Primary sensory neurons with dichotomizing axons projecting to the facet joint and the sciatic nerve in rats

Study Design, Dorsal root ganglion (DRG) neurons that have dichotomizing ax ons to the lumbar facet joint and to the sciatic nerve were investigated in rats using a double fluorescent labeling technique. Objectives. To clarify the existence of DRG neurons with dichotomizing axon s projecting to the lumbar facet joint and to the sciatic nerve in rats. Summary of Background Data. DRG neurons having dichotomizing axons have bee n reported in several species and are considered to be related to referred pain. However, such DRG neurons have not been investigated in the lumbar sp ine. Clinically, pain from the lumbar facet joint is sometimes referred to the lower extremities innervated by the sciatic nerve. Methods, Two kinds of neurotracers (Dil and FG) were used in the present do uble-labeling study. Dil crystals were placed in the left L5-L6 facet joint , and FG was applied to the ipsilateral sciatic nerve or along the midline of the L5 dermatome. Bilateral DRGs T13-S1 were observed by fluorescence mi croscope. Results. DRG neurons double labeled with Dil and FG were recognized only in the ipsilateral DRGs from L3 to L6 levels. Approximately 3% of DRG neurons innervating the L5-L6 facet joint had other axons to the sciatic nerve. By contrast, no double-labeled neurons were observed after FG was applied to the L5 dermatome. Conclusions. In rats approximately 3% of DRG neurons innervating the lumbar facet joints have dichotomized axons projecting to the sciatic nerve.