A role of supraspinal galanin in behavioural hyperalgesia in the rat

Baccharis dracunculifolia DC Consumption Improves Nociceptive and Depressive-like Behavior in Rats with Experimental Osteoarthritis

Osteoarthritis (OA) persistently activates nociceptors, leading to chronic pain, which is often accompanied by the comorbid development of emotional impairments (anxiety and depression), an effect associated with microgliosis. Baccharis dracunculifolia DC (Asteraceae), a Brazilian edible plant, is an important source of active compounds with anti-inflammatory abilities. Thus, we evaluated its ability to reverse OA-induced nociceptive and emotional-like impairments in osteoarthritic ovariectomized female rats using the kaolin/carrageenan (K/C) model. Four weeks after OA induction, mechanical hyperalgesia was confirmed, and the treatment started. Control animals (SHAMs) were treated with phosphate-buffered saline (PBS), while arthritic animals (ARTHs) either received PBS or B. dracunculifolia 50 mg/kg (Bd50) and 100 mg/kg (Bd100), via gavage, daily for five weeks. At the end of the treatment, anxiety-like behavior was assessed using the Open Field Test (OFT), anhedonia was assessed using the Sucrose Preference Test (SPT), and learned helplessness was assessed using the Forced Swimming Test (FST). After occision, microglia were stained with IBA-1 and quantified in brain sections of target areas (prefrontal cortex, amygdala, and periaqueductal grey matter). Treatment with B. dracunculifolia extract reversed OA-induced mechanical hyperalgesia and partly improved depressive-like behavior in OA animals' concomitant to a decrease in the number of M1 microglia. Our findings suggest that B. dracunculifolia extracts can potentially be used in the food industry and for the development of nutraceuticals and functional foods.

An Ascending Excitatory Circuit from the Dorsal Raphe for Sensory Modulation of Pain

The dorsal raphe nucleus (DRN) is an important nucleus in pain regulation. However, the underlying neural pathway and the function of specific cell types remain unclear. Here, we report a previously unrecognized ascending facilitation pathway, the DRN to the mesoaccumbal dopamine (DA) circuit, for regulating pain. Chronic pain increased the activity of DRN glutamatergic, but not serotonergic, neurons projecting to the ventral tegmental area (VTA) (DRNGlu-VTA) in male mice. The optogenetic activation of DRNGlu-VTA circuit induced a pain-like response in naive male mice, and its inhibition produced an analgesic effect in male mice with neuropathic pain. Furthermore, we discovered that DRN ascending pathway regulated pain through strengthened excitatory transmission onto the VTA DA neurons projecting to the ventral part of nucleus accumbens medial shell (vNAcMed), thereby activated the mesoaccumbal DA neurons. Correspondingly, optogenetic manipulation of this three-node pathway bilaterally regulated pain behaviors. These findings identified a DRN ascending excitatory pathway that is crucial for pain sensory processing, which can potentially be exploited toward targeting pain disorders.

A New Gal in Town: A Systematic Review of the Role of Galanin and Its Receptors in Experimental Pain

Galanin is a neuropeptide expressed in a small percentage of sensory neurons of the dorsal root ganglia and the superficial lamina of the dorsal horn of the spinal cord. In this work, we systematically reviewed the literature regarding the role of galanin and its receptors in nociception at the spinal and supraspinal levels, as well as in chronic pain conditions. The literature search was performed in PubMed, Web of Science, Scopus, ScienceDirect, OVID, TRIP, and EMBASE using "Galanin" AND "pain" as keywords. Of the 1379 papers that were retrieved in the initial search, we included a total of 141 papers in this review. Using the ARRIVE guidelines, we verified that 89.1% of the works were of good or moderate quality. Galanin shows a differential role in pain, depending on the pain state, site of action, and concentration. Under normal settings, galanin can modulate nociceptive processing through both a pro- and anti-nociceptive action, in a dose-dependent manner. This peptide also plays a key role in chronic pain conditions and its antinociceptive action at both a spinal and supraspinal level is enhanced, reducing animals' hypersensitivity to both mechanical and thermal stimulation. Our results highlight galanin and its receptors as potential therapeutic targets in pain conditions.

Evidence that increased cholecystokinin (CCK) in the periaqueductal gray (PAG) facilitates changes in Resident-Intruder social interactions triggered by peripheral nerve injury

Individual differences in the effects of a chronic neuropathic injury on social behaviours characterize both the human experience and pre-clinical animal models. The impacts of these changes to the well-being of the individual are often underappreciated. Earlier work from our laboratory using GeneChip® microarrays identified increased cholecystokinin (CCK) gene expression in the periaqueductal gray (PAG) of rats that showed persistent changes in social interactions during a Resident-Intruder encounter following sciatic nerve chronic constriction injury (CCI). In this study, we confirmed these gene regulation patterns using RT-PCR and identified the anatomical location of the CCK-mRNA as well as the translated CCK peptides in the midbrains of rats with a CCI. We found that rats with persistent CCI-induced changes in social behaviours had increased CCK-mRNA in neurons of the ventrolateral PAG and dorsal raphe nuclei, as well as increased CCK-8 peptide expression in terminal boutons located in the lateral and ventrolateral PAG. The functional significance of these changes was explored by microinjecting small volumes of CCK-8 into the PAG of uninjured rats and observing their Resident-Intruder social interactions. Disturbances to social interactions identical to those observed in CCI rats were evoked when injection sites were located in the rostral lateral and ventrolateral PAG. We suggest that CCI-induced changes in CCK expression in these PAG regions contributes to the disruptions to social behaviours experienced by a subset of individuals with neuropathic injury.

Activation of central galanin receptor 2 mitigated insulin resistance in adipocytes of diabetic rats

PURPOSE

Our and other's studies showed that administration of neuropeptide galanin may mitigate insulin resistance via promoting glucose transporter 4 (GLUT4) expression and translocation in rats. The objective of this study is to investigate whether galanin receptor 2 (GAL2-R) in brain mediates the ameliorative effect of galanin on insulin resistance in adipose tissues of type 2 diabetic rats.

METHODS

In this study galanin, GAL2-R agonist M1145 and GAL2-R antagonist M871 were respectively or cooperatively injected into intracerebroventricles of type 2 diabetic rats once a day for successive fifteen days. Then the plasma and fat tissues of rats were used to estimate the alterations of insulin resistance indexes.

RESULTS

The central administration of galanin enhanced 2-deoxy-[³H]-D-glucose, peroxisome proliferator-activated receptor γ and adiponectin levels, food intake and body weight, GLUT4 mRNA expression and GLUT4 concentration in plasma membranes, as well as homeostasis model assessment-insulin resistance index. Those effects of galanin may be blocked by M817, and imitated by M1145 except for food intake and body weight.

CONCLUSION

Those results suggest that central GAL2-R mediates the beneficial effects of galanin on insulin sensitivity in type 2 diabetic rats. GAL2-R agonist may be taken as a potential antidiabetic agent to treat insulin resistance and type 2 diabetes.

Involvement of galanin and galanin receptor 1 in nociceptive modulation in the central nucleus of amygdala in normal and neuropathic rats

The present study was performed to explore the role of galanin and galanin receptor 1 (GalR 1) in nociceptive modulation in the central nucleus of amygdala (CeA) in normal rats and rats with neuropathy, and the involvement of GalR 1 and PKC was also investigated. The hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulations were increased in a dose-dependent manner after intra-CeA injection of galanin in both normal rats and rats with neuropathy. The increased HWLs were significantly attenuated by intra-CeA injection of galanin receptor antagonist M40, indicating an involvement of galanin receptor in nociceptive modulation in CeA. Furthermore, intra-CeA administration of the GalR 1 agonist M 617 induced increases in HWLs in normal rats, suggesting that GalR 1 may be involved in galanin-induce antinociception in CeA. Additionally, intra-CeA injection of the PKC inhibitor inhibited galanin-induced antinociception, showing an involvement of PKC in galanin-induced antinociception in CeA of normal rats. Moreover, there was a significant increase in GalR1 content in CeA in rats with neuropathy than that in normal rats. These results illustrated that galanin induced antinociception in CeA in normal rats and rats with neuropathy, and there is an up-regulation of GalR1 expression in rats with neuropathy.

Chronic stress-induced mechanical hyperalgesia is controlled by capsaicin-sensitive neurones in the mouse

BACKGROUND

Clinical studies demonstrated peripheral nociceptor deficit in stress-related chronic pain states, such as fibromyalgia. The interactions of stress and nociceptive systems have special relevance in chronic pain, but the underlying mechanisms including the role of specific nociceptor populations remain unknown. We investigated the role of capsaicin-sensitive neurones in chronic stress-related nociceptive changes.

METHOD

Capsaicin-sensitive neurones were desensitized by the capsaicin analogue resiniferatoxin (RTX) in CD1 mice. The effects of desensitization on chronic restraint stress (CRS)-induced responses were analysed using behavioural tests, chronic neuronal activity assessment in the central nervous system with FosB immunohistochemistry and peripheral cytokine concentration measurements.

RESULTS

Chronic restraint stress induced mechanical and cold hypersensitivity and increased light preference in the light-dark box test. Open-field and tail suspension test activities were not altered. Adrenal weight increased, whereas thymus and body weights decreased in response to CRS. FosB immunopositivity increased in the insular cortex, dorsomedial hypothalamic and dorsal raphe nuclei, but not in the spinal cord dorsal horn after the CRS. CRS did not affect the cytokine concentrations of hindpaw tissues. Surprisingly, RTX pretreatment augmented stress-induced mechanical hyperalgesia, abolished light preference and selectively decreased the CRS-induced neuronal activation in the insular cortex. RTX pretreatment alone increased the basal noxious heat threshold without influencing the CRS-evoked cold hyperalgesia and augmented neuronal activation in the somatosensory cortex and interleukin-1α and RANTES production.

CONCLUSIONS

Chronic restraint stress induces hyperalgesia without major anxiety, depression-like behaviour or peripheral inflammatory changes. Increased stress-induced mechanical hypersensitivity in RTX-pretreated mice is presumably mediated by central mechanisms including cortical plastic changes.

SIGNIFICANCE

These are the first data demonstrating the complex interactions between capsaicin-sensitive neurones and chronic stress and their impact on nociception. Capsaicin-sensitive neurones are protective against stress-induced mechanical hyperalgesia by influencing neuronal plasticity in the brain.

Involvement of galanin and galanin receptor 2 in nociceptive modulation in anterior cingulate cortex of normal rats and rats with mononeuropathy

The present study was performed to explore the role of galanin and galanin receptor 2 in nociceptive modulation in anterior cingulate cortex (ACC) of normal rats and rats with mononeuropathy. Intra-ACC injection of galanin induced significant increases in hindpaw withdrawal latencies (HWLs) to thermal and mechanical stimulations in both normal rats and rats with mononeuropathy, the increased HWLs were attenuated significantly by intra-ACC injection of galanin receptor 2 antagonist M871, indicating an involvement of galanin receptor 2 in nociceptive modulation in ACC. Interestingly, the galanin-induced HWL was significant higher in rats with mononeuropathy than that in normal rats tested by Randall Selitto test. Furthermore, both the galanin mRNA expression and galanin content increased significantly in ACC in rats with mononeuropathy than that in normal rats. Moreover, both the mRNA levels of galanin receptor 2 and the content of galanin receptor 2 in ACC increased significantly in rats with mononeuropathy than that in normal rats. These results found that galanin induced antinociception in ACC in both normal rats and rats with mononeuropathy. And there may be plastic changes in the expression of galanin and galanin receptor 2 in rats with mononeuropathy, as well as in the galanin-induced antinociception.

Galanin-Mediated Behavioural Hyperalgesia from the Dorsomedial Nucleus of the Hypothalamus Involves Two Independent Descending Pronociceptive Pathways

Activation of the dorsomedial nucleus of the hypothalamus (DMH) by galanin (GAL) induces behavioural hyperalgesia. Since DMH neurones do not project directly to the spinal cord, we hypothesized that the medullary dorsal reticular nucleus (DRt), a pronociceptive region projecting to the spinal dorsal horn (SDH) and/or the serotoninergic raphe-spinal pathway acting on the spinal 5-HT3 receptor (5HT3R) could relay descending nociceptive facilitation induced by GAL in the DMH. Heat-evoked paw-withdrawal latency (PWL) and activity of SDH neurones were assessed in monoarthritic (ARTH) and control (SHAM) animals after pharmacological manipulations of the DMH, DRt and spinal cord. The results showed that GAL in the DMH and glutamate in the DRt lead to behavioural hyperalgesia in both SHAM and ARTH animals, which is accompanied particularly by an increase in heat-evoked responses of wide-dynamic range neurons, a group of nociceptive SDH neurones. Facilitation of pain behaviour induced by GAL in the DMH was reversed by lidocaine in the DRt and by ondansetron, a 5HT3R antagonist, in the spinal cord. However, the hyperalgesia induced by glutamate in the DRt was not blocked by spinal ondansetron. In addition, in ARTH but not SHAM animals PWL was increased after lidocaine in the DRt and ondansetron in the spinal cord. Our data demonstrate that GAL in the DMH activates two independent descending facilitatory pathways: (i) one relays in the DRt and (ii) the other one involves 5-HT neurones acting on spinal 5HT3Rs. In experimental ARTH, the tonic pain-facilitatory action is increased in both of these descending pathways.