Electrophysiological studies of the GABAA receptor ligand, 4-PIOL, on cultured hippocampal neurones.
1. Whole-cell, patch-clamp recordings from cultured hippocampal neurones have been used to characterize the action of the GABAA ligand, 5-(4-piperidyl)isoxazol-3-ol (4-PIOL). The action of 4-PIOL was compared with that of the established GABAA agonist, isoguvacine. 2. With a symmetrical Cl- gradient across the membrane and a holding potential of -60mV, both isoguvacine and 4-PIOL evoked an inward current. The reversal potentials of the responses to both agents were identical (+8.8 mV, n = 4) and the current/voltage relationships showed outward-going rectification. 3. The response to 300 microM 4-PIOL was completely blocked by the GABAA antagonist, bicuculline methobromide (BMB, 10 microM). The pA2 of BMB was greater than 6.46. With 2 mM 4-PIOL about 15% of the response remained in the presence of 100 microM BMB. This may represent a non-specific component of the response to large concentrations of 4-PIOL. 4. 4-PIOL was about 200 times less potent as an agonist than isoguvacine. because of the rapid fade (desensitization) of isoguvacine-induced currents, the maximum response to this agonist was not determined. However, the response to 2 mM 4-PIOL was only a small fraction of that evoked by submaximal concentrations of isoguvacine. 5. Setting the response to 1 mM 4-PIOL as maximum, the EC50 for 4-PIOL was 91 microM (95% confidence limits:73-114 microM). 6. 4-PIOL antagonized the response to isoguvacine with a parallel shift to the right of the dose-response curve. The antagonist action of 4-PIOL was about 30 times weaker than that of BMB. When allowance was made for the intrinsic agonist action of 4-PIOL, the Ki was 116p microM (95% confidence limits: 102-130 microM). This was not significantly different from EC5, (P = 0.86; non-parametric Mann-Whitney test).7. It is concluded that 4-PIOL is a partial agonist at the GABAA receptor on cultured hippocampal neurones.
Kristiansen
U
U
Department of Biology, Royal Danish School of Pharmacy, Copenhagen.
Lambert
J D
JD
Falch
E
E
Krogsgaard-Larsen
P
P
D000818:Animals; D001640:Bicuculline / Q000031:analogs & derivatives / Q000494:pharmacology; D002478:Cells, Cultured; D005260:Female; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D007539:Isonicotinic Acids / Q000494:pharmacology; D007555:Isoxazoles / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D010880:Piperidines / Q000494:pharmacology*; D011247:Pregnancy; D051381:Rats; D011963:Receptors, GABA-A / Q000187:drug effects*; D013116:Spinal Cord / Q000166:cytology
D007539:Isonicotinic Acids; D007555:Isoxazoles; D010880:Piperidines; D011963:Receptors, GABA-A; C073009:5-(4-piperidyl)isoxazol-3-ol; C036615:bicuculline methobromide; D001640:Bicuculline; C015300:isoguvacine