guanosine-diphosphate and 1-amino-1-3-dicarboxycyclopentane

In neocortex glutamate activates ionotropic and metabotropic receptors (mGluRs). Whole-cell current-clamp recordings in the in vitro rat auditory cortex at 32 degrees C were used to explore the role that mGluRs have in regulation of AMPA/kainate, NMDA, and GABA receptor-mediated synaptic transmission. Our findings are: (a) The fast EPSP (AMPA/kainate), slow EPSP (NMDA), and IPSPs (GABAA, GABAB), elicited in pyramidal neurons are reduced in the presence of (1S,3R)-ACPD (mGluR agonist) with greatest effect on the slow IPSP>fast IPSP>>fast EPSP. The effect is likely the result of ACPD acting at presynaptic mGluRs because the probability of release of glutamate and GABA is reduced in the presence of ACPD, intracellular infusion of a G protein antagonist (GDPPS) did not block the effect of ACPD, nor were iontophoretic kainic acid or NMDA-induced depolarizations reduced by ACPD. (b) The slow EPSP is enhanced following washout of ACPD and enhancement is not due to disinhibition because it is present in the absence of IPSPs, but if IPSPs are present, its magnitude can be influenced. Iontophoretic NMDA responses are enhanced in the presence of ACPD, an effect blocked by GDPbetaS and heparin (intracellular inositol 1,4,5-trisphosphate receptor antagonist). Taken together, this evidence suggests that enhancement is a result of group I postsynaptic mGluR activation. (c) In fast-spiking cells ACPD reduces the EPSP (AMPA/kainate and NMDA-mediated). This action is likely presynaptic because it persists when GDPbetaS is in the cells. (d) The rate of spike discharge recorded from fast-spiking cells is accelerated in ACPD but does not change in the presence of GDPbetaS, suggesting a postsynaptic effect. Our data indicate that mGluRs can influence neocortical synaptic transmission in complex ways by acting presynaptically and postsynaptically.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Auditory Cortex; Cycloleucine; Excitatory Postsynaptic Potentials; GluK2 Kainate Receptor; Guanosine Diphosphate; Interneurons; Male; Neocortex; Pyramidal Cells; Rats; Rats, Sprague-Dawley; Receptors, Kainic Acid; Receptors, Metabotropic Glutamate; Synaptic Transmission; Thionucleotides

Dual whole-cell recordings were made in layer 2/3 of the rat neocortex in synaptically connected pyramidal cells and fast-spiking non-accommodating (FSN) interneurons. In 75% of cell pairs (n = 80), the cells formed reciprocal synaptic connections. Trains of backpropagating action potentials in pyramidal cells induced Ca2+ transients in dendrites followed by inhibition of unitary IPSPs. IPSP depression was prevented by loading pyramidal cells with 5 mM BAPTA or EGTA. IPSP depression was mimicked by the metabotropic glutamate receptor (mGluR) agonist ACPD and was prevented by a mixture of the mGluR antagonists CPCCOEt and EGLU.IPSP depression was prevented by loading pyramidal cells with the antagonists of vesicular exocytosis botulinum toxin D (light chain) and GDP-beta-S. It is concluded that Ca2+-dependent release of a retrograde messenger, most probably glutamate, from pyramidal cell dendrites suppresses the inhibition of pyramidal neurons via activation of mGluRs located in FSN interneuron nerve terminals.

Topics: Action Potentials; Animals; Botulinum Toxins; Calcium; Cycloleucine; Dendrites; Egtazic Acid; Electrophysiology; Glutamic Acid; Guanosine Diphosphate; Interneurons; Neocortex; Neural Inhibition; Pyramidal Cells; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Synapses; Synaptic Transmission; Thionucleotides

Glutamate is the predominant excitatory neurotransmitter in the vertebrate CNS. Ionotropic glutamate receptors mediate fast excitatory actions whereas metabotropic glutamate receptors (mGluRs) mediate a variety of slower effects. For example, mGluRs can mediate presynaptic inhibition, postsynaptic excitation, or, more rarely, postsynaptic inhibition. We previously described an unusually slow form of postsynaptic inhibition in one class of projection neuron in the song-control nucleus HVc of the songbird forebrain. These neurons, which participate in a circuit that is essential for vocal learning, exhibit an inhibitory postsynaptic potential (IPSP) that lasts several seconds. Only a portion of this slow IPSP is mediated by GABA(B) receptors. Since these cells are strongly hyperpolarized by agonists of mGluRs, we used intracellular recording from brain slices to investigate the mechanism of this hyperpolarization and to determine whether mGluRs contribute to the slow synaptic inhibition. We report that mGluRs hyperpolarize these HVc neurons by activating G protein-coupled, inwardly-rectifying potassium (GIRK) channels. MGluR antagonists blocked this response and the slow synaptic inhibition. Thus, glutamate can combine with GABA to mediate slow synaptic inhibition by activating GIRK channels in the CNS.

Topics: Amino Acids, Dicarboxylic; Animals; Baclofen; Chelating Agents; Cycloleucine; Egtazic Acid; Electrophysiology; Excitatory Amino Acid Antagonists; G Protein-Coupled Inwardly-Rectifying Potassium Channels; GABA Agonists; Guanosine 5'-O-(3-Thiotriphosphate); Guanosine Diphosphate; Male; Membrane Potentials; Neural Inhibition; Neuroprotective Agents; Potassium Channels; Potassium Channels, Inwardly Rectifying; Receptors, Metabotropic Glutamate; Songbirds; Synapses; Tetraethylammonium; Tetrodotoxin; Thionucleotides

Metabotropic glutamate receptors (mGluRs) have been shown to modulate adenylate cyclase activity via G-proteins. In the present study we report similar results to the previously observed in the literature, showing that glutamate and the metabotropic agonists, 1S,3R-ACPD or quisqualate induced cAMP accumulation in hippocampal slices of young rats. Moreover, guanine nucleotides GTP, GDP or GMP, inhibited the glutamate-induced cAMP accumulation. By measuring LDH activity in the buffer surrounding the slices, we showed that the integrity of the slices was maintained, indicating that the effect of guanine nucleotides was extracellular. GMP, GDPbeta-S or Gpp(NH)p abolished quisqualate-induced cAMP accumulation. GDPbeta-S or Gpp(NH)p but not GMP inhibited 1S,3R-ACPD-induced cAMP accumulation. The response evoked by glutamate was also abolished by the mGluR antagonists: L-AP3 abolished glutamate-induced cAMP accumulation in a dose-dependent manner and MCPG was effective only at the 2 mM dose. DNQX was ineffective. We are reporting here, an inhibition induced by guanine nucleotides, via an extracellular site (s), similar to the observed with classical glutamate antagonists on a cellular response evoked by mGluR agonists.

Topics: Animals; Cyclic AMP; Cycloleucine; Glutamic Acid; Guanine Nucleotides; Guanosine Diphosphate; Guanosine Monophosphate; Guanosine Triphosphate; Guanylyl Imidodiphosphate; Hippocampus; Neurotoxins; Quisqualic Acid; Rats; Rats, Wistar; Receptors, Metabotropic Glutamate; Thionucleotides

The amyloid beta protein (25-35) stimulated appearance of 3H-inositol phosphates from [3H]inositol-prelabeled LA-N-2 cells was investigated. This stimulation was unaltered by extra- and intracellular calcium chelators in a calcium-free medium or by several protein kinase inhibitors. This phospholipase C stimulation by amyloid beta protein appeared to be pertussis toxin sensitive. It is possible that this phospholipase C stimulation by amyloid beta protein is a receptor-mediated process. This possibility is based on two related observations. The stimulation is ablated by the presence of conventional antagonists for metabotropic, adrenergic, and bombesin agonists. The IC50 values were 12 microM for propranolol, 15 microM for AP-3, and 25 nM for [Tyr4,D-Phe12]bombesin. Additional support comes from results of desensitization and resensitization experiments. Amyloid beta protein stimulation of phospholipase C was absent from LA-N-2 cells previously treated with norepinephrine, trans-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD), bombesin, or amyloid beta peptide. In a similar manner, LA-N-2 cells previously treated with amyloid beta protein were no longer responsive to norepinephrine, t-ACPD, or bombesin. The responsiveness to amyloid beta protein returned, subsequent to a period of resensitization for the individual agonists. It is suggested that this observed amyloid beta protein stimulation of phospholipase C may be responsible for the elevated quantity of inositol seen in the brains of Alzheimer's disease patients.

Topics: Adrenergic Agonists; Adrenergic alpha-Agonists; Amyloid beta-Peptides; Bombesin; Calcium; Chelating Agents; Cholera Toxin; Cycloleucine; Egtazic Acid; Enzyme Inhibitors; Epinephrine; Guanosine 5'-O-(3-Thiotriphosphate); Guanosine Diphosphate; Guanosine Triphosphate; Humans; Neuroblastoma; Neuroprotective Agents; Norepinephrine; Peptide Fragments; Pertussis Toxin; Protein Kinase Inhibitors; Protein Kinases; Sensitivity and Specificity; Thionucleotides; Tumor Cells, Cultured; Type C Phospholipases; Virulence Factors, Bordetella

Intracellular recordings were obtained from neocortical brain slices of adult rats maintained in vitro. The effect of metabotropic glutamate receptor activation on spike frequency adaptation in regular spiking layer II and III neurons was determined. Putative metabotropic glutamate receptor agonists and antagonists, as well as inhibitors of intracellular signaling systems, were tested. Activation of metabotropic glutamate receptors by bath applied (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD; 50-200 microM) reduced the first interspike interval and increased action potential frequency at all current intensities. This effect was not blocked by ionotropic glutamate receptor antagonists. Under these recording conditions, quisqualate (1-10 microM) similarly reduced spike frequency adaptation. Neither 1R,3S-ACPD, L-2-carboxycyclopropylglycine-I nor the putative presynaptic metabotropic glutamate receptor agonist, L-2-amino-4-phosphonobutyrate, mimicked the effects of 1S,3R-ACPD or quisqualate. Bath application of the putative metabotropic glutamate receptor antagonist, alpha-methyl-4-carboxyphenylglycine, competitively antagonized the excitatory actions of 1S,3R-ACPD. Another putative antagonist, L-2-amino-3-phosphonopropionate, failed to antagonize the reduction in spike frequency adaptation. Intracellular injection of guanosine-5'-O-(2-thiodiphosphate), a non-hydrolysable analog of GTP, inhibited the postsynaptic metabotropic glutamate receptor-mediated effects. However, the depression of synaptic transmission by 1S,3R-ACPD was not antagonized by this compound. The decrease in spike frequency adaptation by 1S,3R-ACPD was not prevented by prior exposure to the non-specific protein kinase inhibitors H-7 or H-8 (10 microM), the protein kinase A inhibitor H-89 (0.25 microM) or the protein kinase C inhibitor staurosporine (0.10 microM). These data suggest that the metabotropic glutamate receptor-mediated reduction in spike adaptation requires the activation of specific G-protein-coupled metabotropic glutamate receptor subtypes located on postsynaptic sites. The increase in neuronal excitability observed in the adult neocortex may be mediated either by an unidentified G-protein-coupled second messenger or via a membrane-delimited G-protein action.

Topics: 1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Adult; Alanine; Amino Acids, Dicarboxylic; Animals; Benzoates; Cycloleucine; Enzyme Inhibitors; Frontal Lobe; Glycine; GTP-Binding Proteins; Guanosine Diphosphate; Humans; Isoquinolines; N-Methylaspartate; Nerve Tissue Proteins; Neurons; Protein Kinase Inhibitors; Protein Kinases; Quisqualic Acid; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Signal Transduction; Staurosporine; Sulfonamides; Thionucleotides