Enhanced Calcium Uptake by CA1 Pyramidal Cell Dendrites in the Postischemic Phase despite Subnormal Evoked Field Potentials: Excitatory Amino Acid Receptor Dependency and Relationship to Neuronal Damage

After 6–12 h of recovery from transient cerebral ischemia, the pyramidal cells of the hipppocampal CA1 region take up excessive amounts of calcium upon electrical stimulation, which has been suggested to be important for the development of delayed neuronal death. The aim of this study was to further characterize this enhanced calcium uptake with respect to time-course of development, relationship to neuronal damage, and amplitude of evoked field potentials as well as the dependency on N-methyl-d-aspartate (NMDA) and non-NMDA receptors. Adult Wistar rats were used and calcium-sensitive microelectrodes were placed in the stratum radiatum of the CA1 hippocampus for recording of the extracellular calcium concentration ([Ca2+]ec) during 20 min of ischemia and for 6 h of reflow. High-frequency stimulation of the perforant pathway elicited burst firing in CA1 and a transient decrease in [Ca2+]ec which reflects neuronal uptake. Shifts in [Ca2+]ec could not be evoked 0–1 h after ischemia. However, from 1–2 h burst firing could be evoked and the accompanying shift in [Ca2+]ec increased thereafter in amplitude with prolonged reflow, exceeded preischemic levels after 4 h, and reached 250 ± 116% (mean ± SD) of control after 6 h of reflow ( p < 0.05). The extracellular reference potential shift during electrical stimulation and the amplitude of evoked field potentials were still subnormal after 6 h [85 ± 25% and 83 ± 25%, respectively (mean ± SD)]. There was a significant correlation between the degree of stimulated calcium uptake at 6 h postischemia and the extent of CA1 damage evaluated 7 days after the ischemic insult ( r = 0.849; p < 0.001). The shifts in [Ca2+]ec were reduced by the NMDA antagonist MK-801 (0.5–2 mg/kg, i.v.) to approximately 50% of the initial level during both control and postischemic conditions ( p < 0.01). The non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo[ F]quinoxaline (NBQX) (42 ± 13 mg/kg, i.p.; mean ± SD) decreased the amplitude of the evoked field potentials (to 30 ± 28% of control, p < 0.05) and completely abolished the evoked shifts in [Ca2+]ec. In conclusion, the uptake of calcium into CA1 pyramidal cells during electrical stimulation was enhanced already 4 h after ischemia in spite of the fact that other measures of excitability were subnormal. This calcium uptake correlated to the extent of CA1 pyramidal cell damage and was dependent on both NMDA and non-NMDA receptor activation.