ROLE OF NMDA RECEPTORS AND VOLTAGE-ACTIVATED CALCIUM CHANNELS IN AN INVITRO MODEL OF CEREBRAL-ISCHEMIA

In an in vitro model of cerebral ischemia we investigated the function al consequences of repeated hypoxias and the potential protective effe ct of the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5- phosphonovaleric acid (D-APV) and the calcium channel blocker verapami l in preventing the expression of pathophysiological activity. Rat neo cortical slices were exposed to nitrogen for 2-13 min and the hypoxia- induced functional modifications were monitored in layer II/III by rec ording the extracellular DC potential, the extracellular calcium conce ntration ([Ca2+]o) and the stimulus-evoked synaptic responses. Hypoxia caused a reversible 2.4-24.6 mV negative shift in the extracellular D C potential associated with a [Ca2+]o decrease from 1.2 to 0.2 mM and a complete loss of synaptic responsiveness. Repeating hypoxias induced an increase in the amplitude of this anoxic depolarization (AD) and a significant decrease in the AD onset latency. Synaptic responses part ially recovered at 20 and 60 min intervals between subsequent hypoxic periods, indicating that the initial AD did not induce any short-term irreparable functional deficits. Verapamil (50 muM) caused an increase in the AD onset latency. However, in comparison to untreated controls , verapamil induced a reduction of excitatory and inhibitory responses during hypoxia probably by blocking voltage-activated calcium conduct ances. In addition, verapamil did not have any significant effect on t he hypoxia-induced reduction of [Ca2+]o. Bath application Of D-APV (30 muM) prevented the significant reduction in the AD onset latency to t he second hypoxia, but had no significant effect on the AD amplitude a nd duration. The hypoxia-induced decrease in [Ca2+]o was not altered a fter addition Of D-APV to the bathing medium. These data indicate that the influx of calcium through voltage-activated calcium channels and the NMDA receptor-gated ionophore does not significantly contribute to the massive depolarization observed under hypoxic conditions.