Biochemical characterization of putative central purinergic receptors by using 2-chloro[3H]adenosine, a stable analog of adenosine.

After pretreatment of rat brain synaptic membranes with adenosine deaminase to remove endogenous adenosine, 2-chloro[3H]adenosine, a stable analog of adenosine, binds to two sites with Kd values of 1.3 and 16 nM and corresponding Bmax values of 207 and 380 fmol/mg of protein. Binding is reversible, and the highest density of sites occurs in enriched synaptosomal fractions. In peripheral tissue, negligible binding is observed in heart, kidney, and liver, while testicle has 11 fmol of binding sites/mg of protein. In brain, caudate and hippocampus have the highest density of sites, and spinal cord and hypothalamus have the lowest. This high-affinity binding is stereospecific; the L diasteromer of N6-phenylisopropyladenosine is approximately 30-times more potent as a displacer of 2-chloro[3H]adenosine than the D isomer and is also sensitive to theophylline (IC50 = 8.8 microM) and other purine-related compounds. Several putative neurotransmitters, neurotransmitter antagonists, and other centrally active compounds have no effect on binding. The data are consistent with the hypothesis that 2-chloro[3H]adenosine is binding to central purinergic receptors.