C‐fiber (Remak) bundles contain both isolectin B4‐binding and calcitonin gene‐related peptide‐positive axons

Abstract

Unmyelinated nerve fibers (Remak bundles) in the rodent sciatic nerve typically contain multiple axons. This study asked whether C-fiber bundles contain axons arising from more than one type of neuron. Most small neurons of the lumbar dorsal root ganglion (DRG) are either glial cell line-derived neurotrophic factor dependent or nerve growth factor dependent, binding either isolectin B4 (IB4) or antibodies to calcitonin gene-related peptide (CGRP), respectively. Injection of IB4-conjugated horseradish peroxidase into a lumbar DRG resulted in intense labeling of IB4 axons, with very low background. Visualized by confocal fluorescence, IB4-binding and CGRP-positive nerve fibers originating from different DRG neurons came together and remained closely parallel over long distances, suggesting that these two types of axon occupy the same Remak bundle. With double-labeling immunogold electron microscopy (EM), we confirmed that IB4 and CGRP axons were distinct and were found together in single Remak bundles. Previous studies indicate that some DRG neurons express both CGRP and IB4 binding. To ensure that our immunogold results were not a consequence of coexpression, we studied large populations of unmyelinated axons by using quantitative single-label EM. Tetramethylbenzidine, a chromogen with strong intrinsic signal amplification of IB4-horseradish peroxidase, labeled as many as 52% of unmyelinated axons in the dorsal root. Concomitantly, 97% of the Remak bundles with more than one axon contained at least one IB4-labeled axon. Probabilistic modeling using binomial distribution functions rejected the hypothesis that IB4 axons segregate into IB4-specific bundles (P < 0.00001). We conclude that most Remak bundle Schwann cells simultaneously support diverse axon types with different growth factor dependences.

Postnatal changes in the Rexed lamination and markers of nociceptive afferents in the superficial dorsal horn of the rat

“…It may reflect two separate things; one is that, as in the mouse, CGRP-and IB4-positive DRG neurons are separate populations (Molliver et al, 1997;Zwick et al, 2002), yet certain parts of their terminal fields in the dorsal horn are tightly intertwined (see Luo et al, 2007 for more details). The other is that some CGRPpositive neurons also bind IB4, which would be consistent with findings in the DRG (Petruska et al, 2000(Petruska et al, , 2002Murinson et al, 2005;Price and Flores, 2006). The overlap in terminal fields may reflect the subpopulation of doublelabeled afferents as suggested by the observation of thin optical sections at high magnifications (Fig.…”

Chronic nerve compression injury induces a phenotypic switch of neurons within the dorsal root ganglia

“…It is interesting to note that Remak bundles in the normal nerve milieu have been profiled as containing CGRP and IB4 representatives (Murinson et al, 2005). This fact is consistent with the physical character of these neurons, which generally range in size from medium to small and thus comprise a large proportion of the unmyelinated population.…”

Calcitonin gene-related peptide (CGRP) and its receptor components in human and rat spinal trigeminal nucleus and spinal cord at C1-level

“…More neurons expressing both markers are found in dorsal root ganglia compared to the trigeminal ganglion of rat [33]. Electron microscopy showed that IB4 and CGRP expressing axons were distinct, but both could be present in the same bundle of un-myelinated fibers [34]. …”