α5GABAA Receptors Mediate the Amnestic But Not Sedative-Hypnotic Effects of the General Anesthetic Etomidate

Rotman Research Institute, University of Toronto, Toronto, Ontario, Canada
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 05/2006; 26(14):3713-20. DOI: 10.1523/JNEUROSCI.5024-05.2006
Source: PubMed


A fundamental objective of anesthesia research is to identify the receptors and brain regions that mediate the various behavioral components of the anesthetic state, including amnesia, immobility, and unconsciousness. Using complementary in vivo and in vitro approaches, we found that GABAA receptors that contain the alpha5 subunit (alpha5GABAARs) play a critical role in amnesia caused by the prototypic intravenous anesthetic etomidate. Whole-cell recordings from hippocampal pyramidal neurons showed that etomidate markedly increased a tonic inhibitory conductance generated by alpha5GABAARs, whereas synaptic transmission was only slightly enhanced. Long-term potentiation (LTP) of field EPSPs recorded in CA1 stratum radiatum was reduced by etomidate in wild-type (WT) but not alpha5 null mutant (alpha5-/-) mice. In addition, etomidate impaired memory performance of WT but not alpha5-/- mice for spatial and nonspatial hippocampal-dependent learning tasks. The brain concentration of etomidate associated with memory impairment in vivo was comparable with that which increased the tonic inhibitory conductance and blocked LTP in vitro. The alpha5-/- mice did not exhibit a generalized resistance to etomidate, in that the sedative-hypnotic effects measured with the rotarod, loss of righting reflex, and spontaneous motor activity were similar in WT and alpha5-/- mice. Deletion of the alpha5 subunit of the GABAARs reduced the amnestic but not the sedative-hypnotic properties of etomidate. Thus, the amnestic and sedative-hypnotic properties of etomidate can be dissociated on the basis of GABAAR subtype pharmacology.

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    • "It was shown previously that genetic and pharmacologic manipulations that reduce or eliminate inhibitory current carried by GABA A R α5-subunits (α5-KO) are resistant to etomidate's suppression of long term potentiation (LTP) in vitro and learning and memory in vivo (Cheng et al., 2006). Because the majority of α5-GABA A Rs are located extrasynaptically on pyramidal cells, where they mediate a persistent conductance termed " tonic inhibition " (Caraiscos et al., 2004), and tonic inhibition is strongly enhanced by amnestic drugs, it was proposed that the effect of etomidate on synaptic plasticity is due to its enhancement of tonic inhibition (Cheng et al., 2006; Orser, 2007; Martin et al., 2009). Since α5-subunits preferentially partner with β3- subunits (Luddens et al., 1994; Sur et al., 1998), we hypothesized that mice carrying the N265M mutation in the β3-subunit would similarly resist etomidate's enhancement of tonic inhibition and suppression of LTP in vitro and learning in vivo. "
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    ABSTRACT: Enhancement of tonic inhibition mediated by extrasynaptic α5-subunit containing GABAA receptors (GABAARs) has been proposed as the mechanism by which a variety of anesthetics, including the general anesthetic etomidate, impair learning and memory. Since α5 subunits preferentially partner with β3 subunits, we tested the hypothesis that etomidate acts through β3-subunit containing GABAARs to enhance tonic inhibition, block LTP, and impair memory. We measured the effects of etomidate in wild type mice and in mice carrying a point mutation in the GABAAR β3-subunit (β3-N265M) that renders these receptors insensitive to etomidate. Etomidate enhanced tonic inhibition in CA1 pyramidal cells of the hippocampus in wild type but not in mutant mice, demonstrating that tonic inhibition is mediated by β3-subunit containing GABAARs. However, despite its inability to enhance tonic inhibition, etomidate did block LTP in brain slices from mutant mice as well as in those from wild type mice. Etomidate also impaired fear conditioning to context, with no differences between genotypes. In studies of recombinant receptors expressed in HEK293 cells, α5β1γ2L GABAARs were insensitive to amnestic concentrations of etomidate (1 μM and below), whereas α5β2γ2L and α5β3γ2L GABAARs were enhanced. We conclude that etomidate enhances tonic inhibition in pyramidal cells through its action on α5β3-containing GABAA receptors, but blocks LTP and impairs learning by other means - most likely by modulating α5β2-containing GABAA receptors. The critical anesthetic targets underlying amnesia might include other forms of inhibition imposed on pyramidal neurons (e.g. slow phasic inhibition), or inhibitory processes on non-pyramidal cells (e.g. interneurons).
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    • "The effect of THIP on spatial memory was first examined in the Morris water maze, as performance of this task is hippocampusdependent and is regulated by GABA A receptor activity (D'Hooge and De Deyn, 2001; Collinson et al., 2002; Myhrer, 2003; Cheng et al., 2006). WT and Gabrd −/− mice were treated with THIP (4 mg/kg, i.p.) or vehicle 30 min before being trained over 4 trials to locate a hidden platform for memory acquisition. "
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    • "In adult CA1 hippocampus, α5β3γ2 GABARs have high expression extrasynaptically where they localize to the dendritic shaft and to the base of the dendritic spine (Brunig et al., 2002). Both knock-out and knock-down of this receptor, with the use of a selective inverse agonist, prevents impairments in learning following administration of the anesthetic etomidate (Cheng et al., 2006). These procedures also enhance fear conditioning (Collinson et al., 2002; Crestani et al., 2002; Chambers et al., 2003). "
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