Makoto Takenoshita

The α and γ subunit-dependent effects of local anesthetics on recombinant GABAA receptors

Although convulsions due to local anesthetic systemic toxicity are thought to be due to inhibition of GABA(A) receptor-linked currents in the central nervous system, the mechanism of action remains unclear. We therefore examined the effects of local anesthetics on gamma-aminobutyric acid (GABA)-induced currents using recombinant GABA(A) receptors with specific combinations of subunits. Murine GABA(A) receptors were expressed by injection of cRNAs encoding each subunit into Xenopus oocytes. The effects of local anesthetics (lidocaine, bupivacaine, procaine and tetracaine) on GABA-induced currents of receptors expressing different subunit combinations (alpha1beta2, alpha1beta2gamma2s, alpha4beta2gamma2s and beta2) were examined via the two electrode voltage clamp method. At alpha1beta2, alpha1beta2gamma2s and alpha4beta2gamma2s GABA(A) receptors, all local anesthetics inhibited GABA-induced currents in a dose-dependent manner. The presence of the gamma2s subunit resulted in a greater inhibition by all local anesthetics, but the presence of the alpha4 subunit resulted in less inhibition. At beta2 homomeric receptors, local anesthetics directly induced an outward current similar to that of picrotoxin. These data indicated that (1) the alpha and gamma subunits of GABA(A) receptors modulated the inhibitory effects of local anesthetics on GABA(A) function, and (2) local anesthetics can activate the beta2 subunit and may block the GABA(A) receptor channel pore.

Gamma subunit dependent modulation by nitric oxide (no) in recombinant Gabaa receptor

The effect of nitric oxide (NO) on GABA induced Cl− current of recombinant GABAA receptors was studied. Either α1β2γ2s or α1β2 subunit mRNAs synthesized from cDNA of mouse brain were injected into Xenopus oocytes and functional GABAA receptors were expressed. GABA-induced Cl− current was measured with the two electrode voltage clamp technique. The NO donor NOC-18 reduced the GABA-induced Cl− current in the α1β2γ2s subunit receptor in a dose-dependent manner. In α1,β2 subunit receptor, NOC-18 had no effects on GABA-induced currents at low concentrations but showed potentiation at high concentration. These effects were antagonized by the NO extinguisher, carboxy-PTIO. The cGMP analogue 8-Br-cGMP failed to induce NO-like effects. NO directly acts at the GABAA receptor and the γ2s subunit is involved in its action.

The effects of a point mutation of the β2 subunit of GABAA receptor on direct and modulatory actions of general anesthetics

The gamma-aminobutyric acid type A receptor (GABA(A) receptor) sites involved in the direct and modulatory actions of general anesthetics remain to be elucidated. The mutation of tyrosine at position 157 in the beta2 GABA(A) receptor subunit was reported to reduce sensitivity to activation by GABA, but not pentobarbital. We examined whether this mutation of the beta2 subunit (Tyr157-->Phe) affects the direct and modulatory actions of other general anesthetics such as propofol and etomidate. Using the two-electrode voltage clamp method, we recorded Cl- current in Xenopus oocytes expressing alpha1beta2gamma2s and alpha1-mutated beta2gamma2s subunits. The mutation of the beta2 subunit reduced the apparent affinity for propofol. However, the mutation had no effect on both the direct actions of pentobarbital and etomidate or on the modulatory actions of pentobarbital, propofol and etomidate. These results suggest that unique loci may exist for the direct action of propofol and that the GABA binding site may not mediate the modulatory actions of general anesthetics at GABA(A) receptors.

Extracorporeal membrane oxygenation and tracheobronchial foreign body in an infant

Inadvertent aspiration of foreign bodies into the tracheobronchial region occurs most often in children between 1 and 3 years old [1,2]. In the anesthetic management of removal of a tracheobronchial foreign body, difficulty arises when the ventilation route coexists with the surgical field in the narrow lumen of the tracheobronchial tree. It is most important to choose a ventilation technique which does not disturb the surgical procedure, because lethal asphyxia may occur if the foreign body occludes the trachea or main bronchus. We report the successful use of extracorporeal membrane oxygenation (ECMO) for the removal of a tracheal foreign body which led to severe hypoxia.