Acetylsalicylic acid potentiates the antinociceptive effect of morphine in the rat: involvement of the central serotonergic system

Monoamines as Drug Targets in Chronic Pain: Focusing on Neuropathic Pain

Monoamines are involved in regulating the endogenous pain system and indeed, peripheral and central monoaminergic dysfunction has been demonstrated in certain types of pain, particularly in neuropathic pain. Accordingly, drugs that modulate the monaminergic system and that were originally designed to treat depression are now considered to be first line treatments for certain types of neuropathic pain (e.g., serotonin and noradrenaline (and also dopamine) reuptake inhibitors). The analgesia induced by these drugs seems to be mediated by inhibiting the reuptake of these monoamines, thereby reinforcing the descending inhibitory pain pathways. Hence, it is of particular interest to study the monoaminergic mechanisms involved in the development and maintenance of chronic pain. Other analgesic drugs may also be used in combination with monoamines to facilitate descending pain inhibition (e.g., gabapentinoids and opioids) and such combinations are often also used to alleviate certain types of chronic pain. By contrast, while NSAIDs are thought to influence the monoaminergic system, they just produce consistent analgesia in inflammatory pain. Thus, in this review we will provide preclinical and clinical evidence of the role of monoamines in the modulation of chronic pain, reviewing how this system is implicated in the analgesic mechanism of action of antidepressants, gabapentinoids, atypical opioids, NSAIDs and histaminergic drugs.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

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.

Assessment of Antioxidant Effect of 2,5-Dihydroxybenzoic Acid and Vitamin A in Brains of Rats with Induced Hyperoxia

The aim of this study was to evaluate the effect of 2,5-dihydroxybenzoic acid, a salicylate derived from Acetyl salicylic acid (ASA) and vitamin A (vit A) on Na(+), K(+) ATPase enzyme and GSH levels in brain of rats exposed to hyperoxia (Hyp) as oxidant protocol. Rats were treated as follow: group I (control), group II (Hyp), group III (Hyp, ASA), group IV (vit A), group V (Hyp, vit A), group VI (Hyp, vit A, ASA). Vit A was given 5 days before and during Hyp, aspirin at the end of Hyp. Na(+),K(+) ATPase and total ATPase activity was significantly increased in group V. Levels of GSH showed a significant increase in group III, besides, levels of 2,5-dihydroxybenzoic acid as salicylate in plasma were significantly increased in group II. These results elucidate differences in the biochemical response of animal towards intake of various types of antioxidant substances, with increased GSH and salicylate in hyperoxia.

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.

Meloxicam, a specific COX-2 inhibitor, does not enhance the isoflurane minimum alveolar concentration reduction produced by morphine in the rat

A synergistic effect of nonselective cyclooxygenase (COX) inhibitors on morphine-induced decrease of isoflurane minimum alveolar concentration (MAC(ISO)) has been observed in the rat. We studied the influence of specific COX-2 inhibitors on this decrease of MAC. Sixty-four female rats were anesthetized with isoflurane in oxygen. The animals were grouped into saline solution, aspirin (30 mg/kg), morphine (1 mg/kg), morphine (1 mg/kg) + aspirin (30 mg/kg), meloxicam (1 and 3 mg/kg), and morphine (1 mg/kg) + meloxicam (1 and 3 mg/kg). Then the MAC(ISO) was determined from alveolar gas samples at the time of tail clamp. The groups treated with saline solution, aspirin, and 1 and 3 mg/kg meloxicam did not express any statistically relevant changes among them. The administration of morphine + meloxicam 1 or 3 mg/kg significantly reduced the MAC(ISO) just as in the group where only morphine was administered (morphine 1.35% +/- 0.07%, morphine + 1 mg/kg meloxicam 1.36% +/- 0.04%, and morphine + 3 mg/kg meloxicam 1.37% +/- 0.08%). The greatest reduction of MAC(ISO) was after administration of morphine + aspirin (1.19% +/- 0.05%). The administration of meloxicam does not potentiate the morphine-induced decrease of MAC(ISO) in the rat.

IMPLICATIONS

A synergistic effect between morphine and aspirin on isoflurane minimum alveolar concentration has been observed in the rat--an effect that does not occur between morphine and meloxicam.

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.

Effects of acetylsalicylic acid and morphine on neurons of the rostral ventromedial medulla in rat

Morphine exerts its analgesic effect via the endogenous pain control system consisting of the periaqueductal grey (PAG) and the rostral ventromedial medulla (RVM). Acetylsalicylic acid (ASA) may also act via this system, but so far this has only been demonstrated for the inhibitory effect on the tail-flick reflex with extremely high doses (200-300 mg/kg). Both drugs show synergistic effects on PAG neurons in vitro. It is unclear whether this mechanism accounts for the well-known analgesic synergism of these drugs in vivo. Thus, the effects of ASA (30 mg/kg) and morphine on off- and on-cells in the RVM and the jaw-opening reflex (JOR) were investigated in anesthetized rats. Under morphine, off-cell activity increased (+34%), on-cell activity decreased (-98%) and the reflex was suppressed (-53%). ASA increased off-cell activity (+20%) and decreased the activity of on-cells (-52%). After preceding ASA administration, the effects of morphine on off- and on-cells and on the reflex did not alter statistically. The experiments document the modulatory effect of a clinically relevant dose of ASA on RVM cells. This effect resembles that of morphine. The results do not support the hypothesis of a mediation of the analgesic synergism of morphine and ASA by the PAG-RVM-network in vivo.

Synergistic effects between codeine and diclofenac after local, spinal and systemic administration

This study was designed to evaluate the extent of the antinociceptive interaction between codeine and diclofenac at the local, spinal and systemic level. The effects of individual and fixed-ratio combinations of locally, spinally or orally given codeine and diclofenac were assayed using the formalin test in rats. Isobolographic analysis was employed to characterize the synergism produced by the combinations. Codeine, diclofenac and fixed-ratio codeine-diclofenac combinations produced a dose-dependent antinociceptive effect when administered locally, spinally or systemically. ED(30) values were estimated for the individual drugs and isobolograms were constructed. Theoretical ED(30) values for the combination estimated from the isobolograms were 422.2+/-50.5 microg/paw, 138.5+/-9.2 microg/rat, and 9.3+/-1.1 mg/kg for the local, spinal and oral routes, respectively. These values were significantly higher than the actually observed ED(30) values which were 211.1+/-13.6 microg/paw, 45.9+/-3.9 microg/rat, and 2.5+/-0.2 mg/kg, indicating a synergistic interaction. Systemic administration resulted in the highest increase in potency, being about fourfold, while spinal and local administration increased potency in two- and threefold, respectively. The fact that the highest synergism was observed after systemic administration suggests that the interaction is occurring at several anatomical sites. The results support the clinical use of this combination in pain management.

Low doses of nitroparacetamol or dexketoprofen trometamol enhance fentanyl antinociceptive activity

We have reported that subeffective doses of the nonsteroidal anti-inflammatory drug (NSAID) dexketoprofen trometamol enhances micro-opioid receptor agonist fentanyl antinociception. The aim of this study was to assess if this effect can also be observed with other new cyclooxygenase-inhibitors such as nitroparacetamol, and in responses to high intensity electrical stimulation (wind-up). Single motor units were recorded in male Wistar rats under alpha-chloralose anaesthesia. The antinociceptive effect of fentanyl was studied alone and in the presence of subeffective doses of dexketoprofen trometamol or nitroparacetamol. In responses to noxious mechanical stimulation, the potency of fentanyl was enhanced by more than threefold in the presence of the NSAIDs and no significant recovery was observed after 45 min. The opioid antagonist naloxone and the alpha(2)-adrenoceptor antagonist atipamezol did not reverse the effect. The enhancement of the effect of fentanyl in wind-up was lower though significant. We conclude that the co-administration of subeffective doses of new cyclooxygenase-inhibitors and the micro-opioid receptor agonist fentanyl should be considered as a potential pain therapy.

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.

Chronic daily headache: a scientist's perspective

Certain features of chronic daily headache, namely, increased headache frequency, expansion of headache area, and cutaneous allodynia, may imply sensitization of central nociceptive neurons in the trigeminal pathway. Repetitive activation of the trigeminal nerve can lead to a biologic and functional change in trigeminal nucleus caudalis neurons, characterized by a decrease in nociceptive threshold and receptive field expansion. Suppression of the endogenous pain control system can facilitate the process of central sensitization. Evidence of such suppression in patients with chronic daily headache includes decreased platelet serotonin, up-regulation of 5-HT2A receptors, increased platelet nitric oxide production, and increased levels of substance P and nerve growth factor in the cerebrospinal fluid. Results from a number of animal experiments have indicated that chronic analgesic exposure leads to changes in serotonin content and density of 5-HT2A receptors in the central nervous system. This plasticity of the serotonin-dependent pain control system may accelerate the process of sensitization; a biologic outcome that is expressed clinically by the development of chronic daily headache associated with analgesic overuse.

Facilitation of enkephalins catabolism inhibitor-induced antinociception by drugs classically used in pain management

The aim of this study was to investigate the facilitatory effects of subanalgesic or low doses of different drugs (acetylsalicylic acid, ibuprofen and morphine) on the antinociceptive responses induced by the endogenous opioid peptides, enkephalins, protected from their catabolism by the dual enkephalin-degrading enzymes inhibitor RB101. According to the analgesic profile of the three studied compounds different antinociceptive assays were used: the hot plate and formalin tests in mice, and the tail flick and paw pressure tests on inflamed paws in rats and polyarthritic rats. Facilitatory effects of subanalgesic doses of acetylsalicylic acid and ibuprofen on RB101-induced antinociceptive responses were observed in the early and late phases of the formalin test, respectively. In the hot plate, tail flick and paw pressure tests, the dose-dependent analgesic effects of RB101 were strongly potentiated by subanalgesic doses of morphine (0.5 mg/kg), while in these tests, acetylsalicylic acid and ibuprofen were unable to modify the RB101-induced antinociceptive responses. The synergism in antinociceptive effects observed with the combination of RB101 and morphine supported by isobolographic analysis, may have interesting clinical implications, considering both the lack of opiate drawbacks observed with RB101 and the high potentiation of its antinociceptive effects with very low doses of morphine.

[Aspirin, pain and inflammation]

SUBJECT

Acetylsalicylic acid (ASA) is among the most commonly analgesic, antipyretic and anti-inflammatory used drugs. The anti-inflammatory effects of ASA are mediated by the inhibition of cyclooxygenase enzymes with the subsequent decrease of prostaglandin synthesis.

NEW DATA

However, since this discovery of Vane in 1971, much of other mechanisms of anti-inflammatory action, without relation with cyclooxygenases, have been proposed. ASA has peripheric analgesic properties by reducing prostaglandin biosynthesis. But there is evidence that the analgesic effects could be mediated by central mechanisms with changes in the monoaminergic and opioid systems. ASA is essentially used in moderate pains with an inflammatory component (rheumatological disorders, headaches, dental and postoperative pains).

PERSPECTIVES

The clinical use of ASA at anti-inflammatory dose is less frequent because the other non steroidal anti-inflammatory drugs are as effective as ASA, but they are associated with less side effects. Nevertheless, the synergism of ASA and morphine association and the possible involvement of the central serotonergic and opiatergic systems in the antinociceptive activity of ASA could confer a greater role of ASA in pain management.