I’d never heard of Serotonin until I was diagnosed with Neuroendocrine Cancer in 2010. It is frequently discussed, often with contrasting views from the respondents. One common assumption/question is that it is responsible for many things that can go wrong with Neuroendocrine Cancer patients who have serotonin-producing tumours. “It’s the hormones” is an easy assumption to make or an easy answer to give in response to a complex set of circumstances. It’s difficult to get a definitive answer and the science behind the behaviour of our hormones isn’t really 100% tied down.
You may see serotonin referred to as a ‘neurotransmitter’, a ‘chemical’ and a ‘hormone’ – this is complex but it is my understanding that it can add context in respect the role/location of the serotonin, e.g. chemical and hormone are essentially synonymous and are endocrine related whereas neurotransmitter is concerned with the nervous system (the neuro in neuroendocrine) and the brain (more on this below). Consequently, I’ll keep this as basic as I can (author’s note on completion – it was not easy!).
Serotonin and NETs
One thing which is widely accepted and agreed…… Serotonin is definitely involved in Neuroendocrine Tumours, in particular, those resulting in carcinoid syndrome which can manifest as a number of symptoms including but not limited to flushing and diarrhea. Although serotonin is one of the main ‘hormones’ released in excess by certain NETs (mainly midgut), it is not thought to be the main culprit behind some of the symptoms produced by Carcinoid Syndrome. For example, flushing, the most common symptom (and a cardinal one) is thought to be caused by a number of hormones/peptides – too many to list but the main ones are histamine (particularly foregut), tachykinins (Substance P), bradykinins, prostaglandins …….. and I’m sure serotonin’s in there too! It does, however, appear to be massively guilty in causing carcinoid syndrome diarrhoea, desmoplasia, and carcinoid heart issues.
Where does Serotonin come from?
Serotonin’s technical name is 5-hydroxyltryptamine (5-HT). It is converted from 5-Hydrotryptophan (5-HTP) which is also known as oxitriptan. 5-HTP is a naturally occurring amino acid and chemical precursor as well as a metabolic intermediate in the biosynthesis of serotonin (…..and melatonin) from tryptophan. Tryptophan is interesting as that brings in one of the missing pieces of the jigsaw – food! Tryptophan cannot be manufactured in the body, it must be brought in via diet. There is no serotonin in food, it is only manufactured in the body.
Tryptophan in food enters the body and serotonin is created by a biochemical conversion process which combines tryptophan (essentially a protein) with tryptophan hydroxylase (TPH), a chemical reactor. I suspect other substances might be involved in that process. There are two forms of tryptophan hydroxylase – TPH1 and TPH2, which are encoded on two independent genes. TPH1 is linked to peripheral serotonin while TPH2 is related to brain serotonin.
While serotonin cannot cross the blood-brain barrier, tryptophan can, and almost all of it is converted to serotonin. Just to emphasise that NET dietitians do not say to avoid foods containing tryptophan other than at the time of marker testing (see below and nutrition Blog 4).
The introduction of Somatostatin analogues (SSAs) such as Octreotide and Lanreotide, help reduce the secretion of “tumour-derived serotonin” by binding to its receptors on the outside of the cell. If you ever wondered why receptors are important, please check out my blog on this subject (click here).
I mentioned tryptophan hydroxylase (TPH) above and that is actually very interesting as this is how Telotristat Ethyl (XERMELO) is able to help with the symptoms of Carcinoid Syndrome diarrhea (not adequately controlled by SSAs) or where patients are unable to be treated by somatostatin analogues for whatever reason. It’s a potent inhibitor of TPH which will disrupt the manufacturing of tumour-derived serotonin. There is also evidence that it can help reduce the effects or halt the growth of the fibrosis leading to carcinoid heart disease. Slight digression but useful to aid/enhance understanding at this point. Read about Telotristat Ethyl here.
Serotonin and the Brain
There is constant discussion and assumption that serotonin-producing tumours are somehow causing depression, anxiety and rage. If you think about the role of serotonin, to my simple way of thinking, there doesn’t appear to be any concrete evidence to back up this suspicion. Certain NETs can overproduce serotonin in the gut but the issues concerning depression and anxiety are normally associated with low levels of serotonin in the brain.
“Cancer anger” is a normal response to fear, despair and grief – a range of feelings which cancer brings into our lives. It can show as frustration, irritability, emotional withdrawal or aggression. You can feel it whether you have been diagnosed or you are a relative or friend. Cancer anger can happen at any stage of the illness, even years after treatment.
I know many people with cancer who suffer from depression, anxiety and rage but they do not have serotonin-producing tumours. What they do have is a life threatening and/or life changing condition which is bound to have an effect on mind as well as body. Serotonin is a natural substance found in the body and not just there to service NETs. If you didn’t have any, you wouldn’t be able to get out of bed according to one of my ‘favs’ Dr Gene Woltering.
Serotonin is separately manufactured in the brain (~10%) and in the gastrointestinal tract (~90%). The serotonin in the brain must be manufactured in the brain, it cannot be directly increased or reduced external to the brain, i.e. it cannot be directly reinforced by gut serotonin (peripheral serotonin). It follows that ‘brain serotonin’ and ‘gut serotonin’ are held in separate stores, they are manufactured in those stores and remain in those stores – there is no cross-pollination. This is managed by something called the blood-brain-barrier (BBB). Therefore, excess serotonin from NETs does not infiltrate the brain. As low-level of ‘brain serotonin’ is often linked to depression, it also follows that it’s possible to have high levels of serotonin in the gut but low levels in the brain.
My simple way of thinking about such things as outlined above, is that low levels of tryptophan in the brain might be contributing to low levels of serotonin in the brain. To clarify that, I researched the reasons why there could be low serotonin in the brain.
First, let’s dismiss any connection that the type of anti-depressant is called Selective serotonin reuptake inhibitors (SSRIs) is involved. It’s thought that SSRIs work by increasing serotonin levels in the brain. Serotonin is a neurotransmitter (a messenger chemical that carries signals between nerve cells in the brain). We already discussed that it’s thought to have a good influence on mood, emotion and sleep. After carrying a message, serotonin is usually reabsorbed by the nerve cells (known as “reuptake”). SSRIs work by blocking (“inhibiting”) reuptake, meaning more serotonin is available to pass further messages between nearby nerve cells. So tryptophan or peripheral serotonin not really involved.
It would be too simplistic to say that depression and related mental health conditions are caused by low serotonin levels (in the brain), but a rise in serotonin levels (in the brain) can improve symptoms and make people more responsive to other types of treatment, such as Cognitive Behaviour Therapy (CBT).
It should also be noted that the precursor to serotonin, tryptophan, does pass through the BBB and it is therefore possible that tryptophan depletion can lead to less availability in the brain for the manufacture of brain serotonin. Tryptophan depletion can be caused by dietary restrictions (i.e. lack of tryptophan foods) and also by the effects of certain types of tumours as excess serotonin is made leading to less availability of tryptophan. Both could lead to low serotonin in the brain as less tryptophan gets there.
Measuring Serotonin levels
Measuring levels of serotonin is important in both diagnosis and management of certain NETs – although it’s probably sensible to test all potential NET patients during diagnosis when the type of tumour is not yet known. Testing for tumour markers will differ between countries and within countries but the most common standard for testing Serotonin appears to be 5-HIAA (5-hydroxyindoleacetic acid) either via a 24-hour urine test or via a plasma version (mainly used in USA but now creeping into UK). 5-HIAA is the output (metabolite) of 5-HT (Serotonin). Not to be confused with the less reliable ‘serum serotonin’ which is a different test.
Another frequently asked question about serotonin tests is whether they are testing the amount in the brain or the gut. The answer is …… they are testing the levels in the blood. Furthermore, if you are measuring serotonin as an indicator for Carcinoid Syndrome, it has to be remembered that the majority of serotonin is in the gut, so even if serotonin levels in the brain were being measured alongside the gut levels, I don’t believe it would influence the result in any significant way (but I have no science to back that up). It also has to be remembered that serum serotonin and 5HIAA are not absolute tests, they are not 100% sensitive, they are simply indicators of a potential problem. There are methods of measuring brain serotonin but it is very complex and beyond the purposes of this article. However, I would just add that it is the reuptake of Serotonin in the brain (plus some other stuff) that can cause depression, not the actual level or amount in the brain.
I intentionally did not mention the other common test (Chromogranin A) or other markers as they are measuring different things but you can read about in my Testing for Markers blog.
I did say it was a difficult jigsaw!
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