Quantitative study of the Purkinje cell dendritic spines in the rat cerebellum

Meshed neuronal mitochondrial networks empowered by AI-powered classifiers and immersive VR reconstruction

Mitochondrial networks are defined as a continuous matrix lumen, but the morphological feature of neuronal mitochondrial networks is not clear due to the lack of suitable analysis techniques. The aim of the present study is to develop a framework to capture and analyze the neuronal mitochondrial networks by using 4-step process composed of 2D and 3D observation, primary and secondary virtual reality (VR) analysis, with the help of artificial intelligence (AI)-powered Aivia segmentation an classifiers. In order to fulfill this purpose, we first generated the PCs-Mito-GFP mice, in which green fluorescence protein (GFP) could be expressed on the outer mitochondrial membrane specifically on the cerebellar Purkinje cells (PCs), thus all mitochondria in the giant neuronal soma, complex dendritic arborization trees and long projection axons of Purkinje cells could be easily detected under a laser scanning confocal microscope. The 4-step process resolved the complicated neuronal mitochondrial networks into discrete neuronal mitochondrial meshes. Second, we measured the two parameters of the neuronal mitochondrial meshes, and the results showed that the surface area (μm²) of mitochondrial meshes was the biggest in dendritic trees (45.30 ± 53.21), the smallest in granular-like axons (3.99 ± 1.82), and moderate in soma (27.81 ± 22.22) and silk-like axons (17.50 ± 15.19). These values showed statistically different among different subcellular locations. The volume (μm³) of mitochondrial meshes was the biggest in dendritic trees (9.97 ± 12.34), the smallest in granular-like axons (0.43 ± 0.25), and moderate in soma (6.26 ± 6.46) and silk-like axons (3.52 ± 4.29). These values showed significantly different among different subcellular locations. Finally, we found both the surface area and the volume of mitochondrial meshes in dendritic trees and soma within the Purkinje cells in PCs-Mito-GFP mice after receiving the training with the simulating long-term pilot flight concentrating increased significantly. The precise reconstruction of neuronal mitochondrial networks is extremely laborious, the present 4-step workflow powered by artificial intelligence and virtual reality reconstruction could successfully address these challenges.

Ultrastructural effects of sleep and wake on the parallel fiber synapses of the cerebellum

Multiple evidence in rodents shows that the strength of excitatory synapses in the cerebral cortex and hippocampus is greater after wake than after sleep. The widespread synaptic weakening afforded by sleep is believed to keep the cost of synaptic activity under control, promote memory consolidation, and prevent synaptic saturation, thus preserving the brain's ability to learn day after day. The cerebellum is highly plastic and the Purkinje cells, the sole output neurons of the cerebellar cortex, are endowed with a staggering number of excitatory parallel fiber synapses. However, whether these synapses are affected by sleep and wake is unknown. Here, we used serial block face scanning electron microscopy to obtain the full 3D reconstruction of more than 7000 spines and their parallel fiber synapses in the mouse posterior vermis. This analysis was done in mice whose cortical and hippocampal synapses were previously measured, revealing that average synaptic size was lower after sleep compared to wake with no major changes in synapse number. Here, instead, we find that while the average size of parallel fiber synapses does not change, the number of branched synapses is reduced in half after sleep compared to after wake, corresponding to ~16% of all spines after wake and ~8% after sleep. Branched synapses are harbored by two or more spines sharing the same neck and, as also shown here, are almost always contacted by different parallel fibers. These findings suggest that during wake, coincidences of firing over parallel fibers may translate into the formation of synapses converging on the same branched spine, which may be especially effective in driving Purkinje cells to fire. By contrast, sleep may promote the off-line pruning of branched synapses that were formed due to spurious coincidences.

A stable proportion of Purkinje cell inputs from parallel fibers are silent during cerebellar maturation

Cerebellar Purkinje neurons integrate information transmitted at excitatory synapses formed by granule cells. Although these synapses are considered essential sites for learning, most of them appear not to transmit any detectable electrical information and have been defined as silent. It has been proposed that silent synapses are required to maximize information storage capacity and ensure its reliability, and hence to optimize cerebellar operation. Such optimization is expected to occur once the cerebellar circuitry is in place, during its maturation and the natural and steady improvement of animal agility. We therefore investigated whether the proportion of silent synapses varies over this period, from the third to the sixth postnatal week in mice. Selective expression of a calcium indicator in granule cells enabled quantitative mapping of presynaptic activity, while postsynaptic responses were recorded by patch clamp in acute slices. Through this approach and the assessment of two anatomical features (the distance that separates adjacent planar Purkinje dendritic trees and the synapse density), we determined the average excitatory postsynaptic potential per synapse. Its value was four to eight times smaller than responses from paired recorded detectable connections, consistent with over 70% of synapses being silent. These figures remained remarkably stable across maturation stages. According to the proposed role for silent synapses, our results suggest that information storage capacity and reliability are optimized early during cerebellar maturation. Alternatively, silent synapses may have roles other than adjusting the information storage capacity and reliability.

Towards a Bio-Inspired Real-Time Neuromorphic Cerebellum

This work presents the first simulation of a large-scale, bio-physically constrained cerebellum model performed on neuromorphic hardware. A model containing 97,000 neurons and 4.2 million synapses is simulated on the SpiNNaker neuromorphic system. Results are validated against a baseline simulation of the same model executed with NEST, a popular spiking neural network simulator using generic computational resources and double precision floating point arithmetic. Individual cell and network-level spiking activity is validated in terms of average spike rates, relative lead or lag of spike times, and membrane potential dynamics of individual neurons, and SpiNNaker is shown to produce results in agreement with NEST. Once validated, the model is used to investigate how to accelerate the simulation speed of the network on the SpiNNaker system, with the future goal of creating a real-time neuromorphic cerebellum. Through detailed communication profiling, peak network activity is identified as one of the main challenges for simulation speed-up. Propagation of spiking activity through the network is measured, and will inform the future development of accelerated execution strategies for cerebellum models on neuromorphic hardware. The large ratio of granule cells to other cell types in the model results in high levels of activity converging onto few cells, with those cells having relatively larger time costs associated with the processing of communication. Organizing cells on SpiNNaker in accordance with their spatial position is shown to reduce the peak communication load by 41%. It is hoped that these insights, together with alternative parallelization strategies, will pave the way for real-time execution of large-scale, bio-physically constrained cerebellum models on SpiNNaker. This in turn will enable exploration of cerebellum-inspired controllers for neurorobotic applications, and execution of extended duration simulations over timescales that would currently be prohibitive using conventional computational platforms.

Differential synapse density between Purkinje cell dendritic spine and parallel fiber varicosity in the rat cerebellum among the phylogenic lobules

The cerebellum is a region of the brain that plays an important role in motor control. It is classified phylogenetically into archicerebellum, paleocerebellum and neocerebellum. The Purkinje cells are lined in a row called Purkinje cell layer and it has a unique dendritic branches with many spines.

The previous study reported that there is a difference of synapse density according to the lobules based on large two-dimensional data. However, recent study with high voltage electron microscopy showed there was no differences in dendritic spine density of the Purkinje cell according to its phylogenetic lobule. We analyzed Purkinje cell density in the II, VI and X lobules by stereological modules and synaptic density was estimated by double disector based on Purkinje cell density in the molecular layer of each lobule.

The results showed that there was significant difference in the Purkinje cell density and synapse number according to their phylogenetic lobules. The number of Purkinje cell in a given volume was larger in the archicerebellum, but synapse density was higher in the neocerebellum.

These data suggest that cellular and synaptic organization of the Purkinje cell is different according to their phylogenetic background.

Long-Term Depression of Intrinsic Excitability Accompanied by Synaptic Depression in Cerebellar Purkinje Cells

Long-term depression (LTD) at the parallel fiber (PF)-to-cerebellar Purkinje cell (PC) synapse is implicated in the output of PCs, the sole output of the cerebellar cortex. In addition to synaptic plasticity, intrinsic excitability is also one of the components that determines PC output. Although long-term potentiation of intrinsic excitability (LTP-IE) has been suggested, it has yet to be investigated how PF-PC LTD modifies intrinsic excitability of PCs. Here, we show that pairing of the PF and climbing fiber (CF) for PF-PC LTD induction evokes LTD-IE in cerebellar PCs from male C57BL/6 mice. Interestingly, this intrinsic plasticity showed different kinetics from synaptic plasticity, but both forms of plasticity share Ca²⁺ signaling and protein kinase C pathway as their underlying mechanism. Although small-conductance Ca²⁺-activated K⁺ channels play important roles in LTP-IE, no direct implication has been found. After PF-PC LTD induction, neither the temporal summation of dendritic EPSP nor the power of spike frequency adaptation is changed, indicating that cerebellar LTD executes the information processing in a quantitative way without quality changes of synaptic integration and generation of output signals. Our results suggest that LTD-IE may have a synergistic effect with synaptic depression on the total net output of neurons by amplifying the modification of PF synaptic transmission.SIGNIFICANCE STATEMENT Although the output of Purkinje cells (PCs) is a critical component of cerebellum-dependent learning and memory, the changes of PC excitability when synaptic LTD occurs are unclear. Here, we show that the induction of PF-PC LTD evokes LTD-IE in PCs. Our observation complements previous intrinsic plasticity phenomenon of long-term potentiation of intrinsic excitability (LTP-IE), providing evidence for the idea that intrinsic plasticity has bidirectionality as synaptic plasticity. LTD-IE occurs together with synaptic LTD and both phenomena are dependent on the Ca²⁺ signaling pathway. Furthermore, our findings raise the prospect that this synaptic and intrinsic plasticity acts synergistically in PCs to modify neuronal activity in the same direction when learning occurs.

Mitochondrial fission protein Drp1 regulates mitochondrial transport and dendritic arborization in cerebellar Purkinje cells

Mitochondria dynamically change their shape by repeated fission and fusion in response to physiological and pathological conditions. Recent studies have uncovered significant roles of mitochondrial fission and fusion in neuronal functions, such as neurotransmission and spine formation. However, the contribution of mitochondrial fission to the development of dendrites remains controversial. We analyzed the function of the mitochondrial fission GTPase Drp1 in dendritic arborization in cerebellar Purkinje cells. Overexpression of a dominant-negative mutant of Drp1 in postmitotic Purkinje cells enlarged and clustered mitochondria, which failed to exit from the soma into the dendrites. The emerging dendrites lacking mitochondrial transport remained short and unstable in culture and in vivo. The dominant-negative Drp1 affected neither the basal respiratory function of mitochondria nor the survival of Purkinje cells. Enhanced ATP supply by creatine treatment, but not reduced ROS production by antioxidant treatment, restored the hypomorphic dendrites caused by inhibition of Drp1 function. Collectively, our results suggest that Drp1 is required for dendritic distribution of mitochondria and thereby regulates energy supply in growing dendritic branches in developing Purkinje cells.

General principals of miRNA biogenesis and regulation in the brain

MicroRNAs (miRNAs) are small, noncoding RNAs that mediate posttranscriptional gene suppression in a sequence-specific manner. The ability of a single miRNA species to target multiple messenger RNAs (mRNAs) makes miRNAs exceptionally important regulators of various cellular functions. The regulatory capacity of miRNAs is increased further by the miRNA ability to suppress gene expression using multiple mechanisms that range from translational inhibition to mRNA degradation. The high miRNA diversity multiplied by the large number of individual miRNA targets generates a vast regulatory RNA network than enables flexible control of mRNA expression. The gene-regulatory capacity and diversity of miRNAs is particularly valuable in the brain, where functional specialization of neurons and persistent flow of information requires constant neuronal adaptation to environmental cues. In this review we will summarize the current knowledge about miRNA biogenesis and miRNA expression regulation with a focus on the role of miRNAs in the mammalian nervous system.

Virtual NEURON: a strategy for merged biochemical and electrophysiological modeling

Because of its highly branched dendrite, the Purkinje neuron requires significant computational resources if coupled electrical and biochemical activity are to be simulated. To address this challenge, we developed a scheme for reducing the geometric complexity; while preserving the essential features of activity in both the soma and a remote dendritic spine. We merged our previously published biochemical model of calcium dynamics and lipid signaling in the Purkinje neuron, developed in the Virtual Cell modeling and simulation environment, with an electrophysiological model based on a Purkinje neuron model available in NEURON. A novel reduction method was applied to the Purkinje neuron geometry to obtain a model with fewer compartments that is tractable in Virtual Cell. Most of the dendritic tree was subject to reduction, but we retained the neuron's explicit electrical and geometric features along a specified path from spine to soma. Further, unlike previous simplification methods, the dendrites that branch off along the preserved explicit path are retained as reduced branches. We conserved axial resistivity and adjusted passive properties and active channel conductances for the reduction in surface area, and cytosolic calcium for the reduction in volume. Rallpacks are used to validate the reduction algorithm and show that it can be generalized to other complex neuronal geometries. For the Purkinje cell, we found that current injections at the soma were able to produce similar trains of action potentials and membrane potential propagation in the full and reduced models in NEURON; the reduced model produces identical spiking patterns in NEURON and Virtual Cell. Importantly, our reduced model can simulate communication between the soma and a distal spine; an alpha function applied at the spine to represent synaptic stimulation gave similar results in the full and reduced models for potential changes associated with both the spine and the soma. Finally, we combined phosphoinositol signaling and electrophysiology in the reduced model in Virtual Cell. Thus, a strategy has been developed to combine electrophysiology and biochemistry as a step toward merging neuronal and systems biology modeling.

3D electron microscopic reconstruction of segments of rat cerebellar Purkinje cell dendrites receiving ascending and parallel fiber granule cell synaptic inputs

Growing physiological evidence suggests that there are functional differences between synapses made by the ascending and parallel fiber segments of the granule axon on cerebellar Purkinje cells. Supporting this view, our previous electron microscopic studies suggested that these synapses also contacted different regions of the Purkinje cell dendrite, and in particular that ascending segment synapses are made exclusively on the smallest diameter Purkinje cell dendrites. In the current study we used serial electron microscopic techniques to reconstruct Purkinje cell dendritic segments up to almost 10 mum in length. Using a combination of anatomical and immunological labeling techniques we identified the ascending or parallel fiber origins of the excitatory synaptic inputs onto dendritic spines, as well as the location of inhibitory synapses made directly on the dendritic shaft. The results confirmed that there are regions of the Purkinje cell dendrite receiving exclusively ascending or parallel fiber synapses and that ascending segment synapses are only found on small-diameter dendrites. In addition, we describe for the first time small-diameter dendritic regions contacted by both types of excitatory synapses. While our data suggest that the majority of inhibitory inputs to the Purkinje cell tree are associated with parallel fiber synaptic inputs, we also found inhibitory inputs on dendritic regions with mixed ascending and parallel fiber inputs, or exclusively parallel fiber inputs. The finding that ascending and parallel fiber inputs can be segregated on the Purkinje cell dendritic tree provides further evidence that these excitatory granule cell synaptic inputs may be functionally distinct.

Implications on cerebellar function from information coding

One function of the cerebellar cortex is to process information. There are at least two types of information. Temporal information is encoded in the timing pattern of action and synaptic potentials, whereas structural information is encoded in the spatial pattern of the cerebellar synaptic circuitry. Intuitively, analysis of highly complex information in the time domain would require a cerebellar cortex with structural complexity to match. Information theory offers a way to estimate quantitatively both types of information and thereby helps to test hypotheses or advance theories of cerebellar neurobiology. These estimates suggest: (i) That the mossy-fiber-granule-cell system carries far more (temporal) information than the climbing fiber system, (ii) that Purkinje cells extract only a fraction of the (temporal) information from their afferents, and (iii) that the cerebellar cortex has a large (spatial) information coding capacity. Concerning information, one can argue that the cerebellar cortex analyzes temporal information in its afferents as a search engine, in search of coincidental mossy fiber events based on timing cues provided by climbing fiber events. Results of successive searches are continuously being converted into structural information encoded in the spatial distribution pattern of granule-cell-Purkinje-cell synapses along granule cell axons, thereby providing an adaptive and indeed self-correcting dimension to the structural information code. The search engine operation involves cellular mechanisms acting on temporal events and is part of an associative learning process. The conversion and generation of structural information involves neuroplasticity mechanisms acting at the synaptic level, with electrophysiological as well as structural consequences, and may be part of the short- and long-term memory process. These and other attributes qualify the cerebellar cortex as a dynamic information processing center, contributing to memory and learning while linking motor output with sensory events.

A cost-benefit analysis of neuronal morphology

Over hundreds of millions of years, evolution has optimized brain design to maximize its functionality while minimizing costs associated with building and maintenance. This observation suggests that one can use optimization theory to rationalize various features of brain design. Here, we attempt to explain the dimensions and branching structure of dendritic arbors by minimizing dendritic cost for given potential synaptic connectivity. Assuming only that dendritic cost increases with total dendritic length and path length from synapses to soma, we find that branching, planar, and compact dendritic arbors, such as those belonging to Purkinje cells in the cerebellum, are optimal. The theory predicts that adjacent Purkinje dendritic arbors should spatially segregate. In addition, we propose two explicit cost function expressions, falsifiable by measuring dendritic caliber near bifurcations.

Nitric oxide regulates input specificity of long-term depression and context dependence of cerebellar learning

Recent studies have shown that multiple internal models are acquired in the cerebellum and that these can be switched under a given context of behavior. It has been proposed that long-term depression (LTD) of parallel fiber (PF)–Purkinje cell (PC) synapses forms the cellular basis of cerebellar learning, and that the presynaptically synthesized messenger nitric oxide (NO) is a crucial “gatekeeper” for LTD. Because NO diffuses freely to neighboring synapses, this volume learning is not input-specific and brings into question the biological significance of LTD as the basic mechanism for efficient supervised learning. To better characterize the role of NO in cerebellar learning, we simulated the sequence of electrophysiological and biochemical events in PF–PC LTD by combining established simulation models of the electrophysiology, calcium dynamics, and signaling pathways of the PC. The results demonstrate that the local NO concentration is critical for induction of LTD and for its input specificity. Pre- and postsynaptic coincident firing is not sufficient for a PF–PC synapse to undergo LTD, and LTD is induced only when a sufficient amount of NO is provided by activation of the surrounding PFs. On the other hand, above-adequate levels of activity in nearby PFs cause accumulation of NO, which also allows LTD in neighboring synapses that were not directly stimulated, ruining input specificity. These findings lead us to propose the hypothesis that NO represents the relevance of a given context and enables context-dependent selection of internal models to be updated. We also predict sparse PF activity in vivo because, otherwise, input specificity would be lost.

The cerebellum is essential for coordinated movements. The skills for executing such movements are acquired in modules of the cerebellum, and the appropriate modules in which to store the skill for a certain movement are selected according to the environment, or the context, where the movement is made. We are interested in the molecular mechanisms that enable context-dependent cerebellar learning. In search of the key molecules, we combined established simulation models of Purkinje cells, the only output neurons in the cerebellar cortex, and constructed a new model. Using computer simulation, we found that nitric oxide is likely to have a pivotal role in context-dependent learning. Our simulation also provides insights into how sparse sensory information is coded in the cerebellar cortex. These findings have led us to propose the experimentally testable hypothesis that the relevance of a given context to learning modules is represented by the concentration of nitric oxide.

Cerebellar circuitry as a neuronal machine

Shortly after John Eccles completed his studies of synaptic inhibition in the spinal cord, for which he was awarded the 1963 Nobel Prize in physiology/medicine, he opened another chapter of neuroscience with his work on the cerebellum. From 1963 to 1967, Eccles and his colleagues in Canberra successfully dissected the complex neuronal circuitry in the cerebellar cortex. In the 1967 monograph, "The Cerebellum as a Neuronal Machine", he, in collaboration with Masao Ito and Janos Szentágothai, presented blue-print-like wiring diagrams of the cerebellar neuronal circuitry. These stimulated worldwide discussions and experimentation on the potential operational mechanisms of the circuitry and spurred theoreticians to develop relevant network models of the machinelike function of the cerebellum. In following decades, the neuronal machine concept of the cerebellum was strengthened by additional knowledge of the modular organization of its structure and memory mechanism, the latter in the form of synaptic plasticity, in particular, long-term depression. Moreover, several types of motor control were established as model systems representing learning mechanisms of the cerebellum. More recently, both the quantitative preciseness of cerebellar analyses and overall knowledge about the cerebellum have advanced considerably at the cellular and molecular levels of analysis. Cerebellar circuitry now includes Lugaro cells and unipolar brush cells as additional unique elements. Other new revelations include the operation of the complex glomerulus structure, intricate signal transduction for synaptic plasticity, silent synapses, irregularity of spike discharges, temporal fidelity of synaptic activation, rhythm generators, a Golgi cell clock circuit, and sensory or motor representation by mossy fibers and climbing fibers. Furthermore, it has become evident that the cerebellum has cognitive functions, and probably also emotion, as well as better-known motor and autonomic functions. Further cerebellar research is required for full understanding of the cerebellum as a broad learning machine for neural control of these functions.

Kinetic, pharmacological and activity-dependent separation of two Ca2+ signalling pathways mediated by type 1 metabotropic glutamate receptors in rat Purkinje neurones

Type 1 metabotropic glutamate receptors (mGluR1) in Purkinje neurones (PNs) are important for motor learning and coordination. Here, two divergent mGluR1 Ca2+-signalling pathways and the associated membrane conductances were distinguished kinetically and pharmacologically after activation by 1-ms photorelease of L-glutamate or by bursts of parallel fibre (PF) stimulation. A new, mGluR1-mediated transient K+ conductance was seen prior to the slow EPSC (sEPSC). It was seen only in PNs previously allowed to fire spontaneously or held at depolarized potentials for several seconds and was slowly inhibited by agatoxin IVA, which blocks P/Q-type Ca2+ channels. It peaked in 148 ms, had well-defined kinetics and, unlike the sEPSC, was abolished by the phospholipase C (PLC) inhibitor U73122. It was blocked by the BK Ca2+-activated K+ channel blocker iberiotoxin and unaffected by apamin, indicating selective activation of BK channels by PLC-dependent store-released Ca2+. The K+ conductance and underlying transient Ca2+ release showed a highly reproducible delay of 99.5 ms following PF burst stimulation, with a precision of 1-2 ms in repeated responses of the same PN, and a subsequent fast rise and fall of Ca2+ concentration. Analysis of Ca2+ signals showed that activation of the K+ conductance by Ca2+ release occurred in small dendrites and subresolution structures, most probably spines. The results show that PF burst stimulation activates two pathways of mGluR1 signalling in PNs. First, transient, PLC-dependent Ca2+ release from stores with precisely reproducible timing and second, slower Ca2+ influx in the cation-permeable sEPSC channel. The priming by prior Ca2+ influx in P/Q-type Ca2+ channels may determine the path of mGluR1 signalling. The precise timing of PLC-mediated store release may be important for interactions of PF mGluR1 signalling with other inputs to the PN.

Organization of spines on the dendrites of Purkinje cells

Dendritic spines have been investigated intensively over recent years; however, little is yet known about how they organize on the cell surface to make synaptic contacts with appropriate axons. Here we investigate spine distributions along the distal dendrites of cerebellar Purkinje cells, after biolistic labeling of intact tissue with a lipid-soluble dye. We show that the spines have a preference to form regular linear arrays and to trace short-pitch helical paths. The helical ordering is not determined by external factors that may influence how individual spines develop, because the same periodicities were present in fish and mammalian Purkinje cells, including those of weaver mice, which are depleted of the normal presynaptic partners for the spines. The ordering, therefore, is most likely an inherent property of the dendrite. Image reconstruction of dendrites from the different tissues showed that the helical spine distributions invariably lead to approximately equal sampling of surrounding space by the spineheads. The purpose of this organization may therefore be to maximize the opportunity of different spines to interact with different axons.

Quantitative analysis of granule cell axons and climbing fiber afferents in the turtle cerebellar cortex

The turtle cerebellar cortex is a single flat sheet of gray matter that greatly facilitates quantitative analysis of biotylinated dextran amine labeled granule cell and olivocerebellar axons and Nissl-stained granule and Purkinje neurons. On average, ascending granule cell axons are relatively thicker than their parallel fiber branches (mean +/- SD: 0.84 +/- 0.17 vs 0.64 +/- 0.12 microm, respectively). Numerous en passant swellings, the site of presynaptic contact, were present on both ascending and parallel fiber granule cell axons. The swellings on ascending axons (1.82 +/- 0.34 microm, n = 52) were slightly larger than on parallel fibers (1.43 +/- 0.24 microm, n = 430). In addition, per unit length (100 microm) there were more swellings on ascending axons (11.2 +/- 4.2) than on parallel fibers (9.7 +/- 4.2). Each parallel fiber branch from an ascending axon is approximately 1.5 mm long. Olivocerebellar climbing fiber axons followed the highly tortuous dendrites of Purkinje cells in the inner most 15-20% of the molecular layer. Climbing fibers displayed relatively fewer en passant swellings. The spatial perimeter of climbing fiber arbors (area) increased 72% from anteriorly (1797 microm2) to posteriorly (3090 microm2) and 104% from medially (1690 microm2) to laterally (3450 microm2). Differences in the size and spacing of en passant swellings on granule cell axons suggest that ascending axons may have a functionally more significant impact on the excitability of a limited number of radially overlying Purkinje cells than the single contacts by parallel fiber with multiple orthogonally aligned Purkinje cell dendrites. The spatially restricted distribution of climbing fibers to the inner most molecular layer, the paucity of en passant swellings, and different terminal arbor areas are enigmatic. Nevertheless, these finding provide important anatomical information for future optical imaging and electrophysiological experiments.

Spontaneous electrical activity and dendritic spine size in mature cerebellar Purkinje cells

Previous experiments have shown that in the mature cerebellum both blocking of spontaneous electrical activity and destruction of the climbing fibres by a lesion of the inferior olive have a similar profound effect on the spine distribution on the proximal dendrites of the Purkinje cells. Many new spines develop that are largely innervated by parallel fibers. Here we show that blocking electrical activity leads to a significant decrease in size of the spines on the branchlets. We have also compared the size of the spines of the proximal dendritic domain that appear during activity block and after an inferior olive lesion. In this region also, the spines in the absence of activity are significantly smaller. In the proximal dendritic domain, the new spines that develop in the absence of activity are innervated by parallel fibers and are not significantly different in size from those of the branchlets, although they are shorter. Thus, the spontaneous activity of the cerebellar cortex is necessary not only to maintain the physiological spine distribution profile in the Purkinje cell dendritic tree, but also acts as a signal that prevents spines from shrinking.

Optimal information storage and the distribution of synaptic weights: perceptron versus Purkinje cell

It is widely believed that synaptic modifications underlie learning and memory. However, few studies have examined what can be deduced about the learning process from the distribution of synaptic weights. We analyze the perceptron, a prototypical feedforward neural network, and obtain the optimal synaptic weight distribution for a perceptron with excitatory synapses. It contains more than 50% silent synapses, and this fraction increases with storage reliability: silent synapses are therefore a necessary byproduct of optimizing learning and reliability. Exploiting the classical analogy between the perceptron and the cerebellar Purkinje cell, we fitted the optimal weight distribution to that measured for granule cell-Purkinje cell synapses. The two distributions agreed well, suggesting that the Purkinje cell can learn up to 5 kilobytes of information, in the form of 40,000 input-output associations.

Alcohol-Induced Neurodegeneration: When, Where and Why?

This manuscript reviews the proceedings of a symposium organized by Drs. Antonio Noronha and Fulton Crews presented at the 2003 Research Society on Alcoholism meeting. The purpose of the symposium was to examine recent findings on when alcohol induced brain damage occurs, e.g., during intoxication and/or during alcohol withdrawal. Further studies investigate specific brain regions (where) and the mechanisms (why) of alcoholic neurodegeneration. The presentations were (1) Characterization of Synaptic Loss in Cerebella of Mature and Senescent Rats after Lengthy Chronic Ethanol Consumption, (2) Ethanol Withdrawal Both Causes Neurotoxicity and Inhibits Neuronal Recovery Processes in Rat Organotypic Hippocampal Cultures, (3) Binge Drinking-Induced Brain Damage: Genetic and Age Related Effects, (4) Binge Ethanol-Induced Brain Damage: Involvement of Edema, Arachidonic Acid and Tissue Necrosis Factor alpha (TNFalpha), and (5) Cyclic AMP Cascade, Stem Cells and Ethanol. Taken together these studies suggest that alcoholic neurodegeneration occurs through multiple mechanisms and in multiple brain regions both during intoxication and withdrawal.

Role of glial amino acid transporters in synaptic transmission and brain energetics

This article reviews how the uptake of neurotransmitter by glial amino acid transporters limits the spatial spread of transmitter to preserve the independent operation of nearby synapses, temporally shapes postsynaptic currents, and regulates the effects of tonic transmitter release. We demonstrate the importance of amino acid uptake and recycling mechanisms for preventing the loss of energetically costly neurotransmitter from the brain, and also examine the suggestion that glutamate uptake into glia plays a key role in regulating the energy production of the brain. Finally, we assess the role of glial amino acid transporters in transmitter recycling pathways.

The role of glial glutamate transporters in maintaining the independent operation of juvenile mouse cerebellar parallel fibre synapses

There is controversy over the extent to which glutamate released at one synapse can escape from the synaptic cleft and affect receptors at other synapses nearby, thereby compromising the synapse-specificity of information transmission. Here we show that the glial glutamate transporters GLAST and GLT-1 limit the activation of Purkinje cell AMPA receptors produced by glutamate diffusion between parallel fibre synapses in the cerebellar cortex of juvenile mice. For a single stimulus to the cerebellar molecular layer of wild-type mice, increasing the number of activated parallel fibres prolonged the parallel fibre EPSC, demonstrating an interaction between different synapses. Knocking out GLAST, or blocking GLT-1 in the absence of GLAST, prolonged the EPSC when many parallel fibres were stimulated but not when few were stimulated. When spatially separated parallel fibres were activated by granular layer stimulation, the EPSC prolongation produced by stimulating more fibres or reducing glutamate transport was greatly reduced. Thus, GLAST and GLT-1 curtail the EPSC produced by a single stimulus only when many nearby fibres are simultaneously activated. However when trains of stimuli were applied, even to a small number of parallel fibres, knocking out GLAST or blocking GLT-1 in the absence of GLAST greatly prolonged and enhanced the AMPA receptor-mediated current. These results show that glial cell glutamate transporters allow neighbouring synapses to operate more independently, and control the postsynaptic response to high frequency bursts of action potentials.

Properties of unitary granule cell-->Purkinje cell synapses in adult rat cerebellar slices

The cerebellar cortex contains huge numbers of synapses between granule cells and Purkinje cells. These synapses are thought to be a major storage site for information required to execute coordinated movements. To obtain a quantitative description of this connection, we recorded unitary synaptic responses between granule cell and Purkinje cell pairs in adult rat cerebellar slices. Our results are consistent with parallel fiber-->Purkinje cell synapses having high release probabilities and modest paired pulse facilitation. However, a wide range of response amplitudes was observed. Indeed, we detected many fewer parallel fiber connections (7% of the granule cells that were screened) than expected (54%), leading us to suggest that up to 85% of parallel fiber-->Purkinje cell synapses do not generate detectable electrical responses. We also investigated the possible role of granule cell ascending axons by recording granule cells near the Purkinje cell. A high proportion (up to 50%) of local granule cells generated detectable synaptic responses. However, most of these connections were indistinguishable from parallel fiber connections, suggesting that powerful ascending axon connections are rare. The existence of many very weak synapses would provide a mechanism for Purkinje cells to extract information selectively from the mass provided by parallel fibers.

Structure and function of dendritic spines

Spines are neuronal protrusions, each of which receives input typically from one excitatory synapse. They contain neurotransmitter receptors, organelles, and signaling systems essential for synaptic function and plasticity. Numerous brain disorders are associated with abnormal dendritic spines. Spine formation, plasticity, and maintenance depend on synaptic activity and can be modulated by sensory experience. Studies of compartmentalization have shown that spines serve primarily as biochemical, rather than electrical, compartments. In particular, recent work has highlighted that spines are highly specialized compartments for rapid large-amplitude Ca(2+) signals underlying the induction of synaptic plasticity.

Compartmental models of rat cerebellar Purkinje cells based on simultaneous somatic and dendritic patch-clamp recordings

1. Simultaneous dendritic and somatic patch-clamp recordings were made from Purkinje cells in cerebellar slices from 12- to 21-day-old rats. Voltage responses to current impulses injected via either the dendritic or the somatic pipette were obtained in the presence of the selective I(h) blocker ZD 7288 and blockers of spontaneous synaptic input. Neurons were filled with biocytin for subsequent morphological reconstruction. 2. Four neurons were reconstructed and converted into detailed compartmental models. The specific membrane capacitance (C(m)), specific membrane resistance (R(m)) and intracellular resistivity (R(i)) were optimized by direct fitting of the model responses to the electrophysiological data from the same cell. Mean values were: C(m), 0.77 +/- 0.17 microF cm(-2) (mean +/- S.D.; range, 0.64-1.00 microF cm(-2)), R(m), 122 +/- 18 kOmega cm(2) (98-141 kOmega cm(2)) and R(i), 115 +/- 20 Omega cm (93-142 Omega cm). 3. The steady-state electrotonic architecture of these cells was compact under the experimental conditions used. However, somatic voltage-clamp recordings of parallel fibre and climbing fibre synaptic currents were substantially filtered and attenuated. 4. The detailed models were compared with a two-compartment model of Purkinje cells. The range of synaptic current kinetics that can be faithfully recorded using somatic voltage clamp is predicted fairly well by the two-compartment model, even though some of its underlying assumptions are violated. 5. A model of I(h) was constructed based on voltage-clamp data, and inserted into the passive compartmental models. Somatic EPSP amplitude was substantially attenuated compared to the amplitude of dendritic EPSPs at their site of generation. However, synaptic efficacy of the same quantal synaptic conductance, as measured by the somatic EPSP amplitude, was only weakly dependent on synaptic location on spiny branchlets. 6. The passive electrotonic structure of Purkinje cells is unusual in that the steady-state architecture is very compact, while voltage transients such as synaptic potentials and action potentials are heavily filtered.

Cerebellar long-term depression: characterization, signal transduction, and functional roles

Cerebellar Purkinje cells exhibit a unique type of synaptic plasticity, namely, long-term depression (LTD). When two inputs to a Purkinje cell, one from a climbing fiber and the other from a set of granule cell axons, are repeatedly associated, the input efficacy of the granule cell axons in exciting the Purkinje cell is persistently depressed. Section I of this review briefly describes the history of research around LTD, and section II specifies physiological characteristics of LTD. Sections III and IV then review the massive data accumulated during the past two decades, which have revealed complex networks of signal transduction underlying LTD. Section III deals with a variety of first messengers, receptors, ion channels, transporters, G proteins, and phospholipases. Section IV covers second messengers, protein kinases, phosphatases and other elements, eventually leading to inactivation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-selective glutamate receptors that mediate granule cell-to-Purkinje cell transmission. Section V defines roles of LTD in the light of the microcomplex concept of the cerebellum as functionally eliminating those synaptic connections associated with errors during repeated exercises, while preserving other connections leading to the successful execution of movements. Section VI examines the validity of this microcomplex concept based on the data collected from recent numerous studies of various forms of motor learning in ocular reflexes, eye-blink conditioning, posture, locomotion, and hand/arm movements. Section VII emphasizes the importance of integrating studies on LTD and learning and raises future possibilities of extending cerebellar research to reveal memory mechanisms of implicit learning in general.

Quantification of spread of cerebellar long-term depression with chemical two-photon uncaging of glutamate

Localized, chemical two-photon photolysis of caged glutamate was used to map the changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-type glutamate receptors caused by long-term synaptic depression (LTD) in cerebellar Purkinje cells. LTD produced by pairing parallel fiber activity with depolarization was accompanied by a decline in the response of Purkinje cells to uncaged glutamate that accounted for both the time course and magnitude of LTD. This depression of glutamate responses was observed not only at the site of parallel fiber stimulation but also at more distant sites. The amount of LTD decreased with distance and was half-maximal 50 microm away from the site of parallel fiber activity. Estimation of the number of parallel fibers active during LTD induction indicates that LTD modified glutamate receptors not only at active synapses but also at 600 times as many inactive synapses on a single Purkinje cell. Therefore, both active and inactive parallel fiber synapses can undergo changes at a postsynaptic locus as a result of associative pre- and postsynaptic activity.

Ascending granule cell axon: an important component of cerebellar cortical circuitry

Physiologic evidence suggests that local activation of the cerebellar granule cell layer produces a much more restricted spatial activation of overlying Purkinje cells than would be expected from the parallel fiber system. These results have led to the suggestion that synapses associated with the ascending granule cell axon may provide a large, direct, excitatory input to Purkinje cells, whereas parallel fiber synapses may be more modulatory in nature. In the current experiments, serial electron microscopy was used to reconstruct synapses associated with these two segments of the granule cell axons in the cerebellar cortex of albino rats. The results indicate that there are significantly more presynaptic vesicles in ascending segment synapses than in parallel fiber synapses. Furthermore, a first-order linear regression analysis revealed positive correlations between all measures of pre- and postsynaptic morphology for parallel fibers, but not for ascending segment synapses. Perhaps most surprisingly, serial reconstructions of postsynaptic spines and their associated dendrites demonstrated that spines contacted by ascending segment synapses are located exclusively on the smallest diameter distal regions of the Purkinje cell dendrites, whereas parallel fiber synapses are found exclusively on intermediate- and large-diameter regions of the spiny branchlets. Based on two independent calculations, we estimate that 20% of the granule cell synapses onto a Purkinje cell are actually made by the ascending segment. By using computer simulations of a single Purkinje cell dendrite, we have also demonstrated that synchronous activation of these distal ascending segment inputs could produce a substantial somatic response. Taken together, these results suggest that the two different regions of granule cell axons may play very different physiologic roles in cerebellar cortex.

The number of glutamate transporter subtype molecules at glutamatergic synapses: chemical and stereological quantification in young adult rat brain

The role of transporters in shaping the glutamate concentration in the extracellular space after synaptic release is controversial because of their slow cycling and because diffusion alone gives a rapid removal. The transporter densities have been measured electrophysiologically, but these data are from immature brains and do not give precise information on the concentrations of the individual transporter subtypes. Here we show by quantitative immunoblotting that the numbers of the astroglial glutamate transporters GLAST (EAAT1) and GLT (EAAT2) are 3200 and 12,000 per micrometer3 tissue in the stratum radiatum of adult rat hippocampus (CA1) and 18,000 and 2800 in the cerebellar molecular layer, respectively. The total astroglial cell surface is 1.4 and 3.8 m2/cm3 in the two regions, respectively, implying average densities of GLAST and GLT molecules in the membranes around 2300 and 8500 micrometer-2 in the former and 4700 and 740 micrometer-2 in the latter region. The total concentration of glial glutamate transporters in both regions corresponds to three to five times the estimated number of glutamate molecules in one synaptic vesicle from each of all glutamatergic synapses. However, the role of glial glutamate transporters in limiting synaptic spillover is likely to vary between the two regions because of differences in the distribution of astroglia. Synapses are completely ensheathed and separated from each other by astroglia in the cerebellar molecular layer. In contrast, synapses in hippocampus (stratum radiatum) are only contacted by astroglia and are often found side by side without intervening glial processes.

The glutamate transporter EAAT4 in rat cerebellar Purkinje cells: a glutamate-gated chloride channel concentrated near the synapse in parts of the dendritic membrane facing astroglia

Antibodies to an excitatory amino acid transporter (EAAT4) label a glycoprotein of approximately 65 kDa strongly in the cerebellum and weakly in the forebrain. Cross-linking of cerebellar proteins with bis(sulfosuccinimidyl) suberate before solubilization causes dimer bands of EAAT4 and both dimer and trimer bands of the other glutamate transporters GLAST (EAAT1) and GLT (EAAT2) to appear on immunoblots. In contrast to GLAST, GLT, and EAAC (EAAT3), EAAT4 is unevenly distributed in the cerebellar molecular layer, being strongly expressed in parasagittal zones. It is located in cerebellar Purkinje cells, and the highest concentrations are seen in ones expressing high levels of zebrin II (aldolase C). The labeling of Purkinje cell spines and thin dendrites is stronger than that of large diameter dendrites and cell bodies. EAAT4 is present at low concentrations in the synaptic membrane, but is highly enriched in the parts of the dendritic and spine membranes facing astrocytes (which express GLAST and GLT) compared with parts facing neuronal membranes, suggesting a functional relationship with glial glutamate transporters. The presence of EAAT4 in intracellular cisterns and multivesicular organelles may reflect turnover of transporter in the plasma membrane. The total Purkinje cell spine surface and the EAAT4 concentration were found to be 1.1 m2/cm3 and 0.2 mg/cm3, respectively, in the molecular layer, corresponding to 1800 molecules/microm2. The juxtasynaptic location of EAAT4 may explain electrophysiological observations predicting the presence of a neuronal glutamate transporter near the release site at a Purkinje cell spine synapse. EAAT4 may function as a combined transporter and inhibitory glutamate receptor.

Signal transmission in the parallel fiber-Purkinje cell system visualized by high-resolution imaging

We investigated the synaptic transmission in the parallel fiber-Purkinje cell system at high spatio-temporal resolution by using voltage-sensitive dyes and an imaging system. In rat cerebellar slices, cut in the frontal plane or in a plane of the cerebellar surface, local electrical stimulation induced volleys of action potentials in the parallel fibers; subsequent postsynaptic responses from Purkinje cells were observed along the volleys' entire trajectories. Furthermore, the formation of an ordered spatial gradient in parallel fiber conduction velocity across the depth of the molecular layer during postnatal development was observed. In preparations of adult, but not of immature rats, the conduction velocity of parallel fibers in the deep molecular layer was faster than in its more superficial regions. Our observations demonstrate that parallel fibers can mediate Purkinje cell excitation effectively and over considerable distances in a well-organized spatio-temporal manner, thus supporting the classical view of the physiological role assigned to the parallel fibers.

Physiology, morphology and detailed passive models of guinea-pig cerebellar Purkinje cells

1. Purkinje cells (PCs) from guinea-pig cerebellar slices were physiologically characterized using intracellular techniques. Extracellular caesium ions were used to linearize the membrane properties of PCs near the resting potential. Under these conditions the average input resistance, RN, was 29 M omega, the average system time constant, tau 0, was 82 ms and the average cable length, LN, was 0.59. 2. Three PCs were fully reconstructed following physiological measurements and staining with horseradish peroxidase. Assuming that each spine has an area of 1 micron 2 and that the spine density over the spiny dendrites is ten spines per micrometre length, the total membrane area of each PC is approximately 150,000 microns 2, of which approximately 100,000 microns 2 is in the spines. 3. Detailed passive cable and compartmental models were built for each of the three reconstructed PCs. Computational methods were devised to incorporate globally the huge number of spines into these models. In all three cells the models predict that the specific membrane resistivity, Rm, of the soma is much lower than the dendritic Rm (approximately 500 and approximately 100,000 omega cm2 respectively). The specific membrane capacitance, Cm, is estimated to be 1.5-2 muF cm-2 and the specific cytoplasm resistivity, Ri, is 250 omega cm. 4. The average cable length of the dendrites according to the model is 0.13 lambda, suggesting that under caesium conditions PCs are electrically very compact. Brief somatic spikes, however, are expected to attenuate 30-fold when spreading passively into the dendritic terminals. A simulated 200 Hz train of fast, 90 mV somatic spikes produced a smooth 12 mV steady depolarization at the dendritic terminals. 5. A transient synaptic conductance increase, with a 1 nS peak at 0.5 ms and a driving force of 60 mV, is expected to produce approximately 20 mV peak depolarization at the spine head membrane. This EPSP then attenuates between 200- and 900-fold into the soma. Approximately 800 randomly distributed and synchronously activated spiny inputs are required to fire the soma. 6. The passive model of the PC predicts a poor resolution of the spatio-temporal pattern of the parallel fibre input. An equally sized, randomly distributed group of approximately 1% of the parallel fibres, activated within a time window of a few milliseconds, would result in approximately the same composite EPSP at the soma.

Synaptic currents evoked in Purkinje cells by stimulating individual granule cells

Each cerebellar Purkinje cell receives input from about 160,000 glutamatergic granule cells. In anatomically intact preparations this input has hitherto been studied only as a compound synaptic potential or current. Presented here are simultaneous recordings in cerebellar slices of synaptically connected granule cell-Purkinje cell pairs. The mean amplitude of the excitatory synaptic currents evoked by stimulation of individual granule cells ranged from 2 to 60 pA, whereas the great majority of the spontaneous glutamatergic currents in Purkinje cells in the presence of tetrodotoxin were < 40 pA. In several cases, stimulation of a single granule cell evoked a disynaptic inhibitory current. It is estimated that on the order of 50 simultaneously active granule cells are sufficient to excite a Purkinje cell.

Propagation of parallel fiber volleys in the cerebellar cortex: a computer simulation

We have studied the dynamic changes occurring during the propagation of active volleys through the parallel fiber (PF) tract in a portion of molecular layer of the cerebellar cortex, by means of a realistic computer simulation of the fibers (random distribution of PF origin; correlation between length and conduction velocity throughout depth). Surface electrical stimulations as well as mossy fiber stimulations of the PFs were mimicked. From a quantitative analysis of the propagation of the different types of volleys it appears that the constraints enforced by the morphological and functional characteristics of the PFs lead to an important spatial and temporal dispersion of the action potentials during their displacement along the folium. In particular, the time relationship between activity in the deep and superficial fibers as well as the number and location of active fibers at a given time (i.e. locus) will considerably vary during propagation. Moreover, in the case of a mossy fiber stimulation, there exists a striking asymmetry of the volleys travelling on each side of the site of stimulation. Therefore any information coded by the conjunctive activation of more than a few parallel fibers will be spatially and temporally dispersed in such a manner that successive postsynaptic Purkinje cells will receive very different combinations of inputs. This seems in favor of the hypothesis that these neurons function as coincidence detectors.

Quantitative study of granule and Purkinje cells in the cerebellar cortex of the rat

The numerical densities of granule and Purkinje cells in the cerebellar cortex of the rat were determined by stereological methods. The density of Purkinje cells in our fixed material was 1,018 +/- 39 per mm2 (mean +/- s.e.m.) of Purkinje cell layer and that of granule cells 1.92 +/- 0.03 x 10(6) per microliter of granular layer. The total area of Purkinje cell layer was 332 mm2 and the volume of granular layer was 48 microliters. The rat cerebellum therefore contains 3.38 x 10(5) Purkinje cells and 9.2 x 10(7) granule cells, from which there are 274 granule cells for each Purkinje cell. The density of granule cells and the density of parallel fibers in the molecular layer observed in a companion study indicate that the average length of a parallel fiber is around 5 mm.

Number of parallel fiber synapses on an individual Purkinje cell in the cerebellum of the rat

In the present study, stereological techniques applied to electron micrographs of the molecular layer of the rat cerebellum have been used to estimate the number of parallel fiber synapses on the dendritic tree of a single Purkinje cell. Quantitative features of the parallel fiber to Purkinje cell dendritic spine synapses and of the parallel fibers were investigated as a preliminary to estimating the number of synapses. Parallel fiber to Purkinje cell synapses are flattened disclike structures with a mean axial ratio of 14.7 and a mean diameter of 319 microns in fixed tissue. The density of synapses in our fixed material was 8.17 x 10(8) per microliters of molecular layer. Determination of the length density of the synapses per unit area of micrograph indicated a synapse density of 8.03 x 10(8) per microliters. These densities give a total number of synapses per Purkinje cell of 1.74 x 10(5) and 1.71 x 10(5), respectively. Estimation of the number of parallel fiber varicosities and of varicosity length gave a density of 9.31 x 10(8) varicosities per microliters of molecular layer and determining the mean number of parallel fiber to Purkinje cell synapses per varicosity gave a synapse density of 9.82 x 10(8) per microliters, equivalent to 2.09 x 10(5) per Purkinje cell. The reasons why this estimate is likely to be too high are discussed. We conclude that there are some 175,000 parallel fiber synapses on an individual Purkinje cell dendritic tree in the cerebellar cortex of the rat, considerably more than previously reported.