Electrophysiological studies of the GABA_A receptor ligand, 4‐PIOL, on cultured hippocampal neurones

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. 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. The response to 300 μm 4‐PIOL was completely blocked by the GABA_A antagonist, bicuculline methobromide (BMB, 10 μm). The pA₂ of BMB was >6.46. With 2 mm 4‐PIOL about 15% of the response remained in the presence of 100 μm BMB. This may represent a non‐specific component of the response to large concentrations of 4‐PIOL. 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. Setting the response to 1 mm 4‐PIOL as maximum, the EC₅₀ for 4‐PIOL was 91 μm (95% confidence limits: 73–114 μm). 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 μm (95% confidence limits: 102–130 μm). This was not significantly different from EC₅₀ (P = 0.86; non‐parametric Mann‐Whitney test). It is concluded that 4‐PIOL is a partial agonist at the GABA_A receptor on cultured hippocampal neurones. 1991 British Pharmacological Society