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.

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.

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.

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.

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.

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.

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.

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.

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.