Mossy Fiber-granule Cell Synapses Research Articles

Facial Stimulation Induces Long-Term Potentiation of Mossy Fiber-Granule Cell Synaptic Transmission via GluN2A-Containing N-Methyl-D-Aspartate Receptor/Nitric Oxide Cascade in the Mouse Cerebellum.

Long-term synaptic plasticity in the cerebellar cortex is a possible mechanism for motor learning. Previous studies have demonstrated the induction of mossy fiber-granule cell (MF-GrC) synaptic plasticity under in vitro and in vivo conditions, but the mechanisms underlying sensory stimulation-evoked long-term synaptic plasticity of MF-GrC in living animals are unclear. In this study, we investigated the mechanism of long-term potentiation (LTP) of MF-GrC synaptic transmission in the cerebellum induced by train of facial stimulation at 20 Hz in urethane-anesthetized mice using electrophysiological recording, immunohistochemistry techniques, and pharmacological methods. Blockade of GABAA receptor activity and repetitive facial stimulation at 20 Hz (240 pulses) induced an LTP of MF-GrC synapses in the mouse cerebellar cortical folium Crus II, accompanied with a decrease in paired-pulse ratio (N2/N1). The facial stimulation-induced MF-GrC LTP was abolished by either an N-methyl-D-aspartate (NMDA) receptor blocker, i.e., D-APV, or a specific GluNR2A subunit-containing NMDA receptor antagonist, PEAQX, but was not prevented by selective GluNR2B or GluNR2C/D subunit-containing NMDA receptor blockers. Application of GNE-0723, a selective and brain-penetrant-positive allosteric modulator of GluN2A subunit-containing NMDA receptors, produced an LTP of N1, accompanied with a decrease in N2/N1 ratio, and occluded the 20-Hz facial stimulation-induced MF-GrC LTP. Inhibition of nitric oxide synthesis (NOS) prevented the facial stimulation-induced MF-GrC LTP, while activation of NOS produced an LTP of N1, with a decrease in N2/N1 ratio, and occluded the 20-Hz facial stimulation-induced MF-GrC LTP. In addition, GluN2A-containing NMDA receptor immunoreactivity was observed in the mouse cerebellar granular layer. These results indicate that facial stimulation at 20 Hz induced LTP of MF-GrC synaptic transmission via the GluN2A-containing NMDA receptor/nitric oxide cascade in mice. The results suggest that the sensory stimulation-evoked LTP of MF-GrC synaptic transmission in the granular layer may play a critical role in cerebellar adaptation to native mossy fiber excitatory inputs and motor learning behavior in living animals.

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

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

Loss of Piccolo Function in Rats Induces Cerebellar Network Dysfunction and Pontocerebellar Hypoplasia Type 3-like Phenotypes.

Piccolo, a presynaptic active zone protein, is best known for its role in the regulated assembly and function of vertebrate synapses. Genetic studies suggest a further link to several psychiatric disorders as well as Pontocerebellar Hypoplasia type 3 (PCH3). We have characterized recently generated Piccolo KO (Pclogt/gt ) rats. Analysis of rats of both sexes revealed a dramatic reduction in brain size compared with WT (Pclowt/wt ) animals, attributed to a decrease in the size of the cerebral cortical, cerebellar, and pontine regions. Analysis of the cerebellum and brainstem revealed a reduced granule cell layer and a reduction in size of pontine nuclei. Moreover, the maturation of mossy fiber afferents from pontine neurons and the expression of the α6 GABAA receptor subunit at the mossy fiber-granule cell synapse are perturbed, as well as the innervation of Purkinje cells by cerebellar climbing fibers. Ultrastructural and functional studies revealed a reduced size of mossy fiber boutons, with fewer synaptic vesicles and altered synaptic transmission. These data imply that Piccolo is required for the normal development, maturation, and function of neuronal networks formed between the brainstem and cerebellum. Consistently, behavioral studies demonstrated that adult Pclogt/gt rats display impaired motor coordination, despite adequate performance in tasks that reflect muscle strength and locomotion. Together, these data suggest that loss of Piccolo function in patients with PCH3 could be involved in many of the observed anatomical and behavioral symptoms, and that the further analysis of these animals could provide fundamental mechanistic insights into this devastating disorder.SIGNIFICANCE STATEMENT Pontocerebellar Hypoplasia Type 3 is a devastating developmental disorder associated with severe developmental delay, progressive microcephaly with brachycephaly, optic atrophy, seizures, and hypertonia with hyperreflexia. Recent genetic studies have identified non-sense mutations in the coding region of the PCLO gene, suggesting a functional link between this disorder and the presynaptic active zone. Our analysis of Piccolo KO rats supports this hypothesis, formally demonstrating that anatomical and behavioral phenotypes seen in patients with Pontocerebellar Hypoplasia Type 3 are also exhibited by these Piccolo deficient animals.

Angiotensin-Receptor-Associated Protein Modulates Ca2+ Signals in Photoreceptor and Mossy Fiber cells

Fast, precise and sustained neurotransmission requires graded Ca2+ signals at the presynaptic terminal. Neurotransmitter release depends on a complex interplay of Ca2+ fluxes and Ca2+ buffering in the presynaptic terminal that is not fully understood. Here, we show that the angiotensin-receptor-associated protein (ATRAP) localizes to synaptic terminals throughout the central nervous system. In the retinal photoreceptor synapse and the cerebellar mossy fiber-granule cell synapse, we find that ATRAP is involved in the generation of depolarization-evoked synaptic Ca2+ transients. Compared to wild type, Ca2+ imaging in acutely isolated preparations of the retina and the cerebellum from ATRAP knockout mice reveals a significant reduction of the sarcoendoplasmic reticulum (SR) Ca2+-ATPase (SERCA) activity. Thus, in addition to its conventional role in angiotensin signaling, ATRAP also modulates presynaptic Ca2+ signaling within the central nervous system.

Hebbian Spike-Timing Dependent Plasticity at the Cerebellar Input Stage.

Spike-timing-dependent plasticity (STDP) is a form of long-term synaptic plasticity exploiting the time relationship between postsynaptic action potentials (APs) and EPSPs. Surprisingly enough, very little was known about STDP in the cerebellum, although it is thought to play a critical role for learning appropriate timing of actions. We speculated that low-frequency oscillations observed in the granular layer may provide a reference for repetitive EPSP/AP phase coupling. Here we show that EPSP-spike pairing at 6 Hz can optimally induce STDP at the mossy fiber-granule cell synapse in rats. Spike timing-dependent long-term potentiation and depression (st-LTP and st-LTD) were confined to a ±25 ms time-window. Because EPSPs led APs in st-LTP while APs led EPSPs in st-LTD, STDP was Hebbian in nature. STDP occurred at 6-10 Hz but vanished >50 Hz or <1 Hz (where only LTP or LTD occurred). STDP disappeared with randomized EPSP/AP pairing or high intracellular Ca2+ buffering, and its sign was inverted by GABA-A receptor activation. Both st-LTP and st-LTD required NMDA receptors, but st-LTP also required reinforcing signals mediated by mGluRs and intracellular calcium stores. Importantly, st-LTP and st-LTD were significantly larger than LTP and LTD obtained by modulating the frequency and duration of mossy fiber bursts, probably because STDP expression involved postsynaptic in addition to presynaptic mechanisms. These results thus show that a Hebbian form of STDP occurs at the cerebellum input stage, providing the substrate for phase-dependent binding of mossy fiber spikes to repetitive theta-frequency cycles of granule cell activity.SIGNIFICANCE STATEMENT Long-term synaptic plasticity is a fundamental property of the brain, causing persistent modifications of neuronal communication thought to provide the cellular basis of learning and memory. The cerebellum is critical for learning the appropriate timing of sensorimotor behaviors, but whether and how appropriate spike patterns could drive long-term synaptic plasticity remained unknown. Here, we show that this can actually occur through a form of spike-timing-dependent plasticity (STDP) at the cerebellar inputs stage. Pairing presynaptic and postsynaptic spikes at 6-10 Hz reliably induced STDP at the mossy fiber-granule cell synapse, with potentiation and depression symmetrically distributed within a ±25 ms time window. Thus, STDP can bind plasticity to the mossy fiber burst phase with high temporal precision.

The Active and Periactive Zone Organization and the Functional Properties of Small and Large Synapses.

The arrival of an action potential (AP) at a synaptic terminal elicits highly synchronized quanta release. Repetitive APs produce successive synaptic vesicle (SV) fusions that require management of spent SV components in the presynaptic membrane with minimum disturbance of the secretory apparatus. To this end, the synaptic machinery is structured accordingly to the strength and the range of frequencies at which each particular synapse operates. This results in variations in the number and dimension of Active Zones (AZs), amount and distribution of SVs, and probably, in the primary endocytic mechanisms they use. Understanding better how these structural differences determine the functional response in each case has been a matter of long-term interest. Here we review the structural and functional properties of three distinct types of synapses: the neuromuscular junction (NMJ; a giant, highly reliable synapse that must exocytose a large number of quanta with each stimulus to guarantee excitation of the postsynaptic cell), the hippocampal excitatory small synapse (which most often has a single release site and a relatively small pool of vesicles), and the cerebellar mossy fiber-granule cell synapse (which possesses hundreds of release sites and is able to translocate, dock and prime vesicles at high speed). We will focus on how the release apparatus is organized in each case, the relative amount of vesicular membrane that needs to be accommodated within the periAZ upon stimulation, the different mechanisms for retrieving the excess of membrane and finally, how these factors may influence the functioning of the release sites.

Chapter 2 - The Organization of Plasticity in the Cerebellar Cortex: From Synapses to Control

The cerebellum is thought to play a critical role in procedural learning, but the relationship between this function and the underlying cellular and synaptic mechanisms remains largely speculative. At present, at least nine forms of long-term synaptic and nonsynaptic plasticity (some of which are bidirectional) have been reported in the cerebellar cortex and deep cerebellar nuclei. These include long-term potentiation (LTP) and long-term depression at the mossy fiber-granule cell synapse, at the synapses formed by parallel fibers, climbing fibers, and molecular layer interneurons on Purkinje cells, and at the synapses formed by mossy fibers and Purkinje cells on deep cerebellar nuclear cells, as well as LTP of intrinsic excitability in granule cells, Purkinje cells, and deep cerebellar nuclear cells. It is suggested that the complex properties of cerebellar learning would emerge from the distribution of plasticity in the network and from its dynamic remodeling during the different phases of learning. Intrinsic and extrinsic factors may hold the key to explain how the different forms of plasticity cooperate to select specific transmission channels and to regulate the signal-to-noise ratio through the cerebellar cortex. These factors include regulation of neuronal excitation by local inhibitory networks, engagement of specific molecular mechanisms by spike bursts and theta-frequency oscillations, and gating by external neuromodulators. Therefore, a new and more complex view of cerebellar plasticity is emerging with respect to that predicted by the original "Motor Learning Theory," opening issues that will require experimental and computational testing.

TARPs γ‐2 and γ‐7 are essential for AMPA receptor expression in the cerebellum

AbstractThe α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA)‐type glutamate receptors require auxiliary subunits termed transmembrane AMPA receptor regulatory proteins (TARPs), which promote receptor trafficking to the cell surface and synapses and modulate channel pharmacology and gating. Of six TARPs, γ‐2 and γ‐7 are the two major TARPs expressed in the cerebellum. In the present study, we pursued their roles in synaptic expression of cerebellar AMPA receptors. In the cerebellar cortex, γ‐2 and γ‐7 were preferentially localized at various asymmetrical synapses. Using quantitative Western blot and immunofluorescence, we found severe reductions in GluA2 and GluA3 and mild reduction in GluA4 in γ‐2‐knockout (KO) cerebellum, whereas GluA1 and GluA4 were moderately reduced in γ‐7‐KO cerebellum. GluA2, GluA3 and GluA4 were further reduced in γ‐2/γ‐7 double‐KO (DKO) cerebellum. The large losses of GluA2 and GluA3 in γ‐2‐KO mice and further reductions in DKO mice were confirmed at all asymmetrical synapses examined with postembedding immunogold. Most notably, the GluA2 level in the postsynaptic density fraction, GluA2 labeling density at parallel fiber–Purkinje cell synapses, and AMPA receptor‐mediated currents at climbing fiber–Purkinje cell synapses were all reduced to approximately 10% of the wild‐type levels in DKO mice. On the other hand, the reduction in GluA4 in γ‐7‐KO granular layer reflected its loss at mossy fiber–granule cell synapses, whereas that of GluA1 and GluA4 in γ‐7‐KO molecular layer was caused, at least partly, by their loss in Bergmann glia. Therefore, γ‐2 and γ‐7 cooperatively promote synaptic expression of cerebellar AMPA receptors, and the latter also promotes glial expression.

Differential induction of bidirectional long‐term changes in neurotransmitter release by frequency‐coded patterns at the cerebellar input

Sensory stimulation conveys spike discharges of variable frequency and duration along the mossy fibres of cerebellum raising the question of whether and how these patterns determine plastic changes at the mossy fibre-granule cell synapse. Although various combinations of high-frequency bursts and membrane depolarization can induce NMDA receptor-dependent long-term depression (LTD) and long-term potentiation (LTP), the effect of different discharge frequencies remained unknown. Here we show that low-frequency mossy fibre stimulation (100 impulses1 Hz) induces mGlu receptor-dependent LTD. For various burst frequencies, the plasticity-[Ca(2+)](i) relationship was U-shaped resembling the Bienenstok-Cooper-Munro (BCM) learning rule. Moreover, LTD expression was associated with increased paired-pulse ratio, coefficient of variation and failure rate, and with a decrease in release probability, therefore showing changes opposite to those characterizing LTP. The plasticity-[Ca(2+)](i) relationship and the changes in neurotransmitter release measured by varying induction frequencies were indistinguishable from those obtained by varying high-frequency burst duration. These results suggest that different glutamate receptors converge onto a final common mechanism translating the frequency and duration of mossy fibre discharges into a regulation of the LTP/LTD balance, which may play an important role in adapting spatio-temporal signal transformations at the cerebellar input stage.

Event-driven simulation of cerebellar granule cells

Around half of the neurons of a human brain are granule cells (approximately 10 11granule neurons) [Kandel, E.R., Schwartz, J.H., Jessell, T.M., 2000. Principles of Neural Science. McGraw-Hill Professional Publishing, New York]. In order to study in detail the functional role of the intrinsic features of this cell we have developed a pre-compiled behavioural model based on the simplified granule-cell model of Bezzi et al. [Bezzi, M., Nieus, T., Arleo, A., D’Angelo, E., Coenen, O.J.-M.D., 2004. Information transfer at the mossy fiber—granule cell synapse of the cerebellum. 34th Annual Meeting. Society for Neuroscience, San Diego, CA, USA]. We can use an efficient event-driven simulation scheme based on lookup tables (EDLUT) [Ros, E., Carrillo, R.R., Ortigosa, E.M., Barbour, B., Ags, R., 2006. Event-driven simulation scheme for spiking neural networks using lookup tables to characterize neuronal dynamics. Neural Computation 18 (12), 2959–2993]. For this purpose it is necessary to compile into tables the data obtained through a massive numerical calculation of the simplified cell model. This allows network simulations requiring minimal numerical calculation. There are three major features that are considered functionally relevant in the simplified granule cell model: bursting, subthreshold oscillations and resonance. In this work we describe how the cell model is compiled into tables keeping these key properties of the neuron model.

Tactile Stimulation Evokes Long-Term Synaptic Plasticity in the Granular Layer of Cerebellum

Several forms of long-term synaptic plasticity [long-term potentiation (LTP) and long-term depression (LTD)] have been reported in the cerebellar circuit in vitro, but their determination in vivo was still lacking in most cases. Here we show that, in the urethane-anesthetized rat, appropriate patterns of facial tactile stimulation as well as intracerebellar electrical stimulation can induce LTP and LTD in local field potentials recorded from the granular layer of Crus-IIa. LTD prevailed in control conditions, whereas LTP prevailed during local application of gabazine. No relevant plasticity was observed when gabazine and APV were coapplied. The pharmacological and kinetic properties of LTP and LTD in vivo were compatible with those reported in the granule cell layer in vitro (Mapelli and D'Angelo, 2007), suggesting that NMDA receptor-dependent plasticity was generated at the mossy fiber-granule cell synapse under the inhibitory control of the Golgi cell circuit. Interestingly, LTP and LTD were able to regulate the response latency to tactile stimulation, as expected from computational modeling of the expression mechanisms (Nieus et al., 2006). This result suggests that LTP and LTD could regulate the spatiotemporal pattern of granular layer responses to mossy fiber inputs.

High-fidelity transmission of sensory information by single cerebellar mossy fibre boutons

Understanding the transmission of sensory information at individual synaptic connections requires knowledge of the properties of presynaptic terminals and their patterns of firing evoked by sensory stimuli. Such information has been difficult to obtain because of the small size and inaccessibility of nerve terminals in the central nervous system. Here we show, by making direct patch-clamp recordings in vivo from cerebellar mossy fibre boutons-the primary source of synaptic input to the cerebellar cortex-that sensory stimulation can produce bursts of spikes in single boutons at very high instantaneous firing frequencies (more than 700 Hz). We show that the mossy fibre-granule cell synapse exhibits high-fidelity transmission at these frequencies, indicating that the rapid burst of excitatory postsynaptic currents underlying the sensory-evoked response of granule cells can be driven by such a presynaptic spike burst. We also demonstrate that a single mossy fibre can trigger action potential bursts in granule cells in vitro when driven with in vivo firing patterns. These findings suggest that the relay from mossy fibre to granule cell can act in a 'detonator' fashion, such that a single presynaptic afferent may be sufficient to transmit the sensory message. This endows the cerebellar mossy fibre system with remarkable sensitivity and high fidelity in the transmission of sensory information.

Desensitization Properties of AMPA Receptors at the Cerebellar Mossy Fiber–Granule Cell Synapse

Native AMPA receptors (AMPARs) exhibit rapid and profound desensitization in the sustained presence of glutamate. Desensitization therefore contributes to short-term depression at synapses in which glutamate accumulates. At synapses that do not exhibit desensitization-dependent depression, AMPARs are thought to be protected against prolonged or repetitive exposure to synaptically released glutamate. At the cerebellar mossy fiber to granule cell (GC) synapse, in which high release probability and glutamate spillover produce a substantial buildup of glutamate concentration in the cleft ([Glut]cleft) during high-frequency transmission, only moderate desensitization of the phasic AMPAR EPSC occurs. To investigate how such currents are produced, we examined the kinetic properties of synaptic AMPARs in GCs using glutamate uncaging. Photolysis of 4-methoxy-7-nitroindolinyl-caged L-glutamate with large illumination spots produced step-like increases in [Glut]cleft that could be used to systematically probe AMPAR kinetics. At low levels of activation, synaptic AMPARs exhibited little desensitization. With larger activations, the desensitization time course became faster, but the level of desensitization was only weakly dependent on receptor occupancy. Indeed, a substantial desensitization-resistant current component remained (17%) in saturating glutamate. Photolysis with small illumination spots produced brief [Glut]cleft waveforms and transient AMPAR activations, similar to the EPSC current components. Paired-pulse uncaging with such spots revealed little desensitization after spillover-like activations and modest depression after activations that mimicked quantal and spillover components together. Our results show that GC AMPARs exhibit a resistance to desensitization at low occupancies and that this property is crucial for sustaining high-frequency transmission at a synapse in which glutamate accumulates.

Increased Ethanol Resistance and Consumption in Eps8 Knockout Mice Correlates with Altered Actin Dynamics

Dynamic modulation of the actin cytoskeleton is critical for synaptic plasticity, abnormalities of which are thought to contribute to mental illness and addiction. Here we report that mice lacking Eps8, a regulator of actin dynamics, are resistant to some acute intoxicating effects of ethanol and show increased ethanol consumption. In the brain, the N-methyl-D-aspartate (NMDA) receptor is a major target of ethanol. We show that Eps8 is localized to postsynaptic structures and is part of the NMDA receptor complex. Moreover, in Eps8 null mice, NMDA receptor currents and their sensitivity to inhibition by ethanol are abnormal. In addition, Eps8 null neurons are resistant to the actin-remodeling activities of NMDA and ethanol. We propose that proper regulation of the actin cytoskeleton is a key determinant of cellular and behavioral responses to ethanol.

Selective Engagement of Plasticity Mechanisms for Motor Memory Storage

The number and diversity of plasticity mechanisms in the brain raises a central question: does a neural circuit store all memories by stereotyped application of the available plasticity mechanisms, or can subsets of these mechanisms be selectively engaged for specific memories? The uniform architecture of the cerebellum has inspired the idea that plasticity mechanisms like cerebellar long-term depression (LTD) contribute universally to memory storage. To test this idea, we investigated a set of closely related, cerebellum-dependent motor memories. In mutant mice lacking Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV), the maintenance of cerebellar LTD is abolished. Although memory for an increase in the gain of the vestibulo-ocular reflex (VOR) induced with high-frequency stimuli was impaired in these mice, memories for decreases in VOR gain and increases in gain induced with low-frequency stimuli were intact. Thus, a particular plasticity mechanism need not support all cerebellum-dependent memories, but can be engaged selectively according to the parameters of training.

Signaling across the synapse: a role for Wnt and Dishevelled in presynaptic assembly and neurotransmitter release

Proper dialogue between presynaptic neurons and their targets is essential for correct synaptic assembly and function. At central synapses, Wnt proteins function as retrograde signals to regulate axon remodeling and the accumulation of presynaptic proteins. Loss of Wnt7a function leads to defects in the localization of presynaptic markers and in the morphology of the presynaptic axons. We show that loss of function of Dishevelled-1 (Dvl1) mimics and enhances the Wnt7a phenotype in the cerebellum. Although active zones appear normal, electrophysiological recordings in cerebellar slices from Wnt7a/Dvl1 double mutant mice reveal a defect in neurotransmitter release at mossy fiber–granule cell synapses. Deficiency in Dvl1 decreases, whereas exposure to Wnt increases, synaptic vesicle recycling in mossy fibers. Dvl increases the number of Bassoon clusters, and like other components of the Wnt pathway, it localizes to synaptic sites. These findings demonstrate that Wnts signal across the synapse on Dvl-expressing presynaptic terminals to regulate synaptic assembly and suggest a potential novel function for Wnts in neurotransmitter release.

Increased neurotransmitter release during long-term potentiation at mossy fibre-granule cell synapses in rat cerebellum.

During long-term potentiation (LTP) at mossy fibre-granule cell synapses in rat cerebellum synaptic transmission and granule cell intrinsic excitability are enhanced. Although it is clear that changes in granule cell excitability are mediated postsynaptically, there is as yet no direct evidence for the site and mechanism of changes in transmission. To approach this problem, evoked postsynaptic currents (EPSCs) and miniature synaptic currents (mEPSCs) were recorded by patch-clamp in cerebellar slices obtained from P17-P23 rats. LTP was induced by theta-burst stimulation paired with depolarization. During LTP, the EPSCs showed a significant decrease in the coefficient of variation (CV; 28.9 +/- 5.2%, n= 8; P < 0.002), the number of failures (87.1 +/- 41.9%, n= 8; P < 0.04), and the paired-pulse ratio (PPR; 25.5 +/- 4.1% n= 5; P < 0.02). Similar changes were observed by increasing neurotransmitter release (extracellular solutions with high Ca(2+)/Mg(2+) ratio), whereas increases in CV, numbers of failures and PPR occurred when release was decreased (extracellular solutions with low Ca(2+)/Mg(2+) ratio; 10 microm Cl-adenosine). No changes followed modifications of postsynaptic holding potentials, while CV and failures were reduced when the number of active synapses was increased. LTP was prevented by use of solutions with high Ca(2+)/Mg(2+) ratio. Moreover, LTP and the associated CV decrease were observed in the spillover-mediated component of AMPA EPSCs and in NMDA EPSCs. During LTP, mEPSCs did not change in amplitude or variability but significantly increased in frequency (47.6 +/- 16%, n= 4; P < 0.03). By binomial analysis changes in EPSCs were shown to be due to increased release probability (from 0.6 +/- 0.08 to 0.73 +/- 0.06, n= 7; P < 0.02) with a constant number of three to four releasing sites. These observations provide evidence for increased neurotransmitter release during LTP at mossy fibre-granule cell synapses.

5-Hydroxyindole potentiates human α7 nicotinic receptor-mediated responses and enhances acetylcholine-induced glutamate release in cerebellar slices

The effects of 5-hydroxyindole (5-HI) have been investigated on human α7 nicotinic acetylcholine receptors (nAChRs) expressed in Xenopus oocytes and GH4 cells, on native α7 nAChRs expressed by IMR-32 cells and on α7 nAChR-mediated events in mossy fibre–granule cell synapses in rat cerebellar slices. In oocytes expressing α7 nAChRs, 5-HI potentiated sub-maximal, 60 μM ACh-induced ion currents in a concentration-dependent manner, the threshold effective concentration being 30 μM. 5-HI itself did not act as an agonist on α7 nAChRs. A maximum potentiation of 12 times the control was observed at 20 mM 5-HI. The effect of 1 mM 5-HI on the concentration–response curve for ACh revealed that 5-HI increased the potency as well as the efficacy of ACh on α7 nAChRs. 5-HI also potentiated α7-mediated increases in intracellular free calcium levels in both mammalian cells heterologously expressing human α7 nAChRs and in human IMR-32 neuroblastoma cells expressing native α7 nAChRs. At mossy fibre–granule cell synapses, application of 1 mM ACh induced glutamate-evoked excitory post-synaptic currents (EPSCs). Co-application of 1 mM 5-HI with 1 mM ACh further increased the frequency of the EPSCs. The ACh-induced release, as well as the 5-HI-induced enhancement of release, were blocked by 1–10 nM methyllycaconitine or 200 nM α-bungarotoxin, demonstrating that both effects were mediated by presynaptic α7 nAChRs. The results demonstrate that responses mediated by α7 nAChRs are strongly potentiated by 5-HI.

Presynaptic current changes at the mossy fiber-granule cell synapse of cerebellum during LTP.

The involvement of presynaptic mechanisms in the expression of long-term potentiation (LTP), an enhancement of synaptic transmission suggested to take part in learning and memory in the mammalian brain, has not been fully clarified. Although evidence for enhanced vesicle cycling has been reported, it is unknown whether presynaptic terminal excitability could change as has been observed in invertebrate synapses. To address this question, we performed extracellular focal recordings in cerebellar slices. The extracellular current consisted of a pre- (P(1)/N(1)) and postsynaptic (N(2)/SN) component. In ~50% of cases, N(1) could be subdivided into N(1a) and N(1b). Whereas N(1a) was part of the fiber volley (P(1)/N(1a)), N(1b) corresponded to a Ca(2+)-dependent component accounting for 40-50% of N(1), which could be isolated from individual mossy fiber terminals visualized with fast tetramethylindocarbocyanine perchlorate (DiI). The postsynaptic response, given its timing and sensitivity to glutamate receptor antagonists [N(2) was blocked by 10 microM [1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX) and SN by 100 microM APV +50 microM 7-Cl-kyn], corresponded to granule cell excitation. N(2) and SN could be reduced by 1) Ca(2+) channel blockers, 2) decreasing the Ca(2+) to Mg(2+) ratio, 3) paired-pulse stimulation, and 4) adenosine receptor activation. However, only the first two manipulations, which modify Ca(2+) influx, were associated with N(1) (or N(1b)) reduction. LTP was induced by theta-burst mossy fiber stimulation (8 trains of 10 impulses at 100 Hz separated by 150-ms pauses). Interestingly, during LTP, both N(1) (or N(1b)) and N(2)/SN persistently increased, whereas P(1) (or P(1)/N(1a)) did not change. Average changes were N(1) = 38.1 +/- 31.9, N(2) = 49.6 +/- 48.8, and SN = 59.1 +/- 35.5%. The presynaptic changes were not observed when LTP was prevented by synaptic inhibition, by N-methyl-D-aspartate and metabotropic glutamate receptor blockage, or by protein kinase C blockage. Moreover, the presynaptic changes were sensitive to Ca(2+) channel blockers (1 mM Ni(2+) and 5 microM omega-CTx-MVIIC) and occluded by K(+) channel blockers (1 mM tetraethylammmonium). Thus regulation of presynaptic terminal excitability may take part in LTP expression at a central mammalian synapse.

A Simplified Model of Long-Term Plasticity in Cerebellar Mossy Fiber–Granule Cell Synapses

We simulated synaptic transmission and modified a simple model of long-term potentiation and long-term depression in order to describe the long-term plasticity-related changes in cerebellar mossy fiber–granule cell synapses. In our model, protein autophosphorylation, leading to the maintenance of long-term plasticity, is controlled by Ca2+ entry through the NMDA receptor channels. The observed nonlinearity in the development of long-term changes of EPSP in granule cells is explained by the difference in the rate constants of two independent autocatalytic processes.