Functional uncoupling between the receptor and G-protein as the result of PKC activation, observed in Aplysia neurons

Application of either acetylcholine (ACh), dopamine (DA), histamine (HA), or Phe-Met-Arg-Phe-NH₂ (FMRFamide) induces a K⁺-current response in the identified neurons of Aplysia under voltage clamp. This type of response is mediated by a pertussis toxin (PTX)-sensitive G-protein, G(i) or G(o). Extracellular application of 60μM phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC), to these cells markedly depressed all the K⁺-current responses to ACh, DA, HA, and FMRFamide. The depressing effect of PDBu lasted for at least 60 min despite continuous washing with the normal perfusing medium. Application of PKC inhibitors such as 100 μM H-7 or 10 μM staurosporine and PKCl_19-31 prior to the application of PDBu significantly decreased the depressing effects of PDBu. In contrast, an intracellular injection of okadaic acid (OA), an inhibitor of protein phosphatase 1 and 2A, significantly augmented the blocking effect of PDBu. Intracellular injection of the PKC catalytic subunit induced a similar depressing effect as observed with PDBu. The dose-response curves obtained with different transmitters all shifted downward after the activation of PKC, but the ED₅₀ of each transmitter remained unchanged. Furthermore, the K⁺-current responses induced by the intracellular application of GTPγS were not depressed at all, even after the receptor-induced K⁺-current responses of the same cell were markedly depressed. These results strongly suggest that PKC phosphorylated a certain coupling site between the receptor and G-protein, and impaired the signal transduction necessary for triggering the K⁺ channel opening.