Abstract
Clinical and preclinical studies have shown that patients with Diabetic Neuropathy Pain (DNP) present with increased tumor necrosis factor alpha (TNF-α) serum concentration, whereas studies with diabetic animals have shown that TNF-α induces an increase in NaV1.7 sodium channel expression. This is expected to result in sensitization of nociceptor neuron terminals, and therefore the development of DNP. For further study of this mechanism, dissociated dorsal root ganglion (DRG) neurons were exposed to TNF-α for 6 h, at a concentration equivalent to that measured in STZ-induced diabetic rats that developed hyperalgesia. Tetrodotoxin sensitive (TTXs), resistant (TTXr) and total sodium current was studied in these DRG neurons. Total sodium current was also studied in DRG neurons expressing the collapsin response mediator protein 2 (CRMP2) SUMO-incompetent mutant protein (CRMP2-K374A), which causes a significant reduction in NaV1.7 membrane cell expression levels. Our results show that TNF-α exposure increased the density of the total, TTXs and TTXr sodium current in DRG neurons. Furthermore, TNF-α shifted the steady state activation and inactivation curves of the total and TTXs sodium current. DRG neurons expressing the CRMP2-K374A mutant also exhibited total sodium current increases after exposure to TNF-α, indicating that these effects were independent of SUMOylation of CRMP2. In conclusion, TNF-α sensitizes DRG neurons via augmentation of whole cell sodium current. This may underlie the pronociceptive effects of TNF-α and suggests a molecular mechanism responsible for pain hypersensitivity in diabetic neuropathy patients.
Highlights
The World Health Organization (WHO) defines diabetes as a chronic disease that results from poor insulin production or the inability of the body to use it efficiently
Diabetic rats develop hyperalgesia and increasing in TNFα serum concentration Induction of diabetes by intraperitoneal (i.p.) STZ injection resulted in sustained hyperglycemia of diabetic rats for at least 60 days (Fig. 1a and Table 1)
On day 60, the diabetic rats showed an elevation in tumor necrosis factor α (TNF-α) serum concentration level (Control, 340.3 ± 16.0 pg/ml vs Diabetic, 624.9 ± 97.8 pg/ ml, day 60, Fig. 1c and Table 3)
Summary
The World Health Organization (WHO) defines diabetes as a chronic disease that results from poor insulin production or the inability of the body to use it efficiently. The NaV1.7 isoform has been directly linked to diabetic neuropathy and the release of proinflammatory cytokines [11, 15,16,17]. These channels are mostly expressed in small diameter Aδ and C fibers [18] and, not surprisingly, in 85% of functionally identified nociceptors [19]. NaV1.7 expression is increased in DRG neurons of STZ-induced diabetic rats, a change that contributes to pain-related hypersensitivity [11, 16, 20]. In PDN, increased tumor necrosis factor α (TNF-α) expression in dorsal root ganglion (DRG) neurons was linked to increased Nav 1.7 levels and nociceptive behaviors
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