Target-cell specificity of kainate autoreceptor and Ca2+-store-dependent short-term plasticity at hippocampal mossy fiber synapses

Short term plasticity at hippocampal mossy fiber synapses

Short-term synaptic plasticity refers to the regulation of synapses by their past activity on time scales of milliseconds to minutes. Hippocampal mossy fiber synapses onto CA3 pyramidal cells (Mf-CA3 synapses) are endowed with remarkable forms of short-term synaptic plasticity expressed as facilitation of synaptic release by a factor of up to ten-fold. Three main forms of short-term plasticity are distinguished: 1) Frequency facilitation, which includes low frequency facilitation and train facilitation, operating in the range of tens of milliseconds to several seconds; 2) Post-tetanic potentiation triggered by trains of high frequency stimulation, which lasts several minutes; 3) Finally, depolarization-induced potentiation of excitation, based on retrograde signaling, with an onset and offset of several minutes. Here we describe the proposed mechanisms for short-term plasticity, mainly based on the kinetics of presynaptic Ca2+ transients and the Ca2+ sensor synaptotagmin 7, on cAMP-dependent mechanisms and readily releasable vesicle pool, and on the regulation of presynaptic K+ channels. We then review evidence for a physiological function of short-term plasticity of Mf-CA3 synapses in information transfer between the dentate gyrus and CA3 in conditions of natural spiking, and discuss how short-term plasticity counteracts robust feedforward inhibition in a frequency-dependent manner. Although DG-CA3 connections have long been proposed to play a role in memory, direct evidence for an implication of short-term plasticity at Mf-CA3 synapses is mostly lacking. The mechanistic knowledge gained on short-term plasticity at Mf-CA3 synapses should help in designing future experiments to directly test how this evolutionary conserved feature controls hippocampal circuit function in behavioural conditions.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Modeling analysis of subthreshold voltage signaling along hippocampal mossy fiber axons

Axons are classically thought of as electrically well isolated from other parts of the neurons due to the shape of a long cable-like structure. In contrast to this classical view on axonal compartmentalization, recent studies revealed that subthreshold depolarization of soma and dendrite passively propagates to the axons for a substantial distance, as demonstrated in some experimentally accessible axons including hippocampal mossy fibers and cortical pyramidal cell axons. Passive propagation of subthreshold dendritic EPSPs to the axons, defined as EPreSPs (excitatory presynaptic potentials), has been demonstrated to affect transmitter release from the axon terminals. To further characterize and explore the functional significance of passive subthreshold voltage signaling along the axons, the model of EPreSPs along hippocampal mossy fibers, proposed by Alle and Geiger, was reconstructed on the NEURON simulator. To test the effect of EPreSPs on action potentials and transmitter release from the axon terminals, additional conductances were incorporated into the previous passive propagation model. These include the axonal sodium, potassium, and leak channels as well as presynaptic calcium channels composed of P/Q-, N-, and R-types, which are reconstructed from the properties of those recorded from mossy fiber boutons experimentally. In this revised model, the preceding subthreshold EPreSPs slightly reduced the action potential-evoked presynaptic calcium currents by a decrease in the amplitude of action potentials due to the slow depolarization. It should be mentioned that EPreSPs by themselves elicited small calcium currents during subthreshold depolarization through these high-voltage activated calcium channels. Since the previous experimental study by simultaneous pre and postsynaptic recordings demonstrated that EPreSPs enhanced action potential-evoked transmitter release from the mossy fiber terminals, it has been suggested that different mechanisms from the enhancement of action potential-evoked presynaptic calcium entry may involve enhanced transmitter release by EPreSP. Small calcium entry by subthreshold EPreSPs may enhance transmitter release from the mossy fiber terminals by acting as high-affinity calcium sensors for enhancing transmitter release. Another form of axonal subthreshold voltage signaling, GABA-EPreSPs elicited by a spillover of GABA from surrounding interneurons, was also explored. Functional consequences of the two modes of axonal subthreshold voltage signaling were discussed with the simulation results.

mRNA editing of kainate receptor subunits: what do we know so far?

Kainate receptors (KARs) are considered one of the key modulators of synaptic activity in the mammalian central nervous system. These receptors were discovered more than 30 years ago, but their role in brain functioning remains unclear due to some peculiarities. One such feature of these receptors is the editing of pre-mRNAs encoding GluK1 and GluK2 subunits. Despite the long history of studying this phenomenon, numerous questions remain unanswered. This review summarizes the current data about the mechanism and role of pre-mRNA editing of KAR subunits in the mammalian brain and proposes a perspective of future investigations.

Quantal analysis estimates docking site occupancy determining short-term depression at hippocampal glutamatergic synapses

Before fusion, synaptic vesicles (SVs) pause at discrete release/docking sites. During repetitive stimulation, the probability of site occupancy changes following SV fusion and replenishment. The occupancy probability is considered to be one of the crucial determinants of synaptic strength, but it is difficult to estimate separately because it usually blends with other synaptic parameters. Thus, the contribution of site occupancy to synaptic function, particularly to synaptic depression, remains elusive. Here, we directly estimated the occupancy probability at the hippocampal mossy fibre-CA3 interneuron synapse showing synaptic depression, using statistics of counts of vesicular events detected by deconvolution. We found that this synapse had a particularly high occupancy (∼0.85) with a high release probability of a docked SV (∼0.8) under 3 mm external calcium conditions. Analyses of quantal amplitudes and SV counts indicated that quantal size reduction decreased the amplitudes of all responses in a train to a similar degree, whereas release/docking site number was unchanged during trains, suggesting that quantal size and release/docking site number had little influence on the extent of synaptic depression. Model simulations revealed that the initial occupancy with high release probability and slow replenishment determined the time course of synaptic depression. Consistently, decreasing external calcium concentration reduced both the occupancy and release probability, and the reductions in turn produced less depression. Based on these results, we suggest that the occupancy probability is a crucial determinant of short-term synaptic depression at glutamatergic synapses in the hippocampus. KEY POINTS: The occupancy probability of a release/docking site by a synaptic vesicle at presynaptic terminals is considered to be one of the crucial determinants of synaptic strength, but it is difficult to estimate separately from other synaptic parameters. Here, we directly estimate the occupancy probability at the hippocampal mossy fibre-interneuron synapse using statistics of vesicular events detected by deconvolution. We show that the synapses have particularly high occupancy (0.85) with high release probability (0.8) under high external calcium concentration ([Ca²⁺ ]_o ) conditions, and that both parameter values change with [Ca²⁺ ]_o , shaping synaptic depression. Analyses of the quantal amplitudes and synaptic vesicle counts suggest that quantal sizes and release/docking site number have little influence on the extent of synaptic depression. The results suggest that the occupancy probability is a crucial determinant of short-term synaptic depression at glutamatergic synapses in the hippocampus.

Kainate receptors and synaptic plasticity

Synaptic plasticity has classically been characterized to involve the NMDA and AMPA subtypes of glutamate receptors, with NMDA receptors providing the key trigger for the induction of long-term plasticity leading to changes in AMPA receptor expression. Here we review the more subtle roles played by kainate receptors, which contribute critical postsynaptic signalling as well as playing major presynaptic auto-receptor roles. We focus on two research areas: plasticity of kainate receptors themselves and the contribution they make to the plasticity of synaptic transmission. This article is part of the special issue on Glutamate Receptors - Kainate receptors.

Kainate receptor modulation of glutamatergic synaptic transmission in the CA2 region of the hippocampus

Kainate (KA) receptors (KARs) are important modulators of synaptic transmission. We studied here the role of KARs on glutamatergic synaptic transmission in the CA2 region of the hippocampus where the actions of these receptors are unknown. We observed that KA depresses glutamatergic synaptic transmission at Schaffer collateral-CA2 synapses; an effect that was antagonized by NBQX (a KA/AMPA receptors antagonist) under condition where AMPA receptors were previously blocked. The study of paired-pulse facilitation ratio, miniature responses, and fluctuation analysis indicated a presynaptic locus of action for KAR. Additionally, we determined the action mechanism for this depression of glutamate release mediated by the activation of KARs. We found that inhibition of protein kinase A suppressed the effect of KAR activation on evoked excitatory post-synaptic current, an effect that was not suppressed by protein kinase C inhibitors. Furthermore, in the presence of Pertussis toxin, the depression of glutamate release mediated by KAR activation was not present, invoking the participation of a G_i/o protein in this modulation. Finally, the KAR-mediated depression of glutamate release was not suppressed by treatments that affect calcium entry trough voltage-dependent calcium channels or calcium release from intracellular stores. We conclude that KARs present at these synapses mediate a depression of glutamate release through a mechanism that involves the activation of G protein and protein kinase A.

Metabotropic actions of kainate receptors modulating glutamate release

Presynaptic kainate (KA) receptors (KARs) modulate GABA and glutamate release in the central nervous system of mammals. While some of the actions of KARs are ionotropic, metabotropic actions for these receptors have also been seen to modulate both GABA and glutamate release. In general, presynaptic KARs modulate glutamate release through their metabotropic actions in a biphasic manner, with low KA concentrations producing an increase in glutamate release and higher concentrations of KA driving weaker release of this neurotransmitter. Different molecular mechanisms are involved in this modulation of glutamate release, with a G-protein independent, Ca²⁺-calmodulin adenylate cyclase (AC) and protein kinase A (PKA) dependent mechanism facilitating glutamate release, and a G-protein, AC and PKA dependent mechanism mediating the decrease in neurotransmitter release. Here, we describe the events underlying the KAR modulation of glutamatergic transmission in different brain regions, addressing the possible functions of this modulation and proposing future research lines in this field.

Regulation and dysregulation of neuronal circuits by KARs

Kainate receptors (KARs) constitute a family of ionotropic glutamate receptors (iGluRs) with distinct physiological roles in synapses and neuronal circuits. Despite structural and biophysical commonalities with the other iGluRs, AMPA receptors and NMDA receptors, their role as post-synaptic receptors involved in shaping EPSCs to transmit signals across synapses is limited to a small number of synapses. On the other hand KARs regulate presynaptic release mechanisms and control ion channels and signaling pathways through non-canonical metabotropic actions. We review how these different KAR-dependent mechanisms concur to regulate the activity and plasticity of neuronal circuits in physiological conditions of activation of KARs by endogenous glutamate (as opposed to pharmacological activation by exogenous agonists). KARs have been implicated in neurological disorders, based on genetic association and on physiopathological studies. A well described example relates to temporal lobe epilepsy for which the aberrant recruitment of KARs at recurrent mossy fiber synapses takes part in epileptogenic neuronal activity. In conclusion, KARs certainly represent an underestimated actor in the regulation of neuronal circuits, and a potential therapeutic target awaiting more selective and efficient genetic tools and/or ligands.

Kainate receptors: from synaptic activity to disease

Kainate receptors (KARs) are glutamate receptors that participate in the postsynaptic transmission of information and in the control of neuronal excitability, as well as presynaptically modulating the release of the neurotransmitters GABA and glutamate. These modulatory effects, general follow a biphasic pattern, with low KA concentrations provoking an increase in GABA and glutamate release, and higher concentrations mediating a decrease in the release of these neurotransmitters. In addition, KARs are involved in different forms of long- and short-term plasticity. Importantly, altered activity of these receptors has been implicated in different central nervous system diseases and disturbances. Here, we describe the pre- and postsynaptic actions of KARs, and the possible role of these receptors in disease, a field that has seen significant progress in recent years.

Kainate receptors in the developing neuronal networks

Kainate receptors (KARs) are highly expressed in the immature brain and have unique developmentally regulated functions that may be important in linking neuronal activity to morphogenesis during activity-dependent fine-tuning of the synaptic connectivity. Altered expression of KARs in the developing neural network leads to changes in glutamatergic connectivity and network excitability, which may lead to long-lasting changes in behaviorally relevant circuitries in the brain. Here, we summarize the current knowledge on physiological and morphogenic functions described for different types of KARs at immature neural circuitries, focusing on their roles in modulating synaptic transmission and plasticity as well as circuit maturation in the rodent hippocampus and amygdala. Finally, we discuss the emerging evidence suggesting that malfunction of KARs in the immature brain may contribute to the pathophysiology underlying developmentally originating neurological disorders.

Potential mechanism of GABA secretion in response to the activation of GluK1-containing kainate receptors

Hippocampal GABAergic neurons are subdivided into more than 20 subtypes that are distinguished by features and functions. We have previously described the subpopulation of GABAergic neurons, which can be identified in hippocampal cell culture by the calcium response to the application of domoic acid (DoA), an agonist of kainate receptors (KARs). Here, we investigate the features of DoA-sensitive neurons and their GABA release mechanism in response to KARs activation. We demonstrate that DoA-sensitive GABAergic neurons express GluK1-containing KARs because ATPA, a selective agonist of GluK1-containing receptors, induces the calcium response exclusively in these GABAergic neurons. Our experiments also show that NASPM, previously considered a selective antagonist of calcium-permeable AMPARs, blocks calcium-permeable KARs. We established using NASPM that GluK1-containing receptors of the studied population of GABAergic neurons are calcium-permeable, and their activation is required for GABA release, at least in particular synapses. Notably, GABA release occurs even in the presence of tetrodotoxin, indicating that propagation of the depolarizing stimulus is not required for GABA release in this case. Thus, our data demonstrate that the activation of GluK1-containing calcium-permeable KARs mediates the GABA release by the studied subpopulation of GABAergic neurons.

The auxiliary glutamate receptor subunit dSol-1 promotes presynaptic neurotransmitter release and homeostatic potentiation

Presynaptic glutamate receptors (GluRs) modulate neurotransmitter release and are physiological targets for regulation during various forms of plasticity. Although much is known about the auxiliary subunits associated with postsynaptic GluRs, far less is understood about presynaptic auxiliary GluR subunits and their functions. At the Drosophila neuromuscular junction, a presynaptic GluR, DKaiR1D, localizes near active zones and operates as an autoreceptor to tune baseline transmission and enhance presynaptic neurotransmitter release in response to diminished postsynaptic GluR functionality, a process referred to as presynaptic homeostatic potentiation (PHP). Here, we identify an auxiliary subunit that collaborates with DKaiR1D to promote these synaptic functions. This subunit, dSol-1, is the homolog of the Caenorhabditis elegans CUB (Complement C1r/C1s, Uegf, Bmp1) domain protein Sol-1. We find that dSol-1 functions in neurons to facilitate baseline neurotransmission and to enable PHP expression, properties shared with DKaiR1D Intriguingly, presynaptic overexpression of dSol-1 is sufficient to enhance neurotransmitter release through a DKaiR1D-dependent mechanism. Furthermore, dSol-1 is necessary to rapidly increase the abundance of DKaiR1D receptors near active zones during homeostatic signaling. Together with recent work showing the CUB domain protein Neto2 is necessary for the homeostatic modulation of postsynaptic GluRs in mammals, our data demonstrate that dSol-1 is required for the homeostatic regulation of presynaptic GluRs. Thus, we propose that CUB domain proteins are fundamental homeostatic modulators of GluRs on both sides of the synapse.

Differential Effect on Hippocampal Synaptic Facilitation by the Presynaptic Protein Mover

Neurotransmitter release relies on an evolutionarily conserved presynaptic machinery. Nonetheless, some proteins occur in certain species and synapses, and are absent in others, indicating that they may have modulatory roles. How such proteins expand the power or versatility of the core release machinery is unclear. The presynaptic protein Mover/TPRGL/SVAP30 is heterogeneously expressed among synapses of the rodent brain, suggesting that it may add special functions to subtypes of presynaptic terminals. Mover is a synaptic vesicle-attached phosphoprotein that binds to Calmodulin and the active zone scaffolding protein Bassoon. Here we use a Mover knockout mouse line to investigate the role of Mover in the hippocampal mossy fiber (MF) to CA3 pyramidal cell synapse and Schaffer collateral to CA1. While Schaffer collateral synapses were unchanged by the knockout, the MFs showed strongly increased facilitation. The effect of Mover knockout in facilitation was both calcium- and age-dependent, having a stronger effect at higher calcium concentrations and in younger animals. Increasing cyclic adenosine monophosphate (cAMP) levels by forskolin equally potentiated both wildtype and knockout MF synapses, but occluded the increased facilitation observed in the knockout. These discoveries suggest that Mover has distinct roles at different synapses. At MF terminals, it acts to constrain the extent of presynaptic facilitation.

Kainate Receptor-Mediated Depression of Glutamate Release Involves Protein Kinase A in the Cerebellum

Kainate (KA) receptors (KAR) have important modulatory roles of synaptic transmission. In the cerebellum, the action mechanisms of KAR-mediated glutamatergic depression are unknown. We studied these mechanisms by recording evoked excitatory postsynaptic currents (eEPSCs) from cerebellar slices using the whole-cell configuration of the patch-clamp technique. We observed that 3 μM KA decreased the amplitude of eEPSCs and increased the number of failures at the synapses established between parallel fibers (PF) and Purkinje neurons, and the effect was antagonized by NBQX under the condition where AMPA receptors were previously blocked. The inhibition of protein kinase A (PKA) suppressed the effect of KAR activation on eEPSC, and effect was not prevented by protein kinase C inhibitors. Furthermore, in the presence of Pertussis toxin, the depression of glutamate release mediated by KAR activation was prevented, invoking the participation of a G_i/o protein in this modulation. Finally, the KAR-mediated depression of glutamate release was not prevented by blocking calcium-permeable KARs or by treatments that affect calcium release from intracellular stores. We conclude that KARs present at these synapses mediate an inhibition of glutamate release through a mechanism that involves the activation of G-protein and protein kinase A.

Kainate Receptors: Role in Epilepsy

Kainate (KA) is a potent neurotoxin that has been widely used experimentally to induce acute brain seizures and, after repetitive treatments, as a chronic model of temporal lobe epilepsy (TLE), with similar features to those observed in human patients with TLE. However, whether KA activates KA receptors (KARs) as an agonist to mediate the induction of acute seizures and/or the chronic phase of epilepsy, or whether epileptogenic effects of the neurotoxin are indirect and/or mediated by other types of receptors, has yet to be satisfactorily elucidated. Positing a direct involvement of KARs in acute seizures induction, as well as a direct pathophysiological role of KARs in the chronic phase of TLE, recent studies have examined the specific subunit compositions of KARs that might underly epileptogenesis. In the present mini-review, we discuss the use of KA as a convulsant in the experimental models of acute seizures of TLE, and consider the involvement of KARs, their subunit composition and the mode of action in KAR-mediated epilepsy. In acute models, evidence points to epileptogenesis being precipitated by an overall depression of interneuron GABAergic transmission mediated by GluK1 containing KARs. On glutamatergic principal cell in the hippocampus, GluK2-containing KARs regulate post-synaptic excitability and susceptibility to KA-mediated epileptogenesis. In chronic models, a role GluK2-containing KARs in the hippocampal CA3 region provokes limbic seizures. Also observed in the hippocampus, is a ‘reactive plasticity’, where MF sprouting is seen with target granule cells at aberrant synapses recruiting de novo GluR2/GluR5 heteromeric KARs. Finally, in human epilepsy and animal models, astrocytic expression of GluK1, 2, 4, and 5 is reported.

Cerebellar Kainate Receptor-Mediated Facilitation of Glutamate Release Requires Ca2+-Calmodulin and PKA

We elucidated the mechanisms underlying the kainate receptor (KAR)-mediated facilitatory modulation of synaptic transmission in the cerebellum. In cerebellar slices, KA (3 μM) increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) at synapses between axon terminals of parallel fibers (PF) and Purkinje neurons. KA-mediated facilitation was antagonized by NBQX under condition where AMPA receptors were previously antagonized. Inhibition of protein kinase A (PKA) suppressed the effect of KA on glutamate release, which was also obviated by the prior stimulation of adenylyl cyclase (AC). KAR-mediated facilitation of synaptic transmission was prevented by blocking Ca²⁺ permeant KARs using philanthotoxin. Furthermore, depletion of intracellular Ca²⁺ stores by thapsigargin, or inhibition of Ca²⁺-induced Ca²⁺-release by ryanodine, abrogated the synaptic facilitation by KA. Thus, the KA-mediated modulation was conditional on extracellular Ca²⁺ entry through Ca²⁺-permeable KARs, as well as and mobilization of Ca²⁺ from intracellular stores. Finally, KAR-mediated facilitation was sensitive to calmodulin inhibitors, W-7 and calmidazolium, indicating that the increased cytosolic [Ca²⁺] sustaining KAR-mediated facilitation of synaptic transmission operates through a downstream Ca²⁺/calmodulin coupling. We conclude that, at cerebellar parallel fiber-Purkinje cell synapses, presynaptic KARs mediate glutamate release facilitation, and thereby enhance synaptic transmission through Ca²⁺-calmodulin dependent activation of adenylyl cyclase/cAMP/protein kinase A signaling.

Non-canonical Mechanisms of Presynaptic Kainate Receptors Controlling Glutamate Release

A metabotropic modus operandi for kainate receptors (KARs) was first discovered in 1998 modulating GABA release. These receptors have been also found to modulate glutamate release at different synapses in several brain regions. Mechanistically, a general biphasic mechanism for modulating glutamate release by presynaptic KARs with metabotropic actions has emerged, with low KA concentrations invoking an increase in glutamate release, whereas higher concentrations of KA mediate a decrease in the release of this neurotransmitter. The molecular mechanisms underpinning the opposite modulation of glutamate release are distinct, with a G-protein-independent, adenylate cyclase (AC)- and protein kinase A (PKA)-dependent mechanism mediating the facilitation of glutamate release, while a G-protein dependent mechanism (with or without protein kinase recruitment) is involved in the decrease of neurotransmitter release. In the present review, we revisit the mechanisms underlying the non-canonical modus operandi of KARs effecting the bimodal control of glutamatergic transmission in different brain regions, and address the possible functions that this modulation may support.

A Presynaptic Glutamate Receptor Subunit Confers Robustness to Neurotransmission and Homeostatic Potentiation

Homeostatic signaling systems are thought to interface with other forms of plasticity to ensure flexible yet stable levels of neurotransmission. The role of neurotransmitter receptors in this process, beyond mediating neurotransmission itself, is not known. Through a forward genetic screen, we have identified the Drosophila kainate-type ionotropic glutamate receptor subunit DKaiR1D to be required for the retrograde, homeostatic potentiation of synaptic strength. DKaiR1D is necessary in presynaptic motor neurons, localized near active zones, and confers robustness to the calcium sensitivity of baseline synaptic transmission. Acute pharmacological blockade of DKaiR1D disrupts homeostatic plasticity, indicating that this receptor is required for the expression of this process, distinct from developmental roles. Finally, we demonstrate that calcium permeability through DKaiR1D is necessary for baseline synaptic transmission, but not for homeostatic signaling. We propose that DKaiR1D is a glutamate autoreceptor that promotes robustness to synaptic strength and plasticity with active zone specificity.

Monitoring Ca2+ elevations in individual astrocytes upon local release of amyloid beta in acute brain slices

The pathogenesis of Alzheimer's disease (AD) is thought to involve acute neurotoxic effects exerted by oligomeric forms of amyloid-β 1-42 (Aβ). Application of Aβ oligomers in physiological concentrations have been shown to transiently elevate internal Ca2+ in cultured astroglia. While the cellular machinery involved has been extensively explored, to what degree this important signalling cascade occurs in organised brain tissue has remained unclear. Here we adapted two-photon excitation microscopy and calibrated time-resolved imaging (FLIM), coupled with patch-clamp electrophysiology, to monitor Ca2+ concentration ([Ca2+]) inside individual astrocytes and principal neurons in acute brain slices. Inside the slice tissue local micro-ejection of Aβ in sub-micromolar concentrations triggered prominent [Ca2+] elevations in an adjacent astrocyte translated as an approximately two-fold increase (averaged over ∼5min) in basal [Ca2+]. This elevation did not spread to neighbouring cells and appeared comparable in amplitude with commonly documented spontaneous [Ca2+] rises in astroglia. Principal nerve cells (pyramidal neurons) also showed Ca2+ sensitivity, albeit to a lesser degree. These observations shed light on the extent and dynamics of the acute physiological effects of Aβ on brain cells in situ, in the context of AD.

Excitatory Synaptic Input to Hilar Mossy Cells under Basal and Hyperexcitable Conditions

Hilar mossy cells (HMCs) in the hippocampus receive glutamatergic input from dentate granule cells (DGCs) via mossy fibers (MFs) and back-projections from CA3 pyramidal neuron collateral axons. Many fundamental features of these excitatory synapses have not been characterized in detail despite their potential relevance to hippocampal cognitive processing and epilepsy-induced adaptations in circuit excitability. In this study, we compared pre- and postsynaptic parameters between MF and CA3 inputs to HMCs in young and adult mice of either sex and determined the relative contributions of the respective excitatory inputs during in vitro and in vivo models of hippocampal hyperexcitability. The two types of excitatory synapses both exhibited a modest degree of short-term plasticity, with MF inputs to HMCs exhibiting lower paired-pulse (PP) and frequency facilitation than was described previously for MF–CA3 pyramidal cell synapses. MF–HMC synapses exhibited unitary excitatory synaptic currents (EPSCs) of larger amplitude, contained postsynaptic kainate receptors, and had a lower NMDA/AMPA receptor ratio compared to CA3–HMC synapses. Pharmacological induction of hippocampal hyperexcitability in vitro transformed the abundant but relatively weak CA3–HMC connections to very large amplitude spontaneous bursts of compound EPSCs (cEPSCs) in young mice (∼P20) and, to a lesser degree, in adult mice (∼P70). CA3–HMC cEPSCs were also observed in slices prepared from mice with spontaneous seizures several weeks after intrahippocampal kainate injection. Strong excitation of HMCs during synchronous CA3 activity represents an avenue of significant excitatory network generation back to DGCs and might be important in generating epileptic networks.

Calcium-engaged Mechanisms of Nongenomic Action of Neurosteroids

BACKGROUND

Neurosteroids form the unique group because of their dual mechanism of action. Classically, they bind to specific intracellular and/or nuclear receptors, and next modify genes transcription. Another mode of action is linked with the rapid effects induced at the plasma membrane level within seconds or milliseconds. The key molecules in neurotransmission are calcium ions, thereby we focus on the recent advances in understanding of complex signaling crosstalk between action of neurosteroids and calcium-engaged events.

METHODS

Short-time effects of neurosteroids action have been reviewed for GABAA receptor complex, glycine receptor, NMDA receptor, AMPA receptor, G protein-coupled receptors and sigma-1 receptor, as well as for several membrane ion channels and plasma membrane enzymes, based on available published research.

RESULTS

The physiological relevance of neurosteroids results from the fact that they can be synthesized and accumulated in the central nervous system, independently from peripheral sources. Fast action of neurosteroids is a prerequisite for genomic effects and these early events can significantly modify intracellular downstream signaling pathways. Since they may exert either positive or negative effects on calcium homeostasis, their role in monitoring of spatio-temporal Ca2+ dynamics, and subsequently, Ca2+-dependent physiological processes or initiation of pathological events, is evident.

CONCLUSION

Neurosteroids and calcium appear to be the integrated elements of signaling systems in neuronal cells under physiological and pathological conditions. A better understanding of cellular and molecular mechanisms of nongenomic, calcium-engaged neurosteroids action could open new ways for therapeutic interventions aimed to restore neuronal function in many neurological and psychiatric diseases.

Extended Synaptotagmin Localizes to Presynaptic ER and Promotes Neurotransmission and Synaptic Growth in Drosophila

The endoplasmic reticulum (ER) is an extensive organelle in neurons with important roles at synapses including the regulation of cytosolic Ca²⁺, neurotransmission, lipid metabolism, and membrane trafficking. Despite intriguing evidence for these crucial functions, how the presynaptic ER influences synaptic physiology remains enigmatic. To gain insight into this question, we have generated and characterized mutations in the single extended synaptotagmin (Esyt) ortholog in Drosophila melanogaster Esyts are evolutionarily conserved ER proteins with Ca²⁺-sensing domains that have recently been shown to orchestrate membrane tethering and lipid exchange between the ER and plasma membrane. We first demonstrate that Esyt localizes to presynaptic ER structures at the neuromuscular junction. Next, we show that synaptic growth, structure, and homeostatic plasticity are surprisingly unperturbed at synapses lacking Esyt expression. However, neurotransmission is reduced in Esyt mutants, consistent with a presynaptic role in promoting neurotransmitter release. Finally, neuronal overexpression of Esyt enhances synaptic growth and the sustainment of the vesicle pool during intense activity, suggesting that increased Esyt levels may modulate the membrane trafficking and/or resting Ca²⁺ pathways that control synapse extension. Thus, we identify Esyt as a presynaptic ER protein that can promote neurotransmission and synaptic growth, revealing the first in vivo neuronal functions of this conserved gene family.

Hippocampal circuit dysfunction in the Tc1 mouse model of Down syndrome

Hippocampal pathology is likely to contribute to cognitive disability in Down syndrome, yet the neural network basis of this pathology and its contributions to different facets of cognitive impairment remain unclear. Here we report dysfunctional connectivity between dentate gyrus and CA3 networks in the transchromosomic Tc1 mouse model of Down syndrome, demonstrating that ultrastructural abnormalities and impaired short-term plasticity at dentate gyrus-CA3 excitatory synapses culminate in impaired coding of new spatial information in CA3 and CA1 and disrupted behavior in vivo. These results highlight the vulnerability of dentate gyrus-CA3 networks to aberrant human chromosome 21 gene expression and delineate hippocampal circuit abnormalities likely to contribute to distinct cognitive phenotypes in Down syndrome.

Subcellular localization of K+ channels in mammalian brain neurons: remarkable precision in the midst of extraordinary complexity

Potassium channels (KChs) are the most diverse ion channels, in part due to extensive combinatorial assembly of a large number of principal and auxiliary subunits into an assortment of KCh complexes. Their structural and functional diversity allows KChs to play diverse roles in neuronal function. Localization of KChs within specialized neuronal compartments defines their physiological role and also fundamentally impacts their activity, due to localized exposure to diverse cellular determinants of channel function. Recent studies in mammalian brain reveal an exquisite refinement of KCh subcellular localization. This includes axonal KChs at the initial segment, and near/within nodes of Ranvier and presynaptic terminals, dendritic KChs found at sites reflecting specific synaptic input, and KChs defining novel neuronal compartments. Painting the remarkable diversity of KChs onto the complex architecture of mammalian neurons creates an elegant picture of electrical signal processing underlying the sophisticated function of individual neuronal compartments, and ultimately neurotransmission and behavior.

Interplay between synchronization of multivesicular release and recruitment of additional release sites support short-term facilitation at hippocampal mossy fiber to CA3 pyramidal cells synapses

Synaptic short-term plasticity is a key regulator of neuronal communication and is controlled via various mechanisms. A well established property of mossy fiber to CA3 pyramidal cell synapses is the extensive short-term facilitation during high-frequency bursts. We investigated the mechanisms governing facilitation using a combination of whole-cell electrophysiological recordings, electrical minimal stimulation, and random-access two-photon microscopy in acute mouse hippocampal slices. Two distinct presynaptic mechanisms were involved in short-term facilitation, with their relative contribution dependent on extracellular calcium concentration. The synchronization of multivesicular release was observed during trains of facilitating EPSCs recorded in 1.2 mM external Ca(2+) ([Ca(2+)]e). Indeed, covariance analysis revealed a gradual augmentation in quantal size during trains of EPSCs, and application of the low-affinity glutamate receptor antagonist γ-D-glutamylglycine showed an increase in cleft glutamate concentration during paired-pulse stimulation. Whereas synchronization of multivesicular release contributed to the facilitation in 1.2 mM [Ca(2+)]e, variance-mean analysis showed that recruitment of more release sites (N) was likely to account for the larger facilitation observed in 2.5 mM [Ca(2+)]e. Furthermore, this increase in N could be promoted by calcium microdomains of heterogeneous amplitudes observed in single mossy fiber boutons. Our findings suggest that the combination of multivesicular release and the recruitment of additional release sites act together to increase glutamate release during burst activity. This is supported by the compartmentalized spatial profile of calcium elevations in boutons and helps to expand the dynamic range of mossy fibers information transfer.

Kainate receptors in the hippocampus

Kainate receptors (KARs) consist of a family of ionotropic glutamate receptors composed of the combinations of five subunits, GluK1-GluK5. Although KARs display close structural homology with AMPA receptors, they serve quite distinct functions. A great deal of our knowledge of the molecular and functional properties of KARs comes from their study in the hippocampus. This review aims at summarising the functions of KARs in the regulation of the activity of hippocampal synaptic circuits at the adult stage and throughout development. We focus on the variety of roles played by KARs in physiological conditions of activation, at pre- and postsynaptic sites, in different cell types and through either metabotropic or ionotropic actions. Finally, we present some of the few attempts to link the role of KARs in the regulation of local hippocampal circuits to the behavioural functions of the hippocampus in health and diseases.

Information processing and synaptic plasticity at hippocampal mossy fiber terminals

Granule cells of the dentate gyrus receive cortical information and they transform and transmit this code to the CA3 area via their axons, the mossy fibers (MFs). Structural and functional complexity of this network has been extensively studied at various organizational levels. This review is focused on the anatomical and physiological properties of the MF system. We will discuss the mechanism by which dentate granule cells process signals from single action potentials (APs), short bursts and longer stimuli. Various parameters of synaptic interactions at different target cells such as quantal transmission, short- and long-term plasticity (LTP) will be summarized. Different types of synaptic contacts formed by MFs have unique sets of rules for information processing during different rates of granule cell activity. We will investigate the complex interactions between key determinants of information transfer between the dentate gyrus and the CA3 area of the hippocampus.

Kainate receptors in health and disease

Our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology has progressed significantly over the last 30 years. A plethora of studies indicate that kainate receptors are important mediators of the pre- and postsynaptic actions of glutamate, although the mechanisms underlying such effects are still often a topic for discussion. Three clear fields related to their behavior have emerged: there are a number of interacting proteins that pace the properties of kainate receptors; their activity is unconventional since they can also signal through G proteins, behaving like metabotropic receptors; they seem to be linked to some devastating brain diseases. Despite the significant progress in their importance in brain function, kainate receptors remain somewhat puzzling. Here we examine discoveries linking these receptors to physiology and their probable implications in disease, in particular mood disorders, and propose some ideas to obtain a deeper understanding of these intriguing proteins.

Target-cell-specific short-term plasticity in local circuits

Short-term plasticity (STP) denotes changes in synaptic strength that last up to tens of seconds. It is generally thought that STP impacts information transfer across synaptic connections and may thereby provide neurons with, for example, the ability to detect input coherence, to maintain stability and to promote synchronization. STP is due to a combination of mechanisms, including vesicle depletion and calcium accumulation in synaptic terminals. Different forms of STP exist, depending on many factors, including synapse type. Recent evidence shows that synapse dependence holds true even for connections that originate from a single presynaptic cell, which implies that postsynaptic target cell type can determine synaptic short-term dynamics. This arrangement is surprising, since STP itself is chiefly due to presynaptic mechanisms. Target-specific synaptic dynamics in addition imply that STP is not a bug resulting from synapses fatiguing when driven too hard, but rather a feature that is selectively implemented in the brain for specific functional purposes. As an example, target-specific STP results in sequential somatic and dendritic inhibition in neocortical and hippocampal excitatory cells during high-frequency firing. Recent studies also show that the Elfn1 gene specifically controls STP at some synapse types. In addition, presynaptic NMDA receptors have been implicated in synapse-specific control of synaptic dynamics during high-frequency activity. We argue that synapse-specific STP deserves considerable further study, both experimentally and theoretically, since its function is not well known. We propose that synapse-specific STP has to be understood in the context of the local circuit, which requires combining different scientific disciplines ranging from molecular biology through electrophysiology to computer modeling.

Pre-synaptic kainate receptor-mediated facilitation of glutamate release involves PKA and Ca(2+) -calmodulin at thalamocortical synapses

We have investigated the mechanisms underlying the facilitatory modulation mediated by kainate receptor (KAR) activation in the cortex, using isolated nerve terminals (synaptosomes) and slice preparations. In cortical nerve terminals, kainate (KA, 100 μM) produced an increase in 4-aminopyridine (4-AP)-evoked glutamate release. In thalamocortical slices, KA (1 μM) produced an increase in the amplitude of evoked excitatory post-synaptic currents (eEPSCs) at synapses established between thalamic axon terminals from the ventrobasal nucleus onto stellate neurons of L4 of the somatosensory cortex. In both, synaptosomes and slices, the effect of KA was antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted after pre-treatment with a cocktail of antagonists of other receptors whose activation could potentially have produced facilitation of release indirectly. Mechanistically, the observed effects of KA appear to be congruent in synaptosomal and slice preparations. Thus, the facilitation by KA of synaptosomal glutamate release and thalamocortical synaptic transmission were suppressed by the inhibition of protein kinase A and occluded by the stimulation of adenylyl cyclase. Dissecting this G-protein-independent regulation further in thalamocortical slices, the KAR-mediated facilitation of synaptic transmission was found to be sensitive to the block of Ca(2+) permeant KARs by philanthotoxin. Intriguingly, the synaptic facilitation was abrogated by depletion of intracellular Ca(2+) stores by thapsigargin, or inhibition of Ca(2+) -induced Ca(2+) -release by ryanodine. Thus, the KA-mediated modulation was contingent on both Ca(2+) entry through Ca(2+) -permeable KARs and liberation of intracellular Ca(2+) stores. Finally, sensitivity to W-7 indicated that the increased cytosolic [Ca(2+) ] underpinning KAR-mediated regulation of synaptic transmission at thalamocortical synapses, requires downstream activation of calmodulin. We conclude that neocortical pre-synaptic KARs mediate the facilitation of glutamate release and synaptic transmission by a Ca(2+) -calmodulin dependent activation of an adenylyl cyclase/cAMP/protein kinase A signalling cascade, independent of G-protein involvement.

Testosterone depletion in adult male rats increases mossy fiber transmission, LTP, and sprouting in area CA3 of hippocampus

Androgens have dramatic effects on neuronal structure and function in hippocampus. However, androgen depletion does not always lead to hippocampal impairment. To address this apparent paradox, we evaluated the hippocampus of adult male rats after gonadectomy (Gdx) or sham surgery. Surprisingly, Gdx rats showed increased synaptic transmission and long-term potentiation of the mossy fiber (MF) pathway. Gdx rats also exhibited increased excitability and MF sprouting. We then addressed the possible underlying mechanisms and found that Gdx induced a long-lasting upregulation of MF BDNF immunoreactivity. Antagonism of Trk receptors, which bind neurotrophins, such as BDNF, reversed the increase in MF transmission, excitability, and long-term potentiation in Gdx rats, but there were no effects of Trk antagonism in sham controls. To determine which androgens were responsible, the effects of testosterone metabolites DHT and 5α-androstane-3α,17β-diol were examined. Exposure of slices to 50 nm DHT decreased the effects of Gdx on MF transmission, but 50 nm 5α-androstane-3α,17β-diol had no effect. Remarkably, there was no effect of DHT in control males. The data suggest that a Trk- and androgen receptor-sensitive form of MF transmission and synaptic plasticity emerges after Gdx. We suggest that androgens may normally be important in area CA3 to prevent hyperexcitability and aberrant axon outgrowth but limit MF synaptic transmission and some forms of plasticity. The results also suggest a potential explanation for the maintenance of hippocampal-dependent cognitive function after androgen depletion: a reduction in androgens may lead to compensatory upregulation of MF transmission and plasticity.

Presynaptic kainate receptor-mediated bidirectional modulatory actions: mechanisms

Kainate receptors (KARs) are members of the glutamate receptor family, which also includes two other ionotropic subtypes, i.e. NMDA- and AMPA-type receptors, and types I, II and III metabotropic glutamate receptors. KARs mediate synaptic transmission postynaptically through their ionotropic capacity, while presynaptically, they modulate the release of both GABA and glutamate through operationally diverse modus operandi. At hippocampal mossy fiber (MF)-CA3 synapses, KARs have a biphasic effect on glutamate release, such that, depending on the extent of their activation, a facilitation or depression of glutamate release can be observed. This modulation is posited to contribute to important roles of KARs in short- and long-term plasticity. Elucidation of the modes of action of KARs in their depression and facilitation of glutamate release is beginning to gather impetus. Here we will focus on the cellular mechanisms involved in the modulation of glutamate release by presynaptic KAR activation at MF-CA3 synapses, a field that has seen significant progress in recent years.

Cannabinoid- and lysophosphatidylinositol-sensitive receptor GPR55 boosts neurotransmitter release at central synapses

G protein-coupled receptor (GPR) 55 is sensitive to certain cannabinoids, it is expressed in the brain and, in cell cultures, it triggers mobilization of intracellular Ca(2+). However, the adaptive neurobiological significance of GPR55 remains unknown. Here, we use acute hippocampal slices and combine two-photon excitation Ca(2+) imaging in presynaptic axonal boutons with optical quantal analysis in postsynaptic dendritic spines to find that GPR55 activation transiently increases release probability at individual CA3-CA1 synapses. The underlying mechanism involves Ca(2+) release from presynaptic Ca(2+) stores, whereas postsynaptic stores (activated by spot-uncaging of inositol 1,4,5-trisphosphate) remain unaffected by GPR55 agonists. These effects are abolished by genetic deletion of GPR55 or by the GPR55 antagonist cannabidiol, a constituent of Cannabis sativa. GPR55 shows colocalization with synaptic vesicle protein vesicular glutamate transporter 1 in stratum radiatum. Short-term potentiation of CA3-CA1 transmission after a short train of stimuli reveals a presynaptic, Ca(2+) store-dependent component sensitive to cannabidiol. The underlying cascade involves synthesis of phospholipids, likely in the presynaptic cell, but not the endocannabinoids 2-arachidonoylglycerol or anandamide. Our results thus unveil a signaling role for GPR55 in synaptic circuits of the brain.

Kainate receptors: multiple roles in neuronal plasticity

Ionotropic glutamate receptors of the N-methyl-d-aspartate (NMDA)- and AMPA-type, as well as metabotropic glutamate receptors have been extensively invoked in plasticity. Until relatively recently, however, kainate-type receptors (KARs) had been the most elusive to study because of the lack of appropriate pharmacological tools to specifically address their roles. With the development of selective glutamate receptor antagonists, and knockout mice with specific KAR subunits deleted, the functions of KARs in neuromodulation and synaptic transmission, together with their involvement in some types of plasticity, have been extensively probed in the central nervous system. In this review, we summarize the findings related to the roles of KARs in short- and long-term forms of plasticity, primarily in the hippocampus, where KAR function and synaptic plasticity have received avid attention.

Presynaptic kainate receptor-mediated facilitation of glutamate release involves Ca2+-calmodulin and PKA in cerebrocortical synaptosomes

We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors (KARs) in the cortex using isolated nerve terminals (synaptosomes). Kainate (KA) produced an increase on glutamate release at 100 μM. The effect of KA was antagonized by NBQX (with AMPA receptors blocked by GYKI53655). This facilitation was suppressed by the inhibition of PKA activation by Rp-Br-cAMP and H-89. Moreover, the facilitation of glutamate release mediated by KAR requires the mobilization of intrasynaptosomal Ca(2+) stores and the formation of a Ca(2+)-calmodulin complex. We conclude that KARs present on presynaptic terminals in the neocortex mediate the facilitation of glutamate release through a mechanism involving an increase in cytosolic Ca(2+) to activate a Ca(2+)-calmodulin-AC/cAMP/PKA signaling cascade.

Ionotropic receptors at hippocampal mossy fibers: roles in axonal excitability, synaptic transmission, and plasticity

Dentate granule cells process information from the enthorinal cortex en route to the hippocampus proper. These neurons have a very negative resting membrane potential and are relatively silent in the slice preparation. They are also subject to strong feed-forward inhibition. Their unmyelinated axon or mossy fiber ramifies extensively in the hilus and projects to stratum lucidum where it makes giant en-passant boutons with CA3 pyramidal neurons. There is compelling evidence that mossy fiber boutons express presynaptic GABA_A receptors, which are commonly found in granule cell dendrites. There is also suggestive evidence for the presence of other ionotropic receptors, including glycine, NMDA, and kainate receptors, in mossy fiber boutons. These presynaptic receptors have been proposed to lead to mossy fiber membrane depolarization. How this phenomenon alters the excitability of synaptic boutons, the shape of presynaptic action potentials, Ca²⁺ influx and neurotransmitter release has remained elusive, but high-resolution live imaging of individual varicosities and direct patch-clamp recordings have begun to shed light on these phenomena. Presynaptic GABA_A and kainate receptors have also been reported to facilitate the induction of long-term potentiation at mossy fiber—CA3 synapses. Although mossy fibers are highly specialized, some of the principles emerging at this connection may apply elsewhere in the CNS.

Independent regulation of basal neurotransmitter release efficacy by variable Ca²+ influx and bouton size at small central synapses

Concurrent imaging of vesicular release and calcium dynamics in small presynaptic boutons shows that the fusion probability of readily releasable vesicles is a major determinant of the overall variability in release probability.

The efficacy of action potential evoked neurotransmitter release varies widely even among synapses supplied by the same axon, and the number of release-ready vesicles at each synapse is a major determinant of this heterogeneity. Here we identify a second, equally important, mechanism for release heterogeneity at small hippocampal synapses, the inter-synaptic variation of the exocytosis probability of release-ready vesicles. Using concurrent measurements of vesicular pool sizes, vesicular exocytosis rates, and presynaptic Ca²⁺ dynamics, in the same small hippocampal boutons, we show that the average fusion probability of release-ready vesicles varies among synapses supplied by the same axon with the size of the spike-evoked Ca²⁺ concentration transient. We further show that synapses with a high vesicular release probability exhibit a lower Ca²⁺ cooperativity, arguing that this is a direct consequence of increased Ca²⁺ influx at the active zone. We conclude that variability of neurotransmitter release under basal conditions at small central synapses is accounted for not only by the number of release-ready vesicles, but also by their fusion probabilities, which are set independently of bouton size by variable spike-evoked presynaptic Ca²⁺ influx.

Presynaptic kainate receptor-mediated facilitation of glutamate release involves Ca2+ -calmodulin at mossy fiber-CA3 synapses

Presynaptic kainate receptors (KARs) modulate the release of glutamate at synapses established between mossy fibers (MF) and CA3 pyramidal cells in the hippocampus. The activation of KAR by low, nanomolar, kainate concentrations facilitates glutamate release. KAR-mediated facilitation of glutamate release involves the activation of an adenylate cyclase/cyclic adenosine monophosphate/protein kinase A cascade at MF-CA3 synapses. Here, we studied the mechanisms by which KAR activation produces this facilitation of glutamate release in slices and synaptosomes. We find that the facilitation of glutamate release mediated by KAR activation requires an increase in Ca(2+) levels in the cytosol and the formation of a Ca(2+) -calmodulin complex to activate adenylate cyclase. The increase in cytosolic Ca(2+) underpinning this modulation is achieved, both, by Ca(2+) entering via Ca(2+) -permeable KARs and, by the mobilization of intraterminal Ca(2+) stores. Finally, we find that, congruent with the Ca(2+) -calmodulin support of KAR-mediated facilitation of glutamate release, induction of long-term potentiation at MF-CA3 synapses has an obligate requirement for Ca(2+) -calmodulin activity.

Differences in kainate receptor involvement in hippocampal mossy fibre long-term potentiation depending on slice orientation

Long-term potentiation (LTP) is a well-established experimental model used to investigate the synaptic basis of learning and memory. LTP at mossy fibre - CA3 synapses in the hippocampus is unusual because it is normally N-methyl-d-aspartate (NMDA) receptor-independent. Instead it seems that the trigger for mossy fibre LTP involves kainate receptors (KARs). Although it is generally accepted that pre-synaptic KARs play an essential role in frequency facilitation and LTP, their subunit composition remains a matter of significant controversy. We have reported previously that both frequency facilitation and LTP can be blocked by selective antagonism of GluK1 (formerly GluR5/Glu(K5))-containing KARs, but other groups have failed to reproduce this effect. Moreover, data from receptor knockout and mRNA expression studies argue against a major role of GluK1, supporting a more central role for GluK2 (formerly GluR6/Glu(K6)). A potential reason underlying the controversy in the pharmacological experiments may reside in differences in the preparations used. Here we show differences in pharmacological sensitivity of synaptic plasticity at mossy fibre - CA3 synapses depend critically on slice orientation. In transverse slices, LTP of fEPSPs was invariably resistant to GluK1-selective antagonists whereas in parasagittal slices LTP was consistently blocked by GluK1-selective antagonists. In addition, there were pronounced differences in the magnitude of frequency facilitation and the sensitivity to the mGlu2/3 receptor agonist DCG-IV. Using anterograde labelling of granule cells we show that slices of both orientations possess intact mossy fibres and both large and small presynaptic boutons. Transverse slices have denser fibre tracts but a smaller proportion of giant mossy fibre boutons. These results further demonstrate a considerable heterogeneity in the functional properties of the mossy fibre projection.

5-HT7R/G12 signaling regulates neuronal morphology and function in an age-dependent manner

The common neurotransmitter serotonin controls different aspects of early neuronal differentiation, although the underlying mechanisms are poorly understood. Here we report that activation of the serotonin 5-HT(7) receptor promotes synaptogenesis and enhances synaptic activity in hippocampal neurons at early postnatal stages. An analysis of Gα(12)-deficient mice reveals a critical role of G(12)-protein for 5-HT(7) receptor-mediated effects in neurons. In organotypic preparations from the hippocampus of juvenile mice, stimulation of 5-HT(7)R/G(12) signaling potentiates formation of dendritic spines, increases neuronal excitability, and modulates synaptic plasticity. In contrast, in older neuronal preparations, morphogenetic and synaptogenic effects of 5-HT(7)/G(12) signaling are abolished. Moreover, inhibition of 5-HT(7) receptor had no effect on synaptic plasticity in hippocampus of adult animals. Expression analysis reveals that the production of 5-HT(7) and Gα(12)-proteins in the hippocampus undergoes strong regulation with a pronounced transient increase during early postnatal stages. Thus, regulated expression of 5-HT(7) receptor and Gα(12)-protein may represent a molecular mechanism by which serotonin specifically modulates formation of initial neuronal networks during early postnatal development.

Kainate receptor-induced retrograde inhibition of glutamatergic transmission in vasopressin neurons

Presynaptic kainate receptors (KARs) exert a modulatory action on transmitter release. We here report that applications of agonists of GluK1-containing KARs in the rat supraoptic nucleus has an opposite action on glutamatergic transmission according to the phenotype of the postsynaptic neuron. Whereas glutamate release was facilitated in oxytocin (OT) neurons, it was inhibited in vasopressin (VP) cells. Interestingly, an antagonist of GluK1-containing KARs caused an inhibition of glutamate release in both OT and VP neurons, revealing the existence of tonically activated presynaptic KARs that are positively coupled to transmitter release. We thus postulated that the inhibition of glutamate release observed with exogenous applications of GluK1 agonists on VP neurons could be indirect. In agreement with this hypothesis, we first showed that functional GluK1-containing KARs were present postsynaptically on VP neurons but not on OT cells. We next showed that the inhibitory effect induced by exogenous GluK1 receptor agonist was compromised when BAPTA was added in the recording pipette to buffer intracellular Ca2+ and block the release of a putative retrograde messenger. Under these conditions, GluK1-containing KAR agonist facilitates glutamatergic transmission in VP neurons in a manner similar to that observed for OT neurons and that resulted from the activation of presynaptic GluK1 receptors. GluK1-mediated inhibition of glutamate release in VP neurons was also blocked by a κ-opioid receptor antagonist. These findings suggest that activation of postsynaptic GluK1-containing KARs on VP neurons leads to the release of dynorphin, which in turn acts on presynaptic κ-opioid receptors to inhibit glutamate release.

Astrocytes as regulators of synaptic function: a quest for the Ca2+ master key

The emerging role of astrocytes in neural communication represents a conceptual challenge. In striking contrast to the rapid and highly space- and time-constrained machinery of neuronal spike propagation and synaptic release, astroglia appear slow and imprecise. Although a large body of independent experiments documents active signal exchange between astrocytes and neurons, some genetic models have raised doubts about the major Ca2+ -dependent molecular mechanism routinely associated with release of "gliotransmitters." A limited understanding of astrocytic Ca2+ signaling and the imperfect compatibility between physiology and experimental manipulations seem to have contributed to this conceptual bottleneck. Experimental approaches providing mechanistic insights into the diverse mechanisms of intra-astrocyte Ca2+ signaling on the nanoscale are needed to understand Ca2+ -dependent astrocytic function in vivo. This review highlights limitations and potential advantages of such approaches from the current methodological perspective.

Synergistic interactions between kainate and mGlu receptors regulate bouton Ca signalling and mossy fibre LTP

It is currently unknown why glutamatergic presynaptic terminals express multiple types of glutamate receptors. We have addressed this question by studying both acute and long-term regulation of mossy fibre function in the hippocampus. We find that inhibition of both mGlu₁ and mGlu₅ receptors together can block the induction of mossy fibre LTP. Furthermore, mossy fibre LTP can be induced by the pharmacological activation of either mGlu₁ or mGlu₅ receptors, provided that kainate receptors are also stimulated. Like conventional mossy fibre LTP, chemically-induced mossy fibre LTP (chem-LTPm) depends on Ca²⁺ release from intracellular stores and the activation of PKA. Similar synergistic interactions between mGlu receptors and kainate receptors were observed at the level of Ca²⁺ signalling in individual giant mossy fibre boutons. Thus three distinct glutamate receptors interact, in both an AND and OR gate fashion, to regulate both immediate and long-term presynaptic function in the brain.

Upregulation of KCC2 activity by zinc-mediated neurotransmission via the mZnR/GPR39 receptor

Vesicular Zn(2+) regulates postsynaptic neuronal excitability upon its corelease with glutamate. We previously demonstrated that synaptic Zn(2+) acts via a distinct metabotropic zinc-sensing receptor (mZnR) in neurons to trigger Ca(2+) responses in the hippocampus. Here, we show that physiological activation of mZnR signaling induces enhanced K(+)/Cl(-) cotransporter 2 (KCC2) activity and surface expression. As KCC2 is the major Cl(-) outward transporter in neurons, Zn(2+) also triggers a pronounced hyperpolarizing shift in the GABA(A) reversal potential. Mossy fiber stimulation-dependent upregulation of KCC2 activity is eliminated in slices from Zn(2+) transporter 3-deficient animals, which lack synaptic Zn(2+). Importantly, activity-dependent ZnR signaling and subsequent enhancement of KCC2 activity are also absent in slices from mice lacking the G-protein-coupled receptor GPR39, identifying this protein as the functional neuronal mZnR. Our work elucidates a fundamentally important role for synaptically released Zn(2+) acting as a neurotransmitter signal via activation of a mZnR to increase Cl(-) transport, thereby enhancing inhibitory tone in postsynaptic cells.

Short- and long-term depression at glutamatergic synapses on hippocampal interneurons by group I mGluR activation

Group I metabotropic glutamate receptors (mGluRs) are expressed by many interneurons of the hippocampus. Although they have been implicated in short- and long-term synaptic plasticity of glutamatergic transmission, their roles in modulating transmission to interneurons are incompletely understood. The selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) acutely depressed transmission at synapses in the feed-forward inhibitory pathway made by Schaffer collaterals on interneurons in the rat hippocampal CA1 sub-field. DHPG elicited a qualitatively similar depression at synapses made by pyramidal neuron axon collaterals on interneurons in the feedback circuit in stratum oriens. Selective blockers revealed a link from mGluR1 to reversible, and mGluR5 to long-lasting, depression. The acute DHPG-induced depression was consistently accompanied by an elevation in paired-pulse ratio, implying a presynaptic decrease in release probability. However, it was also attenuated by blocking G-protein and Ca(2+) signalling within the postsynaptic neuron, arguing for a retrograde signalling cascade. The DHPG-evoked depression was unaffected by antagonists of CB1 and GABA(B) receptors but was occluded when presynaptic P/Q-type Ca(2+) channels were blocked. Finally, high-frequency stimulation delivered to an independent conditioning pathway evoked a heterosynaptic reversible depression, which was sensitive to group I mGluR antagonists. Group I mGluRs thus powerfully modulate synaptic excitation of hippocampal interneurons and mediate inter-synaptic cross-talk. This article is part of a Special Issue entitled 'Synaptic Plasticity & Interneurons'.

Hippocampal mossy fiber long-term depression in Grm2/3 double knockout mice

Group II metabotropic glutamate receptors (mGluR2, encoded by Grm2, and mGluR3, encoded by Grm3) are inhibitory autoreceptors that negatively modulate the adenylate cyclase signaling cascade. Within the hippocampus, mGluR2 is believed to play a key role in the induction of long-term depression (LTD) at mossy fiber-CA3 synapses. Here, we used Grm2/3 double knockout (dko) mice to investigate to what extent group II mGluRs are necessary for mossy fiber LTD. Surprisingly, we found that these mice displayed prominent mossy fiber LTD. However, the induction of this form of LTD was sensitive to the external Ca(2+) concentration. Mossy fiber LTD in Grm2/3 dko mice was indistinguishable from that in wild-type mice at 4 mM Ca(2+) , but largely absent at 2 mM external Ca(2+) . Mossy fiber LTD in Grm2/3 dko mice was not blocked by the N-methyl-D-aspartic acid (NMDA) receptor antagonist D-AP5, confirming that the observed response did not reflect NMDA receptor-dependent LTD in contaminating associational-commissural fibers, and enabling us to use the NMDA receptor-mediated EPSC to monitor mossy fiber LTD. Using whole-cell recordings, we demonstrated that LTD of the NMDA receptor-mediated EPSC in Grm2/3 dko mice was not affected by intracellular application of BAPTA and CsF to block postsynaptic Ca(2+) and G-protein-mediated effects. This presynaptic LTD was, however, blocked by the AMPA/kainate receptor antagonist, NBQX. Thus, an activity-dependent, external Ca(2+) concentration-sensitive form of mossy fiber LTD can be induced in Grm2/3 dko mice. Two mGluR antagonists also failed to block mossy fiber LTD under 4 mM conditions in wild-type mice, strengthening the conclusion that group II mGluRs are not obligatory for mossy fiber LTD.

Kainate receptors coming of age: milestones of two decades of research

Two decades have passed since the first report of the cloning of a kainate-type glutamate receptor (KAR) subunit. The intervening years have seen a rapid growth in our understanding of the biophysical properties and function of KARs in the brain. This research has led to an appreciation that KARs play very distinct roles at synapses relative to other members of the glutamate-gated ion channel receptor family, despite structural and functional commonalities. The surprisingly diverse and complex nature of KAR signaling underlies their unique impact upon neuronal networks through their direct and indirect effects on synaptic transmission, and their prominent role in regulating cell excitability. This review pieces together highlights from the two decades of research subsequent to the cloning of the first subunit, and provides an overview of our current understanding of the role of KARs in the CNS and their potential importance to neurological and neuropsychiatric disorders.