Fig. 6.

Blockers of I_NaPsuppress subthreshold ADPs in low [Ca²⁺]_o. A–C, Recordings of subthreshold ADPs evoked by 3 msec threshold-straddling stimuli were made in three CA1 PCs in slices perfused sequentially with standard saline (2 Ca²⁺), Ca²⁺-free saline (0 Ca²⁺), and Ca²⁺-free saline containing a drug known to suppressI_NaP. In the three PCs, when the stimulus failed to trigger a spike, it evoked a slow depolarizing potential that declined more slowly to resting potential than expected from passive membrane charging. Lowering [Ca²⁺]_o induced a subthreshold ADP in these PCs. The latter potential was suppressed by 0.2 μm TTX (20 min; A), 50 μm phenytoin (30 min; B), and 2 μm PDB (40 min; C).