Quantitative [¹²³I]FP-CIT pinhole SPECT imaging predicts striatal dopamine levels, but not number of nigral neurons in different mouse models of Parkinson's disease

Abstract

Single photon emission computed tomography (SPECT) using [¹²³I]FP-CIT as radioligand for the dopamine transporter has become a widely used tool to monitor the integrity of the nigrostriatal dopaminergic projection in Parkinson's disease (PD). Previous studies with pinhole SPECT in small animals have demonstrated that the striatal [¹²³I]FP-CIT binding indeed correlates with the striatal dopamine transporter protein level. It is unclear, however, if there is a stable relationship between the striatal [¹²³I]FP-CIT binding and other functionally important parameters of the nigrostriatal system, such as the striatal dopamine levels and the number of dopaminergic neurons in the substantia nigra. To assess this question experimentally, we studied two different mouse models of PD, namely a mild 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine intoxication paradigm, to model mild nigrostriatal damage and the intrastriatal 6-hydroxydopamine paradigm to model more advanced nigrostriatal damage. Our data demonstrate that the striatal [¹²³I]FP-CIT binding measured by SPECT in vivo precisely predicts the striatal dopamine concentrations, but does not necessarily correlate with the nigral dopaminergic cell number. Thus, the present work underscores that FP-CIT SPECT does only allow judging the integrity of the striatal dopaminergic nerve terminals, but not the nigral dopaminergic cells in PD. This finding may have significant impact on the use of [¹²³I]FP-CIT SPECT as a surrogate marker for clinical trials aimed at measuring neuroprotection.

Introduction

Parkinson's disease (PD) is the second most common neurodegenerative disease. Its core clinical features result from depletion in the neurotransmitter dopamine (DA) secondary to a degeneration of dopaminergic neurons in the pars compacta of the substantia nigra (SN) (Dauer and Przedborski, 2003, Dawson and Dawson, 2003). Since the dopamine transporter (DAT) is specifically expressed in dopaminergic neurons, PET- and SPECT-based methods to image DAT-binding radiotracers ([¹²³I]FP-CIT, [¹²³I]β-CIT or [^99mTc]TRODAT-1) are widely used to assess the integrity of the nigrostriatal projection in vivo. Such techniques are helpful to establish the diagnosis of PD or to monitor disease progression (Benamer et al., 2000, Parkinson Study Group, 2002, Ravina et al., 2005). Indeed, experiments in parkinsonian primates (Ma et al., 2002) and in PD patients (Eshuis et al., 2006) showed a strong correlation between the specific uptake of DAT-binding radiotracers in the striatum, as assessed by SPECT, and the [¹⁸F]DOPA uptake, a parameter of the DA-storage capacity of the dopaminergic nerve terminals in the striatum, as assessed by PET. Furthermore, it has been shown in rodents that the striatal [¹²³I]FP-CIT SPECT signal in vivo correlates with the DAT protein levels (Andringa et al., 2005). Thus, SPECT with DAT-binding radiotracers appears to be a valid biomarker to assess the integrity of the striatal dopaminergic innervation in vivo. However, since DAT is a highly regulated protein with a high turnover rate (Ahlskog, 2003), and since DAT expression appears to be reduced in dopaminergic neurons in PD (Uhl et al., 1994, Counihan and Penney, 1998), it is not clear how the in vivo SPECT signal relates to functionally important parameters of the nigrostriatal system, such as the striatal DA levels and the number of dopaminergic neurons in the SN.

In recent years the development of high-resolution pinhole SPECT systems has enabled SPECT studies experimentally in small laboratory animals (Myers and Hume, 2002, Acton and Kung, 2003, Andringa et al., 2005). Several studies with gamma cameras equipped with pinhole collimators showed that the density of the DAT in the striata of rats or mice can be determined by this technique with great precision (Habraken et al., 2001, Scherfler et al., 2002, Booij et al., 2002, Andringa et al., 2005). To assess the relation of the FP-CIT SPECT signal to striatal DA levels and nigral cell numbers we studied two different mouse models of PD. We used the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model in a mild intoxication paradigm to model mild nigrostriatal damage and 6-hydroxydopamine (6-OHDA) model with intrastriatal toxin injection to model more advanced nigrostriatal damage (Höglinger et al., 2004). In difference to previous studies, we used a multi-pinhole collimator with ten apertures in order to increase sensitivity and decrease acquisition time and compared the striatal FP-CIT binding in vivo with postmortem histological and biochemical parameters describing the integrity of the nigrostriatal system.