Analgesic effects of naringenin in rats with spinal nerve ligation-induced neuropathic pain

Abstract

Naringenin, a flavonoid abundant in citrus fruits, such as grapefruits, has been reported to possess anti-inflammatory properties. The present study aimed to investigate the analgesic potential of naringenin in L5 spinal nerve ligation (SNL)-induced peripheral neuropathic pain and the underlying mechanisms associated with neuroinflammation. Different doses of naringenin or saline were administered intrathecally once daily for 11 consecutive days, from 3 days prior to surgery to 7 days after surgery. Pain development was assessed 1 day prior to and 7-14 days after surgery in terms of mechanical withdrawal threshold and thermal withdrawal latency. Astrocytic and microglial activation and production of inflammatory mediators were determined on day 14 after surgery. The results demonstrated that naringenin dose-dependently attenuated the mechanical allodynia and thermal hyperalgesia induced by SNL. Furthermore, naringenin significantly inhibited SNL-induced activation of glial cells (astrocytes and microglia). Morover, the upregulated expression of inflammatory mediators in neuropathic pain was significantly inhibited by naringenin. Our findings suggested that repeated administration of naringenin may alleviate neuropathic pain, possibly through inhibiting neuroinflammation.

Keywords: naringenin; neuroinflammation; neuropathic pain.

Figure 1

Repeated administration of naringenin (Nar) prevented spinal nerve ligation (SNL)-induced mechanical allodynia and thermal hyperalgesia. SNL injury resulted in (A) prominent mechanical allodynia and (B) thermal hyperalgesia on day 7 after surgery. The intrathecal administration of Nar (50, 100 or 200 mg/kg, once per day, from 3 days prior to surgery to 7 days after surgery) significantly inhibited SNL-induced mechanical allodynia and thermal hyperalgesia in a dose-dependent manner. The values are expressed as means ± standard error of the mean (n=6). ^*P<0.05, compared to the SNL + vehicle (Veh) group.

Figure 2

Naringenin (Nar) prevents spinal nerve ligation (SNL)-induced spinal glial activation. The expression of (A) astrocytic marker glial fibrillary acidic protein (GFAP) and (B) microglial marker macrophage antigen-1 (Mac-1) were measured by quantitative reverse trancription-polymerase chain reactions in the lumbar spinal cord of rats on day 14 after surgery. SNL increased GFAP and Mac-1 expression, whereas Nar dose-dependently downregulated the expression of these markers. The values are expressed as means ± standard error of the mean (n=6). ^*P<0.05 compared to the sham + vehicle (Veh) group. ^#P<0.05 compared to the SNL + Veh group.

Figure 3

Effects of intrathecal naringenin (Nar) on pro-inflammatory cytokine levels. The protein levels of the pro-inflammatory cytokines (A) tumor necrosis factor-α (TNF-α), (B) interleukin-1β (IL-1β) and (C) monocyte chemoattractant protein-1 (MCP-1) were determined in the spinal dorsal horn of rats in different treatments groups using ELISA. Nar significantly prevented spinal nerve ligation (SNL)-induced increases in the levels of these inflammatory mediators. The values are expressed as means ± standard error of the mean (n=6). ^*P<0.05 compared to the sham + vehicle (Veh) group. ^#P<0.05 compared to the SNL + Veh group. The levels of the pro-inflammatory cytokines are expressed as relative fold-changes compared to the sham + Veh group.