Effect of oxidative stress induced by intracranial iron overload on central pain after spinal cord injury

Abstract

Background: Central pain (CP) is a common clinical problem in patients with spinal cord injury (SCI). Recent studies found the pathogenesis of CP was related to the remodeling of the brain. We investigate the roles of iron overload and subsequent oxidative stress in the remodeling of the brain after SCI.

Methods: We established a rat model of central pain after SCI. Rats were divided randomly into four groups: SCI, sham operation, SCI plus deferoxamine (DFX) intervention, and SCI plus nitric oxide synthase (NOS) inhibitor treatment. Pain behavior was observed and thermal pain threshold was measured regularly, and brain levels of iron, transferrin receptor 1 (TfR1), ferritin (Fn), and lactoferrin (Lf), were detected in the different groups 12 weeks after establishment of the model.

Results: Rats demonstrated self-biting behavior after SCI. Furthermore, the latent period of thermal pain was reduced and iron levels in the hind limb sensory area, hippocampus, and thalamus increased after SCI. Iron-regulatory protein (IRP) 1 levels increased in the hind limb sensory area, while Fn levels decreased. TfR1 mRNA levels were also increased and oxidative stress was activated. Oxidative stress could be inhibited by ferric iron chelators and NOS inhibitors.

Conclusions: SCI may cause intracranial iron overload through the NOS-iron-responsive element/IRP pathway, resulting in central pain mediated by the oxidative stress response. Iron chelators and oxidative stress inhibitors can effectively relieve SCI-associated central pain.

Keywords: Central pain; Iron; Oxidative stress; Spinal cord injury.

Fig. 1

Paw-withdrawal latency (PWL) in different groups. a Rat PWL. *P < 0.05 compared with control group; △P < 0.05 compared with sham operation group. b Rat PWLs in different groups

Fig. 2

Iron levels in whole-brain and functional brain areas. a Whole-brain iron levels and b iron levels in the hind limb sensory cortex, thalamus, and hippocampus. *Compared with control group, P < 0.05; △compared with sham operation group, P < 0.05

Fig. 3

IRP1, Fn, and LF protein expression in the functional brain areas by Western blot. a Western blot of IRP1, Fn, and LF proteins in the hind limb sensory area of rats. b The gray ratio of IRP1 and the loading control β-actin were plotted. Data represent the average of three experiments. c Fn. d Lf. *Compared with SCI group, P < 0.05; △compared with sham operation group, P < 0.05

Fig. 4

TfR1 and Fn mRNA expression in the functional brain areas by ELISA. TfR1 and Fn levels in the hind limb sensory area, thalamus, and hippocampus of rats. a TfR1. b Fn. *Compared with SCI group, P < 0.05; △compared with sham operation group, P < 0.05

Fig. 5

TfR1 and Fn mRNA expression in the functional brain areas by RT-PCR. a Melting curve of TfR1. b Hind limb sensory area. c Hippocampus. d Thalamus. *Compared with SCI group, P < 0.05; △compared with sham operation group, P < 0.05

Fig. 6

SOD and MDA levels in the functional brain areas. a SOD levels in rat brain. b MDA levels in rat brain. *Compared with SCI group, P < 0.05; △compared with sham operation group, P < 0.05