Anoxic Depression of Excitatory and Inhibitory Postsynaptic Potentials in Rat Neocortical Slices

1. The effects of brief anoxia (4-6 min replacement of O2 by N2) on synaptic potentials evoked from layer IV and/or the white matter were studied in pyramidal neurons of layers II-III from rat neocortical slices. 2. The early and late components of excitatory postsynaptic potentials (EPSPs) showed differential sensitivity to anoxia: within 2 min the late EPSP (lEPSP) disappeared, whereas the amplitude of the early EPSP (eEPSP) decreased by 70% at 5 min of anoxia. Recovery was complete within 4-11 min. 3. Both fast and slow inhibitory postsynaptic potentials (IPSPs) were extremely sensitive to lack of O2 and were abolished earlier than the lEPSP evoked by the same stimulus. As well, recovery of the IPSPs was always more delayed than that of the EPSPs. 4. A transient increase in excitability during early anoxia and/or midrecovery, manifested as enhanced probability of spiking in 25% of neurons, is attributed to the higher sensitivity of IPSPs compared with EPSPs. 5. The anoxic-induced depression of the lEPSP and IPSPs, which are generated close to the soma, is not due to depolarization-induced occlusion; however, occlusion may cause an attenuation of the eEPSP at dendritic sites. 6. The depression of the EPSPs is not a result of a decreased transmembrane Na+ gradient after inactivation of Na-K-adenosine triphosphatase (Na-K-ATPase). Although ouabain induced a depolarization similar to that of anoxia, it did not affect EPSP amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)