Antinociception by systemically-administered acetaminophen (paracetamol) involves spinal serotonin 5-HT₇ and adenosine A₁ receptors, as well as peripheral adenosine A₁ receptors

Acetaminophen (paracetamol) is a widely used analgesic, but its sites and mechanisms of action remain incompletely understood. Recent studies have separately implicated spinal adenosine A₁ receptors (A₁Rs) and serotonin 5-HT₇ receptors (5-HT₇Rs) in the antinociceptive effects of systemically administered acetaminophen. In the present study, we determined whether these two actions are linked by delivering a selective 5-HT₇R antagonist to the spinal cord of mice and examining nociception using the formalin 2% model. In normal and A₁R wild type mice, antinociception by systemic (i.p.) acetaminophen 300mg/kg was reduced by intrathecal (i.t.) delivery of the selective 5-HT₇R antagonist SB269970 3μg. In mice lacking A₁Rs, i.t. SB269970 did not reverse antinociception by systemic acetaminophen, indicating a link between spinal 5-HT₇R and A₁R mechanisms. We also explored potential roles of peripheral A₁Rs in antinociception by acetaminophen administered both locally and systemically. In normal mice, intraplantar (i.pl.) acetaminophen 200μg produced antinociception in the formalin test, and this was blocked by co-administration of the selective A₁R antagonist DPCPX 4.5μg. Acetaminophen administered into the contralateral hindpaw had no effect, indicating a local peripheral action. When acetaminophen was administered systemically, its antinociceptive effect was reversed by i.pl. DPCPX in normal mice; this was also observed in A₁R wild type mice, but not in those lacking A₁Rs. In summary, we demonstrate a link between spinal 5-HT₇Rs and A₁Rs in the spinal cord relevant to antinociception by systemic acetaminophen. Furthermore, we implicate peripheral A₁Rs in the antinociceptive effects of locally- and systemically-administered acetaminophen.