Parallel activation of multiple spinal opiate systems appears to mediate 'non-opiate' stress-induced analgesias.
Watkins LR, Wiertelak EP, Grisel JE, Silbert LH, Maier SF. Parallel activation of multiple spinal opiate systems appears to mediate 'non-opiate' stress-induced analgesias. Brain Res. 1992 Oct 23; 594(1):99-108.