[3H]5'-N-ethylcarboxamide adenosine binds to both Ra and Ri adenosine receptors in rat striatum

Adenosine analogs such as 5'-N- ethylcarboxamide adenosine and N6-cyclohexyladenosine stimulate or inhibit adenosine cyclase activity in preparations of rat striatum depending on the assay conditions. N6-cyclohexyladenosine inhibits but does not stimulate adenosine cyclase activity in preparations of hippocampus. These findings suggest that the striatum contains both Ra (stimulatory) and Ri (inhibitory) adenosine receptors while the hippocampus contains only Ri receptors. We have previously shown that [3H]N6-cyclohexyladenosine binds to Ri receptors in rat hippocampus ( Yeung and Green 1983). Comparisons of the characteristics of [3H]5'-N- ethylcarboxamide adenosine and [3H]N6-cyclohexyladenosine binding to hippocampus show that [3H]5'-N- ethylcarboxamide adenosine also binds to Ri receptors with high affinity. [3H]5'-N- ethylcarboxamide adenosine binds to Ri receptors in the striatum and to a second site that is present in striatum but not hippocampus. High affinity binding of both ligands to Ri receptors can be blocked by treatments with N-ethylmaleimide that do not markedly affect [3H]5'-N- ethylcarboxamide adenosine binding to the second site in the striatum. The pharmacological characteristics of the second site indicate that it is the Ra adenosine receptor.