Ionotropic and metabotropic kainate receptor signalling regulates Cl- homeostasis and GABAergic inhibition

Potassium-chloride co-transporter 2 (KCC2) plays a critical role in the regulation of chloride (Cl-) homeostasis within mature neurons. KCC2 is a secondarily active transporter that extrudes Cl- from the neuron, which maintains a low intracellular Cl- concentration [Cl-]. This results in a hyperpolarized reversal potential of GABA (E-GABA), which is required for fast synaptic inhibition in the mature central nervous system. KCC2 also plays a structural role in dendritic spines and at excitatory synapses, and interacts with 'excitatory' proteins, including the GluK2 subunit of kainate receptors (KARs). KARs are glutamate receptors that display both ionotropic and metabotropic signalling. We show that activating KARs in the hippocampus hyperpolarizes E-GABA, thus strengthening inhibition. This hyperpolarization occurs via both ionotropic and metabotropic KAR signalling in the CA3 region, whereas it is absent in the GluK1/2(-/-) mouse, and is independent of zinc release from mossy fibre terminals. The metabotropic signalling mechanism is dependent on KCC2, although the ionotropic signalling mechanism produces a hyperpolarization of E-GABA even in the absence of KCC2 transporter function. These results demonstrate a novel functional interaction between a glutamate receptor and KCC2, a transporter critical for maintaining inhibition, suggesting that the KAR:KCC2 complex may play an important role in excitatory:inhibitory balance in the hippocampus. Additionally, the ability of KARs to regulate chloride homeostasis independently of KCC2 suggests that KAR signalling can regulate inhibition via multiple mechanisms. Activation of kainate-type glutamate receptors could serve as an important mechanism for increasing the strength of inhibition during periods of strong glutamatergic activity.