The overall objective of this proposal is to understand factors that determine the extracellular levels of endogenous adenosine during conditions of cerebral hypoxia and/or ischemia.Adenosine acts as a potent endogenous neuroprotectant and augmentation of adenosinergic tone appears to be a reasonable basis for therapeutic intervention in cerebral ischemia. To model various states of metabolic stress, the rat hippocampal slice is exposed to conditions of hypoxia, hypoglycemia and in vitro ischemia which is combined hypoxia + hypoglycemia. We correlate electrophysiological indices of neuronal responsiveness with HPLC and isotope measurements of adenosine influx. In the last grant period, we show that a moderate elevation in levels of the inhibitory neuromodulator adenosine contributes to the early, reversible depression of synaptic transmission during hypoxia, hypoglycemia and in vitro ischemia. When exposed to extended conditions of in vitro ischemia, adenosine efflux abruptly rises four to sixfold in close temporal association with an anoxic depolarization whose occurrence is strongly correlated with the irreversible loss of synaptic transmission. The stimulus or trigger for this relatively large release of adenosine is not known. In vitro ischemia is associated with a number of events including: '1) increased intracellular Ca2+,2) excessive release of excitatory amino acids, particularly glutamate, 2) an anoxic depolarization, and 4) ATP depletion. Under normoxic conditions each of this stimuli may affect extracellular adenosine levels. Based on our work and that of others we propose the following working, hypothesis: The anoxic depolarization induces the release of glutamate which, through activation of glutamate receptors, initiates a large Ca2+ influx. The Ca2+ influx triggers, perhaps through ATP depletion, a substantial release of adenosine. The proposed studies will provide insight into the acute neuronal response to ischemic conditions and will provide an increased understanding into adenosine's role as an endogenous neuroprotectant and its potential use in therapeutic 'intervention of cerebral ischemia.
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