Conductance of GABA_A channels activated by pentobarbitone in hippocampal neurons from newborn rats

Neurons were obtained from the CA1 region of the hippocampus of newborn rats and maintained in culture. Channels were activated by pentobarbitone in cell-attached, inside-out or outside-out patches, normally by applying pentobarbitone in flowing bath solution. Currents were outwardly rectifying and blocked by bicuculline, properties of GABA_A channels in these cells. Maximum channel conductance increased as pentobarbitone concentration was increased to 500 μM but conductance then decreased as pentobarbitone concentration was raised further. The best fit of a Hill-type equation to the relationship between maximum channel conductance and pentobarbitone concentration (up to 500 μM) gave an EC₅₀ of 41 μM, a maximum conductance of 36 pS and a Hill coefficient of 1.6. Bicuculline decreased the maximum conductance of the channels activated by pentobarbitone, with an IC₅₀ of 224 μM. Diazepam increased channel conductance, with a maximum effect being obtained with 1 μM diazepam. Diazepam (1 μM) decreased the EC₅₀ of the pentobarbitone effect on channel conductance from 41 μM to 7.2 μM and increased maximum conductance to 72 pS. We conclude that GABA_A channel conductance is related to the concentration of the allosteric agonist pentobarbitone.