Evidence Against a Role for Protein Kinase C in the Inhibition of the Calcium‐activated Potassium Current I_AHP by Muscarinic Stimulants in Rat Hippocampal Neurons

The possible role of protein kinase C activation in the inhibitory action of cholinergic transmitters on the slow Ca‐dependent afterhyperpolarizing current (I_AHP) in hippocampal CA3 pyramidal neurons was investigated using hippocampal slice cultures. I_AHP was inhibited reversibly by methacholine (100–600 nM) and irreversibly by the protein kinase C activator, phorbol‐12,13‐dibutyrate (PDBu, 10 nM to 1 μM). The inhibitory action of PDBu was antagonized by prior (15–60 min) exposure to staurosporin (1 μM). In contrast, the inhibitory effect of methacholine on I_AHP was not reduced after up to 3 h of exposure to this compound. In addition, methacholine produced a reversible inward current at the holding potential, which was augmented by staurosporin. However, prior exposure to PDBu reduced the effect of methacholine on I_AHP and occluded the methacholine‐induced inward current. This effect of PDBu was also observed in the presence of staurosporin, suggesting that it might be exerted through a protein kinase C‐independent pathway. Noradrenalin (2–5 μM) and 8‐bromo cyclic adenosine 3′,5′monophosphate (8‐Br‐cAMP, 1 mM) also produced a reversible block of I_AHP. Their action was antagonized by staurosporin, probably via its effect on protein kinase A. Thus the present experiments suggest that the action of muscarinic agonists on I_AHP cannot be explained by an effect on protein kinase C, but support a role for protein kinase A in mediating the action of noradrenalin.