Involvement of Adrenergic, Serotonergic, and Opioid Mechanisms in Tramadol-Induced Antinociception in Mice

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.

An isobolographic analysis of the adrenergic modulation of diclofenac antinociception

We evaluated the noradrenergic modulation of the antinociceptive activity of diclofenac in mice using the acetic acid writhing test. Dose-response curves were obtained for the antinociceptive effect of diclofenac, phenylephrine, clonidine, desipramine, prazosin, and yohimbine administered both systemically and intrathecally, and ED(50)s were calculated. Noradrenergic modulation was evaluated by performing an isobolographic analysis of the systemic or intrathecal coadministration of fixed-ratio combinations of diclofenac with each adrenergic drug. The systemic, but not the intrathecal, combinations of diclofenac with phenylephrine or clonidine showed supraadditivity, suggesting that the activation of alpha(1) and alpha(2) adrenoceptors interfered with the nociceptive transmission at spinal and supraspinal levels. Supraadditive effects were not demonstrated for the intrathecal injection of diclofenac combined with phenylephrine, clonidine and a selective norepinephrine uptake inhibitor (desipramine) or adrenergic antagonists. We conclude that interaction between adrenoceptors and diclofenac can modulate antinociception by activating common or different mechanisms. Diclofenac has an antinociceptive activity that, in addition to cyclooxygenase inhibition, can be modulated by additive and supraadditive interactions with adrenergic drugs.

IMPLICATIONS

Diclofenac analgesia in mice can be modulated by interaction with adrenergic drugs. The systemic but not the intrathecal administration of phenylephrine and clonidine produced supraadditive interactions. For desipramine, prazosin, and yohimbine, supraadditive interactions were not statistically demonstrated. The coadministration of drugs inducing supraadditive effects could be clinically relevant for the treatment of chronic pain because of reduction of doses and side effects.