The discovery that
paracetamol is metabolized to the potent TRPV1 activator N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (
AM404) and that this metabolite contributes to
paracetamol's antinociceptive effect in rodents via activation of TRPV1 in the central nervous system (CNS) has provided a potential strategy for developing novel
analgesics. Here we validated this strategy by examining the metabolism and antinociceptive activity of the de-acetylated
paracetamol metabolite
4-aminophenol and 4-hydroxy-3-methoxybenzylamine (
HMBA), both of which may undergo a
fatty acid amide hydrolase (FAAH)-dependent biotransformation to potent TRPV1 activators in the brain. Systemic administration of
4-aminophenol and
HMBA led to a dose-dependent formation of
AM404 plus N-(4-hydroxyphenyl)-9Z-octadecenamide (HPODA) and
arvanil plus
olvanil in the mouse brain, respectively. The order of potency of these
lipid metabolites as TRPV1 activators was
arvanil =
olvanil>>
AM404> HPODA. Both
4-aminophenol and
HMBA displayed antinociceptive activity in various rodent
pain tests. The formation of
AM404,
arvanil and
olvanil, but not HPODA, and the antinociceptive effects of
4-aminophenol and
HMBA were substantially reduced or disappeared in FAAH null mice. The activity of
4-aminophenol in the mouse
formalin, von Frey and tail immersion tests was also lost in TRPV1 null mice. Intracerebroventricular injection of the TRPV1 blocker
capsazepine eliminated the antinociceptive effects of
4-aminophenol and
HMBA in the mouse
formalin test. In the rat, pharmacological inhibition of FAAH, TRPV1,
cannabinoid CB1 receptors and spinal 5-HT3 or 5-HT1A receptors, and chemical deletion of bulbospinal serotonergic pathways prevented the antinociceptive action of
4-aminophenol. Thus, the pharmacological profile of
4-aminophenol was identical to that previously reported for
paracetamol, supporting our suggestion that this
drug metabolite contributes to
paracetamol's
analgesic activity via activation of bulbospinal pathways. Our findings demonstrate that it is possible to construct novel antinociceptive drugs based on
fatty acid conjugation as a metabolic pathway for the generation of TRPV1 modulators in the CNS.