Localization and function of pre- and postsynaptic kainate receptors in the rat globus pallidus

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.

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.

Kainate Receptors Play a Role in Modulating Synaptic Transmission in the Olfactory Bulb

Glutamate is the neurotransmitter used at most excitatory synapses in the mammalian brain, including those in the olfactory bulb (OB). There, ionotropic glutamate receptors including N-methyl-d-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) play a role in processes such as reciprocal inhibition and glomerular synchronization. Kainate receptors (KARs) represent another type of ionotropic glutamate receptor, which are composed of five (GluK1-GluK5) subunits. Whereas KARs appear to be heterogeneously expressed in the OB, evidence as to whether these KARs are functional, found at synapses, or modify synaptic transmission is limited. In the present study, coapplication of KAR agonists (kainate, SYM 2081) and AMPAR antagonists (GYKI 52466, SYM 2206) demonstrated that functional KARs are expressed by OB neurons, with a subset of receptors located at synapses. Application of kainate and the GluK1-selective agonist ATPA had modulatory effects on excitatory postsynaptic currents (EPSCs) evoked by stimulation of the olfactory nerve layer. Application of kainate and ATPA also had modulatory effects on reciprocal inhibitory postsynaptic currents (IPSCs) evoked using a protocol that evokes dendrodendritic inhibition. The latter finding suggests that KARs, with relatively slow kinetics, may play a role in circuits in which the relatively brief duration of AMPAR-mediated currents limits the role of AMPARs in synaptic transmission (e.g., reciprocal inhibition at dendrodendritic synapses). Collectively, our findings suggest that KARs, including those containing the GluK1 subunit, modulate excitatory and inhibitory transmission in the OB. These data further suggest that KARs participate in the regulation of synaptic circuits that encode odor information.

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 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.

Histamine H3 receptor activation counteracts adenosine A2A receptor-mediated enhancement of depolarization-evoked [3H]-GABA release from rat globus pallidus synaptosomes

High levels of histamine H3 receptors (H3Rs) are found in the globus pallidus (GP), a neuronal nucleus in the basal ganglia involved in the control of motor behavior. By using rat GP isolated nerve terminals (synaptosomes), we studied whether H3R activation modified the previously reported enhancing action of adenosine A2A receptor (A2AR) stimulation on depolarization-evoked [(3)H]-GABA release. At 3 and 10 nM, the A2AR agonist CGS-21680 enhanced [(3)H]-GABA release induced by high K(+) (20 mM) and the effect of 3 nM CGS-21680 was prevented by the A2AR antagonist ZM-241385 (100 nM). The presence of presynaptic H3Rs was confirmed by the specific binding of N-α-[methyl-(3)H]-histamine to membranes from GP synaptosomes (maximum binding, Bmax, 1327 ± 79 fmol/mg protein; dissociation constant, Kd, 0.74 nM), which was inhibited by the H3R ligands immepip, clobenpropit, and A-331440 (inhibition constants, Ki, 0.28, 8.53, and 316 nM, respectively). Perfusion of synaptosomes with the H3R agonist immepip (100 nM) had no effect on K(+)-evoked [(3)H]-GABA release, but inhibited the stimulatory action of A2AR activation. In turn, the effect of immepip was blocked by the H3R antagonist clobenpropit, which had no significant effect of its own on K(+)-induced [(3)H]-GABA release. These data indicate that H3R activation selectively counteracts the facilitatory action of A2AR stimulation on GABA release from striato-pallidal projections.

Localization and function of dopamine receptors in the subthalamic nucleus of normal and parkinsonian monkeys

The subthalamic nucleus (STN) receives a dopaminergic innervation from the substantia nigra pars compacta, but the role of this projection remains poorly understood, particularly in primates. To address this issue, we used immuno-electron microscopy to localize D1, D2, and D5 dopamine receptors in the STN of rhesus macaques and studied the electrophysiological effects of activating D1-like or D2-like receptors in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys. Labeling of D1 and D2 receptors was primarily found presynaptically, on preterminal axons and putative glutamatergic and GABAergic terminals, while D5 receptors were more significantly expressed postsynaptically, on dendritic shafts of STN neurons. The electrical spiking activity of STN neurons, recorded with standard extracellular recording methods, was studied before, during, and after intra-STN administration of the dopamine D1-like receptor agonist SKF82958, the D2-like receptor agonist quinpirole, or artificial cerebrospinal fluid (control injections). In normal animals, administration of SKF82958 significantly reduced the spontaneous firing but increased the rate of intraburst firing and the proportion of pause-burst sequences of firing. Quinpirole only increased the proportion of such pause-burst sequences in STN neurons of normal monkeys. In MPTP-treated monkeys, the D1-like receptor agonist also reduced the firing rate and increased the proportion of pause-burst sequences, while the D2-like receptor agonist did not change any of the chosen descriptors of the firing pattern of STN neurons. Our data suggest that dopamine receptor activation can directly modulate the electrical activity of STN neurons by pre- and postsynaptic mechanisms in both normal and parkinsonian states, predominantly via activation of D1 receptors.

Pre- and postsynaptic modulations of hypoglossal motoneurons by α-adrenoceptor activation in wild-type and Mecp2(-/Y) mice

Hypoglossal motoneurons (HNs) control tongue movement and play a role in maintenance of upper airway patency. Defects in these neurons may contribute to the development of sleep apnea and other cranial motor disorders including Rett syndrome (RTT). HNs are modulated by norepinephrine (NE) through α-adrenoceptors. Although postsynaptic mechanisms are known to play a role in this effect, how NE modulates the synaptic transmissions of HNs remains poorly understood. More importantly, the NE system is defective in RTT, while how the defect affects HNs is unknown. Believing that information of NE modulation of HNs may help the understanding of RTT and the design of new therapeutical interventions to motor defects in the disease, we performed these studies in which glycinergic inhibitory postsynaptic currents and intrinsic membrane properties were examined in wild-type and Mecp2(-/Y) mice, a mouse of model of RTT. We found that activation of α1-adrenoceptor facilitated glycinergic synaptic transmission and excited HNs. These effects were mediated by both pre- and postsynaptic mechanisms. The latter effect involved an inhibition of barium-sensitive G protein-dependent K(+) currents. The pre- and postsynaptic modulations of the HNs by α1-adrenoceptors were not only retained in Mecp2-null mice but also markedly enhanced, which appears to be a compensatory mechanism for the deficiencies in NE and GABAergic synaptic transmission. The existence of the endogenous compensatory mechanism is an encouraging finding, as it may allow therapeutical modalities to alleviate motoneuronal defects in RTT.

GABA transporter subtype 1 and GABA transporter subtype 3 modulate glutamatergic transmission via activation of presynaptic GABA(B) receptors in the rat globus pallidus

The intra-pallidal application of γ-aminobutyric acid (GABA) transporter subtype 1 (GAT-1) or GABA transporter subtype 3 (GAT-3) transporter blockers [1-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A) or 1-[2-[tris(4-methoxyphenyl)methoxy]ethyl]-(S)-3-piperidinecarboxylic acid (SNAP 5114)] reduces the activity of pallidal neurons in monkey. This effect could be mediated through the activation of presynaptic GABA(B) heteroreceptors in glutamatergic terminals by GABA spillover following GABA transporter (GAT) blockade. To test this hypothesis, we applied the whole-cell recording technique to study the effects of SKF 89976A and SNAP 5114 on evoked excitatory postsynaptic currents (eEPSCs) in the presence of gabazine, a GABA(A) receptor antagonist, in rat globus pallidus slice preparations. Under the condition of postsynaptic GABA(B) receptor blockade by the intra-cellular application of N-(2,6-dimethylphenylcarbamoylmethyl)-triethylammonium bromide (OX314), bath application of SKF 89976A (10 μM) or SNAP 5114 (10 μM) decreased the amplitude of eEPSCs, without a significant effect on its holding current and whole cell input resistance. The inhibitory effect of GAT blockade on eEPSCs was blocked by (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid, a GABA(B) receptor antagonist. The paired-pulse ratio of eEPSCs was increased, whereas the frequency, but not the amplitude, of miniature excitatory postsynaptic currents was reduced in the presence of either GAT blocker, demonstrating a presynaptic effect. These results suggest that synaptically released GABA can inhibit glutamatergic transmission through the activation of presynaptic GABA(B) heteroreceptors following GAT-1 or GAT-3 blockade. In conclusion, our findings demonstrate that presynaptic GABA(B) heteroreceptors in putative glutamatergic subthalamic afferents to the globus pallidus are sensitive to increases in extracellular GABA induced by GAT inactivation, thereby suggesting that GAT blockade represents a potential mechanism by which overactive subthalamopallidal activity may be reduced in parkinsonism.

Physiology and pathology of calcium signaling in the brain

Calcium (Ca(2+)) plays fundamental and diversified roles in neuronal plasticity. As second messenger of many signaling pathways, Ca(2+) as been shown to regulate neuronal gene expression, energy production, membrane excitability, synaptogenesis, synaptic transmission, and other processes underlying learning and memory and cell survival. The flexibility of Ca(2+) signaling is achieved by modifying cytosolic Ca(2+) concentrations via regulated opening of plasma membrane and subcellular Ca(2+) sensitive channels. The spatiotemporal patterns of intracellular Ca(2+) signals, and the ultimate cellular biological outcome, are also dependent upon termination mechanism, such as Ca(2+) buffering, extracellular extrusion, and intra-organelle sequestration. Because of the central role played by Ca(2+) in neuronal physiology, it is not surprising that even modest impairments of Ca(2+) homeostasis result in profound functional alterations. Despite their heterogeneous etiology neurodegenerative disorders, as well as the healthy aging process, are all characterized by disruption of Ca(2+) homeostasis and signaling. In this review we provide an overview of the main types of neuronal Ca(2+) channels and their role in neuronal plasticity. We will also discuss the participation of Ca(2+) signaling in neuronal aging and degeneration.

Differential localization and function of GABA transporters, GAT-1 and GAT-3, in the rat globus pallidus

GABA transporter subtype 1 (GAT-1) and GABA transporter subtype 3 (GAT-3) are the main transporters that regulate inhibitory GABAergic transmission in the mammalian brain through GABA reuptake. In this study, we characterized the ultrastructural localizations and determined the respective roles of these transporters in regulating evoked inhibitory postsynaptic currents (eIPSCs) in globus pallidus (GP) neurons after striatal stimulation. In the young and adult rat GP, GAT-1 was preferentially expressed in unmyelinated axons, whereas GAT-3 was almost exclusively found in glial processes. Except for rare instances of GAT-1 localization, neither of the two transporters was significantly expressed in GABAergic terminals in the rat GP. 1-(4,4-Diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A) (10 μm), a GAT-1 inhibitor, significantly prolonged the decay time, but did not affect the amplitude, of eIPSCs induced by striatal stimulation (15-20 V). On the other hand, the semi-selective GAT-3 inhibitor 1-(2-[tris(4-methoxyphenyl)methoxy]ethyl)-(S)-3-piperidinecarboxylic acid (SNAP 5114) (10 μm) increased the amplitude and prolonged the decay time of eIPSCs. The effects of transporter blockade on the decay time and amplitude of eIPSCs were further increased when both inhibitors were applied together. Furthermore, SKF 89976A or SNAP 5114 blockade also increased the amplitude and frequency of spontaneous IPSCs, but did not affect miniature IPSCs. Significant GABA(A) receptor-mediated tonic currents were induced in the presence of high concentrations of both SKF 89976A (30 μm) and SNAP 5114 (30 μm). In conclusion, these data indicate that GAT-1 and GAT-3 represent different target sites through which GABA reuptake may subserve complementary regulation of GABAergic transmission in the rat GP.

Responses of developing pedunculopontine neurons to glutamate receptor agonists

The pedunculopontine nucleus (PPN) is involved in the generation and maintenance of waking and rapid eye movement (REM) sleep, forming part of the reticular activating system. The PPN receives glutamatergic afferents from other mesopontine nuclei, and glutamatergic input is believed to be involved in the generation of arousal states. We tested the hypothesis that, from postnatal days 9 to 17 in the rat, there are developmental changes in the glutamate receptor subtypes that contribute to the responses of PPN neurons. Whole cell patch-clamp recordings were conducted using brainstem slices from 9- to 17-day-old rats. All cells (types I, II, and III; randomly selected or thalamic-projecting) responded to bath application of the glutamate receptor agonists N-methyl-d-aspartic acid (NMDA) and kainic acid (KA). A developmental decrease in the contribution of the NMDA receptor and developmental increase in the contribution of the KA receptor was observed following electrical stimulation-induced glutamate input. These changes were also observed following bath application in different cell types (randomly selected vs. thalamic-projecting). KA bath application produced an increase in the paired-pulse ratio (PPR) and a decrease in the frequency of miniature excitatory postsynaptic currents (mEPSCs), suggesting that presynaptic KA autoreceptors may decrease the probability of synaptic glutamate input. In contrast, NMDA application produced no changes in the PPR or mEPSCs. Changes in glutamatergic excitability of PPN cell types could underlie the developmental decrease in REM sleep.

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.

Postnatal change of GluR5 kainate receptor expression in the substantia gelatinosa neuron of the trigeminal subnucleus caudalis in mice

The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) has been implicated in the processing of nociceptive information from the orofacial region. Kainate receptors (KARs) play an important role in sensory transmission. Five different KAR subunits have been cloned and the expression of the KAR subunits showed developmental changes. In this study, RT-PCR, western blotting, immunohistochemistry and a patch clamp technique were used examine the functional expression of the GluR5 subunit in the SG of the Vc in juvenile, peripubertal and/or adult mice. The levels of mRNA and protein expression of the GluR5 subunit in the SG of the Vc were higher in the juvenile mice than in the peripubertal or adult mice. In addition, the KA and ATPA, a GluR5 KAR agonist, induced membrane depolarization on the SG neurons in both juvenile and adult mice in a concentration-dependent manner. However, the juvenile SG neurons showed a stronger response to KA and ATPA than those of adults. The membrane depolarization by KA was suppressed slightly in the presence of the AMPA receptor antagonist, GYKI 52466. These results show that the GluR5 KAR subunits are expressed functionally on the SG neurons of the Vc in mice, and the expression levels of the GluR5 subunits decrease with postnatal development. These postnatal changes in the GluR5 KAR subunit may be a possible mechanism for age-dependent pain processing.

Chronic ethanol and withdrawal effects on kainate receptor-mediated excitatory neurotransmission in the rat basolateral amygdala

Withdrawal (WD) anxiety is a significant factor contributing to continued alcohol abuse in alcoholics. This anxiety is extensive, long-lasting, and develops well after the obvious physical symptoms of acute WD. The neurobiological mechanisms underlying this prolonged WD-induced anxiety are not well understood. The basolateral amygdala (BLA) is a major emotional center in the brain and regulates the expression of anxiety. New evidence suggests that increased glutamatergic function in the BLA may contribute to WD-related anxiety following chronic ethanol exposure. Recent evidence also suggests that kainate-type ionotropic glutamate receptors are inhibited by intoxicating concentrations of acute ethanol. This acute sensitivity suggests potential (KA-R) contributions by these receptors to the increased glutamatergic function seen during chronic exposure. Therefore, we examined the effect of chronic intermittent ethanol (CIE) and WD on KA-R-mediated synaptic transmission in the BLA of Sprague-Dawley rats. Our study showed that CIE, but not WD, increased synaptic responses mediated by KA-Rs. Interestingly, both CIE and WD occluded KA-R-mediated synaptic plasticity. Finally, we found that BLA field excitatory postsynaptic potential responses were increased during CIE and WD via a mechanism that is independent of glutamate release from presynaptic terminals. Taken together, these data suggest that KA-Rs might contribute to postsynaptic increases in glutamatergic synaptic transmission during CIE and that the mechanisms responsible for the expression of KA-R-dependent synaptic plasticity might be engaged by chronic ethanol exposure and WD.

Ligands for ionotropic glutamate receptors

Marine-derived small molecules and peptides have played a central role in elaborating pharmacological specificities and neuronal functions of mammalian ionotropic glutamate receptors (iGluRs), the primary mediators of excitatory synaptic transmission in the central nervous system (CNS). As well, the pathological sequelae elicited by one class of compounds (the kainoids) constitute a widely-used animal model for human mesial temporal lobe epilepsy (mTLE). New and existing molecules could prove useful as lead compounds for the development of therapeutics for neuropathologies that have aberrant glutamatergic signaling as a central component. In this chapter we discuss natural source origins and pharmacological activities of those marine compounds that target ionotropic glutamate receptors.

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.

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.

Endoplasmic reticulum is a key organella in bradykinin-triggered ATP release from cultured smooth muscle cells

ATP has broad functions as an autocrine/paracrine molecule. The mode of ATP release and its intracellular source, however, are little understood. Here we show that bradykinin via B(2)-receptor stimulation induces the extracellular release of ATP via the inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)]-signaling pathway in cultured taenia coli smooth muscle cells. It was found that bradykinin also increased the production of Ins(1,4,5)P(3) and 2-APB-inhibitable [Ca(2+)](i). The evoked release of ATP was suppressed by the Ca(2+)-channel blockers, nifedipine, and verapamil. Moreover, the extracellular release of ATP was elicited by photoliberation of Ins(1,4,5)P(3). Bradykinin caused a quick and transient accumulation of intracellular ATP from cells treated with 1% perchloric acid solution (PCA), but not with the cell lysis buffer. Peak accumulation was prevented by 2-APB and thapsigargin, but not by nifedipine or verapamil, inhibitors of extracellular release of ATP. These findings suggest that bradykinin elicits the extracellular release of ATP that is mediated by the Ins(1,4,5)P(3)-induced Ca(2+) signaling and, finally, leads to a Ca(2+)-dependent export of ATP from the cells. Furthermore, the bradykinin-induced transient accumulation of ATP in the cells treated with PCA may imply a possible release of ATP from the endoplasmic reticulum.

Differential contribution of kainate receptors to excitatory postsynaptic currents in superficial layer neurons of the rat medial entorhinal cortex

Although in situ hybridization studies have revealed the presence of kainate receptor (KAR) mRNA in neurons of the rat medial entorhinal cortex (mEC), the functional presence and roles of these receptors are only beginning to be examined. To address this deficiency, whole cell voltage clamp recordings of locally evoked excitatory postsynaptic currents (EPSCs) were made from mEC layer II and III neurons in combined entorhinal cortex-hippocampal brain slices. Three types of neurons were identified by their electroresponsive membrane properties, locations, and morphologies: stellate-like "Sag" neurons in layer II (S), pyramidal-like "No Sag" neurons in layer III (NS), and "Intermediate Sag" neurons with varied morphologies and locations (IS). Non-NMDA EPSCs in these neurons were composed of two components, and the slow decay component in NS neurons had larger amplitudes and contributed more to the combined EPSC than did those observed in S and IS neurons. This slow component was mediated by KARs and was characterized by its resistance to either 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466, 100 microM) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[lsqb]f[rsqb]quinoxaline-7-sulfonamide (NBQX, 1 microM), relatively slow decay kinetics, and sensitivity to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-50 microM). KAR-mediated EPSCs in pyramidal-like NS neurons contributed significantly more to the combined non-NMDA EPSC than did those from S and IS neurons. Layer III neurons of the mEC are selectively susceptible to degeneration in human temporal lobe epilepsy (TLE) and animal models of TLE such as kainate-induced status epilepticus. Characterizing differences in the complement of postsynaptic receptors expressed in injury prone versus injury resistant mEC neurons represents an important step toward understanding the vulnerability of layer III neurons seen in TLE.

The expression and binding of kainate receptors is modified in different brain regions by glutamate neurotoxicity during postnatal rat development

Kainic acid receptor (KA-R) subunits are differentially expressed during brain development, and they modulate both neural growth and survival. High concentrations of glutamate in the brain can induce neuronal injury through these receptors, altering normal development. However, it is unclear whether KAR subunit expression itself is also modified by neonatal exposure to high glutamate. To analyze this, monosodium glutamate (4mg/g of body weight) was subcutaneously administered on postnatal days 1, 3, 5 and 7, and the expression of GluR5, GluR6, KA1 and KA2, as well as [(3)H]-kainic acid (KA-R) binding, was evaluated on postnatal days 14, 21, 30 and 60 in different regions of rat brain. As a result, high levels of GluR5 expression associated with strong [(3)H]-kainic acid binding were observed on postnatal days 30 and 60 in the cerebral cortex of rats exposed to glutamate. Similarly, the changes induced by glutamate administration in the expression of the KA1 and KA2 subunits were paralleled by those of [(3)H]-kainic acid binding in the striatum at postnatal days 21 and 30. In contrast, while KAR subunits were over expressed in the hippocampus, no changes were observed in [(3)H]-kainic acid binding in adult rats that had been exposed to glutamate. Therefore, glutamate modifies both the expression of kainic acid receptor subunits and kainic acid binding in a determined spatial and temporal manner, which may be indicative of a regional susceptibility to glutamate neurotoxicity.

Glutamate and GABA receptors and transporters in the basal ganglia: what does their subsynaptic localization reveal about their function?

GABA and glutamate, the main transmitters in the basal ganglia, exert their effects through ionotropic and metabotropic receptors. The dynamic activation of these receptors in response to released neurotransmitter depends, among other factors, on their precise localization in relation to corresponding synapses. The use of high resolution quantitative electron microscope immunocytochemical techniques has provided in-depth description of the subcellular and subsynaptic localization of these receptors in the CNS. In this article, we review recent findings on the ultrastructural localization of GABA and glutamate receptors and transporters in monkey and rat basal ganglia, at synaptic, extrasynaptic and presynaptic sites. The anatomical evidence supports numerous potential locations for receptor-neurotransmitter interactions, and raises important questions regarding mechanisms of activation and function of synaptic versus extrasynaptic receptors in the basal ganglia.