The Absence of Sensory Axon Bifurcation Affects Nociception and Termination Fields of Afferents in the Spinal Cord.

Philip Tröster,York Winter,Gary R Lewin,Achim Schmidtko,Jonas Petersen,Stefanie Peters,Robert Feil,Frederick Schwaller,Hannes Schmidt,Susanne Feil,Fritz G Rathjen,Oliver Drees,Gohar Ter-Avetisyan,Julia Haseleu

doi:10.3389/fnmol.2018.00019

Abstract

A cGMP signaling cascade composed of C-type natriuretic peptide, the guanylyl cyclase receptor Npr2 and cGMP-dependent protein kinase I (cGKI) controls the bifurcation of sensory axons upon entering the spinal cord during embryonic development. However, the impact of axon bifurcation on sensory processing in adulthood remains poorly understood. To investigate the functional consequences of impaired axon bifurcation during adult stages we generated conditional mouse mutants of Npr2 and cGKI (Npr2fl/fl;Wnt1Cre and cGKIKO/fl;Wnt1Cre) that lack sensory axon bifurcation in the absence of additional phenotypes observed in the global knockout mice. Cholera toxin labeling in digits of the hind paw demonstrated an altered shape of sensory neuron termination fields in the spinal cord of conditional Npr2 mouse mutants. Behavioral testing of both sexes indicated that noxious heat sensation and nociception induced by chemical irritants are impaired in the mutants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are not affected. Recordings from C-fiber nociceptors in the hind limb skin showed that Npr2 function was not required to maintain normal heat sensitivity of peripheral nociceptors. Thus, the altered behavioral responses to noxious heat found in Npr2fl/fl;Wnt1Cre mice is not due to an impaired C-fiber function. Overall, these data point to a critical role of axonal bifurcation for the processing of pain induced by heat or chemical stimuli.

Highlights

The primary sensory representation of the body within the central nervous system is based on the intricate innervation patterns of dorsal root ganglion (DRG) neurons into the spinal cord
Previous studies using knockout mice and axon tracing methods have implicated a cGMP signaling cascade composed of the ligand C-type natriuretic peptide (CNP), the receptor guanylyl cyclase Npr2 and the cGMPdependent kinase I in bifurcation of DRG and cranial sensory ganglia (Schmidt and Rathjen, 2010; Gibson and Ma, 2011; Ter-Avetisyan et al, 2014)
Since constitutive knockouts of Npr2 and cGMP-dependent protein kinase I (cGKI) are not suitable for further anatomical and behavioral studies on the consequences of the primary defect, the branching error at the dorsal root entry zone (DREZ), we generated mouse models that allowed an inactivation of Npr2 or cGKI in embryonic DRGs at very early developmental stages before sensory axons enter the spinal cord

Summary

Introduction

The primary sensory representation of the body within the central nervous system is based on the intricate innervation patterns of dorsal root ganglion (DRG) neurons into the spinal cord. This projection represents an attractive system to study the branching of axons and has enabled the characterization of a cGMP-dependent signaling cascade essential for the bifurcation of sensory axons. In the absence of any one of these components sensory axons no longer bifurcate and instead turn either in a rostral or caudal direction Consistent with these observations is the timing and pattern of localization of CNP in the dorsal spinal cord and Npr and cGKI in sensory neurons (Schmidt et al, 2009). Collateral sprouting originating from the stem axons as well as branch formation in the periphery of the body are not affected by the absence of Npr2-mediated cGMP signaling (Schmidt et al, 2007, 2009; Ter-Avetisyan et al, 2014)

Methods

Results

Conclusion