Characterisation of Peripheral Neuropathy and Neuronal and Endothelial Markers of Neuropathic Pain: a Comparative Study Across Preclinical Models of Neuropathic Pain

Abstract

Neuropathic pain is pain caused by a lesion or disease to the somatosensory pathways. A leading cause is diabetic peripheral neuropathy which effects over half of all diabetic patients. This neuronal damage commonly leads to sensory disturbances such as dysesthesia, allodynia, and hyperalgesia. These unpleasant sensory experiences are often accompanied by secondary symptoms such as anxiety and depression; all of which negatively effects the suffers quality of life. The underlying pathology behind diabetic peripheral neuropathy and other neuropathic pain conditions is not well understood and requires further elucidating. The lack of understanding poses significant problems for diagnosis and optimal treatment strategies, resulting in only a small proportion of sufferers benefitting from current pharmacological treatment. Without biomarkers that can accurately predict neuropathy and neuropathic pain, it is hard to successfully identify preclinical treatments that will translate to optimal treatment strategies for neuropathic pain sufferers. Potential biomarkers of peripheral neuropathic pain include intra-epidermal nerve fibre density (IENFD), Langerhans cells (LCs) density, calcitonin gene-related peptide (cGRP) and vascular endothelial growth factor (VEGF). Plantar skin biopsies were collected from male Wister Han ISG STZ-induced type 1 diabetic rats with a stable mechanical allodynia after 7 weeks and from male and female high fat diet (HFD) induced type diabetic C57 BI6 mice. An immunohistochemistry (IHC) protocol was developed to quantify IENFs, LCs and cGRP density using anti PGP 9.5 and cGRP antibodies. There was found to be a significant decrease in IENF density in STZ induced type 1 diabetic rats at 2- (p<0.001), and 7- weeks (p<0.001) which correlated with an increase in LC expression (r= -0.6214, p= <0.002). There was also a proportional increase in cGRP expression in STZ induced type 1 diabetic rats at 2-, 4-, and 7-weeks, the increase was only significant at 2 weeks (p=0.012). No time course effects in IENFD decrease, LCs increase, or cGRP increase were found in 2-, 4-, and 7-week STZ induced type 1 diabetic rats. VEGF expression levels were quantified, using western blot, in control and STZ induced type 1 diabetes rat retina however no signal was produced with the anit-VEGF antibody, suggesting an issue with the antibody or protocol. In the HFD induced type 2 diabetes model there was a significant decrease in IENFD between male mice fed control chow and those fed 60% fat diet (p < 0.001) as well as between male mice fed 42% and 60% fat diet (p=0.010). In female mice there was a significant decrease in IENFD from the control chow group in both the 42% fat diet group (p=0.002) and the 60% fat diet group (p < 0.001). However no significant change was found in LC expression or proportional levels of cGRP in the HFD induced type 2 diabetes model. This study demonstrated that the quantification of IENFD can be reliably and comparatively measured in plantar skin of healthy and diabetic type 1 and 2 neuropathic rodents. The observed increase in LC expression levels in combination with IENFD have the potential to assess the development and potential severity of the neuropathy, as well as predict the extent of neuropathic pain. The use of CGRP as a reliable biomarker was inconclusive in this study however further investigation is warranted into its use as a translatable neuropathy biomarker, as well as the role it plays in the generation of neuropathic pain, and mechanism for analgesia.