TREK-1 and Best1 Channels Mediate Fast and Slow Glutamate Release in Astrocytes upon GPCR Activation

Astrocytes release glutamate upon activation of various GPCRs to exert important roles in synaptic functions. However, the molecular mechanism of release has been controversial. Here, we report two kinetically distinct modes of nonvesicular, channel-mediated glutamate release. The fast mode requires activation of G_αi, dissociation of G_βγ, and subsequent opening of glutamate-permeable, two-pore domain potassium channel TREK-1 through direct interaction between G_βγ and N terminus of TREK-1. The slow mode is Ca²⁺ dependent and requires G_αq activation and opening of glutamate-permeable, Ca²⁺-activated anion channel Best1. Ultrastructural analyses demonstrate that TREK-1 is preferentially localized at cell body and processes, whereas Best1 is mostly found in microdomains of astrocytes near synapses. Diffusion modeling predicts that the fast mode can target neuronal mGluR with peak glutamate concentration of 100 μM, whereas slow mode targets neuronal NMDA receptors at around 1 μM. Our results reveal two distinct sources of astrocytic glutamate that can differentially influence neighboring neurons.