Research Abstract |
Dissociated vertebrate (frogs & rats) nerve cells were voltage-clamped in the whole-cell configuration. In bull-frog sympathetic and primary afferent neurons, a cationic inward rectifier (H-current, I_H) was identified as the one that can be regulated in a manner connected with phosphorylation of ion channel by cyclic AMP-dependent protein kinase (A-kinase). Thus, bath application of forskolin (1-10 muM) not only increased the maximum H-conductance but caused a depolarizing shift in the I_H activation curve. The basal activity of A-kinase could be inferred from a hyperpolarizing shift in the activation curve when a protein kinase inhibitor, H-8, or A_1-receptor agonists (e. g. N^6-cyclohexyladenosine) was added to the superfusate. Volatile anesthetics (e. g. enflurane, 0.2-0.8 mM) mimicked the H-8 actions on I_H implying that general anesthetics may impair the cyclic AMP-dependence of I^H. In contrast, ion channels responsible for a M-type potassium current (I_M) may have to be phosphorylated by calmodulin-dependent protein kinase in order for them to be open ; these open channels can be closed when they are phosphorylated by calcium-activated phospholipiddependent protein kinase (C-kinase). Any of three types of protein kinase described above did not participate in the regulation of a delayed rectifier potassium current (I_K) ; instead, forskolin (1-10 muM) inhibited I_K in a manner unconnected with adenylate cyclase. In slice preparations for the rat dorsolateral septal nucleus, the release of glutamate and GABA was markedly enhanced following the A-kinase activation (e. g. by forskolin) ; at the same time, the cell membrane often showed a depolarization which was due presumably to the I_H activation. Since we have succeeded in obtaining dissociated septal neurons, possible mechanisms underlying cyclic AMP-dependent events described above have now been subjected to further investigation by means of the whole-cell patch-clamp.
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