Measuring the integrity of the dopamine system in rats with [¹¹C]dihydrotetrabenazine: MicroPET vs phosphor imaging autoradiography

Positron emission tomography (PET) is a powerful tool for in vivo imaging of brain function in human and non-human primates and more recently in small animals such as rodents. As in human subjects, in vivo microPET can be used to evaluate longitudinally the extent of lesions or the effects of pharmacological challenges or potential therapies in animal models of diseases. It is however important to validate and correlate the in vivo data against currently used methods, either behavioural or post mortem assessments. Using the 6-OHDA model of Parkinson's Disease and [11C]()dihydro-tetrabenazine (DTBZ), a tracer of the striatal dopaminergic (DA) terminals not sensitive to regulation as an index of lesion severity, we compared, in the same animals, in vivo data from a Concord microPET R4 scanner to in vitro autoradiography with the same tracer and to in vivo behavioral evaluation with a test known to be sensitive to moderate to severe nigro-striatal lesions and not requiring pharmacological challenges, the tapered ledged beam walking test (TLBWT). Adult male Sprague-Dawley rats with a wide range of 6-OHDA-induced DA nigro-striatal lesions were tested in the TLBWT test, scanned with DTBZ (100 mC/100 gr body weight; SA>500 Ci/mmole) under ketamine anesthesia, before being sacrificed for phosphor imaging autoradiography. For microPET imaging, the ratio of striatal/cerebellar activity was calculated between 30 and 60 min post tracer injection. In vitro binding was performed by incubating 20 um coronal sections in 5 nM DTBZ. Specific binding was calculated as (total binding)-(non-specific binding). The data were converted in pmol/cc tissue using standard curves and the specific activity at incubation 1. The number of hindlimb errors in the narrow beam of TLBWT was used as the behavioral measure of interest. The in vivo and in vitro DTBZ binding were significantly correlated (r = 0.71). Neither in vivo nor in vitro DTBZ binding correlated significantly with the behavioral scores, likely because behavioral deficits were detected in only the most severely lesioned animals. In summary, although microPET DTBZ binding was more sensitive to mild lesions than the behavioral scores, the mildest lesions were more easily identified with in vitro DTBZ binding. In addition, as expected due to its greater spatial resolution, autoradiography allowed better structure (caudate vs putamen vs accumbens for example) identification than microPET. However, the good correlation between in vivo and in vitro binding data suggests that DTBZ/microPET imaging is an excellent tool to assess in vivo the severity of lesions of the DA system in animal models of PD and associated disorders. It also suggests that post-mortem evaluation remains a needed confirmation when actual extent and location of the lesions are important parameters of a study.

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