hallmark rebound spiking at the stimulus offset, SPON neurons were characterized by spiking patterns termed onset, adapting, and burst in response to depolarizing stimuli in vitro. Cells with burst spiking had some morphological differences compared with other SPON neurons and were localized to the dorsolateral region of the nucleus. Both membrane and spiking properties underwent strong developmental regulation, becoming more temporally precise with age for both onset and offset spiking. Single-unit recordings obtained in young mice demonstrated that SPON neurons respond with temporally precise onset spiking upon tone stimulation in vivo, in addition to the typical offset spiking. Taken together, the results of the present study demonstrate that SPON neurons develop sharp on-off spiking, which may confer sensitivity to sound amplitude modulations or abrupt sound transients. These findings are consistent with the proposed involvement of the SPON in the processing of temporal sound structure, relevant for encoding communication cues.