Regulation of cerebellar network development by granule cells and their molecules

The well-organized cerebellar structures and neuronal networks are likely crucial for their functions in motor coordination, motor learning, cognition, and emotion. Such cerebellar structures and neuronal networks are formed during developmental periods through orchestrated mechanisms, which include not only cell-autonomous programs but also interactions between the same or different types of neurons. Cerebellar granule cells (GCs) are the most numerous neurons in the brain and are generated through intensive cell division of GC precursors (GCPs) during postnatal developmental periods. While GCs go through their own developmental processes of proliferation, differentiation, migration, and maturation, they also play a crucial role in cerebellar development. One of the best-characterized contributions is the enlargement and foliation of the cerebellum through massive proliferation of GCPs. In addition to this contribution, studies have shown that immature GCs and GCPs regulate multiple factors in the developing cerebellum, such as the development of other types of cerebellar neurons or the establishment of afferent innervations. These studies have often found impairments of cerebellar development in animals lacking expression of certain molecules in GCs, suggesting that the regulations are mediated by molecules that are secreted from or present in GCs. Given the growing recognition of GCs as regulators of cerebellar development, this review will summarize our current understanding of cerebellar development regulated by GCs and molecules in GCs, based on accumulated studies and recent findings, and will discuss their potential further contributions.

Cav2.2 Channels Sustain Vesicle Recruitment at a Mature Glutamatergic Synapse

The composition of voltage-gated Ca2+ channel (Cav) subtypes that gate action potential (AP)-evoked release changes during the development of mammalian CNS synapses. Cav2.2 and Cav2.3 lose their function in gating-evoked release during postnatal synapse maturation. In mature boutons, Cav2.1 currents provide the almost exclusive trigger for evoked release, and Cav2.3 currents are required for the induction of presynaptic long-term potentiation. However, the functional significance of Cav2.2 remained elusive in mature boutons, although they remain present at active zones and continue contributing significantly to presynaptic Ca2+ influx. Here, we addressed the functional significance of Cav2.2 and Cav2.3 at mature parallel-fiber (PF) to Purkinje neuron synapses of mice of either sex. These synapses are known to exhibit the corresponding developmental Cav subtype changes in gating release. We addressed two hypotheses, namely that Cav2.2 and Cav2.3 are involved in triggering spontaneous glutamate release and that they are engaged in vesicle recruitment during repetitive evoked release. We found that spontaneous miniature release is Ca2+ dependent. However, experiments with Cav subtype-specific blockers excluded the spontaneous opening of Cavs as the Ca2+ source for spontaneous glutamate release. Thus, neither Cav2.2 nor Cav2.3 controls spontaneous release from PF boutons. Furthermore, vesicle recruitment during brief bursts of APs was also independent of Ca2+ influx through Cav2.2 and Cav2.3. However, Cav2.2, but not Cav2.3, currents significantly boosted vesicle recruitment during sustained high-frequency synaptic transmission. Thus, in mature PF boutons Cav2.2 channels are specifically required to sustain synaptic transmission during prolonged neuronal activity.SIGNIFICANCE STATEMENT At young CNS synapses, action potential-evoked release is gated via three subtypes of voltage-gated Ca2+ channels: Cav2.1, Cav2.2, and Cav2.3. During postnatal maturation, Cav2.2 and Cav2.3 lose their function in gating evoked release, such that at mature synapses Cav2.1 provides the almost exclusive source for triggering evoked release. Cav2.3 currents are required for the induction of presynaptic long-term potentiation. However, the function of the still abundant Cav2.2 in mature boutons remained largely elusive. Here, we studied mature cerebellar parallel-fiber synapses and found that Cav2.2 does not control spontaneous release. However, Ca2+ influx through Cav2.2 significantly boosted vesicle recruitment during trains of action potentials. Thus, Cav2.2 in mature parallel-fiber boutons participate in sustaining synaptic transmission during prolonged activity.

How and Why the Cerebellum Recodes Input Signals: An Alternative to Machine Learning

Mossy fiber input to the cerebellum is received by granule cells where it is thought to be recoded into internal signals received by Purkinje cells, which alone carry the output of the cerebellar cortex. In any neural network, variables are contained in groups of signals as well as signals themselves—which cells are active and how many, for example, and statistical variables coded in rates, such as the mean and range, and which rates are strongly represented, in a defined population. We argue that the primary function of recoding is to confine translation to an effect of some variables and not others—both where input is recoded into internal signals and the translation downstream of internal signals into an effect on Purkinje cells. The cull of variables is harsh. Internal signaling is group coded. This allows coding to exploit statistics for a reliable and precise effect despite needing to work with high-dimensional input which is a highly unpredictably variable. An important effect is to normalize eclectic input signals, so that the basic, repeating cerebellar circuit, preserved across taxa, does not need to specialize (within regional variations). With this model, there is no need to slavishly conserve or compute data coded in single signals. If we are correct, a learning algorithm—for years, a mainstay of cerebellar modeling—would be redundant.

GluD2- and Cbln1-mediated competitive interactions shape the dendritic arbors of cerebellar Purkinje cells

The synaptotrophic hypothesis posits that synapse formation stabilizes dendritic branches, but this hypothesis has not been causally tested in vivo in the mammalian brain. The presynaptic ligand cerebellin-1 (Cbln1) and postsynaptic receptor GluD2 mediate synaptogenesis between granule cells and Purkinje cells in the molecular layer of the cerebellar cortex. Here we show that sparse but not global knockout of GluD2 causes under-elaboration of Purkinje cell dendrites in the deep molecular layer and overelaboration in the superficial molecular layer. Developmental, overexpression, structure-function, and genetic epistasis analyses indicate that these dendrite morphogenesis defects result from a deficit in Cbln1/GluD2-dependent competitive interactions. A generative model of dendrite growth based on competitive synaptogenesis largely recapitulates GluD2 sparse and global knockout phenotypes. Our results support the synaptotrophic hypothesis at initial stages of dendrite development, suggest a second mode in which cumulative synapse formation inhibits further dendrite growth, and highlight the importance of competition in dendrite morphogenesis.

Synergism of type 1 metabotropic and ionotropic glutamate receptors in cerebellar molecular layer interneurons in vivo

Type 1 metabotropic glutamate receptors (mGluR1s) are key elements in neuronal signaling. While their function is well documented in slices, requirements for their activation in vivo are poorly understood. We examine this question in adult mice in vivo using 2-photon imaging of cerebellar molecular layer interneurons (MLIs) expressing GCaMP. In anesthetized mice, parallel fiber activation evokes beam-like Ca_i rises in postsynaptic MLIs which depend on co-activation of mGluR1s and ionotropic glutamate receptors (iGluRs). In awake mice, blocking mGluR1 decreases Ca_i rises associated with locomotion. In vitro studies and freeze-fracture electron microscopy show that the iGluR-mGluR1 interaction is synergistic and favored by close association of the two classes of receptors. Altogether our results suggest that mGluR1s, acting in synergy with iGluRs, potently contribute to processing cerebellar neuronal signaling under physiological conditions.

Integration of Swimming-Related Synaptic Excitation and Inhibition by olig2+ Eurydendroid Neurons in Larval Zebrafish Cerebellum

The cerebellum influences motor control through Purkinje target neurons, which transmit cerebellar output. Such output is required, for instance, for larval zebrafish to learn conditioned fictive swimming. The output cells, called eurydendroid neurons (ENs) in teleost fish, are inhibited by Purkinje cells and excited by parallel fibers. Here, we investigated the electrophysiological properties of glutamatergic ENs labeled by the transcription factor olig2. Action potential firing and synaptic responses were recorded in current clamp and voltage clamp from olig2⁺ neurons in immobilized larval zebrafish (before sexual differentiation) and were correlated with motor behavior by simultaneous recording of fictive swimming. In the absence of swimming, olig2⁺ ENs had basal firing rates near 8 spikes/s, and EPSCs and IPSCs were evident. Comparing Purkinje firing rates and eurydendroid IPSC rates indicated that 1-3 Purkinje cells converge onto each EN. Optogenetically suppressing Purkinje simple spikes, while preserving complex spikes, suggested that eurydendroid IPSC size depended on presynaptic spike duration rather than amplitude. During swimming, EPSC and IPSC rates increased. Total excitatory and inhibitory currents during sensory-evoked swimming were both more than double those during spontaneous swimming. During both spontaneous and sensory-evoked swimming, the total inhibitory current was more than threefold larger than the excitatory current. Firing rates of ENs nevertheless increased, suggesting that the relative timing of IPSCs and EPSCs may permit excitation to drive additional eurydendroid spikes. The data indicate that olig2⁺ cells are ENs whose activity is modulated with locomotion, suiting them to participate in sensorimotor integration associated with cerebellum-dependent learning.SIGNIFICANCE STATEMENT The cerebellum contributes to movements through signals generated by cerebellar output neurons, called eurydendroid neurons (ENs) in fish (cerebellar nuclei in mammals). ENs receive sensory and motor signals from excitatory parallel fibers and inhibitory Purkinje cells. Here, we report electrophysiological recordings from ENs of larval zebrafish that directly illustrate how synaptic inhibition and excitation are integrated by cerebellar output neurons in association with motor behavior. The results demonstrate that inhibitory and excitatory drive both increase during fictive swimming, but inhibition greatly exceeds excitation. Firing rates nevertheless increase, providing evidence that synaptic integration promotes cerebellar output during locomotion. The data offer a basis for comparing aspects of cerebellar coding that are conserved and that diverge across vertebrates.

Rack1 Controls Parallel Fiber-Purkinje Cell Synaptogenesis and Synaptic Transmission

Purkinje cells (PCs) in the cerebellum receive two excitatory afferents including granule cells-derived parallel fiber (PF) and the climbing fiber. Scaffolding protein Rack1 is highly expressed in the cerebellar PCs. Here, we found delayed formation of specific cerebellar vermis lobule and impaired motor coordination in PC-specific Rack1 conditional knockout mice. Our studies further revealed that Rack1 is essential for PF–PC synapse formation. In addition, Rack1 plays a critical role in regulating synaptic plasticity and long-term depression (LTD) induction of PF–PC synapses without changing the expression of postsynaptic proteins. Together, we have discovered Rack1 as the crucial molecule that controls PF–PC synaptogenesis and synaptic plasticity. Our studies provide a novel molecular insight into the mechanisms underlying the neural development and neuroplasticity in the cerebellum.

Control of Presynaptic Parallel Fiber Efficacy by Activity-Dependent Regulation of the Number of Occupied Release Sites

Parallel fiber (PF) synapses show pronounced and lasting facilitation during bursts of high-frequency activity. They typically connect to their target neurons via a single active zone (AZ), harboring few release sites (~2-8) with moderate initial vesicular release probability (~0.2-0.4). In light of these biophysical characteristics, it seems surprising that PF synapses can sustain facilitation during high-frequency periods of tens of action potentials (APs). Recent findings suggest an increase in the number of occupied release sites due to ultra-rapid (~180 s^-1), Ca²⁺ dependent recruitment of synaptic vesicles (SVs) from replenishment sites as major presynaptic mechanism of this lasting facilitation. On the molecular level, Synaptotagmin 7 or Munc13s have been suggested to be involved in mediating facilitation at PF synapses. The recruitment of SVs from replenishment sites appears to be reversible on a slower time-scale, thereby, explaining that PF synapses rapidly depress and ultimately become silent during low-frequency activity. Hence, PF synapses show high-frequency facilitation (HFF) but low-frequency depression (LFD). This behavior is explained by regulation of the number of occupied release sites at the AZ by AP frequency.

Long-term depression of presynaptic cannabinoid receptor function at parallel fibre synapses

KEY POINTS

Inhibition of synaptic responses by activation of presynaptic cannabinoid type-1 (Cb1) receptors is reduced at parallel fibre synapses in the cerebellum following 4 Hz stimulation. Activation of adenylyl cyclase is necessary and sufficient for down-regulation of Cb1 receptors induced by 4 Hz stimulation. 4 Hz stimulation reduces Cb1 receptor function by (i) increasing the rate of endocannabinoid clearance from the synapse and (ii) decreasing expression of Cb1 receptors.

ABSTRACT

Cannabinoid type-1 receptors (Cb1R) are expressed in the presynaptic membrane of many synapses, including parallel fibre-Purkinje cell synapses in the cerebellum, where they are involved in short- and long-term plasticity of synaptic responses. We show that Cb1R expression itself is a plastic property of the synapse regulated by physiological activity patterns. We made patch clamp recordings from Purkinje cells in cerebellar slices and assessed Cb1R activity by measuring depolarization-induced suppression of excitation (DSE). We find that DSE is normally stable at parallel fibre synapses but, following 4 Hz stimulation, DSE is persistently reduced and recovers more rapidly. Using a combination of electrophysiology, pharmacology and biochemistry, we show that changes in DSE are a result of the reduced expression of Cb1Rs and increased degradation of endocannabinoids by monoacylglycerol lipase. Long-term changes in presynaptic Cb1R expression may alter other forms of Cb1R-dependent plasticity at parallel fibre synapses, priming or inhibiting the circuit for associative learning.

Direction of action of presynaptic GABAA receptors is highly dependent on the level of receptor activation

Many synapses, including parallel fiber synapses in the cerebellum, express presynaptic GABA_A receptors. However, reports of the functional consequences of presynaptic GABA_A receptor activation are variable across synapses, from inhibition to enhancement of transmitter release. We find that presynaptic GABA_A receptor function is bidirectional at parallel fiber synapses depending on GABA concentration and modulation of GABA_A receptors in mice. Activation of GABA_A receptors by low GABA concentrations enhances glutamate release, whereas activation of receptors by higher GABA concentrations inhibits release. Furthermore, blocking GABA_B receptors reduces GABA_A receptor currents and shifts presynaptic responses toward greater enhancement of release across a wide range of GABA concentrations. Conversely, enhancing GABA_A receptor currents with ethanol or neurosteroids shifts responses toward greater inhibition of release. The ability of presynaptic GABA_A receptors to enhance or inhibit transmitter release at the same synapse depending on activity level provides a new mechanism for fine control of synaptic transmission by GABA and may explain conflicting reports of presynaptic GABA_A receptor function across synapses. NEW & NOTEWORTHY GABA_A receptors are widely expressed at presynaptic terminals in the central nervous system. However, previous reports have produced conflicting results on the function of these receptors at different synapses. We show that presynaptic GABA_A receptor function is strongly dependent on the level of receptor activation. Low levels of receptor activation enhance transmitter release, whereas higher levels of activation inhibit release at the same synapses. This provides a novel mechanism by which presynaptic GABA_A receptors fine-tune synaptic transmission.

Electrophysiological Excitability and Parallel Fiber Synaptic Properties of Zebrin-Positive and -Negative Purkinje Cells in Lobule VIII of the Mouse Cerebellar Slice

Heterogeneous populations of cerebellar Purkinje cells (PCs) are arranged into separate longitudinal stripes, which have different topographic afferent and efferent axonal connections presumably involved in different functions, and also show different electrophysiological properties in firing pattern and synaptic plasticity. However, whether the differences in molecular expression that define heterogeneous PC populations affect their electrophysiological properties has not been much clarified. Since the expression pattern of many of such molecules, including glutamate transporter EAAT4, replicates that of aldolase C or zebrin II, we recorded from PCs of different "zebrin types" (zebrin-positive = aldolase C-positive = Z+; and Z-) in identified neighboring stripes in vermal lobule VIII, in which Z+ and Z- stripes occupy similar widths, in the Aldoc-Venus mouse cerebellar slice preparation. Regarding basic cellular electrophysiological properties, no significant differences were observed in input resistance or in occurrence probability of types of firing patterns between Z+ and Z- PCs. However, the firing frequency of the tonic firing type was higher in Z- PCs than in Z+ PCs. In the case of parallel fiber (PF)-PC synaptic transmission, no significant differences were observed between Z+ and Z- PCs in interval dependency of paired pulse facilitation or in time course of synaptic current measured without or with the blocker of glutamate receptor desensitization. These results indicate that different expression levels of the molecules that are associated with the zebrin type may affect the intrinsic firing property of PCs but not directly affect the basic electrophysiological properties of PF-PC synaptic transmission significantly in lobule VIII. The results suggest that the zebrin types of PCs in lobule VIII is linked with some intrinsic electrophysiological neuronal characteristics which affect the firing frequency of PCs. However, the results also suggest that the molecular expression differences linked with zebrin types of PCs does not much affect basic electrophysiological properties of PF-PC synaptic transmission in a physiological condition in lobule VIII.

Propofol Inhibits Cerebellar Parallel Fiber-Purkinje Cell Synaptic Transmission via Activation of Presynaptic GABAB Receptors in vitro in Mice

Propofol is a widely used intravenous sedative-hypnotic agent, which causes rapid and reliable loss of consciousness via activation of γ -aminobutyric acid A (GABA_A) receptors. We previously found that propofol inhibited cerebellar Purkinje cells (PC) activity via both GABA_A and glycine receptors in vivo in mice. We here examined the effect of propofol on the cerebellar parallel fiber (PF)-PC synaptic transmission in mouse cerebellar slices by whole-cell recording technique and pharmacological methods. We found that following blockade of GABA_A and glycine receptors activity, propofol reversely decreased the amplitude of PF-PC excitatory postsynaptic currents (PF-PC EPSCs), and significantly increased paired-pulse ratio (PPR). The propofol-induced decrease in amplitude of PF-PC EPSCs was concentration-dependent. The half-inhibitory concentration (IC₅₀) of propofol for inhibiting PF-PC EPSCs was 4.7 μM. Notably, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by GABA_B receptor antagonist, saclofen (10 μM), but not blocked by N-methyl-D-aspartate receptor (NMDA) receptor antagonist, D-APV (50 μM). Application of the GABA_B receptor agonist baclofen induced a decrease in amplitude and an increase in PPR of PF-PC EPSCs, as well masked the propofol-induced changes in PF-PC EPSCs. Moreover, the propofol-induced changes in amplitude and PPR of PF-PC EPSCs were abolished by a specific protein kinase A (PKA) inhibitor, KT5720. These results indicate that application of propofol facilitates presynaptic GABA_B receptors, resulting in a depression of PF-PC synaptic transmission via PKA signaling pathway in mouse cerebellar cortex. The results suggest that the interaction with GABA_B receptors may contribute to the general anesthetic action of propofol.

Different types of multiple-synapse boutons in the cerebellar cortex between physically enriched and ataxic mutant mice

Experience-dependent synapse remodeling is associated with information storage in the nervous system. Neuronal synapses show alteration in various neurological and cognitive disorders in their structure and function. At the ultrastructural level, parallel fiber boutons contacting multiple spines of Purkinje cells in the cerebellar cortex are commonly observed in physiologically enriched animals as well as pathological ataxic mutants. However, the dendritic origin of those spines on parallel fiber multiple-synapse boutons (MSBs) has been poorly understood. Here, we investigated this issue by 3-dimensional ultrastructural analysis to determine synaptic connectivity of MSBs in both mice housed in physically enriched environment and cerebellar ataxic mutants. Our results demonstrated that environmental enrichment selectively induced MSBs to contact spines from the same parent dendrite, indicating focal strengthening of synapse through the simultaneous activation of two adjacent spines. In contrast, ataxic mutants displaying impaired motor coordination had significantly more MSBs involving spines originating from different neighboring dendrites compared to both wild-type and environmentally enriched animals, suggesting that compromising multiple synapse formation may lead to abnormal motor behavior in the mutant mice. These findings propose that environmental stimulation in normal animals mainly involves the refinement of preexisting synaptic networks, whereas pathological ataxic conditions may results from less-selective but compromising multiple synaptic formation. This study underscores that different types of multiple synapse boutons may have disparate effects on cerebellar synaptic transmission.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

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.

Glutamate transporter GLAST controls synaptic wrapping by Bergmann glia and ensures proper wiring of Purkinje cells

Astrocytes regulate synaptic transmission through controlling neurotransmitter concentrations around synapses. Little is known, however, about their roles in neural circuit development. Here we report that Bergmann glia (BG), specialized cerebellar astrocytes that thoroughly enwrap Purkinje cells (PCs), are essential for synaptic organization in PCs through the action of the l-glutamate/l-aspartate transporter (GLAST). In GLAST-knockout mice, dendritic innervation by the main ascending climbing fiber (CF) branch was significantly weakened, whereas the transverse branch, which is thin and nonsynaptogenic in control mice, was transformed into thick and synaptogenic branches. Both types of CF branches frequently produced aberrant wiring to proximal and distal dendrites, causing multiple CF-PC innervation. Our electrophysiological analysis revealed that slow and small CF-evoked excitatory postsynaptic currents (EPSCs) were recorded from almost all PCs in GLAST-knockout mice. These atypical CF-EPSCs were far more numerous and had significantly faster 10-90% rise time than those elicited by glutamate spillover under pharmacological blockade of glial glutamate transporters. Innervation by parallel fibers (PFs) was also affected. PF synapses were robustly increased in the entire dendritic trees, leading to impaired segregation of CF and PF territories. Furthermore, lamellate BG processes were retracted from PC dendrites and synapses, leading to the exposure of these neuronal elements to the extracellular milieus. These synaptic and glial phenotypes were reproduced in wild-type mice after functional blockade of glial glutamate transporters. These findings highlight that glutamate transporter function by GLAST on BG plays important roles in development and maintenance of proper synaptic wiring and wrapping in PCs.

GluD2 Endows Parallel Fiber-Purkinje Cell Synapses with a High Regenerative Capacity

Although injured axons usually do not regenerate in the adult CNS, parallel fibers (PFs) regenerate synaptic connections onto cerebellar Purkinje cells (PCs). In this study, we investigated the role of GluD2 in this regenerative process after PF transection using GluD2-knock-out (KO) mice. All dendritic spines on distal dendrites were innervated by PFs in sham-operated wild-type controls, whereas one-third were devoid of innervation in GluD2-KO mice. In both genotypes, a steep drop in the number of PF synapses occurred with a reciprocal surge in the number of free spines on postlesion day 1, when the PF territory aberrantly expanded toward the proximal dendrites. In wild-type mice, the territory and number of PF synapses were nearly fully restored to normal on postlesion day 7, although PF density remained low. Moreover, presynaptic and postsynaptic elements were markedly enlarged, and the PF terminal-to-PC spine contact ratio increased from 1:1 to 1:2 at most synapses. On postlesion day 30, the size and contact ratio of PF synapses returned to sham-operated control values and PF density recovered through the sprouting and elongation of PF collaterals. However, GluD2-KO mice showed neither a hypertrophic response nor territorial restoration 7 d postlesion, nor the recovery of PF axons or synapses on postlesion day 30. This suggests that PF wiring regenerates initially by inducing hypertrophic responses in surviving synaptic elements (hypertrophic phase), followed by collateral formation by PF axons and retraction of PF synapses (remodeling phase). Without GluD2, no transition to these regenerative phases occurs.

SIGNIFICANCE STATEMENT

The glutamate receptor GluD2 expressed at parallel fiber (PF)-Purkinje cell (PC) synapses regulates the formation and maintenance of the synapses. To investigate the role of GluD2 in their extraordinarily high regenerative capacity, the process after surgical transection of PFs was compared between wild-type and GluD2-knock-out mice. We discovered that, in wild-type mice, PF synapses regenerate initially by inducing hypertrophic responses in surviving synaptic elements, and then by sprouting and elongation of PF collaterals. Subsequently, hypertrophied PF synapses remodel into compact synapses. In GluD2-knock-out mice, PF wiring remains in the degenerative phase, showing neither a hypertrophic response nor recovery of PF axons or synapses. Our finding thus highlights that synaptic connection in the adult brain can regenerate with aid of GluD2.

Numbers of presynaptic Ca2+ channel clusters match those of functionally defined vesicular docking sites in single central synapses

Many central synapses contain a single presynaptic active zone and a single postsynaptic density. Vesicular release statistics at such "simple synapses" indicate that they contain a small complement of docking sites where vesicles repetitively dock and fuse. In this work, we investigate functional and morphological aspects of docking sites at simple synapses made between cerebellar parallel fibers and molecular layer interneurons. Using immunogold labeling of SDS-treated freeze-fracture replicas, we find that Ca_v2.1 channels form several clusters per active zone with about nine channels per cluster. The mean value and range of intersynaptic variation are similar for Ca_v2.1 cluster numbers and for functional estimates of docking-site numbers obtained from the maximum numbers of released vesicles per action potential. Both numbers grow in relation with synaptic size and decrease by a similar extent with age between 2 wk and 4 wk postnatal. Thus, the mean docking-site numbers were 3.15 at 2 wk (range: 1-10) and 2.03 at 4 wk (range: 1-4), whereas the mean numbers of Ca_v2.1 clusters were 2.84 at 2 wk (range: 1-8) and 2.37 at 4 wk (range: 1-5). These changes were accompanied by decreases of miniature current amplitude (from 93 pA to 56 pA), active-zone surface area (from 0.0427 μm² to 0.0234 μm²), and initial success rate (from 0.609 to 0.353), indicating a tightening of synaptic transmission with development. Altogether, these results suggest a close correspondence between the number of functionally defined vesicular docking sites and that of clusters of voltage-gated calcium channels.

Modulation, Plasticity and Pathophysiology of the Parallel Fiber-Purkinje Cell Synapse

The parallel fiber-Purkinje cell (PF-PC) synapse represents the point of maximal signal divergence in the cerebellar cortex with an estimated number of about 60 billion synaptic contacts in the rat and 100,000 billions in humans. At the same time, the Purkinje cell dendritic tree is a site of remarkable convergence of more than 100,000 parallel fiber synapses. Parallel fiber activity generates fast postsynaptic currents via α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and slower signals, mediated by mGlu₁ receptors, resulting in Purkinje cell depolarization accompanied by sharp calcium elevation within dendritic regions. Long-term depression (LTD) and long-term potentiation (LTP) have been widely described for the PF-PC synapse and have been proposed as mechanisms for motor learning. The mechanisms of induction for LTP and LTD involve different signaling mechanisms within the presynaptic terminal and/or at the postsynaptic site, promoting enduring modification in the neurotransmitter release and change in responsiveness to the neurotransmitter. The PF-PC synapse is finely modulated by several neurotransmitters, including serotonin, noradrenaline and acetylcholine. The ability of these neuromodulators to gate LTP and LTD at the PF-PC synapse could, at least in part, explain their effect on cerebellar-dependent learning and memory paradigms. Overall, these findings have important implications for understanding the cerebellar involvement in a series of pathological conditions, ranging from ataxia to autism. For example, PF-PC synapse dysfunctions have been identified in several murine models of spino-cerebellar ataxia (SCA) types 1, 3, 5 and 27. In some cases, the defect is specific for the AMPA receptor signaling (SCA27), while in others the mGlu₁ pathway is affected (SCA1, 3, 5). Interestingly, the PF-PC synapse has been shown to be hyper-functional in a mutant mouse model of autism spectrum disorder, with a selective deletion of Pten in Purkinje cells. However, the full range of methodological approaches, that allowed the discovery of the physiological principles of PF-PC synapse function, has not yet been completely exploited to investigate the pathophysiological mechanisms of diseases involving the cerebellum. We, therefore, propose to extend the spectrum of experimental investigations to tackle this problem.

AMPA receptor mediated synaptic excitation drives state-dependent bursting in Purkinje neurons of zebrafish larvae

Purkinje neurons are central to cerebellar function and show membrane bistability when recorded in vitro or in vivo under anesthesia. The existence of bistability in vivo in awake animals is disputed. Here, by recording intracellularly from Purkinje neurons in unanesthetized larval zebrafish (Danio rerio), we unequivocally demonstrate bistability in these neurons. Tonic firing was seen in depolarized regimes and bursting at hyperpolarized membrane potentials. In addition, Purkinje neurons could switch from one state to another spontaneously or with current injection. While GABAAR or NMDAR were not required for bursting, activation of AMPARs by climbing fibers (CFs) was sufficient to trigger bursts. Further, by recording Purkinje neuron membrane potential intracellularly, and motor neuron spikes extracellularly, we show that initiation of motor neuron spiking is correlated with increased incidence of CF EPSPs and membrane depolarization. Developmentally, bistability was observed soon after Purkinje neuron specification and persists at least until late larval stages.

Parallel fiber to Purkinje cell synaptic impairment in a mouse model of spinocerebellar ataxia type 27

Genetically inherited mutations in the fibroblast growth factor 14 (FGF14) gene lead to spinocerebellar ataxia type 27 (SCA27), an autosomal dominant disorder characterized by heterogeneous motor and cognitive impairments. Consistently, genetic deletion of Fgf14 in Fgf14 (-/-) mice recapitulates salient features of the SCA27 human disease. In vitro molecular studies in cultured neurons indicate that the FGF14 (F145S) SCA27 allele acts as a dominant negative mutant suppressing the FGF14 wild type function and resulting in inhibition of voltage-gated Na(+) and Ca(2+) channels. To gain insights in the cerebellar deficits in the animal model of the human disease, we applied whole-cell voltage-clamp in the acute cerebellar slice preparation to examine the properties of parallel fibers (PF) to Purkinje neuron synapses in Fgf14 (-/-) mice and wild type littermates. We found that the AMPA receptor-mediated excitatory postsynaptic currents evoked by PF stimulation (PF-EPSCs) were significantly reduced in Fgf14 (-/-) animals, while short-term plasticity, measured as paired-pulse facilitation (PPF), was enhanced. Measuring Sr(2+)-induced release of quanta from stimulated synapses, we found that the size of the PF-EPSCs was unchanged, ruling out a postsynaptic deficit. This phenotype was corroborated by decreased expression of VGLUT1, a specific presynaptic marker at PF-Purkinje neuron synapses. We next examined the mGluR1 receptor-induced response (mGluR1-EPSC) that under normal conditions requires a gradual build-up of glutamate concentration in the synaptic cleft, and found no changes in these responses in Fgf14 (-/-) mice. These results provide evidence of a critical role of FGF14 in maintaining presynaptic function at PF-Purkinje neuron synapses highlighting critical target mechanisms to recapitulate the complexity of the SCA27 disease.

Rapid development of Purkinje cell excitability, functional cerebellar circuit, and afferent sensory input to cerebellum in zebrafish

The zebrafish has significant advantages for studying the morphological development of the brain. However, little is known about the functional development of the zebrafish brain. We used patch clamp electrophysiology in live animals to investigate the emergence of excitability in cerebellar Purkinje cells, functional maturation of the cerebellar circuit, and establishment of sensory input to the cerebellum. Purkinje cells are born at 3 days post-fertilization (dpf). By 4 dpf, Purkinje cells spontaneously fired action potentials in an irregular pattern. By 5 dpf, the frequency and regularity of tonic firing had increased significantly and most cells fired complex spikes in response to climbing fiber activation. Our data suggest that, as in mammals, Purkinje cells are initially innervated by multiple climbing fibers that are winnowed to a single input. To probe the development of functional sensory input to the cerebellum, we investigated the response of Purkinje cells to a visual stimulus consisting of a rapid change in light intensity. At 4 dpf, sudden darkness increased the rate of tonic firing, suggesting that afferent pathways carrying visual information are already active by this stage. By 5 dpf, visual stimuli also activated climbing fibers, increasing the frequency of complex spiking. Our results indicate that the electrical properties of zebrafish and mammalian Purkinje cells are highly conserved and suggest that the same ion channels, Nav1.6 and Kv3.3, underlie spontaneous pacemaking activity. Interestingly, functional development of the cerebellum is temporally correlated with the emergence of complex, visually-guided behaviors such as prey capture. Because of the rapid formation of an electrically-active cerebellum, optical transparency, and ease of genetic manipulation, the zebrafish has great potential for functionally mapping cerebellar afferent and efferent pathways and for investigating cerebellar control of motor behavior.

Long-term potentiation at cerebellar parallel fiber-Purkinje cell synapses requires presynaptic and postsynaptic signaling cascades

Long-term depression (LTD) and long-term potentiation (LTP) at cerebellar parallel fiber-Purkinje cell (PF-PC) synapses play critical roles in motor learning. The 1 Hz stimulation at PF-PC synapses induces a postsynaptically expressed LTP that requires a postsynaptic Ca(2+) transient, phosphatases, and nitric oxide (NO). However, the mechanism underlying 1 Hz PF-LTP remains unclear because none of the known events is related to each other. Here, we demonstrated that 1 Hz PF-LTP requires postsynaptic cytosolic phospholipase A2 α (cPLA2α)/arachidonic acid (AA) signaling and presynaptic endocannabinoid receptors. Using patch-clamp recording in cerebellar slices, we found that 1 Hz PF-LTP was abolished in cPLA2α-knock-out mice. This deficit was effectively rescued by the conjunction of 1 Hz PF stimulation and the local application of AA. 2-Arachidonoylglycerol and the retrograde activation of cannabinoid receptor 1 (CB1R) were also involved in 1 Hz LTP because it was blocked by the hydrolysis of 2-AG or by inhibiting CB1Rs. The amount of NO released was detected using an NO electrode in cultured granule cells and PF terminals. Our results showed that the activation of CB1Rs at PF terminals activated NO synthetase and promoted NO production. The 1 Hz PF-stimuli evoked limited NO, but 100 Hz PF stimulation generated a large amount. Therefore, 1 Hz PF-LTP, distinct from classical postsynaptically expressed plasticity, requires concurrent presynaptic and postsynaptic activity. In addition, NO of sufficient amplitude decides between the weakening and strengthening of PF-PC synapses.

Long-term depression as a model of cerebellar plasticity

Long-term depression (LTD) here concerned is persistent attenuation of transmission efficiency from a bundle of parallel fibers to a Purkinje cell. Uniquely, LTD is induced by conjunctive activation of the parallel fibers and the climbing fiber that innervates that Purkinje cell. Cellular and molecular processes underlying LTD occur postsynaptically. In the 1960s, LTD was conceived as a theoretical possibility and in the 1980s, substantiated experimentally. Through further investigations using various pharmacological or genetic manipulations of LTD, a concept was formed that LTD plays a major role in learning capability of the cerebellum (referred to as "Marr-Albus-Ito hypothesis"). In this chapter, following a historical overview, recent intensive investigations of LTD are reviewed. Complex signal transduction and receptor recycling processes underlying LTD are analyzed, and roles of LTD in reflexes and voluntary movements are defined. The significance of LTD is considered from viewpoints of neural network modeling. Finally, the controversy arising from the recent finding in a few studies that whereas LTD is blocked pharmacologically or genetically, motor learning in awake behaving animals remains seemingly unchanged is examined. We conjecture how this mismatch arises, either from a methodological problem or from a network nature, and how it might be resolved.

Quantification of the density of cooperative neighboring synapses required to evoke endocannabinoid signaling

The spatial pattern of synapse activation may impact on synaptic plasticity. This applies to the synaptically-evoked endocannabinoid-mediated short-term depression at the parallel fiber (PF) to Purkinje cell synapse, the occurrence of which requires close proximity between the activated synapses. Here, we determine quantitatively this required proximity, helped by the geometrical organization of the cerebellar molecular layer. Transgenic mice expressing a calcium indicator selectively in granule cells enabled the imaging of action potential-evoked presynaptic calcium rise in isolated, single PFs. This measurement was used to derive the number of PFs activated within a beam of PFs stimulated in the molecular layer, from which the density of activated PFs (input density) was calculated. This density was on average 2.8 μm(-2) in sagittal slices and twice more in transverse slices. The synaptically-evoked endocannabinoid-mediated suppression of excitation (SSE) evoked by ten stimuli at 200 Hz was determined from the monitoring of either postsynaptic responses or presynaptic calcium rise. The SSE was significantly larger when recorded in transverse slices, where the input density is larger. The exponential description of the SSE plotted as a function of the input density suggests that the SSE is half reduced when the input density decreases from 6 to 2 μm(-2). We conclude that, although all PFs are truncated in an acute sagittal slice, half of them remain respondent to stimulation, and activated synapses need to be closer than 1.5 μm to synergize in endocannabinoid signaling.

High frequency burst firing of granule cells ensures transmission at the parallel fiber to purkinje cell synapse at the cost of temporal coding

Cerebellar granule cells (GrCs) convey information from mossy fibers (MFs) to Purkinje cells (PCs) via their parallel fibers (PFs). MF to GrC signaling allows transmission of frequencies up to 1 kHz and GrCs themselves can also fire bursts of action potentials with instantaneous frequencies up to 1 kHz. So far, in the scientific literature no evidence has been shown that these high-frequency bursts also exist in awake, behaving animals. More so, it remains to be shown whether such high-frequency bursts can transmit temporally coded information from MFs to PCs and/or whether these patterns of activity contribute to the spatiotemporal filtering properties of the GrC layer. Here, we show that, upon sensory stimulation in both un-anesthetized rabbits and mice, GrCs can show bursts that consist of tens of spikes at instantaneous frequencies over 800 Hz. In vitro recordings from individual GrC-PC pairs following high-frequency stimulation revealed an overall low initial release probability of ~0.17. Nevertheless, high-frequency burst activity induced a short-lived facilitation to ensure signaling within the first few spikes, which was rapidly followed by a reduction in transmitter release. The facilitation rate among individual GrC-PC pairs was heterogeneously distributed and could be classified as either “reluctant” or “responsive” according to their release characteristics. Despite the variety of efficacy at individual connections, grouped activity in GrCs resulted in a linear relationship between PC response and PF burst duration at frequencies up to 300 Hz allowing rate coding to persist at the network level. Together, these findings support the hypothesis that the cerebellar granular layer acts as a spatiotemporal filter between MF input and PC output (D’Angelo and De Zeeuw, 2009).

Non-Hebbian spike-timing-dependent plasticity in cerebellar circuits

Spike-timing-dependent plasticity (STDP) provides a cellular implementation of the Hebb postulate, which states that synapses, whose activity repeatedly drives action potential firing in target cells, are potentiated. At glutamatergic synapses onto hippocampal and neocortical pyramidal cells, synaptic activation followed by spike firing in the target cell causes long-term potentiation (LTP)—as predicted by Hebb—whereas excitatory postsynaptic potentials (EPSPs) evoked after a spike elicit long-term depression (LTD)—a phenomenon that was not specifically addressed by Hebb. In both instances the action potential in the postsynaptic target neuron is an instructive signal that is capable of supporting synaptic plasticity. STDP generally relies on the propagation of Na⁺ action potentials that are initiated in the axon hillhock back into the dendrite, where they cause depolarization and boost local calcium influx. However, recent studies in CA1 hippocampal pyramidal neurons have suggested that local calcium spikes might provide a more efficient trigger for LTP induction than backpropagating action potentials. Dendritic calcium spikes also play a role in an entirely different type of STDP that can be observed in cerebellar Purkinje cells. These neurons lack backpropagating Na⁺ spikes. Instead, plasticity at parallel fiber (PF) to Purkinje cell synapses depends on the relative timing of PF-EPSPs and activation of the glutamatergic climbing fiber (CF) input that causes dendritic calcium spikes. Thus, the instructive signal in this system is externalized. Importantly when EPSPs are elicited before CF activity, PF-LTD is induced rather than LTP. Thus, STDP in the cerebellum follows a timing rule that is opposite to its hippocampal/neocortical counterparts. Regardless, a common motif in plasticity is that LTD/LTP induction depends on the relative timing of synaptic activity and regenerative dendritic spikes which are driven by the instructive signal.

Molecular mechanism of parallel fiber-Purkinje cell synapse formation

The cerebellum receives two excitatory afferents, the climbing fiber (CF) and the mossy fiber-parallel fiber (PF) pathway, both converging onto Purkinje cells (PCs) that are the sole neurons sending outputs from the cerebellar cortex. Glutamate receptor δ2 (GluRδ2) is expressed selectively in cerebellar PCs and localized exclusively at the PF-PC synapses. We found that a significant number of PC spines lack synaptic contacts with PF terminals and some of residual PF-PC synapses show mismatching between pre- and postsynaptic specializations in conventional and conditional GluRδ2 knockout mice. Studies with mutant mice revealed that in addition to PF-PC synapse formation, GluRδ2 is essential for synaptic plasticity, motor learning, and the restriction of CF territory. GluRδ2 regulates synapse formation through the amino-terminal domain, while the control of synaptic plasticity, motor learning, and CF territory is mediated through the carboxyl-terminal domain. Thus, GluRδ2 is the molecule that bridges synapse formation and motor learning. We found that the trans-synaptic interaction of postsynaptic GluRδ2 and presynaptic neurexins (NRXNs) through cerebellin 1 (Cbln1) mediates PF-PC synapse formation. The synaptogenic triad is composed of one molecule of tetrameric GluRδ2, two molecules of hexameric Cbln1 and four molecules of monomeric NRXN. Thus, GluRδ2 triggers synapse formation by clustering four NRXNs. These findings provide a molecular insight into the mechanism of synapse formation in the brain.

Climbing fiber signaling and cerebellar gain control

The physiology of climbing fiber signals in cerebellar Purkinje cells has been studied since the early days of electrophysiology. Both the climbing fiber-evoked complex spike and the role of climbing fiber activity in the induction of long-term depression (LTD) at parallel fiber-Purkinje cell synapses have become hallmark features of cerebellar physiology. However, the key role of climbing fiber signaling in cerebellar motor learning has been challenged by recent reports of forms of synaptic and non-synaptic plasticity in the cerebellar cortex that do not involve climbing fiber activity, but might well play a role in cerebellar learning. Moreover, cerebellar LTD does not seem to strictly require climbing fiber activity. These observations make it necessary to re-evaluate the role of climbing fiber signaling in cerebellar function. Here, we argue that climbing fiber signaling is about adjusting relative probabilities for the induction of LTD and long-term potentiation (LTP) at parallel fiber synapses. Complex spike-associated, dendritic calcium transients control postsynaptic LTD and LTP induction. High calcium transients, provided by complex spike activity, do not only favor postsynaptic LTD induction, but simultaneously trigger retrograde cannabinoid signaling, which blocks the induction of presynaptic LTP. Plasticity of the climbing fiber input itself provides additional means to fine-tune complex spike associated calcium signaling and thus to adjust the gain of heterosynaptic climbing fiber control. In addition to dendritic calcium transients, climbing fiber activity leads to the release of the neuropeptide corticotropin-releasing factor (CRF), which facilitates LTD induction at both parallel fiber and climbing fiber synapses.

Cerebellar climbing fibers modulate simple spikes in Purkinje cells

Purkinje cells have two action potentials: Climbing fiber responses (CFRs) and simple spikes (SSs). CFRs reflect the discharge of a single climbing fiber at multiple synaptic sites on the proximal dendrite of the Purkinje cell. SSs reflect the summed action of a subset of parallel fiber synapses on Purkinje cell dendritic spines. Because mossy fiber afferents terminate on granule cells, the ascending axons of which bifurcate, giving rise to parallel fibers, the modulation of SSs has been attributed to mossy fiber afferent signals. This inference has never been tested. Conversely, the low discharge frequency of CFRs has led many to conclude that they have a unique and intermittent role in cerebellar signal processing. We examine the relative potency of vestibularly modulated mossy fiber and climbing fiber signals in evoking CFRs and SSs in Purkinje cells of the uvula-nodulus in chloralose-urethane-anesthetized rabbits. Vestibular primary afferents were blocked by unilateral labyrinthectomy (UL). A UL destroys the vestibular primary afferent signal to the ipsilateral uvula-nodulus, while leaving intact the vestibular climbing fiber signal from the contralateral inferior olive. After UL, vestibular stimulation modulated CFRs and SSs in ipsilateral uvula-nodular Purkinje cells, demonstrating that the primary vestibular afferent mossy fiber input to the ipsilateral uvula-nodulus was not necessary for SS modulation. Unilateral microlesions of the caudal half of the beta-nucleus of the inferior olive reduced a modulated climbing fiber signal to the contralateral uvula-nodulus, causing loss of both vestibularly modulated CFRs and SSs in contralateral Purkinje cells. Vestibular climbing fibers not only evoke low-frequency CFRs, but also indirectly modulate higher-frequency SSs. This modulation must be attributed to cerebellar interneurons. Golgi cell inhibition of granule cells may provide the interneuronal mechanism for CFR-induced SS modulation.

Calcium dependence of retrograde inhibition by endocannabinoids at synapses onto Purkinje cells

Many types of neurons release endocannabinoids from their dendrites in response to elevation of intracellular calcium levels. Endocannabinoids then activate presynaptic cannabinoid receptors, thereby inhibiting neurotransmitter release for tens of seconds. A crucial step in understanding the physiological role of this retrograde signaling is to determine its sensitivity to elevations of postsynaptic calcium. Here we determine and compare the calcium dependence of endocannabinoid-mediated retrograde inhibition at three types of synapses onto cerebellar Purkinje cells. Previous studies have shown that Purkinje cell depolarization results in endocannabinoid-mediated retrograde inhibition of synapses received from climbing fibers, granule cell parallel fibers, and inhibitory interneurons. Using several calcium indicators with a range of affinities, we performed a series of in situ and in vitro calibrations to quantify calcium levels in Purkinje cells. We found that postsynaptic calcium levels of approximately 15 microM are required for half-maximal retrograde inhibition at all of these synapses. In contrast, previous studies had suggested that endocannabinoid release could occur with slight elevations of calcium above resting levels, which implies that inhibition should be widespread and continuously modulated by subtle changes in intracellular calcium levels. However, our results indicate that such small changes in intracellular calcium are not sufficient to evoke endocannabinoid release. Instead, because of its high requirement for calcium, retrograde inhibition mediated by calcium-dependent endocannabinoid release from Purkinje cells will occur under more restricted conditions and with greater spatial localization than previously appreciated.

Evidence for protein tyrosine phosphatase, tyrosine kinase, and G-protein regulation of the parallel fiber metabotropic slow EPSC of rat cerebellar Purkinje neurons

The slow EPSP (sEPSP) or slow EPSC (sEPSC) at parallel fiber to Purkinje neuron synapses is attributable to a nonselective cation channel coupled to activation of metabotropic type 1 glutamate receptors (mGluR1s). Photorelease of L-glutamate in 1 msec from 4-methoxy-7-nitroindolinyl-or 7-nitroindolinyl-caged glutamate in cerebellar slices was used to isolate and study postsynaptic mechanisms coupling mGluR1 to the cation channel. L-Glutamate immediately activated a glutamate transporter current, followed by the slow mGluR1-activated conductance. Inhibitors of kinases, phosphatases, and G-proteins were tested on the peak glutamate-evoked currents. No effects of the inhibitors were seen on the initial glutamate transporter currents. In contrast, the later mGluR1 currents were either unaffected or enhanced by the protein tyrosine kinase (PTK) inhibitors PP1, K252a, and staurosporine were diminished or blocked by phosphatase inhibitors but were unaffected by inhibitors of serine-threonine kinases PKA, PKC, or PKG. The selective src-PTK inhibitor PP1 (10 microm intracellularly) potentiated submaximal mGluR1 currents evoked by low L-glutamate concentrations but had no effect on maximal responses (80 or 160 microm L-glutamate). L-Glutamate-evoked mGluR1 currents and parallel fiber sEPSCs were reversibly and completely inhibited by protein tyrosine phosphatase (PTP) inhibitor bpV(phen) (50-200 microm) and by nonselective phosphatase inhibitor orthovanadate (0.5 or 1 mm). mGluR1 currents were completely inhibited by GDPbetaS applied intracellularly (5 mm). The results confirm a role for a GTPase postsynaptically, show that tyrosine phosphorylation inhibits mGluR1 coupling to the channel, and show that PTPs increase activation by tyrosine dephosphorylation most likely upstream of the sEPSP cation channel.

Glutamate receptor delta2 subunit in activity-dependent heterologous synaptic competition

In the adult cerebellum, the glutamate receptor delta2 subunit (GluRdelta2) is selectively targeted to the spines of the distal Purkinje cell dendrites, the spiny branchlets, that are innervated by the parallel fibers. Although GluRdelta2 has no known channel function, it is presumed to be involved in the formation and stabilization of these synapses. After block of electrical activity by tetrodotoxin, GluRdelta2s appear in the postsynaptic densities of the proximal dendritic spines, which then lose their contact with climbing fibers and become ectopically innervated by parallel fibers. This phenomenon suggests that climbing fiber activity prevents GluRdelta2 targeting to proximal dendrites and that GluRdelta2s admitted to the postsynaptic density of the spine cause withdrawal of the silent climbing fiber. To test this hypothesis, we studied the distribution of GluRdelta2s in the rat cerebellum by immunoelectron microscopy during the recovery period that follows removal of the electrical block, and during the sprouting of climbing fibers that follows subtotal deletion of the parent inferior olivary neurons by administration of the drug 3-acetylpyridine. We found that after removal of the electrical block, the climbing fibers reinnervate proximal spines that bear GluRdelta2s and these subunits are successively repressed. Similarly, after subtotal lesion of the inferior olive, reinnervation of denervated Purkinje cells occurs on spines bearing GluRdelta2s. Thus, GluRdelta2s are not responsible for displacing silent climbing fibers. We propose instead that GluRdelta2s are associated with climbing fiber-to-Purkinje cell synapses, during development or at early stages of climbing fiber regeneration or sprouting, and are downregulated during the process of synapse maturation.

Optical imaging of long-term depression in the mouse cerebellar cortex in vivo

Conjunctive stimulation of climbing fiber and parallel fiber inputs results in long-term depression (LTD) at parallel fiber-Purkinje cell synapses. Although hypothesized to play a major role in cerebellar motor learning, there has been no characterization of the cellular and molecular mechanisms of LTD in the whole animal, let alone its spatial properties, both of which are critical to understanding the role of LTD in cerebellar function. Neutral red optical imaging of the cerebellar cortex in the anesthetized mouse was used to visualize the spatial patterns of activation. Stimulation of the parallel fibers evoked a transverse beam of optical activity, and stimulation of the contralateral inferior olive evoked parasagittal bands. Conjunctive stimulation of parallel fibers and climbing fibers induced a long-term decrease (at least 1 hr) in the optical response to subsequent parallel fiber activation confined to the region of interaction between these two inputs. Activation of climbing fibers alone failed to induce the long-term decrease. Field potential recordings confirmed that the depression is postsynaptic and restricted to the interaction site. The long-term depression in the beam was prevented by a group 1 metabotropic glutamate receptor (mGluR(1)) antagonist and was absent in transgenic mice selectively expressing an inhibitor of protein kinase C (PKC) in Purkinje cells. Conversely, the long-term depression occurred in the mGluR(4) knock-out mouse, consistent with its postsynaptic origin. In addition to providing the first visualization of parallel fiber-Purkinje cell LTD in the cerebellar cortex, this study demonstrates the spatial specificity of LTD and its dependence on mGluR(1) and PKC in vivo.

Distal extension of climbing fiber territory and multiple innervation caused by aberrant wiring to adjacent spiny branchlets in cerebellar Purkinje cells lacking glutamate receptor delta 2

Organized synapse formation on to Purkinje cell (PC) dendrites by parallel fibers (PFs) and climbing fibers (CFs) is crucial for cerebellar function. In PCs lacking glutamate receptor delta2 (GluRdelta2), PF synapses are reduced in number, numerous free spines emerge, and multiple CF innervation persists to adulthood. In the present study, we conducted anterograde and immunohistochemical labelings to investigate how CFs innervate PC dendrites under weakened synaptogenesis by PFs. In the GluRdelta2 knock-out mouse, CFs were distributed in the molecular layer more closely to the pial surface compared with the wild-type mouse. Serial electron microscopy demonstrated that CFs in the knock-out mouse innervated all spines protruding from proximal dendrites of PCs, as did those in the wild-type mouse. In the knock-out mouse, however, CF innervation extended distally to spiny branchlets, where nearly half of the spines were free of innervation in contrast to complete synapse formation by PFs in the wild-type mouse. Furthermore, from the end point of innervation, CFs aberrantly jumped to form ectopic synapses on adjacent spiny branchlets, whose proximal portions were often innervated by different CFs. Without GluRdelta2, CFs are thus able to expand their territory along and beyond dendritic trees of the target PC, resulting in persistent surplus CFs by innervating the distal dendritic segment. We conclude that GluRdelta2 is essential to restrict CF innervation to the proximal dendritic segment, by which territorized innervation by PFs and CFs is properly structured and the formation of excess CF wiring to adjacent PCs is suppressed.

Roles of glutamate receptor delta 2 subunit (GluRdelta 2) and metabotropic glutamate receptor subtype 1 (mGluR1) in climbing fiber synapse elimination during postnatal cerebellar development

Climbing fiber (CF) synapse formation onto cerebellar Purkinje cells (PCs) is critically dependent on the synaptogenesis from parallel fibers (PFs), the other input to PCs. Previous studies revealed that deletion of the glutamate receptor delta2 subunit (GluRdelta2) gene results in persistent multiple CF innervation of PCs with impaired PF synaptogenesis, whereas mutation of the metabotropic glutamate receptor subtype 1 (mGluR1) gene causes multiple CF innervation with normal PF synaptogenesis. We demonstrate that atypical CF-mediated EPSCs (CF-EPSCs) with slow rise times and small amplitudes coexisted with typical CF-EPSCs with fast rise times and large amplitudes in PCs from GluRdelta2 mutant cerebellar slices. CF-EPSCs in mGluR1 mutant and wild-type PCs had fast rise times. Atypical slow CF responses of GluRdelta2 mutant PCs were associated with voltage-dependent Ca(2+) signals that were confined to PC distal dendrites. In the wild-type and mGluR1 mutant PCs, CF-induced Ca(2+) signals involved both proximal and distal dendrites. Morphologically, CFs of GluRdelta2 mutant mice extended to the superficial regions of the molecular layer, whereas those of wild-type and mGluR1 mutant mice did not innervate the superficial one-fifth of the molecular layer. It is therefore likely that surplus CFs of GluRdelta2 mutant mice form ectopic synapses onto distal dendrites, whereas those of wild-type and mGluR1 mutant mice innervate proximal dendrites. These findings suggest that GluRdelta2 is required for consolidating PF synapses and restricting CF synapses to the proximal dendrites, whereas the mGluR1-signaling pathway does not affect PF synaptogenesis but is involved in eliminating surplus CF synapses at the proximal dendrites.

Three-dimensional comparison of ultrastructural characteristics at depressing and facilitating synapses onto cerebellar Purkinje cells

Cerebellar Purkinje cells receive two distinctive types of excitatory inputs. Climbing fiber (CF) synapses have a high probability of release and show paired-pulse depression (PPD), whereas parallel fiber (PF) synapses facilitate and have a low probability of release. We examined both types of synapses using serial electron microscopic reconstructions in 15-d-old rats to look for anatomical correlates of these differences. PF and CF synapses were distinguishable by their overall ultrastructural organization. There were differences between PF and CF synapses in how many release sites were within 1 microm of a mitochondrion (67 vs 84%) and in the degree of astrocytic ensheathment (67 vs 94%). However, the postsynaptic density sizes for both types of synapses were similar (0.13-0.14 microm(2)). For both types of synapses, we counted the number of docked vesicles per release site to test whether this number determines the probability of release and synaptic plasticity. PF and CF synapses had the same number of anatomically docked vesicles (7-8). The number of docked vesicles at the CF does not support a simple model of PPD in which release of a single vesicle during the first pulse depletes the anatomically docked vesicle pool at a synapse. Alternatively, only a fraction of anatomically docked vesicles may be release ready, or PPD could result from multivesicular release at each site. Similarities in the number of docked vesicles for PF and CF synapses indicate that differences in probability of release are unrelated to the number of anatomically docked vesicles at these synapses.

Discoidin domain receptor 1 functions in axon extension of cerebellar granule neurons

In the developing cerebellum, granule neuron axon outgrowth is a key step toward establishing proper connections with Purkinje neurons, the principal output neuron of the cerebellum. During a search for genes that function in this process, we identified a receptor tyrosine kinase discoidin domain receptor 1 (DDR1) expressed in granule cells throughout their development. Overexpression of a dominant-negative form of DDR1 in immature granule cells results in severe reduction of neurite outgrowth in vitro, in dissociated primary culture, and in vivo, in organotypic slices of neonatal cerebellum. Granule cells that fail to extend axons are positive for differentiation markers such as TAG-1 and the neuron-specific class III beta-tubulin, suggesting that development is affected after granule cells commit to terminal differentiation. DDR1 activation appears to be mediated by its ligand, collagen, which is localized to the pial layer of the developing cerebellum, thereby leading to granule cell parallel fiber extension. Our results therefore indicate that collagen-DDR1 signaling is essential for granule neuron axon formation and further suggest a unique role of pia in cerebellar cortex histogenesis.

Prolonged synaptic currents and glutamate spillover at the parallel fiber to stellate cell synapse

Although neurons often fire in bursts, most of what is known about glutamate signaling and postsynaptic receptor activation is based on experiments using single stimuli. Here we examine the activation of ionotropic glutamate receptors by bursts at the parallel fiber to stellate cell synapse. We show that brief stimulus trains generate prolonged AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated EPSCs recorded in whole-cell voltage clamp. These EPSCs contrast with the rapid AMPAR-mediated EPSC evoked by a single stimulus. The prolonged AMPAR-mediated EPSC is promoted by high-frequency and high-intensity trains and can persist for hundreds of milliseconds. This EPSC is also increased by l-trans-2,4-PDC, an inhibitor of glutamate transporters, suggesting that these transporters usually limit the synaptic response to trains. These prolonged EPSCs reflect both receptor properties and a long-lasting glutamate signal. In addition, several experiments demonstrate that glutamate spillover can contribute to receptor activation. First, imaging stimulus-evoked changes in presynaptic calcium establishes that distinct parallel fiber bands can be activated. Second, activation of parallel fibers that do not directly synapse onto a given stellate cell can evoke indirect AMPAR- and NMDAR-mediated EPSCs in that cell. Third, experiments using the use-dependent NMDAR blocker MK-801 show that these indirect EPSCs reflect glutamate spillover in response to trains. Together, these findings indicate that stimulus trains can generate a sustained and widespread glutamate signal that can in turn evoke large and prolonged EPSCs mediated by ionotropic glutamate receptors. These synaptic properties may have important functional consequences for stellate cell firing.

Olivocerebellar climbing fibers in the granuloprival cerebellum: morphological study of individual axonal projections in the X-irradiated rat

Elimination of cerebellar granule cells early during postnatal development produces abnormal neural organization that retains immature characteristics in the adult, including innervation of each Purkinje cell by multiple climbing fibers from the inferior olive. To elucidate mechanisms underlying development of the olivocerebellar projection, we studied light-microscopic morphology of single olivocerebellar axons labeled with biotinylated dextran amine in adult rats rendered agranular by a single postnatal X-irradiation. Each reconstructed olivocerebellar axon gave off approximately 12 climbing fibers, approximately twice as many as in normal rats. Terminal arborizations of climbing fibers made irregular tufts in most areas, whereas they were arranged vertically in a few mildly affected areas. Each climbing fiber terminal arborization innervated only part of the dendritic arbor of a Purkinje cell, and multiple climbing fibers innervated a single Purkinje cell. These climbing fibers originated either from the same olivocerebellar axon (pseudomultiple innervation) or from distinct axons (true multiple innervation). Abundant non-climbing fiber thin collaterals projected to all cortical layers. Although the longitudinal pattern of the zonal olivocerebellar projection was generally observed, lateral branching, including bilateral projections, was relatively frequent. These results suggest that the granule cell-parallel fiber system induces several important features of olivocerebellar projection: (1) organization of the climbing fiber terminal arborization tightly surrounding Purkinje cell dendrites, (2) elimination of pseudo- and true multiple innervations establishing one-to-one innervation, (3) retraction of non-climbing fiber thin collaterals from the molecular layer, and (4) probable refinement of the longitudinal projection domains by removing aberrant transverse branches.

Afferent-target cell interactions in the cerebellum: negative effect of granule cells on Purkinje cell development in lurcher mice

Lurcher (Lc) is a gain-of-function mutation in the delta2 glutamate receptor gene that results in a large, constitutive inward current in the cerebellar Purkinje cells of +/Lc mice. +/Lc Purkinje cells fail to differentiate fully and die during postnatal development. In normal mice, interactions with granule cells promote Purkinje cell dendritic differentiation. Partial destruction of the granule cell population in young +/Lc mice by x irradiation resulted in a significant increase in Purkinje cell dendritic growth and improved cytoplasmic structure but did not prevent Purkinje cell death. These results indicate two components to Purkinje cell abnormalities in +/Lc mice: a retardation/blockade of dendritic development that is mediated by interactions with granule cells and the death of the cell. Thus, the normal trophic effects of granule cell interaction on Purkinje cell development are absent in the +/Lc cerebellum, suggesting that granule cells are powerful regulators of Purkinje cell differentiation.

Expression pattern and neurotrophic role of the c-fms proto-oncogene M-CSF receptor in rodent Purkinje cells

To investigate whether the c-fms proto-oncogene plays a role in the CNS, we examined its expression in mouse brain. We found that c-fms-positive Purkinje cells first appeared in caudal cerebellum at postnatal day 0 (P0) arranged in a parasagittal manner, and most Purkinje cells gradually became positive by P6. This differential expression was not seen from P7 to adulthood, and the parasagittal pattern until P5 was different from those of L7, zebrins, and the integrin beta1 subunit. No neuronal expression of c-fms was found in the other brain regions examined. In both reeler and weaver mutant mice in the adult stage, all Purkinje cells were positive for c-fms as in the wild-type controls; however, the parasagittal bands of c-fms-positive Purkinje cells were observed even in the adult staggerer mutant. To check the neurotrophic effect of macrophage colony-stimulating factor (M-CSF), we immunostained cerebella derived from osteopetrotic mutant mice, that is, those devoid of active M-CSF. We found that the number of calbindin-positive Purkinje cells in a given cerebellum began to decrease substantially during the initial 4-5 weeks of the postnatal period. In addition, cultured Purkinje cells were dependent on M-CSF for their survival. These data suggest that expression of the c-fms gene is intrinsically programmed in the Purkinje cells and never affected by the afferent synaptic input and that neuronal survival of Purkinje cells is dependent on M-CSF after weaning. Therefore, c-fms is considered to be a new developmental marker for Purkinje cells.

Glutamate receptor targeting to synaptic populations on Purkinje cells is developmentally regulated

Selective targeting of neurotransmitter receptors to specific synapse populations occurs in adult neurons, but little is known about the development of these receptor distribution patterns. In this study, we demonstrate that a specific developmental switch occurs in the targeting of a receptor to an identified synapse population. Localization of delta and AMPA glutamate receptors at parallel and climbing fiber synapses on the developing Purkinje cells was studied using postembedding immunogold. Delta receptors were found to be abundant on postsynaptic membranes at parallel fiber synapses from postnatal day 10 (P10) to adult. In contrast, delta receptors were found to be high at climbing fiber synapses only at P10 and P14. Thus, a major finding of this paper is that high levels of delta receptors are transiently expressed in climbing fiber synapses in the second postnatal week. Labeling of synapses with anti-delta receptor antibody at P10 was limited to the postsynaptic membrane of excitatory synapses and was absent from GABAergic synapses. Unlike delta receptor immunolabeling, AMPA receptor immunolabeling (GluR2/3 and GluR2 antibodies) was high in the postsynaptic membranes of synapses at early postnatal ages (P2 and P5) and was higher in climbing fiber synapses than in parallel fiber synapses from P10 to adult. The present study shows that synapse-specific targeting of glutamate receptors in Purkinje cells is developmentally regulated, with the postsynaptic receptor composition established during synapse maturation. This composition is not dependent on the nature of the initial establishment of synaptic connections.

Control of neurotransmitter release by presynaptic waveform at the granule cell to Purkinje cell synapse

The effect of changes in the shape of the presynaptic action potential on neurotransmission was examined at synapses between granule and Purkinje cells in slices from the rat cerebellum. Low concentrations of tetraethylammonium were used to broaden the presynaptic action potential. The presynaptic waveform was monitored with voltage-sensitive dyes, the time course and amplitude of presynaptic calcium entry were determined with fluorescent calcium indicators, and EPSCs were measured with a whole-cell voltage clamp. Spike broadening increased calcium influx primarily by prolonging calcium entry without greatly affecting peak presynaptic calcium currents, indicating that the majority of calcium channels reach maximal probability of opening in response to a single action potential and that spike broadening increases the open time of these channels. EPSCs were exquisitely sensitive to elevations of calcium influx produced by spike broadening; there was a high power relationship between calcium influx and release such that a 23% increase in spike width led to a 25% increase in total calcium influx, which in turn doubled synaptic strength. The finding that even small changes in spike width influence neurotransmitter release suggests that altering the presynaptic waveform may be an important means of modifying the strength of this synapse. Waveform changes do not, however, contribute significantly to presynaptic modulation via activation of adenosine A1 or GABAB receptors. Furthermore, greatly reducing presynaptic calcium influx did not alter the presynaptic waveform, indicating that calcium channels and calcium-activated channels do not participate in shaping the presynaptic waveform.

Impaired cerebellar synaptic plasticity and motor performance in mice lacking the mGluR4 subtype of metabotropic glutamate receptor

The application of the glutamate analog L-2-amino-4-phosphonobutyric acid (L-AP4) to neurons produces a suppression of synaptic transmission. Although L-AP4 is a selective ligand at a subset of metabotropic glutamate receptors (mGluRs), the precise physiological role of the L-AP4-activated mGluRs remains primarily unknown. To provide a better understanding of the function of L-AP4 receptors, we have generated and studied knockout (KO) mice lacking the mGluR4 subtype of mGluR that displays high affinity for L-AP4. The mGluR4 mutant mice displayed normal spontaneous motor activity and were unimpaired on the bar cross test, indicating that disruption of the mGluR4 gene did not cause gross motor abnormalities, impairments of novelty-induced exploratory behaviors, or alterations in fine motor coordination. However, the mutant mice were deficient on the rotating rod motor-learning test, suggesting that mGluR4 KO mice may have an impaired ability to learn complex motor tasks. Patch-clamp and extracellular field recordings from Purkinje cells in cerebellar slices demonstrated that L-AP4 had no effect on synaptic responses in the mutant mice, whereas in the wild-type mice 100 microM L-AP4 produced a 23% depression of synaptic responses with an EC50 of 2.5 microM. An analysis of presynaptic short-term synaptic plasticity at the parallel fiber-->Purkinje cell synapse demonstrated that paired-pulse facilitation and post-tetanic potentiation were impaired in the mutant mice. In contrast, long-term depression (LTD) was not impaired. These results indicate that an important function of mGluR4 is to provide a presynaptic mechanism for maintaining synaptic efficacy during repetitive activation. The data also suggest that the presence of mGluR4 at the parallel fiber-->Purkinje cell synapse is required for maintaining normal motor function.

Determinants of the time course of facilitation at the granule cell to Purkinje cell synapse

Short-term facilitation is a widely observed form of synaptic enhancement that is not well understood. Although presynaptic calcium has long been implicated in this process, its role is unclear, particularly at synapses in the mammalian brain. We tested the role of presynaptic residual free calcium ([Ca]res) in facilitation of synapses between granule cells and Purkinje cells in rat cerebellar slices. Paired-pulse facilitation of synaptic currents resulted in an approximately 2.5-fold enhancement that decayed with a time constant of approximately 200 msec, as assessed by voltage-clamp recordings. Measurements of [Ca]res using fluorescent calcium-sensitive indicators revealed that [Ca]res decayed more rapidly than did facilitation. Manipulation of [Ca]res dynamics by introducing EGTA into presynaptic terminals sped the decays of [Ca]res and facilitation in a dose-dependent manner. When [Ca]res was reduced to a brief impulse lasting several milliseconds, facilitation was still present, although reduced in amplitude and duration. Facilitation decayed with an intrinsic time constant of approximately 40 msec. These results suggest that facilitation at this synapse is produced by a calcium-driven process with a high affinity and a slow effective off-rate. A combination of [Ca]res dynamics and the properties of a calcium-driven reaction determine the time course and amplitude of facilitation.