Paroxetine Augments while Naloxone Abolishes the Analgesic Effect of Paracetamol in Acute Nociceptive Pain in Mice

The mechanism(s) of analgesic action of paracetamol (acetaminophen; N-acetyl-p-aminophenol) remains controversial. Previous studies on rats suggested that the antinociceptive action of paracetamol might involve the central descending inhibitory pain pathways recruiting both a serotoninergic and an opioidergic system. This study explores this issue in mice using paroxetine, the most potent selective serotonin re-uptake inhibitor, and the nonselective opioid pure antagonist naloxone. Animals were divided into two main groups for two separate experiments, each subdivided into 3 subgroups. In both experiments; the first group served as control, the second group received paracetamol (200 mg/kg, i.p). In one experiment, the third group received paroxetine (20 mg/kg p.o for 7 days) before paracetamol. In the other experiment, animals of the third group were pretreated with naloxone (5 mg/kg, i.p) 30 min before paracetamol. The antinociceptive effect of paracetamol was tested using the hot plate test. Paracetamol displayed a significant antinociceptive activity that was augmented by pretreatment with paroxetine as was shown by maintenance of its effect beyond that shown by paracetamol alone. On the other hand, pretreatment with naloxone abolished paracetamol’s antinociceptive activity in the hot-plate test. These results extended the previous observation in rats that the antinociceptive effect of paracetamol involved activation of a central descending pain inhibitory pathway with serotonin and opioidergic peptides being potential mediators recruited.