Brain endorphins: possible role in reward and memory formation.

A role for enkephalin in the mediation of behavioral reinforcement is supported by several lines of evidence: i) central injections of enkephalin serve as reinforcement for self-administration behavior, ii) electrical stimulation of many enkephalin-rich regions serves as reinforcement for self-stimulation behavior, which is blocked by moderate doses of naloxone, and iii) long-term retention of a passive avoidance response is facilitated by immediate post-learning injections of methionine-enkephalin and morphine.