Subunit-dependent block by isoflurane of wild-type and mutant alpha(1)S270H GABA(A) receptor currents in Xenopus oocytes.
The volatile anesthetic isoflurane both prolongs and reduces the amplitude of GABA-mediated inhibitory postsynaptic currents (IPSCs) recorded in neurons. To explore the latter effect, we investigated isoflurane-induced inhibition of steady-state desensitized GABA currents in Xenopus oocytes expressing wild-type alpha(1)beta(2), alpha(1)beta(2)gamma(2s), mutant alpha(1)(S270H)beta(2) (serine to histidine at residue 270) or alpha(1)(S270H)beta(2)gamma(2s) receptors. The alpha(1) serine 270 site in TM2 (second transmembrane domain of the subunit) is postulated as a binding site for some volatile agents and is critical for positive modulation of sub-maximal GABA responses by isoflurane. For all receptor combinations, at < or =0.6 mM isoflurane (< or =2 minimum alveolar concentration (MAC)) current inhibitions were not pronounced ( approximately 10%) with block reaching half-maximal levels at supraclinical concentrations ( approximately 2 mM isoflurane, 6 MAC). Comparisons with other GABA(A) receptor blockers indicated that isoflurane blocks in a similar manner to picrotoxin, possibly via the pore of the receptor. The extent of isoflurane-induced inhibition was significantly attenuated by inclusion of the gamma(2s)-subunit but was unaffected by introduction of the S270H mutation in the alpha(1)-subunit. In conclusion, isoflurane binds with low affinity and with subunit-specificity to an inhibitory site on the GABA(A) receptor that is distinct from the site that facilitates positive modulation at the extracellular end of the pore.
Hall
Adam C
AC
Neuroscience and Biochemistry Programs, Department of Biological Sciences, Clark Science Center, Smith College, Northampton, MA 01063, USA. ahall@science.smith.edu
Stevens
Renna J N
RJ
Betts
Brooke A
BA
Yeung
Wing-Yee
WY
Kelley
Jill C
JC
Harrison
Neil L
NL
D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D018756:GABA Antagonists / Q000494:pharmacology; D007530:Isoflurane / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects* / Q000502:physiology; D009154:Mutation; D009865:Oocytes / Q000187:drug effects; D018408:Patch-Clamp Techniques; D010852:Picrotoxin / Q000494:pharmacology; D021122:Protein Subunits / Q000187:drug effects* / Q000378:metabolism; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000187:drug effects; D013379:Substrate Specificity; D014981:Xenopus
D018685:Anesthetics, Inhalation; D001640:Bicuculline; D018756:GABA Antagonists; D007530:Isoflurane; D010852:Picrotoxin; D021122:Protein Subunits; D011963:Receptors, GABA-A; D011994:Recombinant Proteins