Morphine acts in vitro to directly prime nociceptors.

Hyperalgesic priming is a preclinical model of the transition from acute to chronic pain characterized by a leftward shift in the dose-response curve for and marked prolongation of prostaglandin E₂ (PGE₂)-induced mechanical hyperalgesia, in vivo. In vitro, priming in nociceptors is characterized by a leftward shift in the concentration dependence for PGE₂-induced nociceptor sensitization. In the present in vitro study we tested the hypothesis that a mu-opioid receptor (MOR) agonist opioid analgesic, morphine, can produce priming by its direct action on nociceptors. We report that treatment of nociceptors with morphine, in vitro, produces a leftward shift in the concentration dependence for PGE₂-induced nociceptor sensitization. Our findings support the suggestion that opioids act directly on nociceptors to induce priming.