GDNF differentially regulates neuronal excitability in DRGs from normal and diabetic mice

Diabetic polyneuropathy (DPN) is among the most common long-term complications of diabetes mellitus, affecting up to 50% of diabetic patients of which 15-25% have chronic neuropathic pain. Although this painful signal has believed to originate in the peripheral nervous system, the precise cellular mechanism of chronic pain associated with DPN is still poorly understood. Evidence has accumulated that abnormal excitability of nociceptive primary sensory neurons contributes to the pathology. Neurotrophic factors, such as glial-derived neurotrophic factor (GDNF), are key regulators of sensory neurons excitability and their altered levels has been associated to pathological conditions. Here, we investigated the effect of GDNF in controlling sensory neuron activity in normal and diabetic mice. To mimic type 1 diabetes, four week-old male mice were injected intraperitoneally with streptozotocin (STZ, 150 mg/kg) that selectively kills insulin-producing pancreatic β-cells. DRGs were acutely excised and treated with collagenase (7 mg/mL, 1 hour at 35°C). Whole-cell patch clamp recordings were obtained from visually identified neurons within intact DRGs before and after the administration of GDNF (100 ng/mL). In current clamp, GDNF induced a depolarizing shift of the firing threshold (from -24 to -18 mV) and delayed the firing onset (from 14 to 66 ms) in control conditions, particularly in small neurons (< 25 µm). Conversely, in diabetic mice GDNF was less effective. Since these findings are consistent with the involvement of voltage-dependent K+ channels, we next verified this hypothesis in voltage clamp after minimizing Na+- and Ca2+-mediated currents. By applying a hyperpolarizing step protocol, GDNF induced a significant increase of inward K+ conductance, causing a change in the slope of the current-voltage relationship. These effects were mostly evoked in small neurons from control mice, while little effect was observed in diabetic conditions. Our data indicate that GDNF exerts an inhibitory control on small DRG neurons through the activation of K+ conductances and that such control is attenuated in diabetes. Restoring this inhibitory control in sensory neurons from diabetic patients may represent a novel strategies for mitigating the symptoms of painful diabetic neuropathy.