According to our data, troglitazone appears to promote fat accumulation in the subcutaneous adipose tissue rather than in the visceral adipose tissue in mildly obese Japanese people with type 2 diabetes. This shift of energy accumulation from the visceral to subcutaneous adipose tissue may greatly contribute to the troglitazone-mediated amelioration of insulin resistance.

The expression and localization of the insulin receptor (IR) was examined in rat dorsal root ganglia (DRG) and spinal cord using Western blotting, in situ hybridization and immunocytochemistry. Western blotting showed that the molecular weight of the IR beta subunit was higher in PNS than that found in CNS. Both IR mRNA and protein expressions were highest in small-sized sensory DRG neurons and myelinated sensory root fibers expressed higher levels of IR protein than myelinated anterior root fibers. In the spinal cord, IR immunoreactive neurons were present in lateral lamina V and in lamina X, suggesting the presence of IR in nociceptive pathways. Electronmicroscopy of DRGs revealed a polarized localization of the IR in abaxonal Schwann cell membranes, outer mesaxons in close vicinity to tight junctions of both myelinating and non-myelinating Schwann cells and to plasma membranes of sensory neurons. From these findings, we speculate that insulin may play a role in sensory fibers involved in nociceptive function often perturbed in diabetic neuropathy. The high expression of IR localizing to tight junctions of dorsal root mesaxons of DRGs may suggest a regulatory role on barrier functions compensating for the lack of a blood-nerve barrier in dorsal root ganglia. This is consistent with the colocalization of IR with tight junctions of the paranodal barrier and endoneurial endothelial cells in peripheral nerve.

Peripheral nerve expresses predominantly the high affinity IR, which is localized to strategic structures associated with the blood-nerve barrier and the paranodal ion-channel barrier.

To explore the molecular abnormalities underlying the degeneration of the node of Ranvier, a characteristic aberration of type 1 diabetic neuropathy, we examined in type 1 BB/Wor and type 2 BBZDR/Wor rats changes in expression of key molecules that make up the nodal and paranodal apparatus of peripheral nerve. Their posttranslational modifications were examined in vitro. Their responsiveness to restored insulin action was examined in type 1 animals replenished with proinsulin C-peptide. In sciatic nerve, the expression of contactin, receptor protein tyrosine phosphatase ␤, and the Na ؉ -channel ␤ 1 subunit, paranodal caspr and nodal ankyrin G was unaltered in 2-month type 1 diabetic BB/Wor rats but significantly decreased after 8 months of diabetes. These abnormalities were prevented by C-peptide administered to type 1 BB/Wor rats and did not occur in duration-and hyperglycemia-matched type 2 BBZDR/ Wor rats. The expression of the ␣-Na ؉ -channel subunit was unaltered. In SH-SY5Y cells, only the combination of insulin and C-peptide normalized posttranslational O-linked N-acetylglucosamine modifications and maximized serine phosphorylation of ankyrin G and p85 binding to caspr. The beneficial effects of C-peptide resulted in significant normalization of the nerve conduction deficits. These data describe for the first time the progressive molecular aberrations underlying nodal and paranodal degenerative changes in type 1 diabetic neuropathy and demonstrate that they are preventable by insulinomimetic C-peptide. Diabetes 53

The more severe molecular, functional and morphometric abnormalities of nociceptive C-fibers in type 1 insulinopenic rats compared to type 2 hyperinsulinemic rats suggest that impaired insulin action may play a more important pathogenetic role than hyperglycemia per se.