Neuropathic pain

Question 1

Congenital insensitivity to pain (CIP)

Answer 1

o Sufferers can’t feel physically pain
o Rare condition
o Pain is needed to protect us

Question 2

What is pain?

Answer 2

o An unpleasant sensory and emotional experience associated with potential or actual tissue damage
o Nociception – physical process of transduction that give rise to pain

Question 3

3 steps of pain pathway

Answer 3

Transduction
Transmission to CNS
Central processing

Question 4

Transduction

Answer 4

 If you do a skin biopsy on a patient with CIP there are no free nerve endings
 These free nerve endings belong to a neuron called the AFFERENT neuron
 Nociceptor/ sensory neuron/ afferent neuron part of peripheral nervous system
 Activated by noxious stimuli (might cause tissue damage)
 Stimuli can be mechanical, thermal or chemical
 Types of nociceptors: mechanical Aδ fibres, thermal Aδ fibres and polymodal C fibres
 Aδ means that the neurons are myelinated means they transmit signal faster (responsible for first sharp wave of pain in graph)
 C-fibre responsible for the second wave – the dull pain

There are different molecules in the axon to sensor the stimuli e.g. TRPV1 ion channel opens in response to heat

Question 5

Transmission to CNS

Answer 5

Action Potential travels along afferent neurons to spinal cord
 AP travels through dorsal root ganglia into dorsal horn of spinal cord
 Signal transmitted along axons
 Main transmitter released is glutamate

Question 6

Central processing

Answer 6

Central processing – signal to brain, so pain is perceived
 Perception
 Modulation – descending ;pathway from brain modulate the signal - dampening or amplification of AP

Question 7

Neuropathic pain

Answer 7

o Pain caused by a lesion or disease of the somatosensory system (nervous system)
o 7-10% of the general population affected

Question 8

3. Nociceptive pain

Answer 8

o Pain caused by a noxious stimulus (that might cause tissue damage)

Question 9

4. Causes of neuropathic pain

Answer 9

o	Mechanical trauma
	Post- Surgery (e.g. post hernia repair) 
	Amputation – Phantom limb pain 
	Peripheral nerve injury
	Spinal cord injury
o	Diabetics
o	Cancer
	Nervous system tumor
	Compression by tumor
	Chemotherapy induced neuropathic pain
o	Stroke
o	Infection – shingles and HIV
o	Genetics – channelopathies
	Erythromelalgia
	Paroxysmal extreme pain disorder
o	Multiple sclerosis

Question 10

5. Signs and symptoms of neuropathic pain

Answer 10

o Spontaneous pain (without stimulation)
o Evoked pain
 Allodynia – pain brought on by normally non-painful stimuli such as cold, gentle brushing against the skin, pressure, etc.
 Hyperalgesia – increase of pain by normally painful stimuli such as pinpricks and heat

o	Associated symptoms
	Reduced activity 
	Poor sleep 
	Depression 
	Anxiety

Question 11

6. Pharmacological treatment

Answer 11

o Nociceptive pain
 NSAIDs, paracetamol
 Weak opioids e.g. codeine
 Strong opioids e.g. morphine

o Neuropathic pain – not responsive to NSAIDS/opioids
 Tricyclic antidepressants – amitriptyline
 Anticonvulsants – gabapentin, carbamazepine
 Serotonin-norepinephrine reuptake inhibitors – duloxetine

Question 12

Limitation of currently treatment for neuropathic pain

Answer 12

o Achieve only 30-50% reduction of pain severity
o Poor tolerability
o Work only in 30% of patients
o Treatment response highly variable between patients

Need for:
• New therapies
• Better patient stratification and personalised treatment

Question 13

8. Personalised treatment for neuropathic pain

Answer 13

o Pharmacogenomics
 Study of how a person’s unique genetic makeup (genome) influences his or her response to medications
 Give drugs in function of the genetic makeup
o Use of patient derived iPSC (induced Pluripotent Stem Cells)

What gene contribute to neuropathic pain?
 Nav1.7 (gene name SCN9A) – voltage gated sodium channel
 This channel is particularly enriched in nociceptors
 Has important function in initiating and proper firing of action potentials

Question 14

What gene contribute to neuropathic pain?

Answer 14

Nav1.7 (gene name SCN9A) – voltage-gated sodium channel
 This channel is particularly enriched in nociceptors
 Has important function in initiating and proper firing of action potentials

Question 15

9. Erythromelalgia - what is it

Answer 15

o Man on fire syndrome
o Redness of the skin
o Warm or moderate exercise triggers severe burning pain

o GAIN OF FUNCTION variant in Nav1.7

Question 16

study on erythromelagia

Answer 16

o Got a family of patients with erythromelalgia and did genetic screening
 They found a variant in Nav1.7
 Variant is called S241T
 This means that the amino acid 241 had serine which was replaced with threonine
 So basically, there was a mutation in these patients from s to t

o They decided to experiment the function of this variant
o So they expressed human Nav1.7 channel in DRG neurons of mice
o Expression of WT Nav1.7 and S241T Nav1.7
o Patch clamp to asses’ neuronal function
o They notice that for normal channel there were 2 AP and for mutant channel there were more Aps
o This means that the S241T Nav1.7 neurons are more excitable, and this explains the pain caused in people

Question 17

how to treat erythromelagia

Answer 17

o Venlafaxine and gabapentin did not provide relief
o Less action potential with carbamazepine (CBZ)
o Something interesting about patient – warmth triggers episodes of pain SO:
o Used multielectrode array (grid of tightly spaced electrodes which capture electrical current) to see effect of CBZ when warm
o They checked electrical current in patients at 33 and 40 degrees without CBZ
o More electrical activity in patient at 40 degrees
o The electrical activity reduced when added CBZ for both temperatures

Question 18

what is small fibre neuropathy

Answer 18

o A disorder in which only the small sensory cutaneous nerves are affected
o The response to treatment is highly variable
o Initial clinical trials treating neuropathic patients => limited efficacy of lacosamide (Nav1.7 channel blocker)

o GAIN OF FUNCTION variant in Nav1.7

Question 19

Study on small fiber neuropathy

Answer 19

 Recruited only those with Nav1.7 variant
 Randomised Control Trial with placebo vs lacosamide (8 weeks)
 Placebo = 21.7% vs lacosamide = 58.3% decreased average pain
 Primary outcome = Efficacy = Proportion of patients with 1-point average pain score reduction compared to baseline

Question 20

Why only 58% of patients with lacosamide has a reduction in pain? Why not everyone?

Answer 20

A separate study tried to answer this question:
o They selected 2 Nav1.7 variants from responsive patients and 3 Nav1.7 variants from non-responsive patients
o Express the variants in cultured cells
o Did patch clamp to see effect of lacosamide on channel
o Lacosamide selectively enhances fast inactivation of the channel only in variants from responders
o That means lacosamide only works on certain variants
o Provides a possible explanation for why not all patients responded to lacosamide treatment

Question 21

Pharmacogenomics for neuropathic pain

Answer 21

Genetics of neuropathic pain:
Which gene variant is involved in neuropathic pain? Nav1.7 (gene name SCN9A)

Use of genetic information to target treatment:
o Erythromelalgia patients carrying the Nav1.7 S231T variant respond to carbamazepine
o Small fiber neuropathy patients carrying certain Nav1.7 variants respond to carbamazepine

Question 22

what is iPSC

Answer 22

o Pluripotent stem cells = undifferentiated cells that can differentiate into various types of cells

Question 23

how to induce iPSC

Answer 23

o For example, we take skin biopsy from patient
o Isolate fibroblasts (a type of cell)and reprogramme to induce state of non-differentiation
o Then we differentiate them into nociceptors
o We can test different drugs on these nociceptors and see what works
o Then give these drugs to patient

Question 24

Example of iPSC study

Answer 24

o 69-year-old female Norwegian Caucasian suffering from small fibre neuropathy for 10 years took part in study
o She had severe continuous burning pain
o Pain was most severe in the evenings => which caused insomnia
o Major impact on quality of life

Previous treatment
 Gabapentin – limited effect
 Pregabalin - limited effect
 Amitriptyline – discontinued due to side effects
 NSAIDs (acetylsalicylic acid) – ineffective

Did a genetic screening but found no variant

o In order to identify potential treatment options, they did iPSC
o Took a sample of her skin, did iPSC, differentiated cells into neurones
o They did patch clamp and multielectrode ray to test function of sensory neurones
o Compare iPSC from patient and control
o Then they tested a drug e.g. lacosamide

What did they find out?

Figure 3: used patch clamp – compare ipsc patient to control
o A more AP with increased voltage
o B showed that there’s no difference in threshold needed to generate AP in control and patient cell
o C higher percentage of spontaneous active neurons for SFN (patient) iPSC derived nociceptors

Figure 4: used multielectrode array
o Higher number of active neurons for SFN patient ipsc derived nociceptors
o Lacosamide inhibits the electrical activity of SFN patient iPSC derived nociceptors

So:
o They decided to give lacosamide to patient
o Reduction of pain, even after 6 months of treatment

Question 25

EMerging studies

Answer 25

iPSC is an emerging study - proof of concept | need to apply to a larger scale

Question 26

Patch clamp

Answer 26

The patch clamp technique is used to study ionic currents in individual isolated living cells

Question 27

Multielectrode array

Answer 27

A multi-electrode array is a grid of tightly spaced microscopic electrodes embedded in the bottom of each well in a multi-well MEA plate. MEAs record the electrical activity of neurons