Parallel Fiber Terminals Research Articles

Coexpression of calcineurin A and B subunits in various subcellular and synaptic compartments of cerebellar neurons and glia with particular abundance at parallel fiber-Purkinje cell synapses

Calcineurin (CN) is a Ca2+/calmodulin-dependent serine/threonine protein phosphatase consisting of catalytic CNA and regulatory CNB subunits, and links activity-dependent Ca2+ signals to various neural functions. Here we studied CN expression in the mouse brain by producing subunit-specific probes and antibodies. Of five CN subunits. CNAα, CNAβ, and CNB1 mRNAs were predominantly expressed over the brain from early embryonic to adult stage, and all were high in the telencephalon and cerebellum. Protein localization was examined in the cerebellum by immunofluorescence with cellular and terminal markers and by preembedding silver-enhanced immunogold microscopy. CNB1 and CNAβ were co-distributed in subcellular and synaptic elements of various cerebellar neurons and glia, whereas CNAα was exclusive in granule cell elements, including parallel fiber terminals. The present study thus discloses that CNB1 subunit well coexists with one or two CNA subunits in various cerebellar compartments. Moreover, high CN contents are provided to parallel fiber-Purkinje cell synapses, i.e., CNAα, CNAβ, and CNB1 in their presynaptic side and CNAβ and CNB1 in their postsynaptic side. These findings will be the anatomical basis, at least partly, for the known regulatory roles of postsynaptic CNs in long-term depression and presynaptic CNs in transmitter release function.

Cellular Diversity and Differential Subcellular Localization of the G-Protein Gαo Subunit in the Mouse Cerebellum.

Heterotrimeric guanine nucleotide-binding proteins (G proteins) transduce signals from G protein-coupled receptors (GPCRs) to effector ion channels and enzymes Gαo, a member of the pertussis toxin-sensitive Gi/o family, is widely expressed in the brain, although its role within a neuronal context remains largely unknown. Using immunohistochemical and quantitative immunoelectron microscopy techniques, we have investigated the expression, cellular and subcellular localization of Gαo in the cerebellar cortex. Histoblot revealed that Gαo is expressed in many brain regions, including the cerebellum. At the cellular level, Gαo protein was distributed in Purkinje cells, basket cells, stellate cells, granule cells and Golgi cells. At the subcellular level, pre-embedding immunoelectron microscopy revealed mainly a postsynaptic localization of Gαo along the extrasynaptic plasma membrane of Purkinje cell dendritic shafts and spines, and dendrites of basket, stellate and granule cells. To a lesser extent, immunolabeling for Gαo was localized in different types of axon terminals establishing excitatory synapses. Moreover, post-embedding immunoelectron microscopy revealed the synaptic localization of Gαo on PSDs of glutamatergic synapses between Purkinje cell spines and parallel fiber terminals and its co-localization with GABAB1 in the same spines. Quantitative analysis of Gαo immunoparticles revealed they preferentially localized on the cytoplasmic face of the plasma membrane. Furthermore, the analysis revealed a high concentration of Gαo around excitatory synapses on Purkinje cell dendritic spines, but a uniform distribution in granule cell dendrites. These molecular-anatomical findings suggest that Gαo is a major signal transducer of specific GPCRs in different neuronal populations in the cerebellum.

Deletion of the cannabinoid CB1 receptor impacts on the ultrastructure of the cerebellar parallel fiber-Purkinje cell synapses.

The cannabinoid CB1 receptor localizes to the glutamatergic parallel fiber (PF) terminals of the cerebellar granule cells and participates in synaptic plasticity, motor control and learning that are impaired in CB1 receptor knockout (CB 1 -KO) mice. However, whether ultrastructural changes at the PF-Purkinje cell (PC) synapses occur in CB 1 -KO remains unknown. We studied this in the vermis of the spinocerebellar lobule V and the vestibulocerebellar lobule X of CB 1 -KO and wild-type (CB 1 -WT) mice by electron microscopy. Lobule V, but not lobule X, of CB 1 -KO had significantly less and longer synapses than in CB 1 -WT. PF terminals were significantly larger in both lobules of CB 1 -KO with no changes in PC dendritic spines. The PF terminals in lobule V of CB 1 -KO contained less synaptic vesicles and lower vesicle density; by contrast, vesicle density in lobule X of CB 1 -KO remained unchangeable relative to CB 1 -WT. There were as many vesicles in lobule V of CB 1 -KO as in CB 1 -WT, but their distribution decreased drastically at 300 nm of the active zone. In lobule X of CB 1 -KO, less vesicles were found within 150 nm from the presynaptic membrane; however, no vesicles were at 450-600 nm of the active zone. A significant higher amount of synaptic vesicles close to the active zone in lobule V and X of CB 1 -KO was observed. In conclusion, the absence of CB1 receptors strikingly and distinctively impacts on the ultrastructural architecture of the PF-PC synapses located in cerebellar lobules that differ in vulnerability to damage and motor functions.

Merlin modulates process outgrowth and synaptogenesis in the cerebellum.

Neurofibromatosis type 2 (NF2) patients are prone to develop glial-derived tumors in the peripheral and central nervous system (CNS). The Nf2 gene product -Merlin is not only expressed in glia, but also in neurons of the CNS, where its function still remains elusive. Here, we show that cerebellar Purkinje cells (PCs) of isoform-specific Merlin-deficient mice were innervated by smaller vGluT2-positive clusters at presynaptic terminals than those of wild-type mice. This was paralleled by a reduction in frequency and amplitude of miniature excitatory postsynaptic currents (mEPSC). On the contrary, in conditional transgenic mice in which Merlin expression was specifically ablated in PCs (L7Cre;Nf2fl/fl), we found enlarged vGluT2-positive clusters in their presynaptic buttons together with increased amplitudes of miniature postsynaptic currents. The presynaptic terminals of these PCs innervating neurons of the deep cerebellar nuclei were also enlarged. When exploring mice with Merlin-deficient granule cells (GCs) (Math1Cre;Nf2fl/fl), we found cerebellar extracts to contain higher amounts of vGluT1 present in parallel fiber terminals. In parallel, mEPSC frequency was increased in Math1Cre;Nf2fl/fl mice. On the contrary, VGluT2 clusters in cerebellar glomeruli composed of NF2-deficient presynaptic Mossy fiber terminals and NF2-deficient postsynaptic GC were reduced in size as shown for isoform-specific knockout mice. These changes in Math1Cre;Nf2fl/fl-deficient mice were paralleled by an increased activation of Rac1-Cofilin signaling which is known to impact on cytoskeletal reorganization and synapse formation. Consistent with the observed synaptic alterations in these transgenic mice, we observed altered ultrasonic vocalization, which is known to rely on proper cerebellar function. No gross morphological changes or motor coordination deficits were observed in any of these transgenic mice. We therefore conclude that Merlin does not regulate overall cerebellar development, but impacts on pre- and post-synaptic terminal organization.

Correction: Distribution of vesicle pools in cerebellar parallel fibre terminals after depression of ectopic transmission

[This corrects the article DOI: 10.1371/journal.pone.0200937.].

Distribution of vesicle pools in cerebellar parallel fibre terminals after depression of ectopic transmission.

At parallel fibre terminals in the cerebellar cortex, glutamate released outside of the active zone can activate AMPA receptors on juxtaposed Bergmann glial cell processes. This process is termed “ectopic” release, and allows for directed transmission to astroglial cells that is functionally independent of synaptic transmission to postsynaptic Purkinje neurons. The location of ectopic sites in presynaptic terminals is uncertain. Functional evidence suggests that stimulation of parallel fibres at 1 Hz exhausts ectopic transmission due to a failure to rapidly recycle vesicles to the ectopic pool, and so would predict a loss of vesicles in the near vicinity of extrasynaptic glial processes. In this study we used this stimulation protocol to investigate whether the distribution of vesicles within the presynaptic terminal is altered after suppression of ectopic release. Stimulation at 1 Hz had only a minor impact on the distribution of vesicles in presynaptic terminals when analysed with electron microscopy. Vesicle number and terminal size were unaffected by 1 Hz stimulation, but the relative abundance of vesicles in close proximity to the active zone was marginally reduced. In contrast, the fraction of vesicles facing glial membranes was unchanged after suppression of ectopic transmission. 1 Hz stimulation also resulted in a small but statistically-significant increase in the distance between glial membrane and presynaptic terminal, suggesting withdrawal of glial membranes from synapses is detectable in ultrastructural anatomy within minutes. These results raise doubts about the location of ectopic release sites, but indicate that neuron-glial association varies on a dynamic time scale.

Regulation of spinogenesis in mature Purkinje cells via mGluR/PKC-mediated phosphorylation of CaMKIIβ

Dendritic spines of Purkinje cells form excitatory synapses with parallel fiber terminals, which are the primary sites for cerebellar synaptic plasticity. Nevertheless, how density and morphology of these spines are properly maintained in mature Purkinje cells is not well understood. Here we show an activity-dependent mechanism that represses excessive spine development in mature Purkinje cells. We found that CaMKIIβ promotes spine formation and elongation in Purkinje cells through its F-actin bundling activity. Importantly, activation of group I mGluR, but not AMPAR, triggers PKC-mediated phosphorylation of CaMKIIβ, which results in dissociation of the CaMKIIβ/F-actin complex. Defective function of the PKC-mediated CaMKIIβ phosphorylation promotes excess F-actin bundling and leads to abnormally numerous and elongated spines in mature IP3R1-deficient Purkinje cells. Thus, our data suggest that phosphorylation of CaMKIIβ through the mGluR/IP3R1/PKC signaling pathway represses excessive spine formation and elongation in mature Purkinje cells.

Immunohistochemical characterization of axon terminals of the adult rat cerebellar cortex

Aim of the present study was to immunohystochemically characterize the glutamatergic terminals in the cerebellar cortex. The study was carried out on adult rat, using immunohistochemistry for the vesicular transporters of glutamate, VGLUT1 e VGLUT2, and synaptophysin. Results. Terminals positive for VGLUT1 e VGLUT2 were observed in the molecular layer (ML) and granular layer (GL). ML. VGLUT1-positive terminals appeared very numerous, homogeneously distributed throughout the layer with a distribution pattern resembling that of the parallel fiber terminals. VGLUT2-positive terminals appeared displaced along distinct spiral lines extending from the deeper part of the layer up to the its superficial part, likely corresponding to synapses between climbing fibers and Purkinje neuron dendrites. No colocalization of VGLUT1 and VGLUT2 was observed in ML. Double labelling for VGLUT1 and synaptophysin and, respectively, VGLUT2 and synaptophysin revealed colocalization, suggesting the axon terminal nature of the immunolabelled elements. Interestingly, some elements appeared synaptophysin positive, but negative for VGLUT1 and VGLUT2. In GL, VGLUT1 and VGLUT2 positive terminals displayed similar distribution patterns, They appeared clustered in restricted regions of the layer, scattered within granule neurons, at the level of synaptic glomeruli. Most of these terminals showed a colocalization of VGLUT1 and VGLUT2. However a part of them shows positivity for VGLUT2 and negativity for VGLUT1. All the VGLUT1 and VGLUT2 positive elements also displayed positivity for synaptophysin. Finally, like in ML, some elements appeared synaptophysin positive, but negative for VGLUT1 and VGLUT2. Conclusion. The results indicate that the glutamatergic terminals in the cerebellar cortex may be differentiated combining Immunohistochemistry for VGLUT1 and VGLUT2. Moreover, the results identify subpopulations within terminals of the parallel, climbing and mossy fibers. In particular, a subpopulation of mossy fiber terminals, displaying positivity for VGLUT2 and negativity for VGLUT1, are different from the vast majority of mossy fiber terminals, which display for both VGLUT1 and VGLUT2, but similar to climbing fiber terminals. It is intriguing to hypothesize that these mossy fibers may constitute a contingent of mossy fiber originated in the inferior olivary nuclear complex.

Editorial: Determinants of Synaptic Information Transfer: From Ca(2+) Binding Proteins to Ca(2+) Signaling Domains.

Editorial: Determinants of Synaptic Information Transfer: From Ca(2+) Binding Proteins to Ca(2+) Signaling Domains.

Caffeine Modulates Vesicle Release and Recovery at Cerebellar Parallel Fibre Terminals, Independently of Calcium and Cyclic AMP Signalling.

BackgroundCerebellar parallel fibres release glutamate at both the synaptic active zone and at extrasynaptic sites—a process known as ectopic release. These sites exhibit different short-term and long-term plasticity, the basis of which is incompletely understood but depends on the efficiency of vesicle release and recycling. To investigate whether release of calcium from internal stores contributes to these differences in plasticity, we tested the effects of the ryanodine receptor agonist caffeine on both synaptic and ectopic transmission.MethodsWhole cell patch clamp recordings from Purkinje neurons and Bergmann glia were carried out in transverse cerebellar slices from juvenile (P16-20) Wistar rats.Key ResultsCaffeine caused complex changes in transmission at both synaptic and ectopic sites. The amplitude of postsynaptic currents in Purkinje neurons and extrasynaptic currents in Bergmann glia were increased 2-fold and 4-fold respectively, but paired pulse ratio was substantially reduced, reversing the short-term facilitation observed under control conditions. Caffeine treatment also caused synaptic sites to depress during 1 Hz stimulation, consistent with inhibition of the usual mechanisms for replenishing vesicles at the active zone. Unexpectedly, pharmacological intervention at known targets for caffeine—intracellular calcium release, and cAMP signalling—had no impact on these effects.ConclusionsWe conclude that caffeine increases release probability and inhibits vesicle recovery at parallel fibre synapses, independently of known pharmacological targets. This complex effect would lead to potentiation of transmission at fibres firing at low frequencies, but depression of transmission at high frequency connections.

A use-dependent increase in release sites drives facilitation at calretinin-deficient cerebellar parallel-fiber synapses.

Endogenous Ca2+-binding proteins affect synaptic transmitter release and short-term plasticity (STP) by buffering presynaptic Ca2+ signals. At parallel-fiber (PF)-to-Purkinje neuron (PN) synapses in the cerebellar cortex loss of calretinin (CR), the major buffer at PF terminals, results in increased presynaptic Ca2+ transients and an almost doubling of the initial vesicular releases probability (pr). Surprisingly, however, it has been reported that loss of CR from PF synapses does not alter paired-pulse facilitation (PPF), while it affects presynaptic Ca2+ signals as well as pr. Here, we addressed this puzzling observation by analyzing the frequency- and Ca2+-dependence of PPF at unitary PF-to-PN synapses of wild-type (WT) and CR-deficient (CR−/−) mice using paired recordings and computer simulations. Our analysis revealed that PPF in CR−/− is indeed smaller than in the WT, to a degree, however, that indicates that rapid vesicle replenishment and recruitment of additional release sites dominate the synaptic efficacy of the second response. These Ca2+-driven processes operate more effectively in the absence of CR, thereby, explaining the preservation of robust PPF in the mutants.

Localization of Presynaptic Plasticity Mechanisms Enables Functional Independence of Synaptic and Ectopic Transmission in the Cerebellum.

In the cerebellar molecular layer parallel fibre terminals release glutamate from both the active zone and from extrasynaptic “ectopic” sites. Ectopic release mediates transmission to the Bergmann glia that ensheathe the synapse, activating Ca2+-permeable AMPA receptors and glutamate transporters. Parallel fibre terminals exhibit several forms of presynaptic plasticity, including cAMP-dependent long-term potentiation and endocannabinoid-dependent long-term depression, but it is not known whether these presynaptic forms of long-term plasticity also influence ectopic transmission to Bergmann glia. Stimulation of parallel fibre inputs at 16 Hz evoked LTP of synaptic transmission, but LTD of ectopic transmission. Pharmacological activation of adenylyl cyclase by forskolin caused LTP at Purkinje neurons, but only transient potentiation at Bergmann glia, reinforcing the concept that ectopic sites lack the capacity to express sustained cAMP-dependent potentiation. Activation of mGluR1 caused depression of synaptic transmission via retrograde endocannabinoid signalling but had no significant effect at ectopic sites. In contrast, activation of NMDA receptors suppressed both synaptic and ectopic transmission. The results suggest that the signalling mechanisms for presynaptic LTP and retrograde depression by endocannabinoids are restricted to the active zone at parallel fibre synapses, allowing independent modulation of synaptic transmission to Purkinje neurons and ectopic transmission to Bergmann glia.

Morphometric analysis of progressive changes in hereditary cerebellar cortical degenerative disease (abiotrophy) in rabbits caused by abnormal synaptogenesis.

We previously investigated rabbit hereditary cerebellar cortical degenerative disease, called cerebellar cortical abiotrophy in the veterinary field, and determined that the pathogenesis of this disease is the result of failed synaptogenesis between parallel fibers and Purkinje cells. In this study, longitudinal changes in the development and atrophy of the cerebellum of rabbits with hereditary abiotrophy after birth were morphometrically examined (postnatal day [PD] 15 and 42) using image analysis. Although development of the cerebellum in rabbits with abiotrophy was observed from PD 15 to PD 42, the growth rate of the cerebellum was less than that in normal rabbits. In rabbits with abiotrophy, the number of granular cells undergoing apoptosis was significantly higher at PD 15 and dramatically decreased at PD 42. The number of granular cells did not increase from PD 15 to 42. The synaptogenesis peak at PD 15 occurred when the largest number of apoptotic granular cells in rabbits with abiotrophy was observed. Although 26% to 36% of parallel fiber terminals formed synaptic junctions with Purkinje cell spines, the remainder did not at PD 15 and 42. The rate of failure of synaptogenesis in the present study might be specific to this case of abiotrophy. Morphometric analysis revealed detailed changes in development and atrophy in animals with postnatal cerebellar disease occurring soon after birth.

Mutant Ataxin-3 with an Abnormally Expanded Polyglutamine Chain Disrupts Dendritic Development and Metabotropic Glutamate Receptor Signaling in Mouse Cerebellar Purkinje Cells

Spinocerebellar ataxia type 3 (SCA3) is caused by the abnormal expansion of CAG repeats within the ataxin-3 gene. Previously, we generated transgenic mice (SCA3 mice) that express a truncated form of ataxin-3 containing abnormally expanded CAG repeats specifically in cerebellar Purkinje cells (PCs). Here, we further characterize these SCA3 mice. Whole-cell patch-clamp analysis of PCs from advanced-stage SCA3 mice revealed a significant decrease in membrane capacitance due to poor dendritic arborization and the complete absence of metabotropic glutamate receptor subtype1 (mGluR1)-mediated retrograde suppression of synaptic transmission at parallel fiber terminals, with an overall preservation of AMPA receptor-mediated fast synaptic transmission. Because these cerebellar phenotypes are reminiscent of retinoic acid receptor-related orphan receptor α (RORα)-defective staggerer mice, we examined the levels of RORα in the SCA3 mouse cerebellum by immunohistochemistry and found a marked reduction of RORα in the nuclei of SCA3 mouse PCs. To confirm that the defects in SCA3 mice were caused by postnatal deposition of mutant ataxin-3 in PCs, not by genome disruption via transgene insertion, we tried to reduce the accumulation of mutant ataxin-3 in developing PCs by viral vector-mediated expression of CRAG, a molecule that facilitates the degradation of stress proteins. Concomitant with the removal of mutant ataxin-3, CRAG-expressing PCs had greater numbers of differentiated dendrites compared to non-transduced PCs and exhibited retrograde suppression of synaptic transmission following mGluR1 activation. These results suggest that postnatal nuclear accumulation of mutant ataxin-3 disrupts dendritic differentiation and mGluR-signaling in SCA3 mouse PCs, and this disruption may be caused by a defect in a RORα-driven transcription pathway.

Early Morphological Changes of Hereditary Cerebellar Cortical Abiotrophy in Rabbits

We previously investigated the hereditary cerebellar cortical abiotrophy in littermates at postnatal day (PD) 25-31 delivered from a pair of rabbits. To estimate the onset time and incipient lesions associated with the cerebellar cortical abiotrophy of the cases, we mated the same pair again and examined early stages of the disease in F1 rabbit showing ataxia (PD 15), finding evidence that the ataxia is passed to subsequent generations via autosomal recessive inheritance. Clinical signs of the affected rabbit showed early-onset dysstasia and ataxia. The affected rabbit showed apoptotic granular cells before and after migration completion, degeneration (swelling) of parallel fiber terminals, abnormal junction (invaginated junction) of the parallel fiber-Purkinje cell synapses and irregular orientation of the Purkinje dendritic arbor in the molecular layer. Additionally, a reduced number of synaptic junctions between parallel fibers and Purkinje cells were detected, as well as at PD 25-31. Secondary changes, such as reduction or degeneration of Purkinje cells and granular cells were not yet observed at early stages. As synapse abnormality preceded the degeneration or reduction of Purkinje and granular cells at early stages, we concluded that the pathogenesis of the present cerebellar lesion was caused by failed synaptogenesis during postnatal cerebellar development.

Synapse type-independent degradation of the endocannabinoid 2-arachidonoylglycerol after retrograde synaptic suppression

The endocannabinoid 2-arachidonoylglycerol (2-AG) mediates retrograde synaptic suppression. Although the mechanisms of 2-AG production are well characterized, how 2-AG is degraded is less clearly understood. Here we found that expression of the 2-AG hydrolyzing enzyme monoacylglycerol lipase (MGL) was highly heterogeneous in the cerebellum, being rich within parallel fiber (PF) terminals, weak in Bergman glia (BG), and absent in other synaptic terminals. Despite this highly selective MGL expression pattern, 2-AG-mediated retrograde suppression was significantly prolonged at not only PF-Purkinje cell (PC) synapses but also climbing fiber-PC synapses in granule cell-specific MGL knockout (MGL-KO) mice whose cerebellar MGL expression was confined to the BG. Virus-mediated expression of MGL into the BG of global MGL-KO mice significantly shortened 2-AG-mediated retrograde suppression at PF-PC synapses. Furthermore, contribution of MGL to termination of 2-AG signaling depended on the distance from MGL-rich PFs to inhibitory synaptic terminals. Thus, 2-AG is degraded in a synapse-type independent manner by MGL present in PFs and the BG. The results of the present study strongly suggest that MGL regulates 2-AG signaling rather broadly within a certain range of neural tissue, although MGL expression is heterogeneous and limited to a subset of nerve terminals and astrocytes.

A new signalling pathway for parallel fibre presynaptic type 4 metabotropic glutamate receptors (mGluR4) in the rat cerebellar cortex

In the rodent cerebellum, pharmacological activation of mGluR4 acutely depresses excitatory synaptic transmission at parallel fibre–Purkinje cell synapses. This depression involves the inhibition of presynaptic calcium (Ca2+) influx that ultimately controls glutamate release. In this study, we investigate the molecular basis of mGluR4-mediated inhibition of presynaptic Ca2+ transients. Our results demonstrate that the mGluR4 effect does not depend on selective inhibition of a specific type of presynaptic voltage-gated Ca2+ channel, but rather involves modulation of all classes of Ca2+ channels present in the presynaptic terminals. In addition, this inhibitory effect does not involve the activation of G protein-activated inwardly rectifying potassium channels, TEA-sensitive potassium channels or two-pore-domain potassium channels. Furthermore, this inhibition does not require pertussis toxin-sensitive G proteins, and is independent of any effect on adenylyl cyclases, protein kinase A, mitogen-activated protein kinases or phosphoinositol-3 kinase activity. Interestingly we found that mGluR4 inhibition of presynaptic Ca2+ influx employs a newly defined signalling pathway, notably that involving the activation of phospholipase C and ultimately protein kinase C.

Pre and post synaptic NMDA effects targeting Purkinje cells in the mouse cerebellar cortex.

N-methyl-D-aspartate (NMDA) receptors are associated with many forms of synaptic plasticity. Their expression level and subunit composition undergo developmental changes in several brain regions. In the mouse cerebellum, beside a developmental switch between NR2B and NR2A/C subunits in granule cells, functional postsynaptic NMDA receptors are seen in Purkinje cells of neonate and adult but not juvenile rat and mice. A presynaptic effect of NMDA on GABA release by cerebellar interneurons was identified recently. Nevertheless whereas NMDA receptor subunits are detected on parallel fiber terminals, a presynaptic effect of NMDA on spontaneous release of glutamate has not been demonstrated. Using mouse cerebellar cultures and patch-clamp recordings we show that NMDA facilitates glutamate release onto Purkinje cells in young cultures via a presynaptic mechanism, whereas NMDA activates extrasynaptic receptors in Purkinje cells recorded in old cultures. The presynaptic effect of NMDA on glutamate release is also observed in Purkinje cells recorded in acute slices prepared from juvenile but not from adult mice and requires a specific protocol of NMDA application.

Mechanisms underlying input-specific expression of endocannabinoid-mediated synaptic plasticity in the dorsal cochlear nucleus

A hallmark of brain organization is the integration of primary and modulatory pathways by principal neurons. Primary sensory inputs are usually not plastic, while modulatory inputs converging to the same principal neuron can be plastic. However, the mechanisms determining this input-specific expression of synaptic plasticity remain unknown. We investigated this problem in the dorsal cochlear nucleus (DCN), where principal cells integrate primary auditory nerve input with plastic, parallel fiber input. Our previous DCN studies have shown that parallel fiber inputs exhibit short- and long-term plasticities mediated by endocannabinoid signaling. Here we show that auditory nerve inputs to principal cells do not show short- or long-term endocannabinoid-mediated synaptic plasticity. Electrophysiological and electron microscopy studies indicate that input specificity arises from selective expression of presynaptic cannabinoid (CB1) receptors in parallel fiber terminals, but not in auditory nerve terminals. However, pairing of parallel fiber activity with auditory nerve activity elicits plasticity in parallel fiber inputs, thus suggesting a role for synaptic plasticity in multisensory integration.

Cerebellar Long-term Depression is Deficient in Niemann–Pick Type C Disease Mice

Niemann-Pick type C disease (NPC) is an autosomal recessive lipidosis characterized by progressive neurodegeneration. Although several studies have revealed unusual accumulation of unesterfied cholesterol in astrocytic lysosome of NPC, pathophysiological basis of cerebellar neuronal dysfunction remains unclear. We compared parallel fiber-Purkinje cell synaptic transmission and long-term depression (LTD) in +/+npc (nih) (npc(+/+)) and -/-npc(nih) (npc(-/-)) mice. Our data showed that adenosine A1 receptor agonists decreased parallel fiber excitatory postsynaptic current (EPSC) amplitude and mEPSC frequency while its antagonists increased EPSC amplitude and mEPSC frequency in wild type and mutant mice. Furthermore, parallel fiber LTD was deficient in npc(-/-) mice and supplement of adenosine triphosphate (ATP) rescued the impaired LTD. Taken together, these experiments suggest that synaptic strength and LTD are altered in npc(-/-) mice due to the decrease of ATP/adenosine release and deactivation of A1 receptors in parallel fiber terminals. The enhanced synaptic transmission and the decreased LTD might result in progressive neurotoxicity of Purkinje cells in npc(-/-) mice.