Once upon a time, Parkinson's Disease was believed to be a purely neurological problem, caused by dopamine producing cell death in a bean sized part of the brain called the Substantia Nigra. We now know that this neurodegeneration is not necessarily casual, but an effect resulting from more systemic issues, which become worse over a sustained period of years prior to diagnosis. Today, it is widely accepted that problems with the gut and digestive system, including nutritional deficits - due to resulting impaired absorption, are not only integral to PD, but are also likely to play major roles in the true causal factors.
I have undertaken a great deal of investigation and research into this area, based on which I've now markedly reduced my own symptoms, including the physical pain and mental anguish, through application of the understandings I've gleaned. So, in this article, I share the main findings of this research, in case this knowledge helps other people with PD ease their suffering too.
The Enteric Nervous System
Here are my executive summary notes and commentary, based on the Wikipedia article.
"The enteric nervous system (ENS) consists of neurons which govern the function of the gastrointestinal tract. It is now usually referred to as separate from the spinal/brain's nervous system since it has its own independent reflex activity. The ENS is also called the second brain."
"The ENS in humans consists of some 500 million neurons, one two-hundredth of the number of neurons in the brain, five times as many as the one hundred million neurons in the human spinal cord, and about 70% of those in the whole nervous system of a cat. The enteric nervous system is embedded in the lining of the gastrointestinal system, beginning in the esophagus and extending down to the anus."
So our "gut instinct" is nearly as "intelligent" as that of a cat!
"The neurons of the ENS are collected in the muscular layer responsible for gut movement such as smooth muscle movements in the intestinal wall, which move food along the tract."
"The ENS can operate autonomously, but it normally communicates with the autonomic nervous system's 'rest & digest' mode via the Vagus Nerve and 'fight-flight-or-freeze' mode of the nervous systems. However, vertebrate studies show that when the Vagus Nerve is severed, the enteric nervous system continues to function."
for detailed articles about the central role of the Vagus Nerve and fight-flight-freeze stress responses in Parkinson's Disease. Note also the studies which show that, when the Vagus Nerve was surgical severed, this provided a significant protective factor for preventing Parkinson's Disease - strongly suggesting that the communication of gut problems via the ENS to the autonomic nervous system is linked to PD onset.
"The ENS is capable of carrying reflexes and acting as an integrating center completely independently. The sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons control peristalsis and churning of intestinal contents. Other neurons control the secretion of enzymes."
"The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in the brain, such as acetylcholine, dopamine, and serotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the body's dopamine."
When I first read this, I was shocked. No scientist, neurologist, doctor or nurse had ever informed me that dopamine, the chemical my system is supposedly in deficit of, has a role outside the brain/Substantia Nigra, let alone that, in fact the majority of it is produced and used outside the brain (it turns out that dopamine is also crucial in the eye for vision, for example)! I immediately asked myself what the implications of this are. Does it mean, when there are only limited supplies of the neurotransmitters present in both brain and gut, such as dopamine in PD or serotonin in depression, that there's a competition between the central and enteric nervous system for which gets the lion's share of these limited supplies?
Dopamine in the Gut
I wanted to know what dopamine is actually doing in the gut in the first place. With a little digging, I found a grant application from the 1980's which showed people were starting to look into this back then:
"Physiological studies suggest that dopaminergic mechanisms are important in the regulation of gastrointestinal motility. However, little is known about dopamine receptors in the gastrointestinal (GI) tract. In man, dopamine inhibits motility in the upper gut, but stimulates motility in the colon. These contrasting effects on motor activity suggest the presence of different populations of dopaminergic receptors in the GI tract."
It seems at least someone was asking the right questions, but I was shocked that, even though this was known about in the science of the eighties, it still isn't very common knowledge decades later. Very few in the medical, neurological and scientific circles in which I inquired could tell me anything more!
The next major source I found is much more recent. It was a very interesting article in a 2012 edition of The New Scientist magazine called
which re-presented some of these questions and ideas as if they were new:
"What are these neurotransmitters doing in the gut?"
"In the brain, dopamine is a signalling molecule associated with pleasure and the reward system. It acts as a signalling molecule in the gut too, transmitting messages between neurons that coordinate the contraction of muscles in the colon."
The article goes on to say:
"The general consensus is that neurotransmitters produced in the gut cannot get into the brain - although, theoretically, they could enter small regions that lack a blood-brain barrier, including the hypothalamus."
A caveat on the this latter statement, however, is given in my article
which explains how both the blood brain barrier and the gut lining can lose their protective functions in a variety of ways. Given that leaky gut and leaky brain are both heavily implicated in the disease, the road blocks between chemicals produced in the gut reaching the brain is not so assured for people with Parkinson's. For example, unusually high levels of histamine, which is naturally produced in the gut as part of digestion, have been found in the brains of people with PD
Intriguingly, however, the New Scientist article also revealed that other hormone activity in the gut can indirectly influence the production of dopamine in the brain too:
"Stress also leads the gut to increase its production of ghrelin, a hormone that, as well as making you feel more hungry, reduces anxiety and depression. Ghrelin stimulates the release of dopamine in the brain both directly, by triggering neurons involved in pleasure and reward pathways, and indirectly by signals transmitted via the vagus nerve."
Again, none of my health professionals have ever mentioned ghrelin to me before - I will write a sequel post on what I discovered about this. However, here I would like to underline the point that many people with PD experience "blocking" of their medication after large or hard to digest meals, and now the above helps make sense of this: hunger is a way to stimulate the Vagus Nerve and hence to promote the relaxational "feel good" state of the body, increasing the system's call on conversion of PD drugs into dopamine in the brain.
"Nevertheless, nerve signals sent from the gut to the brain do appear to affect mood. Indeed, research published in 2006 indicates that stimulation of the vagus nerve can be an effective treatment for chronic depression that has failed to respond to other treatments (The British Journal of Psychiatry, vol 189, p 282)."
The New Scientist article links these findings directly to PD - because the gut as well as the brain becomes short of dopamine supplies (and, I discovered later, so do the eyes):
"A growing realisation that the nervous system in our gut is not just responsible for digestion is partly fuelled by discoveries that this 'second brain' is implicated in a wide variety of brain disorders. In Parkinson’s disease, it has been found that the damage, also shows up in dopamine-producing neurons in the gut."
Implications for Parkinson's Disease
The 2012 New Scientist article referred to above also states:
"If nothing else, the discovery that problems with the ENS are implicated in all sorts of conditions means the second brain deserves a lot more recognition than it has had in the past. 'Its aberrations are responsible for a lot of suffering,' ... a better understanding of the second brain could pay huge dividends in our efforts to control all sorts of conditions, from obesity and diabetes to problems normally associated with the brain such as Alzheimer’s and Parkinson’s."
"Yet the number of researchers investigating the second brain remains small. 'Given it’s potential, it’s astonishing how little attention has been paid to it,' "
Unfortunately, this is what I found to be the case, too. Nevertheless, I hope by sharing what has been gleaned so far, this will help inform people with PD that they do need to pay particular attention to digestive health as well, and to be very careful about what they eat!