The work is aimed at determining the early events leading to block of synaptic transmission during hypoxia. The system in which this is studied is the perforant path to dentate granule cell synapse of the in vitro rat hippocampus. It has been quite well established that the decline in the evoked potential in this preparation and in others results from depolarzation of neuronal processes. I shall analyze events in the outer 2/3 of the molecular layer of the dentate gyrus -the region where the perforant path-dentate granule cell synapses occur - in order to determine events leading to this depolarization. The basic approach will be two-fold: 1) to show a temporal correlation between changes in a possible causal agent and the decay in evoked potential and 2) to show that alterations in this agent will lead to transmissson block and in some cases accelerate the rate of block during hypoxia. Thus, ATP concentration in the synaptic region will be analyzed to see if it falls prior to the decay; Calcium efflux will be studied to see if intracellular free calcium increases prior to the decay. Each of these three possible agents will be examined more closely if the kinetics are consistent with their involvement. Na efflux will be studied to determine whether inhibition of the Na-pump or increased Na permeability is leading to the depolarization. All measurements will be made on the same region in which the elecrophysiological lesion is occurring. ATP and PCr will be determined fluorimetrically. Changes in pHi will be determined from the reactant concentrations in both the creatine kinase and HCO3 negative -CO2 equilibria. It will also be estimated by measuring lactate production. Ion fluxes will be measured by withdrawing fluid from a region just above the synaptic region between the perforant path and dentate granule cell dendrites. The work will establish whether: 1) a decreased ATP, 2) decreased intracellular pH or 3) increased cytosolic Ca2 ions is causing hypoxic transmission block. It will also determine whether that block results from Na pump inhibition or an Na permeability increase.