Electrophysiological studies of the GABA A 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 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 μ m 4‐PIOL was completely blocked by the GABA A antagonist, bicuculline methobromide (BMB, 10 μ m ). The pA 2 of BMB was >6.46. With 2 m m 4‐PIOL about 15% of the response remained in the presence of 100 μ 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 m m 4‐PIOL was only a small fraction of that evoked by submaximal concentrations of isoguvacine. 5 Setting the response to 1 m m 4‐PIOL as maximum, the EC 50 for 4‐PIOL was 91 μ m (95% confidence limits: 73–114 μ 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 μ m (95% confidence limits: 102–130 μ m ). This was not significantly different from EC 50 ( 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.