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 microM D-tubocurarine. Concentrations of nifedipine (10 microM) 
and nimodipine (3 microM) that maximally inhibit L-type calcium channels reduced 
the sI(AHP) by 30 and 50%, respectively. However, higher concentrations of 
nimodipine (10 microM) 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 nM), irreversibly inhibited the sI(AHP) by 37%. 
Together, omega-conotoxin (100 nM) and nifedipine (10 microM) inhibited the 
sI(AHP) by 70%. 10 microM 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.