5-HT receptor subtypes involved in the spinal antinociceptive effect of acetaminophen in rats.

The present study was designed to investigate which subtype of spinal 5-HT receptors were involved in acetaminophen-induced antinociception using the paw-pressure test. Propacetamol (prodrug of acetaminophen, 400 mg/kg, injected intravenously, corresponding to 200 mg/kg of acetaminophen) produced a significant antinociceptive effect in this test. This effect was at least partially inhibited by intrathecal (i.t.) pretreatment with the 5-HT(1B) (penbutolol), 5-HT(2A) (ketanserin), 5-HT(2C) (mesulergine) receptor antagonists, but not by the 5-HT(1A) (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride, WAY 100635) and 5-HT(3) (granisetron) receptor antagonists. This profile was very close to that obtained recently with 5-HT, which suggests an implication of 5-HT in the spinal antinociceptive effect of acetaminophen. These results, the lack of binding of acetaminophen to 5-HT receptors and the increase of central 5-HT levels induced by this drug suggest that acetaminophen-induced antinociception could be indirectly mediated by 5-HT.