Chimeric peptide of Met-enkephalin and FMRFa induces antinociception and attenuates development of tolerance to morphine antinociception.

A synthetic chimeric peptide of Met-enkephalin and FMRFamide (YGGFMKKKFMRFa), based on MERF was synthesized. This peptide was tested for possible antinociceptive effects using the tail flick test in mice. The effect of the chimeric peptide on morphine antinociception and development of tolerance to the antinociceptive action of morphine was also investigated. The chimeric peptide produced significant, dose-dependent antinociception (40, 60 and 90 mg/kg) in the tail flick test. Pretreatment with naloxone (5 mg/kg, IP) significantly attenuated the antinociceptive effect induced by the chimeric peptide (90 mg/kg, IP), indicating involvement of an opioidergic mechanism. In combination experiments with morphine, the antinociceptive dose of the chimeric peptide (60 mg/kg, IP) potentiated morphine (7 mg/kg, IP) antinociception. A low dose of the chimeric peptide (10 mg/kg, IP), that did not produce significant antinociception on its own, also potentiated morphine antinociception. In the tolerance studies, male albino mice received twice daily injections of morphine (20 mg/kg, IP) followed by either saline (0.1 ml) or chimeric peptide (80 mg/kg, IP) for a period of 4 days. A control group received twice daily injections of saline (0.1 ml) for the same period. When tested on Day 5, tolerance to antinociceptive action of morphine (15 mg/kg, IP) was evidenced by decreased response in chronic morphine plus saline treated mice compared to control group. Concurrent administration of chimeric peptide (80 mg/kg, IP) with morphine significantly attenuated the development of tolerance to the antinociceptive action of morphine. The preliminary results of this study demonstrate that peripherally administered chimeric peptide can produce dose dependent, naloxone reversible, antinociception; potentiate morphine antinociception and attenuate morphine tolerance, indicating a possible role of these type of amphiactive sequences in antinociception and its modulation. These chimeric peptides may also prove to be useful tools for further ascertaining the role of FMRFa family of peptides in mechanisms leading to opiate tolerance and dependence.     

Isobolographic analysis of the dual-site synergism in the antinociceptive response of tramadol in the formalin test in rats

Tramadol is an atypical opioid with a complex mechanism of action including a synergistic interaction between the parent drug and an active metabolite. The local action of the parent drug is poorly documented. This study was designed to evaluate the site-site interaction of the antinociception produced by tramadol given by two different routes. The effects of individual and fixed-ratio combinations of intraplantar (i.pl.) and intraperitoneal (i.p.) tramadol were evaluated using the formalin test in rats. Isobolographic analysis was employed to identify the synergy produced by combinations. In both first and second phases of the formalin test, tramadol was active not only by the systemic (ED50 10.2+/-2.1 and 7.1+/-0.5 mg/kg i.p.) but also by the local route (ED50 171.0+/-44.8 and 134.6 microg/paw i.pl.). The isobolographic analysis revealed a "self-synergism" in the antinociceptive effect between the two routes of administration, as the experimental ED50 (211.1+/-13.6 and 45.9+/-3.9 "dose units" phase 1 and 2, respectively) of the combination was significantly lower than the theoretical ED50 (422.2+/-50.5 and 138.5+/-9.2 "dose units"). The mechanism underlying this self-synergism appears to be partially opioid since systemic but not local naloxone reversed the potentiation. The observed dual-site interaction in the antinociceptive action of tramadol provides insights for alternatives in the management of pain.     

Electroacupuncture Effects in a Rat Model of Complete Freund’s Adjuvant-Induced Inflammatory Pain: Antinociceptive Effects Enhanced and Tolerance Development Accelerated

We have previously shown that electroacupuncture (EA) produced antinociception through the release of endogenous opioid peptides to activate opioid receptors during acute nociception. EA produced tolerance after its prolonged application. It has reported that 100 Hz EA could reduce mechanical hyperalgesia in complete Freund’s adjuvant (CFA)-induced inflammatory nociception rats. The present study aims to investigate the antinociceptive effect of EA and the development of EA tolerance in chronic inflammatory nociception rats with CFA injection into the hind paw plantar. The results showed that the antinociceptive effect of 100 Hz EA was significantly enhanced in CFA-induced inflammatory nociception rats. Naloxone at 20 mg/kg could significantly block this antinociceptive effect. Chronic tolerance to EA was developed faster in CFA-induced inflammatory nociception rats than in normal rats. Therefore, 100 Hz EA could enhance antinociceptive effects and accelerate tolerance development in CFA-induced inflammatory nociception rats. The enhancement of EA antinociceptive effect in CFA-induced inflammatory nociception rats might involve the endogenous opioid peptides such as dynorphin. Special issue article in honor of Dr. Ji-Sheng Han.     

Analysis of interaction between etoricoxib and tramadol against mechanical hyperalgesia of spinal cord injury in rats

Drug combinations have the potential advantage of greater analgesia over monotherapy. The present study was aimed to assess any possible interaction (additive or potentiation) in the antinociceptive effects of etoricoxib; a novel cyclooxygenase-2 inhibitor, and tramadol; a typical opioid agonist when administered in combination against mechanical hyperalgesia induced by spinal cord injury in rats. The nature of interaction was analyzed using surface of synergistic interaction (SSI) analysis and an isobolographic analysis. Etoricoxib or tramadol when administered alone to rats, exhibited different antihyperalgesic potencies (ED50 etoricoxib: 0.58+/-0.19 mg/kg, po; ED50 tramadol: 9.85+/-0.57 mg/kg, po). However, both the drugs were found to be long acting against this model of hyperalgesia. Further, etoricoxib and tramadol were co-administered in fixed ratios of ED50 fractions. One combination (0.29/4.79 mg/kg, po: etoricoxib/tramadol) exhibited additivity and other three combinations (0.15/2.39, 0.08/1.19, and 0.04/0.59 mg/kg, po: etoricoxib/tramadol) resulted in potentiation when analyzed by SSI. The SSI was calculated from the total antihyperalgesic effect produced by the combination after the subtraction of the antihyperalgesic effect produced by each of the individual drug. In the isobolographic analysis, the experimental ED50 was found to be far below the line of additivity also indicating a significant (P < 0.05) synergistic antihyperalgesic effect when etoricoxib and tramadol was co-administered to rats. The synergistic antihyperalgesic effect of etoricoxib and tramadol combination suggests that these combinations may have clinical utility in mechanical hyperalgesia associated with spinal injury.     

The effects of NMDA receptor antagonists on acute morphine antinociception in mice

Summary.  Antagonists of the N-methyl-d-aspartate (NMDA) receptor complex inhibit the development of tolerance to antinociceptive effects of morphine and upon acute administration, influence morphine antinociceptive activity. The analysis of numerous studies investigating acute interaction between NMDA receptor antagonists and morphine in mice indicate a variety of procedural differences and reveal that these compounds may potentiate, attenuate and produce no effect on morphine antinociception. The conditions responsible for such conflicting experimental outcome of acute interaction remain unclear. It appears that the effects of NMDA receptor antagonists on morphine tolerance are not causally related to their acute effects on morphine antinociception. Received July 6, 2001 Accepted August 6, 2001 Published online August 9, 2002     

In vivo characterization of the effects of ghrelin on the modulation of acute pain at the supraspinal level in mice

Ghrelin, an acylated peptide produced in the stomach, increases food intake and growth hormone secretion, inhibits pro-inflammatory cascade, etc. Ghrelin and its receptor (GHS-R1a) mRNA were found in the area related to the regions for controlling pain transmission, such as the hypothalamus, the midbrain, the spinal cord, etc. Ghrelin has been shown to have antinociceptive activity and also anti-inflammatory properties in inflammatory pain and chronic neuropathic pain. Therefore, the aim of the present study was to investigate the effects of ghrelin for the first time in the acute pain modulation at the supraspinal level, using the tail withdrawal test and hot-plate test in mice. Intracerebroventricular (i.c.v.) administration of ghrelin (mouse, 0.1–3 nmol) produced a dose- and time-related antinociceptive effect in the tail withdrawal test and hot-plate test, respectively. Antinociceptive effect elicited by ghrelin (i.c.v., 1 nmol) was significantly antagonized by opioid receptor antagonist naloxone (i.c.v., 10 nmol co-injection or i.p., 10 mg/kg, 10 min prior to ghrelin) in both tail withdrawal test and hot-plate test. At these doses, naloxone significantly antagonized the antinociceptive effect induced by morphine (i.c.v., 3 nmol). Ghrelin (i.c.v., 1 nmol)-induced antinociception was significantly antagonized by co-injection with 10 nmol [d-Lys3]-GHRP-6, the selective antagonist of GHS-R1a identified more recently, while [d-Lys3]-GHRP-6 (10 nmol) alone induced neither hyperalgesia nor antinociception. Overall this data indicate that ghrelin could produce antinociception through an interaction with GHS-R1a and with the central opioid system. Thus ghrelin may be a promising peptide for developing new analgesic drugs.     

Antinociceptive cross-tolerance between [D-Arg2]-dermorphin tetrapeptide analogs and morphine

Cross-tolerance between [D-Arg2]-dermorphin tetrapeptide analogs and morphine with respect to antinociception was examined in the present set of experiments. Systemic administration of H-Tyr-D-Arg-Phe-Gly-NH2 (TDAPG-NH2), H-Tyr-D-Arg-Phe-beta-Ala-OH (TDAPA) or morphine over a period of 5 days produced the development of tolerance. In the cross-tolerance study, antinociception after subcutaneous (SC), intracerebroventricular (ICV) and intrathecal (IT) administrations of TDAPG-NH2 and TDAPA in morphine-tolerant mice was not significantly different from their respective effects in saline-pretreated control mice. A marked tolerance to SC- and ICV-administered morphine was seen in mice made tolerant to TDAPG-NH2 and TDAPA. However, IT administration of morphine produced no significant decrement in the antinociceptive activity in mice made tolerant to TDAPG-NH2 and TDAPA. These data indicate that [D-Arg2]-dermorphin tetrapeptide analogs can produce significant antinociception in morphine-tolerant mice.     

Morphine and Clonidine Combination Therapy Improves Therapeutic Window in Mice: Synergy in Antinociceptive but Not in Sedative or Cardiovascular Effects

Opioids are used to manage all types of pain including acute, cancer, chronic neuropathic and inflammatory pain. Unfortunately, opioid-related adverse effects such as respiratory depression, tolerance, physical dependence and addiction have led to an underutilization of these compounds for adequate pain relief. One strategy to improve the therapeutic utility of opioids is to co-administer them with other analgesic agents such as agonists acting at α₂-adrenergic receptors (α₂ARs). Analgesics acting at α₂ARs and opioid receptors (ORs) frequently synergize when co-administered in vivo. Multimodal analgesic techniques offer advantages over single drug treatments as synergistic combination therapies produce analgesia at lower doses, thus reducing undesired side effects. This inference presumes, however, that the synergistic interaction is limited to the analgesic effects. In order to test this hypothesis, we examined the effects of α₂AR/OR combination therapy in acute antinociception and in the often-undesired side effects of sedation and cardiovascular depression in awake unrestrained mice. Morphine, clonidine or their combination was administered by spinal or systemic injection in awake mice. Antinociception was determined using the warm water tail flick assay (52.5°C). Sedation/motor impairment was evaluated using the accelerating rotarod assay and cardiovascular function was monitored by pulse oximetry. Data were converted to percent maximum possible effect and isobolographic analysis was performed to determine if an interaction was subadditive, additive or synergistic. Synergistic interactions between morphine and clonidine were observed in the antinociceptive but not in the sedative/motor or cardiovascular effects. As a result, the therapeutic window was improved ∼200-fold and antinociception was achieved at non-sedating doses with little to no cardiovascular depression. In addition, combination therapy resulted in greater maximum analgesic efficacy over either drug alone. These data support the utility of combination adrenergic/opioid therapy in pain management for antinociceptive efficacy with reduced side-effect liability.     

The involvement of nitric oxide in the analgesic effects of ketamine

We investigated the contribution of NO-cyclic GMP (cGMP) pathway to the antinociceptive effects of ketamine in mice by using the nitric oxide synthase inhibitor, nitro(g)- L-arginine methyl ester (L-NAME). Intraperitoneal (i.p.) (1, 5 or 10 mg/kg) or intrathecal (i.th.) (10, 30 or 60 microg/mouse) administration of ketamine produced dose-dependent antinociceptive effects in the acetic acid-induced writhing and formalin tests but not in the tail-flick nor in hot-plate tests. Pretreatment of mice with L-NAME (10 mg/kg, i.p.) which produced no antinociception on its own, significantly inhibited the antinociceptive effect of ketamine (1, 5 or 10 mg/kg, i.p.). However, L-NAME (30 microg/mouse) was given intrathecally, it neither modified the antinociceptive effect of i.th. ketamine (10, 30 or 60 microg/mouse) nor did it produce an antinociceptive effect alone. These data suggest that the activation of the NO-cGMP pathway probably at the supraspinal level, but not spinal level, contributes to the antinociceptive effects of ketamine.     

Examination of the interaction between peripheral diclofenac and gabapentin on the 5% formalin test in rats

It has been shown that the association of diclofenac with other analgesic agents can increase its antinociceptive activity, allowing the use of lower doses and thus limiting side effects. Therefore, the aim of the present study was to examine the possible pharmacological interaction between diclofenac and gabapentin at the peripheral level in the rat using the 5% formalin test and isobolographic analysis. Diclofenac, gabapentin or a fixed-dose ratio diclofenac-gabapentin combination were administrated locally in the formalin-injured paw and the antinociceptive effect was evaluated using the 5% formalin test. All treatments produced a dose-dependent antinociceptive effect. ED30 values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical ED30 for the diclofenac-gabapentin combination was 597.5+/-87.5 microg/paw, being significantly higher than the actually observed experimental value, 170.9+/-26.07 microg/paw. These results correspond to a synergistic interaction between diclofenac and gabapentin at the peripheral level, potency being about three times higher with regard to that expected from the addition of the effects of the individual drugs. Data suggest that low doses of the diclofenac-gabapentin combination can interact synergistically at the peripheral level and therefore this drug association may represent a therapeutic advantage for the clinical treatment of inflammatory pain.     

Modulation of acute morphine tolerance by corticotropin-releasing factor and dynorphin A in the mouse spinal cord.

Previously, we have demonstrated that intrathecally (i.t.) administered corticotropin-releasing factor (CRF) in mice produces stimulus-specific antinociception and modulation of morphine-induced antinociception by mechanisms involving spinal kappa opioid receptors. Recently, we also have found that CRF releases immunoreactive dynorphin A, a putative endogenous kappa opioid receptor agonist, from superfused mice spinal cords in vitro. Dynorphin A administered intracerebroventricularlly (i.c.v.) to mice has been shown to modulate the expression of morphine tolerance. In the present study, the possible modulatory effects of i.t. administered CRF as well as dynorphin A on morphine tolerance were studied in an acute tolerance model. Subcutaneous administration of 100 mg/kg of morphine sulfate (MS) to mice caused an acute tolerance to morphine-induced antinociception. The antinociceptive ED50 of MS was increased from 4.4 mg/kg (naive mice) to 17.9 mg/kg (4 hours after the injection of 100 mg/kg MS). To study the modulatory effects of spinally administered CRF and dynorphin A on the expression of morphine tolerance, CRF and dynorphin A were injected i.t. at 15 min and 5 min, respectively, before testing the tolerant mice by the tail-flick assay. The antinociceptive ED50 of MS in tolerant mice was decreased to 8.8 mg/kg and 7.1 mg/kg, respectively, after i.t. administration of CRF (0.1 nmol) and dynorphin A (0.2 nmol). In contrast, 0.5 nmol of alpha-helical CRF (9-41), a CRF antagonist and 0.4 nmol of norbinaltorphimine, a highly selective kappa opioid receptor antagonist, when administered i.t. at 15 min before the tail-flick test in tolerant mice, increased the antinociceptive ED50 of MS to 56.6 mg/kg and 88.8 mg/kg, respectively. These data confirmed the modulatory effect of dynorphin A on morphine tolerance and suggested that CRF, which releases dynorphin A in several central nervous system regions, also plays a modulatory role in the expression of morphine tolerance.     

Substance P and antinociception

Substance P (SP)-induced antinociception is still a topic of controversy. Some investigators have failed to see an antinociceptive effect of SP, particularly following intraperitoneal administration. In the present experiments SP induced significant hot plate antinociception in male mice, following intraperitoneal administration. SP exhibited a bell-shaped dose response curve, and the antinociceptive effect was dependent on the pH of the vehicle. The antinociceptive effect of SP lasted for at least 1 hr and was naloxone-reversible. The antinociceptive effect of SP could be prevented by housing subjects collectively rather than individually during the experiment. In conclusion, the bell-shaped dose response curve, the solution pH and different testing procedures all influence the effects of SP on nociception. Given this complexity, it is not surprising that some experiments fail to demonstrate antinociception following SP administration.     

Antinociceptive effect of topiramate in models of acute pain and diabetic neuropathy in rodents

This study assesses the antinociceptive effect induced by different dosages of topiramate (TP), an anticonvulsant drug that is orally administered in models of neuropathic pain and acute pain in rats and mice, respectively. Orally administered TP (80 mg/Kg) in mice causes antinociception in the first and second phases of a formalin test, while in doses of 20 and 40 mg/Kg it was only effective in the second phase. TP (80 mg/Kg, p.o) also exhibited antinociceptive action in the hot plate test, however, it did not have an effect in the capsaicin test in mice, nor in the model of neuropathic pain in diabetic rats. The antinociceptive effect caused by TP (80 mg/Kg, p.o) in the formalin test was reversed by prior treatment with naloxone (opioid antagonist), but not with glibenclamide (antagonist of the potassium channel), ondansetron (antagonist of the serotonin 5HT3 receptor) or cyproheptadine (antagonist of the serotonin 5HT2A receptor).The data show that TP has an important antinociceptive effect in the models of nociception induced by chemical (formalin) or thermal (hot plate) stimuli, and that the opioid system plays a part in the antinociceptive effect, as shown by formalin.     

Effects of neomycin on the development of tolerance to morphine antinociception.

The effects of neomycin on the development of tolerance to morphine antinociception were examined in mice. Because neomycin did not readly cross blood brain barrier, we examined the effects of neomycin following systemic, intracerebroventricular (i.c.v.) and intrathecal (i.t.) injections on the morphine tolerance. Daily subcutaneous (s.c.), i.c.v. and i.t. injections of morphine produced tolerance regardless of route of administration. Both i.c.v. and i.t. neomycin, which alone produced no changes in the basal tail flick latencies, significantly attenuated the development of tolerance to antinociception produced by repeated systemic morphine, while intraperitoneal (i.p.) administration of neomycin did not affect morphine tolerance. Further, i.c.v. and i.t. neomycin attenuated the development of tolerance to antinociception produced by repeated i.c.v. and i.t. morphine, respectively, which were not attenuated by systemic neomycin. This results indicate a potential role for neomycin-sensitive Ca2+ channels on the development of tolerance to the morphine antinoception.     

The role of T-type calcium channels in morphine analgesia,development of antinociceptive tolerance and dependence to morphine,and morphine abstinence syndrome

Involvement of T-type voltage dependent Ca2+ channels (VDCCs) on morphine antinociception, in the development of tolerance and dependence to morphine, and naloxone-precipitated abstinence syndrome in morphine dependent mice was examined by using mibefradil, a T-type VDCCs blocker. Mice were rendered tolerant and dependent on morphine by subcutaneous (s.c.) implantation of a morphine pellet containing 75 mg of morphine base for 72 hr. The tail-flick test was used to assess the nociceptive threshold. Coadministration of acute mibefradil (10 mg/kg, i.p.) with morphine enhanced the antinociceptive effects of acute morphine. Repeated mibefradil administration (10 mg/kg, i.p., just before, 24 and 48 hr after morphine pellet implantation) completely blocked the development of tolerance to the antinociceptive effect of morphine and even by this effect reached supersensitivity to morphine. However, repeated mibefradil treatment did not alter the development of dependence to morphine assessed by the A(50) values of naloxone (s.c.) required to precipitate withdrawal jumping 72 hr after morphine pellet. But, acute mibefradil (10, 30, and 50 mg/kg, i.p.) dose dependently decreased the expression of morphine abstinence syndrome when given directly 30 min prior to naloxone (0,05 mg/kg, s.c.) 72 hr after morphine pellet. These results indicate a critical role of T-type VDCCs in morphine antinociception, the development of tolerance to the antinociceptive effects of morphine and in morphine abstinence syndrome.     

Comparison of the antinociceptive effect of celecoxib, diclofenac and resveratrol in the formalin test

The peripheral antinociceptive effect of the selective COX-2 inhibitor celecoxib in the formalin-induced inflammatory pain was compared with that of resveratrol (COX-1 inhibitor) and diclofenac (non-selective COX inhibitor). Rats received local pretreatment with saline, celecoxib, diclofenac or resveratrol followed by 50 microl of either 1% or 5% formalin. Peripheral administration of celecoxib did not produce antinociception at either formalin concentration. In contrast, diclofenac and resveratrol produced a dose-dependent antinociceptive effect in the second phase of both 1% and 5% formalin test. The peripheral antinociception produced by diclofenac or resveratrol was due to a local action, as drug administration in the contralateral paw was ineffective. Results indicate that the selective COX-2 inhibitor celecoxib does not produce peripheral antinociception in formalin-induced inflammatory pain. In contrast, selective COX-1 and non-selective COX inhibitors (resveratrol and diclofenac, respectively) are effective drugs in this model of pain.     

Intrathecal substance P augments morphine-induced antinociception: Possible relevance in the production of substance P N-terminal fragments

The present study sought to examine the mechanism of substance P to modulate the antinociceptive action of intrathecal (i.t.) morphine in paw-licking/biting response evoked by subcutaneous injection of capsaicin into the plantar surface of the hindpaw in mice. The i.t. injection of morphine inhibited capsaicin-induced licking/biting response in a dose-dependent manner. Substance P (25 and 50 pmol) injected i.t. alone did not alter capsaicin-induced nociception, whereas substance P at a higher dose of 100 pmol significantly reduced the capsaicin response. Western blots showed the constitutive expression of endopeptidase-24.11 in the dorsal and ventral parts of lumbar spinal cord of mice. The N-terminal fragment of substance P (1–7), which is known as a major product of substance P by endopeptidase-24.11, was more effective than substance P on capsaicin-induced nociception. Combination treatment with substance P (50 pmol) and morphine at a subthreshold dose enhanced the antinociceptive effect of morphine. The enhanced effect of the combination of substance P with morphine was reduced significantly by co-administration of phosphoramidon, an inhibitor of endopeptidase-24.11. Administration of d-isomer of substance P (1–7), [d-Pro², d-Phe⁷]substance P (1–7), an inhibitor of [³H] substance P (1–7) binding, or antisera against substance P (1–7) reversed the enhanced antinociceptive effect by co-administration of substance P and morphine. Taken together these data suggest that morphine-induced antinociception may be enhanced through substance P (1–7) formed by the enzymatic degradation of i.t. injected substance P in the spinal cord.     

Synthesis and antinociceptive effects of endomorphin-1 analogs with C-terminal linked by oligoarginine

Endomorphins (EMs) cannot be delivered into the central nervous system (CNS) in sufficient quantity to elicit antinociception when given systemically because they are severely restricted by the blood-brain barrier (BBB). In the present study, we investigated herein a series of EM-1 analogs with C-terminal linked by oligoarginine in order to improve the brain delivery and antinociception after systemic administration. Indeed, all these analogs decreased the opioid receptor affinity and in vitro pharmacological activity. Moreover, analogs 4, 7-9 produced a less potent antinociceptive activity after intracerebroventricular (i.c.v.) administration, with the ED₅₀ values about 11- to 13-fold lower potencies than that of EM-1. Nevertheless, our results revealed that EM-1 failed to induce any significant antinociception at a dose of 50 μmol/kg after subcutaneous (s.c.) administration, whereas equimolar dose of these four analogs produced a little low but significant antinociceptive effects. Naloxone (10 nmol/kg, i.c.v.) significantly blocked the antinociceptive effects, indicating an opioid and central mechanism. These results demonstrated that C-terminal of EM-1 linked to oligoarginine improved the brain delivery, eliciting potent antinociception following peripheral administration.     

The antinociceptive effect of acetylsalicylic acid is differently affected by a CB1 agonist or antagonist and involves the serotonergic system in rats

AimsCombinations of non-steroidal anti-inflammatory drugs (NSAIDs) and cannabinoids are promising because of their potential synergistic effects in analgesia, resulting in a reduction in dosage and minimizing adverse reactions. The analgesic effect of acetylsalicylic acid (ASA), probably due to a central mechanism, also implicates changes in the central monoaminergic system. Therefore, we decided to evaluate the antinociceptive interaction between the CB₁ receptor agonist, HU210, and ASA in tests involving central pain in rats as well as the implication of the central serotonergic system thereon.Main methodsThe selective CB₁ antagonist SR141716A and the potent cannabinoid agonist HU210 were evaluated alone and in combination with ASA in both algesimetric tests (hot-plate and formalin tests) and for 5-HT activity and 5-HT₂ receptor density and affinity.Key findingsASA or HU210 alone showed a dose-dependent effect in both tests. HU210, at an inactive dose, significantly increased the antinociceptive effect of the sub-active dose of ASA. SR141716A (1.5 mg/kg i.p.) was ineffective per se and failed to modify antinociception induced by the HU210 plus ASA combination in either test. HU210 plus ASA significantly decreased the 5-HIAA/5-HT ratio and the 5-HT₂ receptor density in the frontal cortex, changes not antagonized by SR141716A.SignificanceThe present study provides evidence that mutual potentiation of the antinociceptive effects of HU210 and ASA may, at least partly, depend on serotonergic mechanisms, with an indirect participation of cannabinodiergic mechanism. In conclusion, combinations of low doses of cannabinoids and NSAIDs may be of interest from the therapeutic point of view.     

Environmental and intraperitoneal ethanol influences morphine antinociceptive effect in mice

The ability of acute environmental or intraperitoneal (i.p.) ethanol to influence morphine antinociceptive effect was studied in mice. In order to induce tolerance to morphine analgesia, mice received daily injections of 10 mg/Kg morphine over a period of 10 days. Mice were divided into three groups: i.p. ethanol (E), environmental ethanol (E*), and control saline (M). During the induction of tolerance these groups were treated identically except on days 1 and 11. On these days, 10 minutes prior to morphine injection, mice received either i.p. ethanol (1g/Kg), environmental ethanol (a bottle of 10% ethanol placed next to the animals cage during the experiments), or an equivalent volume of saline. Analgesia was assessed using a standard hot plate protocol and dose-response cumulative curves for morphine analgesia were obtained on days 1 and 11. On day 1, both the i.p. and environmental administration of ethanol showed similar morphine-potentiation effects [Mean Effective Dose: ED50 (M1)=4.5 mg/kg; ED50 (E1)=2.4 mg/kg; ED50 (E*1)=2.1 mg/kg]. On day 11, control group mice showed a reduction of morphine analgesia at test [ED50 (M11)=14.1 mg/kg]. Mice receiving i.p. and environmental ethanol again showed a leftward shift in dose-response cumulative curves for morphine antinociception with respect to controls [ED50 (E11)=9.1 mg/kg; ED50 (E*11)=4.7 mg/kg]. I.p. ethanol administration at non-antinociceptive doses enhances the morphine antinociception effect similarly in tolerant and non-tolerant (naive) mice. The presence of environmental ethanol can also induce a similar pattern of increase in morphine antinociception effect.