Injury-related behavior and neuronal plasticity: an evolutionary perspective on sensitization, hyperalgesia, and analgesia

A conserved gastropod withdrawal circuit in Biomphalaria glabrata, an intermediate host for schistosomiasis

Neuronal signals mediated by the biogenic amine serotonin (5-HT) underlie critical survival strategies across the animal kingdom. This investigation examined serotonin-like immunoreactive neurons in the cerebral ganglion of the panpulmonate snail Biomphalaria glabrata, a major intermediate host for the trematode parasite Schistosoma mansoni. Five neurons comprising the cerebral serotonergic F (CeSF) cluster of B. glabrata shared morphological characteristics with neurons that contribute to withdrawal behaviors in numerous heterobranch species. The largest member of this group, designated CeSF-1, projected an axon to the tentacle, a major site of threat detection. Intracellular recordings demonstrated repetitive activity and electrical coupling between the bilateral CeSF-1 cells. In semi-intact preparations, the CeSF-1 cells were not responsive to cutaneous stimuli but did respond to photic stimuli. A large FMRF-NH2-like immunoreactive neuron, termed C2, was also located on the dorsal surface of each cerebral hemiganglion near the origin of the tentacular nerve. C2 and CeSF-1 received coincident bouts of inhibitory synaptic input. Moreover, in the presence of 5-HT they both fired rhythmically and in phase. As the CeSF and C2 cells of Biomphalaria share fundamental properties with neurons that participate in withdrawal responses in Nudipleura and Euopisthobranchia, our observations support the proposal that features of this circuit are conserved in the Panpulmonata.NEW & NOTEWORTHY Neuronal signals mediated by the biogenic amine serotonin underlie critical survival strategies across the animal kingdom. This investigation identified a group of serotonergic cells in the panpulmonate snail Biomphalaria glabrata that appear to be homologous to neurons that mediate withdrawal responses in other gastropod taxa. It is proposed that an ancient withdrawal circuit has been highly conserved in three major gastropod lineages.

Impact of homeostatic body hydration status, evaluated by hemodynamic measures, on different pain sensitization paths to a chronic pain syndrome

Contrasting findings on the mechanisms of chronic pain and hypertension development render the current conventional evidence of a negative relationship between blood pressure (BP) and pain severity insufficient for developing personalized treatments. In this interdisciplinary study, patients with fibromyalgia (FM) exhibiting clinically normal or elevated BP, alongside healthy participants were assessed. Different pain sensitization responses were evaluated using a dynamic 'slowly repeated evoked pain' (SREP) measure, as well as static pain pressure threshold and tolerance measures. Cardiovascular responses to clino-orthostatic (lying-standing) challenges were also examined as acute re- and de-hydration events, challenging cardiovascular and cerebrovascular homeostasis. These challenges involve compensating effects from various cardiac preload or afterload mechanisms associated with different homeostatic body hydration statuses. Additionally, hair cortisol concentration was considered as a factor with an impact on chronic hydration statuses. Pain windup (SREP) and lower pain threshold in FM patients were found to be related to BP rise during clinostatic (lying) rehydration or orthostatic (standing) dehydration events, respectively. These events were determined by acute systemic vasoconstriction (i.e., cardiac afterload response) overcompensating for clinostatic or orthostatic cardiac preload under-responses (low cardiac output or stroke volume). Lower pain tolerance was associated with tonic blood pressure reduction, determined by permanent hypovolemia (low stroke volume) decompensated by permanent systemic vasodilation. In conclusion, the body hydration status profiles assessed by (re)activity of systemic vascular resistance and effective blood volume-related measures can help predict the risk and intensity of different pain sensitization components in chronic pain syndrome, facilitating a more personalized management approach.

Reconceptualizing sensitization in pain: back to basics

Fillingim RB. Redefining sensitization could be a sensitive issue. PAIN Rep 2024;9:e1126.

Pain: Behavioural expression and response in an evolutionary framework

An evolutionary perspective offers insights into the major public health problem of chronic (persistent) pain; behaviours associated with it perpetuate both pain and disability. Pain is motivating, and pain-related behaviours promote recovery by immediate active or passive defence; subsequent protection of wounds; suppression of competing responses; energy conservation; vigilance to threat; and learned avoidance of associated cues. When these persist beyond healing, as in chronic pain, they are disabling. In mammals, facial and bodily expression of pain is visible and identifiable by others, while social context, including conspecifics' responses, modulate pain. Studies of responses to pain emphasize onlooker empathy, but people with chronic pain report feeling disbelieved and stigmatized. Observers frequently discount others' pain, best understood in terms of cheater detection-alertness to free riders that underpins the capacity for prosocial behaviours. These dynamics occur both in everyday life and in clinical encounters, providing an account of the adaptiveness of pain-related behaviours.

Behavioral changes in the first 3 weeks after disbudding in dairy calves

Hot-iron disbudding, the practice of cauterizing horn bud tissue to prevent horn growth in dairy calves, results in behavioral changes indicative of pain in the first few days after the procedure. However, few studies have quantified behavioral changes in the following weeks, while the burn wounds are still healing. Female Holstein calves were disbudded with a heated iron and pain relief (5.5 mL lidocaine cornual nerve block and 1 mg/kg oral meloxicam) at 4 to 10 d of age (n = 19) or not disbudded (n = 19). Calves wore ear tag accelerometers that reported the dominant behavior being performed at 1-min intervals from 3 to 21 d after disbudding. Compared with age-matched controls, disbudded calves tended to spend more time inactive throughout the observation period, ruminated less in the first 3 to 11 d after disbudding, and sucked more from a milk bottle beginning 5 d after disbudding until the end of the 21-d observation period. In addition to the accelerometer data, live observations of sleeping (using a behavioral proxy), lying, and ruminating were collected using instantaneous sampling at 5-min intervals for 24-h periods 3, 10, and 17 d after disbudding. Disbudded calves slept with their head down more on all live observation days and spent more time lying on the 17th d after disbudding, but ruminating did not differ compared with controls, in contrast to the accelerometer results. More time spent inactive, sleeping, and lying, and less time spent ruminating (as indicated by the accelerometer) can be interpreted as attempts to reduce painful stimulation of the disbudding wounds and allocate energy to healing. It is unclear whether the greater amount of sucking in the disbudded calves is nutritive (milk present) or non-nutritive (milk absent), as the algorithm did not distinguish the type of sucking, and further research is needed to explore the factors underlying this effect. We conclude that disbudding alters daily behavior patterns for at least 3 wk, far beyond the duration of recommended pain medication, raising additional welfare concerns about the procedure.

Trust Is for the Strong: How Health Status May Influence Generalized and Personalized Trust

In the trust-health relationship, how trusting other people in society may promote good health is a topic often examined. However, the other direction of influence-how health may affect trust-has not been well explored. In order to investigate this possible effect, we employed the Bayesian Mindsponge Framework (BMF) analytics to go deeper into the information processing mechanisms underlying the expressions of trust. Conducting a Bayesian analysis on a dataset of 1237 residents from Cali, Colombia, we found that general health status is positively associated with generalized trust, but recent experiences of illnesses/injuries have a negative moderating effect. Personalized trust is largely unchanged across different general health conditions, but the trust level becomes higher with recent experiences of illnesses/injuries. Psychophysiological mechanisms of increasing information filtering intensity toward unfamiliar sources during a vulnerable state of health is a plausible explanation of found patterns in generalized trust. Because established personal relationships are reinforced information channels, personalized trust is not affected as much. Rather, the results suggest that people may rely even more on loved ones when they are in bad health conditions. This exploratory study shows that the trust-health relationship can be examined from a different angle that may provide new insights.

Repetitive nociceptive stimulation elicits complex behavioral changes in Hirudo: evidence of arousal and motivational adaptations

Appropriate responses to real or potential damaging stimuli to the body (nociception) are critical to an animal's short- and long-term survival. The initial goal of this study was to examine habituation of withdrawal reflexes (whole-body and local shortening) to repeated mechanical nociceptive stimuli (needle pokes) in the medicinal leech, Hirudo verbana, and assess whether injury altered habituation to these nociceptive stimuli. While repeated needle pokes did reduce shortening in H. verbana, a second set of behavior changes was observed. Specifically, animals began to evade subsequent stimuli by either hiding their posterior sucker underneath adjacent body segments or engaging in locomotion (crawling). Animals differed in terms of how quickly they adopted evasion behaviors during repeated stimulation, exhibiting a multi-modal distribution for early, intermediate and late evaders. Prior injury had a profound effect on this transition, decreasing the time frame in which animals began to carry out evasion and increasing the magnitude of these evasion behaviors (more locomotory evasion). The data indicate the presence in Hirudo of a complex and adaptive defensive arousal process to avoid noxious stimuli that is influenced by differences in internal states, prior experience with injury of the stimulated areas, and possibly learning-based processes.

Post-injury pain and behaviour: a control theory perspective

Injuries of various types occur commonly in the lives of humans and other animals and lead to a pattern of persistent pain and recuperative behaviour that allows safe and effective recovery. In this Perspective, we propose a control-theoretic framework to explain the adaptive processes in the brain that drive physiological post-injury behaviour. We set out an evolutionary and ethological view on how animals respond to injury, illustrating how the behavioural state associated with persistent pain and recuperation may be just as important as phasic pain in ensuring survival. Adopting a normative approach, we suggest that the brain implements a continuous optimal inference of the current state of injury from diverse sensory and physiological signals. This drives the various effector control mechanisms of behavioural homeostasis, which span the modulation of ongoing motivation and perception to drive rest and hyper-protective behaviours. However, an inherent problem with this is that these protective behaviours may partially obscure information about whether injury has resolved. Such information restriction may seed a tendency to aberrantly or persistently infer injury, and may thus promote the transition to pathological chronic pain states.

Persistent nociceptor hyperactivity as a painful evolutionary adaptation

Chronic pain caused by injury or disease of the nervous system (neuropathic pain) has been linked to persistent electrical hyperactivity of the sensory neurons (nociceptors) specialized to detect damaging stimuli and/or inflammation. This pain and hyperactivity are considered maladaptive because both can persist long after injured tissues have healed and inflammation has resolved. While the assumption of maladaptiveness is appropriate in many diseases, accumulating evidence from diverse species, including humans, challenges the assumption that neuropathic pain and persistent nociceptor hyperactivity are always maladaptive. We review studies indicating that persistent nociceptor hyperactivity has undergone evolutionary selection in widespread, albeit selected, animal groups as a physiological response that can increase survival long after bodily injury, using both highly conserved and divergent underlying mechanisms.

Next generation behavioral sequencing for advancing pain quantification

With symptoms such as spontaneous pain and pathologically heightened sensitivity to stimuli, chronic pain accounts for about 20% of physician visits and up to 2/3 of patients are dissatisfied with current treatments. Much of our knowledge on pain processing and analgesics has emerged from behavioral studies performed on animals presenting the same symptoms under pathological conditions. While humans can verbally describe their pain, studies on rodents have relied on behavioral assays providing non-exhaustive characterization or altering animals' original sensitivity through repetitive stimulations. The emergence of what we term "next-generation behavioral sequencing" is now permitting us to quantitatively describe behavioral features on millisecond to minutes long timescales that lie beyond easy detection with the unaided eye. Here, we summarize emerging videography and computational based behavioral approaches that have the potential to significantly improve pain research.

Behavioral changes in senescent giant Pacific octopus (Enteroctopus dofleini) are associated with peripheral neural degeneration and loss of epithelial tissue

Most species of octopus experience extreme physical decline after a single reproductive bout which extends over a period of days, weeks, or months before eventual death. Although outward indicators of senescence are widely recognized, comparatively little is known about physiological and neural changes accompanying terminal decline in octopuses. Here, we measured changes in behavioral response to nociceptive stimuli across the lifespan in giant Pacific octopus (GPO), Enteroctopus dofleini, held in public aquariums in the USA. Post-euthanasia, tissue was collected from arm tips, and neural and epithelial cell degeneration was quantified and compared with biopsies of arm tips from healthy, pre-reproductive GPOs. Behavioral assays showed significant changes both in low threshold mechanosensory responses and nociceptive behavioral responses beginning early in senescence and extending until euthanasia. Histology data showed that while the ratio of apoptotic cells to total cell number stayed constant between healthy and senescent GPOs, overall neural and epithelial cell density was significantly lower in terminally senescent octopuses compared with healthy controls. Our data provide new insight into the time-course and causes of sensory dysfunction in senescent cephalopods and suggest proactive welfare management should begin early in the senescence phase, well before animals enter terminal decline.

Pain sensitivity is associated with general attitudes towards pain: Development and validation of a new instrument for pain research and clinical application

BACKGROUND

With regard to attitudes towards pain, many questionnaires have been developed. Although undoubtedly useful, they were specifically designed for the use in chronic pain and are less suitable for the assessment in the general population. The purpose of the present paper was to develop a measure for the assessment of general attitudes towards pain applicable in the general population, regardless of clinical condition, and to test its psychometric properties.

METHODS

We developed the General Attitudes Towards Pain Inventory and conducted two studies in order to provide psychometric data: In a general population sample (N = 362, study one), participants were asked to complete a questionnaire battery. To assess test-retest reliability, participants were contacted again after four weeks (retest sample: N = 101). For the evaluation of criterion validity (study two), a sample with sadomasochistic sexual preference (N = 68) was additionally recruited.

RESULTS

Statistical analyses revealed, overall, acceptable internal consistencies and test-retest reliabilities. A ten-factor model showed acceptable fit and was superior to alternative models. The inventory demonstrated convergent and divergent validity. In this context, we found pain sensitivity to be associated with pain attitudes. Finally, compared to the general population sample, individuals with sadomasochistic sexual preference showed significantly higher scores on fascination-pleasure and challenge subscales.

CONCLUSIONS

In the present paper, we introduce a new and comprehensive instrument for pain research and provide evidence for its reliability and validity. In addition, we present new insights into how interindividual differences in pain sensitivity relate to pain attitudes.

Approaches to studying injury-induced sensitization and the potential role of an endocannabinoid transmitter

Endocannabinoids are traditionally thought to have an analgesic effect. However, it has been shown that while endocannabinoids can depress nociceptive signaling, they can also enhance non-nociceptive signaling. Therefore, endocannabinoids have the potential to contribute to non-nociceptive sensitization after an injury. Using Hirudo verbana (the medicinal leech), a model of injury-induced sensitization was developed in which a reproducible piercing injury was delivered to the posterior sucker of Hirudo. Injury-induced changes in the non-nociceptive threshold of Hirudo were determined through testing with Von Frey filaments and changes in the response to nociceptive stimuli were tested by measuring the latency to withdraw to a nociceptive thermal stimulus (Hargreaves apparatus). To test the potential role of endocannabinoids in mediating injury-induced sensitization, animals were injected with tetrahydrolipstatin (THL), which inhibits synthesis of the endocannabinoid transmitter 2-arachidonoylglycerol (2-AG). Following injury, a significant decrease in the non-nociceptive response threshold (consistent with non-nociceptive sensitization) and a significant decrease in the response latency to nociceptive stimulation (consistent with nociceptive sensitization) were observed. In animals injected with THL, a decrease in non-nociceptive sensitization in injured animals was observed, but no effect on nociceptive sensitization was observed.

Sensitized by a sea slug: site-specific short-term and general long-term sensitization in Aplysia following Navanax attack

Neurobiological studies of the model species, Aplysia californica (Mollusca, Gastropoda, Euopisthobranchia), have helped advance our knowledge of the neural bases of different forms of learning, including sensitization, a non-associative increase in withdrawal behaviors in response to mild innocuous stimuli However, our understanding of the natural context for this learning has lagged behind the mechanistic studies. Because previous studies of sensitization used electric shock, or other artificial stimulus to produce sensitization, they left unaddressed the question of what stimuli in nature might cause sensitization, until our laboratory demonstrated short and long-term sensitization after predatory attack by spiny lobsters. In the present study, we tested for sensitization after attack by a very different predator, the predacious sea-slug, Navanax inermis (Mollusca, Gastropoda, Euopisthobranchia). Unlike the biting and prodding action of lobster attack, Navanax uses a rapid strike that sucks and squeezes its prey in an attempt to swallow it whole. We found that Navanax attack to the head of Aplysia caused strong immediate sensitization of head withdrawal, and weaker, delayed, sensitization of tail-mantle withdrawal. By contrast, attack to the tail of Aplysia resulted in no sensitization of either reflex. We also developed an artificial attack stimulus that allowed us to mimick a more consistently strong attack. This artificial attack produced stronger but qualitatively similar sensitization: Strong immediate sensitization of head withdrawal and weaker sensitization of tail-mantle withdrawal after head attack, immediate sensitization in tail-mantle withdrawal, but no sensitization of head withdrawal after tail attack. We conclude that Navanax attack causes robust site-specific sensitization (enhanced sensitization near the site of attack), and weaker general sensitization (sensitization of responses to stimuli distal to the attack site). We also tested for long-term sensitization (lasting longer than 24 hours) after temporally-spaced delivery of four natural Navanax attacks to the head of subject Aplysia. Surprisingly, these head attacks, any one of which strongly sensitizes head withdrawal in the short term, failed to sensitize head-withdrawal in the long term. Paradoxically, these repeated head attacks produced long-term sensitization in tail-mantle withdrawal. These experiments and observations confirm that Navanax attack causes short, and long-term sensitization of withdrawal reflexes of Aplysia. Together with the observation of sensitization after lobster attack, they strongly support the premise that sensitization in Aplysia is an adaptive response to sub-lethal predator attack. They also add site-specific sensitization to the list of naturally induced learning phenotypes, as well as paradoxical long-term sensitization of tail-mantle withdrawal (but not head withdrawal) after multiple head attacks.

BKCa Channel Inhibition by Peripheral Nerve Injury Is Restored by the Xanthine Derivative KMUP-1 in Dorsal Root Ganglia

This study explored whether KMUP-1 improved chronic constriction injury (CCI)-induced BKCa current inhibition in dorsal root ganglion (DRG) neurons. Rats were randomly assigned to four groups: sham, sham + KMUP-1, CCI, and CCI + KMUP-1 (5 mg/kg/day, i.p.). DRG neuronal cells (L4-L6) were isolated on day 7 after CCI surgery. Perforated patch-clamp and inside-out recordings were used to monitor BKCa currents and channel activities, respectively, in the DRG neurons. Additionally, DRG neurons were immunostained with anti-NeuN, anti-NF200 and anti-BKCa. Real-time PCR was used to measure BKCa mRNA levels. In perforated patch-clamp recordings, CCI-mediated nerve injury inhibited BKCa currents in DRG neurons compared with the sham group, whereas KMUP-1 prevented this effect. CCI also decreased BKCa channel activity, which was recovered by KMUP-1 administration. Immunofluorescent staining further demonstrated that CCI reduced BKCa-channel proteins, and KMUP-1 reversed this. KMUP-1 also changed CCI-reduced BKCa mRNA levels. KMUP-1 prevented CCI-induced neuropathic pain and BKCa current inhibition in a peripheral nerve injury model, suggesting that KMUP-1 could be a potential agent for controlling neuropathic pain.

Sensory neuron-associated macrophages as novel modulators of neuropathic pain

The peripheral nervous system comprises an infinity of neural networks that act in the communication between the central nervous system and the most diverse tissues of the body. Along with the extension of the primary sensory neurons (axons and cell bodies), a population of resident macrophages has been described. These newly called sensory neuron-associated macrophages (sNAMs) seem to play an essential role in physiological and pathophysiological processes, including infection, autoimmunity, nerve degeneration/regeneration, and chronic neuropathic pain. After different types of peripheral nerve injury, there is an increase in the number and activation of sNAMs in the sciatic nerve and sensory ganglia. The activation of sNAMs and their participation in neuropathic pain development depends on the stimulation of pattern recognition receptors such as Toll-like receptors and Nod-like receptors, chemokines/cytokines, and microRNAs. On activation, sNAMs trigger the production of critical inflammatory mediators such as proinflammatory cytokines (eg, TNF and IL-1β) and reactive oxygen species that can act in the amplification of primary sensory neurons sensitization. On the other hand, there is evidence that sNAMs can produce antinociceptive mediators (eg, IL-10) that counteract neuropathic pain development. This review will present the cellular and molecular mechanisms behind the participation of sNAMs in peripheral nerve injury-induced neuropathic pain development. Understanding how sNAMs are activated and responding to nerve injury can help set novel targets for the control of neuropathic pain.

Anti-Inflammatory Effects of Sulfated Polysaccharide from Sargassum Swartzii in Macrophages via Blocking TLR/NF-Κb Signal Transduction

This study involves enzymatic extraction of fucoidan from Sargassum swartzii and further purification via ion-exchange chromatography. The chemical and molecular characteristics of isolated fucoidan is evaluated concerning its anti-inflammatory potential in RAW 264.7 macrophages under LPS induced conditions. Structural properties of fucoidan were assessed via FTIR and NMR spectroscopy. NO production stimulated by LPS was significantly declined by fucoidan. This was witnessed to be achieved via fucoidan acting on mediators such as iNOS and COX-2 including pro-inflammatory cytokines (TNF-α, IL-6, and IL-1β), with dose dependent down-regulation. Further, the effect is exhibited by the suppression of TLR mediated MyD88, IKK complex, ultimately hindering NF-κB and MAPK activation, proposing its therapeutic applications in inflammation related disorders. The research findings provide an insight in relation to the sustainable utilization of fucoidan from marine brown algae S. swartzii as a potent anti-inflammatory agent in the nutritional, pharmaceutical, and cosmeceutical sectors.

Restraint stress activates defensive behaviors in male rats depending on age and housing condition

Restraint is a widely used experimental stress manipulation in animal models. It is still unclear, however, whether restraint is associated with physical fatigue leading to overall behavioral inhibition, or if it induces activation of defensive behaviors and strategies to protect against subsequent challenges. The aim of this study was to systematically investigate restraint effects in rats based on housing condition (isolation- vs. pair-housed) and age at the time of testing, both of which are relevant to the expression of defensive strategies. Restraint induced behavioral inhibition in male rats younger than postnatal day 65 in an open-field paradigm, while it activated defensive behaviors in adult rats, depending on their housing condition; thereby pair-housed adult rats exhibited a heightened stretch-attend postures (SAPs) and it was suppressed by restraint, while isolation-housed adult rats displayed lower SAPs but it was enhanced by restraint. Restraint also enhanced pain tolerance, but not pain sensitivity, across all ages, regardless of housing conditions. These results suggest that restraint stress activates defensive systems of male rats, including sensory defenses and exploratory strategies in a novel environment, and these expression patterns vary with age from overall inhibition to changes in defensive behavior strategies. Understanding differential changes in these models could lead to greater consistency and better standardization of rodent models commonly used to assess the impact of stress on anxiety and defensive behaviors.

The Effect of Pain Catastrophizing on Endogenous Inhibition of Pain and Spinal Nociception in Native Americans: Results From the Oklahoma Study of Native American Pain Risk

BACKGROUND

Conditioned pain modulation (CPM) is a task that involves measuring pain in response to a test stimulus before and during a painful conditioning stimulus (CS). The CS pain typically inhibits pain elicited by the test stimulus; thus, this task is used to assess endogenous pain inhibition. Moreover, less efficient CPM-related inhibition is associated with chronic pain risk. Pain catastrophizing is a cognitive-emotional process associated with negative pain sequelae, and some studies have found that catastrophizing reduces CPM efficiency.

PURPOSE

The current study examined the relationship between catastrophizing (dispositional and situation specific) and CPM-related inhibition of pain and the nociceptive flexion reflex (NFR; a marker of spinal nociception) to determine whether the catastrophizing-CPM relationship might contribute to the higher risk of chronic pain in Native Americans (NAs).

METHODS

CPM of pain and NFR was assessed in 124 NAs and 129 non-Hispanic Whites. Dispositional catastrophizing was assessed at the beginning of the test day, whereas situation-specific catastrophizing was assessed in response to the CS, as well as painful electric stimuli.

RESULTS

Situation-specific, but not dispositional, catastrophizing led to less NFR inhibition but more pain inhibition. These effects were not moderated by race, but mediation analyses found that: (a) the NA race was associated with greater situation-specific catastrophizing, which led to less NFR inhibition and more pain inhibition, and (b) situation-specific catastrophizing was associated with greater CS pain, which led to more pain inhibition.

CONCLUSIONS

Catastrophizing may contribute to NA pain risk by disrupting descending inhibition.

Evolution of mechanisms and behaviour important for pain

Our understanding of the biology of pain is limited by our ignorance about its evolution. We know little about how states in other species showing various degrees of apparent similarity to human pain states are related to human pain, or how the mechanisms essential for pain-related states evolved. Nevertheless, insights into the evolution of mechanisms and behaviour important for pain are beginning to emerge from wide-ranging investigations of cellular mechanisms and behavioural responses linked to nociceptor activation, tissue injury, inflammation and the environmental context of these responses in diverse species. In February 2019, an unprecedented meeting on the evolution of pain hosted by the Royal Society brought together scientists from disparate fields who investigate nociception and pain-related behaviour in crustaceans, insects, leeches, gastropod and cephalopod molluscs, fish and mammals (primarily rodents and humans). Here, we identify evolutionary themes that connect these research efforts, including adaptive and maladaptive features of pain-related behavioural and neuronal alterations-some of which are quite general, and some that may apply primarily to humans. We also highlight major questions, including how pain should be defined, that need to be answered as we seek to understand the evolution of pain. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Adaptive mechanisms driving maladaptive pain: how chronic ongoing activity in primary nociceptors can enhance evolutionary fitness after severe injury

Chronic pain is considered maladaptive by clinicians because it provides no apparent protective or recuperative benefits. Similarly, evolutionary speculations have assumed that chronic pain represents maladaptive or evolutionarily neutral dysregulation of acute pain mechanisms. By contrast, the present hypothesis proposes that chronic pain can be driven by mechanisms that evolved to reduce increased vulnerability to attack from predators and aggressive conspecifics, which often target prey showing physical impairment after severe injury. Ongoing pain and anxiety persisting long after severe injury continue to enhance vigilance and behavioural caution, decreasing the heightened vulnerability to attack that results from motor impairment and disfigurement, thereby increasing survival and reproduction (fitness). This hypothesis is supported by evidence of animals surviving and reproducing after traumatic amputations, and by complex specializations that enable primary nociceptors to detect local and systemic signs of injury and inflammation, and to maintain low-frequency discharge that can promote ongoing pain indefinitely. Ongoing activity in nociceptors involves intricate electrophysiological and anatomical specializations, including inducible alterations in the expression of ion channels and receptors that produce persistent hyperexcitability and hypersensitivity to chemical signals of injury. Clinically maladaptive chronic pain may sometimes result from the recruitment of this powerful evolutionary adaptation to severe bodily injury. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Persistence of pain in humans and other mammals

Evolutionary models of chronic pain are relatively undeveloped, but mainly concern dysregulation of an efficient acute defence, or false alarm. Here, a third possibility, mismatch with the modern environment, is examined. In ancestral human and free-living animal environments, survival needs urge a return to activity during recovery, despite pain, but modern environments allow humans and domesticated animals prolonged inactivity after injury. This review uses the research literature to compare humans and other mammals, who share pain neurophysiology, on risk factors for pain persistence, behaviours associated with pain, and responses of conspecifics to behaviours. The mammal populations studied are mainly laboratory rodents in pain research, and farm and companion animals in veterinary research, with observations of captive and free-living primates. Beyond farm animals and rodent models, there is virtually no evidence of chronic pain in other mammals. Since evidence is sparse, it is hard to conclude that it does not occur, but its apparent absence is compatible with the mismatch hypothesis. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

The Landscape of Chronic Pain: Broader Perspectives

Chronic pain is a global health concern. This special issue on matters related to chronic pain aims to draw on research and scholarly discourse from an eclectic mix of areas and perspectives. The purpose of this non-systematic topical review is to précis an assortment of contemporary topics related to chronic pain and its management to nurture debate about research, practice and health care policy. The review discusses the phenomenon of pain, the struggle that patients have trying to legitimize their pain to others, the utility of the acute-chronic dichotomy, and the burden of chronic pain on society. The review describes the introduction of chronic primary pain in the World Health Organization's International Classification of Disease, 11th Revision and discusses the importance of biopsychosocial approaches to manage pain, the consequences of overprescribing and shifts in service delivery in primary care settings. The second half of the review explores pain perception as a multisensory perceptual inference discussing how contexts, predictions and expectations contribute to the malleability of somatosensations including pain, and how this knowledge can inform the development of therapies and strategies to alleviate pain. Finally, the review explores chronic pain through an evolutionary lens by comparing modern urban lifestyles with genetic heritage that encodes physiology adapted to live in the Paleolithic era. I speculate that modern urban lifestyles may be painogenic in nature, worsening chronic pain in individuals and burdening society at the population level.

Physical Cue Influences Children's Empathy for Pain: The Role of Attention Allocation

Empathy for pain is evolutionally important and context-dependent. The current study explored the effect of physical cue on 4- to 5-year-old children’s empathy for pain with two experiments. Experiment 1 investigated the effect of valid and invalid physical cue as compared to baseline (without cue) in pain evaluation task (evaluating the pain intensity of a facial expression, N = 28). Experiment 2 employed eye-tracking to investigate the attentional process in valid and baseline conditions (evaluating the pain intensity of a body image with an apparently injured arm or leg, N = 65). We found the evaluation of pain intensity was the highest in the valid condition, and higher in baseline condition than invalid. As for eye-tracking results, children fixated more quickly, had more fixations and longer total fixation duration in valid-cue condition. Of attention allocation, compared with baseline condition, children fixated on arm/leg more quickly, more frequently and for longer time in valid condition. Additionally, eye-tracking results were significantly related to their evaluation of pain intensity.

Isolated nociceptors reveal multiple specializations for generating irregular ongoing activity associated with ongoing pain

Ongoing pain has been linked to ongoing activity (OA) in human C-fiber nociceptors, but rodent models of pain-related OA have concentrated on allodynia rather than ongoing pain, and on OA generated in non-nociceptive Aβ fibers rather than C-fiber nociceptors. Little is known about how ongoing pain or nociceptor OA is generated. To define neurophysiological alterations underlying nociceptor OA, we have used isolated dorsal root ganglion neurons that continue to generate OA after removal from animals displaying ongoing pain. We subclassify OA as either spontaneous activity generated solely by alterations intrinsic to the active neuron or as extrinsically driven OA. Both types of OA were implicated previously in nociceptors in vivo and after isolation following spinal cord injury, which produces chronic ongoing pain. Using novel automated algorithms to analyze irregular changes in membrane potential, we have found, in a distinctive, nonaccommodating type of probable nociceptor, induction by spinal cord injury of 3 alterations that promote OA: (1) prolonged depolarization of resting membrane potential, (2) a hyperpolarizing shift in the voltage threshold for action potential generation, and (3) an increase in the incidence of large depolarizing spontaneous fluctuations (DSFs). Can DSFs also be enhanced acutely to promote OA in neurons from uninjured animals? A low dose of serotonin failed to change resting membrane potential but lowered action potential threshold. When combined with artificial depolarization to model inflammation, serotonin also strongly potentiated DSFs and OA. These findings reveal nociceptor specializations for generating OA that may promote ongoing pain in chronic and acute conditions.

Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain

Important insights into the selection pressures and core molecular modules contributing to the evolution of pain-related processes have come from studies of nociceptive systems in several molluscan and arthropod species. These phyla, and the chordates that include humans, last shared a common ancestor approximately 550 million years ago. Since then, animals in these phyla have continued to be subject to traumatic injury, often from predators, which has led to similar adaptive behaviors (e.g., withdrawal, escape, recuperative behavior) and physiological responses to injury in each group. Comparisons across these taxa provide clues about the contributions of convergent evolution and of conservation of ancient adaptive mechanisms to general nociceptive and pain-related functions. Primary nociceptors have been investigated extensively in a few molluscan and arthropod species, with studies of long-lasting nociceptive sensitization in the gastropod, Aplysia, and the insect, Drosophila, being especially fruitful. In Aplysia, nociceptive sensitization has been investigated as a model for aversive memory and for hyperalgesia. Neuromodulator-induced, activity-dependent, and axotomy-induced plasticity mechanisms have been defined in synapses, cell bodies, and axons of Aplysia primary nociceptors. Studies of nociceptive sensitization in Drosophila larvae have revealed numerous molecular contributors in primary nociceptors and interacting cells. Interestingly, molecular contributors examined thus far in Aplysia and Drosophila are largely different, but both sets overlap extensively with those in mammalian pain-related pathways. In contrast to results from Aplysia and Drosophila, nociceptive sensitization examined in moth larvae (Manduca) disclosed central hyperactivity but no obvious peripheral sensitization of nociceptive responses. Squid (Doryteuthis) show injury-induced sensitization manifested as behavioral hypersensitivity to tactile and especially visual stimuli, and as hypersensitivity and spontaneous activity in nociceptor terminals. Temporary blockade of nociceptor activity during injury subsequently increased mortality when injured squid were exposed to fish predators, providing the first demonstration in any animal of the adaptiveness of nociceptive sensitization. Immediate responses to noxious stimulation and nociceptive sensitization have also been examined behaviorally and physiologically in a snail (Helix), octopus (Adopus), crayfish (Astacus), hermit crab (Pagurus), and shore crab (Hemigrapsus). Molluscs and arthropods have systems that suppress nociceptive responses, but whether opioid systems play antinociceptive roles in these phyla is uncertain.

Interpersonal behavior in anticipation of pain: a naturalistic study of behavioral mimicry prior to surgery

Current and anticipated pain relate to mimicry behavior in divergent ways, which may bear upon the elicitation of social support during the experience of pain.

Introduction:

Social relationships facilitate coping with pain, but research suggests that it may be difficult to galvanize social support during an episode of acute pain.

Objectives:

The current research examined whether social connections are optimized in the anticipation of pain by observing patients' mimicry of an interaction partner prior to surgery. We hypothesized that when controlling for their current experience of pain, patients' anticipation of pain would be associated with greater mimicry of an interaction partner.

Methods:

Sixty-five patients were interviewed in the waiting room of a maxillofacial surgery unit prior to the removal of an impacted wisdom tooth. Patients' spontaneous mimicry of an interviewer was observed. Patients then rated the quality and intensity of their anticipated pain, as well as the intensity of their current pain and their affective distress.

Results:

Anticipated pain, current pain, and affective distress were positively correlated. Current pain was associated with less frequent mimicry of an interaction partner. The zero-order correlation between anticipated pain and mimicry did not reach conventional levels of significance; however, when controlling for current pain, anticipated pain predicted more frequent mimicry of an interaction partner. The relationship between anticipated pain and mimicry was not explained by affective distress.

Conclusion:

This is the first study to demonstrate that anticipated and current pain relate to behavioral mimicry in divergent ways. Further research is needed to investigate whether the current pattern of results generalizes to other interpersonal behaviors that facilitate social bonds.

Central neural alterations predominate in an insect model of nociceptive sensitization

Many organisms respond to noxious stimuli with defensive maneuvers. This is noted in the hornworm, Manduca sexta, as a defensive strike response. After tissue damage, organisms typically display sensitized responses to both noxious or normally innocuous stimuli. To further understand this phenomenon, we used novel in situ and in vitro preparations based on paired extracellular nerve recordings and videography to identify central and peripheral nerves responsible for nociception and sensitization of the defensive behavior in M. sexta. In addition, we used the in vivo defensive strike response threshold assayed with von Frey filaments to examine the roles that N-methyl-D-aspartate receptor (NMDAR) and hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels play in this nociceptive sensitization using the inhibitors MK-801 and AP5 (NMDAR), and ivabradine and ZD7288 (HCN). Using our new preparations, we found that afferent activity evoked by noxious pinch in these preparations was conveyed to central ganglia by axons in the anterior- and lateral-dorsal nerve branches, and that sensitization induced by tissue damage was mediated centrally. Furthermore, sensitization was blocked by all inhibitors tested except the inactive isomer L-AP5, and reversed by ivabradine both in vivo and in vitro. Our findings suggest that M. sexta's sensitization occurs through central signal amplification. Due to the relatively natural sensitization method and conserved molecular actions, we suggest that M. sexta may be a valuable model for studying the electrophysiological properties of nociceptive sensitization and potentially related conditions such as allodynia and hyperalgesia in a comparative setting that offers unique experimental advantages. J. Comp. Neurol. 525:1176-1191, 2017. © 2016 Wiley Periodicals, Inc.

How is chronic pain related to sympathetic dysfunction and autonomic dysreflexia following spinal cord injury?

Autonomic dysreflexia (AD) and neuropathic pain occur after severe injury to higher levels of the spinal cord. Mechanisms underlying these problems have rarely been integrated in proposed models of spinal cord injury (SCI). Several parallels suggest significant overlap of these mechanisms, although the relationships between sympathetic function (dysregulated in AD) and nociceptive function (dysregulated in neuropathic pain) are complex. One general mechanism likely to be shared is central sensitization - enhanced responsiveness and synaptic reorganization of spinal circuits that mediate sympathetic reflexes or that process and relay pain-related information to the brain. Another is enhanced sensory input to spinal circuits caused by extensive alterations in primary sensory neurons. Both AD and SCI-induced neuropathic pain are associated with spinal sprouting of peptidergic nociceptors that might increase synaptic input to the circuits involved in AD and SCI pain. In addition, numerous nociceptors become hyperexcitable, hypersensitive to chemicals associated with injury and inflammation, and spontaneously active, greatly amplifying sensory input to sensitized spinal circuits. As discussed with the aid of a preliminary functional model, these effects are likely to have mutually reinforcing relationships with each other, and with consequences of SCI-induced interruption of descending excitatory and inhibitory influences on spinal circuits, with SCI-induced inflammation in the spinal cord and in DRGs, and with activity in sympathetic fibers within DRGs that promotes local inflammation and spontaneous activity in sensory neurons. This model suggests that interventions selectively targeting hyperactivity in C-nociceptors might be useful for treating chronic pain and AD after high SCI.

Comparative biology of pain: What invertebrates can tell us about how nociception works

The inability to adequately treat chronic pain is a worldwide health care crisis. Pain has both an emotional and a sensory component, and this latter component, nociception, refers specifically to the detection of damaging or potentially damaging stimuli. Nociception represents a critical interaction between an animal and its environment and exhibits considerable evolutionary conservation across species. Using comparative approaches to understand the basic biology of nociception could promote the development of novel therapeutic strategies to treat pain, and studies of nociception in invertebrates can provide especially useful insights toward this goal. Both vertebrates and invertebrates exhibit segregated sensory pathways for nociceptive and nonnociceptive information, injury-induced sensitization to nociceptive and nonnociceptive stimuli, and even similar antinociceptive modulatory processes. In a number of invertebrate species, the central nervous system is understood in considerable detail, and it is often possible to record from and/or manipulate single identifiable neurons through either molecular genetic or physiological approaches. Invertebrates also provide an opportunity to study nociception in an ethologically relevant context that can provide novel insights into the nature of how injury-inducing stimuli produce persistent changes in behavior. Despite these advantages, invertebrates have been underutilized in nociception research. In this review, findings from invertebrate nociception studies are summarized, and proposals for how research using invertebrates can address questions about the fundamental mechanisms of nociception are presented.

Pain expressiveness and altruistic behavior: an exploration using agent-based modeling

Predictions which invoke evolutionary mechanisms are hard to test. Agent-based modeling in artificial life offers a way to simulate behaviors and interactions in specific physical or social environments over many generations. The outcomes have implications for understanding adaptive value of behaviors in context. Pain-related behavior in animals is communicated to other animals that might protect or help, or might exploit or predate. An agent-based model simulated the effects of displaying or not displaying pain (expresser/nonexpresser strategies) when injured and of helping, ignoring, or exploiting another in pain (altruistic/nonaltruistic/selfish strategies). Agents modeled in MATLAB interacted at random while foraging (gaining energy); random injury interrupted foraging for a fixed time unless help from an altruistic agent, who paid an energy cost, speeded recovery. Environmental and social conditions also varied, and each model ran for 10,000 iterations. Findings were meaningful in that, in general, contingencies that evident from experimental work with a variety of mammals, over a few interactions, were replicated in the agent-based model after selection pressure over many generations. More energy-demanding expression of pain reduced its frequency in successive generations, and increasing injury frequency resulted in fewer expressers and altruists. Allowing exploitation of injured agents decreased expression of pain to near zero, but altruists remained. Decreasing costs or increasing benefits of helping hardly changed its frequency, whereas increasing interaction rate between injured agents and helpers diminished the benefits to both. Agent-based modeling allows simulation of complex behaviors and environmental pressures over evolutionary time.

TMD and chronic pain: a current view

This review aims at presenting a current view on the physiopathologic mechanisms associated with temporomandibular disorders (TMDs). While joint pain is characterized by a well-defined inflammatory process mediated by tumor necrosis factor-α and interleukin, chronic muscle pain presents with enigmatic physiopathologic mechanisms, being considered a functional pain syndrome similar to fibromyalgia, irritable bowel syndrome, interstitial cystitis and chronic fatigue syndrome. Central sensitization is the common factor unifying these conditions, and may be influenced by the autonomic nervous system and genetic polymorphisms. Thus, TMDs symptoms should be understood as a complex response which might get worse or improve depending on an individual's adaptation.

Peripheral injury alters schooling behavior in squid, Doryteuthis pealeii

Animals with detectable injuries are at escalated threat of predation. The anti-predation tactic of schooling reduces individual predation risk overall, but it is not known how schooling behavior affects injured animals, or whether risks are reduced equally for injured animals versus other school members. In this laboratory study we examined the effects of minor fin injury on schooling decisions made by squid. Schooling behavior of groups of squid, in which one member was injured, was monitored over 24h. Injured squid were more likely to be members of a school shortly after injury (0.5-2h), but there were no differences compared with sham-injured squid at longer time points (6-24h). Overall, the presence of an injured conspecific increased the probability that a school would form, irrespective of whether the injured squid was a member of the school. When groups containing one injured squid were exposed to a predator cue, injured squid were more likely to join the school, but their position depended on whether the threat was a proximate visual cue or olfactory cue. We found no evidence that injured squid oriented themselves to conceal their injury from salient threats. Overall we conclude that nociceptive sensitization after injury changes grouping behaviors in ways that are likely to be adaptive.

Is blood glucose associated with descending modulation of spinal nociception as measured by the nociceptive flexion reflex?

Objectives

Prior research has shown a relationship between blood glucose levels and some forms of self-regulation (eg, executive function), with low blood glucose levels associated with impaired self-regulation. Further, engagement in self-regulation tasks depletes blood glucose. Given these relationships, the present study examined whether blood glucose is associated with another form of self-regulation, ie, descending pain modulatory processes.

Methods

Forty-seven (32 female) pain-free participants were recruited and completed testing. Blood glucose was measured from finger sticks and a digital meter before and after experimental pain tests. Pain tests included the nociceptive flexion reflex (NFR) threshold to assess descending modulation of spinal nociception, but also electric pain threshold to assess perceptual pain detection. The Stroop color word naming test was also assessed before and after pain testing to examine changes in executive function.

Results

Results indicated that mean blood glucose levels decreased after pain testing, but Stroop performance did not significantly change. Importantly, changes in blood glucose were correlated with NFR threshold, such that decreases in blood glucose were associated with lower NFR thresholds (reduced descending inhibition). Changes in blood glucose were unrelated to pain threshold or executive function.

Conclusion

This study suggests that glucose depletion may impair performance of descending inhibitory processes, without impacting the perceptual detection of pain (pain threshold). Although findings need to be replicated, maintaining adequate glucose levels may be necessary to support inhibition of spinal nociception.

Can Pain or Hyperalgesia Be a Classically Conditioned Response in Humans? A Systematic Review and Meta-Analysis

BACKGROUND

Clinical scenarios of repeated pain usually involve both nociceptive and non-nociceptive input. It is likely that associations between these stimuli are learned over time. Such learning may underlie subsequent amplification of pain, or evocation of pain in the absence of nociception.

METHODS

We undertook a systematic review and meta-analysis to evaluate the evidence that allodynia or hyperalgesia can be a classically conditioned response. A sensitive search of the literature covered Medline, Embase, CINAHL, AMED, PubMed, Scopus, PsycArticles, PsycINFO, Cochrane Library, and Web of Science. Additional studies were identified by contacting experts and searching published reviews. Two reviewers independently assessed studies for inclusion, evaluated risk of bias, and extracted data. Studies were included if they aimed to elicit or amplify pain using a classical conditioning procedure in healthy, adult humans. Studies were excluded if they did not distinguish between classical conditioning and explicit verbal suggestion as learning sources, or did not use experiential learning.

RESULTS

Thirteen studies, with varying risk of bias, were included. Ten studies evaluated classically conditioned hyperalgesia: nine found hyperalgesia; one did not. Pooled effects (n = 8 with full data) showed a significant pain increase after conditioning (mean difference of 7.40 [95%CI: 4.00-10.80] on a 0-100 pain scale). Three studies evaluated conditioned allodynia and found conflicting results.

CONCLUSION

The existing literature suggests that classical conditioning can amplify pain. No conclusions can be drawn about whether or not classical conditioning can elicit pain. Rigorous experimental conditioning studies with nociceptive unconditioned stimuli are needed to fill this gap in knowledge.

Use of von Frey filaments to assess nociceptive sensitization in the hornworm, Manduca sexta

BACKGROUND

The hornworm Manduca sexta exhibits a defensive strike to noxious assaults, a response that is robust and is easily observed by experimenters. Von Frey filaments and methods typical for studying nociception in other animals were used to assess the strike response in M. sexta.

NEW METHODS

A series of von Frey filaments was applied to the body wall in ascending order and the data generated were used to determine the strike threshold by (i) the up-and-down method, (ii) the first response method, and (iii) the simplified up-and-down order method (SUDO). The effect of a noxious pinch on strike threshold was assessed.

COMPARISON WITH EXISTING METHODS

To our knowledge none of these methods has been used on M. sexta previously, making the use of the up-and-down and SUDO methods the first in an invertebrate. The use of the first response method has been used in other invertebrates, and the method appears equally suited to M. sexta.

RESULTS

All three methods were successful in monitoring the threshold sensitivity to touch, which was lowered (sensitized) by tissue damage induced with a pinch. Sensitization lasted 19h.

CONCLUSIONS

All three methods of assessing nociception were successfully applied to quantify the defensive strike response in M. sexta, although the SUDO method required empirical assessment of which filament to start the test sequence with. The results revealed both short- and long-term sensitization. These methods should prove to be useful for quantifying sensitization in M. sexta.

New insights of nociceptor sensitization in bone cancer pain

INTRODUCTION

Numerous studies have shown that an intact CNS is required for the conscious perception of cancer-induced bone pain (CIBP) and that changes in the CNS are clearly evident. Accordingly, the blockage of nociceptive stimulus into the CNS can effectively relieve or markedly attenuate CIBP, revealing the clinical implication of the blockage of ongoing peripheral inputs for the control of CIBP.

AREAS COVERED

In this review, the heterogeneity and excitability of nociceptors in bone are covered. Furthermore, their role in initiating and maintaining CIBP is also described.

EXPERT OPINION

Developing mechanistic therapies to treat CIBP is a challenge, but they have the potential to fundamentally change our ability to effectively block/relieve CIBP and increase the functional status and quality of life of patients with bone metastasis. Further studies are desperately needed at both the preclinical and clinical levels to determine whether the targets as mentioned in this review are viable and feasible for patient populations.

Acute psychosocial stress reduces pain modulation capabilities in healthy men

Anecdotes on the ability of individuals to continue to function under stressful conditions despite injuries causing excruciating pain suggest that acute stress may induce analgesia. However, studies exploring the effect of acute experimental stress on pain perception show inconsistent results, possibly due to methodological differences. Our aim was to systematically study the effect of acute stress on pain perception using static and dynamic, state-of-the-art pain measurements. Participants were 29 healthy men who underwent the measurement of heat-pain threshold, heat-pain intolerance, temporal summation of pain, and conditioned pain modulation (CPM). Testing was conducted before and during exposure to the Montreal Imaging Stress Task (MIST), inducing acute psychosocial stress. Stress levels were evaluated using perceived ratings of stress and anxiety, autonomic variables, and salivary cortisol. The MIST induced a significant stress reaction. Although pain threshold and pain intolerance were unaffected by stress, an increase in temporal summation of pain and a decrease in CPM were observed. These changes were significantly more robust among individuals with stronger reaction to stress ("high responders"), with a significant correlation between the perception of stress and the performance in the pain measurements. We conclude that acute psychosocial stress seems not to affect the sensitivity to pain, however, it significantly reduces the ability to modulate pain in a dose-response manner. Considering the diverse effects of stress in this and other studies, it appears that the type of stress and the magnitude of its appraisal determine its interactions with the pain system.

Emotional modulation of pain and spinal nociception in persons with severe insomnia symptoms

BACKGROUND

Impaired sleep enhances pain, perhaps by disrupting pain modulation.

PURPOSE

Given that emotion modulates pain, the present study examined whether emotional modulation of pain and nociception is impaired in persons with severe insomnia symptoms relative to controls.

METHODS

Insomnia group (n = 12) met the International Classification of Diseases, tenth revision symptoms for primary insomnia and controls (n = 13) reported no sleep impairment. Participants were shown emotionally evocative pictures (mutilation, neutral, and erotica) during which suprathreshold pain stimuli were delivered to evoke pain and the nociceptive flexion reflex (NFR; physiological correlate of spinal nociception).

RESULTS

Emotional responses to pictures were similar in both groups, except that subjective valence/pleasure ratings were blunted in insomnia. Emotional modulation of pain and NFR was observed in controls, but only emotional modulation of NFR was observed in insomnia.

CONCLUSIONS

Consistent with previous findings, pain modulation is disrupted in insomnia, which might promote pain. This may stem from disrupted supraspinal circuits not disrupted brain-to-spinal cord circuits.

Nociceptive sensitization reduces predation risk

Sublethal injury triggers long-lasting sensitization of defensive responses in most species examined, suggesting the involvement of powerful evolutionary selection pressures [1]. In humans, this persistent nociceptive sensitization is often accompanied by heightened sensations of pain and anxiety [2]. While experimental [3] and clinical [4] evidence support the adaptive value of immediate nociception during injury, no direct evidence exists for adaptive benefits of long-lasting sensitization after injury. Recently, we showed that minor injury produces long-term sensitization of behavioral and neuronal responses in squid, Doryteuthis pealei [5, 6]. Here we tested the adaptive value of this sensitization during encounters between squid and a natural fish predator. Locomotion and other spontaneous behaviors of squid that received distal injury to a single arm (with or without transient anesthesia) showed no measurable impairment 6 hr after the injury. However, black sea bass given access to freely swimming squid oriented toward and pursued injured squid at greater distances than uninjured squid, regardless of previous anesthetic treatment. Once targeted, injured squid began defensive behavioral sequences [7, 8] earlier than uninjured squid. This effect was blocked by brief anesthetic treatment that prevented development of nociceptive sensitization [6, 9]. Importantly, the early anesthetic treatment also reduced the subsequent escape and survival of injured, but not uninjured, squid. Thus, while minor injury increases the risk of predatory attack, it also triggers a sensitized state that promotes enhanced responsiveness to threats, increasing the survival (Darwinian fitness) of injured animals during subsequent predatory encounters.

The Positive Consequences of Pain: A Biopsychosocial Approach

Pain is mostly thought of as a problem-as debilitating or harmful. Despite its unpleasantness, however, under some conditions pain can be associated with positive consequences. In this review, we explore these positive biological, psychological, and social consequences of pain. We highlight three different domains in which pain may be considered to have positive consequences. First, pain facilitates pleasure by providing an important contrast for pleasurable experiences, increasing sensitivity to sensory input, and facilitating self-rewarding behavior. Second, pain augments self-regulation and enhancement by increasing cognitive control, reducing rumination, and demonstrating virtue. Third, pain promotes affiliation by arousing empathy from others, motivating social connection, and enhancing group formation. Drawing on evidence scattered across a range of academic fields, we provide for reflection on how pain is represented, generate insights into pain-seeking behavior, and draw attention to the role of painful experiences in maximizing positive outcomes.