Thermal Hyperalgesia In Rats Research Articles

Potential role of Tamoxifen as a secondary analgesic for chemotherapy induced neuropathic pain

Pharmacological modulation of TRPV1 receptor is a promising way of producing analgesia in neuropathic pain models. in the present study we investigated the effect of Tamoxifen, PCK inhibitor ,on thermal hyperalgesia in rats after sciatic nerve injury induced by vincristine. Vincristine administration decreased the withdrawal latencies assessed by tail immersion and hot plate tests ,also it increased the level of inflammatory mediators , spinal NO and serum TNFα levels. Tamoxifen ,administered orally every 2 days one hour before vincristine injection till 7 times, resulted in the increase in withdrawal latencies compared to that of vincristine, also showed a decrease in spinal NO and serum TNFα levels. these findings suggested that TRPV1 receptor has a potential role in vincristine induced painful neuropathy and open the door for tamoxifen to be used as secondary analgesic in painful neuropathies.

The Orally Active Glutamate Carboxypeptidase II Inhibitor E2072 Exhibits Sustained Nerve Exposure and Attenuates Peripheral Neuropathy

Peripheral neuropathy from nerve trauma is a significant problem in the human population and often constitutes a dose-limiting toxicity in patients receiving chemotherapy. (3-2-Mercaptoethyl)biphenyl-2,3-dicarboxylic acid (E2072) is a potent (K(i) = 10 nM), selective, and orally available inhibitor of glutamate carboxypeptidase II (GCPII). Here, we report that E2072 attenuates hyperalgesia and nerve conduction velocity deficits in preclinical rodent models of neuropathic pain and oxaliplatin-induced neuropathy. In the chronic constrictive injury model, orally administered E2072 reversed pre-existing thermal hyperalgesia in rats in a dose-dependent fashion with a minimally effective dose of 0.1 mg/kg/day. It is noteworthy that multiple days of dosing of E2072 were required before analgesia was realized even though GCPII inhibitory exposures were achieved on the first day of dosing. In addition, analgesia was found to persist for up to 7 days after cessation of dosing, consistent with E2072's pharmacokinetic profile and sustained exposure. Furthermore, in a chronic oxaliplatin-induced neuropathy model (6 mg/kg i.p. oxaliplatin twice weekly for 4 weeks), female BALB/c mice receiving daily oral E2072 at 1.0 and 0.1 mg/kg displayed no deficits in either caudal or digital velocity compared with significant deficits observed in mice treated with oxaliplatin alone (12 ± 3 and 9 ± 2%, respectively). Similar findings were seen with oxaliplatin-induced digital and caudal amplitude deficits. It is noteworthy that E2072 showed no interference with the antineoplastic efficacy of oxaliplatin in mice bearing leukemia (L1210), even at doses 100 times its neuroprotective/analgesic dose, indicating a selective effect on neuropathy. These data support the therapeutic utility of GCPII inhibitors in neuropathy and neuropathic pain.

Minocycline and fluorocitrate suppress spinal nociceptive signaling in intrathecal IL‐1β–induced thermal hyperalgesic rats

We previously demonstrated that intrathecal IL-1β caused thermal hyperalgesia in rats. This study was conducted to examine the effects and cellular mechanisms of glial inhibitors on IL-1β-induced nociception in rats. The effects of minocycline (20 μg), fluorocitrate (1 nmol), and SB203580 (5 μg) on IL-1β (100 ng) treatment in rats were measured by nociceptive behaviors, western blotting of p38 mitogen-activated protein kinase (MAPK) and inducible nitric oxide synthase (iNOS) expression, cerebrospinal fluid nitric oxide (NO) levels, and immunohistochemical analyses. The results demonstrated that intrathecal IL-1β activated microglia and astrocytes, but not neurons, in the dorsal horn of the lumbar spinal cord, as evidenced by morphological changes and increased immunoreactivity, phosphorylated p38 (P-p38) MAPK, and iNOS expression; the activation of microglia and astrocytes peaked at 30 min and lasted for 6 h. The immunoreactivities of microglia and astrocytes were significantly increased at 30 min (6.6- and 2.7-fold, respectively) and 6 h (3.3- and 4.0-fold, respectively) following IL-1β injection, as compared with saline controls at 30 min (all P < 0.01). IL-1β induced P-p38 MAPK and iNOS expression predominantly in microglia and less in astrocytes. Minocycline, fluorocitrate, or SB203580 pretreatment suppressed this IL-1β-upregulated P-p38 MAPK mainly in microglia and iNOS mainly in astrocytes; minocycline exhibited the most potent effect. Minocycline and fluorocitrate pretreatment abrogated IL-1β-induced NO release and thermal hyperalgesia in rats. In conclusion, minocycline, fluorocitrate, and SB203580 effectively suppressed the IL-1β-induced central sensitization and hyperalgesia in rats.

GABA, but not opioids, mediates the anti-hyperalgesic effects of 5-HT7 receptor activation in rats suffering from neuropathic pain

Among receptors mediating serotonin actions in pain control, the 5-HT7R is of special interest because it is expressed by primary afferent fibers and intrinsic GABAergic and opioidergic interneurons within the spinal dorsal horn. Herein, we investigated whether GABA and/or opioids contribute to 5-HT7R-mediated control of neuropathic pain caused by nerve ligation. Acute administration of 5-HT7R agonists (AS-19, MSD-5a, E-55888) was found to markedly reduce mechanical and thermal hyperalgesia in rats with unilateral constriction injury to the sciatic nerve (CCI-SN). In contrast, mechanical hypersensitivity caused by unilateral constriction injury to the infraorbital nerve was essentially unaffected by these ligands. Further characterization of the anti-hyperalgesic effect of 5-HT7R activation by the selective agonist E-55888 showed that it was associated with a decrease in IL-1ß mRNA overexpression in ipsilateral L4–L6 dorsal root ganglia and lumbar dorsal horn in CCI-SN rats. In addition, E-55888 diminished CCI-SN-associated increase in c-Fos immunolabeling in superficial laminae of the lumbar dorsal horn and the locus coeruleus, but increased c-Fos immunolabeling in the nucleus tractus solitarius and the parabrachial area in both control and CCI-SN rats. When injected intrathecally (i.t.), bicuculline (3 μg i.t.), but neither phaclofen (5 μg i.t.) nor naloxone (10 μg i.t.), significantly reduced the anti-hyperalgesic effects of 5-HT7R activation (E-55888, 10 mg/kg s.c.) in CCI-SN rats. These data support the idea that 5-HT7R-mediated inhibitory control of neuropathic pain is underlain by excitation of GABAergic interneurons within the dorsal horn. In addition, 5-HT7R activation-induced c-Fos increase in the nucleus tractus solitarius and the parabrachial area suggests that supraspinal mechanisms might also be involved.

Further evidence on a role of chemokines in injury‐related pain hypersensitivity

Further evidence on a role of chemokines in injury‐related pain hypersensitivity

Anti-hyperalgesic effects of anti-serotonergic compounds on serotonin- and capsaicin-evoked thermal hyperalgesia in the rat

The peripheral serotonergic system has been implicated in the modulation of an array of pain states, from migraine to fibromyalgia; however, the mechanism by which serotonin (5HT) induces pain is unclear. Peripherally released 5HT induces thermal hyperalgesia, possibly via modulation of the transient receptor potential V1 (TRPV1) channel, which is gated by various noxious stimuli, including capsaicin. We previously reported in vitro that 5HT increases calcium accumulation in the capsaicin-sensitive population of sensory neurons with a corresponding increase in proinflammatory neuropeptide release, and both are antagonized by pretreatment with 5HT2A and 5HT3 antagonists, as well as the anti-migraine drug sumatriptan. In the current study, we extended these findings in vivo using the rat hind paw thermal assay to test the hypothesis that peripheral 5HT enhances TRPV1-evoked thermal hyperalgesia that can be attenuated with 5HT2A and 5HT3 receptor antagonists, as well as sumatriptan. Thermal hyperalgesia and edema were established by 5HT injection (0.1–10 nmol/100 μl) into the rat hind paw, and the latency to paw withdrawal (PWL) from noxious heat was determined. Rats were then pretreated with either 5HT before capsaicin (3 nmol/10 μl), the 5HT2A receptor antagonist ketanserin or the 5HT3 receptor antagonist granisetron (0.0001–0.1 nmol/100 μl) before 5HT and/or capsaicin, or the 5HT1B/1D receptor agonist sumatriptan (0.01–1 nmol/100 μl) before capsaicin, and PWL was determined. We report that 5HT pretreatment enhances TRPV1-evoked thermal hyperalgesia, which is attenuated with local pretreatment with ketanserin, granisetron, or sumatriptan. We also report that peripheral 5HT induced a similar magnitude of thermal hyperalgesia in male and female rats. Overall, our results provide in vivo evidence supporting an enhancing role of 5HT on TRPV1-evoked thermal hyperalgesia, which can be attenuated by peripheral serotonergic intervention.

Relationship of Axonal Voltage-gated Sodium Channel 1.8 (NaV1.8) mRNA Accumulation to Sciatic Nerve Injury-induced Painful Neuropathy in Rats

Painful peripheral neuropathy is a significant clinical problem; however, its pathological mechanism and effective treatments remain elusive. Increased peripheral expression of tetrodotoxin-resistant voltage-gated sodium channel 1.8 (NaV1.8) has been shown to associate with chronic pain symptoms in humans and experimental animals. Sciatic nerve entrapment (SNE) injury was used to develop neuropathic pain symptoms in rats, resulting in increased NaV1.8 mRNA in the injured nerve but not in dorsal root ganglia (DRG). To study the role of NaV1.8 mRNA in the pathogenesis of SNE-induced painful neuropathy, NaV1.8 shRNA vector was delivered by subcutaneous injection of cationized gelatin/plasmid DNA polyplex into the rat hindpaw and its subsequent retrograde transport via sciatic nerve to DRG. This in vivo NaV1.8 shRNA treatment reversibly and repeatedly attenuated the SNE-induced pain symptoms, an effect that became apparent following a distinct lag period of 3-4 days and lasted for 4-6 days before returning to pretreatment levels. Surprisingly, apparent knockdown of NaV1.8 mRNA occurred only in the injured nerve, not in the DRG, during the pain alleviation period. Levels of heteronuclear NaV1.8 RNA were unaffected by SNE or shRNA treatments, suggesting that transcription of the Scn10a gene encoding NaV1.8 was unchanged. Based on these data, we postulate that increased axonal mRNA transport results in accumulation of functional NaV1.8 protein in the injured nerve and the development of painful neuropathy symptoms. Thus, targeted delivery of agents that interfere with axonal NaV1.8 mRNA may represent effective neuropathic pain treatments.

The effect of intrathecal mu, delta, kappa, and alpha-2 agonists on thermal hyperalgesia induced by mild burn on hind paw in rats

Mild cutaneous thermal injury, leading to a first-degree burn, induces a sensation of burning pain and enhances the pain sensitivity of the skin. Opioid and α(2) receptor agonists are commonly used to reduce such hyperalgesia. We investigated conditions that induced adequate thermal hyperalgesia in rats and compared the effects of μ, δ, κ, and α(2) receptors at the level of the spinal cord in this model. A total of 149 male Sprague-Dawley rats were submitted to this study. A first-degree burn injury was induced in the hind paw by contact with a hot plate. The nociceptive threshold was determined by measuring the time from the application of a light beam to the hind paw to the withdrawal response (paw withdrawal latency, PWL). Various hot-plate exposure times and light beam intensities were tested to determine the conditions that induced adequate hyperalgesia. We also tested the effects of intrathecal morphine (μ agonist), DPDPE ([D-Pen2, D-Pen5] enkephalin, a δ agonist), U50488H (trans(+)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzacetamide methane sulfonate salt, a κ agonist), and ST-91 (2-[2,6-diethyl-phenylamino]-2-imidazoline, an α(2) agonist) on PWL. A first-degree burn was induced by contact with the hot plate for 45 s. Using current of 5.0 A, PWL was reduced by 40% from baseline. Intrathecally administered morphine, DPDPE, and ST-91, but not U50488H, showed dose-dependent antinociceptive effects in both injured and normal paws. Based on these findings, we could find adequate conditions for thermal hyperalgesia model. In this experimental model, μ, δ, and α(2) receptor agonists produced antinociceptive effects at the level of the spinal cord, but the κ receptor agonist did not.

High-frequency stimulation in the ventral posterolateral thalamus reverses electrophysiologic changes and hyperalgesia in a rat model of peripheral neuropathic pain

Chronic neuropathic pain is associated with long-term changes at multiple levels of the neuroaxis, including in the brain, where electrical stimulation has been used to manage severe pain conditions. However, the clinical outcome of deep brain stimulation is often mixed, and the mechanisms are poorly understood. By means of electrophysiologic methods, we sought to characterize the changes in neuronal activity in the ventral posterolateral nucleus of the thalamus (VPL) in a rat model of peripheral neuropathic pain, and to reverse these changes with low-voltage, high-frequency stimulation (HFS) in the VPL. Extracellular single-unit neuronal activity was recorded in naive rats and in those with sciatic chronic constriction injury (CCI). Seven days after CCI, brush- and pinch-evoked firing, as well as spontaneous firing and afterdischarge, were significantly increased compared to naive rats. Spontaneous rhythmic oscillation in neuronal firing was also observed in rats with CCI. HFS decreased neuronal firing rates in rats with CCI up to ∼50% except for spontaneous activity, whereas low-frequency stimulation had no effect. Compared to naive rats, burst firing properties (burst events, percentage of spikes in burst, and mean interburst time) were altered in rats with CCI, whereas these changes were reversed to near normal after HFS. Thermal hyperalgesia in rats with CCI was significantly attenuated by HFS. Therefore, this study demonstrates that electrical stimulation within the VPL can effectively modulate some nociceptive phenomena associated with peripheral neuropathic pain.

Blocking TNF-α with infliximab alleviates ovariectomy induced mechanical and thermal hyperalgesia in rats

Studies have proved an increased expression of tumor necrosis factor alpha (TNF-α) in estrogen deficiency animals, and TNF-α also plays a role in inflammation and neuropathic pain. This study aimed to explore the relationship between TNF-α and ovariectomy induced hyperalgesia. 36 female Sparague-Dawley were included, estrogen depletion models were established by ovariectomy. Then infliximab (a TNF-α blocker) was administrated to the ovariectomized rats for 8 weeks. Pain behavioral tests were performed once a week. The bone mineral density (BMD), serum estradiol and TNF-α level were determined at the 8th week after ovariectomy. The expression of TNF-α in lumbar 5 dorsal root ganglions (L5 DRGs) was examined by immunofluorescence method. Significant hyperalgesia to mechanical and thermal stimuli in groups Ovx-1 and Ovx-2 was observed 1 week after the operation. After treated with infliximab, the pain threshold of Ovx-2 was partially restored, although still lower than the Sham group. The serum TNF-α level of Ovx-1 was significantly higher than Sham and Ovx-2. TNF-α immunofluorescence indicated a significant increase in the expression of TNF-α at L5 DRGs in group Ovx-1 when compared with groups Sham and Ovx-2. The BMD of group Ovx-2 was significantly higher than group Ovx-1 and lower than group Sham. In conclusion, TNF-α plays an important role in estrogen deficiency induced mechanical and thermal hyperalgesia, and DRG may be one site on which TNF-α acts to cause hyperalgesia. Blocking the effect of TNF-α could partially alleviate the estrogen deficiency induced hyperalgesia in rats. Thus, TNF-α may contribute to chronic pain in postmenopausal women.

Repeated Cisplatin Treatments inhibit Gastrointestinal Motility and Induces Baroreflex Changes and Mechanical Hyperalgesia in Rats

Cisplatin is a chemotherapy agent known for its neurotoxicity. We evaluated the effect of cisplatin on the gastric emptying (GE), gastrointestinal (GI) transit of liquid, baroreflex function, thermal, and mechanical withdrawal latencies in rats. Cisplatin increased the GE of liquid with doses ≥2 mg.kg−1 by 59.7–77.4%. This GE delay was not present two weeks after the treatment with five doses of cisplatin at 1 mg.kg−1. Cisplatin also enhanced baroreflex gain possibly by increasing sympathetic activity. Our results demonstrated that cisplatin (2–10 mg.kg−1) causes autonomic neuropathy with GI and baroreflex changes and mechanical but not thermal hyperalgesia in rats.

The antinociceptive potency of N-arachidonoyl-dopamine (NADA) and its interaction with endomorphin-1 at the spinal level

The endogenous N-arachidonoyl-dopamine (NADA) activates both transient receptor potential vanilloid1 (TRPV1) and cannabinoid-1 (CB1) receptors. The goal of this study was to characterize the antinociceptive potential of NADA on inflammatory thermal hyperalgesia in rats at spinal level, and to determine its interaction with endomorphin-1 (EM) at the spinal level.The effects of NADA and EM on thermal hyperalgesia were evaluated in rats with a unilateral hind paw carrageenan-induced inflammation. Intrathecal injection of either EM (0.03–10μg) or NADA (1.5–50μg) caused dose-dependent antihyperalgesia, but NADA was 5.4 times less potent than EM. The antihyperalgesia caused by 15μg NADA was inhibited by the TRPV1 antagonist AMG9810, but not by CB1 antagonist/inverse agonist AM 251, whereas the effect of 50μg NADA was decreased by both drugs. Co-administration of EM with NADA in 1:15 and 1:50 ratios produced a short-lasting potentiation, but isobolographic analysis for the whole investigated period revealed additive interaction between the two endogenous ligands.The results show that both TRPV1 and CB1 receptor activation play a substantial role in the antinociceptive effects of NADA at spinal level, while co-administration of NADA with EM did not show potentiation.

Involvement of Peripheral Ionotropic Glutamate Receptors in Orofacial Thermal Hyperalgesia in Rats

BackgroundThe purpose of the present study was to elucidate the mechanisms that may underlie the sensitization of trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) neurons to heat or cold stimulation of the orofacial region following glutamate (Glu) injection.ResultsGlu application to the tongue or whisker pad skin caused an enhancement of head-withdrawal reflex and extracellular signal-regulated kinase (ERK) phosphorylation in Vc-C2 neurons. Head-withdrawal reflex and ERK phosphorylation were also enhanced following cold stimulation of the tongue but not whisker pad skin in Glu-injected rats, and the head-withdrawal reflex and ERK phosphorylation were enhanced following heat stimulation of the tongue or whisker pad skin. The enhanced head-withdrawal reflex and ERK phosphorylation after heat stimulation of the tongue or whisker pad skin, and those following cold stimulation of the tongue but not whisker pad skin were suppressed following ionotropic glutamate receptor antagonists administration into the tongue or whisker pad skin. Furthermore, intrathecal administration of MEK1/2 inhibitor PD98059 caused significant suppression of enhanced head-withdrawal reflex in Glu-injected rats, heat head-withdrawal reflex in the rats with Glu injection into the tongue or whisker pad skin and cold head-withdrawal reflex in the rats with Glu injection into the tongue.ConclusionsThe present findings suggest that peripheral Glu receptor mechanisms may contribute to cold hyperalgesia in the tongue but not in the facial skin, and also contribute to heat hyperalgesia in the tongue and facial skin, and that the mitogen-activated protein kinase cascade in Vc-C2 neurons may be involved in these Glu-evoked hyperalgesic effects.

Botulinum neurotoxin type A counteracts neuropathic pain and facilitates functional recovery after peripheral nerve injury in animal models

A growing interest was recently focused on the use of Botulinum neurotoxin serotype A (BoNT/A) for fighting pain. The aim of this study was to investigate the effects of BoNT/A on neuropathic pain. It was observed that BoNT/A is able to counteract neuropathic pain induced by chronic constriction injury (CCI) to the sciatic nerve both in mice and in rats. This effect is already present after a single intraplantar (i.pl.) or intrathecal (i.t.) neurotoxin administration that significantly reduces the sciatic nerve ligation-induced mechanical allodynia in mice and rats and thermal hyperalgesia in rats. This effect was evident starting 24 h after the administration of BoNT/A and it was long-lasting, being present 81 or 25 days after i.pl. injection of the higher dose in mice (15 pg/paw) and rats (75 pg/paw), respectively, and 35 days after i.t. injection in rats (75 pg/rat). Moreover, BoNT/A-injected mice showed a quicker recovery of the walking pattern and weight bearing compared to control groups. The behavioral improvement was accompanied by structural modifications, as revealed by the expression of cell division cycle 2 (Cdc2) and growth associated protein 43 (GAP-43) regeneration associated proteins, investigated by immunofluorescence and Western blotting in the sciatic nerve, and by the immunofluorescence expression of S100β and glial fibrillary acidic protein (GFAP) Schwann cells proteins. In conclusion, the present research demonstrate long-lasting anti-allodynic and anti-hyperalgesic effects of BoNT/A in animal models of neuropathic pain together with an acceleration of regenerative processes in the injured nerve, as evidenced by both behavioral and immunohistochemistry/blotting analysis. These results may have important implications in the therapy of neuropathic pain.

Inhibition of Inflammatory Pain by Activating B-Type Natriuretic Peptide Signal Pathway in Nociceptive Sensory Neurons

B-type natriuretic peptide (BNP) has been known to be secreted from cardiac myocytes and activate its receptor, natriuretic peptide receptor-A (NPR-A), to reduce ventricular fibrosis. However, the function of BNP/NPR-A pathway in the somatic sensory system has been unknown. In the present study, we report a novel function of BNP in pain modulation. Using microarray and immunoblot analyses, we found that BNP and NPR-A were expressed in the dorsal root ganglion (DRG) of rats and upregulated after intraplantar injection of complete Freund's adjuvant (CFA). Immunohistochemistry showed that BNP was expressed in calcitonin gene-related peptide (CGRP)-containing small neurons and IB4 (isolectin B4)-positive neurons, whereas NPR-A was present in CGRP-containing neurons. Application of BNP reduced the firing frequency of small DRG neurons in the presence of glutamate through opening large-conductance Ca2+-activated K+ channels (BKCa channels). Furthermore, intrathecal injection of BNP yielded inhibitory effects on formalin-induced flinching behavior and CFA-induced thermal hyperalgesia in rats. Blockade of BNP signaling by BNP antibodies or cGMP-dependent protein kinase (PKG) inhibitor KT5823 [(9S,10R,12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester] impaired the recovery from CFA-induced thermal hyperalgesia. Thus, BNP negatively regulates nociceptive transmission through presynaptic receptor NPR-A, and activation of the BNP/NPR-A/PKG/BKCa channel pathway in nociceptive afferent neurons could be a potential strategy for inflammatory pain therapy.

The amino-terminal region of hemokinin-1 regulates the induction of thermal hyperalgesia in rats

It is known that intrathecal administration of substance P (SP) induces thermal hyperalgesia, whereas hemokinin-1 (HK-1), a member of the same tachykinin family as SP, hardly induces thermal hyperalgesia; however, the underlying mechanism remains to be elucidated. Therefore, we aimed to clarify which amino acid of these peptides contributes to the induction of thermal hyperalgesia. When two chimera peptides between the N-terminal region of SP and the C-terminal region of HK-1, and vice versa, SP (1-5)/HK-1 and HK-1 (1-5)/SP, were intrathecally administered, SP (1-5)/HK-1 induced thermal hyperalgesia whereas HK-1 (1-5)/SP had hardly any effect; furthermore, thermal hyperalgesia was induced by only C-terminal fragments of HK-1 and SP. These findings indicate that the N-terminal region of HK-1 is involved in the non-induction of thermal hyperalgesia. Next, we synthesized and intrathecally administered these chimera peptides in which part of the N-terminal region of HK-1 was replaced with that of SP, and vice versa, and all synthesized peptides induced thermal hyperalgesia. Both SP (1-2)/HK-1 and HK-1 (1-4)/SP certainly induced thermal hyperalgesia, although HK-1 and HK-1 (1-5)/SP had hardly any effect; therefore, it is probable that Ser at the 2nd position and Arg at the 5th position of HK-1 may be involved in the non-induction of thermal hyperalgesia. Furthermore, peptides in which amino acid at the 3rd and/or 4th positions of HK-1 was replaced with that of SP were synthesized. Intrathecal administration of HK-1 (1-2,4-5)/SP, but not HK-1 (1-2,5)/SP and HK-1 (1-3,5)/SP, hardly induced thermal hyperalgesia. These findings indicate that three amino acids, Ser, Thr and Arg at the 2nd, 4th and 5th positions of HK-1, respectively, regulate the induction of thermal hyperalgesia by HK-1.

A cyclic peptide targeted against PSD-95 blocks central sensitization and attenuates thermal hyperalgesia

Post-synaptic density protein PSD-95 is emerging as a valid target for modulating nociception in animal studies. Based on the key role of PSD-95 in neuronal plasticity and the maintenance of pain behavior, we predicted that CN2097, a peptide-based macrocycle of nine residues that binds to the PSD-95 Discs large, Zona occludens 1 (PDZ) domains of PSD-95, would interfere with physiologic phenomena in the spinal cord related to central sensitization. Furthermore, we tested whether spinal intrathecal injection of CN2097 attenuates thermal hyperalgesia in a rat model of sciatic neuropathy. Results demonstrate that spinal CN2097 reverses hyperexcitability of wide dynamic range (WDR) neurons in the dorsal horn of neuropathic rats and decreases their evoked responses to peripheral stimuli (brush, low caliber von Frey and pressure), whereas CN5125 (“negative control”) has no effect. CN2097 also blocks C-fiber long-term potentiation (LTP) in the dorsal horn, which is linked to neuronal plasticity and central sensitization. At a molecular level, CN2097 attenuates the increase in phosphorylated p38 MAPK, a key intracellular signaling pathway in neuropathic pain. Moreover, spinal injection of CN2097 blocks thermal hyperalgesia in neuropathic rats. We conclude that CN2097 is a small molecule peptide with putative anti-nociceptive effects that modulates physiologic phenomena related to central sensitization under conditions of chronic pain.

Spinal translocator protein (TSPO) modulates pain behavior in rats with CFA-induced monoarthritis

Translocator protein 18 kDa (TSPO), previously known as the peripheral benzodiazepine receptor (PBR), is predominantly located in the mitochondrial outer membrane and plays an important role in steroidogenesis, immunomodulation, cell survival and proliferation. Previous studies have shown an increased expression of TSPO centrally in neuropathology, as well as in injured nerves. TSPO has also been implicated in modulation of nociception. In the present study, we examined the hypothesis that TSPO is involved in the initiation and maintenance of inflammatory pain using a rat model of Complete Freund's Adjuvant (CFA)-induced monoarthritis of the tibio-tarsal joint. Immunohistochemistry was performed using Iba-1 (microglia), NeuN (neurons), anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) and anti-PBR (TSPO) on Days 1, 7 and 14 after CFA-induced arthritis. Rats with CFA-induced monoarthritis showed mechanical allodynia and thermal hyperalgesia on the ipsilateral hindpaw, which correlated with the increased TSPO expression in ipsilateral laminae I–II on all experimental days. Iba-1 expression in the ipsilateral dorsal horn was also increased on Days 7 and 14. Moreover, TSPO was colocalized with Iba-1, GFAP and NeuN within the spinal cord dorsal horn. The TSPO agonist Ro5-4864, given intrathecally, dose-dependently retarded or prevented the development of mechanical allodynia and thermal hyperalgesia in rats with CFA-induced monoarthritis. These findings provide evidence that spinal TSPO is involved in the development and maintenance of inflammatory pain behaviors in rats. Thus, spinal TSPO may present a central target as a complementary therapy to reduce inflammatory pain.

Effects of analgesics on the plantar incision-induced drop of the noxious heat threshold measured with an increasing-temperature water bath in the rat

The behavioural noxious heat threshold i.e. the lowest temperature evoking nocifensive behaviour was previously shown to decrease in short-lasting, but not in sustained, inflammatory thermal hyperalgesias. The aim of this study was to examine whether the surgical incision-induced lasting heat hyperalgesia involves a drop of the heat threshold and to assess the effects of conventional opioid and non-opioid analgesics in this model. One of the hind paws of rats was immersed into a water bath whose temperature was near-linearly increased from 30 °C until the animal withdrew its paw from the water. The corresponding bath temperature was considered as the behavioural noxious heat threshold. Hyperalgesia to heat was induced by a standardized plantar surgical incision performed under pentobarbital anaesthesia which led to a 5−7 °C decrease of the noxious heat threshold for seven days. Morphine, diclofenac, and paracetamol administered intraperitoneally 18 h after incision dose-dependently inhibited the drop of heat threshold with minimum effective doses of 0.3, 1, and 100 mg/kg, respectively, as assessed 20, 30 and 40 min after treatment. Thermal hyperalgesia was also decreased by intraplantar treatment with morphine (10 µg) or diclofenac (100 μg). In conclusion, the incision-induced sustained thermal hyperalgesia in rats involves a drop of the heat threshold suggesting that mechanisms of postsurgical pain are distinct from those of pure inflammatory pain. The thermal antihyperalgesic actions of systemically and/or locally applied morphine, diclofenac and paracetamol could be detected with high temporal resolution and sensitivity in this model.

Intrathecal Raf-1-selective siRNA attenuates sustained morphine-mediated thermal hyperalgesia

Studies have demonstrated that long-term opioid treatment leads to an increased sensitivity to painful (hyperalgesia) or normally innocuous (allodynia) stimuli. The molecular mechanisms that lead to paradoxical pain sensitization upon chronic opioid treatment are not completely understood. Enhanced excitatory pain neurotransmitter (such as calcitonin gene-related peptide (CGRP)) release in the dorsal horn of the spinal cord may play a role in sustained morphine-mediated paradoxical pain. Recently we have demonstrated that inhibition of Raf-1 attenuates sustained morphine treatment-mediated augmentation of CGRP release in vitro, in cultured primary sensory neurons. In the present study, we show that knockdown of spinal Raf-1 levels in vivo by intrathecal administration of Raf-1-specific siRNA attenuates sustained morphine-mediated thermal hyperalgesia in rats.