GABA_A receptor modulation by the novel intravenous general anaesthetic E-6375

E-6375 (4-butoxy-2-[4-(2-cyanobenzoyl)-1-piperazinyl] pyrimidine hydrochloride) is a new intravenous general anaesthetic with an anaesthetic potency, in mice, comparable to propofol, or etomidate. Here, we examined the effect of E-6375 upon the GABA_A receptor, a putative target of intravenous anaesthetic action. E-6375 reversibly enhanced GABA-evoked currents mediated by recombinant GABA_A (α₁β ₂γ_2L) receptors expressed in Xenopus laevis oocytes, with little effect on NMDA- and kainate-evoked currents mediated by NR1a/NR2A and GluR1o/GluR2o glutamate receptors, respectively. E-6375 prolonged the decay of GABA-evoked miniature inhibitory postsynaptic currents recorded from rat Purkinje neurones demonstrating the anaesthetic also enhanced the activity of synaptic GABA_A receptors. The GABA enhancing action of E-6375 on recombinant GABA_A receptors was unaffected by the subtype of the α isoform (i.e. α_xβ₂γ _2L; x=1-3) within the receptor, but was increased by the omission of the γ_2L subunit. Receptors incorporating β₂, or β₃, subunits were more sensitive to modulation by E-6375 than those containing the β₁ subunit. The selectivity of E-6375 was largely governed by the identity (serine or asparagine) of a single amino acid residue within the second transmembrane domain of the β-subunit. The various in vivo actions of general anaesthetics may be mediated by GABA _A receptor isoforms that have a differential distribution within the CNS. The identification of agents, such as E-6375, that discriminate between GABA_A receptor subtypes may augur the development of general anaesthetics with an improved therapeutic profile.