Roles of different subtypes of opioid receptors in mediating the ventrolateral orbital cortex opioid-induced inhibition of mirror-neuropathic pain in the rat

Previous studies have demonstrated that opioid receptors in the prefrontal ventrolateral orbital cortex (VLO) are involved in anti-nociception. The aim of this current study was to examine whether opioid receptors in the VLO have effects on the hypersensitivity induced by contralateral L5 and L6 spinal nerve ligation (SNL), termed as mirror neuropathic pain (MNP) in the male rat. Morphine (1.0, 2.5, 5.0 μg) microinjected into the VLO contralateral to the SNL depressed the mechanical paw withdrawal assessed by von Frey filaments and the cold plate (4 °C)-induced paw lifting in a dose-dependent manner on the side without SNL. These effects were antagonized by microinjection of the non-selective opioid receptor antagonist naloxone (1.0 μg) into the same VLO site. Microinjection of endomorphin-1 (5.0 μg), a highly selective μ-opioid receptor agonist, and [d-Ala2, d-Leu5]-enkephalin (DADLE, 10 μg), a δ-/μ-receptor agonist, also depressed the MNP. The effects of both drugs were blocked by selective μ-receptor antagonist β-funaltrexamine (β-FNA, 3.75 μg), but the effect of the DADLE was not influenced by the selective δ-receptor antagonist naltrindole (5.0 μg). Microinjection of the κ-opioid receptor agonist spiradoline mesylate salt (U-62066) (100 μg) had no effect on the MNP. These results suggest that the VLO is involved in opioid-induced inhibition of the MNP and the effect is mediated by μ- (but not δ- and κ-) opioid receptors.