Ca2+-activated nonselective cationic current (I-CAN) in turtle motoneurons

The presence of a calcium-activated nonspecific cationic (CAN) current in t urtle motoneurons and its involvement in plateau potentials, bistability, a nd windup was investigated by intracellular recordings in a spinal cord sli ce preparation. In the presence of tetraethylammonium (TEA) and tetrodotoxi n (TTX), calcium action potentials evoked by depolarizing current pulses we re always followed by an afterdepolarization associated with a decrease in input resistance. The presence of the afterdepolarization depended on the c alcium spike and not on membrane potential. Replacement of extracellular so dium by choline or N-methyl-D-glucamine (NMDG) reduced the afterdepolarizat ion, confirming that it was mediated by a CAN current. Plateau potentials a nd windup were evoked in response to intracellular current pulses in the pr esence of agonist for different metabotropic receptors. Replacement of extr acellular sodium by choline or NMDG did not abolish the generation of plate au potentials, bistability, or windup, showing that Na+ was not the princip al charge carrier. It is concluded that plateau potentials, bistability and windup in turtle motoneurons do not depend on a CAN current even though it s presence can be detected.