Effect of acute and repeated administration of paracetamol on opioidergic and serotonergic systems in rats

Objective and design: We investigated the antinociceptive effect of paracetamol or morphine after repeated administration and the changes in the characteristics of central mu-, kappa- and 5-HT2 receptors. Treatment: Male rats were injected twice a day for seven days with paracetamol (400 mg/kg, i. p.) or morphine (5 mg/ kg, s.c.). Methods: The antinociceptive effect was evaluated 30 min after single and multiple doses of paracetamol and morphine through the hot-plate test. Binding techniques were used to evaluate the receptor characteristics in the frontal cortex. Results: Both paracetamol and morphine induced an antinociceptive effect on day 1 but only paracetamol maintained this effect for seven days while morphine did not. The number of mu-opioid receptors decreased on days 1, 3, and 7 by a similar percentage after paracetamol administration (by 29, 31 and 34 %, respectively), while morphine produced a progressive decrease in comparison with controls (by 37, 49 and 60 %, respectively) and kappa-opioid receptors were unaffected. Both drugs similarly decreased the 5-HT2 receptor number on all days of treatment (by about 30 %). Conclusions: The opioidergic and serotonergic systems are involved in different ways in the induction and maintenance of antinociception after paracetamol or morphine treatment.