Objective: The presence of elevated dopamine (DA) and its major metabolites in the cytosol of neurons has been associated with their vulnerability in Parkinson's disease (PD). Over 99% of the cell's amines are confined to secretory vesicles (SVs), making these structures fundamental in the regulation of cytosolic DA levels. SVs of platelets use similar, if not the same mechanisms to accumulate serotonin in SVs as dopaminergic neurons do to store DA. Hence, any functional defects in platelets probably mirrors events in DA neurons.Methods: We have isolated fresh platelets from the blood of 75 PD patients, 116 matched controls and 24 patients with Parkinsonism, assaying serotonin handling (basal content, accumulation, secretion and spontaneous leakage).Results: We found a dramatic decrease in the serotonin content and uptake by SVs, as well as decreased thrombin‐induced release by platelets from PD patients but not in those from most Parkinsonism cases. Platelets from PD patients also failed to retain serotonin in SVs.Interpretation: These findings indicate a functional impairment in the handling of amines by SVs in PD patients. This defect may serve as a biomarker of PD, and the approach described here may be potentially used for the subclinical detection of PD and to establish a platform to assay disease modifying drugs. ANN NEUROL 2022

Platelets are probably the most accessible human cells to study exocytosis by amperometry. These cell fragments accumulate biological amines, serotonin in particular, using similar if not the same mechanisms as those employed by sympathetic, serotoninergic, and histaminergic neurons. Thus, platelets have been widely recognized as a model system to study certain neurological and psychiatric diseases. Platelets release serotonin by exocytosis, a process that entails the fusion of a secretory vesicle to the plasma membrane and that can be monitored directly by classic single cell amperometry using carbon fiber electrodes. However, this is a tedious technique because any given platelet releases only 4–8 secretory δ-granules. Here, we introduce and validate a diamond-based multielectrode array (MEA) device for the high-throughput study of exocytosis by human platelets. This is probably the first reported study of human tissue using an MEA, demonstrating that they are very interesting laboratory tools to assess alterations to exocytosis in neuropsychiatric diseases. Moreover, these devices constitute a valuable platform for the rapid testing of novel drugs that act on secretory pathways in human tissues.

Amperometry is arguably the most widely used technique for studying the exocytosis of biological amines. However, the scarcity of human tissues, particularly in the context of neurological diseases, poses a challenge for exocytosis research. Human platelets, which accumulate 90% of blood serotonin, release it through exocytosis. Nevertheless, single-cell amperometry with encapsulated carbon fibers is impractical due to the small size of platelets and the limited number of secretory granules on each platelet. The recent technological improvements in amperometric multi-electrode array (MEA) devices allow simultaneous recordings from several high-performance electrodes. In this paper, we present a comparison of three MEA boron-doped diamond (BDD) devices for studying serotonin exocytosis in human platelets: (i) the BDD-on-glass MEA, (ii) the BDD-on-silicon MEA, and (iii) the BDD on amorphous quartz MEA (BDD-on-quartz MEA). Transparent electrodes offer several advantages for observing living cells, and in the case of platelets, they control activation/aggregation. BDD-on-quartz offers the advantage over previous materials of combining excellent electrochemical properties with transparency for microscopic observation. These devices are opening exciting perspectives for clinical applications.

Amperometry is arguably the most widely used technique for studying the exocytosis of biological amines, allowing the study of the dynamics and kinetics of this cellular mechanism in real time

We have demonstrated that platelets from Parkinson's disease patients exhibit impaired serotonin uptake and release. This seems to be caused by a dysfunction in secretory vesicles (SecVs), which share the mechanisms of accumulation with dopaminergic neurons. As a working hypothesis, this dysfunction may create an excess of dopamine causing neurotoxicity a process absent in other conditions like Essential Tremor or Essential Tremor Plus. The serotonin functional analysis in platelets is a potential tool to reduce misdiagnosis. To validate this, we performed a functional study using liquid chromatography (UHPLC) to analyze serotonin content, uptake, and secretion in platelets from 50 Parkinson's disease patients, 23 essential tremor, 17 essential tremor plus, and 50 controls. Parkinson's disease patients show impaired platelet serotonin content and uptake vs . Controls ( P < 0.001), essential tremor ( P < 0.01) and essential tremor plus ( P < 0.05), ANOVA. Notably, no significant differences were found between controls and essential tremor nor essential tremor plus; although, essential tremor plus manifested mild serotonin storage dysfunction. Platelet serotonin dysfunction reflects distinct pathological origins between Parkinson's disease and other tremor syndromes, suggesting that platelet serotonin analysis is a viable and objective tool for differential diagnosis between Parkinson's disease and tremor syndromes.

About 90% of blood serotonin is stored in secretory granules of platelets and is released by exocytosis