Effects of naloxone on opioid receptors in spinal cord dorsal horn of neuropathic pain rat models

Objective To investigate the effects of naloxone on the levels of opioid receptors in spinal cord dorsal horn of neuropathic pain rat models, established with chronic constriction injury(CCI). Methods Male SD rats (200-250 g) were subjected to CCI to establish neuropathic pain models. The rats were divided into 4 groups, and respectively received sham operation and saline treatment (Sham-S group, n=8), CCI and saline treatment (CCI-S group, n=8), CCI and naloxone treatment (CCI-N group, n=24), and CCI and morphine treatment (CCI-M group, n=8). Drug treatments (subcutaneous injection of normal saline, 10 mg·kg-1·d-1 naloxone, or 10 mg·kg-1·d-1 morphine in hindpaw) started from 7th day after surgery for 8 consecutive days. Paw withdrawal mechanical threshold (PWMT) and paw withdrawalthermal latency(PWTL) respectively were measured with von Frey tests and thermal plantar tests before and after CCI. Spinal cord dorsal horn at L3-L5 levels from naloxone-treated CCI rats were collected 10, 12, and 14 days after surgery, and western blot assay was employed to measure the levels of three opioid receptor subtypes. Results CCI rats exhibited ipsilateral hindpaw hyperalgesia, showing reduced PWMT and shortened PWTL (P 0.05) . Conclusions Naloxone, an opioid receptor antagonist, showed antinociceptive effects, similar to morphine, an opioid receptor agonist, on CCI neuropathic pain. This effect probably is relevant to naloxone-induced elevation of DOR in spinal cord dorsal horn. Key words: Naloxone; Neuropathic pain; Opioid receptors