In Vivo Measurement of Density and Affinity of the Monoamine Vesicular Transporter in a Unilateral 6-Hydroxydopamine Rat Model of PD

Abstract

This is the first in vivo determination of the vesicular monoamine transporter (VMAT2) density (B max ) and ligand-transporter affinity (K d app ) in six unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats using micro-positron emission tomography (PET) imaging with [ 11 C]-( + )-a-dihydrotetrabenazine (DTBZ). A multiple ligand concentration transporter assay (MLCTA) was used to determine a B max value of 178632 pmol/mL and a K d app of 47.769.3 pmol/mL for the non-lesioned side and 30.5265.84 and 43.4615.52 pmol/mL for the lesioned side, respectively. While B max was significantly different between the two sides, no significant difference was observed for the K d app . In addition to demonstrating the feasibility of in vivo Scatchard analysis in rats, these data confirm the expectation that a 6-OHDA lesion does not affect the affinity; a much simpler binding potential (BP) measure can thus be used as a marker of lesion severity (LS) in this rat model of Parkinson's disease. A transporter occupancy curve demonstrated negligible transporter occupancy (B1%) at a specific activity (SA) of 1100 nCi/pmol (assuming an injected dose of 100 lCi/100 g), while 10% occupancy was estimated at 100 nCi/pmol. An indirect measurement indicated that the degree of occupancy as a function of SA is independent of LS. Finally, BP measurement reproducibility was assessed and found to be 11%67% for the healthy and 8%612% for the lesioned side. Quantitative PET results can thus be obtained even for severely lesioned animals with the striatum on one side not clearly visible provided accurate image analysis methods are used.

In Vivo Dopamine Transporter Imaging in a Unilateral 6-Hydroxydopamine Rat Model of Parkinson Disease Using 11C-Methylphenidate PET

“…Although a significant difference was found for B max between the lesioned and control sides, no such difference was observed for K app d , indicating that the decreased value of BP ND as estimated from the high SA (B/F) e on the lesion side indicates a reduction in transporter density, rather than being reflective of competitive endogenous ligand changes. The finding that chronic degeneration of the nigral dopamine terminals affects primarily B max is consistent with studies using other pre-and postsynaptic ligands in both human and nonhuman primates with raclopride (29,30) or in rodents with 11 C-dyhydrotetrabenazine (26). This might in turn suggest that changes in K app d may occur mostly as a mechanism to adapt rapidly to acute changes in the receptor or transporter environment.…”

“…Similar occupancy studies performed with 11 C-dyhydrotetrabenazine on unilaterally 6-hydroxydopamine-lesioned rats demonstrated very different characteristics: no dependence of DD severity estimate on tracer SA and excellent linearity of the Scatchard plots over a wide range of free (or bound) tracer concentrations, thus presenting no evidence of a difference in the concentration of the nonspecifically bound tracer between the striatum and reference region (26). Because the impact of the NS term is particularly relevant to a situation of high denervation severity, care must be taken when comparing outcomes of multitracer studies in such a situation.…”

Increased Vesicular Monoamine Transporter Binding during Early Abstinence In Human Methamphetamine Users: Is VMAT2 a Stable Dopamine Neuron Biomarker?

“…11 C]DTBZ binding in living Lee et al, 2000; this study) and postmortem brain of patients with Parkinson's disease (Wilson et al, 1996a) and in animal models of DA neuron damage (Vander Borght et al, 1995a;Strome et al, 2006;Sossi et al, 2007). However, our PET data in MA users, together with similar findings in DRD (De La Fuente-Fernández et al, 2003), and supported by our new animal data (Tong et al, 2008), suggest that (ϩ)[…”

In-vivo Measurement of LDOPA Uptake, Dopamine Reserve and Turnover in the Rat Brain Using [18F]FDOPA PET

“…From this DVR, the BP of DTBZ (BP ND ) was calculated as BP ND ¼ DVRÀ1. In the 6-OHDA lesion model, changes in DTBZ BP ND reflect changes in VMAT2 density (B max ), 30 which is linearly correlated with tyrosine hydroxylase activity. 29 The denervation severity (DS DTBZ ) caused by the lesioning was calculated from the asymmetry in BP ND via:…”