Action Potential Firing Patterns Research Articles

Ventricular hyperinnervation in a mouse model of catecholaminergic polymorphic ventricular tachycardia

Abstract Funding Acknowledgements Type of funding sources: Foundation. Main funding source(s): European Society of Cardiology Basic Research Fellowship. Dutch Heart Foundation Dekker Grant Background Inherited arrhythmia syndromes are a leading cause of sudden cardiac death in young and otherwise healthy patients. They are often caused by mutations in genes that encode ion channels or transporters, leading to conditions such as Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT). CPVT is caused by mutations in RYR2 (encoding ryanodine receptor 2, RyR2). In arrhythmia research, investigation into the effects of these mutations have exclusively been performed in cardiac cells. However, RyR2 is also present in neuronal tissue, including intracardiac neurons [1] (which modulate cardiac function). Moreover, patients often present with clinical signs of autonomic nervous system (ANS) dysfunction, and treatment strategies frequently target the ANS. Aim Our novel research line aims to investigate the neuronal phenotype induced by RyR2 mutations, using a CPVT mouse model to assess functional alterations as well as broader autonomic remodelling. Results Using immunocytochemistry, we reveal for the first time that RyR2 is also expressed in mouse stellate ganglia (crucial autonomic modulators of cardiac function). Through analysis of published RNA sequencing datasets, we find that RYR2 is more abundantly expressed in both stellate ganglion and intracardiac neurons than characteristic neuronal markers such as CHAT, SYN1 and SCN8A [2,3]. Functional patch clamp investigation of isolated stellate ganglion neurons have revealed two distinct populations based on their patterns of action potential (AP) firing, identifying both phasic-firing and tonic-firing type neurons. Investigation of an established CPVT mouse model (RyR2-R2474S) has thus far not revealed any differences in the AP firing frequency of isolated stellate ganglion neurons during sustained depolarising stimuli of various amplitudes. Assessment of calcium homeostasis and neurotransmitter release is currently ongoing. We have furthermore investigated broader autonomic remodelling using immunohistochemistry in mouse heart cryosections. Neuronal innervation density was assessed by staining 10 µm sections of mid-ventricular myocardium for the sympathetic neuron marker tyrosine hydroxylase (TH). Quantification of the TH-positive area revealed a significant increase in the percentage of sympathetic neuronal innervation in ventricular tissue of CPVT mice (5.7% wild-type vs 7.2% RyR2-R2474S, p=0.0253, N=6). Conclusions The neurons that modulate heart function (stellate ganglion and intracardiac neurons) abundantly express RyR2. The RyR2-R2474S mutation leads to significant sympathetic hyper-innervation of ventricular myocardium, which may have important consequences for neurotransmitter release and hence cardiac (electrical) function and arrhythmogenesis.

Muscle Sympathetic Action Potential Firing Patterns Following Intranasal Insulin Administration in Healthy Adults

Objective: Systemic insulin increases muscle sympathetic nervous system activity (MSNA) in healthy adults by augmenting firing frequency of medium-sized sympathetic action potentials (AP) and recruiting previously latent, larger axons. Whether these neural coding patterns are due to central or peripheral effects of insulin is unclear. We examined the impact of elevated central insulin on AP firing patterns in healthy adults. We hypothesized intranasal insulin administration, which increases central insulin levels, would increase MSNA via elevated firing frequency of APs and recruitment of previously latent, larger axons. Methods: Ten participants were assigned to time control [TC, n=6 (3M/3F)] or 160 IU of intranasal insulin [n=4 (3M/1F)] administered over 5 min. MSNA (fibular microneurography) was assessed at baseline and for 30 min following insulin administration. Sympathetic APs were identified using a matched mother wavelet and continuous wavelet transform. APs were divided into the proportion of those firing in small, medium, or large amplitude clusters. AP firing frequency, percent of APs firing within an MSNA burst (synchronous), and the probability of clusters firing more than once within an MSNA burst were identified. Data are reported as median (interquartile range). Results: MSNA burst frequency increased 15 min after intranasal insulin administration [24(20) to 31(23) bursts/min, p=0.03]. At this time, the percent of synchronous APs [72(27) to 90(24)%, p=0.02] and the probability of medium-sized AP clusters firing more than once within an MSNA burst [13(14) to 19(17)%, p=0.03] increased. No changes in MSNA burst frequency [23(14) to 25(13) bursts/min, p=0.27], percent of synchronous APs [74(15) to 75(34), p=0.48], nor firing probability of medium-sized AP clusters [6(12) to 8(7)%, p=0.97] occurred in TC. The total number of detected clusters did not change in either group (p>0.05). Conclusions: Intranasal insulin increases MSNA burst frequency due to a higher percent of synchronous APs, particularly medium-sized AP clusters; however, we did not observe recruitment of latent, larger axons. These exploratory data highlight potential differences in the sympathetic response to central versus peripheral insulin exposure in healthy adults. CAFNR Joy of Discovery (JKL, JP). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Electrophysiological properties of dorsal root ganglion neurons cultured on 3D silicon micro-pillar substrates

BackgroundSilicon-based micro-pillar substrates (MPS), as three-dimensional cell culture platforms with vertically aligned micro-patterned scaffolding structures, are known to facilitate high-quality growth and morphology of dorsal root ganglion (DRG) sensory neurons, promote neurite outgrowth and enhance neurite alignment. However, the electrophysiological aspects of DRG neurons cultured on silicon MPSs have not been thoroughly investigated, which is of greatest importance to ensure that such substrates do not disrupt neuronal homeostasis and function before their widespread adoption in diverse biomedical applications. New methodWe conducted whole-cell patch-clamp recordings to explore the electrophysiological properties of DRG neurons cultured on MPS arrays, utilizing a custom-made upright patch-clamp setup. ResultsOur findings revealed that DRG neurons exhibited similar electrophysiological responses on patterned MPS samples when compared to the control planar glass surfaces. Notably, there were no significant differences observed in the action potential parameters or firing patterns of action potentials between neurons grown on either substrate. Comparison with existing methodsIn the current study we for the first time confirmed that successful electrophysiological recordings can be obtained from the cells grown on MPS. ConclusionOur results imply that, despite the potential alterations caused by the cumulative trauma of tissue harvest and cell dissociation, essential functional cell properties of DRG neurons appear to be relatively maintained on MPS surfaces. Therefore, vertically aligned silicon MPSs could be considered as a potentially effective three-dimensional system for supporting a controlled cellular environment in culture.

Inhibition of 14-3-3 proteins increases the intrinsic excitability of mouse hippocampal CA1 pyramidal neurons.

14-3-3 proteins are a family of regulatory proteins that are abundantly expressed in the brain and enriched at the synapse. Dysfunctions of these proteins have been linked to neurodevelopmental and neuropsychiatric disorders. Our group has previously shown that functional inhibition of these proteins by a peptide inhibitor, difopein, in the mouse brain causes behavioural alterations and synaptic plasticity impairment in the hippocampus. Recently, we found an increased cFOS expression in difopein-expressing dorsal CA1 pyramidal neurons, indicating enhanced neuronal activity by 14-3-3 inhibition in these cells. In this study, we used slice electrophysiology to determine the effects of 14-3-3 inhibition on the intrinsic excitability of CA1 pyramidal neurons from a transgenic 14-3-3 functional knockout (FKO) mouse line. Our data demonstrate an increase in intrinsic excitability associated with 14-3-3 inhibition, as well as reveal action potential firing pattern shifts after novelty-induced hyperlocomotion in the 14-3-3 FKO mice. These results provide novel information on the role 14-3-3 proteins play in regulating intrinsic and activity-dependent neuronal excitability in the hippocampus.

Sex-specific differences in the circadian pattern of action potential firing by rat suprachiasmatic nucleus vasopressin neurons.

The suprachiasmatic nucleus (SCN) of the hypothalamus serves as the master circadian clock in mammals. Most SCN neurons express the inhibitory neurotransmitter GABA (gamma amino butyric acid) along with a peptide cotransmitter. Notably, the neuropeptides vasopressin (VP) and vasoactive intestinal peptide (VIP) define two prominent clusters within the SCN: those located in the ventral core (VIP) and those forming the dorsomedial "shell" of the nucleus (VP). Axons emerging from VP neurons in the shell are thought to mediate much of the SCN's output to other brain regions as well as VP release into the cerebrospinal fluid (CSF). Previous work has shown that VP release by SCN neurons is activity dependent and SCN VP neurons fire action potentials at a higher rate during the light phase. Accordingly, CSF VP levels are higher during daytime. Interestingly, the amplitude of the CSF VP rhythm is greater in males than females, suggesting the existence of sex differences in the electrical activity of SCN VP neurons. Here we investigated this hypothesis by performing cell-attached recordings from 1070 SCN VP neurons across the entire circadian cycle in both sexes of transgenic rats that express green fluorescent protein (GFP) driven by the VP gene promoter. Using an immunocytochemical approach we confirmed that >60% of SCN VP neurons display visible GFP. Recordings in acute coronal slices revealed that VP neurons display a striking circadian pattern of action potential firing, but the characteristics of this activity cycle differ in males and females. Specifically, neurons in males reached a significantly higher peak firing frequency during subjective daytime compared to females and the acrophase occurred ~1 h earlier in females. Peak firing rates in females were not significantly different at various phases of the estrous cycle.

T-type calcium channels as therapeutic targets in essential tremor and Parkinson's disease.

Neuronal action potential firing patterns are key components of healthy brain function. Importantly, restoring dysregulated neuronal firing patterns has the potential to be a promising strategy in the development of novel therapeutics for disorders of the central nervous system. Here, we review the pathophysiology of essential tremor and Parkinson's disease, the two most common movement disorders, with a focus on mechanisms underlying the genesis of abnormal firing patterns in the implicated neural circuits. Aberrant burst firing of neurons in the cerebello-thalamo-cortical and basal ganglia-thalamo-cortical circuits contribute to the clinical symptoms of essential tremor and Parkinson's disease, respectively, and T-type calcium channels play a key role in regulating this activity in both the disorders. Accordingly, modulating T-type calcium channel activity has received attention as a potentially promising therapeutic approach to normalize abnormal burst firing in these diseases. In this review, we explore the evidence supporting the theory that T-type calcium channel blockers can ameliorate the pathophysiologic mechanisms underlying essential tremor and Parkinson's disease, furthering the case for clinical investigation of these compounds. We conclude with key considerations for future investigational efforts, providing a critical framework for the development of much needed agents capable of targeting the dysfunctional circuitry underlying movement disorders such as essential tremor, Parkinson's disease, and beyond.

Bringing to light the physiological and pathological firing patterns of human induced pluripotent stem cell-derived neurons using optical recordings.

Human induced pluripotent stem cells (hiPSCs) are a promising approach to study neurological and neuropsychiatric diseases. Most methods to record the activity of these cells have major drawbacks as they are invasive or they do not allow single cell resolution. Genetically encoded voltage indicators (GEVIs) open the path to high throughput visualization of undisturbed neuronal activity. However, conventional GEVIs perturb membrane integrity through inserting multiple copies of transmembrane domains into the plasma membrane. To circumvent large add-ons to the plasma membrane, we used a minimally invasive novel hybrid dark quencher GEVI to record the physiological and pathological firing patterns of hiPSCs-derived sensory neurons from patients with inherited erythromelalgia, a chronic pain condition associated with recurrent attacks of redness and swelling in the distal extremities. We observed considerable differences in action potential firing patterns between patient and control neurons that were previously overlooked with other recording methods. Our system also performed well in hiPSC-derived forebrain neurons where it detected spontaneous synchronous bursting behavior, thus opening the path to future applications in other cell types and disease models including Parkinson's disease, Alzheimer's disease, epilepsy, and schizophrenia, conditions associated with disturbances of neuronal activity and synchrony.

Mechanosensory encoding dysfunction emerges from cancer-chemotherapy interaction.

Persistent sensory, motor and cognitive disabilities comprise chemotherapy-induced neural disorders (CIND) that limit quality of life with little therapeutic relief for cancer survivors. Our recent preclinical study provides new insight into a condition impacting the severity of chronic CIND. We find that sensorimotor disability observed following cancer treatment exceeds that attributable to chemotherapy alone. A possible explanation for intensified disability emerged from evidence that codependent effects of cancer and chemotherapy amplify defective firing in primary sensory neurons supplying one type of low threshold mechanosensory receptor (LTMR). Here we test whether cancer's modification of chemotherapy-induced sensory defects generalizes across eight LTMR submodalities that collectively generate the signals of origin for proprioceptive and tactile perception and guidance of body movement. Preclinical study enabled controlled comparison of the independent contributions of chemotherapy and cancer to their clinically relevant combined effects. We compared data sampled from rats that were otherwise healthy or bearing colon cancer and treated, or not, with human-scaled, standard-of-care chemotherapy with oxaliplatin. Action potential firing patterns encoding naturalistic mechanical perturbations of skeletal muscle and skin were measured electrophysiologically in vivo from multiple types of LTMR neurons. All expressed aberrant encoding of dynamic and/or static features of mechanical stimuli in healthy rats treated with chemotherapy, and surprisingly also by some LTMRs in cancer-bearing rats that were not treated. By comparison, chemotherapy and cancer in combination worsened encoding aberrations, especially in slowly adapting LTMRs supplying both muscle and glabrous skin. Probabilistic modeling best predicted observed encoding defects when incorporating interaction effects of cancer and chemotherapy. We conclude that for multiple mechanosensory submodalities, the severity of encoding defects is modulated by a codependence of chemotherapy side effects and cancer's systemic processes. We propose that the severity of CIND might be reduced by therapeutically targeting the mechanisms, yet to be determined, by which cancer magnifies chemotherapy's neural side effects as an alternative to reducing chemotherapy and its life-saving benefits.

Sympathetic Action Potential Firing and Recruitment Patterns Are Abnormal in Gestational Hypertension.

We tested the hypothesis that women who develop gestational hypertension (GH) display abnormal sympathetic action potential (AP) discharge patterns during late pregnancy (32-36 weeks), both at supine rest and during postural stress. Thirteen nonpregnant, female controls (nonpregnant controls [CTRL]) and 32 pregnant women participated; 14 had low-risk (no personal history of GH) normal pregnancies (LR-NP), 10 had high-risk (personal history of GH) normal pregnancies (HR-NP), and 8 developed GH. We measured heart rate, blood pressure, and muscle sympathetic nerve activity (microneurography) at supine rest and 60° head-up tilt. Sympathetic AP patterns were studied using wavelet-based methodology. At rest, muscle sympathetic nerve activity burst frequency was elevated in LR-NP, HR-NP, and GH versus CTRL (all P≤0.01); however, the AP content per integrated burst was augmented only in GH (20±5 spikes/burst), compared with CTRL (8±3 spikes/burst), LR-NP (9±2 spikes/burst) and HR-NP (11±4 spikes/burst; all P<0.0001). Thus, total AP firing frequency was greater in GH versus each of CTRL, LR-NP, and HR-NP (all P<0.0001). In pregnancy, AP frequency is related directly to systolic (R2=46%) and diastolic (R2=20%) blood pressure (both P≤0.01). Unlike CTRL (both P<0.01), women who developed GH were unable to increase within-burst AP firing (P=0.71) or recruit latent subpopulations of larger-sized APs (P=0.72) in response to head-up tilt, perhaps related to a ceiling-effect; however, total AP firing frequency in the upright posture was elevated in the GH cohort versus CTRL, LR-NP, and HR-NP (all P<0.05). Women who develop GH display aberrant sympathetic AP firing patterns in both the supine and upright postures.

Development of Neuronal Excitability of the Bushy Cells in Anteroventral Cochlear Nucleus of Rats

The ultrafast and precise single-onset action potential (AP) of the bushy cells (BCs) in the anteroventral cochlear nucleus (AVCN) plays an important role in precise processing of temporal auditory information for localizing sound sources and communication cues. The specialized properties of high conductance of the low-voltage-activated potassium (K⁺_LVA) channel contribute to generate ultrafast and precise single-onset APs in BCs. However, the developmental changes of K⁺_LVA distribution and their contributions to shape neuronal excitability of BCs remain unclear. Therefore, we investigated the developmental changes in neuronal excitability of BCs and K⁺_LVA distribution at different developmental periods. Using electrophysiological recording, we first characterized the firing pattern of BCs in response to a sequence of current injections at different developmental periods. The expression of the K⁺_LVA subunit Kv1.1 in AVCN was examined with Western blot. The results indicated that BCs showed single-onset AP firing patterns and paused multiple APs firing patterns at the postnatal time of day 7 (P7) and were then refined into single-onset firing patterns at P14 and P21. With development, the active membrane properties, including latency and half-width of AP, and passive membrane properties, including capacitance, input resistance, and time constant, were significantly decreased. Furthermore, the refinement of firing patterns in BCs was correlated with the upregulation of the Kv1.1 channel in AVCN. In summary, the present study indicated that BCs optimize precise and single-onset firing with development, possibly driven by the changes in membrane properties and upregulation of Kv1.1 in AVCN.

Experimental Protocols and Analytical Procedures for Studying Synaptic Transmission in Rodent Spinal Cord Dorsal Horn.

Synaptic modulation and plasticity are key mechanisms underlying pain transmission in the spinal cord and supra-spinal centers. The study and understanding of these phenomena are fundamental to investigating both acute nociception and maladaptive changes occurring in chronic pain. This article describes experimental protocols and analytical methods utilized in electrophysiological studies to investigate synaptic modulation and plasticity at the first station of somatosensory processing, the spinal cord dorsal horn. Protocols useful for characterizing the nature of synaptic inputs, the site of modulation (pre- versus postsynaptic), and the presence of short-term synaptic plasticity are presented. These methods can be employed to study the physiology of acute nociception, the pathological mechanisms of persistent inflammatory and neuropathic pain, and the pharmacology of receptors and channels involved in pain transmission. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Spinal cord dissection and acute slice preparation Basic Protocol 2: Stimulation of the dorsal root and extracellular recording (compound action potentials and field potentials) Basic Protocol 3: Patch-clamp recording from dorsal horn neurons: action potential firing patterns and evoked synaptic inputs Basic Protocol 4: Analysis of parameters responsible for changes in synaptic efficacy Basic Protocol 5: Recording and analysis of currents mediated by astrocytic glutamate.

Chemical and Biological Tools for the Study of Voltage-Gated Sodium Channels in Electrogenesis and Nociception.

The malfunction and misregulation of voltage-gated sodium channels (NaV s) underlie in large part the electrical hyperexcitability characteristic of chronic inflammatory and neuropathic pain. NaV s are responsible for the initiation and propagation of electrical impulses (action potentials) in cells. Tissue and nerve injury alter the expression and localization of multiple NaV isoforms, including NaV 1.1, 1.3, and 1.6-1.9, resulting in aberrant action potential firing patterns. To better understand the role of NaV regulation, localization, and trafficking in electrogenesis and pain pathogenesis, a number of chemical and biological reagents for interrogating NaV function have been advanced. The development and application of such tools for understanding NaV physiology are the focus of this review.

A different mechanism of C-type inactivation in the Kv-like KcsA mutant E71V

C-type inactivation is of great physiological importance particularly in voltage-activated K+ channels (Kv), affecting the firing patterns of neurons and shaping cardiac action potentials. While understanding the molecular basis of inactivation has a direct impact on human health, its structural basis remains unresolved. Knowledge about C-type inactivation has been largely deduced from the pH-activated bacterial K+ channel KcsA, whose selectivity filter under inactivating conditions adopts a constricted conformation at the level of the central glycine (TTVGYGD) that is stabilized by tightly bound water molecules.

Dendritic Architecture Predicts in vivo Firing Pattern in Mouse Ventral Tegmental Area and Substantia Nigra Dopaminergic Neurons.

The firing activity of ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) dopaminergic (DA) neurons is an important factor in shaping DA release and its role in motivated behavior. Dendrites in DA neurons are the main postsynaptic compartment and, along with cell body and axon initial segment, contribute to action potential generation and firing pattern. In this study, the organization of the dendritic domain in individual VTA and SNc DA neurons of adult male mice, and their relationship to in vivo spontaneous firing, are described. In comparison with dorsal VTA DA neurons, ventrally located VTA neurons (as measured by cell body location) possess a shorter total dendritic length and simpler dendritic architecture, and exhibit the most irregular in vivo firing patterns among DA neurons. In contrast, for DA neurons in the SNc, the higher irregularity of firing was related to a smaller dendritic domain, as measured by convex hull volumes. However, firing properties were also related to the specific regional distribution of the dendritic tree. Thus, VTA DA neurons with a larger extension of their dendritic tree within the parabrachial pigmented (PBP) nucleus fired more regularly compared with those with relatively more dendrites extending outside the PBP. For DA neurons in the SNc, enhanced firing irregularity was associated with a smaller proportion of dendrites penetrating the substantia nigra pars reticulata. These results suggest that differences in dendritic morphology contribute to the in vivo firing properties of individual DA neurons, and that the existence of region-specific synaptic connectivity rules that shape firing diversity.

Mechanism and function of phasic firing in vasopressin-releasing magnocellular neurosecretory cells.

Magnocellular neurosecretory cells that release vasopressin (MNCVP ) from axon terminals in the neurohypophysis display a unique pattern of action potential firing termed phasic firing. Under basal conditions, only a small proportion of MNCVP display spontaneous phasic firing. However, acute and chronic conditions that stimulate vasopressin release, such as hemorrhage and dehydration, greatly enhance the number of MNCVP that fire phasically. Phasic firing optimizes VP neurosecretion at axon terminals by allowing action potential broadening to promote calcium-dependent frequency-facilitation, at the same time as preventing the secretory fatigue caused by spike inactivation that occurs during prolonged continuous stimulation. This review provides an update on our mechanistic understanding of these processes and highlights important gaps in our knowledge that must be addressed in future experiments.

Activity of Cerebellar Nuclei Neurons Correlates with ZebrinII Identity of Their Purkinje Cell Afferents.

Purkinje cells (PCs) in the cerebellar cortex can be divided into at least two main subpopulations: one subpopulation that prominently expresses ZebrinII (Z+), and shows a relatively low simple spike firing rate, and another that hardly expresses ZebrinII (Z–) and shows higher baseline firing rates. Likewise, the complex spike responses of PCs, which are evoked by climbing fiber inputs and thus reflect the activity of the inferior olive (IO), show the same dichotomy. However, it is not known whether the target neurons of PCs in the cerebellar nuclei (CN) maintain this bimodal distribution. Electrophysiological recordings in awake adult mice show that the rate of action potential firing of CN neurons that receive input from Z+ PCs was consistently lower than that of CN neurons innervated by Z– PCs. Similar in vivo recordings in juvenile and adolescent mice indicated that the firing frequency of CN neurons correlates to the ZebrinII identity of the PC afferents in adult, but not postnatal stages. Finally, the spontaneous action potential firing pattern of adult CN neurons recorded in vitro revealed no significant differences in intrinsic pacemaking activity between ZebrinII identities. Our findings indicate that all three main components of the olivocerebellar loop, i.e., PCs, IO neurons and CN neurons, operate at a higher rate in the Z– modules.

Communication sound recognition and response modification in the secondary auditory cortex of female mice

To study the response characteristics of the secondary auditory cortex (A2) to wriggling calls (WC) and the mechanism of response modulation in female mice. We used patch-clamp and immunofluorescence labeling technique to mark and record the action potential and cell type of A2 neurons. Female C57 mice were stimulated with pure tone and white noise (control), 4.5 kHz, 9 kHz, or 13.5 kHz sound waves extracted from WC (single-frequency simulation group), the combinations of every two of the 3 single-frequency sound waves (two-tone frequency simulation group), or the combinations of 4.5 kHz+7.7 kHz+13.5 kHz and 4.5 kHz+9 kHz+13.5 kHz sound waves (three-frequency simulation group). The firing pattern, firing number, threshold, and latency of the action potential of the A2 neurons were recorded in response to the stimulations. By comparison of the spikes elicited by different sound stimulations, we identified 3 types of neurons with different sensitivities to WC. The WC-sensitive neurons had a significantly greater number of spikes in response to WC than to other sounds and noise (P < 0.001). Comparison of the latency and threshold revealed significantly longer latencies of the WC-sensitive neurons and WC-insensitive neurons in response to WC stimulation than to pure tone stimulation (P=0.002), but their latencies to WC and noise stimulation were similar (P=0.093). The WC-sensitive neurons also had lower threshold to WC than to pure tone and noise stimulations (P=0.02). Analysis of the firing patterns of action potentials showed that the WCsensitive neurons consisted mainly of parvalbumin interneurons. The results of immunofluorescence labeling indicated that inhibitory interneurons were present in the A2 neurons that responded to WC. A2 contains 3 types of neurons with different sensitivities to WC. Among them, the WC-sensitive neurons is mainly PV neurons, whose response characteristics to different types of sounds can help to explain the mechanism of communication sound recognition and response modification in A2.

Ketamine Action on Astrocytes Provides New Insights into Rapid Antidepressant Mechanisms.

Ketamine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, exerts rapid, potent and long-lasting antidepressant effect already after a single administration of a low dose into depressed individuals. Apart from targeting neuronal NMDARs essential for synaptic transmission, ketamine also interacts with astrocytes, the principal homoeostatic cells of the central nervous system. The cellular mechanisms underlying astrocyte-based rapid antidepressant effect are incompletely understood. Here we overview recent data that describe ketamine-dependent changes in astrocyte cytosolic cAMP activity ([cAMP]i) and ketamine-induced modifications of stimulus-evoked Ca2+ signalling. The latter regulates exocytotic release of gliosignalling molecules and stabilizes the vesicle fusion pore in a narrow configuration that obstructs cargo discharge or vesicle membrane recycling. Ketamine also instigates rapid redistribution of cholesterol in the astrocyte plasmalemma that may alter flux of cholesterol to neurones, where it is required for changes in synaptic plasticity. Finally, ketamine attenuates mobility of vesicles carrying the inward rectifying potassium channel (Kir4.1) and reduces the surface density of Kir4.1 channels that control extracellular K+ concentration, which tunes the pattern of action potential firing in neurones of lateral habenula as demonstrated in a rat model of depression. Thus, diverse, but not mutually exclusive, mechanisms act synergistically to evoke changes in synaptic plasticity leading to sustained strengthening of excitatory synapses necessary for rapid antidepressant effect of ketamine.

The Input-Output Relation of Primary Nociceptive Neurons is Determined by the Morphology of the Peripheral Nociceptive Terminals.

The output from the peripheral terminals of primary nociceptive neurons, which detect and encode the information regarding noxious stimuli, is crucial in determining pain sensation. The nociceptive terminal endings are morphologically complex structures assembled from multiple branches of different geometry, which converge in a variety of forms to create the terminal tree. The output of a single terminal is defined by the properties of the transducer channels producing the generation potentials and voltage-gated channels, translating the generation potentials into action potential (AP) firing. However, in the majority of cases, noxious stimuli activate multiple terminals; thus, the output of the nociceptive neuron is defined by the integration and computation of the inputs of the individual terminals. Here, we used a computational model of nociceptive terminal tree to study how the architecture of the terminal tree affects the input-output relation of the primary nociceptive neurons. We show that the input-output properties of the nociceptive neurons depend on the length, the axial resistance (Ra), and location of individual terminals. Moreover, we show that activation of multiple terminals by a capsaicin-like current allows summation of the responses from individual terminals, thus leading to increased nociceptive output. Stimulation of the terminals in simulated models of inflammatory or neuropathic hyperexcitability led to a change in the temporal pattern of AP firing, emphasizing the role of temporal code in conveying key information about changes in nociceptive output in pathologic conditions, leading to pain hypersensitivity.SIGNIFICANCE STATEMENT Noxious stimuli are detected by terminal endings of primary nociceptive neurons, which are organized into morphologically complex terminal trees. The information from multiple terminals is integrated along the terminal tree, computing the neuronal output, which propagates toward the CNS, thus shaping the pain sensation. Here, we revealed that the structure of the nociceptive terminal tree determines the output of nociceptive neurons. We show that the integration of noxious information depends on the morphology of the terminal trees and how this integration and, consequently, the neuronal output change under pathologic conditions. Our findings help to predict how nociceptive neurons encode noxious stimuli and how this encoding changes in pathologic conditions, leading to pain.

Persistent and resurgent Na+ currents in vestibular calyx afferents.

Vestibular afferent neurons convey information from hair cells in the peripheral vestibular end organs to central nuclei. Primary vestibular afferent neurons can fire action potentials at high rates and afferent firing patterns vary with the position of nerve terminal endings in vestibular neuroepithelia. Terminals contact hair cells as small bouton or large calyx endings. To investigate the role of Na+ currents (INa) in firing mechanisms, we investigated biophysical properties of INa in calyx-bearing afferents. Whole cell patch-clamp recordings were made from calyx terminals in thin slices of gerbil crista at different postnatal ages: immature [postnatal day (P)5-P8, young (P13-P15), and mature (P30-P45)]. A large transient Na+ current (INaT) was completely blocked by 300 nM tetrodotoxin (TTX) in mature calyces. In addition, INaT was accompanied by much smaller persistent Na+ currents (INaP) and distinctive resurgent Na+ currents (INaR), which were also blocked by TTX. ATX-II, a toxin that slows Na+ channel inactivation, enhanced INaP in immature and mature calyces. 4,9-Anhydro-TTX (4,9-ah-TTX), which selectively blocks Nav1.6 channels, abolished the enhanced INa in mature, but not immature, calyces. Therefore, Nav1.6 channels mediate a component of INaT and INaP in mature calyces, but are minimally expressed at early postnatal days. INaR was expressed in less than one-third of calyces at P6-P8, but expression increased with development, and in mature cristae INaR was frequently found in peripheral calyces. INaR served to increase the availability of Na+ channels following brief membrane depolarizations. In current clamp, the rate and regularity of action potential firing decreased in mature peripheral calyces following 4,9-ah-TTX application. Therefore, Nav1.6 channels are upregulated during development, contribute to INaT, INaP, and INaR, and may regulate excitability by enabling higher mean discharge rates in a subpopulation of mature calyx afferents.NEW & NOTEWORTHY Action potential firing patterns differ between groups of afferent neurons innervating vestibular epithelia. We investigated the biophysical properties of Na+ currents in specialized vestibular calyx afferent terminals during postnatal development. Mature calyces express Na+ currents with transient, persistent, and resurgent components. Nav1.6 channels contribute to resurgent Na+ currents and may enhance firing in peripheral calyx afferents. Understanding Na+ channels that contribute to vestibular nerve responses has implications for developing new treatments for vestibular dysfunction.