Intestinal Inflammation and Morphine Tolerance Alter the Interaction between Morphine and Clonidine on Gastrointestinal Transit in Mice

Background Morphine and clonidine show synergy or antagonism inhibiting gastrointestinal transit depending on their proportion and level of effect. Their interaction during morphine tolerance and intestinal inflammation were assessed. Methods Gastrointestinal transit in mice was evaluated with charcoal and antitransit effects expressed as percent mean values +/- SEM. Tolerance was induced with a morphine pellet (75 mg) implanted for 72 h, and inflammation with intragastric croton oil. Dose-response curves for morphine and clonidine alone and combined at a 1:1 potency ratio were obtained, and doses producing a 50% and 60% inhibition were calculated (ED50, ED60). Interaction was established by isobolograms, interaction indexes, and analysis of variance. Results In naive and tolerant mice, the combination induced linear dose-response curves up to the ED60 and then reached a plateau. In naive mice, ED50 values were as follows: morphine 1.52 +/- 0.15 mg/kg, clonidine 0.09 +/- 0.008 mg/kg, and combined 0.506 +/- 0.084 mg/kg (0.478 +/- 0.08 mg/kg morphine plus 0.028 +/- 0.004 mg/kg clonidine). During tolerance, ED50 values were as follows: morphine 9.73 +/- 0.8 mg/kg, clonidine 0.09 +/- 0.007 mg/kg, combination 0.131 +/- 0.09 mg/kg (morphine 0. 13 +/- 0.09 mg/kg plus clonidine 0.0013 +/- 0.0005 mg/kg). In both groups, the interaction was synergistic up to the ED60 and antagonistic thereafter; synergy was enhanced during tolerance. During inflammation, ED50 values were as follows: morphine 0.17 +/- 0.04 mg/kg, clonidine 0.015 +/- 0.006 mg/kg, combined 0.62 +/- 0.04 mg/kg (morphine 0.568 +/- 0.04 mg/kg plus clonidine 0.052 +/- 0.004 mg/kg); thus, potencies of morphine and clonidine increased 9.3 and 7.1 times, while the combination remained unaltered. Moreover, inflammation transformed synergy into antagonism. Conclusions The interaction between morphine and clonidine was significantly altered during tolerance and inflammation. During tolerance, synergy was present up to 60% effect and then became antagonistic. Inflammation converted synergy to antagonism. A common pathway in signal transduction could partially explain the results.