Hindpaw Withdrawal Latency Research Articles

Pathogenesis of spinal cord injury induced edema and neuropathic pain: expression of multiple isoforms of wnk1

Neuropathic pain (NP) is a common occurrence following spinal cord injury (SCI). Identification of specific molecular pathways that are involved in pain syndromes has become a major priority in current SCI research. We have investigated the role of a cation-dependent chloride transporter, Cl-regulatory protein Na(+)-K(+)-Cl(-) 1 (NKCC1), phosphorylation profile of NKCC1 and its specific involvement in neuropathic pain following contusion SCI (cSCI) using a rat model. Administration of the NKCC1 inhibitor bumetanide (BU) increases the mean hindpaw withdrawal latency time (WLT), thermal hyperalgesia (TH) following cSCI. These results demonstrate implication of NKCC1 co-transporter and BUin SCI-induced neuropathic pain. The with-no-lysine (K)-1 (WNK1) kinase has been shown to be an important regulator of NKCC1 phosphorylation in many systems, including nocioception. Mutations in a neuronal-specific exon of WNK1 (HSN2) was identified in patients that have hereditary sensory neuropathy type II (HSANII) also implicates WNK1 in nocioception, such that these patients have loss of perception to pain, touch and heat. In our ongoing research we proposed two studies utilizing our contusion SCI (cSCI) NP model of rat. Study 1 aimed at NKCC1 expression and activity is up-regulated following cSCI in the early edema and chronic neuropathic pain phases. Study 2 aimed at identifying the expression profile of alternatively spliced WNK1 isoforms in animals exhibiting thermal hyperalgesia (TH) following cSCI. Adult male Sprague Dawley rats (275-300 g) following laminectomy received cSCI at T9 with the NYU impactor-device II by dropping 10 g weight from the height of 12.5 mm. Control rats obtained laminectomy but no impaction. Following injury, functional recovery was assessed by BBB locomotor scores on day 1, 7, 14, 21, 35, and 42 and development of thermal hyperalgesia on day 21, 28, 35, and 42 day of injury by monitoring hind paw withdraw latency time (WLT) in seconds compared with the baseline data before injury. Increased NKCC1 may explain observed increase in magnetic resonance imaging (MRI) T2, exhibiting NKCC1 localization in neurons. This data supports NKCC1's role in the pathogenesis of acute and chronic phases of injury, namely spinal cord edema and chronic phase neuropathic pain. NKCC1 dependent chloride influx requires the phosphorylation at specific residues. Probing for the HSN2 exon of WNK1 reveals two key findings: i) the HSN2 exon is found in alternatively spliced neuronal isoforms found at 250 kDa and 230 kDa; ii) the 250 kDa isoform is found only in tissue that is injured. This data implicates the NKCC1/WNK1/WNK1HSN2 involvement in post-injury response that contributes to the development of neuropathic pain. Targeting this system may have therapeutic benefit.

Relationship Between Blood Levels and the Anti‐Hyperalgesic Effect of Ketoprofen in the Rat

The relationship between blood levels of ketoprofen and its anti-hyperalgesic effects was examined in rat using the carrageenan-evoked thermal hyperalgesia model. Female adult Wistar rats were injected with carrageenan into the plantar surface of the right hind paw. Immediately after, rats were administered with ketoprofen po and hindpaw withdrawal latency measured and micro-whole blood samples were obtained over six hours via a cannula inserted in the caudal artery. Ketoprofen levels were measured by HPLC. Ketoprofen concentration increased in a dose-dependent manner and was reflected in dose-dependent anti-hyperalgesic effect. The pharmacokinetic and pharmacodynamic parameters expressed as mean ± s.e.m. following administration of 1, 3.2, and 10 mg/kg ketoprofen were: Cmax 1.27 ± 0.08, 3.44 ± 0.20 and 11.76 ± 0.81 μg/mL; AUClast 4.16 ± 0.17, 11.63 ± 0.65 and 28.15 ± 1.32 μg h/mL; and Emax observed (AUCE ): 65.41 ± 7.79, 92.06 ± 6.46 and 98.42 ± 7.53%. A direct relationship between blood concentrations and the anti-hyperalgesic effect of ketoprofen followed a maximum effect model equation. The results indicate that the anti-hyperalgesic effect of ketoprofen in the carrageenan pain model can be predicted by the pharmacokinetic properties of ketoprofen.

Minocycline, a microglial inhibitor, blocks spinal CCL2-induced heat hyperalgesia and augmentation of glutamatergic transmission in substantia gelatinosa neurons

BackgroundSeveral lines of evidence suggest that CCL2 could initiate the hyperalgesia of neuropathic pain by causing central sensitization of spinal dorsal horn neurons and facilitating nociceptive transmission in the spinal dorsal horn. The cellular and molecular mechanisms by which CCL2 enhances spinal pain transmission and causes hyperalgesia remain unknown. The substantia gelatinosa (lamina II) of the spinal dorsal horn plays a critical role in nociceptive transmission. An activated spinal microglia, which is believed to release pro-inflammatory cytokines including TNF-α, plays an important role in the development of neuropathic pain, and CCL2 is a key mediator for spinal microglia activation. In the present study, we tested the hypothesis that spinal CCL2 causes the central sensitization of substantia gelatinosa neurons and enhances spinal nociceptive transmission by activating the spinal microglia and augmenting glutamatergic transmission in lamina II neurons.MethodsCCL2 was intrathecally administered to 2-month-old male rats. An intrathecal injection of CCL2 induced heat hyperalgesia, which was assessed using the hot plate test. Whole-cell voltage-clamp recordings substantia gelatinosa neurons in spinal cord slices were performed to record glutamatergic excitatory postsynaptic currents (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs).ResultsThe hot plate test showed that 1 day after the intrathecal injection of CCL2 (1 μg), the latency of hind-paw withdrawal caused by a heat stimulus was significantly reduced in rats. One day after the intrathecal administration of CCL2, the amplitude of the evoked glutamatergic EPSCs and the frequency of spontaneous glutamatergic miniature EPSCs (mEPSCs) were significantly increased in outer lamina II neurons. Intrathecal co-injection of minocycline, a specific inhibitor of microglial activation, and CCL2 blocked the CCL2-induced reduction in the latency of hind-paw withdrawal and thermal hyperalgesia. Following intrathecal co-administration of CCL2 and minocycline, CCL2 failed to increase the frequency of glutamatergic mEPSCs and failed to promote glutamine release in lamina II neurons. Intrathecal co-injection of WP9QY, a selective TNF-α antagonist, and CCL2 completely inhibited CCL2-induced heat hyperalgesia and inhibited the increase in the frequency of glutamatergic mEPSCs in substantia gelatinosa neurons.ConclusionIn summary, our results suggest that an intrathecal injection of CCL2 causes thermal hyperalgesia by augmenting the excitatory glutamatergic transmission in substantia gelatinosa neurons through a presynaptic mechanism and facilitating nociceptive transmission in the spinal dorsal horn. Further studies show that intrathecal co-administration of minocycline, a specific inhibitor of microglial activation, or WP9QY, a selective TNF-α antagonist, completely inhibited CCL2 potentiation of glutamatergic transmission in substantia gelatinosa neurons and CCL2-induced heat hyperalgesia. The results of the present study suggest that peripheral nerve injury-induced upregulation of the spinal CCL2 level causes the central sensitization of substantia gelatinosa neurons by activating spinal microglia and that TNF-α mediates CCL2-induced thermal hyperalgesia and augmentation of glutamatergic transmission in lamina II neurons.

Nicotine dependence produces hyperalgesia: Role of corticotropin-releasing factor-1 receptors (CRF1Rs) in the central amygdala (CeA)

Because tobacco use has a large negative health and financial impact on society, it is critical to identify the factors that drive excessive use. These factors include the aversive withdrawal symptoms that manifest upon cessation of tobacco use, and may include increases in nociceptive processing. Corticotropin-releasing factor (CRF) signalling in the central amygdala (CeA) has been attributed an important role in: (1) central processing of pain, (2) excessive nicotine use that results in nicotine dependence, and (3) in mediating the aversive symptoms that manifest following cessation of tobacco exposure. Here, we describe three experiments in which the main hypothesis was that CRF/CRF1 receptor (CRF1R) signalling in the CeA mediates nicotine withdrawal-induced increases in nociceptive sensitivity in rats that are dependent on nicotine. In Experiment 1, nicotine-dependent rats withdrawn from chronic intermittent (14-h/day) nicotine vapor exhibited decreased hind paw withdrawal latencies in response to a painful thermal stimulus in the Hargreaves test, and this effect was attenuated by systemic administration of the CRF1R antagonist, R121919. In Experiment 2, nicotine-dependent rats withdrawn from nicotine vapor exhibited robust increases in mRNA for CRF and CRF1Rs in CeA. In Experiment 3, intra-CeA administration of R121919 reduced thermal nociception only in nicotine-dependent rats. Collectively, these results suggest that nicotine dependence increases CRF/CRF1R signalling in the CeA that mediates withdrawal-induced increases in sensitivity to a painful stimulus. Future studies will build on these findings by exploring the hypothesis that nicotine withdrawal-induced reduction in pain thresholds drive excessive nicotine use via CRF/CRF1R signalling pathways.

Antinociceptive effects of sensory stimulation involve dynorphin B supraspinally in rats

The aim was to investigate the mechanisms behind sensory stimulation which can be used to desensitize CNS in patients with atypical orofacial pain. Earlier studies have shown that the kappa-receptor in the periaqueductal gray (PAG) is involved in sensory stimulation induced antinociception. A possible antinociceptive role for dynorphin B (DynB) in supraspinal regions was tested. The behavioral effect of sensory stimulation in conscious rats, by stroking the fur, was tested using the nociceptive test hotplate and the hindpaw withdrawal latency (HWL) was measured. In anesthetized rats sensory stimulation during different modalities, stroking or pinching was performed and the microdialysis technique was used to determine the extra cellular level of DynB in the ventrolateral PAG. To evaluate the antinociception after sensory stimulation DynB was microinjected into the PAG and the effect was measured with the HWL to heat. The results showed that sensory stimulation in conscious rats significantly increased the HWL as an antinociceptive effect. Innocuous sensory stimulation such as stroking the fore paw significantly elevated the DynB level in the PAG compared to internal control. After pinching a tendency to delayed release of DynB was seen and a possible discharge of the nerve terminals could be speculated upon. The blood pressure did significantly increase after pinching but not after stroking. An intra-PAG injection of DynB into the PAG increased the HWL to heat after 24h compared to basal level of HWL and to saline treated animals. In conclusion, DynB is involved in the antinociception that is triggered by sensory stimulation.

Antinociceptive effects induced by intra-periaqueductal grey injection of the galanin receptor 1 agonist M617 in rats with morphine tolerance

The present study was performed to investigate the antinociceptive effects of M617, a selective galanin receptor 1 agonist, and M1145, a selective galanin receptor 2 agonist, in the periaqueductal grey (PAG) in rats with morphine tolerance. Intra-PAG injection of 0.1nmol, 0.5nmol and 1nmol of M617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats with morphine tolerance. Nevertheless, intra-PAG injection of 5nmol of the selective galanin receptor 2 agonist M1145 showed no significant influences on HWLs to noxious thermal and mechanical stimulations in rats with morphine tolerance. The results demonstrated that it is the selective galanin receptor 1 agonist M617, not the selective galanin receptor 2 agonist M1145, induced significant antinociceptive effects in morphine-tolerant rats, indicating that galanin receptor 1 is involved in nociceptive modulation in the PAG of morphine-tolerant rats.

Analgesic Efficacy of Low Intensity Laser Therapy in a Monosodium Iodoacetate-induced Osteoarthritic Rat Model

[Purpose] This study evaluated the analgesic efficacy of low-intensity laser therapy (LILT) in a monosodium iodoacetate (MIA)-induced arthritis rat model. [Subjects and Method] Thirty Sprague–Dawley rats were randomly divided into 3 groups with 10 rats each: the normal, control, and LILT groups. The LILT group was treated with LILT using a gallium–aluminum–arsenide diode laser during the 3 weeks. Each group had 10 rats. All treatments were applied once a day, 5 days a week, for a total experimental period of 21 days. Weight-bearing shift, paw withdrawal threshold (PWT), and paw withdrawal latency were used as outcome measures. [Results] The hind paw weight-bearing shift, PWT, and paw withdrawal latency of rats in the control group were significantly lower than those in the normal group. In the LILT group, the weight-bearing shift, PWT, and paw withdrawal latency were significantly greater than those in the control group. [Conclusion] LILT reduces pain related behaviors of MIA-induced osteoarthritis in rats.

Effects of Simvastatin on Experimental Neuropathic Pain Model and the Role of Nitric Oxide

The aim of the present study was to evaluate putative antinociceptive effects of simvastatin and contribution of nitric oxide to experimental neuropathic pain model in rats. 48 Male, Sprague-Dawley rats (200-250g) were divided into seven groups. Simvastatin (10-20 mg/kg i.p.) was administered intraperitoneally for ten days to rats which were injured by main root ligation of sciatic nerve in chronic construction injury model. The antinociceptive activity of simvastatin was evaluated by using mechanical allodynia, thermal hyperalgesia and cold allodynia on the days of 0, 3, 7, 10. The same tests were performed to assess the role of nitric oxide, by using simvastatin 20mg/kg with 10mg/kg L-NAME or L-Arginine. Tramadol, used routinely in neuropathic pain treatment, was evaluated as a positive control. In all tests, hindpaw withdrawal thresholds and latencies were obtained for ligated, control and sham operated groups 30 minutes after drug or vehicle injections. Simvastatin (10-20 mg/kg) and tramadol 10 mg/kg groups significantly increased paw withdrawal latency on third day in mechanical allodynia test. Simvastatin 20mg/kg and tramadol groups significantly increased antinociceptive effect on tenth day. L-NAME+simvastatin 20mg/kg combination showed significant effect on the tenth day. L-Arginine+simvastatin 20mg/kg significantly decreased paw withdrawal threshold as compared to simvastatin (10-20 mg/kg) on the third day. There was no significant difference between groups in terms of thermal hyperalgesia. Simvastatin 20mg/kg significantly increased paw withdrawal latency on the tenth day as compared to control group in cold allodynia test. Simvastatin 20mg/kg significantly decreased the neuropathic pain in the present study. As a conclusion it seems that nitric oxide pathway may contribute to antinociceptive effect of simvastatin.

Antinociceptive effects induced by injection of the galanin receptor 1 agonist M617 into central nucleus of amygdala in rats

The present study was performed to explore the antinociceptive effects of M617, a selective galanin receptor 1 agonist, in the central nucleus of amygdala (CeA) of rats. Intra-CeA injection of 0.1nmol, 0.5nmol and 1nmol of M617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Furthermore, rats received intra-CeA administration of M617 and galanin. The HWL to noxious thermal and mechanical stimulations increased markedly, and there were no significant differences in HWLs of rats received intra-CeA administration of M617 and galanin. The results demonstrated that intra-CeA injection of M617 induced significant antinociceptive effects in CeA of rats, indicating that galanin receptor 1 may be involved in M617-induced antinociception in the CeA of rats.

Antinociceptive effects of galanin in the nucleus accumbens of rats

It has been demonstrated that galanin plays important roles in the modulation of nociceptive information in rats. The present study is performed to investigate the regulating role of galanin in nociception in the nucleus accumbens (NAc) of rats. Intra-NAc administration of galanin induces dose-dependent increases in the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in rats. Furthermore, the galanin-induced antinociceptive effects are blocked by following intra-NAc injection of the galanin receptor antagonist galantide. The results demonstrate that galanin induces antinociceptive effects in the NAc of rats, and galanin receptors are involved in the galanin-induced antinociception effects.

Distinct Susceptibility to Inoculated Melanoma and Sensitivity to Cancer Pain in Mouse Lines With High and Low Sensitivity to Stress

Approximately 30 years ago, we developed 2 mouse lines with enhanced and decreased opioid system activity using bidirectional selection for high (high analgesia [HA] line) and low (low analgesia [LA] line) swim stress-induced analgesia. These mouse lines differ substantially in pain sensitivity, measured as hind paw withdrawal latency in a hot plate test. Moreover, compared with the LA mice, the HA mice exhibited reduced energy expenditure under stress and different depression-like behavior as well as higher sensitivity to mutagens and the high frequency of spontaneous and carcinogen-induced tumors. In the current study, we observed distinct differences in the growth rate of orthotopically implanted melanoma and the onset of cancer pain. Whereas the HA line was prone to tumors and carcinogenesis was rapid in all specimens, the LA mice either did not develop tumors (70%) or developed tumors that often regressed spontaneously (30%). Animals from both lines developed robust thermal hypersensitivity in the tumor-bearing paw compared with animals that were injected with saline. However, we found that hyperalgesia in tumor-bearing mice persists for a much shorter time in the HA than in LA mice. Naltrexone, given subcutaneously, restored hyperalgesia in the HA mice, whereas it was ineffective in the LA mice. The results suggest that activity of the opioid system may influence carcinogenesis and the intensity of cancer pain and indicates that HA and LA mice are good models for such studies.

Involvement of Spinal Serotonin Receptors in Electroacupuncture Anti-Hyperalgesia in an Inflammatory Pain Rat Model

We previously showed that electroacupuncture (EA) activates medulla-spinal serotonin-containing neurons. The present study investigated the effects of intrathecal 5,7-dihydroxytryptamine creatinine sulfate, a selective neurotoxin for serotonergic terminals, the 5-hydroxytryptamine 1A receptor (5-HT1AR) antagonist NAN-190 hydrobromide and the 5-HT2C receptor (5-HT2CR) antagonist SB-242,084 on EA anti-hyperalgesia. EA was given twice at acupoint GB30 after complete Freund's adjuvant (CFA) injection into hind paw. CFA-induced hyperalgesia was measured by assessing hind paw withdrawal latency (PWL) to a noxious thermal stimulus 30 min post-EA. Serotonin depletion and the 5-HT1AR antagonist blocked EA anti-hyperalgesia; the 5-HT2CR antagonist did not. Immunohistochemical staining showed that spinal 5-HT1AR was expressed and that 5-HT2CR was absent in naive and CFA-injected animals 2.5 h post-CFA. These results show a correlation between EA anti-hyperalgesia and receptor expression. Collectively, the data show that EA activates supraspinal serotonin neurons to release 5-HT, which acts on spinal 5-HT1AR to inhibit hyperalgesia.

Central and Local Administration of Gingko Biloba Extract EGb 761® Inhibits Thermal Hyperalgesia and Inflammation in the Rat Carrageenan Model

Oral administration of the standardized Ginkgo biloba extract EGb 761® has been shown to inhibit thermal hyperalgesia in rodent models of inflammatory and postsurgical pain, but the mechanism underlying these effects is not known. We sought to determine the site of action of EGb 761 by investigating the antihyperalgesic and antiinflammatory properties of EGb 761 after local and central drug administration in the rat carrageenan model of inflammation. Adult male Wistar rats received an intraplantar injection of carrageenan (3%) or saline into the left hindpaw followed 3 hours later by an intraplantar injection of EGb 761 (30, 100, or 300 μg) or vehicle into the left paw; or intrathecal injection of EGb 761 (0.5, 1, 3, 10, or 100 μg) or vehicle into the lumbar spinal cord region. Diclofenac (100 μg) was administered as a positive control. Hindpaw withdrawal latency (in seconds) to thermal stimulation, response threshold (in grams) to mechanical stimulation, and paw volume were measured at 0, 2, 4, 6, and 24 hours after carrageenan injection. Both intraplantar (30, 100, and 300 μg) and intrathecal (0.5 and 1 μg) EGb 761 significantly inhibited carrageenan-induced thermal hyperalgesia and were equally as effective as diclofenac, but had no effect on mechanical hypersenitivity. Application ≥3 μg EGb 761 to the spinal cord induced adverse behavioral effects, which precluded further nociceptive testing. Intraplantar (300 μg) and intrathecal (1 μg) EGb 761 also significantly reduced paw edema. These studies show that EGb 761 acts both at the site of inflammation and centrally at the spinal cord level to inhibit inflammation and thermal hyperalgesia, and may be useful in the treatment of inflammatory pain.

Antinociceptive effects of intracerebroventricular injection of the galanin receptor 1 agonist M 617 in rats

Previous studies in our laboratory demonstrated that galanin and its receptors play important roles in nociceptive modulation in the central nervous system. The present study was performed to explore the antinociceptive effects of the galanin receptor 1 agonist M 617 in the central nervous system of rats. Intracerebroventricular injection of 0.1nmol, 0.5nmol, 1nmol or 2nmol of M 617 induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Furthermore, both intracerebroventricular injection of M 617 and galanin induced significant increases in HWLs in rats. Interestingly, there were no significant differences between the antinociceptive effects induced by M 617 and galanin, indicating that galanin receptor 1 plays main roles in galanin-induced antinociceptive effects in the brain of rats.

Naturally occurring monoepoxides of eicosapentaenoic acid and docosahexaenoic acid are bioactive antihyperalgesic lipids

Beneficial physiological effects of long-chain n-3 polyunsaturated fatty acids are widely accepted but the mechanism(s) by which these fatty acids act remains unclear. Herein, we report the presence, distribution, and regulation of the levels of n-3 epoxy-fatty acids by soluble epoxide hydrolase (sEH) and a direct antinociceptive role of n-3 epoxy-fatty acids, specifically those originating from docosahexaenoic acid (DHA). The monoepoxides of the C18:1 to C22:6 fatty acids in both the n-6 and n-3 series were prepared and the individual regioisomers purified. The kinetic constants of the hydrolysis of the pure regioisomers by sEH were measured. Surprisingly, the best substrates are the mid-chain DHA epoxides. We also demonstrate that the DHA epoxides are present in considerable amounts in the rat central nervous system. Furthermore, using an animal model of pain associated with inflammation, we show that DHA epoxides, but neither the parent fatty acid nor the corresponding diols, selectively modulate nociceptive pathophysiology. Our findings support an important function of epoxy-fatty acids in the n-3 series in modulating nociceptive signaling. Consequently, the DHA and eicosapentaenoic acid epoxides may be responsible for some of the beneficial effects associated with dietary n-3 fatty acid intake.

Involvement of opioid receptors in the CGRP-induced antinociception in the nucleus accumbens of rats

It has been demonstrated that calcitonin gene-related peptide (CGRP) plays important roles in the modulation of nociception in the nucleus accumbens (NAc) of rats. The present study is performed to explore the possible involvement of opioid receptors in the CGRP-induced antinociception in the NAc of rats. Intra-NAc administration of CGRP induces significant increases in the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in rats. Interestingly, the CGRP-induced antinociceptive effects are inhibited by following intra-NAc injection of the opioid receptor antagonist naloxone, suggesting that the opioid receptors are involved in the CGRP-induced antinociception in the NAc of rats. Furthermore, the CGRP-induced antinociception is attenuated by intra-NAc injection of mu-opioid receptor (MOR) antagonist β-funaltrexamine (β-FNA) and kappa-opioid receptor (KOR) antagonist nor-binaltorphimine (nor-BNI), but not by delta-receptor (DOR) antagonist naltrindole. In the present study, we also demonstrated that there was no significant difference between the CGRP-induced antinociception and the morphine-induced antinociception in the NAc in rats. The results of the present study demonstrate that both mu- and kappa-opioid receptors are involved in the CGRP-induced antinociception in the NAc of rats.

A polymorphism in exon 2 of the δ-opioid receptor affects nociception in response to specific agonists and antagonists in mice selectively bred for high and low analgesia

This study searched for polymorphic sites in the murine μ-, δ- and κ-opioid receptors that presumably influence pain perception. We employed two mouse lines divergently bred for high (HA, high analgesia line) and low (LA, low analgesia line) swim stress-induced analgesia (SSIA). These mouse lines differ substantially in pain sensitivity, measured as hind paw withdrawal latency in the hot-plate test. We found a novel C320T transition in exon 2 of the δ-opioid receptor gene, resulting in an A107V substitution in the first extracellular loop (EL1) of the peptide chain. Using hot-plate and tail-flick tests, we found a significant association between the genotype of this locus and basal nociception and SSIA magnitude. Moreover, this transition affects the pharmacological effects of two specific δ-opioid receptor ligands, the agonist SNC80 and the antagonist naltrindole. The impact of the C320T polymorphism on the magnitude of SSIA was partially mediated by endogenous opioids. The effectiveness of exogenous δ-opioid receptor ligands was greater in the HA than in the LA line, and was greater in C320C homozygotes than in C320T heterozygotes within each line. Our results indicate that the C320T polymorphism in the δ-opioid receptor gene is at least partly responsible for the divergent nociceptive thresholds in HA and LA mice under both basal and post-stress conditions.

Antinociceptive effects of galanin in the central nucleus of amygdala of rats, an involvement of opioid receptors

The central nucleus of amygdala (CeA) is a very important brain structure involved in multiple physiological functions, especially in pain modulation. There are high densities of galanin and galanin receptors found in the CeA. The present study was performed to explore the antinociceptive effects of galanin in the CeA of rats, and possible involvements of opioid receptors in the galanin-induced antinociception. Intra-CeA injection of galanin induced dose-dependent increases in hindpaw withdrawal latencies (HWLs) to noxious thermal and mechanical stimulations in rats. Interestingly, the amtinociceptive effect induced by intra-CeA injection of galanin was blocked by intra-CeA injection of naloxone, a common opioid receptor antagonist, indicating an involvement of opioid receptors in the galanin-induced antinociception in the CeA of rats. Moreover, intra-CeA injection of either selective mu-opioid receptor antagonist β-funaltrexamine (β-FNA) or delta-opioid receptor antagonist naltrindole, but not kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI), significantly attenuated the galanin-induced increases in HWLs in the CeA of rats. Taken together, the results demonstrate that galanin induces antinociceptive effects in the CeA of rats, and both mu- and delta-opioid receptors are involved in the galanin-induced antinociception.

Reduced Pain and Inflammation in Juvenile and Adult Rats Fed a Ketogenic Diet

The ketogenic diet is a high-fat, low-carbohydrate regimen that forces ketone-based rather than glucose-based cellular metabolism. Clinically, maintenance on a ketogenic diet has been proven effective in treating pediatric epilepsy and type II diabetes, and recent basic research provides evidence that ketogenic strategies offer promise in reducing brain injury. Cellular mechanisms hypothesized to be mobilized by ketone metabolism and underlying the success of ketogenic diet therapy, such as reduced reactive oxygen species and increased central adenosine, suggest that the ketolytic metabolism induced by the diet could reduce pain and inflammation. To test the effects of a ketone-based metabolism on pain and inflammation directly, we fed juvenile and adult rats a control diet (standard rodent chow) or ketogenic diet (79% fat) ad libitum for 3–4 weeks. We then quantified hindpaw thermal nociception as a pain measure and complete Freund's adjuvant-induced local hindpaw swelling and plasma extravasation (fluid movement from the vasculature) as inflammation measures. Independent of age, maintenance on a ketogenic diet reduced the peripheral inflammatory response significantly as measured by paw swelling and plasma extravasation. The ketogenic diet also induced significant thermal hypoalgesia independent of age, shown by increased hindpaw withdrawal latency in the hotplate nociception test. Anti-inflammatory and hypoalgesic diet effects were generally more robust in juveniles. The ketogenic diet elevated plasma ketones similarly in both age groups, but caused slowed body growth only in juveniles. These data suggest that applying a ketogenic diet or exploiting cellular mechanisms associated with ketone-based metabolism offers new therapeutic opportunities for controlling pain and peripheral inflammation, and that such a metabolic strategy may offer significant benefits for children and adults.

Involvement of Protein Kinase C in Morphine Tolerance at Spinal Levels of Rats

The present study was performed to investigate the possible role of protein kinase C (PKC) in morphine tolerance at spinal levels of rats. Intrathecal injection of 10 μg of morphine induced increases in the hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in rats. After intrathecal injections of 10 μg of morphine (twice a day) lasted for 5 days, the antinociceptive effects induced by intrathecal injections of morphine decreased significantly in rats. Interestingly, we found that there were significant increases in the content of PKC in the dorsal horn of the spinal cord and the dorsal root ganglion, but not in the ventral horn of the spinal cord, in rats with morphine tolerance determined by Western blot, suggesting that PKC is involved in morphine tolerance at spinal levels of rats. Furthermore, our results demonstrated that chronic intrathecal injection of the PKC inhibitor significantly inhibited the development of morphine tolerance. Moreover, we found that the maintenance of morphine tolerance was blocked by intrathecal administration of a PKC inhibitor in rats, and the inhibitory effects of the PKC inhibitor on morphine tolerance lasted for more than two days. Taken together, the present study clearly showed that PKC is involved in morphine tolerance at the spinal level of rats and that intrathecal administration of a PKC inhibitor can block the development and maintenance of morphine tolerance.