MHD8 Gut-brain axis Flashcards
Breifly describe what the gut-brain axis is
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.
What does the gut-brain axis include?
Central nervous system Autonomic nervous system Enteric nervous system Hypothalamic pituitary adrenal (HPA) axis Gut microbiome
Where is sensory information relayed?
What happens here?
Central nervous system, which is responsible for integrating the sensory information and directing the necessary response
Name some monoamine (biogenic amines) neurotransmitters
Serotonin Histamine Dopamine Epinephrine Norepinephrine
Name some amino acid neurotransmitters
Glutamate (excitatory)
gamma-aminobutyric acid (GABA) (inhibitory)
Glycine (inhibitory)
Name some peptide neurotransmitters
Opioids – group of peptide neurotransmitters (e.g. endorphin)
Name a neurotransmitter that is not a monoamine, peptide or an amino acid
acetylcholine
What is the CNS encased in and bathed in?
encased in bone and bathed in cerebralspinal fluid CSF
What is CSF
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
How is the chemical environment of the brain maintained?
By the blood-brain barrier: relatively impermeable membranes of capillaries in the CNS
What is the ANS
The motor subdivision of the peripheral nervous system (PNS), which controls activities automatically. The ANS is further divided into the sympathetic and parasympathetic divisions.
What does the ANS relay?
- afferent signals arising from the lumen of the gut to the CNS
- efferent signals from CNS to the intestinal wall
Discuss the details of vagus nerve
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).
What is the Enteric nervous system (ENS)?
The local nervous system of the digestive system: it’s the largest component of the autonomic nervous system
What do the two plexuses of the ENS?
- 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
What is considered the core stress efferent axis?
Hypothalamic pituitary-adrenal (HPA) axis
Discuss the hypothalamic pituitary-adrenal (HPA) axis
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.
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?
Both neural and hormonal lines of communication combine to allow the brain to do so
How does cross-talk between the microbes in the GI tract and the CNS occur?
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.
What does the HPA axis coordinates adaptive responses against?
Stress including activation of memory and emotional centres in the limbic system of the brain.
What does modulation of the CNS by the microbiota occur primarily through?
What is this mediated by?
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.
What do short chain fatty acids (SCFAs), secondary bile acids and tryptophan metabolites interact with, during modulation of the CNS?
Enteroendocrine cells (EECs), enterochromaffin cells (ECCs) and the mucosal immune system
How many bioactive molecules exert a central effect on the brain?
They can potentially cross the intestinal barrier, enter the systemic circulation, and may cross the blood-brain barrier
How many types of neuropeptides do Enteroendocrine cells (EECs) contain?
20+
Where are EECs found?
They are interspersed between gut epithelial cells throughout the length of the gut
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
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.
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.
What are L cells located at the distal ileum activated by? What do they subsequently secrete?
SCFA
Secrete PYY and GLP-1
How does acetate, produced in the gut, regulate appetite?
Through a central metabolic process
What are the most common type of neuroendocrine cells in the GI tract?
Enterochromaffin cells (ECCs)
Where are ECCs located?
at the base of intestinal crypts
What do ECCs produce?
Serotonin
How much of the body’s serotonin is stored in ECCs and enteric neurones? Where is the rest of it stored?
95%
CNS
What does serotonin have a central role in?
Regulating GI motility and secretion
SCFA and secondary bile acids can regulate what?
A significant percentage of ECC serotonin synthesis and release
What is the precursor for serotonin and a number of other metabolites that contribute to neuroendocrine signalling?
The essential amino acid tryptophan
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.
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
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.
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).