Abstract
Opioid-induced nausea and vomiting (OINV) is reported to occur at a lower dose level than the analgesic dosage, whereas it is not clear whether this pharmacological association is also found under the presence of acute pain. In the present study, we investigated the relationship between analgesic dose of morphine and OINV in rat model of postoperative acute pain. A simple laparotomy with a 1-cm midline incision was performed under isoflurane anesthesia (postoperative pain rats). Control rats were given only anesthesia (n=5). Rats were subcutaneously injected with physiological saline (5 ml/kg) or 0.1, 0.5, or 1.0 mg/kg morphine (n=5-6, each dose). The degree of postoperative pain and morphine-induced OINV were quantified using the rat grimace scale (RGS) and kaolin intake, respectively. In laparotomy, but not control, rats, a significant increase in RGS was observed compared to baseline, which was inhibited by morphine in dose-dependent manner; half-maximal analgesic dose was 0.4 mg/kg. One the other hand, administered of morphine increased in kaolin intake dose-dependently in control rats from the lowest dose (0.1 mg/kg) and in postoperative pain rats from 0.5 mg/kg. Furthermore, regression analysis indicates a negative correlation between RGS and kaolin intake in postoperative pain rats. Our results indicate that OINV tends to occur when opioids are used at doses greater than needed to relieve postoperative acute pain.
