Richard Robitaille

Gliotransmitters Travel in Time and Space

The identification of the presence of active signaling between astrocytes and neurons in a process termed gliotransmission has caused a paradigm shift in our thinking about brain function. However, we are still in the early days of the conceptualization of how astrocytes influence synapses, neurons, networks, and ultimately behavior. In this Perspective, our goal is to identify emerging principles governing gliotransmission and consider the specific properties of this process that endow the astrocyte with unique functions in brain signal integration. We develop and present hypotheses aimed at reconciling confounding reports and define open questions to provide a conceptual framework for future studies. We propose that astrocytes mainly signal through high-affinity slowly desensitizing receptors to modulate neurons and perform integration in spatiotemporal domains complementary to those of neurons.

Strategic location of calcium channels at transmitter release sites of frog neuromuscular synapses

The localization of Ca2+ channels relative to the position of transmitter release sites was investigated at the frog neuromuscular junction (NMJ). Ca2+ channels were labeled with fluorescently tagged omega-conotoxin GVIA, an irreversible Ca2+ channel ligand, and observed with a confocal laser scanning microscope. The Ca2+ channel labeling almost perfectly matched that of acetylcholine receptors which were labeled with fluorescent alpha-bung-arotoxin. This indicates that groups of Ca2+ channels are localized exclusively at the active zones of the frog NMJ. Cross sections of NMJs showed that Ca2+ channels are clustered on the presynaptic membrane adjacent to the postsynaptic membrane.

Astrocytes Are Endogenous Regulators of Basal Transmission at Central Synapses

Basal synaptic transmission involves the release of neurotransmitters at individual synapses in response to a single action potential. Recent discoveries show that astrocytes modulate the activity of neuronal networks upon sustained and intense synaptic activity. However, their ability to regulate basal synaptic transmission remains ill defined and controversial. Here, we show that astrocytes in the hippocampal CA1 region detect synaptic activity induced by single-synaptic stimulation. Astrocyte activation occurs at functional compartments found along astrocytic processes and involves metabotropic glutamate subtype 5 receptors. In response, astrocytes increase basal synaptic transmission, as revealed by the blockade of their activity with a Ca(2+) chelator. Astrocytic modulation of basal synaptic transmission is mediated by the release of purines and the activation of presynaptic A(2A) receptors by adenosine. Our work uncovers an essential role for astrocytes in the regulation of elementary synaptic communication and provides insight into fundamental aspects of brain function.

GABAergic Network Activation of Glial Cells Underlies Hippocampal Heterosynaptic Depression

Tetanus-induced heterosynaptic depression in the hippocampus is a key cellular mechanism in neural networks implicated in learning and memory. A growing body of evidence indicates that glial cells are important modulators of synaptic functions, but very little is known about their role in heterosynaptic plasticity. We examined the role of glial cells in heterosynaptic depression, knowing that tetanization and NMDA application caused depression of synaptic field responses (fEPSPs) and induced Ca2+ rise in glial cells. Here we report that chelating Ca2+ in a glial syncytium interfered with heterosynaptic depression and NMDA-induced fEPSP depression, suggesting that Ca2+ activation of glial cells is necessary for heterosynaptic depression. The NMDA-induced Ca2+ rise in glial cells was sensitive to tetrodotoxin and reduced by the GABAB antagonist CGP55845. Both heterosynaptic depression and simultaneous Ca2+ activation of glial cells were prevented by CGP55845, suggesting an involvement of the GABAergic network in glial activation and heterosynaptic depression. Also, the GABAB agonist baclofen caused both a Ca2+ rise in glial cells and fEPSP depression. Heterosynaptic depression, as well as NMDA- and baclofen-induced depression, were attenuated by an A1 antagonist, cyclopentyl-theophylline, whereas glial cell activation was not, indicating a role of adenosine downstream of glial activation. Finally, heterosynaptic depression requires ATP degradation because ectonucleotidase inhibitors reduced this plasticity. Our work indicates that Ca2+ activation of glial cells is necessary for heterosynaptic depression, which involves the sequential interaction of Schaffer collaterals, the GABAergic network, and glia. Thus, glial and neuronal networks are functionally associated during the genesis of heterosynaptic plasticity at mammalian central excitatory synapses.

Modulation of Synaptic Efficacy and Synaptic Depression by Glial Cells at the Frog Neuromuscular Junction

The ability of perisynaptic glial cells to modulate transmitter release and synaptic depression was studied at the frog neuromuscular junction (nmj). Injection of GTPgammaS in perisynaptic Schwann cells (PSCs), glial cells at this synapse, induced a reduction in the amplitude of nerve-evoked synaptic responses but had no effect on the frequency, the amplitude, or the duration of the miniature endplate currents (MEPCs). Also, paired pulse facilitation was not affected. The reduction in transmitter release was mediated by pertussis toxin-(PTX) sensitive and insensitive G proteins. Blockade of G proteins in PSCs with GDPbetaS reduced synaptic depression induced by high frequency trains of stimuli, whereas activation of G proteins occluded it. Hence, the activation by endogenous neurotransmitters of G proteins in PSCs induced a profound depression in neurotransmitter release.

Glial Cells and Neurotransmission: An Inclusive View of Synaptic Function

Glial cells throughout the nervous system are closely associated with synapses. Accompanying these anatomical couplings are intriguing functional interactions, including the capacity of certain glial cells to respond to and modulate neurotransmission. Glial cells can also help establish, maintain, and reconstitute synapses. In this review, we discuss evidence indicating that glial cells make important contributions to synaptic function.

Transmitter release increases intracellular calcium in perisynaptic schwann cells in situ

Glial cells isolated from the nervous system are sensitive to neurotransmitters and may therefore be involved in synaptic transmission. The sensitivity of individual perisynaptic Schwann cells to activity of a single synapse was investigated, in situ, at the frog neuromuscular junction by monitoring changes in intracellular Ca2+ in the Schwann cells. Motor nerve stimulation induced an increase in intracellular Ca2+ in these Schwann cells; this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of the cotransmitters acetylcholine and ATP evoked Ca2+ responses even in the absence of extracellular Ca2+. Successive trains of nerve stimuli or applications of transmitters resulted in progressively smaller Ca2+ responses. We conclude that transmitter released during synaptic activity can evoke release of intracellular Ca2+ in perisynaptic Schwann cells. This Ca2+ signal may play a role in the maintenance or modulation of a synapse. These data show that synaptic transmission involves three cellular components with both postsynaptic and glial components responding to transmitter secretion.

Differential Regulation of Transmitter Release by Presynaptic and Glial Ca2+ Internal Stores at the Neuromuscular Synapse

The differential regulation of synaptic transmission by internal Ca2+ stores of presynaptic terminals and perisynaptic Schwann cells (PSCs) was studied at the frog neuromuscular junction. Thapsigargin (tg), an inhibitor of Ca2+-ATPase pumps of internal stores, caused a transient Ca2+ elevation in PSCs, whereas it had no effect on Ca2+ stores of presynaptic terminals at rest. Tg prolonged presynaptic Ca2+ responses evoked by single action potentials with no detectable increase in the resting Ca2+ level in nerve terminals. However, Ca2+ accumulation was observed during high frequency stimulation. Tg induced a rapid rise in endplate potential (EPP) amplitude, accompanied by a delayed and transient increase. The effects appeared presynaptic, as suggested by the lack of effects of tg on the amplitude and time course of miniature EPPs (MEPPs). However, MEPP frequency was increased when preparations were stimulated tonically (0.2 Hz). The delayed and transient increase in EPP amplitude was occluded by injections of the Ca2+chelator BAPTA into PSCs before tg application, whereas a rise in intracellular Ca2+ in PSCs induced by inositol 1,4,5-triphosphate (IP3) injections potentiated transmitter release. Furthermore, increased Ca2+buffering capacity after BAPTA injection in PSCs resulted in a more pronounced synaptic depression induced by high frequency stimulation of the motor nerve (10 Hz/80 sec). It is concluded that presynaptic Ca2+ stores act as a Ca2+clearance mechanism to limit the duration of transmitter release, whereas Ca2+ release from glial stores initiates Ca2+-dependent potentiation of synaptic transmission.

Glial cells in synaptic plasticity.

Plasticity of synaptic transmission is believed to be the cellular basis for learning and memory, and depends upon different pre- and post-synaptic neuronal mechanisms. Recently, however, an increasing number of studies have implicated a third element in plasticity; the perisynaptic glial cell. Originally glial cells were thought to be important for metabolic maintenance and support of the nervous system. However, work in the past decade has clearly demonstrated active involvement of glia in stability and overall nervous system function as well as synaptic plasticity. Through specific modulation of glial cell function, a wide variety of roles for glia in synaptic plasticity have been uncovered. Furthermore, interesting circumstantial evidence suggests a glial involvement in multiple other types of plasticity. We will discuss recent advances in neuron-glial interactions that take place during synaptic plasticity and explore different plasticity phenomena in which glial cells may be involved.

Perisynaptic Glia Discriminate Patterns of Motor Nerve Activity and Influence Plasticity at the Neuromuscular Junction

In the nervous system, the induction of plasticity is coded by patterns of synaptic activity. Glial cells are now recognized as dynamic partners in a wide variety of brain functions, including the induction and modulation of various forms of synaptic plasticity. However, it appears that glial cells are usually activated by stereotyped, sustained neuronal activity, and little attention has been given to more subtle changes in the patterns of synaptic activation. To this end, we used the mouse neuromuscular junction as a simple and useful model to study glial modulation of synaptic plasticity. We used two patterns of motor nerve stimulation that mimic endogenous motor-neuronal activity. A continuous stimulation induced a post-tetanic potentiation and a phasic Ca2+ response in perisynaptic Schwann cells (PSCs), glial cells at this synapse. A bursting pattern of activity induced a post-tetanic depression and oscillatory Ca2+ responses in PSCs. The different Ca2+ responses in PSCs indicate that they decode the pattern of synaptic activity. Furthermore, the chelation of glial Ca2+ impaired the production of the sustained plasticity events indicating that PSCs govern the outcome of synaptic plasticity. The mechanisms involved were studied using direct photo-activation of PSCs with caged Ca2+ that mimicked endogenous plasticity. Using specific pharmacology and transgenic knock-out animals for adenosine receptors, we showed that the sustained depression was mediated by A1 receptors while the sustained potentiation is mediated by A2A receptors. These results demonstrate that glial cells decode the pattern of synaptic activity and subsequently provide bidirectional feedback to synapses.