Neuropathic pain

Question 1

Define pain

Answer 1

Pain is an unpleasant physical sensation associated with actual or potential tissue damage.

Question 2

Define Nociception

Answer 2

Nociception is the physical process of transduction that gives rise to pain. It is the sensory nervous system’s response to certain harmful or potentially harmful stimuli.

Question 3

Define nociceptors

Answer 3

Nocioreceptors are specialised sensory neurons that detect and respond to noxious stimuli. They have a protective role by triggering the sensation of pain.

Question 4

State the different types of nociceptors

Answer 4

Mechanical nocioreceptors (A-delta fibres

Thermal nocioreceptors (A-delta fibres)

Polymodal nocioreceptors (C-fibres)

Question 5

Describe Mechanical nocioreceptors

Answer 5

respond to strong mechanical stimulation such as pinching, crushing or cutting.

Question 6

Describe thermal nocioreceptors

Answer 6

detect extreme temperatures. Cold nociceptors are activated by very low temperatures. Heat nociceptors respond to potentially damaging high temperatures.

Question 7

Describe polymodal nocioreceptors

Answer 7

sensitive to multiple types of damaging stimuli, including mechanical, thermal and chemical nociceptive signals (Slow aches and pains)

Question 8

Define afferent neurons

Answer 8

Afferent neurons (sensory neurons): responsible for sensory and pain transmission from tissue and organs towards the CNS.

Question 9

Types of afferent neurons involved in pain transmission:

Answer 9

• A-delta fibres: These are myelinated, fast-conducting fibres that transmit sharp, immediate pain. (mechanoreceptor and nociceptor)
• C fibres: unmyelinated fibres  slow pain and aches. Stimulates deeper tissues (muscles). This pain takes longer to reach the brain. (nociceptor, thermoreceptor and mechanoreceptor)
• A-beta fibres: myelinated fibres  transmits low intensity stimuli (brush, touch). (mechanoreceptors)

Question 10

Define neuropathic pain

Answer 10

Pain caused by a lesion or disease (or dysfunction) of the somatosensory system (nervous system)

It involves abnormal processing of pain signals and pathways in the peripheral or CNS.

Question 11

Compare and contrast neuropathic pain and nociceptive pain

Answer 11

Neuropathic Pain is caused by a lesion or disease (or dysfunction) of the somatosensory system (nervous system)

It involves abnormal processing of pain signals and pathways in the peripheral or CNS.

Whereas, Nociceptive pain is caused by noxious stimulus (that may cause tissue damage)

Question 12

Describe possible causes for neuropathic pain

Answer 12

Diabetes: Can lead to diabetic neuropathy, where high blood sugar levels cause damage to peripheral nerves, especially in the legs and feet.

Erythromelalgia a genetic channelopathy caused by a gain of function Nav1.7 variant. leading to episodes of burning pain.

Postherpetic neuralgia: Follows a shingles outbreak, where the reactivation of the varicella-zoster virus damages nerve fibers, leading to long-term pain.

Question 13

Describe signs/symptoms of neuropathic pain

Answer 13

Burning or shooting pain: Often described as an electric shock-like sensation.

Numbness or tingling: Patients may experience these sensations in the areas affected by nerve damage.

Increased sensitivity to touch: A condition known as allodynia, where pain is caused by stimuli that normally do not provoke pain, such as light touches or clothing.

Stabbing sensation

Pins and needles

Question 14

define pharmacogenomics

Answer 14

Study of how a person’s unique genetic makeup (genome) influences their response to medications. This can be utilised in the treatment of neuropathic pain for more ‘personalised therapy’.

Question 15

Which gene variants contribute to neuropathic pain?

Answer 15

Variants in Nav1.7 (voltage gated sodium channel (gene name SCN9A)

Nav1.7 is predominantly expressed in the peripheral nervous system, particularly in the pain pathways of dorsal root ganglia (DRG) neurons.

Variants in the gene encoding Nav1.7 can significantly affect the channel’s function, potentially leading to altered pain perception.

Question 16

What is the function of Nav1.7

Answer 16

Nav1.7 is enriched in nociceptors. It has important functions in the initiation and proper firing of action potentials.

Question 17

Variants in the gene encoding Nav1.7

Answer 17

Variants in the gene encoding Nav1.7 can significantly affect the channel’s function, potentially leading to altered pain perception.

Gain of function variants in NAv1.7:
Erythromelalgia (IEM)
Small Fiber Neuropathy (SFN)
Painful Diabetic Neuropathy
Paroxysmal Extreme Pain Disorder (PEPD)

Loss of function Nav1.7 variants:
Congenital Insensitivity to Pain (CIP)

Question 18

Erythromelalgia (Man on fire syndrome)

Answer 18

Nav1.7 variant

This condition is characterised by episodes of burning pain, warmth, and redness in the extremities.

Question 19

Erythromelalgia symptoms

Answer 19

condition is characterised by episodes of burning pain, warmth, and redness in the extremities.

Question 20

What treatment does a specific subgroup of Erythromelalgia patients respond to?

Answer 20

patients carrying the Nav1.7 S241T variant respond to carbamazepine

Question 21

Erythromelalgia key research findings

Answer 21

Key Research Findings (Yang et al., 2012)
* Genetic Screening: Patients with erythromelalgia were found to have the S241T mutation in Nav1.7.
* Functional Assessment of S241T Variant:
o Human Nav1.7 channels, including both the wild-type (WT) and the S241T variant, were expressed in Dorsal root ganglion (DRG) neurons of mice.
o Patch-clamp techniques were used to assess the functional properties of these channels. Patch clamping is a laboratory technique that allows the study of single or multiple ion channels in cells.
* Outcome of Functional Assessments:
o Neurons expressing the S241T variant of Nav1.7 showed increased excitability compared to those with the wild type. This heightened excitability of DRG neurons can lead to increased pain perception, as seen in erythromelalgia.
Treatment Challenges: Patients with this specific genetic mutation often do not respond well to typical pain management strategies like venlafaxine and gabapentin, underscoring the need for targeted treatment options.

Question 22

current first line pharmacological treatment for neuropathic pain

Answer 22

Tricyclic antidepressants ( Amitriptyline)
Anticonvulsants (Gabapentin, Carbamazepine)
Serotonin-norepinephrine reuptake inhibitors (SNRI) (Duloxetine)

Question 23

Describe the pain pathway

Answer 23

Transduction:
Painful physical or chemical stimulus is transformed into a signal (action potential)

Afferent neurons (sensory neurons): responsible for sensory and pain transmission from tissue and organs towards the CNS.

Transmission of the action potential to the CNS (Dorsal horn in the spinal cord)
This action potential is propagated to the dorsal horn of the spinal cord.

Dorsal horn of the spinal cord:

When nociceptive afferent neurons (A-delta and C-fibres) are stimulated in the dorsal horn, 2 neurotransmitters are released:

Glutamate
Substance P

o Central processing
* Perception
* Modulation (dampening or amplification of the action potential)

Question 24

If first line treatment for neuropathic pain isn’t effective what is used/given?

Answer 24

Nortriptyline, Tramadol (Opioids), topical treatments (Capsaicin)

Question 25

What is the therapeutic recommendations for patients with the S241T variant of NaV1.7

Answer 25

Carbamazepine

Question 26

How was the effectiveness of carbamazepine identified?

Answer 26

o Electrophysiological Studies: Using a patch-clamp technique, researchers observed the effect of CBZ on neurons expressing the S241T mutation.
Results show how CBZ effectively reduces the number of action potentials in response to a stimulus.

Temperature-Specific Response: The study further evaluated the impact of CBZ at different temperatures using a multielectrode array (MEA) to measure neuronal activity.
CBZ was found to reduce electrical activity at higher temperatures, which are relevant to the symptoms experienced by erythromelalgia patients.

Therapeutic Recommendation: Based on the reduced neuronal excitability observed in laboratory settings, the researchers recommend administering CBZ to patients with the S241T mutation, noting significant relief from the intense pain symptoms associated with erythromelalgia.

Question 27

Which gene encodes NaV1.7

Answer 27

voltage gated sodium channel (SCN9A)

Question 28

Small fibre neuropathy

Answer 28

a condition characterized by damage to the small fibres of the nervous system, leading to severe pain, typically described as a burning sensation.

Small fiber neuropathy occurs when damage to the peripheral nerves predominantly or entirely affects the small myelinated (Aδ) fibers or unmyelinated C fibers.

Question 29

State a treatment for small fibre neuropathy for patients with a higher number of active neurons

Answer 29

Lacosamide - is known to modulate sodium channels

Lacosamide was found to inhibit the electrical activity of these nociceptors effectively

Question 30

why is Lacosamide effective for patients with a higher number of active neurons

Answer 30

Multi-electrode array recordings revealed a higher number of active neurons in cultures derived from the SFN patient compared to controls, confirming increased neuronal excitability in the disease model.

Treatment with Lacosamide, a drug known to modulate sodium channel activity, effectively reduced the abnormal electrical activity in SFN patient-derived neurons. This was evident as a decrease in both the frequency and intensity of neuronal firing.

These results collectively indicate that while the fundamental capacity to generate action potentials remains unchanged in SFN, the affected neurons exhibit heightened baseline activity. Lacosamide has shown effectiveness in dampening this excessive activity, aligning with its potential use in treating the symptoms of SFN.

Question 31

State the limitations of the current pharmacological treatment for neuropathic pain

Answer 31

Achieves only 30-50% reduction of pain severity.
Poor tolerability by patients (due to side effects)
Treatment response is highly variable between patients.

Hence the need for new therapies and patient stratification and personalised treatments…..

Question 32

describe a study (aims, methods, results) where new treatment strategies for personalised treatment of neuropathic pain was employed (for small fibre neuropathy)

Answer 32

Namer et al., 2019

Aims: to identify treatment options using induced pluripotent stem cells (iPSCs) from a patient’s skin biopsy for small fibre neuropathy

Methods:
Skin Biopsy and iPSC Generation: Fibroblasts were isolated from a skin biopsy of the patient and reprogrammed into iPSCs.
Differentiation: These iPSCs were then differentiated into sensory neurons/nociceptors, which are relevant to the patient’s condition.
Testing and Analysis: Through techniques like patch-clamp and multi-electrode arrays, these neurons were analyzed for electrical activity, particularly in response to potential treatments.

Results:
iPSC Derivation and Application:
Drug Testing: anticonvulsant drug Lacosamide, which is known to modulate sodium channels, was tested on the patient’s iPSC-derived nociceptors. Lacosamide was found to inhibit the electrical activity of these nociceptors effectively.

Clinical Outcome

• Treatment Administration: Based on the lab results, Lacosamide was administered to the patient.
• Effectiveness: The treatment led to a tremendous reduction in the patient’s pain, illustrated by a series of smiley faces representing the patient’s pain levels before and after treatment over a period of 6 months. The smileys indicate a significant improvement in pain and presumably quality of life.
  Patient’s pain tremendously reduced.

Question 33

describe a study (aims, methods, results) where new treatment strategies for personalised treatment of neuropathic pain was employed (for erthromyalgia)

Answer 33

Key Research Findings (Yang et al., 2012)
* Genetic Screening: Patients with erythromelalgia were found to have the S241T mutation in Nav1.7.
* Functional Assessment of S241T Variant:
o Human Nav1.7 channels, including both the wild-type (WT) and the S241T variant, were expressed in Dorsal root ganglion (DRG) neurons of mice.
o Patch-clamp techniques were used to assess the functional properties of these channels. Patch clamping is a laboratory technique that allows the study of single or multiple ion channels in cells.
* Outcome of Functional Assessments:
o Neurons expressing the S241T variant of Nav1.7 showed increased excitability compared to those with the wild type. This heightened excitability of DRG neurons can lead to increased pain perception, as seen in erythromelalgia.
Treatment Challenges: Patients with this specific genetic mutation often do not respond well to typical pain management strategies like venlafaxine and gabapentin, underscoring the need for targeted treatment options.

Carbamazepine (CBZ) Effectiveness:
o Electrophysiological Studies: Using a patch-clamp technique, researchers observed the effect of CBZ on neurons expressing the S241T mutation.
Results show how CBZ effectively reduces the number of action potentials in response to a stimulus.
Temperature-Specific Response: The study further evaluated the impact of CBZ at different temperatures using a multielectrode array (MEA) to measure neuronal activity.
CBZ was found to reduce electrical activity at higher temperatures, which are relevant to the symptoms experienced by erythromelalgia patients.

Question 34

Techniques to study the electrical activity of neurons

Answer 34

Patch-Clamp Technique and Multi-electrode Array (MEA)

Question 35

Describe the Patch clamp technique

Answer 35

Patch-Clamp Technique: method is used to study the electrical properties of neurons. It measures voltage changes or electrical currents that pass through the neuron’s plasma membrane, crucial for understanding how neurons communicate and how diseases might affect this communication.

Question 36

Describe the Multi-electrode Array (MEA):

Answer 36

Multi-electrode Array (MEA): technique allows for the recording of electrical activity from multiple points within a network of neurons simultaneously. It’s particularly useful for observing how neuronal activity is altered in disease states or in response to drugs.

Question 37

Patch clamp technique findings in SFN trial

Answer 37

• The electrical studies using the patch-clamp technique showed that neurons derived from a patient with Small Fiber Neuropathy (SFN) have the same action potential threshold as control neurons, indicating no difference in the basic mechanism of action potential initiation.
• However, a significantly higher percentage of SFN patient-derived neurons displayed spontaneous activity compared to controls. This suggests that these neurons are more likely to be inherently active, potentially contributing to the neuropathic pain experienced by patients.

Question 38

Multi-electrode Array findings in SFN trial

Answer 38

• Multi-electrode array recordings revealed a higher number of active neurons in cultures derived from the SFN patient compared to controls, confirming increased neuronal excitability in the disease model.
• Treatment with Lacosamide, a drug known to modulate sodium channel activity, effectively reduced the abnormal electrical activity in SFN patient-derived neurons. This was evident as a decrease in both the frequency and intensity of neuronal firing.

Question 39

what did trial results indicate for lacosamide

Answer 39

results collectively indicate that while the fundamental capacity to generate action potentials remains unchanged in SFN, the affected neurons exhibit heightened baseline activity. Lacosamide has shown effectiveness in dampening this excessive activity, aligning with its potential use in treating the symptoms of SFN.

Question 40

Generating induced pluripotent stem cells (iPSCs) from a patient’s skin biopsy

Answer 40

. Skin Biopsy: Starting Material: A small sample of skin is taken from a patient. This sample contains various cell types, including fibroblasts, which are commonly used because they are easy to culture and reprogram.
. Fibroblasts
* Isolation: Fibroblasts are isolated from the skin biopsy. These cells are then cultured in the lab to increase their number before the reprogramming process begins.

Reprogramming to iPSCs
* Process: The isolated fibroblasts are genetically reprogrammed to revert them to a pluripotent stem cell state.
* Result: The reprogrammed cells, now iPSCs, have the ability to differentiate into any cell type in the body, mimicking the capabilities of embryonic stem cells.
4. iPSC Culture
* Expansion and Maintenance: iPSCs are cultured under specific conditions that maintain their pluripotent state. They can be expanded to produce many more cells, all genetically identical to the original fibroblast cells but with the capability to differentiate into any cell type.
5. Differentiation
* Directed Differentiation: Through the application of various growth factors and signalling molecules in a controlled manner, iPSCs are directed to differentiate into the specific type of cells required for research or therapy. For instance, to study a neurological disease, iPSCs can be differentiated into neurons.
* Confirmation: The differentiated cells are typically confirmed through morphological assessment and the expression of cell-type-specific markers to ensure they have correctly transformed into the desired cell type.

6. Application in Drug Testing and Therapies
   * Drug Testing: The differentiated cells can be used in pharmaceutical testing to observe how drugs affect them, identifying potential treatments or side effects.
   * Regenerative Medicine

Question 41

What are fibroblasts (characteristics)

Answer 41

Characteristics: Fibroblasts are connective tissue cells which produce collagen and other fibres. They play a critical role in wound healing and are integral to the skin’s structure.