Inflammatory Mediators in Normal Brain Function Flashcards
What is the cytokine profile in the normal brain?
In the normal brain:
• Barely detectable, pM range, of cytokines (IL-1, IL-6, TNFa) – usual only increased as a response to pathogens
• Low levels have important regulatory roles for homostasis (neuro-immune modulators)
• Evidence obtained from gene-deficient mice and recombinant cytokine administration
What are ‘normal’ brain functions?
• Those that contribute to everyday processes:
− Development → formation of the brain, synapse formation in learning
− Sleeping
− Feeding
− Cognition
• Abnormal brain function occurs in pathophysiologies such as schizophrenia, depression, alzheimers etc…
How would you identify a role for a particular cytokine in the brain?
- Expression pattern in the CNS → temporal and spatial. Is the molecule that you think has a role in a certain process actually expressed in the CNS? Can you measure it? Is it expressed where and when you would think? Eg) if you think it has a role in sleep, you would expect it to be expressed when sleep is brought on.
- Exogenous administration of the recombinant cytokine → Administer the molecule to try and mimic the process you think it affects. This is artificial, eg) if something works at a synapse, and you inject it subcut., it isn’t a true physiological representation of its effects.
- Endogenous over-expression (transgenic)
- Inhibition of the endogenous protein → Antagonist/antibody. Knockout. Transgenic over-expression of inhibitor. This is the best way to measure its true physiological role.
What is synaptic plasticity.
- Originally thought the brain was hard-wired, this is not true.
- Synaptic plasticity → the ability of synapses to strengthen or weaken over time, in response to increases or decreases in their activity. Plastic changes also result from changes in number of neurotransmitter receptors on a synapse.
- Fundamental for the brain to adapt and respond to different stimuli.
Describe long-term potentiation
• The idea that synaptic changes underlie experienced-induced adaptations in the brain was proposed more than 60 years ago – suggested that the synapse between two neurons is strengthened if they are activated simultaneously.
• Support was proved by the demonstration that high frequency stimulation of hippocampal afferents resulted in persistent augmentation of their synaptic strength – a phenomenon called LTP.
− If you stimulate at high frequency (tetanus) – the response from the post-synaptic cells is enhanced, even when the stimulus returns to normal.
− AMPA receptors are responsible for allowing Na+ through to allow potentiation of the action potential.
− NMDA receptors allow Ca2+ in
− After HFS, NMDA receptors lose the magnesium block, allowing Ca2+ to flow into the synapse
− This allows recruitment of AMPA receptors – and this stay, giving a potentiated AMPA-mediated response.
• Research demonstrated this underlies memory storage.
Describe the experimental evidence to suggest a role for IL-1 in LTP.
- 8 hours after triggering LTP in an animal, show high levels of IL-1B cDNA
- Inducing LTP also results in increases of other cytokines including IL-18 and IL-6
Is increased IL-1 a consequence of, or necessary for, LTP?
• Trigger LTP, then 2 hours later administer IL-1RA
• This abolishes LTP down to amost baseline levels
→ Suggest IL-1 is necessary to maintain LTP.
Is IL-1 needed to induce LTP?
• IL-1RA administered at the start of the experiment, and then removed later
• LTP does not occur until IL-1RA is removed
→ Suggests that in addition to maintaining LTP, IL-1 is needed to induce LTP.
Describe the experimental evidence suggesting a role for IL-1 in spatial learning.
• The previous experiments were performed on brain slices – not really relevant at the whole animal level
• Real in vivo effects of IL-1 can be assess using the Morris Water Maze test
− Treatment of animals with IL-1RA resulted in longer latency to reach the platform, suggesting the animals are poorer learniers.
− Admin of IL-1 has no effect – it doesn’t enhance learning, it stays the same (this is not always the case, see later).
− IL-1R KO mice show reduced LTP in vivo recordings from the dentate gyrus
− These IL-1R KO mice show no improvements in latency to reach the platform in the MWM taste – showing they are not learning from their visual cues.
− Over-expression of IL-1RA show now improvements in latency to reach the platform – not learning
Contextual learning:
• Use fear conditioning – put mouse in a chamber, and pair a shock with light or sound. Then give them the light without the shock, and measure their freezing (expectancy to receive a shock) – if they don’t freeze for as long, they have lost that association.
• IL-1RA over-expressing mice don’t freeze for as long – they have a diminished fear response, suggesting they have lost their association of the light with the shock → indicating impaired memory.
Brain changes in IL-1RA transgenics:
• Enlarged ventricles and a smaller hippocampus in mice overexpressing IL-1RA
Deletion of the P2X7 ATP receptor gives impaired memory:
• This receptor is essential for IL-1B production by hippocampal glia
• Deletion results in no IL-1B expression → gives memory impairment and abrogation of hippocampal neural activation.
• These findings support a critical role for P2X7 receptor mediated IL-1B expression in memory processes.
Describe the differences between exogenous and endogenous administration of IL-1 on LTP. What does this mean?
• Exogenous admin is at high doses (ng or ug range) – would never see this in a physiological system, usually acts at the fm range.
• Important to consider the differences between exogenous admin and the blocking of endogenous IL-1
− Indeed, in one study, exogenous admin of IL-1 inhibits LTP → this suggests IL-1 is detrimental
− In other situations, administration of IL-1 produces memory facilitation
− Study showed that i.c.v admin of a relatively low dose of IL-1b both before and after both passive and active avoidance training resulted in better memory and/or increased the number of avoidance responses.
IL-1 effects on memory are dose-dependent:
• People now believe IL-1 has a U-shaped dose response
• At high levels, such as those seen with exogenous admin, or that may be seen pathologically – IL-1 has a detrimental response in the learning test.
• But at low levels, as found in normal physiology (at that we inhibit with endogenous inhibitors) – IL-1 has an enhancement of memory
Describe a role for IL-6 in synaptic plasticity.
• Research indicates high levels of IL-6, particularly in the context of ageing, are associated with poor memory functioning over time.
• However, role is complex, and IL-6 may influence learning in memory in different ways depending on conditions
• In both chronic fatigue sufferes and healthy controls, exogenous admin of IL-6 did not produce any memory disturbance, and patients had improved performance in the test → although investigators attribute this to practice effect, as they didn’t include a saline-control group so couldn’t say for certain.
• In SLE patients, higher levels of IL-6 in the plasma were associated with higher learning scores
• In one study, a 20-fold increase in IL-6 was measured 4 hours following in vivo LTP induction
− This increase was only found in rats where LTP was maintained for 8 hours, but not when it subsided after 3
− Admin of anti-IL6 antibodies 90min following LTP still resulted in longer maintenance of LTP, but when injected 5 min following LTP, no change was seen.
− → Suggests IL-6 involved in a specific phase of LTP consolidation.
Describe a role for prostaglandins in synaptic plasticity.
- COX-2 is expressed in neurons, glia and endothelial cells
- i.c.v admin of non-selective COX inhibiyots indomethacin and ibuprofen, as well as selective COX-2 inhibitor celecoxib, impairs MWM performance.
- COX-2 activity is upregulated following HFS associated with LTP induction
- Inhibition of COX activity results in impaired hippocampal LTP
Describe a role for TNFa in synaptic plasticity.
- Extensive literature on detrimental effects of TNFa in learning and memory during inflammation however:
- In passive avoidance paradigm, i.p injection of TNFa 24h before training resulted in increased number of avoidance responses.
- TNFa selectively secreted by astrocytes enhances synaptic efficiency by increasing surface expression of AMPA receptors.
- Doesn’t seem to be involved in acute plasticity however, as there is still normal LTP in mice with genetically impaired TNF signaling.
Describe a role for T cells in synaptic plasticity.
- Based on findings that CD4+ T cells against brain self-antigen can be neuroprotective, hypothesized that T cells play a general supportive role in brain and mind functioning.
- Experimental evidence provided by mice with SCID (no B or T cells) as well as nude mice (no T cells) display impariments in spatial learning and memory in the MWM test.
- Repleneshing T cells in nude mice improved their learning and memory
- The T cells that accumulate in the meninges following learning in the MWM test express high levels of IL-4
- IL-4 deficient mice have cognitive impairments.
Describe the role of microglia in sculpting circuits during development.
• Microglia engulf presynaptic and postsynaptic elements (axon terminals and dendritic spines) during developmental periods of circuit refinement
• Several ‘find me’ and ‘eat me’ pathways identified → CX3CL1 acts as a chemokine, binds CX3CR1 expressed exclusively by microglia
− CX3CR1 KO mice have transient increase in the density of dendritic spines
• Complement has a role – C3 and C3R KO mice have reduced microglial engulfment of synapses → complement proteins at developing synapses lead to targeted engulfment by microglia.
• Microglia-mediates synaptic elimination depends on neuronal activity:
− Microglia preferentially phagocytose less active presynaptic inputs
− Complement may therefore be activity dependent ‘eat me’ signals on less active inputs
→ Complemnt therefore has parallel functions in the nervous and immune systems – mark unwanted elements for elimination. However, in the CNS, this occurs in the absence of neuroinflammation.
Describe the role of microglia in surveying the adult brain during health.
- Dynamic process motility allows microglia to survey surrounding synapses and respond accordingl.
- In the visual cortex, 94% of microglial processes touch synaptic elements at any given point in time → associated with both growth and elimination of small spines
- Decreasing synaptic input to the visual cortex during dark adaptation causes decreased microglial process motility.
- Evidence that ATP is released during high neuronal activity to promote enhanced microglial surveillance.
Describe the interactions of microglia and chemokines that can modify synaptic plasticity, and how this can affect higher brain processes.
• Interaction of microglial CX3CR1 and its CNS ligand, neuronal CX3CL1 allows for precise communication between neurons and microglia
• Application of CX3CL1 causes a dose-dependent depression of synaptic transmission → thus a chemokine expressed by neurons acts on microglia to reduce neuronal activitity.
• In addition to acute actions, chronic reduction of this pathway by deletion of CX3CR1 leads to elevated levels of inflammatory IL-1B, leading to a reduction of LTP (remember, increased levels are detrimental).
• This can also effect higher brain processes:
− In CX3CR1 KO mice, both juveniles and adults show decreased social interaction, as well as increased repetitive behaviours, trains often linked with autism spectrum disorder.
