6-cyano-7-nitroquinoxaline-2-3-dione and nipecotic-acid

Nitric oxide (NO) modulates the uptake and/or release of neurotransmitters through a variety of cellular mechanisms. However, the pharmacological and biochemical processes underlying these neurochemical effects of NO often remain unclear. In our study, we used immunocytochemical methods to study the effects of NO, cyclic guanosine monophosphate (cGMP), and peroxynitrite on the uptake and release of gamma-aminobutyric acid (GABA) and glycine in the turtle retina. In addition, we examined the involvement of glutamate receptors, calcium, and the GABA transporter in this GABA uptake and release. We also tested for interactions between the GABAergic and glycinergic systems. In general, we show that NO stimulated GABA release and inhibited glycine release. The NO-stimulated GABA release involved calcium-dependent or calcium-independent synaptic release or reversal of the GABA transporter. Some effects of NO on GABA release involved glutamate, cGMP, or peroxynitrite. NO promoted glycine uptake and inhibited its release, and this inhibition of glycine release was influenced by GABAergic modulation. These findings indicate that NO modulates the levels of the inhibitory transmitters GABA and glycine through several specific biochemical mechanisms in different retinal cell types and layers. Thus it appears that some of the previously described reciprocal interactions between GABA and glycine in the retina function through specific NO signaling pathways.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Bicuculline; Cadmium; Citrulline; Cyclic GMP; DEET; Dizocilpine Maleate; Drug Interactions; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Free Radical Scavengers; GABA Antagonists; gamma-Aminobutyric Acid; Glycine; Immunohistochemistry; In Vitro Techniques; Molsidomine; Neural Inhibition; Nipecotic Acids; Nitric Oxide; Potassium; Retina; Silver Staining; Thiourea; Turtles; Vigabatrin

GABAergic inhibition in the brain can be classified as either phasic or tonic. gamma-Aminobutyric acid (GABA) uptake by GABA transporters (GATs) can limit the time course of phasic currents arising from endogenous and exogenous GABA, as well as decrease a tonically active GABA current. GABA transporter subtypes 1 and 3 (GAT-1 and GAT-3) are the most heavily expressed of the four known GAT subtypes. The role of GATs in shaping GABA currents in the neocortex has not been explored. We obtained patch-clamp recordings from layer II/III pyramidal cells and layer I interneurons in rat sensorimotor cortex. We found that selective GAT-1 inhibition with NO711 decreased the amplitude and increased the decay time of evoked inhibitory postsynaptic currents (IPSCs) but had no effect on the tonic current or spontaneous IPSCs (sIPSCs). GAT-2/3 inhibition with SNAP-5114 had no effect on IPSCs or the tonic current. Coapplication of NO711 and SNAP-5114 substantially increased tonic currents and synergistically decreased IPSC amplitudes and increased IPSC decay times. sIPSCs were not resolvable with coapplication of NO711 and SNAP-5114. The effects of the nonselective GAT antagonist nipecotic acid were similar to those of NO711 and SNAP-5114 together. We conclude that synaptic GABA levels in neocortical neurons are controlled primarily by GAT-1, but that GAT-1 and GAT-2/3 work together extrasynaptically to limit tonic currents. Inhibition of any one GAT subtype does not increase the tonic current, presumably as a result of increased activity of the remaining transporters. Thus neocortical GAT-1 and GAT-2/3 have distinct but overlapping roles in modulating GABA conductances.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Analysis of Variance; Animals; Animals, Newborn; Anisoles; Bicuculline; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Drug Synergism; Electric Stimulation; GABA Agonists; GABA Antagonists; GABA Plasma Membrane Transport Proteins; gamma-Aminobutyric Acid; In Vitro Techniques; Membrane Potentials; Membrane Transport Modulators; Membrane Transport Proteins; Models, Neurological; Muscimol; Neocortex; Neural Inhibition; Neurons; Neurotransmitter Uptake Inhibitors; Nipecotic Acids; Oximes; Patch-Clamp Techniques; Rats; Rats, Sprague-Dawley; Receptors, GABA-A; Time Factors

Taurine application in the CA1 area of rat hippocampal slices induces a long-lasting potentiation of excitatory synaptic transmission that has some mechanistic similitude with the late phase of long-term potentiation (L-LTP). Previous indirect evidence such as temperature and sodium dependence indicated that taurine uptake is one of the primary steps leading to the taurine-induced synaptic potentiation. We show that taurine-induced potentiation is not related to the intracellular accumulation of taurine and is not impaired by 2-guanidinoethanesulphonic acid, a taurine transport inhibitor that is a substrate of taurine transporter. We have found that taurine uptake in hippocampal synaptosomes was inhibited by SKF 89976A, a GABA uptake blocker that is not transportable by GABA transporters. SKF 89976A prevents the induction of synaptic potentiation by taurine application. This effect is neither mimicked by nipecotic acid, a broad inhibitor of GABA transporters that does not affect taurine uptake, nor by NO-711, a specific and potent inhibitor of GABA transporter GAT-1. In addition, L-LTP induced by trains of high-frequency stimulation is also inhibited by SKF 89976A, and taurine, at a concentration that does not change basal synaptic transmission, overcomes such inhibition. We conclude that taurine induces synaptic potentiation through the activation of a system transporting taurine and that taurine uptake is required for the induction of synaptic plasticity phenomena such as L-LTP.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Carrier Proteins; Dose-Response Relationship, Drug; Electric Stimulation; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Female; GABA Antagonists; Guanidines; Hippocampus; In Vitro Techniques; Intracellular Space; Long-Term Potentiation; Membrane Glycoproteins; Membrane Transport Proteins; Nipecotic Acids; Rats; Rats, Sprague-Dawley; Synapses; Synaptosomes; Taurine

In this study we explored the effect of the stimulation of nicotinic acetylcholine receptors located on interneurons by measuring 4-amino-n-[2,3-(3)H]butyric acid ([(3)H]GABA) release and monitoring [Ca (2+)](i) in superfused hippocampal slices. In the presence of 6-cyano-7-nitroquinoxaline-2,3-dione, (+/-)-2-amino-5-phosphonopentanoic acid, and atropine, i.e., under the blockade of N-methyl-D-aspartate and non-N-methyl-D-aspartate glutamate and muscarinic receptors, nicotine did not alter the spontaneous outflow of [(3)H]GABA, but significantly increased the stimulation-evoked [(3)H]GABA efflux. This effect of nicotine depended on the time interval between nicotine treatment and electrical stimulus, the concentration of nicotine (1-100 microM), and the parameters of electrical depolarization. Acetylcholine (0.03-3 mM), and the alpha 7 subtype-selective agonist choline (0.1-10 mM), also potentiated stimulus-evoked release of [(3)H]GABA, whereas 1,1-dimethyl-4-phenilpiperazinium iodide failed to increase the tritium outflow significantly. The effect of nicotine treatment was prevented by tetrodotoxin (1 microM) and by the nicotinic acetylcholine receptor antagonist mecamylamine (10 microM), and the alpha 7 subtype-selective antagonists alpha-bungarotoxin (100 nM) and methyllycaconitine (10 nM), whereas dihidro-beta-erythroidine (20 nM) was without effect. Perfusion of 100 microM nicotine caused a [Ca(2+)](i) transient in about one-third of the tested interneurons; however, the response to subsequent electrical stimulation remained unchanged. Inhibition of the GABA transporter system by nipecotic acid (1 mM) or by decreasing the bath temperature to 12 degrees C abolished completely the effect of nicotine to potentiate the stimulation-evoked release of GABA. These findings indicate that the activation of alpha 7-type nicotinic receptors of hippocampal interneurons results in a long-lasting ability of these cells to respond to depolarization with an increased release of GABA mediated by the transporter system.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Atropine; Calcium; Carrier Proteins; Chloride Channels; Drug Synergism; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA Plasma Membrane Transport Proteins; gamma-Aminobutyric Acid; Hippocampus; Male; Membrane Proteins; Membrane Transport Proteins; Muscarinic Antagonists; Neurons; Nicotine; Nicotinic Agonists; Nipecotic Acids; Organic Anion Transporters; Perfusion; Proline; Protein Kinase C; Rats; Rats, Wistar; Receptors, Nicotinic; Sodium Channel Blockers; Tritium

Kainate (KA), quisqualate (QA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) stimulated gamma-aminobutyric acid [3H]gamma-aminobutyric acid (GABA) release from cultured cerebellar type 2 astrocytes and from their bipotential precursors. The evoked release was prevented by the antagonist 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX). AMPA and QA applied together with KA at concentrations around or above their EC50S (20-50 microM) antagonized the stimulatory effect of KA on [3H]GABA release. On the other hand, the releasing action of KA was potentiated by concentrations of QA in the low micromolar range (2-5 microM), particularly when the concentration of KA was at the borderline of effectiveness (10 microM). KA and QA did not elevate intracellular cyclic GMP levels in astrocyte cultures, although guanylate cyclase was present in both type 2 and type 1 astrocytes. The inability of KA to elevate cyclic GMP levels in astrocytes was the only major difference in the behavior of this glutamate agonist between astroglial and neuronal cultures. The GABA transport inhibitor nipecotic acid or replacement of NaCl with LiCl abolished [3H]GABA uptake and also KA- and QA-induced release of preaccumulated [3H]GABA. Therefore, [3H]GABA was released from type 2 astrocytes and their progenitors through its Na(+)-dependent transport system, operating in an outward direction when the cells were depolarized by non-NMDA receptor agonists.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Astrocytes; Carrier Proteins; Cells, Cultured; Cerebellum; Chlorides; Cyclic GMP; GABA Plasma Membrane Transport Proteins; gamma-Aminobutyric Acid; Ibotenic Acid; Ion Channel Gating; Kainic Acid; Kynurenic Acid; Lithium; Lithium Chloride; Membrane Potentials; Membrane Proteins; Membrane Transport Proteins; Nerve Tissue Proteins; Neurons; Nipecotic Acids; Nitroprusside; Organic Anion Transporters; Proline; Quinoxalines; Quisqualic Acid; Rats; Receptors, AMPA; Receptors, Kainic Acid; Receptors, Neurotransmitter; Secretory Rate; Sodium; Stimulation, Chemical