Impaired glucose tolerance and insulinopenia in the GK-rat causes peripheral neuropathy

Recent studies indicate that impaired glucose tolerance (IGT) in man is a causative factor in idiopathic sensory neuropathy, and that insulinopenia may contribute substantially to the severity of diabetic peripheral neuropathy. The effect of sustained IGT and progressive insulinopenia in the absence of overt hyperglycemia on peripheral nerve abnormalities was examined in the Goto-Kakizaki (GK)-rat.

Two and eighteen-month-old GK rats with decreased glucose tolerance and overt insulinopenia, respectively, were examined with respect to nerve function, structure, morphometry and molecular integrity, and were compared to age-matched control rats.

Both 2-(p < 0.001) and 18-month-old (p < 0.001) GK rats showed reduced body weight. Blood glucose levels following glucose tolerance tests were elevated in both the 2-month and the 18-month-old GK rats. Fasting plasma insulin levels in the 2-month GK rats were increased threefold (p < 0.05) but decreased by 71% (p < 0.001) in the 18-month GK rats. The two-month GK rats showed a normal nerve conduction velocity, whereas in the 18-month GK rats it was reduced to 76% (p < 0.001) of control values. No morphometric abnormalities were found in the 2-month GK rats, whereas the 18-month GK rats showed loss of small myelinated fibers (p < 0.001), atrophy and loss of unmyelinated axons (p < 0.05) and an increased (p < 0.01) frequency of regenerating fibers. In the older GK rats, both mRNA and protein expression of nerve growth factor (NGF) in the sciatic nerve were significantly reduced (p < 0.001 and p < 0.05), and NGFR TrkA (high affinity NGF receptor) and NGFRp75 (low affinity NGF-receptor) protein expression was reduced in dorsal root ganglia (DRG) (both p < 0.05). These changes were accompanied by significantly reduced protein expressions of substance P (SP) and calcitonin gene-related protein (CGRP) in DRG's (both p < 0.001) as well as a 40% (p < 0.001) decrease in SP and a 62% (p < 0.001) decrease in CGRP-positive DRG neurons. In the sciatic nerve, SP and CGRP protein expression was decreased by 71% (p < 0.01) and 79% (p < 0.01), respectively.

IGT combined with hyperinsulinemia for 2 months have no detectable effect on peripheral nerve function or structure. In contrast, IGT and subsequent insulinopenia result in a functional and structural neuropathy associated with impaired NGF support and neuropeptide synthesis. We suggest that these abnormalities are mainly due to insulinopenia rather than hyperglycemia.