Striatal acetylcholine-dopamine imbalance in Parkinson's disease: in vivo neuroimaging study with dual-tracer PET and dopaminergic PET-informed correlational tractography

Previous studies of animal models of Parkinson's disease (PD) suggest an imbalance between striatal acetylcholine (ACh) and dopamine (DA), although other studies have questioned this. To our knowledge, there are no previous in vivo neuroimaging studies examining striatal ACh-DA imbalance in PD patients. Using cholinergic and dopaminergic PET (¹⁸F-FEOBV and ¹¹C-DTBZ, respectively) and correlational tractography, our aim was to investigate the ACh-DA interaction at two levels of dopaminergic loss in PD subjects: integrity loss of the nigrostriatal dopaminergic white matter tract; and loss at the presynaptic-terminal level. Methods: The study involved 45 subjects with mild to moderate PD (36 men, 9 women; mean age, 66.3 ± 6.3 years, disease duration, 5.8 ± 3.6; Hoehn and Yahr stage, 2.2 ± 0.6) and 15 control subjects (9 men, 6 women; mean age, 69.1 ± 8.6 years). PET imaging was performed using standard protocols. We first estimated the integrity of the dopaminergic nigrostriatal white matter tracts in PD subjects by incorporating molecular information from striatal ¹¹C-DTBZ PET into the fiber tracking process using correlational tractography (based on quantitative anisotropy, QA; a measure of tract integrity). Subsequently, we used voxel-based correlation to test the association of the mean QA of the nigrostriatal tract of each cerebral hemisphere with striatal ¹⁸F-FEOBV distribution volume ratio (DVR) in PD subjects. The same analysis was performed for ¹¹C-DTBZ DVR in 12 striatal subregions (presynaptic-terminal level). Results: Unlike ¹¹C-DTBZ DVR in striatal subregions, the mean QA of the nigrostriatal tract of the most affected (MA) hemisphere showed a negative correlation with a striatal cluster of ¹⁸F-FEOBV DVR in PD subjects (p corrected= 0.039). We also found that the mean ¹⁸F-FEOBV DVR within this cluster was higher in the PD group compared to the control group (P = 0.01). Cross-validation analyses confirmed these findings. We also found an increase of bradykinesia ratings associated with increased ACh-DA imbalance in the MA hemisphere (r=0.41, P = 0.006). Conclusion: Our results provide evidence for the existence of striatal ACh-DA imbalance in early PD and may provide an avenue for testing in vivo effects of therapeutic strategies aimed at restoring striatal ACh-DA imbalance in PD.

Dopaminergic Nigrostriatal Connectivity in Early Parkinson Disease: In Vivo Neuroimaging Study of 11C-DTBZ PET Combined with Correlational Tractography

Previous histopathologic and animal studies have shown axonal impairment and loss of connectivity of the nigrostriatal pathway in Parkinson disease (PD). However, there are conflicting reports from in vivo human studies. ¹¹C-dihydrotetrabenazine (¹¹C-DTBZ) is a vesicular monoamine type 2 transporter PET ligand that allows assessment of nigrostriatal presynaptic dopaminergic terminal integrity. Correlational tractography based on diffusion MRI can incorporate ligand-specific information provided by ¹¹C-DTBZ PET into the fiber-tracking process. The purpose of this study was to assess the in vivo association between the integrity of the nigrostriatal tract (defined by correlational tractography) and the degree of striatal dopaminergic denervation based on ¹¹C-DTBZ PET. Methods: The study involved 30 subjects with mild to moderate PD (23 men and 7 women; mean age, 66 ± 6.2 y; disease duration, 6.4 ± 4.0 y; Hoehn and Yahr stage, 2.1 ± 0.6; Movement Disorder Society [MDS]-revised Unified Parkinson Disease Rating Scale [UPDRS] [I-III] total score, 43.4 ± 17.8) and 30 control subjects (18 men and 12 women; mean age, 62 ± 10.3 y). ¹¹C-DTBZ PET was performed using standard synthesis and acquisition protocols. Correlational tractography was performed to assess quantitative anisotropy (QA; a measure of tract integrity) of white matter fibers correlating with information derived from striatal ¹¹C-DTBZ data using the DSI Studio toolbox. Scans were realigned according to least and most clinically affected cerebral hemispheres. Results: Nigrostriatal tracts were identified in both hemispheres of PD patients. Higher mean QA values along the identified tracts were significantly associated with higher striatal ¹¹C-DTBZ distribution volume ratios (least affected: r = 0.57, P = 0.001; most affected: r = 0.44, P = 0.02). Lower mean QA values of the identified tract in the LA hemisphere associated with increased severity of bradykinesia sub-score derived from MDS-UPDRS part III (r = -0.42; P = 0.02). Cross-validation revealed the generalizability of these results. Conclusion: These findings suggest that impaired integrity of dopaminergic nigrostriatal nerve terminals is associated with nigrostriatal axonal dysfunction in mild to moderate PD. Assessment of nigrostriatal tract integrity may be suitable as a biomarker of early- or even prodromal-stage PD.

Neuroimaging and clinical features in adults with a 22q11.2 deletion at risk of Parkinson's disease

The recurrent 22q11.2 deletion is a genetic risk factor for early-onset Parkinson's disease. Adults with the associated 22q11.2 deletion syndrome (22q11.2DS) may exhibit phenotypes that could help identify those at highest risk and reveal disease trajectories. We investigated clinical and neuroimaging features relevant to Parkinson's disease in 26 adults: 13 with 22q11.2DS at genetic risk of Parkinson's disease (mean age = 41.5 years, standard deviation = 9.7), 12 healthy age and sex-matched controls, and a 22q11.2DS patient with l-DOPA-responsive early-onset Parkinson's disease. Neuroimaging included transcranial sonography and positron emission tomography using 11C-dihydrotetrabenazine (11C-DTBZ), a radioligand that binds to the presynaptic vesicular monoamine transporter. The 22q11.2DS group without Parkinson's disease demonstrated significant motor and olfactory deficits relative to controls. Eight (61.5%) were clinically classified with parkinsonism. Transcranial sonography showed a significantly larger mean area of substantia nigra echogenicity in the 22q11.2DS risk group compared with controls (P = 0.03). The 22q11.2DS patient with Parkinson's disease showed the expected pattern of severely reduced striatal 11C-DTBZ binding. The 22q11.2DS group without Parkinson's disease however showed significantly elevated striatal 11C-DTBZ binding relative to controls (∼33%; P < 0.01). Results were similar within the 22q11.2DS group for those with (n = 7) and without (n = 6) psychotic illness. These findings suggest that manifestations of parkinsonism and/or evolution to Parkinson's disease in this genetic at-risk population may include a hyperdopaminergic mechanism. Adequately powered longitudinal studies and animal models are needed to evaluate the relevance of the observed clinical and imaging phenotypes to Parkinson's disease and other disorders that are more prevalent in 22q11.2DS, such as schizophrenia.