Parallel fiber plasticity

Cerebellar long-term depression (LTD) is classically observed when climbing fibers, originating from the inferior olive, and parallel fibers, axons of granule cells, are activated repetitively and synchronously. On the basis that the climbing fiber signals errors in motor performance, LTD provides a mechanism of learning whereby inappropriate motor signals, relayed to the cerebellar cortex by parallel fibers, are selectively weakened through their repeated, close temporal association with climbing fiber activity. LTD therefore provides a cellular substrate for error-driven motor learning in the cerebellar cortex. In recent years, it has become apparent that depression at this synapse can also occur without the need for concurrent climbing fiber activation provided the parallel fibers are activated in such a way as to mobilize calcium within the Purkinje cell. A form of long-term potentiation (LTP) has also been uncovered at this synapse, which similarly relies only upon parallel fiber activation. In brain slice preparations and contrary to expectation, each of these forms of parallel fiber induced plasticity, as well as classical LTD, does not remain confined to activated parallel fibers as previously thought, but both depression and potentiation have the capacity to spread to neighboring parallel fiber synapses several tens of microns away from the activated fibers. Here, the cellular mechanisms responsible for the induction and heterosynaptic spread of parallel fiber LTP and LTD are compared to those involved in classical LTD and the physiological implications that the heterosynaptic spread of plasticity may have on cerebellar signal processing are discussed.     

LTD,RP, and Motor Learning

Long-term depression (LTD) at excitatory synapses between parallel fibers and a Purkinje cell has been regarded as a critical cellular mechanism for motor learning. However, it was demonstrated that normal motor learning occurs under LTD suppression, suggesting that cerebellar plasticity mechanisms other than LTD also contribute to motor learning. One candidate for such plasticity is rebound potentiation (RP), which is long-term potentiation at inhibitory synapses between a stellate cell and a Purkinje cell. Both LTD and RP are induced by the increase in postsynaptic Ca²⁺ concentration, and work to suppress the activity of a Purkinje cell. Thus, LTD and RP might work synergistically, and one might compensate defects of the other. RP induction is dependent on the interaction between GABA_A receptor and GABA_A receptor binding protein (GABARAP). Transgenic mice expressing a peptide which inhibits binding of GABARAP and GABA_A receptor only in Purkinje cells show defects in both RP and adaptation of vestibulo-ocular reflex (VOR), a motor learning paradigm. However, another example of motor learning, adaptation of optokinetic response (OKR), is normal in the transgenic mice. Both VOR and OKR are reflex eye movements suppressing the slip of visual image on the retina during head movement. Previously, we reported that delphilin knockout mice show facilitated LTD induction and enhanced OKR adaptation, but we recently found that VOR adaptation was not enhanced in the knockout mice. These results together suggest that animals might use LTD and RP differently depending on motor learning tasks.     

Calcium Channel-Dependent Induction of Long-Term Synaptic Plasticity at Excitatory Golgi Cell Synapses of Cerebellum

Golgi cells, together with granule cells and mossy fibers, form a neuronal microcircuit regulating information transfer at the cerebellum input stage. Despite theoretical predictions, little was known about long-term synaptic plasticity at Golgi cell synapses. Here, we have used whole-cell patch-clamp recordings and calcium imaging to investigate long-term synaptic plasticity at excitatory synapses impinging on Golgi cells. In acute mouse cerebellar slices, mossy fiber theta-burst stimulation (TBS) could induce either long-term potentiation (LTP) or long-term depression (LTD) at mossy fiber-Golgi cell and granule cell-Golgi cell synapses. This synaptic plasticity showed a peculiar voltage dependence, with LTD or LTP being favored when TBS induction occurred at depolarized or hyperpolarized potentials, respectively. LTP required, in addition to NMDA channels, activation of T-type Ca²⁺ channels, while LTD required uniquely activation of L-type Ca²⁺ channels. Notably, the voltage dependence of plasticity at the mossy fiber-Golgi cell synapses was inverted with respect to pure NMDA receptor-dependent plasticity at the neighboring mossy fiber-granule cell synapse, implying that the mossy fiber presynaptic terminal can activate different induction mechanisms depending on the target cell. In aggregate, this result shows that Golgi cells show cell-specific forms of long-term plasticity at their excitatory synapses, that could play a crucial role in sculpting the response patterns of the cerebellar granular layer.SIGNIFICANCE STATEMENT This article shows for the first time a novel form of Ca²⁺ channel-dependent synaptic plasticity at the excitatory synapses impinging on cerebellar Golgi cells. This plasticity is bidirectional and inverted with respect to NMDA receptor-dependent paradigms, with long-term depression (LTD) and long-term potentiation (LTP) being favored at depolarized and hyperpolarized potentials, respectively. Furthermore, LTP and LTD induction requires differential involvement of T-type and L-type voltage-gated Ca²⁺ channels rather than the NMDA receptors alone. These results, along with recent computational predictions, support the idea that Golgi cell plasticity could play a crucial role in controlling information flow through the granular layer along with cerebellar learning and memory.     

The role of calcium in synaptic plasticity and motor learning in the cerebellar cortex

The cerebellum is important for motor coordination, as well as motor learning and memories. Learning is believed to occur in the cerebellar cortex, in the form of synaptic plasticity. Central to motor learning theory are Purkinje cells (PCs), which are the sole output neurons of the cerebellar cortex. Motor memories are postulated to be stored in the form of long-term depression (LTD) at parallel fiber synapses with PCs, once thought to be the only plastic synapse in the cerebellar cortex. However, in the past few decades many studies have demonstrated that several other synapses in the cerebellar cortex are indeed plastic, and that LTD or long-term potentiation at these various synapses could affect the overall output signal of PCs from the cerebellar cortex. Almost all of these forms of synaptic plasticity are dependent on calcium to some extent. In the current review we discuss various types of synaptic plasticity in the cerebellar cortex and the role of calcium in these forms of plasticity.     

Regulation and Interaction of Multiple Types of Synaptic Plasticity in a Purkinje Neuron and Their Contribution to Motor Learning

There are multiple types of plasticity at both excitatory glutamatergic and inhibitory GABAergic synapses onto a cerebellar Purkinje neuron (PN). At parallel fiber to PN synapses, long-term depression (LTD) and long-term potentiation (LTP) occur, while at molecular layer interneuron to PN synapses, a type of LTP called rebound potentiation (RP) takes place. LTD, LTP, and RP seem to contribute to motor learning. However, each type of synaptic plasticity might play a different role in various motor learning paradigms. In addition, defects in one type of synaptic plasticity could be compensated by other forms of synaptic plasticity, which might conceal the contribution of a particular type of synaptic plasticity to motor learning. The threshold stimulation for inducing each type of synaptic plasticity and the induction conditions are different for different plasticity mechanisms, and they change depending on the state of an animal. Facilitation and/or saturation of synaptic plasticity occur after certain behavioral experiences or in some transgenic mice. Thus, the regulation and roles of synaptic plasticity are complicated. Toward a comprehensive understanding of the respective roles of each type of synaptic plasticity and their possible interactions during motor learning processes, I summarize induction conditions, modulations, interactions, and saturation of synaptic plasticity and discuss how multiple types of synaptic plasticity in a PN might work together in motor learning processes.     

Depressed by Learning—Heterogeneity of the Plasticity Rules at Parallel Fiber Synapses onto Purkinje Cells

Climbing fiber-driven long-term depression (LTD) of parallel fiber synapses onto cerebellar Purkinje cells has long been investigated as a putative mechanism of motor learning. We recently discovered that the rules governing the induction of LTD at these synapses vary across different regions of the cerebellum. Here, we discuss the design of LTD induction protocols in light of this heterogeneity in plasticity rules. The analytical advantages of the cerebellum provide an opportunity to develop a deeper understanding of how the specific plasticity rules at synapses support the implementation of learning.     

Properties of 4 Hz stimulation‐induced parallel fiber–Purkinje cell presynaptic long‐term plasticity in mouse cerebellar cortex in vivo

Cerebellar parallel fiber–Purkinje cell (PF–PC) long‐term synaptic plasticity is important for the formation and stability of cerebellar neuronal circuits, and provides substrates for motor learning and memory. We previously reported both presynaptic long‐term potentiation (LTP) and long‐term depression (LTD) in cerebellar PF–PC synapses in vitro. However, the expression and mechanisms of cerebellar PF–PC synaptic plasticity in the cerebellar cortex in vivo are poorly understood. In the present study, we studied the properties of 4 Hz stimulation‐induced PF–PC presynaptic long‐term plasticity using in vivo the whole‐cell patch‐clamp recording technique and pharmacological methods in urethane‐anesthetised mice. Our results demonstrated that 4 Hz PF stimulation induced presynaptic LTD of PF–PC synaptic transmission in the intact cerebellar cortex in living mice. The PF–PC presynaptic LTD was attenuated by either the N‐methyl‐D‐aspartate receptor antagonist, D‐aminophosphonovaleric acid, or the group 1 metabotropic glutamate receptor antagonist, JNJ16259685, and was abolished by combined D‐aminophosphonovaleric acid and JNJ16259685, but enhanced by inhibition of nitric oxide synthase. Blockade of cannabinoid type 1 receptor activity abolished the PF–PC LTD and revealed a presynaptic PF–PC LTP. These data indicate that both endocannabinoids and nitric oxide synthase are involved in the 4 Hz stimulation‐induced PF–PC presynaptic plasticity, but the endocannabinoid‐dependent PF–PC presynaptic LTD masked the nitric oxide‐mediated PF–PC presynaptic LTP in the cerebellar cortex in urethane‐anesthetised mice.     

Simple spike and complex spike activity of floccular Purkinje cells during the optokinetic reflex in mice lacking cerebellar long-term depression

Cerebellar long-term depression (LTD) at parallel fibre-Purkinje cell (P-cell) synapses is thought to embody neuronal information storage for motor learning. Transgenic L7-protein kinase C inhibitor (PKCI) mice in which cerebellar LTD is selectively blocked do indeed exhibit impaired adaptation in the vestibulo-ocular reflex (VOR) while their default oculomotor performance is unaffected. Although supportive, these data do not definitively establish a causal link between memory storage required for motor learning and cerebellar LTD. As the L7-PKCI transgene is probably activated from the early stages of P-cell development, an alternative could be that P-cells develop abnormal signals in L7-PKCI mutants, disturbing mechanisms of motor learning that rely on proper P-cell outputs. To test this alternative hypothesis, we studied simple spike (SS) and complex spike (CS) activity of vertical axis P-cells in the flocculus of L7-PKCI mice and their wild-type littermates during sinusoidal optokinetic stimulation. Both SS and CS discharge dynamics appeared to be very similar in wild-type and transgenic P-cells at all stimulus frequencies (0.05-0.8 Hz). The CS activity of all vertical axis cells increased with contralateral stimulus rotation and lagged ipsiversive eye velocity by 165-180 degrees. The SS modulation was roughly reciprocal to the CS modulation and lagged ipsiversive eye velocity by approximately 15 degrees. The baseline SS and CS discharge characteristics were indistinguishable between the two genotypes. We conclude that the impaired VOR learning in L7-PKCI mutants does not reflect fundamental aberrations of the cerebellar circuitry. The data thus strengthen the evidence that cerebellar LTD is implicated in rapid VOR learning but not in the development of normal default response patterns.     

Input minimization: a model of cerebellar learning without climbing fiber error signals.

The cerebellum is critical for motor learning. Current cerebellar learning models follow the Marr/Albus paradigm, in which climbing fibers provide error signals that shape plastic synapses between parallel fibers and Purkinje cells. However, climbing fibers have slow and largely random discharge, and seem unlikely to provide error signals with resolution sufficient to guide cerebellar learning. Parallel fibers carry error signals and could direct the plasticity of their own synapses, but the error signals are carried along with other signals. This report presents the new input minimization (InMin) model, in which Purkinje cells reduce error by minimizing their overall parallel fiber input. The slowly, randomly firing climbing fiber provides only synchronization pulses. InMin offers an alternative that can unify cerebellar findings.     

The effects of fear conditioning on cerebellar LTP and LTD

Long-term potentiation (LTP) and depression (LTD) at parallel fibre-Purkinje cell synapses have been described in vitro in the cerebellar cortex, but the physiological roles of these two forms of plasticity have not been well defined. Here we show that, in cerebellar slices taken from rats that had undergone fear conditioning, there was a significant occlusion of electrically induced LTP at parallel fibre-Purkinje cell synapses. This effect was long-lasting and related to associative processes, as LTP was not occluded in unpaired animals. Notably, in conditioned animals the LTP-inducing protocol produced LTD in some cells instead of LTP. Conversely, synaptic depression induced by conjunctive stimulation of parallel fibers and climbing fibres was impaired in tissue taken immediately following aversive stimulation in both paired and unpaired subjects. This effect was not, however, long-lasting as the incidence and extent of LTD returned to normal levels 24 h after behavioural testing. These findings suggest that LTP takes part in the mechanisms underlying aversive associative memories in the cerebellum.     

Mechanisms of locomotor control in the cerebellum

Animals as well as humans adapt their locomotor patterns to suit different situations. To perform smooth and stable locomotion, they coordinate not only parts of a limb but also different limbs. The cerebellum is important for sensorimotor control and plays a crucial role in intra- and inter-limb coordination. Cerebellar gait ataxia is characterized by postural deficiencies and decomposition of movements. During locomotion, the vermis and the intermediate region of the cerebellum receive information through the spinocerebellar pathways about the ongoing activities in the spinal stepping generator and the somatosensory receptors. The information is conveyed by mossy fiber afferents to Purkinje neurons via granule cells and their axons, i.e., parallel fibers. Purkinje neurons transform the mossy fiber input signals to output signals that in turn modulate activities in the brainstem descending tract neurons of the brainstem that are involved in locomotion. Further, Purkinje neurons receive enhanced climbing fiber signals during perturbed locomotion. These climbing fiber signals may induce synaptic plasticity at the parallel fiber-Purkinje neuron synapses. Long-term depression (LTD) occurs in parallel fiber-Purkinje neuron synapses and is regarded as the cellular basis for the learning mechanism of the cerebellar neuronal circuit. The activation of parallel fibers releases glutamate and nitric oxide, and the released glutamate activates the glutamate receptors in the Purkinje neurons. mGluR1, a subtype of the metabotropic glutamate receptors, is highly expressed in Purkinje neurons. In addition, delta 2 glutamate receptor is expressed in only Purkinje neurons throughout the brain. Genetically targeted mice for these glutamate receptors and/or pharmacological blocking studies have been promoted to determine the functional linkage between the molecules at the cellular level and the adaptability of locomotion at the behavioral level. This article highlights some recent advances in the understanding of the role played by the cerebellum in the adaptive control of locomotion.     

Asymmetries in Cerebellar Plasticity and Motor Learning

Synaptic plasticity at the parallel fiber to Purkinje cell synapse has long been considered a cellular correlate for cerebellar motor learning. Functionally, long-term depression and long-term potentiation at these synapses seem to be the reverse of each other, with both pre- and post-synaptic expression occurring in both. However, different cerebellar motor learning paradigms have been shown to be asymmetric and not equally reversible. Here, we discuss the asymmetric reversibility shown in the vestibulo-ocular reflex and eyeblink conditioning and suggest that different cellular plasticity mechanisms might be recruited under different conditions leading to unequal reversibility.     

Role of β3-adrenergic receptor in the modulation of synaptic transmission and plasticity in mouse cerebellar cortex

Convergent lines of evidence have recently highlighted β3-adrenoreceptors (ARs) as a potentially critical target in the regulation of nervous and behavioral functions, including memory consolidation, anxiety, and depression. Nevertheless, the role of β3-ARs in the cerebellum has been never investigated. To address this issue, we first examined the effects of pharmacological manipulation of β3-ARs on motor learning in mice. We found that blockade of β3-ARs by SR 59230A impaired the acquisition of the rotarod task with no effect on general locomotion. Since the parallel fiber–Purkinje cell (PF–PC) synapse is considered to be the main cerebellar locus of motor learning, we assessed β3-AR modulatory action on this synapse as well as its expression in cerebellar slices. We demonstrate, for the first time, a strong expression of β3-ARs on Purkinje cell soma and dendrites. In addition, whole-cell patch-clamp recordings revealed that bath application of β3-AR agonist CL316,243 depressed the PF–PC excitatory postsynaptic currents via a postsynaptic mechanism mediated by the PI3K signaling pathway. Application of CL316,243 also interfered with the expression of PF long-term potentiation, whereas SR 59230A prevented the induction of LTD at PF–PC synapse. These results underline the critical role of β3-AR on cerebellar synaptic transmission and plasticity and provide a new mechanism for adrenergic modulation of motor learning.     

The Ins and Outs of GluD2—Why and How Purkinje Cells Use the Special Glutamate Receptor

The δ2 glutamate receptor (GluD2) is predominantly expressed in cerebellar Purkinje cells and plays crucial roles in cerebellar functions. Indeed, the number of synapses between parallel fibers (PFs) and Purkinje cells is specifically and severely reduced in GluD2-null cerebellum. In addition, long-term depression (LTD) at PF-Purkinje cell synapses is impaired in these mice. Nevertheless, the mechanism by which GluD2 regulate these two functions-morphological and functional synaptic plasticity at PF synapses-has remained unclear. Recently, we found that Cbln1, a glycoprotein released from granule cells, was bound to the N-terminal domain of GluD2 and regulated formation and maintenance of PF-Purkinje cell synapses. Furthermore, we found that D: -Ser released from Bergmann glia bound the ligand-binding domain of GluD2 and mediated LTD in a manner dependent on the C-terminus. These findings indicate how GluD2 is activated and regulates functions at PF-Purkinje cell synapses. A hypothesis about why GluD2 is employed by PF synapses is also discussed.     

Cerebellar long-term potentiation under suppressed postsynaptic Ca2+ activity.

To study the roles of postsynaptic Ca2+ activity in cerebellar synaptic plasticity, we used a patch-recording technique in Purkinje cell dendrites. While the combination of parallel fibre stimulation and 8-bromo cyclic guanosine monophosphate (Br-cGMP) application produced long-term depression (LTD) of the parallel fibre/Purkinje cell transmission, the same stimuli evoked long-term potentiation (LTP) during postsynaptic injection of Ethyleneglycol-bis-(beta-amino-ethylether)-N, N, N', N'-tetraacetate (EGTA). Furthermore, in the presence of alpha-aminobutyric acid (GABA), parallel fibre stimulation plus Br-cGMP produced LTP of extracellular K+ increases following parallel fibre stimulation. These results suggest that postsynaptic Ca2+ activity in Purkinje cells is negatively correlated to the direction of plastic changes and that the Ca2+ changes and cGMP play distinct roles in cerebellar synaptic plasticity.     

Adaptive-filter Models of the Cerebellum: Computational Analysis

Many current models of the cerebellar cortical microcircuit are equivalent to an adaptive filter using the covariance learning rule. The adaptive filter is a development of the original Marr–Albus framework that deals naturally with continuous time-varying signals, thus addressing the issue of 'timing' in cerebellar function, and it can be connected in a variety of ways to other parts of the system, consistent with the microzonal organization of cerebellar cortex. However, its computational capacities are not well understood. Here we summarise the results of recent work that has focused on two of its intrinsic properties. First, an adaptive filter seeks to decorrelate its (mossy fibre) inputs from a (climbing fibre) teaching signal. This procedure can be used both for sensory processing, e.g. removal of interference from sensory signals, and for learning accurate motor commands, by decorrelating an efference copy of those commands from a sensory signal of inaccuracy. As a model of the cerebellum the adaptive filter thus forms a natural link between events at the cellular level, such as forms of synaptic plasticity and the learning rules they embody, and intelligent behaviour at the system level. Secondly, it has been shown that the covariance learning rule enables the filter to handle input and intrinsic noise optimally. Such optimality may underlie the recently described role of the cerebellum in producing accurate smooth pursuit eye movements in the face of sensory noise. Moreover, it has the consequence of driving most input weights to very small values, consistent with experimental data that many parallel-fibre synapses are normally silent. The effectiveness of silent synapses can only be altered by LTP, so learning tasks depending on a reduction of Purkinje cell firing require the synapses to be embedded in a second, inhibitory pathway from parallel fibre to Purkinje cell. This pathway and the appropriate climbing-fibre related plasticity have been described experimentally, and its presence has implications for asymmetries and hysteresis in behavioural learning rates that are also consistent with experimental observations. These computational properties of the adaptive filter suggest that it is both powerful and realistic enough to be a suitable candidate model of the cerebellar cortical microcircuit.     

Purkinje Neurons: Development,Morphology, and Function

Cerebellar Purkinje neurons are arguably some of the most conspicuous neurons in the vertebrate central nervous system. They have characteristic planar fan-shaped dendrites which branch extensively and fill spaces almost completely with little overlap. This dendritic morphology is well suited to receiving a single or a few excitatory synaptic inputs from each of more than 100,000 parallel fibers which run orthogonally to Purkinje cell dendritic trees. In contrast, another type of excitatory input to a Purkinje neuron is provided by a single climbing fiber, which forms some hundreds to thousands of synapses with a Purkinje neuron. This striking contrast between the two types of synaptic inputs to a Purkinje neuron has attracted many neuroscientists. It is also to be noted that Purkinje neurons are the sole neurons sending outputs from the cerebellar cortex. In other words, all computational results within the cortex are transmitted by Purkinje cell axons, which inhibit neurons in the cerebellar or vestibular nucleus. Notably, Purkinje neurons show several forms of synaptic plasticity. Among them, long-term depression (LTD) at parallel fiber synapses has been regarded as a putatively essential mechanism for cerebellum-dependent learning. In this special issue on Purkinje neurons, you will find informative reviews and original papers on the development, characteristics and functions of Purkinje neurons, or related themes contributed by outstanding researchers.     

Cerebellar Synaptic Plasticity and the Credit Assignment Problem

The mechanism by which a learnt synaptic weight change can contribute to learning or adaptation of brain function is a type of credit assignment problem, which is a key issue for many parts of the brain. In the cerebellum, detailed knowledge not only of the local circuitry connectivity but also of the topography of different sources of afferent/external information makes this problem particularly tractable. In addition, multiple forms of synaptic plasticity and their general rules of induction have been identified. In this review, we will discuss the possible roles of synaptic and cellular plasticity at specific locations in contributing to behavioral changes. Focus will be on the parts of the cerebellum that are devoted to limb control, which constitute a large proportion of the cortex and where the knowledge of the external connectivity is particularly well known. From this perspective, a number of sites of synaptic plasticity appear to primarily have the function of balancing the overall level of activity in the cerebellar circuitry, whereas the locations at which synaptic plasticity leads to functional changes in terms of limb control are more limited. Specifically, the postsynaptic forms of long-term potentiation (LTP) and long-term depression (LTD) at the parallel fiber synapses made on interneurons and Purkinje cells, respectively, are the types of plasticity that mediate the widest associative capacity and the tightest link between the synaptic change and the external functions that are to be controlled.     

The extreme C-terminus of GluRdelta2 is essential for induction of long-term depression in cerebellar slices

Long-term depression (LTD) of parallel fibre (PF)-Purkinje cell synapses in the cerebellum is recognized as a cellular substrate of motor learning. Although the delta2 glutamate receptor (GluRdelta2) has been shown to be crucial for LTD, the mechanisms by which GluRdelta2 functions remain elusive. In this study, we developed a virus vector-based gene transfer approach to rescue impaired LTD in GluRdelta2-null Purkinje cells in cerebellar slice preparations. We demonstrated that LTD was restored in GluRdelta2-null Purkinje cells transduced with wild-type but not with mutant GluRdelta2, which lacked the PDZ-ligand domain in the C-terminus. Immunohistochemical analysis revealed no difference in expression levels or spine localization patterns between virally introduced wild-type and mutant GluRdelta2 proteins. Similarly, LTD was abrogated in Purkinje cells that had been acutely perfused with peptides, hampering the interaction of GluRdelta2 with PDZ proteins such as PSD-93, PTPMEG and S-SCAM but not with delphilin. Together, these results indicate that PDZ proteins that bind to the C-terminus of GluRdelta2 are not essential for localizing GluRdelta2 at synapses but are crucial for conveying signals necessary for the induction of LTD.     

Around LTD Hypothesis in Motor Learning

Long-term depression (LTD) at parallel fiber-Purkinje neuron synapses has been regarded as a primary cellular mechanism for motor learning. However, this hypothesis has been challenged. Demonstration of normal motor learning under LTD-suppressed conditions suggested that motor learning can occur without LTD. Synaptic plasticity mechanisms other than LTD have been found at various synapses in the cerebellum. Animals may achieve motor learning using several types of synaptic plasticity in the cerebellum including LTD.