ABSTRACT
BACKGROUND: Opioids have been used for the management of pain and coadministration of two opioids may induce synergism. In a model of tonic pain, the acetic acid writhing test and in a phasic model, the hot plate, the antinociceptive interaction between fentanyl, methadone, morphine, and tramadol was evaluated. RESULTS: The potency of opioids in the writhing test compared to the hot plate assay was from 2.5 (fentanyl) to 15.5 (morphine) times, respectively. The ED50 was used in a fixed ratio for each of the six pairs of opioid combinations, which, resulted in a synergistic antinociception except for methadone/tramadol and fentanyl/tramadol which were additive, in the hot plate. The opioid antagonists naltrexone, naltrindole and nor-binaltorphimine, suggests that the synergism of morphine combinations are due to the activation of MOR subtypes with partially contribution of DOR and KOR, however fentanyl and methadone combinations are partially due to the activation of MOR and DOR subtypes and KOR lack of participation. The antinociceptive effects of tramadol combinations, are partially due to the activation of MOR, DOR and KOR opioid subtypes. CONCLUSION: These results suggets that effectiveness and magnitude of the interactions between opioids are dependent on pain stimulus intensity.
Subject(s)
Analgesics, Opioid/pharmacology , Fentanyl/pharmacology , Morphine/pharmacology , Nociceptive Pain , Receptors, Opioid/metabolism , Animals , Drug Synergism , Fentanyl/agonists , Male , Mice , Morphine/agonists , Narcotic Antagonists/pharmacology , Nociceptive Pain/drug therapy , Nociceptive Pain/mortality , Nociceptive Pain/physiopathologyABSTRACT
Intramuscular (i. m.) administration in the minimum effective dose (MED) of central analgesics of fentanyl and dipyrone, polyamine agonist spermine and also IEM-1460 (IEM-1460 is AMPA receptors antagonist and agonist of the NMDA polyamine receptor site) causes the maximal analgesic effect in the tail flick test in rats. The combined i.m. administration of dipyrone with IEM-1460 and spermine in threshold, noneffective alone doses according 1/5 part from their MED leads to decrease of MED dipyrone in the combination with IEM-1460 in 120 times, and MED dipyrone in combination with spermine--in 10 times. The combined i.m. administration of fentanyl with IEM-1460 and spermine in above mentioned threshold doses leads to decrease of MED fentanyl in the combination with IEM-1460 in 150 times, and MED fentanyl in the combination with spermine--in 15 times.
Subject(s)
Adamantane/analogs & derivatives , Anesthetics, Intravenous/pharmacology , Anti-Inflammatory Agents, Non-Steroidal/pharmacology , Dipyrone/pharmacology , Fentanyl/pharmacology , Spermine/pharmacology , Adamantane/agonists , Adamantane/pharmacology , Animals , Anti-Inflammatory Agents, Non-Steroidal/agonists , Dipyrone/agonists , Drug Synergism , Fentanyl/agonists , Male , Rats , Spermine/agonistsABSTRACT
RATIONALE: Discriminative stimulus effects of mu opioids vary systematically as a function of training dose. Differences among training doses may arise from multiple mechanisms. OBJECTIVES: In vivo apparent pA(2) analyses were used to examine the contributions of opioid mechanisms to stimulus control by low and high training doses of the mu opioid fentanyl. METHODS: Saline and one of two doses of fentanyl, administered s.c., were established as discriminative stimuli in two groups of rats (low training dose group: 0.01 mg/kg; high training dose group: 0.04 mg/kg). Generalization tests and in vivo apparent pA(2) analyses were used to evaluate receptor mechanisms of stimulus control. RESULTS: Fentanyl, etonitazene, methadone, and morphine evoked full fentanyl generalization in both groups but were more potent in the low-dose group. Spiradoline and d-amphetamine did not evoke generalization in either group. Naltrexone antagonized stimulus and rate-altering effects of fentanyl in both groups, with apparent pA(2) values of 7. 6 in the low-dose group and 7.5 in the high-dose group. Nalbuphine and nalorphine evoked full generalization in the low-dose group but less than 40% generalization in the high-dose group. In the high-dose group, nalbuphine and nalorphine antagonized the stimulus and rate-altering effects of fentanyl with apparent pA(2) values of 5.3 and 6.1, respectively, demonstrating lower efficacy mu actions. CONCLUSIONS: Changes in fentanyl training dose preserved the mu opioid selectivity of stimulus control but altered the intensity of the transduced mu opioid stimulus required for generalization. These differences in intensity of the fentanyl stimulus determined whether low efficacy mu opioids would evoke or antagonize fentanyl generalization.
