It has been well demonstrated that the analgesic effects of acupuncture are mediated by an endogenous opioid pathway (Stux et al., 1995; Diehl, 1999).
Acupuncture impulses are relayed via collateral connections to the PAG, which contains the endogenous opioids. PAG sends inhibitory information to the spinal dorsal horn and blocks noxious signal inputs from the periphery (descending inhibition; #1).
Others showed that electrical stimulation of peripheral nerves results in an inhibition to the pain pathways at the spinal cord level (Hanai, 2000), suggesting that acupuncture may also inhibit the pain pathway at the spinal cord levels as well (#2).
In addition to opioids, hypothalamic OXT has anti-nociceptive effects (Yang et al., 2007). Acupuncture can stimulate OXT expression at the PVN (Yoshimoto et al. 2012), indicating that anti-nociceptive effects of acupuncture is mediated by the combined effects of opioid and OXT.