Glucagon-like peptide 1, insulin, sensory neurons, and diabetic neuropathy

Like insulin, glucagon-like peptide 1 (GLP-1) may have direct trophic actions on the nervous system, but its potential role in supporting diabetic sensory neurons is uncertain. We identified wide expression of GLP-1 receptors on dorsal root ganglia sensory neurons of diabetic and nondiabetic mice. Exendin-4, a GLP-1 agonist, increased neurite outgrowth of adult sensory neurons in vitro. To determine the effects ofexendin-4 in comparison with continuous low- or high-dose insulin in vivo, we evaluated parallel cohorts of type 1 (streptozotocin-induced) and type 2 (db/db) mice of 2 months' diabetes duration with established neuropathy during an additional month of treatment. High-dose insulin alone reversed hyperglycemia in type 1 diabetic mice, partly reversed thermal sensory loss, improved epidermal innervation but failed to reverse electrophysiological abnormalities. Exendin-4 improved both sensory electrophysiology and behavioral sensory loss. Low-dose insulin was ineffective. In type 2 diabetes, hyperglycemia was uncorrected, and neither insulin nor exendin-4 reversed sensory electrophysiology, sensory behavior, or loss of epidermal axons. However, exendin-4 alone improved motor electrophysiology. Receptor for advanced glycosylated end products and nuclear factor-κB neuronal expression were not significantly altered by diabetes or treatment. Taken together, these results suggest that although GLP-1 agonists and insulin alone are insufficient to reverse all features of diabetic neuropathy, in combination, they might benefit some aspects of established diabetic neuropathy.