Unilateral lesions of mesostriatal dopaminergic pathway alters the withdrawal response of the rat hindpaw to mechanical stimulation

Effects of hippocampal damage on pain perception in a rat model of Alzheimer's disease induced by amyloid-β and ibotenic acid injection into the hippocampus

Patients with Alzheimer's disease (AD) present with a variety of symptoms, including core symptoms as well as behavioral and psychological symptoms. Somatosensory neural systems are generally believed to be relatively unaffected by AD until late in the course of the disease; however, somatosensory perception in patients with AD is not yet well understood. One factor that may complicate the assessment of somatosensory perception in humans centers on individual variations in pathological and psychological backgrounds. It is therefore necessary to evaluate somatosensory perception using animal models with uniform status. In the current study, we focused on the hippocampus, the primary site of AD. We first constructed a rat model of AD model using bilateral hippocampal injections of amyloid-β peptide 1-40 and ibotenic acid; sham rats received saline injections. The Morris water maze test was used to evaluate memory impairment, and the formalin test (1 % or 4 % formalin) and upper lip von Frey test were performed to compare pain perception between AD model and sham rats. Finally, histological and immunohistochemical methods were used to evaluate tissue damage and neuronal activity, respectively, in the hippocampus. AD model rats showed bilateral hippocampal damage and had memory impairment in the Morris water maze test. Furthermore, AD model rats exhibited significantly less pain-related behavior in phase 2 (the last 50 min of the 60-minute observation) of the 4 % formalin test compared with the sham rats. However, no significant changes were observed in the von Frey test. Immunohistochemical observations of the trigeminal spinal subnucleus caudalis after 4 % formalin injection revealed significantly fewer c-Fos-immunoreactive cells in AD model rats than in sham rats, reflecting reduced neuronal activity. These results indicate that AD model rats with hippocampal damage have reduced responsiveness to persistent inflammatory chemical stimuli to the orofacial region.

Pain in Parkinson's disease: a neuroanatomy-based approach

Parkinson's disease is a progressive neurodegenerative disorder characterized by the deposition of misfolded alpha-synuclein in different regions of the central and peripheral nervous system. Motor impairment represents the signature clinical expression of Parkinson's disease. Nevertheless, non-motor symptoms are invariably present at different stages of the disease and constitute an important therapeutic challenge with a high impact for the patients' quality of life. Among non-motor symptoms, pain is frequently experienced by patients, being present in a range of 24-85% of Parkinson's disease population. Moreover, in more than 5% of patients, pain represents the first clinical manifestation, preceding by decades the exordium of motor symptoms. Pain implies a complex biopsychosocial experience with a downstream complex anatomical network involved in pain perception, modulation, and processing. Interestingly, all the anatomical areas involved in pain network can be affected by a-synuclein pathology, suggesting that pathophysiology of pain in Parkinson's disease encompasses a 'pain spectrum', involving different anatomical and neurochemical substrates. Here the various anatomical sites recruited in pain perception, modulation and processing are discussed, highlighting the consequences of their possible degeneration in course of Parkinson's disease. Starting from peripheral small fibres neuropathy and pathological alterations at the level of the posterior laminae of the spinal cord, we then describe the multifaceted role of noradrenaline and dopamine loss in driving dysregulated pain perception. Finally, we focus on the possible role of the intertwined circuits between amygdala, nucleus accumbens and habenula in determining the psycho-emotional, autonomic and cognitive experience of pain in Parkinson's disease. This narrative review provides the first anatomically driven comprehension of pain in Parkinson's disease, aiming at fostering new insights for personalized clinical diagnosis and therapeutic interventions.

Insight gained from using animal models to study pain in Parkinson's disease

Pain is one of the key non-motor symptoms experienced by a large proportion of people living with Parkinson's disease (PD), yet the mechanisms behind this pain remain elusive and as such its treatment remains suboptimal. It is hoped that through the study of animal models of PD, we can start to unravel some of the contributory mechanisms, and perhaps identify models that prove useful as test beds for assessing the efficacy of potential new analgesics. However, just how far along this journey are we right now? Is it even possible to model pain in PD in animal models of the disease? And have we gathered any insight into pain mechanisms from the use of animal models of PD so far? In this chapter we intend to address these questions and in particular highlight the findings generated by others, and our own group, following studies in a range of rodent models of PD.

Dynamics of Lateral Habenula-Ventral Tegmental Area Microcircuit on Pain-Related Cognitive Dysfunctions

Chronic pain is a health problem that affects the ability to work and perform other activities, and it generally worsens over time. Understanding the complex pain interaction with brain circuits could help predict which patients are at risk of developing central dysfunctions. Increasing evidence from preclinical and clinical studies suggests that aberrant activity of the lateral habenula (LHb) is associated with depressive symptoms characterized by excessive negative focus, leading to high-level cognitive dysfunctions. The primary output region of the LHb is the ventral tegmental area (VTA), through a bidirectional connection. Recently, there has been growing interest in the complex interactions between the LHb and VTA, particularly regarding their crucial roles in behavior regulation and their potential involvement in the pathological impact of chronic pain on cognitive functions. In this review, we briefly discuss the structural and functional roles of the LHb-VTA microcircuit and their impact on cognition and mood disorders in order to support future studies addressing brain plasticity during chronic pain conditions.

Altered parabrachial nucleus nociceptive processing may underlie central pain in Parkinson's disease

The presence of central neuropathic pain in Parkinson's disease suggests that the brain circuits that allow us to process pain could be dysfunctional in the disorder. However, there is to date no clear pathophysiological mechanism to explain these symptoms. In this work, we present evidence that the dysfunction of the subthalamic nucleus and/or substantia nigra pars reticulata may impact nociceptive processing in the parabrachial nucleus (PBN), a low level primary nociceptive structure in the brainstem, and induce a cellular and molecular neuro-adaptation in this structure. In rat models of Parkinson's disease with a partial dopaminergic lesion in the substantia nigra compacta, we found that the substantia nigra reticulata showed enhanced nociceptive responses. Such responses were less impacted in the subthalamic nucleus. A total dopaminergic lesion produced an increase in the nociceptive responses as well as an increase of the firing rate in both structures. In the PBN, inhibited nociceptive responses and increased expression of GABA_A receptors were found following a total dopaminergic lesion. However, neuro-adaptations at the level of dendritic spine density and post-synaptic density were found in both dopaminergic lesion groups. These results suggest that the molecular changes within the PBN following a larger dopaminergic lesion, such as increased GABA_A expression, is a key mechanism to produce nociceptive processing impairment, whilst other changes may protect function after smaller dopaminergic lesions. We also propose that these neuro-adaptations follow increased inhibitory tone from the substantia nigra pars reticulata and may represent the mechanism generating central neuropathic pain in Parkinson's disease.

Blockade of the orexin receptors in the ventral tegmental area could attenuate the stress-induced analgesia: A behavioral and molecular study

Exposure to stressful stimuli induces various physiological and behavioral responses, affects pain perception, and alters gene expression. Stress elicits an analgesic effect in laboratory animals, termed the "stress-induced analgesia" (SIA). Orexin neuropeptides, processed from pre-pro-orexin in the hypothalamus, release during stress and are known to be antinociceptive. The current study examined the modulatory role of the ventral tegmental area (VTA) orexinergic system in the restraint SIA and extracellular signal-regulated kinase (ERK) activation in the nucleus accumbens (NAc). Adult male Wistar rats were subjected to intra-VTA injection of orexin-1 and -2 receptor antagonists (SB334867 and TCS OX2 29; 1, 3, 10, and 30 nmol/0.3 μl, respectively) five min before a 3-h period of exposure to restraint stress (RS). Western blot analysis was also used to assess the levels of ERK and phosphorylated ERK (p-ERK) in the NAc tissues. RS exposure produced an analgesic response to the thermal pain model (Tail-flick test). RS-induced antinociception was inhibited by intra-VTA administration of SB334867 and TCS OX2 29. Moreover, in the molecular study, exposure to forced swim stress (FSS) and RS significantly enhanced the p-ERK/ERK ratio. Blockade of both orexin receptors diminished the p-ERK/ERK ratio, but this decrease was significant only in the FSS group of animals that received TCS OX2 29. Collectively, the present findings suggested the functional roles of intra-VTA orexin receptors and ERK signaling in the SIA.

Behavioral, Cellular and Molecular Responses to Cold and Mechanical Stimuli in Rats with Bilateral Dopamine Depletion in the Mesencephalic Dopaminergic Neurons

Pain is the major non-motor symptom in Parkinson's disease (PD). Preclinical studies have mostly investigated mechanical pain by considering the decrease in a nociceptive threshold. Only a few studies have focused on thermal pain in animal models of PD. Therefore, the goal of this study was to assess the thermal nociceptive behavior of rats subjected to 6-hydroxydopamine (6-OHDA) administration, which constitutes an animal model of PD. Thermal plate investigation demonstrated significant thermal sensitivity to cold temperatures of 10 °C and 15 °C, and not to higher temperatures, in 6-OHDA-lesioned rats when compared with sham. 6-OHDA-lesioned rats also showed cold allodynia as demonstrated by a significant difference in the number of flinches, latency and reaction time to acetone stimulus. Ropinirole administration, a dopamine receptor 2 (D2R) agonist, blocked the acetone-induced cold allodynia in 6-OHDA-lesioned rats. In addition, mechanical hypersensitivity and static allodynia, as demonstrated by a significant difference in the vocalization threshold and pain score respectively, were noticed in 6-OHDA-lesioned rats. Acetone stimulus induced a significant increase in extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) phosphorylation, a pain process molecular marker, in the spinal dorsal horn (SDH), the insular and cingulate cortices in 6-OHDA-lesioned rats when compared to sham. In 6-OHDA-lesioned rats, there was a significant augmentation in the expression of both protein kinase C gamma (PKCγ) and glutamate decarboxylase 67 (GAD67) in the SDH. This highlighted an increase in excitation and a decrease in inhibition in the SDH. Overall, the present study demonstrated a clear cold thermal hypersensitivity, in addition to a mechanical one, in 6-OHDA-lesioned rats.

The Role of Mesostriatal Dopamine System and Corticostriatal Glutamatergic Transmission in Chronic Pain

There is increasing recognition of the involvement of the nigrostriatal and mesolimbic dopamine systems in the modulation of chronic pain. The first part of the present article reviews the evidence indicating that dopamine exerts analgesic effects during persistent pain by stimulating the D2 receptors in the dorsal striatum and nucleus accumbens (NAc). Thereby, dopamine inhibits striatal output via the D2 receptor-expressing medium spiny neurons (D2-MSN). Dopaminergic neurotransmission in the mesostriatal pathways is hampered in chronic pain states and this alteration maintains and exacerbates pain. The second part of this article focuses on the glutamatergic inputs from the medial prefrontal cortex to the NAc, their activity changes in chronic pain, and their role in pain modulation. Finally, interactions between dopaminergic and glutamatergic inputs to the D2-MSN are considered in the context of persistent pain. Studies using novel techniques indicate that pain is regulated oppositely by two independent dopaminergic circuits linking separate parts of the ventral tegmental area and of the NAc, which also interact with distinct regions of the medial prefrontal cortex.

Nigrostriatal dopamine depletion promoted an increase in inhibitory markers (parvalbumin, GAD67, VGAT) and cold allodynia

Pain constitutes the major non-motor symptom in Parkinson's disease (PD). Its mechanism is still poorly understood although an increase in excitation or a decrease in inhibition have been reported in preclinical studies. The aim of this study was to investigate gamma aminobutyric acid (GABA) inhibition in the 6-hydroxydopamine (6-OHDA) PD rat model. Therefore, the expression of three inhibitory markers parvalbumin, glutamate decarboxylase 67 (GAD67) and vesicular GABA transporter (VGAT) was evaluated, besides cold allodynia, in bilateral 6-OHDA lesioned rat. There was a significant increase in the expression of the three markers labeling within the spinal dorsal horn (SDH) of 6-OHDA lesioned rats. In parallel, there was also an increase of the excitatory marker protein kinase C gamma (PKCγ) . PKCγ cells have a crucial role in pain chronicity and are regulated by GABAergic influences. Central dopamine depletion induced an increase in excitation as reveled by an increase in cFOS expression upon acetone stimulus and the presence of cold allodynia. In addition, dopamine depletion induced increased expression in inhibitory markers, which may reflect a disinhibition or a decreased inhibition in 6-OHDA lesioned rats.

Nociception alterations precede motor symptoms in a progressive model of parkinsonism induced by reserpine in middle-aged rats

Nociception alterations are frequent non-motor symptoms of the prodromal phase of Parkinson's disease (PD). The period for the onset of symptoms and the pathophysiological mechanisms underlying these alterations remain unclear. We investigated the course of nociception alterations in a progressive model of parkinsonism induced by reserpine (RES) in rats. Male Wistar rats (6-7 months) received 5 or 10 subcutaneous injections of RES (0.1 mg/kg) or vehicle daily for 20 days. Motor evaluation and nociceptive assessment were performed throughout the treatment. At the end of the treatment rats were euthanized, the brains removed and processed for immunohistochemical analysis (TH and c-Fos). The RES-treated rats exhibited an increased nociceptive response to mechanical and chemical stimulation in the electronic von Frey and formalin tests, respectively. Moreover, these alterations preceded the motor impairment observed in the catalepsy test. In addition, the RES treatment reduced the TH-immunoreactivity in the ventral tegmental area (VTA) and increased the c-Fos expression in the ventral-lateral periaqueductal gray (vlPAG), rostral ventral medulla (RVM) and dorsal raphe nucleus (DRN) after noxious stimuli induced by formalin. Taken together, our results reinforce that nociceptive changes are one of the early signs of PD and monoamine depletion in basal ganglia can be involved in the abnormal processing of nociceptive information in PD.

Therapeutic effects of Wharton's jelly-derived Mesenchymal Stromal Cells on behaviors, EEG changes and NGF-1 in rat model of the Parkinson's disease

Human Wharton's jelly-derived Mesenchymal Stromal Cells (hWJ-MSCs) have shown beneficial effects in improving the dopaminergic cells in the Parkinson's disease (PD). In the present study, the effects of hWJ-MSCs on hyperalgesia, anxiety deficiency and Pallidal local electroencephalogram (EEG) impairment, alone and combined with L-dopa, were examined in a rat model of PD. Adult male Wistar rats were divided into five groups: 1) sham, 2) PD, 3) PD + C (Cell therapy), 4) PD + C+D (Drug), and 5) PD + D. PD was induced by injection of 6-OHDA (16 μg/2 μl into medial forebrain bundle (MFB)). PD + C group received hWJ-MSCs (1 × 10⁶ cells, intravenous (i.v.)) twice post PD induction. PD + C+D groups received hWJ-MSCs combined with L-Dopa/Carbidopa, (10/30 mg/kg, intraperitoneally (i.p.)). PD + D group received L-Dopa/Carbidopa alone. Four months later, analgesia, anxiety-like behaviors, were evaluated and Pallidal local EEG was recorded. Level of insulin-like growth factor 1 (IGF-1) was measured in the striatum and dopaminergic neurons were counted in substantia nigra (SNc). According to data, MFB-lesioned rats showed hyperalgesia in tail flick, anxiety-like symptoms in cognitive tests, impairment of electrical power of pallidal local EEG as field potential, count of dopaminergic neurons in SNc and level of IGF-1 in striatum. These complications restored significantly by MSCs treatment (p < 0.001). Our findings confirm that chronic treatment with hWJ-MSC, alone and in combination with L-Dopa, improved nociception and cognitive deficit in PD rats which may be the result of increasing IGF-1 and protect the viability of dopaminergic neurons.

Synergistic antiallodynic and antihyperalgesic interaction between L-DOPA and celecoxib in parkinsonian rats is mediated by NO-cGMP-ATP-sensitive K+ channel

Pain is a usual and troublesome non-motor symptom of Parkinson's disease, with a prevalence of 29-82%. Therefore, it's vital to find pharmacological treatments for managing PD-associated pain symptoms, to improve patients' quality of life. For this reason, we tested the possible synergy between L-DOPA and celecoxib in decreasing allodynia and hyperalgesia induced by unilateral lesioning with 6-OHDA into the SNpc in rats. We also tested whether the antiallodynic and antihyperalgesic effect induced by combination of L-DOPA and celecoxib is mediated by the NO-cGMP-ATP-sensitive K⁺ channel pathway. Tactile allodynia and mechanical hyperalgesia were evaluated using von Frey filament. Isobolographic analyses were employed to define the nature of the drug interaction using a fixed dose ratio (0.5: 0.5). We found that acute and sub-acute (10-day) treatment with a single dose of L-DOPA (3-25 mg/kg, i. p.) or celecoxib (2.5-20 mg/kg, i. p.) induced a dose-dependent antiallodynic and antihyperalgesic effect in parkinsonian rats. Isobolographic analysis revealed that the ED₅₀ values obtained by L-DOPA + celecoxib combination was significantly less than calculated additive values, indicating that co-administration of L-DOPA with celecoxib produces synergistic interactions in its antiallodynic and antihyperalgesic effect in animals with nigrostriatal lesions. Moreover, the antiallodynic and antihyperalgesic effects induced by L-DOPA + celecoxib combination were blocked by intrathecal pre-treatment with L-NAME, ODQ, and glibenclamide. Taken together, the data suggest that L-DOPA + celecoxib combination produces an antiallodynic and antihyperalgesic synergistic interaction at the systemic level, and these effects are mediated, at the central level, through activation of the NO-cGMP-ATP-sensitive K⁺ channel pathway.

SIRT1 Mediates Neuropathic Pain Induced by Sciatic Nerve Chronic Constrictive Injury in the VTA-NAc Pathway

Background

Mounting evidence has shown that sirtuin 1 (SIRT1), a class III histone deacetylase, alleviated several types of neuropathic pain in the spinal cord and dorsal root ganglion and regulated some aberrant behaviors in the ventral tegmental area (VTA) and the nucleus accumbens (NAc).

Methods

In this context, the effect of SIRT1 on neuropathic pain in the VTA-NAc pathway was investigated in the model of chronic constrictive injury (CCI).

Results

SIRT1 was localized in the VTA neurons in naive mice. The expression of SIRT1 was decreased in the contralateral VTA of CCI mice. After microinjection of SRT1720 (an activator of SIRT1) in the contralateral VTA of CCI mice, the established thermal hyperalgesia was attenuated. However, it was further exacerbated by EX-527 (an inhibitor of SIRT1). The elevated level of acetyl-histone 3 was reduced by SRT1720 but further elevated by EX-527 in the contralateral VTA of CCI mice. The increased expression of Fos in both VTA and NAc was downregulated by SRT1720 but further upregulated by EX-527 in CCI mice.

Conclusions

The discovery of the effect of SIRT1 on neuropathic pain in the VTA represents an important step forward in understanding the analgesic mechanisms of the VTA-NAc pathway.

Intrastriatal administration of coenzyme Q10 enhances neuroprotection in a Parkinson's disease rat model

Parkinson's disease is a neurodegenerative disorder, and no treatment has been yet established to prevent disease progression. Coenzyme Q10, an antioxidant, has been considered a promising neuroprotective agent; however, conventional oral administration provides limited efficacy due to its very low bioavailability. In this study, we hypothesised that continuous, intrastriatal administration of a low dose of Coenzyme Q10 could effectively prevent dopaminergic neuron degeneration. To this end, a Parkinson's disease rat model induced by 6-hydroxydopamine was established, and the treatment was applied a week before the full establishment of this disease model. Behavioural tests showed a dramatically decreased number of asymmetric rotations in the intrastriatal Coenzyme Q10 group compared with the no treatment group. Rats with intrastriatal Coenzyme Q10 exposure also exhibited a larger number of dopaminergic neurons, higher expression of neurogenetic and angiogenetic factors, and less inflammation, and the effects were more prominent than those of orally administered Coenzyme Q10, although the dose of intrastriatal Coenzyme Q10 was 17,000-times lower than that of orally-administered Coenzyme Q10. Therefore, continuous, intrastriatal delivery of Coenzyme Q10, especially when combined with implantable devices for convection-enhanced delivery or deep brain stimulation, can be an effective strategy to prevent neurodegeneration in Parkinson's disease.

Potentiation of morphine-induced antinociception by harmaline: involvement of μ-opioid and ventral tegmental area NMDA receptors

RATIONAL

Morphine is one of the most well-known and potent analgesic agents; however, it can also induce various side effects. Thus, finding drugs and mechanisms which can potentiate the analgesic effects of low doses of morphine will be a good strategy for pain management.

OBJECTIVE

The involvement of μ-opioid receptors and ventral tegmental area (VTA) glutamatergic system in harmaline and morphine combination on the nociceptive response were investigated. Also, we examined reward efficacy and tolerance expression following the drugs.

METHODS

Animals were bilaterally cannulated in the VTA by stereotaxic instrument. A tail-flick (TF) apparatus and conditioned place preference (CPP) paradigm were used to measure nociceptive response and rewarding effects in male NMRI mice respectively.

RESULTS

Morphine (2 mg/kg, i.p.) had no effect in TF test. Also, harmaline (1.25 and 5 mg/kg, i.p.) could not change pain threshold. Combination of a non-effective dose of harmaline (5 mg/kg) and morphine (2 mg/kg) produced antinociception and also prevented morphine tolerance but had no effect on the acquisition of CPP. Systemic administration of naloxone (0.5 and 1 mg/kg) and intra-VTA microinjection of NMDA (0.06 and 0.1 μg/mouse) before harmaline (5 mg/kg) plus morphine (2 mg/kg) prevented antinociception induced by the drugs. D-AP5 (0.5 and 1 μg/mouse, intra-VTA) potentiated the effect of low-dose harmaline (1.25 mg/kg) and morphine (2 mg/kg) and induced antinociception. Microinjection of the same doses of NMDA or D-AP5 into the VTA alone had no effect on pain threshold.

CONCLUSION

The findings showed that harmaline potentiated the analgesic effect of morphine and reduced morphine tolerance. Glutamatergic and μ-opioidergic system interactions in the VTA seem to have a modulatory role in harmaline plus morphine-induced analgesia.

Potential of animal models for advancing the understanding and treatment of pain in Parkinson's disease

Pain is a commonly occurring non-motor symptom of Parkinson’s disease (PD). Treatment of pain in PD remains less than optimal and a better understanding of the underlying mechanisms would facilitate discovery of improved analgesics. Animal models of PD have already proven helpful for furthering the understanding and treatment of motor symptoms of PD, but could these models offer insight into pain in PD? This review addresses the current position regarding pain in preclinical models of PD, covering the face and predictive validity of existing models and their use so far in advancing understanding of the mechanisms contributing to pain in PD. While pain itself is not usually measured in animals, nociception in the form of thermal, mechanical or chemical nociceptive thresholds offers a useful readout, given reduced nociceptive thresholds are commonly seen in PD patients. Animal models of PD including the reserpine-treated rat and neurodegenerative models such as the MPTP-treated mouse and 6-hydroxydopamine (6-OHDA)-treated rat each exhibit reduced nociceptive thresholds, supporting face validity of these models. Furthermore, some interventions known clinically to relieve pain in PD, such as dopaminergic therapies and deep brain stimulation of the subthalamic nucleus, restore nociceptive thresholds in one or more models, supporting their predictive validity. Mechanistic insight gained already includes involvement of central and spinal dopamine and opioid systems. Moving forward, these preclinical models should advance understanding of the cellular and molecular mechanisms underlying pain in PD and provide test beds for examining the efficacy of novel analgesics to better treat this debilitating non-motor symptom.

Changes in Monoaminergic Neurotransmission in an Animal Model of Osteoarthritis: The Role of Endocannabinoid Signaling

Chronic pain is a main symptom of osteoarthritis (OA). Moreover, a high percentage of OA patients suffer from mental health problems. The endocannabinoid (EC) system has attracted attention as an emerging drug target for pain treatment together with its activity on the mesolimbic reward system. Understanding the circuits that govern the reward of pain relief is crucial for the search for effective analgesics. Therefore, we investigated the role of the EC system on dopamine (DA) and noradrenaline (NA) in an animal model of OA-related chronic pain. OA rats exhibited significant decreases in DA metabolism in the nucleus accumbens (NAc), striatum (STR) and hippocampus (HC). NA metabolism was also significantly decreased by chronic pain in OA rats; however, this disruption was limited to the frontal cortex (FCx) and HC. URB597 (an inhibitor of EC metabolism) treatment completely reversed the decreased DA metabolism, especially in the brain reward system and the HC. Furthermore, administration of URB597 normalized the impairment of NA activity in the HC but potentiated the decreased NA levels in the FCx. Our results demonstrated that chronic pain in OA rats was reflected by the inhibition of mesolimbic and mesocortical dopaminergic transmission, and may indicate the pro-pain role of NA in the FCx. The data provide understanding about changes in neurotransmission in chronic pain states and may explain the clinical improvement in perceived life quality following cannabinoid treatment. Additional mechanistic studies in preclinical models examining the intersection between chronic pain and reward circuits may offer new approaches for improving pain therapy.

Chronic pain and pain processing in Parkinson's disease

Pain is experienced by the vast majority of patients living with Parkinson's disease. It is most often of nociceptive origin, but may also be ascribed to neuropathic (radicular or central) or miscellaneous sources. The recently validated King's Parkinson's Disease Pain Scale is based on 7 domains including musculoskeletal pain, chronic body pain (central or visceral), fluctuation-related pain, nocturnal pain, oro-facial pain, pain with discolouration/oedema/swelling, and radicular pain. The basal ganglia integrate incoming nociceptive information and contribute to coordinated motor responses in pain avoidance and nocifensive behaviors. In Parkinson's disease, nigral and extra-nigral pathology, involving cortical areas, brainstem nuclei, and spinal cord, may contribute to abnormal central nociceptive processing in patients experiencing pain or not. The dopamine deficit lowers multimodal pain thresholds that are amenable to correction following levodopa dosing. Functional brain imaging with positron emission tomography following administration of H₂¹⁵O revealed abnormalities in the sensory discriminative processing of pain (insula/SII), as well as in the affective motivational processing of pain (anterior cingulate cortex, prefrontal cortex). Pain management is dependent on efforts invested in diagnostic accuracy to distinguish nociceptive from neuropathic pain. Treatment requires an integrated approach including strategies to lessen levodopa-related response fluctuations, in addition to other pharmacological and non-pharmacological options such as deep brain stimulation and rehabilitation.

Pain in Extrapyramidal Neurodegenerative Diseases

OBJECTIVE

Pain is one of the most common nonmotor symptoms of Parkinson disease (PD) and other Parkinson plus syndromes, with a major effect on quality of life. The aims of the study were to examine the prevalence and characteristics of pain in PD and other Parkinson plus syndromes and patient use and response to pain medications.

METHODS

The cohort consisted of 371 patients: 300 (81%) with PD and 71 (19%) with Parkinson plus syndromes. Data on clinical parameters and pain were collected by questionnaire. Disease severity was measured with the Unified PD Rating Scale for patients with PD.

RESULTS

Pain was reported by 277 patients (74%): 241 with PD and 36 with Parkinson plus syndromes. The prevalence of pain was significantly higher in the patients with PD than in the patients with Parkinson plus syndromes (80% vs. 50%, P<0.001) and higher in patients with synucleinopathies than in patients with tauopathies (70% vs. 40%, P<0.001). In the synucleinopathies, the most common pain was central pain (32%), whereas in the tauopathies only 4% of patients had central pain. Anti-Parkinson treatment relieved the pain in 21% of the patients with PD. Only 114 patients (48%) who experienced pain were treated with pain medications. The most beneficial analgesics were nonsteroidal anti-inflammatory drugs and medical cannabis.

CONCLUSION

Pain is prevalent among patients with PD and Parkinsonian plus syndromes. Pain relief can be achieved by more intensive anti-Parkinson medications or pain medications.

Chronic treatment with carvacrol improves passive avoidance memory in a rat model of Parkinson's disease

ABSTRACT The present study investigated the effects of carvacrol on motor and memory deficits as well as hyperalgesia in the 6-OHDA-lesioned rat model of Parkinson's disease. The animals were subjected to unilateral microinjection of 6-OHDA into the medial forebrain bundle and treated with carvacrol (25, 50 and 100 mg/kg, ip) for six weeks after surgery. The 6-OHDA-lesioned rats showed contralateral rotations towards the lesion side, which was accompanied by learning and memory deficits in a passive avoidance test and a decrease in tail withdrawal latency in a tail flick test at the end of week 6. The results also showed that treatment with carvacrol at a dose of 25 mg/kg ameliorated memory deficits, with no effect on rotations and hyperalgesia in lesioned rats. In conclusion, carvacrol improves memory impairments in rats with Parkinson's disease; therefore, it may serve as an adjunct therapy for the alleviation of memory deficits in Parkinson's disease patients.

Intrastriatal Chromospheres' Transplant Reduces Nociception in Hemiparkinsonian Rats

The present study evaluates the possible antinociceptive effect of chromosphere transplants in rats injected with 6-hydroxydopamine (6-OHDA), a model of Parkinson's disease. Male adult Wistar rats received 40μg/0.5μl of 6-OHDA or 0.5μl of vehicle into the left substantia nigra (SNc). Rats were evaluated for mechanical allodynia, cold allodynia, thermal hyperalgesia and formalin. Rats with altered nociceptive threshold were transplanted with chromospheres. After transplant, rats were evaluated every week. Our results confirm that 6-OHDA injection into rat's SNc reduces mechanical, thermal, and chemical thresholds. Interestingly, chromospheres' transplant reverted 6-OHDA-induced allodynia and hyperalgesia. The antinociceptive effect induced by chromospheres was dopamine D2- and opioid-receptor dependent since sulpiride or naltrexone reverted its effect.

Manganese-enhanced magnetic resonance imaging depicts brain activity in models of acute and chronic pain: A new window to study experimental spontaneous pain?

Application of functional imaging techniques to animal models is vital to understand pain mechanisms, but is often confounded by the need to limit movement artefacts with anaesthesia, and a focus on evoked responses rather than clinically relevant spontaneous pain and related hyperalgesia. The aim of the present study was to investigate the potential of manganese-enhanced magnetic resonance imaging (MEMRI) to measure neural responses during on-going pain that underpins hyperalgesia in pre-clinical models of nociception. As a proof of concept that MEMRI is sensitive to the neural activity of spontaneous, intermittent behaviour, we studied a separate positive control group undergoing a voluntary running wheel experiment. In the pain models, pain behaviour (weight bearing asymmetry and hindpaw withdrawal thresholds (PWTs)) was measured at baseline and following either intra-articular injection of nerve growth factor (NGF, 10µg/50µl; acute pain model, n=4 rats per group), or the chondrocyte toxin monosodium iodoacetate (MIA, 1mg/50µl; chronic model, n=8 rats per group), or control injection. Separate groups of rats underwent a voluntary wheel running protocol (n=8 rats per group). Rats were administered with paramagnetic ion Mn²⁺ as soluble MnCl₂ over seven days (subcutaneous osmotic pump) to allow cumulative activity-dependent neural accumulation in the models of pain, or over a period of running. T1-weighted MR imaging at 7T was performed under isoflurane anaesthesia using a receive-only rat head coil in combination with a 72mm volume coil for excitation. The pain models resulted in weight bearing asymmetry (NGF: 20.0 ± 5.2%, MIA: 15 ± 3%), and a reduction in PWT in the MIA model (8.3 ± 1.5g) on the final day of assessment before undergoing MR imaging. Voxel-wise and region-based analysis of MEMRI data did not identify group differences in T1 signal. However, MnCl₂ accumulation in the VTA, right Ce amygdala, and left cingulate was negatively correlated with pain responses (greater differences in weight bearing), similarly MnCl₂ accumulation was reduced in the VTA in line with hyperalgesia (lower PWTs), which suggests reduced regional activation as a result of the intensity and duration of pain experienced during the 7 days of MnCl₂ exposure. Motor cortex T1-weighted signal increase was associated with the distance ran in the wheel running study, while no between group difference was seen. Our data suggest that on-going pain related signal changes identified using MEMRI offers a new window to study the neural underpinnings of spontaneous pain in rats.

Subthalamic deep brain stimulation alters neuronal firing in canonical pain nuclei in a 6-hydroxydopamine lesioned rat model of Parkinson's disease

INTRODUCTION

Chronic pain is one of the most common non-motor symptoms of Parkinson's disease (PD) affecting up to 85% of patients. Previous studies have established that reduced mechanical and thermal thresholds occur in both idiopathic PD patients and animal models of PD, suggesting that changes may occur in sensory processing circuits. Improvements in sensory thresholds are achieved using subthalamic nucleus (STN) deep brain stimulation (DBS), however the mechanism by which this occurs remains unresolved.

MATERIALS AND METHODS

We examined unilateral medial forebrain bundle 6-hydroxydopamine (6OHDA) rat model of PD to determine whether STN DBS alters neuronal firing rates in brain areas involved in ascending and descending pain processing. Specifically, single unit in vivo recordings were conducted in the anterior cingulate cortex (ACC), the periaqueductal grey (PAG), and the ventral posteriolateral nucleus of the thalamus (VPL), before, during and after stimulation was applied to the STN at 50 or 150Hz.

RESULTS

Sham and 6OHDA lesioned animals have similar neuronal firing activity in the VPL, ACC and PAG before stimulation was applied (p>0.05). In 6OHDA lesioned rats, both low frequency stimulation (LFS) (p<0.01) and high frequency stimulation (HFS) (p<0.05) attenuated firing frequency in the ACC. In shams, only LFS decreased firing frequency. A subset of neurons in the PAG was significantly attenuated in both sham and 6OHDA lesioned animals during HFS and LFS (p<0.05), while another subset of PAG neuronal activity significantly increased in 6OHDA lesioned rats during HFS (p<0.05). Finally, low or high frequency STN DBS did not alter neuronal firing frequencies in the VPL.

CONCLUSIONS

Our results suggest that STN DBS alters neuronal firing in descending pain circuits. We hypothesize that STN DBS attenuates excitatory projections from the ACC to the PAG in 6OHDA lesioned rats. Following this, neurons in the PAG respond by either increasing (during HFS only) or decreasing (during both LFS and HFS), which may modulate descending facilitation or inhibition at the level of the spinal cord. Future work should address specific neuronal changes in the ACC and PAG that occur in a freely moving parkinsonian animal during a pain stimulus treated with STN DBS.

Nigrostriatal dopaminergic depletion increases static orofacial allodynia

Background

This study investigated mesencephalic dopamine depletion effects on static mechanical allodynia (SMA) elicited by chronic constriction of the infraorbitary nerve (CCI-IoN).

Methods

Dopamine depletion (6-OHDA administration into the medial forebrain bundle) effects on CCI-IoN-induced SMA were explored using behavioral (nocifensive behavior score upon non-noxious stimuli using von Frey filament), pharmacological (bromocriptine injections) and immunohistochemical (PKCγ and pERK1/2) techniques.

Results

The central dopamine depletion increased significantly the SMA score. Intraperitoneal and intracisternal injections of bromocriptine alleviated the allodynic behavior observed in both CCI-IoN and CCI-IoN + 6-OHDA animal groups. At the cellular level, dopamine depletion induced a significant increase in PKCγ expression in the medullary dorsal horn (MDH) in rat with CCI-IoN + 6-OHDA when compared to sham animals (CCI-IoN only). Similarly, after static non-noxious stimuli, the expression of pain marker proteins pERK1/2 within the MDH revealed significantly a higher number of positive cells in CCI-IoN + 6-OHDA rats when compared to the CCI-IoN group.

Conclusion

This study demonstrates that nigrostriatal dopamine depletion exacerbates the neuropathic pain resulting from CCI-IoN. This effect is probably due to an action through descending pain inhibitory systems which increased pain sensitization at the MDH level. It demonstrates also an analgesic effect elicited by D2R activation at the segmental level.

Electronic supplementary material

The online version of this article (doi:10.1186/s10194-016-0607-z) contains supplementary material, which is available to authorized users.

Stimulation of the ventral tegmental area increased nociceptive thresholds and decreased spinal dorsal horn neuronal activity in rat

Deep brain stimulation has been found to be effective in relieving intractable pain. The ventral tegmental area (VTA) plays a role not only in the reward process, but also in the modulation of nociception. Lesions of VTA result in increased pain thresholds and exacerbate pain in several pain models. It is hypothesized that direct activation of VTA will reduce pain experience. In this study, we investigated the effect of direct electrical stimulation of the VTA on mechanical, thermal and carrageenan-induced chemical nociceptive thresholds in Sprague-Dawley rats using our custom-designed wireless stimulator. We found that: (1) VTA stimulation itself did not show any change in mechanical or thermal threshold; and (2) the decreased mechanical and thermal thresholds induced by carrageenan injection in the hind paw contralateral to the stimulation site were significantly reversed by VTA stimulation. To further explore the underlying mechanism of VTA stimulation-induced analgesia, spinal cord dorsal horn neuronal responses to graded mechanical stimuli were recorded. VTA stimulation significantly inhibited dorsal horn neuronal activity in response to pressure and pinch from the paw, but not brush. This indicated that VTA stimulation may have exerted its analgesic effect via descending modulatory pain pathways, possibly through its connections with brain stem structures and cerebral cortex areas.

The effects of subthalamic deep brain stimulation on mechanical and thermal thresholds in 6OHDA-lesioned rats

Chronic pain is a major complaint for up to 85% of Parkinson's disease patients; however, it often not identified as a symptom of Parkinson's disease. Adequate treatment of motor symptoms often provides analgesic effects in Parkinson's patients but how this occurs remains unclear. Studies have shown both Parkinson's patients and 6-hydroxydopamine-lesioned rats exhibit decreased sensory thresholds. In humans, some show improvements in these deficits after subthalamic deep brain stimulation, while others report no change. Differing methods of testing and response criteria may explain these varying results. We examined this effect in 6-hydroxydopamine-lesioned rats. Sprague-Dawley rats were unilaterally implanted with subthalamic stimulating electrodes in the lesioned right hemisphere and sensory thresholds were tested using von Frey, tail-flick and hot-plate tests. Tests were done during and off subthalamic stimulation at 50 and 150 Hz to assess its effects on sensory thresholds. The 6-hydroxydopamine-lesioned animals exhibited lower mechanical (left paw, P < 0.01) and thermal thresholds than shams (hot plate, P < 0.05). Both 50 and 150 Hz increased mechanical (left paw; P < 0.01) and thermal thresholds in 6-hydroxydopamine-lesioned rats (hot-plate test: 150 Hz, P < 0.05, 50 Hz, P < 0.01). Interestingly, during von Frey testing, low-frequency stimulation provided a more robust improvement in some 6OHDA lesioned rats, while in others, the magnitude of improvement on high-frequency stimulation was greater. This study shows that subthalamic deep brain stimulation improves mechanical allodynia and thermal hyperalgesia in 6-hydroxydopamine-lesioned animals at both high and low frequencies. Furthermore, we suggest considering using low-frequency stimulation when treating Parkinson's patients where pain remains the predominant complaint.

Neural mechanism underlying hyperalgesic response to orofacial pain in Parkinson's disease model rats

To investigate the neural mechanism of pain originating from the orofacial region in PD patients, we used PD model rats produced by unilateral injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. We investigated effects of nigrostriatal lesions on the behavioral response (face rubbing) to formalin injection into the upper lip. We also examined expression of c-Fos and phosphorylated extracellular signal-regulated kinase (pERK) in the trigeminal spinal subnucleus caudalis (Vc) and expression of c-Fos in the periaqueductal gray matter (PAG). Face rubbings following formalin injection showed a biphasic profile, with the first phase for the first 5 min and the second phase from 10 to 90 min. Rats with 6-OHDA lesions showed increased face rubbings in the second phase when formalin was injected ipsilaterally to the lesion, and c-Fos expression in the Vc increased. When formalin was injected contralaterally, face rubbings were reduced in the first phase, however, expression levels of c-Fos and pERK in the Vc were unchanged. No significant difference was found in c-Fos expression in the PAG between 6-OHDA- and saline-injected rats. These results suggest that unilateral dopamine depletion in the nigrostriatal pathway may be involved in hypersensitivity to noxious stimulation delivered to the orofacial region.

Lesion of the dopaminergic nigrostriatal pathway induces trigeminal dynamic mechanical allodynia

BACKGROUND

Pain constitutes the major non motor syndrome in Parkinson's disease (PD) and includes neuropathic pain; however current drug therapies used to alleviate it have only limited efficacy. This is probably due to poor understanding of the mechanisms underlying it.

AIMS

We investigated a major class of trigeminal neuropathic pain, dynamic mechanical allodynia (DMA), in a rat model of PD and in which a bilateral 6-hydroxy dopamine (6-OHDA) injection was administered to produce a lesion of the nigrostriatal dopaminergic pathway.

RESULTS AND DISCUSSION

Lesioned animals presented significant DMA in the orofacial area that occurred from 4 days to 5 weeks post-injury. To investigate a segmental implication in the neuropathic pain induced by dopamine depletion, the expression of the isoform gamma of the protein kinase C (PKCg) and phosphorylated extracellular signal-regulated kinases 1/2 (pERK1/2) was explored in the medullary dorsal horn (MDH). There was a high increase in PKCg expression in the III and IIi laminae of the MDH of lesioned-animals compared to shams. pERK1/2 expression was also significantly high in the ipsilateral MDH of lesioned rats in response to non-noxious tactile stimulus of the orofacial region. Since pERK1/2 is expressed only in response to nociceptive stimuli in the dorsal spinal horn, the current study demonstrates that non-noxious stimuli evoke allodynic response. Intraperitoneal and intracisternal administrations of bromocriptine, a dopamine 2 receptor (D2R) agonist, significantly decreased DMA compared to control rats injected with saline. These data demonstrate for the first time that nigrostriatal dopaminergic depletion produces trigeminal neuropathic pain that at least involves a segmental mechanism. In addition, bromocriptine was shown to have a remarkable analgesic effect on this neuropathic pain symptom.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

A key role of the basal ganglia in pain and analgesia--insights gained through human functional imaging

The basal ganglia (BG) are composed of several nuclei involved in neural processing related to the execution of motor, cognitive and emotional activities. Preclinical and clinical data have implicated a role for these structures in pain processing. Recently neuroimaging has added important information on BG activation in conditions of acute pain, chronic pain and as a result of drug effects. Our current understanding of alterations in cortical and sub-cortical regions in pain suggests that the BG are uniquely involved in thalamo-cortico-BG loops to integrate many aspects of pain. These include the integration of motor, emotional, autonomic and cognitive responses to pain.

Role of central dopaminergic circuitry in pain processing and nitroglycerin-induced hyperalgesia

Experimental evidence shows that dopaminergic transmission within the basal ganglia is involved in the modulation of nociceptive information. Epidemiological studies show that in some disease states inherent pathophysiological mechanisms that involve degenerative changes (Parkinson's disease; PD) can also impact negatively on other unrelated functional systems (i.e. nociception). Delayed Fos expression in response to nitroglycerin (NTG) administration is a procedure used to identify the neuroanatomical substrates of the migraine condition. In this study, we investigated the influence of dopaminergic nigrostriatal denervation, obtained by intrastriatal injection of 6-hydroxydopamine (6-OHDA), on this response in Sprague-Dawley rats. We also explored the effects on the NTG-induced hyperalgesic response to painful stimuli (formalin and tail-flick tests). Nigrostriatal lesion prevented the neuronal activation typically induced by NTG in sub-cortical areas involved in pain perception, autonomic control and neuroendocrine functions, such as hypothalamic nuclei, periaqueductal grey, parabrachial nucleus and the medullary nuclei. In addition, 6-OHDA-induced lesion inhibited NTG-induced hyperalgesia. Our data show that integrity of central dopaminergic neurotransmission is required for the NTG-induced activation of sub-cortical areas involved in the expression of migraine symptoms, as well as for the hyperalgesic response to painful stimuli elicited by the drug.

Nociceptive behavioral responses to chemical, thermal and mechanical stimulation after unilateral, intrastriatal administration of 6-hydroxydopamine

The basal ganglia are involved not only with motor processes such as posture, pre-movement planning and movement initiation, but also with the processing and modulation of nociceptive somatosensory information. In the current studies, unilateral, intrastriatal 6-hydroxydopamine (6-OHDA) was used to investigate how dopamine depletion alters nociceptive behavioral responses to chemical, thermal and mechanical stimulation in rats. Compared to control rats injected with intrastriatal saline, rats depleted of dopamine displayed increased nociceptive responses to chemical stimulation of the face and hyperalgesic responses to thermal stimulation of the hind paw without alterations in rearing behavior or body weight gain. Minor changes were observed in the response to mechanical stimulation of the hind paws and face. These data provide further evidence that the dopaminergic nigrostriatal pathway plays a role in the modulation of nociceptive information.

Behavioral responses and Fos activation following painful stimuli in a rodent model of Parkinson's disease

In Parkinson's disease (PD), the motor dysfunction caused by degeneration of the nigrostriatal pathway is often associated with alterations of pain perception. This is likely related to the role that the nigrostriatal system may play in the processing of noxious, somatosensory stimuli. To further address this issue, we used a rodent model of PD, based on the unilateral, intrastriatal injection of neurotoxin 6-hydroxydopamine (6-OHDA). We investigated the effects of the nigrostriatal lesion on behavioral responses to pain tests designed to explore different aspects of nociception, such as the formalin test and the tail flick test; we also explored modifications in the expression of Fos protein, a marker of neuronal activation, in supraspinal nuclei involved in the integration of pain perception and stress-related behavior. Rats bearing the nigrostriatal lesion showed complex alterations in pain perception, including hyperalgesic responses to the tonic, inflammatory pain elicited by formalin injection, but only when the stimulus was delivered ipsilaterally to the lesion. This phenomenon was associated with delayed responses to the phasic, thermal stimulus induced by the tail flick test. The hyperalgesic response to the formalin test was accompanied by reduced Fos expression in the paraventricular nucleus of the hypothalamus, which is part of a network (the medial pain system) that mediates motivational-affective aspects of pain. Our results confirm that a unilateral alteration of central dopaminergic transmission disrupts the neural mechanisms underlying proper integration of painful stimuli, particularly in the hemibody ipsilateral to the dopaminergic denervation.

Striatal dopamine D2 receptors attenuate neuropathic hypersensitivity in the rat

Earlier studies indicate that striatal dopamine D(2) receptors are involved in pain regulation in non-neuropathic conditions. We assessed whether striatal dopamine D(2) receptors contribute to pain regulation also in neuropathic conditions. The spared nerve injury model of neuropathy was induced by unilateral ligation of the tibial and common peroneal nerves in the rat. In awake nerve-injured animals, pain-related withdrawal responses to calibrated monofilaments or noxious heating were attenuated following striatal administration of a dopamine D(2) receptor agonist quinpirole. Pain-related responses were attenuated only in the nerve-injured limb ipsilateral to the injection and in the midline (tail). In unoperated controls, striatal administration of quinpirole at an antihypersensitive dose did not influence withdrawal responses to mechanical stimulation. Attenuation of pain-related responses induced by striatal administration of quinpirole was reversed by intrathecal administration of a dopamine D(2) receptor antagonist (eticlopride) or a non-selective 5-HT receptor antagonist (methysergide), but not by an alpha(2)-adrenoceptor antagonist (atipamezole). In the rostroventromedial medulla of lightly anesthetized neuropathic animals, striatal administration of quinpirole significantly decreased the activity of presumably pronociceptive cells that are activated by noxious stimulation. The innocuous H-reflex in lightly anesthetized control animals was not suppressed by striatal administration of quinpirole at an antihypersensitive dose. The results indicate that striatal dopamine D(2) receptors attenuate neuropathic hypersensitivity. The antihypersensitive effect induced by striatal dopamine D(2) receptors in peripheral neuropathy involves suppression of impulse discharge of presumably pronociceptive neurons in the rostroventromedial medulla, and a descending influence acting on spinal 5-HT and dopamine D(2) receptors.