Phylogenetic distribution of [³H]cyclohexyladenosine binding sites in nervous tissue

The specific binding of the A₁ adenosine receptor ligand, [³H]CHA, was investigated in membrane fractions prepared from brains of eleven vertebrate species and ganglia of four invertebrate species. Substantial amounts of specific [³H]CHA binding sites were demonstrated in brain membranes of all vertebrate species examined; however, [³H]CHA binding sites were not detectable in nervous tissue of the invertebrate species studied. The densities of [³H]CHA binding sites in vertebrate brains increase in higher vertebrates. Moreover, the pharmacological characteristics of the site labeled by [³H]CHA in two divergent classes of vertebrates were similar. The broad phylogenetic distribution of A₁ adenosine receptors in primitive as well as advanced vertebrate species suggests a fundamental role for adenosine in neuronal modulation.