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Dorsal root ganglion (DRG) neurons with dichotomizing axons have been reported in several species and are thought to be related to referred pain. However, these neurons, which have dichotomizing axons to the lumbar muscles and to the knee, have not been investigated. Clinically, pain from the lumbar muscles is sometimes referred to the lower extremities. Two kinds of neurotracers [1,1′-dioctadecyl-3,3,3′,3′-tetramethyl-indocarbocyanine perchlorate (DiI) and fluoro-gold (FG)] were used in the present double-labelling study. DiI crystals were placed in the left lower back muscle, and FG was applied to the medial side of the knee. Bilateral DRGs from L1 through L6 were immunoreacted with calcitonin gene-related peptide (CGRP) antibodies and observed under a fluorescence microscope. DRG neurons double-labelled with DiI and FG were recognized only in the ipsilateral DRGs from levels L1 to L6. Approximately 1% of DRG neurons innervating the low back muscles had other axons to the medial side of the knee. In double-labelled neurons, the ratio of CGRP-immunoreactive DRG neurons was 60%. This finding provides a possible neuroanatomical explanation for referred knee pain from the lower back since CGRP is a marker of sensory neurons typically involved with pain perception. However, these neurons are rare, and mechanisms of referred pain may be explained by the convergence–projection hypothesis. [ABSTRACT FROM AUTHOR]

The distribution of sensory neurons innervating the peritoneum was studied using axonal transport of fluoro-gold. The tracer was injected into parietal peritoneum, diaphragm, mesentery, mesocolon, visceral peritoneum covering the stomach, small intestine, colon, liver, spleen, kidney, urinary bladder or uterus. After ten days of survival bilateral dorsal root ganglia from C2 to S6, and the nodose ganglia were dissected. The cryostat sections of these ganglia were mounted on glass slides and observed with a fluorescence microscope. In cases where the tracer was placed on the peritoneum covering the abdominal wall, labeled neurons were observed only in the ipsilateral dorsal root ganglia. A small number of neurons in nodose and cervical dorsal root ganglia of both sides were labeled after placing the tracer on the central part of the diaphragm. When fluoro-gold was applied to the peripheral part of the diaphragm, nodose ganglion was negative, and dorsal root ganglia from T6 to T12 were positive. Many neurons in the nodose ganglia in addition to somata in the dorsal root ganglia from T4 to T13 were labeled when the tracer was placed on the peritoneum lining the stomach, small intestine or caecum. After applying the tracer onto the colon, labeled neurons were observed in the dorsal root ganglia from T13 to L2 and L5 to S1. Ganglion cells in the nodose and dorsal root ganglia from T5 to T13 were positive when fluoro-gold was placed on the mesentery. No labeled neurons were observed in any ganglia when the tracer was applied to the peritoneum covering the spleen, kidney, uterus, urinary bladder and liver. These results suggest that most of the parietal peritoneum receives sensory nerves from dorsal root ganglia and the visceral peritoneum from both spinal nerves and the vagus nerve. [ABSTRACT FROM AUTHOR]

Summary: The rat L5/6 disc is innervated from T13 to L6 dorsal root ganglia (DRGs) multisegmentally. Sensory fibers from T13, L1 and L2 DRGs have been reported to innervate through the paravertebral sympathetic trunks, whereas those from L3 to L6 DRGs innervate directly through sinuvertebral nerves on the posterior longitudinal ligament (PLL). The presence of substance P (SP)- and calcitonin gene-related peptide (CGRP)-immunoreactive (ir) nerve fibers has been demonstrated in the lumbar intervertebral discs, but their percentages in DRG neurons have not been studied. Fluoro-gold (F-G) labeled neurons innervating the L5/6 disc were distributed throughout DRGs from T13 to L6 levels. Of F-G labeled neurons innervating the L5/6 disc, the percentage of SP-ir T13 to L6 DRG neurons was 30%, and that of CGRP-ir neurons was 47%. The mean cross-sectional area of the cell of SP-ir neurons was 696 ± 66 ?m2 (mean ± S.E.), and that of CGRP-ir neurons was 695 ± 72 ?m2 (mean ± S.E.). SP- and CGRP-ir were mainly observed in small neurons. The percentages of SP-or CGRP-ir neurons in L1 and L2 DRGs innervating the L5/6 disc were not different from those in L3, L4 or L5 DRGs. In the physiological condition in rats, DRG neurons at all levels may have the same significant role in pain sensation of the disc. [Copyright &y& Elsevier]

: Background contextAn annular tear extending to the outer one-third of the annulus is thought to be one of the causes of low back pain. However, some patients have bilateral low back symptoms, even if the annular tear is localized in the lateral disc. Because nociceptive information from the lateral disc is transmitted by the dorsal root ganglion (DRG) neurons innervating the lateral disc, we investigated the distribution of the DRG neurons innervating the lateral portion of the disc.: PurposeTo clarify the distribution and pathway of the DRG neurons innervating the lateral portion of the L5–L6 disc in rats.: Study design/settingUsing the retrograde tracing method, we studied the innervation pattern of the lateral portion of the L5–L6 intervertebral disc in rats.: MethodsThe retrograde transport of Fluoro-Gold (F-G; Fluorochrome, Denver, CO) was used in 22 rats. Subjects included a nontreated group (n = 16) and a sympathectomized group (n = 6). Seven days after the application of F-G crystals to the left lateral portion of the L5–L6 disc, bilateral T12–L6 DRGs were observed by fluorescent microscopy.: ResultsIn the nontreated group, of all the F-G–labeled neurons, 93.1% were present in the left DRGs and 6.9% were in the right DRGs. The number of labeled neurons was largest in the left L2 DRGs. In the sympathectomized group, the numbers of labeled neurons in the T13, L1 and L2 DRGs were significantly lower than the numbers in the nontreated group.: ConclusionResults of this study indicate that DRG neurons innervating the lateral portion of the disc are distributed mainly in the ipsilateral side but also in the contralateral side. The DRG neurons in T13, L1 and L2 innervate the lateral portion of the L5–L6 disc through the paravertebral sympathetic trunks. [Copyright &y& Elsevier]

The P2X3 receptor is normally localized in a sub-population of small-diameter dorsal root ganglion (DRG) neurons, and is thought to be related to pain perception. The aim of this study in rats was to examine P2X3-immunoreactivity in DRG neurons innervating the lumbar disc and in DRG neurons innervating cutaneous tissues. Fluoro-Gold was applied to the L5–L6 disc, the plantar skin of the hind paw (L4–L5 dermatomes), and the back skin (L1–L2 dermatomes). It has been reported that the L5–L6 disc is innervated by T13–L5 DRG neurons. We performed immunostaining using antibodies against the P2X3 receptor of T13–L5 DRGs to examine the L5–L6 disc, L4 and L5 DRGs to examine plantar skin and L1 and L2 DRGs to examine back skin. The P2X3-immunoreactivity was detected in 22.0 and 22.8% of neurons, labeled by Fluoro-Gold applied to plantar and back skin, respectively. However, P2X3-immunoreactivity was detected in only 4.0% of the neurons projecting to the L5–L6 disc. The proportion of P2X3-immunoreactive neurons was significantly larger in the DRG neurons innervating the plantar or the back skin, than in the DRG neurons innervating the lumbar disc. These results suggest that the P2X3 receptors are abundant in DRG neurons innervating cutaneous tissues, but not in neurons innervating the lumbar disc. It is likely therefore that the P2X3 receptor is less related to the mechanism of discogenic pain, than to cutaneous tissue pain. [Copyright &y& Elsevier]