1 Neural mechanism of painMEDI6100 Sydney Broome Fremantle

2 Post-doctoral pain research fellowProf Eric Visser Dr Natalie Morellini Post-doctoral pain research fellow

3 Student learning outcomesDescribe 'pain pathways' in depth Outline central nervous system [CNS] control over pain perception See learning suggestions at the end of this slide show Main reference to read Schug SA, et al. APM:SE Working Group of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine (2015), Acute Pain Management: Scientific Evidence (4th edn) ANZCA & FPM, Melbourne, pages 1-7

4 EJ Visser Churack Chair UNDA 2017 copyrightWhat is pain? ‘Pain is an unpleasant sensory & emotional experience associated with actual or potential tissue damage’ (IASP 2012) Pain is generated by the conscious brain in response to perceived tissue damage Pain is more than just a sensation Pain is a subjective, multidimensional, whole-person experience Pain is what the person-in-pain says it is Pain always occurs in a ‘context’ -bio-medical-psycho-social-environmental EJ Visser Churack Chair UNDA 2017 copyright

5 Nociception ‘The process of encoding & transmitting noxious stimuli in the nervous system’ (IASP 2012) Converting the ‘energy’ released by tissue damage into electro-chemical signals in the nervous system -chemical (‘inflammatory soup’) -mechanical -thermal Pain & nociception are not the same thing Nociception is the sensory processing ‘bit’ (a brain input) Pain is the sensory & emotional experience (a brain output) Nociception is the (main) trigger & driver of pain CAN have pain without nociception & vice versa

6 Pain is nature’s tissue-damage ‘alarm’

7 Pain alarm Pain motivates us to avoid tissue damage in real timeAcute stress response Pain behaviours

8 Pain pathways Rene Descartes

9 Nociceptive (pain) pathwaysLimbic system Brainstem Dorsal horn

10 EJ Visser Churack Chair UNDA 2017 copyrightPeripheral nociception EJ Visser Churack Chair UNDA 2017 copyright

11 Inflammation Dolor (pain) Calor (heat) Rubor (redness)Galen & Celsus Dolor (pain) Calor (heat) Rubor (redness) Tumor (swelling) Functio laesa (loss of function)

12 Inflammatory soup Visser UNDA pain 2017

13 Nociceptors transductionConverting the (chemical, thermal, mechanical) energy of tissue damage into electro-chemical nerve signals Heat Acid (H+) TRPV1 DRG Cold TRPV8 Mechanical

14 TRPV1 transduction

15 EJ Visser Churack Chair UNDA 2017 copyrightA & C fibres transmission A fibres Thick Myelinated Fast Localised ‘Sharp’ pain C fibres Thin Unmyelinated Slow Poorly localised Dull, aching or burning pain EJ Visser Churack Chair UNDA 2017 copyright

16 Voltage-gated Na+ channels transmissionLocal anaesthetics Anticonvulsants Neuroma ectopics E Visser Churack Chair UNDA 2016

17 E Visser Churack Chair UNDA 2016Na+ channels polymorphisms SCN9A gene Nav1.7 Erythromelalgia Congenital insensitivity to pain E Visser Churack Chair UNDA 2016

18 The ‘CPU’ of nociceptive systemDorsal horn modulation The ‘CPU’ of nociceptive system

19 Dorsal horn

20 Dorsal horn first-order synapse Nuclear changes

21 Glutamate Most abundant neurotransmitter in body (90%)Excitatory (neurotoxic in high amounts) Amplification Sensitization Learning Memory Wind-up Long term potentiation Glia mops up glutamate

22 It’s the nervous system’s ‘chemical transistor’ (amplifier)NMDA channel It’s the nervous system’s ‘chemical transistor’ (amplifier) Mediates central sensitization -’wind-up’ -memory (hippocampus) Glutamate is the agonist Blocked by ketamine Visser MED 200 UNDA pain pharmacology 2016

23 Central sensitization amplifierIncreased nociceptive output for a given nociceptive input Amplifier effect Capacitance effect ‘memory’ NMDA Hz lllllllllllllllllllll 50 Hz Wind-up Dorsal horn Visser MED 200 UNDA pain pharmacology 2016 23

24 Wind-up amplifier effectIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII Stimulus 3 Hz

25 Allodynia ‘touch pain’Greek for ‘other pain’ Allodynia is the clinical sign for central sensitization If any of these non-painful stimuli feels painful, you have detected allodynia… ...and therefore central sensitization 25

27 Conditioned pain modulationDescending inhibition of nociception Flight (or fight) Neuroplasticity Inhibitory transmitters -norepinephrine -serotonin -endorphins CPM allows us to escape danger CPM allows us to sit on our bottoms -45 kg/cm2 pressure Visser MED 200 UNDA pain pharmacology 2016

28 Pain gate in dorsal horn

29 Biphasic pain responseAnalgesia: allows ‘flight’ from danger Hyperalgesia Tissue damage Analgesia Hyperalgesia -rests injured body part (healing) -teaches ‘caveman’ not to get injured again EJ Visser Churack Chair UNDA 2017 copyright Simonnet & Rivat Neuroreport 2003

30 EJ Visser Churack Chair UNDA 2017 copyrightNociceptive (pain) modulation Pain Inhibitory Control “Damping” Central Sensitization “Amplifier” EJ Visser Churack Chair UNDA 2017 copyright 30

31 E Visser Churack Chair UNDA 2016Red heads & pain MCR 1 receptor gene (RR) (CS16) MSH receptor polymorphism Less pricking & pressure pain More cold sensitive More sensitive to morphine Less sensitive to GA, LA E Visser Churack Chair UNDA 2016

32 Nociceptive (pain) pathwaysDescending inhibition DRG NE Aδ & C fibres Dorsal horn Central sensitization ‘amplification’ transduction transmission modulation tissue damage TRPV channels Inflammatory soup voltage gated Na ion channels Visser UNDA pain 2017 copyright

33 Spinal cord pathways Thalamocortical system Limbic system© Future Neurol 2007 Thalamocortical system Limbic system Primitive emotional brain Spinal-midbrain-limbic pathway midline tract slower transmission poorly localised pain visceral > somatic pain evolution: primitive Spinothalamic pathway lateral tract fast transmission well localised pain somatic > visceral pain evolution: ‘newer’ © Eric J. Visser 2017 UNDA. All rights reserved

34 Descending inhibitionNE © Eric J. Visser 2017 UNDA. All rights reserved

35 The bane of pain is mainly in the brainLoesser E Visser Churack Chair UNDA 2015

36 EJ Visser Churack Chair UNDA 2017 copyrightPain in the brain ‘matrix’ 22 loci Somatosensory cortex Pre frontal cortex Amygdala RAS EJ Visser Churack Chair UNDA 2017 copyright

37 Emotional & autonomic responsePain ‘relay station’ Autonomic Cingulate gyrus Fear & anxiety panic Memory of pain © Eric J. Visser 2017 UNDA. All rights reserved

38 Somatosensory cortex localization Sensory homunculus (body-map)© Eric J. Visser 2017 UNDA. All rights reserved

39 Questions?

40 Learning suggestions Know the definitions of pain & nociceptionUnderstand differences between them Appreciate that pain is a complex, multi-dimensional experience generated by the conscious brain, and not just a sensation Understand that pain is a tissue damage ‘alarm’ system Know that a person can experience pain when there’s NO tissue damage Understand the basic anatomy of nociceptive (‘pain’) pathways Understand the transduction, transmission & modulation of nociceptive signals in the nervous system

41 Visser MED 200 UNDA pain pharmacology 2016Learning suggestions Understand the concept of the ‘pain gate’ in the dorsal horn Understand the concept of neuroplasticity & its role in pain Understand that central sensitization ‘amplifies’ nociception Understand that conditioned pain modulation ‘dampens’ nociception Understand the concept of pain ‘wind-up’ Know that allodynia (touch pain) is the main clinical sign of CS Understand that nociception activates the (emotional) limbic system Understand that there is no specific ‘pain centre’ in the brain Understand the roles of the neuroimmune (glia) & adrenergic systems in nociception & pain Visser MED 200 UNDA pain pharmacology 2016

42 Glia and nociception Glia are non-neuronal cells in the central (CNS) and peripheral (PNS) nervous systems, that maintain homeostasis, form myelin, and provide support and protection for neurons. Glia in PNS Schwann cells- nerve Satellite cells-DRG Glia in CNS Astrocytes- involved in “normal” nociception Microglia Oligodendrocytes Ependymal cells Glia can be activated by; Peripheral nerve injury Trauma- physical or psychological Hypoxia Infection * Activated after PNS injury Glia are non-neuronal cells… In the PNS, there are schwann cells in the peripheral nerves and satellite cells located in the DRG . In the CNS, in the brain and spinal cord there are astrocytes that are involved in neuronal physiology and we can say regulate almost all aspects on neuronal functioning, including having a role in normal nociception. Here we have an astrocyte whose role is continuous reuptake of neurotransmitters excitatory glutamate and GABA around synapses between sensory afferents and inhibitory interneurons respectively allows a fast and regulated nociceptive neurotransmission toward upper brain regions. In the spinal cord and brain we also have microglia, but this is not activated during acute nociceptive responses. This is the same for other glia, such as oligos and ependymal cells. However, in certain circumstances such as peripheral nerve injury, these glia can be activated. Glia can also be activated after trauma, hypoxia etc Gosselin et al., (2010) Neuroscientist. 16(5): 519–531

43 Immune and glial response after nerve injurySatellite cells Schwann cells Resident Microglia and Astrocytes Active Microglia Transient activation Promotes healing Limits injury I have given an example of what happens after a peripheral nerve injury. Nerve injury recruits and activates immune cells at sight of lesion in DRG and in ventral and dorsal horns of spinal cord. Macrophages, T lymphocytes and mast cells invade lesion, schwann cells proliferate, an dedifferentiate and form bands to guide regenerating axons. In the DRG, macrophage and Tcells proliferate and macrophages also move within the sheath that satellite cells form around cell bodies of sensory neurons. and the satellite cells also proliferate and become active. About one week after injury, in the spinal cord, dense clusters of microglia that surround sensory and motor neurons become activated, and astrocytes are also activated which lead to release of inflammatory mediators that contribute to pain. In normal healing glia return to inactive state, however sometimes there can be long lasting changes which include structural alterations, cell proliferation, loss of neurotransmitter or ion buffering capacities, release of proinflammatory or proalgesic mediators and neurotoxicity.  Pro-inflammatory events Chronic activation Neuronal death Chronic pain Scholz and Woolf (2007) Nature Neuroscience 10:11; 1361

44 Pro-inflammatory mediatorsNeuroinflammation Neuroinflammation is defined as inflammation of a nerve or parts of the nervous system (CNS and PNS) Caused by activation of glia and infiltration of immune cells Can modulate excitatory and inhibitory synaptic transmission leading to enhanced chronic pain Pro-inflammatory mediators This overlaps with neuroinflammation occurs in the CNS and PNS and characterized by infiltration of leukocytes and increased production of inflammatory mediators at these sites. In particular, neuroinflammation manifests as activation of glial cells as mentioned before, activation of glial cells leads to the production pf proinflamm mediators that can powerfully modulate excitatory and inhibitory synaptic transmission, leading to central sensitization and enhanced chronic pain states.  Another system that is involved in nociception and chronic pain is the adrenergic receptors Increased release of glutamate and ATP leads to disturbances in Ca2+ signalling, increased production of cytokines and free radicals, attenuation of the astrocyte glutamate transport capacity, and conformational changes in the astrocytic cytoskeleton, the actin filaments, which can lead to formation and rebuilding of new synapses. New neuronal contacts are established for maintaining and spreading pain sensation with the astrocytic networks as bridges. Thereby the glial cells can maintain the pain sensation even after the original injury has healed, and convert the pain into long-term by altering neuronal excitability. It can even be experienced from other parts of the body.  Chronic pain

45 Adrenergic receptors G-protein coupled receptorsBind catecholamines- e.g. adrenaline, noradrenaline Several types- beta and alpha expressed throughout body Alpha- 1 and 2 expressed in neurons and implicated in nociception and pain Alpha-2 agonists are used to treat pain – inhibit release of noradrenaline Alpha-1 receptors expressed in keratinocytes, nerve fibres and blood vessels of skin- implicated in maintenance of chronic pain Noradrenaline Other systems that are implicated in pain are the Adrenergic receptors. Adrenergic receptors are G-protein couple receptors that bind catecholamines such as adrenaline and noradrenaline. You might be familiar with their involvement in modulating smooth muscle contraction. There are several types including beta and alpha adrenergic receptors, but alpha adrenergic receptors are expressed in neurons and implicated in nociception and pain. Alpha 2 receptors are expressed pre-synaptically and activation results in a negative feedback loop that supresses release of noradrenaline. Alpha-2 agonists such as clonidine are often used to treat pain as it binds and inhibits release of noradrenaline. Alpha-1 receptors are located on the post-synaptic neuron.

46 Alpha1-adrenergic receptors and neuropathic painPeripheral nerve injury Up-regulation of alpha1-adrenoceptors on nociceptive nerves and keratinocytes Activation of sympathetic nervous system Heightened nociceptor excitability and inflammation Functional changes in sympathetic activity Release of inflammatory mediators from keratinocytes Breaching of blood-nerve barrier permits alpha1- adrenoceptor activation on nociceptive afferents

47 Terminator II-Judgment Day (1991)Nociception vs pain explained in a movie Terminator II-Judgment Day (1991) John Connor “Does it hurt when you get shot?” The Terminator “I sense injuries… The data could be called pain.” EJ Visser Churack Chair UNDA 2017 copyright

48 Extra slides and information

49 EJ Visser Churack Chair UNDA 2017 copyrightAcute pain Pain of recent onset & limited duration (<3M) Clear relationship to injury or disease Physiological pain Nociceptive, inflammatory Protective & adaptive Pain ≈ amount of tissue damage Pain improves with healing EJ Visser Churack Chair UNDA 2017 copyright

50 Pain is nature’s tissue damage alarmAcute pain signals a ‘tissue damage emergency’ Unpleasant sensory & emotion experience -aversive conditioning (learning) Pain behaviours -escape -signals risk of tissue damage to others Pain (nociception) has protected Earth’s life-forms for millions of years Evolutionary survival advantage Highly preserved in phylogeny

51 Pain alarm Pain conditions us (and others in our social group) to avoid future tissue damage Learn from our mistakes Especially in childhood Nocebo response

52 Fear (anxiety) is also an alarmPain ≈ fear (anxiety) Both protect tissues from damage Shared facial expressions & behaviours Shared neurochemistry -serotonin, nor-epinephrine Shared neuroanatomy & function Anxiety = chronic pain & disability -catastrophizing, hypervigilance -3Ps: panic, PTSD, phobias EJ Visser Churack Chair UNDA 2017 copyright

53 EJ Visser Churack Chair UNDA 2017Pain behaviours EJ Visser Churack Chair UNDA 2017

54 EJ Visser Churack Chair UNDA 2017 copyrightPain behaviours Signals ‘distress’ Defensive: fight & flight, feign & freeze (play-dead) Protective: splinting, limping, rubbing Adaptive: crawl to cave, call an ambulance Social signalling: facial expressions, crying, swearing ‘I need help’, ‘stay away from danger’ EJ Visser Churack Chair UNDA 2017 copyright

55 Pain pathways Rene Descartes"Particles of heat activate a spot of skin attached by a fine thread to a valve in the brain… this opens the valve allowing animal spirits to flow from a cavity into the muscles causing them to flinch, and turn the head and eyes toward the affected body part, also moving the hand and turn the body protectively.”

56 https://previews.123rf.com/images/hfsimaging/hfsimaging1411/hfsimaging / Noxious-and-pain-receptors-in- skin-and-the-nerve-pathways-to-the-brain-via-the-spinal-cord-and-thala-Stock-Vector.jpg

57 © Eric J. Visser 2017 UNDA. All rights reservedAfferent & efferent pathways Afferents Arrive Carry CNS inputs Efferents Exit Carry CNS outputs © Eric J. Visser 2017 UNDA. All rights reserved

58 Pain gate dorsal horn

59 Comparing visceral & somatic pain Visceral pain Somatic painFew nociceptors (15%) C-fibres Many nociceptors (75%) A-δ fibres Function: homeostasis > nociception Function: nociception > homeostasis Transmission with autonomic nerves Transmission in somatic nerves Terminates in superficial dorsal horn (I,II,V) Terminates in deeper dorsal horn (II,V,X) Medial (slow) spinal cord pathways Spino-parabrachial-limbic, dorsal columns Lateral (fast) spinal cord pathway Spinothalamic Terminates in limbic brain ‘Primitive’ mid-brain Limbic-hypothalamic-autonomic system Termination in thalamus & cortex Neocortex Pain: slow onset & offset burning, aching, gripping, colicky Pain: fast onset & offset sharp, throbbing Pain ≠ tissue damage Pain ≈ tissue damage © Eric J. Visser 2017 UNDA. All rights reserved

60 Comparing visceral & somatic pain Visceral pain Somatic painPain is poorly localised (midline, torso) Localised with flat of hand Pain is well localised (‘lateralized’) Localised with tip of finger Referred pain Minimal referred pain ↑ Emotion & pain behaviour (distress) (Limbic system) ↓ Emotion & pain behaviour Pain more likely to be remembered Pain less likely to be remembered ↑ Autonomic response Sympathetic & parasympathetic ↓ Autonomic response ‘Vegetative’ motor response ‘Rest & recover’ (rest & digest) ‘Reactive’ motor response Flight, run away from danger Evolutionary primitive pain system Evolutionary advanced pain system Monitors internal environment Monitors external environment © Eric J. Visser 2017 UNDA. All rights reserved

61 Referred pain Definition: Pain experienced in a different part of the body to the site of nociception Sensory afferents from viscera & somatic tissues share a common connection in spinal dorsal horn Dorsal horn convergence © Eric J. Visser 2017 UNDA. All rights reserved

62 Dorsal horn convergenceT2/3 dorsal horn level T2/3 dermatome © Eric J. Visser 2017 UNDA. All rights reserved

63 Immune modulation of nociception in the nervous systemInterleukin 1, 6, TNF Need this to happen to have chronic pain. Toll-like receptor 4