Opioid receptors and acetaminophen (paracetamol).

We report that the acetaminophen (paracetamol)-induced spinal (intrathecal, i.t.)/supraspinal (intracerebroventricular, i.c.v.) site/site antinociceptive 'self-synergy' in mice is attenuated by the opioid receptor subtype selective antagonists beta-funaltrexamine hydrochloride (beta-FNA; mu), naltrindole (delta), and nor-binaltorphine hydrochloride (nor-BNI; kappa). These findings further implicate endogenous opioids (viz., endorphins, enkephalins, and dynorphins) and their pathways as contributors to the central antinociceptive action of acetaminophen.