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

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Abstract
Summary 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 R a (stimulatory) and R i (inhibitory) adenosine receptors while the hippocampus contains only R i receptors. We have previously shown that [3H]N6-cyclohexyladenosine binds to R i 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 R i receptors with high affinity. [3H]5′-N-ethylcarboxamide adenosine binds to R i receptors in the striatum and to a second site that is present in striatum but not hippocampus. High affinity binding of both ligands to R i 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 R a adenosine receptor.