Thermodynamic differences between agonist and antagonist interactions with binding sites for [3H]spiroperidol in rat...

The characteristics of the binding of the dopamine receptor antagonist [3H]spiroperidol to rat striatal membranes were examined at six different incubation temperatures ranging from 1 degree to 37 degrees. Although the number of receptors labeled at each temperature was identical, the affinity of the receptor for [3H]spiroperidol decreased 10-fold as the incubation temperature was lowered from 37 degrees to 1 degree. The binding of [3H]spiroperidol was entropy-driven (delta S degree = +80 cal/mole-deg), endothermic (delta H degree = +10 kcal/mole), and exergonic (delta G degree = -13 kcal/mole). Qualitatively similar results were found for (+)-butaclamol, another dopamine receptor antagonist. The binding of the agonists dopamine and (+/-)-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene to sites labeled by [3H]spiroperidol in the striatum also appeared to be entropy-driven (delta S degree = +35 cal/mole-deg). In contrast to the results obtained in studies with antagonists, however, the affinity of the receptor for agonists was independent of the incubation temperature between 8 degrees and 37 degrees. Competition curves for the inhibition of [3H]spiroperidol binding by agonists became increasingly complex as the incubation temperature was lowered. The addition of GTP reduced the affinity of the receptor for agonists at all temperatures but did not simplify interpretation of these complex curves. At 1 degree there was a decrease in the affinity of the receptor for dopamine, and the effect of GTP was abolished.