Extrasynaptic GABA(A) channels activated by THIP are modulated by diazepam in CA1 pyramidal neurons in the rat brain hippocampal slice.

Single-channel currents were activated by THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) in cell-attached patches on CA1 pyramidal neurons in the rat hippocampal slice preparation. THIP activated GABA(A) channels after a delay that was concentration-dependent and decreased by 1 microM diazepam. The currents showed outward rectification. Channels activated at depolarized 40 mV relative to the chloride reversal potential had low conductance (<40 pS) but the conductance increased with time, resulting in high-conductance channels (>40 pS). The average maximal-channel conductances for 2 and 100 microM THIP were 59 and 62 pS (-Vp = 40 mV), respectively, whereas in 2 microM THIP plus 1 microM diazepam, it was 71 pS. The results show that in hippocampal neurons THIP activates channels with characteristics similar to those of channels activated by low concentrations (0.5-5 microM ) of GABA. The increase in the inhibitory conductance with membrane depolarization permits gradation of the shunt pathway relative to the level of the excitatory input.