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Are Exogenous Opioids Considered To Be Neuropeptides?

  1. Jan 2, 2015 #1
    One class of neuromodulator are the neuropeptides.

    An example of enodogenous neuropeptides are the opioids like enkephalin, endorphin, and dynorphin, which the body produces naturally. But can exogenous opioids like morphine and heroine be classified as "neuropeptides"?

    Thank you.
    Last edited: Jan 2, 2015
  2. jcsd
  3. Jan 3, 2015 #2


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    No. Neuropeptides are classified as such because they have the chemical structure of peptides (polymers of amino acids). Thus, this is most of a classification based on the structure of the molecule than its function.

    Many exogenous opiods like heroin and morphine manage to mimic the shape and biological activity of these neuropeptides while being quite distinct chemically. So despite their ability to act like neuropeptides, heroin and morphine are not classified as neuropeptides.
  4. Jan 3, 2015 #3


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    I take a different approach than Ygggdrasil. Canonically, there are four criteria for defining a neurotransmitter (of which neuropeptides are a particular class); they are mostly bio-functional criteria [1], not particularly chemical structure based; the first is:

    So the drug being exogenous, by definition, disqualifies it from being a neurotransmitter in the first place.

  5. Jan 3, 2015 #4
    Thank you for your responses.

    Neurmodulators can fall into the categories of neuropeptides and neurotransmitters. Neuropeptides are proteins, while neurotransmitters are small molecules. So morphine and heroine (small molecules) wouldn't fall into the category of neuropeptides because they aren't proteins, and as Pythagorean pointed out, they wouldn't fall into the category of neurtransmitters either.

    So another question is: Can exogenous opioids like morphine and heroine be considered "neuromodulators"? I have a feeling "no", because even though they "modulate" neuroactivity, similar to what Ygggdrasil said, it is not a matter of function, but structure of the compounds.

    Is this correct?

    Thank you.
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