Perioperative stroke remains a major risk during surgery. Volatile anesthetics protect against experimental brain ischemia but the mechanism is not defined. We hypothesize that volatile anesthetics potentiate GABAergic neurotransmission and enhance CI- influx. This hyperpolarizes neurons delaying time to ischemic depolarization and Ca2++ influx. This hypothesis is derived from observations that volatile anesthetics potentiate GABAA receptors and bicuculline, a GABAA antagonist, reverses isoflurane protection in vitro. We also hypothesize that volatile anesthetic GABAergic properties are more important to protection than glutamate receptor antagonistic properties. We propose these Specific Aims: 1) Define a dose-response for isoflurane protection during rat forebrain ischemia/Compare with efficacy of muscimol, a GABAA receptor agonist/Compare with time to onset of ischemic depolarization and pre-ischemic cerebral metabolic rate; 2) Determine if isoflurane neuroprotection against severe forebrain ischemia is permanent; 3) Compare the relative neuroprotective effect of selective NMDA/AMPA receptor antagonism to isoflurane, which also possesses GABAergic potentiation; 4) Determine the role of GABAA potentiation in isoflurane protection against severe forebrain ischemia.
For Specific Aim #4, we will examine: a) if isoflurane protection against forebrain ischemia or striatal NMDA microinjections is reversed by GABAA antagonists (flurothyl, bicuculline, flumazenil), b) if isoflurane delays time to ischemia induced Ca2++ influx and if this is reversed by GABAA antagonists, c) if correction for this delay, by extending ischemia duration, equivalently reverses neuroprotection, d) effects of GABAA beta subunit-targeted deletion (knockout) or striatal antisense oligonucleotide microinjection on in vivo isoflurane protection, e) the extent to which isoflurane provides protection in organotypic hippocampal slices against NMDA excitotoxicity or oxygen/glucose deprivation and respective effects on CI- uptake, f) the extent to which this is reversed by antagonists of the GABAA, GABAB, and strychnine sensitive glycine receptors, and g) relationships between isoflurane and GABAA antagonists on CI- and Ca ++uptake in NMDA stimulated synaptoneurosomes. We believe that GABAAergic pharmacologic properties of volatile anesthetics known to be critical for anesthesia are the same properties that confer cerebral protection.
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