Metamizol potentiates morphine effects on visceral pain and evoked c-Fos immunoreactivity in spinal cord

Inherent Dangers of Using Non-US Food and Drug Administration-Approved Substances of Abuse

Use of US Food and Drug Administration-approved substances of abuse has innate risks due to pharmacologic and pharmacokinetic properties of the medications, but the risk when using nonapproved drug products is much greater. Unbeknownst to the user, the dose of active ingredients in substances of abuse can vary substantially between different products because of manufacturing practices or improper storage. Even naturally occurring substances of abuse can have extensive dosage variability because of effects of the growing season and conditions, or differences in harvesting, storage, or manufacture of the finished products. Many illicit substances are adulterated, to make up for intentional underdosing or to enhance the effect of the intended active ingredient. These adulterants can be dangerous and produce direct cardiovascular, neurologic, hematologic, or dermatologic reactions or obscure adverse effects. Finally, an illicit substance can be contaminated or substituted for another one during its manufacture, leading to differences in adverse events, adverse event severity, or the drug interaction profile. Substances can be contaminated with microbes that induce infections or heavy metals that can damage organs or cause cancer. This milieu of undisclosed substances can also induce drug interactions. For reasons that are discussed, individuals who use substances of abuse are at increased risk of morbidity or mortality if they develop coronavirus disease 2019. Health professionals who treat patients with acute, urgent events associated with substances of abuse, or those treating the chronic manifestations of addiction, need to appreciate the complex and variable composition of substances of abuse and their potential health effects.

Mechanisms involved in antinociception induced by a polysulfated fraction from seaweed Gracilaria cornea in the temporomandibular joint of rats

Temporomandibular disorder is a common clinical condition involving pain in the temporomandibular joint (TMJ) region. This study assessed the antinociceptive effects of a polysulfated fraction from the red seaweed Gracilaria cornea (Gc-FI) on the formalin-induced TMJ hypernociception in rats and investigated the involvement of different mechanisms. Male Wistar rats were pretreated with injection (sc) of saline or Gc-FI 1h before intra- TMJ injection of formalin to evaluate the nociception. The results showed that pretreatment with Gc-FI significantly reduced formalin-induced nociceptive behavior. Moreover, the antinociceptive effect of the Gc-FI was blocked by naloxone (a non-selective opioid antagonist), suggesting the involvement of opioids selective receptors. Thus, the pretreatment with selective opioids receptors antagonists, reversed the antinociceptive effect of the Gc-FI in the TMJ. The Gc-FI antinociceptive effect depends on the nitric oxide/cyclic GMP/protein kinase G/ATP-sensitive potassium channel (NO/cGMP/PKG/K⁺_ATP) pathway because it was prevented by pretreatment with inhibitors of nitric oxide synthase, guanylate cyclase enzyme, PKG and a K⁺_ATP blocker. In addition, after inhibition with a specific heme oxygenase-1 (HO-1) inhibitor, the antinociceptive effect of the Gc-FI was not observed. Collectively, these data suggest that the antinociceptive effect induced by Gc-FI is mediated by μ/δ/κ-opioid receptors and by activation NO/cGMP/PKG/K⁺_ATP channel pathway, besides of HO-1.

Therapeutic potential of cyclooxygenase-3 inhibitors in the management of glioblastoma

In this study we investigated the expression of COX-1, COX-2 and COX-3 mRNA in C6 glioblastoma and normal brain tissues and the effects of acetaminophen, indomethacin or metamizole treatments on the development of C6 glioblastoma in relation with COX inhibition. Glioblastoma cells were inoculated intracerebrally into frontal lobe of adult male Wistar albino rats. 10 days after inoculation, rats were treated with 150 mg/kg acetaminophen, 10 mg/kg indomethacin or 150 mg/kg metamizole. The tumor size was measured histologically and total RNA was isolated from tumor or normal brain tissue and mRNA levels of COX isoforms were determined by qRT-PCR. Our results showed the presence of COX-1, COX-2 and COX-3 expressions in both C6 glioblastoma and normal brain tissues. In tumor tissues COX-3 expression was significantly higher than normal brain tissue (p < 0.05) while there was no significant difference in COX-1 and COX-2 expressions. Acetaminophen and indomethacin decreased the tumor size by 71 and 43 % by inhibiting COX-3 mRNA expression around 87 and 91 % respectively. For the first time our study proposes a possible relationship between COX-3 mRNA expression and C6 glioblastoma development. We also suggested that the inhibition of COX-3 enzyme may be responsible for decrease in tumor size in part, the mechanism by which acetaminophen and indomethacin decreased rat C6 glioblastoma growth. However, the molecular events responsible for COX-3 effects on tumor development are still unresolved as these drugs exert their anti-cancer effect via both COX-3 dependent and independent mechanisms.

DREAM regulates BDNF-dependent spinal sensitization

Background

The transcriptional repressor DREAM (downstream regulatory element antagonist modulator) controls the expression of prodynorphin and has been involved in the modulation of endogenous responses to pain. To investigate the role of DREAM in central mechanisms of pain sensitization, we used a line of transgenic mice (L1) overexpressing a Ca²⁺- and cAMP-insensitive DREAM mutant in spinal cord and dorsal root ganglia.

Results

L1 DREAM transgenic mice showed reduced expression in the spinal cord of several genes related to pain, including prodynorphin and BDNF (brain-derived neurotrophic factor) and a state of basal hyperalgesia without change in A-type currents. Peripheral inflammation produced enhancement of spinal reflexes and increased expression of BDNF in wild type but not in DREAM transgenic mice. The enhancement of the spinal reflexes was reproduced in vitro by persistent electrical stimulation of C-fibers in wild type but not in transgenic mice. Exposure to exogenous BDNF produced a long-term enhancement of dorsal root-ventral root responses in transgenic mice.

Conclusions

Our results indicate that endogenous BDNF is involved in spinal sensitization following inflammation and that blockade of BDNF induction in DREAM transgenic mice underlies the failure to develop spinal sensitization.

Design and microwave-assisted synthesis of 5-trifluoromethyl-4,5-dihydro-1H-pyrazoles: Novel agents with analgesic and anti-inflammatory properties

In this work, we reported the synthesis and evaluation of the analgesic and anti-inflammatory properties of novel 3- or 4-substituted 5-trifluoromethyl-5-hydroxy-4,5-dihydro-1H-1-carboxyamidepyrazoles (where 3-/4-substituent=H/H, Me/H, Et/H, Pr/H, i-Pr/H, Bu/H, t-Bu/H, Ph/H, 4-Br-Ph/H and H/Me) designed in the exploration of the bioisosteric replacement of benzene present in salicylamide with a 5-trifluoromethyl-4,5-dihydro-1H-pyrazole scaffold. Target compounds were synthesized from the cyclocondensation of 4-alkoxy-1,1,1-trifluoromethyl-3-alken-2-ones with semicarbazide hydrochloride through a rapid one-pot reaction via microwave irradiation. In addition to spectroscopic data, the structure of the compounds was supported by X-ray diffraction. Subcutaneous administration of the 5-trifluoromethyl-4,5-dihydro-1H-pyrazoles decreased pain-related behavior during neurogenic and inflammatory phases of the formalin test in mice. Moreover, the more active analgesic compounds (3-/4-=Et/H and H/Me) significantly decreased carrageenan-induced paw edema in mice. The data obtained in this work suggest that the synthesized compounds could be promising candidates for the future development of novel analgesic and anti-inflammatory agents.

The effects of intrathecal cyclooxygenase-1, cyclooxygenase-2, or nonselective inhibitors on pain behavior and spinal Fos-like immunoreactivity

BACKGROUND

Prostaglandins are synthesized by cyclooxygenase (COX) and are thought to play an important role in nociceptive transmission in the spinal cord. Fos expression is an indicator of spinal neuron activation. We examined the role of intrathecal selective and nonspecific COX inhibitors on spinal C-Fos expression.

METHODS

To evaluate the relative contribution of COX-1 and COX-2 in nociceptive transmission in the spinal cord, we assessed the effects of the selective COX-1 inhibitor SC 560, the selective COX-2 inhibitor celecoxib, and the nonselective COX inhibitor ketorolac on formalin-evoked behavior and spinal c-Fos-like immunoreactivity (FLI). Rats received each of the drugs (30, 60, or 90 microg) intrathecally before the subcutaneous administration of formalin (5%, 50 microL) to the plantar surface of a hindpaw. The control group received vehicle intrathecally before the administration of formalin.

RESULTS

Phase 1 flinching behavior decreased in rats given celecoxib or ketorolac 90 mug. Phase 2 flinching behavior decreased in rats given all doses of ketorolac or celecoxib 90 microg (P < 0.05). The FLI was significantly reduced in rats given celecoxib or ketorolac 90 microg for laminae I-II (P < 0.05). By contrast, for laminae V-VI, only the ketorolac 60 or 90 microg treatment group demonstrated a larger decrease in FLI (P < 0.05). The FLI expression in laminae V-VI had a significant correlation with phase 2 flinching behavior (P < 0.05).

CONCLUSIONS

A dual inhibitor of COX-1 and COX-2 suppressed both responses of formalin-evoked behaviors and FLI expression of whole laminae in the lumbar spinal cord. FLI expression of laminae I-II alone may not be a good indicator of the ability to produce anti-hypersensitivity; however, the FLI of laminae V-VI correlates with phase 2 responses.

Antinociceptive and anti-inflammatory effects of compounds isolated from Scaphyglottis livida and Maxillaria densa

Oral administration of a CH(2)Cl(2)-MeOH (1:1) extract of Scaphyglottis livida produced dose-dependent antinociceptive and anti-inflammatory effects when tested in mice and rats using the hot-plate (150-600 mg/kg) and carrageenan-induced inflammation (150-600 mg/kg) models, respectively. Morphine (1.5-6 mg/kg, p.o.) and indomethacin (10-40 mg/kg, p.o.) were used as positive controls, respectively. Four compounds were isolated from the active extract of Scaphyglottis livida, namely 5alpha-lanosta-24,24-dimethyl-9(11),25-dien-3beta-ol (LDD), 24,24,dimethyl-9,19-cyclolanosta-9(11),25-dien-3-one (cyclobalanone), gigantol and 3,4'-dihydroxy-3',4,5-trimethoxybibenzyl (DTB). LDD and gigantol (25-100 mg/kg, p.o.) significantly increased the hot-plate latency in comparison to vehicle-treated mice and decreased carrageenan-induced inflammation in rats. The antinociception provoked by LDD and gigantol was partially blocked by naloxone (1mg/kg, i.p.). However, pretreatment with L-NAME (100 mg/kg, i.p.) and glibenclamide (10 mg/kg, i.p.) did not affect the antinociceptive response induced by LDD or gigantol suggesting that their pharmacological effect could be partially due to activation of opioid receptors. Moreover, a CH(2)Cl(2)-MeOH (1:1) extract of Maxillaria densa reduced acetic acid-induced abdominal writhes but was not able to produce antinociception in the hot-plate assay. Two compounds were isolated from the active extract of Maxillaria densa, namely fimbriol A and erianthridin. Both compounds partially reduced acetic acid-induced writhes. The results tend to support the popular use of this species in folk medicine for treatment of painful complaints.

Co-administration of rofecoxib and tramadol results in additive or sub-additive interaction during arthritic nociception in rat

Over the decades, nonsteroidal anti-inflammatory drugs (NSAIDs) and opioids are the most commonly used analgesics in the management of acute and chronic pain. In order to assess a possible antinociceptive interactions, the antinociceptive effects of rofecoxib p.o., a preferential inhibitor of cyclooxygenase-2, and tramadol-hydrochloride p.o., an atypical opioid analgesic, administered either separately or in combination, were determined using a rat model of arthritic pain. The data were interpreted using the surface of synergistic interaction (SSI) analysis and an isobolographic analysis to establish the nature of the interaction. The SSI was calculated from the total antinociceptive effect produced by the combination after subtraction of the antinociceptive effect produced by each individual drug. Female rats received orally rofecoxib alone (1.0, 1.8, 3.2, 5.6, 10.0, 17.8, 31.6 and 56.2 mg/kg), tramadol alone (1.8, 3.2, 5.6, 10.0, 17.8, 31.6 and 56.2 mg/kg) or 12 different combinations of rofecoxib plus tramadol. Five combinations exhibited various degrees of sub-additive (i.e. less than the sum of the effects produced by the each drug alone) antinociceptive effects (3.2 mg/kg tramadol with 7.8 mg/kg rofecoxib; 5.6 mg/kg tramadol with either 10.0 or 17.8 mg/kg rofecoxib; 10.0 mg/kg tramadol with either 10.0 or 17.8 mg/kg rofecoxib), whereas the other 7 combinations showed additive antinociceptive effects (i.e. the sum of the effects produced by each agent alone). Three combination of rofecoxib+tramadol (10.0+5.6, 10.0+10.0, and 17.8+5.6 mg/kg respectively) presented high sub-additive interactions (P<0.002: Q=9.5). The combination rofecoxib (17.8 mg/kg)+tramadol (10.0 mg/kg) caused gastric injuries less severe than those observed with indomethacin, but more severe than those obtained with rofecoxib or tramadol in single administration. The antinociceptive interaction of rofecoxib and tramadol suggests that combinations with these drugs may have no clinical utility in pain therapy.

Endogenous opioids are involved in morphine and dipyrone analgesic potentiation in the tail flick test in rats

The combined administration of low doses of opiates with non-steroidal anti-inflammatory drugs can produce additive or supra-additive analgesic effects while reducing unwanted side effects. We have recently reported that co-administration of morphine with dipyrone (metamizol) produces analgesic potentiation both in naïve and in morphine-tolerant rats. The purpose of this work was to determine the role of opioids on the acute potentiation observed between morphine and dipyrone i.v. in the rat tail flick test. To do this, two experiments were done. In the first one, naloxone was administered 10 min before morphine (3.1 mg/kg), dipyrone (600 mg/kg) or their combination at the same doses. Control animals received saline instead of naloxone. In the second experiment, naloxone (or saline) was given 2 min after reaching the maximal peak effect with each individual analgesic treatment. When naloxone was i.v. administered prior to analgesics, it completely blocked morphine effects, partially prevented morphine/dipyrone antinociception and delayed dipyrone-induced nociception. At 3.1 mg/kg, naloxone produced an increased nociception. When naloxone was given after analgesics, it dose-dependently blocked the effects of morphine alone and in combination with dipyrone but with different potency in each case. As to dipyrone, naloxone delayed the time to antinociceptive peak effect. Taken together, these results support the notion that endogenous opioids are involved in the analgesic potentiation observed with the combination of morphine plus dipyrone.

Characterization of oxalic acid derivatives as new metabolites of metamizol (dipyrone) in incubated hen's egg and human

Metamizol (dipyrone, 1), a widely used drug with effective analgesic and antispasmodic properties, shows severe side effects like agranulocytosis and anaphylactic shock reactions, the reasons of which are not known until today. After oral administration 1 is completely metabolized. All hitherto known metabolites have an intact pyrazolinone ring structure like the parent compound and are completely extractable from urine with polar organic solvents. However, only a fractional amount of the applied dosage can be recovered by this procedure. To clarify the reason of this deficit of unknown metabolites we followed the hypothesis of oxidative rupture of the heterocyclic ring during metabolism of 1. On the basis of former in vitro results we now were able to identify in quality three oxalic acid derivatives and one acetic acid phenylhydrazide as new metabolites of metamizol in the allantoic fluid (AF) of incubated hen's eggs as well as in human urine by means of GC-MS analysis and comparison with unequivocally synthesized authentic reference compounds. Whereas the oxamazide 7, the phenylhydrazide 8 and N-methyloxamic acid 9 are only present in trace concentrations and therefore cannot account for the deficit in the balance of metabolites, the oxalic acid monohydrazide 11 seems to be excreted in higher amount. But quantitative determination of this new metabolite would be required to answer the open questions concerning the biotransformation of metamizol and thereby to detect new facts about mode of action and side effects of this drug.

Spinal synergy between nonselective cyclooxygenase inhibitors and morphine antinociception in mice

The antinociception induced by the intrathecal coadministration of combinations of morphine with the nonsteroidal anti-inflammatory drugs (NSAIDs) naproxen, piroxicam, metamizol, diclofenac and ketoprofen was studied by isobolographic analysis in the acetic acid writhing test of mice. The effective dose that produced 50% antinociception (ED(50)) was calculated from the log dose-response curve of intrathecally administered fixed ratio combinations of morphine with each NSAID. By isobolographic analysis, this ED(50) was compared to the theoretical additive ED(50) calculated from the ED(50) of morphine and of each NSAID alone. As shown by isobolograms, all the combinations were synergistic, the experimental ED(50)'s being significantly smaller than the theoretically calculated ED(50)'s. The results of this study demonstrate potent interactions between morphine and NSAIDs and validate the clinical use of the combinations of opioids and NSAIDs in pain treatment, even by the intrathecal route.

Enhancement of fentanyl antinociception by subeffective doses of nitroparacetamol (NCX-701) in acute nociception and in carrageenan-induced monoarthritis

We have reported that subanalgesic doses of new generation non-steroidal anti-inflammatory drugs (NSAIDs) enhance the antinociceptive activity of the mu-opiate fentanyl, and the duration of its effect, in acute nociception. Since this therapy is intended for situations of hyperalgesia, we have compared the antinociceptive activity of fentanyl in the absence and in the presence of subeffective doses of NCX-701 (nitroparacetamol) in normal animals and in animals with carrageenan-induced monoarthritis. Subanalgesic dose of NCX-701 did not modify any of the nociceptive responses on its own but reduced the ID50 of fentanyl more than two-fold in both the normal and sensitized states. When administered alone, full recovery from fentanyl was always observed within 15 to 20 minutes, however, full recovery was not observed in the presence of NCX-701. Naloxone was unable to reverse the effect, suggesting a possible reduction of other opiate-mediated secondary effects. We therefore studied the possibility that combining administration of fentanyl and nitroparacetamol (NCX-701) would reduce the development of acute tolerance to fentanyl in behavioral experiments. Acute tolerance to fentanyl in behavioral nociceptive reflexes was developed within 72 h after the constant infusion of the drug, whereas in animals treated with small doses of NCX-701 tolerance was not observed. In summary, our results, both in normal animals and in animals with hyperalgesia, show that fentanyl antinociception can be strongly potentiated with subanalgesic doses of the NSAID NCX-701 and that the development of acute tolerance to fentanyl in normal animals is prevented by this combination of drugs.

Dipyrone potentiates morphine-induced antinociception in dipyrone-treated and morphine-tolerant rats

Coadministration of morphine and dipyrone produces acute and chronic antinociceptive potentiation in drug-naive rats. In this work, the effectiveness of the combination was determined in rats pretreated with morphine or dipyrone. Nine groups of male rats received (i.v.) 3.1 mg/kg morphine, 600 mg/kg dipyrone, or the morphine-dipyrone combination twice a day for five administrations (three groups per treatment). From the 6th to the 10th administration, one group out of each treatment continued without change, while the other two were switched to one of the other two possible treatments. In morphine-tolerant rats, morphine plus dipyrone produced a transient antinociceptive potentiation. In dipyrone-treated animals, this combination produced a long-lasting potentiation. In animals only treated with the combination, antinociception was clear since the beginning, although it decreased after the 6th injection. No cross-tolerance was seen between morphine and dipyrone. These data suggest that dipyrone potentiates morphine-induced antinociception in dipyrone-treated as well as in morphine-tolerant rats.

Subanalgesic doses of dexketoprofen and HCT-2037 (nitrodexketoprofen) enhance fentanyl antinociception in monoarthritic rats

Subanalgesic doses of the non-steroidal antiinflammatory drugs (NSAID) dexketoprofen trometamol and nitroparacetamol (NCX-701) enhance mu-opiate fentanyl effect in acute nociception. It is not known if a similar combination of drugs is effective in situations of spinal cord sensitization. The aim of this study was to assess if the enhancement of fentanyl antinociception can be observed in carrageenan-induced monoarthritis, when combined with dexketoprofen (DKT) or nitrodexketoprofen (HCT-2037). Withdrawal reflexes were recorded as single motor units in male Wistar rats anesthetized with alpha-chloralose. Fentanyl was studied alone and in the presence of 0.4, 0.8 micromol/kg of DKT or 0.3 micromol/kg of HCT-2037. In responses to noxious mechanical stimulation, the ID50 of fentanyl was enhanced twofold by 0.8 micromol/kg DKT and more than fourfold by HCT-2037 and no significant recovery was observed 45 min later. DKT 0.4 micromol/kg was, however, very little effective. The opioid antagonist naloxone did not reverse the effect. Enhancement of fentanyl effect on wind-up was only observed with HCT-2037 but not with DKT. We conclude that the combined administration of subanalgesic doses of dexketoprofen derivatives, specially its nitroderivative, and the mu-opiate fentanyl is an effective antinociceptive therapy in situations of articular inflammation involving a naloxone-independent mechanism of action.

Synergy between the antinociceptive effects of morphine and NSAIDs

The intraperitoneal administration of morphine, diclofenac, ketoprofen, meloxicam, metamizol, paracetamol and piroxicam induced dose-dependent antinociception in mice tested with the acetic acid writhing test. The isobolographic analysis of the simultaneous intraperitoneal administration of fractions of the ED50's of morphine with each nonsteroidal anti-inflammatory drug (NSAID) demonstrated the existence of a supra-additive interaction (synergy). The selective antagonist of µ-opioid receptors naltrexone partially reversed the supra-additive interactions to additive interactions; however, the combinations of morphine/metamizol and morphine/paracetamol were completely antagonized, resulting in subadditive interactions. The selective antagonist of δ-opioid receptors naltrindole failed to significantly attenuate the combinations of morphine with ketoprofen, meloxicam and piroxicam, but decreased the activity of the combinations of morphine with diclofenac, metamizol and paracetamol, transforming the interactions from supra-additive to additive. Nor-binaltorphimine was used to evaluate the involvement of κ-opioid receptors. Nor-binaltor phimine did not modify the supra-additive interaction of morphine and NSAIDs and the additive interaction of the co-administration of morphine and metamizol. The synergy between morphine and NSAIDs could be related to different pathways of pain transmission, probably related to the different intracellular signal transduction mechanisms of action of opioid and non-opioid agents.Key words: writhing test, NSAIDs, morphine, antinociception, isobolographic analysis, synergy.

Analysis of the analgesic interactions between ketorolac and tramadol during arthritic nociception in rat

The potential advantage of using combination therapy is that analgesia can be maximized while minimizing the incidence of adverse effects. In order to assess a possible synergistic antinociceptive interactions, the antinociceptive effects of ketorolac tromethamine, p.o., a nonsteroidal anti-inflammatory drug (NSAID), and tramadol hydrochloride, p.o., an atypical opioid analgesic, administered either separately or in combination, were determined using a rat model of arthritic pain. The data were interpreted using the surface of synergistic interaction (SSI) analysis and an isobolographic analysis to establish the nature of the interaction. The surface of synergistic interaction was calculated from the total antinociceptive effect produced by the combination after subtraction of the antinociceptive effect produced by each individual drug. Female rats received orally ketorolac alone (0.18, 0.32, 0.56, 1.0, 1.78, 3.16, and 5.62 mg/kg), tramadol alone (3.16, 5.62, 10.0, 17.78, 31.62, 56.23, and 100.0 mg/kg), or 24 different combinations of ketorolac plus tramadol. Ten combinations exhibited various degrees of potentiation of antinociceptive effects (17.78 mg/kg tramadol with either 0.18, 0.32, or 0.56 mg/kg ketorolac; 10.0 mg/kg tramadol with either 0.18, 0.32, 0.56, or 1.8 mg/kg ketorolac; 5.6 mg/kg tramadol with either 0.32 or 0.56 mg/kg ketorolac; and 3.16 mg/kg tramadol with 0.32 mg/kg ketorolac), whereas the other 14 combinations showed additive antinociceptive effects. Three combinations of ketorolac+tramadol (1.0+17.78, 1.78+10, and 1.78+17.78, mg/kg respectively) produced the maximum antinociceptive effects, and two combinations (0.32+10.0 and 0.56+10.0 mg/kg, respectively) presented effects of high potentiation (P<0.001). This combination caused gastric injuries less severe than those observed with indomethacin. The synergistic antinociceptive effects of ketorolac and tramadol were important and suggest that combinations with these drugs may have clinical utility in pain therapy.

Analgesia induced by dietary restriction is mediated by the kappa-opioid system

Progress in the control and treatment of pain may be facilitated by a better understanding of mechanisms underlying nociceptive processing. Here we show that mice subjected to an intermittent fasting diet (IFD) display markedly reduced responses in models of thermal and visceral pain compared with mice fed ad libitum (AL). Pharmacological analyses suggest that a change in the endogenous kappa-opioid system underlies IFD-induced analgesia. The levels of prodynorphin mRNA and kappa-opioid receptors in the spinal cord are higher in IFD than in AL mice. Furthermore, in spinal cord nuclear protein extracts, the activity of the transcriptional repressor DREAM (downstream regulatory element antagonist modulator), the main regulator of prodynorphin expression, is lower in IFD than in AL mice. Finally, c-Fos expression in dorsal spinal cord after noxious stimulation is significantly lower in IFD than in AL animals, indicating that dynorphin could block nociceptive information at the spinal cord. These results suggest that dietary restriction together with administration of kappa-opioid agonists could be useful as a new therapeutic approach for pain relief.

Interaction between metamizol and tramadol in a model of acute visceral pain in rats

Tramadol (TRM) and metamizol (MTZ) are drugs with complex mechanisms of action, extensively used in combination in pain management. In the present investigation we have evaluated the interaction between MTZ:TRM in the ethacrinic acid writhing test in rats. Dose-response curves (s.c.) were obtained for each drug individually, combined in fixed potency ratios (1:0.3, 1:1, 1:3), and for MTZ in presence of a fixed-dose of TRM (3.5 mg/kg). Interactions were analysed using isobolograms, interaction indexes (INT-I) and ANOVA. We used naloxone (1 mg/kg s.c.) to determine the opioid-component of the effects (ED80). Isobolograms demonstrated antagonism at the ED20, for 1:0.3 and 1:3 mixtures (p<0.01), whereas 1:1 was additive. At the ED50 and ED80 all combinations showed synergy. Fixed-dose experiments demonstrated that treatment (p<0.0001), dose (p<0.0001), and their interaction (p<0.0001) were statistically significant. Naloxone partially antagonized TRM (67%), but not MTZ; the percentage reversal of the combinations was directly related to the dose of TRM in the combination. The results show that the MTZ:TRM interaction on antinociception is synergistic or antagonistic depending on the level of effect. Synergy is demonstrated at 50% or higher levels, thus supporting the results obtained in humans by our group. Below the ED50 antagonism or additivity is present depending on the ratio of the combination. The mechanisms of the interaction remain unknown.

Morphine and dipyrone co-administration delays tolerance development and potentiates antinociception

This work analyses the time course of tolerance development and antinociceptive potentiation throughout repeated co-administration of morphine (an opioid receptor agonist) plus dipyrone (a non-steroidal anti-inflammatory drug) in the tail-flick test. Male Wistar rats were i.v. injected with morphine (3.1 mg/kg), dipyrone (600 mg/kg) or the combination morphine/dipyrone twice a day for 5 days. Dipyrone produced antinociceptive effects with a trend towards tolerance development at the end of the treatment. Morphine was initially effective, but complete tolerance developed after its fifth administration. The combination of morphine plus dipyrone produced a significant potentiation and longer duration of antinociceptive effects. The antinociceptive efficacy of morphine and dipyrone co-administration gradually decreased after the sixth injection. An additional group of rats treated with dipyrone for 11 days developed complete tolerance after the 19th administration. These data suggest that repeated co-administration of morphine plus dipyrone results in a delay of tolerance development and in a potentiation of their individual antinociceptive effects.

Enhancement of antinociception by co-administration of an opioid drug (morphine) and a preferential cyclooxygenase-2 inhibitor (rofecoxib) in rats

Synergism has been used to obtain analgesia at doses at which side effects are minimal. In addition, it has been demonstrated that inhibition of cyclooxygenase-2 is responsible for the therapeutic effects of nonsteroidal anti-inflammatory drugs (NSAIDs). The aim of this study was to evaluate the antinociceptive interaction between the preferential COX-2 inhibitor, rofecoxib and morphine. Several combinations were evaluated using the pain-induced functional impairment model (PIFIR), a rat model of arthritic pain. Surface of synergistic interaction (SSI) analysis and an isobolographic method were used to detect the antinociceptive potency of the drugs, given either individually or in combination. The surface of synergistic interaction was calculated from the total antinociceptive effect produced by the combination after subtraction of the antinociceptive effect produced by each individual drug. Male rats received orally morphine alone (10, 17.8, 31.6, 56.2 and 100.0 mg/kg), rofecoxib alone (3.2, 5.6, 10, 31.6, 56.2 and 74.0 mg/kg) or 12 different combinations of morphine and rofecoxib. Three combinations exhibited potentiation of antinociceptive effects (10 mg/kg of morphine with either 5.6, 10 or 31.6 mg/kg of rofecoxib), whereas the other nine combinations showed additive antinociceptive effects. The combination of morphine, 56.2 mg/kg (p.o.), and rofecoxib, 31.6 mg/kg (p.o.), produced the maximum antinociceptive effect (P<0.05). This combination caused gastric injuries less severe than those observed with indomethacin, i.e. it reduced ulcers and erosion formation. The synergistic antinociceptive effects of rofecoxib and morphine are important and suggest that combinations with drugs may decrease the side effects associated with the use of nonselective NSAIDs. Furthermore, the present results suggest that combinations containing opioid drugs and selective COX-2 inhibitors may have clinical utility in pain therapy.

mGlu5 receptor antagonist decreases Fos expression in spinal neurons after noxious visceral stimulation

In this study we examined the effects of the glutamate metabotropic subtype 5 (mGlu5) receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) on Fos expression in the spinal cord in a model of visceral pain in the rat. We show that noxious stimulation increases the number of Fos-positive neurons in the dorsal horn of the thoracic and lumbar spinal cord, and that pretreatment with MPEP significantly reduces the number of Fos-positive neurons in these areas. These data indicate that mGlu5 is involved in the transmission of visceral pain in the spinal cord.

Metamizol potentiates morphine antinociception but not constipation after chronic treatment

This work evaluates the antinociceptive and constipating effects of the combination of 3.2 mg/kg s.c. morphine with 177.8 mg/kg s.c. metamizol in acutely and chronically treated (once a day for 12 days) rats. On the 13th day, antinociceptive effects were assessed using a model of inflammatory nociception, pain-induced functional impairment model, and the charcoal meal test was used to evaluate the intestinal transit. Simultaneous administration of morphine with metamizol resulted in a markedly antinociceptive potentiation and an increasing of the duration of action after a single (298+/-7 vs. 139+/-36 units area (ua); P<0.001) and repeated administration (280+/-17 vs. 131+/-22 ua; P<0.001). Antinociceptive effect of morphine was reduced in chronically treated rats (39+/-10 vs. 18+/-5 au) while the combination-induced antinociception was remained similar as an acute treatment (298+/-7 vs. 280+/-17 au). Acute antinociceptive effects of the combination were partially prevented by 3.2 mg/kg naloxone s.c. (P<0.05), suggesting the partial involvement of the opioidergic system in the synergism observed. In independent groups, morphine inhibited the intestinal transit in 48+/-4% and 38+/-4% after acute and chronic treatment, respectively, suggesting that tolerance did not develop to the constipating effects. The combination inhibited intestinal transit similar to that produced by morphine regardless of the time of treatment, suggesting that metamizol did not potentiate morphine-induced constipation. These findings show a significant interaction between morphine and metamizol in chronically treated rats, suggesting that this combination could be useful for the treatment of chronic pain.

Visceral antinociception produced by bee venom stimulation of the Zhongwan acupuncture point in mice: role of alpha(2) adrenoceptors

The goal of the present study was to determine whether bee venom (BV) injection into the Zhongwan acupoint (CV12), compared to injection into a non-acupoint, produced antinociception in an acetic acid-induced visceral pain model. This was accomplished by injecting BV subcutaneously into the Zhongwan acupoint or into a non-acupoint 30 min before intraperitoneal injection of acetic acid in ICR mice. BV injection into the acupoint produced a dose dependent suppression of acetic acid-induced abdominal stretches and of acetic acid-induced Fos expression in the spinal cord and the nucleus tractus solitarii. In contrast BV injection into the non-acupoint only produced antinociception at the highest dose of BV tested. Naloxone pretreatment did not alter the antinociceptive effect of BV acupoint injection on the abdominal stretch reflex. On the other hand, pretreatment with the alpha 2-adrenoceptor antagonist, yohimbine completely blocked the antinociceptive effect of BV acupoint injection. These results imply that BV acupoint stimulation can produce visceral antinociception that is associated with activation of alpha 2-adrenoceptors, but not with naloxone-sensitive opioid receptors.

Neuronal nitric oxide synthase (nNOS) mRNA is down-regulated, and constitutive NOS enzymatic activity decreased, in thoracic dorsal root ganglia and spinal cord of the rat by a substance P N-terminal metabolite

Nitric oxide (NO) in the spinal cord plays a role in sensory and autonomic activity. Pain induced by acetic acid in the abdominal stretch (writhing) assay and hyperalgesia associated with chronic pain are highly sensitive to NO synthase (NOS) inhibitors. Because substance P (SP) is released and up-regulated in some models of chronic pain, we hypothesized that an accumulation of SP metabolites may influence NOS expression and activity. To test this hypothesis, we examined the effect of intrathecally (i.t.) injected substance P (1-7) [SP(1-7)], the major metabolite of SP in the rat, on neuronal NOS (nNOS) mRNA in the thoracic and lumbar spinal cord, dorsal root ganglia (DRG) and on the corresponding constitutive NOS (cNOS) enzyme activity. Detected using quantitative RT-PCR, nNOS mRNA content in the thoracic spinal cord was decreased 6 h after injection of 5 micromol of SP(1-7) and returned to control 2 days later. In thoracic DRG, nNOS mRNA was reduced 48 h after SP(1-7). The cNOS enzymatic activity in thoracic spinal tissue was gradually decreased to a minimum at 72 h. Down-regulation of NOS by SP(1-7) in the thoracic area appears to be highly associated with capsaicin-sensitive primary afferent neurons. No similar changes in either parameter were measured in the lumbar area after SP(1-7). These data suggest that N-terminal SP fragments, which are known to cause long-term antinociception in the writhing assay, may do so by their ability to down-regulate NO synthesis along nociceptive pathways.

3-Methyl-5-hydroxy-5-trichloromethyl-1H-1-pyrazolcarboxyamide induces antinociception

The antinociceptive action of a novel pyrazole-derived compound, 3-methyl-5-hydroxy-5-trichloromethyl-1H-1-pyrazolcarboxyamide (MPCA) was evaluated using the formalin and tail-immersion tests in mice. Anti-inflammatory activity was assessed by paw plethysmometry in adult rats using the carrageenin-induced paw edema test. Subcutaneous administration of MPCA (22, 66, and 200 mg/kg) induced a dose-dependent decrease in the time spent licking during the neurogenic and inflammatory phases of the formalin test, and preadministration of naloxone (1 mg/kg, sc) did not prevent MPCA-induced (200 mg/kg, sc) antinociception. Naloxone decreased the spontaneous locomotor activity of mice, while MPCA had no effect on locomotion. In contrast, administration of the opioid antagonist caused a significant increase in the locomotor behavior of mice previously injected with MPCA. MPCA was devoid of antinociceptive action by the tail-immersion test and of anti-inflammatory activity. Moreover, MPCA had no effect on the motor performance of mice in the rotarod test. These results suggest that MPCA induces antinociception in the neurogenic and inflammatory phases of the formalin test, an effect that does not involve opioid receptors.

Suppressive effects of Neiting acupuncture on toothache: an experimental analysis on Fos expression evoked by tooth pulp stimulation in the trigeminal subnucleus pars caudalis and the periaqueductal gray of rats

To clarify the antinociceptive mechanism of acupuncture on acute pain, c-fos protein (Fos) expression induced by tooth pulp stimulation was immunohistochemically examined in the spinal trigeminal subnucleus pars caudalis (spVc) and the periaqueductal gray (PAG) of rats with or without Neiting acupuncture. The central projection of trigeminal ganglion neurons innervating in the tooth pulp was examined by tract-tracing method with horseradish peroxidase-conjugated wheat germ agglutinin (WGA-HRP). Central terminals from the first maxillary molar tooth were labeled transganglionically in the dorsomedial part of spVc with WGA-HRP. Numerous numbers of Fos-immunoreactive (Fos-ir) cells were found in the spVc and PAG by stimulation of the tooth pulp with acetic acid or saline. Neiting acupuncture significantly reduced the Fos expression in the spVc induced by tooth pulp stimulation. On the other hand, Neiting acupuncture evoked many Fos-ir cells in the PAG. The present results suggest that Neiting acupuncture activated PAG neurons that sent descending inhibitory fibers to medullo-spinal nociceptive neurons, and reduced the number of Fos-expressed neurons in the trigeminal subnucleus pars caudalis mediating noxious information from teeth to the higher central nervous system.

Induction of glycerol phosphate dehydrogenase gene expression during seizure and analgesia

Using mRNA differential display, we found that the gene for NAD(+)-dependent glycerol phosphate dehydrogenase (GPDH; EC 1.1.1.8) is induced in rat brain following seizure activity. Northern blot and in situ hybridization analysis confirmed the differential display results; they also showed, in a separate model of neuronal activation, that after thermal noxious stimulation of the hind-paws, a similar increase in GPDH mRNA occurs in the areas of somatotopic projection in the lumbar spinal cord. Surprisingly, administration of analgesic doses of morphine or the nonsteroidal antiinflammatory drugs aspirin, metamizol (dipyrone), and indomethacin also increased GPDH mRNA levels in rat spinal cord. The opioid receptor antagonist naloxone completely blocked morphine induction of GPDH but had no effect on GPDH induction by noxious heat stimulation or metamizol treatment, implicating different mechanisms of GPDH induction. Nevertheless, in all cases, induction of the GPDH gene requires adrenal steroids and new protein synthesis, as the induction was blocked in adrenalectomized rats and by cycloheximide treatment, respectively. Our results suggest that the induction of the GPDH gene upon peripheral noxious stimulation is related to the endogenous response to pain as it is mimicked by exogenously applied analgesic drugs.

Evidence for a peripheral mechanism of action for the potentiation of the antinociceptive effect of morphine by dipyrone

The potentiation of the antinociceptive effect of morphine by dipyrone (metamizol) and the possible participation of a peripheral mechanism on such synergism were studied with the use of the formalin test in the rat. Nociception was induced by the intraplantar injection of diluted formalin (1%) in the right hind paw. Local administration of either dipyrone or morphine in the site of injury produced a dose-dependent antinociceptive effect. When combined, noneffective doses of morphine (1.25 microg/paw) and dipyrone (100 microg/paw) produced a significantly greater antinociceptive effect compared with either drug alone or saline. The opioid antagonist naloxone partly reversed the effect of the dipyrone-morphine combination. On the other hand, the inhibitor of nitric oxide (NO) synthesis, N(G)-L-nitro-arginine methylester (L-NAME), but not its inactive isomer, D-NAME, completely antagonized the effect of the dipyrone-morphine combination. These results suggest that the potentiation of morphine-induced antinociception by dipyrone in the formalin test requires an important participation of local release of NO, activating the NO-cyclic GMP pathway at the peripheral level.

Regulation of cyclooxygenase activity by metamizol

The ability of metamizol to inhibit cyclooxygenase-1 and cyclooxygenase-2 activities has been evaluated using different cyclooxygenase sources. Metamizol inhibited purified cyclooxygenase-1 and cyclooxygenase-2 with an IC50 of about 150 microg/ml. A similar IC50 value for cyclooxygenase-2 was obtained in lipopolysaccharide-activated broken murine macrophages. Consistent with these findings, molecular models of the complexes between cyclooxygenase-1 or cyclooxygenase-2 with 4-methylaminoantipyrine, the major active derivative of metamizol, suggested a common binding mode to both isoforms. In intact cells, however, the inhibition profiles were markedly different. The IC50 values of metamizol for cyclooxygenase-1 in intact bovine aortic endothelial cells (BAEC) cells and human platelets were 1730 +/- 150 microg/ml and 486 +/- 56 microg/ml, respectively. Inhibition of cyclooxygenase-2 activity in murine macrophages and primary human leukocytes activated by lipopolysaccharide yielded IC50 values of 12 +/- 1.8 microg/ml and 21 +/- 2.9 microg/ml, respectively. These data indicate that the IC50 values obtained with purified enzymes or disrupted cells cannot always be extrapolated to the cyclooxygenase inhibitory activity of nonsteroidal antiinflammatory drugs (NSAIDs) in intact cells. The data presented here also indicate that cyclooxygenase-2 inhibition could play an important role in the pharmacological effects of metamizol.