Peripheral Neuroinflammation Flashcards
1
Q
What nerve is important for pain perception?
A
The Sciatic Nerve
2
Q
What are the 5 steps of pain perception as labelled by Descartes?
A
- Detection
- Transduction (heat to electrical)
- Conduction (across te peripheral nerve to the spinal cord)
- Trasmission
- Perception
3
Q
What are nociceptors?
A
High threshold densly innovated sensory neurons of the peripheral somatosensory nervous system activated by noxious stimuli
4
Q
What is the somatosensory nervous system?
A
- Comprised of the dorsal route ganglion
- Many sensory neurons which lie in pairs in the DRG
5
Q
What are the somatosensory sensory neurons?
A
A heterogenous population and can be divided by a host of criteria such as sense of modality, size and receptors used
6
Q
What is the diameter, myelination, nerve conduction and function of the Aa/Ab sensory neuron?
A
- Large 40-80micrometers
- Is myelinated
- Fast 40-120 m/s
- Proprioception, Low threshold mechansoreception such as to innovate muscle spindles and joints
7
Q
What is the diameter, myelination, nerve conduction and function of the Adelta sensory neuron?
A
- Small-medium 15-20 micrometers
- Thinly myelinated
- Medium 10-30m/s
- High threshold mechanoreception, thermoreception (nociceptive)
8
Q
What is the diameter, myelination, nerve conduction and function of the C sensory neurons?
A
- Small 10-25 micrometers
- No myelination
- Slow 0.5-2 m/s
- Thermoreception, high threshold mechanoreception, thermoreception, chemoreception and are silent polymodal nociceptors
9
Q
What are silent polymodal nociceptors?
A
Only stimulated when inflammation is around
10
Q
What is the IASP definition of a nociceptor?
A
A high-threshold sensory receptor of the peripheral somatosensory nervous system that is capable of transducing and encoding noxious stimuli
11
Q
How is the nociceptor receptor sensitivity studies?
A
Through patch clamp recordings or electrophysiological assessment
12
Q
What did Ardem Patapoutian do?
A
Won the Nobel prize for describing the role of TRP in sensory and temp processing
13
Q
What receptors does mechanical stimuli activate?
A
- P2X/P2Y via ATP
- TRPA1, TRP?
14
Q
What receptors does H+ stimulate?
A
- ASIC
- TRPV1
15
Q
What type of receptors does heat stimulate?
A
- TRPV1
- TRPV3 which further stimulates ?
16
Q
What type of receptors does cold and mint/cinnamon/eucalyptus stimulate?
A
TRPM8
17
Q
What does TRP stand for?
A
Transient receptor potential channel
18
Q
What does ASIC stand for?
A
Amiloride-sensitive cation channel
19
Q
What does P2X stand for?
A
Ionotropic purinoceptor
20
Q
What does P2Y stand for?
A
GPC pyrimidinergic receptor
21
Q
What occurs once the sensory receptors for nociception are stimulated?
A
They generate an generator potential and then an action potential along the sensory nerve terminal
22
Q
Where are the different nociceptor receptors located in the nerve?
A
They can be located at different point of the sensory axon
23
Q
Outline transduction phase of the primary afferent input to the spinal cord?
A
- Stimulus to the transducer channels/receptors (TRP for example)
- Stimulus to other cells as neurons and axons do not live in a bubble. Surrounded by things like glial, accessory cells and keratinocytes which will release different factors causing actions on the sensory axon
24
Q
Outline conduction phase of the primary afferent input to the spinal cord?
A
The receptors cause a generator potential and an AP along the DRG in the dorsal horn
25
Outline transmission phase of the primary afferent input to the spinal cord?
- The AP reaches the central terminals and there is presynaptic modulation and Ca influx
- Neurotransmitters are released such as glutamate, substance P and CGRP causing transmission in the spinal cord
26
Outline nociceptive pain
- Pain that arises from actual or threatened damage to non-neuronal tissue and is due to the activation of nociceptors
27
What type of stimuli is nociceptive?
- High-threshold stimulus-dependent pain
- Thermal, mechanical or chemical stimuli
28
How does nociceptive pain occur?
Pain evoked in a graded response by appropriate high intensity stimuli
Is a good pain, is adaptive and serves a purpose
29
Outline inflammatory pain
- Active inflammation
- Sensitisation occurs
- Pain evoked by low and high intensity stimuli (innocuous stimuli causes a response)
- Is adaptive, protective during the healing response and reversible
30
What is neuropathic pain?
Pain caused by a lesion or disease of the somatosensory nervous system (chronic pain)
31
What is an example of traumatic neuropathic pain?
Nerve entrapment or injury
32
What is an example of central neuropathic pain?
Stroke, spinal cord injury, MS
33
What is an example of neurotoxic neuropathic pain?
Microtubule-stabalising agen (MTSA) induced neuropathy which is a drug acting within the nervous system
34
What is an example of infectious neuropathic pain?
HIV-neuropathy and post-herpatic neuralgia (shingles)
35
What is an example of metabolic neuropathic pain?
Diabetic neuropathy and alcohol-induced neuropathy
36
What is an example of idiopathic neuropathic pain?
Chronic neuropathic pain with unknown and multiple sources
37
Outline neuropathic pain
- Marked neuroimmune component
- Causes sensitisation
- Spontaneous, and evoked by low and high intensity stimuli
- Abnormal amplification
- Serves no useful purpose
- Maladaptive and persistent
- Not well-managed
38
What is sensitisation?
- Recruitment of a neuronal response to normally subthreshold stimuli- may change the activation thresholds of the axons
- Increased responsiveness of nociceptive neurons to their normal input, channels can stay open for longer
- Increases in receptive field size may occur
- Spontaneous discharge
39
What are the clinical features of neuropathic pain?
- Sensitisation is usually inferred indirectly from phenomena
- Stimulus-evoked pain
- Spontaneous pains
- Associated with co-morbidities such as anxiety, depression and sleep disturbance
40
What are the two stimulus-evoked pains of neuropathic pain?
- Hyperalgesia- increased pain from a stimulus that normally provokes pain
- Allodynia- pain due to a stimulus that does not normally provoke pain
41
What are spontaneous pains?
Often described as burning, tightness accompanied with parasthesia, tingling, shooting or stabbing pains
42
How can clinical pain be assessed in humans?
Through questionnaires or pain diaries and pain distress logs such as the short-form McGill questionnaire and the 0-10 VAS numeric pain distress scale
43
What are some emerging electrophysiological markers of pain?
- Pain evoked potentials
- Nerve conduction studies
- Microneurography
- Nerve excitability testing
- H-Reflex RDD
44
What are pain evoked potentials?
Biomarker of pathology of Adelta nerve fiber pathways in the DPN
45
What are nerve conduction studies for pain sensing?
E.g. peroneal amplitude and conduciton velocity biomarker of large fibre neuropathy in DPN
46
What is microneurography for pain sensing?
Direct recording of c-fibre nociceptors potential biomarker of nociceptor hyperexcitibility in pDPN
47
What is nerve excitability testing in pain sensing?
Potential biomarker of large fibre axonal excitability in DPN
48
What is the H-Reflex RDD in pain sensing?
Potential biomarker of spinal disinhibition in pDPN
49
What is a limitation of the questionnaire methodoloy for pain detecting?
- Is an indirect measure
- Can now start to map the neuronal methods to see if there is sensitisation for example
- Using electrophysiological markers
50
What are the stats for pain relief in neuropathic pain?
Treatments such as pregabalin offer only 30-40% of patients around 40-50% pain relied.
Showing that we are trying to treat the symptoms rather than the underlying mechanisms
51
What do experimental models of neuropathic pain usually do?
Involves unilateral injury to rodent peripheral/sciatic nerve through a number of ways
52
In what different ways is the peripheral nerve damaged in rodent models of neuropathic pain?
- Chronic constriction injury
- Full or partial sciatic nerve ligation/crush
- Spinal nerve ligation model
- Nerve transection (axotomy)
Can then see if the animal is more or less sensitive and see if medication improves
53
What are some other animal models used for neuropathic pain that do not involve the damage of the sciatic nerve?
- Specific neurotoxic
- Infectious
- Metabolic neuropathies
- Such as STZ-induced diabetes for diabetic neuropathy, paclitaxol (chemotherapy neuropathies)
54
What are the 3 ways to assess the preclinical models of neuropathic pain?
- Mechanical sensitivity using von-frey filaments
- Thermal sensitivity using hargreaves IR/hot plate
- Non stimulus-evoked measures
55
Outline how the stimulus-evoked thresholds are measured in assessing preclinical models for neuropathic pain
- Measure paw withdrawal thresholds or latency such as the size of the von frey filament or time due to thermal stimuli which tests for hypo/hypersensitivity
- Need controls such as uninjured vs injured comparisons of thresholds for unilateral nerve injury and/or sham operated, vehicle treated or naive animal (depending on the model)
56
Outline how the non stimulus-evoked thresholds are measured in assessing preclinical models for neuropathic pain
- Important but more challenging to quantify in animal models as cannot ask them
- Behavioural changes such as thigmotaxis/open field, locomotion, anxiety, changes in gait or burrowing, facial expression and place preference tests
57
What are the 4 mechanisms of neuropathic pain?
- Increased inflammatory cells and mediators in the PNS and CNS (sensitisation is responsible for this)
- Altered nociceptor thresholds and activity (receptor/ion channel expression)
- Altered spinal processing:sensitisation, synaptic reorganisation
- Altered central processing, descending inhibtion
58
How does mast cell activation and degranulation occur in neuropathic pain?
- Tissue damage occurs
- Mast cells, resident immune systems can degranulate and release histamines and immune mediators
- These bind to receptors on the primary afferent terminals and detect pain
- Also collateral branches leading to the release of neuropeptides in which can bind to the mast cells
- Causes neurogenic inflammation, increase in blood flow, vascular permeability and cell recruitment
59
What does preventing mast cell degranulation do?
- Decreases histamine compound release which induced inflammatory response and burning-like pain
- Decreases hypersensitivity and the recruitment of further pro-inflammatory cells and mediators at sites of nerve injury
60
What is an example of something that can prevent mast cell degranulation?
- Sodium cromoglycate
- Which is in eyedrops to dampen inflammation
61
What does an increase in peripheral inflammatory mediators elicit in terms of neuronal excitability?
- Short and long term effects
- Direct receptor-mediated excitatory effects of **histamine**, bradykinin, PGE2 and **TNFa**
- Increases firing after noxious pressure stimulus is applied to peripheral receptive field
- Also amplifies the response
- This is the direct receptor stimulating effect of the inflammatory mediators
62
How is TNFa used to assess sensory thresholds in preclinical models?
- TNF-neutralising antibodies administered
- This anti-TNFa can reduce sensitivity following in vivo nerve injury model
- AntiTNFR1 (not R2) can reduce sensitivity
- Hyperalgesia is induced by TNFa injection, and is reduced in TNFR1 KO mice and by antiTNF and etanercept (TNF-sequestering drug)
63
What are the roles of neutrophils after injury?
- Are the first cells to infiltrate th damaged tissue
- Depletion of neutrophils following nerve injury such as using an anti-neutrophil antibody decrease hypersensitivity
- They dampen the pain response
64
What roles do the neutrophils fill?
- Increasing proinflammatory fators such as TNF and chemokines which activate and attract other inflammatory cell types including macrophages, T lymphocytes
- Macrophages and mast cells release prostaglandins and the cytokines which break the BBB
- Essentially causes an inflammatory soup
65
What factors increase at the sites of peripheral nerve injury?
IL1b and TNFa after sciatic nerve injury, increase in message and protein measured following
66
How do IL-1B nd TNF contribute to hypersensitivity after peripheral nerve injury?
- In KO animal, perform a nerve injury
- In WT, become hypersensitive
- In TNFa KO are less responsive
- If treat the KO animal with a cytokine, get the hypersensitive response back
67
What are the direct and indirect actions in the neuron afer injury?
- Direct receptor-mediated excitatory effects such as histamine and TNFa acting on their receptors
- Indirect roles include increase in vascular permeability and increased cell recruitment
- Direct and indirect modulation of receptor and ion channels
68
What is a key channel involved in neuropathic pain?
- Na channel activation
- Normally activated through PKA and MAPK pathwways which can phosphorylate and change the dynamics of the Na channels
69
Where do nociceptive fibres terminate?
Top laminae (small and tightly packed in the top two laminae- as defined by Rexed)
70
What is the organisation of the lamina?
1-X based on the size and packinf density of the neurons
71
What is WDR?
Wide dynamic range
72
What are WDR activated by?
- Brush
- Touch
- Tap
- Pinprick
- Pinch
- Pressure
73
What are the nociceptive specifc nerves sensitive to?
- Pinprick
- Pinch
- Pressure
- (only activated bby more high threshold stimuli)
74
How does NS become more like WDR after inflammation?
- If peripheral inflammation in a peripheral field
- NS has a different profile, is hyperresponsive
- Histamine increases the responsiveness
- It then acts more like a WDR, activated by the innocuous stimuli
- Something has happened to the periphery driving the central changes
75
What does repetative stimulation of the nociceptors cause?
Progressive increase in action potential discharge and prolonged excitability of neurones in the spinal cord following the stimulis (spinal wind up)
76
How does normal to facilitated firing occur in spinal processing of neuropathic pain?
- Input from the periphery
- Through the dorsal horn and second order neuronal activation (NMDAR governed)
- activates through the spinothalamic tract to the thalamus where is perceived as noxious
- When lots of glutamate release at the second order neurons, causes an increased responsiveness, amplifying and facilitating the response
77
What is the Gate control theory of pain?
- Is a balance of large AB and small C fibre activity
- The small fiber comes in and terminates in the top of the laminae in the brain
- The larger fibres terminate lower down and the interneurons connect the two.
- The activity of the second order neurons is dependent on the balance of the information
- Large fibres can cause inhibitory interneurons to inhibit the pain (if stimulate low threshold)
- This floods AB input and gates the pain
78
What is glutamate's response to neuropathic pain?
- Calcium influx and neurotransmitter release acting on their receptors
- So much glutamate is being released that the Mg block is overcome (phosphorylation of the NMDA)
- Glutamate comes in and a massive Ca influx occurs changing the excitability of the second order spinal cord neuron
- At the same time, astrocytes and microglia are infiltrating with T cells, causing inflammatroy mediators and impacting on different activity profiles
79
Outline the diference in the normal pain processing pathway and in pathological conditions
- Normal involves the stimulation of a receptor, sodium channels get inserted and removed from the membrane leading to normal excitation
- Pathological has this inflammatory soup with prolonged activation. There is increased sigalling through the signal transduction pathways altering the sodium pathway (altered trafficking/expression levels at membrane leads to influx and hyperexcitabiliy) and changes the peripheral sensitisation
80
What is a feature of neuropathic pain that can be used to differentiate it from other types of pain?
The prescence of spontaneous pain
81
What happens in the peripheral nerve to Schwann cells following injury?
- De-differentation
- Lose their sheath and form a proliferating phenotype
- Forms streams allowing the regeneration of injured peripheral axons
82
Where can neuropeptides influence?
Impact the blood vessels
83
What is a vasicular effect created by the release of substance P from afferent terminals?
Vasodilation as more blood flows to the area to recruit more cells
84
What do matrix metalloproteinases do?
- Break down the blood-nerve barrier
- Cleave and activate a number of cytokines and growth factors
- Can be inhibited by minocycline, a broad spectrum antibiotic
85
What is wallerian degeneration?
Inflammatory changes associated with nerve injury
86
What cell type facilitates wallerian degeneration following peripheral nerve injury?
- Schwann cells
- Due to de-differentation and proliferation
- Paves way for clearing of factors such as the release of neutrophils
87
What type of relationship is there between Schwann cells and the axon?
- Reciprocal
- The axons can release factors such as neuregulin
- Schwann cells can release factors to bind the receptors to the membrane
- Can promote axonal regeneration and cell survival
- But may also trigger persistant bad pain
88
In peripheral injury what receptor does TNFa bind to?
- TNFR1 causing an increase in firing rate
- TNFR2 is not expressed in sensory neurons
89
What can TNFa increase the levels of which potentiates the nociceptor activity?
TTX-resistant sodium channels
90
What is hyperalgesia reduced by?
Etanercept which is a TNF receptor fusion protein
91
Which glutamate receptor activation is particularly important for central sensitisation?
- NMDA
- Increased activity means overcomming the mg block
92
How does allodynia occur in terms of altered spinal processing?
- The AB fibres, low threshold mechanoreceptors which terminate lower in the dorsal horm than the smaller c fibres
- Can get small branches which can branch onto a neuron
- Also have interneurons throughout the dorsal horn, could input onto a different nociceptive neuron and influence activity
- Balance of output could be changed due to flooding with AB perhaps or with aberrant touch, may activate a nociceptive neuron
93
Spinal microglial cells share a myeloid lineage and have similar features to which type of peripheral cell type?
Macrophages
94
When there is spinal hyperexcitability, where is there the most microglial activation?
- The ipsilateral side to the injury
- Packed amoeboid type in the superficial dorsal horn
95
Microglial activation in the spinal dorsal horn following nerve injury is accompanied by what?
- An invasion of T-lymphocytes
- Activation of astrocytes
- Astrocyte proliferation
96
Outline chemokines' role in spinal hyperexcitability
- Large family of chemotactic cytokines (4 sub families) recruitment of proinflammatory cells to the site of injury/inflammation
- Mediate the effects via interactions with 7 transmembrane GPCRs
- Chemokines increase Ca2+, are excitatory, stimulate the release of Substance P and CGRP from sensory neurons
- Can modulate TRP/NaV channel expression and activity
97
What happens to chemokine CCL2 levels following a nerve injury?
- Stained sections through the DRG in a mouse with a peripheral nerve injury
- CCL2 is upregulated following injury: in small diameter sensory neurons, there is a rapid incrase
- In the dorsal horn, in spinal astrocytes, CCL2 also increases in the injury side
98
How do spinal chemokines potentiate signal hyperexcitability?
- CCL2 potentiates ERK phosphorylation and gluatamterfic signalling on the second order neurons
- An injection of CCL2 or overexpression of transgenic version increases nociceptive responses
- Transgenic mice lacking CCR2 have reduced pain response
- Implies the CCL2 is important in the pain response through the NMDA link
99
What occurs to the CX3CR1 fractalkine receptor following a unilateral nerve injury?
- Fractalkine can signal in either a membrane-bound state or be cleaved into soluble signalling form (via the action of a protease cathepsin S- released from activated microglia)
- Upregulated other pro-inflammatory mediators increasing spinal excitability.
100
Why is there not only a superficial dorsal horn response to unilateral nerve injury but a response lower down too?
- Because there are also motor axons too
- Getting an injury response around all these cell bodies
101
What does the chemokine fractalkine act on?
Microglia and astrocytes
102
What is an example of a drug which inhibits glial activation/function in preclinical models of neuropathic pain?
Duloxetine
103
What is clinical evidence of the activation of microglia from spinal injury?
- Post mortem sponal cord patients with shinges show microgliosis in the dorsal horn observed in dermatomal region of the rash
- Thalamic glial activation visualised using PET in chronic lower back pain patients
104
What are problems in therapies targetting neuroimmine interaction?
- Pharmacokinetics
- Getting the compound to the right place, CNS penetrance; BBB
- Also selectivity and different receptor types
- How the drug is metabolised
105
What is the sensory discriminate part of pain?
Identifies the painful stimulus according to location, time and intensity
106
What is the behavioural and emotional response to pain?
Activation in the amygdala and thalamus in the midbrain
107
What is the descending inhibition pathways of pain?
Modulates the pain experience through the periaqueductal grey through the release of noradrenaline
108
What is the pain matrix?
- Revealed by PET and fRMI studies
- Many regions are acivated by pain
- Makes chronic pain challanging to treat as is part of a personal experience also
