ELECTROPHYSIOLOGICAL STUDIES OF THE GABA(A) RECEPTOR LIGAND, 4-PIOL, ON CULTURED HIPPOCAMPAL-NEURONS

1 Whole-cell, patch-clamp recordings from cultured hippocampal neurones have been used to characterize the action of the GABA(A) ligand, 5-(4-piperidyl)isoxazol-3-ol (4-PIOL). The action of 4-PIOL was compared with that of the established GABA(A) agonist, isoguvacine. 2 With a symmetrical Cl- gradient across the membrane and a holding potential of -60 mV, 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-mu-M 4-PIOL was completely blocked by the GABA(A) antagonist, bicuculline methobromide (BMB, 10-mu-M). The pA2 of BMB was > 6.46. With 2 mM 4-PIOL about 15% of the response remained in the presence of 100-mu-M 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-mu-M (95% confidence limits: 73-114-mu-M). 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 K(i) was 116-mu-M (95% confidence limits: 102-130-mu-M). This was not significantly different from EC50 (P = 0.86; non-parametric Mann-Whitney test). 7 It is concluded that 4-PIOL is a partial agonist at the GABA(A) receptor on cultured hippocampal neurones.