Differences in innervation and innervated neurons between hip and inguinal skin

Abstract

Pain originating from the hip may be referred to the groin and anterior thigh. We investigated sensory dorsal root ganglion neurons innervating the hip and the inguinal skin in rats using retrograde neurotransport and immunohistochemistry. A retrograde neurotracer Fluoro-Gold was injected into the left hip or inguinal skin of rats. Seven days later, we harvested bilateral dorsal root ganglions and counted the number of Fluoro-Gold-labeled neurons positive for calcitonin gene-related peptide, a marker of nerve growth factor-dependent neurons, or isolectin B4, a marker of glial cell line-derived neurotrophic factor-dependent neurons. In the hip group, Fluoro-Gold-labeled neurons were distributed throughout the left dorsal root ganglions from T13 to L5, primarily at L1, L2, L3, and L4, and the percentage of calcitonin gene-related peptide-positive neurons was higher than that of isolectin B4-binding neurons. In the inguinal skin group, Fluoro-Gold-labeled neurons were distributed throughout the left dorsal root ganglions from T13 to L3, primarily at L1, L2, and L3, and the percentage of isolectin B4-binding neurons was higher than that of calcitonin gene-related peptide-positive neurons. These data suggest the sensory innervation pattern and characteristics of the sensory nerve of the rat hip are different from those of inguinal skin.

Fig. 1A–C

Fluor-Gold labeling and CGRP-IR or IB4-binding of neurons in the rat inguinal skin are shown. (A) Neurons (arrows) indicate FG-labeled neurons innervating the inguinal skin. Scale bar = 100 μm (Stain, FG-labeled; original magnification, x100). (B) Neurons (arrows) do not bind CGRP-IR (Stain, double, CGRP and Alexa Fluor® 594; original magnification, x100). (C) Conversely, neurons (arrows) show IB4 binding (Stain, double stain, IB4 and Alexa Fluor® 488; original magnification, x100).

Fig. 2

The graph shows the distribution of the mean number of FG-labeled DRG neurons innervating the hip. There is a difference in the number of FG-labeled neurons in each DRG (p = 6.6 × 10⁻⁹). In post hoc testing the numbers of FG-labeled neurons in L1, L2, L3, and L4 were higher than the numbers of labeled neurons in T12, T13, L5, and L6.

Fig. 3

Distribution of the mean number of FG-labeled DRG neurons innervating inguinal skin is shown. There is a difference in the number of FG-labeled neurons in each DRG (p = 4.19 × 10⁻¹⁰). In post hoc testing the largest number of labeled neurons was in L1, and the numbers of FG-labeled neurons in L1, L2, and L3 were higher than the numbers of labeled neurons in T12, T13, and L4.

Fig. 4

A comparison of the CGRP-IR and IB4-binding neurons innervating the hip is shown. In the hip group, the number of FG-labeled CGRP-positive neurons was higher than the numbers of FG-labeled neurons binding IB4 in L1 (p = 0.0005), L2 (p = 0.0124), L3 (p = 0.0001), and L4 (p = 0.0070).

Fig. 5

A comparison of the CGRP-IR and IB4-binding neurons innervating inguinal skin is shown. In the inguinal skin group, the number of FG-labeled neurons binding IB4 was higher than the number of FG-labeled CGRP-positive neurons in L2 (p = 0.0070).

Fig. 6

The percentage of CGRP-IR neurons innervating the hip relative to the total number of neurons was higher than the mean percentage innervating inguinal skin (p = 3.03 × 10⁻⁶). In contrast, the percentage of IB4-binding neurons innervating inguinal skin relative to the total number of neurons was higher than the mean percentage innervating the hip (p = 9.41 × 10⁻⁹).