Modulation of recombination human gamma-aminobutyric acidA receptors by isoflurane: influence of the delta subunit.
BACKGROUND
The gamma-aminobutyric acid (GABA)A receptor/chloride channel has a broad-spectrum anesthetic sensitivity and is a key regulator of arousal. Each receptor/channel complex is an assembly of five protein subunits. Six subunit classes have been identified, each containing one to six members; many combinations are expressed throughout the brain. Benzodiazepines and intravenous anesthetic agents are clearly subunit dependent, but the literature to date suggests that volatile anesthetics are not. The physiological role of the delta subunit remains enigmatic, and it has not been examined as a determinant of anesthetic sensitivity.


METHODS
Combinations of GABA(A) receptor subunit cDNAs were injected into Xenopus laevis oocytes: alpha1beta1, alpha1beta1gamma2L, alpha1beta1delta, and alpha1beta1gamma2Ldelta. Expression of functional ion channels with distinct signalling and pharmacologic properties was demonstrated within 1-4 days by established electrophysiological methods.


RESULTS
Co-expression of the delta subunit produced changes in receptor affinity; current density; and the modulatory efficacy of diazepam, zinc, and lanthanum; it also produced subtle changes in the rate of desensitization in response to GABA. Isoflurane enhanced GABA-induced responses from all combinations: alphabeta delta (>10-fold) > alphabeta > alphabeta gamma > or = alphabeta gammadelta (approximately 5-fold). Dose-response plots were bell shaped. Compared with alphabeta gamma receptors (EC50 = 225 microM), both alphabeta delta (EC50 = 372 microM) and alphabeta gammadelta (EC50 = 399 microM) had a reduced affinity for isoflurane. Isoflurane (at a concentration close to the EC50 for each subunit) increased the affinity of GABA for its receptor but depressed the maximal response (alphabeta gamma and alphabeta gammadelta). In contrast, the small currents through alphabeta delta receptors were enhanced, even at saturating agonist concentrations.


CONCLUSIONS
Delta subunit expression alters GABA(A) receptor function but is not an absolute determinant of anesthetic sensitivity.
Lees
G
G
Academic Department of Anesthetics, Imperial College of Science, Technology and Medicine at St. Mary's Hospital Medical School, London, United Kingdom. george.lees@sunderland.ac.uk
Edwards
M D
MD
D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D001921:Brain / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D007530:Isoflurane / Q000494:pharmacology*; D011963:Receptors, GABA-A / Q000737:chemistry / Q000187:drug effects*; D011994:Recombinant Proteins / Q000187:drug effects; D014982:Xenopus laevis
D018685:Anesthetics, Inhalation; D011963:Receptors, GABA-A; D011994:Recombinant Proteins; D007530:Isoflurane