Evidence for endogenous inhibition of autotomy by galanin in the rat after sciatic nerve section: demonstrated by chronic intrathecal infusion of a high affinity galanin receptor antagonist

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.

Crosstalk between exercise and galanin system alleviates insulin resistance

Studies have demonstrated that aerobic exercise can enhance insulin sensitivity, however, the precise mechanism for this outcome is not entirely identified. Emerging evidences point out that exercise can upregulate galanin protein and mRNA expression, resulting in improvement of insulin sensitivity via an increase in translocation of glucose transporter 4 and subsequent glucose uptake in myocytes and adipocytes of healthy and type 2 diabetic rats, which may be blocked by galanin antagonist. In return, galanin can exert the exercise-protective roles to prevent excessive movement of skeletal muscle and to accelerate exercise trauma repair in exercise-relative tissues. Studies also implicated that combination of aerobic exercise and activation of galanin system may make more significant improvement in insulin sensitivity than that of either one did. These suggest that galanin system is essential for physical activity to alleviate insulin resistance, namely, the beneficial effect of physical activity on glucose uptake is at least partly mediated by galanin system. Besides, co-treatment with galanin and exercise is an effective therapeutic strategy for reducing insulin resistance.

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.

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.

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.

Galanin receptor-expressing dorsal horn neurons: role in nociception

Galanin, along with enkephalins and neuropeptide Y, has been hypothesized to negatively modulate nociception in the superficial dorsal horn of the spinal cord. In the present study, we sought to determine the role of presumably excitatory dorsal horn galanin receptor-expressing neurons in nociception by selectively destroying GalR1-expressing superficial dorsal horn interneurons using lumbar intrathecal injections of the targeted cytotoxin, galanin-saporin (Gal-sap). Lumbar intrathecal injection of Gal-sap (500 ng) reduced immunoperoxidase staining for GalR1 in the superficial dorsal horn without affecting primary afferent neurons in lumbar dorsal root ganglia. Lumbar intrathecal Gal-sap also: 1--reduced nocifensive reflex responding on the thermal plate at 0.3 °C, 44 °C, and 47 °C; 2--increased hot side occupancy in a thermal preference task (15 °C vs 45 °C); and, 3--decreased escape from 44 °C and 47 °C, but not 20 °C. Thus, similar to lesions of mu opiate receptor-expressing dorsal horn interneurons, selective destruction of GalR1-expressing superficial dorsal horn neurons produces heat hypo-algesia, likely due to loss of GalR1-expressing excitatory interneurons leading to reduced activation of nociceptive projection neurons in response to aversive heat. These results are different than those seen with intrathecal neuropeptide Y-saporin and suggest the potential value of selectively targeting GalR1-expressing dorsal horn neurons to control pain.

Galanin and spinal pain mechanisms: past, present, and future

Since the discovery of galanin in 1983, one of the most frequently suggested physiological function for this peptide is pain modulation at the level of the spinal cord. This notion, initially based on the preferential distribution of galanin in dorsal spinal cord, has been supported by results from a large number of morphological, molecular, and functional studies. It is generally agreed that spinally applied galanin produces a biphasic, dose-dependent effect on spinal nociception through activation of GalR1 (inhibitory) or GalR2 (excitatory) receptors. Galanin also appears to have an endogenous inhibitory role, particularly after peripheral nerve injury when the synthesis of galanin is increased in sensory neurons. In recent years, small molecule ligands of galanin receptors have been developed, which may lead to the development of analgesic drugs, which affects the galanin system at the spinal cord level.

Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin

Galanin modulates spinal nociceptive processing by interacting with two receptors, GalR1 and GalR2. The underlying neurophysiological mechanisms were examined by whole-cell recording from identified neurons in the substantia gelatinosa of young adult rats. GalR1 was activated with a 'cocktail' containing the GalR1/2 agonist, AR-M 961 (0.5 microM), in the presence of the GalR2 antagonist, M871 (1.0-2.5 microM). GalR2 was activated with the selective agonist, AR-M 1896 (0.5-1.0 microM). Application of the 'GalR1 agonist cocktail' often activated an inwardly-rectifying conductance in delay firing (excitatory) and tonically firing (inhibitory) neurons. This conductance was not activated by AR-M 1896 which instead decreased or increased an outwardly-rectifying conductance at voltages positive to -70 mV. Despite this variability in its actions on current-voltage relationships, AR-M 1896 very consistently decreased membrane excitability, as measured by cumulative action potential latency in response to a depolarizing current ramp. This strong GalR2-mediated effect was seen in neurons where membrane conductance was decreased, and where membrane excitability might be predicted to increase. GalR2 was also located presynaptically, as AR-M 1896 increased the interevent interval of spontaneous EPSCs in both delay and tonic cells. By contrast, the 'GalR1 agonist cocktail' had little effect on spontaneous EPSCs, suggesting that presynaptic terminals do not express GalR1. These diverse actions of GalR1 and GalR2 activation on both inhibitory and excitatory neurons are discussed in relation to the known spinal antinociceptive and pro-nociceptive actions of galanin, to the possible association of GalR1 with the inhibitory G-protein, G(i/o) and to report that GalR2 activation suppresses Ca2+ channel currents.

Neurotrophin-3 attenuates galanin expression in the chronic constriction injury model of neuropathic pain

We have recently shown that exogenous neurotrophin-3 (NT-3) acts antagonistically to nerve growth factor (NGF) in regulation of nociceptor phenotype in intact neurons and suppresses thermal hyperalgesia and expression of molecules complicit in this behavioral response induced by chronic constriction injury (CCI) of the sciatic nerve. The present study examines whether there is a global influence of NT-3 in mitigating alterations in peptide and NGF receptor expression; molecules believed to also contribute to CCI-associated pain. Thus, the influence of NT-3 on phenotypic changes in dorsal root ganglion (DRG) neurons in rats coincident with CCI was examined using in situ hybridization. Seven days following injury, the incidence of expression of the neuropeptides galanin and pituitary adenylate cyclase-activating polypeptide (PACAP) was increased in L5 sensory neurons ipsilateral to the injury from 12% to 60% and 16% to 37% respectively, in addition to an increased level of expression. In contrast, there was no consistent significant change in tropomyosin-related kinase A (trkA) expression following CCI. Intrathecal infusion of NT-3 globally mitigated both the increased incidence and elevated levels of galanin messenger RNA (mRNA) expression observed following CCI, reducing the former from 60% to 39%. NT-3 infusion resulted in a limited reduction in the incidence and level of neuronal PACAP in medium to large size, but not small size, DRG neurons. NT-3 had no significant net effect on CCI-induced alterations in trkA mRNA expression.

Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

Galanin over-expression decreases the development of neuropathic pain-like behaviors in mice after partial sciatic nerve injury

The neuropeptide galanin may have a role in modulation of nociception, particularly after peripheral nerve injury. Here we assessed the development of neuropathic pain-like behaviors in mice overexpressing galanin under the dopamine beta-hydroxylase promoter. Unoperated galanin over-expressing mice exhibited a moderately reduced sensitivity to noxious heat. Both galanin over-expressing mice and wild-type controls developed mechanical and heat hypersensitivity after photochemically induced partial sciatic nerve ischemic injury. The magnitude and persistence of such pain-like behaviors were significantly less, and recovery was faster in galanin over-expressing mice compared to wild types. However, the recovery from toe-spread deficits did not differ between galanin over-expressing and wild-type mice after a crush injury to the sciatic nerve. Thus, early recovery in pain-like response is unlikely to result from accelerated regeneration in the galanin over-expressing mice. Immunohistochemical analysis showed that galanin is over-expressed both in small and large dorsal root ganglion cells in the transgene mouse, whereas large galanin-positive neurons were never seen in wild-type mice. The present results in general support an inhibitory role of galanin in nociception and indicate that increased availability of galanin in spinal dorsal horn at the time or shortly after nerve injury may reduce the development of pain-like behaviors in mice.

[125I]-Galanin binding in brain of wildtype, and galanin- and GalR1-knockout mice: Strain and species differences in GalR1 density and distribution

Widespread production of knockout and transgenic mice has led to an increased use of mice as animal models for studies of normal- and patho-physiology. Hence, the precise mapping of central transmitter/peptide systems in the mouse has become essential for the interpretation of functional studies and for the correct correlation with findings obtained in the rat, primates and/or human. In this regard, the current study reports the autoradiographic localization of [(125)I]-galanin (GAL) binding sites in brain of the common C57BL/6J and 129OlaHsd mouse strains, as well as in GAL and galanin receptor-1 (GalR1) knockout (KO) mice. In C57BL/6J and 129OlaHsd mice, [(125)I]-GAL binding sites were detected throughout the brain, including moderate-high relative densities in the basal ganglia (caudate putamen, nucleus [n.] accumbens, olfactory tubercle, substantia nigra), limbic regions (septum, bed n. stria terminalis, ventral hippocampus, amygdala), cingulate, retrosplenial, entorhinal cortex, centro-lateral/medial thalamic n., preoptic/lateral hypothalamus, midbrain (superior colliculus, periaqueductal gray), pons/medulla oblongata (parabrachial, pontine reticular and solitary tract n.) and cerebellar cortex. [(125)I]-GAL binding levels were low or absent in main olfactory bulb, neocortex, ventrolateral/geniculate thalamic n., dorsal hippocampus, inferior colliculus and cranial motor n. In simultaneous determinations, relative [(125)I]-GAL binding site densities in brain were generally lower in C57BL/6J than in 129OlaHsd mice, while the density and distribution of central binding in the GAL-KO mouse was essentially identical to that in its background-129OlaHsd strain. In contrast, no specific [(125)I]-GAL binding was detected in any region of GalR1-KO mouse brain, revealing that under the experimental conditions used, the peptide ligand binding is predominantly (exclusively) to the GalR1 subtype. This evaluation of GAL receptor site distribution in mouse brain has revealed similarities and some differences with the equivalent system in rat and provides a valuable reference for future comparative studies of central GAL transmission.

Heightened pain and delayed regeneration of galanin axons in diabetic mice

Galanin peptide in primary sensory neurons may confer analgesia following injury. Its presence in regenerative axon sprouts where pain may be initiated has not been examined. We examined very early outgrowth of peptidergic axon sprouts after sciatic nerve crush in mice with experimental streptozotocin-induced diabetes. Diabetic mice had a retarded wave of outgrowing galanin axons, but those expressing calcitonin gene-related peptide grew normally. Diabetic mice also developed early, then persistent excessive autotomy behaviour, an index of pain behaviour in complete nerve lesions. Diabetes is associated with variations in the early outgrowth of peptide-containing axons. A relative delay in galanin axon outgrowth could contribute to heightened neuropathic pain in diabetes.

Systemic galnon, a low-molecular weight galanin receptor agonist, reduces heat hyperalgesia in rats with nerve injury

We have examined the effect of systemically administered galnon, a novel low-molecular weight agonist of galanin receptors, on neuropathic pain-like behaviors in rats after photochemically induced partial nerve injury. Galnon is a galanin receptor ligand with moderate affinity to spinal cord membranes (K(D) of 6+/-0.6 microM). While intraperitoneally applied galnon produced no significant effect on mechanical or cold hypersensitivity, it dose-dependently prolonged heat withdrawal latency in nerve-injured rats. The effect of galnon was more potent on the injured side which has significantly shorter latency than the contralateral side. The anti-hyperalgesic effect of galanon was prevented by intrathecal M35, a galanin receptor antagonist. No side effects, such as sedation or motor impairment, were seen following systemic galnon treatment at the doses used. It is concluded that systemic galnon alleviated heat-hyperalgesic response in rats with partial sciatic nerve injury. This effect was likely to be mediated by activation of spinal galanin receptors.

Hyperalgesia and increased neuropathic pain-like response in mice lacking galanin receptor 1 receptors

The neuropeptide galanin may have a role in modulation of nociception, particularly after peripheral nerve injury. The effect of galanin is mediated by at least three subtypes of receptors. In the present study, we assessed the nociceptive sensitivity in mice lacking the galanin receptor 1 gene (Galr1) and the development of neuropathic pain-like behaviours after photochemically induced partial sciatic nerve ischaemic injury. Under basal condition, Galr1 knock-out (Galr1(-/-)) mice had shortened response latency on the hot plate, but not tail flick and paw radiant heat, tests. The mechanical sensitivity was not different between Galr1(-/-) and wild type (Galr1(+/+)) mice, whereas the cold response was moderately enhanced in Galr1(-/-) mice. Both Galr1(-/-) mice and Galr1(+/+) controls developed mechanical and heat hypersensitivity after partial sciatic nerve injury. The duration of such pain-like behaviours was significantly increased in Galr1(-/-). The Galr1(-/-) mice and Galr1(+/+) mice did not differ in their recovery from deficits in toe-spread after sciatic nerve crush. The results provide some evidence for an inhibitory function for the neuropeptide galanin acting on galanin receptor 1 (GALR1) in nociception and neuropathic pain after peripheral nerve injury in mice.

Transgenic overexpression of galanin in the dorsal root ganglia modulates pain-related behavior

The neuropeptide galanin is expressed in the dorsal root ganglia (DRG) and spinal cord and is thought to be involved in the modulation of pain processing. However, its mechanisms of action are complex and poorly understood, as both facilitatory and inhibitory effects have been described. To understand further the role played by galanin in nociception, we have generated two transgenic lines that overexpress galanin in specific populations of primary afferent DRG neurons in either an inducible or constitutive manner. In the first line, a previously defined enhancer region from the galanin locus was used to target galanin to the DRG (Gal-OE). Transgene expression recapitulates the spatial endogenous galanin distribution pattern in DRG neurons and markedly overexpresses the peptide in the DRG after nerve injury but not in the uninjured state. In the second line, an enhancer region of the c-Ret gene was used to constitutively and ectopically target galanin overexpression to the DRG (Ret-OE). The expression of this second transgene does not alter significantly after nerve injury. Here, we report that intact Ret-OE, but not Gal-OE, animals have significantly elevated mechanical and thermal thresholds. After nerve damage, using a spared nerve-injury model, mechanical allodynia is attenuated markedly in both the Gal-OE and Ret-OE mice compared with WT controls. These results support an inhibitory role for galanin in the modulation of nociception both in intact animals and in neuropathic pain states.

Reduced spinal cord sensitization to C-fibre stimulation in mice over-expressing galanin

The neuropeptide galanin may have a role in spinal nociception. In this study, we examined the excitability of the flexor reflex and its sensitization by repetitive stimulation of nociceptive C-fibres in anaesthetized mice that over-express galanin. No difference was seen between over-expressing galanin and wild-type mice in the magnitude of the baseline flexor reflex. Repetitive conditioning stimulation of C-fibres (10 stimuli at 1 Hz) produced a gradual increase in reflex magnitude during the conditioning stimulation (wind-up), as well as an increase in spinal reflex excitability after the termination of the stimulus train (central sensitization) in wild-type mice. Although the wind-up did not differ between over-expressing galanin and wild-type mice, the magnitude of central sensitization was significantly reduced in the over-expressing galanin mice (24 +/- 13% peak increase compared with 164 +/- 65% in the wild-type). Intrathecal administration of M35, a galanin receptor antagonist, markedly enhanced central sensitization in over-expressing galanin mice in association with C-fibre conditioning stimulation, while having no effect in wild-type mice. These results provide further electrophysiological evidence for an inhibitory function of galanin in modulation of central sensitization in response to C-fibre stimulation.

Galanin-immunoreactive cells and their relation to calcitonin gene-related peptide-, substance P- and somatostatin-immunoreactive cells in rat lumbar dorsal root ganglia

We report upon the distribution of galanin-immunoreactive (GAL-IR) cells in the lumbar dorsal root ganglia (DRG) of the rat, and upon the distribution of GAL-IR cells, which also contain calcitonin gene-related peptide (CGRP)-, substance P (SP)- and somatostatin (SOM)-immunoreactivity. Neuropeptide-immunoreactive lumbar DRG cells were 55.8% for CGRP, 12.7% for SP, and 6.5% for GAL in lumbar DRG cells. There was no significant difference between the right and left DRGs (L1-L6) for any neuropeptide-immunoreactive cell (P < 0.01). In terms of size distribution, CGRP-immunoreactive cells were identified below 1500 microm2, and SP-, and GAL-IR cells below 600 microm2. Neuropeptide immunoreactive cells showed various immunoreactivities in the cytoplasm according to each neuropeptide. CGRP and SP immunoreactive cells were colocalized with GAL immunoreactive cells in the serial sections about 83.3 and 60% respectively, but SOM colocalizing with GAL-IR cells were not in evidence. The current results confirm and extend previous results, and show that neuropeptides can coexist in single sensory neurones of the rat DRG. In addition, our results demonstrate that the normal distribution of some neurotransmitters modulating sensory action in Wistar Kyoto rat, make this model more prone to develop neuropathic pain than Sprague-Dawley rat.

Mu opioid receptors and analgesia at the site of a peripheral nerve injury

Opioid ligands may exert antinociception through receptors expressed on peripheral afferent axons. Whether local opioid receptors might attenuate neuropathic pain is uncertain. In this work, we examined the function and expression of local mu opioid receptors (MORs) associated with the chronic constriction injury (CCI) model of sciatic neuropathic pain in rats. Low-dose morphine or its carrier were percutaneously superfused over the CCI site with the injector blinded to the identity of the injectate. Morphine, but not its carrier, and not equimolar systemic doses of morphine reversed thermal hyperalgesia in a dose-related, naloxone-sensitive fashion. Moreover, analgesia was conferred at both 48 hours and 14 days after CCI, times associated with very different stages of nerve repair. Equimolar local DAGO ([D-Ala2, N-Me-Phe4, Gly5-(ol)] enkephalin), a selective MOR ligand, provided similar analgesia. Local morphine also attenuated mechanical allodynia. MOR protein was expressed in axonal endbulbs of Cajal just proximal to the injury site, in aberrantly regenerating small axons in the epineurial sheath around the CCI site and in residual small axons distal to the CCI lesion. Sensory neurons ipsilateral to CCI had an increase in the proportion of neurons expressing MOR. We suggest that local MOR expressed in axons may be exploited to modulate some forms of neuropathic pain.

The second galanin receptor GalR2 plays a key role in neurite outgrowth from adult sensory neurons

Expression of the neuropeptide galanin is markedly upregulated within the adult dorsal root ganglion (DRG) after peripheral nerve injury. We demonstrated previously that the rate of peripheral nerve regeneration is reduced in galanin knock-out mice, with similar deficits observed in neurite outgrowth from cultured mutant DRG neurons. Here, we show that the addition of galanin peptide significantly enhanced neurite outgrowth from wild-type sensory neurons and fully rescued the observed deficits in mutant cultures. Furthermore, neurite outgrowth in wild-type cultures was reduced to levels observed in the mutants by the addition of the galanin antagonist M35 [galanin(1-13)bradykinin(2-9)]. Study of the first galanin receptor (GalR1) knock-out animals demonstrated no differences in neurite outgrowth compared with wild-type animals. Similarly, use of a GalR1-specific antagonist had no effect on neuritogenesis. In contrast, use of a GalR2-specific agonist had equipotent effects on neuritogenesis to galanin peptide, and inhibition of PKC reduced neurite outgrowth from wild-type sensory neurons to that observed in galanin knock-out cultures. These results demonstrate that adult sensory neurons are dependent, in part, on galanin for neurite extension and that this crucial physiological process is mediated by activation of the GalR2 receptor in a PKC-dependent manner.

The participation of galanin in pain processing at the spinal level

Galanin, a 29-amino-acid peptide expressed in dorsal root ganglia (DRG) and spinal dorsal horn interneurones, is regulated by nerve injury and peripheral inflammation. The functional significance of such regulation has been subject to intense studies, including the analysis of galanin null mice, with the production of apparently conflicting results. Here, we suggest that upregulation of galanin in DRG neurones following nerve injury results in antinociception via stimulation of galanin GAL1 receptors on dorsal horn neurones, and that the pro-nociceptive effect of galanin is related to presynaptic galanin GAL2 receptors on primary afferents. A selective GAL1 receptor agonist could therefore be valuable for the treatment of neuropathic pain.

Targeted disruption of galanin: new insights from knock-out studies

The neuropeptide galanin has a widespread but no means ubiquitous expression pattern in the nervous and endocrine systems. Profound changes in the levels and distribution of the peptide occur in a range of path-physiological situations including nerve injury or damage and alterations in the circulating levels of a number of hormones. There is now a substantial body of work to indicate that galanin plays an important biological role as a regulator of neurotransmitter and hormone release in the adult. The recent generation of mice carrying a loss-of-function mutation within the galanin gene has allowed us new insights into the physiological actions of galanin. In this manuscript we detail three sets of data relating to the major phenotypic effects thus far delineated, putting them in the context of existing published data. These studies demonstrate that galanin acts as a developmental and trophic factor to subsets of neurons in the nervous and neuroendocrine systems.

Long-term intrathecal catheterization in the rat

We report an intrathecal (i.t.) catheter system that permits repeated administration of volumes of 10 microl or more in the awake rat over many months. A small skin incision is made and a 32 ga polyurethane catheter is inserted in the sacral subarachnoid space using a modified 22 ga needle. The other end of the catheter is tunneled subcutaneously to the flank and exteriorized through a titanium port. The device is well tolerated, does not cause sensory or motor deficits, and does not interfere with behavioral testing. Rats equipped with this device can be housed with other rats. Over the 9 month observation period the function of the catheter was verified by repeated injection of 15 microl of 2% lidocaine that caused temporary paraplegia. Out of 12 implanted rats, the number of fully functional catheters was 10 at 3 months, seven at 6 months, and six at 9 months. At 3 months, i.t. injection of anti-dopamine-beta-hydroxylase antibodies conjugated to saporin (DBH-SAP, 5 microg/10 microl) resulted in noradrenergic denervation of the spinal cord in all rats (n=10). We propose that intrathecal catheterization is well suited for long term behavioral and pharmacological studies.

Central galanin and N-terminal galanin fragment induce c-Fos immunoreactivity in the medulla oblongata of the anesthetized rat

This immunohistochemical study analyzed the c-Fos expression (c-Fos-ir) induced by galanin injections. Galanin and N-terminal galanin fragment (1-15) induced a significant increase of c-Fos expression (c-ir) within the medulla oblongata 90 min and 6 h. after intracisternal injections. This expression has been studied mainly in the nucleus of the solitary tract and in the ventrolateral medulla showing different temporal profiles for both peptides. The presence of c-Fos-ir in TH-positive cells was analyzed in all the groups. These results may be relevant to understand the role of galanin in several functions including central cardiovascular control.

The microenvironment of injured and regenerating peripheral nerves

Local events in the milieu of injured peripheral nerve trunks may have an important influence on the likelihood of regenerative success or the development of neuropathic pain. Injury-related changes in the microcirculation of this milieu have provided some evidence that axonal endbulbs, structures that form at the proximal end of transected axons, dump peptides and other molecules into the injury milieu where they may exert local actions, including those on microvessels. During a later phase of nerve repair, macrophage influx and pancellular proliferative events appear to develop in a coordinated fashion. Nitric oxide is probably an important and prominent player in the injured nerve trunk, both at early and later stages of the repair process. A better understanding of the injured peripheral nerve microenvironment may allow therapeutic approaches that can enhance regeneration and diminish pain.

Mice over-expressing galanin have elevated heat nociceptive threshold

The neuropeptide galanin may have a role in modulation of nociceptive input at spinal level. Here we report that mice over-expressing galanin exhibit significant elevation of nociceptive threshold to thermal stimulation in comparison to wild-type mice as assessed by the tail flick and paw heat irradiation tests. No change in response to mechanical or cold stimulation was seen. The elevated heat nociceptive threshold in the galanin over-expressing mice was reversed by intrathecal application of the putative galanin receptor antagonist M-35, galanin-(1-12)-pro-bradykinin-(2-9). The results thus support that galanin has an inhibitory function in rodent spinal cord.

Effect of intrathecal galanin and its putative antagonist M35 on pain behavior in a neuropathic pain model

There is currently some debate over a possible role of galanin in pain processing. It was recently reported that the levels of galanin in dorsal root ganglia (DRGs) seem related to development of allodynia after unilateral sciatic nerve constriction injury. In our present study, we aimed at characterizing the effect of exogenous and endogenous galanin on pain behavior in allodynic and non-allodynic rats in which the levels of galanin in DRG neurons are low and high, respectively [28]. The results show that in allodynic rats, the mechanical threshold increases dose-dependently after intrathecal (i.t.) injection of galanin, while no significant changes were observed in groups treated with the putative galanin antagonist M35 or saline. In non-allodynic rats i.t. injection of M35 induced a significant mechanical allodynic state, which did not occur after injection of galanin, bradykinin, the bradykinin fragment(2-9) or saline. The results suggest that in the present experimental paradigm exogenous galanin has an anti-allodynic effect in the allodynic rats, and that endogenous galanin has a tonic inhibitory effect in the non-allodynic group.

The galanin receptor antagonist M40 blocks the central cardiovascular actions of the galanin N-terminal fragment (1-15)

It has been shown that galanin plays a role in central cardiovascular regulation. Galanin administered centrally induces an increase of heart rate and a weak vasodepressor response, whereas the N-terminal galanin fragment (1-15) elicits vasopressor effects and tachycardia. Furthermore, it has been shown that galanin-(1-15), but not galanin-(1-29), decreases the baroreceptor reflex sensitivity. Since these data demonstrate that both galanin and its N-terminal fragment (1-15) exert a different modulation on central cardiovascular control, the aim of this work has been to study if the specific galanin receptor antagonist Galanin-(1-12)-Pro-(Ala-Leu)(2)-Ala]-amide (M40) could modulate their cardiovascular actions. Urethane anaesthetized rats were injected intracisternally and the changes in mean arterial pressure and heart rate were monitored. Two doses of M40 alone have been tested for their cardiovascular effects. With the dose of 1.0 nmol, a significant tachycardia was observed (P<0.001), but 0.1 nmol was ineffective. This suggests a possible agonistic effect for the higher doses of M40. The galanin receptor antagonist M40 at the dose of 0.1 nmol failed to modify the weak vasodepressor effects and tachycardia induced by 3.0 nmol of galanin-(1-29). However, the same dose completely blocked the vasopressor and tachycardic responses elicited by 3.0 nmol of galanin-(1-15). These data show that M40 differentially counteracts the central cardiovascular responses of the galanin fragment and give a functional support for the existence of galanin receptor subtypes within the brainstem. Therefore, the present findings can be explained on the basis that the cardiovascular actions of galanin-(1-29) could be mediated by one type of galanin receptor, whereas a galanin receptor subtype that recognizes N-terminal fragments of galanin may mediate the actions of galanin-(1-15).

Multimeric CREB-binding sites in the promoter regions of a family of G-protein-coupled receptors related to the vertebrate galanin and nociceptin/orphanin-FQ receptor families

Four related genes encoding a family of G-protein-coupled receptors (GPCRs) have been isolated from the mollusc Lymnaea stagnalis. The coding regions of this family of receptors share 97-99% sequence similarity at both the protein and nucleotide level, and they also share high sequence identity with vertebrate galanin and orphanin-FQ/nociceptin GPCR families. Analysis of the promoter regions reveals shared domains, some of which encode highly conserved repeating units. One 27-bp repeating unit, which encodes a c-AMP response element (CRE) and binds CREB protein, is repeated 14 times in one promoter. In situ hybridization showed expression of these receptors in identified neurons of several behaviourly important networks including those involved in feeding and ion and water regulation. These Lymnaea receptors are likely to represent members of a novel family of invertebrate neuropeptide receptors extensively regulated in response to intracellular signalling cascades.

Antinociceptive role of galanin in periaqueductal grey of rats with experimentally induced mononeuropathy

The present study was performed in rats with experimentally induced mononeuropathy after left common sciatic nerve ligation. The hindpaw withdrawal latencies to thermal and mechanical stimulation increased significantly after intra-periaqueductal grey injection of 2 or 3nmol, but not 1nmol of galanin in rats with mononeuropathy. Intraperitoneal administration of 4.5mg/kg morphine induced significant increases in hindpaw withdrawal latencies to both noxious stimulation, which were attenuated by following intra-periaqueductal grey injection of 2nmol of the galanin antagonist galantide. Furthermore, the antinociceptive effect induced by intra-periaqueductal grey injection of 26.6nmol of morphine was attenuated significantly by following intra-periaqueductal gray administration of 2nmol of galantide. The results demonstrated that in periaqueductal grey galanin plays an antinociceptive role in rats with mononeuropathy and galanin is involved in the mechanisms of opioid-induced antinociception.

Galanin and spinal nociceptive mechanisms: recent advances and therapeutic implications

Galanin is a peptide consisting of 29 or 30 (in humans) amino acids that is present in sensory and spinal dorsal horn neurons. Endogenous galanin may have an important modulatory function on nociceptive input at the spinal level. In addition, exogenously administered galanin exerts complex effects on spinal nociceptive transmission, where inhibitory action appears to predominate. Peripheral nerve injury and inflammation, conditions associated with chronic pain, upregulate the synthesis of galanin in sensory neurons and spinal cord neurons, respectively. Hence, the sensory effect of galanin may be increased under these conditions, raising the possibility that modulation of the activity of the galanin system may produce antinociception.

Galanin knockout mice reveal nociceptive deficits following peripheral nerve injury

The neuropeptide galanin has been identified as a potential neurotransmitter/neuromodulator within the central nervous system. In the present study, the role of endogenous galanin in nociceptive processing in the nervous system has been analysed by using mice carrying a targeted mutation in the galanin gene. Supporting this, the effect of chronic administration of exogenous galanin on nociceptive sensory inputs has been assayed in adult rats. In the absence of peripheral nerve injury, the sensitivity to threshold noxious stimuli is significantly higher in galanin mutant mice than wild-type controls. Following peripheral nerve injury, in conditions under which endogenous galanin levels are elevated, spontaneous and evoked neuropathic pain behaviours are compromised in mutant mice. Conversely, chronic intrathecal delivery of exogenous galanin to nerve-intact adult rats is associated with persistent behavioural hypersensitivity, a significant increase in c-fos expression and an increase in PKCgamma immunoreactivity within the spinal cord dorsal horn. The present results demonstrate that a relationship exists between the degree of nerve injury-induced galanin expression and the degree of behavioural hypersensitivity, and show that galanin may play a role in nociceptive processing in the spinal cord, with interrelated inhibitory and excitatory effects.

Regulation of galanin and neuropeptide Y in dorsal root ganglia and dorsal horn in rat mononeuropathic models: possible relation to tactile hypersensitivity

The expression of galanin and neuropeptide Y in rat lumbar 5 (L5) dorsal root ganglia and dorsal horn (L4-5) was studied after four types of peripheral nerve injury using immunohistochemistry and in situ hybridization. The possible correlation between these two peptides and tactile allodynia-like behaviour was analysed as well. The models employed were the Gazelius (photochemical lesion) and Seltzer and Bennett (constriction lesions) models, as well as complete sciatic nerve transection (axotomy). Two weeks after surgery, the Gazelius model rats more frequently displayed a greater tactile allodynia than the rats from the Seltzer and Bennett models. Tactile allodynia was not observed in any of the axotomized rats. A marked increase in the number of galanin-immunoreactive and galanin messenger RNA-positive neuron profiles was observed in ipsilateral dorsal root ganglia in all types of models. The increase in allodynic rats (Gazelius, Seltzer and Bennett models) was less pronounced than that after axotomy. In addition, in the Bennett model the number of galanin-immunoreactive neurons was significantly lower in allodynic rats as compared to non-allodynic rats, and the same tendency, but less obvious was found in the Seltzer model. Furthermore, an increase in galanin-immunoreactive fibres was found in the superficial laminae of the ipsilateral dorsal horn in all lesion models, especially in lamina II. A dramatic increase in the number of neuropeptide Y and neuropeptide Y messenger RNA-positive neuron profiles was also found in the ipsilateral dorsal root ganglia in all models, but no significant difference was found in peptide levels between allodynic and non-allodynic rats in any of the models. The present results suggest that the levels of endogenous galanin may play a role in whether or not allodynia develops in the Bennett model.

Galanin and galanin receptors

The development of a strain of galanin knockout mice has provided confirmation of a neuroendocrine role for galanin, as well as supporting results of previous physiological investigations indicating a role for galanin in analgesia and neuropathic pain, and potentially in neuronal growth and regeneration processes. Whether elevation of galanin expression in neurodegenerative disorders such as Alzheimer's disease represents a survival response or exacerbates functional deficit in afflicted individuals remains to be determined. More detailed analysis of the phenotype of the galanin knockout mouse should provide insights into the physiological role of galanin in memory and learning processes, as well as in hypothalamic function and other aspects of neuroendocrine regulation. Biochemical and molecular cloning efforts have demonstrated that the multiplicity of actions of galanin is matched by complexity in the distribution and regulation of galanin and its receptors. A focus on characterisation of galanin receptors has resulted in the molecular cloning of three receptor subtypes to date. The distribution and functional properties of these receptors have not yet been fully elucidated, currently precluding assignment of discrete functions of galanin to any one receptor subtype. It is not currently possible to reconcile available pharmacological data using analogs of galanin and chimeric peptides in functional assay systems with the pharmacological properties of cloned receptor subtypes. This highlights the value of further knockout approaches targeting galanin receptor subtypes, but also raises the possibility of the existence of additional receptor subtypes that have yet to be cloned, or that receptor activity may be modulated by regulatory molecules that remain to be identified. The development of receptor subtype-specific compounds remains a high priority to advance work in this area. The ability to selectively modulate the many different actions of galanin, through a clearer understanding of receptor structure-function relationships and neuronal distribution, promises to provide important insights into the molecular and cellular basis of galanin action in normal physiology, and may provide lead compounds with therapeutic application in the prevention and treatment of a range of disorders.

Intra-periaqueductal grey injection of galanin increases the nociceptive response latency in rats, an effect reversed by naloxone

The nociceptive response latencies were increased significantly after intra-periaqueductal grey (PAG) administration of 1.0 or 3.0 nmol of galanin, but not 0.3 nmol, in rats. The effect of galanin was attenuated by following injection of 5.5 nmol of naloxone into PAG. These results indicate an anti-nociceptive role of galanin, and a possible interaction between galanin and opioid peptides in PAG in rats.

Effects of intrathecal galanin on nociceptive responses in rats with mononeuropathy

The present study was performed on rats with experimental mononeuropathy induced by left common sciatic nerve loose ligation. Unilateral sciatic nerve loose ligation induced decreases of the hindpaw withdrawal latency to the hot-plate test, cold-plate test and the Randall Selitto test. Sciatic nerve loose ligation induced hyperesponsiveness to touch at room temperature also. Intrathecal administration of either 3 or 6 nmol of galanin, but not 1 nmol, induced significant bilateral increases in hindpaw withdrawal latencies to the hot-plate test, cold-plate test and the Randall Selitto tests in rats with left mononeuropathy. The results indicate that galanin may play important roles in transmission of presumed nociceptive information in the spinal cord of mononeuropathic rats.

Discovery of a receptor related to the galanin receptors

We report the isolation of a cDNA clone named GPR54, which encodes a novel G protein-coupled receptor (GPCR). A PCR search of rat brain cDNA retrieved a clone partially encoding a GPCR. In a library screening this clone was used to isolate a cDNA with an open reading frame (ORF) encoding a receptor of 396 amino acids long which shared significant identities in the transmembrane regions with rat galanin receptors GalR1 (45%), GalR3 (45%) and GalR2 (44%). Northern blot and in situ hybridization analyses revealed that GPR54 is expressed in brain regions (pons, midbrain, thalamus, hypothalamus, hippocampus, amygdala, cortex, frontal cortex, and striatum) as well as peripheral regions (liver and intestine). In COS cell expression of GPR54 no specific binding was observed for 125I-galanin. A recent BLAST search with the rat GPR54 ORF nucleotide sequence recovered the human orthologue of GPR54 in a 3.5 Mb contig localized to chromosome 19p13.3.

Intrathecal galanin alleviates allodynia-like behaviour in rats after partial peripheral nerve injury

We have previously suggested that the neuropeptides galanin and galanin message-associated peptide (GMAP) may have an inhibitory role in spinal nociception. The present study examined the effects of intrathecal (i.t.) administration of these two peptides on allodynia-like behaviours in response to mechanical and cold stimulation in rats after photochemically induced ischaemic peripheral nerve injury. I.t. galanin significantly alleviated the mechanical- and cold-allodynia-like behaviours in nerve injured rats, and was not associated with motor impairment or sedation. I.t. GMAP relieved mechanical allodynia much less than galanin. I.t. M-35, a high-affinity galanin receptor antagonist, did not significantly alter the response of the rats to mechanical or cold stimulation. At 1 or 2 weeks postinjury, around 15% of dorsal root ganglion (DRG) neuron profiles showed galanin-like immunoreactivity. These profiles were mostly small sized. Although the number of galanin positive cells was thus increased in the DRG in the present model, the increase was substantially less than after complete sciatic nerve section, as previously shown. The present results showed that spinal administration of galanin inhibited some abnormal pain-like behaviours in rats after partial peripheral nerve injury. These results further support an inhibitory function for galanin in nociception. However, endogenous galanin may not play a significant role in suppressing nociceptive input after partial ischaemic peripheral nerve injury, as the upregulation of galanin is moderate.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

Galanin in somatosensory function

Galanin-like immunoreactivity and galanin receptors are found in dorsal root ganglion (DRG) cells and in dorsal horn interneurons, suggesting that this neuropeptide may have a role in sensory transmission and modulation at the spinal level. Expression of galanin or galanin receptors in the DRG and spinal cord are altered, sometimes in a dramatic fashion, by peripheral nerve injury or inflammation. Under normal conditions, galanin occurs in a small population of primary sensory neurons as well as in spinal interneurons. However, following peripheral nerve injury or inflammation, expression of galanin in primary afferents and spinal cord is upregulated. We examined the role of galanin in spinal processing of nociceptive information under normal and pathologic conditions in a large series of electrophysiologic and behavioral studies. Results suggest that under normal conditions galanin exerts tonic inhibition of nociceptive input to the central nervous system. After peripheral nerve injury the inhibitory control exerted by endogenous galanin, probably released from DRG neurons, is increased. During inflammation, galanin presumably released from dorsal horn interneurons also exerts an inhibitory function. Thus, stable galanin agonists may be useful in the treatment of inflammatory and neuropathic pain.

Galanin expression in neuropathic pain: friend or foe?

We investigated a possible link between galanin expression and evoked pain accompanying painful partial sciatic nerve lesions. Increased galanin immunoreactivity (IR) in the dorsal horn, in gracile nucleus, and in sensory neurons following chronic constriction injury (CCI) compared to complete sciatic transection suggested a facilitatory role in thermal and mechanical hypersensitivity (allodynia). We therefore investigated the effects of endogenous interleukin-6 (IL-6) and nerve growth factor (NGF) on allodynia and neuropeptide expression. IL-6 knockout mice showed decreased allodynia and galanin-IR compared to wild-type mice, but also decreased substance P (SP)-IR in the dorsal horn. Anti-NGF-treated rats with CCI also showed decreased allodynia and SP-IR, but increased galanin-IR in the dorsal horn. These results suggest that evoked pain is more tightly linked to SP than to galanin expression. If galanin's effects are inhibitory as the bulk of the literature suggests, its effects are subordinate to those of SP and to other changes following CCI.

Cloning and evaluation of the role of rat GALR-2, a novel subtype of galanin receptor, in the control of pain perception

We have identified a novel subtype of galanin receptor (GALR-2) in rat dorsal root ganglia and spinal cord. The open reading frame of GALR-2 is 1116 nucleotides long, encoding a protein of 372 amino acids with a theoretical molecular mass of 40.7 kD. Membranes prepared from stable pools of 293 cells expressing GALR-2, but not wild-type 293 cells, demonstrated high affinity galanin binding sites. Rat galanin and galanin-related peptides M40, C7, M15, and galanin effectively competed for binding; peptide C7 demonstrated a lower affinity for rGALR-2, and all these peptides were agonists at rGALR-2 when assessed on a microphysiometer. Studies on the expression of GALR-2 in various tissues by Northern and in situ hybridization analyses suggest a low abundance but wide distribution of GALR-2 mRNA, including several discrete areas in brain and spinal cord and a high abundance in the dorsal root ganglia.