Endorphins were nature's painkillers, produced in the pituitary gland and hypothalamus of all humanoid vertebrates. They regulated pain and hunger and were connected to the production of sex hormones.
The Cardassian Obsidian Order provided its operatives with cranial implants that stimulated the production of endorphins, thus allowing the wearer to resist torture. (DS9: "The Wire")
The naturally occuring neurochemical endorphins were described by the Star Trek Encyclopedia (4th ed., vol. 1, p. 241) as opiate peptides similar to the drug morphine, which acted upon the nervous system to affect sensations of pain and pleasure.
References
- TNG:
- "Shades of Gray"
- "Realm of Fear"
- DS9:
- Star Trek: Insurrection