Scintillation Proximity Assay: The human PDE10A full length assay was performed in 96-well micro titer plates. The reaction mixture of 50 μl contained 20 mM HEPES pH=7.5/10 mM MgCl2/0.05 mg/ml BSA (Sigma cat. # A-7906), 50 nM cGMP (Sigma, cat. # G6129) and 50 nM [3H]-cGMP (GE Healthcare, cat. # TRK392 S.A. 13.2 Ci/mmol), 3.75 ng/well PDE10A enzyme (Enzo Life Science, Lausen, Switzerland cat # SE-534) with or without a specific test compound. A range of concentrations of the potential inhibitor was used to generate data for calculating the concentration of inhibitor resulting in 50% of the effect (e.g. IC50, the concentration of the competitor inhibiting PDE10A activity by 50%). Non-specific activity was tested without the enzyme. The reaction was initiated by the addition of the substrate solution (cGMP and [3H]-cGMP) and allowed to progress for 20 minutes at room temperature. The reaction was terminated by adding 25 μl of YSi-SPA scintillation beads (GE Healthcare, cat. # RPNQ0150) in 18 mM zinc sulphate solution (stop reagent). After 1 h under shaking, the plate was centrifuged one minute at 170 g to allow beads to settle. Afterwards, radioactive counts were measured on a Perkin Elmer Top-Count Scintillation plate reader.