The opioid, dopamine and serotonin receptors are crucial molecules in drug addictions. Previous studies of signal transduction pathways from these receptors have shown that their activation alters membrane conductances for K+ and Ca*+ and levels of second messengers such as CAMP and inositol 1, 4, 5-triphosphate, each through G-protein activation, and ultimately results in the activation or inhibition of neural firing and of neurotransmitter release.'In the signal transduction from these receptors, the G-protein-activated K+ channel (GIRK channel), a member of a family of inward-rectifier K+ channels, is considered to play an important r~ le.~,~ Recent molecular biological studies have revealed the presence of at least three GIRK channels in the brain, GIRK1, GIRK2 and GIRK3 channels. 5-* However, details of these GIRK channels including those regarding their in vivo relationships among each other and their functional coupling with various receptors remain largely unknown. a This work was supported in part by research grants from the Ministry of Education, Science and Culture of Japan and the Frontier Research Program, RIKEN. Corresponding author: Kazutaka Ikeda, Laboratory for Synaptic Function, Frontier Research Program, the Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama 351-01, Japan. Tel.:+ 81-48-462-1111 (ext: 3102); Fax:+ 81-48-462-4798;