Effects of Heating-needle Stimulation in Restoration of Weakened Descending Inhibition of Nociception in a Rat Model of Parkinson's Disease

Antinociceptive role of the thalamic dopamine D3 receptor in descending modulation of intramuscular formalin-induced muscle nociception in a rat model of Parkinson's disease

Pain in Parkinson's disease (PD) has been validated as one of the major non-motor dysfunctions affecting the quality of life and subsequent rehabilitation. In the present study, we investigated the role of the dopamine D3 receptor in the thalamic mediodorsal (MD) and ventromedial (VM) nuclei mediated descending control of nociception and intramuscular (i.m.) 2.5% formalin-induced persistent muscle nociception. Paw withdrawal reflexes were measured in naive rats and rats subjected to PD induced by unilateral microinjection of 6 μg 6-OHDA into the rat striatum. Formalin-induced muscle nociception in phase 1, inter-phase, and phase 2 was significantly greater in PD rats compared to naive and vehicle-treated rats (P < 0.001). PD rats exhibited bilaterally mechanical hyperalgesia and heat hypoalgesia in formalin-induced muscle nociception. Microinjection of SK609, a dopamine D3 receptor agonist, at various doses (2.5-7.5 nmol/0.5 μl) into the thalamic VM nucleus dose-dependently prolonged heat-evoked paw withdrawal latencies in both naive and PD rats. Administration of SK609 to either the MD or VM nuclei had no effect on noxious mechanically evoked paw withdrawal reflexes. Pre-treatment of the thalamic MD nucleus with SK609 significantly attenuated formalin-induced nociception, and reversed mechanical hyperalgesia, but not heat hypoalgesia. Pre-treatment of the thalamic VM nucleus with SK609 inhibited formalin-induced nociception in the late phase of phase 2 (30-75 min) and heat hypoalgesia, but not mechanical hyperalgesia (P < 0.05). It is suggested that the dopamine D3 receptors in the thalamus play an antinociceptive role in the descending modulation of nociception. Activation of D3 receptors within the thalamic MD and VM nuclei attenuates descending facilitation and enhances descending inhibition in rats during PD.

Pathological pain: Non-motor manifestations in Parkinson disease and its treatment

In addition to motor symptoms, non-motor manifestations of Parkinson's disease (PD), i.e. pain, depression, sleep disturbance, and autonomic disorders, have received increasing attention. As one of the non-motor symptoms, pain has a high prevalence and is considered an early pre-motor symptom in the development of PD. In relation to pathological pain and its management in PD, particularly in the early stages, it is hypothesized that the loss of dopaminergic neurons causes a functional deficit in supraspinal structures, leading to an imbalance in endogenous descending modulation. Deficits in dopaminergic-dependent pathways also affect non-dopaminergic neurotransmitter systems that contribute to the pathological processing of nociceptive input, the integration, and modulation of pain in PD. This review examines the onset and progression of pain in PD, with a particular focus on alterations in the central modulation of nociception. The discussion highlights the importance of abnormal endogenous descending facilitation and inhibition in PD pain, which may provide potential clues to a better understanding of the nature of pathological pain and its effective clinical management.

Thalamus: The 'promoter' of endogenous modulation of pain and potential therapeutic target in pathological pain

More recently, the thalamic mediodorsal (MD) and ventromedial (VM) nuclei have been revealed to be functioned as 'nociceptive discriminator' in discriminating noxious and innocuous peripheral afferents, and exhibits distinct different descending controls of nociception. Of particularly importance, the function of thalamic nuclei in engaging descending modulation of nociception is 'silent' or inactive during the physiological state as well as in condition exposed to insufficient noxious stimulation. Once initiation by sufficient noxious or innocuous C-afferents associated with temporal and spatial summation, the thalamic MD and VM nuclei exhibit salient, different effects: facilitation and inhibition, on noxious mechanically and heat evoked nociception, respectively. Based on series of experimental evidence, we here summarize a novel hypothesis involving thalamic MD and VM nuclei functioned as 'promoter' in initiating descending facilitation and inhibition of pain with specific spatiotemporal characteristics. We further hypothesize that clinical remedy in targeting thalamic VM nucleus by enhancing its activities in recruiting inhibition alone or decreasing thalamic MD nucleus induced facilitation may provide promising way in effectively control of pathological pain.