Nrf2 activation ameliorates mechanical allodynia in paclitaxel-induced neuropathic pain

Abstract

Paclitaxel-induced neuropathic pain (PINP) is refractory to currently used analgesics. Previous studies show a pivotal role of oxidative stress in PINP. Because the nuclear factor erythroid-2-related factor 2 (Nrf2) has been considered as the critical regulator of endogenous antioxidant defense, we here explored whether activation of Nrf2 could attenuate PINP. A rat model of PINP was established by intraperitoneal injection of paclitaxel (2 mg/kg) every other day with a final cumulative dose of 8 mg/kg. Hind paw withdrawal thresholds (PWTs) in response to von Frey filament stimuli were used to assess mechanical allodynia. We showed that a single dose of Nrf2 activator, oltipraz (10, 50, and 100 mg/kg), dose-dependently attenuated established mechanical allodynia, whereas repeated injection of oltipraz (100 mg· kg^-1· d^-1, i.p. from d 14 to d 18) almost abolished the mechanical allodynia in PINP rats. The antinociceptive effect of oltipraz was blocked by pre-injection of Nrf2 inhibitor trigonelline (20 mg/kg, i.p.). Early treatment with oltipraz (100 mg· kg^-1· d^-1, i.p. from d 0 to d 6) failed to prevent the development of the PINP, but delayed its onset. Western blot and immunofluorescence analysis revealed that the expression levels of Nrf2 and HO-1 were significantly upregulated in the spinal cord of PINP rats. Repeated injection of oltipraz caused further elevation of the expression levels of Nrf2 and HO-1 in the spinal cord of PINP rats, which was reversed by pre-injection of trigonelline. These results demonstrate that oltipraz ameliorates PINP via activating Nrf2/HO-1-signaling pathway in the spinal cord.

Keywords: HO-1; Nrf2; neuropathic pain; oltipraz; oxidative stress; paclitaxel.

Fig. 1

Schematic illustration of the experimental design. a To determine whether a single dose of an Nrf2 activator can alleviate established PINP, oltipraz (10, 50, or 100 mg/kg, i.p.) was given on day 14 (d 14) after the first injection of paclitaxel. The behavioral test was conducted before oltipraz injection, and 0.5, 1, 2, and 4 h after injection. b To determine whether repetitive injections of an Nrf2 activator can reverse established PINP, oltipraz (100 mg/kg, i.p.) was given once daily from d 14 to d 18. The behavioral test was conducted on d 13 and 1 h after oltipraz injection each day. c To determine whether an Nrf2 inhibitor can reverse the analgesic effect of oltipraz, trigonelline (20 mg/kg, i.p.) was given 30 min before oltipraz. The behavioral test was conducted before trigonelline injection, and 0.5, 1, 2, and 4 h after oltipraz injection. d To determine whether early treatment with an Nrf2 activator can suppress the development of PINP, oltipraz (100 mg/kg, i.p.) was given once daily from d 0 to d 6. The behavioral test was conducted on d 0 before the first injection of paclitaxel, and on d 3, d 7, d 14, and d 21

Fig. 2

Mechanical allodynia induced by intraperitoneal injections of paclitaxel. The PWT was significantly decreased from d 7 to d 21 after the first injection of paclitaxel (***P < 0.001 compared with the vehicle group, n = 6 in each group). In contrast, no significant change in the PWT was observed in vehicle-treated rats during the observation period

Fig. 3

Therapeutic and preventive effects of oltipraz on mechanical allodynia in PINP rats. a No significant change in the PWT was observed after a single intraperitoneal injection of oltipraz at a dose of 10 mg/kg compared with vehicle administration (P > 0.05). However, oltipraz (50 and 100 mg/kg, i.p.) considerably increased the PWT of PINP rats; this change began at 0.5 h, peaked at 1 h, and lasted for ~2 h (**P < 0.01, ***P < 0.001 compared with the vehicle group, n = 6 in each group). b Repeated injections of Olti (100 mg/kg, i.p.) from d 14 to d 18 significantly reversed mechanical allodynia in PINP rats (***P < 0.001 compared with the vehicle group, n = 6 in each group). c The analgesic effect of oltipraz (Olti, 100 mg/kg, i.p.) in PINP was completely abolished by the Nrf2 inhibitor trigonelline (Trig, ***P < 0.001 compared with the vehicle group, ^###P < 0.001 compared with the PINP + 100 mg/kg Olti + 20 mg/kg Trig group, n = 6 in each group). d Early treatment with oltipraz (Olti, 100 mg/kg, i.p.) 30 min before the injection of paclitaxel from d 0 to d 6 significantly elevated the PWT of PINP rats on d 7, but not on d 14 or d 21 (*P < 0.05 compared with the vehicle group, n = 6 in each group)

Fig. 4

Oltipraz increased the expression of Nrf2 in the spinal cords of PINP rats. a The spinal expression of Nrf2 was significantly increased in PINP rats (*P < 0.05 compared with the vehicle group, n = 6 in each group). Moreover, oltipraz treatment further increased the spinal expression of Nrf2 in PINP rats, which was reversed by the preadministration of trigonelline (*P < 0.05, ***P < 0.001 compared with the vehicle-treated group, ^###P < 0.001 compared with the oltipraz-treated group). b Nrf2 (green) was localized in neurons (as indicated by NeuN, a neuronal marker, red)

Fig. 5

Oltipraz increased the expression of HO-1 in the spinal cords of PINP rats. a The spinal expression of HO-1 was significantly increased in PINP rats (**P < 0.01 compared with the vehicle group, n = 6 in each group). Moreover, oltipraz treatment further increased the spinal expression of HO-1 in PINP rats, which was reversed by the preadministration of trigonelline (*P < 0.05, ***P < 0.001 compared with the vehicle group, ^##P < 0.01, ^###P < 0.001 compared with the oltipraz-treated group). b HO-1 (green) was localized in neurons (as indicated by NeuN, a neuronal marker, red)