Effects of brief anoxia on the electrophysiology of neocortical neurons in vitro.

Abstract

Although the function of all mammalian central neurons is destroyed by prolonged lack of O$\sb2$, during briefer exposures neurons from different regions show differential vulnerability. Pyramidal neurons from the neocortex are reported as some of the most sensitive to anoxia. Using intracellular recording techniques in current clamp mode, the effects of brief anoxia (4-6 min replacement of O$\sb2$ by N$\sb2$) were studied in pyramidal neurons of layers II-III from adult rat neocortical slices. Changes of passive membrane properties (V$\sb{\rm R}$ and R$\sb{\rm N}$) and of synaptic potentials (EPSPs and IPSPs) induced by anoxia have been investigated together with the possible mechanisms underlying these changes. Characteristically, brief anoxia induced in all tested neurons a moderate depolarization (AD) (3-5 mV), accompanied by a small to moderate (20%) decrease in R$\sb{\rm N}$. The ionic substrate of AD has not been entirely elucidated, but the results of experiments involving the Na$\sp+$ channel blocker TTX, Na$\sp+$ substitution, and excitatory amino acid (EAA) antagonists (KYN, DL-APV, MCPG) strongly suggest that a major factor responsible for AD is a Na$\sp+$ influx, mainly through EAA-activated channels of the "non-NMDA" (AMPA/kainate) type. However, since it is most probable that AD is a mixed ionic phenomenon, other conductances and intracellular cation accumulation are likely to be involved. Anoxia invariably induced a reversible depression of the evoked post-synaptic potentials, with both fast and slow IPSPs showing an increased sensitivity as compared to the EPSPs. Among the latter, the polysynaptic late EPSP component was more sensitive (reversible block) than the monosynaptic early EPSP (60-70% reduction). The mechanisms responsible for PSP depression were shown to be both pre- and post-synaptic in origin, with a more important contribution of the former--the process most likely involved being presynaptic inhibition mediated by adenosine activation of A$\sb1$ receptors, as shown by the protective effect of the A$\sb1$ antagonist, 8-CPT. Among other protective antianoxic measures investigated, creatine pre-incubation and attempts to activate the ATP-sensitive K$\sp+$ channels with diazoxide were shown to be ineffective. Changes in temperature between 26 and 37.5$\sp\circ$C (from the control temperature of 33.5$\sp\circ$C) showed, however, a good correlation with the anoxic events (i.e., warming accentuated and cooling diminished the effects of anoxia). Therefore, relative hypothermia (26$\sp\circ$C) proved to be the only measure capable of fully stabilizing the membrane potential and preserving the PSP amplitude during the anoxic challenge.