1. VOIE DE SIGNALISATION ET D'INTEGRATION DE LA DOULEUR VISCERALE
Symposium Mayoly sur le Syndrome de l'Intestin Irritable.
Organisateur: Pr. Boucekkine
VOIE DE SIGNALISATION ET D'INTEGRATION
DE LA DOULEUR VISCERALE
L. Buéno
Unité de Neurogastroentérologie INRA
Toulouse, France
Alger, 25 Avril 2010

2. Processing of Sensory Signals in Spinal Cord, Brain Stem, and Brain
Gracilis and Cuneatus
Brain Stem
Dorsal column nociceptive pathway
Lat. spinothalamic tract
Spinal Cord
Spinoreticular tract
Dorsal Root
Abdominal
Viscus

3. Ascending Pain Pathways
IBS - Ascending Visceral Pain Pathway
Ascending Pain Pathways

4. Descending Pain Modulation
ACC
Thalamus
PAG
Locus coeruleus
Caudal raphe nucleus
Amygdala
Noradrenergic
Rostral ventral medulla
Spinal visceral nociceptive transmission is subject to descending modulatory influences from supraspinal structures (e.g., periaqueductal gray, nucleus raphe magnus, locus ceruleus, nuclei reticularis gigantocellularis, and the ventrobasal complex of the thalamus). Descending modulation can be inhibitory, facilitatory or both depending on the context of the visceral stimulus or the intensity of the descending signal. Serotonergic, noradrenergic, dopaminergic and opioid projections are major components of descending modulatory pathways.
The descending influence from the ventromedial medulla is mediated mainly by pathways traveling in the dorsolateral spinal cord and can be inhibitory or facilitatory based on stimulus intensity. In contrast, descending control from the thalamus is context-specific in that it may facilitate or inhibit spinal nociceptive processing depending upon the presence or absence of central sensitization.
Aside from descending inhibitory influences, spinal visceral input is subject to tonically active supraspinal facilitating modulation, which may be a mechanism that enhances conscious perception of visceral sensations in the absence of noxious stimulation. Exaggeration of descending facilitating signals from the brain might also partly explain the visceral hypersensitivity that is found in a subset of IBS patients.
1. Zhuo M, Gebhart GF. Facilitation and attenuation of a visceral nociceptive reflex from the rostroventral medulla in the rat. Gastroenterology 2002;122:
Serotonergic
Test balloon
Opioid
Spinal cord
Spinal afferent
Rectosigmoid

5. IBS MCC pACC Brain Activation with Noxious Visceral Stimulation
Prefrontal Cortex
Cing Cx included
MCC
pACC
Thalamus
Pf, Re, Cl, Li
Locus coeruleus
Subnucleus reticularis dorsalis
Spinal cord
Lamina 1
IBS

6. Convergence of Somatic and Visceral Afferents in the Spinal Cord
Brain
Somatic afferent
Dorsal root ganglion
Spinothalamic tract
Visceral afferent
Gallbladder

7. Divergence of Somatic and Visceral Afferents in the Spinal Cord
(Wolf et al. 1965)

8. (mechanosensitivity)
Colonic Referred Pain
(mechanosensitivity)
Increased pain intensity

9. Mechanoreceptors of the Digestive tract - localisation
Vascular
(8/10 high threshold)
Serosal
(1/3 low threshold)
Tonic & Phasic
distensions
Tonic
Distension
(act in series)
Mucosal
(8/10 low threshold)
Phasic
Distension (act in parallel)

10. Possible sites of alterations in gut sensitivity1
Afferent nerve terminals
Brain 2
Dorsal horn + -
Raphe
Cortex
Limbic
system 3
Brain integrative nuclei
Nuclei 4
Diffuse noxious inhibitory system
5-HT
5-HT + -
Thalamus 4
Gut
Wall
Dorsal root
Ganglia 3
SPINAL +
--- SP
Nociceptive
stimulus
EAA
CORD - DH PN + - + 2 1

11. Spinal Gating for Three Classes of Visceral Nociceptors (Low, High and Silent) Account for Normal Regulatory Functions, Acute and Chronic Pain
Synaptic terminal, second order, active neuron
Synaptic terminal, spinal and inactive neuron
This slide illustrates the concept that changes in levels of firing in low- and high-threshold hold afferent and silent afferents underlie enteric sensory neurophysiology. Low-threshold afferents accounts for the sensory information necessary for central regulation of autonomic functions and for non-painful sensations from the digestive tract. Information coded by the low-threshold afferents is relayed by second order spinal neurons to processing centers in the brain. Normal sensory function without pain is associated with activation of these afferents. Stimulation of sufficient intensity (e.g., distension or strong muscular contaction) activates both low- and high-threshold mechanosensitive afferents and the second order spinal neurons that relay the information from both kinds of receptors to processing centers in the brain. The recruitment of second order high-threshold pathways is interpreted by the brain and relayed to consciousness as acute pain of variable intensity depending on the frequency of firing in the combined pathways. Mediators released by inflammatory/immune cells sensitize both low- and high-threshold afferents to fire at higher frequency than normal in response to a given stimulus strength. Mediators associated with inflammation also sensitize normally silent afferents. This leads to increased intensity of activity transmitted by spinal pathways to processing centers in the brain that may be projected to consciousness as chronic abdominal pain.
1. Modeled after: Cervero F, Janig W. Visceral nociceptors: a new world order? Trends Neurosci 1992; 15:
Threshold
Threshold
Threshold
Low
High
Silent
Low
High
Silent
Low
High
Silent
Normal sensation
(No pain)
High intensity
(Acute pain)
Inflammation
(Chronic pain)

12. Postsynaptic receptors at dorsal horn level: - NMDA, AMPA
- NK1, NK2, NK3
- CGRP1, SST3
- 5-HT1, 5-HT3
- opioid (µ,d,k)
- a2 adrenergic
- GABAA
Presynaptic receptors at
dorsal horn level:
- CCK
- GABAA
- 5-HT3
- µ,d,k
- a2 adrenergic
Mediators contained in visceral
primary afferents :
- glutamate, aspartate
- SP, NKA, NKB
- CGRP
- somatostatin
- VIP, galanin
(Bueno et al. 1996)

13. Receptors at nerve terminals of nociceptive fibers as putative
targets to reduce inflammation-induced hyperalgesia
Other putative targets
(spinal cord level)
TRPV1….5
mGluR1,5
Sub.P
CB1, CB2
2
opioid
Nociceptive terminal
NK1,2,3
CCK1
and….. others H+
ATP
NGF
IL-1ß
PGE2
Hist.
Tryp.
5-HT
Pressure
Heat
Bradykinin
Other putative targets
(periphery)
Tissue
damage
Mast
cell
LIF
IL6
TNFa
CRF
NGF
Proteases
CRF-R1, CRF-R2
PAR1, PAR2
TrkA, p75

14. Role of Mast cell in mucosal inflammation-induced
visceral hyperalgesia
Stress
Mast
cell
CRF
Degranulation
minutes
Allergy
Histamine
Leukotrienes
Cytokines*
proteases
NGF
IgE, SP,5-HT
PAF, ATP
Inflammation
hours
GM-CSF, IL-1
Secretion
Infection
NGF, NPY
Cytokines*
Chemokines
Sympathetic
activation *
Cytokines: TNFa, IL-3, IL-5, IL-4, IL-13, IL-10, GM-CSF
(adapted from Shakoory et al,2004, Penicci et al.2003)

15. Endothelium/ Epithelium Mast cell Eosinophil/ neutrophils
BIDIRECTIONAL REGULATION OF MAST-CELL-EOSINOPHIL FUNCTION SP
SCF
Endothelium/
Epithelium
IL-4
PAR2
tryptase
PAR2
Mast
cell
Cytokines
chymase
Eotaxin
PAR2
SCF
Cytokines
GM-CSF
Chemokines
IL-1 / TNF-a
CCR3
Eosinophil/
neutrophils
Adapted from Shakoory et al. 2004

16. P P P P ROLE OF MAST CELLS IN VISCERAL PAIN:
(degranulation in response to mechanical stimuli)
Sensitization
Density
Normal barosensitivity
Normal barosensitivity P P
Post-infection
Pain
Post-stress
Pain P P
MC degranulation
MC degranulation

17. Role of mast cell in favorising increase in colonic gut permeability
and subsequent increase in visceral sensitivity to distension
STRESS
CRF
Mast cell
activation
5-HT, tryptase
NGF
activation
Visceral
hyperalgesia
Cytokines
Chemokines
Inflammatory
mediators
sensory nerve
terminal
sensitization
T-lymphocyte
Neutrophils
TJ OPENING
IFN 
MLC
MLCK
MLCP
(Ait-Belgnaoui et al. Pain 2005)

18. Repetitive Stimulation Sensitizes the Spinal Cord
Dorsal root ganglion
Wind-up
Sensitized spinal circuits
Repeated balloon distention
Mechanosensitive afferent
Repeated inflation of a balloon to noxious levels in the recto-sigmoid region leads to lowered perception thresholds for discomfort and pain in IBS patients. This phenomenon is believed to be due to centrally mediated hyperalgesia. Centrally mediated hyperalgesia reflects hyperexcitability of second order sensory neurons in the dorsal horn of the spinal cord. Under conditions of severe and persistent nociceptive stimulation, small-diameter, nonmyelinated C fibers fire repetitively and their input progressively increases responses of second order dorsal horn neurons. “Wind-up” of the second order neurons is generated by the release of the neurotransmitter glutamate from the C fibers. Noxious stimulation of the type applied in balloon distention studies in IBS patients can produce long-term changes in the excitability of dorsal horn neurons. This phenomenon is called central sensitization to distinguish it from the sensitization that occurs at the endings of the sensory neurons in the intestinal wall during release of inflammatory mediators. Long-lasting changes in the excitability of dorsal horn neurons impart a memory for nociceptive input from C fibers. Upregulation in the expression of neuropeptides and neurotransmitters and their receptors presumably underlie the phenomenon of central sensitization. Elevated excitability in nociceptive dorsal horn neurons underlies spinally mediated hyperalgesia, which is termed central sensitization to distinguish it from sensitization that occurs at nociceptive terminals in the periphery. In conditions of severe tissue injury and persistent injury, nociceptive C fibers fire repetitively and the excitability of the second order neurons in the dorsal horn increases progressively in response to the elevated synaptic input. This effect is sometimes called wind-up and reflects the synaptic release of glutamate from the incoming C fibers and activation of N-methyl-D-aspartate (NMDA)-type glutamate receptors expressed by the second order neurons. One piece of suggestive evidence for wind-up hypersensitivity is the finding that second order neurons in the dorsal horn show induction of the early gene c-fos in response to noxious balloon distention of the colon in rats (96).
These long-lasting changes in the excitability of dorsal horn neurons are like a memory imprint of the nociceptive input. Accumulating evidence suggests that the spinal wind-up phenomenon may be operational and be an underlying factor in the intestinal hypersensitivity associated with IBS.
1. Basbaum AI, Jessell TM. The perception of pain. In: Principles of neuroscience 4th ed. Kandel ER, Schwartz JH, Jessell TM (eds.) New York: McGraw-Hill 2000:
2. Munakata J, Naliboff B, Harraf F, Kodner A, Lembo T, Chang L, Silverman DHS, Mayer EA. Repetitive sigmoid stimulation induces rectal hyperalgesia in patients with irritable bowel syndrome. Gastroenterology 1997;112:55-63.
3. Traub RJ, Pechman P, Iadarola MJ, Gebhart GF. Fos-like proteins in the lumbosacral spinal cord following noxious and non-noxious colorectal distention in the rat. Pain 1992;49:

19. MODULATION OF NEURONAL NOCICEPTIVE TRANSMISSION
AT SPINAL CORD LEVEL BY GLIAL CELLS
(from Svensson et al. 2010)

20. ? ? GLIAL CELL NEUROMEDIATORS INVOLVED IN
NOCICEPTIVE TRANSMISSION AT SPINAL CORD LEVEL ? ?
(from Svensson et al. 2010)

21. SPINAL MICROGLIA ACTIVATION IN STRESS -INDUCED VISCERAL HYPERALGESIA
Bradesi et al. 2009

22. SPINAL MICROGLIA ACTIVATION IN “NARCOTIC BOWEL SYNDROME”
Agostini et al (in press)

23. CONCLUSIONS
La douleur d'origine digestive est le plus souvent associée à une sensibilisation des mécano-récepteurs pariétaux principalement par les produits de dégranulation des mastocytes.
La présence d'une micro-inflammation associée à une redistribution des terminaisons est un facteur majeur de l'hypersensibilité viscérale
observée dans le SII.
La douleur viscérale chronique est déclenchée par à une hypersensibilité locale mais facilité et entretenue par une facilitation et amplification spinale des messages nociceptifs
Outre les facteurs locaux, les cellules gliales et en particulier la microglie participent à la facilitation spinale de cette transmission nociceptive