Background Diabetic neuropathy accompanies both Type 1 and Type 2 diabetes, although it shows in both humans and animal models distinct differences between the two types of diabetes. Progressive paranodal degenerations occurring in Type 1, but not in Type 2, diabetes is believed to account for the more severe functional de®cits in Type 1 diabetic rats. This suggests that factors other than hyperglycemia, such as insulin de®ciency, may play a pathogenetic role. In this study, we investigated the immunolocalization of the insulin receptor (IR) and the expression of its two alternatively spliced isoforms in adult rat peripheral nerve.
Methods Adult male Wistar rats 6±8 months of age were examined. Both light and ultrastructural immunohistochemistry was employed for localization of IR. The antibody was a mouse monoclonal antibody raised against the b-subunit of human IR. Reverse transcription polymerase chain reaction (RT-PCR) was used to identify the two IR isoforms in peripheral nerve and seven other organs. Localization of the mRNA message was assessed by in situ hybridization.
Results IR was localized to paranodal terminal Schwann cell loops and microvilli and to the paranodal axolemma. Furthermore, IR immunoreactivity was also present in Schmidt-Lantermann incisures. Endoneurial vessels showed IR localization on plasma membranes and in endocytotic vesicles of endothelial cells and pericytes. A high intensity of immunostained IR was found in close proximity to interendothelial tight junctions. Peripheral nerve showed, like the brain, predominantly the high af®nity IR lacking exon 11. The mRNA message was localized to Schwann cells, endothelial cells and pericytes.
Conclusion Peripheral nerve expresses predominantly the high af®nity IR, which is localized to strategic structures associated with the blood±nerve barrier and the paranodal ion-channel barrier.