Pertussis toxin prevents presynaptic inhibition by kainate receptors of rat hippocampal [(3)H]GABA release

Two Signaling Modes Are Better than One: Flux-Independent Signaling by Ionotropic Glutamate Receptors Is Coming of Age

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamatergic transmission can be mediated by ionotropic glutamate receptors (iGluRs), which mediate rapid synaptic depolarization that can be associated with Ca2+ entry and activity-dependent change in the strength of synaptic transmission, as well as by metabotropic glutamate receptors (mGluRs), which mediate slower postsynaptic responses through the recruitment of second messenger systems. A wealth of evidence reported over the last three decades has shown that this dogmatic subdivision between iGluRs and mGluRs may not reflect the actual physiological signaling mode of the iGluRs, i.e., α-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA) receptors (AMPAR), kainate receptors (KARs), and N-methyl-D-aspartate (NMDA) receptors (NMDARs). Herein, we review the evidence available supporting the notion that the canonical iGluRs can recruit flux-independent signaling pathways not only in neurons, but also in brain astrocytes and cerebrovascular endothelial cells. Understanding the signaling versatility of iGluRs can exert a profound impact on our understanding of glutamatergic synapses. Furthermore, it may shed light on novel neuroprotective strategies against brain disorders.

GluK2 Q/R editing regulates kainate receptor signaling and long-term potentiation of AMPA receptors

Q/R editing of the kainate receptor (KAR) subunit GluK2 radically alters recombinant KAR properties, but the effects on endogenous KARs in vivo remain largely unexplored. Here, we compared GluK2 editing-deficient mice that express ∼95% unedited GluK2(Q) to wild-type counterparts that express ∼85% edited GluK2(R). At mossy fiber-CA3 (MF-CA3) synapses GluK2(Q) mice displayed increased postsynaptic KAR function and KAR-mediated presynaptic facilitation, demonstrating enhanced ionotropic function. Conversely, GluK2(Q) mice exhibited reduced metabotropic KAR function, assessed by KAR-mediated inhibition of slow after-hyperpolarization currents (ISAHP). GluK2(Q) mice also had fewer GluA1-and GluA3-containing AMPA receptors (AMPARs) and reduced postsynaptic AMPAR currents at both MF-CA3 and CA1-Schaffer collateral synapses. Moreover, long-term potentiation of AMPAR-mediated transmission at CA1-Schaffer collateral synapses was reduced in GluK2(Q) mice. These findings suggest that GluK2 Q/R editing influences ionotropic/metabotropic balance of KAR signaling to regulate synaptic expression of AMPARs and plasticity.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

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.

Losing balance: Kainate receptors and psychiatric disorders comorbidities

Cognition and behavior are tightly linked to synaptic function. A growing body of evidence suggests that aberrant neurotransmission, caused by changes in synaptic protein expression levels, may be a major cause underlying different brain disorders. These changes in expression result in abnormal synaptic organization or function, leading to impaired neurotransmission and unbalanced circuit operations. Here, we review the data supporting the involvement of mutations in genes coding for kainate receptor (KAR) subunits in the pathogenesis of psychiatric disorders and Down syndrome (DS). We show that most of these mutations do not affect the biophysical properties or the receptors, but rather alter subunit expression levels. On the basis of reports studying KAR genes mutations in mouse models of autism spectrum disorders and DS, we illustrate how deviations from the physiological regulatory role that these receptors play in neurotransmitter release and plasticity give rise to synaptic alterations that lead to behavioral and cognitive deficits underlying these disorders.

Kainate receptor regulation of synaptic inhibition in the hippocampus

Kainate receptors (KARs) are glutamate-type receptors that mediate both canonical ionotropic currents and non-canonical metabotropic signalling. While KARs are expressed widely throughout the brain, synaptic KAR currents have only been recorded at a limited set of synapses, and the KAR currents that have been recorded are relatively small and slow, which has led to the question, what is the functional significance of KARs? While the KAR current itself is relatively modest, its impact on inhibition in the hippocampus can be profound. In the CA1 region of the hippocampus, presynaptic KAR activation bidirectionally regulates γ-aminobutyric acid (GABA) release in a manner that depends on the glutamate concentration; lower levels of glutamate facilitate GABA release via an ionotropic pathway, while higher levels of glutamate depress GABA release via a metabotropic pathway. Postsynaptic interneuron KAR activation increases spike frequency through an ionotropic current, which in turn can strengthen inhibition. In the CA3 region, postsynaptic KAR activation in pyramidal neurons also strengthens inhibition, but in this case through a metabotropic pathway which regulates the neuronal chloride gradient and hyperpolarizes the reversal potential for GABA (E_GABA ). Taken together, the evidence for KAR-mediated regulation of the strength of inhibition via pre- and postsynaptic mechanisms provides compelling evidence that KARs are ideally positioned to regulate excitation-inhibition balance - through sensing the excitatory tone and concomitantly tuning the strength of inhibition.

Kainate Receptors, Homeostatic Gatekeepers of Synaptic Plasticity

Extensive research over the past decades has characterized multiple forms of synaptic plasticity, identifying them as key processes that allow the brain to operate in a dynamic manner. Within the wide variety of synaptic plasticity modulators, kainate receptors are receiving increasing attention, given their diversity of signaling mechanisms and cellular expression profile. Here, we summarize the experimental evidence about the involvement of kainate receptor signaling in the regulation of short- and long-term plasticity, from the perspective of the regulation of neurotransmitter release. In light of this evidence, we propose that kainate receptors may be considered homeostatic modulators of neurotransmitter release, able to bidirectionally regulate plasticity depending on the functional history of the synapse.

Non-canonical Signaling, the Hidden Life of Ligand-Gated Ion Channels

Neurotransmitter receptors are responsible for the transfer of information across the synapse. While ionotropic receptors form ion channels and mediate rapid membrane depolarization, so-called metabotropic receptors exert their action though slower, less direct intracellular signaling pathways. Glutamate, GABA, and acetylcholine can activate both ionotropic and metabotropic receptors, yet the distinction between these "canonical" signaling systems has become less clear since ionotropic receptors were proposed to also activate second messenger systems, defining a "non-canonical" signaling pathway. How these alternative pathways affect neuronal circuit activity is not well understood, and their influence could be more significant than previously anticipated. In this review, we examine the evidence available that supports the existence of parallel and unsuspected signaling pathways used by ionotropic neurotransmitter receptors.

A proteomic analysis reveals the interaction of GluK1 ionotropic kainate receptor subunits with Go proteins

Kainate receptors (KARs) are found ubiquitously in the CNS and are present presynaptically and postsynaptically regulating synaptic transmission and excitability. Functional studies have proven that KARs act as ion channels as well as potentially activating G-proteins, thus indicating the existance of a dual signaling system for KARs. Nevertheless, it is not clear how these ion channels activate G-proteins and which of the KAR subunits is involved. Here we performed a proteomic analysis to define proteins that interact with the C-terminal domain of GluK1 and we identified a variety of proteins with many different functions, including a Go α subunit. These interactions were verified through distinct in vitro and in vivo assays, and the activation of the Go protein by GluK1 was validated in bioluminescence resonance energy transfer experiments, while the specificity of this association was confirmed in GluK1-deficient mice. These data reveal components of the KAR interactome, and they show that GluK1 and Go proteins are natural partners, accounting for the metabotropic effects of KARs.

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.

Metabotropic actions of kainate receptors in dorsal root ganglion cells

Kainate receptors are widely distributed in the CNS, but also in the PNS. Dorsal root ganglia are enriched in this subtype of glutamate ionotropic receptors. In addition to their activity as ligand-gated ion channels, kainate receptors exhibit other properties already characterized in other systems, such as hippocampus, i.e., their ability to induce a metabotropic cascade signalling, through G-protein and PKC activation. With a very similar actuation mechanism as formerly described in the CNS, kainate receptors in the DRG also present other differentiated features, such as the Ca(2+) channel blockade and a self-regulation property. The peculiarity of these neurons has served to progress the study of kainate receptors. Nevertheless, many other physiological functions of these receptors remain unclear, as does the related molecular nature of the metabotropic cascade and the involvement of this signalling pathway with sensory transmission of pain.

Pharmacological activation of kainate receptors drives endocannabinoid mobilization

Activation of both presynaptic metabotropic cannabinoid type 1 receptors (CB(1)s) and ionotropic kainate receptors (KARs) can efficiently modulate GABA release at many synapses of the CNS. The inhibitory effect of kainic acid (KA) has been ascribed to metabotropic actions, and KAR-induced release of secondary neuromodulatory agents may partly mediate these actions. Here, we investigated the involvement of the endocannabinoid system in the modulation of GABAergic synaptic transmission by pharmacological activation of KARs with KA in CA1 pyramidal neurons of the mouse hippocampus. We show that the depression of GABAergic synaptic transmission induced by KA (3 μm) is strongly inhibited by the simultaneous blockade of CB(1) and GABA(B) receptors with SR141716A (5 μm) and CGP55845 (5 μm), respectively. KA induces a calcium-dependent mobilization of the endocannabinoid anandamide (AEA) by activation of GluK2-containing KARs in postsynaptic pyramidal neurons. Consistently, the effect of KA is prolonged by the inhibitor of AEA degradation URB597 (1 μm) in a CB(1)-dependent manner, but it is not altered by blockade of degradation or synthesis of the other main endocannabinoid 2-arachidonoylglycerol (2AG). Hence, our work reveals that the pharmacological activation of KARs leads to the stimulation of secondary metabotropic signaling systems. In addition, these data further underline the profound mechanistic differences between exogenous and endogenous activation of KARs in the hippocampus.

In the developing hippocampus kainate receptors control the release of GABA from mossy fiber terminals via a metabotropic type of action

Kainate receptors (KARs) are glutamate-gated ion channels assembled from various combinations of GluK1-GluK5 subunits with different physiological and pharmacological properties. In the hippocampus, KARs expressed at postsynaptic sites mediate a small component of excitatory postsynaptic currents while at presynaptic sites they exert a powerful control on transmitter release at both excitatory and inhibitory connections. KARs are developmentally regulated and play a key role in several developmental processes including neuronal migration, differentiation and synapse formation. Interestingly, they can signal through a canonical ionotropic pathway but also through a noncanonical modality involving pertussis toxin-sensitive G proteins and downstream signaling molecules.In this Chapter some of our recent data concerning the functional role of presynaptic KARs in regulation of transmitter release from immature mossy fiber terminals and in synaptic plasticity processes will be reviewed. Early in postnatal development, MFs release into their targeted neurons mainly GABA which is depolarizing and excitatory. Endogenous activation of GluK1 KARs localized on MF terminals by glutamate present in the extracellular space down regulates GABA release, leading sometimes to synapse silencing. The depressant effect of GluK1 on MF responses is mediated by a metabotropic process, sensitive to pertussis toxin and phospholipase C (PLC) along the transduction pathway downstream to G protein activation. Blocking PLC with the selective antagonist U73122, unmasks the potentiating effect of GluK1 on MF-evoked GABAergic currents, which probably depend on the ionotropic type of action of these receptors.In addition, GluK1 KARs dynamically regulate the direction of spike-time dependent plasticity, a particular form of Hebbian type of learning which consists in bidirectional modifications in synaptic strength according to the temporal order of pre and postsynaptic spiking. At immature MF-CA3 synapses pairing MF stimulation with postsynaptic spiking and vice versa induces long term depression of MF-evoked GABAergic currents. In the case of positive pairing synaptic depression can be switched into spike-time dependent potentiation by blocking GluK1 KARs with UBP 302. The depressant action exerted by GluK1 KARs on MF responses would prevent the excessive activation of the CA3 associative network by the excitatory action of GABA early in postnatal development.

Localization and functions of kainate receptors in the basal ganglia

Kainate receptors (KARs) are one of the three subtypes of ionotropic glutamate receptors in the CNS. These receptors are widely expressed pre- and postsynaptically throughout the brain. Thus, kainate receptor activation mediates a large variety of pre- and postsynaptic effects on either glutamatergic or GABAergic synaptic transmission. Although ionotropic functions for KAR have been described in multiple brain regions, there is considerable evidence from various CNS regions that KARs activation modulates GABA release through either G-protein dependent metabotropic pathway or secondary activation of G-protein coupled receptors. In the present chapter, we provide further evidence supporting that these two pathways are also involved in the modulation of GABA release in specific basal ganglia nuclei. Because of their more subtle effects on neurotransmisison regulation than other ionotropic glutamate receptors, KARs represent interesting targets for the future development of pharmacotherapy for basal ganglia diseases.

Presynaptic kainate receptors increase GABAergic neurotransmission in rat periaqueductal gray neurons

Neurons within the periaqueductal gray (PAG) have been implicated in the central regulation of pain signals by affecting the descending inhibitory pathway. Here we report on the functional role of presynaptic kainate receptors within the PAG. Using a conventional whole-cell patch clamp technique, we recorded GABAergic spontaneous miniature inhibitory postsynaptic currents (mIPSCs) from mechanically isolated rat PAG neurons in the presence of 300nM tetrodotoxin and 20microM DL-2-amino-5-phosphonovaleric acid under voltage-clamp conditions. Kainic acid at a 10microM concentration significantly increased the frequency of GABAergic mIPSCs without affecting their amplitude, suggesting that kainic acid acts presynaptically to enhance spontaneous GABA release. The kainic acid-induced increase in mIPSC frequency was completely blocked by CNQX, a selective AMPA/kainate receptor antagonist. While neither AMPA nor NMDA affected GABAergic mIPSC frequency, ATPA, a selective agonist of GluR5-containing kainate receptors, increased GABAergic mIPSC frequency in a concentration-dependent manner. The kainic acid-induced increase in mIPSC frequency was completely suppressed either in the presence of 100microM Cd(2+), a general voltage-dependent Ca(2+) channel (VDCC) blocker, or in the Na(+)-free external solution. These results suggest that presynaptic kainate receptors have a low permeability to Ca(2+), and that their activation elicits a presynaptic depolarization large enough to activate presynaptic VDCCs. Presynaptic kainate receptors on GABAergic nerve terminals appear to modulate GABAergic transmission, and in doing so may play an important role in the regulation of PAG neuron excitability.

In the developing rat hippocampus, endogenous activation of presynaptic kainate receptors reduces GABA release from mossy fiber terminals

Presynaptic kainate receptors regulate synaptic transmission in several brain areas but are not known to have this action at immature mossy fiber (MF) terminals, which during the first week of postnatal life release GABA, which exerts into targeted cells a depolarizing and excitatory action. Here, we report that, during the first week of postnatal life, endogenous activation of GluK1 receptors by glutamate present in the extracellular space severely depresses MF-mediated GABAergic currents [GABA(A)-mediated postsynaptic currents (GPSCs)]. Activation of GluK1 receptors was prevented by treating the slices with enzymatic glutamate scavengers that enhanced the clearance of glutamate from the extracellular space. The depressant effect of GluK1 on MF-GPSCs was mediated by a metabotropic process sensitive to pertussis toxin. In the presence of U73122 (1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), a selective inhibitor of phospholipase C, along the transduction pathway downstream to G-protein, GluK1 activation increased the probability of GABA release, thus unveiling the ionotropic action of this receptor. In line with this type of action, we found that GluK1 enhanced MF excitability by directly depolarizing MF terminals via calcium-permeable cation channels. Furthermore, GluK1 dynamically regulated the direction of spike time-dependent plasticity occurring by pairing MF stimulation with postsynaptic spiking and switched spike time-dependent potentiation into depression. The GluK1-induced depression of MF-GPSCs would prevent excessive activation of the CA3 associative network by the excitatory action of GABA and the emergence of seizures in the immature brain.

Glia-dependent switch of kainate receptor presynaptic action

Presynaptic kainate receptors (KARs) exert a modulatory action on transmitter release. This effect can be switched from facilitation to inhibition by an increased concentration of KAR agonists. We here report that activation of presynaptic GluK1-containing KARs facilitates GABA release on oxytocin and vasopressin neurons in the supraoptic nucleus of the hypothalamus. Increase in ambient levels of glutamate associated with the physiological reduction of astrocytic coverage of oxytocin neurons in lactating rats switches this KAR-mediated facilitation to inhibition of GABAergic transmission. This effect was reproduced in both oxytocin and vasopressin neurons of virgin rats when glutamate transporters were blocked pharmacologically, thereby establishing that enhanced levels of extracellular glutamate induce the switch in KAR-mediated action. The facilitation of GABA release was inhibited with philanthotoxin, a Ca(2+)-permeable KAR antagonist, suggesting that this effect was associated with an ionotropic mode of action. Conversely, KAR-mediated inhibition was compromised in the presence of U73122, a phospholipase C inhibitor, in agreement with the involvement of a metabotropic pathway. We thus reveal that physiological astrocytic plasticity modifies the mode of action of presynaptic KARs, thereby inversing their coupling with GABA release.

Age-dependent enhancement of inhibitory synaptic transmission in CA1 pyramidal neurons via GluR5 kainate receptors

Changes in hippocampal synaptic networks during aging may contribute to age-dependent compromise of cognitive functions such as learning and memory. Previous studies have demonstrated that GABAergic synaptic transmission exhibits age-dependent changes. To better understand such age-dependent changes of GABAergic synaptic inhibition, we performed whole-cell recordings from pyramidal cells in the CA1 area of acute hippocampal slices on aged (24-26 months old) and young (2-4 months old) Brown-Norway rats. We found that the frequency and amplitude of spontaneous inhibitory postsynaptic current (IPSCs) were significantly increased in aged rats, but the frequency and amplitude of mIPSCs were decreased. Furthermore, the regulation of GABAergic synaptic transmission by GluR5 containing kainate receptors was enhanced in aged rats, which was revealed by using LY382884 (a GluR5 kainate receptor antagonist) and ATPA (a GluR5 kainate receptor agonist). Moreover, we demonstrated that vesicular glutamate transporters are involved in the kainate receptor dependent regulation of sIPSCs. Taken together, these results suggest that GABAergic synaptic transmission is potentiated in aged rats, and GluR5 containing kainate receptors regulate the inhibitory synaptic transmission through endogenous glutamate. These alterations of GABAergic input with aging could contribute to age-dependent cognitive decline.

Kainate receptors with a metabotropic modus operandi

Kainate receptors (KARs), together with AMPA and NMDA, are typically described as ionotropic glutamate receptors. The functions of KARs have begun to be elucidated only in the last decade. Although some the actions of KARs are classically ionotropic, surprisingly others seem to involve the activation of second-messenger cascades and invoke metabotropic roles for this type of glutamate receptor. In this review, we describe these metabotropic actions of KARs in relation to the putative signalling cascades involved. Although it is still a mystery how KARs activate G proteins to stimulate second-messenger cascades, intriguingly, in very recent studies, specific subunits of KARs have been demonstrated to associate with G proteins. Altogether, the body of evidence supports the hypothesis that, together with the canonical ionotropic operation, KARs expedite long-lasting signalling by novel metabotropic modes of action.

Activation of presynaptic kainate receptors suppresses GABAergic synaptic transmission in the rat globus pallidus

The globus pallidus (GP) plays a central integrative role in the basal ganglia circuitry. It receives strong GABAergic inputs from the striatum (Str) and significant glutamatergic afferents from the subthalamic nucleus (STN). The change in firing rate and pattern of GP neurons is a cardinal feature of Parkinson's disease pathophysiology. Kainate receptor (KAR) GluR6/7 subunit immunoreactivity is expressed presynaptically in GABAergic striatopallidal terminals which provides a substrate for regulation of GABAergic transmission in GP. To test this hypothesis, we recorded GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) in the GP following electrical stimulation of the Str. Following blockade of AMPA and N-methyl-d-aspartate receptors with selective antagonists, bath application of kainate (KA) (0.3-3 microM) reduced significantly the amplitude of evoked IPSCs. This inhibition was associated with a significant increase in paired-pulse facilitation ratio and a reduction of the frequency, but not amplitude, of miniature inhibitory postsynaptic currents (mIPSCs), suggesting a presynaptic site of KA action. The KA effects on striatopallidal GABAergic transmission were blocked by the G-protein inhibitor, N-ethylmaleimide (NEM), or protein kinase C (PKC) inhibitor calphostin C. Our results demonstrate that KAR activation inhibits GABAergic transmission through a presynaptic G protein-coupled, PKC-dependent metabotropic mechanism in the rat GP. These findings open up the possibility for the development of KA-mediated pharmacotherapies aimed at decreasing the excessive and abnormally regulated inhibition of GP neurons in Parkinson's disease.

PKC-dependent autoregulation of membrane kainate receptors

Agonists of kainate receptors (KARs) cause both the opening of the associated ion channels and the activation of signalling pathways driven by G-proteins and PKC. Here we report the existence of an unknown mechanism of KAR autoregulation, involving the interplay of this two signalling mechanisms. Repetitive activation of native KARs evoked the rundown of the ionotropic responses in a manner that was dependent on the activation of PKC. Experiments on recombinant GluR5 expressed in neuroblastoma cells indicated that KARs trigger the activation of PKC and induce the internalization of membrane receptors. This phenomenon depends on the PKC-mediated phosphorylation of serines 879 and 885 of the GluR5-2b subunits, since mutation of these two residues abolished internalization. These results reveal that the non-canonical signalling of KARs is associated with a sensitive mechanism that detects afferent activity. Such a mechanism represents an active way to limit overactivation of the KAR system, by regulating the number of KARs in the cell membrane.

Metabotropic actions of kainate receptors in the CNS

Kainate receptors (KARs), together with NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors (AMPA), are typically described as ionotropic glutamate receptors. Although ionotropic functions for KARs are beginning to be characterized in multiple brain regions, both, in the pre- and post-synaptic compartments of the synapse, there is accumulating evidence that KARs mediate some of their effects without invoking ion-fluxes. Thus, since 1998, when the first metabotropic action of KARs was described in the modulation of GABA release in hippocampal interneurons, there have been increasing reports that some of the functions of KARs involve the participation of intracellular signalling cascades and depend on G protein activation. These surprising observations, attesting metabotropic actions of KARs, akin to those usually attributed to seven transmembrane region G protein-coupled receptors, make the physiological classification and description of glutamate receptors more complex. In the present review, we describe the metabotropic roles of KARs in the CNS and discuss the intriguing properties of this receptor which, structurally shows all the facets of a typical ionotropic receptor, but appears to express a metabotropic remit at some key synapses.

Presynaptic inhibition by kainate receptors converges mechanistically with presynaptic inhibition by adenosine and GABAB receptors

Kainate receptors are widely reported to regulate the release of neurotransmitter in the CNS, but the mechanisms involved remain controversial. Previous studies have found that the kainate receptor agonist ATPA, which selectively activates Glu(K5)-containing kainate receptors, depresses glutamate release at Schaffer-collateral synapses in the hippocampus. In the present study, we provide pharmacological evidence that this depressant effect is mediated by Glu(K5)-containing heteromers, but is distinct from a similar depressant effect engaged by the kainate receptor agonist domoate. The depressant effect of ATPA is insensitive to antagonists for GABA(A), GABA(B), and adenosine receptors, and is also unaffected by lowering the release probability by reducing extracellular calcium. However, the effect of ATPA is partly occluded by prior activation of GABA(B) receptors and completely occluded by prior activation of adenosine receptors, suggesting a mechanistic convergence of heteromeric Glu(K5) kainate receptor signaling with GABA(B) receptors and adenosine receptors. The effects of domoate are partially occluded by both adenosine and GABA(B) receptor agonists, indicating at least a partial convergence of Glu(K5)-lacking kainate receptor signaling with these other pathways. The depressant effect of ATPA is not blocked by inhibition of serine/threonine protein kinases. These results suggest that ATPA and domoate inhibit glutamate release through mechanisms that converge with those of classical metabotropic receptor agonists, although they do so through different receptors.

Kainate receptors

Kainate receptors form a family of ionotropic glutamate receptors that appear to play a special role in the regulation of the activity of synaptic networks. This review first describes briefly the molecular and pharmacological properties of native and recombinant kainate receptors. It then attempts to outline the general principles that appear to govern the function of kainate receptors in the activity of synaptic networks under physiological conditions. It subsequently describes the way that kainate receptors are involved in synaptic integration, synaptic plasticity, the regulation of neurotransmitter release and the control of neuronal excitability, and the manner in which they might play an important role in synaptogenesis and synaptic maturation. These functions require the proper subcellular localization of kainate receptors in specific functional domains of the neuron, necessitating complex cellular and molecular trafficking events. We show that our comprehension of these mechanisms is just starting to emerge. Finally, this review presents evidence that implicates kainate receptors in pathophysiological conditions such as epilepsy, excitotoxicity and pain, and that shows that these receptors represent promising therapeutic targets.

Distinct subunits in heteromeric kainate receptors mediate ionotropic and metabotropic function at hippocampal mossy fiber synapses

Heteromeric kainate receptors (KARs) containing both glutamate receptor 6 (GluR6) and KA2 subunits are involved in KAR-mediated EPSCs at mossy fiber synapses in CA3 pyramidal cells. We report that endogenous glutamate, by activating KARs, reversibly inhibits the slow Ca2+-activated K+ current I(sAHP) and increases neuronal excitability through a G-protein-coupled mechanism. Using KAR knockout mice, we show that KA2 is essential for the inhibition of I(sAHP) in CA3 pyramidal cells by low nanomolar concentrations of kainate, in addition to GluR6. In GluR6(-/-) mice, both ionotropic synaptic transmission and inhibition of I(sAHP) by endogenous glutamate released from mossy fibers was lost. In contrast, inhibition of I(sAHP) was absent in KA2(-/-) mice despite the preservation of KAR-mediated EPSCs. These data indicate that the metabotropic action of KARs did not rely on the activation of a KAR-mediated inward current. Biochemical analysis of knock-out mice revealed that KA2 was required for the interaction of KARs with Galpha(q/11)-proteins known to be involved in I(sAHP) modulation. Finally, the ionotropic and metabotropic actions of KARs at mossy fiber synapses were differentially sensitive to the competitive glutamate receptor ligands kainate (5 nM) and kynurenate (1 mM). We propose a model in which KARs could operate in two modes at mossy fiber synapses: through a direct ionotropic action of GluR6, and through an indirect G-protein-coupled mechanism requiring the binding of glutamate to KA2.

Ionotropic glutamate receptors are expressed in GABAergic terminals in the rat superficial dorsal horn

Ionotropic glutamate receptors (IGR), including NMDA, AMPA, and kainate receptors, are expressed in terminals with varied morphology in the superficial laminae (I-III) of the dorsal horn of the spinal cord. Some of these terminals can be identified as endings of primary afferents, whereas others establish symmetric synapses, suggesting that they may be gamma-aminobutyric acid (GABA)-ergic. In the present study, we used confocal and electron microscopy of double immunostaining for GAD65, a marker for GABAergic terminals, and for subunits of IGRs to test directly whether IGRs are expressed in GABAergic terminals in laminae I-III of the dorsal horn. Although colocalization is hard to detect with confocal microscopy, electron microscopy reveals a substantial number of terminals immunoreactive for GAD65 also stained for IGRs. Among all GAD65-immunoreactive terminals counted, 37% express the NMDA receptor subunit NR1; 28% are immunopositive using an antibody for the GluR2/4 subunits of the AMPA receptor; and 20-35% are immunopositive using antibodies for the kainate receptor subunits GluR5, GluR6/7, KA1, or KA2. Terminals immunoreactive for IGR subunits and GAD65 establish symmetric synapses onto dendrites and perikarya and can be presynaptic to primary afferent terminals within both type 1 and type 2 synaptic glomeruli. Activation of presynaptic IGR may reduce neurotransmitter release. As autoreceptors in terminals of Adelta and C afferent fibers in laminae I-III, presynaptic IGRs may play a role in inhibiting nociception. As heteroreceptors in GABAergic terminals in the same laminae, on the other hand, presynaptic IGRs may have an opposite role and even contribute to central sensitization and hyperalgesia.

A mosaic of functional kainate receptors in hippocampal interneurons

Although some physiological functions of kainate receptors (KARs) still remain unclear, recent advances have highlighted a role in synaptic physiology. In hippocampal slices, kainate depresses GABA-mediated synaptic inhibition and increases the firing rate of interneurons. However, the sensitivity to agonists of these responses differs, suggesting that the presynaptic and somatic KARs have a distinct molecular composition. Hippocampal interneurons express several distinct KAR subunits that can assemble into heteromeric receptors with a variety of pharmacological properties and that, in principle, could fulfill different roles. To address which receptor types mediate each of the effects of kainate in interneurons, we used new compounds and mice deficient for specific KAR subunits. In a recombinant assay, 5-carboxyl-2,4-di-benzamido-benzoic acid (NS3763) acted exclusively on homomeric glutamate receptor subunit 5 (GluR5), whereas 3S,4aR,6S,8aR-6-((4-carboxyphenyl)methyl) 1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884) antagonized homomeric GluR5 and any heteromeric combination containing GluR5 subunits. In hippocampal slices, LY382884, but not NS3763, was able to prevent kainate-induced depression of evoked IPSC. In contrast, neither prevented the concomitant increase in spontaneous IPSC frequency. The selectivity of these compounds was seen additionally in knock-out mice, such that they were inactive in GluR5-/- mice but completely effective in GluR6-/- mice. Our data indicate that in wild-type mice, CA1 interneurons express heteromeric GluR6 -KA2 receptors in their somatic compartments and GluR5-GluR6 or GluR5-KA2 at presynaptic terminals. However, functional compensation appears to take place in the null mutants, a new pharmacological profile emerging more compatible with the activity of homomeric receptors in both compartments: GluR5 in GluR6-/- mice and GluR6 in GluR5-/- mice.

Kainate receptor agonists and antagonists mediate tolerance to kainic acid and reduce high-affinity GTPase activity in young, but not aged, rat hippocampus

Domoic acid acts at both kainic acid (KA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-sensitive glutamate receptors and induces tolerance against subsequent domoic acid insult in young but not aged rat hippocampus. To determine the receptor specificity of this effect, tolerance induction was examined in hippocampal slices from young and aged rats. Slices were preconditioned by exposure to low-dose KA to activate kainate receptors, or the AMPA-receptor selective agonist (S)-5-fluorowillardiine (FW), and following washout, tolerance induction was assessed by administration of high concentrations of KA or FW (respectively). FW preconditioning failed to induce tolerance to subsequent FW challenges, while KA-preconditioned slices were significantly resistant to the effects of high-dose KA. KA preconditioning failed to induce tolerance in aged CA1. Given the lasting nature of the tolerance effect, we examined G-protein-coupled receptor function. A number of ionotropic KA receptor agonists and antagonists significantly reduced constitutive GTPase activity in hippocampal membranes from young but not aged rats. Furthermore, in young CA1, low concentrations of the AMPA/KA blocker GYKI-52466 also induced tolerance to high-dose KA. Our findings suggest that tolerance is triggered by a selective reduction in constitutive KA-sensitive G-protein activity, and that this potential neuroprotective mechanism is lost with age.

Presynaptic kainate receptors modulating glutamatergic transmission in the rat hippocampus are inhibited by arachidonic acid

Kainate receptors are ionotropic glutamate receptors located postsynaptically, mediating frequency-dependent transmission, and presynaptically, modulating transmitter release. In contrast to the excitatory postsynaptic kainate receptors, presynaptic kainate receptor can also be inhibitory and their effects may involve a metabotropic action. Arachidonic acid (AA) modulates most ionotropic receptors, in particular postsynaptic kainate receptor-mediated currents. To further explore differences between pre- and postsynaptic kainate receptors, we tested if presynaptic kainate receptors are affected by AA. Kainate (0.3-3 microM) and the kainate receptor agonist, domoate (60-300 nM), inhibited by 19-54% the field excitatory postsynaptic potential (fEPSP) slope in rat CA1 hippocampus, and increased by 12-32% paired-pulse facilitation (PPF). AA (10 microM) attenuated by 37-72% and by 62-66% the domoate (60-300 nM)-induced fEPSP inhibition and paired-pulse facilitation increase, respectively. This inhibition by AA was unaffected by cyclo- and lipo-oxygenase inhibitors, indomethacin (20 microM) and nordihydroguaiaretic acid (NDGA, 50 microM) or by the free radical scavenger, N-acetyl-L-cysteine (0.5 mM). The K+ (20 mM)-evoked release of [3H]glutamate from superfused hippocampal synaptosomes was inhibited by 18-39% by domoate (1-10 microM), an effect attenuated by 35-63% by AA (10 microM). Finally, the KD (40-55 nM) of the kainate receptor agonist [3H]-(2S,4R)-4-methylglutamate ([3H]MGA) (0.3-120 nM) binding to hippocampal synaptosomal membranes was increased by 151-329% by AA (1-10 microM). These results indicate that AA directly inhibits presynaptic kainate receptor controlling glutamate release in the CA1 area of the rat hippocampus.

Electroencephalographic, behavioral, and c-fos responses to acute domoic acid exposure

Domoic acid, a potent excitotoxic analogue of glutamate and kainate, may cause seizures, amnesia, and sometimes death in humans consuming contaminated shellfish. Continuous behavioral observations and recordings of the electrocorticogram (ECoG, via bipolar, epidural electrodes) were obtained from nonanesthetized rats for 2 h after intraperitoneal injection with either saline, 2.2, or 4.4 mg/kg of domoic acid. Rats were then sacrificed for c-fos immunohistochemistry. Fast Fourier transformation (FFT) of the ECoG data to obtain the voltage as a function of frequency indicated that the lower frequency bands (theta, 4.75-6.75 Hz and delta, 1.25-4.50 Hz) were the first to respond, with a significant elevation by 30 min after the high dose of domoic acid. The lower dose of domoic acid also caused a significant elevation of ECoG voltage, but not until later in the session. Sixty minutes after dosing, the behavioral biomarkers of "ear scratching" and "rearing, praying" (RP) seizures became significantly elevated in the high-dose rats. The low-dose rats showed no significant alterations in behavior at any time during the session. In postmortem brains obtained immediately after the sessions, c-fos was activated in the anterior olfactory nucleus by both the low and high doses of domoic acid. However, only the high dose increased c-fos immunoreactivity in the hippocampus, affecting both the granule and pyramidal neurons. These data indicate that electroencephalographic and c-fos responses can be obtained at a dose of domoic acid that fails to activate the behavioral response most commonly used as a bioassay for this marine toxin: ear scratching with the ipsilateral foot.

Kainate receptors and synaptic transmission

Excitatory glutamatergic transmission involves a variety of different receptor types, each with distinct properties and functions. Physiological studies have identified both post- and presynaptic roles for kainate receptors, which are a subtype of the ionotropic glutamate receptors. Kainate receptors contribute to excitatory postsynaptic currents in many regions of the central nervous system including hippocampus, cortex, spinal cord and retina. In some cases, postsynaptic kainate receptors are co-distributed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, but there are also synapses where transmission is mediated exclusively by postsynaptic kainate receptors: for example, in the retina at connections made by cones onto off bipolar cells. Modulation of transmitter release by presynaptic kainate receptors can occur at both excitatory and inhibitory synapses. The depolarization of nerve terminals by current flow through ionotropic kainate receptors appears sufficient to account for most examples of presynaptic regulation; however, a number of studies have provided evidence for metabotropic effects on transmitter release that can be initiated by activation of kainate receptors. Recent analysis of knockout mice lacking one or more of the subunits that contribute to kainate receptors, as well as studies with subunit-selective agonists and antagonists, have revealed the important roles that kainate receptors play in short- and long-term synaptic plasticity. This review briefly addresses the properties of kainate receptors and considers in greater detail the physiological analysis of their contributions to synaptic transmission.

Presynaptic modulation controlling neuronal excitability and epileptogenesis: role of kainate, adenosine and neuropeptide Y receptors

Based on the idea that seizures may arise from an overshoot of excitation over inhibition, all substances that may decrease glutamatergic function while having no effect or even increasing GABAergic neurotransmission are likely to be effective anticonvulsants. We now review the possible role of three such neuromodulators, kainate, adenosine, and neuropeptide Y receptors in controlling hyperexcitability and epileptogenesis. Particular emphasis is given on the robust neuromodulatory role of these three groups of receptors on the release of glutamate in the hippocampus, a main focus of epilepsy. Moreover, we also give special attention to the mechanisms of receptor activation and coupled signaling events that can be explored as attractive targets for the treatment of epilepsy and excitotoxicity. The present paper is a tribute to Arsélio Pato de Carvalho who has been the main driving force for the development of Neuroscience in Portugal, notably with a particular emphasis on the presynaptic mechanisms of modulation of neurotransmitter release.

Noncanonical signaling by ionotropic kainate receptors

The potent neurotoxin kainate activates ion channel-forming receptors. However, it can also activate a G protein-coupled signaling pathway to inhibit transmitter release in central neurons. It remains unclear whether the same receptor complex is involved in both signaling activities. Here we show that in a population of dorsal root ganglion cells, exposure to kainate elicits a G protein-dependent increase in intracellular Ca2+. Furthermore, in these cells a brief exposure to kainate inhibited the K+-induced Ca2+ increase, a process that was sensitive to the G protein inhibitor Pertussis toxin and inhibitors of protein kinase C. This metabotropic action did not require ion channel activity and was not observed in neurons prepared from mice deficient for the ion channel-forming subunit GluR5. These results indicate that GluR5, an ion channel-forming subunit, signals through a second messenger cascade, inhibiting voltage-dependent Ca2+ channels. Thus, such a system represents a noncanonical signaling route of ion channel-forming receptors.

Cobalt staining of hippocampal neurons mediated by non-desensitizing activation of AMPA but not kainate receptors

Activation of calcium permeable glutamate receptors is likely to be important for neuronal death associated with brain trauma, stroke and neurodegenerative diseases. Cobalt uptake can be used to identify cells containing Ca2+-permeable non-NMDA ionotropic glutamate receptors. However, the relative contribution of AMPA and kainate receptors, and also the role of receptor desensitization on the influx of Co2+, remain to be established. We found that the selective non-desensitizing activation of AMPA receptors was efficient in promoting Co2+ staining. However, the selective activation of kainate receptors, even under non-desensitizing conditions, did not result in Co2+ staining. Taken together, our results show that non-desensitizing stimulation of AMPA, but not of kainate receptors, mediates the influx of Co2+ in cultured rat hippocampal neurons.

Bidirectional modulation of GABA release by presynaptic glutamate receptor 5 kainate receptors in the basolateral amygdala

The activation of kainate receptors modulates GABAergic synaptic transmission, but the mechanisms are currently a matter of intense debate. In the basolateral amygdala (BLA), the glutamate receptor 5 (GluR5) subunit of kainate receptors is heavily expressed, and GluR5 antagonists block a novel form of synaptic plasticity; yet little is known about the role of GluR5-containing kainate receptors in the physiology of the amygdala. Here we show that GluR5 agonists bidirectionally modulate the strength of synaptic transmission from GABAergic interneurons to pyramidal cells in a concentration-dependent manner. Low concentrations of (RS)-S-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) (ATPA) (0.3 microm) or glutamate (5 microm) reduced the number of failures of GABAergic synaptic transmission and enhanced the frequency of miniature IPSCs (mIPSCs). High concentrations of ATPA (10 microm) or glutamate (200 microm) increased the number of synaptic failures and reduced the frequency of mIPSCs. The facilitation or suppression of GABAergic transmission by the GluR5 agonists did not require activation of voltage-gated calcium channels or presynaptic GABA(B) receptors. It was also found that extracellular, endogenous glutamate tonically reduces the rate of failures of GABAergic transmission. These results suggest that the terminals of GABAergic neurons in the BLA carry two subtypes of GluR5-containing kainate receptors, which have different agonist affinities and activate opposing mechanisms of action. The GluR5-mediated, bidirectional modulation of GABA release by glutamate in the BLA may play an important role in the regulation of synaptic plasticity and neuronal excitability in this structure, under normal and pathological conditions.

N-Methyl-D-aspartate receptor activation inhibits protein synthesis in cortical neurons independently of its ionic permeability properties

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate, strongly depresses protein synthesis. We have previously demonstrated in cortical neurons that a glutamate-induced increase in intracellular Ca(2+) is likely responsible for the blockade of the elongation step of protein synthesis. In this study, we provide evidence indicating that NMDA mobilizes a thapsigargin-sensitive pool of intracellular Ca(2+). Exposure of cortical neurons to NMDA, in the absence of external Ca(2+), produced a transient rise in intracellular Ca(2+) that was suppressed by pretreatment with thapsigargin. This rise in intracellular Ca(2+) did not result from an influx of Na(+) via reversal of the mitochondrial Na(+)/Ca(2+) exchanger since it persisted in a Na(+)-free medium or in the presence of CGP 37157, an inhibitor of the exchanger. Moreover, the NMDA-induced increase in intracellular Ca(2+) required the presence of D-serine, was blocked by D(-)-2-amino-5-phosphonopentanoic acid, but was not reduced in the presence of external Mg(2+). This unexpected non-ionotropic effect of NMDA was associated with an inhibition of protein synthesis that was also insensitive to the absence of external Ca(2+) or Na(+), or presence of Mg(2+). NMDA treatment resulted in an increase in the phosphorylation of eEF-2 in the absence or presence of external Ca(2+). The initiation step of protein synthesis was not blocked by NMDA since the phosphorylation of initiation factor eIF-2alpha subunit was not altered by NMDA treatment. In conclusion, we provide evidence indicating that NMDA can inhibit protein synthesis in cortical neurons through a process that involves the mobilization of intracellular Ca(2+) stores via a mechanism that is not linked to the ionic properties of NMDA receptors.

Kainate receptor-dependent presynaptic modulation and plasticity

Kainate-type ionotropic glutamate receptors (KARs) distribute widely and heterogenously throughout the central nervous system (CNS). There is now increasing evidence showing that, in addition to conventional action to mediate postsynaptic excitation, KAR also exerts presynaptic action modulating the amount of transmitter release at certain synapses in the CNS. The mechanism and physiological function of presynaptic KARs have been studied most extensively at the hippocampal mossy fiber (MF)-CA3 synapse, one of the CNS regions where the highest density of KAR subunits is expressed. One unique feature of presynaptic KARs is that their activation modulates transmitter release bi-directionally; weak activation enhances glutamate release, while strong activation leads to inhibition. These findings may be explained by their possible ionotropic action leading to axonal depolarization, which in turn regulates several voltage-dependent channels involved in action potential-dependent Ca2+ entry processes. Furthermore, physiological activation of presynaptic KAR involves an activity-dependent process. Large frequency facilitation, a form of short-term plasticity characteristic of the MF-CA3 synapse, is mediated, at least partly, by presynaptic KAR. Bi-directional and activity-dependent regulation of transmitter release by kainate autoreceptors might have physiological significance in information processing in the hippocampus and other CNS regions, as well as its well-known pathological action contributing to epileptogenesis.

Presynaptic kainate receptors in the hippocampus: slowly emerging from obscurity

Kainate receptor agonists depress transmitter release at several synapses in the hippocampus. Distinct mechanisms appear to underlie this phenomenon at different synapses. Recently, it has emerged that presynaptic kainate receptors can also potentiate the release of both GABA and glutamate and that axonal kainate receptors can trigger ectopic action potentials in interneurons. Because synaptically released glutamate mimics many of the actions of exogenous agonists, presynaptic kainate receptors potentially play an extensive role in hippocampal signaling.

Molecular physiology of kainate receptors

A decade ago, our understanding of the molecular properties of kainate receptors and their involvement in synaptic physiology was essentially null. A plethora of recent studies has altered this situation profoundly such that kainate receptors are now regarded as key players in the modulation of transmitter release, as important mediators of the postsynaptic actions of glutamate, and as possible targets for the development of antiepileptic and analgesic drugs. In this review, we summarize our current knowledge of the properties of kainate receptors focusing on four key issues: 1) their structural and biophysical features, 2) the important progress in their pharmacological characterization, 3) their pre- and postsynaptic mechanisms of action, and 4) their involvement in a series of physiological and pathological processes. Finally, although significant progress has been made toward the elucidation of their importance for brain function, kainate receptors remain largely an enigma and, therefore, we propose some new roads that should be explored to obtain a deeper understanding of this young, but intriguing, class of proteins.

Kainate receptors depress excitatory synaptic transmission at CA3-->CA1 synapses in the hippocampus via a direct presynaptic action

Kainate receptor activation depresses synaptic release of neurotransmitter at a number of synapses in the CNS. The mechanism underlying this depression is controversial, and both ionotropic and metabotropic mechanisms have been suggested. We report here that the AMPA/kainate receptor agonists domoate (DA) and kainate (KA) cause a presynaptic depression of glutamatergic transmission at CA3-->CA1 synapses in the hippocampus, which is not blocked by the AMPA receptor antagonist GYKI 53655 but is blocked by the AMPA/KA receptor antagonist CNQX. Neither a blockade of interneuronal discharge nor antagonists of several neuromodulators affect the depression, suggesting that it is not the result of indirect excitation and subsequent release of a neuromodulator. Presynaptic depolarization, achieved via increasing extracellular K(+), caused a depression of the presynaptic fiber volley and an increase in the frequency of miniature EPSCs. Neither effect was observed with DA, suggesting that DA does not depress transmission via a presynaptic depolarization. However, the effects of DA were abolished by the G-protein inhibitors N-ethylmaleimide and pertussis toxin. These results suggest that KA receptor activation depresses synaptic transmission at this synapse via a direct, presynaptic, metabotropic action.

Presynaptic kainate receptors that enhance the release of GABA on CA1 hippocampal interneurons

We report that kainate receptors are present on presynaptic GABAergic terminals contacting interneurons and that their activation increases GABA release. Application of kainate increased the frequency of miniature inhibitory postsynaptic currents recorded in CA1 interneurons. Local applications of glutamate but not of AMPA or NMDA also increased GABA quantal release. Application of kainate as well as synaptically released glutamate reduced the number of failures of GABAergic neurotransmission between interneurons. Thus, activation of presynaptic kainate receptors increases the probability of GABA release at interneuron-interneuron synapses. Glutamate may selectively control the communication between interneurons by increasing their mutual inhibition.