title: A new class of adenosine receptors in brain. Characterization by 2-chloro[3H]adenosine binding.
abstract: Micromolar concentrations of adenosine and its analogs have profound depressant effects on neuronal firing and synaptic transmission in many brain areas. Using the adenosine agonist 2-chloro[3H]adenosine (Cl[3H]Ado), we have identified a distinct class of micromolar-affinity adenosine binding sites in rat forebrain membranes. Specific Cl[3H]Ado binding was reversible and saturable with an apparent KD of 9.1 microM and a Bmax of 61 pmoles/mg protein. The present studies were conducted using washed brain membrane fractions not treated with adenosine deaminase. Specific Cl[3H]Ado binding under these conditions was insensitive to (-)-N6-(R-phenylisopropyl)adenosine ((-)PIA) and treatment with 3 mM N-ethylmaleimide, unlike high-affinity A1 adenosine receptor binding. Treatment of membranes with adenosine deaminase revealed an additional population of binding sites sensitive to (-)PIA. Inhibition of Cl[3H]Ado binding by adenosine analogs exhibited an order of potency ClAdo greater than 5'-N-ethylcarboxamide adenosine (NECA) greater than (-)PIA which differs from that of both A1 and A2 adenosine receptors. The potent A1 and A2 receptor antagonist 8-phenyltheophylline had no significant effect on binding up to 10 microM. Specific binding, however, was inhibited by the adenosine antagonists 8(p-sulfophenyl)theophylline, isobutylmethylxanthine, theophylline, and caffeine. Micromolar Cl[3H]Ado binding was highly selective for adenosine agonists and antagonists. These results suggest that the micromolar-affinity Cl[3H]Ado binding sites may represent a novel central purinergic receptor, distinct from the A1 and A2 adenosine receptors involved in the regulation of adenylate cyclase.
mesh_terms: D015762:2-Chloroadenosine; D000241:Adenosine / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000243:Adenosine Deaminase / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D001931:Brain Mapping; D005033:Ethylmaleimide / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D051381:Rats; D011956:Receptors, Cell Surface / Q000145:classification / Q000187:drug effects / Q000378:metabolism*; D011983:Receptors, Purinergic; D013347:Subcellular Fractions / Q000378:metabolism; D013570:Synaptic Membranes / Q000378:metabolism; D013696:Temperature
chemical_list: D011956:Receptors, Cell Surface; D011983:Receptors, Purinergic; D015762:2-Chloroadenosine; D000243:Adenosine Deaminase; D000241:Adenosine; D005033:Ethylmaleimide