Fig. 5.

Blockers of I_NaPsuppress spike ADPs in low [Ca²⁺]_o.A–C, Recordings of single spikes evoked by 3 msec 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 the native spike was followed by an active ADP. Lowering [Ca²⁺]_o caused spike broadening, blocked the fast AHP, and enhanced the active ADP. The latter potential was completely depressed after 20 min exposure to 0.2 μm TTX (A). Likewise, it was markedly reduced after 30 min exposure to 50 μmphenytoin (B) or 5 μm PDB (C).