Anti-exudative effects of opioid receptor agonists in a rat model of carrageenan-induced acute inflammation of the paw

Efficacy of the Multi-Target Compound E153 in Relieving Pain and Pruritus of Different Origins

Itch and pain are closely related but distinct sensations that share largely overlapping mediators and receptors. We hypothesized that the novel, multi-target compound E153 has the potential to attenuate pain and pruritus of different origins. After the evaluation of sigma receptor affinity and pharmacokinetic studies, we tested the compound using different procedures and models of pain and pruritus. Additionally, we used pharmacological tools, such as PRE-084, RAMH, JNJ 5207852, and S1RA, to precisely determine the role of histamine H3 and sigma 1 receptors in the analgesic and antipruritic effects of the compound. In vitro studies revealed that the test compound had potent affinity for sigma 1 and sigma 2 receptors, moderate affinity for opioid kappa receptors, and no affinity for delta or μ receptors. Pharmacokinetic studies showed that after intraperitoneal administration, the compound was present at high concentrations in both the peripheral tissues and the central nervous system. The blood-brain barrier-penetrating properties indicate its ability to act centrally at the levels of the brain and spinal cord. Furthermore, the test compound attenuated different types of pain, including acute, inflammatory, and neuropathic. It also showed a broad spectrum of antipruritic activity, attenuating histamine-dependent and histamine-independent itching. Finally, we proved that antagonism of both sigma 1 and histamine H3 receptors is involved in the analgesic activity of the compound, while the antipruritic effect to a greater extent depends on sigma 1 antagonism.

Contribution of opioid and neurotensin receptors in the anti-inflammatory activity of PK20 hybrid compound in murine airways

PK20 is an anti-inflammatory hybrid compound, composed of an endomorphin-2-like and neurotensin-like fragments. The aim of the present study is to assess the contribution of particular pharmacophores to the activity of the hybrid tested. For this purpose, airway hyperresponsiveness, accumulation of inflammatory cells in bronchoalveolar lavage fluid (BALF), concentration of mouse mast cell protease, malondialdehyde and secretory phospholipase 2 activity in lung tissue, as well as production of pro-inflammatory cytokines in BALF and lung were determined by using murine model of non-atopic asthma. Blocking either neurotensin receptors or mu opioid receptors did not alter the potential of PK20 in reducing airway hyperresponsiveness. In studies of inflammatory cells, the beneficial effect of the entire peptide occurs to be mediated by the stimulation of neurotensin receptors. However, regarding cytokine and biochemical assays, pretreatment with both receptor antagonists resulted in a different effect on its activity depending on the parameter studied. To conclude, the activation of both the opioid and neurotensin receptors seems to be necessary to induce the full anti-inflammatory activity of the hybrid compound.

Antinociceptive and anti-arthritic effects of aqueous whole plant extract of Trianthema portulacastrum in rodents: Possible mechanisms of action

ETHNOPHARMACOLOGICAL RELEVANCE

Trianthema portulacastrum L. (Aizoaceae) is used in traditional African Medicine for the treatment of various illnesses including dropsy, inflammation and rheumatism.

AIM OF THE STUDY

This study was designed to investigate the anti-nociceptive and anti-arthritic properties of the aqueous whole plant extract of Trianthema portulacastrum (AETP), possible mechanisms of action and characterize some of the active constituents.

MATERIALS AND METHODS

Antinociceptive activity was evaluated using the acetic acid-induced writhing and hot plate tests in mice. The carrageenan test was used to induce a transient inflammation while arthritis was induced with complete Freund's adjuvant (CFA) in rats. On completion of CFA-induced arthritis macroscopic observations, the rats were euthanized to isolate the spleen, liver and limbs for estimation of oxidative stress and histological analysis.

RESULTS

AETP (10, 50, or 250 mg/kg; p.o.) produced significant (p < 0.05) and dose-dependent inhibition (41.10, 50.40, and 67.10%, respectively) of writhing response elicited by acetic acid. Also, increased pain threshold of supraspinally mediated nociceptive behaviour, with peak maximum possible effect (MPE) obtained at 250 mg/kg (22.98%; 30 min post-treatment). However, the pre-treatment of mice with Nitro-L-arginine (L-NNA) or naloxone reversed AETP-induced antinociception. In another experiment, AETP produced time course inhibition of carrageenan-induced paw oedema with peak effect (50.60%) at 250 mg/kg as well as significant reduction in CFA-induced arthritis by 58.56%, on day 27 and arthritic index (26.84%). Similarly, AETP attenuated CFA-induced MDA generation and deficit in antioxidant enzyme activities. Histological analysis of rat joints revealed a reduction in the synovial hyperplasia and mononuclear infiltration induced by CFA in AETP treated groups.

CONCLUSION

Findings from this study showed that T. portulacastrum possesses anti-nociceptive action through nitrergic and opioidergic signalling as well as anti-arthritic effect through enhancement of antioxidant defense system and inhibition of release or actions of inflammatory mediators.

Aryl-1,3,5-triazine ligands of histamine H4 receptor attenuate inflammatory and nociceptive response to carrageen, zymosan and lipopolysaccharide

Objective and design

Histamine H₄ receptor (H₄R) offers a great potential for new therapeutic strategies for the treatment of inflammation-based diseases. The aim of this study is to present the pharmacological profile of two recently synthesized ligands of H₄R with particular reference to their anti-inflammatory and analgesic activity.

Materials and subjects

We used mice and rats in the in vivo tests. We also used murine RAW 264.7 cells and isolated guinea-pig ileum in in vitro test.

Treatments

In the in vivo tests, animals were pre-treated with the increasing doses of investigated compounds (12.5, 25 and 50 mg/kg) and reference compounds: JNJ7777120 (25 mg/kg), indomethacin (10 mg/kg). Macrophages were pre-treated with two concentrations of tested compounds 100 and 10 µM.

Methods

We examined anti-inflammatory and analgesic effects of the new H₄R antagonists in the in vivo models of inflammation induced by carrageenan or zymosan. We assessed the level of cAMP and release of cytokines, ROS and NO in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. Moreover, we assessed the affinity of the investigated compounds for histamine H₁ receptor in functional studies.

Results

Both investigated compounds reduced paw edema, mechanical and thermal hyperalgesia in the carrageenan-induced acute inflammation. Moreover, administration of the investigated compounds resulted in decreased granulocyte influx and attenuated nociceptive reaction in the zymosan-induced peritonitis model. In the same model of inflammation, the investigated compounds reduced vascular permeability; however, this effect was observed only after the highest applied dose. Furthermore, the test compounds had no impact on cell viability in the experiments on RAW 264.7 macrophages. In these cells, stimulated with LPS, the test compounds decreased reactive oxygen species (ROS) production. They increased the cellular concentration of cAMP and attenuated the production of inflammatory cytokines such as TNFα and IL-1β. All results were comparable to those obtained for the reference compound JNJ7777120 with the exception of the impact on NO production. Nevertheless, this effect was similar to that obtained for the other reference compound rolipram, which is a phosphodiesterase 4 (PDE 4) inhibitor. Further experiments revealed that both of the investigated compounds possessed relatively low affinity for histamine H₁ receptor and do not inhibit the activity of the PDE 4B1 enzyme. In addition, all the effects of the investigated compounds in in vivo experiments were observed at doses that did not cause neurologic deficits in rotarod test and did not reduce spontaneous locomotor activity.

Conclusions

Our results demonstrate the anti-inflammatory and analgesic activity of the new aryl-1,3,5-triazine derivatives, which are primarily H₄R–dependent.

Anti-inflammatory and blood stasis activities of essential oil extracted from Artemisia argyi leaf in animals

Artemisia argyi leaf is a well-known species in traditional Chinese medicine. However, the anti-inflammatory and activating blood stasis activities of its essential oil (AAEO) have not been explored in vivo. The present study measured the contents of three chemical components by gas chromatography (GC). The anti-acute inflammatory effects of AAEO were investigated in dimethyl benzene, glacial acetic acid and carrageenan-induced animals through skin administration or by oral gavage, respectively. The effects of AAEO on haemorheology were studied in a rat acute blood stasis model. The contents of eucalyptol, camphor and borneol in AAEO were 254.4, 51.6 and 58.7 mg/g, respectively. All dosages of AAEO by skin administration significantly decreased the swelling in dimethyl benzene-induced ear oedema and carrageenan-induced paw oedema, and reduced the permeability in glacial acetic acid-induced abdominal blood capillary (p < 0.01). Meanwhile, haemorheology indexes such as whole blood viscosity and the erythrocyte aggregation index significantly decreased only in the high dosage group. In addition, the effects of AAEO by oral gavage were weaker than skin administration at the medium dose in the experiments. It suggests that AAEO has better absorption bioavailability and pharmacological effects through skin administration due to the better skin permeability of essential oil than gastrointestinal absorption.

Antinociceptive, anti-inflammatory and smooth muscle relaxant activities of the pyrrolo[3,4-d]pyridazinone derivatives: Possible mechanisms of action

The aim of this study was to evaluate the analgesic as well as anti-inflammatory activities of the new pyrrolo[3,4-d]pyridazinone derivatives. Moreover, the present study attempted to assess some of the mechanisms involved in the pharmacological activity of these compounds. In the previous studies it was shown that these compounds were highly active in the phenylbenzoquinone-induced 'writhing syndrome' test and had much lower activity in the hot plate, which indicates that mainly peripheral mechanisms of analgesia are involved in their effects. In these extended studies the analgesic activity of two tested compounds (4c, 4f) was confirmed in some animal models of pain. The studied compounds showed a significant and dose-related antinociceptive effect in the models of pain induced by formalin, capsaicin and glutamic acid. Both compounds decreased the edema formation and one of them (4c) attenuated mechanical hyperalgesia in carrageenan-induced paw inflammation in rats. Furthermore, both compounds inhibited cell migration, plasma exudation and nociceptive reaction in zymosan A-induced mouse peritonitis. In the subsequent studies, including experiments on isolated organs (ileum, trachea, aorta), radioligand assays and biochemical tests, it was demonstrated that analgesic and anti-inflammatory effects of the investigated structures are largely due to their competitive antagonism for histamine H1 receptor. The influence on the level of cAMP in inflammatory cells (shown in RAW 264.7 macrophages) and subsequent inhibition of cytokine (TNFα, IL-1β) release can also be one of the important mechanisms of their action. Moreover some additional mechanisms may also be involved in the eventual analgesic effect of tested pyrrolo[3,4-d]pyridazinone derivatives.

Targeting inflammation and wound healing by opioids

Opioid receptors are expressed on peripheral sensory nerve endings, cutaneous cells, and immune cells; and local application of opioids is used for the treatment of inflammatory pain in arthritis, burns, skin grafts, and chronic wounds. However, peripherally active opioids can also directly modulate the inflammatory process and wound healing. Here, we discuss the underlying mechanisms of opioid action and the conceivable therapeutic approaches for opioid treatment, as investigated in experimental and clinical studies. A large number of in vitro experiments and animal model investigations have produced evidence that peripherally active opioids can reduce plasma extravasation, vasodilation, proinflammatory neuropeptides, immune mediators, and tissue destruction. In contrast to currently available anti-inflammatory agents, opioids have not demonstrated organ toxicity, thus making them interesting candidates for drug development. Few clinical studies have tapped into this potential to date.

Anti-inflammatory and antinociceptive effects of salbutamol on acute and chronic models of inflammation in rats: involvement of an antioxidant mechanism

The possible role of β-2 adrenergic receptors in modulation of inflammatory and nociceptive conditions suggests that the β-2 adrenergic receptor agonist, salbutamol, may have beneficial anti-inflammatory and analgesic effects. Therefore, in this study, we induced inflammatory and nociceptive responses with carrageenan-induced paw edema or cotton-pellet-induced granuloma models, both of which result in oxidative stress. We hypothesized that salbutamol would prevent inflammatory and nociceptive responses by stimulating β-2 adrenergic receptors and the prevention of generation of ROS during the acute inflammation process in rats. Both doses of salbutamol used in the study (1 and 2 mg/kg) effectively blocked the acute inflammation and inflammatory nociception induced by carrageenan. In the cotton-pellet-induced granuloma test, both doses of salbutamol also significantly decreased the weight of granuloma tissue on the cotton pellets when compared to the control. Anti-inflammatory and analgesic effects of salbutamol were found to be comparable with those of indomethacin. Salbutamol decreased myeloperoxidase (MPO) activity and lipid peroxidation (LPO) level and increased the activity of superoxide dismutase (SOD) and level of glutathione (GSH) during the acute phase of inflammation. In conclusion, salbutamol can decrease acute and chronic inflammation, possibly through the stimulation of β-2 adrenergic receptors. This anti-inflammatory effect may be of significance in asthma treatment, where inflammation also takes part in the etiopathology. This study reveals that salbutamol has significant antioxidative effects, which at least partially explain its anti-inflammatory capabilities. These findings presented here may also shed light on the roles of β-2 adrenergic receptors in inflammatory and hyperalgesic conditions.

Analgesic and Anti-Inflammatory Activities ofTorenia concolorLindley var.formosanaYamazaki and Betulin in Mice

The present study was intended to examine the analgesic effect of the 70% methanol extract of Torenia concolor Lindley var. formosana Yamazaki (TCMeOH) and betulin using models of acetic acid-induced writhing response and formalin test. In addition, we investigated the anti-inflammatory effect of TCMeOHand betulin using model of λ-carrageenan-induced paw edema. We observed the activities of antioxidant enzymes (SOD, GPx and GR) in the liver and the levels of malondialdehyde (MDA) and nitric oxide (NO) in the edema paw. The results showed that TCMeOH(1.0 and 2.0 g/kg) and betulin (30 and 90 mg/kg), significantly inhibited the acetic acid-induced writhing response. TCMeOH(2.0 g/kg) and betulin (30 and 90 mg/kg) significantly inhibited formalin-induced licking time during both the early and late phases. TCMeOH(0.5, 1.0 and 2.0 g/kg) and betulin (30 and 90 mg/kg) also significantly decreased the paw edema at the 4th hour after λ-carrageenan injection. Furthermore, TCMeOHand betulin treatment also significantly increased the activities of SOD, GR and GPx in the liver while decreasing the level of MDA in the edema paw. Finally, betulin (30 and 90 mg/kg) also caused considerable reduction of NO level in the edema paw. Taken together, the present results indicated that TCMeOHand betulin possessed analgesic and anti-inflammatory effects. The anti-inflammatory mechanism of TCMeOHand betulin may be related to decreasing the levels of MDA and NO in the edema paw by increasing the activities of antioxidant enzymes in the liver.

Adjuvant effect of caffeine on acetylsalicylic acid anti-nociception: Prostaglandin E2 synthesis determination in carrageenan-induced peripheral inflammation in rat

In the present study, we report a synergistic interaction between acetylsalicylic acid (ASA) and caffeine (CAF) on the inhibition of nociception in a model of peripheral inflammation in rat; on the contrary no interaction have been found on anti-inflammatory effects and peripheral prostaglandin E2 (PGE-2) synthesis inhibition. Acute inflammation was induced by the subplantar injection of carrageenan into the right hind paw, and the effects of the drugs were evaluated from 0 to 5h. Nociception was assessed using the Randall & Selitto test, and the inflammatory response by plethismometry. Oral administration of ASA (10-400mg/kg) induced dose-related anti-nociceptive and anti-inflammatory effects. On the other hand, oral CAF administration (5-50mg/kg) did not show a dose-related inhibitory effect, neither on the inhibition of nociception nor on the inflammatory response. To analyze a possible interaction between both drugs a dose-response curve to ASA plus a fixed dose of CAF (5mg/kg) was obtained 3h after the injection of carrageenan, when the inflammatory pain peaked. A fixed dose of CAF was able to improve the anti-nociceptive, but not the anti-inflammatory, effects of ASA. We also assessed, by enzyme immunoassay, the effects of the combination on peripheral PGE-2 levels: CAF did not alter the inhibitory effect of ASA on PGE-2 synthesis. Our results corroborate the well-known clinical effects of combining ASA and CAF; on the other hand, we rule out that prostaglandin synthesis inhibition at peripheral sites would be the mechanism responsible of the adjuvant anti-nociceptive effect of CAF.

Pharmacology of peripheral opioid receptors

PURPOSE OF REVIEW

Since the detection of morphine by the pharmacologist Friedrich Sertürner in 1806, opioids have been used as potent centrally acting analgesics. In addition to the central site of action, peripheral endogenous opioid analgesic systems have been extensively studied, especially in the past two decades. This review is not only mentioned to give a brief summary in this well investigated field of peripheral opioid receptors, but also to highlight the role of peripheral opioid receptors in other physiological and pathophysiological conditions.

RECENT FINDINGS

A number of studies, which initially focused on nociception, also revealed an important role of the peripheral opioid receptor system in opioid-induced bowel dysfunction and pruritus, as well as in wound healing, cardioprotection, and the analgesic effects of celecoxib.

SUMMARY

Efforts continue to develop opioid analgesics unable to cross the blood-brain barrier, which act only peripherally in low doses, thus providing adequate analgesia without central and systemic side-effects.The awareness of the influence of peripheral opioid receptors beyond nociception may also have therapeutic ramifications on the other fields mentioned above. For example, the treatment of opioid-induced bowel dysfunction by methylnaltrexone is one of the major findings in the previous years.

Antinociceptive and anti-exudative synergism between dexketoprofen and tramadol in a model of inflammatory pain in mice

Preclinical studies have demonstrated antinociceptive synergism between dexketoprofen (DEX) and tramadol (TRM) in acute animal models of nociception. The aim of the present study was to investigate the type of interaction between DEX and TRM in a chronic musculoskeletal pain model in mice, which fairly replicates the characteristics of chronic osteoarticular pain in humans. Inflammation was induced by a subplantar injection of complete Freund's adjuvant (CFA) in male CF1 mice. Nociceptive thresholds were evaluated using the hot plate, the nocifensive spontaneous behavior and the acetone tests, while plasma extravasation (PE) was assessed with Evan's blue. We used the following experimental groups: control (no inflammation), acute (1 day after CFA injection), and chronic inflammation (7 days after CFA). Dose-response curves for DEX and TRM, individually and combined in a 1 : 1 proportion based on their potency were obtained, and the doses that produced a 50% inhibition calculated. The isobolographic analysis revealed that in all groups of study (no inflammation, acute, and chronic inflammation), the combination of DEX : TRM was synergistic, for both the inhibition of nociception and the PE. The results suggest that the DEX : TRM (1 : 1) combination could be useful in the management of acute and chronic inflammatory musculoskeletal pains in humans; in addition, the synergistic interaction between the drugs observed both during acute and chronic inflammation suggests that less doses would be required of each drug to obtain effective analgesia.

Indirect role of beta2-adrenergic receptors in the mechanism of analgesic action of nonsteroidal antiinflammatory drugs

OBJECTIVE

Adrenal gland hormones have been shown to have a role in the antiinflammatory effect mechanism of nonsteroidal antiinflammatory drugs. This study investigates whether the analgesic effects of indomethacin, diclofenac sodium, aspirin, and nimesulide (IDAN; upper case letters of the four drugs we used) are also related to adrenal gland hormones.

DESIGN

The analgesic effects of IDAN were studied in the carrageenan-induced inflammatory pain model using both intact and adrenalectomized rats. Paw withdrawal tests were performed in adrenalectomized rats that had been pretreated with phenoxybenzamine, propranolol, and metoprolol.

SETTING

This study was performed in Pharmacology and Biochemistry Laboratories of Faculty of Medicine.

PATIENTS/SUBJECTS

A total of 306 (114 intact and 192 adrenalectomized) male Albino Wistar rats were used.

INTERVENTIONS

Adrernalectomy, drug administrations, pain model induction and pain threshold measurements were performed during the study.

MEASUREMENTS AND MAIN RESULTS

Although the analgesic effects of nonsteroidal antiinflammatory drugs were lost in adrenalectomized rats, they exerted significant analgesia in adrenalectomized rats that had been pretreated with prednisolone and adrenalin. All these drugs were found to decrease serum adrenalin concentration but did not change serum cortisole (corticosterone in rats) concentration. Prednisolone and adrenalin inhibited carrageenan-induced hyperalgesia in adrenalectomized rat groups pretreated with metoprolol or phenoxybenzamine, but not in rats given propranolol. Propranolol also negated the analgesic effects of IDAN in intact rats. The analgesic effects provided by either prednisolone or adrenalin could not be inhibited by the alpha1, alpha2, or beta1 blockers but disappeared when beta2 receptors were blocked.

CONCLUSIONS

The analgesic effects of nonsteroidal antiinflammatory drugs appear to be related to endogenous adrenalin and cortisole. We have demonstrated that adrenalin and prednisolone play important roles in the analgesic effect mechanism of IDAN. Prednisolone and adrenalin produce analgesic effects through beta2-adrenergic receptors, suggesting an indirect role for beta2-adrenergic receptors in the analgesic effect mechanism of the nonsteroidal antiinflammatory drugs mentioned.

Morphine-induced early delays in wound closure: involvement of sensory neuropeptides and modification of neurokinin receptor expression

Dose-limiting side effects of centrally acting opioid drugs have led to the use of topical opioids to reduce the pain associated with chronic cutaneous wounds. However, previous studies indicate that topical morphine application impairs wound healing. This study was designed to elucidate the mechanisms by which morphine delays wound closure. Rats were depleted of sensory neuropeptides by treatment with capsaicin, and full-thickness 4-mm diameter wounds were excised from the intrascapular region. Wounds were treated topically twice daily with 5mM morphine sulfate, 1mM substance P, 1mM neurokinin A, or 5mM morphine combined with 1mM substance P or neurokinin A and wound areas assessed. During closure, wound tissue was taken 1, 3, 5, and 8 days post-wounding from control and morphine-treated rats and immunostained for neurokinin receptors and markers for macrophages, myofibroblasts, and vasculature. Results obtained from capsaicin-treated animals demonstrated a significant delay in the early stages of wound contraction that was reversed by neuropeptide application. Treatment of capsaicin-treated rats with topical morphine did not further delay wound closure, suggesting that topical opioids impair wound closure via the inhibition of peripheral neuropeptide release into the healing wound. Morphine application altered neurokinin-1 and neurokinin-2 receptor expression in inflammatory and parenchymal cells essential for wound healing in a cell-specific manner, demonstrating a direct effect of morphine on neurokinin receptor regulation within an array of cells involved in wound healing. These data provide evidence indicating a potentially detrimental effect of topical morphine application on the dynamic wound healing process.

Intrathecally injected morphine inhibits inflammatory paw edema: the involvement of nitric oxide and cyclic-guanosine monophosphate

BACKGROUND

Morphine can inhibit inflammatory edema in experimental animals. The mechanisms and sites by which opioids exert this effect are still under debate. Since the spinal level is a site for modulation of the neurogenic component of inflammation, we investigated the effect of intrathecal (i.t.) administration of morphine, and the involvement of spinal nitric oxide (NO)/cyclic-guanosine monophosphate-GMP pathway in carrageenan (CG)-induced paw edema.

METHODS

Male Wistar rats received i.t. injections of drugs (20 microL) 30 min before paw stimulation with CG (150 microg). Edema was measured as paw volume increase (mL), and neutrophil migration was evaluated indirectly by myeloperoxidase (MPO) assay.

RESULTS

Morphine (37, 75, and 150 nmol) inhibited inflammatory edema, but had no effect on MPO activity. Coinjection with naloxone (64 nmol) reversed the effect of morphine. The corticosteroid synthesis inhibitor, aminoglutethimide (50 mg/kg, v.o.), administered 90 min before morphine injection did not modify its antiedematogenic effect. Low doses of the NO synthase inhibitor, N(omega)-nitro-L-arginine (L-NNA; 10 and 30 pmol) increased, while higher doses (3 and 30 nmol) inhibited edema. The guanylate cyclase inhibitor 1H-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 21 and 42 nmol) increased, while the phosphodiesterase type 5 inhibitor sildenafil (0.15 and 1.5 nmol) inhibited paw edema. Coadministration of a subeffective dose of L-NNA (3 pmol) or ODQ (10 nmol) with morphine prevented its antiedematogenic effect, but sildenafil (0.15 nmol) rendered a subeffective dose of morphine effective (18 nmol). ODQ also prevented the antiedematogenic effect of the NO donor S-nitroso-N-acetyl-penicilamine.

CONCLUSION

These results support the idea that morphine can act on opioid receptors at the spinal level to produce antiedematogenic, and that the NO/cGMP pathway seems to be an important mediator in this effect.

Intramuscular administration of morphine reduces mustard-oil-induced craniofacial-muscle pain behavior in lightly anesthetized rats

The present study investigated the role of peripheral opioid receptors in mustard oil-induced nociceptive behavior and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing between 300 and 400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for the intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle. Intramuscularly-administered morphine significantly reduced shaking behavior but not MO-induced inflammation. Intramuscular pretreatment with naloxone, an opioid receptor antagonist, reversed antinociception produced by intramuscularly-administered morphine, while intracisternal administration of naloxone did not affect the antinociception of peripheral morphine. Pretreatment with d-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a mu opioid receptor antagonist, but not naltrindole, a delta opioid receptor antagonist, nor norbinaltorphimine (nor-BNI), a kappa opioid receptor antagonist, reversed intramuscularly-administered morphine-induced antinociception. These results indicate that intramuscularly-administered morphine produces antinociception in craniofacial muscle nociception and that this intramuscularly-administered morphine-induced antinociception is mediated by a peripheral mu opioid receptor. Our observations further support the clinical approach of administering opioids in the periphery for the treatment of craniofacial muscle nociception.

Inhibitory effect of PACAP-38 on acute neurogenic and non-neurogenic inflammatory processes in the rat

Inhibitory actions of pituitary adenylate cyclase activating polypeptide (PACAP) have been described on cellular/vascular inflammatory components, but there are few data concerning its role in neurogenic inflammation. In this study we measured PACAP-like immunoreactivity with radioimmunoassay in the rat plasma and showed a two-fold elevation in response to systemic stimulation of capsaicin-sensitive sensory nerves by resiniferatoxin, but not after local excitation of cutaneous afferents. Neurogenic plasma extravasation in the plantar skin induced by intraplantar capsaicin or resiniferatoxin, as well as carrageenan-induced paw edema were significantly diminished by intraperitoneal PACAP-38. In summary, these results demonstrate that PACAP is released from activated capsaicin-sensitive afferents into the systemic circulation. It diminishes acute pure neurogenic and mixed-type inflammatory reactions via inhibiting pro-inflammatory mediator release and/or by acting at post-junctional targets on the vascular endothelium.

Tolerance to the Antinociceptive and Antiexudative Effects of Morphine in a Murine Model of Peripheral Inflammation

Opioids are used in humans in the management of chronic osteoarticular pains, but the development of tolerance to the analgesic effects after continuous administration is still not well understood. Our aim was to characterize morphine tolerance in a murine model of arthritis that mimics the sequence of events occurring in humans. Inflammation was induced by the intraplantar injection of complete Freund's adjuvant (CFA) and tolerance by the implantation of a 75-mg morphine pellet. We assessed the antihyperalgesic (plantar and Randall-Selitto tests), antiallodynic (Von Frey test), and antiexudative (Evans blue) effects of morphine, the mu-opioid receptor (MOR) mRNA levels in dorsal root ganglia (DRG), and MOR protein levels in DRG and plantar tissue. Inflammation induced plasma extravasation, and it significantly increased the antihyperalgesic effects of morphine (p < 0.05). Morphine pellet implantation decreased morphine potency in all tests. ED(50) values decreased 4.4 and 7.3 times in the absence and presence of inflammation in the plantar test and 2.7 and 5.3 times in the Randall-Selitto test, whereas plasma extravasation decreased 4.2 times. MOR mRNA levels in the DRG were not affected 7 days after inflammation, whereas chronic morphine administration induced a discrete increase (p < 0.05). MOR protein in the DRG or the paw was unchanged. The results show that inflammation enhances the development of tolerance to the antihyperalgesic and antiexudative effects of morphine. At the molecular level, our results suggest that these effects are not mediated by changes in MOR expression but by other changes in receptor activation/internalization.

Roles of capsaicin-sensitive primary afferents in differential rat models of inflammatory pain: a systematic comparative study in conscious rats

To characterize the role of capsaicin-sensitive primary afferents in inflammatory pain, the effects of subcutaneous (s.c.) injection of 0.15% capsaicin on different chemical irritants-induced pathological nociception including persistent spontaneous nociception, primary thermal and mechanical hyperalgesia, and inflammatory response were systematically investigated in unanesthetized conscious rats. Four different animal models of inflammatory pain: the bee venom (BV) test, the formalin test, the carrageenan model, and the complete Freund's adjuvant (CFA) model, were employed and compared. Local pre-treatment with capsaicin produced a significant inhibition on the s.c. BV and formalin induced long-lasting persistent spontaneous nociception. However, this capsaicin-induced inhibitory effect on spontaneous nociception in the BV test was only found within the late phase (tonic nociception; 11-60 min), but not the early phase (acute nociception; 0-10 min). A complete preventing effect of capsaicin on the decreased thermal paw withdrawal latency was found in the BV, carrageenan, and CFA models. Nevertheless, pre-treatment with capsaicin only produced complete blocking effects on the decreased mechanical paw withdrawal threshold in the BV and carrageenan models, but not in the CFA model. For inflammatory response, a significant inhibition of the BV-elicited paw swelling was found following capsaicin treatment. In marked contrast, capsaicin did not produce any effects on the paw inflammation during exposure to carrageenan, CFA, and formalin. These data suggest that capsaicin-sensitive primary afferents may play differential roles in the induction and development of pathological nociception in differential inflammatory pain models. In contrast to other chemical irritants, BV-induced long-term spontaneous nociception, facilitated nociceptive behavior, and inflammation are modulated by peripheral capsaicin-sensitive afferents.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Pharmacological profile of a novel phosphodiesterase 7A and -4 dual inhibitor, YM-393059, on acute and chronic inflammation models

YM-393059 is a novel phosphodiesterase (PDE) 7A and PDE4 dual inhibitor that inhibits both Th1 [interleukin (IL)-2 and interferon-gamma] and Th2 (IL-4) cytokines in vitro [Yamamoto, S., Sugahara, S., Naito, R., Ichikawa, A., Ikeda, K., Yamada, T., Shimizu, Y., 2006. The effects of a novel phosphodiesterase 7A and -4 dual inhibitor, YM-393059, on T-cell-related cytokine production in vitro and in vivo. Eur. J. Pharmacol. 541, 106-114]. To characterize the pharmacological profile of YM-393059, its effects on several acute and chronic inflammation models were examined. In acute inflammation models, YM-393059 significantly suppressed the delayed-type hypersensitivity reaction to sheep red blood cells in mice with an ED(50) value of 17.1 mg/kg. YM-393059 failed to suppress paw edema in the carrageenin-induced edema model in rats. These pharmacological effects were similar to those of cyclosporine, a typical T-cell immunosuppressant. However, YM-393059, but not cyclosporine, significantly inhibited zymosan-induced neutrophil accumulation in mice with an ED(50) value of 25.7 mg/kg. In mouse toluene-2,4-diisocyanate-induced contact dermatitis, a chronic inflammation model, YM-393059 and cyclosporine significantly suppressed ear edema at doses of 30 and 20 mg/kg, respectively. In this model, YM-393059 also tended to reduce the serum immunoglobulin E antibody level, whereas cyclosporine dramatically potentiated it. These results suggest that YM-393059 inhibits both Th1- and Th2-cell-dependent reactions and also the function of neutrophils.

Beta-endorphin differentially affects inflammation in two inbred rat strains

It has been shown that inflammation of rat paws elicits accumulation of opioid peptide beta-endorphin-containing immune cells in the inflamed subcutaneous tissue, contributing to immunocyte-produced pain suppression. However, the possible mechanisms involved in the pharmacological application of beta-endorphin in rat paw inflammation have not been investigated. The present study was set up to explore the effects of intraplantar injection of beta-endorphin on Concanavalin A-induced paw edema in two inbred rat strains, Albino Oxford (AO) and Dark Agouti (DA). Both high dose-induced suppression and low dose-induced potentiation of edema development in AO and DA rats, respectively, were blocked with antagonists specific for delta (naltrindole) and kappa (nor-binaltorphimine) opioid receptors. beta-endorphin in vitro decreased phagocytosis and increased nitric oxide (NO) production in air pouch granulocytes obtained from AO rats. However, in cells from DA rat strain beta-endorphin modulated both phagocytosis and NO production in a concentration-dependent manner. It could be concluded that the strain-dependent opposing effects of beta-endorphin on paw inflammation are mediated through delta and kappa opioid receptors and probably involve changes in the production of reactive oxygen species by inflammatory cells. Our results point to the importance of genotype for pharmacological manipulations and the development of inflammation.

Synergistic interaction between fentanyl and the histamine H3 receptor agonist R-(α)-methylhistamine, on the inhibition of nociception and plasma extravasation in mice

Here we report a synergistic interaction between fentanyl and the histamine H(3) receptor agonist R-(alpha)-methylhistamine on the inhibition of nociception and plasma extravasation in mice. Chronic inflammation was induced by subplantar injection of Complete Freund's Adjuvant into the right hind paw, and the effect of the drugs was evaluated 7 days later. Nociception and plasma extravasation were assessed by hot-plate and Evans blue tests respectively. Subcutaneous administration of fentanyl (0.01-0.1 mg/kg) induced dose-related anti-nociceptive and anti-extravasation effects (E(max)=100% and 62%, respectively). R-(alpha)-methylhistamine administration (0.3-3 mg/kg) showed a dose-related inhibitory effect on extravasation (E(max)=65%) but not on nociception. To analyze possible interaction between these two drugs, a dose-response curve to fentanyl plus a fixed dose of R-(alpha)-methylhistamine (0.5 mg/kg) was obtained. The dose-response curve for the combined treatment showed a shift to the left compared with that for fentanyl alone. Our results confirm that fentanyl and R-(alpha)-methylhistamine interact in a synergic way, inhibiting nociception and plasma extravasation.

Antiexudative Effects of Opioids and Expression of κ- and δ- Opioid Receptors during Intestinal Inflammation in Mice: Involvement of Nitric Oxide

The study evaluates the effects of kappa- (KOR), delta- (DOR), and mu-opioid receptor (MOR) agonists on the inhibition of plasma extravasation during acute and chronic intestinal inflammation in mice. The antiexudative effects of KOR and DOR agonists in animals treated with nitric oxide synthase (NOS) inhibitors and their protein levels in the gut (whole jejunum and mucosa) and spinal cord of mice with chronic intestinal inflammation were also measured. Inflammation was induced by the intragastric administration of one (acute) or two (chronic) doses of croton oil. Plasma extravasation was measured using Evans blue and protein levels by Western blot and immunoprecipitation. Plasma extravasation was significantly increased 2.7 times during chronic inflammation. The potency of the KOR agonist trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolydinyl)cyclohexyl]-benzeneazetamine (U50,488H) inhibiting plasma extravasation was enhanced 26.3 times during chronic compared with acute inflammation. [d-Pen(2),d-Pen(5)]-Enkephalin (DPDPE) (a DOR agonist) was also 11.8 times more potent during chronic inflammation, whereas the antiexudative effects of fentanyl (a MOR agonist) were not significantly altered. Receptor-specific antagonists reversed the effects. Protein levels of KOR and DOR in the whole jejunum and mucosa were significantly increased after chronic inflammation. Treatment with NOS inhibitors N(omega)-nitro-l-arginine methyl ester or l-N(6)-(1-iminoethyl)-lysine hydrochloride diminished plasma extravasation and inhibited the increased antiexudative effects of U50,488H and DPDPE during chronic intestinal inflammation. The data show that the enhanced antiexudative effects of KOR and DOR agonists could be related to an increased expression of KOR and DOR in the gut and that the release of nitric oxide may play a role augmenting the effects of opioids during chronic inflammation.

The Role of Central and Peripheral μ Opioid Receptors in Inflammatory Pain and Edema: A Study Using Morphine and DiPOA ([8-(3,3-Diphenyl-propyl)-4-oxo-1-phenyl-1,3,8-triaza-spiro[4.5]dec-3-yl]-acetic Acid)

The role of opioid receptors located in the central nervous system (CNS) and peripheral nervous system in inflammatory pain is well established. In contrast, although it is has been shown that mu agonists can reduce other manifestations of inflammation, such as edema, the mechanism of action remains unclear. In this study, we have activated mu receptors located centrally, those located peripherally, and those located both centrally and peripherally and compared the effects on pain and edema using the rat carrageenan model of acute inflammation. Activation of mu receptors located only in the periphery, by administration of the peripheralized mu agonist [8-(3,3-diphenyl-propyl)-4-oxo-1-phenyl-1,3,8-triaza-spiro[4.5]dec-3-yl]-acetic acid (DiPOA) or local administration of morphine, resulted in antihyperalgesia (30 mg/kg DiPOA, 83% inhibition; 100 microg/rat morphine, 75% inhibition) without affecting edema. In contrast, activation of both central and peripheral mu receptors using systemically administered morphine resulted in antihyperalgesia (1 mg/kg, 80% inhibition) and inhibition of edema (10 mg/kg, 54% inhibition). Finally, activation of only receptors located in the CNS, by central administration of DiPOA or systemic administration of morphine after block of only the peripheral mu receptors using q-naltrexone, resulted in a significant reduction in edema. Our findings confirm the role of peripheral mu receptors in the pathology of pain associated with acute inflammation and argue against the involvement of these receptors in edema formation. Furthermore, our data demonstrate that activation of mu receptors in the brain inhibits carrageenan-induced edema and suggest that the antiedematous effect of morphine is due to action at central receptors alone.