Evolution of mechanisms and behaviour important for pain

Because chronic pain is maladaptive for patients, pain researchers often assume that its underlying mechanisms are mainly pathological. For example, chronic neuropathic pain has been attributed to death of inhibitory interneurons, to aberrant re-expression in pain pathways of Na+ channels that normally function embryonically, and to "ectopic" generation of action potentials in the cell bodies of primary nociceptors. However, some forms of clinically maladaptive pain might result from evolutionarily adaptive responses. This might occur, for example, after traumatic injury severe enough to leave damaged tissue chronically weakened and without adequate sensory reinnervation. Continuing protective attention to a severely injured region (eg, post-amputation pain) may be achieved by persistently increased responsiveness of surviving sensory neurons and by spontaneous activity in intact and axotomized nociceptors representing that region. In several phyla, peripheral injury induces persistent nociceptor hyperexcitability and behavioral hypersensitivity. Experimentally preventing nociceptive sensitization (and nociceptor hyperactivity) induced by peripheral injury has been shown to decrease survival of squid during attacks by predators. In mammals, painful consequences of both peripheral and central neural injury result from induction of persistent nociceptor hyperactivity. Mechanisms underlying persistent hyperactive states in nociceptors that can drive clinically maladaptive pain under various conditions may have been selected during evolution to enhance survival after severe traumatic injury.

Professor Edgar Terry Walters, University of Texas Health Science Center, USA

Professor Edgar Terry Walters, University of Texas Health Science Center, USA

Edgar (Terry) Walters is Professor of Integrative Biology and Pharmacology and holder of the Fondren Chair in Cellular Signaling at the McGovern Medical School in Houston, Texas, USA. He received his PhD in physiology in 1980 from Columbia University. For more than 35 years, his research has addressed the functions and mechanisms of adaptive neuronal responses to axonal or tissue injury in diverse species (the sea slug Aplysia, squid, moth larvae, rats, and mice), often from comparative and evolutionary perspectives. His discoveries in Aplysia and rodents have defined functional alterations and previously unrecognized cellular mechanisms in primary nociceptors contributing to apparently adaptive reactions to tissue damage and nerve injury, as well as to aversive learning. Using squid, his group provided the first direct evidence for a survival benefit of nociceptive sensitization, along with indirect evidence that nociceptor hyperactivity is evolutionarily adaptive. Current projects are defining the electrophysiological and cell signaling mechanisms associated with persistent hyperexcitability and ongoing electrical activity in rodent nociceptors during neuropathic and inflammatory pain. His group is also refining operant tests to better reveal behavioral functions of persistent neuronal alterations associated with injury and pain. He is delighted to help organise this first international meeting on the evolution of mechanisms and behavior important for pain, which was made possible by generous support from the Royal Society.

What is the purpose of pain? A standard answer is that it serves a protective function to alert an organism to potential or actual damage. A standard example is the effect of touching a dangerously hot object which activates peripheral nociceptors, that in turn can drive flexion reflexes. These reactions are essentially all neuronal and pain has become associated with purely neuronal phenomena. But there are other recognised functions of pain. One is to promote recovery and repair after injury and a standard example here is the prolonged reduced motility and rest associated with, say, a broken leg. Another recognised function of pain is promoting learning about the dangers of the world and this too often takes place over a prolonged time course. It is now clear that these prolonged pain states are inevitably associated with the recruitment of another defence system of the body – and that is the immune system. There are almost no circumstances where repetitive and maintained activation of the nociceptive system does not also lead to activation of the innate or adaptive immune system. There is growing evidence, which will be reviewed in this lecture, that these two systems do not operate independently but are closely co-ordinated in their actions. Thus, there is ample evidence that a major activator of the nociceptive system is the immune system and we have many example of immune mediators acting as pain mediators (eg NGF). Some of these mediators are closely associated with reparative functions. For instance, there is growing evidence that the fatigue associated with many chronic pain states is driven by immune factors like IL6 acting on the nervous system. Perhaps more surprisingly, there is also a growing body of evidence that the sensitivity of the immune system is also regulated by activity in the nervous system, sometimes with dramatic functional consequences. These too will be reviewed in this lecture.

Dr Stephen McMahon, King’s College London, UK

Dr Stephen McMahon, King’s College London, UK

Stephen McMahon is Sherrington Professor of Physiology at King’s College London. He is a Fellow of the Academy of Medical Sciences. He trained with Professor PD Wall at University College London and since then has run his own research laboratory in London. His major research interest is pain mechanisms. He has a long-standing interest in identifying pain mediators and studying their neurobiological actions. He has worked extensively on the role of NGF (neutralizing antibodies now in multiple phase III trials), and ATP acting at P2X3 receptors (receptor antagonists now in multiple phase II and III trials). His current research is focused on neuro-immune interactions, particularly the neurobiology of chemokines, and the genetics and epigenetics of pain.

Professor McMahon currently directs the Wellcome Trust Pain Consortium, and prior to this, the London Pain Consortium, a collection of leading pain researchers working to better understand chronic pain mechanisms and improve treatments. He was academic lead on a EU-IMI consortium called Europain, a collaboration of scientists working in academia and industry, 2009-2015. He is also deputy Chair of the MRC’s Neuroscience and Mental Health Board. He has published more than 300 research articles in scientific journals including, Nature, Nature Medicine, Nature Neuroscience, Cell, Neuron and the Journal of Neuroscience and has an H-index of 102.

Nociceptor/immune connections are found in animals across phyla. For example, local inflammation and/or damage results in increased nociceptive sensitivity in the affected area in mammals and insects (Drosophila). However, in mammals, systemic illness can increase peripheral nociceptive sensitivity even in the absence of damage in the periphery. This phenomenon has not, to our knowledge, been found in other animal groups. We found that the caterpillar Manduca sexta also shows increased nociceptor sensitivity in the periphery when fed heat-killed pathogens. The ingested pathogens induced an immune response in the midgut and fat body (an important immune organ in insects). There was no evidence of immunopathology in the periphery. The increase in nociceptive sensitivity is probably part of the reconfiguration of physiological networks that occurs during an immune response. The increase in nociceptive sensitivity may help compensate for the declines in anti-predator behaviour that occurs as molecular resources are shifted towards the immune system.

Dr Shelley Adamo, Dalhousie University, Canada

Dr Shelley Adamo, Dalhousie University, Canada

Shelley Adamo received her BSc in Zoology at the University of Toronto and her PhD from McGill University. Her first post doctoral position, in the department of Neurobiology and Behavior at Cornell University with Ron Hoy, emphasized the interconnections between physiology and behaviour. Her second post doc, with Nancy Beckage at the University of California-Riverside, developed her interests in parasitic manipulators as a means of understanding brain function. Parasitic manipulators often control their hosts by exploiting immune/neural connections. This work has led to the present research on the ancient signalling systems between the immune and nervous systems. She is presently a full professor at Dalhousie University in Halifax.

Animals face hazards that cause tissue damage and have immediate nociceptive reflexes that protect them from such damage. In addition, some taxa have evolved the capacity for pain experience. The function of pain appears to be linked to long-term changes in motivation brought about by the unpleasant nature of the pain experience. Pain presumably enhances long-term protection through behaviour modification based, in part, on memory of the unpleasant feeling. The talk considers behavioural and physiological criteria that might help to distinguish nociception from pain in crustaceans. Rapid avoidance learning and prolonged memory indicate central processing rather than mere reflexes and are consistent with the experience of pain. Complex, prolonged grooming or rubbing may be beyond mere reflex and demonstrate an awareness of the specific site of stimulus application. Trade-offs with other motivational systems indicate central processing, and a noxious experience might affect behaviour for at least 24 hours. Recent evidence of fitness enhancing anxiety-like states is also consistent with the idea of pain. Physiological changes in response to noxious stimuli mediate some of the behavioural change, and some of these physiological changes are due to the noxious stimulus not the behavioural response. Thus, available data go beyond the idea of just nociception but the impossibility of total proof of pain that is similar to our own feelings, means that pain in crustaceans is still disputed. Pain in animals should not be defined on the basis of human experience.

Professor Bob Elwood, Queen's University Belfast, UK

Professor Bob Elwood, Queen's University Belfast, UK

Bob Elwood is Emeritus Professor of Animal Behaviour, Queen’s University in Belfast. He has broad interests in behaviour, particularly factors that control paternal care and inhibit infanticide in mammals, how animals gather information, and how animals make decisions in contests for resources. The latter used various crustacean species. His interest in potential pain in crustaceans followed a chance meeting with a well-known seafood chef, who asked if they felt pain. Bob has published over 200 papers and book chapters, and was Treasurer and then President of the Association for the Study of Animal Behaviour. He has held editorial roles with Animal Behaviour, Biology Letters and Journal of Zoology. He supervised more than 40 PhD students and devised and taught a masters programme in animal behaviour and welfare. He was elected to the Royal Irish Academy, awarded the ASAB medal, and the BSAS/RSPCA prize for innovative developments in animal welfare.

Evolutionary medicine has encouraged recognising the utility of aversive responses, distinguishing them carefully from diseases, and using the smoke detector principle to understand normal but unnecessarily aversive responses. In recent years, recognition has also been growing that natural selection has shaped mechanisms that adjust defensive responses as a function of experience. Repeated elicitation of a defensive response often indicates that prior responses have been inadequate to provide protection or that the environment is extremely dangerous; in such situations, the utility of more rapid or intense responses can shape mechanisms that adjust thresholds and response strength. However, such mechanisms are intrinsically prone to dysregulation from positive feedback, offering a possible evolutionary explanation for vulnerability to chronic pain. Evolutionary thinking has also increasingly considered connections between physical pain and mental pain. They share common phylogenetic origins, functions, regulation mechanisms, and vulnerability to dysregulation. Both evolve to subtypes for coping with different kinds of situations. For physical pain the situations are different kinds of tissue damage. For mental pain, the relevant situations are social dangers, losses, and efforts in pursuit of unreachable goals. Further exploration of these connections offers major opportunities for understanding the utility of low mood and anxiety, and their pathological counterparts in depression and anxiety disorders. This perspective also has implications for prevention and treatment, especially in light of the shared neural modulators and pathways that regulate physical and mental pain.

Dr Randolph Nesse, Arizona State University, USA

Dr Randolph Nesse, Arizona State University, USA

Randolph M Nesse, MD is Professor of Life Sciences at Arizona State University, where he moved in 2014 from the University of Michigan to become the Founding Director of the Center for Evolution Medicine. His research on evolution and aging led to a collaboration with the evolutionary biologist George Williams that initiated much new work in evolutionary medicine. His current research is on how selection shapes mechanisms that regulate defenses such as pain, fever, anxiety and low mood, and how asymmetric fitness landscapes make genetic diseases inevitable. He is a Fellow of the AAAS, a Distinguished Life Fellow of the American Psychiatric Association, and President of the International Society for Evolution, Medicine & Public Health, where he advances his mission of making evolutionary biology a basic science for medicine.

Sublethal injury triggers long-lasting sensitization of defensive responses in many species, suggesting that powerful evolutionary selection pressures have driven this widespread pattern. In humans, persistent nociceptive sensitization is often accompanied by heightened sensations of pain and anxiety. While considerable experimental and clinical evidence supports the adaptive value of immediate nociception during injury, identifying the function of long-lasting sensitization after injury in mammalian models has been challenging. Cephalopod molluscs have the largest and most complex brains of all the invertebrates, and are promising comparative models of neurobiology and behavior. Previous work has shown that cephalopods express short- and long-term behavioral and neural sensitization after minor injury, expressed as decreased response thresholds and escalated defensive behaviors. In recent experiments that place injured cephalopods into naturalistic experimental settings with predators, those that expressed nociceptive sensitization after injury survived predatory encounters at higher rates that those for which nociceptive sensitization was blocked. Ongoing studies examining adaptive functions for nociceptive sensitization are focusing on foraging, cognition and mate choice, to identify fitness benefits that extend both across behavioral domains and across the lifetime of the injured animal.

Dr Robyn Crook, San Francisco State University, USA

Dr Robyn Crook, San Francisco State University, USA

Robyn Crook is an Assistant Professor of Physiology at San Francisco State University. She received her undergraduate degree in Zoology with first class honors from the University of Melbourne, Australia, and a PhD in Ecology, Evolution and Behavior from the City University of New York. Her research focuses on conserved mechanisms of injury-induced sensitization, with a particular emphasis on behavioral plasticity that underlies compensatory behaviors that enhance fitness. Using cephalopod molluscs, her research has demonstrated conserved patterns of nociceptive sensitization in cephalopods and mammals, and that anxiety-like behaviors induced by tissue injury are adaptive. She is an expert in the field of invertebrate animal welfare, and her research on anesthesia and euthanasia in cephalopod molluscs has had wide-ranging impacts on legislative efforts to regulate the use of invertebrates in research.

In the medicinal leech, Hirudo verbana, it is possible to identify and record from polymodal and mechanical nociceptors, non-nociceptive touch- and pressure-sensitive neurons, and many of the postsynaptic neurons these afferents target. This makes Hirudo a useful species for studying synaptic modulation within nociceptive circuits and the functional relevance of such neuromodulation at the behavioral level. In this presentation, I will discuss three examples of synaptic plasticity and their potential behavioral significance not just to Hirudo, but to animals in general. First, we have shown that repetitive stimulation of non-nociceptive afferents elicits persistent depression in nociceptive synapses that is endocannabinoid-dependent. This endocannabinoid-mediated mechanism may provide an alternative explanation for how repetitive, non-painful stimulation can have persistent anti-nociceptive effects. Second, we have found that noxious stimuli potentiates non-nociceptive synapses via a mechanism that is also endocannabinoid-dependent and involves a disinhibitory mechanism. This contributes to understanding how endocannabinoids can have both pro- and anti-nociceptive effects. Finally, we have observed a potential interaction between NMDA receptor mediated long-term potentiation in nociceptive synapses and endocannabinoid-mediated depression that points to crosstalk between pro- and anti-nociceptive modulatory mechanisms. Together these studies demonstrate the strength of using comparative approaches to understand nociceptive mechanism from the cellular to behavioral level.

Professor Brian D Burrell, University of South Dakota, USA

Professor Brian D Burrell, University of South Dakota, USA

Brian D Burrell’s lab is interested in the basic biology of how nociception is modulated from the synaptic to the behavioral level. They utilise the medicinal leech, Hirudo verbana, as the model system of choice. The nervous system in Hirudo is very well characterised in terms of the identity, functional role, and synaptic connections of many neurons including the nociceptive and non-nociceptive sensory cells. Therefore, it is possible to carry out detailed analyses of pre- and postsynaptic cellular mechanisms mediating synaptic plasticity and to link plasticity in individual neurons or synapses to changes at the behavioral level. Professor Brian D Burrell and colleagues are currently focusing on the role of a class of lipid neurotransmitters, the endocannabinoids, and how these modulatory transmitters can have both anti-nociceptive and pro-nociceptive effects.

Nerve injury can lead to devastating pain that is difficult to treat, in part because we have an incomplete understanding of the biology driving disease. To identify core disease mechanisms we investigated neuropathic pain in Drosophila. We show that peripheral injury triggers a loss of central inhibition which was necessary and sufficient to develop neuropathic sensitization. Similar changes in central inhibitory tone have been described in mammalian pain and this may represent a core disease pathology that is amenable to therapy. To this end, we generated inhibitory neurons derived from human induced pluripotent stem cells (hiPSC) and transplanted these neurons into the spinal cord of neuropathic mice. Remarkably, hiPSC-derived inhibitory transplants survive long-term, show integration, and promoted lasting pain relief without side effects. Together, these data highlight that central disinhibition is a core mechanism driving neuropathic pain, and argue that hiPSC-derived transplants may be an effective and non-addictive pain therapy.

Dr Greg Neely, University of Sydney, Australia

Dr Greg Neely, University of Sydney, Australia

Greg Neely completed a PhD in human immunology in Calgary Canada. His work in functional genomics began in 2003 in Vienna, Austria, where he combined high throughput in vivo screening in fruit flies with transgenic mice and human genetics to find new pain and heart disease genes. Greg moved to Sydney, Australia, to start his own lab in 2011. A continuing interest of the lab is high throughput phenotypic screening using fruit flies or cell culture systems, and a major focus is using these tools to find new genes and mechanisms that control pain perception or chronic pain.

The Galko laboratory is interested in how tissue damage and other stimuli sensitize peripheral nociceptors. Historically, this topic has been studied in vertebrate systems. The approach of this laboratory is to use the genetically tractable model organism, Drosophila melanogaster (a fruit fly) to interrogate the molecular/genetic basis of injury-induced changes in nociceptive responsiveness. To do this they combine tissue damage assays with behavioral tests of nociceptive responsiveness. An overview of the lab’s work identifying conserved signaling pathways that are required for acute injury-induced thermal nociceptive sensitization will be presented. Ongoing efforts to establish nociception assays for all sensory modalities (heat, cold, touch, chemical) will be reviewed. A goal of having assays for all sensory modalities is so that one can fully understand how different types of injury (UV irradiation, physical wounding), disease states (diabetes or cancer) and disease treatment (chemotherapy) impact the ability of an animal to perceive and properly respond to noxious stimuli. Progress recently identifying signaling pathways (insulin signaling) that regulate the duration or persistence of nociceptive sensitization will be presented as will data connecting this finding to the etiology of nociceptive sensory changes that accompany diabetes. Finally, recent progress using mouse genetics approaches to test whether some of the conserved pathways identified in flies act in similar ways during nociceptive sensitization in vertebrates will be covered.

Dr Michael Galko, University of Texas MDAnderson Cancer Center, USA

Dr Michael Galko, University of Texas MDAnderson Cancer Center, USA

The Galko laboratory is interested in how organisms respond to tissue damage. His group has used the fruit fly, Drosophila, to establish novel tissue repair (wound healing) and nociception assays. At the level of sensory neurons his group is interested in evolutionarily conserved mechanisms by which nociceptors become hypersensitive to sensory stimuli (temperature, touch, chemicals) following injury or disease.