Are Exogenous Opioids Considered To Be Neuropeptides?

[FredericChopin]

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.

 

[Ygggdrasil]

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.

 

[Pythagorean]

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:

1. The substance must be present within the presynaptic neuron. Clearly, a chemical cannot be secreted from a presynaptic neuron unless it is present there. Because elaborate biochemical pathways are required to produce neurotransmitters, showing that the enzymes and precursors required to synthesize the substance are present in presynaptic neurons provides additional evidence that the substance is used as a transmitter. Note, however, that since the transmitters glutamate, glycine, and aspartate are also needed for protein synthesis and other metabolic reactions in all neurons, their presence is not sufficient evidence to establish them as neurotransmitters.

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

http://www.ncbi.nlm.nih.gov/books/NBK10957/box/A377/?report=objectonly

 

[FredericChopin]

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.