Thomsen, B. L. C. | bioRxiv Science

bioRxiv2026

Striatal Dopaminergic Dysfunction Constrains Motor Invigoration in Parkinson's Disease

Bradykinesia is a cardinal motor feature of Parkinson's disease and is characterised by a reduced ability to generate movements with normal speed and force. Although nigrostriatal dopamine loss is regarded as the primary cause, direct human evidence linking dopaminergic degeneration to striatal dysfunction and impaired motor output and vigour remains limited. To investigate this relationship, we developed a grip-force paradigm that quantified movement vigour as the ability to rapidly generate force. Fifty-three individuals with Parkinson's disease, 30 with levodopa-induced dyskinesia and 23 without, and 25 age-matched healthy controls performed the task during 3T functional MRI. Thirty-five patients additionally underwent dopamine transporter PET imaging to assess associations among nigrostriatal dopaminergic integrity, striatal activity and motor performance. Patients with Parkinson's disease showed marked reductions in initial movement vigour accompanied by reduced bilateral putaminal activation during force generation. Lower putaminal dopamine transporter binding was associated with both poorer movement vigour and weaker grip-related putaminal activation during functional MRI, establishing a direct link between nigrostriatal degeneration, impaired striatal recruitment and motor slowing. Functional MRI further revealed reduced reward-related responses in the nucleus accumbens. Across participants, stronger reward-related ventral striatal activity was associated with greater movement vigour, suggesting that motivational processes contribute independently to motor performance. Patients with levodopa-induced dyskinesia exhibited more severe putaminal dopaminergic denervation than patients without dyskinesia but showed no additional impairment of movement vigour or striatal task responses. These findings provide direct in vivo evidence that dorsal nigrostriatal dopaminergic degeneration constrains movement vigour in Parkinson's disease through reduced striatal recruitment during action generation. Impaired reward-related signalling in ventral striatum emerges as an additional, partly independent mechanism influencing motor performance, highlighting distinct motor and motivational contributions to bradykinesia.

neuroscience↗

bioRxiv2025

Blunted response of caudal locus coeruleus to arousing stimuli in Parkinson's Disease

Parkinsons disease (PD) causes progressive degeneration of noradrenergic neurons in the locus coeruleus (LC), contributing to non-motor symptoms. Using neuromelanin-sensitive ultra-high field (7T) MRI, we previously identified a reduction in neuromelanin signal in the caudal LC, indicating a rostro-caudal gradient of noradrenergic cell loss. Caudal LC degeneration was associated with greater severity of non-motor symptoms such as orthostatic hypotension and apathy. In the current study, we expanded the PD cohort to further validate the structure-symptom relationships within the LC and investigate how degeneration along the rostro-caudal LC axis affects arousal-related functional responsivity. To this end, 71 people with PD in the ON-medication state and 40 age- and sex-matched healthy controls underwent clinical assessments and 7T magnetization transfer-weighted (MTw) MRI to quantify structural changes along the rostro-caudal LC axis. A subgroup of 30 people with PD and 27 controls underwent 7T fMRI to assess LC responsivity to arousing auditory and visual stimuli in two fMRI sessions on separate days. Healthy controls were scanned twice without medication, while people with PD were studied on and off dopaminergic medication in counterbalanced order. In the PD group, MTw MRI confirmed a significant reduction of the regional neuromelanin signal in caudal LC relative to the control group (P = 0.0099). This structural disintegration correlated with orthostatic hypotension (P = 0.0087) and cognitive impairment (P = 0.036), corroborating its clinical relevance. Functional MRI revealed reduced activation of the caudal LC to arousing visual and auditory stimuli in people with PD relative to controls (P = 0.012). This difference reached statistical significance only in the ON-medication state, with a similar but non-significant trend in the OFF-medication state (P = 0.10). In an exploratory analysis of a smaller sub-sample, structural and functional caudal LC signals were significantly correlated in both people with PD and healthy controls (P = 0.0069). Together, the findings provide evidence for a rostro-caudal gradient of LC pathology in PD at both structural and functional levels. While structural MRI provides fine-grained insights into spatial gradients of disease-related pathology, functional MRI captures impaired functional responsivity of caudal LC. The presence of arousal-induced hypoactivation of caudal LC in the ON-medication state indicates that LC dysfunction extends beyond dopamine deficits in PD, highlighting complex interactions between dopaminergic and noradrenergic systems.

neuroscience↗