Abstract

Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate induced a continuous decrease in the γ-aminobutyric acid (GABA)-activated current amplitude from recombinant GABA receptors (formed by ρ1 or αβγ subunits) expressed inXenopus oocytes. This decline was due to internalization of receptors from the plasma membrane as confirmed by a decrease in surface fluorescence with green fluorescence protein-tagged receptors as well as a concomitant decrease in surface [³H]GABA binding. PMA specifically caused internalization of GABA receptors, but not neuronal acetylcholine receptors (α₇ or α₄β₂), indicating the internalization was not a general, nonspecific phenomenon. Mutation of ρ1 PKC phosphorylation sites, identified by in vitro phosphorylation, did not prevent GABA receptor internalization, nor did coexpression of the ρ1 M3-M4 intracellular loop along with ρ1 GABA receptors. It is likely that PKC-mediated phosphorylation of other proteins, rather than ρ1 itself, was required for the internalization. Both ρ1 and αβγ receptors did not degrade after phorbol 12-myristate 13-acetate-induced internalization, but returned to the membrane surface within 24 h. These data suggest internalized receptors can exist in an intracellular compartment that can be delivered back to the plasma membrane. Thus, by regulating GABA receptor surface expression, PKC may play a key role in the regulation of GABA-mediated inhibition.