Figure 2.

Competition binding experiments on striatal membrane homogenates with [³H]methylspiperone and dopamine in the absence (●) and presence (Δ) of guanine triphosphate (GTP). High affinity agonist binding was observed in the absence of GTP, but the proportion of high affinity binding was similar in heterozygous (A) and DBH knockout (B) mice. In the presence of GTP, one low-affinity binding site was observed. All experiments were performed in triplicate and repeated three times. K_I values and proportion of high affinity agonist binding are shown in table I.