Long-Term Diabetic Microenvironment Augments the Decay Rate of Capsaicin-Induced Currents in Mouse Dorsal Root Ganglion Neurons.

Xingjuan Chen,Fletcher White,Ashley Riley,Maria Grant,Alexander Obukhov,Megan Welch,Yaqian Duan

doi:10.3390/molecules24040775

Abstract

Individuals with end-stage diabetic peripheral neuropathy present with decreased pain sensation. Transient receptor potential vanilloid type 1 (TRPV1) is implicated in pain signaling and resides on sensory dorsal root ganglion (DRG) neurons. We investigated the expression and functional activity of TRPV1 in DRG neurons of the Ins2+/Akita mouse at 9 months of diabetes using immunohistochemistry, live single cell calcium imaging, and whole-cell patch-clamp electrophysiology. 2′,7′-Dichlorodihydrofluorescein diacetate (DCFH-DA) fluorescence assay was used to determine the level of Reactive Oxygen Species (ROS) in DRGs. Although TRPV1 expressing neuron percentage was increased in Ins2+/Akita DRGs at 9 months of diabetes compared to control, capsaicin-induced Ca2+ influx was smaller in isolated Ins2+/Akita DRG neurons, indicating impaired TRPV1 function. Consistently, capsaicin-induced Ca2+ influx was decreased in control DRG neurons cultured in the presence of 25 mM glucose for seven days versus those cultured with 5.5 mM glucose. The high glucose environment increased cytoplasmic ROS accumulation in cultured DRG neurons. Patch-clamp recordings revealed that capsaicin-activated currents decayed faster in isolated Ins2+/Akita DRG neurons as compared to those in control neurons. We propose that in poorly controlled diabetes, the accelerated rate of capsaicin-sensitive TRPV1 current decay in DRG neurons decreases overall TRPV1 activity and contributes to peripheral neuropathy.

Highlights

Individuals diagnosed with diabetes mellitus are at increased risk of developing microcomplications of the disease, including diabetic retinopathy, renal failure, and peripheral neuropathy
The transient receptor potential vanilloid type 1 (TRPV1) channel is a Ca2+ permeable plasma membrane cation channel that can be activated by heat, acid, and capsaicin [4,5,6] and is expressed robustly in the nociceptive sensory neurons of dorsal root ganglion (DRG) [7,8]
There were 20 to 150 isolated DRG neurons per coverslip from tested groups (p = 0.027). * indicates p < 0.05 and *** indicates p < 0.001. +/Akita one mouse assessed in each experiment

Summary

Introduction

Individuals diagnosed with diabetes mellitus are at increased risk of developing microcomplications of the disease, including diabetic retinopathy, renal failure, and peripheral neuropathy. Peripheral neuropathy is one of the most common complications of end-stage diabetes, which manifests as symmetrically decreased pain sensation in the lower extremities and is associated with a high incidence of foot ulceration. Despite the high prevalence of this condition, the mechanisms underlying this dysfunction are poorly understood, limiting the development of new therapeutic strategies. TRPV1 is known to be upregulated in a number of clinical disease-associated pain conditions [9,10,11,12]. Animal models of induced diabetes have demonstrated increased TRPV1 expression that correlates to hyperalgesia and decreased TRPV1 expression in hypoalgesia [13], reflecting earlier and later manifestations of diabetic neuropathy, respectively

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Discussion

Conclusion