Formalin pain is expressed in decerebrate rats but not attenuated by morphine

Between Scylla and Charybdis: assessing the multidimensional aspects of pain behaviors in rats using a double avoidance place preference paradigm

ABSTRACT

Although the behavioral response to pain is complex and involves supraspinal processes, assessment of pain symptoms in animal models still mainly relies on reflex-based nociceptive tests, which do not account for the affective-motivational nor cognitive components of pain. We introduce a double avoidance place preference paradigm, an integrated testing procedure in freely moving rats that relies on the conflict between the avoidance of a dark compartment in which a thermal ramp is activated, and the escape towards an aversive brightly lit compartment. We were able to differentiate the first nociceptive threshold from the temperature of definitive escape from the dark compartment, conveying information on the adaptive behavior of animals. Measures were repeated after an hour to evaluate the adaptive learning response upon reexposure. In naive animals, there was a significant decrease in the time spent in the dark compartment at all stages of the testing paradigm upon reexposure, leading to a final escape before the flood had reached nociceptive values. This adaptive behavior was blunted by anxiolytic treatment. In animals exhibiting hyperalgesia following intraplantar complete Freund adjuvant injection, escape thresholds were significantly higher than that of control animals, hinting at a maladaptive affective-motivational response to noxious stimulation. However, in cuff animals, we failed to reveal any hot nociceptive hypersensitivity, but animals exhibited a strong adaptive response to cold simulation upon reexposure. Overall, the proposed paradigm allows for an integrated cortical response leading to a proactive avoidance behavior, while fully complying with ethical standards in animal experimentation.

An electroencephalogram biomarker of fentanyl drug effects

Opioid drugs influence multiple brain circuits in parallel to produce analgesia as well as side effects, including respiratory depression. At present, we do not have real-time clinical biomarkers of these brain effects. Here, we describe the results of an experiment to characterize the electroencephalographic signatures of fentanyl in humans. We find that increasing concentrations of fentanyl induce a frontal theta band (4 to 8 Hz) signature distinct from slow-delta oscillations related to sleep and sedation. We also report that respiratory depression, quantified by decline in an index of instantaneous minute ventilation, occurs at ≈1700-fold lower concentrations than those that produce sedation as measured by reaction time. The electroencephalogram biomarker we describe could facilitate real-time monitoring of opioid drug effects and enable more precise and personalized opioid administration.

What Is It Like to Be a Bass? Red Herrings, Fish Pain and the Study of Animal Sentience

Debates around fishes' ability to feel pain concern sentience: do reactions to tissue damage indicate evaluative consciousness (conscious affect), or mere nociception? Thanks to Braithwaite's discovery of trout nociceptors, and concerns that current practices could compromise welfare in countless fish, this issue's importance is beyond dispute. However, nociceptors are merely necessary, not sufficient, for true pain, and many measures held to indicate sentience have the same problem. The question of whether fish feel pain - or indeed anything at all - therefore stimulates sometimes polarized debate. Here, we try to bridge the divide. After reviewing key consciousness concepts, we identify "red herring" measures that should not be used to infer sentience because also present in non-sentient organisms, notably those lacking nervous systems, like plants and protozoa (P); spines disconnected from brains (S); decerebrate mammals and birds (D); and humans in unaware states (U). These "S.P.U.D. subjects" can show approach/withdrawal; react with apparent emotion; change their reactivity with food deprivation or analgesia; discriminate between stimuli; display Pavlovian learning, including some forms of trace conditioning; and even learn simple instrumental responses. Consequently, none of these responses are good indicators of sentience. Potentially more valid are aspects of working memory, operant conditioning, the self-report of state, and forms of higher order cognition. We suggest new experiments on humans to test these hypotheses, as well as modifications to tests for "mental time travel" and self-awareness (e.g., mirror self-recognition) that could allow these to now probe sentience (since currently they reflect perceptual rather than evaluative, affective aspects of consciousness). Because "bullet-proof" neurological and behavioral indicators of sentience are thus still lacking, agnosticism about fish sentience remains widespread. To end, we address how to balance such doubts with welfare protection, discussing concerns raised by key skeptics in this debate. Overall, we celebrate the rigorous evidential standards required by those unconvinced that fish are sentient; laud the compassion and ethical rigor shown by those advocating for welfare protections; and seek to show how precautionary principles still support protecting fish from physical harm.

Nociception in the Glycine Receptor Deficient Mutant Mouse Spastic

Glycine receptors (GlyRs) are the primary mediators of fast inhibitory transmission in the mammalian spinal cord, where they modulate sensory and motor signaling. Mutations in GlyR genes as well as some other genes underlie the hereditary disorder hyperekplexia, characterized by episodic muscle stiffness and exaggerated startle responses. Here, we have investigated pain-related behavior and GlyR expression in the spinal cord of the GlyR deficient mutant mouse spastic (spa). In spastic mice, the GlyR number is reduced due to a β subunit gene (Glrb) mutation resulting in aberrant splicing of GlyRβ transcripts. Via direct physical interaction with the GlyR anchoring protein gephyrin, this subunit is crucially involved in the postsynaptic clustering of heteromeric GlyRs. We show that the mutation differentially affects aspects of the pain-related behavior of homozygous Glrb^spa/Glrb^spa mice. While response latencies to noxious heat were unchanged, chemically induced pain-related behavior revealed a reduction of the licking time and an increase in flinching in spastic homozygotes during both phases of the formalin test. Mechanically induced nocifensive behavior was reduced in spastic mice, although hind paw inflammation (by zymosan) resulted in allodynia comparable to wild-type mice. Immunohistochemical staining of the spinal cord revealed a massive reduction of dotted GlyRα subunit immunoreactivity in both ventral and dorsal horns, suggesting a reduction of clustered receptors at synaptic sites. Transcripts for all GlyRα subunit variants, however, were not reduced throughout the dorsal horn of spastic mice. These findings suggest that the loss of functional GlyRβ subunits and hence synaptically localized GlyRs compromises sensory processing differentially, depending on stimulus modality.

A functional subdivision within the somatosensory system and its implications for pain research

Somatosensory afferents are traditionally classified by soma size, myelination, and their response specificity to external and internal stimuli. Here, we propose the functional subdivision of the nociceptive somatosensory system into two branches. The exteroceptive branch detects external threats and drives reflexive-defensive reactions to prevent or limit injury. The interoceptive branch senses the disruption of body integrity, produces tonic pain with strong aversive emotional components, and drives self-caring responses toward to the injured region to reduce suffering. The central thesis behind this functional subdivision comes from a reflection on the dilemma faced by the pain research field, namely, the use of reflexive-defensive behaviors as surrogate assays for interoceptive tonic pain. The interpretation of these assays is now being challenged by the discovery of distinct but interwoven circuits that drive exteroceptive versus interoceptive types of behaviors, with the conflation of these two components contributing partially to the poor translation of therapies from preclinical studies.

Innovations and advances in modelling and measuring pain in animals

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.

Comparison of morphine, oxycodone and the biased MOR agonist SR-17018 for tolerance and efficacy in mouse models of pain

The mu opioid receptor-selective agonist, SR-17018, preferentially activates GTPγS binding over βarrestin2 recruitment in cellular assays, thereby demonstrating signaling bias. In mice, SR-17018 stimulates GTPγS binding in brainstem and produces antinociception with potencies similar to morphine. However, it produces much less respiratory suppression and mice do not develop antinociceptive tolerance in the hot plate assay upon repeated dosing. Herein we evaluate the effects of acute and repeated dosing of SR-17018, oxycodone and morphine in additional models of pain-related behaviors. In the mouse warm water tail immersion assay, an assessment of spinal reflex to thermal nociception, repeated administration of SR-17018 produces tolerance as does morphine and oxycodone. SR-17018 retains efficacy in a formalin-induced inflammatory pain model upon repeated dosing, while oxycodone does not. In a chemotherapeutic-induced neuropathy pain model SR-17018 is more potent and efficacious than morphine or oxycodone, moreover, this efficacy is retained upon repeated dosing of SR-17018. These findings demonstrate that, with the exception of the tail flick test, SR-17018 retains efficacy upon chronic treatment across several pain models.

Anti-hypernociceptive and anti-inflammatory effects of JM-20: A novel hybrid neuroprotective compound

The present study examines the possible effect of the novel hybrid molecule JM-20 (3-ethoxycarbonyl-2-methyl-4-(2-nitrophenyl)-411-dihydro-1H-pyrido[2,3-b] [1,5] benzodiazepine) on pain-related behaviours in a persistent pain model (5% formalin test) and in the neutrophil migration events during the inflammatory process. It further introduces JM-20 in a chronic constriction injury (CCI) model to clarify the possible subjacent mechanisms with its consequent clinical relevance. A single administration of JM-20 (20 or 40 mg/kg, per os [p.o.]) decreased licking/biting exclusively in the tonic phase of the formalin test in a GABA/benzodiazepine (BZD) receptor antagonist flumazenil-sensitive manner. JM-20 reduced in vivo neutrophil migration, rolling and adhesion to the endothelium induced by intraperitoneal administration of carrageenan in mice. In addition, plasma extravasation and tumour necrosis factor alpha production in the peritoneal fluid were decreased. Treatment with JM-20 (20 mg/kg, p.o.) for 7 days after CCI reduced mechanical hypersensitivity in a NG-monomethyl-l-arginine (L-NMMA)/methylene blue/glibenclamide-sensitive manner. Histopathological signs of Wallerian degeneration (WD) of the sciatic nerve were also attenuated, as well as interleukin-1 beta release in the spinal cord. The nitrate/nitrite concentration was increased centrally and did not show differences at the peripheral nerve level. The findings of this study suggest JM-20 can decrease persistent pain. A transient activity of its BDZ portion on nociceptive pathways mediated by GABA/BDZ receptors in association with its anti-inflammatory properties could be at least partially involved in this effect. JM-20 decreased CCI-induced mechanical hypersensitivity via the l-arginine/nitric oxide (NO)/cyclic GMP-sensitive ATP-sensitive potassium channel pathway. Its neuroprotective ability by preventing WD could be implicated in its anti-neuropathic mechanisms.

Automated and rapid self-report of nociception in transgenic mice

There are currently no rapid, operant pain behaviors in rodents that use a self-report to directly engage higher-order brain circuitry. We have developed a pain detection assay consisting of a lick behavior in response to optogenetic activation of predominantly nociceptive peripheral afferent nerve fibers in head-restrained transgenic mice expressing ChR2 in TRPV1 containing neurons. TRPV1-ChR2-EYFP mice (n = 5) were trained to provide lick reports to the detection of light-evoked nociceptive stimulation to the hind paw. Using simultaneous video recording, we demonstrate that the learned lick behavior may prove more pertinent in investigating brain driven pain processes than the reflex behavior. Within sessions, the response bias of transgenic mice changed with respect to lick behavior but not reflex behavior. Furthermore, response similarity between the lick and reflex behaviors diverged near perceptual threshold. Our nociceptive lick-report detection assay will enable a host of investigations into the millisecond, single cell, neural dynamics underlying pain processing in the central nervous system of awake behaving animals.

Developing Improved Translational Models of Pain: A Role for the Behavioral Scientist

The effective management of pain is a longstanding public health concern. Although opioids have been frontline analgesics for decades, they also have well-known undesirable effects that limit their clinical utility, such as abuse liability and respiratory depression. The failure to develop better analgesics has, in some ways, contributed to the escalating opioid epidemic that has claimed tens of thousands of lives and has cost hundreds of billions of dollars in health-care expenses. A paradigm shift is needed in the pharmacotherapy of pain management that will require extensive efforts throughout biomedical science. The purpose of the present review is to highlight the critical role of the behavioral scientist to devise improved translational models of pain for drug development. Despite high heterogeneity of painful conditions that involve cortical-dependent pain processing, current models often feature an overreliance on simple reflex-based measures and an emphasis on the absence, rather than presence, of behavior as evidence of analgesic efficacy. Novel approaches should focus on the restoration of operant and other CNS-mediated behavior under painful conditions.

Mangiferin: Possible uses in the prevention and treatment of mixed osteoarthritic pain

Osteoarthritis (OA) pain has been proposed to be a mixed pain state, because in some patients, central nervous system factors are superimposed upon the more traditional peripheral factors. In addition, a considerable amount of preclinical and clinical evidence has shown that, accompanying the central neuroplasticity changes and partially driven by a peripheral nociceptive input, a real neuropathic component occurs that are particularly linked to disease severity and progression. Hence, innovative strategies targeting neuroprotection and particularly neuroinflammation to prevent and treat OA pain could be introduced. Mangiferin (MG) is a glucosylxanthone that is broadly distributed in higher plants, such as Mangifera indica L. Previous studies have documented its analgesic, anti-inflammatory, antioxidant, neuroprotective, and immunomodulatory properties. In this paper, we propose its potential utility as a multitargeted compound for mixed OA pain, even in the context of multimodal pharmacotherapy. This hypothesis is supported by three main aspects: the cumulus of preclinical evidence around this xanthone, some preliminary clinical results using formulations containing MG in clinical musculoskeletal or neuropathic pain, and by speculations regarding its possible mechanism of action according to recent advances in OA pain knowledge.

Calf aversion to hot-iron disbudding

Dairy calves are routinely disbudded by cauterization with a hot iron. To mitigate the intra-operative and initial post-operative pain associated with this procedure some farmers provide calves general and local anesthetics, but it is unknown if the procedure remains aversive. We used a place-conditioning paradigm to assess aversion caused by hot-iron cautery with a local anesthetic compared to a sham procedure. A test area was divided into three equally sized pens: two 'treatment' pens with distinct visual cues were connected by a central 'neutral' pen. Each calf went through the disbudding procedure and a 6-h recovery period in one treatment pen and the control procedure in the other treatment pen. In three tests (48, 72 and 96 h after the second treatment), calves could freely roam among the pens until they chose to lie down, ending the session. Calves spent less time in either of the treatment pens compared to the central pen. When only comparing the two treatment pen, calves spent less time in the disbudding pen, especially during the first test. Calves were also less likely to lie down in the pen associated with the disbudding procedure. We conclude that even with the use of a local anesthetic, hot-iron disbudding is salient and aversive for calves, indicating the need to refine or avoid the procedure.

Why mu-opioid agonists have less analgesic efficacy in neuropathic pain?

Injury to peripheral nerves often leads to abnormal pain states (hyperalgesia, allodynia and spontaneous pain), which can remain long after the injury heals. Although opioid agonists remain the gold standard for the treatment of moderate to severe pain, they show reduced efficacy against neuropathic pain. In addition to analgesia, opioid use is also associated with hyperalgesia and analgesia tolerance, whose underlying mechanisms share some commonalities with nerve injury-induced hypersensitivity. Here, we reviewed up-to-day research exploring the contribution of mu-opioid receptor (MOR) on the pathophysiology of neuropathic pain and on analgesic opioid actions under these conditions. We focused on the specific contributions of MOR populations at peripheral, spinal and supraspinal level. Moreover, evidences of neuroplastic changes that may underlie the low efficacy of MOR agonists under neuropathic pain conditions are reviewed and discussed. Sensitization processes leading to pain hypersensitivity, molecular changes in signalling pathways triggered by MOR and glial activation are some of these mechanisms elicited by both nerve injury and opioid exposure. Nerve injury-induced pain hypersensitivity might be masking the initial analgesic effects of opioid agonists, and alternatively, sustained opioid treatment to individuals already suffering from neuropathic pain could aggravate their pathophysiological state. Finally, some combined therapies that can increase opioid analgesic effectiveness in neuropathic pain treatment are highlighted. SIGNIFICANCE: This review provides evidence of the low benefit of opioid monotherapy in neuropathic pain and analyses the reasons of this reduced effectiveness. Opioid agonists along with drugs targeted to block the sensitization processes induced by MOR stimulation might result in a better management of neuropathic pain.

Neuropathic Pain After Spinal Cord Injury: Challenges and Research Perspectives

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that remains difficult to treat because underlying mechanisms are not yet fully understood. In part, this is due to limitations of evaluating neuropathic pain in animal models in general, and SCI rodents in particular. Though pain in patients is primarily spontaneous, with relatively few patients experiencing evoked pains, animal models of SCI pain have primarily relied upon evoked withdrawals. Greater use of operant tasks for evaluation of the affective dimension of pain in rodents is needed, but these tests have their own limitations such that additional studies of the relationship between evoked withdrawals and operant outcomes are recommended. In preclinical SCI models, enhanced reflex withdrawal or pain responses can arise from pathological changes that occur at any point along the sensory neuraxis. Use of quantitative sensory testing for identification of optimal treatment approach may yield improved identification of treatment options and clinical trial design. Additionally, a better understanding of the differences between mechanisms contributing to at- versus below-level neuropathic pain and neuropathic pain versus spasticity may shed insights into novel treatment options. Finally, the role of patient characteristics such as age and sex in pathogenesis of neuropathic SCI pain remains to be addressed.

Burst Spinal Cord Stimulation in Peripherally Injured Chronic Neuropathic Rats: A Delayed Effect

OBJECTIVE

Two well-known spinal cord stimulation (SCS) paradigms, conventional (Con) and burst SCS, are hypothesized to exert their antinociceptive effects through different stimulation-induced mechanisms. We studied the course of the behavioral antinociceptive effect during 60 minutes of SCS and 30 minutes post-SCS in a rat model of chronic neuropathic pain.

METHODS

Animals underwent a unilateral partial sciatic nerve ligation, after which quadripolar electrodes were implanted into the epidural space at vertebral level T13 (n = 43 rats). While receiving either Con SCS or biphasic burst SCS, the pain behavior of the rats was assessed by means of paw withdrawal thresholds (WTs) in response to the application of von Frey monofilaments.

RESULTS

After 15 minutes of Con SCS (n = 21), WTs significantly differed from baseline (P = 0.04), whereas WTs of the burst SCS group (n = 22) did not. After 30 minutes of SCS, WTs of the Con SCS and burst SCS groups reached similar levels, both significantly different from baseline, indicating a comparable antinociceptive effect for these SCS paradigms. Yet, the WTs of the burst SCS group were still significantly increased compared with baseline at 30 minutes post-stimulation, whereas the WTs of the Con SCS group were not.

CONCLUSIONS

To conclude, biphasic burst SCS results in a delayed antinociceptive effect after onset of the stimulation, as compared with Con SCS, in a chronic neuropathic pain model. Furthermore, biphasic burst SCS seems to exhibit a delayed wash-out of analgesia after stimulation is turned off.

Conventional-SCS vs. Burst-SCS and the Behavioral Effect on Mechanical Hypersensitivity in a Rat Model of Chronic Neuropathic Pain: Effect of Amplitude

OBJECTIVE

Various spinal cord stimulation (SCS) modes are used in the treatment of chronic neuropathic pain disorders. Conventional (Con) and Burst-SCS are hypothesized to exert analgesic effects through different stimulation-induced mechanisms. Preclinical electrophysiological findings suggest that stimulation intensity is correlated with the effectiveness of Burst-SCS. Therefore, we aimed to investigate the relation between amplitude (charge per second) and behavioral effects in a rat model of chronic neuropathic pain, for both Conventional Spinal Cord Stimulation (Con-SCS) and biphasic Burst-SCS.

MATERIALS AND METHODS

Animals (n = 12 rats) received a unilateral partial sciatic nerve ligation, after which they were implanted with quadripolar electrodes in the epidural space at thoracic level 13. Mechanical hypersensitivity was assessed using paw withdrawal thresholds (WTs) to von Frey monofilaments, at various SCS intensities (amplitudes) and multiple time points during 60 minutes of stimulation and 30 minutes post stimulation.

RESULTS

Increasing amplitude was shown to improve the efficacy of Con-SCS, whereas the efficacy of Burst-SCS showed a non-monotonic relation with amplitude. Con-SCS at 66% MT (n = 5) and Burst-SCS at 50% MT (n = 6) were found to be equally effective in normalizing mechanical hypersensitivity. However, in the assessed time period Burst-SCS required significantly more mean charge per second to do so (p < 0.01). When applied at comparable mean charge per second, Con-SCS resulted in a superior behavioral outcome (p < 0.01), compared with Burst-SCS.

CONCLUSION

Biphasic Burst-SCS requires significantly more mean charge per second in order to achieve similar pain relief, as compared with Con-SCS, in an experimental model of chronic neuropathic pain.

The role of peripheral vasopressin 1A and oxytocin receptors on the subcutaneous vasopressin antinociceptive effects

BACKGROUND

Vasopressin (AVP) seems to play a role as an antinociceptive neurohormone, but little is known about the peripheral site of action of its antinociceptive effects. Moreover, AVP can produce motor impairment that could be confused with behavioural antinociception. Finally, it is not clear which receptor is involved in the peripheral antinociceptive AVP effects.

METHODS

In anaesthetized rats with end-tidal CO₂ monitoring, extracellular unitary recordings were performed, measuring the evoked activity mediated by Aβ-, Aδ-, C-fibres and post-discharge. Behavioural nociception and motor impairment were evaluated under subcutaneous AVP (0.1-10 μg) using formalin and rotarod tests. Selective antagonists to vasopressin (V_1A R) or oxytocin receptors (OTR) were used. Additionally, vasopressin and oxytocin receptors were explored immunohistochemically in skin tissues.

RESULTS

Subcutaneous AVP (1 and 10 μg/paw) induced antinociception and a transitory reduction of the end-tidal CO₂ . The neuronal activity associated with Aδ- and C-fibre activation was diminished, but no effect was observed on Aβ-fibres. AVP also reduced paw flinches in the formalin test and a transitory locomotor impairment was also found. The AVP-induced antinociception was blocked by the selective antagonist to V_1A R (SR49059) or OTR (L368,899). Immunohistochemical evidence of skin VP and OT receptors is given.

CONCLUSIONS

Subcutaneous AVP produces antinociception and behavioural analgesia. Both V1a and OTR participate in those effects. Our findings suggest that antinociception could be produced in a local manner using a novel vasopressin receptor located in cutaneous sensorial fibres. Additionally, subcutaneous AVP also produces important systemic effects such as respiratory and locomotor impairment.

SIGNIFICANCE

Our findings support that AVP produces peripheral antinociception and behavioural analgesia in a local manner; nevertheless, systemic effects are also presented. Additionally, this is the first detailed electrophysiological analysis of AVP antinociceptive action after subcutaneous administration. The results are reasonably explained by the demonstration of V_1A R and OTR in cutaneous fibres.

Dissociation of morphine analgesic effects in the sensory and affective components of formalin-induced spontaneous pain in male and female rats

Sex differences in the analgesic effects of morphine have been previously reported in various models that represent the sensory component of pain. However, pain sensation is a complex process that consists of both sensory and affective components. It is presently unclear whether the analgesic effects of morphine between the sensory and affective components of pain are sexually dimorphic. Moreover, differences in morphine dose-response in the two components of pain have not been examined in male and female rats. Therefore, we examined the analgesic effects of morphine on the sensory and affective components of formalin-induced pain behaviors in male and female rats. To discern the sensory component, rats were pretreated with varying doses of morphine and then intraplantar formalin-induced paw flinches were measured. Morphine reduced the number of formalin-induced paw flinches at a treatment dose of 4.0mg/kg. Morphine analgesia was similar across the sexes in the early (phase 1) and late phase (phase 2) of the formalin test. To examine the affective component, rats were pretreated with varying doses of morphine, and then intraplantar formalin-induced conditioned place aversion (CPA) was examined. Formalin produced CPA, which was blocked by morphine at doses of 1.0mg/kg and higher in male and female rats. Lastly, formalin-induced cFos expression and the effects of systemic morphine were examined in the superficial dorsal horn of the spinal cord. Intraplantar formalin produced robust expression of cFos; however, morphine did not attenuate the cFos expression. These results demonstrate a notable dissociation of the analgesic effects of morphine by detecting a fourfold shift in the minimum effective dose between the sensory and affective components of formalin-induced spontaneous pain, that were similar between male and female rats. The findings further suggest disparate mechanisms involved in systemic morphine-induced analgesia in the two components of formalin-induced pain.

Comparison of operant escape and reflex tests of nociceptive sensitivity

Testing of reflexes such as flexion/withdrawal or licking/guarding is well established as the standard for evaluating nociceptive sensitivity and its modulation in preclinical investigations of laboratory animals. Concerns about this approach have been dismissed for practical reasons - reflex testing requires no training of the animals; it is simple to instrument; and responses are characterized by observers as latencies or thresholds for evocation. In order to evaluate this method, the present review summarizes a series of experiments in which reflex and operant escape responding are compared in normal animals and following surgical models of neuropathic pain or pharmacological intervention for pain. Particular attention is paid to relationships between reflex and escape responding and information on the pain sensitivity of normal human subjects or patients with pain. Numerous disparities between results for reflex and operant escape measures are described, but the results of operant testing are consistent with evidence from humans. Objective reasons are given for experimenters to choose between these and other methods of evaluating the nociceptive sensitivity of laboratory animals.

The pain of pain: challenges of animal behavior models

Berend Olivier has had a long-standing interest in the utility of animal models for a wide variety of therapeutic indications. His work has spanned multiple types of models, blending ethological, or species typical and naturalistic behaviors, along with methodologies based on learned behavior. He has consistently done so, from an analytical as well as predictive perspective, and has made multiple contributions while working in both the pharmaceutical industry and within an academic institution. Although focused primarily on psychiatric disorders, Berend has conducted research in the area of pain in humans and in animals, demonstrating an expansive appreciation for the breadth, scope and significance of the science and applications of the discipline of pharmacology to these diverse areas. This review focuses on the use of animal models in pain research from the perspective of the long-standing deficiencies in the development of therapeutics in this area and from a preclinical perspective where the translational weaknesses have been quite problematic. The challenges confronting animal models of pain, however, are not unique to this area of research, as they cut across several therapeutic areas. Despite the deficiencies, failures and concerns, existing animal models of pain continue to be of widespread use and are essential to progress in pain research as well as in other areas. Although not focusing on specific animal models of pain, this paper seeks to examine general issues facing the use of these models. It does so by exploring alternative approaches which capture recent developments, which build upon principles and concepts we have learned from Berend's contributions, and which provide the prospect of helping to address the absence of novel therapeutics in this area.

Fish do not feel pain and its implications for understanding phenomenal consciousness

Phenomenal consciousness or the subjective experience of feeling sensory stimuli is fundamental to human existence. Because of the ubiquity of their subjective experiences, humans seem to readily accept the anthropomorphic extension of these mental states to other animals. Humans will typically extrapolate feelings of pain to animals if they respond physiologically and behaviourally to noxious stimuli. The alternative view that fish instead respond to noxious stimuli reflexly and with a limited behavioural repertoire is defended within the context of our current understanding of the neuroanatomy and neurophysiology of mental states. Consequently, a set of fundamental properties of neural tissue necessary for feeling pain or experiencing affective states in vertebrates is proposed. While mammals and birds possess the prerequisite neural architecture for phenomenal consciousness, it is concluded that fish lack these essential characteristics and hence do not feel pain.

Operant nociception in nonhuman primates

The effective management of pain is a longstanding public health concern. Morphine-like opioids have long been front-line analgesics, but produce undesirable side effects that can limit their application. Slow progress in the introduction of novel improved medications for pain management over the last 5 decades has prompted a call for innovative translational research, including new preclinical assays. Most current in vivo procedures (eg, tail flick, hot plate, warm water tail withdrawal) assay the effects of nociceptive stimuli on simple spinal reflexes or unconditioned behavioral reactions. However, clinical treatment goals may include the restoration of previous behavioral activities, which can be limited by medication-related side effects that are not measured in such procedures. The present studies describe an apparatus and procedure to study the disruptive effects of nociceptive stimuli on voluntary behavior in nonhuman primates, and the ability of drugs to restore such behavior through their analgesic actions. Squirrel monkeys were trained to pull a cylindrical thermode for access to a highly palatable food. Next, sessions were conducted in which the temperature of the thermode was increased stepwise until responding stopped, permitting the determination of stable nociceptive thresholds. Tests revealed that several opioid analgesics, but not d-amphetamine or Δ(9)-THC, produced dose-related increases in threshold that were antagonist sensitive and efficacy dependent, consistent with their effects using traditional measures of antinociception. Unlike traditional reflex-based measures, however, the results also permitted the concurrent evaluation of response disruption, providing an index with which to characterize the behavioral selectivity of antinociceptive drugs.

Improving the translation of analgesic drugs to the clinic: animal models of neuropathic pain

Neuropathic pain remains an area of considerable unmet clinical need. Research based on preclinical animal models has failed to deliver truly novel treatment options, questioning the predictive value of these models. This review addresses the shortcomings of rodent in vivo models commonly used in the field and highlights approaches which could increase their predictivity, including more clinically relevant assays, outcome measures and animal characteristics. The methodological quality of animal studies also needs to be improved. Low internal validity and incomplete reporting lead to a waste of valuable research resources and animal lives, and ultimately prevent an objective assessment of the true predictivity of in vivo models.

Pain assessment in animal models: do we need further studies?

In the last two decades, animal models have become important tools in understanding and treating pain, and in predicting analgesic efficacy. Although rodent models retain a dominant role in the study of pain mechanisms, large animal models may predict human biology and pharmacology in certain pain conditions more accurately. Taking into consideration the anatomical and physiological characteristics common to man and pigs (median body size, digestive apparatus, number, size, distribution and communication of vessels in dermal skin, epidermal-dermal junctions, the immunoreactivity of peptide nerve fibers, distribution of nociceptive and non-nociceptive fiber classes, and changes in axonal excitability), swines seem to provide the most suitable animal model for pain assessment. Locomotor function, clinical signs, and measurements (respiratory rate, heart rate, blood pressure, temperature, electromyography), behavior (bright/quiet, alert, responsive, depressed, unresponsive), plasma concentration of substance P and cortisol, vocalization, lameness, and axon reflex vasodilatation by laser Doppler imaging have been used to assess pain, but none of these evaluations have proved entirely satisfactory. It is necessary to identify new methods for evaluating pain in large animals (particularly pigs), because of their similarities to humans. This could lead to improved assessment of pain and improved analgesic treatment for both humans and laboratory animals.

Effects of salt-loading hypertension on nociception in rats

Background

There is on going controversy on the effect of experimentally induced hypertension on nociception. The effect of salt-loading-induced hypertension on pain was studied in male rats.

Method

Twenty-four male Sprague-Dawley rats (160–280 g) were divided into two groups. Group A (n = 12) was treated with normal-feed diet (control), while group B (n = 12) was treated with 8% salt-loaded diet for 10 weeks. After 10 weeks of the treatment, six rats each from groups A and B were used for blood pressure measurement, while the remaining six rats were used for both the tail-flick and formalin tests. Thermal and chemical pain test were assessed using tail immersion test (tail flick) and formalin test pain paradigms at onset of salt-loading diet and after 10 weeks of salt loading.

Results

Chronic administration of salt-loading diet caused significant increases (P < 0.001) in systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure. Moreover, salt-loading-induced hypertension was found to significantly reduce pain sensitivity in the tail-immersion test (P < 0.001) and in the early and late phase of the formalin test (P < 0.01). However, the hypoalgesia was higher in the late phase (94.8%) than in the early phase (56.8%) of the formalin test.

Conclusion

The present study suggests that high salt-loading-induced hypertension causes hypoalgesia in rats, which might be due more to reduction in inflammatory response.

Evaluation of reward from pain relief

The human experience of pain is multidimensional and comprises sensory, affective, and cognitive dimensions. Preclinical assessment of pain has been largely focused on the sensory features that contribute to nociception. The affective (aversive) qualities of pain are clinically significant but have received relatively less mechanistic investigation in preclinical models. Recently, operant behaviors such as conditioned place preference, avoidance, escape from noxious stimulus, and analgesic drug self-administration have been used in rodents to evaluate affective aspects of pain. An important advance of such operant behaviors is that these approaches may allow the detection and mechanistic investigation of spontaneous neuropathic or ongoing inflammatory/nociceptive (i.e., nonevoked) pain that is otherwise difficult to assess in nonverbal animals. Operant measures may allow the identification of mechanisms that contribute differentially to reflexive hypersensitivity or to pain affect and may inform the decision to progress novel mechanisms to clinical trials for pain therapy. Additionally, operant behaviors may allow investigation of the poorly understood mechanisms and neural circuits underlying motivational aspects of pain and the reward of pain relief.

Maternal prenatal stress in rats influences c-fos expression in the spinal cord of the offspring

Previous studies in humans have reported a link between maternal stress and disturbed infant physiological behavior. The objective of our study was to examine in experimental rats how maternal prenatal stress induced by a forced swim test affects offspring afferent spinal responses mediated by stimulation of vaginocervical receptors. The activation of spinal cord neurons showing c-fos expression was examined following vaginocervical mechanical stimulation in adult rats, which were the offspring of dams exposed to gestational stress from E10 until delivery. Vaginocervical stimulation of both prenatal-stressed and non-prenatal-stressed rats induced an increase in immunoreactive protein in the spinal cord ranging from T12 to S1 segmental levels. However, a significantly higher (40%) increase in the expression of Fos-immunoreactive neurons was observed in vaginocervical stimulated prenatally stressed rats than in non-stimulated prenatally stressed ones. This increase was higher in L5-S1 levels than in T12-L4. When the regional distribution was examined, results showed that up to 80% of activated neurons were located in the dorsal horn in both non-stimulated prenatally stressed and stimulated prenatally stressed groups, with a significantly higher density in the latter. Our results demonstrate that maternal prenatal stress can have consequences on vaginocervical responses conveyed to the spinal cord. The increase in Fos labeled neurons in T12-S1 in prenatally stressed rats induced by vaginocervical stimulation suggests the hypersensitivity of the genital tract associated with activation of spinal circuits spanning multiple segments.

Increase in formalin-induced tonic pain by 5alpha-reductase and aromatase inhibition in female rats

Little is known about the role of steroidogenic enzymes in pain modulation. This study examined the effects of 5α-reductase and aromatase inhibition on formalin-induced tonic pain (FITP) in adult female rats. The animals received subcutaneous injection (5 mg/kg) of finasteride (an inhibitor of 5α-reductase) and letrozole (an inhibitor of aromatase), either separately or in combination, 15 min before formalin injection at a low (0.25%) and high (2.5%) concentration. Pretreatment with inhibitors increased FITP evoked by injection of 0.25% formalin, but they were not effective on 2.5% formalin pain. The enhancing effects of finasteride and letrozole on FITP induced by 2.5% formalin was demonstrated by inhibitory actions of these drugs on morphine (7 and 10 mg/kg, intraperitoneal) induced antinociception. The nervous system could be considered as the main target of the enzymes inhibition, since the pronociceptive effect was also observed after administration of inhibitors to ovariectomized rats. Altogether, these findings suggest that the biological activity of the enzymes 5α-reductase and aromatase modulates FITP and may help to develop effective therapeutic strategies to counteract pain.

Evidence for a role of NTS2 receptors in the modulation of tonic pain sensitivity

BACKGROUND

Central neurotensin (NT) administration results in a naloxone-insensitive antinociceptive response in animal models of acute and persistent pain. Both NTS1 and NTS2 receptors were shown to be required for different aspects of NT-induced analgesia. We recently demonstrated that NTS2 receptors were extensively associated with ascending nociceptive pathways, both at the level of the dorsal root ganglia and of the spinal dorsal horn. Then, we found that spinally administered NTS2-selective agonists induced dose-dependent antinociceptive responses in the acute tail-flick test. In the present study, we therefore investigated whether activation of spinal NTS2 receptors suppressed the persistent inflammatory pain symptoms observed after intraplantar injection of formalin.

RESULTS

We first demonstrated that spinally administered NT and NT69L agonists, which bind to both NTS1 and NTS2 receptors, significantly reduced pain-evoked responses during the inflammatory phase of the formalin test. Accordingly, pretreatment with the NTS2-selective analogs JMV-431 and levocabastine was effective in inhibiting the aversive behaviors induced by formalin. With resolution at the single-cell level, we also found that activation of spinal NTS2 receptors reduced formalin-induced c-fos expression in dorsal horn neurons. However, our results also suggest that NTS2-selective agonists and NTS1/NTS2 mixed compounds differently modulated the early (21-39 min) and late (40-60 min) tonic phase 2 and recruited endogenous pain inhibitory mechanisms integrated at different levels of the central nervous system. Indeed, while non-selective drugs suppressed pain-related behaviors activity in both part of phase 2, intrathecal injection of NTS2-selective agonists was only efficient in reducing pain during the late phase 2. Furthermore, assessment of the stereotypic pain behaviors of lifting, shaking, licking and biting to formalin also revealed that unlike non-discriminative NTS1/NTS2 analogs reversing all nociceptive endpoint behaviors, pure NTS2 agonists specifically inhibited paw lifting, supporting a role of NTS2 in spinal modulation of persistent nociception.

CONCLUSION

The present study provides the first demonstration that activation of NTS2 receptors produces analgesia in the persistent inflammatory pain model of formalin. The dichotomy between these two classes of compounds also indicates that both NTS1 and NTS2 receptors are involved in tonic pain inhibition and implies that these two NT receptors modulate the pain-induced behavioral responses by acting on distinct spinal and/or supraspinal neural circuits. In conclusion, development of NT agonists targeting both NTS1 and NTS2 receptors could be useful for chronic pain management.

Naltrexone fails to increase pain affect in response to inflammatory pain in a novel escape/avoidance paradigm

The non-specific opioid antagonist naltrexone has traditionally been used as treatment for opioid overdose, as well as in research settings as an antagonist to examine opioid and non-opioid mediated analgesia. However, the mechanisms by which this drug operates are not well understood, and its exact effects on sensory and affective pain processes remain uncertain. Various studies have demonstrated that naltrexone behaves in a paradoxical manner, leading to analgesia, no discernable changes, or an increase in pain, depending on the circumstances of the study. This imprecise spectrum of effects leads to difficulty in interpreting results in studies where naltrexone was utilized as an antagonist. Therefore, the purpose of this experiment was to further examine whether naltrexone elicits dose-dependent effects in behavioral tests designed to quantify the sensory and affective components of pain. Naltrexone was not expected to have an effect on carrageenan-induced inflammatory pain in sensory pain measures, but a dose-dependent increase was predicted in behavior related to the affective component of pain. Eighty-eight male Sprague-Dawley rats were used to test these hypotheses by measuring Mechanical Paw Withdrawal Thresholds before and after naltrexone injection and by assessing performance in the Place Escape Avoidance Paradigm test, a novel paradigm to test pain affect, in which naltrexone had not been utilized. The results demonstrated that naltrexone failed to increase place/escape avoidance behavior as anticipated, but rather produced a slight, but non-significant, decrease in escape avoidance behavior. Further research is needed to elucidate the differential effects of naltrexone on various aspects of pain-related behavior.

Animal models of pain: progress and challenges

Many are frustrated with the lack of translational progress in the pain field, in which huge gains in basic science knowledge obtained using animal models have not led to the development of many new clinically effective compounds. A careful re-examination of animal models of pain is therefore warranted. Pain researchers now have at their disposal a much wider range of mutant animals to study, assays that more closely resemble clinical pain states, and dependent measures beyond simple reflexive withdrawal. However, the complexity of the phenomenon of pain has made it difficult to assess the true value of these advances. In addition, pain studies are importantly affected by a wide range of modulatory factors, including sex, genotype and social communication, all of which must be taken into account when using an animal model.

Examining the role of the medial thalamus in modulating the affective dimension of pain

The purpose of this project was to explore the role of the medial thalamus (MT), including the medial dorsal thalamus (MD) and associated midline nuclei in pain processing. Experiment 1 explored the role of electrolytic lesions to the MT in the formalin test. It was hypothesized that animals with electrolytic lesions to the MT would have attenuated paw licking behavior during the second phase of the formalin tests as compared to sham lesion controls. This hypothesis was based on evidence of projections from the MD to the ACC, and previous research demonstrating attenuation of paw licking behavior in the second phase of the formalin test in animals with ACC lesions. Experiment 2 tested the effects of electrolytic MT lesions on mechanical paw withdrawal thresholds in the L5 nerve ligation model. It was hypothesized that lesions of the MT would not alter mechanical paw withdrawal thresholds. Experiment 3 tested the effects of electrolytic MT lesions on escape/avoidance behavior in the place escape avoidance paradigm. For experiment 1, animals with MT lesions were found to have slightly elevated paw licking behavior, but only across two time points. No differences in mechanical paw withdrawal thresholds and in escape/avoidance behavior were detected as compared to the sham lesion group. These results indicate a limited role for the medial thalamic nuclei in coding for pain intensity and the affective dimension of pain. Additional research is needed to explore the role of individual medial nuclei in pain processing.

Anesthetics and cerebral metabolism

PURPOSE OF REVIEW

This review focuses on the utilization of the effects of general anesthetics on cerebral metabolism as revealed by imaging for therapeutic and preventive purposes, for understanding mechanisms of anesthetic action, and for elucidating mechanisms of cerebral processing in humans.

RECENT FINDINGS

General anesthetics suppress cerebral metabolism significantly. This effect has been used for neuroprotection during inadequate cerebral blood flow. With the advent of noninvasive imaging techniques, this suppression has also been used to image and map the sites of anesthetic action in the living human brain. Volatile agents, intravenous anesthetics, and analgesics have all begun to be explored using mostly positron emission tomography. The ability of anesthetics to change global baseline brain metabolism has created the opportunity to examine the relevance of global baseline (resting) brain activity in terms of region-specific cerebral processing.

SUMMARY

Anesthetics experimentally appear to be useful for neuroprotection, at least during the early post-ischemic period. Identification of the cerebral sites of anesthetic action by in vivo human brain imaging provides new insights into the mechanism of action of these agents. Anesthetic-related manipulation of baseline brain metabolism demonstrates the significant contribution of this global activity to regional cerebral processing.

Opioid modulation of reflex versus operant responses following stress in the rat

In pre-clinical models intended to evaluate nociceptive processing, acute stress suppresses reflex responses to thermal stimulation, an effect previously described as stress-induced "analgesia." Suggestions that endogenous opioids mediate this effect are based on demonstrations that stress-induced hyporeflexia is enhanced by high dose morphine (>5 mg/kg) and is reversed by naloxone. However, reflexes and pain sensations can be modulated differentially. Therefore, in the present study direct comparisons were made of opioid agonist and antagonist actions, independently and in combination with acute restraint stress in Long Evans rats, on reflex lick-guard (L/G) and operant escape responses to nociceptive thermal stimulation (44.5 degrees C). A high dose of morphine (>8 mg/kg) was required to reduce reflex responding, but a moderate dose of morphine (1 mg/kg) significantly reduced escape responding. The same moderate dose (and also 5 mg/kg) of morphine significantly enhanced reflex responding. Naloxone (3 mg/kg) significantly enhanced escape responding but did not affect L/G responding. Restraint stress significantly suppressed L/G reflexes (hyporeflexia) but enhanced escape responses (hyperalgesia). Stress-induced hyperalgesia was significantly reduced by morphine and enhanced by naloxone. In contrast, stress-induced hyporeflexia was blocked by both naloxone and 1 mg/kg of morphine. Thus, stress-induced hyperalgesia was opposed by endogenous opioid release and by administration of morphine. Stress-induced hyporeflexia was dependent upon endogenous opioid release but was counteracted by a moderate dose of morphine. These data demonstrate a differential modulation of reflex and operant outcome measures by stress and by separate or combined opioid antagonism or administration of morphine.

Expansion of formalin-evoked Fos-immunoreactivity in rats with a spinal cord injury

Peripheral tissue injury as well as spinal cord injury (SCI) may lead to sensitization of dorsal horn neurons and alterations in nociceptive processing. Thus, peripheral injuries experienced by SCI patients, even if not initially perceived, could result in a persistent and widespread activation of dorsal horn neurons and emerge as chronic pain with interventive repair or modest recovery from SCI. To visualize the spinal neuron response to peripheral tissue injury following complete SCI in rats, the neural transcription factor Fos was quantitated in the spinal cord. Two weeks following either a complete transection of the spinal cord at the level of T8 or a sham surgery (laminectomy), rats were injected with formalin into the left hind paw. Sham-operated rats demonstrated biphasic hind paw pain-related behavior following formalin injection, but transected rats displayed fewer behaviors in the second (tonic) phase. Stereological analysis of the sham group revealed that the extent of formalin-induced Fos expression was within the lumbar dorsal horn, with numerous Fos-like immunoreactive profiles in the ipsilateral dorsal horn and some contralateral immunoreactive profiles. In contrast, the level of Fos-like immunoreactivity in the transected group was significantly elevated and expanded in range compared to the sham group, with increases observed in the normal laminar distribution regions, as well as multi-segmentally through sacral levels and increases in the contralateral dorsal horn segments. The data demonstrate that widespread activation of spinal, especially dorsal horn, neurons following peripheral insult can occur in the injured spinal cord, despite reduced pain responsiveness, and suggests that exaggerated pain may emerge as spinal recovery or repair progresses.

Sex differences in opioid-mediated pain inhibitory mechanisms during the interphase in the formalin test

Many chronic pain conditions are more prevalent in women than men and both fundamental and clinical research supports the implication of endogenous pain inhibitory mechanisms. The goal of this study was to verify if sex differences on endogenous pain inhibitory mechanisms during the formalin test are opioidergic and modulated by sex hormones. Formalin tests were performed with naloxone hydrochloride, a non-selective opioid antagonist in intact and gonadectomized Sprague-Dawley rats of both sexes. Considering the sexual dimorphisms we found, where naloxone preferentially blocked the interphase in female rats, injections of all the possible combinations of mu- (naltrexone hydrochloride), delta- (naltrindole hydrochloride) and kappa-selective antagonists (norbinaltorphimine dihydrochloride) were given to evaluate the contribution of these opioid-receptor subtypes to the inhibitory mechanism during the interphase in intact females. Finally, the systemic administration of naloxone methiodide and intrathecal administration of naloxone hydrochloride in intact females allowed us to verify if the action of endogenous opioids that are liberated during the interphase takes place at the periphery or spinally, respectively. The results show that the interphase was almost completely inhibited by naloxone in females while it produced only a slight blockade in males. These results permitted us to conclude that opioids play a major role in the pain inhibitory mechanism of the interphase in females while a non-opioid mechanism seems to be responsible for this inhibitory pathway in males. Using gonadectomized animals of both sexes, we demonstrated the modulation of the opioidergic system of the interphase by sex hormones. The administration of different combinations of selective antagonists for mu-, kappa- and delta-opioid receptors in intact females permitted us to conclude that only the combination of kappa- and delta-selective antagonists significantly blocked the interphase. The same result was obtained with the combination of the three antagonists, confirming the results with systemic naloxone hydrochloride. Finally, intrathecal administration permitted us to support that the action of naloxone is primarily at the spinal level, even if a supraspinal action cannot be ruled out. These results are of particular interest in showing sexual dimorphisms in endogenous pain modulation mechanisms during the interphase of the formalin test. A clearer understanding of the difference between male and female endogenous pain inhibitory pathways should lead to a better understanding of the role of endogenous pain modulation deficits in certain chronic pain conditions.

Increased formalin-induced pain and expression of fos neurons in the lumbar spinal cord of prenatally stressed infant rats

When pregnant dams are stressed, there is a resultant alteration in brain development and behavior in their offspring. Prior work has shown increased nociceptive responses in adolescent or adult rats born of stressed dams. However, the age at which those changes first occur is not known. The aim of the present study was to evaluate the effect of prenatal stress on pain sensitivity in the formalin test in 7-day-old rats, behaviorally and by fos-like immunoreactivity (Fos-LI) in the lumbar spinal cord dorsal horn. The behavioral response to intraplantar injection of formalin is represented by two nociceptive phases separated by an interphase during which nociceptive responses decrease; the interphase is not seen until the start of the third postnatal week and appears as descending inhibitory monoaminergic systems develop. Prenatally stressed infants showed increased nociceptive responses in the second, tonic phase and a large increase in the number of formalin-induced Fos-LI neurons in the lumbar dorsal horn, a result consistent with the behavioral data. The increased nociception in prenatally stressed 7-day-old pups may be associated with the decrease in the intensity of serotonin-like immunoreactivity and density of serotonergic cells in the lumbar spinal cord dorsal horn and the dorsal raphe nucleus reported earlier.