Objective: To explore the innervation and biochemical characteristics associated with acupoint "Sanyinjiao" (SP 6) in the rat by using neural tracing and immunofluorescence techniques.
Methods: Five male SD rats were used in the present study. Alexa Fluor 488 conjugated cholera toxin subunit B (AF 488-CTB, 5 L, 1%) was injected into the site superior to the prominence of the medial malleolus, a corresponding site to acupoint SP 6 in the human body. After 3 surviving days, the rat's local tissue of SP 6 region, spinal cord and dorsal root ganglia (DRGs) at lumbar (L) segments (L 1 - S 1) were dissected following perfusion with 4% paraformaldehyde, cut into sections (20 μm for DRG and SP 6, and 40 μm for spinal cord) and observed under light microscope. In addition, substance P (SP) immunoreaction in the spinal cord, DRG and SP 6 region tissues was examined with immunofluorescence.
Results: All the labeling of AF 488-CTB appeared on the ipsilateral side of the injection. The labeled sensory and motor neurons distributed in L 3 - L 6 DRGs and spinal ventral horns with high concentration in the L 4 and L 5 segments. In addition, SP-positive neural labeling was found in small sensory neurons in DRG, axonal terminals in lamina I - II of spinal dorsal horns, and free nerve fibers in local subcutaneous tissues. Additionally, 24. 1 % of AF 488-CTB labeled sensory neurons simultaneously presented a SP-positive expression.
Conclusion: These findings reveal that the sensory and motor neurons associated with acupoint SP 6 distribute in L 3- L 6 DRGs and spinal ventral horns of rats, concentrating in the L 4 and L 5 segments, and 24.1% sensory neurons show SP-positive, suggesting a possible involvement of the primary SP-positive sensory neurons in the transmission of acupuncture stimulation signals.