Evaluation of Analgesic Actions of Morphine in Various Pain Models in Experimental Animals
Interestingly, administration of the opioid antagonist, naloxone, alone into inflamed paw has been shown to antagonize the inflammatory pain produced by the intraplantar injection of carrageenan in rats. This paradoxical analgesic action is mediated by stereospecific opiate receptors and is explained due to a peripheral action on cutaneous opiate receptors and/or local production of morphinomimetic metabolites locally formed by the dealkylation of naloxone. The locus of tail stimulation in the tail flick assay is an important parameter in determining the analgesic effects, as the distal tail section has been reported to be more sensitive to the analgesic effects of morphine than the proximal sections. All the chronic pain models, with the aim of mimicking the human conditions of chronic pain, lead to the production of pain, which the animals cannot control. Despite the difficult tasks of assessing pain, anxiety, and euphoria in experimental animals, the characterization and evaluation of analgesics remains relatively easy at the present time.