Injection of Fluoro-Gold into the tibial nerve leads to prolonged but reversible functional deficits in rats

Daguo Mi,Jiahui Niu,Yumin Yang,Junying Yan,Yaxian Wang,Yanping Zhang,Wen Hu,Ying Yuan

doi:10.1038/s41598-019-46285-7

Abstract

Tract tracing with neuronal tracers not only represents a straightforward approach to identify axonal projection connection between regions of the nervous system at distance but also provides compelling evidence for axonal regeneration. An ideal neuronal tracer meets certain criteria including high labeling efficacy, minimal neurotoxicity, rapid labeling, suitable stability in vivo, and compatibility to tissue processing for histological/immunohistochemical staining. Although labeling efficacy of commonly used fluorescent tracers has been studied extensively, neurotoxicity and their effect on neural functions remains poorly understood. In the present study, we comprehensively evaluated motor and sensory nerve function 2–24 weeks after injection of retrograde tracer Fluoro-Gold (FG), True Blue (TB) or Fluoro-Ruby (FR) in the tibial nerve in adult Spague-Dawley rats. We found that motor and sensory nerve functions were completely recovered by 24 weeks after tracer exposure, and that FG lead to a more prolonged delay in functional recovery than TB. These findings shed light on the long-term effect of tracers on nerve function and peripheral axonal regeneration, and therefore have implications in selection of appropriate tracers in relevant studies.

Highlights

Tract tracing with neuronal tracers provides compelling evidence for identification of anatomical axonal projection and characterization of integrity/restoration of axonal connection[1,2]
We recently found that injection of retrograde tracers Fluoro-Gold (FG), True Blue (TB) and Fluoro-Ruby (FR) in the tibial nerve in rats resulted in varying levels of axonal degeneration and functional impairment: FG lead to chemical severance of the nerve, and resulted in functional deficits almost as acute and complete as that caused by surgical transection of the nerve; TB caused functional impairment and axonal degeneration distal to injection, but to a lesser degree than FG; whereas the effect of intra-neural injection of FR on nerve function and axonal integrity was negligible[21]
We further investigated how tracer-induced functional deficits evolve over time by using a battery of well-established functional assessments, and we found that motor and sensory nerve functions were completely recovered by 24 weeks after injection of tracers in the tibial nerve, despite a prolonged delay in the case of FG

Summary

Introduction

Tract tracing with neuronal tracers provides compelling evidence for identification of anatomical axonal projection and characterization of integrity/restoration of axonal connection[1,2]. Fluorescent retrograde tracers are popular in neuroscience studies since tracer exposure in the nerve or axonal pathway distal to injury provides a unique opportunity to directly visualize neurons which have regenerated axons across the injury site[2,9,10,11]. Labeling efficacy of retrograde fluorescent tracers has been extensively studied[12,13,14,15,16]; toxicity of tracers to the labeled neurons/axons, which is another important property to be considered, was seldom investigated and poorly understood[17,18,19,20]. Did not cause significant function deficits after injected even at a greater amount into the tibial nerve, similar to the vehicle control (saline)[21]

Methods

Results

Conclusion