6-cyano-7-nitroquinoxaline-2-3-dione and selfotel

In earlier studies it was found that glutamatergic transmission within the nucleus accumbens septi is involved in the performance of a learned visual shape discrimination in pigeons. This study examines what effects several kinds of glutamate and dopamine antagonists have on the same task. Pigeons were trained with the relevant discrimination, bilaterally implanted with cannulas into the nucleus accumbens and tested after various transmission blockers had been administered intracerebrally. SCH-23390, a D1 dopamine antagonist, at the dose used, had no effect, and Spiperone, a D2-dopamine and 5HT2a-serotonine antagonist, significantly decreased the error repeat trials. CNQX, a non-NMDA glutamate receptor antagonist, and Cycloleucine, an antagonist of the glycine allosteric site of NMDA receptors, had no effect. CGS-19755, a selective competitive NMDA antagonist, significantly impaired performance by significantly decreasing the percent correct trials and increasing the error repeat trials. CPPG, a II/III metabotropic glutamate antagonist, remarkably improved performance. MMPG, a III/II metabotropic glutamate antagonist, at the dose used, did not have any significant effect. The preparation employed may be a useful animal model of perceptual disturbances in schizophrenia.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Alanine; Animals; Benzazepines; Cognition; Columbidae; Cycloleucine; Discrimination Learning; Dopamine Antagonists; Dopamine D2 Receptor Antagonists; Excitatory Amino Acid Antagonists; Glycine; Nucleus Accumbens; Pipecolic Acids; Random Allocation; Receptor, Serotonin, 5-HT2A; Receptors, Dopamine D1; Receptors, Dopamine D2; Receptors, Metabotropic Glutamate; Receptors, N-Methyl-D-Aspartate; Serotonin 5-HT2 Receptor Antagonists; Spiperone

Systemic nicotine stimulates dopamine (DA) release in the nucleus accumbens (NAcc), and N-methyl-D-aspartate (NMDA) receptors in the ventral tegmental area (VTA) appear to be involved. However, it is not known whether the secretion of DA elicited by nicotine depends on the tonic and/or phasic activation of NMDA receptors by glutamate (Glu). To clarify this, in vivo microdialysis was conducted in freely moving, alert rats to measure DA and Glu overflows in the NAcc and Glu in the VTA. Nicotine (0.065, 0.09, or 0.135 mg/kg delivered i.v. at 0.09 mg/kg/60 s via a jugular cannula) dose dependently stimulated NAcc DA secretion (P <.05). However, 0.065 mg/kg nicotine failed to stimulate Glu release in the VTA, whereas higher doses of nicotine (> or =0.09 mg/kg) were effective (P <.05). Administering the competitive NMDA receptor antagonists, 2-amino-5-phosphonopentanoic acid (AP-5; 1 mM) or 0.2 mM cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS 19755) through the VTA probe, abolished NAcc DA release after 0.065 mg/kg nicotine (P <.01) and reduced the response to 0.09 mg/kg nicotine. Therefore, the NAcc DA response to a relatively low dose of nicotine depends on the tonic activation of NMDA receptors in the VTA. In contrast, infusing 1 mM 2-amino-5-phosphonopentanoic acid or 1 mM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor antagonist, into the NAcc through the microdialysis probe had no effect on NAcc DA secretion in response to 0.09 mg/kg nicotine. These findings, coupled with data showing that Glu secretion in the VTA was stimulated only by higher doses of nicotine, indicate that the phasic release of VTA Glu is involved in the NAcc DA response to higher doses of nicotine (> or =0.09 mg/kg).

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Dopamine; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Glutamic Acid; Male; Microdialysis; Nicotine; Nicotinic Agonists; Nucleus Accumbens; Pipecolic Acids; Rats; Rats, Sprague-Dawley; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Stimulation, Chemical; Ventral Tegmental Area

Microglia- and macrophage-induced neuronal death may underlie a number of neurodegenerative diseases. The effects of factors secreted by monocytic cells were studied on glutamatergic synaptic transmission between cultured rat hippocampal neurons. Conditioned media from differentiated human U937 cells was collected after 24 h and applied to neurons (0.5%-30% dilution). Unactivated U937 cells spontaneously released factors that when applied to neuronal cultures evoked bursts of action potentials and elicited neuronal death (29+/-4%). Conditioned media collected from U937 cells evoked intracellular calcium ([Ca(2+)](i)) spiking (0.5%-2% dilution) and at higher concentrations evoked sustained increases in intracellular calcium (3%-30% dilution), as measured by indo-1-based photometry in single neurons. Activation of the U937 cells with zymosin A (500 microg/ml) enhanced the potency of the conditioned media to increase intraneuronal [Ca(2+)](i) as indicated by a leftward shift in the concentration-response curve. Selective antagonists to voltage-gated Na(+) and Ca(2+) channels and NMDA-gated channels (tetrodotoxin, nimodipine, and (+/-)-2-amino-5-phosphonopentanoic acid, respectively) blocked the calcium transients elicited by unactivated and zymosin -A-treated conditioned media. This pharmacologic profile is consistent with U937-released factors that excite the synaptic network that forms between cultured hippocampal neurons.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Calcium; Calcium Channel Blockers; Cell Death; Cells, Cultured; Culture Media, Conditioned; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Excitatory Amino Acids; Fetus; Glutamic Acid; Hippocampus; Humans; Membrane Potentials; Monocytes; Neurons; Neurotoxins; Nimodipine; Patch-Clamp Techniques; Pipecolic Acids; Rats; Receptors, N-Methyl-D-Aspartate; Synapses; Tetrodotoxin; U937 Cells; Zymosan

Activation of cannabinoid receptors inhibits voltage-gated Ca2+ channels and activates K+ channels, reminiscent of other G-protein-coupled signaling pathways that produce presynaptic inhibition. We tested cannabinoid receptor agonists for effects on excitatory neurotransmission between cultured rat hippocampal neurons. Reducing the extracellular Mg2+ concentration to 0.1 mM elicited repetitive, transient increases in intracellular Ca2+ concentration ([Ca2+]i spikes) that resulted from bursts of action potentials, as measured by combined whole-cell current clamp and indo-1-based microfluorimetry. Pharmacological characterization indicated that the [Ca2+]i spikes required glutamatergic synaptic transmission. Cannabinoid receptor ligands inhibited stereoselectively the frequency of [Ca2+]i spiking in the rank order of potency: CP 54,939 > CP 55,940 > Win 55,212-2 > anandamide, with EC50 values of 0.36, 1.2, 2.7, and 71 nM, respectively. CP 55,940 was potent, but not efficacious, and reversed the inhibition produced by Win 55,212-2, indicating that it is a partial agonist. Inhibition of [Ca2+]i spiking by Win 55,212-2 was prevented by treatment of cultures with active, but not heat-treated, pertussis toxin. Win 55,212-2 (100 nM) inhibited stereoselectively CNQX-sensitive excitatory postsynaptic currents (EPSCs) elicited by presynaptic stimulation with an extracellular electrode, but did not affect the presynaptic action potential or currents elicited by direct application of kainate. Consistent with a presynaptic site of action, Win 55,212-2 increased both the number of response failures and the coefficient of variation of the evoked EPSCs. In contrast, cannabimimetics did not affect bicuculline-sensitive inhibitory postsynaptic currents. Thus, activation of cannabinoid receptors inhibits the presynaptic release of glutamate via an inhibitory G-protein.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Cells, Cultured; Dose-Response Relationship, Drug; Excitatory Amino Acid Antagonists; Glutamic Acid; Hippocampus; Magnesium; Patch-Clamp Techniques; Pipecolic Acids; Rats; Receptors, Cannabinoid; Receptors, Drug; Synaptic Transmission

Several amphetamine analogues are reported to increase striatal glutamate efflux in vivo, whereas other data indicate that glutamate is capable of stimulating the efflux of dopamine (DA) in the striatum via a glutamate receptor-dependent mechanism. Based on these findings, it has been proposed that the ability of glutamate receptor-blocking drugs to antagonize the effects of amphetamine may be explained by their capacity to inhibit DA release induced by glutamate. To examine this possibility further, we investigated in vivo the ability of glutamate antagonists to inhibit DA release induced by either methamphetamine (METH) or 3,4-methylenedioxymethamphetamine (MDMA). Both METH and MDMA increased DA efflux in the rat striatum and, in animals killed 1 week later, induced persistent depletions of DA and serotonin in tissue. Pretreatment with MK-801 or CGS 19755 blocked the neurotoxic effects of METH and MDMA but did not significantly alter striatal DA efflux induced by either stimulant. Infusion of 6-cyano-7-nitro-quinoxaline-2,3-dione into the striatum likewise did not alter METH-induced DA overflow, and none of the glutamatergic antagonists affected the basal release of DA when given alone. The findings suggest that the neuroprotective effects of NMDA antagonists do not involve an inhibition of DA release, nor do the data support the proposal that glutamate tonically stimulates striatal DA efflux in vivo. Whether phasic increases in glutamate content might stimulate DA release, however, remains to be determined.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Amines; Animals; Corpus Striatum; Dizocilpine Maleate; Dopamine; Excitatory Amino Acid Antagonists; Male; Methamphetamine; N-Methyl-3,4-methylenedioxyamphetamine; Pipecolic Acids; Rats; Rats, Sprague-Dawley

The density of N-methyl-D-aspartate (NMDA) receptors on membranes prepared from cultured cortical neurons was determined using binding assays with [125I]I-MK-801 after exposure of cultures to antagonists of the NMDA receptor complex. The density of binding sites for [125I]I-MK-801 was increased by 40-80% after exposure to D-2-amino-5-phosphonopentanoic acid (D-AP5), with no change in the number or viability of neurons. The effect of D-AP5 was concentration dependent, with an EC50 of 10 microM. Up-regulation of NMDA receptors was observed after 2-7 days but not after 1 day of exposure to 100 microM D-AP5. The density of NMDA receptors was also increased after exposure of cells to CGS 19755 and MK-801 but not after exposure to the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. The binding of [3H]AMPA was unaltered after exposure to D-AP5. These results demonstrate that the density of NMDA receptors on cultured neurons can be selectively up-regulated by exposure to NMDA receptor antagonists. Increases in the density of NMDA receptors occurring in vivo could complicate therapeutic approaches to the treatment of neurological disorders.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Cells, Cultured; Cerebral Cortex; Dizocilpine Maleate; Neurons; Pipecolic Acids; Quinoxalines; Rats; Receptors, N-Methyl-D-Aspartate; Up-Regulation

The function of astrocytes in cerebral cortex may be studied by comparing the properties of conventional, astrocyte-rich cultures with astrocyte-poor cultures in which astrocyte proliferation has been stringently suppressed. Exposure of astrocyte-poor, but not astrocyte-rich, cultures to fresh medium containing 2 mM glutamine resulted in the death of most neurons within 24 h. This study was undertaken to understand the basis for the apparent toxicity of glutamine in astrocyte-poor cultures. The toxicity of glutamine was found to be mediated by glutamate, which demonstrated an LD50 as a neurotoxin in astrocyte-poor cultures of 2 microM. Exposure of astrocyte-poor (but not astrocyte-rich) cultures to fresh medium containing glutamine for 17.5-24 h resulted in the accumulation of substantial quantities of glutamate (255 +/- 158 microM; mean +/- standard deviation) coincident with the death of neurons in the cultures. Exposure of astrocyte-poor cultures to glutamate in the absence of glutamine did not result in the accumulation of extracellular glutamate. Both the neuronal death and the extracellular glutamate accumulation in astrocyte-poor cultures exposed to glutamine could be blocked by N-methyl-D-aspartate (NMDA) antagonists. These observations suggest that astrocytes as well as glutamine may play an important role in the pathogenesis of glutamate neurotoxicity in the central nervous system.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Astrocytes; Cell Survival; Cells, Cultured; Cerebral Cortex; Extracellular Space; Glutamates; Glutamic Acid; Glutamine; N-Methylaspartate; Neurons; Pipecolic Acids; Quinoxalines; Rats; Rats, Inbred Strains