The Analgesic Effect of Morphine on Peripheral Opioid Receptors: An Experimental Research

Opioids represent one of the key pillars in postoperative pain management, but their use has been associated with a variety of serious side effects. Thus, it is crucial to investigate the timing and course of opioid administration in order to ensure a best efficacy to side-effect profile. The aim of our article was to investigate the analgesic effects of locally administered morphine sulfate (intraplantar) in a carrageenan-induced inflammation model in rats. After carrageenan administration, the rats were divided into 10 equal groups and were injected with either morphine 5 mg/kg or 0.9% saline solution at different time intervals, depending on the assigned group. The analgesic effect was assessed through thermal stimulation. Our results showed that paw withdrawal time was significantly higher in rats treated with morphine compared to those in the control group 9.18 ± 3.38 compared to 5.14 ± 2.21 seconds, p=0.012). However, differences were more pronounced at certain time intervals post-carrageenan administration (at 180 minutes compared to 360 minutes, p=0.003 and at 180 minutes compare to 1440 minutes p<0.001), indicating that efficacy varies depending on the timing of treatment. In conclusion, our findings support the hypothesis that locally administered morphine may alleviate pain under inflammatory conditions and underscores the importance of considering treatment timing when evaluating the analgesic effect.