NMDA-induced 45Ca release in the dentate gyrus of newborn rats:: in vivo microdialysis study

This in vivo study, aimed at detecting the N-methyl-D-aspartate (NMDA) evoked Ca2+-induced Ca2+ release from intracellular stores in the neonatal rat brain, demonstrates that the application of 5 mM N-methyl-D-aspartate via a microdialysis probe for 20 min to the dentate gyrus (DG) of halotane-anesthetized 7 day-old (postnatal day 7, PND 7) rats induces a prolonged decrease in Ca2+ concentration in an initially calcium-free dialysis medium, indicative of a drop in the extracellular concentration of Ca2+ and Ca2+ influx to neurons. In parallel experiments, a huge NMDA-evoked release of Ca-45 from the pre-labeled endogenous Ca2+ pool was observed and interpreted as the expression of intracellular Ca2+ release. Dantrolene (100 mu M) significantly inhibited the NMDA-induced Ca-45 release, whereas 250 mu M ryanodine exerted an unspecific biphasic effect. Autoradiographic and immunocytochemical detection of ryanodine receptors and calbindin D-28K respectively, in the hippocampal region of PND 7 rats displayed a pronounced expression of [H-3]ryanodine binding sites in the DG, but only a slight immunoreactivity of calbindin D-28K. Plastic changes in neurons or excitotoxic neuronal damage induced by the activation of NMDA receptors are mediated by Ca2+ signals, resulting from an influx of extracellular Ca2+, and also in some neurons, from the release of intracellular Ca2+. Our previous in vivo microdialysis experiments visualized NMDA-evoked Ca-45 release in the adult rat dentate gyrus, attributable to Ca2+-induced Ca2+ release from the ryanodine-sensitive pool. An additional role of calbindin in the mechanism of this phenomenon has been suggested. This aspect has not been studied in vivo in newborn rats. Our present results indicate that the release of Ca-45 from the prelabeled intracellular, dantrolene-sensitive Ca2+ pool in the DG neurons of immature rats, most probably representing a phenomenon of Ca2+-induced Ca2+ release, significantly participates in the generation of NMDA receptor-mediated intracellular Ca2+ signals, whereas the role of calbindin D-28K in the mechanism of Ca-45 release is negligible. (C) 2000 Elsevier Science Ltd. All rights reserved.