ly-341495 and 2-amino-4-phosphonobutyric-acid

Group III metabotropic glutamate receptors (mGluRs) reduce synaptic transmission at the Schaffer collateral-CA1 (SC-CA1) synapse in rats by a presynaptic mechanism. Previous studies show that low concentrations of the group III-selective agonist, L-AP4, reduce synaptic transmission in slices from neonatal but not adult rats, whereas high micromolar concentrations reduce transmission in both age groups. L-AP4 activates mGluRs 4 and 8 at much lower concentrations than those required to activate mGluR7, suggesting that the group III mGluR subtype modulating transmission is a high affinity receptor in neonates and a low affinity receptor in adults. The previous lack of subtype selective ligands has made it difficult to test this hypothesis. We have measured fEPSPs in the presence of novel subtype selective agents to address this question. We show that the effects of L-AP4 can be blocked by LY341495 in both neonates and adults, verifying that these effects are mediated by mGluRs. In addition, the selective mGluR8 agonist, DCPG, has a significant effect in slices from neonatal rats but does not reduce synaptic transmission in adult slices. The mGluR4 selective allosteric potentiator, PHCCC, is unable to potentiate the L-AP4-induced effects at either age. Taken together, our data suggest that group III mGluRs regulate transmission at the SC-CA1 synapse throughout development but there is a developmental regulation of the subtypes involved so that both mGluR7 and mGluR8 serve this role in neonates whereas mGluR7 is involved in regulating transmission at this synapse throughout postnatal development.

Topics: Age Factors; Amino Acids; Aminobutyrates; Animals; Animals, Newborn; Cell Line, Transformed; Dose-Response Relationship, Drug; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Hippocampus; Humans; In Vitro Techniques; Male; Neural Pathways; Neurons; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Synapses; Synaptic Transmission; Transfection; Xanthenes

While group II metabotropic glutamate receptors (mGluRs) are known to be expressed in the rat globus pallidus (GP), their functions remain poorly understood. We used standard patch clamping technique in GP slices to determine the effect of group II mGluR activation on excitatory transmission in this region. Activation of group II mGluRs with the group-selective agonist DCG-IV or APDC reduced the amplitude of the evoked excitatory postsynaptic currents (EPSCs) and significantly increased the paired pulse ratio suggesting a presynaptic site of action. This was further supported by double-labeling electron microscopy data showing that group II mGluRs (mGluR2 and 3) immunoreactivity is localized in glutamatergic pre-terminal axons and terminals in the GP. Furthermore, we found that LY 487379, an mGluR2-specific allosteric modulator, significantly potentiated the inhibitory effect of DCG-IV on the excitatory transmission in the GP. Co-incubation with 30 microM LY 487379 increased the potency of DCG-IV about 10-fold in the GP. We were thus able to pharmacologically isolate the mGluR2-mediated function in the rat GP using an mGluR2-specific allosteric modulator. Therefore, our findings do not only shed light on the functions of group II mGluRs in the GP, they also illustrate the therapeutic potential of mGluR-targeting allosteric modulators in neurological disorders such as Parkinson's disease.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Amino Acids; Aminobutyrates; Anesthetics, Local; Animals; Animals, Newborn; Cyclopropanes; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Drug Interactions; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Globus Pallidus; Glycine; In Vitro Techniques; Lidocaine; Membrane Potentials; Methoxyhydroxyphenylglycol; Neurons; Patch-Clamp Techniques; Proline; Pyridines; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Sulfonamides; Synaptic Transmission; Xanthenes

We have studied the effects of groups II and III metabotropic glutamate receptor (mGluR) activation on excitatory responses recorded from hippocampal interneurons of CA1 stratum lacunosum moleculare (SLM). Excitatory postsynaptic currents (EPSCs) evoked by stimulation of the perforant pathway were reduced either by the group II mGluR agonist LY354740 (50-100 nM, 49.1+/-5.7% of control) or by the group III mGluR agonist l-2-amino-4-phosphonobutyric acid (l-AP4) (50 microM, 36.8+/-4.4% of control). Both drugs significantly enhanced paired-pulse facilitation of the EPSCs. Furthermore, both 100 nM LY354740 and 50 microM l-AP4 reduced the frequency, but not the amplitude, of miniature excitatory synaptic currents (mEPSCs), recorded in the presence of 1 microM TTX and 50 microM picrotoxin, or EPSCs evoked by perforant pathway stimulation in the presence of 2.5 mM Sr2+. The broad-spectrum mGluR antagonist LY341495 (10-50 microM) did not affect test EPSCs elicited 210 ms after stimulation at 100 Hz. At network level, 1-5 microM LY354740 significantly reduced the power of gamma frequency oscillations induced by 20 microM carbachol, 600 nM kainate and 5 mM K+ in hippocampal CA1 area. Our results show powerful modulation of excitatory transmission impinging on interneurons of CA1 SLM by presynaptic group II or III mGluRs.

Topics: Amino Acids; Aminobutyrates; Analysis of Variance; Animals; Animals, Newborn; Bridged Bicyclo Compounds; Electric Stimulation; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Hippocampus; In Vitro Techniques; Interneurons; Neural Inhibition; Perforant Pathway; Presynaptic Terminals; Rats; Rats, Sprague-Dawley; Receptors, Metabotropic Glutamate; Strontium; Tetrodotoxin; Time Factors; Xanthenes

A threshold-like nonlinearity in signal transfer from mouse rod photoreceptors to rod bipolar cells dramatically improves the absolute sensitivity of the rod signals. The work described here reaches three conclusions about the mechanisms generating this nonlinearity. (1) The nonlinearity is caused primarily by saturation of the feedforward rod-to-rod bipolar synapse and not by feedback from horizontal or amacrine cells. This saturation renders the rod bipolar current insensitive to small changes in transmitter release from the rod. (2) Saturation occurs within the G protein cascade that couples receptors to channels in the rod bipolar dendrites, with little or no contribution from presynaptic mechanisms or saturation of the postsynaptic receptors. (3) Between 0.5 and 2 bipolar transduction channels are open in darkness at each synapse, compared to the approximately 30 channels open at the peak of the single photon response.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Amino Acids; Aminobutyrates; Animals; Binding Sites; Darkness; Drug Interactions; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Glycine; Guanosine 5'-O-(3-Thiotriphosphate); In Vitro Techniques; Light; Membrane Potentials; Mice; Mice, Inbred C57BL; Neurons; Patch-Clamp Techniques; Photons; Retina; Retinal Rod Photoreceptor Cells; Synapses; Synaptic Transmission; Valine; Xanthenes

I have compared the effects of group II or III metabotropic glutamate receptor (mGluR) activation on monosynaptic excitatory responses recorded intracellularly from CA1 pyramidal neurons of rat hippocampus and evoked by perforant pathway stimulation in vitro. The excitatory postsynaptic currents (EPSCs) were reduced either by the group II mGluR agonist LY354740 (500 nM, 31 +/- 6% of control) or by the group III agonist L-AP4 (400 microM, 53 +/- 5% of control). Both drugs enhanced EPSC paired-pulse facilitation (range 125-189% of control). These effects were blocked by the broad-spectrum mGluR antagonist LY341495 (1 or 20 microM) which when applied alone did not significantly change the EPSCs elicited at low (0.1-0.2 Hz) or higher (1-100 Hz) frequency of stimulation. Prior reduction of the EPSCs induced by L-AP4 did not occlude the subsequent inhibition elicited by LY354740. The effect of LY354740, but not that of L-AP4, was blocked in the presence of the cAMP analogue Sp-cAMPS (20 microM) and with the K(+) channel antagonist alpha-dendrotoxin (125 nM). In contrast, the effect of L-AP4, but not that of LY354740, was prevented by the calmodulin inhibitor ophiobolin A (25 microM) and with the N-type Ca(2+) channel antagonist omega-conotoxin-GVIA (1 microM). In the presence of the P/Q type Ca(2+) channel antagonist omega-agatoxin-IVA (400 nM), the EPSCs were depressed either by LY354740 or by L-AP4. Groups II and III mGluRs are segregated at the presynaptic terminal, and there are distinct differences between the properties of the presynaptic inhibition mediated by these two groups of receptors.

Topics: Amino Acids; Aminobutyrates; Animals; Bridged Bicyclo Compounds; Calcium Channel Blockers; Cyclic AMP; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Drug Interactions; Elapid Venoms; Electric Stimulation; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; Hippocampus; In Vitro Techniques; Membrane Potentials; Neural Inhibition; omega-Conotoxin GVIA; Perforant Pathway; Phosphinic Acids; Propanolamines; Quinoxalines; Rats; Receptors, Metabotropic Glutamate; Statistics, Nonparametric; Synaptic Transmission; Thionucleotides; Time Factors; Xanthenes

1. The group II metabotropic glutamate (mGlu) receptor antagonist (2S,1'S,2'S)-2-(2-carboxycyclopropyl)-2-(9H-xanthen-9-yl)glycine (LY341495) also has activity at group I and III mGlu receptors at higher concentrations and can be used to discriminate between mGlu receptor subtypes. We report the antagonist action of LY341495 on glutamate receptors expressed in the neonatal rat spinal cord preparation and the use of this antagonist to investigate the group III mGlu receptor subtypes responsible for mediating the depression of synaptic transmission in the spinal cord mediated by the group III mGlu receptor agonists (S)-2-amino-4-phosphonobutanoic acid ((S)-AP4) and (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid (ACPT-I). 2. LY341495 antagonised mGlu receptor agonist-induced responses in the spinal cord with a rank order of potency of group II > group III > group I, which is the same as that observed in human cloned mGlu receptor cell lines. Antagonism of group II and III mGlu receptor-mediated effects were time dependent when low-nanomolar concentrations of LY341495 were used. Although the rank order of potency of LY341495 was the same on native rat and cloned human mGlu receptors, there was a compression in the selectivity between group II and III mGlu receptors, expressed in the spinal cord. 3. In agreement with a previous study on cloned ionotropic glutamate receptors 100 microM LY341495 had little or no effect on N-methyl-D-aspartate, (S)-2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl) propionic acid or kainate receptor-mediated responses on motoneurones. 4. LY341495 exhibited low-nanomolar potency antagonist activity against (S)-AP4 and ACPT-I suggesting that these agonists are activating predominantly mGlu8 and that mGlu4 receptors do not play a role in modulating synaptic transmission in the pathways stimulated in the experiments described here.

Topics: Amino Acids; Aminobutyrates; Animals; Animals, Newborn; Cyclopentanes; In Vitro Techniques; Motor Neurons; Rats; Receptors, AMPA; Receptors, Kainic Acid; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Spinal Cord; Synaptic Transmission; Time Factors; Tricarboxylic Acids; Xanthenes

The GABAergic projection neurons of the substantia nigra pars reticulata (SNr) exert an important influence on the initiation and control of movement. The SNr is a primary output nucleus of the basal ganglia (BG) and is controlled by excitatory inputs from the subthalamic nucleus (STN) and inhibitory inputs from the striatum and globus pallidus. Changes in the output of the SNr are believed to be critically involved in the development of a variety of movement disorders. Anatomical studies reveal that metabotropic glutamate receptors (mGluRs) are highly expressed throughout the BG. Interestingly, mRNA for group III mGluRs are highly expressed in STN, striatum, and globus pallidus, and immunocytochemical studies have shown that the group III mGluR proteins are present in the SNr. Thus it is possible that group III mGluRs play a role in the modulation of synaptic transmission in this nucleus. We performed whole cell patch-clamp recordings from nondopaminergic SNr neurons to investigate the effect of group III mGluR activation on excitatory and inhibitory transmission in the SNr. We report that activation of group III mGluRs by the selective agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 100 microM) decreases inhibitory synaptic transmission in the SNr. Miniature inhibitory postsynaptic currents studies and paired-pulse studies reveal that this effect is mediated by a presynaptic mechanism. Furthermore we found that L-AP4 (500 microM) also reduces excitatory synaptic transmission at the STN-SNr synapse by action on presynaptically localized group III mGluRs. The finding that mGluRs modulate the major inputs to SNr neurons suggests that these receptors may play an important role in motor function and could provide new targets for the development of pharmacological treatments of movement disorders.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Amino Acids; Aminobutyrates; Animals; Bicuculline; Drug Design; Electric Stimulation; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; gamma-Aminobutyric Acid; Glutamic Acid; Glycine; Kainic Acid; Nerve Tissue Proteins; Patch-Clamp Techniques; Phosphoserine; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, GABA-A; Receptors, Metabotropic Glutamate; Receptors, Presynaptic; Substantia Nigra; Xanthenes