Activation of the galanin receptor 2 in the periphery reverses nerve injury-induced allodynia

Neuroendocrine and neuroimmune mechanisms underlying comorbidity of pain and obesity

Pain and obesity, as well as their associated impairments, are major health concerns. Understanding the relationship between the two is the focus of a growing body of research. However, early researches attribute increased mechanical stress from excessive weight as the main factor of obesity-related pain, which not only over-simplify the association, but also fail to explain some controversial outcomes arising from clinical investigations. This review focuses on neuroendocrine and neuroimmune modulators importantly involved in both pain and obesity, analyzing nociceptive and anti-nociceptive mechanisms based on neuroendocrine pathways including galanin, ghrelin, leptin and their interactions with other neuropeptides and hormone systems which have been reported to play roles in pain and obesity. Mechanisms of immune activities and metabolic alterations are also discussed, due to their intense interactions with neuroendocrine system and crucial roles in the development and maintenance of inflammatory and neuropathic pain. These findings have implications for health given rising rates of obesity and pain-related diagnoses, by providing novel weight-control and analgesic therapies targeted on specific pathways.

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.

The partial saphenous nerve injury model of pain impairs reward-related learning but not reward sensitivity or motivation

ABSTRACT

Chronic pain is highly comorbid with affective disorders, including major depressive disorder. A core feature of major depressive disorder is a loss of interest in previously rewarding activities. Major depressive disorder is also associated with negative affective biases where cognitive processes are modulated by the affective state. Previous work from our laboratory has shown that reward-related learning and memory is impaired in rodent models of depression generated through a variety of different manipulations. This study investigated different aspects of reward-related behaviour in a rodent model of chronic pain, the partial saphenous nerve injury (PSNI). Using our reward-learning assay, an impairment in reward learning was observed with no difference in sucrose preference, consistent with a lack of effect on reward sensitivity and similar to the effects seen in depression models. In a successive negative contrast task, chronic pain was not associated with changes in motivation for reward either under normal conditions or when reward was devalued although both sham and PSNI groups exhibited the expected negative contrast effect. In the affective bias test, PSNI rats developed a positive affective bias when treated with gabapentin, an effect not seen in the controls suggesting an association with the antinociceptive effects of the drug inducing a relatively more positive affective state. Together, these data suggest that there are changes in reward-related cognition in this chronic pain model consistent with previous findings in rodent models of depression. The effects seen with gabapentin suggest that pain-associated negative affective state may be remediated by this atypical analgesic.

Beneficial effects of galanin system on diabetic peripheral neuropathic pain and its complications

Diabetic peripheral neuropathic pain (DPNP) is a distal spontaneous pain, caused by lesion of sensory neurons and accompanied by depression and anxiety frequently, which reduce life quality of patients and increase society expenditure. To date, antidepressants, serotonin-noradrenaline reuptake inhibitors and anticonvulsants are addressed as first-line therapy to DPNP, alone or jointly. It is urgently necessary to develop novel agents to treat DPNP and its complications. Evidences indicate that neuropeptide galanin can regulate multiple physiologic and pathophysiological processes. Pain, depression and anxiety may upregulate galanin expression. In return, galanin can modulate depression, anxiety, pain threshold and pain behaviors. This article provides a new insight into regulative effects of galanin and its subtype receptors on antidepressant, antianxiety and against DPNP. Through activating GALR1, galanin reinforces depression-like and anxiogenic-like behaviors, but exerts antinociceptive roles. While via activating GALR2, galanin is referred to as anti-depressive and anti-anxiotropic compounds, and at low and high concentration facilitates and inhibits nociceptor activity, respectively. The mechanism of the galanin roles is relative to increase in K⁺ currents and decrease in Ca²⁺ currents, as well as neurotrophic and neuroprotective roles. These data are helpful to develop novel drugs to treat DPNP and its complications.

Effects of trigeminal nerve injury on the expression of galanin and its receptors in the rat trigeminal ganglion

In the spinal nervous system, the expression of galanin (GAL) and galanin receptors (GALRs) that play important roles in the transmission and modulation of nociceptive information can be affected by nerve injury. However, in the trigeminal nervous system, the effects of trigeminal nerve injury on the expression of GAL are controversy in the previous studies. Besides, little is known about the effects of trigeminal nerve injury on the expression of GALRs. In the present study, the effects of trigeminal nerve injury on the expression of GAL and GALRs in the rat trigeminal ganglion (TG) were investigated by using quantitative real-time reverse transcription-polymerase chain reaction and immunohistochemistry. To identify the nerve-injured and nerve-uninjured TG neurons, activating transcription factor 3 (ATF3, the nerve-injured neuron marker) was stained by immunofluorescence. The levels of GAL mRNA in the rostral half and caudal half of the TG dramatically increased after transection of infraorbital nerve (ION) and inferior alveolar nerve (IAN), respectively. Immunohistochemical labeling of GAL and ATF3 revealed that GAL level was elevated in both injured and adjacent uninjured small and medium-sized TG neurons after ION/IAN transection. In addition, the levels of GAL₂R-like immunoreactivity were reduced in both injured and adjacent uninjured TG neurons after ION/IAN transection, while levels of GAL₁R and GAL₃R-like immunoreactivity remained unchanged. Furthermore, the number of small to medium-sized TG neurons co-expressing GAL- and GAL₁R/GAL₂R/GAL₃R-like immunoreactivity was significantly increased after ION/IAN transection. In line with previous studies in other spinal neuron systems, these results suggest that GAL and GALRs play functional roles in orofacial neuropathic pain and trigeminal nerve regeneration after trigeminal nerve injury.

A preliminary study on DRGs and spinal cord of a galanin receptor 2-EGFP transgenic mouse

The neuropeptide galanin functions via three G-protein coupled receptors, Gal_1-3-R. Both Gal₁-R and ₂-R are involved in pain signaling at the spinal level. Here a Gal₂-R-EGFP transgenic (TG) mouse was generated and studied in pain tests and by characterizing Gal₂-R expression in both sensory ganglia and spinal cord. After peripheral spared nerve injury, mechanical allodynia developed and was ipsilaterally similar between wild type (WT) and TG mice. A Gal₂-R-EGFP-positive signal was primarily observed in small and medium-sized dorsal root ganglion (DRG) neurons and in spinal interneurons and processes. No significant difference in size distribution of DRG neuronal profiles was found between TG and WT mice. Both percentage and fluorescence intensity of Gal₂-R-EGFP-positive neuronal profiles were overall significantly upregulated in ipsilateral DRGs as compared to contralateral DRGs. There was an ipsilateral reduction in substance P-positive and calcitonin gene-related peptide (CGRP)-positive neuronal profiles, and this reduction was more pronounced in TG as compared to WT mice. Moreover, Gal₂-R-EGFP partly co-localized with three pain-related neuropeptides, CGRP, neuropeptide Y and galanin, both in intact and injured DRGs, and with galanin also in local neurons in the superficial dorsal horn. Taken together, the present results provide novel information on the localization and phenotype of DRG and spinal neurons expressing the second galanin receptor, Gal₂-R, and on phenotypic changes following peripheral nerve injury. Gal2-R may also be involved in autoreceptor signaling.

Galaninergic intramural nerve and tissue reaction to antral ulcerations

BACKGROUND

Well-developed galaninergic gastric intramural nerve system is known to regulate multiple stomach functions in physiological and pathological conditions. Stomach ulcer, a disorder commonly occurring in humans and animals, is accompanied by inflammatory reaction. Inflammation can cause intramural neurons to change their neurochemical profile. Galanin and its receptors are involved in inflammation of many organs, however, their direct participation in stomach reaction to ulcer is not known. Therefore, the aim of the study was to investigate adaptive changes in the chemical coding of galaninergic intramural neurons and mRNA expression encoding Gal, GalR1, GalR2, GalR3 receptors in the region of the porcine stomach directly adjacent to the ulcer location.

METHODS

The experiment was performed on 24 pigs, divided into control and experimental groups. In 12 experimental animals, stomach antrum ulcers were experimentally induced by submucosal injection of acetic acid solution. Stomach wall directly adjacent to the ulcer was examined by: (1) double immunohistochemistry-to verify the changes in the number of galaninergic neurons (submucosal, myenteric) and fibers; (2) real-time PCR to verify changes in mRNA expression encoding galanin, GalR1, GalR2, GalR3 receptors.

KEY RESULTS

In the experimental animals, the number of Gal-immunoreactive submucosal perikarya was increased, while the number of galaninergic myenteric neurons and fibers (in all the stomach wall layers) remained unchanged. The expression of mRNA encoding all galanin receptors was increased.

CONCLUSIONS & INTERFERENCES

The results obtained unveiled the participation of galanin and galanin receptors in the stomach tissue response to antral ulcerations.

Elevated galanin receptor type 2 primarily contributes to mechanical hypersensitivity after median nerve injury

In this study, we investigated temporal changes in galanin receptor type 2 (GalR2) expression in NF200-, galanin-, neuropeptide Y (NPY)-, and neuronal nitric oxide synthase (nNOS)-like immunoreactive (LI) dorsal root ganglion (DRG) neurons after median nerve chronic constriction injury (CCI), and the effects of GalR2 on c-Fos expression in the cuneate nucleus (CN). Double immunofluorescence labeling methods were used to appraise changes in GalR2 expression in NF200-LI, galanin-LI, NPY-LI, and nNOS-LI DRG neurons after CCI. The von Frey assay was used to assess the efficiency of intraplantar administration of saline, M871 (a GalR2 antagonist), or AR-M1896 (a GalR2 agonist) on neuropathic signs of rats with CCI. The effects of alterations in c-Fos expression were assessed in all treatments. The percentage of GalR2-LI neurons in lesioned DRGs increased and peaked at 1 week after CCI. We further detected that percentages of GalR2-LI neurons labeled for NF200, galanin, NPY, and nNOS significantly increased following CCI. Furthermore, M871 remarkably attenuated tactile allodynia, but the sensation was slightly aggravated by AR-M1896 after CCI. Consequentially, after electrical stimulation of the CCI-treated median nerve, the number of c-Fos-LI neurons in the cuneate nucleus (CN) was significantly reduced in the M871 group, whereas it increased in the AR-M1896 group. These results suggest that activation of GalR2, probably through NPY or nitric oxide, induces c-Fos expression in the CN and transmits mechanical allodynia sensations to the thalamus.

Preclinical Analgesic and Safety Evaluation of the GalR2-preferring Analog, NAX 810-2

The potential clinical utility of galanin peptidic analogs has been hindered by poor metabolic stability, lack of brain penetration, and hyperglycemia. In addition to possessing potent anticonvulsant efficacy, galanin analogs are analgesic in various assays. The purpose of these studies was to evaluate the lead galanin receptor type 2 (GalR2)-preferring analog, NAX 810-2, in various pain assays, as well as determine any potential for insulin inhibition, growth hormone stimulation, and cognitive impairment. NAX 810-2 was evaluated in mouse (carrageenan, formalin, tail flick, plantar incision) and rat pain models (partial sciatic nerve ligation). NAX 810-2 dose-dependently increased paw withdrawal latency following plantar administration of carrageenan (ED₅₀ 4.7 mg/kg). At a dose of 8 mg/kg, NAX 810-2 significantly attenuated nociceptive behaviors following plantar administration of formalin, and this was observed for both phase I (acute) and phase II (inflammatory) components of the formalin behavioral response. NAX-810-2 was active at higher doses in the mouse tail flick model (ED₅₀ 20.2 mg/kg) and similarly, reduced mechanical allodynia following plantar incision in mice at a dose of 24 mg/kg. NAX 810-2 also reduced mechanical allodynia in the partial sciatic nerve ligation model at a dose of 4 mg/kg. In addition, NAX 810-2 did not impair insulin secretion at doses of 2.5 and 8 mg/kg (acutely) or at a dose of 8 mg/kg given daily for 5 days. Similarly, 8 mg/kg (twice daily, 5 days) of NAX 810-2 did not increase growth hormone levels. These results demonstrate that NAX 810-2 possesses a favorable pre-clinical profile as a novel and first-in-class analgesic.

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.

Beneficial effects of neuropeptide galanin on reinstatement of exercise-induced somatic and psychological trauma

Galanin is a versatile neuropeptide that is distinctly upregulated by exercise in exercise-related tissues. Although benefits from exercise-induced upregulation of this peptide have been identified, many issues require additional exploration. This Review summarizes the information currently available on the relationship between galanin and exercise-induced physical and psychological damage. On the one hand, body movement, exercise damage, and exercise-induced stress and pain significantly increase local and circulatory galanin levels. On the other hand, galanin plays an exercise-protective role to inhibit the flexor reflex and prevent excessive movement of skeletal muscles through enhancing response threshold and reducing acetylcholine release. Additionally, elevated galanin levels can boost repair of the exercise-induced damage in exercise-related tissues, including peripheral nerve, skeletal muscle, blood vessel, skin, bone, articulation, and ligament. Moreover, elevated galanin levels may serve as effective signals to buffer sport-induced stress and pain via inhibiting nociceptive signal transmission and enhancing pain threshold. This Review deepens our understanding of the profitable roles of galanin in exercise protection, exercise injury repair, and exercise-induced stress and pain. Galanin and its agonists may be used to develop a novel preventive and therapeutic strategy to prevent and treat exercise-induced somatic and psychological trauma. © 2016 Wiley Periodicals, Inc.

Characterization of the Rat GAL2R Promoter: Positive Role of ETS-1 in Regulation of the Rat GAL2R Gene in PC12 Cells

Galanin receptor 2 (GAL2R) is a G protein-coupled receptor for the neuropeptide galanin that regulates many important physiological functions and pathological processes. To investigate the molecular mechanism governing GAL2R gene transcription, the rat GAL2R promoter was isolated and analyzed. We found that the region from -320 to -300 of the GAL2R promoter contains two putative ETS-1 elements and plays an important role in regulating GAL2R promoter activity. We also showed that transcription factor ETS-1 bound to this region in vitro and in vivo. Overexpression of ETS-1 significantly increased GAL2R promoter activity and transcription of the GAL2R gene, whereas knockdown of ETS-1 produced the opposite effects. In addition, we showed that ETS-1 recruited co-activator p300 to the GAL2R promoter. These data indicate a role for ETS-1 in the control of the GAL2R gene expression and provide a basis for understanding the transcriptional regulation of the GAL2R gene.

The Influence of Gastric Antral Ulcerations on the Expression of Galanin and GalR1, GalR2, GalR3 Receptors in the Pylorus with Regard to Gastric Intrinsic Innervation of the Pyloric Sphincter

Gastric antrum ulcerations are common disorders occurring in humans and animals. Such localization of ulcers disturbs the gastric emptying process, which is precisely controlled by the pylorus. Galanin (Gal) and its receptors are commonly accepted to participate in the regulation of inflammatory processes and neuronal plasticity. Their role in the regulation of gastrointestinal motility is also widely described. However, there is lack of data considering antral ulcerations in relation to changes in the expression of Gal and GalR1, GalR2, GalR3 receptors in the pyloric wall tissue and galaninergic intramural innervation of the pylorus. Two groups of pigs were used in the study: healthy gilts and gilts with experimentally induced antral ulcers. By double immunocytochemistry percentages of myenteric and submucosal neurons expressing Gal-immunoreactivity were determined in the pyloric wall tissue and in the population of gastric descending neurons supplying the pyloric sphincter (labelled by retrograde Fast Blue neuronal tracer). The percentage of Gal-immunoreactive neurons increased only in the myenteric plexus of the pyloric wall (from 16.14±2.06% in control to 25.5±2.07% in experimental animals), while no significant differences in other neuronal populations were observed between animals of both groups. Real-Time PCR revealed the increased expression of mRNA encoding Gal and GalR1 receptor in the pyloric wall tissue of the experimental animals, while the expression(s) of GalR2 and GalR3 were not significantly changed. The results obtained suggest the involvement of Gal, GalR1 and galaninergic pyloric myenteric neurons in the response of pyloric wall structures to antral ulcerations.

Mechanisms of Perineural Invasion

Perineural invasion (PNI) is the neoplastic invasion of nerves. PNI is widely recognized as an important adverse pathological feature of many malignancies, including pancreatic, prostate, and head and neck cancers and is associated with a poor prognosis. Despite widespread acknowledgment of the clinical significance of PNI, the mechanisms underlying its pathogenesis remain largely unknown. Recent theories of PNI pathogenesis have placed a significant emphasis on the active role of the nerve microenvironment, with PNI resulting from well-orchestrated reciprocal interactions between cancer and host. Elucidating the mechanisms involved in PNI may translate into targeted therapies for this ominous process.

Galanin and its receptor system promote the repair of injured sciatic nerves in diabetic rats

Various studies have reported that galanin can promote axonal regeneration of dorsal root ganglion neurons in vitro and inhibit neuropathic pain. However, little is known about its effects on diabetic peripheral neuropathy, and in vivo experimental data are lacking. We hypothesized that repeated applications of exogenous galanin over an extended time frame may also repair nerve damage in diabetic peripheral neuropathy, and relieve pain in vivo. We found that neuropathic pain occurred in streptozotocin-induced diabetic rats and was more severe after sciatic nerve pinch injury at 14 and 28 days than in diabetic sham-operated rats. Treatment with exogenous galanin alleviated the neuropathic pain and promoted sciatic nerve regeneration more effectively in diabetic rats than in non-diabetic rats after sciatic nerve pinch injury. This was accompanied by changes in the levels of endogenous galanin, and its receptors galanin receptor 1 and galanin receptor 2 in the dorsal root ganglia and the spinal dorsal horn when compared with nerve pinch normal rats. Our results show that application of exogenous galanin daily for 28 days can promote the regeneration of injured sciatic nerves, and alleviate neuropathic pain in diabetic rats.

Cisplatin induced sensory neuropathy is prevented by vascular endothelial growth factor-A

Increased patient survival is a mark of modern anti-cancer therapy success. Unfortunately treatment side-effects such as neurotoxicity are a major long term concern. Sensory neuropathy is one of the common toxicities that can arise during platinum based chemotherapy. In many cases the current poor understanding of the neurological degeneration and lack of suitable analgesia has led to high incidences of patient drop out of treatment. VEGF-A is a prominent neuroprotective agent thus it was hypothesised to prevent cisplatin induced neuropathy. Systemic cisplatin treatment (lasting 3 weeks biweekly) resulted in mechanical allodynia and heat hyperalgesia in mice when compared to vehicle control. PGP9.5 sensory nerve fibre innervation was reduced in the plantar skin in the cisplatin treated group versus vehicle control mice. The cisplatin induced sensory neurodegeneration was associated with increased cleaved caspase 3 expression as well as a reduction in Activating Transcription Factor 3 and pan VEGF-A expression in sensory neurons. VEGF-A165b expression was unaltered between vehicle and cisplatin treatment. rhVEGF-A165a and rhVEGF-A165b both prevented cisplatin induced sensory neurodegeneration. Cisplatin exposure blunts the regenerative properties of sensory neurons thus leading to sensory neuropathy. However, here it is identified that administration of VEGF-A isoform subtypes induce regeneration and prevent cell death and are therefore a possible adjunct therapy for chemotherapy induced neuropathy.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Galanin modulates the neural niche to favour perineural invasion in head and neck cancer

Perineural invasion (PNI) is an indicator of poor survival in multiple cancers. Unfortunately, there is no targeted treatment for PNI since the molecular mechanisms are largely unknown. PNI is an active process, suggesting that cancer cells communicate with nerves. However, nerve-tumour crosstalk is understudied due to the lack of in vivo models to investigate the mechanisms. Here, we developed an in vivo model of PNI to characterise this interaction. We show that the neuropeptide galanin (GAL) initiates nerve-tumour crosstalk via activation of its G-protein-coupled receptor, GALR2. Our data reveal a novel mechanism by which GAL from nerves stimulates GALR2 on cancer cells to induce NFATC2-mediated transcription of cyclooxygenase-2 and GAL. Prostaglandin E2 promotes cancer invasion, and in a feedback mechanism, GAL released by cancer induces neuritogenesis, facilitating PNI. This study describes a novel in vivo model for PNI and reveals the dynamic interaction between nerve and cancer.

Analgesic properties of a peripherally acting and GalR2 receptor-preferring galanin analog in inflammatory, neuropathic, and acute pain models

There are ongoing efforts to develop pain therapeutics with novel mechanisms of action that avoid common side effects associated with other analgesics. The anticonvulsant neuropeptide galanin is a potent regulator of neuronal excitability and has a well established role in pain modulation, making it a potential target for novel therapies. Our previous efforts focused on improving blood-brain-barrier penetration and enhancing the metabolic stability of galanin analogs to protect against seizures. More recently, we designed peripherally acting galanin analogs that reduce pain-related behaviors by acting in the periphery and exhibit preferential binding toward galanin receptor (GalR)2 over GalR1. In this study, we report preclinical studies of a monodisperse oligoethylene glycol-containing galanin analog, NAX 409-9 (previously reported as GalR2-dPEG24), in rodent analgesic and safety models. Results obtained with NAX 409-9 in these tests were compared with the representative analgesics gabapentin, ibuprofen, acetylsalicylic acid, acetaminophen, and morphine. In mice that received intraplantar carrageenan, NAX 409-9 increased paw withdrawal latency with an ED50 of 6.6 mg/kg i.p. NAX 409-9 also increased the paw withdrawal threshold to mechanical stimulation following partial sciatic nerve ligation in rats (2 mg/kg). Conversely, NAX 409-9 had no effect in the tail flick or hot plate assays (up to 24 mg/kg). Importantly, NAX 409-9 did not negatively affect gastrointestinal motility (4-20 mg/kg), respiratory rate (40-80 mg/kg), or bleed time (20 mg/kg). These studies illustrate that this nonbrain-penetrating galanin analog reduces pain behaviors in several models and does not produce some of the dose-limiting toxicities associated with other analgesics.

Galanin receptors possibly modulate the obesity-induced change in pain threshold

Pain threshold may be up-regulated or down-regulated according to gender, age, race/ethnic and psychological state. Previous studies indicated that obesity may change pain threshold, both nociceptive and antinociceptive, which resulted from obesity-reduced variation of neuroendocrine. However there is a limited understanding of its molecular mechanism underlying this variation. A lot of evidence supports that galanin increases food intake and body weight to induce obesity in animals. This peptide may also modulate nociceptive susceptibility via central galanin receptor 1 and peripheral galanin receptor 2 in dorsal root ganglion. Whereas injury and obesity may up-regulate the galanin expression and stimulate its secretion to elevate the plasma levels of subjects. Pain may increase the risk of obesity through reduced physical activity. In this review, we highlighted the multiple bilateral interrelation between obesity and pain sensitivity, between galanin and obesity and between galanin and injure-induced pain. In view of the above, we reasoned that galanin receptors possibly participated in the modulation of the obesity-induced change in pain threshold, which need further direct evidence to support as yet. This review is helpful to explore the mechanism that galanin receptors regulate the obesity-induced change of pain sensitivity and to contribute to our understanding of the relation among galanin, obesity and pain threshold.

Different responses of galanin and calcitonin gene-related peptide to capsaicin stimulation on dorsal root ganglion neurons in vitro

Both galanin (Gal) and calcitonin gene-related peptide (CGRP) are sensory neuropeptides which expressed in dorsal root ganglion (DRG) neurons and are involved in nociceptive processing. Capsaicin (CAP) influences nociceptive processing via influencing the expression of sensory neuropeptides in primary sensory neurons. However, little is known about the alterations of Gal and CGRP expression at the same condition stimulated by CAP. In the present study, primary cultured DRG neurons were used to determine the different responses of Gal and CGRP to CAP stimulation. DRG neurons were cultured for 48 hours and then exposed to CAP (2 μmol/L), capsazepine (CPZ) (2 μmol/L) plus CAP (2 μmol/L), or extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor PD98059 (10 μmol/L) plus CAP (2 μmol/L) for an additional 24hours. The DRG neurons were continuously exposed to culture media as a control. After that, the levels of Gal mRNA and CGRP mRNA of DRG neurons were determined using real time-PCR analysis. Gal and CGRP expression in situ was detected by an immunofluorescent labeling technique. The levels of phosphorylated-ERK1/2 (pERK1/2) protein were detected using a Western blot assay. The results showed that CAP evoked increases of Gal and its mRNA and decreases of CGRP and its mRNA in DRG neurons. Administration of either CPZ or PD98059 blocked the effects of CAP. These data indicate that Gal and CGRP shared different responses to CAP stimulation. Gal and CGRP may have different effects in nociceptive processing during neurogenic inflammation.

Galanin-induced decreases in nucleus accumbens/striatum excitatory postsynaptic potentials and morphine conditioned place preference require both galanin receptor 1 and galanin receptor 2

The neuropeptide galanin has been shown to alter the rewarding properties of morphine. To identify potential cellular mechanisms that might be involved in the ability of galanin to modulate opiate reward, we measured excitatory postsynaptic potentials (EPSPs), using both field and whole-cell recordings from striatal brain slices extracted from wild-type mice and mice lacking specific galanin receptor (GalR) subtypes. We found that galanin decreased the amplitude of EPSPs in both the dorsal striatum and nucleus accumbens. We then performed recordings in slices from knockout mice lacking either the GalR1 or GalR2 gene, and found that the ability of galanin to decrease EPSP amplitude was absent from both mouse lines, suggesting that both receptor subtypes are required for this effect. In order to determine whether behavioral responses to opiates were dependent on the same receptor subtypes, we tested GalR1 and GalR2 knockout mice for morphine conditioned place preference (CPP). Morphine CPP was significantly attenuated in both GalR1 and GalR2 knockout mice. These data suggest that mesolimbic excitatory signaling is significantly modulated by galanin in a GalR1-dependent and GalR2-dependent manner, and that morphine CPP is dependent on the same receptor subtypes.

Incorporation of monodisperse oligoethyleneglycol amino acids into anticonvulsant analogues of galanin and neuropeptide y provides peripherally acting analgesics

Delivery of neuropeptides into the central and/or peripheral nervous systems supports development of novel neurotherapeutics for the treatment of pain, epilepsy and other neurological diseases. Our previous work showed that the combination of lipidization and cationization applied to anticonvulsant neuropeptides galanin (GAL) and neuropeptide Y (NPY) improved their penetration across the blood-brain barrier yielding potent antiepileptic lead compounds, such as Gal-B2 (NAX 5055) or NPY-B2. To dissect peripheral and central actions of anticonvulsant neuropeptides, we rationally designed, synthesized and characterized GAL and NPY analogues containing monodisperse (discrete) oligoethyleneglycol-lysine (dPEG-Lys). The dPEGylated analogues Gal-B2-dPEG(24), Gal-R2-dPEG(24) and NPY-dPEG(24) displayed analgesic activities following systemic administration, while avoiding penetration into the brain. Gal-B2-dPEG(24) was synthesized by a stepwise deprotection of orthogonal 4-methoxytrityl and allyloxycarbonyl groups, and subsequent on-resin conjugations of dPEG(24) and palmitic acids, respectively. All the dPEGylated analogues exhibited substantially decreased hydrophobicity (expressed as logD values), increased in vitro serum stabilities and pronounced analgesia in the formalin and carrageenan inflammatory pain assays following systemic administration, while lacking apparent antiseizure activities. These results suggest that discrete PEGylation of neuropeptides offers an attractive strategy for developing neurotherapeutics with restricted penetration into the central nervous system.

Differential roles of galanin on mechanical and cooling responses at the primary afferent nociceptor

Background

Galanin is expressed in a small percentage of intact small diameter sensory neurons of the dorsal root ganglia and in the afferent terminals of the superficial lamina of the dorsal horn of the spinal cord. The neuropeptide modulates nociception demonstrating dose-dependent pro- and anti-nociceptive actions in the naïve animal. Galanin also plays an important role in chronic pain, with the anti-nociceptive actions enhanced in rodent neuropathic pain models. In this study we compared the role played by galanin and its receptors in mechanical and cold allodynia by identifying individual rat C-fibre nociceptors and characterising their responses to mechanical or acetone stimulation.

Results

Mechanically evoked responses in C-fibre nociceptors from naive rats were sensitised after close intra-arterial infusion of galanin or Gal2-11 (a galanin receptor-2/3 agonist) confirming previous data that galanin modulates nociception via activation of GalR2. In contrast, the same dose and route of administration of galanin, but not Gal2-11, inhibited acetone and menthol cooling evoked responses, demonstrating that this inhibitory mechanism is not mediated by activation of GalR2. We then used the partial saphenous nerve ligation injury model of neuropathic pain (PSNI) and the complete Freund’s adjuvant model of inflammation in the rat and demonstrated that close intra-arterial infusion of galanin, but not Gal2-11, reduced cooling evoked nociceptor activity and cooling allodynia in both paradigms, whilst galanin and Gal2-11 both decreased mechanical activation thresholds. A previously described transgenic mouse line which inducibly over-expresses galanin (Gal-OE) after nerve injury was then used to investigate whether manipulating the levels of endogenous galanin also modulates cooling evoked nociceptive behaviours after PSNI. Acetone withdrawal behaviours in naive mice showed no differences between Gal-OE and wildtype (WT) mice. 7-days after PSNI Gal-OE mice demonstrated a significant reduction in the duration of acetone-induced nociceptive behaviours compared to WT mice.

Conclusions

These data identify a novel galaninergic mechanism that inhibits cooling evoked neuronal activity and nociceptive behaviours via a putative GalR1 mode of action that would also be consistent with a TRP channel-dependent mechanism.

Effects of exogenous galanin on neuropathic pain state and change of galanin and its receptors in DRG and SDH after sciatic nerve-pinch injury in rat

A large number of neuroanatomical, neurophysiologic, and neurochemical mechanisms are thought to contribute to the development and maintenance of neuropathic pain. However, mechanisms responsible for neuropathic pain have not been completely delineated. It has been demonstrated that neuropeptide galanin (Gal) is upregulated after injury in the dorsal root ganglion (DRG) and spinal dorsal horn (SDH) where it plays a predominantly antinociceptive role. In the present study, sciatic nerve-pinch injury rat model was used to determine the effects of exogenous Gal on the expression of the Gal and its receptors (GalR1, GalR2) in DRG and SDH, the alterations of pain behavior, nerve conduction velocity (NCV) and morphology of sciatic nerve. The results showed that exogenous Gal had antinociceptive effects in this nerve-pinch injury induced neuropathic pain animal model. It is very interesting that Gal, GalR1 and GalR2 change their expression greatly in DRG and SDH after nerve injury and intrathecal injection of exougenous Gal. Morphological investigation displays a serious damage after nerve-pinch injury and an amendatory regeneration after exogenous Gal treatment. These findings imply that Gal, via activation of GalR1 and/or GalR2, may have neuroprotective effects in reducing neuropathic pain behaviors and improving nerve regeneration after nerve injury.

Peripheral galanin receptor 2 as a target for the modulation of pain

The neuropeptide galanin is widely expressed in the nervous system and has an important role in nociception. It has been shown that galanin can facilitate and inhibit nociception in a dose-dependent manner, principally through the central nervous system, with enhanced antinociceptive actions after nerve injury. However, following nerve injury, expression of galanin within the peripheral nervous system is dramatically increased up to 120-fold. Despite this striking increase in the peripheral nervous system, few studies have investigated the role that galanin plays in modulating nociception at the primary afferent nociceptor. Here, we summarise the recent work supporting the role of peripherally expressed galanin with particular reference to the dual actions of the galanin receptor 2 in neuropathic pain highlighting this as a potential target analgesic.