¹²⁵I-spiperone: A novel ligand for D₂ dopamine receptors

¹²⁵I-Spiperone binds with high affinity (K_D 0.3 nM) to a single specific site (B_max 34 pmol/g wet weight) in homogenates of rat corpus striatum. Specific binding is about 40-60 percent of total binding and is displaced stereo-specifically by butaclamol and clopenthixol. Neuroleptic drugs of various classes are potent inhibitors of ¹²⁵I-spiperone binding (K_i's 1-10 nM). Selective dopamine antagonists such as sulpiride (K_i 50 nM) are dopamine agonists such as apomorphine (K_i 200 nM) are also potent inhibitors. The drug specificity of ¹²⁵I-spiperone binding correlates well with that of ³H-spiperone binding, providing good evidence that ¹²⁵I-spiperone labels D₂ dopamine receptors in striatal membranes. ¹²⁵I-Spiperone, with its high specific activity (2200 Ci/mmol) may prove to be a useful ligand in studies examining D₂ dopamine receptors in soluble preparations and by autoradiography. Furthermore iodinated spiperone may be useful in radioreceptor assays of neuroleptic drug levels and, in a ¹²³I-labeled form, for imaging of dopamine receptors, in vivo, using single photon tomography.