Eulàlia Planas

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).

Antinociceptive/Anti-Edema Effects of Liposomal Morphine during Acute Inflammation of the Rat Paw

We evaluated the anti-edema/antinociceptive effects of subcutaneous free and liposomal morphine in rats with carrageenan-induced inflammation of the paw. We assessed antinociception by the paw pressure test and edema by plethysmography. Unilamellar liposomes (150–200 nm) with 0.3% morphine hydrochloride were used; encapsulation signifcantly reduced the rate for release of morphine in vitro. During inflammation, the antinociceptive potency of free, but not liposomal morphine increased 2.5 times; moreover, duration of the effects was prolonged by encapsulation (p < 0.001). The anti-edema effects of liposomal morphine were more pronounced (p < 0.001) and of longer duration (p < 0.05). All the effects were reversed by naloxone. The results show that morphine encapsulation enhances the anti-edema effects and prolongs antinociception.

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

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

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

Opioids are used in humans in the management of chronic osteoarticular pains, but the development of tolerance to the analgesic effects after continuous administration is still not well understood. Our aim was to characterize morphine tolerance in a murine model of arthritis that mimics the sequence of events occurring in humans. Inflammation was induced by the intraplantar injection of complete Freunds adjuvant (CFA) and tolerance by the implantation of a 75-mg morphine pellet. We assessed the antihyperalgesic (plantar and Randall-Selitto tests), antiallodynic (Von Frey test), and antiexudative (Evans blue) effects of morphine, the μ-opioid receptor (MOR) mRNA levels in dorsal root ganglia (DRG), and MOR protein levels in DRG and plantar tissue. Inflammation induced plasma extravasation, and it significantly increased the antihyperalgesic effects of morphine (p < 0.05). Morphine pellet implantation decreased morphine potency in all tests. ED50 values decreased 4.4 and 7.3 times in the absence and presence of inflammation in the plantar test and 2.7 and 5.3 times in the Randall-Selitto test, whereas plasma extravasation decreased 4.2 times. MOR mRNA levels in the DRG were not affected 7 days after inflammation, whereas chronic morphine administration induced a discrete increase (p < 0.05). MOR protein in the DRG or the paw was unchanged. The results show that inflammation enhances the development of tolerance to the antihyperalgesic and antiexudative effects of morphine. At the molecular level, our results suggest that these effects are not mediated by changes in MOR expression but by other changes in receptor activation/internalization.

Fentanyl–trazodone–paracetamol triple drug combination: Multimodal analgesia in a mouse model of visceral pain

Multimodal or balanced analgesia is commonly used in the management of acute and chronic pain in humans, in order to achieve the best analgesic/safety profile. Here, by using a model of visceral acute tonic pain, the acetic acid-induced writhing test of mice, we show a synergistic interaction between fentanyl, trazodone and paracetamol on the inhibition of nociception. First of all, once assessed that all drugs induced dose-related antinociceptive effects, they were mixed in fixed ratio (1:1) combinations and a synergistic drug-drug interaction was obtained in all circumstances. Thereafter, we assayed the effects of the triple combination of fentanyl-trazodone-paracetamol and it was demonstrated that they displayed a potent synergistic interaction on the inhibition of acetic acid-mediated nociception. Interestingly, drug dosage reduction permitted to reduce the incidence of possible adverse effects, namely exploratory activity and motor coordination, thus it was demonstrated that it improved the benefit/risk profile of such treatment. Afterwards, we attempted to elucidate the mechanism of action of such interaction, by means of the non-selective opioid receptor antagonist naloxone. Interestingly, naloxone completely antagonized the antinociceptive effects of fentanyl, and it also partially reversed paracetamol and trazodone mediated analgesia. Furthermore, when naloxone was co-administered with the triple-drug treatment it blocked the previously observed enhanced antinociceptive effects of the combination. Thus, these results indicated that the endogenous opioid system played a main role in the present drug-drug interaction. Overall, the triple combination of fentanyl-trazodone-paracetamol induced a potent synergistic antinociceptive effect, which could be of interest for optimal multimodal clinical analgesia.

Histamine H3 receptor activation potentiates peripheral opioid-mediated antinociception: substance P role in peripheral inflammation in mice.

Opioids provide effective analgesia in adult patients with painful inflammatory diseases. The proposed mechanism of action is the activation of peripheral opioid receptors, which may be up-regulated in such conditions. Here, by using a chronic inflammation model, namely subplantar injection of Complete Freunds adjuvant, we show a peripheral synergistic interaction between the histamine H(3) receptor agonist R-(alpha)-methylhistamine and fentanyl on the inhibition of thermal hyperalgesia and of peripheral substance P accumulation. Firstly, dose-related effects obtained for the subplantar antinociceptive effect of fentanyl (0.05-1 microg) in the presence of a fixed dose of R-(alpha)-methylhistamine (12.5 microg) showed a shift to the left when compared to that obtained with fentanyl alone. In a similar way, the subcutaneous administration of fentanyl (0.005-0.1mg/kg) plus a fixed dose of R-(alpha)-methylhistamine (0.5mg/kg) induced a supra additive effect on the inhibition of substance P accumulation in the hind-paw skin of inflamed mice. Interestingly, when a neurokinin-1 receptor antagonist was co-administered, the antinociceptive effects of the combined treatment were potentiated. The peripheral adjuvant effect of R-(alpha)-methylhistamine on fentanyl antinociception and inhibition of substance P accumulation was also demonstrated by means of opioid and histamine H(3) receptors selective antagonists: first, naloxone blockade of fentanyl-mediated effects were partially reversed by co-administration of R-(alpha)-methylhistamine, and second, thioperamide partially antagonised the combined R-(alpha)-methylhistamine/fentanyl effects. Overall, our results clearly show that R-(alpha)-methylhistamine enhances fentanyl effects at peripheral sites, and that the control of substance P levels might be one of the mechanisms responsible of such interaction.

Synergistic interaction between dexamethasone and tramadol in a murine model of acute visceral pain

Tramadol is effective in the management of mild to moderate postoperative pain, but its administration is associated with nausea and vomiting. Patients treated with tramadol, often receive dexamethasone as antiemetic. The aim of our investigation was to assess if the two drugs interact in a murine model of acute visceral pain. Using the acetic acid writhing test in mice, we assessed the antinociceptive effects of tramadol and dexamethasone (a glucocorticoid with antiemetic effect) administrated individually and in a 1 : 1 fixed ratio combination. Tramadol and dexamethasone induced a dose‐dependent inhibition of the writhing response when administered individually, with ED50 values of 2.9 [2.09–4.31, 95% confidence limit (CL)] mg/kg, and 0.13 (0.05–0.29, 95% CL) mg/kg, respectively. The ED50 of the combination was 0.13 (0.01–0.29, 95% CL) mg/kg; the isobolographic and interaction index analysis revealed a synergistic interaction. The results suggest that the combination of tramadol and dexamethasone could be beneficial in the management of postoperative pain in humans.

Effects of Morphine and Liposomal Morphine in a Model of Intestinal Inflammation in Mice

We have investigated the antitransit effects of free and liposomal morphine in a model of intestinal inflammation. Mice received saline or croton oil orally, 3 h prior to evaluation, and gastrointestinal transit was measured 20 min afterwards. Peak/duration of effects, potency (ED50) and antagonism by naloxone and naloxone methiodide were evaluated. Peak effects occurred 30 and 40 min after administration of morphine and liposomal morphine, respectively. Encapsulated morphine had a more pronounced and prolonged effect than morphine. Comparison of the ED50S demonstrated that the potency of liposomal morphine was 3.5 times higher than that of morphine during inflammation; in addition, inflammation increased the potency of morphine and liposomal morphine, 3 and 9.2 times, respectively. The effects of morphine and liposomal morphine in croton oil-treated mice were reversed by naloxone and naloxone methiodide. The results show that during inflammation, the potency and duration of the antitransit effects of morphine are significantly enhanced by encapsulation.