Figure 1.

Schematic presentation of the experiment. The blue rectangles represent the total population of D₂ receptors. At baseline, an unknown proportion of these receptors is occupied by dopamine (shaded area), and only a fraction of receptors are unoccupied and available to [¹²³I]IBZM binding. After α-MPT-induced depletion of endogenous dopamine, all receptors are available to [¹²³I]IBZM binding. Thus, comparing the [¹²³I]IBZM binding potential at baseline and after dopamine depletion allows derivation of the proportion of D₂ receptors that were masked by dopamine at baseline. The SPECT images display distribution of [¹²³I]IBZM in one subject at baseline and after dopamine depletion (note the increase in specific binding in the striatum).