Spinal NK-1 receptor-expressing neurons and descending pathways support fentanyl-induced pain hypersensitivity in a rat model of postoperative pain

The clinically important opioid fentanyl, administered acutely, enhances mechanical hypersensitivity in a model of surgical pain induced by plantar incision. Activity of neurokinin-1 (NK-1) receptor-expressing ascending spinal neurons, descending pathways originating in the rostral ventromedial medulla (RVM), and spinal dynorphin are necessary for the development and maintenance of hyperalgesia during sustained morphine exposure, suggesting that these mechanisms may also be important in opioid enhancement of surgical pain. Therefore, we examined the roles of these mechanisms in sensory hypersensitivity produced by acute fentanyl administration in rats not undergoing surgical incision and in rats undergoing plantar incision. In non-operated rats, fentanyl induced analgesia followed by immediate and long-lasting sensory hypersensitivity, as previously described. Fentanyl also enhanced pain sensitivity induced by plantar incision. Ablation of NK-1-expressing spinal neurons by pre-treatment with substance P-Saporin reduced sensory hypersensitivity in fentanyl-treated rats and, to a lesser extent, in fentanyl-treated rats with a surgical incision. Microinjection of lidocaine into the RVM completely reversed fentanyl-induced sensory hypersensitivity and fentanyl enhancement of incision-induced sensory hypersensitivity. RVM lidocaine injection resulted in a slight reduction of incision-induced sensory hypersensitivity in the absence of fentanyl pre-treatment. Spinal dynorphin content increased by 30 ± 7% and 66 ± 17% in fentanyl- and fentanyl/incision-treated rats. Spinal administration of antiserum to dynorphin attenuated sensory hypersensitivity in fentanyl-treated rats. These data support a partial role of NK-1 receptor-containing ascending pathways and a crucial role of descending facilitatory pathways in fentanyl-induced hyperalgesia and in the enhanced hyperalgesia produced by fentanyl treatment following surgical incision.