| dc.description.abstract | The purpose of the present study was to characterize the antinociceptive effects of tramadol, fentanyl and
morphine,whentwoofthemwere systemically combined in a 1:1 potency ratio, in the hot plate, the acetic
acid writhing, and the formalin tests in mice. Interaction indexes and isobolographic analysis were used
to assess the type of interaction. Fentanyl was the most potent drug, followed by morphine and tramadol,
with the exception in the phase I of formalin test. Synergistic interactions were obtained when tramadol
was combined with fentanyl or with morphine in the writhing and formalin tests. But, in the hot plate only
additive interactions were obtained. Changes were induced on the type of interaction depending on the
level of effect of opioid–opioid combinations. Moreover, co-administration of fentanyl with morphine
showed additivity, regardless of the type of stimulus. Standard rotarod test analysis confirmed intact
motor coordination. The present findings suggest that the type of interaction between opioids is not only
related to the nature of nociceptive stimulus but also to non-opioid analgesic pathways. | |
