Pain Neurobiology Book

Question 1

Most pain neurons in the periphery are. . .

Answer 1

. . . C-type neurons

Question 2

Where are the 1st, 2nd, and 3rd order pain neurons?

Answer 2

1st order: Nucleus at the DRG, axon extending bidirectionally into the spinal cord and the periphery (for sensation). Synapses on 2nd order neuron dendrite and on motor neurons in the anterior horn, which result in reflexive contraction.
2nd order: Nucleus in the dorsal horn of the spinal cord, ipsilateral to the 1st order neuron body. Axon decussates and extends up to the 3rd order neuron in the thalamus. The firing of the 2nd order neuron is the most regulated step in nociception, with several interneurons and adjacent neurons and glia having important input.
3rd order: Nucleus in the thalamus. Relays the signal to the motor cortex.

Question 3

Major cellular component that is sensed by nociceptive neurons and initiates a pain signal

Answer 3

ATP (from cellular breakdown)

There are multiple receptors for ATP on 1st order nociceptive neurons, including P2X receptors, which are ligand-gated ion channels

Question 4

Two main signals produced in response to damage that promote 1st order nociceptive neuron sensitization and firing

Answer 4

1. Bradykinin - Produced by ___. Downstream signaling results in sodium channel phosphorylation. Also activates phospholipases that mobilize arachadonic acid for prostaglandin synthesis.
2. Nerve growth factor (NGF) - Has important roles in neurogenesis during development, but in a growth organism this role is repressed and instead NGF is re-purposed as a pain signaling molecule. NGF activates the RTK TrkA on 1st order nociceptive neurons and mast cells. This results in the release of histamine and serotonin.

Question 5

Vasoactive signals produced in response to acute injury

Answer 5

CGRP (calcitonin gene-related peptide) and Substance P.

Both increase vascular permeability, resulting in swelling and mast cell recruitment. The recruitment of mast cells and other immune and non-immune inflammatory cells will result in the production of further mediators (bradykinin, NGF).

Question 6

Effects of peripheral sensitization by bradykinin and NGF

Answer 6

Allodynia and hyperalgesia

Question 7

Main sodium channels in C-type 1st order nociceptive neurons

Answer 7

NaV1.7 channels make up the majority

A minority are NaV1.8, which are interesting in that they are not affected by tetrodotoxin.

Question 8

Sensation of temperature and thermal pain

Answer 8

Mediated by the TRPV (transient protein receptor vanilloid) channels. They are temperature-gated ion channels that traffic calcium, but also have many ligands (like capsaicin or menthol, but also bradykinin and NGF) that result in opening.

Increased temperature in certain ranges above 104 F results in increased open probability and depolarization.

Question 9

Most pain signaling in 1st, 2nd, and 3rd order pain neurons utilizes ___ as the principal neurotransmitter

Answer 9

Most pain signaling in 1st, 2nd, and 3rd order pain neurons utilizes glutamate as the principal neurotransmitter and AMPA (ligand gated Na channel) as the principal receptor

Question 10

Long term potentiation in the setting of 2nd order pain neurons

Answer 10

This is the result of NDMA receptor signaling (as opposed to AMPA signaling. NDMARs are Ca channels while AMPARs are Na channels).

In response to a depolarization, NDMARs are opened and an inhibitory Mg molecule is released from the Ca channel pocket. This increases the sensitivity of the 2nd order neuron by freeing the channel of Mg.

The Ca also activates Ca-dependent kinases which phosphorylate downstream targets that increase sensitivity. This is the early phase of long-term potentiation. When kinase activity is prolonged, kinases may activate nuclear targets that increase the synthesis of ion channels and receptors. This results in chronically increased sensitivity, or late-phase long-term potentiation.

Question 11

Long-term hyper-excitability

Answer 11

An important mechanism for many types of chronic pain disorder.

LTH is the result of a signaling pathway involving activation and retrograde transport of PKG1α from the axon to the cell body of a 1st order nociceptive neuron. A large enough injury results in nerve damage or in Ca signaling along the axon. This Ca activates an enzyme cascade which results in the production of PKG1α in its active form, also exposing a retrograde transport motif. Once it arrives back at the cell body, it initiates a signaling cascade that changes transcription to induce LTH.

Question 12

NGF in long-term pain signaling

Answer 12

In addition to its role in local sensitization, NGF possesses the ability to alter 1st order nociceptive neuron transcription.

When NGF binds its receptor TrkA on a neuron axon, the active TrkA-NGF complex is internalized in a vesicle that is retrograde trafficked along the axon.

Once at the cell body, this signaling increases neuronal expression of bradykinin receptors, voltage-gated sodium channels, voltage-gated calcium channels, TRPV1, substance-P, and CGRP. The result is potent sensitization of the neuron to a variety of pain signals. Furthermore, the increase in these protein levels may last for months.

Question 13

Neuropathic pain

Answer 13

Neuropathic pain results from some form of neuron injury.

Ca enters the cell and activates the LTH response, but also activates a regenerative response at the cut site of the neuron. This results in the formation of a neuroma (of varying size). These neuromas have a very high expression of all the nociceptive machinery, and as such they are highly susceptible to pain signals.

Question 14

Central pain

Answer 14

Pain originating in a second or third order nociceptive neuron.

This type of pain is the cause of the phantom limb phenomenon.

Question 15

Role of inflammation in pain

Answer 15

IL-1b increases the production of substance P and COX.

TNFa creates a positive reinforcement loop and also results in production of NGF, thereby invoking all of NGF’s downstream effects.

Question 16

Ectopic pain

Answer 16

Name for pain that originates due to receptor activation along the length of the axon rather than at the axon’s terminus.

May also lead to pain mislocalization or referred pain.

Question 17

Dual innervation of visceral surfaces

Answer 17

The parietal wall of the sacs is innervated by branches of the spinal nerves, which can carry nociceptive information.

The visceral wall of the sacs is innervated by visceral nerves, which can carry nociceptive information, but often result in referred pain.

Question 18

Why do visceral nociceptive neurons result in referred pain?

Answer 18

Because they do not have their own dedicated 2nd order nociceptive neurons. Instead, they rely on the closet set of 2nd order SOMATIC nociceptive neurons to where their roots originate and have nuclei in the DRG.

Question 19

Three classes of endorphin-like molecule

Answer 19

Endorphins
Enkephalins (tetrapeptides)
Dynorphins

Question 20

Periaqueductal Gray neurons and their regulation of pain

Answer 20

The periaqueductal gray matter contains neuron cell bodies of regulatory nerve fibers that synapse on 2nd order nociceptive neurons in the dorsal horn.

These neurons are responsible for incorporating modulatory signals from the CNS as well as feedback inhibition from 2nd order nociceptive neurons. They include neurons which respond to norepinephrine, serotonin, and enkephalins.

Question 21

Location of mu opioid receptors

Answer 21

Mu opioid receptors are located on the presynaptic terminal of the 1st order nociceptive neuron. Activation of these receptors results in hyperpolarization of the terminal and inhibition of calcium channels, both of which inhibit the ability of the terminal to activate the post-synaptic 2nd order nociceptive neuron

Question 22

GABA signaling and pain

Answer 22

1st order nociceptive neurons also display receptors for GABA on their pre-synaptic terminals.

GABA signaling enables Cl- to enter the cell, resulting in hyper-polarization and inhibiting depolarization and associated signal transduction to the 2nd order nociceptive neuron.

Unlike many other modulators of this junction, these GABAergic neurons do not have their nuclei within the periaqueductal gray matter.

Question 23

Norepinephrine signaling and pain

Answer 23

1st order nociceptive neurons display adrenergic receptors on their pre-synaptic terminals.

Norepinephrine binding results in downstream inhibition of Ca channel activity, resulting in a diminished ability to activate 2nd order nociceptive neurons.

Question 24

Serotonin signaling and pain

Answer 24

1st order nociceptive neurons display numerous 5HT receptors on their pre-synaptic terminals.

Unlike with norepinephrine and GABA, the signaling with serotonin is quite complex as there are 12 distinct receptors with different functions.

However, as a general rule, serotonin in the synaptic space decreases the ability of 1st degree nociceptive neurons to activate 2nd degree nociceptive neurons, and thus serotonin is considered an analgesic signal.

Question 25

Cannabidiol receptors

Answer 25

CB1 and CB2. These are the main targets of THC.
Anandamide is an endogenous ligand for the CB receptors.

The CB1 receptor is distributed throughout the nervous system while the CB2 receptor is primarily found in the periphery. The CB1 receptor localizes to the periaqueductal gray, DRG, and dorsal spinal cord – co-localizing with many regions important for pain regulation.

These receptors regulate GABAergic neurons and may also modulate the 1st-2nd order nociceptive neuron synapse.

Question 26

Function of Anandamide-CB1 signaling

Answer 26

Anandamide is synthesized in the post-synaptic neuron (here 2nd or 3rd order nociceptive neuron) in response to intracellular Ca. Thus, it is primarily released when the post-synaptic neuron depolarizes.

Anandamide enters the synaptic space and binds to CB1 receptors on the pre-synaptic neuron (retrograde signaling). This binding results in the inhibition of voltage-gated Ca channels on the pre-synaptic neuron, suppressing this neuron’s ability to activate the post-synaptic neuron again.

Anandamide is then removed from the synaptic space by a high-affinity transporter and degraded by fatty acid amide hydrolase.

Question 27

Function of Anandamide-CB2 signaling

Answer 27

CB2 is located on many immune cells and has anti-inflammatory effects.

Anandamide in this context is synthesized by 1st order nociceptive neurons close to their site of sensation. Production of anandamide is stimulated by 1st order nociceptive neuron Ca mobilization. Anandamide is then released into the interstitial space, where it binds CB2 receptors on immune cells.

Interestingly, COX-2 inactivates anandamide in this context.

Question 28

Effects of cannabidiol

Answer 28

Cannabidiol, also found in cannabis, actually blocks binding to CB1 and CB2 receptors. As such, it has no psychoactive or hallucinogenic effects.

This would make you suspect that it increases nociception, however it also inhibits fatty acid amide hydrolase and desensitizes TRPV1 channels, and blocks the reuptake of numerous neurotransmitters: noradrenaline, seortonin, dopamine, and GABA. This makes its effects on pain similar to those of tricyclic antidepressants, which similarly block the reuptake of several neurotransmitters.

Question 29

Pain asymbolia

Answer 29

Dissociation between the awareness of an injury that should cause pain and the experience of this pain.

Observed in some patients following lobotomy.

Question 30

Pain asymbolia

Answer 30

Dissociation between the awareness of an injury that should cause pain and the experience of this pain.

Observed in some patients following lobotomy.

Question 31

Anterior cingulate cortex

Answer 31

Region of cortical brain located on the medial surface of each hemisphere, adjacent to the corpus callosum.

Functions to make us aware of various stimuli coming from elsewhere in the brain, as well as making us aware of pain.

Patients with intractable pain who received cingulotomies reported immediate relief. Specifically, they describe being aware of the pain, but that it was no longer bothersome – they could ignore it.

There are projections from the third order nociceptive neurons of the thalamus to the ACC, and it is believed that this links the somatosensory and affective pain circuits.

Question 32

Nucleus accumbens and pain

Answer 32

The nucleus accumbens, the heart of our pleasure and reward circuits, also plays a role in our perception and awareness of pain.

The nucleus accumbens contains projections to the anterior cingulate cortex and the amygdala, which may reduce the perception of pain when the reward/outcome for experiencing the pain is deemed sufficient.

Question 33

The amygdala and pain

Answer 33

The amygdala connects to both the anterior cingulate cortex which regulates pain perception and the periaqueductal gray neurons of the spine which regulate pain sensation.

This enables the amygdala to increase or decrease both sensation and perception in the setting of a fight or flight response and the emotions of fear, anxiety, or anger.

Question 34

Pain-offset relief

Answer 34

Describes the lack of pain and, in fact, pleasure which may be derived from self-harm in the setting of non-suicidal self injury (NSSI).

Likely due, in part, to the activation of opioidogenic neurons in the periaqueductal gray matter.

Question 35

Salience network

Answer 35

Network of connections between the insula cortex and the anterior cingulate cortex which evaluates the relative significance of different sensations.

These connections determine which sensations/perceptions are at the center of our attention at any given moment.

Question 36

Pain regulation by the prefrontal cortex

Answer 36

The prefrontal cortex inputs context and cognition into the pain circuit.

Neurons from the prefrontal cortex radiate to the anterior cingulate cortex, periaqueductal gray matter, and nucleus accumbens.

The PFC is involved in the mechanism of the placebo effect, the effect of masochism on reducing pain perception, and the effect of religiosity on reducing pain perception. Generally, belief and association have their input via this pathway.

Question 37

Naloxone and the placebo effect in pain

Answer 37

Fascinatingly, the placebo effect’s influence on pain perception disappears in patients who receive naloxone.

This makes sense mechanistically, as inputs from the prefrontal cortex and periaqueductal gray matter rely upon opioidergic neurons.