Saxitoxin differentiates between two types of Na+-dependent potentials in the Retzius cell of hirudinid leeches

The effects of tetrodotoxin (TTX) and saxitoxin (STX) on the action potentials recorded in Ca2+-free solution in the absence of Ca2+ and K+ currents were investigated in the Retzius cell of three hirudinid leech species (Hirudo medicinalis, Macrobdella decora and Poecilobdella granulosa) and in the glossiphoniid leech Haementeria ghilianii. In the four leech species, stimulation of the Retzius cell in the presence of 25 mmol l-1 tetraethylammonium chloride (TEA), 3 mmol l-1 4-aminopyridine (4-AP) and 2mmol l-1 Mn2+ evoked prolonged action potentials consisting of an initial fast-rising spike followed by a plateau lasting several hundreds of milliseconds. The amplitude and duration of both components of action potentials recorded under these conditions were dependent on [Na+]o. In the hirudinid leeches the initial spike was unaffected by TTX and STX whereas the plateau was selectively blocked by micromolar concentration of STX. In Haementeria both the initial spike and the subsequent plateau were sensitive to nanomolar concentrations of STX and TTX with an estimated ED50 of approximately 20 nmol l-1 for inhibition of Vmax of the fast spike. The results suggest that there are two types of ionic currents mediating the two distinct components of Na+-dependent action potentials in the Retzius cell: (1) a fast-inactivating one, presumably underlying the normal spike which is TTX- and STX-resistant in hirudinid leeches but sensitive to both agents in Haementeria and (2) a low-threshold, prolonged current which underlies the plateau recorded from these cells in the absence of Ca2+ and K+ currents and which is selectively blocked by STX in hirudinid leeches but sensitive to both STX and TTX in Haementeria. It is likely that the variable kinetic and pharmacological properties that characterize the various Na+ potentials in these identified homologous neurones may be of functional significance and result from differences in the molecular structure of their Na+ channels.