NSAIDs and opioids antinociception in a thermal murine phasic pain

Pain is an unpleasant sensation that causes mild or severe physical discomfort, by which induces the use of analgesics which are basically of two types: opioids or NSAIDs. The objectives of the present study was to evaluate the antinociception activity induced by NSAIDs and opioids in a thermal model of animal pain, the tail flick assay, and to determine the effect of the MOR antagonist, naltrexone. Antinociception was assessed by the tail flick test using an digital algesiometer. The rank orden of potency was fentanyl > morphine > ibuprofen > tramadol > codeine > meloxicam > paracetamol and the antinociceptive ratio, compared with paracetamol, was between 460 for fentanyl and 1 for meloxicam. Pretreatment with the opioid receptor antagonist, naltrexone, 1 mg/kg i.p., significant reversed the antinociceptive effect of NSAIDs and opioids. The results obtained with naltrexone in this assay confirm the antagonism of this agent on opioids. However, the antagonism over NSAIDs is a new finding since that has not been previously reported. This study test that NSAIDs and opioids induce antinociceptive activity in a thermal murine phasic pain with the following rank orden of potency  fentanyl > morphine > ibuprofen > tramadol > codeine > meloxicam > paracetamol. Although the mechanisms of action of these drugs are different, naltrexone, a MOR antagonist, blocked the effects of both agents, suggesting that inhibition of pain seem partially mediated by MOR with association to central mechanisms.