Guanine nucleotides do not inhibit tritiated dopamine agonist binding to bovine striatal membranes

The addition of GTP (50 μM), MnCl2 (1 mM) or EDTA (2 mM) had no effect on the affinity or capacity of bovine striatal plasma membranes for [3H]spiperone. However, GTP caused a decrease in the potency of dopamine as an inhibitor of [3H]spiperone binding under all conditions tested. Manganese enhanced the potency of dopamine both in the presence and absence of GTP, but NaCl (100 mM) had no effect. Neither manganese nor GTP caused any change in the affinity or capacity of bovine striatal membranes for the tritiated agonists dopamine, apomorphine or ADTN. GPPNHP, a nonhydrolyzable analog of GTP, was also ineffective. However, in identical experiments using rat striatal membranes, 50 μM GTP caused a decrease in affinity for all three tritiated agonists and this effect was observed both in the presence and absence of manganese (1 mM). In addition, binding capacities for [3H]dopamine and [3H]ADTN were doubled when manganese was present. In light of this and other reports that GTP inhibits tritiated agonist binding in rat striatum, it is suggested that the absence of such inhibition in bovine striatal membranes may reflect a fundamental difference between the two species with regard to their receptors for dopamine agonists.