Development of a simultaneous PET/microdialysis method to identify the optimal dose of 11C-raclopride for small animal imaging

Abstract

In the field of small animal positron emission tomography (PET), the assumptions underlying human and primate kinetic models may not be sustained in rodents. That is, the threshold dose at which a pharmacologic response occurs may be lower in small animals. In order to define this relationship, we combined microPET imaging using 11 C-raclopride with microdialysis measures of extracellular fluid (ECF) dopamine (DA). In addition, we performed a series of studies in which a known mass of raclopride was microinfused into one striatum prior to a high specific activity (SA) systemic injection of 11 C-raclopride. This single-injection approach provided a high and low SA region of radiotracer binding in the same animal during the same scanning session. Our data demonstrate that the binding potential (BP) declines above 3.5 pmol/ml (0.35 μg), with an ED 50 of 8.55 ± 5.62 pmol/ ml. These data also provide evidence that BP may be compromised by masses of raclopride below 2.0 pmol/ml (0.326 μg). Increases in ECF DA were produced by mass doses of raclopride over 3.9 pmol/ml (0.329 μg) with an ED 50 of 8.53 ± 2.48 pmol/ml. Taken together, it appears that an optimal range of raclopride mass exists between 2.0 and 3.5 pmol/ml, around which the measured BP can be compromised by system sensitivity, endogenous DA, or excessive competition with unlabeled compound. KeywordsMicroPET; Rodent; 11 C-raclopride; D 2 receptors; Microdialysis; Mass effectThe miniaturization of clinical positron emission tomography (PET) has produced a new generation of commercially available PET scanners, providing a powerful tool for imaging biological processes in small laboratory animals. With the microPET system and other small animal tomographs, a number of laboratories have started to validate animal models of different disease states (Araujo et al., 2000;Hume et al., 1996;Kornblum et al., 2000). Receptor-specific radiotracers have also been used to track gene expression following viral insertion of specific cDNA (MacLaren et al., 1999). This valuable tool has the potential to capture the functional relationship between receptors and neurotransmitters and has been used to measure neurotransmitter regulation in schizophrenia (Abi-Dargham et al., 2000), or the modulation of one neurotransmitter system by another (Dewey et al., 1988(Dewey et al., , 1990 to establish whether the assumptions underlying clinical PET are appropriate for small animal imaging.As the physical size of the animal decreases, the relative mass in a tracer dose becomes more significant, resulting in the possibility that the radiotracer itself may perturb the system under investigation. This, then, becomes a violation of the fundamental premise which unites the activity of a radioactive molecule to a physiologic process. That is, the sensitivity to the mass of injected ligand might distinguish small animal PET from its clinical counterpart, and present an important limitation on the interpretation of rodent PET data. This is especially relevant to radiotracers whose molecula...

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“…Nevertheless, the binding ratios for [ 11 C]-raclopride in the present study were higher than ratios obtained from anesthetized animals in our previous reports (Schiffer et al, 2005a;Patel et al, 2008), and with this, there was a higher between-animal variability (Fig. 3).…”

“…All images were spatially normalized into a standard space, which we identified as Paxinos and Watson stereotaxic space (Paxinos and Watson, 1986). As a template, we used the magnetic resonance imaging (MRI) atlas provided by Schweinhardt et al (2003), which is in stereotaxic space and for which we have previously developed [ 11 C]-raclopride reference images for spatial preprocessing (Schiffer et al, 2005a). Data voxel size in the Schweinhardt atlas (2003) is scaled by a factor of 10 to enable a one-to-one relationship between the coordinates of the Paxinos atlas and the voxel display in the SPM software package.…”

“…Because high-mass doses of radiotracer can occupy sufficient number of D 2 receptors as to change dopamine dynamics (Schiffer et al, 2005a), it is important to ensure that the dose of [ 11 C]-raclopride carries with it the lowest possible mass of unlabeled raclopride, resulting in the lowest possible occupancy. This represents a continual compromise for animal PET experiments, because the injected dose of radioactivity must provide sufficient counting statistics for reproducible image quality-a factor that uniquely affects iteratively reconstructed data (see below)-yet higher doses of radioactivity also carry higher masses of unlabeled ligand, leading to higher occupancy of D 2 receptors.…”

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