Constitutive, agonist-accelerated, recycling and lysosomal degradation of GABAB receptors in cortical neurons

Endocytosis is considered as an important mechanism for regulating cell surface numbers and thereby signaling strength of G protein-coupled receptors. Currently, little is known about the endocytotic pathways of GABAB receptors in neurons. Here we report that GABAB receptors are constitutively internalized presumably via clathrin-dependent endocytosis in cultured cortical neurons. Colocalization of GABAB receptors with endosomal marker proteins indicated sorting of GABAB receptors from early endosomes to recycling endosomes and to lysosomes. Cell surface biotinylation experiments revealed fast constitutive recycling of GABAB receptors as the predominant pathway that was accelerated by the GABAB receptor agonist baclofen. Finally, degradation of GABAB receptors in lysosomes was demonstrated by their intracellular accumulation upon inhibition of lysosomal proteases and by blocking recycling which resulted in the redirection of receptors to lysosomes for degradation. These data imply rapid constitutive – agonist-accelerated – recycling of GABAB receptors presumably via clathrin-coated pits and their final targeting to lysosomes for degradation.