PROPERTIES OF VOLTAGE-ACTIVATED IONIC CURRENTS IN CELLS FROM THE BRAINS OF THE TRICLAD FLATWORM BDELLOURA-CANDIDA

Cells were dispersed from the brains of the triclad flatworm Bdelloura candida and maintained in primary culture for up to 2 weeks. Cultured cells assumed a variety of morphologies consistent with those of neur ones in vivo. Whole-cell voltage-clamp recordings from cultured cells revealed that these cells possess a variety of ionic currents, includi ng a fast transient sodium current, a calcium current and several pota ssium currents. The sodium current does not inactivate completely but instead decays to a steady-state component which has the same physiolo gy and pharmacology as the fast transient component, suggesting that t he two components are carried by the same population of channels. The physiology and pharmacology of these various currents were not remarka ble save for the fact that, contrary to earlier reports, all sodium cu rrents examined were sensitive to tetrodotoxin (TTX). These animals ar e, therefore, the lowest animals known to possess TTX-sensitive sodium currents and, as such, represent a major stage in sodium channel evol ution.