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