Further Evidence for Possible Analgesic Mechanism of Electroacupuncture: Effects on Neuropeptides and Serotonergic Neurons in Rat Spinal Cord

The possible mechanism of electroacupuncture (EAc) in reference to the effects of neuropeptides on serotonergic neurons in rat spinal cord was studied. The tested drugs were administered by intrathecal injection or spinal push-pull perfusion. The results showed that baclofen, substance P (SP) and naloxone administered intrathecally could reduce the tail pressure pain threshold. The pain threshold was increased by met-enkephalin (EK) and EAc. The action of EAc was antagonized by naloxone. The release of 5-HT in the spinal cord evoked by tail pressure pain stimulation (TP) was inhibited by EK, baclofen and EAc. However, naloxone could potentiate the 5-HT release evoked by TP. EAc reversed the naloxone potentiation of TP-evoked 5-HT release. The 5-HT release evoked by exogenous SP, however, was potentiated by EK and EAc. From these results, it is suggested that the influence of EAc on 5-HT release may be due to activation of enkephalin-interneurons, which presynaptically inhibit the primary sensory neurons in the spinal cord.