Acupuncture and
electroacupuncture are used in
pain relief; however, the mechanism underlying the
analgesic effect of acupuncture is unclear. Several lines of evidence propose that the periaqueductal gray (PAG), which is one of the regions that contributes to the endogenous
pain inhibitory system, is involved in the
analgesic effect of acupuncture, and the region receives several neural projections such as
histamine and noradrenalin and contains the
dopamine cell bodies. The current study examined the effects of
electroacupuncture at Zusanli (ST36) and Shangjuxu (ST37)
acupoints, which are used for clinical
pain control, on the release of
neurotransmitters in the PAG in rats.
Histamine and
dopamine release was increased after
pain stimulus, while the changes were completely abolished by
electroacupuncture.
Pain stimulus had no effect on noradrenalin release, but
electroacupuncture increased its release. These findings indicate that acupuncture at Zusanli and Shangjuxu exerts an antinociceptive effect via the activation of neurons in the PAG and that the histaminergic, dopaminergic, and noradrenalinergic systems in the PAG are related to
electroacupuncture-induced
pain relief.