Dopamine D2 receptor imaging and measurement with SPECT.

Only recently it has become possible to label and visualize dopamine D2 receptors in the living human brain using an [123I]labeled ligand and the SPECT technique. Iodobenzamide (S(-)IBZM) is a substance with high affinity and high specificity for D2 receptors and was used in controls and in patients with different extrapyramidal disorders. After the i.v. administration of 5mCi (185 MBq) of [123I] labeled S(-)IBZM data collection was performed with a rotating double head scintillation camera between 60 and 110 minutes. In a semiquantitative approach a ratio was calculated between mean counts/pixel in the striatum and a region in the lateral frontal cortex, which gives an index of receptor density. In a group of 21 controls this ratio is 1.73 +/- 0.09. A highly significant age-related decline is found in controls and in 57 patients with PD. PD patients without L-DOPA or dopamine agonist treatment do not differ from controls (1.72 +/- 0.09; n = 18), whereas patients under dopaminergic therapy show a significantly lower binding ratio (1.65 +/- 0.13; n = 39; p = 0.017) suggesting receptor downregulation. Comparing the striatum ipsi- and contralateral to clinical symptoms in 18 hemiparkinsonian patients shows slightly but significantly higher values on the contralateral side, indicating receptor supersensitivity (1.71 +/- 0.11 vs. 1.66 +/- 0.09; p = 0.0014). No correlation between D2 receptor binding and clinical stage, duration of disease or duration of dopaminergic therapy exists. Markedly reduced ratios are measured in 7 patients with progressive supranuclear palsy and multiple system atrophies and in 18 patients with Huntington's disease (1.40 +/- 0.09 and 1.37 +/- 0.12 respectively; p = 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)