Ca2+ channels involved in the generation of the slow afterhyperpolarization in cultured rat hippocampal pyramidal neurons

The advantages of using isolated cells have led us to develop short-term cultures of hippocampal pyramidal cells, which retain many of the properties of cells in acute preparations and in particular the ability to generate afterhyperpolarizations after a train of action potentials. Using perforated-patch recordings, both medium and slow afterhyperpolarization currents (mI(AHP) and sI(AHP), respectively) could be obtained from pyramidal cells that were cultured for 8-15 days. The sI(AHP) demonstrated the kinetics and pharmacologic characteristics reported for pyramidal cells in slices. In addition to confirming the insensitivity to 100 nM apamin and 1 mM TEA, we have shown that the sI(AHP) is also insensitive to 100 nM charybdotoxin but is inhibited by 100 mu M D-tubocurarine. Concentrations of nifedipine (10 mu M) and nimodipine (3 mu M) that maximally inhibit L-type calcium channels reduced the sI(AHP) by 30 and 50%, respectively. However, higher concentrations of nimodipine (10 mu M) abolished the sI(AHP), which can be partially explained by an effect on action potentials. Both nifedipine and nimodipine at maximal concentrations were found to reduce the HVA calcium current in freshly dissociated neurons to the same extent. The N-type calcium channel inhibitor, omega-conotoxin GVIA (100 mu M), irreversibly inhibited the sI(AHP) by 37%. Together, omega-conotoxin (100 nM) and nifedipine (10 mu M) inhibited the sI(AHP) by 70%. 10 mu M ryanodine also reduced the sI(AHP) by 30%, suggesting a role for calcium-induced calcium release. It is concluded that activation of the sI(AHP) in cultured hippocampal pyramidal cells is mediated by a rise in intracellular calcium involving multiple pathways and not just entry via L-type calcium channels.