Long-term depression of neuron to glial signalling in rat cerebellar cortex

Astrocyte-Neuron Networks: A Multilane Highway of Signaling for Homeostatic Brain Function

Research on glial cells over the past 30 years has confirmed the critical role of astrocytes in pathophysiological brain states. However, most of our knowledge about astrocyte physiology and of the interactions between astrocytes and neurons is based on the premises that astrocytes constitute a homogeneous cell type, without considering the particular properties of the circuits or brain nuclei in which the astrocytes are located. Therefore, we argue that more-sophisticated experiments are required to elucidate the specific features of astrocytes in different brain regions, and even within different layers of a particular circuit. Thus, in addition to considering the diverse mechanisms used by astrocytes to communicate with neurons and synaptic partners, it is necessary to take into account the cellular heterogeneity that likely contributes to the outcomes of astrocyte-neuron signaling. In this review article, we briefly summarize the current data regarding the anatomical, molecular and functional properties of astrocyte-neuron communication, as well as the heterogeneity within this communication.

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.

Glutamine/Glutamate Transporters in Glial Cells: Much More Than Participants of a Metabolic Shuttle

Glial glutamine and glutamate transporters play an important role in glial/neuronal interactions. An excellent model to establish the role of these membrane proteins is the cerebellum. The most abundant glutamatergic synapse in the central nervous system is present in the molecular layer of the cerebellar cortex, and it is entirely wrapped by Bergmann glial cells. The recycling of glutamate involves glutamate and glutamine transporters enriched in these radial glial processes. The functional properties of amino acid glial transporters allow, in an activity-dependent manner, the conformation of protein complexes important for the adequate support of glutamatergic neurotransmission. A detailed description of the most important features of glial glutamate and glutamine transporters follows, and a working model of the molecular mechanisms by which these glutamate and glutamine binding proteins interact, and by these means might modulate cerebellar glutamatergic transactions, is presented.

Activity-Dependent Plasticity of Astroglial Potassium and Glutamate Clearance

Recent evidence has shown that astrocytes play essential roles in synaptic transmission and plasticity. Nevertheless, how neuronal activity alters astroglial functional properties and whether such properties also display specific forms of plasticity still remain elusive. Here, we review research findings supporting this aspect of astrocytes, focusing on their roles in the clearance of extracellular potassium and glutamate, two neuroactive substances promptly released during excitatory synaptic transmission. Their subsequent removal, which is primarily carried out by glial potassium channels and glutamate transporters, is essential for proper functioning of the brain. Similar to neurons, different forms of short- and long-term plasticity in astroglial uptake have been reported. In addition, we also present novel findings showing robust potentiation of astrocytic inward currents in response to repetitive stimulations at mild frequencies, as low as 0.75 Hz, in acute hippocampal slices. Interestingly, neurotransmission was hardly affected at this frequency range, suggesting that astrocytes may be more sensitive to low frequency stimulation and may exhibit stronger plasticity than neurons to prevent hyperexcitability. Taken together, these important findings strongly indicate that astrocytes display both short- and long-term plasticity in their clearance of excess neuroactive substances from the extracellular space, thereby regulating neuronal activity and brain homeostasis.

Plasticity of Neuron-Glial Transmission: Equipping Glia for Long-Term Integration of Network Activity

The capacity of synaptic networks to express activity-dependent changes in strength and connectivity is essential for learning and memory processes. In recent years, glial cells (most notably astrocytes) have been recognized as active participants in the modulation of synaptic transmission and synaptic plasticity, implicating these electrically nonexcitable cells in information processing in the brain. While the concept of bidirectional communication between neurons and glia and the mechanisms by which gliotransmission can modulate neuronal function are well established, less attention has been focussed on the computational potential of neuron-glial transmission itself. In particular, whether neuron-glial transmission is itself subject to activity-dependent plasticity and what the computational properties of such plasticity might be has not been explored in detail. In this review, we summarize current examples of plasticity in neuron-glial transmission, in many brain regions and neurotransmitter pathways. We argue that induction of glial plasticity typically requires repetitive neuronal firing over long time periods (minutes-hours) rather than the short-lived, stereotyped trigger typical of canonical long-term potentiation. We speculate that this equips glia with a mechanism for monitoring average firing rates in the synaptic network, which is suited to the longer term roles proposed for astrocytes in neurophysiology.

Reappraisal of Bergmann glial cells as modulators of cerebellar circuit function

Just as there is a huge morphological and functional diversity of neuron types specialized for specific aspects of information processing in the brain, astrocytes have equally distinct morphologies and functions that aid optimal functioning of the circuits in which they are embedded. One type of astrocyte, the Bergmann glial cell (BG) of the cerebellum, is a prime example of a highly diversified astrocyte type, the architecture of which is adapted to the cerebellar circuit and facilitates an impressive range of functions that optimize information processing in the adult brain. In this review we expand on the function of the BG in the cerebellum to highlight the importance of astrocytes not only in housekeeping functions, but also in contributing to plasticity and information processing in the cerebellum.

Astroglial calcium signaling displays short-term plasticity and adjusts synaptic efficacy

Astrocytes are dynamic signaling brain elements able to sense neuronal inputs and to respond by complex calcium signals, which are thought to represent their excitability. Such signaling has been proposed to modulate, or not, neuronal activities ranging from basal synaptic transmission to epileptiform discharges. However, whether calcium signaling in astrocytes exhibits activity-dependent changes and acutely modulates short-term synaptic plasticity is currently unclear. We here show, using dual recordings of astroglial calcium signals and synaptic transmission, that calcium signaling in astrocytes displays, concomitantly to excitatory synapses, short-term plasticity in response to prolonged repetitive and tetanic stimulations of Schaffer collaterals. We also found that acute inhibition of calcium signaling in astrocytes by intracellular calcium chelation rapidly potentiates excitatory synaptic transmission and short-term plasticity of Shaffer collateral CA1 synapses, i.e., paired-pulse facilitation and responses to tetanic and prolonged repetitive stimulation. These data reveal that calcium signaling of astrocytes is plastic and down-regulates basal transmission and short-term plasticity of hippocampal CA1 glutamatergic synapses.

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

Background

Cerebellar 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.

Methods

Whole cell patch clamp recordings from Purkinje neurons and Bergmann glia were carried out in transverse cerebellar slices from juvenile (P16-20) Wistar rats.

Key Results

Caffeine 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.

Conclusions

We 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.

Ectopic release of glutamate contributes to spillover at parallel fibre synapses in the cerebellum

In the rat cerebellar molecular layer, spillover of glutamate between parallel fibre synapses can lead to activation of perisynaptic receptors that mediate short- and long-term plasticity. This effect is greatest when clusters of fibres are stimulated at high frequencies, suggesting that glutamate clearance mechanisms must be overwhelmed before spillover can occur. However, parallel fibres can also release transmitter directly into the extracellular space, from 'ectopic' release sites. Ectopic transmission activates AMPA receptors on the Bergmann glial cell processes that envelop parallel fibre synapses, but the possible contribution of this extrasynaptic release to intersynaptic communication has not been explored. We exploited long-term depression of ectopic transmission, and selective pharmacology, to investigate the impact of these release sites on the time course of Purkinje neuron excitatory postsynaptic currents (EPSCs). Depletion of ectopic release pools by activity-dependent long-term depression decreased EPSC decay time, revealing a 'late' current that is present when fibres are stimulated at low frequencies. This effect was enhanced when glutamate transporters were inhibited, and reduced when extracellular diffusion was impeded. Blockade of N-type Ca(2+) channels inhibited ectopic transmission to Bergmann glia and decreased EPSC decay time. Similarly, perfusion of the Ca(2+) chelator EGTA-AM into the slice progressively eliminated ectopic transmission to glia and decreased EPSC decay time with closely similar time courses. Collectively, this evidence suggests that ectopically released glutamate contributes to spillover transmission, and that ectopic release therefore degrades the spatial precision of synapses that fire infrequently, and may make them more prone to exhibit plasticity.

How do astrocytes shape synaptic transmission? Insights from electrophysiology

A major breakthrough in neuroscience has been the realization in the last decades that the dogmatic view of astroglial cells as being merely fostering and buffering elements of the nervous system is simplistic. A wealth of investigations now shows that astrocytes actually participate in the control of synaptic transmission in an active manner. This was first hinted by the intimate contacts glial processes make with neurons, particularly at the synaptic level, and evidenced using electrophysiological and calcium imaging techniques. Calcium imaging has provided critical evidence demonstrating that astrocytic regulation of synaptic efficacy is not a passive phenomenon. However, given that cellular activation is not only represented by calcium signaling, it is also crucial to assess concomitant mechanisms. We and others have used electrophysiological techniques to simultaneously record neuronal and astrocytic activity, thus enabling the study of multiple ionic currents and in depth investigation of neuro-glial dialogues. In the current review, we focus on the input such approach has provided in the understanding of astrocyte-neuron interactions underlying control of synaptic efficacy.

Astroglial potassium clearance contributes to short-term plasticity of synaptically evoked currents at the tripartite synapse

Astroglial processes enclose ∼60% of CA1 hippocampal synapses to form the tripartite synapse. Although astrocytes express ionic channels, neurotransmitter receptors and transporters to detect neuronal activity, the nature, plasticity and impact of the currents induced by neuronal activity on short-term synaptic plasticity remain elusive in hippocampal astrocytes. Using simultaneous electrophysiological recordings of astrocytes and neurons, we found that single stimulation of Schaffer collaterals in hippocampal slices evokes in stratum radiatum astrocytes a complex prolonged inward current synchronized to synaptic and spiking activity in CA1 pyramidal cells. The astroglial current is composed of three components sensitive to neuronal activity, i.e. a long-lasting potassium current mediated by Kir4.1 channels, a transient glutamate transporter current and a slow residual current, partially mediated by GABA transporters and Kir4.1-independent potassium channels. We show that all astroglial membrane currents exhibit activity-dependent short-term plasticity. However, only the astroglial glutamate transporter current displays neuronal-like dynamics and plasticity. As Kir4.1 channel-mediated potassium uptake contributes to 80% of the synaptically evoked astroglial current, we investigated in turn its impact on short-term synaptic plasticity. Using glial conditional Kir4.1 knockout mice, we found that astroglial potassium uptake reduces synaptic responses to repetitive stimulation and post-tetanic potentiation. These results show that astrocytes integrate synaptic activity via multiple ionic channels and transporters and contribute to short-term plasticity in part via potassium clearance mediated by Kir4.1 channels.

Origin, lineage and function of cerebellar glia

The glial cells of the cerebellum, and particularly astrocytes and oligodendrocytes, are characterized by a remarkable phenotypic variety, in which highly peculiar morphological features are associated with specific functional features, unique among the glial cells of the entire CNS. Here, we provide a critical report about the present knowledge of the development of cerebellar glia, including lineage relationships between cerebellar neurons, astrocytes and oligodendrocytes, the origins and the genesis of the repertoire of glial types, and the processes underlying their acquisition of mature morphological and functional traits. In parallel, we describe and discuss some fundamental roles played by specific categories of glial cells during cerebellar development. In particular, we propose that Bergmann glia exerts a crucial scaffolding activity that, together with the organizing function of Purkinje cells, is necessary to achieve the normal pattern of foliation and layering of the cerebellar cortex. Moreover, we discuss some of the functional tasks of cerebellar astrocytes and oligodendrocytes that are distinctive of cerebellar glia throughout the CNS. Notably, we report about the regulation of synaptic signalling in the molecular and granular layer mediated by Bergmann glia and parenchymal astrocytes, and the functional interaction between oligodendrocyte precursor cells and neurons. On the whole, this review provides an extensive overview of the available literature and some novel insights about the origin and differentiation of the variety of cerebellar glial cells and their function in the developing and mature cerebellum.

Brain-derived neurotrophic factor and its receptors in Bergmann glia cells

Brain-derived neurotrophic factor is an abundant and widely distributed neurotrophin expressed in the Central Nervous System. It is critically involved in neuronal differentiation and survival. The expression of brain-derived neurotrophic factor and that of its catalytic active cognate receptor (TrkB) has been extensively studied in neuronal cells but their expression and function in glial cells is still controversial. Despite of this fact, brain-derived neurotrophic factor is released from astrocytes upon glutamate stimulation. A suitable model to study glia/neuronal interactions, in the context of glutamatergic synapses, is the well-characterized culture of chick cerebellar Bergmann glia cells. Using, this system, we show here that BDNF and its functional receptor are present in Bergmann glia and that BDNF stimulation is linked to the activation of the phosphatidyl-inositol 3 kinase/protein kinase C/mitogen-activated protein kinase/Activator Protein-1 signaling pathway. Accordingly, reverse transcription-polymerase chain reaction (RT-PCR) experiments predicted the expression of full-length and truncated TrkB isoforms. Our results suggest that Bergmann glia cells are able to express and respond to BDNF stimulation favoring the notion of their pivotal role in neuroprotection.

What can the spinal cord teach us about learning and memory?

The work of recent decades has shown that the nervous system changes continually throughout life. Activity-dependent central nervous system (CNS) plasticity has many different mechanisms and involves essentially every region, from the cortex to the spinal cord. This new knowledge radically changes the challenge of explaining learning and memory and greatly increases the relevance of the spinal cord. The challenge is now to explain how continual and ubiquitous plasticity accounts for the initial acquisition and subsequent stability of many different learned behaviors. The spinal cord has a key role because it is the final common pathway for all behavior and is a site of substantial plasticity. Furthermore, because it is simple, accessible, distant from the rest of the CNS, and directly connected to behavior, the spinal cord is uniquely suited for identifying sites and mechanisms of plasticity and for determining how they account for behavioral change. Experimental models based on spinal cord reflexes facilitate study of the gradual plasticity that makes possible most rapid learning phenomena. These models reveal principles and generate concepts that are likely to apply to learning and memory throughout the CNS. In addition, they offer new approaches to guiding activity-dependent plasticity so as to restore functions lost to injury or disease.

The dynamics of single spike-evoked adenosine release in the cerebellum

The purine adenosine is a potent neuromodulator in the brain, with roles in a number of diverse physiological and pathological processes. Modulators such as adenosine are difficult to study as once released they have a diffuse action (which can affect many neurones) and, unlike classical neurotransmitters, have no inotropic receptors. Thus rapid postsynaptic currents (PSCs) mediated by adenosine (equivalent to mPSCs) are not available for study. As a result the mechanisms and properties of adenosine release still remain relatively unclear. We have studied adenosine release evoked by stimulating the parallel fibres in the cerebellum. Using adenosine biosensors combined with deconvolution analysis and mathematical modelling, we have characterised the release dynamics and diffusion of adenosine in unprecedented detail. By partially blocking K+ channels, we were able to release adenosine in response to a single stimulus rather than a train of stimuli. This allowed reliable sub-second release of reproducible quantities of adenosine with stereotypic concentration waveforms that agreed well with predictions of a mathematical model of purine diffusion. We found no evidence for ATP release and thus suggest that adenosine is directly released in response to parallel fibre firing and does not arise from extracellular ATP metabolism. Adenosine release events showed novel short-term dynamics, including facilitated release with paired stimuli at millisecond stimulation intervals but depletion-recovery dynamics with paired stimuli delivered over minute time scales. These results demonstrate rich dynamics for adenosine release that are placed, for the first time, on a quantitative footing and show strong similarity with vesicular exocytosis.

Ectopic release sites lack fast vesicle recycling mechanisms, causing long-term depression of neuron-glial transmission in rat cerebellum

Classical synaptic transmission occurs at active zones within the synaptic cleft, but increasing evidence suggests that vesicle fusion can also occur outside of these zones, releasing transmitter directly into the extrasynaptic space. The role of such "ectopic" release is unclear, but in the cerebellar molecular layer it is thought to guide the processes of Bergmann glia toward synaptic terminals through activation of glial α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptors. Once surrounding the terminal, the glial process is presumed to limit spillover of neurotransmitter between synapses by rapid uptake of glutamate. We have previously reported that this route for neuron-glial transmission exhibits long-term depression following repetitive stimulation at frequencies in the 0.1-1 Hz range, in ex vivo slices from rat cerebellum. Here, we present evidence that LTD arises because ectopic sites lack the fast recycling mechanisms that operate at the active zone. Consequently, ectopic vesicles constitute an exhaustible pool that is depleted at normal synaptic firing rates and only recovers slowly. This effect is cumulative, meaning that the strength of ectopic transmission provides a read-out of the average frequency of presynaptic firing over several minutes. Glial processes are therefore likely to interact most closely with terminals that fire infrequently; conditions that may promote elimination of, rather than support for, the connection.

Depression of parallel and climbing fiber transmission to Bergmann glia is input specific and correlates with increased precision of synaptic transmission

In the cerebellar cortex, Bergmann glia enclose the synapses of both parallel and climbing fiber inputs to the Purkinje neuron. The glia express Ca(2+)-permeable AMPA receptors, and the GLAST and GLT-1 classes of glutamate transporter, which are activated by glutamate released during synaptic transmission. We have previously reported that parallel fiber to Bergmann glial transmission in rat cerebellar slices exhibits a form of frequency-dependent plasticity, namely long-term depression, following repetitive stimulation at 0.1-1 Hz. Here, we report that this form of plasticity is also present at the climbing fiber input, that climbing and parallel fibers can be depressed independently, that discrete parallel fiber inputs can also be depressed independently, and that depression is maintained when a distributed array of parallel fibers are stimulated (in contrast to several forms of synaptic plasticity at the Purkinje neuron). Depression of glutamate transporter currents does not correlate with a decrease in the stringency with which Purkinje neuron synapses are isolated. Rather, postsynaptic currents in Purkinje neurons decay more rapidly and perisynaptic metabotropic glutamate receptors are activated less effectively after stimulation at 0.2 and 1 Hz, suggesting that depression arises from a decrease in extrasynaptic glutamate concentration and not from impairment of glutamate clearance in and around the synapse. These results indicate that neuron-glial plasticity is activity dependent, input specific and does not require spillover between adjacent synapses to manifest. They also argue against a withdrawal of the glial sheath from synaptic regions as the putative mechanism of plasticity.

Characterization of trans-neuronal trafficking of Cbln1

Cbln1, a glycoprotein secreted from granule cells and GluRdelta2 in the postsynaptic densities of Purkinje cells are components of an incompletely understood pathway essential for integrity and plasticity of parallel fiber-Purkinje cell synapses. We show that Cbln1 undergoes anterograde transport from granule cells to Purkinje cells and Bergmann glia, and enters the endolysosomal trafficking system, raising the possibility that Cbln1 exerts its activity on or within Purkinje cells and Bergmann glia. Cbln1 is absent in Purkinje cells and Bergmann glia of GluRdelta2-null mice, suggesting a mechanistic convergence on Cbln1 trafficking. Ectopic expression of Cbln1 in Purkinje cells of L7-cbln1 transgenic mice reveals Cbln1 undergoes anterograde and retrograde trans-neuronal trafficking even across synapses that lack GluRDelta2, indicating that it is not universally essential for Cbln1 transport. The L7-cbln1 transgene also ameliorates the locomotor deficits of cbln1-null mice, indicating that the presence and/or release of Cbln1 from the postsynaptic neuron has functional consequences.

Ubiquitous plasticity and memory storage

To date, most hypotheses of memory storage in the mammalian brain have focused upon long-term synaptic potentiation and depression (LTP and LTD) of fast glutamatergic excitatory postsynaptic currents (EPSCs). In recent years, it has become clear that many additional electrophysiological components of neurons, from electrical synapses to glutamate transporters to voltage-sensitive ion channels, can also undergo use-dependent long-term plasticity. Models of memory storage that incorporate this full range of demonstrated electrophysiological plasticity are better able to account for both the storage of memory in neuronal networks and the complexities of memory storage, indexing, and recall as measured behaviorally.

Motor learning induces astrocytic hypertrophy in the cerebellar cortex

Motor skill learning, but not mere motor activity, is associated with an increase in both synapse number and glial cell volume within the cerebellar cortex. The increase in synapse number has been shown to persist for at least 4 weeks in the absence of continued training. The present experiment similarly examined how a prolonged interruption in training affects the training-induced increase in astrocytic volume. Adult female rats were randomly allocated to either an acrobatic motor learning condition (AC) or a motor control condition (MC). The AC animals were trained to traverse a complex series of obstacles and each AC animal was pair matched with an MC animal that traversed an obstacle-free runway. These groups were further assigned to one of three training conditions. Animals in the early condition were trained for 10 consecutive days, animals in the delay condition received the same 10 days of training followed by a 28-day period without training, and animals in the continuous condition were trained for the entire 38 days. Unbiased stereological techniques were used to determine that AC animals had a significantly greater volume of astrocytes per Purkinje cell in the cerebellar paramedian lobule than the MC animals, a difference which was reduced (and not statistically detectable) among animals in the delay condition. These findings demonstrate that learning triggers the hypertrophy of astrocytic processes and furthermore that, unlike learning-induced synaptogenesis, astrocytic growth is reduced in the absence of continued training.

Regulation of the Mouse Na+-Dependent Glutamate/Aspartate Transporter GLAST: Putative Role of an AP-1 DNA Binding Site

Appropriate removal of L: -glutamate from the synaptic cleft is important for prevention of the excitotoxic effects of this neurotransmitter. The Na+-dependent glutamate/aspartate transporter GLAST is regulated in the short term, by a transporter-dependent decrease in uptake activity while in the long term, a receptor's-dependent decrease in GLAST protein levels leads to a severe reduction in glutamate uptake. The promoter region of the mouse glast gene harbors an Activator Protein-1 site (AP-1). To gain insight into the molecular mechanisms triggered by Glu-receptors activation involved in GLAST regulation, we took advantage of the neonatal mouse cerebellar prisms model. We characterized the glutamate uptake activity; the glutamate-dependent effect on GLAST protein levels and over the interaction of nuclear proteins with a mouse glast promoter AP-1 probe. A time and dose dependent decrease in transporter activity matching with a decrease in GLAST levels was recorded upon glutamate treatment. Moreover, a significant increase in glast AP-1 DNA binding was found. Pharmacological experiments established that both effects are mediated through alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors, favoring the notion of the critical involvement of glutamate in the regulation of its binding partners: receptors and transporters.

Presynaptic modulation of parallel fibre signalling to Bergmann glia

Transmission at the parallel fibre-Purkinje neurone synapse of the cerebellum can be depressed by a number of presynaptic receptors: endocannabinoid (CB1), metabotropic glutamate (mGluR4), adenosine (A1) and GABA (GABA(B)), which have been implicated in both short- and long-term synaptic plasticity. Stimulation of parallel fibres also activates glutamate receptors and transporters on the Bergmann glial cell that forms a sheath around the synapse. The resulting glial extrasynaptic currents (ESC) exhibit short- and long-term plasticity, which differs from the plasticity of adjacent synapses. This functional independence could arise from differential modulation of presynaptic release sites targeted to synapses or glia, but the sensitivity of glial ESC to these inhibitory pathways is unknown. Here I show that all four presynaptic receptors depress parallel fibre-Bergmann glial cell signalling with similar potency to synaptic transmission. Depression of glial ESC is accompanied by a decrease in paired pulse ratio. However, application of receptor antagonists had no effect on ESC amplitude, indicating that tonic activation of these pathways does not occur, and antagonists failed to block the activity-dependent depression of glial ESC observed during tetanic or low frequency stimulation. These data suggest that modulation of presynaptic glutamate release does not underlie glial plasticity.

Patching the glia reveals the functional organisation of the brain

The neuroglia was initially conceived by Rudolf Virchow as a non-cellular connective tissue holding neurones together. In 1894, Carl Ludwig Schleich proposed a hypothesis of fully integrated and interconnected neuronal-glial circuits as a substrate for brain function. This hypothesis received direct experimental support only hundred years later, after several physiological techniques, and most notably the patch-clamp method, were applied to glial cells. These experiments have demonstrated the existence of active and bi-directional neuronal-glial communications, integrating neuronal networks and glial syncytium into one functional circuit. The data accumulated during last 15 years prompt rethinking of the neuronal doctrine towards more inclusive concept, which regards both neurones and glia as equally responsible for information processing in the brain.

Interactions between Purkinje neurones and Bergmann glia

Throughout the development of the cerebellar cortex, Purkinje neurones interact closely with Bergmann glial cells, a specialized form of astrocyte. This review summarizes the intimate developmental, anatomical and functional relationships between these two cell types, with particular emphasis on recent discoveries regarding glutamate release from climbing and parallel fibres as a pathway for signalling synaptic activity to Bergmann glia.