Early loss of peptidergic intraepidermal nerve fibers in an STZ-induced mouse model of insensate diabetic neuropathy

Abstract

Peptidergic and nonpeptidergic nociceptive neurons represent parallel yet distinct pathways of pain transmission, but the functional consequences of such specificity are not fully understood. Here, we quantified the progression of peptidergic and nonpeptidergic axon loss within the epidermis in the setting of a dying-back neuropathy induced by diabetes. STZ-induced diabetic MrgD mice heterozygous for green fluorescent protein (GFP) in nonpeptidergic DRG neurons were evaluated for sensitivity to mechanical and noxious thermal and chemogenic stimuli 4 or 8 weeks post-STZ. Using GFP expression in conjunction with PGP9.5 staining, nonpeptidergic (PGP+/GFP+) and peptidergic (PGP+/GFP-) intraepidermal nerve fibers (IENFs) were quantified at each time point. At 4 weeks post-STZ, nonpeptidergic epidermal innervation remained unchanged while peptidergic innervation was reduced by 40.6% in diabetic mice. By 8 weeks post-STZ, both nonpeptidergic innervation and peptidergic innervation were reduced in diabetic mice by 34.1% and 43.8%, respectively, resulting in a 36.5% reduction in total epidermal IENFs. Behavioral deficits in mechanical, thermal, and chemogenic sensitivity were present 4 weeks post-STZ, concomitant with the reduction in peptidergic IENFs, but did not worsen over the next 4 weeks as nonpeptidergic fibers were lost, suggesting that the early reduction in peptidergic fibers may be an important driving force in the loss of cutaneous sensitivity. Furthermore, behavioral responses were correlated at the 4 week time point with peptidergic, but not nonpeptidergic, innervation. These results reveal that peptidergic and nonpeptidergic nociceptive neurons are differentially damaged by diabetes, and behavioral symptoms are more closely related to the losses in peptidergic epidermal fibers.

Animal Models of Peripheral Neuropathies

“…Although it is a relatively less invasive method (compared to sural or superficial peroneal nerve biopsies), findings in skin biopsies correlate better with clinical features in DPN patients [28,29], especially when compared to electrophysiological parameters. In recent years, quantitation of intraepidermal nerve fiber density has been applied to both streptozotocin-induced rat [30] and mouse [31,32] models of DPN, and mouse models of peripheral neuropathy seen in type 2 diabetes [33,34]. Use of intraepidermal nerve fiber density as a true morphological correlate of the degree of sensory axonal loss is likely to improve the usefulness of rodent models of DPN, both in mechanistic studies and in preclinical efficacy studies of potential therapeutic drugs.…”

Diabetic Painful and Insensate Neuropathy: Pathogenesis and Potential Treatments

“…51,87 Others found reduced tactile sensitivity, rather than tactile allodynia, in STZ-diabetic C57Bl/6J mice and db/db mice. 58,70 Tactile allodynia also develops in nerve injury models of neuropathic pain 88 -90 (e.g., in the model of sciatic nerve ischemia 90 ). The mechanisms underlying diabetes-induced tactile allodynia have not been studied in detail, although considerable progress has been made in experimental studies performed in the last several years.…”

Mouse models of diabetic neuropathy

“…The SHD-STZ model exhibits a robust and early neuropathic phenotype, and changes in neuropathic function, including increased thermal latency, decreased mechanical sensitivity, decreased NCV, and decreased IENFD, are typically observed by 12 weeks after STZ administration (Demiot et al 2006;Johnson et al 2008;Muller et al 2008;Obrosova et al 2005). However, the SHD-STZ model is associated with severe toxicity and high levels of postinjection mortality (Ventura-Sobrevilla et al 2011).…”