NITRIC-OXIDE DISRUPTS CA2-NEURONS( HOMEOSTASIS IN HIPPOCAMPAL)

Nitric oxide has been recognized in recent years as an important media tor of neuronal toxicity, which in many cases involves alterations of the cytoplasmic Ca2+ concentration ([Ca2+](i)). In [Ca2+](i) fluorimet ric experiments on cultured hippocampal neurons, the nitric oxide-rele asing agent S-nitrosocysteine produced a delayed rise in [Ca2+](i) ove r a 20-min exposure, which was accompanied by a progressive slowing of the kinetics of recovery from depolarization-induced [Ca2+](i) transi ents. These effects were blocked by oxyhemoglobin and by superoxide di smutase, confirming nitric oxide as the responsible agent, and suggest ing that they involved peroxynitrite formation. Similar alterations of [Ca2+](i) homeostasis were produced by the mitochondrial ATP synthase inhibitor oligomycin, and when an ATP-regenerating system was supplie d via the patch pipette in combined whole-cell patch-clamp-[Ca2+](i) f luorimetry experiments, S-nitrosocysteine had no effect on the resting [Ca2+](i) or on the recovery kinetics of [Ca2+](i) transients induced by direct depolarization. We conclude that prolonged exposure to nitr ic oxide disrupts [Ca2+](i) homeostasis in hippocampal neurons by impa iring Ca2+ removal from the cytoplasm, possibly as a result of ATP dep letion. The resulting persistent alterations in [Ca2+](i) may contribu te to the delayed neurotoxicity of nitric oxide.