Actions and interactions of GABA and benzodiazepines in the mouse hippocampal slice

Intracellular recordings have been made from CA1, CA3 and dentate cells of the mouse hippocampal slice. Gamma-aminobutyric acid (GABA) and the water-soluble benzodiazepines, midazolam and flurazepam were applied close to the impaled cell somata by microelectrophoresis. GABA always caused a fall in input resistance, although the associated changes in membrane potential were variable. These were consistent with reports which have defined a hyperpolarizing response to somatic and a depolarizing response to dendritic application of GABA to CA1 and CA3 cells in the rat and guinea-pig. In this study, the phenomenon was seen in CA1, CA3 and dentate cells. The reversal potential for the hyperpolarizing, somatic response to GABA lay between -70 and -75 mV, similar to the reversal potential of the evoked recurrent inhibitory post-synaptic potential (i.p.s.p.). The change in cell input conductance caused by GABA was larger at membrane potentials positive to the resting potential and smaller at hyperpolarized membrane potentials. Extracellular recordings of action potential frequency were made from ten cells in which the application of either midazolam or flurazepam increased the inhibitory potency of GABA. In three of these cells, the benzodiazepine reduced action potential frequency slightly when applied alone. Neither midazolam nor flurazepam had a consistent effect on membrane potential, resting input resistance or the current-voltage (I-V) relations of the cells when ejected alone. In twenty-eight of forty-one cells examined in detail, ejection of either midazolam or flurazepam was found to increase the response to GABA. In six cells, the response to GABA was significantly reduced by the benzodiazepine tested whilst in the remainder, no interaction of the drugs could be demonstrated. Examination of the dose-response relation for GABA alone and in the presence of midazolam or flurazepam showed that the maximal response to GABA was increased by the benzodiazepine in some cells while it was unchanged in others. When intracellular electrodes were filled with potassium chloride, spontaneous depolarizing GABA-mediated post-synaptic potentials (p.s.p.s) were seen. Measurement of the interval and amplitude distributions of these events showed that flurazepam increased their amplitude but not their frequency.