Effects of GABA receptor activation on theV_m of intact CA1 interneurons.V_m of two CA1 interneurons under control conditions and in the presence of bath-applied GABA and TTX (A) and GABA and NBQX/APV (B) monitored from the cell-attached K(V) current reversal. V_m of both interneurons fluctuated spontaneously, which was associated with AP activity (see current trace in A). The fluctuations were largely suppressed by TTX (0.5 μm; A) or by NBQX/APV (20 and 100 μm; B), indicating that they arise from excitatory input to the cell, although the AHPs after action potentials contribute to the hyperpolarizing deflections (A,asterisks). The principal effect of bath-applied GABA in P7–P21 animals is a stabilization of theV_m, usually at a level that is not significantly different (A) or slightly depolarized (B) from the average control level. The inset shows for each condition the reversal of K(V) currents from 10 consecutive cell-attached responses to ramp stimuli applied at 2 sec intervals. C, Summary of GABA effects on interneurons from P7–P21 (left) and P1–P4 rats (right). In the older animals, GABA causedV_m to stabilize at a potential close to control level, whereas in young animals it induced a membrane depolarization. Cells are aligned according to the averageV_m. Minimum and maximumV_m levels are indicated in the presence and absence of GABA. The mean V_m of cells in the two age groups was not significantly different (p > 0.1). The black circlesunder the graph indicate the approximate age of the animal, with each postnatal week corresponding to one circle; an empty half circle indicates 1-to 2-d-old rats. Cells originating from the same animal are identified with symbols.