dizocilpine-maleate and 2-(2-3-dicarboxycyclopropyl)glycine

We tested a proposal that the hyperpolarization-activated cation channel (I(h) channel) is involved in the induction of short- and long-term plasticity at the hippocampal mossy fiber-CA3 synapses. Bath application of a specific I(h) channel blocker ZD 7288, at a concentration at which it blocked I(h) channels, substantially depressed mossy fiber synaptic transmission, and this inhibition was occluded by previous blockade of these channels by CsCl. In addition, ZD 7288 attenuated the amplitude of both AMPA and NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) equally and caused a coincident increase in the failure rate of single-fiber EPSCs and paired-pulse facilitation (PPF). It also blocked long-term potentiation (LTP) induction when applied before high-frequency tetanic stimulation (TS), and reversed LTP when applied afterwards. Continuous application of CsCl, which efficiently blocks I(h) channels, mimicked ZD 7288 in inhibiting LTP. Furthermore, ZD 7288 blocked both forskolin- and Sp-8-CPT-cAMPS-mediated enhancements of synaptic transmission. However, it did not affect the frequency facilitation induced by increasing the stimulus frequency from 0.05-1 Hz and the expression of the long-term depression (LTD) induced by low-frequency stimulation (LFS) or DCG-IV. Perforated patch-clamp recordings from granule cells revealed that the voltage for half-maximal activation (V(1/2)) of I(h) was significantly shifted towards the depolarizing direction following forskolin or Sp-8-CPT-cAMPS treatment. This enhanced I(h) current was not due to persistent activation of protein kinase A (PKA), because PKA inhibitor KT5720 did not abolish the difference between the activation curves. Therefore, we conclude that I(h) channels may contribute to the development and regulation of short- and long-term plasticity at the mossy fiber-CA3 synapses.

Topics: Animals; Carbazoles; Cesium; Chlorides; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Cyclic Nucleotide-Gated Cation Channels; Cyclopropanes; Dizocilpine Maleate; Electrophysiology; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Extracellular Space; Glycine; Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; In Vitro Techniques; Indoles; Ion Channels; Long-Term Potentiation; Membrane Potentials; Mice; Mice, Inbred ICR; Mossy Fibers, Hippocampal; Nerve Tissue Proteins; Neuronal Plasticity; Patch-Clamp Techniques; Potassium Channels; Pyrimidines; Pyrroles; Synapses

Autoradiographical studies revealed that 10 nM [3H]N-acetyl-aspartyl-glutamate (NAAG) labelled grey matter structures, particularly in the hippocamus, cerebral neocortex, striatum, septal nuclei and the cerebellar cortex. The binding was inhibited by (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG IV), an agonist at group II metabotropic glutamate receptors (mGluR II). (RS)-alpha-Methyl-4-tetrazolylphenylglycine (MTPG), (RS)-alpha-cyclopropyl-4-phosphonoglycine (CPPG) and (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE), all antagonists at mGluR II and mGluR III, also inhibited [3H]NAAG binding. Other inhibitors were (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a broad-spectrum mGluR agonist with preference for groups I and II and the mGluR I agonists/mGluR II antagonists (S)-3-carboxy-4-hydroxyphenylglycine (3,4-CHPG) and (S)-4-carboxy-3-hydroxyphenylglycine (4,3-CHPG). Neither the mGluR I specific agonist (S)-dihydroxyphenylglycine nor any of the ionotropic glutamate receptor ligands such as kainate, AMPA and MK-801 had strong effects (except for the competitive NMDA antagonist CGS 19755, which produced 20-40% inhibition at 100 microM) suggesting that, at low nM concentrations, [3H]NAAG binds predominantly to metabotropic glutamate receptors, particularly those of the mGluR II type. Several studies have indicated that NAAG can interact with mGluR II and the present study supports this notion by demonstrating that sites capable of binding NAAG at low concentrations and displaying pharmacological characteristics of mGluR II exist in the central nervous tissue. Furthermore, the results show that autoradiography of [3H]NAAG binding can be used to quantify the distribution of such sites in distinct brain regions and study their pharmacology at the same time.

Topics: Alanine; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Binding Sites; Brain; Brain Chemistry; Carboxypeptidases; Cold Temperature; Cycloleucine; Cyclopropanes; Dipeptides; Dizocilpine Maleate; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Female; Glutamate Carboxypeptidase II; Glycine; Kainic Acid; Male; Nerve Tissue Proteins; Phosphoserine; Pipecolic Acids; Rats; Rats, Sprague-Dawley; Receptors, Glutamate; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Tetrazoles