Nociceptive spinal cord neurons of laminae I-III exhibit oxidative stress damage during diabetic neuropathy which is prevented by early antioxidant treatment with epigallocatechin-gallate (EGCG)

Spinal cord neurons located in laminae I-III respond to nociceptive stimuli and participate in the transmission of painful information to the brain. In the present study we evaluated if nociceptive laminae I-III neurons are affected by oxidative stress damage in a model of diabetic neuropathic pain (DNP), the streptozotocin-induced diabetic rat (STZ rat). Additionally, we evaluated the effects of a preventive antioxidant treatment with epigallocatechin-gallate (EGCG) in nociceptive neuronal activation and behavioural signs of DNP. Three days after diabetes induction, a treatment protocol of STZ rats with an aqueous solution of EGCG in the drinking water was initiated. Ten weeks after the onset of treatment, the spinal cords were immunoreacted against validated markers of oxidative stress damage (8-hydroxy-2'-deoxyguanosine; 8-OHdG) and of nociceptive neuronal activation (Fos). Mechanical hypersensitivity was assessed before and after EGCG treatment. Untreated STZ rats presented increased levels of 8-OHdG immunoreaction, higher numbers of Fos-immunoreacted neurons and high levels of co-localization of 8-OHdG and Fos in laminae I-III. Treatment with EGCG normalized the increase of the above mentioned parameters and ameliorated mechanical hypersensitivity. The present study shows that nociceptive neurons in spinal cord laminae I-III exhibit oxidative stress damage during diabetic neuropathy, which probably affects ascending pain transmission during DNP. The neurobiological mechanisms and translational perspectives of the beneficial effects of a preventive and sustained EGCG treatment in DNP need to be evaluated in the future.

α-Lipoic acid normalizes nociceptive neuronal activity at the spinal cord of diabetic rats

AIM

To evaluate the effects of antioxidant treatment of streptozotocin (STZ)-diabetic rats with α-lipoic acid (α-LA) in neuronal and microglial activation at the spinal cord, an important relay station of nociceptive transmission. Because of the role of the potassium chloride co-transporter 2 (KCC2) in neuronal activation at the spinal cord and the influence of microglia in KCC2 expression, we also evaluated the effects of α-LA in KCC2 expression at the spinal cord.

METHODS

Four weeks after STZ injection, the rats received daily intraperitoneal injections of α-LA (100 mg/kg), during 2 weeks. Mechanical nociception was evaluated before and after α-LA treatment. Spinal cords were immunoreacted against 8-OH-dG (marker of oxidative stress damage), Fos (marker of neuronal activation) and CD11b (marker of microglia). KCC2 expression was evaluated by immunohistochemistry and western blotting.

RESULTS

Treatment with α-LA decreased the 8-OH-dG and Fos expressions to controls' levels, but did not affect CD11b. Treatment with α-LA alleviated mechanical hyperalgesia and partially corrected KCC2 expression.

CONCLUSIONS

This study shows that neuronal hyperactivity at the spinal cord of STZ-diabetic rats can be corrected by α-LA, which may account for alleviation of mechanical hyperalgesia. These effects are probably partially mediated by KCC2, but are independent from microglia.

Minocycline completely reverses mechanical hyperalgesia in diabetic rats through microglia-induced changes in the expression of the potassium chloride co-transporter 2 (KCC2) at the spinal cord

AIM

neuronal hyperactivity at the spinal cord during mechanical hyperalgesia induced by diabetes may result from a decrease in the local expression of the potassium chloride co-transporter 2 (KCC2), which shifts the action of the neurotransmitter γ-amminobutiric acid (GABA) from inhibitory to excitatory. In this study, we evaluated the effects of spinal microglia inhibition or brain-derived neurotrophic factor (BDNF) blockade on KCC2 expression, spinal neuronal activity and mechanically induced pain responses of streptozotocin (STZ)-diabetic rats.

METHODS

four weeks after induction of diabetes, the STZ-diabetic rats received daily intrathecal injections, for 3 days, of minocycline (microglia inhibitor), TrkB/Fc (BDNF sequester) or saline. Behavioural responses to mechanical nociceptive stimulation of STZ-diabetic rats were evaluated by the Randall-Selitto test. The lumbar spinal cord was immunoreacted against the Fos protein (marker of neuronal activation) or KCC2, which was also quantified by western blotting. BDNF levels at the spinal cord were quantified by an enzyme-linked immunosorbent assay (ELISA).

RESULTS

minocycline treatment reversed the mechanical hyperalgesia, increased Fos expression and decreased the KCC2 expression detected in STZ-diabetic rats to control levels. Treatment with TrkB/Fc was less effective, inducing moderate effects in mechanical hyperalgesia and Fos expression and only a partial correction of KCC2 expression. BDNF levels were not increased in STZ-diabetic rats.

CONCLUSIONS

this study demonstrates that the microglial activation at the spinal cord contributes to mechanical hyperalgesia and spinal neuronal hyperactivity induced by diabetes, apparently by regulating the KCC2 expression. These effects do not seem to be mediated by BDNF, which is an important difference from other chronic pain conditions. New targets directed to prevent spinal microglia activation should be considered for the treatment of mechanical hyperalgesia induced by diabetes.