Bilateral increase in striatal dopamine D₂ receptor density in the 6-hydroxydopamine-lesioned rat: a serial in vivo investigation with small animal PET

Unilateral destruction of the substantia nigra by local application of 6-hydroxydopamine (6-OHDA) serves as an animal model for Parkinson's disease. In this study, the changes in neostriatal dopamine D₂ receptor density were investigated with a small animal positron emission tomograph (PET) before and after 6-OHDA lesion. PET scans were performed in 14 rats after injection of the D₂ receptor radioligand [¹⁸F] N-methylbenperidol. After the first scan (day 0), nigrostriatal pathways were lesioned by unilateral injections of 6-OHDA. Further PET scans were performed on days 2 and 14 post-lesion. For both striata, B _max values were determined from saturation binding curves with non-linear regression analysis. In the striatum ipsilateral to the lesion, B _max initially amounted to 19.3±1. 9 fmol/mg (mean±SD) and increased to 19.7±2.2 and 29.9±5.7 fmol/mg on days 2 and 14 post-lesion, respectively. Contralateral B _max values increased from 19.2±2 fmol/mg prior to the lesion to 21.2±2.9 and 28.6±5.7 fmol/mg on days 2 and 14, respectively. On day 14, the ipsilateral saturation binding curve differed from the ipsilateral pre-lesion curve ( P =0.04; F test). When the contralateral pre-lesion saturation binding curve was compared with the contralateral post-lesion curve on day 14, a P value of 0.08 was obtained. This first serial in vivo imaging study of 6-OHDA-lesioned rats showed a time-dependent increase in striatal D₂ receptor density on both sides, the increase being more pronounced ipsilateral to the lesion. This result implies that compensatory mechanisms in the intact hemisphere contribute to regenerative processes following nigrostriatal dopaminergic denervation. Overall, our findings show the feasibility of repetitive in vivo studies of striatal receptor density with a small animal tomograph. Moreover, the applied in vivo saturation binding technique provides a versatile method for the quantification of time-dependent changes in the concentration of receptor binding sites.