This study aimed to investigate the role of ACh in SmI emanating descending modulation of thalamic Pf neurons in acupuncture analgesia.Multi-micropipettes were used for both extracellularly recording responses of thalamic Pf neurons to noxious stimulation of the plantar area and drug application in rats.It was found that (1) lesion of SmI obviously attenuated the inhibitory effect of EA applied at "Zusanli"and "Huantiao"points on nociceptive responses in Pfneurons; (2) after lesion of Sml iontophoretic application of ACh markedly suppressed the nociceptive responses of Pf neurons

which was significantly different from the effect of iontophoretic NaCl (as the control) showing no influence on them; (3)the inhibition induced by iontophoretic ACh applied in the rats with lesion of SmI was similar to that produced by EA applied in those with SmI intact.It is indicated that EA can activate SmI to release ACh to exert descending modulation

in which ACh is involved in SmI originating descending regulation of Pf neurons in acupuncture analgesia