Neurotrophic modulation of myelinated cutaneous innervation and mechanical sensory loss in diabetic mice

Human diabetic patients often lose touch and vibratory sensations, but to date, most studies on diabetes-induced sensory nerve degeneration have focused on epidermal C-fibers. Here, we explored the effects of diabetes on cutaneous myelinated fibers in relation to the behavioral responses to tactile stimuli from diabetic mice. Weekly behavioral testing began prior to streptozotocin (STZ) administration and continued until 8 weeks, at which time myelinated fiber innervation was examined in the footpad by immunohistochemistry using antiserum to neurofilament heavy chain (NF-H) and myelin basic protein (MBP). Diabetic mice developed reduced behavioral responses to non-noxious (monofilaments) and noxious (pinprick) stimuli. In addition, diabetic mice displayed a 50% reduction in NF-H-positive myelinated innervation of the dermal footpad compared with non-diabetic mice. To test whether two neurotrophins nerve growth factor (NGF) and/or neurotrophin-3 (NT-3) known to support myelinated cutaneous fibers could influence myelinated innervation, diabetic mice were treated intrathecally for 2 weeks with NGF, NT-3, NGF and NT-3. Neurotrophin-treated mice were then compared with diabetic mice treated with insulin for 2 weeks. NGF and insulin treatment both increased paw withdrawal to mechanical stimulation in diabetic mice, whereas NT-3 or a combination of NGF and NT-3 failed to alter paw withdrawal responses. Surprisingly, all treatments significantly increased myelinated innervation compared with control-treated diabetic mice, demonstrating that myelinated cutaneous fibers damaged by hyperglycemia respond to intrathecal administration of neurotrophins. Moreover, NT-3 treatment increased epidermal Merkel cell numbers associated with nerve fibers, consistent with increased numbers of NT-3-responsive slowly adapting A-fibers. These studies suggest that myelinated fiber loss may contribute as significantly as unmyelinated epidermal loss in diabetic neuropathy, and the contradiction between neurotrophin-induced increases in dermal innervation and behavior emphasizes the need for multiple approaches to accurately assess sensory improvements in diabetic neuropathy.