Intrathecal co-administration of morphine and nimodipine produces higher antinociceptive effect by synergistic interaction as evident by injecting different doses of each drug in rats

Earlier, we reported that morphine-nimodipine combination produces significantly higher antinociception after intrathecal but not after systemic administration in mice. Different doses of morphine and nimodipine (5 mu g of morphine, 5 mu g of nimodipine, 5 mu g each of morphine and nimodipine, 10 mu g of morphine, 10 mu g of nimodipine, 10 mu g morphine with 5 mu g nimodipine and 5 mu g of morphine with 10 mu g of nimodipine) were now injected intrathecally in Wistar rats to further characterise this antinociceptive effect. The acute antinociceptive effect was measured by the tail-flick test between 15 min to 7 h. The onset of maximum antinociception (100% MPE) was earlier (by 15 min) in nimodipine (5 mu g) than in morphine (5 mu g) treated group (by 30 min). Though earlier in onset, 5 Kg nimodipine produced transient antinociception, which was significantly higher than saline treated controls for the initial 30 min only. Morphine (5 mu g) produced significantly higher antinociception between 15 min to 3:30 h in comparison to control animals. However, co-administration of both morphine and nimodipine led to significantly higher antinociception than morphine alone at 4:00 h and also between 5:00 to 6:30 h. Interestingly, the combined antinociceptive action of morphine and nimodipine was not significantly different from 10 mu g of morphine, which indicated synergistic interaction. Naloxone (5 mg/kg) could reverse this antinociceptive effect of morphine-nimodipine combination though it failed to reverse nimodipine (5 mu g)-mediated antinociception at 15 min. Increasing the dose of either morphine or nimodipine to 10 mu g did not increase antinociception except between 6:30-7:00 h. No obvious side effect was noted after administration of either morphine or nimodipine or both. (c) 2007 Elsevier B.V. All rights reserved.