Separation of different types of afterpotentials following penicillin-induced paroxysmal depolarization shifts of neurons in the motor cortex of the rat.

Afterpotentials of penicillin-induced paroxysmal depolarization shifts (PDS) of neurones in the motor cortex of the rat in vivo were investigated with intracellular recordings. Following the PDS, 4 different types of afterpotentials were observed: fast afterhyperpolarizations with an average duration of 600 ms, fast afterdepolarizations with an average duration of 700 ms, slow afterhyperpolarizations with an average duration of 1.9 s and slow afterdepolarizations with an average duration of 1.8 s. The fast and slow afterpotentials could occur in various combinations with the exception of a fast afterhyperpolarization followed by a slow afterdepolarization. Neurones displaying afterdepolarizations had higher resting membrane potentials than those displaying afterhyperolarizations, i.e. the polarity of the afterpotential depended on membrane potential. Experiments with intracellular Cl- injection indicated that the fast but not the slow afterpotentials are associated with an increase in membrane Cl- conductance. The slow afterhyperpolarizations are suggested to result from a calcium-dependent or a synaptically generated potassium current.