Reversible Synaptic Blockade Caused by Hypoxia of Moderate Degree in Hippocampal Tissues Slices
Oxygen was withdrawn from tissue slices of rat hippocampus maintained submerged under flowing artificial cerebrospinal fluid (ACSF) at 29°C. Synaptic transmission to CA1 region was rapidly blocked as focally recorded excitatory postsynaptic potentials (fEPSP) were depressed and orthodromic population spikes disappeared without failure of presynaptic afferent fiber volleys. Following readmission of oxygen, transmission was restored, population spike amplitude grew beyond prehypoxic control amplitude, and double or triple spikes appeared where pyramidal cells fired only single spikes before. This state of posthypoxic hyperexcitability was transient after mild hypoxia but persistent after moderate hypoxia. This may be explained by impairment of membrane bound Na+K+ ATPase and consequent incomplete retrieval of the K+ lost by neurons during the hypoxic episode. During such mild to moderate hypoxic episodes, interstitial fluid became acid and interstitial potassium concentration rose somewhat, but calcium level remained unchanged and no spreading depression (SD)-like potential shift occurred. The state of these slices seems similar to the zone of “penumbra” in the periphery of a region of an ischemic brain infarct.
KeywordsSpreading Depression Compound Action Potential Population Spike Excitatory Postsynaptic Potential Postsynaptic Cell
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