Dopaminergic modulation of neuropathic pain: analgesia in rats by a D2-type receptor agonist

Selenoprotein T, a potential treatment of attention-deficit/hyperactivity disorder and comorbid pain in neonatal 6-OHDA lesioned mice

pathological pain and Attention-deficit/hyperactivity disorder (ADHD) are two complex multifactorial syndromes. The comorbidity of ADHD and altered pain perception is well documented in children, adolescents, and adults. According to pathophysiological investigations, the dopaminergic system's dysfunction provides a common basis for ADHD and comorbid pain. Growing evidence suggests that oxidative stress may be crucial in both pathologies. Recent studies revealed that a small peptide encompassing the redox-active site of selenoprotein T (PSELT), protects dopaminergic neurons and fibers as well as lesioned nerves in animal models. The current study aims to examine the effects of PSELT treatment on ADHD-like symptoms and pain sensitivity, as well as the role of catecholaminergic systems in these effects. Our results demonstrated that intranasal administration of PSELT reduced the hyperactivity in the open field, decreased the impulsivity displayed by 6-OHDA-lesioned male mice in the 5-choice serial reaction time task test and improved attentional performance. In addition, PSELT treatment significantly increased the nociception threshold in both normal and inflammatory conditions. Furthermore, anti-hyperalgesic activity was antagonized with sulpiride pre-treatment, but not by phentolamine, or propranolol pre-treatments. The present study suggests that PSELT reduces the severity of ADHD symptoms in mice and possesses potent antinociceptive effects which could be related to the involvement of D2/D3 dopaminergic receptors.

Subcutaneous administration of a novel TRPM8 antagonist reverses cold hypersensitivity while attenuating the drop in core body temperature

BACKGROUND AND PURPOSE

We extend the characterization of the TRPM8 antagonist VBJ103 with tests of selectivity, specificity and distribution, therapeutic efficacy of systemic administration against oxaliplatin-induced cold hyperalgesia and the impact of systemic administration on core body temperature (CBT).

EXPERIMENTAL APPROACH

Selectivity at human TRPA1 and TRPV1 as well as in vitro safety profiling was determined. Effects of systemic administration of VBJ103 were evaluated in a model of oxaliplatin-induced cold hyperalgesia. Both peripheral and centrally mediated effects of VBJ103 on CBT were assessed with radiotelemetry.

KEY RESULTS

VBJ103 had no antagonist activity at TRPV1 and TRPA1, but low potency TRPA1 activation. The only safety liability detected was partial inhibition of the dopamine transporter (DAT). VBJ103 delivered subcutaneously dose-dependently attenuated cold hypersensitivity in oxaliplatin-treated mice at 3, 10 and 30 mg·kg-1 (n = 7, P < 0.05). VBJ103 (30 mg·kg-1) antinociception was influenced by neither the TRPA1 antagonist HC-030031 nor the DAT antagonist GBR12909. Subcutaneous administration of VBJ103 (3, 10 and 30 mg·kg-1, but not 100 or 300 mg·kg-1, n = 7) decreased CBT (2°C). Intraperitoneal (i.p.) administration of VBJ103 (3, 10 and 30 mg·kg-1) dose-dependently decreased CBT to an extent larger than that detected with subcutaneous administration. Intracerebroventricular (i.c.v.) administration (306 nmol/1 μL; n = 5) did not alter CBT.

CONCLUSIONS AND IMPLICATIONS

We achieve therapeutic efficacy with subcutaneous administration of a novel TRPM8 antagonist that attenuates deleterious influences on CBT, a side effect that has largely prevented the translation of TRPM8 as a target.

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.

Descending dopaminergic pathway facilitates itch signal processing via activating spinal GRPR+ neurons

A11 dopaminergic neurons regulate somatosensory transduction by projecting from the diencephalon to the spinal cord, but the function of this descending projection in itch remained elusive. Here, we report that dopaminergic projection neurons from the A11 nucleus to the spinal dorsal horn (dopaminergicA11-SDH ) are activated by pruritogens. Inhibition of these neurons alleviates itch-induced scratching behaviors. Furthermore, chemogenetic inhibition of spinal dopamine receptor D1-expressing (DRD1+ ) neurons decreases acute or chronic itch-induced scratching. Mechanistically, spinal DRD1+ neurons are excitatory and mostly co-localize with gastrin-releasing peptide (GRP), an endogenous neuropeptide for itch. In addition, DRD1+ neurons form synapses with GRP receptor-expressing (GRPR+ ) neurons and activate these neurons via AMPA receptor (AMPAR). Finally, spontaneous itch and enhanced acute itch induced by activating spinal DRD1+ neurons are relieved by antagonists against AMPAR and GRPR. Thus, the descending dopaminergic pathway facilitates spinal itch transmission via activating DRD1+ neurons and releasing glutamate and GRP, which directly augments GRPR signaling. Interruption of this descending pathway may be used to treat chronic itch.

Dopamine receptor D2, but not D1, mediates the reward circuit from the ventral tegmental area to the central amygdala, which is involved in pain relief

Pain involves both sensory and affective dimensions. The amygdala is a key player in linking nociceptive stimuli to negative emotional behaviors or affective states. Relief of pain is rewarding and activates brain reward circuits. Whether the reward circuit from the ventral tegmental area (VTA) to the central amygdala (CeA) is involved in pain relief remains unexplored. Using a model of experimental postsurgical pain, we found that pain relief elicited conditioned place preference (CPP), activated CeA-projecting dopaminergic cells in the VTA, and decreased dopaminergic D2 receptor expression in the CeA. Activation of the VTA-CeA neural pathway using optogenetic approaches relieved incisional pain. Administration of a D2 receptor agonist reversed the pain relief elicited by light-induced activation of the VTA-CeA pathway. These findings indicate that the VTA-CeA circuit is involved in pain relief in mice via dopamine receptor D2 in the CeA.

Genetic and epigenetic mechanisms influencing acute to chronic postsurgical pain transitions in pediatrics: Preclinical to clinical evidence

Background

Chronic postsurgical pain (CPSP) in children remains an important problem with no effective preventive or therapeutic strategies. Recently, genomic underpinnings explaining additional interindividual risk beyond psychological factors have been proposed.

Aims

We present a comprehensive review of current preclinical and clinical evidence for genetic and epigenetic mechanisms relevant to pediatric CPSP.

Methods

Narrative review.

Results

Animal models are relevant to translational research for unraveling genomic mechanisms. For example, Cacng2, p2rx7, and bdnf mutant mice show altered mechanical hypersensitivity to injury, and variants of the same genes have been associated with CPSP susceptibility in humans; similarly, differential DNA methylation (H1SP) and miRNAs (miR-96/7a) have shown translational implications. Animal studies also suggest that crosstalk between neurons and immune cells may be involved in nociceptive priming observed in neonates. In children, differential DNA methylation in regulatory genomic regions enriching GABAergic, dopaminergic, and immune pathways, as well as polygenic risk scores for enhanced prediction of CPSP, have been described. Genome-wide studies in pediatric CPSP are scarce, but pathways identified by adult gene association studies point to potential common mechanisms.

Conclusions

Bench-to-bedside genomics research in pediatric CPSP is currently limited. Reverse translational approaches, use of other -omics, and inclusion of pediatric/CPSP endophenotypes in large-scale biobanks may be potential solutions. Time of developmental vulnerability and longitudinal genomic changes after surgery warrant further investigation. Emergence of promising precision pain management strategies based on gene editing and epigenetic programing emphasize need for further research in pediatric CPSP-related genomics.

Altered Pain Processing Associated with Administration of Dopamine Agonist and Antagonist in Healthy Volunteers

Striatal dopamine dysfunction is associated with the altered top-down modulation of pain processing. The dopamine D2-like receptor family is a potential substrate for such effects due to its primary expression in the striatum, but evidence for this is currently lacking. Here, we investigated the effect of pharmacologically manipulating striatal dopamine D2 receptor activity on the anticipation and perception of acute pain stimuli in humans. Participants received visual cues that induced either certain or uncertain anticipation of two pain intensity levels delivered via a CO₂ laser. Rating of the pain intensity and unpleasantness was recorded. Brain activity was recorded with EEG and analysed via source localisation to investigate neural activity during the anticipation and receipt of pain. Participants completed the experiment under three conditions, control (Sodium Chloride), D2 receptor agonist (Cabergoline), and D2 receptor antagonist (Amisulpride), in a repeated-measures, triple-crossover, double-blind study. The antagonist reduced an individuals’ ability to distinguish between low and high pain following uncertain anticipation. The EEG source localisation showed that the agonist and antagonist reduced neural activations in specific brain regions associated with the sensory integration of salient stimuli during the anticipation and receipt of pain. During anticipation, the agonist reduced activity in the right mid-temporal region and the right angular gyrus, whilst the antagonist reduced activity within the right postcentral, right mid-temporal, and right inferior parietal regions. In comparison to control, the antagonist reduced activity within the insula during the receipt of pain, a key structure involved in the integration of the sensory and affective aspects of pain. Pain sensitivity and unpleasantness were not changed by D2R modulation. Our results support the notion that D2 receptor neurotransmission has a role in the top-down modulation of pain.

Dopamine-neurotransmission and nociception in zebrafish: An anti-nociceptive role of dopamine receptor drd2a

Dopamine (DA) is an important modulator in nociception and analgesia. Spinal DA receptors are involved in descending modulation of the nociceptive transmission. Genetic variations within DA neurotransmission have been associated with altered pain sensitivity and development of chronic pain syndromes. The variant rs6277 in dopamine receptor 2 a (drd2a) has been associated with a decreased D₂ receptor availability and increased nociception. The aim of this study is to further characterize the role of DA neurotransmission in nociception and the anti-nociceptive function of drd2a. The phenotype caused by rs6277 was modelled in zebrafish larvae using morpholino's and the effect on nociception was tested using a validated behavioural assay. The anti-nociceptive role of drd2a was tested using pharmacological intervention of D₂ agonist Quinpirole. The experiments demonstrate that a decrease in drd2a expression results in a pro-nociceptive behavioural phenotype (P = 0.016) after a heat stimulus. Furthermore, agonism of drd2a with agonist Quinpirole (0.2 μM) results in dose-dependent anti-nociception (P = 0.035) after a heat stimulus. From these results it is concluded that the dopamine receptor drd2a is involved in anti-nociceptive behaviour in zebrafish. The model allows further screening and testing of genetic variation and treatment involved in nociception.

The Extract of Corydalis yanhusuo Prevents Morphine Tolerance and Dependence

The opioid epidemic was triggered by an overprescription of opioid analgesics. In the treatment of chronic pain, repeated opioid administrations are required which ultimately lead to tolerance, physical dependence, and addiction. A possible way to overcome this conundrum consists of a co-medication that maintains the analgesic benefits of opioids while preventing their adverse liabilities. YHS, the extract of the plant Corydalis yanhusuo, has been used as analgesic in traditional Chinese medicine for centuries. More recently, it has been shown to promote analgesia in animal models of acute, inflammatory, and neuropathic pain. It acts, at least in part, by inhibiting the dopamine D2 receptor, suggesting that it may be advantageous to manage addiction. We first show that, in animals, YHS can increase the efficacy of morphine antinociceptive and, as such, decrease the need of the opioid. We then show that YHS, when coadministered with morphine, inhibits morphine tolerance, dependence, and addiction. Finally, we show that, in animals treated for several days with morphine, YHS can reverse morphine dependence and addiction. Together, these data indicate that YHS may be useful as a co-medication in morphine therapies to limit adverse morphine effects. Because YHS is readily available and safe, it may have an immediate positive impact to curb the opioid epidemic.

The Distinct Functions of Dopaminergic Receptors on Pain Modulation: A Narrative Review

Chronic pain is considered an economic burden on society as it often results in disability, job loss, and early retirement. Opioids are the most common analgesics prescribed for the management of moderate to severe pain. However, chronic exposure to these drugs can result in opioid tolerance and opioid-induced hyperalgesia. On pain modulation strategies, exploiting the multitarget drugs with the ability of the superadditive or synergistic interactions attracts more attention. In the present report, we have reviewed the analgesic effects of different dopamine receptors, particularly D1 and D2 receptors, in different regions of the central nervous system, including the spinal cord, striatum, nucleus accumbens (NAc), and periaqueductal gray (PAG). According to the evidence, these regions are not only involved in pain modulation but also express a high density of DA receptors. The findings can be categorized as follows: (1) D2-like receptors may exert a higher analgesic potency, but D1-like receptors act in different manners across several mechanisms in the mentioned regions; (2) in the spinal cord and striatum, antinociception of DA is mainly mediated by D2-like receptors, while in the NAc and PAG, both D1- and D2-like receptors are involved as analgesic targets; and (3) D2-like receptor agonists can act as adjuvants of μ-opioid receptor agonists to potentiate analgesic effects and provide a better approach to pain relief.

Potential for Kappa-Opioid Receptor Agonists to Engineer Nonaddictive Analgesics: A Narrative Review

A serious adverse effect of prescription opioid analgesics is addiction, both to these analgesics and to illicit drugs like heroin that also activate the µ-opioid receptor (MOR). Opioid use disorder (OUD) and opioid overdose deaths represent a current American health crisis, and the prescription of opioid analgesics has contributed significantly to this crisis. While prescription opioids are highly effective analgesics, there currently exists no facile way to use them for extended periods without the risk of addiction. If addiction caused by MOR-targeting analgesics could be blocked by blending in a new "antiaddiction" ingredient that does not diminish analgesia and does not introduce its own therapeutically limiting side effects, then continued clinical use of prescription opioids for treating pain could be maintained (or even enhanced) instead of curtailed. In this narrative review, we contextualize this hypothesis, first with a brief overview of the current American opioid addiction crisis. The neurobiology of 2 key receptors in OUD development, MOR and the κ-opioid receptor (KOR), is then discussed to highlight the neuroanatomical features and circuitry in which signal transduction from these receptors lie in opposition-creating opportunities for pharmacological intervention in curtailing the addictive potential of MOR agonism. Prior findings with mixed MOR/KOR agonists are considered before exploring new potential avenues such as biased KOR agonists. New preclinical data are highlighted, demonstrating that the G protein-biased KOR agonist nalfurafine reduces the rewarding properties of MOR-targeting analgesics and enhances MOR-targeting analgesic-induced antinociception. Finally, we discuss the recent discovery that a regulator of G protein signaling (namely, RGS12) is a key component of signaling bias at KOR, presenting another drug discovery target toward identifying a single agent or adjuvant to be added to traditional opioid analgesics that could reduce or eliminate the addictive potential of the latter drug.

The dopamine D1-D2DR complex in the rat spinal cord promotes neuropathic pain by increasing neuronal excitability after chronic constriction injury

Dopamine D1 receptor (D1DR) and D2 receptor (D2DR) are closely associated with pain modulation, but their exact effects on neuropathic pain and the underlying mechanisms remain to be identified. Our research revealed that intrathecal administration of D1DR and D2DR antagonists inhibited D1-D2DR complex formation and ameliorated mechanical and thermal hypersensitivity in chronic constriction injury (CCI) rats. The D1-D2DR complex was formed in the rat spinal cord, and the antinociceptive effects of D1DR and D2DR antagonists could be reversed by D1DR, D2DR, and D1-D2DR agonists. Gαq, PLC, and IP3 inhibitors also alleviated CCI-induced neuropathic pain. D1DR, D2DR, and D1-D2DR complex agonists all increased the intracellular calcium concentration in primary cultured spinal neurons, and this increase could be reversed by D1DR, D2DR antagonists and Gαq, IP3, PLC inhibitors. D1DR and D2DR antagonists significantly reduced the expression of p-PKC γ, p-CaMKII, p-CREB, and p-MAPKs. Levo-corydalmine (l-CDL), a monomeric compound in Corydalis yanhusuo W.T. Wang, was found to obviously suppress the formation of the spinal D1-D2DR complex to alleviate neuropathic pain in CCI rats and to decrease the intracellular calcium concentration in spinal neurons. l-CDL-induced inhibition of p-PKC γ, p-MAPKs, p-CREB, and p-CaMKII was also reversed by D1DR, D2DR, and D1-D2DR complex agonists. In conclusion, these results indicate that D1DR and D2DR form a complex and in turn couple with the Gαq protein to increase neuronal excitability via PKC γ, CaMKII, MAPK, and CREB signaling in the spinal cords of CCI rats; thus, they may serve as potential drug targets for neuropathic pain therapy.

D2 receptor activation relieves pain hypersensitivity by inhibiting superficial dorsal horn neurons in parkinsonian mice

Chronic pain is a common and undertreated nonmotor symptom in Parkinson's disease (PD). Although chronic pain is improved by L-dopa in some PD patients, the underlying mechanisms remain unclear. In this study, we established PD mice by unilateral microinjection of 6-OHDA in the medial forebrain bundle to investigate the contribution of spinal cord dopamine receptors to parkinsonian pain hypersensitivity. The von Frey filament tests and thermal pain tests revealed that these PD mice displayed decreased nociceptive thresholds in both hindpaws; intrathecal injection of L-dopa or apomorphine significantly increased the mechanical and thermal nociceptive thresholds, and the analgesic effect was mimicked by ropinirole (a D2 receptor agonist), but not SKF38393 (a D1/D5 receptor agonist), and blocked by sulpiride (a D2 receptor antagonist), but not SKF83566 (a D1/D5 receptor antagonist). Whole-cell recordings in lumber spinal cord slices showed that superficial dorsal horn (SDH) neurons in PD mice exhibited hyperexcitability, including more depolarized resting membrane potentials and more action potentials evoked by depolarizing current steps, which were mitigated by ropinirole. Furthermore, ropinirole inhibited the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in SDH neurons more strongly in PD mice than in control mice. However, sulpiride caused less disinhibition of sEPSCs in PD mice than in control mice. Taken together, our data reveal that pain hypersensitivity in PD mice is associated with hyperexcitability of SDH neurons, and both events are reversed by activation of spinal D2 receptors. Therefore, spinal D2 receptors can be promising therapeutic targets for the treatment of PD pain.

Modulation of intracellular calcium concentration by D2-like DA receptor agonists in non-peptidergic DRG neurons is mediated mainly by D4 receptor activation

Nociceptive stimuli attributes are codified in the periphery; at this level, D2-like dopamine (DA) receptor activation decreases the high voltage-gated Ca²⁺ current predominantly in mechanonociceptive neurons, which explains the presynaptic action mechanism of the antinociception produced by quinpirole when it is intrathecally administered in rats. However, the identity of D2-like DA receptor subtype that mediates this effect remains unknown. To answer this question, we used Fluo-4-based Ca²⁺ microfluorometry to study the depolarization-elicited [Ca²⁺]_i increase in small non-peptidergic DRG neurons (identified by its binding to the Isolectin B₄), and to test the effect of D2-like DA receptor activation by quinpirole in presence of selective antagonists for D2, D3, and D4 DA receptors. The results showed a significantly greater contribution of the D4 DA receptor in the down-modulation of depolarization-elicited [Ca²⁺]_i increase in small non-peptidergic DRG neurons compared to the other receptors. Although the D2 and D3 receptor antagonists also slightly inhibited the effect of quinpirole, their effects were significantly weaker than those of the D4 receptor antagonist. Furthermore, we showed that quinpirole selectively inhibits the Ca_V2.2 Ca²⁺ channels. Our results suggest that the activation of the D4 DA receptors is a promising strategy for pain management at the spinal cord level.

Dopaminergic Modulation of Orofacial Mechanical Hypersensitivity Induced by Infraorbital Nerve Injury

While the descending dopaminergic control system is not fully understood, it is reported that the hypothalamic A11 nucleus is its principle source. To better understand the impact of this system, particularly the A11 nucleus, on neuropathic pain, we created a chronic constriction injury model of the infraorbital nerve (ION-CCI) in rats. ION-CCI rats received intraperitoneal administrations of quinpirole (a dopamine D2 receptor agonist). ION-CCI rats received microinjections of quinpirole, muscimol [a gamma-aminobutyric acid type A (GABA_A) receptor agonist], or neurotoxin 6-hydroxydopamine (6-OHDA) into the A11 nucleus. A von Frey filament was used as a mechanical stimulus on the maxillary whisker pad skin; behavioral and immunohistochemical responses to the stimulation were assessed. After intraperitoneal administration of quinpirole and microinjection of quinpirole or muscimol, ION-CCI rats showed an increase in head-withdrawal thresholds and a decrease in the number of phosphorylated extracellular signal-regulated kinase (pERK) immunoreactive (pERK-IR) cells in the superficial layers of the trigeminal spinal subnucleus caudalis (Vc). Following 6-OHDA microinjection, ION-CCI rats showed a decrease in head-withdrawal thresholds and an increase in the number of pERK-IR cells in the Vc. Our findings suggest the descending dopaminergic control system is involved in the modulation of trigeminal neuropathic pain.

Neuropathic Pain Dysregulates Gene Expression of the Forebrain Opioid and Dopamine Systems

Disturbances in the function of the mesostriatal dopamine system may contribute to the development and maintenance of chronic pain, including its sensory and emotional/cognitive aspects. In the present study, we assessed the influence of chronic constriction injury (CCI) of the sciatic nerve on the expression of genes coding for dopamine and opioid receptors as well as opioid propeptides in the mouse mesostriatal system, particularly in the nucleus accumbens. We demonstrated bilateral increases in mRNA levels of the dopamine D1 and D2 receptors (the latter accompanied by elevated protein level), opioid propeptides proenkephalin and prodynorphin, as well as delta and kappa (but not mu) opioid receptors in the nucleus accumbens at 7 to 14 days after CCI. These results show that CCI-induced neuropathic pain is accompanied by a major transcriptional dysregulation of molecules involved in dopaminergic and opioidergic signaling in the striatum/nucleus accumbens. Possible functional consequences of these changes include opposite effects of upregulated enkephalin/delta opioid receptor signaling vs. dynorphin/kappa opioid receptor signaling, with the former most likely having an analgesic effect and the latter exacerbating pain and contributing to pain-related negative emotional states.

The role of the endogenous neurotransmitters associated with neuropathic pain and in the opioid crisis: The innate pain-relieving system

Neuropathic pain is a chronic pain caused by central and peripheral nerve injury, long-term diabetes or treatment with chemotherapy drugs, and it is dissimilar to other chronic pain conditions. Chronic pain usually seriously affects the quality of life, and its drug treatment may result in increased costs of social and medical care. As in the USA and Canada, in Europe, the demand for pain-relieving medicines used in chronic pain has also significantly increased, but most European countries are not experiencing an opioid crisis. In this review, the role of various endogenous transmitters (noradrenaline, dopamine, serotonin, met- and leu-enkephalins, β-endorphin, dynorphins, cannabinoids, ATP) and various receptors (α₂, μ, etc.) in the innate pain-relieving system will be discussed. Furthermore, the modulation of pain processing pathways by transmitters, focusing on neuropathic pain and the role of the sympathetic nervous system in the side effects of excessive opioid treatment, will be explained.

Bilateral Parkinson's disease model rats exhibit hyperalgesia to subcutaneous formalin administration into the vibrissa pad

We bilaterally injected 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle of rats and developed bilateral Parkinson’s disease (PD) model rats in order to experimentally investigate the neural mechanisms underlying the alteration of nociception in the orofacial region of patients with PD. We explored the effects of dopamine depletion on nociception by investigating behavioral responses (face rubbing) triggered by subcutaneous administration of formalin into the vibrissa pad. We also assessed the number of c-Fos–immunoreactive (c-Fos-IR) cells in the superficial layers of the trigeminal spinal subnucleus caudalis (Vc). Subcutaneous formalin administration evoked a two-phase increase in face rubbing. We observed the first increase 0–5 min after formalin administration (first phase) and the second increase 10–60 min after administration (second phase). The number of face rubbing behaviors of 6OHDA–injected rats did not significantly change compared with saline–injected rats in both phases. Significant increase of c-Fos-IR cells in the Vc was found in 6-OHDA–injected rats after formalin administration compared with those in saline–injected rats after formalin administration. We also assessed expression of c-Fos-IR cells in the paraventricular nucleus (PVN), and significant decrease of c-Fos-IR cells in the PVN of 6-OHDA–injected rats was found. Taken together, these findings suggest that bilateral dopaminergic denervation evoked by 6-OHDA administration causes hyperalgesia in the trigeminal region and the PVN may be involved in the hyperalgesia.

Dopamine D1 and D2 receptors mediate analgesic and hypnotic effects of l-tetrahydropalmatine in a mouse neuropathic pain model

RATIONALE

Levo-tetrahydropalmatine (l-THP), an active ingredient of Corydalis yanhusuo, has been reported to be a partial agonist for dopamine D₁ receptors (D₁R) and an antagonist for D₂R. Although it has been safely used clinically in China for decades as an analgesic with sedative/hypnotic properties, there are few studies that address the mechanisms by which l-THP exerts its beneficial effects in chronic pain-induced sleep disturbance.

OBJECTIVES

To investigate the effects and mechanisms of l-THP on sleep disturbance in a neuropathic pain-like condition.

METHODS

A mouse model of chronic neuropathic pain induced by partial sciatic nerve ligation (PSNL) was employed. The antinociceptive and hypnotic effects of l-THP were evaluated by measurement of mechanical allodynia, thermal hyperalgesia, and electroencephalogram (EEG) recordings in PSNL mice. Pharmacological approaches and c-Fos expression were used to clarify the mechanisms of l-THP.

RESULTS

Intraperitoneal injection of l-THP at 5 and 10 mg/kg not only significantly increased the mechanical threshold by 134.4% and 174.8%, and prolonged the thermal latency by 49.4% and 69.2%, but also increased non-rapid eye movement sleep by 17.5% and 29.6%, and decreased sleep fragmentation in PSNL mice, compared with the vehicle control. Moreover, the antinociceptive effect of l-THP was prevented by D₁R antagonist SCH23390 or D₂R agonist quinpirole; meanwhile, the hypnotic effect of l-THP was blocked by quinpirole rather than by SCH23390. Immunohistochemistry demonstrated that l-THP inhibited c-Fos overexpression induced by PSNL in the cingulate cortex and the periaqueductal gray.

CONCLUSIONS

These findings indicated that l-THP exerted analgesic effects by agonism D₁R and antagonism D₂R, and the antagonism of D₂R mediated the hypnotic effect of l-THP in PSNL mice.

D2-like receptor agonist synergizes the μ-opioid agonist spinal antinociception in nociceptive, inflammatory and neuropathic models of pain in the rat

Opioids are potent analgesic drugs, but their use has been limited due to their side effects. Antinociceptive effects of D2-like receptor agonists such as quinpirole have been shown at the spinal cord level; however, their efficacy is not as high as that of opioids. Dopaminergic agonists are long-prescribed and well-tolerated drugs that have been useful to treat clinically and experimentally painful conditions. Because current pain treatments are not completely effective, the aim of this work was to determine if a D2-like receptor agonist improves the antinociceptive effects of a μ-opioid receptor agonist. Drugs were intrathecally administered in adult rats; mechanonociceptive and thermonociceptive tests were carried out. Intraplantar injection of complete Freund's adjuvant (CFA) and sciatic loose ligation (SLL) were used for inflammatory and neuropathic models of pain, respectively. In intact animals, D-Ala2, N-MePhe4, Gly-ol-enkephalin (DAMGO; a µ-opioid receptor agonist) increased the paw withdrawal latencies (PWL) in thermal and mechanical nociceptive tests in a dose-dependent manner. Quinpirole (D2-like receptor agonist) increased PWL only in mechanonociception. In the presence of quinpirole (1 nmol), the ED₅₀ of the mechanical antinociceptive effect of DAMGO was significantly decreased (8-fold). Coadministration of 1 nmol quinpirole and 30 pmol DAMGO completely reversed hyperalgesia in the CFA model, whereas 100 pmol DAMGO plus 1 nmol quinpirole reversed the allodynia in the SLL model. This work offers evidence about a synergistic antinociceptive effect between opioidergic and dopaminergic drugs. This combination may relieve painful conditions resistant to conventional treatments, and it may reduce the adverse effects of chronic opioid administration.

Role of Dehydrocorybulbine in Neuropathic Pain After Spinal Cord Injury Mediated by P2X4 Receptor

Chronic neuropathic pain is one of the primary causes of disability subsequent to spinal cord injury. Patients experiencing neuropathic pain after spinal cord injury suffer from poor quality of life, so complementary therapy is seriously needed. Dehydrocorybulbine is an alkaloid extracted from Corydalis yanhusuo. It effectively alleviates neuropathic pain. In the present study, we explored the effect of dehydrocorybulbine on neuropathic pain after spinal cord injury and delineated its possible mechanism. Experiments were performed in rats to evaluate the contribution of dehydrocorybulbine to P2X4 signaling in the modulation of pain-related behaviors and the levels of pronociceptive interleukins and proteins after spinal cord injury. In a rat contusion injury model, we confirmed that chronic neuropathic pain is present on day 7 after spinal cord injury and P2X4R expression is exacerbated after spinal cord injury. We also found that administration of dehydrocorybulbine by tail vein injection relieved pain behaviors in rat contusion injury models without affecting motor functions. The elevation in the levels of pronociceptive interleukins (IL-1β, IL-18, MMP-9) after spinal cord injury was mitigated by dehydrocorybulbine. Dehydrocorybulbine significantly mitigated the upregulation of P2X4 receptor and reduced ATP-evoked intracellular Ca2+ concentration. Both P2XR and dopamine receptor2 agonists antagonized dehydrocorybulbine's antinociceptive effects. In conclusion, we propose that dehydrocorybulbine produces antinociceptive effects in spinal cord injury models by inhibiting P2X4R.

The hypothalamic-spinal dopaminergic system: a target for pain modulation

Nociceptive signals conveyed to the dorsal horn of the spinal cord by primary nociceptors are subject to extensive modulation by local neurons and by supraspinal descending pathways to the spinal cord before being relayed to higher brain centers. Descending modulatory pathways to the spinal cord comprise, among others, noradrenergic, serotonergic, γ-aminobutyric acid (GABA)ergic, and dopaminergic fibers. The contributions of noradrenaline, serotonin, and GABA to pain modulation have been extensively investigated. In contrast, the contributions of dopamine to pain modulation remain poorly understood. The focus of this review is to summarize the current knowledge of the contributions of dopamine to pain modulation. Hypothalamic A11 dopaminergic neurons project to all levels of the spinal cord and provide the main source of spinal dopamine. Dopamine receptors are expressed in primary nociceptors as well as in spinal neurons located in different laminae in the dorsal horn of the spinal cord, suggesting that dopamine can modulate pain signals by acting at both presynaptic and postsynaptic targets. Here, I will review the literature on the effects of dopamine and dopamine receptor agonists/antagonists on the excitability of primary nociceptors, the effects of dopamine on the synaptic transmission between primary nociceptors and dorsal horn neurons, and the effects of dopamine on pain in rodents. Published data support both anti-nociceptive effects of dopamine mediated by D2-like receptors and pro-nociceptive effects mediated by D1-like receptors.

Macrophage migration inhibitory factor mediates peripheral nerve injury-induced hypersensitivity by curbing dopaminergic descending inhibition

Our previous works disclosed the contributing role of macrophage migration inhibitory factor (MIF) and dopaminergic inhibition by lysine dimethyltransferase G9a/Glp complex in peripheral nerve injury-induced hypersensitivity. We herein propose that the proinflammatory cytokine MIF participates in the regulation of neuropathic hypersensitivity by interacting with and suppressing the descending dopaminergic system. The lumbar spinal cord (L-SC) and ventral tegmental area (VTA) are two major locations with significant upregulation of MIF after chronic constriction injury (CCI) of the sciatic nerve, and they display time-dependent changes, along with a behavioral trajectory. Correspondingly, dopamine (DA) content shows the reverse characteristic change to MIF with a time-dependent curve in post-surgical behavior. The levels of both MIF and DA are reversed by the MIF tautomerase inhibitor ISO-1, and a negative relationship exists between MIF and DA. The reversed role of ISO-1 also affects tyrosine hydroxylase expression. Furthermore, CCI induces Th promoter CpG site methylation in the L-SC and VTA areas, and this effect could be abated by ISO-1 administration. G9a/SUV39H1 and H3K9me2/H3K9me3 enrichment within the Th promoter region following CCI in the L-SC and VTA was also decreased by ISO-1. In cultured dopaminergic neurons, rMIF enhanced the recruitment of G9a and SUV39H1, followed by an increase in H3K9me2/H3K9me3. These molecular changes correspondingly exhibited alterations in Th promoter CpG site methylation and pain behaviors. In summary, MIF functions as a braking factor in curbing dopaminergic descending inhibition in peripheral nerve injury-induced hypersensitivity by mediating Th gene methylation through G9a/SUV39H1-associated H3K9 methylation.

Blockade of dopamine D2 receptors disrupts intrahippocampal connectivity and enhances pain-related working memory deficits in neuropathic pain rats

BACKGROUND

Dopamine (DA) is thought to be important to local hippocampal networks integrity during spatial working memory (sWM) processing. Chronic pain may contribute to deficient dopaminergic signalling, which may in turn affect cognition. However, the neural mechanisms that determine this impairment are poorly understood. Here, we evaluated whether the sWM impairment characteristic of animal models of chronic pain is dependent on DA D2 receptor (D2r) activity.

METHODS

To address this issue, we implanted multichannel arrays of electrodes in the dorsal and ventral hippocampal CA1 field (dvCA1) of rats and recorded the neuronal activity during a classical delayed food-reinforced T-maze sWM task. Within-subject behavioural performance and patterns of dorsoventral neural activity were assessed before and after the onset of persistent neuropathic pain using the spared nerve injury (SNI) model.

RESULTS

Our results show that the peripheral nerve lesion caused a disruption in sWM and hippocampus spike activity and that disruption was maximized by the systemic administration of the D2r antagonist raclopride. These deficits are strictly correlated with a selective disruption of hippocampal theta-oscillations. Particularly, we found a significant decrease in intrahippocampal CA1 field connectivity level.

CONCLUSIONS

Together, these results suggest that disruption of the dopaminergic balance in the intrahippocampal networks may be important for the development of cognitive deficits experienced during painful conditions.

SIGNIFICANCE

This study provides new insights into the role of D2r in the manifestation of pain-related sWM deficits. Our findings support that selective blockade of D2r produces a significant decrease in intrahippocampal connectivity mediated by theta-oscillations, and amplifies pain-related sWM deficits. These results suggest that further characterization of intrahippocampal dopaminergic modulation may be clinically relevant for the understanding of cognitive impairments that accompanies nociceptive stressful conditions.

Haloperidol Decreases Hyperalgesia and Allodynia Induced by Chronic Constriction Injury

Neuropathic pain has proven to be a difficult condition to treat, so investigational therapy has been sought that may prove useful, such as the use of sigma-1 antagonists. Haloperidol (HAL) is a compound that shows a high affinity with these receptors, acting as an antagonist. Therefore, the objective of this study was to demonstrate its effect in an experimental model of neuropathic pain and corroborate its antagonistic action of the sigma-1 receptors under these conditions. BD-1063 was used as a sigma-1 antagonist control, and gabapentin (Gbp) was used as a positive control. The antihyperalgesic and anti-allodynic effects of the drugs were determined after single-dose trials. In every case, the effects increased in a dose-dependent manner. HAL had the same efficacy as both BD-1063 and Gbp. In the analysis of pharmacological potency, in which the ED50 were compared, HAL was the most potent drug of all. The effect of HAL on chronic constriction injury (CCI) rats was reversed by the sigma-1 agonist (PRE-084). HAL reversed the hyperalgesic and allodynic effects of PRE-084 in naïve rats. The dopamine antagonist, (-)-sulpiride, showed no effect in CCl rats. These results suggest that HAL presents an antinociceptive effect via sigma-1 receptor antagonism at the spinal level in the CCl model.

Potentiation of Morphine-Induced Antinociception by Propranolol: The Involvement of Dopamine and GABA Systems

Tolerance to the analgesic effect of morphine is a major clinical problem which can be managed by co-administration of another drug. This study investigated the ability of propranolol to potentiate the antinociceptive action of morphine and the possible mechanisms underlying this effect. Antinociception was assessed in three nociceptive tests (thermal, hot plate), (visceral, acetic acid), and (inflammatory, formalin test) in mice and quantified by measuring the percent maximum possible effect, the percent inhibition of acetic acid-evoked writhing response, and the area under the curve values of number of flinches for treated mice, respectively. The study revealed that propranolol (0.25-20 mg/Kg, IP) administration did not produce analgesia in mice. However, 10 mg/Kg propranolol, enhanced the antinociceptive effect of sub-analgesic doses of morphine (0.2, 1, and 2 mg/Kg, IP) in the three nociceptive tests. It also shifted the dose response curve of morphine to the left. The combined effect of propranolol and morphine was attenuated by haloperidol (D₂ receptor antagonist, 1.5 mg/Kg, IP), and bicuculline (GABA_A receptor antagonist, 2 mg/Kg, IP). Repeated daily administration of propranolol (10 mg/Kg, IP) did not alter the nociceptive responses in the three pain tests, but it significantly potentiated morphine-induced antinociception in the hot plate, acetic acid-evoked writhing, and in the second phase of formalin tests. Together, the data suggest that a cross-talk exists between the opioidergic and adrenergic systems and implicate dopamine and GABA systems in this synergistic effect of morphine-propranolol combination. Propranolol may serve as an adjuvant therapy to potentiate the effect of opioid analgesics.

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.

Aberrant plasticity of peripheral sensory axons in a painful neuropathy

Neuronal cells express considerable plasticity responding to environmental cues, in part, through subcellular mRNA regulation. Here we report on the extensive changes in distribution of mRNAs in the cell body and axon compartments of peripheral sensory neurons and the 3' untranslated region (3'UTR) landscapes after unilateral sciatic nerve entrapment (SNE) injury in rats. Neuronal cells dissociated from SNE-injured and contralateral L4 and L5 dorsal root ganglia were cultured in a compartmentalized system. Axonal and cell body RNA samples were separately subjected to high throughput RNA sequencing (RNA-Seq). The injured axons exhibited enrichment of mRNAs related to protein synthesis and nerve regeneration. Lengthening of 3'UTRs was more prevalent in the injured axons, including the newly discovered alternative cleavage and polyadenylation of NaV1.8 mRNA. Alternative polyadenylation was largely independent from the relative abundance of axonal mRNAs; but they were highly clustered in functional pathways related to RNA granule formation in the injured axons. These RNA-Seq data analyses indicate that peripheral nerve injury may result in highly selective mRNA enrichment in the affected axons with 3'UTR alterations potentially contributing to the mechanism of neuropathic pain.

Potential functional and pathological side effects related to off-target pharmacological activity

Most pharmaceutical companies test their discovery-stage proprietary molecules in a battery of in vitro pharmacology assays to try to determine off-target interactions. During all phases of drug discovery and development, various questions arise regarding potential side effects associated with such off-target pharmacological activity. Here we present a scientific literature curation effort undertaken to determine and summarize the most likely functional and pathological outcomes associated with interactions at 70 receptors, enzymes, ion channels and transporters with established links to adverse effects. To that end, the scientific literature was reviewed using an on-line database, and the most commonly reported effects were summarized in tabular format. The resultant table should serve as a practical guide for research scientists and clinical investigators for the prediction and interpretation of adverse side effects associated with molecules interacting with components of this screening battery.

Food-Derived Natural Compounds for Pain Relief in Neuropathic Pain

Neuropathic pain, defined as pain caused by a lesion or disease of the somatosensory nervous system, is characterized by dysesthesia, hyperalgesia, and allodynia. The number of patients with this type of pain has increased rapidly in recent years. Yet, available neuropathic pain medicines have undesired side effects, such as tolerance and physical dependence, and do not fully alleviate the pain. The mechanisms of neuropathic pain are still not fully understood. Injury causes inflammation and immune responses and changed expression and activity of receptors and ion channels in peripheral nerve terminals. Additionally, neuroinflammation is a known factor in the development and maintenance of neuropathic pain. During neuropathic pain development, the C-C motif chemokine receptor 2 (CCR2) acts as an important signaling mediator. Traditional plant treatments have been used throughout the world for treating diseases. We and others have identified food-derived compounds that alleviate neuropathic pain. Here, we review the natural compounds for neuropathic pain relief, their mechanisms of action, and the potential benefits of natural compounds with antagonistic effects on GPCRs, especially those containing CCR2, for neuropathic pain treatment.

The Antinociceptive Properties of the Corydalis yanhusuo Extract

Corydalis yanhusuo. W.T. extracts (YHS) are widely used for the treatment of pain and inflammation. There are a few studies that assessed the effects of YHS in pain assays; however, none of these studies has systematically compared its activities in the different pain animal modes namely: acute, inflammatory and chronic pain. Furthermore, little is known about the mechanism of YHS activity in these assays. The aim of this study was to systematically evaluate the antinociceptive properties of YHS by testing it in four standardized pain assays and to investigate its mechanism. YHS antinociceptive properties were analyzed in the tail flick, the formalin paw licking, the von Frey filament and the hot box assays after spinal nerve ligation, which monitors acute nociceptive, persistent inflammatory and chronic neuropathic pain, respectively. YHS pharmacological profile was determined by screening it against a battery of G-protein coupled receptors and its mechanism of action was studied using knock-out mice. Our study shows that YHS, at a non-sedative dose, increases the tail flick latency in the tail flick assay without resulting in development of tolerance. YHS also decreases paw licking time in the formalin assay. Further, YHS increases paw withdraw threshold and latency in the von Frey filament and the hot box assays, respectively. In vitro, YHS exhibits prominent dopamine receptor antagonistic properties. In dopamine D2 receptor knockout mice, its antinociceptive effects are attenuated in acute and neuropathic pain but not inflammatory pain assays. Our results therefore indicate that YHS effectively attenuates acute, inflammatory and neuropathic pain, without causing tolerance. The effects on acute and neuropathic pain, but not inflammatory pain, are at least partially mediated through dopamine D2 receptor antagonism. Since YHS is a dietary supplement commercially available in the United States, our data suggest that it might be a candidate for alternative pain treatment.

Dopaminergic inhibition by G9a/Glp complex on tyrosine hydroxylase in nerve injury-induced hypersensitivity

The neural balance between facilitation and inhibition determines the final tendency of central sensitization. Nerve injury-induced hypersensitivity was considered as the results from the enhanced ascending facilitation and the diminished descending inhibition. The role of dopaminergic transmission in the descending inhibition has been well documented, but its underlying molecular mechanisms are unclear. Previous studies demonstrated that the lysine dimethyltransferase G9a/G9a-like protein (Glp) complex plays a critical role in cocaine-induced central plasticity, and given cocaine’s role in the nerve system is relied on its function on dopamine system, we herein proposed that the reduced inhibition of dopaminergic transmission was from the downregulation of tyrosine hydroxylase expression by G9a/Glp complex through methylating its gene Th. After approval by the Animal Care and Use Committee, C57BL/6 mice were used for pain behavior using von Frey after spared nerve injury, and Th CpG islands methylation was measured using bisulfite sequencing at different nerve areas. The inhibitor of G9a/Glp, BIX 01294, was administered intraventricularly daily with bolus injection. The protein levels of G9a, Glp, and tyrosine hydroxylase were measured with immunoblotting. Dopamine levels were detected using high-performance liquid chromatography. The expression of G9a but not Glp was upregulated in ventral tegmental area at post-injury day 4 till day 49 (the last day of the behavioral test). Correspondingly, the Th CpG methylation is increased, but the tyrosine hydroxylase expression was downregulated and the dopamine level was decreased. After the intracerebroventriclar injection of BIX 01294 since the post-injury days 7 and 14 for consecutive three days, three weeks, and six weeks, the expression of tyrosine hydroxylase was upregulated with a significant decrease in Th methylation and increase in dopamine level. Moreover, the pain after G9a/Glp inhibitor was attenuated significantly. In sum, methytransferase G9a/Glp complex partially controls dopaminergic transmission by methylating Th in peripheral nerve injury-induced neuropathic pain.

Dopaminergic treatment of restless legs syndrome in spinal cord injury patients with neuropathic pain

Recent studies report high incidence of restless legs syndrome (RLS) in patients with spinal cord injury (SCI), who may also present pain and sensory disturbances. In the present manuscript, we examine and discuss diagnostic and treatment challenges of comorbid RLS and neuropathic pain (NP) in SCI. We evaluated seven men with a mean age of 55.6 (s.d.=14.0) years, with chronic complete or incomplete SCI at the thoracic or lumbar level, for complaints of sensory disturbances in the legs, which initially were attributed to drug-resistant NP. Because overlapped RLS was suspected, clinical evaluation of NP and RLS, serum ferritin and iron level assessment, and video polysomnographic (VPSG) studies were conducted. Pramipexole (0.18 mg q.d.^-1) was added to treat RLS, and a follow-up was performed at 2 months. We found that in six subjects the RLS was comorbid with NP and in one subject the symptoms of RLS were misdiagnosed as NP. VPSG revealed periodic limb movements (PLMs) in all patients, including PLMs of the legs, arms or both. Serum ferritin was <50 ng ml^-1 in two patients. RLS improved significantly after 2 months with pramipexole. On the basis of current findings, we recommend physicians to be aware of the comorbidity between RLS and NP secondary to SCI to include suitable diagnostic procedures and effective treatments.

The Role of Spinal Dopaminergic Transmission in the Analgesic Effect of Nefopam on Rat Inflammatory Pain

Background

Nefopam has been known as an inhibitor of the reuptake of monoamines, and the noradrenergic and/or serotonergic system has been focused on as a mechanism of its analgesic action. Here we investigated the role of the spinal dopaminergic neurotransmission in the antinociceptive effect of nefopam administered intravenously or intrathecally.

Methods

The effects of intravenously and intrathecally administered nefopam were examined using the rat formalin test. Then we performed a microdialysis study to confirm the change of extracellular dopamine concentration in the spinal dorsal horn by nefopam. To determine whether the changes of dopamine level are associated with the nefopam analgesia, its mechanism was investigated pharmacologically via pretreatment with sulpiride, a dopaminergic D2 receptor antagonist.

Results

When nefopam was administered intravenously the flinching responses in phase I of the formalin test were decreased, but not those in phase II of the formalin test were decreased. Intrathecally injected nefopam reduced the flinching responses in both phases of the formalin test in a dose dependent manner. Microdialysis study revealed a significant increase of the level of dopamine in the spinal cord by intrathecally administered nefopam (about 3.8 fold the baseline value) but not by that administered intravenously. The analgesic effects of intrathecally injected nefopam were not affected by pretreatment with sulpiride, and neither were those of the intravenous nefopam.

Conclusions

Both the intravenously and intrathecally administered nefopam effectively relieved inflammatory pain in rats. Nefopam may act as an inhibitor of dopamine reuptake when delivered into the spinal cord. However, the analgesic mechanism of nefopam may not involve the dopaminergic transmission at the spinal level.

Effect of Motor Impairment on Analgesic Efficacy of Dopamine D2/3 Receptors in a Rat Model of Neuropathy

Testing the clinical efficacy of drugs that also have important side effects on locomotion needs to be properly designed in order to avoid erroneous identification of positive effects when the evaluation depends on motor-related tests. One such example is the evaluation of analgesic role of drugs that act on dopaminergic receptors, since the pain perception tests used in animal models are based on motor responses that can also be compromised by the same substances. The apparent analgesic effect obtained by modulation of the dopaminergic system is still a highly disputed topic. There is a lack of acceptance of this effect in both preclinical and clinical settings, despite several studies showing that D2/3 agonists induce antinociception. Some authors raised the hypothesis that this antinociceptive effect is enhanced by dopamine-related changes in voluntary initiation of movement. However, the extent to which D2/3 modulation changes locomotion at analgesic effective doses is still an unresolved question. In the present work, we performed a detailed dose-dependent analysis of the changes that D2/3 systemic modulation have on voluntary locomotor activity and response to four separate tests of both thermal and mechanical pain sensitivity in adult rats. Using systemic administration of the dopamine D2/3 receptor agonist quinpirole, and of the D2/3 antagonist raclopride, we found that modulation of D2/3 receptors impairs locomotion and exploratory activity in a dose-dependent manner across the entire range of tested dosages. None of the drugs were able to consistently diminish either thermal or mechanical pain perception when administered at lower concentrations; on the other hand, the larger concentrations of raclopride (0.5–1.0 mg/kg) strongly abolished pain responses, and also caused severe motor impairment. Our results show that administration of both agonists and antagonists of dopaminergic D2/3 receptors affects sensorimotor behaviors, with the effect over locomotion and exploratory activity being stronger than the observed effect over pain responses.

Water avoidance stress induces visceral hyposensitivity through peripheral corticotropin releasing factor receptor type 2 and central dopamine D2 receptor in rats

BACKGROUND

Water avoidance stress (WAS) is reported to induce functional changes in visceral sensory function in rodents, but the results which have been demonstrated so far are not consistent, i.e., hypersensitivity or hyposensitivity. We determined the effect of WAS on visceral sensation and evaluated the mechanisms of the action.

METHODS

Visceral sensation was assessed by abdominal muscle contractions induced by colonic balloon distention, i.e., visceromotor response (VMR), measured electrophysiologically in conscious rats. The electromyogram electrodes were acutely implanted under anesthesia on the day of the experiment. The threshold of VMR was measured before and after WAS for 1 h. To explore the mechanisms of WAS-induced response, drugs were administered 10 min prior to the initiation of WAS.

KEY RESULTS

WAS significantly increased the threshold of VMR, and this effect was no longer detected at 24 h after. Intraperitoneal injection of astressin2 -B (200 μg/kg), a corticotropin releasing factor (CRF) receptor type 2 antagonist abolished the response by WAS. Subcutaneous (sc) injection of sulpiride (200 mg/kg), a dopamine D2 receptor antagonist blocked the response, while sc domperidone (10 mg/kg), a peripheral dopamine D2 receptor antagonist did not alter it. Naloxone (1 mg/kg, sc), an opioid antagonist did not modify it either.

CONCLUSIONS & INFERENCES

WAS induced visceral hyposensitivity through peripheral CRF receptor type 2 and central dopamine D2 receptor, but not through opioid pathways. As altered pain inhibitory system was reported to be observed in the patients with irritable bowel syndrome, CRF and dopamine signaling might contribute to the pathophysiology.

Restless Bladder in an Elderly Woman: An Unusual Feature or a Variant of Restless Legs Syndrome?

We herein report an elderly patient with a history of restless legs syndrome (RLS) who developed unpleasant sensations in the lower abdomen and perineum. The patient' s symptoms occurred during the evening and at rest and forced the patient to void, resulting in some symptom relief. Urological examinations and spinal magnetic resonance imaging findings were unremarkable. The adjunctive use of rotigotine resolved her symptoms. The symptoms were considered to be "restless bladder". Further studies are required to elucidate whether restless bladder is an identical condition to the clinical entities of other RLS variants or whether it represents an unusual feature of RLS.

A D2-like receptor family agonist produces analgesia in mechanonociception but not in thermonociception at the spinal cord level in rats

The administration of dopaminergic drugs produces analgesia in individuals experiencing different types of pain. Analgesia induced by these drugs at the spinal cord level is mediated by D2-like agonists, which specifically inhibit the detection of nociceptive stimuli by sensory afferents. The extent of the analgesia provided by spinal dopamine agonists remains controversial, and the cellular mechanism of this analgesic process is poorly understood. The objective of this study was to evaluate the analgesic effect of quinpirole, a D2-like agonist, based on two nociceptive tests and at various doses that were selected to specifically activate dopamine receptors. We found that intrathecal quinpirole administration produces analgesia of mechanical but not thermal nociception and that the analgesic effect of quinpirole is reversed by a mix of D2, D3, and D4 receptor-specific antagonists, suggesting that the activation of all D2-like receptors is involved in the analgesia produced by intrathecal quinpirole. The differential effect on thermal and mechanical nociception was also tested upon the activation of μ-opioid receptors. As reported previously, low doses of the μ-opioid receptor agonist DAMGO produced analgesia of only thermonociception. This evidence shows that a D2-like receptor agonist administered at the spinal cord level produces analgesia specific to mechanonociception but not thermonociception.