Preferential expression of IGF-I in small DRG neurons and down-regulation following injury

In this study, we examined the expression of insulin-like growth factor I (IGF-I) and its receptor (IGF-IR) in dorsal root ganglia (DRG) neurons in two rodent models of nerve injury: sciatic nerve axotomy and streptozotocin-induced (STZ) painful diabetic neuropathy. We demonstrate that IGF-I and its receptor are preferentially expressed in small (< 25 mu m diameter) DRG neurons. There is a significant down-regulation in the expression of IGF-I and IGF-IR in the small DRG neurons of STZ rats by 59% and 71%, respectively. A parallel reduction in expression is shown in axotomized < 25 mum diameter DRG neurons for IGF-I (47%) but not for IGF-IR. The loss of IGF-I support to a population of predominantly nociceptive neurons may contribute to neuropathic pain observed in these models.