Nursing Pharmacology Chapter 13:  Pain Management:  Opioids

• Endogenous Opioid Peptides: The rationale for endogenous opioid peptides came from the idea that opioid receptors are probably present in the body for the purpose of interacting with endogenous or naturally occurring substances.  A

  • As a consequence, research proceeded to attempt identification of these naturally occurring substances now known as  ß-endorphins and related peptides.

    • Morphine (and related agents) cause analgesia by acting at the brain regions containing peptides which have opioid-like properties

  • Endogenous substances = endogenous opioid peptides

  • Previous used term "endorphin" now refers to ß-endorphins and related peptides derived from the precursor: prepro-opiomelanocortin

  • Most widely distributed opioid analgesic peptides:

    • Pentapeptides

      1. Methionine-enkephalin (met-enkephalin)

      2. Leucine-enkephalin (leu-enkephalin)

  • Three major precursor proteins:

    • Prepro-opiomelanocortin (POMC) consists of:

      • met-enkephalin sequences

      • ß-endorphin sequenceprocess

      Some nonopioid peptides:

      1. ACTH

      2. ß-lipotropin

      3. Melanocyte-stimulating hormone

    • Preproenkephalin (proenkephalin A ) consist of:

      • 6 copies of met-enkephalin

      • 1 copy of leu-enkephalin

    • Preprodynorphin (proenkephalin B) consists of active peptides containing the leu-enkephalin sequence:

      • Dynorphin A

      • Dynorphin B

      • α and ß neoendorphin

  • Endogenous opioid precursors which are localized at pain modulation brain regions are probably released during stress, including pain or pain anticipation.  

    • Also, precursor molecules for endogenous opioids are localized in adrenal medulla and gut neural plexuses.