Neuronal Chloride Regulation via KCC2 Is Modulated through a GABA_BReceptor Protein Complex

<jats:p>GABA<jats:sub>B</jats:sub>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<jats:sub>B</jats:sub>receptor forms part of larger signaling complexes, which enable the receptor to mediate multiple different effects within neurons. Here we report that GABA<jats:sub>B</jats:sub>receptors can physically associate with the potassium-chloride cotransporter protein, KCC2, which sets the driving force for the chloride-permeable ionotropic GABA<jats:sub>A</jats:sub>receptor in mature neurons. Using biochemical, molecular, and functional studies in rodent hippocampus, we show that activation of GABA<jats:sub>B</jats:sub>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.</jats:p><jats:p><jats:bold>SIGNIFICANCE STATEMENT</jats:bold>Synaptic inhibition in the brain is mediated by ionotropic GABA<jats:sub>A</jats:sub>receptors (GABA<jats:sub>A</jats:sub>Rs) and metabotropic GABA<jats:sub>B</jats:sub>receptors (GABA<jats:sub>B</jats:sub>Rs). 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<jats:sub>B</jats:sub>R 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<jats:sub>A</jats:sub>R. We demonstrate that GABA<jats:sub>B</jats:sub>R activation can regulate KCC2 at the cell surface in a manner that alters intracellular chloride and the reversal potential for the GABA<jats:sub>A</jats:sub>R. Our data therefore support an additional mechanism by which GABA<jats:sub>B</jats:sub>Rs are able to modulate fast synaptic inhibition.</jats:p>