Peptides, as seen in the picture alongside, are short chains of amino acids, and there are three types of peptides that serve as neurotransmitters:
- Opioid peptides
- Substance P
Since substance P has a very specific yet detailed function, I’ll be addressing that separately, and you can see that here. In the meantime, here is information regarding the other two types of peptides.
Somatostatin has two other aliases: they include GHIH or growth hormone inhibiting hormone and SRIF or somatotropin release-inhibiting factor. It acts on both the anterior pituitary gland and the gastrointestinal tract as an inhibitory hormone.
The first table shows the hormone functions counteracted by somatostatin within the gastrointestinal tract.
In essence, what somatostatin does is stop the digestion process in its tracks. In the GI tract it does the following:
- Acts as an antacid
- Stops the digestion of fats and proteins
- Decreases peristalsis (movement of the gut)
- Prevents sodium carbonate from being secreted by the action of secretin.
- Acts as a vasoconstrictor
- Lowers the level of insulin production
- Accelerates the stomach emptying process.
The second table illustrates what somatostatin does in the anterior pituitary tract. The
effects of somatostatin are induced by an acidic environment (i.e. a low pH).
The most well-known example of an opioid peptide are endorphins, which are naturally secreted after moderate to vigorous exercise, hence the term ‘endorphin high’. Within the brain, these peptides play vital roles in several key functions, including:
- Food intake
- Attachment behaviour
- Stress and our responses to it
All these peptides act on opioid receptors in the brain. You may have heard of the term opiates because these chemicals are produced naturally in the opium plant, as seen in the picture alongside. Opiates (such as the commonly used codeine and morphine) tend to act as painkillers, but the function of opioids can vary, albeit they can be similar. Some are also discovered within the human genome, but others we tend to get from our food.
Some examples of opioids that have been discovered in our DNA are as follows:
- Haemoglobin-derived opioid peptides (from the blood)
Examples of food products that contain opioid peptides include the following:
- Milk (has casein, which is converted into casomorphin)
- Grains such as wheat, rye and barley (the gluten contains gluten exorphin, and it also contains the peptides gliadorphin and gluteomorphin)
- Soybeans (contain soymorphin-5)
- Spinach (contains rubiscolin)