Neurotensinergic periaqueductal gray‐rostral ventromedial medulla‐spinal dorsal horn pathway

Aim To investigate the neurotensin (NT)‐containing periaqueductal gray (PAG)‐rostral ventromedial medulla (RVM)‐spinal dorsal horn (SDH) descending pathway, which has been shown functionally antinociceptive in nature. Methods Fluoro‐gold (FG) or horseradish peroxidase (HRP) retrograde and biotinylated dextran amine (BDA) anterograde tracing methods and immunohistochemistry combined with tracing methods were used in the rat. Results After FG and BDA was injected into SDH or PAG, respectively, and combined with immunohistochemistry for NT and NT receptor 1 (NTR1), we found that NTR1‐immunoreactive (IR) neurons in the RVM received NT‐IR projections from PAG and also sent projections to the SDH. Then under electron microscope, in the RVM, we further found: (a) NT‐IR terminals made synapses on HRP‐labeled dendrites showing NTR1‐immunoreactivities; (b) BDA‐labeled terminals made synapses on HRP‐labeled dendrites exhibiting NTR1‐immunoreactivities; (c) BDA‐labeled terminals containing NT made synapses on HRP‐labeled dendrites. Conclusions PAG projecting axon terminals contain NT and form asymmetric synapses on NTR1‐IR neurons in the RVM which sent descending fibers to the SDH. They might be involved in the NT‐induced analgesia. This project was supported by grants from the National Natural Science Foundation of China (Nos. 30771133, 30971123, 31010103909).