Search

We have investigated the mechanisms underlying the facilitatory modulation mediated by kainate receptor ( KAR) activation in the cortex, using isolated nerve terminals (synaptosomes) and slice preparations. In cortical nerve terminals, kainate (KA, 100 μM) produced an increase in 4-aminopyridine (4-AP)-evoked glutamate release. In thalamocortical slices, KA (1 μM) produced an increase in the amplitude of evoked excitatory post-synaptic currents (eEPSCs) at synapses established between thalamic axon terminals from the ventrobasal nucleus onto stellate neurons of L4 of the somatosensory cortex. In both, synaptosomes and slices, the effect of KA was antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione, and persisted after pre-treatment with a cocktail of antagonists of other receptors whose activation could potentially have produced facilitation of release indirectly. Mechanistically, the observed effects of KA appear to be congruent in synaptosomal and slice preparations. Thus, the facilitation by KA of synaptosomal glutamate release and thalamocortical synaptic transmission were suppressed by the inhibition of protein kinase A and occluded by the stimulation of adenylyl cyclase. Dissecting this G-protein-independent regulation further in thalamocortical slices, the KAR-mediated facilitation of synaptic transmission was found to be sensitive to the block of Ca2+ permeant KARs by philanthotoxin. Intriguingly, the synaptic facilitation was abrogated by depletion of intracellular Ca2+ stores by thapsigargin, or inhibition of Ca2+-induced Ca2+-release by ryanodine. Thus, the KA-mediated modulation was contingent on both Ca2+ entry through Ca2+-permeable KARs and liberation of intracellular Ca2+ stores. Finally, sensitivity to W-7 indicated that the increased cytosolic [Ca2+] underpinning KAR-mediated regulation of synaptic transmission at thalamocortical synapses, requires downstream activation of calmodulin. We conclude that neocortical pre-synaptic KARs mediate the facilitation of glutamate release and synaptic transmission by a Ca2+-calmodulin dependent activation of an adenylyl cyclase/ cAMP/protein kinase A signalling cascade, independent of G-protein involvement. [ABSTRACT FROM AUTHOR]

Abstract: We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors (KARs) in the cortex using isolated nerve terminals (synaptosomes). Kainate (KA) produced an increase on glutamate release at 100μM. The effect of KA was antagonized by NBQX (with AMPA receptors blocked by GYKI53655). This facilitation was suppressed by the inhibition of PKA activation by Rp-Br-cAMP and H-89. Moreover, the facilitation of glutamate release mediated by KAR requires the mobilization of intrasynaptosomal Ca2+ stores and the formation of a Ca2+–calmodulin complex. We conclude that KARs present on presynaptic terminals in the neocortex mediate the facilitation of glutamate release through a mechanism involving an increase in cytosolic Ca2+ to activate a Ca2+–calmodulin–AC/cAMP/PKA signaling cascade. [Copyright &y& Elsevier]

J. Neurochem. (2012) 122, 891-899. Abstract Presynaptic kainate receptors (KARs) modulate the release of glutamate at synapses established between mossy fibers (MF) and CA3 pyramidal cells in the hippocampus. The activation of KAR by low, nanomolar, kainate concentrations facilitates glutamate release. KAR-mediated facilitation of glutamate release involves the activation of an adenylate cyclase/cyclic adenosine monophosphate/protein kinase A cascade at MF-CA3 synapses. Here, we studied the mechanisms by which KAR activation produces this facilitation of glutamate release in slices and synaptosomes. We find that the facilitation of glutamate release mediated by KAR activation requires an increase in Ca2+ levels in the cytosol and the formation of a Ca2+-calmodulin complex to activate adenylate cyclase. The increase in cytosolic Ca2+ underpinning this modulation is achieved, both, by Ca2+ entering via Ca2+-permeable KARs and, by the mobilization of intraterminal Ca2+ stores. Finally, we find that, congruent with the Ca2+-calmodulin support of KAR-mediated facilitation of glutamate release, induction of long-term potentiation at MF-CA3 synapses has an obligate requirement for Ca2+-calmodulin activity. [ABSTRACT FROM AUTHOR]

Kainate receptors (KARs), together with NMDA and α-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. [ABSTRACT FROM AUTHOR]

We have explored the mechanisms involved in the facilitation of glutamate release mediated by the activation of kainate receptors in the rat hippocampus using isolated nerve terminal (synaptosome) and slice preparations. In hippocampal nerve terminals, kainate (KA) produced an increase of glutamate release at concentrations of agonist ranging from 10 to 1000 µM. In hippocampal slices, KA at low nanomolar concentrations (20-50 nM) also produced an increase of evoked excitatory postsynaptic currents (eEPSCs) at mossy fibre-CA3 synapses. In both, synaptosomes and slices, the effect of KA was antagonized by CNQX, and persisted after pretreatment with a cocktail of antagonists for other receptors whose activation could potentially have produced facilitation of release. These data indicate that the facilitation of glutamate release observed is mediated by the activation of presynaptic glutamate receptors of the kainate type. Mechanistically, the observed effects of KA appear to be the same in synaptosomal and slice preparations. Thus, the effect of KA on glutamate release and mossy fibre--CA3 synaptic transmission was occluded by the stimulation of adenylyl cyclase by forskolin and suppressed by the inhibition of protein kinase A by H-89 or Rp-Br-cAMP. We conclude that kainate receptors present at presynaptic terminals in the rat hippocampus mediate the facilitation of glutamate release through a mechanism involving the activation of an adenylyl cyclase-second messenger cAMP-protein kinase A signalling cascade. [ABSTRACT FROM AUTHOR]

1 The presynaptic interactions between facilitatory &beta-adrenoreceptors and inhibitory 5-hydroxy-tryptamine (5-HT) receptors modulating glutamate release from cerebrocortical nerve terminals were examined. 2 4-Aminopyridine (4-AP, 1 mM)-evoked glutamate release was facilitated by the membrane permeant cyclic-3′,5′-adenosine monophosphate (cAMP) analogue, 8-bromo-cAMP (8-Br-cAMP), used to directly activate cAMP-dependent protein kinase (PKA). 3 The β-adrenoreceptor agonist, isoprenaline (ISO), effected a concentration-dependent potentiation of 4-AP-evoked glutamate release which was abolished by the β-adrenoreceptor antagonist, propranolol, and the PKA inhibitor, Rp-cyclic-3′,5′-adenosine-monophosphothioate (Rp-cAMPS). 4 5-HT receptor activation by 100 μM 5-HT produced an inhibition of 4-AP-evoked glutamate release in nerve terminals. The inhibitory effect of 5-HT could be mimicked by the selective 5-HT[SUB1A] receptor agonist, 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and antagonized by 1-(2-methoxyphenyl)-4-(4-phthalimidobutyl)piperazine (NAN-190). 5 When 5-HT (or 8-OH-DPAT) was used in conjunction with ISO or 8-Br-cAMP, the β-adrenoreceptor- and PKA-mediated potentiation of glutamate release was abrogated. 6 The inhibitry crosstalk of 5-HT[SUB1A] receptors to β-adrenoreceptor-mediated facilitation of glutamate release was abolished in the presence of NAN-190. 7 Examination of voltage-dependent Ca[SUP2+] influx revealed that, while ISO and 5-HT alone caused a respective potentiation and diminution of the 4-AP-evoked increase in [Ca[SUP2+]][SUBe], the co-presence of 5-HT abolished the ISO mediated potentiation of Ca[SUP2+] influx. 8 Together, these results suggest that β-adrenoreceptors and 5-HT[SUB1A] receptors coexist on the cerebrocortical nerve terminals and that the cross-talk between the two receptor signalling pathways occurs at a locus downstream from cAMP production, possibly at the level of... [ABSTRACT FROM AUTHOR]

AbstractThe Ca2+/calmodulin-dependent protein phosphatase 2B (PP2B) also known as calcineurin (CN) has been implicated in the Ca2+-dependent inactivation of Ca2+ channels in several cell types. To study the role of calcineurin in the regulation of Ca2+-channel activity, phosphatase expression was altered in NG108-15 cells by transfection of sense and antisense plasmid constructs carrying the catalytic subunit of human PP2Bβ3. Relative to mock-transfected (wild-type) controls, cells overexpressing calcineurin showed dramatically reduced high-voltage-activated Ca2+ currents which were recoverable by the inclusion of 1 μm FK506 in the patch pipette. Conversely, in cells with reduced calcineurin expression, high-voltage-activated Ca2+ currents were larger relative to controls. Additionally in these cells, low-voltage-activated currents were significantly reduced. Analysis of high-voltage-activated Ca2+ currents revealed that the kinetics of inactivation were significantly accelerated in cells overexpressing calcineurin. Following the delivery of a train of depolarizing pulses in experiments designed to produce large-scale Ca2+ influx across the cell membrane, Ca2+-dependent inactivation of high-voltage-activated Ca2+ currents was increased in sense cells, and this increase could be reduced by intracellular application of 1 mm BAPTA or 1 μm FK506. These data support a role of calcineurin in the negative feedback regulation of Ca2+ entry through voltage-operated Ca2+ channels. [ABSTRACT FROM AUTHOR]

1. The exocytotic release of L-glutamate from guinea-pig cerebral cortical synaptosomes can be extensively inhibited by preincubation with botulinum neurotoxin type A at 37 °C for 1-2 h. 2. The toxin has no effect on synaptosomal respiratory control, respiratory capacity, ATP synthesis, plasma-membrane 86Rb+ permeability or plasma-membrane potential, does not inhibit the entry of 4545Ca2+ into the synaptosome upon depolarization and does not alter the ability of intrasynaptosomal mitochondria to sequester Ca2+. 3. The blockade of Ca2+- dependent glutamate release may be totally reversed by the Ca2+/2 H+-exchange ionophore ionomycin, but not by increasing extracellular Ca2+ concentration. 4. It is suggested (a) that exocytosis is triggered by the penetration of Ca2+ into an intracellular hydrophobic milieu; (b) that this stage is blocked by the toxin and (c) that ionomycin is able to bypass this block and deliver Ca2+ to the exocytotic apparatus. [ABSTRACT FROM AUTHOR]