MHD8 Gut-brain axis Flashcards
1
Q
Breifly describe what the gut-brain axis is
A
Bidirectional communications between the central and the enteric nervous system, linking emotional and cognitive centres of the brain with peripheral intestinal functions. This connection is two-way where the gut can influence the brain and the brain can influence the gut and the GI environment.
2
Q
What does the gut-brain axis include?
A
Central nervous system Autonomic nervous system Enteric nervous system Hypothalamic pituitary adrenal (HPA) axis Gut microbiome
3
Q
Where is sensory information relayed?
What happens here?
A
Central nervous system, which is responsible for integrating the sensory information and directing the necessary response
4
Q
Name some monoamine (biogenic amines) neurotransmitters
A
Serotonin Histamine Dopamine Epinephrine Norepinephrine
5
Q
Name some amino acid neurotransmitters
A
Glutamate (excitatory)
gamma-aminobutyric acid (GABA) (inhibitory)
Glycine (inhibitory)
6
Q
Name some peptide neurotransmitters
A
Opioids – group of peptide neurotransmitters (e.g. endorphin)
7
Q
Name a neurotransmitter that is not a monoamine, peptide or an amino acid
A
acetylcholine
8
Q
What is the CNS encased in and bathed in?
A
encased in bone and bathed in cerebralspinal fluid CSF
9
Q
What is CSF
A
CSF is a colourless fluid produced by specific structures in the brain and provides a special chemical environment for nervous tissue and a buffer against physical damage
10
Q
How is the chemical environment of the brain maintained?
A
By the blood-brain barrier: relatively impermeable membranes of capillaries in the CNS
11
Q
What is the ANS
A
The motor subdivision of the peripheral nervous system (PNS), which controls activities automatically. The ANS is further divided into the sympathetic and parasympathetic divisions.
12
Q
What does the ANS relay?
A
- afferent signals arising from the lumen of the gut to the CNS
- efferent signals from CNS to the intestinal wall
13
Q
Discuss the details of vagus nerve
A
The vagus nerve is the longest nerve of the autonomic nervous system in the human body. It supplies motor parasympathetic fibres to all the organs, from the neck down to the colon.
Afferent spinal and vagal sensory neurons carry feedback from the intestinal end to the brain which in turn engages the hypothalamus and limbic system (responsible for regulation of emotions). While descending projections from the limbic system (activated via stress) influence the autonomic activity of the gut (and the environment of the microbiota).
14
Q
What is the Enteric nervous system (ENS)?
A
The local nervous system of the digestive system: it’s the largest component of the autonomic nervous system
15
Q
What do the two plexuses of the ENS?
A
- myenteric plexus - located between the longitudinal and circular layers of muscle in the tunica muscularis and exerts control primarily over digestive tract motility
- submucous plexus – located in the submucosa with a principle function to sense the environment within the lumen, regulate GI blood flow and control epithelial cell function
16
Q
What is considered the core stress efferent axis?
A
Hypothalamic pituitary-adrenal (HPA) axis
17
Q
Discuss the hypothalamic pituitary-adrenal (HPA) axis
A
it coordinates the adaptive responses of the organism to stressors of any kind. It is part of the limbic system, a crucial zone of the brain predominantly involved in memory and emotional responses.
18
Q
How can the brain to influence the activities of intestinal functional effector cells, such as immune cells, epithelial cells, enteric neurons, smooth muscle cells, interstitial cells of Cajal and enterochromaffin cells?
A
Both neural and hormonal lines of communication combine to allow the brain to do so
19
Q
How does cross-talk between the microbes in the GI tract and the CNS occur?
A
Via endocrine (HPA-axis), immune (cytokines and chemokines) and ANS. The gut brain axis combines the sympathetic and parasympathetic arms of the ANS, which drives afferent and efferent neural signals between the gut and the brain, respectively.
20
Q
What does the HPA axis coordinates adaptive responses against?
A
Stress including activation of memory and emotional centres in the limbic system of the brain.
21
Q
What does modulation of the CNS by the microbiota occur primarily through?
What is this mediated by?
A
Neuroendocrine and neuroimmune mechanisms, often involving the vagus nerve. This is mediated by several microbially derived molecules that include short chain fatty acids (SCFAs), secondary bile acids and tryptophan metabolites.
22
Q
What do short chain fatty acids (SCFAs), secondary bile acids and tryptophan metabolites interact with, during modulation of the CNS?
A
Enteroendocrine cells (EECs), enterochromaffin cells (ECCs) and the mucosal immune system
23
Q
How many bioactive molecules exert a central effect on the brain?
A
They can potentially cross the intestinal barrier, enter the systemic circulation, and may cross the blood-brain barrier
24
Q
How many types of neuropeptides do Enteroendocrine cells (EECs) contain?
A
20+
25
Where are EECs found?
They are interspersed between gut epithelial cells throughout the length of the gut
26
Name some of the neuropeptides produced by EECs, when are these produced?
```
peptide YY (PYY),
neuropeptide Y (NPY),
cholecystokinin,
glucagon-like peptide-1 (GLP-1), GLP-2,
substance P
```
Following stimulation by bacterial products or released in response to chemical or mechanical stimuli
27
What happens to the neuropeptides released from EECs?
These molecules can enter the systemic circulation and reach centres in the CNS involved in ingesting behaviour or act locally and activate closely adjacent afferent vagal terminals in the gut or liver to generate brain signals.
28
What receptors involved in satiety and hunger have been identified on EEC cells?
acetate produced in the gut can reach the brain and regulate appetite through a central metabolic process. This demonstrates how microbial signals can regulate the feeding behaviours of the host.
29
What are L cells located at the distal ileum activated by? What do they subsequently secrete?
SCFA
| Secrete PYY and GLP-1
30
How does acetate, produced in the gut, regulate appetite?
Through a central metabolic process
31
What are the most common type of neuroendocrine cells in the GI tract?
Enterochromaffin cells (ECCs)
32
Where are ECCs located?
at the base of intestinal crypts
33
What do ECCs produce?
Serotonin
34
How much of the body's serotonin is stored in ECCs and enteric neurones? Where is the rest of it stored?
95%
| CNS
35
What does serotonin have a central role in?
Regulating GI motility and secretion
36
SCFA and secondary bile acids can regulate what?
A significant percentage of ECC serotonin synthesis and release
37
What is the precursor for serotonin and a number of other metabolites that contribute to neuroendocrine signalling?
The essential amino acid tryptophan
38
How is tryptophan acquired and accessed?
As the host cannot produce tryptophan it must be acquired from the diet. Dietary tryptophan can be metabolized by the gut microbiota to indole reducing its bioavailability for the host.
39
Even if scientists are able to determine species of bacteria that are able to synthesise certain bacteria, what are they unable to conclude?
Unknown if these neurotransmitters reach the relevant host receptors or achieve sufficient concentrations to elicit a host response
40
Which bacteria can alter what neurotransmission
Lactobacillus acidophilus strain modulates expression of cannabinoid receptors in the spinal cord
Bifidobacterium infantis increases plasma tryptophan concentrations and thereby modulates serotonin
Lactobacillus rhamnosus alters central GABA receptor expression
SCFA have also been demonstrated to influence the development of the blood-brain barrier and increase its integrity.
Lipopolysaccharide (LPS), an inflammatory component of Gram-negative bacterial cell walls, has been found to increase the leakiness of the blood-brain barrier, a feature that can be attenuated by the SCFA such as propionate and butyrate.
41
Give an example of immune cells in the gut synthesising and releasing a hormone in response to bacterial product
Leukocytes can synthesise and release adrenocorticotropic hormone (ACTH) and endorphins in response to bacterial Lipopolysaccharide (LPS).
42
Give an example of a cytokine in the gut having an effect on nerve cells
The pro-inflammatory cytokine IL-1b is able to inhibit the release of norepinephrine from noradrenoceptor axon terminals in the intestine.
43
What can translocation of bacteria or LPS from the gut into the circulation (increased with a “leaky gut”) trigger?
An inflammatory response
44
Upon detection of LPS, the host enzyme___________ is induced, which catalyses the breakdown of tryptophan to ________ to produce a signal ______________that dampens this inflammatory response.
Indoleamine dioxygenase (IDO)
kynurenine
(3-hydroxyanthranilic acid [3HAA])
45
What does increasing the host metabolism of tryptophan lead to?
Increased host metabolism of tryptophan to kynurenine in response to inflammation reduces the availability of tryptophan for serotonin synthesis (referred to as ‘tryptophan steal’) and can result in depressive-like behaviour.
46
What causes the tryptophan steal?
Translocation of bacteria or LPS from the gut into the circulation triggers an inflammatory response., 2. The host enzyme indoleamine dioxygenase (IDO) is induced, which catalyses the breakdown of tryptophan to kynurenine to dampen this inflammatory response, 3. Increased metabolism of tryptophan to kynurenine reduces its availability for serotonin synthesis
47
How is a Morris water maze set up?
- tempera paint added to the water
- hidden platform (1/10 the length of the diameter of the water body) is placed ~1 cm below the water surface
- privacy blinds surround ¾ of the water tank to provide three visual cues
48
How is the Morris water test run?
The animal is added to the water and the escape latency, distance moved, and velocity is recorded during the time spent in the tank. The experiment is repeated over several days and these measures allow learning, memory and spatial working to be studied.
49
What is novel object recognition test used to evaluate?
Cognition, particularly memory in rodent models of CNS disorders
50
What is novel object recognition test based off the presumption of?
Based on the spontaneous tendency of rodents to spend more time exploring a novel object than a familiar one
51
How is the novel object recognition test run?
During habituation, the animals are allowed to explore an empty arena. After this habituation period (24 hours), animals are exposed to the familiar arena with two identical objects placed at an equal distance. The following day, one familiar object is removed and the mice explore the open field in the presence of the remaining familiar object and a novel object to test long-term recognition memory. If memory is impaired equal time will be spent with both the familiar object and the novel object.
52
What two tests are used for memory and learning?
Morris water test
| Novel object test
53
What test can be used to evaluate social behaviour?
Three chamber sociability and social novelty test
54
What exactly does the three chamber sociability and social novelty test, test?
Evaluates cognition in the form of general sociability and interest in social novelty in rodent models
55
What is the three chamber sociability and social novelty test useful for?
Quantifying deficits in social behaviour in transgenic animals exhibiting autistic traits. Rodents typically prefer to spend more time with another rodent (sociability) and will investigate a novel intruder more so than a familiar one (social novelty).
56
How is three chamber sociability and social novelty test run?
In this test there are three sessions using a box divided into three chambers with openings allowing movement between the chambers. After habituation in the empty box, the subject encounters a novel intruder animal placed in a small cage in one of the chambers and an empty cage placed in another chamber (sociability session). In the next session, the subject encounters the first intruder as well as a second new intruder placed in the previously empty second cage (social novelty session). The time spent sniffing each cage, spent in each chamber and the number of entries into each chamber is recorded.
57
Name two tests to evaluate anxiety and depression-like behaviours
Forced swim test
| Elevated plus maze
58
Explain the forced swim test in as much detail as possible
Measures the time spent swimming versus the time spent floating in a tall cylinder filled with water. Rodents will swim in the water looking for an escape route. Eventually the animal ceases this activity and exhibits a characteristic immobility (begin floating). The physical immobility is thought to be an indication of behavioural despair. The amount of time between when the animal is placed in the water and the onset of immobility is recorded as well as the time spent immobile. Rodent models of depression exhibit a decrease in time spent trying to escape.
59
Explain the elevated plus maze in as much detail as possible
Used to assess anxiety-related behaviour in rodents. The apparatus consists of a “+” shaped maze elevated above the floor with two oppositely positioned closed (covered) arms, two oppositely positioned open arms and an open centre area. As the animals explore the maze they are recorded with a video camera. The preference for being in the open arms versus the closed arms is calculated to measure anxiety-like behaviour. More anxious animals will spend a greater amount of time in the closed arms.
60
Did Germ Free (GF) mice or Specific-pathogen Free (SPF - free of pathogens) display increased motor activity and reduced anxiety compared to the other??
GF mice
61
Does GF mice behaviour return to normal following colonisation?
In early mice life yes
| Adult mice no
62
Bacterial infection causes ___________ dysfunction in mice
stress-induced memory
| However this is specific to different types of mice strains
63
What happens to the behavioural and physiological characteristics of rats receiving faeces from depressed and control patients and transferred them by oral gavage to a microbiota-deficient (via antibiotic treatment)?
Rats receiving the ‘depressed’ microbiota exhibited behavioural and physiological features characteristic of depression whilst those receiving the control microbiota did not.
64
How is anhedonia measured in rats?
Sucrose preference test – a task that assesses the animal’s interest in seeking out a sweet rewarding drink relative to plain drinking water
65
What is anhedonia?
The loss of interest in previously rewarding or enjoyable activities and is one of the main symptoms of depression
66
Is there evidence to suggest gut microbiota have a role in depression?
Gut microbiota may play a causal role in the development of features of depression.
67
How are autism-spectrum disorder (ASD) characterised?
It is a group of neurodevelopmental disorders: characterised by deficits in social interactions including verbal and non-verbal behaviours.
68
How is it believed ASD-like behaviours arise?
from a complex interaction between genetic defects and environmental risk factors, including the intestinal microbiota, causing abnormal neurodevelopment during maturation in utero and in early childhood
69
Are microbial alterations observed in patients with ASD are a cause or a consequence of ASD?
It is unknown
70
What gastrointestinal dysfunction is observed in people with ASD?
- diarrhoea
- abdominal pain
- chronic constipation
- gaseousness
- GI inflammation
- GI symptom severity is strongly correlated to ASD symptom severity.
71
What is the correlation between faecal bacteria and behaviour observed in people with ASD?
Analysis of faecal matter from children with ASD identified a correlation between bacteria such as Clostridum, Sutterella and Desulfovibrio and altered neurobehavioural development as observed in ASD.
The microbiota of ASD children has been shown to be less diverse than that of neurotypical children with an imbalance in certain microbial species. This includes a lower abundance of Prevotella, Veilonellaceae and Coprococcus species.
72
What neuroactive microbial metabolites do people with ASF have compared to those without?
Children with autism have different metabolic profiles compared to neurotypical children with microbial-associated pathways found to be altered.
- differences in SCFA
- amino acids
- ASD children excrete higher amounts of p-cresol sulfate.
This suggests that the differences in microbiomic structure result in functional variation and changes in the production of neuroactive microbial metabolites.
73
How are antibiotics involved with ASD?
In a study of children with regressive autism, antibiotics improved ASD symptoms. Such improvements ceased once the course of antibiotics was stopped. This suggests that the gut microbiota is involved in developmental regression associated with this form of autism
74
What is the link between a westernised diet and ASF?
The prevalence of autism is rising exponentially, especially in developed countries where Westernised diet and lifestyle are associated with gut microbial dysbiosis
75
Name the two hypothesises for the development of ASD
Maternal immune activation
| Maternal obesity
76
Explain the maternal immune activation hypothesis
As a consequence of maternal infection, during pregnancy, may contribute to the development of autism. Studies have shown that probiotics (Bacteroides fragilis) can reverse autistic behaviours and gastrointestinal abnormalities in offspring of maternal-immune activation mice. Using another mouse model of autism, treatment with Lactobacillus reuteri was also found to correct social deficits in these mice.
77
Explain the maternal obesity hypothesis
Another hypothesis is that maternal obesity causes a shift in the maternal gut microbiome which increases the risk of the offspring developing autism.
78
Continued perturbations of gut microbiota can lead to what?
Loss of resilience have implications for human health.
79
How can we can modify the structure and function of the gut microbiota to reduce the risk of various diseases or relieve symptoms of diseases?
Dietary modulation:
- prebiotics
- probiotics
- faecal microbiota transplantation
80
What are prebiotics?
are nutritional compounds that are selectively fermented by commensal/symbiotic microorganisms and promote their growth in the GI tract to increase their abundance. Prebiotics can also selectively stimulate the activity of commensal/symbiotic microbes which confers benefit to the host.
81
What do typical prebiotics include?
- non-digestible fibres
- resistant starch
- oligosaccharides
- fructans
- galactans
- inulins
- fructooligosaccharides (FOS)
- galactooligosaccharides (GOS)
- human milk oligosaccharides (HMO)
82
Why are prebiotics more likely to reach large intestine intact?
Since prebiotic compounds are not digested by human enzymes
83
What happens to prebiotics when they reach the large intestine?
They are fermented into beneficial metabolites such as short chain fatty acids, these beneficial metabolites increase the beneficial populations of bacteria and/or stimulate the activity of bacteria that are already present
84
What can HMOs can stimulate the growth of?
Bifidobacterium because HMOs are fermented by these bacteria to produce SCFA, which are important for the healthy of the infant.
85
What are probiotics?
live microorganisms that are believed to confer benefits to the host. Metabolic activity of probiotic bacteria includes fermentation of carbohydrates and production of SCFAs. Probiotics (when ingested) are thought to provide a transient benefit to the host – in contrast to prebiotics, there is not convincing evidence that they change the gut microbiome.
86
What genera are the typical probiotic bacteria from?
Bifidobacterium and Lactobacillus: usually these probiotics contain a consortium of strain
87
What is a good source of probiotics?
Fermented food and drink, such as kefir, yogurt, kimchi, tempeh, kombucha, sauerkraut, miso
88
Why may many probiotic bacteria may die before reaching the large intestine?
Probiotic strains that are ingested must survive the harsh acidic conditions of the stomach
89
Is evidence on whether probiotics change the microbiome clear?
The literature surrounding probiotics is very conflicting. Although it is not clear whether probiotics change the composition of the gut microbiome, it has been shown that they provide beneficial effects to the host, particularly when there is some dysbiosis.
90
Give 3 examples of where pre or probiotics could be used
Inflammatory bowel disease
Patients with inflammatory bowel disease exhibit reductions in Bacteroidetes and Firmicutes, and probiotics are used to treat this dysbiosis although there are conflicting results concerning the efficacy of this treatment.
Autism
Children with autism are likely to have gastrointestinal dysfunction and abnormal dietary patterns. Both probiotics and prebiotics have been used to improve dysbiosis associated with these symptoms. Probiotics have also been shown to reduce anxiety and depression.
Aging
In the elderly population, the diversity of the gut microbiome is decreased. Pre-/pro-biotics have been shown to improve the conditions of elderly people.
91
What is a faecal microbiota transplant?
The administration of a solution of faecal matter from a donor into the intestinal tract of a recipient in order to directly change the recipient’s microbial composition and confer a health benefit.
92
What does the FMT process involve?
1) Selecting a donor
2) Faecal microbiota preparation
3) Administration
93
What does selecting the donor for FMT involve?
Selecting a donor without a family history of autoimmune, metabolic and malignant diseases and screening any potential pathogens.
94
What does faecal microbiota preparation for FMT involve?
Faeces are prepared by mixing with water or normal saline, followed by a filtration step to remove any particulate matter.
95
What doesadministration FMT involve?
Different routes of administration (nasogastric tube; nasojejunal tube; colonoscopy; enema).
96
What has FMT been used to treat?
Recurring Clostridioides difficile infection (CDI), inflammatory bowel disease (IBD), obesity and metabolic syndrome, functional gastrointestinal disorders (FGID) including irritable bowel syndrome (IBS)
97
What have recent studies explored about FMT treatment?
Its use in neurological disorders (depression, anxiety) and psoriasis
98
What has a study found that FMT changed in the gut microbial profile of the children with ASD?
It improved behavioural symptoms as well as GI function. Such changes persisted for at least 8 weeks after the FMT.
99
What is the problem with using antibiotics such as vancomycin and metronidazole to reduce c.diff infections?
A significant portion of patients with CDI go on to develop recurrent CDI, which can lead to significant morbidity and mortality.
100
How does c.difficile establish in human gut?
DI is a leading nosocomial infection. C. difficile is a poor competitor against members of a healthy microbiota. However, disruption of the intestinal microbiota (typically by antibiotics) reduces this competition and the natural barrier of the microbiota against the pathogen.
101
What is the problem when C. difficile establishes?
It produce anti-microbial products (such as p-cresol) to suppress the development and recovery of the microbiota.
102
How does FMT help with CDI? How does it do this?
It restores the gut microbial barrier to outcompete the pathogen in the CDI patients. However, exact mechanisms by which it exerts its therapeutic effects are not fully understood. Some evidence showed that bile acid metabolism and/or sialic acid metabolism may play a role.
Interestingly, a sterilised FMT has also been shown to work indicating that live bacteria may not be required but rather their products or viruses.
103
What are the risks for FMT?
- developed obesity after receiving the faecal microbiota from her overweight daughter
- patient died from peritonitis that may have resulted from nasogastric tube perforation during FMT
- increase the risk of communicable disease transmission due to poor screening processes and the unknown virome for donors
- little known about the long-term safety of the patients receiving FMT
104
What is the current research on FMT and future developments?
Some research work has also been carried out to refine the transplanted bacterial community to a collection of ‘core microbes’, enabling greater control of the preparation, safety and assessment of efficacy. The oral FMT capsules and synthetic stool products with the core microbes are also developed.
