AMPA receptor-mediated regulation of a Gi-protein in cortical neurons

Abstract

Excitatory synaptic transmission in the central nervous system is mediated primarily by the release of glutamate from presynaptic terminals onto postsynaptic channels gated by N -methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors¹,². The myriad intracellular responses arising from the activation of the NMDA and AMPA receptors have previously been attributed to the flow of Ca²⁺ and/or Na ⁺ through these ion channels^1,2,3,4,5,6. Here we report that the binding of the agonist AMPA to its receptor can generate intracellular signals that are independent of Ca²⁺ and Na⁺ in rat cortical neurons. In the absence of intracellular Ca²⁺ and Na⁺, AMPA, but not NMDA, brought about changes in a guanine-nucleotide-binding protein (Gα_i1) that inhibited pertussis toxin-mediated ADP-ribosylation of the protein in an in vitro assay. This effect was observed in intact neurons treated with AMPA as well as in isolated membranes exposed to AMPA, and was also found in MIN6 cells, which express functional AMPA receptors but have no metabotropic glutamate receptors. AMPA also inhibited forskolin-stimulated activity of adenylate cyclase in neurons, demonstrating that G_i proteins were activated. Moreover, both Gβγ blockage and co-precipitation experiments demonstrated that the modulation of the G_i protein arose from the association of Gα_i1 with the glutamate receptor-1 (GluR1) subunit. These results suggest that, as well as acting as an ion channel, the AMPA receptor can exhibit metabotropic activity.