Initial clinical manifestations of Parkinson's disease: features and pathophysiological mechanisms

A direct projection from the subthalamic nucleus to the ventral thalamus in monkeys

The current basal ganglia model considers the internal division of the globus pallidus and the substantia nigra pars reticulata as the sole sources of basal ganglia output to the thalamus. However, following the delivery of retrograde tracers into the ventral anterior/ventral lateral thalamic nuclei, a moderate number of labeled neurons were found within the subthalamic nucleus (STN) in control cases, MPTP-treated monkeys and animals with levodopa-induced dyskinesias. Furthermore, dual tracing experiments showed that subthalamo-thalamic and subthalamo-pallidal projections arise from different subpopulations of STN efferent neurons. Moreover, upregulated expression of the mRNA coding the vesicular glutamate transporter 2 (vGlut2) was found in retrogradely-labeled STN neurons in MPTP-treated monkeys. By contrast, there is a reduction in vGlut2 mRNA expression in subthalamo-thalamic neurons in dyskinetic monkeys. In conclusion, our findings support the presence of a direct projection from the STN to the ventral thalamus that appears to be functionally modulated by dopaminergic activity.

Zonisamide: aspects in neuroprotection

Zonisamide is widely used as an antiepileptic drug. Two studies published recently in Experimental Neurology focus on the drug's neuroprotective effect. In the present commentary, we discuss the significance of their findings and aspects of zonisamide in neuroprotection.

Non-motor dopamine withdrawal syndrome after surgery for Parkinson's disease: predictors and underlying mesolimbic denervation

Apathy has been reported to occur after subthalamic nucleus stimulation, a treatment of motor complications in advanced Parkinson's disease. We carried out a prospective study of the occurrence of apathy and associated symptoms, predictors and mechanisms in the year following subthalamic stimulation. Dopamine agonist drugs were discontinued immediately after surgery and levodopa was markedly reduced within 2 weeks. Apathy and depression were assessed monthly, using the Starkstein apathy scale and the Beck Depression Inventory. Dopamine agonists were re-introduced if patients developed apathy or depression. Preoperative non-motor fluctuations were evaluated using the Ardouin Scale. Depression, apathy and anxiety were evaluated both on and off levodopa. Analysis of predictors of apathy was performed using a Cox proportional hazard model. Twelve patients who developed apathy and a control group of 13 patients who did not underwent [11C]-raclopride positron emission tomography scanning before and after oral intake of methylphenidate. In 63 patients with Parkinson's disease treated with subthalamic stimulation, dopaminergic treatment was decreased by 82% after surgery. Apathy occurred after a mean of 4.7 (3.3-8.2) months in 34 patients and was reversible in half of these by the 12-month follow-up. Seventeen patients developed transient depression after 5.7 (4.7-9.3) months and these fell into the apathy group with one single exception. At baseline, fluctuations in depression, apathy and anxiety scores were greater in the group with apathy. Fluctuations in apathy, depression and anxiety ratings during a baseline levodopa challenge were also significant predictors of postoperative apathy in univariate analysis, but not motor and cognitive states or the level of reduction of dopaminergic medication. The multivariate model identified non-motor fluctuations in everyday life and anxiety score during the baseline levodopa challenge as two independent significant predictors of postoperative apathy. Without methylphenidate, [11C]-raclopride binding potential values were greater in apathetic patients bilaterally in the orbitofrontal, dorsolateral prefrontal, posterior cingulate and temporal cortices, left striatum and right amygdala, reflecting greater dopamine D2/D3 receptor density and/or reduced synaptic dopamine level in these areas. The variations of [11C]-raclopride binding potential values induced by methylphenidate were greater in non-apathetic patients in the left orbitofrontal cortex, dorsolateral prefrontal cortex, thalamus and internal globus pallidus and bilaterally in the anterior and posterior cingulate cortices, consistent with a more important capacity to release dopamine. Non-motor fluctuations are related to mesolimbic dopaminergic denervation. Apathy, depression and anxiety can occur after surgery as a delayed dopamine withdrawal syndrome. A varying extent of mesolimbic dopaminergic denervation and differences in dopaminergic treatment largely determine mood, anxiety and motivation in patients with Parkinson's disease, contributing to different non-motor phenotypes.

Genes regulated in MPTP-treated macaques and human Parkinson's disease suggest a common signature in prefrontal cortex

The presymptomatic phase of Parkinson's disease (PD) is now recognized as a prodromal phase, with compensatory mechanism masking its progression and non-motor early manifestations, such as depression, cognitive disturbances and apathy. Those mechanisms were thought to be strictly dopamine-mediated until recent advances have shed light upon involvement of putative outside-basal ganglia, i.e. cortical, structures. We took advantage of our progressive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated macaque model to monitor whole genome transcriptional changes in several brain areas. Our data reveals that transcriptomic activity changes take place from early stages, suggesting very early compensatory mechanisms or pathological activity outside the basal ganglia, including the PFC. Specific transcriptomic changes occurring in the PFC of fully parkinsonian MPTP-treated macaques have been identified. Interestingly, a large part of these transcriptomic changes were also observed in human post-mortem samples of patients with neurodegenerative diseases analysed by quantitative PCR. These results suggest that the PFC is able to detect the progression of dopamine denervation even at very early time points. There are therefore mechanisms, within the PFC, leading to compensatory alterations and/or participating to pathophysiology of prodromal PD manifestations.

Coordination of grasping and walking in Parkinson's disease

Studies on grasp control underlying manual dexterity in people with Parkinson disease (PD) suggest that anticipatory grasp control is mainly unaffected during discrete tasks using simple two-digit grasp. Nevertheless, impaired hand function during daily activities is one of the most disabling symptoms of PD. As many daily grasping activities occur during functional movements involving the whole body, impairments in anticipatory grasp control might emerge during a continuous dynamic task such as object transport during walking. In this case, grasp control must be coordinated along with multiple body segments. The present study investigated the effect of PD on anticipatory grasp control and intersegmental coordination during walking with a hand-held object. Nine individuals with idiopathic PD (tested OFF and ON medication) and nine healthy age-matched controls carried a grip instrument between their right thumb and index finger during self-paced and fast walking. Although the amplitude of grip forces was higher in standing and walking for subjects with PD, both subjects with PD and control subjects coupled grip and inertial force changes in an anticipatory fashion while walking. However, gait-induced motions of the object relative to that of the trunk (i.e., dampening) was reduced in subjects with PD. Medication increased the dampening in all subjects with PD. We suggest that these differences are associated with impairments in intersegmental coordination.

Toxicity of a serotonin-derived neuromelanin

Postoperative Cognitive Dysfunction (POCD) is associated with increased mortality in the elderly and may occur from lipid peroxidation in aging. We previously showed that sevoflurane sequesters acrolein, which promotes the formation of a novel species of a putative neuromelanin. The current study examined the properties of this serotonin-derived melanoid (SDM). The interaction of SDM with unilamellar vesicles (ULVs) was examined using lipid membrane probes. Vesicle disruption was investigated by leakage of dye from calcein-loaded ULVs. We observed that SDM decreased diphenyl-hexatriene fluorescence anisotropy and increased the temperature-dependent change in anisotropy. SDM changed the absorbance of merocyanin-bound ULVs. SDM increased detergent-mediated calcein leakage. SDM structure was dramatically altered upon interaction with ULVs. We also observed that SDM enhanced detergent-mediated leakage of loaded ULVs, suggesting that SDM may be neurotoxic. We propose that inhalational agents, which sequester acrolein, may promote the production of certain species of neuromelanin that depletes local serotonin and enhances neuronal vulnerability.