Step Current Injection Research Articles

Genetic deletion of STAC2 adaptor protein alters electrical activity of mouse chromaffin cells.

Voltage-gated Cav1.2 and Cav1.3 L-type Ca2+ channels (LTCCs) regulate action potential (AP) firing and catecholamine release in mouse chromaffin cells (MCCs). Src homology 3 and cysteine rich domain adaptor proteins (Stac1-Stac3) have recently been identified as novel regulators of neuronal LTCCs expression and biophysical properties. Upon overexpression in cultured hippocampal neurons Stac2 abolished LTCC Ca2+ dependent inactivation via an allosteric inhibition of calmodulin binding. Additionally, in Drosophila, deletion of the homologous DStac gene resulted in deficient LTCC Ca2+ transients and reduced neuropeptide release. Here we investigated the effect of Stac2 genetic ablation on neuronal LTCC function, electrical activity, and vesicle exocytosis in MCCs. Constitutive deletion of Stac2 does not affect the MCCs resting membrane potential or spontaneous firing frequency. However, the AP depolarization threshold was significantly reduced in Stac2-/- MCCs compared to WT (WT = −35.6 ± 1.2 mV, Stac2-/- = −40.6 ± 0.6 mV; ∗∗ P < 0.01). Additionally, step current injection elicited an electrical activity with higher initial AP firing frequency in Stac2-/- compared to WT that led to earlier depolarization block (F0 ≅2 pA current injection: WT = 0.28 ± 0.28 Hz, Stac2-/- = 0.89 ± 0.40 Hz; ∗ P < 0.05). Surprisingly, initial characterization shows that Stac2 deletion does not alter whole-cell Ca2+ current amplitude or inactivation kinetics but significantly shifts the voltage-dependence of activation to more hyperpolarized potential (V0.5: WT = −5.95 ± 1.65 mV, Stac2-/- = −15.25 ± 2.05 mV; ∗ P < 0.05). We are currently investigating possible effects of Stac2 deletion on other ionic conductances governing MCC excitability and the role on catecholamine vesicle exocytosis. Support: TWF F.18863, LFU 2021-CHEM-8 to SMG; FWF P31434, P36053, DOC30-B30 to PT; P33776 to MC.

Propofol modulates inhibitory inputs in paraventricular thalamic nucleus of mice

The mechanisms of general anaesthetics such as propofol have drawn substantial attention. The effects of propofol on inhibitory postsynaptic currents are not exactly the same in different brain nuclei. Recent studies revealed that the paraventricular thalamic nucleus (PVT) is a critical nucleus modulating wakefulness. However, the effects of propofol on PVT neurons and the mechanisms underlying such effects remain unknown. Here, we performed the whole-cell recording of the PVT neurons in acute brain slices and bath application of propofol. We found that propofol hyperpolarized the membrane potentials of the PVT neurons and suppressed the action potentials induced by step-current injection. Propofol did not affect the spontaneous inhibitory postsynaptic currents (sIPSCs) amplitude or frequency, but prolonged the sIPSCs half-width. Besides, propofol increased miniature inhibitory synaptic currents (mIPSCs) frequency and half-width. Furthermore, propofol could induce GABAA receptors-mediated tonic inhibitory currents dose-dependently. Thus, our results demonstrate that propofol hyperpolarizes PVT neurons by modulating inhibitory currents via GABAA receptors in mice.

Age‐Related Dysregulation of Neuronal Activity in Single Dopaminergic Neurons

Aging is a known risk factor for neurodegenerative diseases, including Parkinson's disease (PD) and Alzheimer's disease. Dopamine neurotransmission has been implicated in the etiology of PD and age‐related decline of motor control, motivation, and cognitive functions. Few studies have pursued cellular and synaptic mechanisms to explain the decline in midbrain dopamine neurotransmission with age. Here, we used in vivo single‐cell electrophysiological recordings in anesthetized mice to characterize changes in dopamine neurotransmission related to age. We used mice of 4, 12, 24, 30 months of age to record midbrain dopamine neuronal firing activity. Our preliminary data indicates a steady decrease of dopamine neuron firing activity with age, particularly in 30 month‐old mice. In a parallel experiment, using ex vivo whole cell patch clamp electrophysiology, we assessed excitability and firing pattern in midbrain dopamine neurons. To evaluate excitability, spontaneous firing was measured and step current injections from 20pA up to 400pA were also applied to induce firing. Our results show an increase in firing rate frequency and a decrease in the number of spikes with higher current injections, starting with 18 months of age, possibly indicating increased sensitivity to depolarization block. We are currently studying synaptic transmission by assessing glutamate receptor‐mediated burst firing in dopamine neurons in young and old mice. These results will provide a comprehensive assessment of the effects of age on firing induced by both intrinsic conductances and external synaptic input.Support or Funding InformationR01 from NIA (R01 AG052606) and T32 training grant from NIA (T32 AG052363)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Intrinsic excitability differs between murine hypoglossal and spinal motoneurons.

Motoneurons differ in the behaviors they control and their vulnerability to disease and aging. For example, brain stem motoneurons such as hypoglossal motoneurons (HMs) are involved in licking, suckling, swallowing, respiration, and vocalization. In contrast, spinal motoneurons (SMs) innervating the limbs are involved in postural and locomotor tasks requiring higher loads and lower movement velocities. Surprisingly, the properties of these two motoneuron pools have not been directly compared, even though studies on HMs predominate in the literature compared with SMs, especially for adult animals. Here we used whole cell patch-clamp recording to compare the electrophysiological properties of HMs and SMs in age-matched neonatal mice (P7-P10). Passive membrane properties were remarkably similar in HMs and SMs, and afterhyperpolarization properties did not differ markedly between the two populations. HMs had narrower action potentials (APs) and a faster upstroke on their APs compared with SMs. Furthermore, HMs discharged APs at higher frequencies in response to both step and ramp current injection than SMs. Therefore, while HMs and SMs have similar passive properties, they differ in their response to similar levels of depolarizing current. This suggests that each population possesses differing suites of ion channels that allow them to discharge at rates matched to the different mechanical properties of the muscle fibers that drive their distinct motor functions.

Subthreshold resonance in biophysically-based models of low- and high-input conductance motoneurons

Resonance in the membrane potential enables a neuron to discriminate its inputs on the basis of their frequency content, so that oscillatory inputs near the resonant frequency produce the largest responses [1]. Recently, Manuel et al. [2] have shown that spinal motoneurons (MNs) exhibit membrane resonance in the frequency range of about 8-14 Hz. Modeling studies of resonance in neural membrane usually use minimal or reduced models, accounting only for the ionic currents sufficient to generate the resonance and plausible to exist in the investigated neuron type. However, in the present study we developed models of low- and a high-input conductance MNs including most of the ionic currents known or hypothesized to exist in mammalian MNs and exhibiting several of the known physiological features. Therefore, instead of investigating resonance as an isolated phenomenon we analyze it as consequence of interactions between the passive properties and the ionic currents in a more realistic fashion. The low-input conductance MN model did not show resonances, whereas the high-input conductance MN model showed resonances dependent on the membrane potential (Figure ​(Figure1).1). These resonances were mainly caused by the hyperpolarization-activated cationic (H) current. However, for membrane potentials near the firing threshold (purple lines in Figure ​Figure1),1), another resonance mainly affected by transient sodium and delayed rectifier currents predominated and its resonance frequency was correlated with the minimum firing rate caused by a step current injection. Figure 1 Dependence of resonance of the high-input conductance MN model on the basal level of injected current. A. somatic potential B. impedance amplitude C. impedance phase D. Quality (Q) factor E. resonance frequency (fr) F. total phase advance (ΦL ...

Decreased Na(+) influx lowers hippocampal neuronal excitability in a mouse model of neonatal influenza infection.

Influenza virus infection is one of common infectious diseases occurring worldwide. The human influenza virus can infect the central nervous system and cause brain dysfunctions affecting cognition and spatial memory. It has been previously shown that infection with the influenza viral protein within the hippocampus decreases Ca2+ influx and reduces excitatory postsynaptic currents. However, the neuronal properties of animals surviving neonatal infection have not been investigated. Using a mouse model of neonatal influenza infection, we performed thorough electrophysiological analyses of hippocampal neurotransmission. We found that animals surviving the infection exhibited reduced spontaneous transmission with no significant defects in evoked neurotransmission. Interestingly, the hippocampus of the infected group conducted synaptic transmission with less fidelity upon repeated stimulations and failed to generate action potentials faithfully upon step current injections primarily due to reduced Na+ influx. The reversal potential for the Na+ current was hyperpolarized and the activation of Na+ channels was slower in the infected group while the inactivation process was minimally disturbed. Taken together, our observations suggest that neonatally infected offsprings exhibit noticeable deficits at rest and severe failures when higher activity is required. This study provides insight into understanding the cellular mechanisms of influenza infection-associated functional changes in the brain.

Modelling the Effects of Electrical Coupling between Unmyelinated Axons of Brainstem Neurons Controlling Rhythmic Activity.

Gap junctions between fine unmyelinated axons can electrically couple groups of brain neurons to synchronise firing and contribute to rhythmic activity. To explore the distribution and significance of electrical coupling, we modelled a well analysed, small population of brainstem neurons which drive swimming in young frog tadpoles. A passive network of 30 multicompartmental neurons with unmyelinated axons was used to infer that: axon-axon gap junctions close to the soma gave the best match to experimentally measured coupling coefficients; axon diameter had a strong influence on coupling; most neurons were coupled indirectly via the axons of other neurons. When active channels were added, gap junctions could make action potential propagation along the thin axons unreliable. Increased sodium and decreased potassium channel densities in the initial axon segment improved action potential propagation. Modelling suggested that the single spike firing to step current injection observed in whole-cell recordings is not a cellular property but a dynamic consequence of shunting resulting from electrical coupling. Without electrical coupling, firing of the population during depolarising current was unsynchronised; with coupling, the population showed synchronous recruitment and rhythmic firing. When activated instead by increasing levels of modelled sensory pathway input, the population without electrical coupling was recruited incrementally to unpatterned activity. However, when coupled, the population was recruited all-or-none at threshold into a rhythmic swimming pattern: the tadpole “decided” to swim. Modelling emphasises uncertainties about fine unmyelinated axon physiology but, when informed by biological data, makes general predictions about gap junctions: locations close to the soma; relatively small numbers; many indirect connections between neurons; cause of action potential propagation failure in fine axons; misleading alteration of intrinsic firing properties. Modelling also indicates that electrical coupling within a population can synchronize recruitment of neurons and their pacemaker firing during rhythmic activity.

Kv2 channels regulate firing rate in pyramidal neurons from rat sensorimotor cortex

The largest outward potassium current in the soma of neocortical pyramidal neurons is due to channels containing Kv2.1 α subunits. These channels have been implicated in cellular responses to seizures and ischaemia, mechanisms for intrinsic plasticity and cell death, and responsiveness to anaesthetic agents. Despite their abundance, knowledge of the function of these delayed rectifier channels has been limited by the lack of specific pharmacological agents. To test for functional roles of Kv2 channels in pyramidal cells from somatosensory or motor cortex of rats (layers 2/3 or 5), we transfected cortical neurons with DNA for a Kv2.1 pore mutant (Kv2.1W365C/Y380T: Kv2.1 DN) in an organotypic culture model to manipulate channel expression. Slices were obtained from rats at postnatal days (P7-P14) and maintained in organotypic culture. We used biolistic methods to transfect neurons with gold 'bullets' coated with DNA for the Kv2.1 DN and green fluorescent protein (GFP), GFP alone, or wild type (WT) Kv2.1 plus GFP. Cells that fluoresced green, contained a bullet and responded to positive or negative pressure from the recording pipette were considered to be transfected cells. In each slice, we recorded from a transfected cell and a control non-transfected cell from the same layer and area. Whole-cell voltage-clamp recordings obtained after 3-7 days in culture showed that cells transfected with the Kv2.1 DN had a significant reduction in outward current (∼45% decrease in the total current density measured 200 ms after onset of a voltage step from -78 to -2 mV). Transfection with GFP alone did not affect current amplitude and overexpression of the Kv2.1 WT resulted in greatly increased currents. Current-clamp experiments were used to assess the functional consequences of manipulation of Kv2.1 expression. The results suggest roles for Kv2 channels in controlling membrane potential during the interspike interval (ISI), firing rate, spike frequency adaptation (SFA) and the steady-state gain of firing. Specifically, firing rate and gain were reduced in the Kv2.1 DN cells. The most parsimonious explanation for the effects on firing is that in the absence of Kv2 channels, the membrane remains depolarized during the ISIs, preventing recovery of Na(+) channels from inactivation. Depolarization and the number of inactivated Na(+) channels would build with successive spikes, resulting in slower firing and enhanced spike frequency adaptation in the Kv2.1 DN cells.

Membrane Properties of Striatal Direct and Indirect Pathway Neurons in Mouse and Rat Slices and Their Modulation by Dopamine

D1 and D2 receptor expressing striatal medium spiny neurons (MSNs) are ascribed to striatonigral (“direct”) and striatopallidal (“indirect”) pathways, respectively, that are believed to function antagonistically in motor control. Glutamatergic synaptic transmission onto the two types is differentially affected by Dopamine (DA), however, less is known about the effects on MSN intrinsic electrical properties. Using patch clamp recordings, we comprehensively characterized the two pathways in rats and mice, and investigated their DA modulation. We identified the direct pathway by retrograde labeling in rats, and in mice we used transgenic animals in which EGFP is expressed in D1 MSNs. MSNs were subjected to a series of current injections to pinpoint differences between the populations, and in mice also following bath application of DA. In both animal models, most electrical properties were similar, however, membrane excitability as measured by step and ramp current injections consistently differed, with direct pathway MSNs being less excitable than their counterparts. DA had opposite effects on excitability of D1 and D2 MSNs, counteracting the initial differences. Pronounced changes in AP shape were seen in D2 MSNs. In direct pathway MSNs, excitability increased across experimental conditions and parameters, and also when applying DA or the D1 agonist SKF-81297 in presence of blockers of cholinergic, GABAergic, and glutamatergic receptors. Thus, DA induced changes in excitability were D1 R mediated and intrinsic to direct pathway MSNs, and not a secondary network effect of altered synaptic transmission. DAergic modulation of intrinsic properties therefore acts in a synergistic manner with previously reported effects of DA on afferent synaptic transmission and dendritic processing, supporting the antagonistic model for direct vs. indirect striatal pathway function.

Hippocampal Pyramidal Neurons Comprise Two Distinct Cell Types that Are Countermodulated by Metabotropic Receptors

Relating the function of neuronal cell types to information processing and behavior is a central goal of neuroscience. In the hippocampus, pyramidal cells in CA1 and the subiculum process sensory and motor cues to form a cognitive map encoding spatial, contextual, and emotional information, which they transmit throughout the brain. Do these cells constitute a single class or are there multiple cell types with specialized functions? Using unbiased cluster analysis, we show that there are two morphologically and electrophysiologically distinct principal cell types that carry hippocampal output. We show further that these two cell types are inversely modulated by the synergistic action of glutamate and acetylcholine acting on metabotropic receptors that are central to hippocampal function. Combined with prior connectivity studies, our results support a model of hippocampal processing in which the two pyramidal cell types are predominantly segregated into two parallel pathways that process distinct modalities of information.

Bistable persistent spiking of layer II and layer V medial entorhinal cortical neurons during theta frequency oscillations in vitro

In the presence of muscarinic cholinergic modulation in vitro, persistent spiking neurons of medial entorhinal cortex (mEC) exhibit bistability in that they can be switched from quiescence to spiking by a brief depolarizing input. The spiking state is thought to be maintained by a positive feedback loop between a nonspecific, calcium-sensitive cationic current (ICAN) carried by canonical transient receptor potential (TRPC) channels and heightened intracellular calcium levels sustained during sufficiently fast spiking. Bistable persistent spiking (PSB) has been observed across layers and cell types in mEC and has been implicated in working memory and grid cell function, so cholinergic modulation of TRPC channels represents a potentially fundamental mechanism shaping the spiking output of mEC neurons. However, spiking of mEC neurons during navigation and goal directed behavior in vivo is strongly theta modulated (4-12 Hz), and it is not known how the PSB mechanism may interact with theta oscillations. In the present study we evaluated the effect of muscarinic modulation on input-output processing of mEC neurons to tonic, slowly varying, and oscillatory inputs. First, we investigated the frequency to injected current (FI) relationship using step and ramp current injection protocols in the presence of the muscarinic agonist carbachol (CCh). FI measures made following suprathreshold pre-pulse current injections exhibited higher spike frequencies than when measured following subthreshold pre-pulses, illustrating a spike-history dependence of the FI relationship. The instantaneous spike frequency measured during a range of current steps showed a positive slope for most PSB neurons, indicating that ICAN acts as a reverse spike frequency adaptation mechanism. In response to linearly increasing ramp current stimuli, mEC neurons showed a steep acceleration followed by a plateau of spike frequency. Spike frequency hysteresis during decreasing ramp stimuli demonstrated significant adaptation for higher input levels, but spiking was often maintained at lower levels of injected current. Next, using different combinations of theta frequency sinewave current injections and DC holding currents, we were able to generate a range of spiking patterns where spikes were elicited on different proportions of theta cycles and at different phases of the theta input. With muscarinic activation, spiking was elicited on a higher proportion of theta cycles and at earlier phases of the oscillatory input for lower levels of tonic current input. A subset of recorded neurons exhibitted repeated sequences of intrinsic phase precessing spiking behavior. Similar to the plateau in spike rate observed with ramp stimuli, in response to stimuli composed of sinewaves superimposed on ramps, spiking phase rapidly advanced and maintained an ‘early-phase plateau’ as DC current continued to increase. Taken together, we demonstrate that muscarinic modulation of TRPC channels determines the gain of the frequency to injected current relationship of mEC neurons, interacts with adaptation mechanisms to generate spike history dependencies of spike rate including hysteresis, and shapes the distribution of spiking phases relative to oscillatory input. We propose that increasing activation of ICAN by spike-triggered calcium influx as an animal enters a firing field may provide a cellular mechanism contributing to theta phase precession of mEC grid cells.

A high-level, simulator independent, Python library for simulating small networks of multicompartmental neurons

We present a high level, simulator independent, Python library for building simulations of small populations of multi-compartmental neurons, in which membrane voltage is calculated from individual ionic current flows. The API allows the user to quickly construct simulations of small populations of neurons and synaptic connections, with particular focus on: (1) allowing simulation specification, with visualisation and analysis in a minimal, clean, human readable language; (2) reduction of complex simulation tool chains to a single python script; (3) promoting reproducible research through automatic self-documentation; (4) encourage the re-use of morphologies, neurons and channels in a way that specific and stochastic variation in parameters is simple; (5) transparent handling of different units; (6) allowing the use of established formats, (e.g. NEURON nmodl files[1]); (7) simplifying the definition and sharing of new channel types, including the possibility to support other libraries and standards easily (e.g. PyNN[2], 9ML, NeuroML[3]). Consider the following example: “We would like to build a network, of 30 neurons. Each soma should be 20(± 3) µm diameter, and axons are 1000µm long, have a hillock of radius 5um for the first 10µm, then taper to a radius 0.3µm. We will use a single compartment for the soma, divide the hillock into compartments of length 1um, and the axon into 100 compartments. We will use the sodium and potassium voltage-gated channels as specified by the equations and parameters in paper X, but use 3 times the density of sodium channels on the hillock. The neurons should be connected by gap junctions, distributed according to algorithm Y, with resistances of 1±0.5G Ohm. The neurons also excite other neurons in the population with AMPA synapses, with a probability of connections between any pair of 20%. We will stimulate this network with NMDA synaptic input to ~50% of the neurons at fixed times, as well as different levels of step current injections to cell 12, and plot graphs of the ionic currents flowing across the membrane of cells 2, 12, and any cells that spike, as well as the membrane voltage of all the neurons. We want to compare with the case when the gap junctions have been blocked.” The library allows the specification of this complete simulation and visualisation of the results in a single Python file of less than 100 lines, only requiring a single command to run. It is designed to make it possible to specify connection algorithms, morphologies, channel descriptions and simulation parameters as high-level objects, such that they can be reused in other simulations, without the need to copy and paste code. Units are handled transparently, reports of the simulation setup are automatically generated, and a clean separation between of simulation and visualisation is achieved through a mechanism of 'tagging' in the framework. The library is based on standard Python libraries, (numpy, matplotlib), making it easy to integrate with other libraries. The framework is under active development, with a NEURON backend, currently supporting everything in the above example. A first release is intended for July 2012.

Are all spinal segments equal: intrinsic membrane properties of superficial dorsal horn neurons in the developing and mature mouse spinal cord

Neurons in the superficial dorsal horn (SDH; laminae I-II) of the spinal cord process nociceptive information from skin, muscle, joints and viscera. Most of what we know about the intrinsic properties of SDH neurons comes from studies in lumbar segments of the cord even though clinical evidence suggests nociceptive signals from viscera and head and neck tissues are processed differently. This ‘lumbar-centric' view of spinal pain processing mechanisms also applies to developing SDH neurons. Here we ask whether the intrinsic membrane properties of SDH neurons differ across spinal cord segments in both the developing and mature spinal cord. Whole cell recordings were made from SDH neurons in slices of upper cervical (C2-4), thoracic (T8-10) and lumbar (L3-5) segments in neonatal (P0-5) and adult (P24-45) mice. Neuronal input resistance (R(IN)), resting membrane potential, AP amplitude, half-width and AHP amplitude were similar across spinal cord regions in both neonates and adults (∼100 neurons for each region and age). In contrast, these intrinsic membrane properties differed dramatically between neonates and adults. Five types of AP discharge were observed during depolarizing current injection. In neonates, single spiking dominated (∼40%) and the proportions of each discharge category did not differ across spinal regions. In adults, initial bursting dominated in each spinal region, but was significantly more prevalent in rostral segments (49% of neurons in C2-4 vs. 29% in L3-5). During development the dominant AP discharge pattern changed from single spiking to initial bursting. The rapid A-type potassium current (I(Ar)) dominated in neonates and adults, but its prevalence decreased (∼80% vs. ∼50% of neurons) in all regions during development. I(Ar) steady state inactivation and activation also changed in upper cervical and lumbar regions during development. Together, our data show the intrinsic properties of SDH neurons are generally conserved in the three spinal cord regions examined in both neonate and adult mice. We propose the conserved intrinsic membrane properties of SDH neurons along the length of the spinal cord cannot explain the marked differences in pain experienced in the limbs, viscera, and head and neck.

Acoustic over-exposure triggers burst firing in dorsal cochlear nucleus fusiform cells

Acoustic over-exposure (AOE) triggers deafness in animals and humans and provokes auditory nerve degeneration. Weeks after exposure there is an increase in the cellular excitability within the dorsal cochlear nucleus (DCN) and this is considered as a possible neural correlate of tinnitus. The origin of this DCN hyperactivity phenomenon is still unknown but it is associated with neurons lying within the fusiform cell layer. Here we investigated changes of excitability within identified fusiform cells following AOE. Wistar rats were exposed to a loud (110 dB SPL) single tone (14.8 kHz) for 4 h. Auditory brainstem response recordings performed 3–4 days after AOE showed that the hearing thresholds were significantly elevated by about 20–30 dB SPL for frequencies above 15 kHz. Control fusiform cells fired with a regular firing pattern as assessed by the coefficient of variation of the inter-spike interval distribution of 0.19 ± 0.11 (n = 5). Three to four days after AOE, 40% of fusiform cells exhibited irregular bursting discharge patterns (coefficient of variation of the inter-spike interval distribution of 1.8 ± 0.6, n = 5; p < 0.05). Additionally the maximal firing following step current injections was reduced in these cells (from 83 ± 11 Hz, n = 5 in unexposed condition to 43 ± 6 Hz, n = 5 after AOE) and this was accompanied by an increased firing gain (from 0.09 ± 0.01 Hz/pA, n = 5 in unexposed condition to 0.56 ± 0.25 Hz/pA, n = 5 after AOE). Current and voltage clamp recordings suggest that the presence of bursts in fusiform cells is related to a down regulation of high voltage activated potassium currents.In conclusion we showed that AOE triggers deafness at early stages and this is correlated with profound changes in the firing pattern and frequency of the DCN major output fusiform cells. The changes here described could represent the initial network imbalance prior to the emergence of tinnitus.

In vivo conditions induce faithful encoding of stimuli by reducing nonlinear synchronization in vestibular sensory neurons.

Previous studies have shown that neurons within the vestibular nuclei (VN) can faithfully encode the time course of sensory input through changes in firing rate in vivo. However, studies performed in vitro have shown that these same VN neurons often display nonlinear synchronization (i.e. phase locking) in their spiking activity to the local maxima of sensory input, thereby severely limiting their capacity for faithful encoding of said input through changes in firing rate. We investigated this apparent discrepancy by studying the effects of in vivo conditions on VN neuron activity in vitro using a simple, physiologically based, model of cellular dynamics. We found that membrane potential oscillations were evoked both in response to step and zap current injection for a wide range of channel conductance values. These oscillations gave rise to a resonance in the spiking activity that causes synchronization to sinusoidal current injection at frequencies below 25 Hz. We hypothesized that the apparent discrepancy between VN response dynamics measured in in vitro conditions (i.e., consistent with our modeling results) and the dynamics measured in vivo conditions could be explained by an increase in trial-to-trial variability under in vivo vs. in vitro conditions. Accordingly, we mimicked more physiologically realistic conditions in our model by introducing a noise current to match the levels of resting discharge variability seen in vivo as quantified by the coefficient of variation (CV). While low noise intensities corresponding to CV values in the range 0.04–0.24 only eliminated synchronization for low (<8 Hz) frequency stimulation but not high (>12 Hz) frequency stimulation, higher noise intensities corresponding to CV values in the range 0.5–0.7 almost completely eliminated synchronization for all frequencies. Our results thus predict that, under natural (i.e. in vivo) conditions, the vestibular system uses increased variability to promote fidelity of encoding by single neurons. This prediction can be tested experimentally in vitro.

Slow integration leads to persistent action potential firing in distal axons of coupled interneurons

The conventional view of neurons is that synaptic inputs are integrated on a timescale of milliseconds to seconds in the dendrites, with action potential initiation occurring in the axon initial segment. In a subset of rodent hippocampal and neocortical interneurons in acute slices prepared from serotonin 5b receptor (Htr5b) BAC transgenic mice [1], we found a much slower form of integration leading to action potential initiation in the distal axon. In approximately 80% of these interneurons (n=214 of 274), and in 23% of hippocampal interneurons in wild-type C57BL/6 mice (n=6 of 26), hundreds of spikes, evoked over a period of minutes, resulted in persistent firing that lasted for a similar duration. Persistent firing was observed in response to step current injections, synaptic stimulation, sine wave current injections or in response to stimulation with natural spike trains [2]. With all of these protocols, multiple stimuli were required to induce persistent firing. While axonal action potential firing was required to trigger persistent firing, somatic depolarization was not; antidromic stimulation of the axon while hyperpolarizing the soma with current injection produced persistent firing. In addition, phase plots of persistent firing revealed that spikes had two components: an initial component represented spiking in the axon and a second component that overlapped with the current-evoked spikes, indicative of a somato-dendritic spike following an initial, axonally initiated spike. In some recordings (n = 11), partial spikes (spikelets) were observed during persistent firing. These spikelets overlapped the first component of the full-amplitude spikes, with the peak of the spikelets corresponding to an inflection on the rising phase seen in the full-amplitude spikes. These observations suggest that the first component of each action potential during persistent firing is an axonal spike, which sometimes fails to evoke a somato-dendritic spike. Furthermore, in some cells (n=3), spikelets were observed if the soma was hyperpolarized during persistent firing. These spikelets were smaller than those observed without hyperpolarization, suggesting that they are caused by propagation failures at a more distal axonal location. Using a stylized computational model constructed with the NEURON simulation environment [3] of a branching axon attached to a soma, we simulated both small- and large amplitude spikelets, as well as full-amplitude spikes, by depolarizing a branch of the axon during somatic hyperpolarization. Large-amplitude spikelets corresponded to failure of the action potential to invade the soma, whereas small-amplitude spikelets corresponded to failures at the axon branches, 40 μm from the soma. Similar results were obtained with a full morphological model of a branching axonal arborization. Additionally, in paired recordings, persistent firing was not restricted to the stimulated neuron; it could also be produced in the unstimulated cell (n=3). None of these pairs exhibited direct electrical coupling, and both glutamate and GABA receptors were blocked. Consolidating these results suggests the existence of a previously unknown operational mode for some mammalian neurons. These interneurons can slowly integrate spiking, share the output across a coupled network of axons and respond with persistent firing even in the absence of input to the soma or dendrites.

Modeling UBC intrinsic excitability

Unipolar brush cells (UBCs) are excitatory glutamatergic interneurons of the cerebellar granular layer receiving both primary and secondary vestibular inputs through mossy fibers (excitatory input) and Golgi axon (inhibitory input). The brush like structure of the dendrite allows to form a giant synapses in the glomerulus and to produce an all or none post synaptic response with short delay and protracted kinetics. The excitable response of UBCs can be either a tonic discharge or a high-frequency burst of action potentials. When injected with progressively increasing depolarizing currents from a negative membrane potential, the UBC generates a burst sustained by a calcium spike (Figure ​(Figure1,1, +20pA) and then a protracted discharge with shorter latency and spike frequency adaption (Figure ​(Figure1,1, +25pA). The intrinsic excitability of UBCs is determined by an H current and by Low Voltage activated and High Voltage activated calcium currents [2,3]. Fast inactivating T-type Calcium channels generate low-threshold spikes and L-type Calcium channel sustain tonic firing. The H current (activated between -60mV and -80mV) produces a slow hyperpolarization characterized by a “sag” in response to a hyperpolarizing step (Figure ​(Figure1,1, -16pA) and an afterhyperpolarization at the end of a depolarizing step. Figure 1 Excitable response of the UBC model to step-current injection.

Early dating influences long‐term synaptic partnerships

Early dating influences long‐term synaptic partnerships

Excitability of paraventricular nucleus neurones that project to the rostral ventrolateral medulla is regulated by small‐conductance Ca2+‐activated K+ channels

Whole cell patch-clamp recordings were performed in brain slices to investigate mechanisms regulating the excitability of paraventricular nucleus (PVN) neurones that project directly to the rostral ventrolateral medulla (RVLM) (PVN-RVLM neurones) of rats. In voltage-clamp recordings, step depolarization elicited a calcium-dependent outward tail current that reversed near E(K). The current was nearly abolished by apamin and by UCL1684, suggesting mediation by small-conductance Ca(2+)-activated K(+) (SK) channels. In current-clamp recordings, depolarizing step current injections evoked action potentials that underwent spike-frequency adaptation (SFA). SK channel blockade with apamin or UCL1684 increased the spike frequency without changing the rate of SFA. Upon termination of step current injection, a prominent medium after-hyperpolarization potential (mAHP) was observed. SK channel blockade abolished the mAHP and revealed an after-depolarization potential (ADP). In response to ramp current injections, the rate of sub-threshold depolarization was increased during SK channel blockade, indicating that depolarizing input resistance was increased. Miniature EPSC frequency, amplitude, and decay kinetics were unaltered by bath application of apamin, suggesting that SK channel blockade likely increased excitability by a postsynaptic action. We conclude that although SK channels play little role in generating SFA in PVN-RVLM neurones, their activation nevertheless does dampen excitability. The mechanism appears to involve activation of a mAHP that opposes a prominent ADP that would otherwise facilitate firing.

Hyped up about the hypothalamus

Hyped up about the hypothalamus