Cooperative regulation by G proteins and Na⁺ of neuronal GIRK2 K⁺ channels

G protein gated inward rectifier K⁺ (GIRK) channels open and thereby silence cellular electrical activity when inhibitory G protein coupled receptors (GPCRs) are stimulated. Here we describe an assay to measure neuronal GIRK2 activity as a function of membrane-anchored G protein concentration. Using this assay we show that four Gβɣ subunits bind cooperatively to open GIRK2, and that intracellular Na⁺ – which enters neurons during action potentials – further amplifies opening mostly by increasing Gβɣ affinity. A Na⁺ amplification function is characterized and used to estimate the concentration of Gβɣ subunits that appear in the membrane of mouse dopamine neurons when GABA_B receptors are stimulated. We conclude that GIRK2, through its dual responsiveness to Gβɣ and Na⁺, mediates a form of neuronal inhibition that is amplifiable in the setting of excess electrical activity.