Extracellular signal-regulated protein kinase activation in spinal cord contributes to pain hypersensitivity in a mouse model of type 2 diabetes

Painful peripheral neuropathy is a common complication of diabetes mellitus. The symptom of pain can become a major factor that decreases the quality of life of patients with diabetes, while effective treatment is lacking. In the present study, we aimed to investigate the changes of pain threshold in the early stage of diabetes in db/db mice, an animal model of type 2 diabetes mellitus, and the underlying molecular mechanisms. We found that （1） db/db mice （with a leptin receptor-null mutation and characterized by obesity and hyperglycemia） showed hypersensitivity to mechanical and thermal stimuli at the early stage of diabetes; （2） phosphorylated extracellular signal- regulated kinase （pERK）, but not total ERK in the spinal cord and dorsal root ganglia in db/db mice significantly increased compared with wild-type mice. The increased pERK immunoreactivity occurred in both NeuN-expressing neurons and GFAP- expressing astrocytes, but not in Iba-l-expressing microglia; （3） both single and consecutive （for 5 days） intrathecal injections of U0126 （2 nmol per day）, a selective MEK （an ERK kinase） inhibitor beginning at 8 weeks of age, attenuated the bilateral mechanical allodynia in the von-Frey test and heat hyperalgesia in Hargreave＇s test; and （4） db/db mice also displayed increased nocifensive behavior during the formalin test, and this was blocked by intrathecal injection of U0126. Also, the expression of pERK1 and pERK2 was upregulated following the formalin injection. Our results suggested that the activation of ERK in spinal neurons and astrocytes is correlated with pain hypersensitivity of the type 2 diabetes animal model. Inhibiting the ERK pathway may provide a new therapy for pain control in type 2 diabetes.

SpinalP2X7R contributes to streptozotocin-induced mechanical allodynia in mice

Painful diabetic neuropathy(PDN)is a diabetes mellitus complication.Unfortunately,the mechanisms underlying PDN are still poorly understood.Adenosine triphosphate(ATP)-gated P2X7 receptor(P2X7R)plays a pivotal role in non-diabetic neuropathic pain,but little is known about its effects on streptozotocin(STZ)-induced peripheral neuropathy.Here,we explored whether spinal cord P2X7R was correlated with the generation of mechanical allodynia(MA)in STZ-induced type 1 diabetic neuropathy in mice.MA was assessed by measuring paw withdrawal thresholds and western blotting.Immunohistochemistry was applied to analyze the protein expression levels and localization of P2X7R.STZ-induced mice expressed increased P2X7R in the dorsal horn of the lumbar spinal cord during MA.Mice injected intrathecally with a selective antagonist of P2X7R and P2X7R knockout(KO)mice both presented attenuated progression of MA.Double-immunofluorescent labeling demonstrated that P2X7R-positive cells were mostly co-expressed with Iba1(a microglia marker).Our results suggest that P2X7R plays an important role in the development of MA and could be used as a cellular target for treating PDN.

CX3CR1 contributes to streptozotocin-induced mechanical allodynia in the mouse spinal cord

Patients with diabetic peripheral neuropathy experience debilitating pain that significantly affects their quality of life(Abbott et al.,2011),by causing sleeping disorders,anxiety,and depression(Dermanovic Dobrota et al.,2014).The primary clinical manifestation of painful diabetic neuropathy(PDN)is mechanical hypersensitivity,also known as mechanical allodynia(MA)(Callaghan et al.,2012).MA’s underlying mechanism remains poorly understood,and so far,based on symptomatic treatment,it has no effective therapy(Moore et al.,2014).