ORIGINAL ARTICLES
RAN Yuhua, WANG Yixian, SHI Liming, LI Zhiping, GAO Xiang, GAO Jing
OBJECTIVE To investigate the analgesic effects and potential mechanisms of the partial agonist of the 5-hydroxytryptamine 1A (5-HT1A) receptor and the selective 5-HT reuptake inhibitor, viladazone (Vil), in various animal models of pain. METHODS ① Mouse acetic acid writhing test: KM mice were divided into the model group, model+morphine 10 mg·kg-1 group, and model+Vil 2, 4, 8 mg·kg-1 groups. Thirty minutes after ig administration of saline (model group) or corresponding drugs, each group was ip injected with a 2% acetic acid aqueous solution (0.01 mL·g-1), and the writhing frequency of the mice was observed and recorded from 5 to 20 min. ② Mouse formalin pain test: KM mice were divided into the model group and model+Vil 2, 4 and 8 mg·kg-1 groups. Thirty minutes after ig administration of saline (model group) or drugs, 20 μL of 5% formalin solution was sc injected into the right plantar region of the mice. The licking time (the sum of the duration of licking and biting the paw) of the mice was observed and recorded during two periods: the acute phase (0-5 min after sc formalin injection) and the delayed phase (15-35 min after sc formalin injection). ③ Rat chronic constriction injury (CCI) of the sciatic nerve experiment: SD rats successfully examined with a paw withdrawal threshold (PWT) <5 g were randomly divided into a CCI model group and a CCI model+Vil 2, 4 and 8 mg·kg-1 group. Solvent (model group) or corresponding drugs were ig administered, and the PWT of the modeled side was measured at 30, 60, 120 and 240 min after the first administration to evaluate the acute analgesic effect of Vil on mechanical pain. Then, Vil was continuously ig administered for 14 d, and the PWT was measured 1 h after Vil administration on the 7th and 14th d to evaluate the long-term analgesic effect of Vil. Immunofluorescence staining was employed to analyze the expression levels of inflammation-related proteins, ionized calcium binding adapter molecule 1 (IBA-1), tumor necrosis factor α (TNF-α), and interleukin 1β (IL-1β), in brain tissues. Enzyme-linked immunosorbent assay (ELISA) was used to detect the levels of IBA-1, TNF-α and IL-1β in the dorsal root ganglion of the spinal cord in the CCI model. RESULTS ① In the mouse model of acetic acid writhing, single ig administration of morphine 10 mg·kg-1 and Vil at varied doses significantly reduced the number of writhings induced by acetic acid compared to the model group. ② In the formalin-induced pain model, the average licking time of the model group was 50.5 s during the acute phase of inflammatory pain (0-5 min after intraplantar injection of 5% formalin), and 347.9 s during the delayed phase of inflammatory pain (25-35 min after formalin injection). Compared to the model group, single ig administration of Vil 2-8 mg·kg-1 reduced chronic pain induced by formalin in mice, and each dose of Vil significantly decreased the licking time of mice, but had no notable impact on the licking duration exhibited by mice during acute phase. ③ In the CCI model, the PWT values of CCI model rats significantly decreased compared with the control group. Pathological damage to varying extents was observed in brain slices, manifested as enlarged intercellular spaces and the appearance of vacuoles. The expression of IBA-1 in brain tissue significantly increased, while TNF-α and IL-1β hardly changed. The levels of IBA-1, TNF-α and IL-1β in the spinal dorsal root ganglion significantly increased. Compared with the CCI model, after single administration of Vil 2-8 mg·kg-1 for 60, 120 and 240 min, Vil significantly reduced the PWT values. After two-week continuous administration, the PWT values in Vil 4 and 8 mg·kg-1 were significantly reduced, and Vil 2-8 mg·kg-1 could alleviate the neuropathic pain to some extent. Vil 8 mg·kg-1 significantly reduced the elevated levels of inflammatory factors compared to CCI rats. CONCLUSION The antidepressant Vil exhibits analgesic effects in mouse models of acetic acid writhing, formalin-induced inflammation, and neuropathic pain induced by CCI in rats, with a more pronounced effect on neuropathic pain. The mechanism of action may be related to the inhibition of inflammatory pathways of IBA-1.