Potent endogenous opioid protein b-Endorphin is derived from propiomelanocortin, beta-Endorphin is a protein found in the brain, anterior pituitary, skin, immune system, and other peripheral sites. beta-endorphin is released in response to painful stimuli. beta-endorphin has potent antinociceptive activity that is mediated through its action on μ receptors in brain and by μ and kappa receptors in the spinal cord.