Painful neuropathy is one of the most common complications of diabetes, one hallmark of which is
tactile allodynia (
pain hypersensitivity to innocuous stimulation). The underlying mechanisms of
tactile allodynia are, however, poorly understood. Emerging evidence indicates that, following nerve injury, activated microglia in the spinal cord play a crucial role in
tactile allodynia. However, it remains unknown whether spinal microglia are activated under diabetic conditions and whether they contribute to diabetes-induced
tactile allodynia. In the present study, using
streptozotocin (STZ)-induced diabetic rats that displayed
tactile allodynia, we found several morphological changes of activated microglia in the dorsal horn. These included increases in Iba1 and OX-42 labeling (markers of microglia), hypertrophic morphology, the thickness and the retraction of processes, and in the number of activated microglia cells. Furthermore, in the dorsal horn of STZ diabetic rats, extracellular signal-regulated
protein kinase (ERK) and an upstream
kinase, Src-family kinase (SFK), both of which are implicated in microglial functions, were activated exclusively in microglia. Moreover, inhibition of ERK phosphorylation in the dorsal horn by intrathecal administration of
U0126, an inhibitor of ERK activation, produced a striking alleviation of existing, long-term
tactile allodynia of diabetic rats. We also found that a single administration of
U0126 reduced the expression of
allodynia. Together, these results suggest that activated dorsal horn microglia may be a crucial component of diabetes-induced
tactile allodynia, mediated, in part, by the ERK signaling pathway. Thus, inhibiting microglia activation in the dorsal horn may represent a therapeutic strategy for treating diabetic
tactile allodynia.