α5GABAA receptors mediate the amnestic but not sedative-hypnotic effects of the general anesthetic etomidate

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 GABA(A) receptors that contain the alpha 5 subunit (alpha 5GABA(A)Rs) 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 alpha 5GABA(A)Rs, 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 alpha 5 null mutant (alpha 5-/-) mice. In addition, etomidate impaired memory performance of WT but not alpha 5-/- 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 alpha 5-/- 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 alpha 5-/- mice. Deletion of the alpha 5 subunit of the GABA(A)Rs 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 GABA(A)R subtype pharmacology.