Expression cloning of rat cerebellar adenosine A1 receptor by coupling to Kir channels

G protein activation of inwardly rectifying K+ (Kir) channels by heptahelical receptors is an important signaling motif in slow synaptic transmission in the mammalian brain. To isolate candidate receptors responsive to the purine nucleoside adenosine, a cerebellar cDNA library was constructed in the vector pSGEM and transcripts were injected into Xenopus Laevis oocytes co-expressing Kir3.1 and/or Kir3.2 subunits. Stepwise fractionation and functional characterization of the library using two-electrode voltage clamp measurements resulted in the identification of a single unique cDNA clone with an open reading frame of 326 amino acids. The pharmacological properties as determined from the responses to cyclopentyl-adenosine (CPA, EC50 = 7 nM) and CGS21680 (EC50 = 2.6 microM) were typical of adenosine A1 receptors. The differential receptor coupling to heteromeric Kir channels composed of Kir3.1-4 subunits provides a useful technique to isolate novel heptahelical receptors.