Localized loss of Ca2+ Homeostasis in neuronal dendrites is a downstream consequence of metabolic compromise during extended NMDA exposures

Excessive Ca2+ loading is central to most hypotheses of excitotoxic neuronal damage. We examined dendritic Ca2+ signals in single CA1 neurons, injected with fluorescent indicators, after extended exposures to a low concentration of NMDA (5 mu M). As shown previously, NMDA produces an initial transient Ca2+ elevation of several micromolar, followed by recovery to submicromolar levels. Then after a delay of similar to 20-40 min, a large Ca2+ elevation appears in apical dendrites and propagates to the soma. We show here that this large delayed Ca2+ increase is required for ultimate loss of membrane integrity. However, transient removal of extracellular Ca2+ for varying epochs before and after NMDA exposure does not delay the propagation of these events. In contrast to compound Ca2+ elevations, intracellular Na+ elevations are monophasic and were promptly reversed by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H- dibenzo [a,d] cyclohepten-5,10-imine maleate]. MK-801 applied after the transient Ca2+ elevations blocked the delayed propagating Ca2+ increase. Even if applied after the propagating response was visualized, MK-801 restored resting Ca2+ levels. Propagating Ca2+ increases in dendrites were delayed or prevented by (1) reducing extracellular Na+, (2) injecting ATP together with the Ca2+ indicator, or (3) provision of exogenous pyruvate. These results show that extended NMDA exposure initiates degenerative signaling generally in apical dendrites. Although very high Ca2+ levels can report the progression of these responses, Ca2+ itself may not be required for the propagation of degenerative signaling along dendrites. In contrast, metabolic consequences of sustained Na+ elevations may lead to failure of ionic homeostasis in dendrites and precede Ca2+-dependent cellular compromise.