Accumulation of methylglyoxal increases the advanced glycation end-product levels in DRG and contributes to lumbar disk herniation-induced persistent pain

Abstract

Lumbar disk herniation (LDH) with discogenic low back pain and sciatica is a common and complicated musculoskeletal disorder. The underlying mechanisms are poorly understood, and there are no effective therapies for LDH-induced pain. In the present study, we found that the patients who suffered from LDH-induced pain had elevated plasma methylglyoxal (MG) levels. In rats, implantation of autologous nucleus pulposus (NP) to the left lumbar 5 spinal nerve root, which mimicked LDH, induced mechanical allodynia, increased MG level in plasma and dorsal root ganglion (DRG), and enhanced the excitability of small DRG neurons (<30 μm in diameter). Intrathecal injection of MG also induced mechanical allodynia, and its application to DRG neurons ex vivo increased the number of action potentials evoked by depolarizing current pulses. Furthermore, inhibition of MG accumulation by aminoguanidine attenuated the enhanced excitability of small DRG neurons and the mechanical allodynia induced by NP implantation. In addition, NP implantation increased levels of advanced glycation end products (AGEs) in DRG, and intrathecal injection of MG-derived AGEs induced the mechanical allodynia and DRG neuronal hyperactivity. Intrathecal injection of MG also significantly increased the expression of AGEs in DRG. Importantly, scavenging of MG by aminoguanidine also attenuated the increase in AGEs induced by NP implantation. These results suggested that LDH-induced MG accumulation contributed to persistent pain by increasing AGE levels. Thus generation of AGEs from MG may represent a target for treatment of LDH-induced pain.NEW & NOTEWORTHY Our study demonstrates that methylglyoxal accumulation via increasing advanced glycation end-product levels in dorsal root ganglion contributes to the persistent pain induced by lumbar disk herniation, which proposed potential targets for the treatment of lumbar disk herniation-induced persistent pain.

Keywords: AGEs; DRG; lumbar disk herniation; methylglyoxal; pain.

Fig. 1.

LDH induced persistent pain and MG upregulation. A: VAS was evaluated in the patients or volunteers (**P < 0.01 vs. normal volunteer group; ##P < 0.01 vs. no-pain volunteer group; n = 20 patients, n = 10 normal volunteers, n = 12 no-pain volunteers). B: the concentration of plasma MG was examined in patients or volunteers (**P < 0.01 vs. normal volunteer group; ##P < 0.01 vs. no-pain volunteer group; n = 20 patients, n = 10 normal volunteers, n = 12 no-pain volunteers). C: analysis of linear correlation between VAS and plasma MG level in patients. D: the paw withdrawal threshold of rats was significantly decreased following NP implantation (**P < 0.01 vs. sham group, n = 12 in each group). E: the MG level of plasma was examined at different time points following NP implantation (**P < 0.01 vs. sham group, n = 12 in each group). F: intraperitoneal injection of MG at a dose of 2 or 5 mg/kg, but not 0.5 mg/kg, significantly induced the mechanical allodynia in rats (**P < 0.01 vs. vehicle group, n = 12 in each group).

Fig. 2.

Upregulated MG in DRG enhanced the excitability of sensory neurons and contributed to the persistent pain induced by NP implantation. A: aminoguanidine (AG; 1 μM/10 μl i.t.) reduced the MG upregulation in DRG induced by NP implantation (**P < 0.01 vs. sham group; ##P < 0.01 vs. corresponding NP implantation group, n = 6 in each group). B: AG (1 μM/10 μl i.t.) reduced the increase in number of action potentials in DRG neurons after NP implantation (**P < 0.01 vs. sham group; ##P < 0.01 vs. corresponding NP implantation group, n = 25 in each group). C: intrathecal injection of MG at a dose of 5 μM/10 μl significantly induced mechanical allodynia (**P < 0.01 vs. vehicle group, n = 12 in each group). D: intrathecal injection of MG at a dose of 5 μM/10 μl significantly increased the number of action potentials of DRG neurons (**P < 0.01 vs. vehicle group, n = 22 in each group). E: AG (1 μM/10 μl i.t.) markedly attenuated the mechanical allodynia induced by NP implantation (**P < 0.01 vs. sham group; ## P < 0.01 vs. corresponding NP implantation group, n = 12 in each group). F: the change of GLO1 activity in DRG following NP implantation (**P < 0.01 vs. sham group, n = 6 in each group).

Fig. 3.

Upregulated AGEs in DRG enhanced the excitability of sensory neurons and contributed to the persistent pain induced by NP implantation. A: representative blots and histogram show the upregulation of AGEs following NP implantation in DRG (**P < 0.01 vs. sham group, n = 6 in each group). B: intrathecal injection of MG-derived AGEs significantly induced the mechanical allodynia (**P < 0.01 vs. vehicle group, n = 12 in each group). C: intrathecal injection of MG-derived AGEs at a dose of 50 μg/10 μl at day 7 significantly increased the number of action potentials (**P < 0.01 vs. vehicle group, n = 22 in each group). D: intrathecal injection of MG (5 μM/10 μl) significantly upregulated the expression of AGEs in DRG (**P < 0.01 vs. vehicle group, n = 6 in each group). E: intrathecal injection of aminoguanidine (AG; 1 μM/10 μl) significantly decreased the upregulation of AGEs in DRG induced by NP implantation (**P < 0.01 vs. sham group; ##P < 0.01 vs. corresponding NP implantation group, n = 6 in each group).