KCNJ9, also known as GIRK3 (G protein-activated inwardly rectifying potassium channel 3), is a crucial subunit that forms inwardly rectifying potassium channels. These channels are primarily activated by inhibitory G proteins (Gi/o) downstream of various G protein-coupled receptors (GPCRs), such as those for neurotransmitters like GABA, acetylcholine, and adenosine. Upon activation, KCNJ9 facilitates potassium efflux, leading to membrane hyperpolarization and a reduction in neuronal excitability. It is widely expressed in the brain, where it plays a key role in regulating synaptic transmission and neuronal circuits, influencing processes ranging from pain perception to reward and motivation.