Phosphorylation and Chronic Agonist Treatment Atypically Modulate GABA B Receptor Cell Surface Stability

Abstract

GABAB receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. The dynamic control of the cell surface stability of GABAB receptors is likely to be of fundamental importance in the modulation of receptor signaling. Presently, however, this process is poorly understood. Here we demonstrate that GABAB receptors are remarkably stable at the plasma membrane showing little basal endocytosis in cultured cortical and hippocampal neurons. In addition, we show that exposure to baclofen, a well characterized GABA B receptor agonist, fails to enhance GABAB receptor endocytosis. Lack of receptor internalization in neurons correlates with an absence of agonist-induced phosphorylation and lack of arrestin recruitment in heterologous systems. We also demonstrate that chronic exposure to baclofen selectively promotes endocytosis-independent GABAB receptor degradation. The effect of baclofen can be attenuated by activation of cAMP-dependent protein kinase or co-stimulation of β-adrenergic receptors. Furthermore, we show that increased degradation rates are correlated with reduced receptor phosphorylation at serine 892 in GABABR2. Our results support a model in which GABABR2 phosphorylation specifically stabilizes surface GABAB receptors in neurons. We propose that signaling pathways that regulate cAMP levels in neurons may have profound effects on the tonic synaptic inhibition by modulating the availability of GABAB receptors.