1. The possibility of long-lasting modifications of glutamatergic responses after anoxic-aglycemic (AA) episodes was investigated in CA1 hippocampal neurons of adult slices. Bicuculline (10 muM) was continuously bath applied to block GABA(A) receptor-mediated currents. AA episodes were induced by brief (1.30-3 min) perfusions with a glucose free artificial-cerebro-spinal-fluid (ACSF) saturated with 95% N2-5% CO2. 2. In presence of (0.6 mM) Mg2+ and a low concentration of the a-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 1 muM), the Schaffer collateral field EPSPs consisted of an early AMPA receptor-mediated component and a late N-methyl-D-aspartate (NMDA) receptor-mediated component. The former was blocked by (10 muM) CNQX and the latter by (50) muM D-2-amino-5-phosphonovalerate (D-APV). The AA episode induced a selective long-term potentiation (LTP) of the NMDA receptor-mediated component [+70 +/- 13% (mean +/- SE), P less-than-or-equal-to 0.008, n = 91 without affecting significantly the AMPA receptor-mediated component (+2 +/- 4, P less-than-or-equal-to 0.86 n = 9). This selective LTP is due to an enhanced efficacy of synaptic transmission and will be referred to as anoxic LTP. 3. In slices perfused with an ACSF containing a physiological concentration of (1.3 mM) Mg2+ and no CNQX, the intracellularly recorded excitatory postsynaptic potential (EPSP) was mixed (AMPA/NMDA) at -65 mV and exclusively mediated by AMPA receptors at -100 mV. At -65 mV, the AA episode induced a persistent potentiation of the EPSP (peak amplitude potentiated by 43 +/- 6%, P less-than-or-equal-to 0.008, n = 9, 1 h after return to control ACSF). This potentiated component of the EPSP was fully sensitive to (50 muM) D-APV. The CNQX-sensitive AMPA receptor-mediated component was not affected by the AA episode (-5.7 +/- 6%, P less-than-or-equal-to 0.123, n = 9). Furthermore, at -100 mV a large APV-sensitive component appeared after the AA episode (+58 +/- 18% of the peak amplitude, P less-than-or-equal-to 0.018, n = 9). Therefore, the AA episode induced a selective LTP of the NMDA receptor-mediated component of the EPSP. 4. A robust LTP (+50.0 +/- 7.5%, P less-than-or-equal-to 0.008, n = 12) of the NMDA receptor-mediated intracellular EPSP was also observed when AMPA receptors were fully and continuously blocked by (15 muM) CNQX. This provides direct evidence that AMPA receptors do not play a role in the induction or expression of anoxic LTP. 5. The pharmacologically isolated NMDA receptor-mediated synaptic current (EPSC) recorded with the single electrode voltage clamp (SEVC) technique (V(H) = -67.5 +/- 4.4 mV, n = 9) was not potentiated by anoxia when the recording was kept in the SEVC mode during and (5 min) after the AA episode (-2.8 +/- 1.5%, P less-than-or-equal-to 0.3, n = 9). This prevented the AA-induced changes in membrane potential (a depolarization followed by an hyperpolarization) from occurring. In contrast, anoxic LTP (+55.9 +/- 11.4%, P less-than-or-equal-to 0.028, n = 6) was observed when the recording was shifted to current clamp mode (Vm = -66.8 +/- 3.7 mV, n = 10) during and shortly after the AA episode. This suggests that induction of anoxic LTP is voltage-dependent. 6. The current evoked by focal pressure application of (300 muM) NMDA was also persistently potentiated by anoxia (+59.3 +/- 7.4%, P less-than-or-equal-to 0.028, VH = -61.6 +/- 1.6 mV, n = 6) in presence of (1 muM) TTX and ( 1 5 muM) CNQX when the recording was shifted to current clamp mode (Vm = -67.5 +/- 2.4 mV, n = 10) during and shortly after the AA episode. 7. These findings provide the first evidence for a selective long-term potentiation of NMDA receptor-mediated currents induced by anoxia combined with aglycemia. The induction of anoxic LTP is voltage-dependent, it also requires the activation of NMDA receptors. The maintenance of anoxic LTP is due to a persistent postsynaptic upregulation of NMDA receptor-mediated currents. The large NMDA receptor-driven currents which will be present after anoxia even at hyperpolarized potentials could play an important role in ischemic delayed cell death.
