GABA(A) receptor phosphorylation and functional modulation in cortical neurons by a protein kinase C-dependent pathway

GABA(A) receptors are critical mediators of fast synaptic inhibition in the brain, and the predominant receptor subtype in the central nervous system is believed to be a pentamer composed of alpha, beta, and gamma subunits. P revious studies on recombinant receptors have shown that protein kinase C ( PRC) and PHA directly phosphorylate intracellular serine residues within th e receptor beta subunit and modulate receptor function. However, the releva nce of this regulation for neuronal receptors remains poorly characterized. To address this critical issue, we have studied phosphorylation and functi onal modulation of GABA(A) receptors in cultured cortical neurons. Here we show that the neuronal beta3 subunit is basally phosphorylated on serine re sidues by a PHC-dependent pathway. PKC inhibitors abolish basal phosphoryla tion, increasing receptor activity, whereas activators of PKC enhance beta3 phosphorylation with a concomitant decrease in receptor activity. PKA acti vators were shown to increase the phosphorylation of the beta3 subunit only in the presence of PKC inhibitors, We also show that the main sites of pho sphorylation within the neuronal beta3 subunit are likely to include Ser-40 8 and Ser-409, residues that are important for the functional modulation of beta3-containing recombinant receptors, Furthermore, PHC activation did no t change the total number of GABA(A) receptors in the plasma membrane, sugg esting that the effects of PKC activation are on the gating or conductance of the channel. Together, these results illustrate that cell-signaling path ways that activate PKC may have profound effects on the efficacy of synapti c inhibition by directly modulating GABA(A) receptor function.