Background and Purpose
Dopaminergic D2-receptors quantification in vivo provides important information in the study of cerebral diseases, especially for degeneration or neural repair follow-up. [11C]raclopride binding in rat striata has been recently fully characterised and all model parameters were identified with a multi-injection approach and a beta-microprobe (Mauger et al., submitted data). Simulations derived from these results showed that a simpler protocol could be used in a PET imaging framework. This study aims at validating a single injection protocol, without any blood sampling, which allows the simultaneous estimation of Bmax and KdVr. This will be performed by estimating Bmax and KdVr on a population of living rats, and correlate these quantities with ex vivo Bmax and Kd obtained on the very same striata.
Method
Each rat (male Sprague-Dawley anesthetized with isoflurane) was placed in a stereotactic frame and underwent 3 to 4 PET scans (microPET® FocusTM 220, Concorde Microsystem) performed on separate days. The protocol is based on a partial saturation method (Delforge et al., 1990). All scans, except one, were performed after an intra-veinous injection of [11C]raclopride (74 to 111 MBq) that aimed at occupying 70% of the receptors. The last scan (pre-saturation) was performed after pre-treatment with a large amount of raclopride (1 mg/kg, i.v.). The animals were pre-treated with cimetidine (150 mg/kg, i.p.) 50 minutes prior to each PET scan, and the acquisition lasted for 60 minutes. In vivo Bmax and KdVr were estimated on late data, showing a “dynamic equilibrium” where the Scatchard equations are still valid (due to [11C]raclopride characteristics). Based on pre-saturation data, cerebellum was used as a reference region, and B/F (bound/free) was plotted against B (bound). A few days after the last scan, rats were sacrificed. Their brain was quickly removed and striata were dissected and frozen for subsequent [3 H]raclopride binding. Binding was performed in duplicate, after incubation of 2 hours at room temperature 3 , on each structure separately, providing an ex vivo estimate of Bmax and Kd. Figure 1 shows a typical Scatchard plot, for left and right striata. Bmax and KdVr values are consistent with those obtained in the former study. Mean values were 22.7 pmol/ml and 8.5 nM, with a relative SD of 16 and 15%, respectively. Preliminary results show that a minimum of 70% of receptor occupation has to be reached in order to achieve a good accuracy of the estimators. Ex vivo binding is currently being carried out.
Conclusion
A non-invasive simple protocol that allows simultaneous quantitation of D2-receptor density and affinity with [11C]raclopride is presented and validated. This will provide a very useful tool for assessing neuro-transmission variations in animal models of neurodegenerative disease.