μ-Opioid Receptor Activation at the Dorsal Reticular Nucleus Shifts Diffuse Noxious Inhibitory Controls to...

The dorsal reticular nucleus is a pain facilitatory area involved in diffuse noxious inhibitory control (DNIC) through opioidergic mechanisms that are poorly understood. The hypothesis was that signaling of μ-opioid receptors is altered in this area with prolonged chronic inflammatory pain and that this accounts for the loss of DNICs occurring in this condition.

Monoarthritis was induced in male Wistar rats (n = 5 to 9/group) by tibiotarsal injection of complete Freund's adjuvant. The immunolabeling of µ-opioid receptors and the phosphorylated forms of µ-opioid receptors and cAMP response element binding protein was quantified. Pharmacologic manipulation of μ-opioid receptors at the dorsal reticular nucleus was assessed in DNIC using the Randall-Selitto test.

At 42 days of monoarthritis, μ-opioid receptor labeling decreased at the dorsal reticular nucleus, while its phosphorylated form and the phosphorylated cAMP response element binding protein increased. [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate (DAMGO) enhanced DNIC analgesia in normal animals (means ± SD: pre-DNIC: 126.9 ± 7.0 g; DNIC - DAMGO: 147.5 ± 8.0 g vs. DNIC + DAMGO: 198.1 ± 19.3 g; P < 0.001), whereas it produced hyperalgesia in monoarthritis (pre-DNIC: 67.8 ± 7.5 g; DNIC - DAMGO: 70.6 ± 7.7 g vs. DNIC + DAMGO: 32.2 ± 2.6 g; P < 0.001). An ultra-low dose of naloxone, which prevents the excitatory signaling of the μ-opioid receptor, restored DNIC analgesia in monoarthritis (DNIC - naloxone: 60.0 ± 6.1 g vs. DNIC + naloxone: 98.0 ± 13.5 g; P < 0.001), compared to saline (DNIC - saline: 62.5 ± 5.2 g vs. DNIC + saline: 64.2 ± 3.8 g). When injected before DAMGO, it restored DNIC analgesia and decreased the phosphorylated cAMP response element binding protein in monoarthritis.

The dorsal reticular nucleus is likely involved in a facilitatory pathway responsible for DNIC hyperalgesia. The shift of μ-opioid receptor signaling to excitatory in this pathway likely accounts for the loss of DNIC analgesia in monoarthritis.