Neuronal Chloride Regulation via KCC2 Is Modulated through a GABA_B Receptor Protein Complex.

GABA_B receptors are G-protein-coupled receptors that mediate inhibitory synaptic actions through a series of downstream target proteins. It is increasingly appreciated that the GABA_B receptor forms part of larger signaling complexes, which enable the receptor to mediate multiple different effects within neurons. Here we report that GABA_B receptors can physically associate with the potassium-chloride cotransporter protein, KCC2, which sets the driving force for the chloride-permeable ionotropic GABA_A receptor in mature neurons. Using biochemical, molecular, and functional studies in rodent hippocampus, we show that activation of GABA_B receptors results in a decrease in KCC2 function, which is associated with a reduction in the protein at the cell surface. These findings reveal a novel "crosstalk" between the GABA receptor systems, which can be recruited under conditions of high GABA release and which could be important for the regulation of inhibitory synaptic transmission.SIGNIFICANCE Synaptic inhibition in the brain is mediated by ionotropic GABA_A receptors (GABA_ARs) and metabotropic GABA_B receptors (GABA_BRs). To fully appreciate the function and regulation of these neurotransmitter receptors, we must understand their interactions with other proteins. We describe a novel association between the GABA_BR and the potassium-chloride cotransporter protein, KCC2. This association is significant because KCC2 sets the intracellular chloride concentration found in mature neurons and thereby establishes the driving force for the chloride-permeable GABA_AR. We demonstrate that GABA_BR activation can regulate KCC2 at the cell surface in a manner that alters intracellular chloride and the reversal potential for the GABA_AR. Our data therefore support an additional mechanism by which GABA_BRs are able to modulate fast synaptic inhibition.