Etomidate does not alter recovery after anoxia of evoked population spikes recorded from the CA1 region of rat hippocampal slices.

BACKGROUND: Etomidate is an anesthetic agent that reduces the cerebral metabolic rate and causes minimal cardiovascular depression. Its ability to improve recovery after anoxia or ischemia is equivocal. An in vitro neuronal preparation was used to examine the action of etomidate on electrophysiologic and biochemical parameters during and after anoxia. METHODS: The Schaffer collateral pathway was stimulated, and a postsynaptic evoked population spike was recorded from the CA1 pyramidal cell layer of rat hippocampal slices. Etomidate or propylene glycol, its solvent, was present 15 min before, during, and 10 min after anoxia. Adenosine triphosphate, sodium, and potassium concentrations were measured at the end of anoxia in tissue treated with etomidate, propylene glycol, or with no added drugs. RESULTS: Etomidate did not alter recovery after 6 min of anoxia. The population spikes from untreated slices recovered to 32% of their preanoxic amplitude, and slices treated with 0.5, 3, and 30 microg/ml etomidate recovered to 24%, 35%, and 13%, respectively. Slices treated with propylene glycol, equivalent to that in 3 and 30 microg/ml etomidate, recovered to 46% and 12%, respectively, and this was not significantly different from untreated slices. Etomidate did not attenuate the decrease in adenosine triphosphate concentrations during anoxia. The increase in sodium and the decrease in potassium during anoxia were significantly attenuated by 30 but not by 3 microg/ml etomidate. CONCLUSIONS: A range of etomidate concentrations did not significantly alter recovery of the evoked population spike after anoxia in rat hippocampal slices. A high concentration of etomidate did attenuate the increase in sodium and the decrease in potassium during anoxia.

Anoxia reduces depolarization induced calcium uptake in the rat hippocampal slice.

Veratridine-induced depolarization caused a large increase in Ca uptake in the rat hippocampal slice (30.2 vs. 9.0 nM/mg dry weight). This uptake was reduced to 18.4 nM/mg when veratridine was combined with anoxia. When compared with veratridine exposure alone, the combination of anoxia and veratridine increased intracellular Na (460 vs. 380 microM/g), decreased intracellular K (30 vs. 40 microM/g) and decreased ATP levels (0.1 vs. 0.8 nM/mg). The changes in Na, K, and ATP should enhance net Ca uptake, yet Ca uptake was reduced. This suggests an effect of anoxia to block Ca channels. In summary anoxia attenuates depolarization-induced Ca uptake. This may represent a mechanism by which neurons are partially protected against anoxic damage which could be more severe if depolarization-induced Ca uptake was not limited.

The barbiturate thiopental reduces ATP levels during anoxia but improves electrophysiological recovery and ionic homeostasis in the rat hippocampal slice.

The barbiturate anesthetic thiopental enhances recovery of the evoked population spike recorded from rat hippocampal slices after short periods of anoxia. Thiopental reduces changes in sodium, potassium and calcium but enhances the fall in ATP levels during anoxia. The postsynaptic population spike recorded from the CA1 pyramidal cell region of the slices treated with thiopental (600 microM) recovered to 67% of the preanoxic amplitude after 3.5 min of anoxia. There was less recovery (24%) when a lower concentration of thiopental (250 microM) was used. Untreated slices recovered to only 10% of their preanoxic amplitude after 3.5 min of anoxia. Other studies have demonstrated that maintaining ATP levels during anoxia may be an important mechanism of protection. In contrast to those studies, thiopental was protective although it enhanced the fall of ATP levels after 3.5 min of anoxia in the CA1 region and after 3.5 and 5 min in the dentate region. Thus enhanced recovery of the population spike with thiopental is not due to its preservation of ATP levels. This result allows a clear separation of improved ATP levels during anoxia from other mechanisms of protection. We therefore looked for other mechanisms of protection. Sodium and potassium levels were measured after 10 min of anoxia. In untreated tissue, sodium levels in the slice rose and potassium levels fell significantly. In thiopental-treated tissue, changes in sodium and potassium caused by anoxia and by veratridine under normoxic conditions were significantly reduced. During anoxia calcium-45 uptake increases; thiopental significantly reduces this uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Midazolam improves electrophysiologic recovery after anoxia and reduces the changes in ATP levels and calcium influx during anoxia in the rat hippocampal slice.

Since blockers of excitatory transmission have been shown to reduce anoxic and ischemic neuronal damage, augmentation of inhibitory transmission by agents such as midazolam might have a similar protective effect. Rat hippocampal slices were maintained in vitro and used to determine whether and by what mechanism midazolam improves recovery of evoked responses after anoxia. The Schaffer collateral pathway in the slice was stimulated electrically, and an extracellular potential, the evoked population spike, was recorded from the CA1 pyramidal cells, which are postsynaptic. The slices were made anoxic by substituting artificial cerebrospinal fluid aerated with 95% nitrogen-5% carbon dioxide for fluid aerated with 95% oxygen-5% carbon dioxide. Percentage recovery was expressed as the amplitude of the evoked population spike 60 min after anoxia divided by its preanoxic amplitude. Protection in this model is defined as a significant (P less than 0.05) improvement in percentage recovery compared to the recovery of untreated slices. There was no recovery of the response recorded from CA1 pyramidal cells after 5 min of anoxia (4 +/- 2%) (mean +/- standard error of the mean [SEM]). Slices were treated with midazolam 10 min before, during, and 10 min after anoxia. Midazolam (1 microM) did not enhance recovery after anoxia when dissolved either in water (3 +/- 3%) or in dimethyl sulfoxide (DMSO) (1 +/- 1%). A higher concentration of midazolam (100 microM) did enhance recovery when dissolved in DMSO (27 +/- 7%) but not when dissolved in water (5 +/- 2%). To test whether prolonged pretreatment with midazolam dissolved in water would enhance recovery, slices were treated for 30 min prior to anoxia.(ABSTRACT TRUNCATED AT 250 WORDS)

Methods for studying the effect of anesthetics on anoxic damage in the rat hippocampal slice.

The rat hippocampal slice was used as a model system to study the effects of anesthetics on anoxic damage. Thiopental, but not isoflurane, allowed recovery of the postsynaptic population spike evoked from the dentate granule cells. Creatine preincubation protected both dentate granule and CA1 pyramidal cells against anoxic damage. Calcium-free, 10 mM magnesium artificial cerebrospinal fluid (ACSF) was shown to protect CA1 pyramidal cells against anoxic damage. The presynaptic population spike recovered to its preanoxic amplitude after anoxia even under conditions where the postsynaptic population spike demonstrated no recovery. Thus the hippocampal slice is a useful system for studying the effects of anesthetics and other pharmacological agents on anoxic damage.