Background

Increased descending pain facilitation accounts for opioid-induced hyperalgesia, but the underlying mechanisms remain elusive. Given the role of µ-opioid receptors in opioid-induced hyperalgesia in animals, the authors hypothesized that the dorsal reticular nucleus, a medullary pain facilitatory area, is involved in opioid-induced hyperalgesia through altered µ-opioid receptor signaling.

Methods

The authors used male Wistar rats (n = 5 to 8 per group), chronically infused with morphine, to evaluate in the dorsal reticular nucleus the expressions of the µ-opioid receptor and phosphorylated cAMP response element-binding, a downstream marker of excitatory µ-opioid receptor signaling. The authors used pharmacologic and gene-mediated approaches. Nociceptive behaviors were evaluated by the von Frey and hot-plates tests.

Results

Lidocaine fully reversed mechanical and thermal hypersensitivity induced by chronic morphine. Morphine-infusion increased µ-opioid receptor, without concomitant messenger RNA changes, and phosphorylated cAMP response element-binding levels at the dorsal reticular nucleus. µ-opioid receptor knockdown in morphine-infused animals attenuated the decrease of mechanical thresholds and heat-evoked withdrawal latencies compared with the control vector (von Frey [mean ± SD]: −17 ± 8% vs. −40 ± 9.0%; P < 0.001; hot-plate: −10 ± 5% vs. −32 ± 10%; P = 0.001). µ-opioid receptor knockdown in control animals induced the opposite (von Frey: −31 ± 8% vs. −17 ± 8%; P = 0.053; hotplate: −24 ± 6% vs. −3 ± 10%; P = 0.001). The µ-opioid receptor agonist (D-ALA2,N-ME-PHE4,GLY5-OL)-enkephalin acetate (DAMGO) decreased mechanical thresholds and did not affect heat-evoked withdrawal latencies in morphine-infused animals. In control animals, DAMGO increased both mechanical thresholds and heat-evoked withdrawal latencies. Ultra-low-dose naloxone, which prevents the excitatory signaling of the µ-opioid receptor, administered alone, attenuated mechanical and thermal hypersensitivities, and coadministered with DAMGO, restored DAMGO analgesic effects and decreased phosphorylated cAMP response element-binding levels.

Conclusions

Chronic morphine shifted µ-opioid receptor signaling from inhibitory to excitatory at the dorsal reticular nucleus, likely enhancing descending facilitation during opioid-induced hyperalgesia in the rat.

Editor’s Perspective
What We Already Know about This Topic
  • The phenomenon of opioid-induced hyperalgesia is supported by descending pain facilitation from brainstem nuclei

  • The dorsal reticular nucleus is modulated by opioids and mediates descending pain facilitation in some settings

What This Article Tells Us That Is New
  • Using a rat model of opioid-induced hyperalgesia, it was shown that reducing dorsal reticular activity with lidocaine blocked nociceptive sensitization from opioid infusion

  • Knockdown of µ-opioid receptors or prevention of excitatory signaling using naloxone in the dorsal reticular nucleus prevented and reversed opioid-induced hyperalgesia

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