Abstract
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.