The distribution of D2/D3 receptor binding in the adolescent rhesus monkey using small animal PET imaging
Introduction
In recent years, in vivo identification of dopamine D2/D3 binding using PET imaging has focused on the extrastriatal regions of the brain, where the D2/D3 receptor density is reduced approximately 10-fold to 100-fold from the striatal regions of the putamen and caudate nucleus. The PET radioligands providing the most favorable imaging characteristics for visualizing the D2/D3 receptors in low density regions are the high affinity radiotracers [F-18]fallypride (Mukherjee et al., 1999) and [C-11]FLB 457 (Halldin et al., 1995). This pursuit of characterizing extrastriatal dopaminergic function is driven by clinical research in neuropsychiatric illness revealing disease specific differences, in diseases such as schizophrenia (Suhara et al., 2002, Talvik et al., 2003, Tuppurainen et al., 2003, Yasuno et al., 2004, Buchsbaum et al., 2006), Parkinson's Disease (Kaasinen et al., 2000, 2004), depression (Klimke et al., 1999) and Tourette Syndrome (Gilbert et al., 2006). There is also a strong interest in measuring dopamine transmission in the extrastriatal regions induced by either pharmacologic manipulation (Riccardi et al., 2006) or performance of a mental task (Aalto et al., 2005, Christian et al., 2006). These findings demonstrate the need for further research in the extrastriatal D2/D3 system and promote the use of animal models to further examine its potential role in behavior, neuropsychiatric pathology and targeted drug development. The rhesus monkey (Macaca mulatta) serves as an excellent model for studying many of the neuroreceptor systems in the brain, including the dopaminergic system. Their anatomy, protein structure, receptor pharmacology and brain chemistry are considered to be similar to a large extent to humans. Neurodevelopmentally, the monkey brain mimics the human brain and many of the CNS tracts are found to be in close proximity in the monkeys and humans. Specific behaviors such as freezing, exploration and self-grooming have served as correlates to human emotional responses known to be related to the dopaminergic system (Pani et al., 2001). The rhesus animal model and PET imaging have also been used to unveil disruptions in the dopaminergic system as a result of moderate levels of fetal alcohol exposure (Roberts et al., 2004).
Neuroimaging of the extrastriatal D2/D3 receptors in the brain requires a high affinity radiotracer that is sufficiently cleared from nonspecific regions of the brain to provide a high target (specific) to background (nonspecific) binding ratio. [F-18]Fallypride possesses these attributes and has been validated in nonhuman primates (Christian et al., 2000 — rhesus; Slifstein et al., 2004 — baboons) and humans (Mukherjee et al., 2002, Siessmeier et al., 2005) to provide a quantitative index of D2/D3 binding. Further, the development of high resolution human and small animal PET imaging systems provides the necessary hardware to fully exploit the precise binding profile of [F-18]fallypride to the D2/D3 receptors throughout the brain. However, the use of high resolution imaging comes at the cost of requiring increased PET signal, i.e. a preserved number of counts per resolution element. A number of investigators have addressed the technical issues arising from small animal PET scanning and the tradeoff between higher resolution and reduced signal to noise ratio (SNR) and the implications on kinetic parameter estimation (Meikle et al., 2000, Sossi et al., 2005). Within the limits of a given scanner configuration, the improved PET signal can be achieved by increasing the amount of injected radiotracer, however, consideration must be given to minimize the competition of “tracer” ligand mass effects (Hume et al., 1998, Jagoda et al., 2004, Kung and Kung, 2005). Attention to mass effects of competing ligand is of particular concern for high affinity PET ligands, such as [F-18]fallypride and [C-11]FLB 457. For humans, it has been reported that doses of unlabeled FLB 457 should be less than 0.5 μg to avoid confounding occupancy of the drug (Olsson et al., 2004). For radiotracers with very low nonspecific uptake, such as [F-18]fallypride, the requirements for adequate PET signal are dictated to a large extent not by the target regions with elevated specific binding, but rather by the regions with low specific binding as well as the reference region. The large difference in rate constants of ligand binding (kon·Bmax) and dissociation (koff) of these radiotracers can require several hours of imaging to yield a stable measure of apparent binding potential (BPND). At these late time points the radiotracer concentration in the cortex and cerebellum is significantly reduced, presenting potential errors in accurately assaying the PET measured concentration. Due to these limits, the proper application of scanner related corrections such as scatter, randoms, attenuation and normalization are imperative to achieving accurate results (Bendriem and Townsend, 1998, Alexoff et al., 2003).
The purpose of this study is to provide a description of the distribution of the D2/D3 dopamine receptors in the adolescent brain of the rhesus monkey. As this is the first such report of large cohort results, a primary emphasis is placed on the methodological issues presented in using a high affinity D2/D3 radiotracer with small animal PET imaging. We focus on a close examination of the effects of ligand mass in our measurement of [F-18]fallypride binding. Also, the kinetics of [F-18]fallypride in the cerebellum are presented in detail to examine the potential effects of small but significant specific binding and low concentration measurement. These data are presented to provide the neuroimaging community with information regarding the expected variation in D2/D3 receptor binding in the rhesus monkey and also to provide baseline data for further large cohort comparisons based on longitudinal studies or drug development research.
Section snippets
NHP colony
A total of 33 M. mulatta (rhesus) underwent [F-18]fallypride PET scans for this work. The cohort has been described in detail in our previous work (Oakes et al., 2007, Kalin et al., 2008). It consisted of 23 female, 10 male; ages 3.2–5.3 years. Rhesus monkeys in captivity have a median life-span of 25 years (Colman and Kemnitz, 1999). Though translation to human years is nonlinear, we can approximate the equivalent age of this cohort to be 12 year old humans. All animals were pair-housed at the
Cerebellum kinetics
The time course of [F-18]fallypride in the region of the cerebellum is shown in Fig. 1 for the decay corrected data. These data reveal high initial uptake of radiotracer followed by relatively rapid clearance with only approximately 0.003% injected dose (I.D.) per cc of tissue at 2 h post-injection. These data were fit to a bi-exponential curve to provide an estimate of the clearance of radiotracer from the cerebellum, with the results given in Table 1.
The figure reveals relatively close
Discussion
Normative databases of PET neuroligand binding have been reported in humans for dopamine synthesis, D1 receptors, D2 receptors, and transporters (Ito et al., 2008) and 5-HT1A receptors (Rabiner et al., 2002, Costes et al., 2005). These provide an important resource to the neuroimaging community not only for exploring specific regional variations in neuroreceptors but also for providing insight into sample size considerations and experimental methodologies. As molecular imaging techniques such
Conclusions
We present the measurement of D2/D3 dopamine receptor binding in a large cohort of rhesus monkeys using small animal PET imaging with [F-18]fallypride. The variance of intersubject DVR measured in this cohort was similar across all regions of the brain, with the highest variability found in the caudate nucleus. This variability was consistent with levels reported by others in normative groups measuring PET neuroreceptor binding in humans. Other than the pituitary, there was no significant
Acknowledgments
The authors would like to thank the following for their contributions to this research: Drs. Jim Holden, Kristin Javaras and Howard Rowley for technical discussions; Joseph Hampel, Elizabeth Smith and Aleem Baktiar for data acquisition and processing; H. Van Valkenberg, Tina Johnson, Kyle Meyer, Elizabeth Zao and the staff at the Harlow Center for Biological Psychology and the Wisconsin National Primate Research Center at the University of Wisconsin (RR000167) for nonhuman primate handling.
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