Burst-Dependent Bidirectional Plasticity in the Cerebellum Is Driven by Presynaptic NMDA Receptors

Associative plasticity of granule cell inputs to cerebellar Purkinje cells

Granule cells of the cerebellum make up to 175,000 excitatory synapses on a single Purkinje cell, encoding the wide variety of information from the mossy fibre inputs into the cerebellar cortex. The granule cell axon is made of an ascending portion and a long parallel fibre extending at right angles, an architecture suggesting that synapses formed by the two segments of the axon could encode different information. There are controversial indications that ascending axon (AA) and parallel fibre (PF) synapse properties and modalities of plasticity are different. We tested the hypothesis that AA and PF synapses encode different information, and that the association of these distinct inputs to Purkinje cells might be relevant to the circuit and trigger plasticity, similar to the coincident activation of PF and climbing fibre inputs. Here, by recording synaptic currents in Purkinje cells from either proximal or distal granule cells (mostly AA and PF synapses, respectively), we describe a new form of associative plasticity between these two distinct granule cell inputs. We show for the first time that synchronous AA and PF repetitive train stimulation, with inhibition intact, triggers long-term potentiation (LTP) at AA synapses specifically. Furthermore, the timing of the presentation of the two inputs controls the outcome of plasticity and induction requires NMDAR and mGluR1 activation. The long length of the PFs allows us to preferentially activate the two inputs independently, and despite a lack of morphological reconstruction of the connections, these observations reinforce the suggestion that AA and PF synapses have different coding capabilities and plasticity that is associative, enabling effective association of information transmitted via granule cells.

Different Numbers of Conjunctive Stimuli Induce LTP or LTD in Mouse Cerebellar Purkinje Cell

Long-term depression (LTD) of synaptic transmission at parallel fiber (PF)-Purkinje cell (PC) synapses plays an important role in cerebellum-related motor coordination and learning. LTD is induced by the conjunction of PF stimulation and climbing fiber (CF) stimulation or somatic PC depolarization, while long-term potentiation (LTP) is induced by PF stimulation alone. Therefore, it is considered that different types of stimulation induce different types of synaptic plasticity. However, we found that a small number of conjunctive stimulations (PF + somatic depolarization of PC) induced LTP, but did not induce LTD of a small size. This LTP was not associated with changes in paired-pulse ratio, suggesting postsynaptic origin. Additionally this LTP was dependent on nitric oxide. This LTP was also induced by a smaller number of physiological conjunctive PF and CF stimuli. These results suggested that a larger number or longer period of conjunctive stimulation is required to induce LTD by overcoming LTP. Ca2+ transients at the PC dendritic region was measured by calcium imaging during LTD-inducing conjunctive stimulation. Peak amplitude of Ca2+ transients increased gradually during repetitive conjunctive stimulation. Instantaneous peak amplitude was not different between the early phase and late phase, but the average amplitude was larger in the later phase than in the early phase. These results show that LTD overcomes LTP, and increased Ca2+ integration or a number of stimulations is required for LTD induction.

Regulation by Presynaptic NMDA Receptors of Non-Linear Postsynaptic Summation of the Cortical Input to CA1 Pyramidal Neurons

Entorhinal cortex (EC) LIII and LII glutamatergic neurons make monosynaptic connections onto distal apical dendrites of hippocampal CA1 and CA2 pyramidal neurons (PNs), respectively, through perforant path (PP) projections. We previously reported that a brief train of PP stimuli evokes strong supralinear temporal summation of excitatory postsynaptic potentials (EPSPs) in CA1 PNs that requires NMDAR activation, with relatively little summation in CA2 PNs in mice of either sex. Here we provide evidence from combined immunogold electron microscopy, cell-type specific genetic deletion and pharmacology that the NMDARs required for supralinear temporal summation of the CA1 PP EPSP are presynaptic, located in the PP terminals. Moreover, we found that the number of NMDARs in PP terminals innervating CA1 PNs is significantly greater than that found in PP terminals innervating CA2 PNs, providing a potential explanation for the difference in temporal summation in these two classes of hippocampal PNs.

Plasticity mechanisms of genetically distinct Purkinje cells

Despite its uniform appearance, the cerebellar cortex is highly heterogeneous in terms of structure, genetics and physiology. Purkinje cells (PCs), the principal and sole output neurons of the cerebellar cortex, can be categorized into multiple populations that differentially express molecular markers and display distinctive physiological features. Such features include action potential rate, but also their propensity for synaptic and intrinsic plasticity. However, the precise molecular and genetic factors that correlate with the differential physiological properties of PCs remain elusive. In this article, we provide a detailed overview of the cellular mechanisms that regulate PC activity and plasticity. We further perform a pathway analysis to highlight how molecular characteristics of specific PC populations may influence their physiology and plasticity mechanisms.

New clues for the role of cerebellum in schizophrenia and the associated cognitive impairment

Schizophrenia (SZ) is a complex neuropsychiatric disorder associated with severe cognitive dysfunction. Although research has mainly focused on forebrain abnormalities, emerging results support the involvement of the cerebellum in SZ physiopathology, particularly in Cognitive Impairment Associated with SZ (CIAS). Besides its role in motor learning and control, the cerebellum is implicated in cognition and emotion. Recent research suggests that structural and functional changes in the cerebellum are linked to deficits in various cognitive domains including attention, working memory, and decision-making. Moreover, cerebellar dysfunction is related to altered cerebellar circuit activities and connectivity with brain regions associated with cognitive processing. This review delves into the role of the cerebellum in CIAS. We initially consider the major forebrain alterations in CIAS, addressing impairments in neurotransmitter systems, synaptic plasticity, and connectivity. We then focus on recent findings showing that several mechanisms are also altered in the cerebellum and that cerebellar communication with the forebrain is impaired. This evidence implicates the cerebellum as a key component of circuits underpinning CIAS physiopathology. Further studies addressing cerebellar involvement in SZ and CIAS are warranted and might open new perspectives toward understanding the physiopathology and effective treatment of these disorders.

Impaired cerebellar plasticity hypersensitizes sensory reflexes in SCN2A-associated ASD

Children diagnosed with autism spectrum disorder (ASD) commonly present with sensory hypersensitivity or abnormally strong reactions to sensory stimuli. Such hypersensitivity can be overwhelming, causing high levels of distress that contribute markedly to the negative aspects of the disorder. Here, we identify a mechanism that underlies hypersensitivity in a sensorimotor reflex found to be altered in humans and in mice with loss of function in the ASD risk-factor gene SCN2A. The cerebellum-dependent vestibulo-ocular reflex (VOR), which helps maintain one's gaze during movement, was hypersensitized due to deficits in cerebellar synaptic plasticity. Heterozygous loss of SCN2A-encoded NaV1.2 sodium channels in granule cells impaired high-frequency transmission to Purkinje cells and long-term potentiation, a form of synaptic plasticity important for modulating VOR gain. VOR plasticity could be rescued in mice via a CRISPR-activator approach that increases Scn2a expression, demonstrating that evaluation of a simple reflex can be used to assess and quantify successful therapeutic intervention.

Silences, spikes and bursts: Three-part knot of the neural code

When a neuron breaks silence, it can emit action potentials in a number of patterns. Some responses are so sudden and intense that electrophysiologists felt the need to single them out, labelling action potentials emitted at a particularly high frequency with a metonym - bursts. Is there more to bursts than a figure of speech? After all, sudden bouts of high-frequency firing are expected to occur whenever inputs surge. The burst coding hypothesis advances that the neural code has three syllables: silences, spikes and bursts. We review evidence supporting this ternary code in terms of devoted mechanisms for burst generation, synaptic transmission and synaptic plasticity. We also review the learning and attention theories for which such a triad is beneficial.

Impaired cerebellar plasticity hypersensitizes sensory reflexes in SCN2A-associated ASD

Children diagnosed with autism spectrum disorder (ASD) commonly present with sensory hypersensitivity, or abnormally strong reactions to sensory stimuli. Such hypersensitivity can be overwhelming, causing high levels of distress that contribute markedly to the negative aspects of the disorder. Here, we identify the mechanisms that underlie hypersensitivity in a sensorimotor reflex found to be altered in humans and in mice with loss-of-function in the ASD risk-factor gene SCN2A. The cerebellum-dependent vestibulo-ocular reflex (VOR), which helps maintain one's gaze during movement, was hypersensitized due to deficits in cerebellar synaptic plasticity. Heterozygous loss of SCN2A-encoded NaV1.2 sodium channels in granule cells impaired high-frequency transmission to Purkinje cells and long-term potentiation, a form of synaptic plasticity important for modulating VOR gain. VOR plasticity could be rescued in adolescent mice via a CRISPR-activator approach that increases Scn2a expression, highlighting how evaluation of simple reflexes can be used as quantitative readout of therapeutic interventions.

Timing to be precise? An overview of spike timing-dependent plasticity, brain rhythmicity, and glial cells interplay within neuronal circuits

In the mammalian brain information processing and storage rely on the complex coding and decoding events performed by neuronal networks. These actions are based on the computational ability of neurons and their functional engagement in neuronal assemblies where precise timing of action potential firing is crucial. Neuronal circuits manage a myriad of spatially and temporally overlapping inputs to compute specific outputs that are proposed to underly memory traces formation, sensory perception, and cognitive behaviors. Spike-timing-dependent plasticity (STDP) and electrical brain rhythms are suggested to underlie such functions while the physiological evidence of assembly structures and mechanisms driving both processes continues to be scarce. Here, we review foundational and current evidence on timing precision and cooperative neuronal electrical activity driving STDP and brain rhythms, their interactions, and the emerging role of glial cells in such processes. We also provide an overview of their cognitive correlates and discuss current limitations and controversies, future perspectives on experimental approaches, and their application in humans.

Depolarization-induced bursts of miniature synaptic currents in individual synapses of developing cerebellum

In central synapses, spontaneous transmitter release observed in the absence of action potential firing is often considered as a random process lacking time or space specificity. However, when studying miniature glutamatergic currents at cerebellar synapses between parallel fibers and molecular layer interneurons, we found that these currents were sometimes organized in bursts of events occurring at high frequency (about 30 Hz). Bursts displayed homogeneous quantal size amplitudes. Furthermore, in the presence of the desensitization inhibitor cyclothiazide, successive events within a burst displayed quantal amplitude occlusion. Based on these findings, we conclude that bursts originate in individual synapses. Bursts were enhanced by increasing either the external potassium concentration or the external calcium concentration, and they were strongly inhibited when blocking voltage-gated calcium channels by cadmium. Bursts were prevalent in elevated potassium concentration during the formation of the molecular layer but were infrequent later in development. Since postsynaptic AMPA receptors are largely calcium permeant in developing parallel fiber-interneuron synapses, we propose that bursts involve presynaptic calcium transients implicating presynaptic voltage-gated calcium channels, together with postsynaptic calcium transients implicating postsynaptic AMPA receptors. These simultaneous pre- and postsynaptic calcium transients may contribute to the formation and/or stabilization of synaptic connections.

Diverse role of NMDA receptors for dendritic integration of neural dynamics

Neurons, represented as a tree structure of morphology, have various distinguished branches of dendrites. Different types of synaptic receptors distributed over dendrites are responsible for receiving inputs from other neurons. NMDA receptors (NMDARs) are expressed as excitatory units, and play a key physiological role in synaptic function. Although NMDARs are widely expressed in most types of neurons, they play a different role in the cerebellar Purkinje cells (PCs). Utilizing a computational PC model with detailed dendritic morphology, we explored the role of NMDARs at different parts of dendritic branches and regions. We found somatic responses can switch from silent, to simple spikes and complex spikes, depending on specific dendritic branches. Detailed examination of the dendrites regarding their diameters and distance to soma revealed diverse response patterns, yet explain two firing modes, simple and complex spike. Taken together, these results suggest that NMDARs play an important role in controlling excitability sensitivity while taking into account the factor of dendritic properties. Given the complexity of neural morphology varying in cell types, our work suggests that the functional role of NMDARs is not stereotyped but highly interwoven with local properties of neuronal structure.

Modified climbing fiber/Purkinje cell synaptic connectivity in the cerebellum of the neonatal phencyclidine model of schizophrenia

Synaptogenesis and neural network remodeling are at their maximum during the perinatal period of human brain development. Perturbations of this highly sensitive stage might underlie the etiology of neurodevelopmental disorders. Subchronic neonatal administration of phencyclidine, a drug of abuse, has been used to model schizophrenia in rodents. In this model, we found specific long-term synaptic changes in Purkinje cells and transient gene expression changes in the cerebellum. While transient increased neuronal activity in the cerebellum, induced using chemogenetics, reproduces some phencyclidine-induced molecular changes, it is insufficient to reproduce the long-term synaptic effects. Our results show the complex mechanism of action of phencyclidine on the development of neuronal connectivity and further highlight the potential contribution of cerebellar defects in psychiatric diseases.

Environmental perturbations during the first years of life are a major factor in psychiatric diseases. Phencyclidine (PCP), a drug of abuse, has psychomimetic effects, and neonatal subchronic administration of PCP in rodents leads to long-term behavioral changes relevant for schizophrenia. The cerebellum is increasingly recognized for its role in diverse cognitive functions. However, little is known about potential cerebellar changes in models of schizophrenia. Here, we analyzed the characteristics of the cerebellum in the neonatal subchronic PCP model. We found that, while the global cerebellar cytoarchitecture and Purkinje cell spontaneous spiking properties are unchanged, climbing fiber/Purkinje cell synaptic connectivity is increased in juvenile mice. Neonatal subchronic administration of PCP is accompanied by increased cFos expression, a marker of neuronal activity, and transient modification of the neuronal surfaceome in the cerebellum. The largest change observed is the overexpression of Ctgf, a gene previously suggested as a biomarker for schizophrenia. This neonatal increase in Ctgf can be reproduced by increasing neuronal activity in the cerebellum during the second postnatal week using chemogenetics. However, it does not lead to increased climbing fiber/Purkinje cell connectivity in juvenile mice, showing the complexity of PCP action. Overall, our study shows that administration of the drug of abuse PCP during the developmental period of intense cerebellar synaptogenesis and circuit remodeling has long-term and specific effects on Purkinje cell connectivity and warrants the search for this type of synaptic changes in psychiatric diseases.

Non-Linear Frequency Dependence of Neurovascular Coupling in the Cerebellar Cortex Implies Vasodilation-Vasoconstriction Competition

Neurovascular coupling (NVC) is the process associating local cerebral blood flow (CBF) to neuronal activity (NA). Although NVC provides the basis for the blood oxygen level dependent (BOLD) effect used in functional MRI (fMRI), the relationship between NVC and NA is still unclear. Since recent studies reported cerebellar non-linearities in BOLD signals during motor tasks execution, we investigated the NVC/NA relationship using a range of input frequencies in acute mouse cerebellar slices of vermis and hemisphere. The capillary diameter increased in response to mossy fiber activation in the 6-300 Hz range, with a marked inflection around 50 Hz (vermis) and 100 Hz (hemisphere). The corresponding NA was recorded using high-density multi-electrode arrays and correlated to capillary dynamics through a computational model dissecting the main components of granular layer activity. Here, NVC is known to involve a balance between the NMDAR-NO pathway driving vasodilation and the mGluRs-20HETE pathway driving vasoconstriction. Simulations showed that the NMDAR-mediated component of NA was sufficient to explain the time course of the capillary dilation but not its non-linear frequency dependence, suggesting that the mGluRs-20HETE pathway plays a role at intermediate frequencies. These parallel control pathways imply a vasodilation-vasoconstriction competition hypothesis that could adapt local hemodynamics at the microscale bearing implications for fMRI signals interpretation.

Presynaptic NMDA Receptors Influence Ca2+ Dynamics by Interacting with Voltage-Dependent Calcium Channels during the Induction of Long-Term Depression

Spike-timing-dependent long-term depression (t-LTD) of glutamatergic layer (L)4-L2/3 synapses in developing neocortex requires activation of astrocytes by endocannabinoids (eCBs), which release glutamate onto presynaptic NMDA receptors (preNMDARs). The exact function of preNMDARs in this context is still elusive and strongly debated. To elucidate their function, we show that bath application of the eCB 2-arachidonylglycerol (2-AG) induces a preNMDAR-dependent form of chemically induced LTD (eCB-LTD) in L2/3 pyramidal neurons in the juvenile somatosensory cortex of rats. Presynaptic Ca²⁺ imaging from L4 spiny stellate axons revealed that action potential (AP) evoked Ca²⁺ transients show a preNMDAR-dependent broadening during eCB-LTD induction. However, blockade of voltage-dependent Ca²⁺ channels (VDCCs) did not uncover direct preNMDAR-mediated Ca²⁺ transients in the axon. This suggests that astrocyte-mediated glutamate release onto preNMDARs does not result in a direct Ca²⁺ influx, but that it instead leads to an indirect interaction with presynaptic VDCCs, boosting axonal Ca²⁺ influx. These results reveal one of the main remaining missing pieces in the signaling cascade of t-LTD at developing cortical synapses.

Heterogeneity of intrinsic plasticity in cerebellar Purkinje cells linked with cortical molecular zones

In the cerebellar cortex, heterogeneous populations of Purkinje cells (PCs), classified into zebrin (aldolase C)-positive (Z+) and -negative (Z-) types, are arranged into separate longitudinal zones. They have different topographic neuronal connections and show different patterns of activity in behavior tasks. However, whether the zebrin type of PCs directly links with the physiological properties of the PC has not been well clarified. Therefore, we applied in vitro whole-cell patch-clamp recording in Z+ and Z- PCs in vermal and hemispheric neighboring zebrin zones in zebrin-visualized mice. Intrinsic excitability is significantly higher in Z- PCs than in Z+ PCs. Furthermore, intrinsic plasticity and synaptic long-term potentiation are enhanced more in Z- PCs than in Z+ PCs. The difference was mediated by different modulation of SK channel activities between Z+ and Z- PCs. The results indicate that cellular physiology differentially tunes to the functional compartmentalization of heterogeneous PCs.

A stable proportion of Purkinje cell inputs from parallel fibers are silent during cerebellar maturation

Cerebellar Purkinje neurons integrate information transmitted at excitatory synapses formed by granule cells. Although these synapses are considered essential sites for learning, most of them appear not to transmit any detectable electrical information and have been defined as silent. It has been proposed that silent synapses are required to maximize information storage capacity and ensure its reliability, and hence to optimize cerebellar operation. Such optimization is expected to occur once the cerebellar circuitry is in place, during its maturation and the natural and steady improvement of animal agility. We therefore investigated whether the proportion of silent synapses varies over this period, from the third to the sixth postnatal week in mice. Selective expression of a calcium indicator in granule cells enabled quantitative mapping of presynaptic activity, while postsynaptic responses were recorded by patch clamp in acute slices. Through this approach and the assessment of two anatomical features (the distance that separates adjacent planar Purkinje dendritic trees and the synapse density), we determined the average excitatory postsynaptic potential per synapse. Its value was four to eight times smaller than responses from paired recorded detectable connections, consistent with over 70% of synapses being silent. These figures remained remarkably stable across maturation stages. According to the proposed role for silent synapses, our results suggest that information storage capacity and reliability are optimized early during cerebellar maturation. Alternatively, silent synapses may have roles other than adjusting the information storage capacity and reliability.

NMDARs in granule cells contribute to parallel fiber-Purkinje cell synaptic plasticity and motor learning

Long-term synaptic plasticity is believed to be the cellular substrate of learning and memory. Synaptic plasticity rules are defined by the specific complement of receptors at the synapse and the associated downstream signaling mechanisms. In young rodents, at the cerebellar synapse between granule cells (GC) and Purkinje cells (PC), bidirectional plasticity is shaped by the balance between transcellular nitric oxide (NO) driven by presynaptic N-methyl-D-aspartate receptor (NMDAR) activation and postsynaptic calcium dynamics. However, the role and the location of NMDAR activation in these pathways is still debated in mature animals. Here, we show in adult rodents that NMDARs are present and functional in presynaptic terminals where their activation triggers NO signaling. In addition, we find that selective genetic deletion of presynaptic, but not postsynaptic, NMDARs prevents synaptic plasticity at parallel fiber-PC (PF-PC) synapses. Consistent with this finding, the selective deletion of GC NMDARs affects adaptation of the vestibulo-ocular reflex. Thus, NMDARs presynaptic to PCs are required for bidirectional synaptic plasticity and cerebellar motor learning.

Spontaneous and action potential-evoked Ca2+ release from endoplasmic reticulum in neocortical synaptic boutons

Although the endoplasmic reticulum (ER) is present throughout axons, and IP3 and ryanodine receptors are widely expressed in nerve terminals, whether Ca2+ release from presynaptic stores contributes to action potential (AP)-evoked Ca2+ transients remains controversial. We investigated the release of Ca2+ from ER stores in boutons en passant of neocortical layer 5 pyramidal neurons. A hallmark of these stores is that they spontaneously release Ca2+ at a low frequency. Using a high-affinity Ca2+ indicator, we documented and characterised such spontaneous Ca2+ transients (sCaTs), which occurred at a rate of ~0.2 per min and raised the intracellular Ca2+ concentration ([Ca2+]i) by ~2 µM in the absence of exogenous buffers. Caffeine increased the average frequency of sCaTs by 90%, without affecting their amplitude and decay kinetics. Therefore, presynaptic ryanodine receptors were likely involved. To determine if presynaptic ER stores contribute to intracellular Ca2+ accumulation during repetitive stimulation, we measured [Ca2+]i during 2 s long trains of APs evoked at 10-50 Hz. We found that for frequencies <20 Hz, [Ca2+]i reached a steady state within ~500 ms after stimulation onset. However, for higher frequencies, [Ca2+]i continued to increase with AP number, suggesting that the rate of Ca2+ entry exceeded the rate of clearance. Comparison between measured and predicted values indicates supralinear summation of Ca2+. Block of the sarco/endoplasmic reticulum Ca2+-ATPase reduced the supralinearity of summation, without reducing the amplitude of a single AP-evoked Ca2+ transient. Together, our results implicate presynaptic ER stores as a source of Ca2+ during repetitive stimulation.

Presynaptic NMDA receptors facilitate short-term plasticity and BDNF release at hippocampal mossy fiber synapses

Neurotransmitter release is a highly controlled process by which synapses can critically regulate information transfer within neural circuits. While presynaptic receptors - typically activated by neurotransmitters and modulated by neuromodulators - provide a powerful way of fine-tuning synaptic function, their contribution to activity-dependent changes in transmitter release remains poorly understood. Here, we report that presynaptic NMDA receptors (preNMDARs) at mossy fiber boutons in the rodent hippocampus can be activated by physiologically relevant patterns of activity and selectively enhance short-term synaptic plasticity at mossy fiber inputs onto CA3 pyramidal cells and mossy cells, but not onto inhibitory interneurons. Moreover, preNMDARs facilitate brain-derived neurotrophic factor release and contribute to presynaptic calcium rise. Taken together, our results indicate that by increasing presynaptic calcium, preNMDARs fine-tune mossy fiber neurotransmission and can control information transfer during dentate granule cell burst activity that normally occur in vivo.

Synaptic plasticity rules with physiological calcium levels

Spike-timing-dependent plasticity (STDP) is considered as a primary mechanism underlying formation of new memories during learning. Despite the growing interest in activity-dependent plasticity, it is still unclear whether synaptic plasticity rules inferred from in vitro experiments are correct in physiological conditions. The abnormally high calcium concentration used in in vitro studies of STDP suggests that in vivo plasticity rules may differ significantly from in vitro experiments, especially since STDP depends strongly on calcium for induction. We therefore studied here the influence of extracellular calcium on synaptic plasticity. Using a combination of experimental (patch-clamp recording and Ca²⁺ imaging at CA3-CA1 synapses) and theoretical approaches, we show here that the classic STDP rule in which pairs of single pre- and postsynaptic action potentials induce synaptic modifications is not valid in the physiological Ca²⁺ range. Rather, we found that these pairs of single stimuli are unable to induce any synaptic modification in 1.3 and 1.5 mM calcium and lead to depression in 1.8 mM. Plasticity can only be recovered when bursts of postsynaptic spikes are used, or when neurons fire at sufficiently high frequency. In conclusion, the STDP rule is profoundly altered in physiological Ca²⁺, but specific activity regimes restore a classical STDP profile.

NMDA receptors in axons: there's no coincidence

In the textbook view, N-methyl-d-aspartate (NMDA) receptors are postsynaptically located detectors of coincident activity in Hebbian learning. However, controversial presynaptically located NMDA receptors (preNMDARs) have for decades been repeatedly reported in the literature. These preNMDARs have typically been implicated in the regulation of short-term and long-term plasticity, but precisely how they signal and what their functional roles are have been poorly understood. The functional roles of preNMDARs across several brain regions and different forms of plasticity can differ vastly, with recent discoveries showing key involvement of unusual subunit composition. Increasing evidence shows preNMDAR can signal through both ionotropic action by fluxing calcium and in metabotropic mode even in the presence of magnesium blockade. We argue that these unusual properties may explain why controversy has surrounded this receptor type. In addition, the expression of preNMDARs at some synapse types but not others can underlie synapse-type-specific plasticity. Last but not least, preNMDARs are emerging therapeutic targets in disease states such as neuropathic pain. We conclude that axonally located preNMDARs are required for specific purposes and do not end up there by accident.

Short-term depression and long-term plasticity together tune sensitive range of synaptic plasticity

Synaptic efficacy is subjected to activity-dependent changes on short- and long time scales. While short-term changes decay over minutes, long-term modifications last from hours up to a lifetime and are thought to constitute the basis of learning and memory. Both plasticity mechanisms have been studied extensively but how their interaction shapes synaptic dynamics is little known. To investigate how both short- and long-term plasticity together control the induction of synaptic depression and potentiation, we used numerical simulations and mathematical analysis of a calcium-based model, where pre- and postsynaptic activity induces calcium transients driving synaptic long-term plasticity. We found that the model implementing known synaptic short-term dynamics in the calcium transients can be successfully fitted to long-term plasticity data obtained in visual- and somatosensory cortex. Interestingly, the impact of spike-timing and firing rate changes on plasticity occurs in the prevalent firing rate range, which is different in both cortical areas considered here. Our findings suggest that short- and long-term plasticity are together tuned to adapt plasticity to area-specific activity statistics such as firing rates.

Synaptic long-term plasticity, the long-lasting change in efficacy of connections between neurons, is believed to underlie learning and memory. Synapses furthermore change their efficacy reversibly in an activity-dependent manner on the subsecond time scale, referred to as short-term plasticity. It is not known how both synaptic plasticity mechanisms—long- and short-term—interact during activity epochs. To address this question, we used a biologically-inspired plasticity model in which calcium drives changes in synaptic efficacy. We applied the model to plasticity data from visual- and somatosensory cortex and found that synaptic changes occur in very different firing rate ranges, which correspond to the prevalent firing rates in both structures. Our results suggest that short- and long-term plasticity act in a well concerted fashion.

Is Purkinje Neuron Hyperpolarisation Important for Cerebellar Synaptic Plasticity? A Retrospective and Prospective Analysis

Two recent studies have demonstrated that the dendritic Ca²⁺ signal associated with a climbing fibre (CF) input to the cerebellar Purkinje neuron (PN) depends on the membrane potential (V_m). Specifically, when the cell is hyperpolarised, this signal is mediated by T-type voltage-gated Ca²⁺ channels; in contrast, when the cell is firing, the CF-PN signal is mediated by P/Q-type voltage-gated Ca²⁺ channels. When the CF input is paired with parallel fibre (PF) activity, the signal is locally amplified at the sites of PF-activated synapses according to the V_m at the time of the CF input, suggesting that the standing V_m is a critical parameter for the induction of PF synaptic plasticity. In this review, I analyse how the V_m can potentially play a role in cerebellar learning focussing, in particular, on the hyperpolarised state that appears to occur episodically, since PNs are mostly firing under physiological conditions. By revisiting the recent literature reporting in vivo recordings and synaptic plasticity studies, I speculate on how a putative role of the PN V_m can provide an interpretation for the results of these studies.

Switching On Depression and Potentiation in the Cerebellum

Long-term depression (LTD) and long-term potentiation (LTP) in the cerebellum are important for motor learning. However, the signaling mechanisms controlling whether LTD or LTP is induced in response to synaptic stimulation remain obscure. Using a unified model of LTD and LTP at the cerebellar parallel fiber-Purkinje cell (PF-PC) synapse, we delineate the coordinated pre- and postsynaptic signaling that determines the direction of plasticity. We show that LTP is the default response to PF stimulation above a well-defined frequency threshold. However, if the calcium signal surpasses the threshold for CaMKII activation, then an ultrasensitive "on switch" activates an extracellular signal-regulated kinase (ERK)-based positive feedback loop that triggers LTD instead. This postsynaptic feedback loop is sustained by another, trans-synaptic, feedback loop that maintains nitric oxide production throughout LTD induction. When full depression is achieved, an automatic "off switch" inactivates the feedback loops, returning the network to its basal state and demarcating the end of the early phase of LTD.

Neuronal Nitric Oxide Synthase in Cultured Cerebellar Bergmann Glia: Glutamate-Dependent Regulation

Glutamate exerts its actions through the activation of membrane receptors expressed in neurons and glia cells. The signaling properties of glutamate transporters have been characterized recently, suggesting a complex array of signaling transactions triggered by presynaptic released glutamate. In the cerebellar molecular layer, glutamatergic synapses are surrounded by Bergmann glia cells, compulsory participants of glutamate turnover and supply to neurons. Since a glutamate-dependent increase in cGMP levels has been described in these cells and the nitric oxide-cGMP signaling cascade increases their glutamate uptake activity, we describe here the Bergmann glia expression of neuronal nitric oxide synthetase. An augmentation of neuronal nitric oxide synthase was found upon glutamate exposure. This effect is mediated by glutamate transporters and is related to an increase in the stability of the enzyme. These results strengthen the notion of a complex regulation of glial glutamate uptake that supports neuronal glutamate signaling.

Short-term plasticity at cerebellar granule cell to molecular layer interneuron synapses expands information processing

Information processing by cerebellar molecular layer interneurons (MLIs) plays a crucial role in motor behavior. MLI recruitment is tightly controlled by the profile of short-term plasticity (STP) at granule cell (GC)-MLI synapses. While GCs are the most numerous neurons in the brain, STP diversity at GC-MLI synapses is poorly documented. Here, we studied how single MLIs are recruited by their distinct GC inputs during burst firing. Using slice recordings at individual GC-MLI synapses of mice, we revealed four classes of connections segregated by their STP profile. Each class differentially drives MLI recruitment. We show that GC synaptic diversity is underlain by heterogeneous expression of synapsin II, a key actor of STP and that GC terminals devoid of synapsin II are associated with slow MLI recruitment. Our study reveals that molecular, structural and functional diversity across GC terminals provides a mechanism to expand the coding range of MLIs.

Complex Spike Wars: a New Hope

The climbing fiber-Purkinje cell circuit is one of the most powerful and highly conserved in the central nervous system. Climbing fibers exert a powerful excitatory action that results in a complex spike in Purkinje cells and normal functioning of the cerebellum depends on the integrity of climbing fiber-Purkinje cell synapse. Over the last 50 years, multiple hypotheses have been put forward on the role of the climbing fibers and complex spikes in cerebellar information processing and motor control. Central to these theories is the nature of the interaction between the low-frequency complex spike discharge and the high-frequency simple spike firing of Purkinje cells. This review examines the major hypotheses surrounding the action of the climbing fiber-Purkinje cell projection, discussing both supporting and conflicting findings. The review describes newer findings establishing that climbing fibers and complex spikes provide predictive signals about movement parameters and that climbing fiber input controls the encoding of behavioral information in the simple spike firing of Purkinje cells. Finally, we propose the dynamic encoding hypothesis for complex spike function that strives to integrate established and newer findings.

Interneuronal NMDA receptors regulate long-term depression and motor learning in the cerebellum

KEY POINTS

NMDA receptors (NMDARs) are required for long-term depression (LTD) at parallel fibre-Purkinje cell synapses, but their cellular localization and physiological functions in vivo are unclear. NMDARs in molecular-layer interneurons (MLIs), but not granule cells or Purkinje cells, are required for LTD, but not long-term potentiation induced by low-frequency stimulation of parallel fibres. Nitric oxide produced by NMDAR activation in MLIs probably mediates LTD induction. NMDARs in granule cells or Purkinje cells are dispensable for motor learning during adaptation of horizontal optokinetic responses.

ABSTRACT

Long-term potentiation (LTP) and depression (LTD), which serve as cellular synaptic plasticity models for learning and memory, are crucially regulated by N-methyl-d-aspartate receptors (NMDARs) in various brain regions. In the cerebellum, LTP and LTD at parallel fibre (PF)-Purkinje cell (PC) synapses are thought to mediate certain forms of motor learning. However, while NMDARs are essential for LTD in vitro, their cellular localization remains controversial. In addition, whether and how NMDARs mediate motor learning in vivo remains unclear. Here, we examined the contribution of NMDARs expressed in granule cells (GCs), PCs and molecular-layer interneurons (MLIs) to LTD/LTP and motor learning by generating GC-, PC- and MLI/PC-specific knockouts of Grin1, a gene encoding an obligatory GluN1 subunit of NMDARs. While robust LTD and LTP were induced at PF-PC synapses in GC- and PC-specific Grin1 (GC-Grin1 and PC-Grin1, respectively) conditional knockout (cKO) mice, only LTD was impaired in MLI/PC-specific Grin1 (MLI/PC-Grin1) cKO mice. Application of diethylamine nitric oxide (NO) sodium, a potent NO donor, to the cerebellar slices restored LTD in MLI/PC-Grin1 cKO mice, suggesting that NO is probably downstream to NMDARs. Furthermore, the adaptation of horizontal optokinetic responses (hOKR), a cerebellar motor learning task, was normally observed in GC-Grin1 cKO and PC-Grin1 cKO mice, but not in MLI/PC-Grin1 cKO mice. These results indicate that it is the NMDARs expressed in MLIs, but not in PCs or GCs, that play important roles in LTD in vitro and motor learning in vivo.

Cerebellar learning using perturbations

The cerebellum aids the learning of fast, coordinated movements. According to current consensus, erroneously active parallel fibre synapses are depressed by complex spikes signalling movement errors. However, this theory cannot solve the credit assignment problem of processing a global movement evaluation into multiple cell-specific error signals. We identify a possible implementation of an algorithm solving this problem, whereby spontaneous complex spikes perturb ongoing movements, create eligibility traces and signal error changes guiding plasticity. Error changes are extracted by adaptively cancelling the average error. This framework, stochastic gradient descent with estimated global errors (SGDEGE), predicts synaptic plasticity rules that apparently contradict the current consensus but were supported by plasticity experiments in slices from mice under conditions designed to be physiological, highlighting the sensitivity of plasticity studies to experimental conditions. We analyse the algorithm's convergence and capacity. Finally, we suggest SGDEGE may also operate in the basal ganglia.

Emergence of spontaneous assembly activity in developing neural networks without afferent input

Spontaneous activity is a fundamental characteristic of the developing nervous system. Intriguingly, it often takes the form of multiple structured assemblies of neurons. Such assemblies can form even in the absence of afferent input, for instance in the zebrafish optic tectum after bilateral enucleation early in life. While the development of neural assemblies based on structured afferent input has been theoretically well-studied, it is less clear how they could arise in systems without afferent input. Here we show that a recurrent network of binary threshold neurons with initially random weights can form neural assemblies based on a simple Hebbian learning rule. Over development the network becomes increasingly modular while being driven by initially unstructured spontaneous activity, leading to the emergence of neural assemblies. Surprisingly, the set of neurons making up each assembly then continues to evolve, despite the number of assemblies remaining roughly constant. In the mature network assembly activity builds over several timesteps before the activation of the full assembly, as recently observed in calcium-imaging experiments. Our results show that Hebbian learning is sufficient to explain the emergence of highly structured patterns of neural activity in the absence of structured input.

NMDA receptor-dependent presynaptic inhibition at the calyx of Held synapse of rat pups

N-Methyl-d-aspartate receptors (NMDARs) play diverse roles in synaptic transmission, synaptic plasticity, neuronal development and neurological diseases. In addition to their postsynaptic expression, NMDARs are also expressed in presynaptic terminals at some central synapses, and their activation modulates transmitter release. However, the regulatory mechanisms of NMDAR-dependent synaptic transmission remain largely unknown. In the present study, we demonstrated that activation of NMDARs in a nerve terminal at a central glutamatergic synapse inhibits presynaptic Ca²⁺ currents (I_Ca) in a GluN2C/2D subunit-dependent manner, thereby decreasing nerve-evoked excitatory postsynaptic currents. Neither presynaptically loaded fast Ca²⁺ chelator BAPTA nor non-hydrolysable GTP analogue GTPγS affected NMDAR-mediated I_Ca inhibition. In the presence of a glutamate uptake blocker, the decline in I_Ca amplitude evoked by repetitive depolarizing pulses at 20 Hz was attenuated by an NMDAR competitive antagonist, suggesting that endogenous glutamate has a potential to activate presynaptic NMDARs. Moreover, NMDA-induced inward currents at a negative holding potential (−80 mV) were abolished by intra-terminal loading of the NMDAR open channel blocker MK-801, indicating functional expression of presynaptic NMDARs. We conclude that presynaptic NMDARs can attenuate glutamate release by inhibiting voltage-gated Ca²⁺ channels at a relay synapse in the immature rat auditory brainstem.

Activity-dependent plasticity of presynaptic GABAB receptors at parallel fiber synapses

Parallel fiber synapses in the cerebellum express a wide range of presynaptic receptors. However, presynaptic receptor expression at individual parallel fiber synapses is quite heterogeneous, suggesting physiological mechanisms regulate presynaptic receptor expression. We investigated changes in presynaptic GABA_B receptors at parallel fiber-stellate cell synapses in acute cerebellar slices from juvenile mice. GABA_B receptor-mediated inhibition of excitatory postsynaptic currents (EPSCs) is remarkably diverse at these synapses, with transmitter release at some synapses inhibited by >50% and little or no inhibition at others. GABA_B receptor-mediated inhibition was significantly reduced following 4 Hz parallel fiber stimulation but not after stimulation at other frequencies. The reduction in GABA_B receptor-mediated inhibition was replicated by bath application of forskolin and blocked by application of a PKA inhibitor, suggesting activation of adenylyl cyclase and PKA are required. Immunolabeling for an extracellular domain of the GABA_B2 subunit revealed reduced surface expression in the molecular layer after exposure to forskolin. GABA_B receptor-mediated inhibition of action potential evoked calcium transients in parallel fiber varicosities was also reduced following bath application of forskolin, confirming presynaptic receptors are responsible for the reduced EPSC inhibition. These data demonstrate that presynaptic GABA_B receptor expression can be a plastic property of synapses, which may compliment other forms of synaptic plasticity. This opens the door to novel forms of receptor plasticity previously confined primarily to postsynaptic receptors.

Loss of Ethanol Inhibition of N-Methyl-D-Aspartate Receptor-Mediated Currents and Plasticity of Cerebellar Synapses in Mice Expressing the GluN1(F639A) Subunit

BACKGROUND

Glutamatergic N-methyl-d-aspartate receptors (NMDARs) are well known for their sensitivity to ethanol (EtOH) inhibition. However, the specific manner in which EtOH inhibits channel activity and how such inhibition affects neurotransmission, and ultimately behavior, remains unclear. Replacement of phenylalanine 639 with alanine (F639A) in the GluN1 subunit reduces EtOH inhibition of recombinant NMDARs. Mice expressing this subunit show reduced EtOH-induced anxiolysis, blunted locomotor stimulation following low-dose EtOH administration, and faster recovery of motor function after moderate doses of EtOH, suggesting that cerebellar dysfunction may contribute to some of these behaviors. In the mature mouse cerebellum, NMDARs at the cerebellar climbing fiber (CF) to Purkinje cell (PC) synapse are inhibited by low concentrations of EtOH and the long-term depression (LTD) of parallel fiber (PF)-mediated currents induced by concurrent activation of PFs and CFs (PF-LTD) requires activation of EtOH-sensitive NMDARs. In this study, we examined cerebellar NMDA responses and NMDA-mediated synaptic plasticity in wild-type (WT) and GluN1(F639A) mice.

METHODS

Patch-clamp electrophysiological recordings were performed in acute cerebellar slices from adult WT and GluN1(F639A) mice. NMDAR-mediated currents at the CF-PC synapse and NMDAR-dependent PF-LTD induction were compared for genotype-dependent differences.

RESULTS

Stimulation of CFs evoked robust NMDA-mediated excitatory postsynaptic currents (EPSCs) in PCs that were similar in amplitude and kinetics between WT and GluN1(F639A) mice. NMDA-mediated CF-PC EPSCs in WT mice were significantly inhibited by EtOH (50 mM) while those in mutant mice were unaffected. Concurrent stimulation of CF and PF inputs induced synaptic depression of PF-PC EPSCs in both WT and mutant mice, and this depression was blocked by the NMDA antagonist DL-APV. The synaptic depression of PF-PC EPSCs in WT mice was also blocked by a low concentration of EtOH (10 mM) that had no effect on plasticity in GluN1(F639A) mice.

CONCLUSIONS

These results demonstrate that inhibition of cerebellar NMDARs may be a key mechanism by which EtOH affects cerebellar-dependent behaviors.

Impact of NMDA Receptor Overexpression on Cerebellar Purkinje Cell Activity and Motor Learning

In many brain regions involved in learning NMDA receptors (NMDARs) act as coincidence detectors of pre- and postsynaptic activity, mediating Hebbian plasticity. Intriguingly, the parallel fiber (PF) to Purkinje cell (PC) input in the cerebellar cortex, which is critical for procedural learning, shows virtually no postsynaptic NMDARs. Why is this? Here, we address this question by generating and testing independent transgenic lines that overexpress NMDAR containing the type 2B subunit (NR2B) specifically in PCs. PCs of the mice that show larger NMDA-mediated currents than controls at their PF input suffer from a blockage of long-term potentiation (LTP) at their PF-PC synapses, while long-term depression (LTD) and baseline transmission are unaffected. Moreover, introducing NMDA-mediated currents affects cerebellar learning in that phase-reversal of the vestibulo-ocular reflex (VOR) is impaired. Our results suggest that under physiological circumstances PC spines lack NMDARs postsynaptically at their PF input so as to allow LTP to contribute to motor learning.

Frequency-dependent mobilization of heterogeneous pools of synaptic vesicles shapes presynaptic plasticity

The segregation of the readily releasable pool of synaptic vesicles (RRP) in sub-pools that are differentially poised for exocytosis shapes short-term plasticity. However, the frequency-dependent mobilization of these sub-pools is poorly understood. Using slice recordings and modeling of synaptic activity at cerebellar granule cell to Purkinje cell synapses of mice, we describe two sub-pools in the RRP that can be differentially recruited upon ultrafast changes in the stimulation frequency. We show that at low-frequency stimulations, a first sub-pool is gradually silenced, leading to full blockage of synaptic transmission. Conversely, a second pool of synaptic vesicles that cannot be released by a single stimulus is recruited within milliseconds by high-frequency stimulation and support an ultrafast recovery of neurotransmitter release after low-frequency depression. This frequency-dependent mobilization or silencing of sub-pools in the RRP in terminals of granule cells may play a role in the filtering of sensorimotor information in the cerebellum.

A method to induce human cortical long-term potentiation by acoustic stimulation

OBJECT

Acoustic stimulation induced LTP in the human auditory cortex was successfully recorded for the first time by electroencephalography (EEG) using a stimulus of 1 kHz pure-tone in 2005. However, it was barely reproduced, given considerable challenges to reliably elicit and accurately record the enhanced potentials in vivo. The purpose of this paper was to explore whether acoustic stimuli other than 1 kHz pure-tone could generate LTP or not.

MEASURES

To answer this question, we proposed a tetanic-stimulation paradigm of pure-tones, narrow-band noises (NBNs) and white noise (WN) to elicit LTP in human subjects.

RESULTS

The results showed that pure-tones with different frequency could elicit LTP in human auditory cortex, and proved for the first time that NBNs and WN could also achieve the same goal. Interestingly, it was also shown that the noises with certain bandwidth induced the greatest LTP and the WN induced LTP had the least variation over time and across subjects in comparison with pure-tones and NBNs.

CONCLUSIONS

In light of the results, we suggested to use the paradigm for broader studies of human in vivo cortical plasticity.

Activation of the CREB/c-Fos Pathway during Long-Term Synaptic Plasticity in the Cerebellum Granular Layer

The induction of long-term potentiation and depression (LTP and LTD) is thought to trigger gene expression and protein synthesis, leading to consolidation of synaptic and neuronal changes. However, while LTP and LTD have been proposed to play important roles for sensori-motor learning in the cerebellum granular layer, their association with these mechanisms remained unclear. Here, we have investigated phosphorylation of the cAMP-responsive element binding protein (CREB) and activation of the immediate early gene c-Fos pathway following the induction of synaptic plasticity by theta-burst stimulation (TBS) in acute cerebellar slices. LTP and LTD were localized using voltage-sensitive dye imaging (VSDi). At two time points following TBS (15 min and 120 min), corresponding to the early and late phases of plasticity, slices were fixed and processed to evaluate CREB phosphorylation (P-CREB) and c-FOS protein levels, as well as Creb and c-Fos mRNA expression. High levels of P-CREB and Creb/c-Fos were detected before those of c-FOS, as expected if CREB phosphorylation triggered gene expression followed by protein synthesis. No differences between control slices and slices stimulated with TBS were observed in the presence of an N-methyl-D-aspartate receptor (NMDAR) antagonist. Interestingly, activation of the CREB/c-Fos system showed a relevant degree of colocalization with long-term synaptic plasticity. These results show that NMDAR-dependent plasticity at the cerebellum input stage bears about transcriptional and post-transcriptional processes potentially contributing to cerebellar learning and memory consolidation.

A role for NMDAR-dependent cerebellar plasticity in adaptive control of saccades in humans

BACKGROUND

Saccade pulse amplitude adaptation is mediated by the dorsal cerebellar vermis and fastigial nucleus. Long-term depression at the parallel fibre-Purkinjie cell synapses has been suggested to provide a cellular mechanism for the corresponding learning process. The mechanisms and sites of this plasticity, however, are still debated.

OBJECTIVE

To test the role of cerebellar plasticity phenomena on adaptive saccade control.

METHODS

We evaluated the effect of continuous theta burst stimulation (cTBS) over the posterior vermis on saccade amplitude adaptation and spontaneous recovery of the initial response. To further identify the substrate of synaptic plasticity responsible for the observed adaptation impairment, subjects were pre-treated with memantine, an N-methyl-d-aspartate receptor (NMDAR) antagonist.

RESULTS

Amplitude adaptation was altered by cTBS, suggesting that cTBS interferes with cerebellar plasticity involved in saccade adaptation. Amplitude adaptation and spontaneous recovery were not affected by cTBS when recordings were preceded by memantine administration.

CONCLUSION

The effects of cTBS are NMDAR-dependent and are likely to involve long-term potentiation or long-term depression at specific synaptic connections of the granular and molecular layer, which could effectively take part in cerebellar motor learning.

Granular Layer Neurons Control Cerebellar Neurovascular Coupling Through an NMDA Receptor/NO-Dependent System

Neurovascular coupling (NVC) is the process whereby neuronal activity controls blood vessel diameter. In the cerebellum, the molecular layer is regarded as the main NVC determinant. However, the granular layer is a region with variable metabolic demand caused by large activity fluctuations that shows a prominent expression of NMDA receptors (NMDARs) and nitric oxide synthase (NOS) and is therefore much more suitable for effective NVC. Here, we show, in the granular layer of acute rat cerebellar slices, that capillary diameter changes rapidly after mossy fiber stimulation. Vasodilation required neuronal NMDARs and NOS stimulation and subsequent guanylyl cyclase activation that probably occurred in pericytes. Vasoconstriction required metabotropic glutamate receptors and CYP ω-hydroxylase, the enzyme regulating 20-hydroxyeicosatetraenoic acid production. Therefore, granular layer capillaries are controlled by the balance between vasodilating and vasoconstricting systems that could finely tune local blood flow depending on neuronal activity changes at the cerebellar input stage.

SIGNIFICANCE STATEMENT

The neuronal circuitry and the biochemical pathways that control local blood flow supply in the cerebellum are unclear. This is surprising given the emerging role played by this brain structure, not only in motor behavior, but also in cognitive functions. Although previous studies focused on the molecular layer, here, we shift attention onto the mossy fiber granule cell (GrC) relay. We demonstrate that GrC activity causes a robust vasodilation in nearby capillaries via the NMDA receptors-neuronal nitric oxide synthase signaling pathway. At the same time, metabotropic glutamate receptors mediate 20-hydroxyeicosatetraenoic acid-dependent vasoconstriction. These results reveal a complex signaling network that hints for the first time at the granular layer as a major determinant of cerebellar blood-oxygen-level-dependent signals.