Studies on the mechanisms of action of picrotoxin, quercetin and pregnanolone at the GABA rho 1 receptor.
1. The mechanisms of action of antagonists of the gamma-aminobutyric acid C (GABA(C)) receptor picrotoxin, quercetin and pregnanolone were studied. 2. Ionic currents (chloride), mediated through human homomeric GABA rho(1) receptors expressed in Xenopus oocytes, were recorded by two-electrode voltage clamp. 3. Dose-response (D-R) curves and kinetic measurements of GABA rho(1) currents were carried out in the presence or absence of antagonists. Use-dependent actions were also evaluated. 4. Picrotoxin, quercetin and pregnanolone exerted noncompetitive actions. 5. IC(50) values measured at the EC(50) for GABA (1 microM) were as follows: picrotoxin 0.6+/-0.1 microM (Hill coefficient n=1.0+/-0.2); quercetin 4.4+/-0.4 microM (n=1.5+/-0.2); pregnanolone 2.1+/-0.5 microM (n=0.8+/-0.1). 6. These antagonists produced changes only in the slope of the linear current-voltage relationships, which was indicative of voltage-independent effects. 7. The effect of picrotoxin on GABA rho(1) currents was use-dependent, strongly relied on agonist concentration and showed a slow onset and offset. The mechanism was compatible with an allosteric inhibition and receptor activation was a prerequisite for antagonism. 8. The effect of quercetin was use-independent, showed relatively fast onset and offset, and resulted in a slowed time course of the GABA-evoked currents. 9. The effect of pregnanolone was use-independent, presented fast onset and a very slow washout, and did not affect current activation. 10. All the antagonists accelerated the time course of deactivation of the GABA rho(1) currents.
Goutman
Juan D
JD
Departamento de Fisiología, Biología Molecular y Celular, Instituto de Investigaciones en Ingeniería Genética y Biología Molecular, Consejo Nacional de Investigaciones Científicas y Técnicas, Universidad de Buenos Aires, República Argentina.
Calvo
Daniel J
DJ
D000818:Animals; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005071:Evoked Potentials / Q000187:drug effects; D018756:GABA Antagonists / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D010852:Picrotoxin / Q000494:pharmacology*; D011280:Pregnanolone / Q000494:pharmacology*; D011794:Quercetin / Q000494:pharmacology*; D018080:Receptors, GABA-B / Q000187:drug effects* / Q000235:genetics; D011994:Recombinant Proteins / Q000187:drug effects; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D018076:DNA, Complementary; D018756:GABA Antagonists; D010852:Picrotoxin; D011280:Pregnanolone; D011794:Quercetin; D018080:Receptors, GABA-B; D011994:Recombinant Proteins; D005680:gamma-Aminobutyric Acid