Activation of protein kinase C induces internalization of the GABA(C) receptors expressed in Xenopus oocytes.

In a previous study, we showed that the protein kinase C (PKC) activator 4beta-phorbol 12-myristate 13-acetate (PMA) inhibited the gamma-aminobutyric acid (GABA)-gated currents in Xenopus oocytes expressing human rho1 GABA(C) receptors. To investigate whether the inhibition of currents was due to a decrease in efficacy or in the potency of rho1 GABA(C) receptor, concentration-response curves for GABA were compared before and after PMA treatment. The EC50 concentrations of GABA obtained during the maximally inhibited period were not statistically different from the concentrations obtained before PMA treatment (1.74 +/- 0.33 and 1.45 +/- 0.28 microM, respectively). These results indicate that the inhibition depends on a change in number or conductance of active receptor channels, but not on a change in affinity for GABA. To allow histochemical detection of rho1 GABA(C) receptors, we constructed a receptor tagged at the C-terminal position with human c-myc epitope. Electrophysiologically, the tagged receptors showed almost the same sensitivities for GABA and PMA as those of wild-type rho1 GABA(C) receptors. Immunohistochemistry with anti-myc antibody detected a dense concentration of tagged receptors at the surface area of Xenopus oocytes. Transient exposure to PMA reduced the density of immunofluorescence at the surface area and increased it in the subsurface area. These results suggest that the stimulation of protein kinase C leads to internalization of rho1 GABA(C) receptors expressed in Xenopus oocytes.