Nervous System Physiology Flashcards
1
Q
What is a neuroimmune interaction?
A
Integration between the immune system and the central nervous system
2
Q
In which direction do neuroimmune interactions occur?
A
Reciprocal and bi-directional flow of information between the brain/CNS and immune system
3
Q
How were neuroimmune interactions discovered?
A
- In the 1930s by Selye (endocrinologist) who worked on non-specific responses of an organism to stress and disease
- During WW1, deaths from infections produced changes in the adrenal cortex
- Discovered the HPA is active during infections
4
Q
Provide a classic pathway example of a neuroimmune interaction (HPA activation)
A
- Infection
- Signals towards to the brain interacting with the hypothalamus, in particular the PVN (a small collection of nuclei)
- This releases CRH which acts in the anterior pituitary
- This causes the release of ACTH which circulates and interacts with the adrenal cortex to release glucocorticoids
- These act on the immune system (immunosupressing) to reduce inflammation
5
Q
What is an example of the brain initiating a neuroimmune interaction?
A
- Neuroendocrine axis
- Where the autonomic nervous system can innervate immune organs such as the spleen, thymus and lymph nodes
- The innate and adaptive immune cells express adrenergic receptors (many immune cells also have neurotransmitters)
6
Q
How can stress activate the endocrine system?
A
- Physical/emotional stress and psychiatric illness activate the endocrine system affecting the immune system, in turn which affects changes in the brain
- Such as when stressed with work, the stress is removed, get a cold
- During the stress response, the immune system is activated. This then gets dampened down and causes more likelihood for infection
7
Q
What is the function of neuroimmune interactions?
A
- Control many bodily functions
- Maintain homeostasis and health such as:
Intestinal physiology, secretory immune function, conception and transfer of immunity to offspring, sleep, recovery from illness and provide host defense against infection and injury
8
Q
What is the host defense response to infection and what does it cause?
A
- Is a coordinated response of an organism to infection in order to eliminate the infection to aid recovery and restore homeostasis
- Charcterised by endocrine, autonomic and behavioural changes
- Causing an acute phase response
9
Q
What is the acute phase response (APR)?
A
- First phase of host defense response to infection
- Induces physiological and behavioural changes which is detreimental to the pathogen and therefore beneficial to us as it cannot get a hold in the body
- Is a temporary suspension of normal homeostasis
- Aids in clearance of the pathogen and return to normal physiology and homeostasis
10
Q
For the acute phase response to be effective what features must it have?
A
- Acute
- Destroy the pathogen
- Limit and repair the damaged tissue within a short time (if prolonged could be detrimental)
11
Q
What is the first phase of the acute phase response?
A
- Infection occurs causing immune response and cytokine release (IL-6)
- The liver will cause hepatic production of different factors such as acute phase proteins
- This is a marker of inflammation but can also increase in other instances (pregnancy)
- It acts in the complement system to produce other factors such as CRP (C-reactive protein) and serum amyloid A (SAA)
- These act locally to help destroy the infection
12
Q
What does C-reactive protein do?
A
Acts as an opsonin, sticks to microbial membrane and aids binding to phagocytes
13
Q
What does serum amyloid A do?
A
Recruits immune cells and induces enzymes that degrade the extracellular matrix which in turn allows the immune ells to get where they need to be more quickly and efficiently
14
Q
What is the second phase of the acute phase response?
A
- Metabolic modifications such as protein catabolism, lipolysis, gluconeogenesis, hypoferremia (low in ion), hypozincaemia (low in zinc), hormonal changes
- Sickness behaviour such as fever, nausea, loss of appetite, loss of body weight, reduced locomotion, disturbed sleep, depression and mild cognitive disorders (memy and attention impairment)
15
Q
Why does hypoferremia and hypozincaemia occur?
A
The low levels of these makes it a poor environment for the pathogen to replicate and function properly
16
Q
Why does fever and loss of appetite occur in sickness behaviour?
A
- Fever causes a rise in core body temp which helps the body fight the infection as bacteria won’t replicate as well, it will aid the immune cells getting where they need to be
- Loss of appetite reduces the vital nutrients for the bacteria
17
Q
What mediates sickness behaviour?
A
The brain via cytokines
18
Q
Why is it bad to prevent sickness behaviour?
A
If force feed mice after an infection, it can decrease survival time and kill the animal in some cases. These changes are there for our benefit
19
Q
How do cytokines signal to the brain through passive diffusion?
A
- The BBB does not normally allow diffustion of most molecules
- There are some refions which are leaky/incomplete- these are the circumventricular organs where the endothelial cells are not as tightly linked so substances can passively diffuse from the blood to the brain
20
Q
What is evidence against passive diffusion?
A
Tanycytes may prevent entry (like astrocytes which form a physical barrier near the CVOs)
21
Q
What are the 4 CVOs?
A
- Subfornical organ (SFO)
- Organum vasculosum of the lamina terminalis (OVLT)
- Median eminence (ME)
- Area postrema (AP)
22
Q
How do cytokines signal to the brain through resident immune cells producing cytokines and diffusing?
A
- There are macrophage like cells in the CVOs and choroid plexus (makes the CSF)
- These resident immune cells have PRRs such as the TLRs which respond to PAMPs on bacteria stimulating cytokines
- Because they are leaky, there is passive diffusion into the brain
23
Q
How do cytokines signal to the brain through receptor/transporter mediated means?
A
- Some cytokines have specific transporters at the BBB
- These bind the cytokine and then transport into the brain, this is a saturable (if too much cytokines, cannot cope) and active transport system requiring energy
24
Q
How do cytokines signal to the brain through indirect actions via secondary mechanisms?
A
- There are cytokine receptors (IL-1R) on perivascular macrophages (reside in between the endothelial cells and the basement membrane also expressing immune receptors) and endothelial cells of the brain venules
- Once bound to the receptor, the cytokine induces secondary messengers such as PGE2 which can cause fever
- These influence neuronal activity in key brain regions known to mediate sickness behaviour
25
How do cytokines signal to the brain through afferent nerves?
- They can be mediated through the activation of peripheral nerves (rapid)
- The end terminals of nerves express cytokine receptors which can signal to the brain when stimulated by locally produced cytokines
- Stimulation of the vagus nerve induces stimulation of several brain areas that are activated by infection
- Transection of the vagus nerve attenuates CNS mediated behavioural responses
26
What type if infections do the vagus nerve and the trigeminal nerve react to?
- Vagus- during abdominal and visceral infections
- Trigeminal- during oro-lingual infections
27
What is the result of peripheral cytokines signalling to the brain?
- Actions of all the immune-brain communication pathways lead to..
- Increased pro-inflammatory cytokines in the brain by diffusion, endogenous production, microglial cells in the brain parenchyma, macrophages in CVOs, meninges and choroid plexus
- Activation of messengers such as PGE2
- Neuroendocrine, metabolic and behavioural changes
28
What is lipopolysaccaride (LPS)?
- An experimental tool to induce sickness, used to model gram negative infections
- Is part of the outer lipid bilayer of gram negative bacteria such as E coli
- Is released during bacterial proliferation and as a by-produce of bacterial lysis
- It enters the circulation and can signal via a LPS receptor complex (TLR4 and MD-2)
- This binds to CD14
- This reliably induces APR and sickness behaviour in rodents
29
What did a intraperitoneal injection of LPS reveal in mice?
- Injection into the abdomen before a transmitter was placed here to measure the core body temp remotely
- Found significant increase in core body temp in 8 hours then recovered
- Reduces food intake significantly
- Reduces body weight not only due to decreased food intake but due to increased metabolic rate and less drinking too
- Recover quickly- around 2 days
30
What are 3 conditions which make one more prone to infection and less likely to recover quickly?
- Diabetes
- Obesity
- Ageing
31
What have animal models of diabetes, obesity and ageing revealed about their response to LPS?
Impaired responses, usually enhanced sickness behaviour
32
How do obese people react to infections?
- Increased prevalence and severity of infections
- Increase morbidity in ovese patients after diseases such as COVID
- Increased incidence of wound infection after surgery
- Higher rates of infection and mortality after burns
- Higher incidence of respiratory, peridontal (gum infection) and skin infection
33
What happens if inject a leptin deficient mouse with LPS?
There is increased food intake compared to non-obese mice
- Also mortality rate is significantly accelerated in dose dependent doses of LPS
34
Why is the leptin deficient mouse not the besr assay for obesity?
- Is an artificial way of inducing obesity and leptin deficiency is rare
- People are more commonly obese due to bad diet
35
How do the diet-induced obese mouse react to LPS injection?
- There is an enhanced decrease in food intake
- Controls recover much faster
- Core body temp is much higher in obese
- There is exacerbated and prolonged response to infection
36
What is leptin?
- 16kDa protein (hormone)
- Obesity gene product
- Produced predominantly in adipose tissue (the more obese, the more leptin one will have)
- Released into plasma in proportio to adipose tissue mass
- Is a negative regulator of food intake and body weight (if inject into mouse, there is a loss of appetite)
37
What are the diet induced obese rodents leptin levels like?
Have increased plasma leptin but become resistant
38
How is leptin involved in the regulation of immunity?
- Is produced by inflammatory-regulatory cells
- It increases during infection and inflammation
- It can regulate the release of several cytokines
39
How may leptin be involved in rodent sickness behaviour enhancement?
- As it is involved in immunity, maybe the resistance is involved
however
- Circulating leptin mediates LPS-induced anorexia in rats
40
What occurs in rodents when given an LPS injection and a leptin antiserum?
- In normal weighted rodents, the animals are given LPS injection which causes sickness behaviour and increased core body temp
- If then give LPS in conjunction with a leptin antiserum this sickness behaviour and temp rise is stopped
- This suggests that leptin mediates the effects of LPS in sickness behaviour
41
What are 2 suggestions that leptin mediation of sickness behaviour leads to?
- As obese mice (ob/ob) are leptin deficient, they should be resistant to LPS but infection causes enhanced sickness behaviour in these mice???
- As diet-induced obese mice are leptin resistant they should also be resistant to LPS but infection also causes nehanced sickness behaviour
- Other mediators must be involved in sickness behaviour in obese rodents
42
How does cytokine release in response to infection change in diet induced obese mice?
- There is an impaired response in terms of how the animals release cytokines and how much
- Need cytokine release to fight.
- In the obese animals there is a blunted response
- It is possible that the obese animals aren't amounting an immune response quite the same and it may be delayed, therefore the pathogen gets a hold more in the obese animals and they are then slower to clear the infection
43
Do Cytokines have a normal function within the brain?
- Yes
- Evidence has been gathered to suggest that these inflammatory molecules play a role in normal cognitive processes and brain function
- If involved in both however, when do we block them and if we do in disease what impact will this have on normal brain function?
44
What are some normal brain functions that control/contribute to everyday processes?
- Development (evidence that maternal infection is a driver of brain changes leading to schizophrenia)
- Sleep (data of cytokines role in sleep, when people are ill, their sleep is disrupted)
- Appetite
- Neural transmission
- Learning and memory
- Cognition (performing higher levels of thinking)
45
What are cytokines normal brain physiology?
- Very low levels (pM rage) of cytokines in normal CNS
- These play an important role in CNS homeostasis and normal functions
- Neurons have cytokine receptors for example in the hippocampus
46
What are the steps for identifying a physiological role for cytokines or any molecule?
- Expression pattern in the brain (looking for the molecule)
- If something might be involved in memory for exmple, look at the hippocampus (area of cell type) finding spatial patterns
- Also can see if expressed at the right time for example if are performing high levels of functioning of cognition, levels would increase at this time (temporal pattern)
- Effects of exogenous administration recombinant protein (if generate a recombinant form, it should recreate the actions involved)
- Effects of endogenous over-expression of the protein (transgenic) (Knock things out, modulate and insert genes to find control)
- Effects of inhibition of endogenous proteins through antagonist/antibody, KO or transgenic over-expression of the inhibitor
47
What is LTP?
- First described in 1973
- Record the activity of a group of neurons and then stimulate with a high frequency stimulation and go back and re-record. The post-synaptic signal was increased
- It is beleived that this strengthening (and weakening) of synapses is important in learning and memory
- LTP could be the mechanistic model underlying this
48
How was it shown that IL-1 levels increase after LTP in vivo?
- Induce LTP from high frequency stimulation to increase the synaptic potentials in animals
- Measure the IL-1
- 8 hours after LTP, IL-1 levels go through a large increase
- If block LTP (AP-5), there is no increase in cytokines
49
How is it shown that IL-1 is required for the maintenance of LTP?
- Treat with IL-1 antagonist (IL-1ra)
- LTP is induced alongside the antagonist- the potentiation is abolished
- It transiently comes back to normal
- The maintenance of LTP is gone, to keep LTP we need IL-1 in the system
50
How was it shown that IL-1 is required for induction of LTP?
- Move the administration of IL-1 antagonist (IL-1ra) before the stimulus and increase the synaptic response
- Higher dose of the antagonist, do not get LTP at a proper physiological temperature
- IL-1 contributes to the induction and maintenance of the process involved in learning and memory
51
How was it shown that IL-1 is necessary for spatial learning?
- Behaviourally
- Use a Morris Water Maze
- Train the animal to find the platform and then submerge the platform (hippocampal driven finding of the platform)
- Measure how long it takes for them to get to the platform
- IL-1 antagonist (IL-1ra) treatment slowed them down, so is necessary for learning
52
How was it shown that LTP and memory were impaired in IL1R1-deficient mice?
- Knockout the receptor for IL-1
- Recorded from the hippocampus
- Do not get LTP when KO
- Also those lacking the receptor cannot learn the Morris water maze task (they have poor capability)
53
How was it shown that over-expression of IL-1Ra impairs memory?
- Transgenic mouse, overexpressing the receptor antagonist
- Cannot learn the morris water maze task or classical (contextual) fear conditioning
54
How does a lack of IL-1 affect brain development?
- Transgenic mice, overexpressing IL-1Ra
- Image the brains of these mice overtime (MR scans)
- Showed decreased whole brain volume and hippocampal and cerebral cortex especially
- Enlarged ventricles
- Shows, IL-1 is needed to form the memory strucutres in the brain properly
55
Why does IL-1 inhibit LTP in CA3 region of the hippocampus when exogenously applied at high doses?
- Applied IL-1 onto slices which inhibited LTP
- Is due to dose dependency, IL-1 at low levels enhances memory but at high levels (and suboptimal) has a detrimental effect
56
How do other cytokines (beside IL-1) act within normal physiology?
- TNF, IL-6 largely do the same thing
- At optimal doses, help memory type processes and plasticity but moving into pathological states become detrimental
57
Why was the immune system suggested to be the seventh sense?
- Kipnis
- Argued the basis that the immune system must sense what is occuring to an organism as the immune system reacts to a pathogen
- It is programmed to sense organisms and microorganisms and must inform the brain for action
58
How was it shown that T cells are required for memory?
- T cell deficinecy leads to cognitive dysfunction
- Morris maze data with scid mice who lack T cells
- These mice do not learn overtime after 4 days and 4 trials a day
- Nude mice are also deficient in T cells but if replace the T cells, they can then do the task
59
How is IL-4 required for memory?
- Regulation of learning and memory by meningeal immunity
- The factor about T cells which are important
- IL-4 KO mice cannot learn a task
- If KO IL-4 in T cells is the same
- So T cell derived IL-4 is imoprtant
60
How was it shown that GABAergic neuronal IL-4R mediates T cell memory effects?
- Adenovirus expression to modulate systems
- Measured contextual fear conditioning and time
- If KO IL-4, the mouse does not freeze for as long (same for T-Cell deficient mice)
- The receptor for IL-4 is removed from GABAergic neurons and it shows tha tthe mice lacking the receptor in the hippocampus do not learn the task as well
61
What is the outcome of GABAergic neuronal IL-4R effects on T cell memory effects?
- T cells produce IL-4 acting on the IL-4R receptor on neurons for behaviours such as contextual fear memory
- Selectively delete the receptor on GABAergic neurons causes defects
- Is required for synaptic function and LTP
- IL-4Ra depletion on GABAergic neurons is sufficient to impair contextual fear memory
- IL-4 delivery into the CSF ameliorates memory defecits in SCID mice
62
Where are cytokines regulating CNS activity derived from?
- The meninges
- May be a major communicating group between the brain and the body
- The dura and subarachnoid space if packed with immune cells which can release cytokines and tranmitters to be transported to the brain and effect function (in terms of cognition)
63
What is the effect of meningeal gamma-delta T cell IL-17 on STM?
- Test of memory using a Y shaped maze (should go through each arm in a logical way as remember where it has previously been)
- In the KO IL-17 mice, STM is affected and also lack T cells
- LTM is not affected
- Gamma-delta T cells resident in the meninges releases IL-17 which is important for short term memory processes
64
What is IFNgamma necessary for?
- Social behaviour
- IFNgamma is derived from T cells in the meninges
- If KO this, the mouse does not show normal social behaviour
- But if return IFNgamma to the KO mouse, then sociability returns
- Is due to changes in the activity of neurons in the prefrontal cortex
- It regulates the activity of this pathway and the firing so when the T cells are low- the pathway becomes hyperactive and there is reduced social behaviour
65
What are microglia's homeostatic functions?
- Activated microglia are associated with disease
- Evolutionary-wise shuold have a positive role
- Found that they survey their environment through their processes within their own niche
- Important for producing proginator cells and have roles in removing weak synapses
- Are important in tissue damage
66
What are important for remodelling of neuronal synapses?
- Microglia
- Synaptic pruning during development and plasticity during adulthood
- Microglia have a baseline survalence and then enhanced survalence where there is a wider spread and more monitoring of the dendrites
67
Besides the dendrites, what other cellular aspect do microglia work on?
- The cell body
- Particularly cholinergic neurons
- They also can contact blood vessels and may have some control over blood flow
68
What happens to the microglia during inflammation?
Microglia proliferate and synthesise cytokines
69
Why is the birth of the microglia unusual?
- Other brain cells come from neuroectodermal or neuroepithelial proginators (neurons, oligodendrocytes, astrocytes etc)
- Microglia come from haematopoietic stem cells (form peripheral immune cells such as lymphocytes and red blood cells)
- They branch off to a common myeloid progenator (forms other peripheral immune cells such as monocytes)
- This leads to extra-embryonic yolk sac myeloid cells during development which form the microglia
70
How do the yolk sac blood islands allow microglia entry into the brain?
The can migrate and colonise in the brain as during development there is an incomplete BBB
71
What are the microglia phenotypes?
- Resiident microglia can become activated to adpot one of many diverse phenotypes
- Historically these were classified as M1 or M2
- M1 are pro-inflammatory whereas M2 release anti-inflammatory cytokines
- This differential is now not generally used as think that they can be on some spectrum between different states
- The way they can adopt the different phenotypes depends on disease type, stage of disease, age of the patient and many other variables
72
How do microglia respond to a laser burn?
- Normally, processes move around
- GFP tagged into a receptor so they fluoresce
- Did a small laser burn causnig stress
- Can see the microglia surrounding this and have the processes rushing towards this
- ATP and other factors will be released which microglia can sense in order to isolate the injury and do something (only the processes)
73
In the normal brain, how to endothelial cells act?
They are part of the BBB that separates the brain tissue from the periphery
74
In the normal brain, how do inflammatory mediators interact with neurons?
Inflammatory mediators can act on neuronal cells to modulate important physiological functions
75
What is Interleukin 1?
Is a potent pro-inflammatory cytokine involved in the induction of fever in response to systemic infection
76
In experiemental settings that use recombinnant IL-1. Why is a heat-treated preparation (boiled at 95 for 30mins) used?
- To inactivate IL-1 in order to test for the presence of contaminants
- Is due to the way that IL-1 is produced, using a tag vector inserted into a bacteria
- The soluton of tagged IL-1 also has residual LPS which activates the same MAPK/NFkB pathways as IL-1
- Therefore heat treat in order to denature IL-1, if there is still a response, then we know it is the residual LPS.
77
What is a neuroimmune interaction?
A bi-directional link between the CNS and the immune system
78
Where is ACTH produced?
- The pituitary gland under the CRF production by the hypothalamus
- CRF in the blood system of the anterior pituitary stalk travel to the pituitary to interact with corticotroph
79
What are the major cytokines that act on the hypothalamus to induce the release of pituitary hormones?
TNFa
IL-1
IL-6
80
What does in situ hybridisation allow the identification of?
- IL-1R1 mRNA expression in the hypothalamus
- Relies on designing an antisense RNA probe complimentary to the RNA in the brain for the IL-1R1
- This is labelled and then when incubate can reveal different things
- Such as IL-1 receptor protein expression using immunohistochemistry to see where something is expressed
- Or IL-1 binding sites using radio-ligand binding
81
Why does mRNA expression not always mean that the protein will be expressed in that area?
Couold be deposited in the synapses in a different brain region to where the cell bodies are
82
How does IL-1 induce the HPA axis?
- Activating noradrenergic neurons in the hypothalamus
- There are two ways in which it can be activated
- One is through the brain indirectly or directly through peripheral infection, having a negative effect due to the release of corticosteroids being immunosuppressive
- The other is through stress, there is upregulation of IL-1 due to glucocorticoids. A chronic period of this can lower immunity.
83
How does noradrenaline induce ACTH release?
alpha-adrenergic receptor and beta-adrenergic receptors
84
How can peripheral IL-1 activate the HPA axis?
Via vagal afferents, passive diffusion and by acting on the brain endothelium
85
What effect does IL-6 have?
A synergistic effect with IL-1 in the induction of the HPA axis
This means that the effect of the two agents is greater than the sum of individual effects
86
Where are glucocorticalsteroids synthesised?
By the adrenal glands to inhibit the hypothalamus and the pituitary as a negative feedback mechanism
