Figure 3. Effect of MTSET and competition with sulpiride on [³H]NMSP binding.

Binding assays were performed in intact cells using [³H]NMSP and quinpirole to induce internalization. A. Surface receptors were inactivated with increasing concentrations of MTSET. Under control conditions (, Q−), MTSET inactivated 90% of [³H]NMSP binding. After quinpirole treatment ( , Q+), MTSET inactivated only 55% of [³H]NMSP binding, indicated that ~45% of D₂ receptor underwent internalization. B. Surface receptors were blocked with the relatively membrane-impermeant antagonist sulpiride. Under control conditions (, Q−), sulpiride competition binding showed a two-site [³H]NMSP binding curve; the majority of D₂ receptors (90%) were accessible to sulpiride and so subject to competition at high affinity (Ki_(s)= 9.1 ± 1.0 nM), while only a small minority (10%), presumably internalized, were relatively inaccessible and so subject to competition at low affinity (Ki_(i)= 142.5 ± 26.8 μM). Following quinpirole treatment ( , Q+), the high affinity [³H]NMSP binding was reduced to 55%, while the low affinity binding increased to 45%, consistent with robust D₂ receptor internalization.