The therapeutic potential of galanin in the management of pain - a review article

Neuropeptides represent the most diverse family of neurotransmitters counting numerous members and even more G protein-coupled receptors, all of which are potential targets for drug development. Here, we focus on galanin and its three receptors by describing their possible involvement in pain and regeneration. Although animal experiments indicate that galanin, together with other molecules, may act as an endogenous system protecting against pain and improving nerve growth, these results have so far not been translated into patient treatments.

Sinomenine facilitates the efficacy of gabapentin or ligustrazine hydrochloride in animal models of neuropathic pain

Management of chronic pain is restricted by the lack of effective tools. This study evaluated the efficacies of sinomenine combined gabapentin or ligustrazine hydrochloride in treating peripheral and central chronic neuropathic pain. The study was conducted in mice with photochemically induced sciatic nerve injury, and in rats with photochemically induced spinal cord injury. For assessing the effectiveness of combined therapy, sinomenine, gabapentin or ligustrazine hydrochloride was injected intraperitoneally (i.p.), and pain behavioral tests were performed. At sub-effective dosages, pre-administration of sinomenine (for 60 min) plus gabapentin or ligustrazine hydrochloride, generated significant anti-allodynic effects in mice or rats with peripheral or central neuropathic pain. However, these effects were abolished when gabapentin or ligustrazine hydrochloride was pre-administered, and then sinomenine was given 60 min later. The combined efficacies of sinomenine and gabapentin or ligustrazine hydrochloride, cannot be blocked or reversed by the naloxone, suggesting the underlying mechanism is not mediated by opioid receptors. Moreover, following repeated treatments, sinomenine and gabapentin combination increased the baseline mechanical threshold, while generating prolonged analgesia, without introducing notable side effects. Sinomenine can enhance the efficacy of gabapentin or ligustrazine hydrochloride in rodent models of peripheral or central neuropathic pain, without introducing tolerance or other notable side effects. Findings of current study suggest that combing sinomenine and gabapentin or ligustrazine hydrochloride could be highly beneficial in neuropathic pain therapies.

Gabapentin and NMDA receptor antagonists interacts synergistically to alleviate allodynia in two rat models of neuropathic pain

Background and aims

The clinical management of neuropathic pain remains a challenge. We examined the interaction between gabapentin and NMDA receptor antagonists dextromethrophan and MK-801 in alleviating neuropathic pain-like behaviors in rats after spinal cord or sciatic nerve injury.

Methods

Female and male rats were produced with Ischemic spinal cord injury and sciatic nerve injury. Gabapentin, dextromethorphan, MK-801 or drug combinations were injected with increasing doses. Mechanical response thresholds were tested with von Frey hairs to graded mechanical touch/pressure, and ethyl chloride spray was applied to assess the cold sensitivity before and after injuries.

Results

In spinally injured rats, gabapentin and dextromethorphan did not affect allodynia-like behaviors at doses of 30 and 20 mg/kg, respectively. In contrast, combination of 15 or 30 mg/kg gabapentin with dextromethorphan at 10 mg/kg produced total alleviation of allodynia to mechanical or cold stimulation. Further reducing the dose of gapapentin to 7.5 mg/kg and dextromethorphan to 5 mg/kg still produced significant effect. MK-801, another NMDA receptor antagonist, also enhanced the effect of gabapentin in spinally injured rats. Similar synergistic anti-allodynic effect between dextromethorphan and gabapentin was also observed in a rat model of partial sciatic nerve injury. No increased side effect was seen following the combination between gabapentin and dextromethorphan.

Conclusions

In conclusion, the present study suggested that combining NMDA receptor antagonists with gabapentin could provide synergistic effect to alleviate neuropathic pain and reduced side effects.

Implications

Combining NMDA receptor antagonists with gabapentin may provide a new approach in alleviating neuropathic pain with increased efficacy and reduced side effects.

Delayed Imatinib Treatment for Acute Spinal Cord Injury: Functional Recovery and Serum Biomarkers

With no currently available drug treatment for spinal cord injury, there is a need for additional therapeutic candidates. We took the approach of repositioning existing pharmacological agents to serve as acute treatments for spinal cord injury and previously found imatinib to have positive effects on locomotor and bladder function in experimental spinal cord injury when administered immediately after the injury. However, for imatinib to have translational value, it needs to have sustained beneficial effects with delayed initiation of treatment, as well. Here, we show that imatinib improves hind limb locomotion and bladder recovery when initiation of treatment was delayed until 4 h after injury and that bladder function was improved with a delay of up to 24 h. The treatment did not induce hypersensitivity. Instead, imatinib-treated animals were generally less hypersensitive to either thermal or mechanical stimuli, compared with controls. In an effort to provide potential biomarkers, we found serum levels of three cytokines/chemokines--monocyte chemoattractant protein-1, macrophage inflammatory protein (MIP)-3α, and keratinocyte chemoattractant/growth-regulated oncogene (interleukin 8)--to increase over time with imatinib treatment and to be significantly higher in injured imatinib-treated animals than in controls during the early treatment period. This correlated to macrophage activation and autofluorescence in lymphoid organs. At the site of injury in the spinal cord, macrophage activation was instead reduced by imatinib treatment. Our data strengthen the case for clinical trials of imatinib by showing that initiation of treatment can be delayed and by identifying serum cytokines that may serve as candidate markers of effective imatinib doses.

Sinomenine alleviates mechanical hypersensitivity in mice with experimentally induced rheumatoid arthritis

Background and aims

We have previously reported that sinomenine, an alkaloid isolated from the root of the plant Sinomenium acutum, had antinociceptive effect in rodent models of acute inflammatory or neuropathic pain. As a traditional medicine, sinomenine is used in China to treat rheumatoid arthritis (RA).

Methods

In the present study, we evaluated the potential antinociceptive effect of sinomenine in a mouse model of RA, collagen type II antibody (CII Ab) induced arthritis (CAIA) after acute and chronic administration.

Results

As single administration, sinomenine at 40 or 80 mg/kg significantly reduced mechanical hypersensitivity both at the time of peak joint inflammation (days 11–19 after CII Ab injection) or during the post-inflammatory phase (days 35–54). No tolerance to the effect of 80 mg/kg sinomenine was observed during repeated injection twice a day for 5 days from day 11 to day 19 or from day 49 to day 53 after CII Ab injection in CAIA mice.

Conclusions

We have shown that sinomenine is effective in alleviating localized and spread hypersensitivities in CAIA mice both during acute inflammation and in post-inflammatory phase. Further, repeated sinomenine administration has elevated the baseline mechanical threshold without producing tolerance.

Implications

Sinomenine may be clinically useful to treat chronic pain in RA, including wide-spread pain which appears to be a difficult clinical problem despite the improvement in the acute treatment of RA by disease modifying agents.

Repeated sinomenine administration alleviates chronic neuropathic pain-like behaviours in rodents without producing tolerance

Background and aims We have previously reported that systemic administration of sinomenine produced antinociception in various experimental pain conditions in rodents, particularly in models of neuropathic pain. In the present study we assessed the effects of repeated administration of sinomenine in two rodent models of neuropathic pain in order to study the development of tolerance. Methods The analgesic effect of sinomenine was tested in female Sprague-Dawley rats that exhibited mechanical and cold hypersensitivity following ischaemic injury to the spinal cord and in male C57/BL6 mice that developed mechanical hypersensitivity after ischaemic injury to the sciatic nerve. Briefly, the animals were anaesthetized and injected i.v. with the photosensitizing dye erythrosine B. Vertebral segments T12 to T13 in rats or the sciatic nerve in mice were exposed and irradiated under an argon ion laser for 10min or 45s, respectively. In rats, mechanical hypersensitivity to pressure with von Frey hairs, the response to brushing and decreasing cold temperature were tested in the flanks or upper back areas. In mice, mechanical hypersensitivity on the hind paw to von Frey hairs and response to cold following a drop of acetone were measured. Sinomenine was administered i.p. in rats and p.o. in mice at 10:00 and 16:00, twice a day for 5 days. Response threshold before and 2h after drug administration at 10.00h was recorded. Results Repeated administration of sinomenine at 10 or 20mg/kg twice a day, doses that have no analgesic effect as single injection, alleviated mechanical, but not cold allodynia in spinally injured rats and the effect was maintained during the 5 day treatment period with no signs of tolerance. Furthermore, the pre-drug response threshold was significantly elevated during repeated treatment with 20mg/kg sinomenine. Sinomenine administered at 40mg/kg twice a day for 5 days significantly reduced mechanical and cold alldoynia, elevated pre-drug response threshold without tolerance development in spinally injured rats. Similarly, sinomenine at 80mg/kg twice a day for 5 days significantly reduced mechanical allodynia in mice with sciatic nerve injury and increased pre-drug response threshold with no sign of tolerance. The effect of sinomenine on response threshold persisted for days after termination of the 5 day drug administration. Conclusions The results suggest that repeated administration of simomenine produced an enhanced anti-allodynic effect without tolerance in rodent models of neuropathic pain. Implications Sinomenine may be tested as a novel analgesic in treating some forms of chronic neuropathic pain in patients.

Activation of TRPM8 cold receptor triggers allodynia-like behavior in spinally injured rats

Aims

Pain in response to innocuous cold stimulation (cold allodynia) is a common symptom in patients with neuropathic pain. Cold allodynia is difficult to treat and its mechanisms are poorly understood. Several transient receptor potential (TRP) channels have been shown to be the molecular sensors for cold stimulation in a temperature-dependent manner, but the contribution of various TRP channels in mediating cold allodynia in neuropathic pain is unclear. We have previously shown that spinally injured rats developed neuropathic pain-like behaviors, including marked cold allodynia. We now assessed the role of TRP channels in mediating cold allodynia in rats after ischemic spinal cord injury.

Methods

Methods: Spinal cord injury was produced using a photochemical method. The mechanical allodynia was assessed by examining the vocalization thresholds to graded mechanical touch/pressure applied with von Frey hairs. Temperature controlled cold stimulation was produced by a Peltier thermode (active surface 25 mm × 50 mm) connected to a MSA Thermal Simulator (Somedic, Sweden) with baseline temperature of 32 °C. The rate of temperature change was 0.5 °C/s. The temperature required to elicit cold allodynia was examined. The responses of the rats to topical application of icilin or menthol, agonists of transient receptor potential melastain 8 (TRPM8), were also studied.

Results

Normal rats did not exhibit nociceptive responses to cooling stimulation to the trunk and back area (minimal temperature +6°C) and they also did not react aversively to topical application of icilin or menthol. After spinal cord injury, the rats developed mechanical allodynia at the trunk and back just rostral to the dermatome of the injured spinal segments. In the same area, rats exhibited significant nociceptive responses to cooling from day 1 after injury, lasting for at least 70 days which is the longest time of observation. For the first two weeks after injury, the majority of spinally injured rats had a nociceptive response to cooling above 17°C. At day 70, about 50% of rats responded to cooling above 17 °C. Topical application of 400 μM icilin or 4mM menthol also elicited pain-like responses in spinally injured rats and these two cold mimetics also significantly exacerbated existing mechanical allodynia.

Conclusion

Our results showed that activation of the TRPM8 channel by menthol or icilin triggers allodynia in spinally injured rats and increases, rather than decreases, mechanical allodynia. TRPM8 channels which respond to cooling above 17 ° C may be involved at least in part in mediating cold allodynia in the rat model of neuropathic spinal cord injury pain.

Implications

The work introduced a method of quantitative testings of responses of rats to cold stimulation and may contribute to the understanding of mechanisms of cold allodynia after injury to the nervous system.

Genetic and sex influence on neuropathic pain-like behaviour after spinal cord injury in the rat

BACKGROUND

Chronic pain of neuropathic nature after spinal cord injury (SCI) is common and its underlying mechanisms are poorly understood. Genes, as well as sex, have been implicated, but not thoroughly investigated in experimental genetic models for complex traits. We have previously found that inbred Dark-Agouti (DA) rats develop more severe SCI pain-like behaviour than a major histocompatibility complex-congenic Piebald Virol Glaxo (PVG)-RT1(av1) strain in a model of photochemically induced SCI.

METHODS

In this study, a genome-wide linkage study in an F2 cross between the susceptible DA and resistant PVG-RT1(av1) strains was performed in order to explore the influence of genes and sex for SCI pain.

RESULTS

A consistent finding was that female rats in parental, F1 and F2 generations displayed increased pain sensitivity at testing before injury and also developed mechanical hypersensitivity more rapidly and to a greater extent than male rats. In addition, we could identify three quantitative trait loci (QTLs) associated with pain-like behaviour: a sex-specific QTL on chromosome 2, one on chromosome 15 and on chromosome 6. Animals carrying DA alleles at each of these loci were more susceptible to development of mechanical hypersensitivity compared with rats with PVG alleles.

CONCLUSION

This is the first whole genome QTL mapping of neuropathic pain-like behaviour in a model of SCI. The results provide strong support for a significant genetic and sex component in development of pain after SCI and provide the basis for further genetic dissection and positional cloning of the underlying genes.

Comparison of the anti-allodynic and antinociceptive effects of systemic; intrathecal and intracerebroventricular morphine in a rat model of central neuropathic pain

The present study has assessed the efficacy and potency of systemic, intrathecal (i.t.) and intracerebroventricular (i.c.v.) morphine in alleviating a chronic mechanical allodynia-like behaviour in a rat model of central pain after spinal cord injury. The anti-allodynic potency of morphine was compared to its antinociceptive potency in both spinally injured and normal rats. Systemic (intraperitoneal or subcutaneous) morphine did not significantly relieve allodynia up to 3 mg/kg cumulative dose, which was sedative. Mechanical allodynia-like behaviour was only significantly relieved by l0 mg/kg morphine which caused severe sedation. Low doses of i.c.v. morphine abolished vocalization of allodynic rats to von Frey hair stimulation. However, other components of abnormal reactions to innocuous mechanical stimulation, such as agitation, jumping and avoidance, were only relieved by i.c.v. morphine at high, sedative doses. In contrast, i.t. morphine dose dependently reversed all components of the allodynic reactions without causing sedation. Morphine administered by all three routes produced powerful antinociception in the tail flick test in normal rats. The antinociceptive potency of morphine was reduced after systemic, i.t. and, particularly, i.c.v. administration in allodynic rats. These results indicated that i.t. morphine relieved the mechanical allodynia-like responses in spinally injured rats, whereas systemic or i.c.v. morphine only increased the response threshold to innocuous mechanical stimulation at high doses, which was associated with sedation, making it difficult to assess the anti-allodynic effect. However, i.c.v. morphine may be particularly effective for the affective component of pain in the present model. The antinociceptive potency of morphine was also reduced in spinally injured rats. It is suggested that morphine may be differentially effective against different types of pain after different routes of administration. Spinal morphine may be a therapeutic alternative in treating central pain after spinal cord injury.

Heparanase overexpression reduces carrageenan-induced mechanical and cold hypersensitivity in mice

Heparanase controls the structure and functions of extracellular matrix (ECM) by degrading heparan sulfate proteoglycans. Heparanase is involved in inflammatory process through modulating the functions of inflammatory cytokines. The present study aimed to find out whether overexpression of heparanase in mice affects carrageenan-induced localized inflammation and inflammatory hyperalgesia. Without challenge, the heparanase overexpression did not significantly affect the mice in response to mechanical, cold and heat stimulation. Unilateral subcutaneous administration of carrageenan produced hypersensitivity to mechanical and cold in both wildtype and the heparanase overexpression (Hpa-tg) mice 24h after treatment. In comparison to wildtype animals, the Hpa-tg mice showed significantly reduced mechanical and cold hypersensitivity. This may, at least partially, due to the reduced mast cell infiltration at the site of inflammation in Hpa-tg mice. These data support a role for heparanase that reduces localized inflammation and inflammatory hyperalgesia in mice.

Peripheral adenosine A2A receptors are involved in carrageenan-induced mechanical hyperalgesia in mice

Here we studied the role of peripheral adenosine A(2A) receptors in mechanical hyperalgesia during inflammation using mice lacking the A(2A) receptors. Unilateral s.c. administration of the local inflammatory agent λ-carrageenan induced profound mechanical hyperalgesia 24 h after administration in the ipsilateral hind paw in wild-type mice. In homozygous mice lacking the A(2A) receptors, carrageenan-induced hyperalgesia was significantly reduced compared to wild type controls. The reduction in inflammatory hyperalgesia seen in A(2A) receptor knock-out mice was not associated with changes in paw edema. CGS 21680, a selective A(2A) receptor agonist, produced significantly more mechanical hyperalgesia in wild type females than in wild type males upon direct s.c. injection into the hindpaw whereas it had no effect upon systemic administration. The hyperalgesic effect of CGS 21680 was markedly reduced in the A(2A) knock-out mice of both sexes. Subcutaneous ZM-241,385, a selective A(2A) receptor antagonist, injected into the hindpaw reduced the mechanical hyperalgesia following carrageenan in female mice, but not in males. The results indicate that activation of peripheral adenosine A(2A) receptors during inflammation is associated with mechanical hyperalgesia, and that this effect is more prominent in females than in males.

Multipotent mesenchymal stromal cells attenuate chronic inflammation and injury-induced sensitivity to mechanical stimuli in experimental spinal cord injury

PURPOSE

Previous reports established that after a contusion injury to the rat spinal cord, locomotor function was enhanced by the transplantation of cells from bone marrow referred to as either mesenchymal stem cells or multipotent mesenchymal stromal cells (MSCs). It has also been established that neural stem cells (NSCs) enhance locomotor function after transplantation into the injured rat spinal cord. However, the beneficial effects of NSCs are limited by graft-induced allodynia-like responses. Little is known about the effects of MSCs on sensory function in spinal cord injury. Therefore, the objective of this research was to determine whether transplantation of MSCs into the injured rat spinal cord induces allodynia-like responses.

METHODS

Contusion injuries of two different severities were induced in rats to examine the effects of transplantation with MSCs on sensorimotor deficits. The effects of MSCs on chronic inflammation were investigated, since inflammation is reported to have a role in the sensorimotor deficits associated with spinal cord injury. In addition, observations in other models suggest that MSCs possess immunosuppressive effects.

RESULTS

We found that in contrast to previous observations with the transplantation of neural stem cells, transplantation of MSCs did not induce allodynia. MSCs attenuated injury-induced sensitivity to mechanical stimuli but had no effect on injury-induced sensitivity to cold stimuli. MSCs also significantly attenuated the chronic inflammatory response as assayed by GFAP immunoreactivity for reactive astrocytes and ED1 immunoreactivity for activated macrophages/microglia. In addition, transplantation of MSCs increased white matter volumes and decreased cyst size in sections of the cord containing the lesion.

CONCLUSION

The results suggest that the sensorimotor enhancements produced by MSCs can at least in part be explained by anti-inflammatory/immunosuppressive effects of the cells, similar to such effects of these cells observed in other experimental models.

Correlational analysis for identifying genes whose regulation contributes to chronic neuropathic pain

BACKGROUND

Nerve injury-triggered hyperexcitability in primary sensory neurons is considered a major source of chronic neuropathic pain. The hyperexcitability, in turn, is thought to be related to transcriptional switching in afferent cell somata. Analysis using expression microarrays has revealed that many genes are regulated in the dorsal root ganglion (DRG) following axotomy. But which contribute to pain phenotype versus other nerve injury-evoked processes such as nerve regeneration? Using the L5 spinal nerve ligation model of neuropathy we examined differential changes in gene expression in the L5 (and L4) DRGs in five mouse strains with contrasting susceptibility to neuropathic pain. We sought genes for which the degree of regulation correlates with strain-specific pain phenotype.

RESULTS

In an initial experiment six candidate genes previously identified as important in pain physiology were selected for in situ hybridization to DRG sections. Among these, regulation of the Na+ channel alpha subunit Scn11a correlated with levels of spontaneous pain behavior, and regulation of the cool receptor Trpm8 correlated with heat hypersensibility. In a larger scale experiment, mRNA extracted from individual mouse DRGs was processed on Affymetrix whole-genome expression microarrays. Overall, 2552 +/- 477 transcripts were significantly regulated in the axotomized L5DRG 3 days postoperatively. However, in only a small fraction of these was the degree of regulation correlated with pain behavior across strains. Very few genes in the "uninjured" L4DRG showed altered expression (24 +/- 28).

CONCLUSION

Correlational analysis based on in situ hybridization provided evidence that differential regulation of Scn11a and Trpm8 contributes to across-strain variability in pain phenotype. This does not, of course, constitute evidence that the others are unrelated to pain. Correlational analysis based on microarray data yielded a larger "look-up table" of genes whose regulation likely contributes to pain variability. While this list is enriched in genes of potential importance for pain physiology, and is relatively free of the bias inherent in the candidate gene approach, additional steps are required to clarify which transcripts on the list are in fact of functional importance.

Both MHC and non-MHC genes regulate development of experimental neuropathic pain in rats

We have previously demonstrated that differences in neuropathic pain-like behaviors after sciatic nerve injury genetically maps to the major histocompatibility complex (MHC) in rats carrying RT1(c) or RT1(av1) haplotypes on the Piebald Virol Glaxo (PVG) background. In order to further explore the genetic contribution to neuropathic pain, we have here examined the MHC-congenic rat strains PVG-RT1(n) and PVG-RT1(av1) and the inbred strains PVG (RT1(c)) and Brown-Norway (BN; RT1(n)). All studied strains developed mechanical hypersensitivity (allodynia-like behavior) of the hind paw after photochemically induced sciatic nerve injury. However, the PVG-RT1(n) and PVG strains displayed significantly more allodynia than PVG-RT1(av1) and BN rats. In addition, the BN strain demonstrated an elevated threshold for the baseline response. The results demonstrate that both MHC and non-MHC genes influence experimental neuropathic pain in rats and also suggest that allelic variation contained in the RT1(av1) haplotype on the PVG background protects against neuropathic pain.

Galanin and spinal pain mechanisms: where do we stand in 2008?

Since the discovery of galanin in 1983, one of the most frequently mentioned possible physiological functions for this peptide is spinal pain modulation. This notion, initially based on the preferential presence of galanin in dorsal spinal cord, has been supported by results from a large number of morphological, molecular and functional studies in the last 25 years. 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 inhibitory role endogenously, 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, raising the hope that drugs affecting galaninergic transmission may be used as analgesics.

Contrasting genetic effects of major histocompatibility complex on ischemic peripheral nerve and spinal cord injury in female rats

We have recently shown that the major histocompatibility complex (MHC) exerts a regulatory influence on the development of neuropathic pain-like behaviors after partial sciatic nerve injury in male rats. In the present study, we assessed the role of the MHC in peripheral nerve injury-induced pain as well as central pain following spinal cord injury in female rats using the following inbred strains: Dark Agouti (DA; RT1(av1)), Piebald Virol Glaxo (PVG; RT1(c)) and in the MHC-congenic strain PVG-RT1(av1). In line with our previous observation in male rats, PVG-RT1(c) displayed more severe allodynia compared to the strains carrying the RT1(av1) haplotype (PVG-RT1(av1) and DA-RT1(av1)) following sciatic nerve injury in female rats. However, the MHC did not seem to impact the development of allodynia following spinal cord injury since the two congenic strains PVG-RT1(c) and PVG-RT1(av) did not differ after spinal cord injury. Interestingly, the DA-RT1(av1) strain displayed significantly more severe allodynia than both PVG strains and this difference was not explained by the extent of spinal cord injury. These results suggest that there are differences in the genetic mechanisms for neuropathic pain development following peripheral or central nervous system injury, both in regarding to the role of the MHC complex as well as non-MHC genes.

Genetic analysis of neuropathic pain-like behavior following peripheral nerve injury suggests a role of the major histocompatibility complex in development of allodynia

Neuropathic pain is a common consequence of damage to the nervous system. We here report a genetic analysis of development of neuropathic pain-like behaviors after unilateral photochemically-induced ischemic sciatic nerve injury in a panel of inbred rat strains known to display different susceptibility to autoimmune neuroinflammation. Pain behavior was initially characterized in Dark-Agouti (DA; RT1(av1)), Piebald Virol Glaxo (PVG; RT1(c)), and in the major histocompatibility complex (MHC)-congenic strain PVG-RT1(av1). All strains developed mechanical hypersensitivity (allodynia) following nerve injury. However, the extent and duration of allodynia varied significantly among the strains, with PVG displaying more severe allodynia compared to DA rats. Interestingly, the response of PVG-RT1(avRT1) was similar to that of DA, suggesting regulation by the MHC locus. This notion was subsequently confirmed in an F2 cohort derived from crossing of the PVG and PVG-RT1(av1)strains, where allodynia was reduced in homozygous or heterozygous carriers of the RT1(av1) allele in comparison to rats homozygous for the RT1(c) allele. These results indicate that certain allelic variants of the MHC could influence susceptibility to develop and maintain neuropathic pain-like behavior following peripheral nerve injury in rats.

Spontaneous pain following spinal nerve injury in mice

Autotomy behavior is frequently observed in rats and mice in which the nerves of the hindlimb are severed, denervating the paw. This is the neuroma model of neuropathic pain. A large body of evidence suggests that this behavior reflects the presence of spontaneous dysesthesia and pain. In contrast, autotomy typically does not develop in partial nerve injury pain models, leading to the belief that these animals develop hypersensibility to applied stimuli (allodynia and hyperalgesia), but not spontaneous pain. We have modified the widely used Chung (spinal nerve ligation [SNL]) model of neuropathic pain in a way that retains the fundamental neural lesion, but eliminates nociceptive sensory cover of the paw. These animals performed autotomy. Moreover, the heritable across strains predisposition to spontaneous pain behavior in this new proximal denervation model (SNN) was highly correlated with pain phenotype in the neuroma model suggesting that the pain mechanism in the two models is the same. Relative reproducibility of strain predispositions across laboratories was verified. These data indicate that the neural substrate for spontaneous pain is present in the Chung-SNL model, and perhaps in the other partial nerve injury models as well, but that spontaneous pain is not expressed as autotomy in these models because there is protective nociceptive sensory cover.

Lacosamide, a new anti-epileptic, alleviates neuropathic pain-like behaviors in rat models of spinal cord or trigeminal nerve injury

The effect of systemic administration of lacosamide, a newly developed anti-epileptic, on neuropathic pain-like behaviors was examined in rats after ischemic injury to the infraorbital nerve or spinal cord using a photochemical method. In rats with infraorbital nerve injury, lacosamide reduced mechanical hypersensitivity and the effect was markedly stronger in female than in male rats. In spinal cord injured female rats 10-20 mg/kg lacosamide dose-dependently alleviated the mechanical and cold allodynia-like behaviors without causing motor impairments or marked sedation. Administration of lacosamide twice daily at 20 mg/kg for 7 days totally alleviated the allodynia-like state in spinally-injured rats with no tolerance. Following treatment cessation the cold and the static allodynia reappeared but the effect on dynamic mechanical allodynia (brushing) was maintained until day 11. Lacosamide also produced hypothermia at antinociceptive doses in rats. It is suggested that this novel compound may be useful as an analgesic for treating central and trigeminal neuropathic pain. Furthermore, there may be a gender difference to the effect of lacosamide with female rats being more responsive to the treatments.

Effect of acute and chronic administration of caffeine on pain-like behaviors in rats with partial sciatic nerve injury

Caffeine, used in many pain medications as an adjuvant analgesic, is an adenosine A1 and A2A receptor antagonist. Here we examined the effects of acute or chronic caffeine administration in rats after partial sciatic nerve injury. The hindpaw response to mechanical or cold stimulation was assessed following photochemically induced sciatic nerve injury which leads to hypersensitivity to these stimuli. Caffeine was administered i.p. acutely or in the drinking water chronically. The mechanical and cold hypersensitivity of sciatic nerve-injured rats was dose-dependently alleviated by acute systemic administration of caffeine (10-80 mg/kg). The effect of caffeine was, however, associated with side effects including locomotor stimulation or depression. Chronic oral administration (average daily doses 27.5 mg/kg/day or 61.5 mg/kg/day for 2 weeks) of caffeine starting at the time of nerve injury did not significantly affect the development of pain-like behaviors. Thus, acute, but not long term, caffeine intake reduced neuropathic pain state in nerve-injured rats, but only at very high doses. The potential hyperalgesic effect of chronic A1 adenosine receptor blockade may have been compensated for by an antinociceptive effect of caffeine through antagonism of A2A receptors and tolerance development.

Phenotyping of sensory and sympathetic ganglion neurons of a galanin-overexpressing mouse--possible implications for pain processing

The distribution of galanin was studied in the lumbar 5 dorsal root ganglia (DRGs) and spinal cord, superior cervical ganglia (SCGs), and skin of transgenic mice overexpressing galanin under the dopamine beta-hydroxylase (DBH) promoter (GalOE-DBH mice) and in wild type (WT) mice. The DRGs and spinal cord were analysed before and after a unilateral, complete transection (axotomy) of the sciatic nerve and after dorsal rhizotomy. Both galanin protein and transcript were studied by, respectively, immunohistochemistry and in situ hybridization. Increased galanin expression was observed in several small, medium-sized and large DRG neuron profiles (NPs) in the naïve transgenic mouse, frequently in neurons lacking calcitonin gene-related peptide (CGRP) and isolectin B4-binding. This lack of coexistence was particularly evident in the medium-sized/large NPs. In the dorsal horn of the spinal cord, no differences were detected between GalOE-DBH and WT mice, both displaying a strong galanin-positive neuropil in the superficial laminae of the dorsal horn, but the transgenic mice showed a more abundant galanin-positive innervation of the ventral horn. A 12-day dorsal rhizotomy, surprisingly, failed to alter the galanin staining patterns in the dorsal (and ventral) dorsal horn. Unilateral axotomy induced upregulation of galanin in DRG NPs of all sizes in both types of mouse. In the hindpaw skin, a profuse galanin-positive fiber plexus was observed in sweat glands and around blood vessels of the transgenic mice, being much more restricted in WT mice. Finally, GalOE mice exhibited a strong galanin-like immunoreactivity in most SCG NPs. The overexpression of the peptide in DRGs and SCGs was paralleled by increased mRNA levels. The present results show that overexpression of galanin under the control of the DBH promoter does not only occur, as expected in these mice, in noradrenline/adrenaline neurons but also in DRG neurons, particularly in large and medium-sized NPs. To what extent and how this overexpression pattern is related to the previously shown elevated pain threshold under normal and lesion conditions is discussed [Grass, S., Crawley, J.N., Xu, X.J., Wiesenfeld-Hallin, Z., 2003a. Reduced spinal cord sensitization to C-fibre stimulation in mice over-expressing galanin. Eur. J. Neurosci. 17, 1829-1832; Hygge-Blakeman, K., Brumovsky, P., Hao, J.X., Xu, X.J., Hökfelt, T., Crawley, J.N., Wiesenfeld-Hallin, Z., 2004. Galanin over-expression decreases the development of neuropathic pain-like behaviour in mice after partial sciatic nerve injury. Brain Res. 1025, 152-158].

Behavioral changes and trigeminal ganglion sodium channel regulation in an orofacial neuropathic pain model

We used a photochemical method to generate a partial ischemic injury to the infraorbital branch of the trigeminal nerve in rats. Following injury, rats developed a bilateral persistent hypersensitivity to mechanical stimulation in the territory innervated by the infraorbital nerve. In addition, spread of mechanical hypersensitivity beyond the facial region was noted. Heat hypersensitivity was also present, although to a lesser extent and of a shorter duration. In some rats, excessive facial grooming/scratching were observed. Morphological examination revealed a graded damage to the irradiated portion of the infraorbital nerve that was related to the duration of laser irradiation. Investigations of gene expression changes in injured trigeminal ganglion neurons of animals with behavioral signs of neuropathic pain demonstrated that the sodium channel alpha-subunit Na(v)1.3-absent in sham-operated animals-was expressed to a limited extent. mRNAs for Na(v)1.8 and Na(v)1.9 were reduced both with respect to proportions of expressing neurons and to intensities, whereas the beta 3 subunit was markedly upregulated. mRNA levels of p11, a regulatory factor that facilitates the surface expression of Na(v)1.8, were unchanged. Previous findings have shown that injury to the trigeminal nerve branches may elicit responses that differ from those of segmental spinal nerves. Despite this we conclude that the key sodium channel regulations that are reported as consequences of nerve damage in the dorsal root ganglia seem to appear also in the trigeminal ganglion. Thus, novel analgesic drugs designed to target the sodium channel subtypes involved could be of use for the treatment of orofacial pain.

Sex differences in pain perception

BACKGROUND

A number of studies have demonstrated a higher prevalence of chronic pain states and greater pain sensitivity among women compared with men. Pain sensitivity is thought to be mediated by sociocultural, psychological, and biological factors.

OBJECTIVE

This article reviews laboratory studies that provide evidence of sex differences in pain sensitivity and the response to analgesic drugs in animals and humans. The biological basis of such differences is emphasized.

METHODS

The literature from this relatively new field was surveyed, and studies that clearly illustrate the differences in pain mechanisms between the sexes are presented. Using the search terms sex, gender, and pain, a review was conducted of English-language literature published on MEDLINE between January 1980 and August 2004.

RESULTS

Although differences in pain sensitivity between women and men are partly attributable to social conditioning and to psychosocial factors, many laboratory studies of humans have described sex differences in sensitivity to noxious stimuli, suggesting that biological mechanisms underlie such differences. In addition, sex hormones influence pain sensitivity; pain threshold and pain tolerance in women vary with the stage of the menstrual cycle. Imaging studies of the brain have shown differences between men and women in the spatial pattern and intensity of response to acute pain. Among rodents, females are more sensitive than males to noxious stimuli and have lower levels of stress-induced analgesia. Male rodents generally have stronger analgesic response to mu-opioid receptor agonists than females. Research on transgenic mice suggests that normal males have a higher level of activity in the endogenous analgesic system compared with normal females, and a human study has found that mu-receptors in the healthy female brain are activated differently from those in the healthy male brain. The response to kappa-opioids, which is mediated by the melanocortin-1 receptor gene in both mice and humans, is also different for each sex.

CONCLUSION

Continued research at the genetic and receptor levels may support the need to develop gender-specific drug therapies.

Galanin and spinal nociceptive mechanisms: recent results from transgenic and knock-out models

Genetically modified mice with transgenic overexpression of galanin or depletion of genes for galanin or galanin receptors have become available, providing a new approach for analyzing the role of galanin in nociception. Mice overexpressing galanin had a moderate heat hypoalgesia, reduced spinal sensitization after repetitive C-fiber stimulation and reduced development of neuropathic pain-like behavior after sciatic nerve injury. On the other hand, mice lacking the GALR1 receptor (Galr1-/-) exhibited only slight increase in heat nociception in the hot plate, but not tail flick, test and showed no increase in spinal sensitization. The duration, but not magnitude, of neuropathic pain-like behaviors, was increased in Galr1-/- mice after nerve injury. These results support an inhibitory effect of galanin on nociception, but the physiological role played by galanin via GALR1 receptors needs to be further studied.

Increased nociceptive response in mice lacking the adenosine A1 receptor

The role of the adenosine A1 receptor in nociception was assessed using mice lacking the A1 receptor (A1R-/-) and in rats. Under normal conditions, the A1R-/- mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A1R-/- mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the mu-opioid ligand DAMGO in the spinal cord. A1R-/- mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A1R-/-mice. Peripheral nerve injury did not change the level of adenosine A1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.

Allodynia limits the usefulness of intraspinal neural stem cell grafts; directed differentiation improves outcome

Several studies have reported functional improvement after transplantation of neural stem cells into injured spinal cord. We now provide evidence that grafting of adult neural stem cells into a rat thoracic spinal cord weight-drop injury improves motor recovery but also causes aberrant axonal sprouting associated with allodynia-like hypersensitivity of forepaws. Transduction of neural stem cells with neurogenin-2 before transplantation suppressed astrocytic differentiation of engrafted cells and prevented graft-induced sprouting and allodynia. Transduction with neurogenin-2 also improved the positive effects of engrafted stem cells, including increased amounts of myelin in the injured area, recovery of hindlimb locomotor function and hindlimb sensory responses, as determined by functional magnetic resonance imaging. These findings show that stem cell transplantation into injured spinal cord can cause severe side effects and call for caution in the consideration of clinical trials.

Comparison of the antinociceptive effect of morphine, methadone, buprenorphine and codeine in two substrains of Sprague-Dawley rats

Sprague-Dawley rats from two different vendors, Möllegård, Denmark and B&K Universal, Sweden, have been tested for the antinociceptive effect of morphine, methadone, buprenorphine and codeine on the hot plate. Morphine and methadone had significantly weaker effect in Möllegård rats compare to B&K rats. In contrast, the effect of buprenorphine was stronger in Möllegård rats than in B&K rats and the effect of codeine was similar in the two substrains. Plasma levels of morphine, morphine-6-glucuronide, morphine-3-glucuronide, buprenorphine and norbuprenorphine were determined at two time points after drug injection. Möllegård rats had significantly lower mean plasma level of morphine and significantly higher ratio of morphine-3-glucuronide/morphine at 30 min, compared to B&K rats. No difference was seen for the metabolism of buprenorphine in the two substrains. The results suggest that Möllegård rats metabolize morphine to morphine-3-glucuronide to a greater extent than B&K rats, and this may at least partly underlie the substrain difference in the effect of morphine. It is also suggested that the antinociceptive mechanisms of buprenorphne may be different from those of morphine and methadone.

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.

High-efficacy 5-HT1A receptor activation causes a curative-like action on allodynia in rats with spinal cord injury

The selective, high-efficacy 5-HT(1A) receptor agonist, (3-chloro-4-fluoro-phenyl)-[4-fluoro-4-[[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl]piperidin-1-yl]-methanone (F 13640) has been reported to produce long-term analgesia in rodent models of chronic nociceptive and neuropathic pain; it also preempts allodynia following spinal cord injury. Here, rats underwent spinal cord injury, fully developed allodynia, and were infused with saline or 0.63 mg/day of F 13640 for 56 days. Infusion was then discontinued, and further assessments of allodynia (vocalization threshold to von Frey filament stimulation, responses to brush and cold) were conducted for another 70 days. F 13640-induced analgesia persisted during this post-treatment period. The data offer initial evidence that high-efficacy 5-HT(1A) receptor activation produces an unprecedented curative-like action on pathological pain.

The very-high-efficacy 5-HT1A receptor agonist, F 13640, preempts the development of allodynia-like behaviors in rats with spinal cord injury

Central neuropathic pain after spinal cord injury (SCI) presents a challenging clinical problem with limited treatment options. [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-([(5-methyl-pyridin-2-ylmethyl)-amino]-methyl)piperidin-1-yl]]-methadone (F 13640) is a recently discovered very-high-efficacy, selective 5-HT1A receptor agonist that produces a remarkably powerful, central analgesia through unprecedented neuroadaptive mechanisms. In a rat model of spinal cord injury pain, we previously found that chronic infusion of F 13640 alleviated pain-like behaviors. Here, we report that infusion of 0.63 mg/day of F 13640 for 8 weeks starting 24 h before the induction of injury significantly attenuates the development of chronic allodynia-like behavior in rats sustaining a photochemically-induced, ischaemic injury of the dorsal laminae of the L3-L5 segments of the spinal cord. Importantly, the preemptive effect of F 13640 persisted for 2 months after treatment was discontinued. The data warrant the study of the possible effects of the early administration of F 13640 in patients sustaining spinal cord injury.

A nitric oxide (NO)-releasing derivative of gabapentin, NCX 8001, alleviates neuropathic pain-like behavior after spinal cord and peripheral nerve injury

1. Nitric oxide (NO) participates, at least in part, to the establishment and maintenance of pain after nerve injury. Therefore, drugs that target the NO/cGMP signaling pathway are of interest for the treatment of human neuropathic pain. Various compounds endowed with NO-releasing properties modulate the expression and function of inducible nitric oxide synthase (iNOS), the key enzyme responsible for sustained NO production under pathological conditions including neuropathic pain. 2. With this background, we synthesized a new chemical entity, [1-(aminomethyl)cyclohexane acetic acid 3-(nitroxymethyl)phenyl ester] NCX8001, which has a NO-releasing moiety bound to gabapentin, a drug currently used for the clinical management of neuropathic pain. We examined the pharmacological profile of this drug with respect to its NO-releasing properties in vitro as well as to its efficacy in treating neuropathic pain conditions (allodynia) consequent to experimental sciatic nerve or spinal cord injuries. 3. NCX8001 (1-30 microm) released physiologically relevant concentrations of NO as it induced a concentration-dependent activation of soluble guanylyl cyclase (EC(50)=5.6 microm) and produced consistent vasorelaxant effects in noradrenaline-precontracted rabbit aortic rings (IC(50)=1.4 microm). 4. NCX8001, but not gabapentin, counteracted in a concentration-dependent fashion lipopolysaccharide-induced overexpression and function of iNOS in RAW264.7 macrophages cell line. Furthermore, NCX8001 also inhibited the release of tumor necrosis factor alpha (TNFalpha) from stimulated RAW264.7 cells. 5. NCX8001 (28-280 micromol x kg(-1), i.p.) reduced the allodynic responses of spinal cord injured rats in a dose-dependent fashion while lacking sedative or motor effects. In contrast, gabapentin (170-580 micromol x kg(-1), i.p.) resulted less effective and elicited marked side effects. 6. NCX8001 alleviated the allodynia-like responses of rats to innocuous mechanical or cold stimulation following lesion of the sciatic nerve. This effect was not shared by equimolar doses of gabapentin. 7. Potentially due to the slow releasing kinetics of NO, NCX8001 alleviated pain-like behaviors in two rat models of neuropathic pain in a fashion that is superior to its parent counterpart gabapentin. This new gabapentin derivative, whose mechanism deserves to be explored further, offers new hopes to the treatment of human neuropathic pain.

Opioid hyperalgesia and tolerance versus 5-HT1A receptor-mediated inverse tolerance

In addition to analgesia, opioids also produce paradoxical hyperalgesic effects following acute and chronic treatment. In this article, we review the occurrence of this hyperalgesia under several conditions, and discuss the potential mechanisms and clinical implications. We also review recent evidence that paradoxical analgesia and inverse tolerance induced by stimulation of 5-HT(1A) receptors, which is a mirror image of opioid-induced hyperalgesia and tolerance, might achieve clinically significant analgesia in chronic pain.

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.

Flexor reflex excitability in mice lacking galanin receptor galanin-R1

This study was conducted to examine the excitability of the nociceptive flexor reflex and its sensitization by repetitive stimulation of C-fibers in anesthetized mice that lack the galanin-R1 receptor. Repetitive stimulation of C-fibers induced a gradual increase in reflex magnitude during the stimulation (wind-up), and a subsequent increase in spinal reflex excitability (central sensitization). This occurred in GAL-R1 -/-, GAL-R1 +/-, and +/+ wild-type controls, with no significant differences observed between genotypes. Intrathecal administration of galanin markedly blocked the sensitization following the repetitive stimulation in all three groups. No differences between wild-type or galanin-R1 receptor knockout mice were seen. These results confirm previous studies in rats, showing that intrathecal galanin reduces the central sensitization following wind-up. The present data indicate that this effect is probably mediated by receptors other than GAL-R1.

Galanin expression in adult human dorsal root ganglion neurons: initial observations

Human dorsal root ganglia (DRGs) were obtained during various procedures and processed for single and double in situ hybridisation using oligonucleotide probes complementary to three peptide mRNAs. Some postmortem ganglia were also analysed. In donor (unlesioned) DRGs 12.5% of the neuron profiles (NPs) were galanin mRNA-positive (mRNA(+)), 47.5% calcitonin gene-related peptide (CGRP) mRNA(+) and 32.7% substance P mRNA(+). The corresponding percentages for cervical/thoracic DRGs from patients suffering from severe brachial plexus injury were 32.8%, 57.4% and 34.5%, respectively. In these DRGs a high proportion of the galanin mRNA(+) NPs contained CGRP mRNA and substance P mRNA. In DRGs from a patient with migraine-like pain a comparatively small proportion expressed galanin, whereas in DRGs from a herpes zoster patient galanin mRNA(+) NPs were comparatively more frequent. The results from human postmortem DRGs revealed only weak peptide mRNA signals. The present results demonstrate that galanin is expressed in DRGs not only in a number of animal species including monkey as previously shown, but also in a considerable proportion of human DRG neurons, often together with CGRP and substance P, and mostly in small neurons. Thus, galanin may play a role in processing of sensory information, especially pain, in human DRGs and dorsal horn. However, to what extent a similarly dramatic upregulation of galanin expression can be seen after peripheral nerve lesion in man, as has been reported for rat, mouse and monkey, remains to be analysed.

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.

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.