Chapter 2 Neurochemistry of Somatosensory and Pain Processing

Question 1

What are the mediators of “inflammatory soup” ?

Answer 1

Bradykinin, Low pH, Serotonin, Histamine, Eicosanoids (the prostaglandins, thromboxanes, and leukotrienes.), Nitric Oxide, Adenosine, Cytokines,

Question 2

What is Bradykinin?

Answer 2

a potent vasodilating peptide, plays a critical
role in inflammatory pain and hyperalgesia via actions on two G-protein–coupled receptors: the constitutively expressed B2 receptor, and the B1 receptor, the expression of which is increased following tissue injury

Question 3

Following injury, how is bradykinin released?

Answer 3

Following injury, bradykinin is released by kininogens and produces acute pain by activation of unmyelinated and myelinated nociceptors.
Bradykinin also produces transient heat hyperalgesia in humans by sensitization of nociceptors through activation of phospholipase C (PLC), protein kinase C (PKC), the production of eicosanoids and nitric oxide (NO), and modulation of the TRPV1 (VR1) channel

Question 4

How does Low pH (Excess free H) contributes to the pain and hyperalgesia?

Answer 4

Low pH selectively causes activation and sensitization of nociceptors to mechanical stimuli by opening dorsal root ganglion neuron specific acid-sensing ion channels

Question 5

How is Serotonin released?

Answer 5

Serotonin is released from platelets in response to platelet activating factor derived from mast cell degranulation, leads to pain by directly activating nociceptors. Serotonin also potentiates bradykinin induced pain and nociceptor activation.

Question 6

How is Histamine released?

Answer 6

Histamine is released from mast cells by Substance P and calcitonin gene–related peptide (CGRP). These neuropeptides are derived from activated nociceptors and produce a variety of responses, including vasodilation and edema.
histamine excites polymodal visceral nociceptors and potentiates the responses of nociceptors to bradykinin and heat.

Question 7

What are Eicosanoids?

Answer 7

Eicosanoids are a large family of arachidonic acid metabolites that include the prostaglandins, thromboxanes, and leukotrienes.

Question 8

How do Eicosanoids work?

Answer 8

Eicosanoids directly activate articular afferents

and sensitize these, as well as those in skin and viscera, to natural stimuli and other endogenous chemicals

Question 9

How do Prostaglandins work?

Answer 9

Prostaglandins, synthesized by the constitutive enzyme, COX-1, and by the inducible enzyme COX-2,13 reduce the activation threshold of tetrodotoxin-resistant Na1 currents in nociceptors, increase intracellular cAMP levels, and increase the excitability of sensory neurons

Question 10

How do Leukotrienes work?

Answer 10

Leukotrienes, metabolites of the lipoxygenase pathway, are released by macrophages and mast cells, contribute to hyperalgesia and sensitization to mechanical stimuli by acting on G-protein–coupled receptors (GPCR) and by serving as chemoattractants for cytokine-producing cells,
and result in further sensitization of primary afferents.

Question 11

How does Nitric Oxide work?

Answer 11

Nitric oxide (NO) released by damaged afferents and acting on soluble guanylyl cyclase (sGC) can further sensitize nearby neurons, augmenting pain and inflammation in both GPCR and non–GPCR-mediated pathways.

Question 12

What is the role of Adenosine?

Answer 12

Adenosine and its mono- and poly-phosphate derivates (AMP, ADP, ATP) are increased in the extracellular space with tissue injury and inflammation.
Adenosine induces pain in humans by direct activation of nociceptors. ATP also induces pain in
humans and activates C-nociceptors in healthy human skin, but does not sensitize C fibers to mechanical or heat stimuli. It is thought that ATP activates nociceptive neurons in normal skin via the purinergic receptors P2X3 and the heteromeric P2X2/P2X3 receptor

Question 13

What are Cytokines?

Answer 13

Cytokines (e.g., interleukin-1b (IL-1b); tumor necrosis factor a (TNFa); interleukin-6 (IL-6)) are released by a variety of cells, such as macrophages, astrocytes, and Schwann cells, to regulate inflammatory cell responses, but also promote pain signaling.
Both IL-1b and TNFa directly excite and sensitize nociceptive afferent fibers to thermal and mechanical stimuli
IL-6 in combination with its soluble IL-6 receptor also sensitizes nociceptors to heat.

Question 14

How do Excitatory amino acid (EAA) receptors play a role in the modulation of nociception?

Answer 14

the presence of such receptors on dorsal root ganglion cells and on the presynaptic terminals of primary afferents

Question 15

How does Nerve growth factor (NGF) may contribute to inflammatory pain ?

Answer 15

Nerve growth factor (NGF) may contribute to inflammatory pain via direct and indirect mechanisms. Inflammatory mediators, such as cytokines, increase NGF production in inflamed tissues. In turn, NGF stimulates mast cells to release histamine and serotonin, which can sensitize primary afferent fibers. Further, NGF itself
may directly sensitize nociceptors and can alter the distribution of A-d fibers such that a greater proportion of fibers have nociceptor properties. Heat hyperalgesia can be induced by NGF acting directly on the peripheral terminals of primary afferent fibers.

Question 16

What are Proteinases?

Answer 16

Proteinases such as thrombin, trypsin, and tryptase, although not traditionally considered part of the inflammatory soup, are mediators of pain and inflammation for their actions on proteinase-activated receptors (PAR).

Question 17

What occurs when proteinase-activated receptors (PAR) are activated?

Answer 17

Activation of PAR1 via thrombin leads to the
release of histamine, substance P, CGRP, and cytokines.
Activation of PAR2 by trypsin and tryptase creates a cascade of inflammatory reactions, including prostaglandin and bradykinin release, which would further sensitize unmyelinated primary afferents. The net effect of activation of these receptors is sensitivity to both mechanical and thermal stimuli

Question 18

What are the numerous mediators released into inflamed or injured tissue that act to limit pain transmission?

Answer 18

Opioids, Acetylcholine, Gamma amino butyric acid (GABA), Somatostatin (SST)

Question 19

How do Opioids work?

Answer 19

The peripheral terminals of afferent fibers contain receptors for opioids, but the number of receptors present is upregulated during inflammation. Further, inflammatory cells such as macrophages,
monocytes, and lymphocytes induced by interleukin 1b and corticotropin-releasing hormone (CRH) originating from the inflamed tissue may serve to increase the amount of endogenous opioids in the tissue.
Peripheral endogenous opioids may also be activated by endothelin-1 (ET-1), which is a potent vasoactive peptide, synthesized and released by epithelia after tissue injury

Question 20

How does Acetylcholine work?

Answer 20

Acetylcholine is released into injured tissue from non-neuronal sources and modulates pain via its effects on nicotinic or muscarinic receptors. Nicotinic agonists have weak excitatory effects on C-nociceptors and induce a mild sensitization to heat but no alterations in mechanical responsiveness.
In contrast, muscarinic agonists desensitizes C-nociceptors to mechanical and heat stimuli.

Question 21

How do Gamma amino butyric acid (GABA) work?

Answer 21

Gamma amino butyric acid (GABA) may have a peripheral role in pain transmission similar to the bimodal actions of acetylcholine.
GABAA receptors are located on unmyelinated
primary afferents and activation of these receptors by low doses of the agonist muscimol decrease pain, whereas high doses potentiate pain.

GABAA receptors have also been found in DRG cells and on their central terminals in the dorsal horn, and direct application of GABA antagonists
to DRG cells decrease hypersensitivity in an animal model of neuropathic pain

Question 22

What is Somatostatin?

Answer 22

Somatostatin (SST) is a peptide commonly associated with the GI system that may also serve as an antinociceptive agent.

Question 23

Peripheral Second Messenger Pathways

Answer 23

Inflammation is associated with the release of a host of chemical mediators
These agents may mediate pain by directly activating nociceptors or they may also produce more enduring changes in the sensory neuron, such as early post-translational changes or even longer-lasting transcription-dependent changes in effector genes in DRG cells

Question 24

How does Peripheral Second Messenger work?

Answer 24

The early post-translational changes include phosphorylation of transducer molecules (e.g., VR1 receptor) and voltage-gated ion channels (e.g., sodium channels) in the peripheral terminals
of nociceptors (peripheral sensitization).

Question 25

Peripheral Second Messenger Pathways

Answer 25

Inflammatory mediators, such as bradykinin and NGF, lower the threshold of TRPV1-mediated, heat-induced currents in DRG neurons and increase the proportion of DRG cells that respond to capsaicin. These changes occur by phospholipase C (PLC)–dependent phosphorylation by protein kinase (PKC), by phosphorylation by protein kinase A (PKA) and by hydrolysis of the membrane phospholipid, phosphatidylinosital-4-5-biphophate (PIP2).
PKA and PKC also induce a short-term sensitization of nociceptors to heat by modulating the activity of tetrodotoxin-resistant sodium currents.

Question 26

How are the anterolateral and dorsal column-medial lemniscal pathways that mediate pain transmission throughout the CNS similar?

Answer 26

Both systems involve three classes of transmitter
compounds, excitatory neurotransmitters, inhibitory
neurotransmitters, and neuropeptides that are found
in three anatomical compartments: sensory afferent terminals, local circuit terminals, and descending (or ascending) modulatory circuit terminals.

Question 27

What are the Excitatory Neurotransmitters?

Answer 27

The amino acids glutamate and aspartate constitute the main excitatory neurotransmitters found at synapses throughout the somatosensory system.

Question 28

What are the four receptor types for glutamate and aspartate in the somatosensory system?

Answer 28

These receptors are named for the synthetic agonists that best activate them; they include the N-methyl-D-aspartate (NMDA),53 the kainate, the AMPA ((R,S)-a-amino-3-hydroxy-5-methlyisoxazole-4- propionic acid) receptors, and the metabotropic receptors.

Question 29

How is the NMDA receptor activated?

Answer 29

The NMDA receptor is recruited only by intense
and/or prolonged somatosensory stimuli that are sufficient to relieve the tonic magnesium block that regulates its divalent cation channel. Persistent activation of NMDA receptors leads to sensitization of dorsal horn neurons that includes an increase in receptive field size, decreased activation threshold, and prolonged depolarization.

Question 30

Multiple factors influence NMDA receptor–related sensitization. What is the effect of Bradykinin on the receptor?

Answer 30

the release of bradykinin leads to increases
in spinal glutamate released by astrocytes and
neurons. This glutamate activates NMDA receptors,
augmenting central sensitization.

Question 31

FIGURE 2-2 Schematic diagram of the neurochemistry of somatosensory processing in the spinal dorsal horn.

Answer 31

See Diagram on page 11

Question 32

What is the role of Adenosine triphosphate (ATP) in somatosensory transmission?

Answer 32

Adenosine triphosphate (ATP) also modulates somatosensory transmission. The primary receptor for ATP is the P2X family of receptors which are present on the central terminals of primary afferent fibers innervating neurons in lamina V and II of the dorsal horn where they function to increase the release of the glutamate. The binding of ATP to P2 receptors on microglia changes the phenotype of these cells to include increased expression of P2 and cytokine receptors. These now activated microglia begin to secrete inflammatory mediators such as cytokines, nerve growth factor, and NO. These factors serve to sustain pain and inflammation.

Question 33

What are the inhibitory neurotransmitters in the somatosensory system?

Answer 33

The amino acids glycine and gamma-amino-butyric acid (GABA) are the chief inhibitory neurotransmitters in the somatosensory system.
Glycine is the chief inhibitory amino acid at spinal levels while GABA predominates at higher levels.

Question 34

Where are the receptor sites for Glycine and GABA?

Answer 34

Two receptor sites for glycine, a chloride-linked, strychninesensitive receptor and a strychnine-insensitive regulatory site on the NMDA glutamate receptors. GABA is found in
local circuit neurons of spinal laminae I, II, and III.

Question 35

What are the three types of GABA receptors identified?

Answer 35

GABA-A receptor is linked to a chloride channel and modulated by barbiturates, benzodiazepines and alcohol. Selective GABAA agonists include muscimol and selective antagonists include gabazine.

GABA-B receptor has been associated with both a potassium ionophore and with a G-protein–linked complex. Baclofen is a selective GABAB receptor agonist and phaclofen is a selective antagonist.

GABA-C receptor has also been described as associated with a potassium channel ionophore.Cis-4-aminocrotonic acid (CACA)
is a selective agonist for this site, but there is no selective antagonist for GABAC receptors.present. GABAC receptors do not appear to have any role in the modulation of somatosensory information.

Question 36

What is the role of Norepinephrine in somatosensory transmission?

Answer 36

Norepinephrine is another abundant inhibitory neurotransmitter, and is especially important in descending brainstem projections to the dorsal horn. The inhibitory effects of norepinephrine in the spinal cord appear to be twofold by directly activating inhibitory GABAergic interneurons and by also inhibiting excitatory interneurons.The adrenergic receptors include two broad classes
termed the alpha- and beta-receptors, each of which in turn have several subtypes. The a2-adrenergic receptor is the primary form found in the spinal dorsal horn that has an inhibitory role on the processing of sensory information.

Question 37

What is the role of Serotonin in somatosensory transmission?

Answer 37

Serotonin is also involved in descending pathways to the spinal dorsal horn, predominantly from the midbrain raphe nuclei. There are multiple serotonin receptor subtypes including 5HT-1, 2, and 3 receptors
Currently, it is thought that the antinociceptive effects are mediated by activation of a-1
adenoreceptors and 5HT2 receptors leading to descending inhibition.

Question 38

What is the role of Adenosine in somatosensory transmission?

Answer 38

Adenosine is another important inhibitory neurotransmitter at spinal levels.
There are at least two types of adenosine
receptors termed the A1 and A2 sites. Occupation of these sites by adenosine results in G-protein–mediated alterations of cyclic AMP levels in target cells. Adenosine may mediate a portion of the analgesia produced by brainstem norepinephrine projections to the spinal cord and appears to have especially robust analgesic properties in neuropathic pain conditions.

Question 39

What is the role of Acetylcholine in somatosensory transmission?

Answer 39

Acetylcholine (Ach) is yet another neurotransmitter that mediates antinociception at the level of the spinal dorsal horn. Stimulation of the vagus nerve results in inhibition of pain transmission, and it is likely that this effect is mediated by Ach. Ach may also contribute to the analgesia produced by the a2-adrenergic receptor agonist clonidine.
The antinociceptive effects of acetylcholine appear mediated by the muscarinic and not by the nicotinic acetylcholine receptor subtypes.

Question 40

What are the excitatory neuropeptides in the somatosensory system?

Answer 40

Substance P and neurokinin A serve as excitatory neuropeptides in the somatosensory system. The receptors for these peptides include the neurokinin 1 and 2 sites, each of which have been associated with elevation of intracellular calcium levels, perhaps through liberation of inositol phosphate.

Question 41

Where are the excitatory neuropeptides in the somatosensory system located?

Answer 41

These two peptides may be present in intrinsic neurons of the spinal dorsal horn and thalamus
but are especially concentrated in primary afferent
fibers. At the spinal level, these peptides are only released following application of noxious stimuli which are sufficient to produce sustained discharges in C-nociceptors, although some small myelinated (Ad) fibers may also contain substance P.

Question 42

What causes increase in release of substance P?

Answer 42

CGRP like substance P synthesis and release are increased by another excitatory peptide, neuropeptide Y. Spinal release of CGRP has an excitatory effect on wide dynamic range neurons,

Question 43

What are the other peptide that have a role in the somatosensory system?

Answer 43

Cholecystokinin (CCK) is a hormone peptide normally involved in digestion; however, it is also involved in the maintenance of pain.

Somatostatin, the enkephalins, and possibly dynorphin, are included as inhibitory neuropeptides at spinal level. These peptides are contained in both intrinsic neurons of the dorsal horn and in the fibers descending to the dorsal horn. from various brainstem nuclei.

Cannabinoids are present in the peripheral and central nervous systems and play a role in inhibiting pain.

Question 44

What are Peroxisome Proliferator-Activated Receptors (PPARs)?

Answer 44

the PPARs represent a class of nuclear receptors
which serve as transcription factors. PPAR stimulation plays an important role in suppressing inflammation,(inflammatory substances such as substance P, CGRP, and cytokines) In turn, mediation of these and other factors allows for inhibition of inflammation and pain and PPAR agonists have been shown to inhibit the development of pain but 
the serious side effects of increased adiposity and fluid retention.

Question 45

What are the four ion channels involved in pain

signal propagation in the CNS?

Answer 45

Sodium, Calcium, Potassium, and Chloride.

Question 46

What is the role of Na channels in pain

signal propagation in the CNS?

Answer 46

Sodium channels serve as the key to propagation of neural impulses throughout the nervous system, as the opening of these channels is the primary event underlying the depolarization of nerve membranes.
The local anesthetics lidocaine and bupivacaine physically block sodium channels, preventing the movement of sodium across the membrane.

Question 47

What is the role of K channels in pain

signal propagation in the CNS?

Answer 47

Potassium is the second main cation of the neuronal action potential.Opening of voltage-gated potassium channels allows outward positive current flow from neurons, such as during repolarization following an action potential. Blockade of
these channels initially prolongs generation of action potentials.
Continued application, however, prevents repolarization and so ultimately produces a failure to generate action potentials. The inwardly rectifying channels establish and regulate the resting membrane potential.

Question 48

What is the role of Ca channels in pain

signal propagation in the CNS?

Answer 48

Calcium ions are not directly involved in action potential propagation, but instead are essential for the release of neurotransmitters following synaptic depolarization. At least four different types of calcium channels, the L-, N-,T-, and P-types, have been identified in dorsal horn neurons.

Question 49

What is the role of Cl- channels in pain

signal propagation in the CNS?

Answer 49

Chloride ions are also a major contributor to signal
propagation, and three major classes of chloride channels have been identified. 
The first class identified was the ligand-gated chloride channels, including those of the gamma-aminobutyric acid type A (GABAA) and glycine
receptors, and these are common in dorsal horn neurons.
The second class, also likely common at spinal levels, is the voltage-gated chloride channel. 

The final chloride channel class is activated by cyclic adenosine monophosphate and may include only the cystic fibrosis transmembrane regulator.

Activation of chloride currents usually results in hyperpolarization of neurons, and facilitation
of these hyperpolarizing currents underlies the mechanisms of many depressant drugs. However, the GABAA receptors on primary afferent terminals gate a chloride channel that allows efflux, instead of the normal influx, of chloride with a net effect therefore of depolarizing primary afferent terminals.