In Vivo Evaluation of ¹¹C-DASB for Quantitative SERT Imaging in Rats and Mice

Abstract

Serotonin, or 5-hydroxytryptamine (5-HT), plays a key role in the central nervous system and is involved in many essential neurologic processes such as mood, social behavior, and sleep. The serotonin transporter ligand 11 C-3-amino-4(2-dimethylaminomethyl-phenylsufanyl)-benzonitrile ( 11 C-DASB) has been used to determine nondisplaceable binding potential (BP ND ), which is defined as the quotient of the available receptor density (B avail ) and the apparent equilibrium dissociation rate constant (1/appK D ) under in vivo conditions. Because of the increasing number of animal models of human diseases, there is a pressing need to evaluate the applicability of 11 C-DASB to rats and mice. Here, we assessed the feasibility of using 11 C-DASB for quantification of serotonin transporter (SERT) density and affinity in vivo in rats and mice. Methods: Rats and mice underwent 4 PET scans with increasing doses of the unlabeled ligand to calculate B avail and appK D using the multiple-ligand concentration transporter assay. An additional PET scan was performed to calculate test-retest reproducibility and reliability. BP ND was calculated using the simplified reference tissue model, and the results for different reference regions were compared. Results: Displaceable binding of 11 C-DASB was found in all brain regions of both rats and mice, with the highest binding being in the thalamus and the lowest in the cerebellum. In rats, displaceable binding was largely reduced in the cerebellar cortex, which in mice was spatially indistinguishable from cerebellar white matter. Use of the cerebellum with fully saturated binding sites as the reference region did not lead to reliable results. Test-retest reproducibility in the thalamus was more than 90% in both mice and rats. In rats, B avail , appK D , and ED 50 were 3.9 ± 0.4 pmol/mL, 2.2 ± 0.4 nM, and 12.0 ± 2.6 nmol/kg, respectively, whereas analysis of the mouse measurements resulted in inaccurate fits due to the high injected tracer mass. Conclusion: Our data showed that in rats, 11 C-DASB can be used to quantify SERT density with good reproducibility. BP ND agreed with the distribution of SERT in the rat brain. It remains difficult to estimate quantitative parameters accurately from mouse measurements because of the high injected tracer mass and underestimation of the binding parameters due to high displaceable binding in the reference region. Ser otonergic neurons accumulate mainly in the brainstem and form the raphe nuclei (1). Although the raphe nuclei consist of only a few neurons, these nuclei innervate nearly all brain regions (1). Under normal physiologic conditions, the membranebound, sodium-chloride-dependent SERT (serotonin transporter, or 5-hydroxytryptamine [5-HT] transporter) is responsible for specific reuptake of released serotonin by cells (2). This transporter is located on the soma, dendritic collaterals, and entire axon of serotonergic neurons (3).One of the most promising noninvasive imaging approaches for visualizing the distribution of SERT is the SERT mark...

Quantitative Rodent Brain Receptor Imaging

“…Through saturation binding experiments for [ 11 C]raclopride, an injected The binding potential (BP ND ) was calculated using the simplified reference tissue model (SRTM) [44] with the cerebellum as the reference tissue dose of G 1 nmol/kg for rats [72] and 0.5 nmol/kg for mice [12] was estimated by others and in our studies. For [ 11 C]DASB, we observed an injected dose of G 1 nmol/kg for rats, while low receptor occupancy could not be obtained in mice using high molar activities up to 116 GBq/μmol [11]. In those experiments, receptor occupancy of less than 5-10 % could not be achieved; therefore, the values were obtained only by interpolation of the occupancy plot and need to be confirmed by in vivo experiments at higher molar activities.…”

“…Numerous small-animal receptor studies have been performed using commercially available preclinical PET systems [10][11][12][13][14][15][16]. However, resolving small structures within rodent brains using PET imaging is feasible but nevertheless challenging due to the relatively low spatial resolution of small-animal PET systems compared to the tiny size of structures in rodent brains.…”

“…Hence, higher numbers of animals are needed to gain statistically reliable results. To assess the influence of these limitations on the estimation of quantitative binding values and to determine the optimal quantification approach for a given PET tracer in small laboratory animals, test-retest experiments have been performed for several CNS PET ligands [11][12][13][75][76][77][78][79][80][81][82][83]. In addition, physiological parameters can also vary within one animal at two different scan time points and may influence receptor availability, limiting the ability of PET to detect differences in an interventional study where a baseline and a treatment scan are compared.…”

“…binding serotonin transporter (SERT) ligand, because of the lack of an appropriate reference region. The entire cerebellum cannot be used in this case due to the presence of serotonergic projections in the cerebellar white matter [11].…”

“…Since the rat brain, and consequently the rat cerebellum, is larger in size, the obstacle of specific binding in the reference region can be surmounted by deriving TACs specifically from the cerebellar cortex, which lacks SERT; this solution, however, is not feasible for the mouse brain [11]. A reference region in the brain is also not available for the synaptic vesicle protein 2A (SV2A) tracer [ 11 C]UCB-J.…”