Whole-Cell Voltage-Clamp Investigation Of The Role Of Pkc In Muscarinic Inhibition Of I-Ahp In Rat Ca1 Hippocampal Neurons

Muscarinic, cholinergic inputs, largely from the medial septum, have pronounced effects on hippocampal cell excitability. A major effect of synaptically released ACh is block of the slow Ca2+-dependent potassium current, called I-AHP. Protein kinase C exists in the hippocampus in high concentrations, its activation blocks I-AHP, and it has been suggested as a mediator of the muscarinic-receptor-(mAChR)-mediated actions. Using conditions that produce a stable postspike afterhyperpolarizing current (I-AHP) in whole-cell recordings from CA1 hippocampal pyramidal neurons in the slice preparation, we have investigated the role of PKC in the cholinergic inhibition of I-AHP mediated by mACHRs. Bath application of the general kinase inhibitor, H17, had no effect on inhibition of I-AHP by carbachol, although H7 dramatically reduced inhibition of I-AHP by the phorbol ester, phorbol-12,13-diacetate (PDA). Another muscarinic response thought to be mediated by PKC-inhibition of GABA(B)-mediated hyperpolarization-was reduced by extracellular H7 treatment, suggesting that the coupling between mAChRs and protein kinase activity was maintained in whole-cell recordings. We also discovered that PDA does not mediate its effects on I-AHP directly. Intracellular perfusion of high concentrations of H7 (10 mM) or the specific PKC inhibitor, PKCI(19-31) (1 mM), did not prevent inhibition of I-AHP by PDA. These results are consistent with an indirect, presynaptic action of phorbol esters on I-AHP, possibly mediated through enhanced release of neurotransmitter from surrounding cells. (C) 1996 Wiley-Liss, Inc.