Objective: We aimed to determine whether the descending 5-HT facilitatory pathway from rostral ventromedial medulla (RVM) and the extracellular 5-HT in RVM are involved in the occurrence of pain in Parkinson’s disease (PD).

Background: Pain is one of the most distressing non-motor symptoms of PD. Despite its impact, the underlying mechanism is still elusive. To investigate the role of serotonergic pathway at the RVM in pain sensory abnormalities, a series of experiments were performed in 6-OHDA-induced PD model.

Methods: Rotarod test was used to evaluate rats moter function. Radiant heat test and von frey test were carried out to detect thermal and mechanical pain thresholds. Immunofluorescence was used to observe the 5-HT neurons and fibers in RVM and spinal cord. High Performance Liquid Chromatography (HPLC) was used to determine 5-HT and 5-HIAA level.

Results: The duration of running time used Rotarod test was significantly reduced in 6-OHDA-induced PD rats. The nociceptive thresholds of both mechanical and heat pain were reduced compared to sham-operated rats. In addition to the degeneration of cell bodies and fibers in the substantia nigra pars compacta (SNpc), we found that the extracellular 5-HT content in RVM was reduced and the number of 5-HT neurons was aslo decreased. Intra-RVM microinjection of 5,7-DHT and citalopram both partially reversed pain hypersensitivity .

Conclusions: Our data suggested that the descending 5-HT facilitatory pathway and the altered extracellular 5-HT content in RVM may play a role in the hyperalgesia of 6-OHDA-induced PD model.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/inhibition-of-nociceptive-responses-by-modulating-serotonergic-signals-at-the-rostral-ventromedial-medulla-in-6-ohda-induced-parkinsons-disease-rat-model/