Changes in peripheral intraepidermal nerve fiber densities in CD137 ligand – mediated neuropathic pain following sciatic nerve crush

Abstract

Abstract Previously, we have shown that the co-stimulatory molecule, CD137L plays a significant role in the development of neuropathic pain-like behaviors following sciatic nerve crush (SNC). Intraepidermal nerve fiber (IENF) density has shown high specificity and reliability in testing for small fiber sensory neuropathy. Here, we investigated the effects of CD137L depletion on the changes of IENFs following SNC. Both wild type (WT) C57BL/6 and B6_CD137LKO mice underwent SNC and sham surgeries. The ipsilateral and contralateral hind paws from individual mice were collected on days 0 (naïve), 3, 7, 14, 28, and 56 following surgery (2 males and 2 females per group per time point). All tissues were processed for fluorescent immunohistochemistry (IHC), which was performed with antibodies against PGP9.5 (pan-neuronal marker for total nerve fibers) and CGRP (peptidergic nociceptive fibers) followed by respective fluorescence-labeled secondary antibodies and fluorescence-labeled IB4 (non-peptidergic nociceptive fibers). The density of fibers was determined based on the respective IENF numbers (PGP9.5+, CGRP+ or IB4+) and the length of corresponding skin tissue. At the baseline, CD137L KO and WT mice had comparable levels of IENF densities for all types of fibers examined. Although surgery (SNC or sham) induced a transient increase in IB4 IENF density in CD137L KO mice at day 28, neither surgery induced significant changes in IENF densities over time. We did not observe significant differences between SNC vs. sham groups in either WT or CD137L KO mice. Therefore CD137L-mediated neuropathic pain behaviors cannot be attributed to reduction of hind paw IENF densities.