We investigated role of
capsaicin-sensitive afferents within and without the areas of Zusanli (ST36)/Shangjuxu (ST37)
acupoints along the stomach (ST) meridian in the perception and modulation of
pain assessed by visual analog scale of
pain and its distribution rated by subjects, pressure pain threshold (PPT), and heat pain threshold (HPT) in humans. Compared with the treatment of non-
acupoint area,
capsaicin (100µg/50µl) administered into either ST36 or ST37
acupoint caused the strongest
pain intensity and the most extensive
pain distribution, followed by rapid onset, bilateral, long-lasting secondary
mechanical hyperalgesia and slower onset secondary heat hypoalgesia (1day after the
capsaicin treatment). Between treatments of different
acupoints,
capsaicin administrated into the ST36
acupoint exhibited the stronger
pain intensity and more widespread
pain distribution compared with the treatment of ST37
acupoint. A period of 30- to 45-min, but not 15-min, 43°C heating-needle stimulation applied to the ST36
acupoint significantly enhanced the HPT, and had no effect on PPT. Upon trapezius muscle
pain elicited by the i.m. injection of 5.8% saline, pre-emptive treatment of the contralateral ST36
acupoint with 43°C heating-needle stimulation alleviated the ongoing
muscle pain, reduced painful area, and reversed the decrease in HPT. It is suggested that (1)
pain elicited from the
acupoint and non-
acupoint areas differs significantly, which are supposed to be dependent on the different distributions and contributions of
capsaicin-sensitive afferents. (2) Non-painful heat stimulation is a valid approach in prevention of ongoing
muscle pain with associated post-effects of peripheral and central sensitization.