Effects of protein kinase C activators and inhibitors on membrane properties, synaptic responses, and cholinergic actions in CA1 subfield of rat hippocampus in situ and in vitro.
Activation and inhibition of protein kinase C (PKC) has been reported to induce several effects in hippocampus in vitro. It has been also proposed that, in hippocampus, phorbol esters mimic the effects of acetylcholine. To test whether the actions produced by PKC activators and inhibitors in situ are comparable to those induced in vitro preparations, we studied, in the CA1 region of the hippocampus both in situ and in vitro, the responses produced by activation and inhibition of protein kinase C. Once the effects of various PKC activators and inhibitors were established their interaction with muscarinic agonists was studied. The main findings were as follows: I) Extracellular studies in situ: 1) Phorbol diacetate (PDAc) enhanced the population spike and dendritic field amplitudes. Unlike ACh, it never induced disfacilitation or disinhibition. 2) The effects produced by muscarinic agonists were not occluded by prolonged PDAc applications. 3) Inhibition of PKC with H-7 induced a strong excitation manifested by induction of multiple spikes and broadening of the dendritic field response. This excitation was associated with blockade of IPSPs, represented by positive waves, at a presynaptic site, which was antagonised by PDAc suggesting the involvement of PKC. 4) Sphingosine, a dual PKC/calcium-calmodulin-dependent kinase inhibitor, did not reproduce H-7-induced responses. However, it did prevent the actions of muscarinic agonists. II) Intracellular studies in vitro: 1) PDAc applications by either iontophoresis or superfusion produced a i) depolarization; ii) increase in input resistance (RN); iii) blockade of the anomalous rectification ("sag"); iv) increase in the fast--but decrease in the slow--afterhyperpolarization (AHP); and v) reduction in excitability, measured by the repetitive firing evoked by depolarizing pulses. 2) During local (iontophoretic) applications of PDAc, the reversal potential of IPSPs was not affected significantly whereas during bath applications, it shifted toward more positive values. 3) Iontophoresis of H-7 caused a decrease in RN, hyperpolarization, and blockade of IPSPs. In conclusion, in the hippocampus, PKC can modulate the IPSPs, the anomalous rectification, and the membrane potential, but PKC is unlikely to be the major intracellular mediator of the excitatory actions of acetylcholine. The possible involvement of calcium-calmodulin-dependent kinase is discussed.
Agopyan
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Department of Physiology, McGill University, Montréal, Québec, Canada.
Agopyan
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D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D004789:Enzyme Activation; D005071:Evoked Potentials / Q000187:drug effects; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007546:Isoquinolines / Q000494:pharmacology*; D008297:Male; D016210:Methacholine Chloride / Q000494:pharmacology*; D010277:Parasympathomimetics / Q000494:pharmacology*; D010703:Phorbol Esters / Q000494:pharmacology*; D010879:Piperazines / Q000494:pharmacology*; D011493:Protein Kinase C / Q000037:antagonists & inhibitors / Q000378:metabolism*; D011712:Pyramidal Tracts / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D011919:Rats, Inbred Strains; D013110:Sphingosine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D013569:Synapses / Q000187:drug effects / Q000502:physiology*
D007546:Isoquinolines; D010277:Parasympathomimetics; D010703:Phorbol Esters; D010879:Piperazines; D016210:Methacholine Chloride; C008874:phorbol-12,13-diacetate; D019307:1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine; D011493:Protein Kinase C; D000109:Acetylcholine; D013110:Sphingosine; C005682:safingol