K+ currents in cultured neurones from a polyclad flatworm

Cells from the brain of the polyclad flatworm Notoplana atomata were disper sed and maintained in primary culture for up to 3 weeks. Whole-cell patch-c lamp of presumed neurones revealed outwardly directed K+ currents that comp rised, in varying proportions, a rapidly activating (time constant tau =0.9 4+/-0.79ms; N=15) and inactivating (tau =26.1+/-1.9ms; N=22) current and a second current that also activated rapidly (tau =1.1+0.2 ms; N=9) (means +/ - S.E.M.) but did not inactivate within 100 ms. Both current types activate d over similar voltage ranges. Activation and steady-state inactivation ove rlap and are markedly rightward-shifted compared with most Shaker-like curr ents (half-activation of 16.9+/-1.9 mV, N=7, half-inactivation of -35.4+/-3 .0mV, N=5). Recovery from inactivation was rapid (50+/-2.5ms at -90mV). Bot h currents were unaffected by tetraethylammonium (25 mmol l(-1)), whereas 4 -aminopyridine (10 mmol l(-1)) selectively blocked the inactivating current . The rapidly inactivating current, like cloned Kf channels from cnidarians and certain cloned Kf channels from molluscs and the Kv3 family of vertebr ate channels, differed from most A-type K+ currents reported to date. These findings suggest that K+ currents in Notoplana atomata play novel roles in shaping excitability properties.