We can think of the gut like a roundabout; it has links to every system in the body. We have already explored the link between the gut and skin health, now we want to take a closer look at what we know as the gut-brain axis or GBA. This concept has gained traction over the last few decades and for good reason, but the question on everyone’s tongue… Does my dog have a gut-brain axis and what does this mean?
Let’s see if we can answer that question.
What is the Gut-Brain Axis?
The gut-brain axis (GBA) consists of bidirectional communication between the nervous system, linking emotional and cognitive centres of the brain with intestinal functions. But research is indicating the importance of gut microbiota in influencing these interactions.
The interaction between microbiota and the GBA appears to be bidirectional, through signalling from gut-microbiota to brain and vice versa. It is thought that this occurs in many ways, including neural, endocrine, immune, and humoral links.
Neural Links
Increasing evidence has found that the vagus nerve, a major neural connection between the gut and brain, plays a key role in facilitating signalling along the microbiota-gut-brain axis.
The vagus nerve is the tenth of twelve pairs of cranial nerves found in the peripheral nervous system. Cranial nerves mainly innervate anatomical structures of the head and neck, but the vagus nerve is the exception to this rule; it extends from the brainstem through the neck and the thorax down to the abdomen. Because of its long reach throughout the body, it is often described as the “wandering nerve.”
In the neck, the vagus nerve innervates the pharynx and larynx which are responsible for swallowing and vocalisation. In the thorax, it is the chief parasympathetic supply to the heart – this is what reduces our heart rate when we are recovering from a stressful stimulus.
In the intestine, the vagus nerve regulates smooth muscle contraction and many secretory functions. The vagus nerve provides a critical link between the central nervous system and the enteric nervous system; the enteric nervous system is like the digestive system’s own nervous system – one that us or our dogs have little conscious control over.
The vagus nerve link was largely established through studies utilising a surgical procedure known as a vagotomy which cuts or removes the vagus nerve to identify any resulting implications.
Surgical vagotomy has been used to investigate the physiological role of the vagus nerve since the early 19th century.
Surgical vagotomy has historically been carried out in humans to treat peptic ulcers. The procedure reduces stomach acid through its denervation of the cells that produce it. In more recent years, these procedures are less common, with patients opting for pharmaceutical alternatives. But what is interesting are the common side effects of vagotomy in humans which include interference with gastric emptying, increased and dysfunctional bowel movements and nutrient malabsorption resulting in deficiency. We almost found out about the role of the vagus nerve in digestive function by accident.
The vagus nerve provides a link between the internal organs, including the gastrointestinal tract, and the central nervous system, with 80-90% of fibres being afferent (transmitting to the central nervous system) and 10-20% transmitting signals in the other direction, being efferent.
The vagotomy procedure has implicated the gut-brain axis in cases of depression, stress resilience and anxiety.
Vagus Nerve Signalling
Within the small and large intestine you will find vagal afferents (nerve endings). These afferents can detect stretch and tension (how much food is in the gut), but they can also detect chemicals being absorbed across the epithelial layer.
But what’s super interesting is that bacteria in the gut produce metabolites and neurotransmitters which can act directly on vagus nerve endings too! These nerve endings notice the metabolites and neurotransmitters and relay to the brain.
We have found this through bacterial supplementation – for example, when certain bacteria are infused directly into the large intestine, vagal firing alters.
So we know we can alter vagal firing through the population of bacteria found in the gut, but in our dogs, massage is gaining great traction at modulating vagus nerve function.
Endocrine Links
There is increasing evidence that suggests the gut as an endocrine organ, largely through its ability to produce and regulate multiple compounds that reach systemic circulation and subsequently act to influence the function of distal organs and systems.
The biochemical complexity of the gut microbiota exceeds that of the brain, and many of the hormones produced by the microbiota are also neurotransmitters within the central nervous system. For example, γ-aminobutyric acid (GABA), the most important inhibitory transmitter in the brain is produced by several lactobacilli bacteria and monoamines such as noradrenaline, dopamine, and serotonin are also produced by certain strains of bacteria.
We must also consider the role of the HPA axis.
The hypothalamic-pituitary-adrenal axis (HPA) is the main stress response system. It is the neuroendocrine link between perceived stress and physiological reactions to stress.
The use of germ-free animals has provided one of the most significant insights into the role of the microbiota in regulating the development of the HPA axis.
It has been found that germ-free animals suffer aberrant responses to stress, but when they are then colonised with specific pathogen free faecal matter, their stress response becomes more balanced.
Researchers have concluded that the microbial content of the gut is critical to the development of an appropriate stress response later in life and that there is a narrow window in early life where colonisation must occur to ensure normal development of the HPA axis.
Immune Links
Numerous studies in recent years investigating the gut-brain axis have demonstrated an important role for the gut microbiota in modulating brain development and function, with the immune system serving as an important coordinator of these interactions.
Germ-free animals have regularly demonstrated that role of the microbiota in proper maturation of the immune system and immune regulation within the central nervous system is mediated by microglia, astrocytes, and oligodendrocytes.
Microglia work to protect the brain against various pathological conditions through immune activation, phagocytosis, and cytokine production.
Astrocytes are the most abundant glial cell population in the brain and, similarly to microglia, have multiple functions in CNS integrity and homeostasis, including BBB integrity, ion gradient balance, neurotransmitter turnover and nutrient transport.
Time and time again, germ-free animals have suffered immature immune cell populations in the brain.
In addition, intestinal bacteria have recently been demonstrated to regulate both foetal and adult neurogenesis (the production of new brain cells).
Research has also shown that high levels of inflammation, perinatally, are associated with abnormal brain development. Furthermore, neonatal exposure to high pathogenic load in the gut is associated with memory impairment.
Overall, long-term illnesses and chronic intestinal inflammation are associated with behavioural disturbances including cognitive impairment, deficits in learning, impaired memory, depression, and anxiety.
Findings Here
What Does All This Mean?
Whilst there are limitations to many rodent studies which include transferability, the wealth of data is indicating that gut health does influence brain function. To this end, we need to support our dog’s gut health as much as possible if we want to subsequently support brain health.
How Can We Optimise Gut Health?
1. Eat whole, unprocessed foods; nutrient-dense and biologically appropriate foods. Look to a freshly cooked and balanced food at home or raw meat and bones with variety. Raw fed dogs often show up with the best microbiome results.
2. Eliminate food intolerances and allergies. Just because you start feeding a fresh wholefood diet doesn’t mean everything your dog is eating is appropriate for him. An elimination diet is a good way to see what foods may be bothering your pet.
3. Treat any infections or overgrowth of bugs. SIBO and yeast overgrowth are more and more prevalent in processed fed pets. In some cases, antibiotics are necessary, but we generally like to look for natural herbs with an antimicrobial effect.
4. Heal the gut lining. We need to support immunity and address over sensitivity. Look to lovely gut healing ingredients such as Slippery Elm, De-Glycerised Licorice, Glutamine and N-Acetyl Glucosamine.
5. Avoid foods and products known to disrupt the gut microbiome – these include rancid fats, pesticide treated foods, toxic cleaning products and more.
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6. Rebuild your ecosystem of friendly bacteria. Adding in probiotics to the regime can be beneficial.
7. Avoid or minimise stress
Can Stress Affect My Dog’s Digestive System?
If you would like any support with your dog’s health, then check out our services to see how we can help.
Thanks for reading,
MPN Team
