6-cyano-7-nitroquinoxaline-2-3-dione and 2-phenyl-4-4-5-5-tetramethylimidazoline-1-oxyl-3-oxide

Nitric oxide (NO) affects neuronal activity of the midbrain periaqueductal gray (PAG). The purpose of this report was to investigate the role of GABA receptors in NO modulation of neuronal activity through inhibitory and excitatory synaptic inputs within the dorsolateral PAG (dl-PAG). First, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) and excitatory postsynaptic currents (mEPSCs) were recorded using whole cell voltage-clamp methods. Increased NO by either S-nitroso-N-acetyl-penicillamine (SNAP, 100 microM) or L-arginine (50 microM) significantly augmented the frequency of mIPSCs of the dl-PAG neurons without altering their amplitudes or decay time constants. The effects were eliminated after bath application of carboxy-PTIO (NO scavenger), and 1-(2-trifluorom-ethylphenyl) imidazole (NO synthase inhibitor). In contrast, SNAP and L-arginine did not alter mEPSCs in dl-PAG neurons. However the frequency of mEPSCs was significantly increased with prior application of the GABA(B) receptors antagonist, CGP55845. In addition, NO significantly decreased the discharge rate of spontaneous action potentials in the dl-PAG neurons and the effect was reduced in the presence of the GABA(A) receptor antagonist, bicuculline. Our data show that within the dl-PAG NO potentiates the synaptic release of GABA, while NO-induced GABA presynaptically inhibits glutamate release through GABA(B) receptors. Overall, NO suppresses neuronal activity of the dl-PAG via a potentiation of GABAergic synaptic inputs and via GABA(A) receptors.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Cyclic N-Oxides; Drug Interactions; Enzyme Inhibitors; Excitatory Amino Acid Antagonists; Female; Free Radical Scavengers; GABA Agents; Imidazoles; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Male; Neural Inhibition; Neurons; Nitric Oxide; Nitric Oxide Donors; Patch-Clamp Techniques; Penicillamine; Periaqueductal Gray; Rats; Rats, Sprague-Dawley; Receptors, GABA

We employed a mouse model of ALS, in which overexpression of a familial ALS-linked Cu/Zn-SOD mutation leads to progressive MN loss and a clinical phenotype remarkably similar to that of human ALS patients, to directly test the excitotoxicity hypothesis of ALS. Under basal culture conditions, MNs in mixed spinal cord cultures from the Cu/Zn-SOD mutant mice exhibited enhanced oxyradical production, lipid peroxidation, increased intracellular calcium levels, decreased intramitochondrial calcium levels, and mitochondrial dysfunction. MNs from the Cu/Zn-SOD mutant mice exhibited greatly increased vulnerability to glutamate toxicity mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors. The increased vulnerability of MNs from Cu/Zn-SOD mutant mice to glutamate toxicity was associated with enhanced oxyradical production, sustained elevations of intracellular calcium levels, and mitochondrial dysfunction. Pretreatment of cultures with vitamin E, nitric oxide-suppressing agents, peroxynitrite scavengers, and estrogen protected MNs from Cu/Zn-SOD mutant mice against excitotoxicity. Excitotoxin-induced degeneration of spinal cord MNs in adult mice was more extensive in Cu/Zn-SOD mutant mice than in wild-type mice. The mitochondrial dysfunction associated with Cu/Zn-SOD mutations may play an important role in disturbing calcium homeostasis and increasing oxyradical production, thereby increasing the vulnerability of MNs to excitotoxicity.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; Animals; Antioxidants; Calcium; Cells, Cultured; Cyclic N-Oxides; Estradiol; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Fluorescent Dyes; Free Radicals; Glutamic Acid; Homeostasis; Humans; Imidazoles; Lipid Peroxidation; Mice; Mice, Transgenic; Mitochondria; Motor Neuron Disease; Motor Neurons; Neurotoxins; NG-Nitroarginine Methyl Ester; Nitrates; Oxidative Stress; Rhodamine 123; Spinal Cord; Superoxide Dismutase; Superoxides; Vitamin E