Enhancement of persistent Na+ current by sea anemone toxin (ATX II) exerts dual action on hippocampal excitability

We used anemone toxin II (ATX II) to study how a selective enhancement of persistent Na+ current (INaP) would affect the excitability of CA1 pyramidal neurons in the hippocampal slice. In whole-cell recordings from CA1 cell somata, local application of ATX II (10 microM) into the stratum pyramidale invariably depolarized the neurons and produced sustained burst discharges with depolarizing plateau potentials of variable amplitude and length. However, the strong excitatory action of ATX II, observed on the single cell level, was not mirrored in field potential recordings from the same hippocampal subfield. The amplitude of the electrically evoked population spike declined, reflecting the decreased availability of fast Na+ channels, and the intracellulary recorded burst discharges were not detected by the field electrode. The lacking synchronization of cellular bursting activity was seen during both local and bath application of ATX II, suggesting that the toxin, in addition to promoting burst discharges of individual neurons, simultaneously dampens network excitability. In fact, ATX II reduced afferent fibre volleys (reflecting axonal excitability) and field excitatory postsynaptic potentials (EPSPs) in a similar fashion. As the expression of different Na+ channel subtypes appears to be compartmentalized within hippocampal neurons, we propose that point mutations leading to pathologically enhanced INaP might exert quite opposite effects, depending on the type and location of the Na+ channel affected. Whereas alterations of somatodendritic Na+ channels would give rise to bursting activity, alterations of axonal Na+ channels would primarily decrease network excitability.