A long-term depression of AMPA currents in cultured cerebellar Purkinje neurons

The Rules of Cerebellar Learning: Around the Ito Hypothesis

As a tribute to Masao Ito, we propose a model of cerebellar learning that incorporates and extends his original model. We suggest four principles that align well with conclusions from multiple cerebellar learning systems. (1) Climbing fiber inputs to the cerebellum drive early, fast, poorly-retained learning in the parallel fiber to Purkinje cell synapse. (2) Learned Purkinje cell outputs drive late, slow, well-retained learning in non-Purkinje cell inputs to neurons in the cerebellar nucleus, transferring learning from the cortex to the nucleus. (3) Recurrent feedback from Purkinje cells to the inferior olive, through interneurons in the cerebellar nucleus, limits the magnitude of fast, early learning in the cerebellar cortex. (4) Functionally different inputs are subjected to plasticity in the cerebellar cortex versus the cerebellar nucleus. A computational neural circuit model that is based on these principles mimics a large amount of neural and behavioral data obtained from the smooth pursuit eye movements of monkeys.

A Late Phase of Long-Term Synaptic Depression in Cerebellar Purkinje Cells Requires Activation of MEF2

The MEF2 family of transcription factors restricts excitatory synapse number in an activity-dependent fashion during development, yet MEF2 has not been implicated in long-term synaptic depression (LTD), which is thought to initiate synapse elimination. Mutations in MEF2 pathways are implicated in autism spectrum disorders, which include cerebellar dysfunction. Here, we test the hypothesis that cerebellar LTD requires postsynaptic activation of MEF2. Knockdown of MEF2D produces suppression of the transcription-dependent late phase of LTD in cultured Purkinje cells. The late phase of LTD is also completely blocked in Purkinje cells derived from MEF2A+MEF2D null mice and rescued with plasmids that drive expression of MEF2D but not phosphatase-resistant mutant MEF2D S444D. Wild-type Purkinje cells transfected with a constitutively active form of MEF2 show no alterations of synaptic strength. Thus, postsynaptic activation of MEF2 by S444 dephosphorylation is necessary, but not sufficient, for the late phase of cerebellar LTD.

No neuron is an island: Homeostatic plasticity and over-constraint in a neural circuit

To support computation the activity of neurons must vary within a useful range, which highlights one potential value of homeostatic plasticity. The interconnectedness of the brain, however, introduces the possibility that combinations of homeostatic mechanisms can produce over-constraint in which not all set points can be satisfied. We use a simulation of the cerebellum to investigate the potential for such conflict and its consequences. In this instance the conflict produces perpetual drift and eventual saturation of synaptic weights. We show that these problems can be resolved for this network by a particular combination of sites and rules for plasticity. We also show that simulations that implement these rules for homeostatic plasticity are more resistant to forgetting. These results illustrate the general principle that homeostatic plasticity within a system must not set up conflicts in which mutually exclusive set points exist and that one consequence can be perpetual induction of plasticity.

Conditioned climbing fiber responses in cerebellar cortex and nuclei

The eyeblink conditioning paradigm captures an elementary form of associative learning in a neural circuitry that is understood to an extraordinary degree. Cerebellar cortical Purkinje cell simple spike suppression is widely regarded as the main process underlying conditioned responses (CRs), leading to disinhibition of neurons in the cerebellar nuclei that innervate eyelid muscles downstream. However, recent work highlights the addition of a conditioned Purkinje cell complex spike response, which at the level of the interposed nucleus seems to translate to a transient spike suppression that can be followed by a rapid spike facilitation. Here, we review the characteristics of these responses at the cerebellar cortical and nuclear level, and discuss possible origins and functions.

Physiology of the cerebellum

The cerebellum is a central brain structure deeply integrated into major loops with the cerebral cortex, brainstem, and spinal cord. The cerebellum shows a complex regional organization consisting of modules with sagittal orientation. The cerebellum takes part in motor control and its lesions cause a movement incoordination syndrome called ataxia. Recent observations also imply involvement of the cerebellum in cognition and executive control, with an impact on pathologies like dyslexia and autism. The cerebellum operates as a forward controller learning to predict the precise timing of correlated events. The physiologic mechanisms of cerebellar functioning are still the object of intense research. The signals entering the cerebellum through the mossy fibers are processed in the granular layer and transmitted to Purkinje cells, while a collateral pathway activates the deep cerebellar nuclei (DCN). Purkinje cells in turn inhibit DCN, so that the cerebellar cortex operates as a side loop controlling the DCN. Learning is now known to occur through synaptic plasticity at multiple synapses in the granular layer, molecular layer, and DCN, extending the original concept of the Motor Learning Theory that predicted a single form of plasticity at the synapse between parallel fibers and Purkinje cells under the supervision of climbing fibers deriving from the inferior olive. Coordination derives from the precise regulation of timing and gain in the different cerebellar modules. The investigation of cerebellar dynamics using advanced physiologic recordings and computational models is now providing new clues on how the cerebellar network performs its internal computations.

Orexin receptors mediate long-term depression of excitatory synaptic transmission in the spinal cord dorsal horn

Neuropeptides orexin-A and -B are related to the regulation of sleep/wakefulness and feeding behaviors. Recently, the peptides have also been shown to yield antinociceptive effects in various pain models. However, it is not clear whether orexins are involved in forms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), the increase and the decrease of synaptic efficacy, respectively. In the present study, we examined whether orexin receptor type 1 (OX1) and 2 (OX2) are involved in the induction or maintenance of LTD of excitatory synaptic transmission using transverse spinal cord slices of young rats. Repetitive electrical stimulation of Lissauer's tract zone at 2Hz for 5min (600 pulses), combined with a holding potential of -30mV, induced LTD of the amplitude of excitatory postsynaptic currents (EPSCs) which are evoked by the activation of primary afferent fibers. The maintenance of LTD was significantly prevented by bath application of SB674042 (1μM), an OX1 antagonist, or EMPA (1μM), an OX2 antagonist. In addition, LTD was dependent on the NMDA receptor, as the NMDA receptor antagonist D-AP5 blocked the maintenance of LTD. Our study suggests that orexins, via activation of both OX1 and OX2, play a significant role in the expression of NMDA-dependent LTD, thereby contributing to the spinal modulation of pain transmission.

Modulation of Complex-Spike Duration and Probability during Cerebellar Motor Learning in Visually Guided Smooth-Pursuit Eye Movements of Monkeys

Activation of an inferior olivary neuron powerfully excites Purkinje cells via its climbing fiber input and triggers a characteristic high-frequency burst, known as the complex spike (CS). The theory of cerebellar learning postulates that the CS induces long-lasting depression of the strength of synapses from active parallel fibers onto Purkinje cells, and that synaptic depression leads to changes in behavior. Prior reports showed that a CS on one learning trial is linked to a properly timed depression of simple spikes on the subsequent trial, as well as a learned change in pursuit eye movement. Further, the duration of a CS is a graded instruction for single-trial plasticity and behavioral learning. We now show across multiple learning paradigms that both the probability and duration of CS responses are correlated with the magnitudes of neural and behavioral learning in awake behaving monkeys. When the direction of the instruction for learning repeatedly was in the same direction or alternated directions, the duration and probability of CS responses decreased over a learning block along with the magnitude of trial-over-trial neural learning. When the direction of the instruction was randomized, CS duration, CS probability, and neural and behavioral learning remained stable across time. In contrast to depression, potentiation of simple-spike firing rate for ON-direction learning instructions follows a longer time course and plays a larger role as depression wanes. Computational analysis provides a model that accounts fully for the detailed statistics of a complex set of data.

Nonspecific synaptic plasticity improves the recognition of sparse patterns degraded by local noise

Many forms of synaptic plasticity require the local production of volatile or rapidly diffusing substances such as nitric oxide. The nonspecific plasticity these neuromodulators may induce at neighboring non-active synapses is thought to be detrimental for the specificity of memory storage. We show here that memory retrieval may benefit from this non-specific plasticity when the applied sparse binary input patterns are degraded by local noise. Simulations of a biophysically realistic model of a cerebellar Purkinje cell in a pattern recognition task show that, in the absence of noise, leakage of plasticity to adjacent synapses degrades the recognition of sparse static patterns. However, above a local noise level of 20%, the model with nonspecific plasticity outperforms the standard, specific model. The gain in performance is greatest when the spatial distribution of noise in the input matches the range of diffusion-induced plasticity. Hence non-specific plasticity may offer a benefit in noisy environments or when the pressure to generalize is strong.

Intracellular Ca(2+) thresholds for induction of excitatory long-term depression and inhibitory long-term potentiation in a cerebellar Purkinje neuron

Synaptic plasticity in the cerebellar cortex contributes to motor learning. In particular, long-term depression at excitatory parallel fiber - Purkinje neuron synapses has been intensively studied as a primary cellular mechanism for motor learning. Recent studies showed that synaptic plasticity other than long-term depression such as long-term potentiation at inhibitory interneuron - Purkinje neuron synapses called rebound potentiation is also involved in motor learning. It was suggested that long-term depression and rebound potentiation might synergistically support motor learning. Here, we have examined induction conditions of long-term depression and rebound potentiation in cultured rat Purkinje neurons, and found that both of them were induced simultaneously by certain patterns of depolarization of a Purkinje neuron. Further, we found that long-term depression was induced by shorter depolarizing pulses causing a smaller intracellular Ca(2+) increase than rebound potentiation. These results support an idea that long-term depression and rebound potentiation synergistically contribute to motor learning, and suggest that long-term depression may play a primary role in wider variety of motor learning paradigms than rebound potentiation.

Induction of long-term potentiation and long-term depression is cell-type specific in the spinal cord

This study shows that the direction of synaptic plastic changes in the spinal cord is cell-type specific in response to nociceptive input.

The underlying mechanism of chronic pain is believed to be changes in excitability in spinal dorsal horn (DH) neurons that respond abnormally to peripheral input. Increased excitability in pain transmission neurons, and depression of inhibitory neurons, are widely recognized in the spinal cord of animal models of chronic pain. The possible occurrence of 2 parallel but opposing forms of synaptic plasticity, long-term potentiation (LTP) and long-term depression (LTD) was tested in 2 types of identified DH neurons using whole-cell patch-clamp recordings in mouse spinal cord slices. The test stimulus was applied to the sensory fibers to evoke excitatory postsynaptic currents in identified spinothalamic tract neurons (STTn) and GABAergic neurons (GABAn). Afferent conditioning stimulation (ACS) applied to primary afferent fibers with various stimulation parameters induced LTP in STTn but LTD in GABAn, regardless of stimulation parameters. These opposite responses were further confirmed by simultaneous dual patch-clamp recordings of STTn and GABAn from a single spinal cord slice. Both the LTP in STTn and the LTD in GABAn were blocked by an NMDA receptor antagonist, AP5, or an intracellular Ca²⁺ chelator, BAPTA. Both the pattern and magnitude of intracellular Ca²⁺ after ACS were almost identical between STTn and GABAn based on live-cell calcium imaging. The results suggest that the intense sensory input induces an NMDA receptor-dependent intracellular Ca²⁺ increase in both STTn and GABAn, but produces opposing synaptic plasticity. This study shows that there is cell type–specific synaptic plasticity in the spinal DH.

Synaptic Plasticity in the Cerebellar Cortex and its Role in Motor Learning

Synaptic plasticity plays a role in the learning capability of brain tissues. Long-term depression (LTD) of parallel fiber synapses in cerebellar Purkinje cells occurs when these synapses are activated in conjunction with climbing fiber synapses. Signal transduction mechanisms underlying LTD have recently been investigated extensively. It has also become apparent that climbing fiber signals encode errors in the motor performance of an animal. It is therefore hypothesized that learning proceeds in cerebellar tissues in such a way that error signals of climbing fibers act to depress by LTD those parallel fiber synapses responsible for the errors. The cerebellum contains a large number of corticonuclear microcomplexes. Each microcomplex is connected to an extracerebellar system and is presumed to endow the system with learning capability. The hypothesis accounts for the adaptation of the vestibuloocular reflex and probably also for other forms of motor and cognitive learning.

Optimization of cerebellar purkinje neuron cultures and development of a plasmid-based method for purkinje neuron-specific, miRNA-mediated protein knockdown

We present a simple and efficient method to knock down proteins specifically in Purkinje neurons (PN) present in mixed mouse primary cerebellar cultures. This method utilizes the introduction via nucleofection of a plasmid encoding a specific miRNA downstream of the L7/Pcp2 promoter, which drives PN-specific expression. As proof-of-principle, we used this plasmid to knock down the motor protein myosin Va, which is required for the targeting of smooth endoplasmic reticulum (ER) into PN spines. Consistent with effective knockdown, transfected PNs robustly phenocopied PNs from dilute-lethal (myosin Va-null) mice with regard to the ER targeting defect. Importantly, our plasmid-based approach is less challenging technically and more specific to PNs than several alternative methods (e.g., biolistic- and lentiviral-based introduction of siRNAs). We also present a number of improvements for generating mixed cerebellar cultures that shorten the procedure and improve the total yield of PNs, and of transfected PNs, considerably. Finally, we present a method to rescue cerebellar cultures that develop large cell aggregates, a common problem that otherwise precludes the further use of the culture.

Integrated plasticity at inhibitory and excitatory synapses in the cerebellar circuit

The way long-term potentiation (LTP) and depression (LTD) are integrated within the different synapses of brain neuronal circuits is poorly understood. In order to progress beyond the identification of specific molecular mechanisms, a system in which multiple forms of plasticity can be correlated with large-scale neural processing is required. In this paper we take as an example the cerebellar network, in which extensive investigations have revealed LTP and LTD at several excitatory and inhibitory synapses. Cerebellar LTP and LTD occur in all three main cerebellar subcircuits (granular layer, molecular layer, deep cerebellar nuclei) and correspondingly regulate the function of their three main neurons: granule cells (GrCs), Purkinje cells (PCs) and deep cerebellar nuclear (DCN) cells. All these neurons, in addition to be excited, are reached by feed-forward and feed-back inhibitory connections, in which LTP and LTD may either operate synergistically or homeostatically in order to control information flow through the circuit. Although the investigation of individual synaptic plasticities in vitro is essential to prove their existence and mechanisms, it is insufficient to generate a coherent view of their impact on network functioning in vivo. Recent computational models and cell-specific genetic mutations in mice are shedding light on how plasticity at multiple excitatory and inhibitory synapses might regulate neuronal activities in the cerebellar circuit and contribute to learning and memory and behavioral control.

Striatal cholinergic interneuron regulation and circuit effects

The striatum plays a central role in motor control and motor learning. Appropriate responses to environmental stimuli, including pursuit of reward or avoidance of aversive experience all require functional striatal circuits. These pathways integrate synaptic inputs from limbic and cortical regions including sensory, motor and motivational information to ultimately connect intention to action. Although many neurotransmitters participate in striatal circuitry, one critically important player is acetylcholine (ACh). Relative to other brain areas, the striatum contains exceptionally high levels of ACh, the enzymes that catalyze its synthesis and breakdown, as well as both nicotinic and muscarinic receptor types that mediate its postsynaptic effects. The principal source of striatal ACh is the cholinergic interneuron (ChI), which comprises only about 1-2% of all striatal cells yet sends dense arbors of projections throughout the striatum. This review summarizes recent advances in our understanding of the factors affecting the excitability of these neurons through acute effects and long term changes in their synaptic inputs. In addition, we discuss the physiological effects of ACh in the striatum, and how changes in ACh levels may contribute to disease states during striatal dysfunction.

Cerebellar learning mechanisms

The mechanisms underlying cerebellar learning are reviewed with an emphasis on old arguments and new perspectives on eyeblink conditioning. Eyeblink conditioning has been used for decades a model system for elucidating cerebellar learning mechanisms. The standard model of the mechanisms underlying eyeblink conditioning is that there two synaptic plasticity processes within the cerebellum that are necessary for acquisition of the conditioned response: (1) long-term depression (LTD) at parallel fiber-Purkinje cell synapses and (2) long-term potentiation (LTP) at mossy fiber-interpositus nucleus synapses. Additional Purkinje cell plasticity mechanisms may also contribute to eyeblink conditioning including LTP, excitability, and entrainment of deep nucleus activity. Recent analyses of the sensory input pathways necessary for eyeblink conditioning indicate that the cerebellum regulates its inputs to facilitate learning and maintain plasticity. Cerebellar learning during eyeblink conditioning is therefore a dynamic interactive process which maximizes responding to significant stimuli and suppresses responding to irrelevant or redundant stimuli. This article is part of a Special Issue entitled SI: Brain and Memory.

Purkinje cell stripes and long-term depression at the parallel fiber-Purkinje cell synapse

The cerebellar cortex comprises a stereotyped array of transverse zones and parasagittal stripes, built around multiple Purkinje cell subtypes, which is highly conserved across birds and mammals. This architecture is revealed in the restricted expression patterns of numerous molecules, in the terminal fields of the afferent projections, in the distribution of interneurons, and in the functional organization. This review provides an overview of cerebellar architecture with an emphasis on attempts to relate molecular architecture to the expression of long-term depression (LTD) at the parallel fiber-Purkinje cell (pf-PC) synapse.

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.

Multitasking on the run

Researchers combine genetics and imaging to reveal that individual granule cells in the cerebellum integrate sensory and motor information.

Calcium as a trigger for cerebellar long-term synaptic depression

Cerebellar long-term depression (LTD) is a form of long-term synaptic plasticity that is triggered by calcium(Ca2+) signals in the postsynaptic Purkinje cell. This Ca2+comes both from IP3-mediated release from intracellular Ca2+ stores, as well as from Ca2+ influx through voltage-gated Ca2+ channels. The Ca2+ signal that triggers LTD occurs locally within dendritic spines and is due to supralinear summation of signals coming from these two Ca2+ sources. The properties of this postsynaptic Ca2+signal can explain several features of LTD, such as its associativity, synapse specificity, and dependence on thetiming of synaptic activity, and can account for the slow kinetics of LTD expression. Thus, from a Ca2+ signaling perspective, LTD is one of the best understood forms of synaptic plasticity.

Calcium input frequency, duration and amplitude differentially modulate the relative activation of calcineurin and CaMKII

NMDA receptor dependent long-term potentiation (LTP) and long-term depression (LTD) are two prominent forms of synaptic plasticity, both of which are triggered by post-synaptic calcium elevation. To understand how calcium selectively stimulates two opposing processes, we developed a detailed computational model and performed simulations with different calcium input frequencies, amplitudes, and durations. We show that with a total amount of calcium ions kept constant, high frequencies of calcium pulses stimulate calmodulin more efficiently. Calcium input activates both calcineurin and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) at all frequencies, but increased frequencies shift the relative activation from calcineurin to CaMKII. Irrespective of amplitude and duration of the inputs, the total amount of calcium ions injected adjusts the sensitivity of the system to calcium input frequencies. At a given frequency, the quantity of CaMKII activated is proportional to the total amount of calcium. Thus, an input of a small amount of calcium at high frequencies can induce the same activation of CaMKII as a larger amount, at lower frequencies. Finally, the extent of activation of CaMKII signals with high calcium frequency is further controlled by other factors, including the availability of calmodulin, and by the potency of phosphatase inhibitors.

Distributed synergistic plasticity and cerebellar learning

Studies on synaptic plasticity in the context of learning have been dominated by the view that a single, particular type of plasticity forms the underlying mechanism for a particular type of learning. However, emerging evidence shows that many forms of synaptic and intrinsic plasticity at different sites are induced conjunctively during procedural memory formation in the cerebellum. Here, we review the main forms of long-term plasticity in the cerebellar cortex that underlie motor learning. We propose that the different forms of plasticity in the granular layer and the molecular layer operate synergistically in a temporally and spatially distributed manner, so as to ultimately create optimal output for behaviour.

The orthosteric agonist 2-chloro-5-hydroxyphenylglycine activates mGluR5 and mGluR1 with similar efficacy and potency

BACKGROUND

The efficacy, potency, and selectivity of the compound 2-Chloro-5-hydroxyphenylglycine (CHPG), a nominally selective agonist for metabotropic glutamate receptor 5 (mGluR5), were examined with select mGluRs by examining their ability to induce modulation of the native voltage dependent ion channels in isolated sympathetic neurons from the rat superior cervical ganglion (SCG). SCG neurons offer a null mGluR-background in which specific mGluR subtypes can be made to express via intranuclear cDNA injection.

RESULTS

Consistent with previous reports, CHPG strongly activated mGluR5b expressed in SCG neurons with an apparent EC50 around 60 μM. Surprisingly, CHPG also activated two mGluR1 splice variants with a similar potency as at mGluR5 when calcium current inhibition was used as an assay for receptor function. No effect of 1 mM CHPG was seen in cells expressing mGluR2 or mGluR4, suggesting that CHPG only activates group I mGluRs (mGluR1 and 5). CHPG was also able to induce modulation of M-type potassium current through mGluR1, but not as consistently as glutamate. Since this channel is modulated through a Gq-dependent pathway, these data indicate that CHPG may exhibit some biased agonist properties on mGluR1. Closer examination of the voltage-independent, Gq-mediated component of mGluR-induced calcium current modulation data confirmed that some biased agonism was evident, but the effect was weak and inconsistent.

CONCLUSIONS

These data contrast with the established literature which suggests that CHPG is a selective mGluR5 agonist. Instead, CHPG appears to act equally well as an agonist at mGluR1. While some weak biased agonism was observed with CHPG acting on mGluR1, but not mGluR5, favoring Gi/o signaling over Gq/11, this effect does not appear sufficient to fully explain the discrepancies in the literature.

Selective and sustained α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor activation in cerebellum induces dystonia in mice

Dystonia is a neurological disorder characterized by involuntary muscle contractions that cause twisting movements and abnormal postures. Functional imaging consistently reveals cerebellar overactivity in dystonic patients regardless of the type or etiology of the disorder. To explore mechanisms that might explain the basis for the cerebellar overactivity in dystonia, normal mice were challenged with intracerebellar application of a variety of agents that induce hyperexcitability. A nonspecific increase in cerebellar excitability, such as that produced by picrotoxin, was not associated with dystonia. Instead, glutamate receptor activation, specifically AMPA receptor activation, was necessary to evoke dystonia. AMPA receptor agonists induced dystonia, and AMPA receptor antagonists reduced the dystonia induced by glutamate receptor agonists. AMPA receptor antagonists also ameliorated the dystonia exhibited by the dystonic mouse mutant tottering, suggesting that AMPA receptors may play a role in some other genetic models of dystonia. Furthermore, AMPA receptor desensitization mediated the expression of dystonia. Preventing AMPA receptor desensitization with cyclothiazide or the nondesensitizing agonist kainic acid exacerbated the dystonic response. These results suggest the novel hypothesis that the cerebellar overactivity observed in neuroimaging studies of patients with dystonia may be an indirect reflection of abnormal glutamate signaling. In addition, these results imply that reducing AMPA receptor activation by blocking AMPA receptors and promoting AMPA receptor desensitization or negative allosteric modulators may prove to be beneficial for treating dystonia.

A late phase of LTD in cultured cerebellar Purkinje cells requires persistent dynamin-mediated endocytosis

Long-term synaptic depression (LTD) of cerebellar parallel fiber-Purkinje cell synapses is a form of use-dependent synaptic plasticity that may be studied in cell culture. One form of LTD is induced postsynaptically through an mGlu1/Ca influx/protein kinase Cα (PKCα) cascade, and its initial expression requires phosphorylation of ser-880 in the COOH-terminal PDZ-ligand region of GluA2 and consequent binding of PICK1. This triggers postsynaptic clathrin/dynamin-mediated endocytosis of GluA2-containing surface AMPA receptors. Cerebellar LTD also has a late phase beginning 45-60 min after induction that is blocked by transcription or translation inhibitors. Here, I have sought to determine the expression mechanism of this late phase of LTD by applying various drugs and peptides after the late phase has been established. Neither bath application of mGluR1 antagonists (JNJ-16259685, LY-456236) nor the PKC inhibitor GF-109203X starting 60-70 min after LTD induction attenuated the late phase. Similarly, achieving the whole cell configuration with a second pipette loaded with the peptide PKC inhibitor PKC(19-36) starting 60 min postinduction also failed to alter the late phase. Late internal perfusion with peptides designed to disrupt PICK1-GLUA2 interaction or PICK1 dimerization failed to impact late phase LTD expression. However, late internal perfusion with two different blockers of dynamin, the drug dynasore and a dynamin inhibitory peptide (QVPSRPNRAP), produced rapid and complete reversal of cerebellar LTD expression. These findings suggest that the protein synthesis-dependent late phase of LTD requires persistent dynamin-mediated endocytosis, but not persistent PICK1-GluA2 binding nor persistent activation of the upstream mGluR1/PKCα signaling cascade.

Neural circuitry and plasticity mechanisms underlying delay eyeblink conditioning

Pavlovian eyeblink conditioning has been used extensively as a model system for examining the neural mechanisms underlying associative learning. Delay eyeblink conditioning depends on the intermediate cerebellum ipsilateral to the conditioned eye. Evidence favors a two-site plasticity model within the cerebellum with long-term depression of parallel fiber synapses on Purkinje cells and long-term potentiation of mossy fiber synapses on neurons in the anterior interpositus nucleus. Conditioned stimulus and unconditioned stimulus inputs arise from the pontine nuclei and inferior olive, respectively, converging in the cerebellar cortex and deep nuclei. Projections from subcortical sensory nuclei to the pontine nuclei that are necessary for eyeblink conditioning are beginning to be identified, and recent studies indicate that there are dynamic interactions between sensory thalamic nuclei and the cerebellum during eyeblink conditioning. Cerebellar output is projected to the magnocellular red nucleus and then to the motor nuclei that generate the blink response(s). Tremendous progress has been made toward determining the neural mechanisms of delay eyeblink conditioning but there are still significant gaps in our understanding of the necessary neural circuitry and plasticity mechanisms underlying cerebellar learning.

An efficient method for the long-term and specific expression of exogenous cDNAs in cultured Purkinje neurons

We present a simple and efficient method for expressing cDNAs in Purkinje neurons (PNs) present in heterogeneous mouse cerebellar cultures. The method combines the transfection of freshly dissociated cerebellar cells via nucleofection with the use of novel expression plasmids containing a fragment of the L7 (Pcp2) gene that, within the cerebellum, drives PN-specific expression. The efficiency of PN transfection (determined 13 days post nucleofection) is approximately 70%. Double and triple transfections are routinely achieved at slightly lower efficiencies. Expression in PNs is obvious after one week in culture and still strong after three weeks, by which time these neurons are well-developed. Moreover, high-level expression is restricted almost exclusively to the PNs present in these mixed cultures, which greatly facilitates the characterization of PN-specific functions. As proof of principle, we used this method to visualize (1) the morphology of living PNs expressing mGFP, (2) the localization and dynamics of the dendritic spine proteins PSD-93 and Homer-3a tagged with mGFP and (3) the interaction of live PNs expressing mGFP with other cerebellar neurons expressing mCherry from a β-Actin promoter plasmid. Finally, we created a series of L7-plasmids containing different fluorescent protein cDNAs that are suited for the expression of cDNAs of interest as N- and C-terminally tagged fluorescent fusion proteins. In summary, this procedure allows for the highly efficient, long-term, and specific expression of multiple cDNAs in differentiated PNs, and provides a favorable alternative to two procedures (viral transduction, ballistic gene delivery) used previously to express genes in cultured PNs.

The conditioned eyeblink reflex: a potential tool for the detection of cerebellar dysfunction in multiple sclerosis

The delayed conditioned eyeblink reflex, in which an individual learns to close the eyelid in response to a conditioned stimulus (e.g. a tone) relies entirely on the functional integrity of a cerebellar motor circuitry that involves the contingent activation of Purkinje cells by parallel and climbing fibres. Molecular changes that disrupt the function of this circuitry, in particular a loss of type-1 metabotropic glutamate receptors (mGlu1 receptors), occur in Purkinje cells of patients with multiple sclerosis and in mice with experimental autoimmune encephalomyelitis as a result of neuroinflammation. mGlu1 receptors are required for cerebellar motor learning associated with the conditioned eyeblink reflex. We propose that the delayed paradigm of the eyeblink conditioning might be particularly valuable for the detection of subtle abnormalities of cerebellar motor learning that are clinically silent and are not associated with demyelinating lesions or axonal damage. In addition, the test might have predictive value following a clinically isolated syndrome, and might be helpful for the evaluation of the efficacy of drug treatment in multiple sclerosis.

Imaging of Ca2+ and related signaling molecules and investigation of their functions in the brain

Intracellular Ca(2+) signaling, and related mechanisms involving inositol 1,4,5-trisphosphate (IP(3)), nitric oxide, and the excitatory neurotransmitter glutamate, play a major role in the regulation of cellular function in the brain. Due to the complex morphology of central neurons, the correct spatiotemporal distribution of signaling molecules is essential. Thus, imaging studies have been particularly useful in elucidating the functions of these signaling molecules. The advancement of imaging methods, together with the development of a new method for the specific inhibition of intracellular IP(3) signaling, have made it possible to identify pathways that are regulated by Ca(2+) signals in the brain, including Ca(2+)-dependent synaptic maintenance and glial cell-dependent neurite growth. Further investigation of Ca(2+)-related signaling is expected to increase our understanding of brain function in the future.

Parasagittally aligned, mGluR1-dependent patches are evoked at long latencies by parallel fiber stimulation in the mouse cerebellar cortex in vivo

The parallel fibers (PFs) in the cerebellar cortex extend several millimeters along a folium in the mediolateral direction. The PFs are orthogonal to and cross several parasagittal zones defined by the olivocerebellar and corticonuclear pathways and the expression of molecular markers on Purkinje cells (PCs). The functions of these two organizations remain unclear, including whether the bands respond similarly or differentially to PF input. By using flavoprotein imaging in the anesthetized mouse in vivo, this study demonstrates that high-frequency PF stimulation, which activates a beamlike response at short latency, also evokes patches of activation at long latencies. These patches consist of increased fluorescence along the beam at latencies of 20-25 s with peak activation at 35 s. The long-latency patches are completely blocked by the type 1 metabotropic glutamate receptor (mGluR(1)) antagonist LY367385. Conversely, the AMPA and NMDA glutamate receptor antagonists DNQX and APV have little effect. Organized in parasagittal bands, the long-latency patches align with zebrin II-positive PC stripes. Additional Ca(2+) imaging demonstrates that the patches reflect increases in intracellular Ca(2+). Both the PLCβ inhibitor U73122 and the ryanodine receptor inhibitor ryanodine completely block the long-latency patches, indicating that the patches are due to Ca(2+) release from intracellular stores. Robust, mGluR(1)-dependent long-term potentiation (LTP) of the patches is induced using a high-frequency PF stimulation conditioning paradigm that generates LTP of PF-PC synapses. Therefore, the parasagittal bands, as defined by the molecular compartmentalization of PCs, respond differentially to PF inputs via mGluR(1)-mediated release of internal Ca(2+).

Learning on multiple timescales in smooth pursuit eye movements

We commonly think of motor learning as a gradual process that makes small, adaptive steps in a consistent direction. We now report evidence that learning in pursuit eye movements could start with large, transient short-term alterations that stoke a more gradual long-term process. Monkeys tracked a target that started moving horizontally or vertically. After 250 ms of motion had produced a preinstruction eye velocity close to target velocity, an orthogonal component of target motion created an instructive change in target direction that was randomly in one of the two directions along the orthogonal axis. The preinstruction eye velocity in each trial expressed single-trial learning as a bias toward the direction of the instruction in the prior trial. The single-trial learning was forgotten within 4 to 10 s. Two observations implied that single-trial learning was not simply cognitive anticipation. First, the magnitude of the trial-over-trial change in eye velocity depended on the ongoing eye velocity at the time of the instruction in the prior trial. Single-trial learning was negligible if the prior trial had provided a well-timed cue without evoking any preinstruction eye velocity. Second, regular alternation of the direction of the instructive target motion caused reactive rather than anticipatory trial-over-trial changes in eye velocity. Humans showed very different responses that appeared to be based on cognitive anticipation rather than learning. We suggest that single-trial learning results from a low-level learning mechanism and may be a necessary prerequisite for longer-term modifications that are more permanent.

Lateral diffusion of inositol 1,4,5-trisphosphate receptor type 1 in Purkinje cells is regulated by calcium and actin filaments

Inositol 1,4,5-trisphosphate receptor type 1 (IP(3) R1) is an intracellular Ca(2+) release channel that plays crucial roles in the functions of Purkinje cells. The dynamics of IP(3) R1 on the endoplasmic reticulum membrane and the distribution of IP(3) R1 in neurons are thought to be important for the spatial regulation of Ca(2+) release. In this study, we analyzed the lateral diffusion of IP(3) R1 in Purkinje cells in cerebellar slice cultures using fluorescence recovery after photobleaching. In the dendrites of Purkinje cells, IP(3) R1 showed lateral diffusion with an effective diffusion constant of approximately 0.30 μm(2) /s, and the diffusion of IP(3) R1 was negatively regulated by actin filaments. We found that actin filaments were also involved in the regulation of IP(3) R1 diffusion in the spine of Purkinje cells. Glutamate or quisqualic acid stimulation, which activates glutamate receptors and leads to a Ca(2+) transient in Purkinje cells, decreased the diffusion of IP(3) R1 and increased the density of actin in spines. These findings indicate that the neuronal activity-dependent augmentation of actin contributes to the stabilization of IP(3) R1 in spines.