Characterization of [3H]nemonapride binding to mouse brain dopamine D2 receptors assessed in vivo and ex vivo for metabolic modeling in PET studies.

We characterized [3H]nemonapride ([3H]NEM, [3H]YM-09151-2) binding to dopamine D2 receptors. In mice given [3H]NEM with and without sulpiride, the in vivo specific binding of the [3H]NEM to the D2 receptors in the striatum was assessed: SBin vivo-1, striatal uptake minus cerebellar uptake; and SBin vivo-2, uptake in the control mice minus uptake in the sulpiride-treated mice. Tissue homogenates were divided into cytosol and the membrane binding fraction (MB). When the MB was incubated in vitro with sulpiride, the dissociated and nondissociated fractions were defined as the ex vivospecific binding (SBex vivo) and the ex vivo nonspecific binding (NBex vivo), respectively. HPLC revealed that most of the radioactivity in the MB was [3H]NEM, whereas metabolites were found in the cytosol. In the striatum, the SBex vivo increased with time (50% of the total tissue uptake at 60 min), and was equivalent to the SBin vivo-2. The SBin vivo-1 was comparable to the MB. In the cerebral cortex and cerebellum, the SBex vivo decreased with time and the SBex vivo/free [3H]NEM ratios were smaller than those in the striatum.