Membrane potential-dependent modulation of recurrent inhibition in rat neocortex

Responses of Cortical Neurons to Intracortical Microstimulation in Awake Primates

Intracortical microstimulation (ICMS) is commonly used in many experimental and clinical paradigms; however, its effects on the activation of neurons are still not completely understood. To document the responses of cortical neurons in awake nonhuman primates to stimulation, we recorded single-unit activity while delivering single-pulse stimulation via Utah arrays implanted in primary motor cortex (M1) of three macaque monkeys. Stimuli between 5 and 50 μA delivered to single channels reliably evoked spikes in neurons recorded throughout the array with delays of up to 12 ms. ICMS pulses also induced a period of inhibition lasting up to 150 ms that typically followed the initial excitatory response. Higher current amplitudes led to a greater probability of evoking a spike and extended the duration of inhibition. The likelihood of evoking a spike in a neuron was dependent on the spontaneous firing rate as well as the delay between its most recent spike time and stimulus onset. Tonic repetitive stimulation between 2 and 20 Hz often modulated both the probability of evoking spikes and the duration of inhibition; high-frequency stimulation was more likely to change both responses. On a trial-by-trial basis, whether a stimulus evoked a spike did not affect the subsequent inhibitory response; however, their changes over time were often positively or negatively correlated. Our results document the complex dynamics of cortical neural responses to electrical stimulation that need to be considered when using ICMS for scientific and clinical applications.

BackEISNN: A deep spiking neural network with adaptive self-feedback and balanced excitatory-inhibitory neurons

Spiking neural networks (SNNs) transmit information through discrete spikes that perform well in processing spatial-temporal information. Owing to their nondifferentiable characteristic, difficulties persist in designing SNNs that deliver good performance. SNNs trained with backpropagation have recently exhibited impressive performance by using gradient approximation. However, their performance on complex tasks remains significantly inferior to that of deep neural networks. By taking inspiration from autapses in the brain that connect spiking neurons with a self-feedback connection, we apply adaptive time-delayed self-feedback to the membrane potential to regulate the precision of the spikes. We also strike a balance between the excitatory and inhibitory mechanisms of neurons to dynamically control the output of spiking neurons. By combining these two mechanisms, we propose a deep SNN with adaptive self-feedback and balanced excitatory and inhibitory neurons (BackEISNN). The results of experiments on several standard datasets show that the two modules not only accelerate the convergence of the network but also increase its accuracy. Our model achieved state-of-the-art performance on the MNIST, Fashion-MNIST, and N-MNIST datasets. The proposed BackEISNN also achieved remarkably good performance on the CIFAR10 dataset while using a relatively light structure that competes against state-of-the-art SNNs.

Spatial and temporal heterogeneity in the lineage progression of fine oligodendrocyte subtypes

Background

Oligodendrocytes are glial cells that support and insulate axons in the central nervous system through the production of myelin. Oligodendrocytes arise throughout embryonic and early postnatal development from oligodendrocyte precursor cells (OPCs), and recent work demonstrated that they are a transcriptional heterogeneous cell population, but the regional and functional implications of this heterogeneity are less clear. Here, we apply in situ sequencing (ISS) to simultaneously probe the expression of 124 marker genes of distinct oligodendrocyte populations, providing comprehensive maps of the corpus callosum, cingulate, motor, and somatosensory cortex in the brain, as well as gray matter (GM) and white matter (WM) regions in the spinal cord, at postnatal (P10), juvenile (P20), and young adult (P60) stages. We systematically compare the abundances of these populations and investigate the neighboring preference of distinct oligodendrocyte populations.

Results

We observed that oligodendrocyte lineage progression is more advanced in the juvenile spinal cord compared to the brain, corroborating with previous studies. We found myelination still ongoing in the adult corpus callosum while it was more advanced in the cortex. Interestingly, we also observed a lateral-to-medial gradient of oligodendrocyte lineage progression in the juvenile cortex, which could be linked to arealization, as well as a deep-to-superficial gradient with mature oligodendrocytes preferentially accumulating in the deeper layers of the cortex. The ISS experiments also exposed differences in abundances and population dynamics over time between GM and WM regions in the brain and spinal cord, indicating regional differences within GM and WM, and we found that neighboring preferences of some oligodendroglia populations are altered from the juvenile to the adult CNS.

Conclusions

Overall, our ISS experiments reveal spatial heterogeneity of oligodendrocyte lineage progression in the brain and spinal cord and uncover differences in the timing of oligodendrocyte differentiation and myelination, which could be relevant to further investigate functional heterogeneity of oligodendroglia, especially in the context of injury or disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01325-z.

Prefrontal Cortical Control of Anxiety: Recent Advances

Dysfunction in the prefrontal cortex is commonly implicated in anxiety disorders, but the mechanisms remain unclear. Approach-avoidance conflict tasks have been extensively used in animal research to better understand how changes in neural activity within the prefrontal cortex contribute to avoidance behaviors, which are believed to play a major role in the maintenance of anxiety disorders. In this article, we first review studies utilizing in vivo electrophysiology to reveal the relationship between changes in neural activity and avoidance behavior in rodents. We then review recent studies that take advantage of optical and genetic techniques to test the unique contribution of specific prefrontal cortex circuits and cell types to the control of anxiety-related avoidance behaviors. This new body of work reveals that behavior during approach-avoidance conflict is dynamically modulated by individual cell types, distinct neural pathways, and specific oscillatory frequencies. The integration of these different pathways, particularly as mediated by interactions between excitatory and inhibitory neurons, represents an exciting opportunity for the future of understanding anxiety.

Past and Future of Analog-Digital Modulation of Synaptic Transmission

Action potentials (APs) are generally produced in response to complex summation of excitatory and inhibitory synaptic inputs. While it is usually considered as a digital event, both the amplitude and width of the AP are significantly impacted by the context of its emission. In particular, the analog variations in subthreshold membrane potential determine the spike waveform and subsequently affect synaptic strength, leading to the so-called analog-digital modulation of synaptic transmission. We review here the numerous evidence suggesting context-dependent modulation of spike waveform, the discovery analog-digital modulation of synaptic transmission in invertebrates and its recent validation in mammals. We discuss the potential roles of analog-digital transmission in the physiology of neural networks.

Lateral inhibition by Martinotti interneurons is facilitated by cholinergic inputs in human and mouse neocortex

A variety of inhibitory pathways encompassing different interneuron types shape activity of neocortical pyramidal neurons. While basket cells (BCs) mediate fast lateral inhibition between pyramidal neurons, Somatostatin-positive Martinotti cells (MCs) mediate a delayed form of lateral inhibition. Neocortical circuits are under control of acetylcholine, which is crucial for cortical function and cognition. Acetylcholine modulates MC firing, however, precisely how cholinergic inputs affect cortical lateral inhibition is not known. Here, we find that cholinergic inputs selectively augment and speed up lateral inhibition between pyramidal neurons mediated by MCs, but not by BCs. Optogenetically activated cholinergic inputs depolarize MCs through activation of ß2 subunit-containing nicotinic AChRs, not muscarinic AChRs, without affecting glutamatergic inputs to MCs. We find that these mechanisms are conserved in human neocortex. Cholinergic inputs thus enable cortical pyramidal neurons to recruit more MCs, and can thereby dynamically highlight specific circuit motifs, favoring MC-mediated pathways over BC-mediated pathways.

Parvalbumin and somatostatin expressing interneurons mediate lateral inhibition between cortical neurons. Here the authors report the mechanisms by which acetylcholine from the basal forebrain selectively augments lateral inhibition via Martinotti cells and show that this is conserved in humans.

Long-latency suppression of auditory and somatosensory change-related cortical responses

Sensory suppression is a mechanism that attenuates selective information. As for long-latency suppression in auditory and somatosensory systems, paired-pulse suppression, observed as 2 identical stimuli spaced by approximately 500 ms, is widely known, though its mechanism remains to be elucidated. In the present study, we investigated the relationship between auditory and somatosensory long-latency suppression of change-related cortical responses using magnetoencephalography. Somatosensory change-related responses were evoked by an abrupt increase in stimulus strength in a train of current-constant square wave pulses at 100 Hz to the left median nerve at the wrist. Furthermore, auditory change-related responses were elicited by an increase in sound pressure by 15 dB in a continuous sound composed of a train of 25-ms pure tones. Binaural stimulation was used in Experiment 1, while monaural stimulation was used in Experiment 2. For both somatosensory and auditory stimuli, the conditioning and test stimuli were identical, and inserted at 2400 and 3000 ms, respectively. The results showed clear suppression of the test response in the bilateral parisylvian region, but not in the postcentral gyrus of the contralateral hemisphere in the somatosensory system. Similarly, the test response in the bilateral supratemporal plane (N100m) was suppressed in the auditory system. Furthermore, there was a significant correlation between suppression of right N100m and right parisylvian activity, suggesting that similar mechanisms are involved in both. Finally, a high test-retest reliability for suppression was seen with both modalities. Suppression revealed in the present study is considered to reflect sensory inhibition ability in individual subjects.

Rapid State-Dependent Alteration in Kv3 Channel Availability Drives Flexible Synaptic Signaling Dependent on Somatic Subthreshold Depolarization

In many neurons, subthreshold depolarization in the soma can transiently increase action-potential (AP)-evoked neurotransmission via analog-to-digital facilitation. The mechanisms underlying this form of short-term synaptic plasticity are unclear, in part, due to the relative inaccessibility of the axon to direct physiological interrogation. Using voltage imaging and patch-clamp recording from presynaptic boutons of cerebellar stellate interneurons, we observed that depolarizing somatic potentials readily spread into the axon, resulting in AP broadening, increased spike-evoked Ca²⁺ entry, and enhanced neurotransmission strength. K_v3 channels, which drive AP repolarization, rapidly inactivated upon incorporation of K_v3.4 subunits. This leads to fast susceptibility to depolarization-induced spike broadening and analog facilitation independent of Ca²⁺-dependent protein kinase C signaling. The spread of depolarization into the axon was attenuated by hyperpolarization-activated currents (I_h currents) in the maturing cerebellum, precluding analog facilitation. These results suggest that analog-to-digital facilitation is tempered by development or experience in stellate cells.

Activity dependent feedback inhibition may maintain head direction signals in mouse presubiculum

Orientation in space is represented in specialized brain circuits. Persistent head direction signals are transmitted from anterior thalamus to the presubiculum, but the identity of the presubicular target neurons, their connectivity and function in local microcircuits are unknown. Here, we examine how thalamic afferents recruit presubicular principal neurons and Martinotti interneurons, and the ensuing synaptic interactions between these cells. Pyramidal neuron activation of Martinotti cells in superficial layers is strongly facilitating such that high-frequency head directional stimulation efficiently unmutes synaptic excitation. Martinotti-cell feedback plays a dual role: precisely timed spikes may not inhibit the firing of in-tune head direction cells, while exerting lateral inhibition. Autonomous attractor dynamics emerge from a modelled network implementing wiring motifs and timing sensitive synaptic interactions in the pyramidal—Martinotti-cell feedback loop. This inhibitory microcircuit is therefore tuned to refine and maintain head direction information in the presubiculum.

Head direction is encoded by cells in the presubiculum, but the role of local circuitry in head direction encoding remains unknown. Here the authors demonstrate how a specific inhibitory neuron type, the Martinotti cell, together with excitatory pyramidal cells supports head direction signals.

Mapping Horizontal Spread of Activity in Monkey Motor Cortex Using Single Pulse Microstimulation

Anatomical studies have demonstrated that distant cortical points are interconnected through long range axon collaterals of pyramidal cells. However, the functional properties of these intrinsic synaptic connections, especially their relationship with the cortical representations of body movements, have not been systematically investigated. To address this issue, we used multielectrode arrays chronically implanted in the motor cortex of two rhesus monkeys to analyze the effects of single-pulse intracortical microstimulation (sICMS) applied at one electrode on the neuronal activities recorded at all other electrodes. The temporal and spatial distribution of the evoked responses of single and multiunit activities was quantified to determine the properties of horizontal propagation. The typical responses were characterized by a brief excitatory peak followed by inhibition of longer duration. Significant excitatory responses to sICMS could be evoked up to 4 mm away from the stimulation site, but the strength of the response decreased exponentially and its latency increased linearly with the distance. We then quantified the direction and strength of the propagation in relation to the somatotopic organization of the motor cortex. We observed that following sICMS the propagation of neural activity is mainly directed rostro-caudally near the central sulcus but follows medio-lateral direction at the most anterior electrodes. The fact that these interactions are not entirely symmetrical may characterize a critical functional property of the motor cortex for the control of upper limb movements. Overall, these results support the assumption that the motor cortex is not functionally homogeneous but forms a complex network of interacting subregions.

Dynamic Control of Neurotransmitter Release by Presynaptic Potential

Action potentials (APs) in the mammalian brain are thought to represent the smallest unit of information transmitted by neurons to their postsynaptic targets. According to this view, neuronal signaling is all-or-none or digital. Increasing evidence suggests, however, that subthreshold changes in presynaptic membrane potential before triggering the spike also determines spike-evoked release of neurotransmitter. We discuss here how analog changes in presynaptic voltage may regulate spike-evoked release of neurotransmitter through the modulation of biophysical state of voltage-gated potassium, calcium and sodium channels in the presynaptic compartment. The contribution of this regulation has been greatly underestimated and we discuss the impact for information processing in neuronal circuits.

Analogue modulation of back-propagating action potentials enables dendritic hybrid signalling

We report that back-propagating action potentials (bAPs) are not simply digital feedback signals in dendrites but also carry analogue information about the overall state of neurons. Analogue information about the somatic membrane potential within a physiological range (from -78 to -64 mV) is retained by bAPs of dentate gyrus granule cells as different repolarization speeds in proximal dendrites and as different peak amplitudes in distal regions. These location-dependent waveform changes are reflected by local calcium influx, leading to proximal enhancement and distal attenuation during somatic hyperpolarization. The functional link between these retention and readout mechanisms of the analogue content of bAPs critically depends on high-voltage-activated, inactivating calcium channels. The hybrid bAP and calcium mechanisms report the phase of physiological somatic voltage fluctuations and modulate long-term synaptic plasticity in distal dendrites. Thus, bAPs are hybrid signals that relay somatic analogue information, which is detected by the dendrites in a location-dependent manner.

Cortical Interneuron Subtypes Vary in Their Axonal Action Potential Properties

The role of interneurons in cortical microcircuits is strongly influenced by their passive and active electrical properties. Although different types of interneurons exhibit unique electrophysiological properties recorded at the soma, it is not yet clear whether these differences are also manifested in other neuronal compartments. To address this question, we have used voltage-sensitive dye to image the propagation of action potentials into the fine collaterals of axons and dendrites in two of the largest cortical interneuron subtypes in the mouse: fast-spiking interneurons, which are typically basket or chandelier neurons; and somatostatin containing interneurons, which are typically regular spiking Martinotti cells. We found that fast-spiking and somatostatin-expressing interneurons differed in their electrophysiological characteristics along their entire dendrosomatoaxonal extent. The action potentials generated in the somata and axons, including axon collaterals, of somatostatin-expressing interneurons are significantly broader than those generated in the same compartments of fast-spiking inhibitory interneurons. In addition, action potentials back-propagated into the dendrites of somatostatin-expressing interneurons much more readily than fast-spiking interneurons. Pharmacological investigations suggested that axonal action potential repolarization in both cell types depends critically upon Kv1 channels, whereas the axonal and somatic action potentials of somatostatin-expressing interneurons also depend on BK Ca(2+)-activated K(+) channels. These results indicate that the two broad classes of interneurons studied here have expressly different subcellular physiological properties, allowing them to perform unique computational roles in cortical circuit operations.

SIGNIFICANCE STATEMENT

Neurons in the cerebral cortex are of two major types: excitatory and inhibitory. The proper balance of excitation and inhibition in the brain is critical for its operation. Neurons contain three main compartments: dendritic, somatic, and axonal. How the neurons receive information, process it, and pass on new information depends upon how these three compartments operate. While it has long been assumed that axons are simply for conducting information from the cell body to the synapses, here we demonstrate that the axons of different types of interneurons, the inhibitory cells, possess differing electrophysiological properties. This result implies that differing types of interneurons perform different tasks in the cortex, not only through their anatomical connections, but also through how their axons operate.

Presynaptic hyperpolarization induces a fast analogue modulation of spike-evoked transmission mediated by axonal sodium channels

In the mammalian brain, synaptic transmission usually depends on presynaptic action potentials (APs) in an all-or-none (or digital) manner. Recent studies suggest, however, that subthreshold depolarization in the presynaptic cell facilitates spike-evoked transmission, thus creating an analogue modulation of a digital process (or analogue–digital (AD) modulation). At most synapses, this process is slow and not ideally suited for the fast dynamics of neural networks. We show here that transmission at CA3–CA3 and L5–L5 synapses can be enhanced by brief presynaptic hyperpolarization such as an inhibitory postsynaptic potential (IPSP). Using dual soma–axon patch recordings and live imaging, we find that this hyperpolarization-induced AD facilitation (h-ADF) is due to the recovery from inactivation of Nav channels controlling AP amplitude in the axon. Incorporated in a network model, h-ADF promotes both pyramidal cell synchrony and gamma oscillations. In conclusion, cortical excitatory synapses in local circuits display hyperpolarization-induced facilitation of spike-evoked synaptic transmission that promotes network synchrony.

'Digital' spike-evoked transmission can be facilitated by slow subthreshold 'analogue' depolarisation of the presynaptic neuron. Here, the authors identify a novel, rapid form of digital-analogue facilitation in mammalian neurons whereby presynaptic hyperpolarisation enables de-inactivation of axonal Nav channels.

Anatomy and physiology of the thick-tufted layer 5 pyramidal neuron

The thick-tufted layer 5 (TTL5) pyramidal neuron is one of the most extensively studied neuron types in the mammalian neocortex and has become a benchmark for understanding information processing in excitatory neurons. By virtue of having the widest local axonal and dendritic arborization, the TTL5 neuron encompasses various local neocortical neurons and thereby defines the dimensions of neocortical microcircuitry. The TTL5 neuron integrates input across all neocortical layers and is the principal output pathway funneling information flow to subcortical structures. Several studies over the past decades have investigated the anatomy, physiology, synaptology, and pathophysiology of the TTL5 neuron. This review summarizes key discoveries and identifies potential avenues of research to facilitate an integrated and unifying understanding on the role of a central neuron in the neocortex.

Modulation of spike-evoked synaptic transmission: The role of presynaptic calcium and potassium channels

Action potentials are usually considered as the smallest unit of neuronal information conveyed by presynaptic neurons to their postsynaptic target. Thus, neuronal signaling in brain circuits is all-or-none or digital. However, recent studies indicate that subthreshold analog variation in presynaptic membrane potential modulates spike-evoked transmission. The informational content of each presynaptic action potential is therefore greater than initially expected. This property constitutes a form of fast activity-dependent modulation of functional coupling. Therefore, it could have important consequences on information processing in neural networks in parallel with more classical forms of presynaptic short-term facilitation based on repetitive stimulation, modulation of presynaptic calcium or modifications of the release machinery. We discuss here how analog voltage shift in the presynaptic neuron may regulate spike-evoked release of neurotransmitter through the modulation of voltage-gated calcium and potassium channels in the axon and presynaptic terminal. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Action potential modulation in CA1 pyramidal neuron axons facilitates OLM interneuron activation in recurrent inhibitory microcircuits of rat hippocampus

Oriens-lacunosum moleculare (O-LM) interneurons in the CA1 region of the hippocampus play a key role in feedback inhibition and in the control of network activity. However, how these cells are efficiently activated in the network remains unclear. To address this question, I performed recordings from CA1 pyramidal neuron axons, the presynaptic fibers that provide feedback innervation of these interneurons. Two forms of axonal action potential (AP) modulation were identified. First, repetitive stimulation resulted in activity-dependent AP broadening. Broadening showed fast onset, with marked changes in AP shape following a single AP. Second, tonic depolarization in CA1 pyramidal neuron somata induced AP broadening in the axon, and depolarization-induced broadening summated with activity-dependent broadening. Outside-out patch recordings from CA1 pyramidal neuron axons revealed a high density of α-dendrotoxin (α-DTX)-sensitive, inactivating K⁺ channels, suggesting that K⁺ channel inactivation mechanistically contributes to AP broadening. To examine the functional consequences of axonal AP modulation for synaptic transmission, I performed paired recordings between synaptically connected CA1 pyramidal neurons and O-LM interneurons. CA1 pyramidal neuron–O-LM interneuron excitatory postsynaptic currents (EPSCs) showed facilitation during both repetitive stimulation and tonic depolarization of the presynaptic neuron. Both effects were mimicked and occluded by α-DTX, suggesting that they were mediated by K⁺ channel inactivation. Therefore, axonal AP modulation can greatly facilitate the activation of O-LM interneurons. In conclusion, modulation of AP shape in CA1 pyramidal neuron axons substantially enhances the efficacy of principal neuron–interneuron synapses, promoting the activation of O-LM interneurons in recurrent inhibitory microcircuits.

Analog modulation of spike-evoked transmission in CA3 circuits is determined by axonal Kv1.1 channels in a time-dependent manner

Synaptic transmission usually depends on action potentials (APs) in an all-or-none (digital) fashion. Recent studies indicate, however, that subthreshold presynaptic depolarization may facilitate spike-evoked transmission, thus creating an analog modulation of spike-evoked synaptic transmission, also called analog-digital (AD) synaptic facilitation. Yet, the underlying mechanisms behind this facilitation remain unclear. We show here that AD facilitation at rat CA3-CA3 synapses is time-dependent and requires long presynaptic depolarization (5-10 s) for its induction. This depolarization-induced AD facilitation (d-ADF) is blocked by the specific Kv1.1 channel blocker dendrotoxin-K. Using fast voltage-imaging of the axon, we show that somatic depolarization used for induction of d-ADF broadened the AP in the axon through inactivation of Kv1.1 channels. Somatic depolarization enhanced spike-evoked calcium signals in presynaptic terminals, but not basal calcium. In conclusion, axonal Kv1.1 channels determine glutamate release in CA3 neurons in a time-dependent manner through the control of the presynaptic spike waveform.

Dopaminergic modulation of axonal potassium channels and action potential waveform in pyramidal neurons of prefrontal cortex

Voltage-gated K(+) (KV) channels play critical roles in shaping neuronal signals. KV channels distributed in the perisomatic regions and thick dendrites of cortical pyramidal neurons have been extensively studied. However, the properties and regulation of KV channels distributed in the thin axons remain unknown. In this study, by performing somatic and axonal patch-clamp recordings from layer 5 pyramidal neurons of prefrontal cortical slices, we showed that the rapidly inactivating A-currents mediated the transient K(+) currents evoked by action potential (AP) waveform command (KAP) at the soma, whereas the rapidly activating but slowly inactivating KV1-mediated D-currents dominated the KAP at the axon. In addition, activation of D1-like receptors for dopamine decreased the axonal K(+) currents, as a result of an increase in the activity of cAMP-PKA pathway. In contrast, activation of D2-like receptors showed an opposite effect on the axonal K(+) currents. Further experiments demonstrated that functional D1-like receptors were expressed at the main axon trunk and their activation could broaden the waveforms of axonal APs. Together, these results show that axonal KV channels were subjected to dopamine modulation, and this modulation could regulate the waveforms of propagating APs at the axon, suggesting an important role of dopaminergic modulation of axonal KV channels in regulating neuronal signalling.

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.

Axonal bleb recording

Patch-clamp recording requires direct accessibility of the cell membrane to patch pipettes and allows the investigation of ion channel properties and functions in specific cellular compartments. The cell body and relatively thick dendrites are the most accessible compartments of a neuron, due to their large diameters and therefore great membrane surface areas. However, axons are normally inaccessible to patch pipettes because of their thin structure; thus studies of axon physiology have long been hampered by the lack of axon recording methods. Recently, a new method of patch-clamp recording has been developed, enabling direct and tight-seal recording from cortical axons. These recordings are performed at the enlarged structure (axonal bleb) formed at the cut end of an axon after slicing procedures. This method has facilitated studies of the mechanisms underlying the generation and propagation of the main output signal, the action potential, and led to the finding that cortical neurons communicate not only in action potential-mediated digital mode but also in membrane potential-dependent analog mode.

What are the mechanisms for analogue and digital signalling in the brain?

Synaptic transmission in the brain generally depends on action potentials. However, recent studies indicate that subthreshold variation in the presynaptic membrane potential also determines spike-evoked transmission. The informational content of each presynaptic action potential is therefore greater than initially expected. The contribution of this synaptic property, which is a fast (from 0.01 to 10 s) and state-dependent modulation of functional coupling, has been largely underestimated and could have important consequences for our understanding of information processing in neural networks. We discuss here how the membrane voltage of the presynaptic terminal might modulate neurotransmitter release by mechanisms that do not involve a change in presynaptic Ca(2+) influx.

Laminarly orthogonal excitation of fast-spiking and low-threshold-spiking interneurons in mouse motor cortex

In motor cortex, long-range output to subcortical motor circuits depends on excitatory and inhibitory inputs converging on projection neurons in layers 5A/B. How interneurons interconnect with these projection neurons, and whether these microcircuits are interneuron and/or projection specific, is unclear. We found that fast-spiking interneurons received strong intralaminar (horizontal) excitation from pyramidal neurons in layers 5A/B including corticostriatal and corticospinal neurons, implicating them in mediating disynaptic recurrent, feedforward, and feedback inhibition within and across the two projection classes. Low-threshold-spiking (LTS) interneurons were instead strongly excited by descending interlaminar (vertical) input from layer 2/3 pyramidal neurons, implicating them in mediating disynaptic feedforward inhibition to both projection classes. Furthermore, in a novel pattern, lower layer 2/3 preferentially excited interneurons in one layer (5A/LTS) and excitatory neurons in another (5B/corticospinal). Thus, these inhibitory microcircuits in mouse motor cortex follow an orderly arrangement that is laminarly orthogonalized by interneuron-specific, projection-nonspecific connectivity.

Enhancement of asynchronous release from fast-spiking interneuron in human and rat epileptic neocortex

Down-regulation of GABAergic inhibition may result in the generation of epileptiform activities. Besides spike-triggered synchronous GABA release, changes in asynchronous release (AR) following high-frequency discharges may further regulate epileptiform activities. In brain slices obtained from surgically removed human neocortical tissues of patients with intractable epilepsy and brain tumor, we found that AR occurred at GABAergic output synapses of fast-spiking (FS) neurons and its strength depended on the type of connections, with FS autapses showing the strongest AR. In addition, we found that AR depended on residual Ca²⁺ at presynaptic terminals but was independent of postsynaptic firing. Furthermore, AR at FS autapses was markedly elevated in human epileptic tissue as compared to non-epileptic tissue. In a rat model of epilepsy, we found similar elevation of AR at both FS autapses and synapses onto excitatory neurons. Further experiments and analysis showed that AR elevation in epileptic tissue may result from an increase in action potential amplitude in the FS neurons and elevation of residual Ca²⁺ concentration. Together, these results revealed that GABAergic AR occurred at both human and rat neocortex, and its elevation in epileptic tissue may contribute to the regulation of epileptiform activities.

Short- and long-term plasticity at the axon initial segment

The axon initial segment (AIS) is a highly specialized neuronal subregion that is the site of action potential initiation and the boundary between axonal and somatodendritic compartments. In recent years, our understanding of the molecular structure of the AIS, its maturation, and its multiple fundamental roles in neuronal function has seen major advances. We are beginning to appreciate that the AIS is dynamically regulated, both over short timescales via adaptations in ion channel function, and long timescales via activity-dependent structural reorganization. Here, we review results from this emerging field highlighting how structural and functional plasticity relate to the development of the initial segment, and to neuronal disorders linked to AIS dysfunction.

Somatic membrane potential and Kv1 channels control spike repolarization in cortical axon collaterals and presynaptic boutons

The shape of action potentials invading presynaptic terminals, which can vary significantly from spike waveforms recorded at the soma, may critically influence the probability of synaptic neurotransmitter release. Revealing the conductances that determine spike shape in presynaptic boutons is important for understanding how changes in the electrochemical context in which a spike is generated, such as subthreshold depolarization spreading from the soma, can modulate synaptic strength. Utilizing recent improvements in the signal-to-noise ratio of voltage-sensitive dye imaging in mouse brain slices, we demonstrate that intracortical axon collaterals and en passant presynaptic terminals of layer 5 pyramidal cells exhibit a high density of Kv1 subunit-containing ion channels, which generate a slowly inactivating K(+) current critically important for spike repolarization in these compartments. Blockade of the current by low doses of 4-aminopyridine or α-dendrotoxin dramatically slows the falling phase of action potentials in axon collaterals and presynaptic boutons. Furthermore, subthreshold depolarization of the soma broadened action potentials in collaterals bearing presynaptic boutons, an effect abolished by blocking Kv1 channels with α-dendrotoxin. These results indicate that action potential-induced synaptic transmission may operate through a mix of analog-digital transmission owing to the properties of Kv1 channels in axon collaterals and presynaptic boutons.