Role of GDNF in diabetes-induced alterations in primary sensory neurons

Diabetic polyneuropathy is among the most common long-term complications of diabetes mellitus and affects up to fifty percent of diabetic patients, some of which (15-25%) displaying chronic neuropathic pain. Nociceptive information is transmitted by a specific subset of peripheral sensory neurons, the nociceptors. The cell bodies of nociceptors are located in the dorsal root ganglia (DRGs) that relay nociceptive stimuli to the central nervous system. Since previous studies showed an involvement (among others) of the glial-derived neurotrophic factor (GDNF) in neuropathic pain, here we investigate the intervention of GDNF in the alterations of DRGs induced by the diabetic neuropathy, combining electrophysiological and morphological approaches. To mimic type 1 diabetes, four week-old male mice are injected intraperitoneally with streptozotocin (STZ, 150 mg/kg) that selectively kills insulin-producing pancreatic β-cells. Patch clamp recordings are carried out on intact DRGs, after treatment with collagenase (5-10 mg/ml). Recorded neurons are intracellularly labelled by Lucifer yellow or Alexa 568 for post-recording identification. Diabetes-related alterations in DRGs are analyzed by immunofluorescence on fixed tissues. The main populations of nociceptors are identified by an IB4 biotin-conjugate and an anti-CGRP antibody that respectively label non-peptidergic and peptidergic nociceptors. An anti-p-ERK antibody is used to report the activation of the MAPK/ERK pathway that plays a critical role in central sensitization and transduction of nociceptive signals. Preliminary results show an increase of p-ERK expression in nociceptors of diabetic mice in comparison to controls. This is paralleled by a decreased firing latency in DRG neurons of diabetic mice. Administration of GDNF during electrophysiological recordings delays firing in a subset of neurons. The phenotype (peptidergic vs non-peptidergic) of recorded neurons is subsequently addressed. Our data suggest a role of GDNF in modulating firing activity in primary sensory neurons. Alterations of GDNF signaling in diabetic mice may underlay the presence of neuropathic pain symptoms.