Hedonic and motivational responses to food reward are unchanged in rats with neuropathic pain

Spared nerve injury decreases motivation in long-access homecage-based operant tasks in mice

ABSTRACT

Neuropathic pain causes both sensory and emotional maladaptation. Preclinical animal studies of neuropathic pain-induced negative affect could result in novel insights into the mechanisms of chronic pain. Modeling pain-induced negative affect, however, is variable across research groups and conditions. The same injury may or may not produce robust negative affective behavioral responses across different species, strains, and laboratories. Here, we sought to identify negative affective consequences of the spared nerve injury model on C57BL/6J male and female mice. We found no significant effect of spared nerve injury across a variety of approach-avoidance conflict, hedonic choice, and coping strategy assays. We hypothesized these inconsistencies may stem in part from the short test duration of these assays. To test this hypothesis, we used the homecage-based Feeding Experimentation Device version 3 to conduct 12-hour, overnight progressive ratio testing to determine whether mice with chronic spared nerve injury had decreased motivation to earn palatable food rewards. Our data demonstrate that despite equivalent task learning, spared nerve injury mice are less motivated to work for a sugar pellet than sham controls. Furthermore, when we normalized behavioral responses across all the behavioral assays we tested, we found that a combined normalized behavioral score is predictive of injury state and significantly correlates with mechanical thresholds. Together, these results suggest that homecage-based operant behaviors provide a useful platform for modeling nerve injury-induced negative affect and that valuable pain-related information can arise from agglomerative data analyses across behavioral assays-even when individual inferential statistics do not demonstrate significant mean differences.

Serotonin and Dopamine Show Different Response Profiles to Acute Stress in the Nucleus Accumbens and Medial Prefrontal Cortex of Rats with Neuropathic Pain

The ability to adaptively guide behaviour requires the integration of external information with internal motivational factors. Decision-making capabilities can be impaired by acute stress and is often exacerbated by chronic pain. Chronic neuropathic pain patients often present with cognitive dysfunction, including impaired decision-making. The mechanisms underlying these changes are not well understood but may include altered monoaminergic transmission in the brain. In this study we investigated the relationships between dopamine, serotonin, and their metabolites in key brain regions that regulate motivated behaviour and decision-making. The neurochemical profiles of the medial prefrontal cortex, orbital prefrontal cortex, and nucleus accumbens were analysed using HPLC in rats that received a chronic constriction injury (CCI) of the right sciatic nerve and an acute stress (15-min restraint), prior to an outcome devaluation task. CCI alone significantly decreased dopamine but not serotonin concentrations in the medial prefrontal cortex. By contrast, restraint stress acutely increased dopamine in the medial prefrontal cortex, and the nucleus accumbens; and increased serotonin in the medial prefrontal cortex 2 h later. The sustained dopaminergic and serotonergic responses to acute stress highlight the importance of an animal's ability to mount an effective coping response. In addition, these data suggest that the impact of nerve injury and acute stress on outcome-devaluation occurs independently of dopaminergic and serotonergic transmission in the medial prefrontal cortex, orbital prefrontal cortex and nucleus accumbens of rats.

Behavioral outcomes of complete Freund adjuvant-induced inflammatory pain in the rodent hind paw: a systematic review and meta-analysis

ABSTRACT

Many analgesics inadequately address the psychiatric comorbidities of chronic and persistent pain, but there is no standard preclinical model of pain-altered behavior to support the development of new therapies. To explore this conflicting and inconclusive literature, we conducted a focused systematic review and meta-analysis on the effect of complete Freund adjuvant-induced (CFA) rodent hind paw inflammation on multiple classical indicators of exploratory behavior, stress coping, and naturalistic behavior. Our primary objective was to define CFA's effect on assays including, but not limited to, the elevated plus maze and forced swim test. Our secondary objective was to discover how variables such as species and strain may influence outcomes in such assays. We searched Ovid MEDLINE, Embase, Scopus, and Web of Science in April and October 2020 for studies with adult rodents injected with CFA into the hind paw and subsequently tested for aspects of "anxiety-like" or "depressive-like" behaviors. Forty-four studies evaluated performance in the elevated plus or zero maze, open field test, light-dark box, place escape and avoidance paradigm, forced swim test, tail suspension test, sucrose preference test, wheel running, and burrowing assay. Complete Freund adjuvant modestly but significantly decreased exploratory behavior, significantly increased passive stress coping in the tail suspension test but not the forced swim test, and significantly decreased preference for sucrose and naturally rewarding activity. Subgroup analyses revealed significant differences between species and animal sourcing. Based on the evidence provided here, we conclude future studies should focus on CFA's effect on natural rewards and naturalistic behaviors.

Chronic pain precedes disrupted eating behavior in low-back pain patients

Chronic pain is associated with anhedonia and decreased motivation. These behavioral alterations have been linked to alterations in the limbic brain and could explain the increased risk for obesity in pain patients. The mechanism of these behavioral changes and how they set in in relation to the development of chronic pain remain however poorly understood. Here we asked how eating behavior was affected in low-back pain patients before and after they transitioned to chronic pain, compared to patients whose pain subsided. Additionally, we assessed how the hedonic perception of fat-rich food, which is altered in chronic pain patients, related to the properties of the nucleus accumbens in this patients' population. We hypothesized that the accumbens would be directly implicated in the hedonic processing of fat-rich food in pain patients because of its well-established role in hedonic feeding and fat ingestion, and its emerging role in chronic pain. Accordingly, we used behavioral assays and structural brain imaging to test sub-acute back pain patients (SBP) and healthy control subjects at baseline and at approximately one-year follow-up. We also studied a sample of chronic low-back pain patients (CLBP) at one time point only. We found that SBP patients who recovered at follow-up (SBPr) and CLBP patients showed disrupted eating behaviors. In contrast, SBP patients who persisted in having pain at follow-up (SBPp) showed intact eating behavior. From a neurological standpoint, only SBPp and CLBP patients showed a strong and direct relationship between hedonic perception of fat-rich food and nucleus accumbens volume. This suggests that accumbens alterations observed in SBPp patients in previous works might protect them from hedonic eating disruptions during the early course of the illness. We conclude that disrupted eating behavior specifically sets in after pain chronification and is accompanied by structural changes in the nucleus accumbens.

Acute pain-related depression of operant responding maintained by social interaction or food in male and female rats

RATIONALE

Clinically relevant pain is often associated with functional impairment and behavioral depression, including depression of social behavior. Moreover, recovery of function is a major goal in pain treatment. We used a recently developed model of operant responding for social interaction in rats to evaluate the vulnerability of social behavior to an experimental pain manipulation and the sensitivity of pain-depressed social behavior to treatment with clinically effective analgesics.

METHODS

Sprague-Dawley male and female rats were trained to lever press for social access to another rat, and responding was evaluated after treatment with (a) intraperitoneal injection of dilute lactic acid (IP acid; 0.18-5.6%) administered alone as a visceral noxious stimulus, (b) the mu-opioid receptor (MOR) agonist morphine (0.32-10 mg/kg) or nonsteroidal anti-inflammatory drug (NSAID) ketoprofen (10 mg/kg) administered alone, or (c) morphine or ketoprofen administered before IP acid. For comparison, the same treatments were evaluated in separate rats trained to lever press for food delivery.

RESULTS

Both IP acid alone and morphine alone more potently decreased responding maintained by social interaction than by food, whereas ketoprofen did not affect responding for either reinforcer. In general, analgesics were most effective to rescue operant responding when relatively low IP acid concentrations produced significant but submaximal behavioral depression; however, morphine was not effective to rescue responding for social interaction.

CONCLUSIONS

Operant responding maintained by social interaction was more sensitive to pain-related disruption and less responsive to opioid analgesic rescue than food-maintained operant responding. Social behavior may be especially vulnerable to depression by pain states.

Effects of chemotherapy on operant responding for palatable food in male and female mice

Patients treated with cancer chemotherapeutics frequently report chemotherapy-induced peripheral neuropathy (CIPN), changes in mood (depression and anxiety) and functional impairments. Rodent models of CIPN elicit limited alterations in functional behaviors, which pose challenges in developing preclinical models of chemotherapy-induced behavioral depression. The study examined the consequences of chemotherapy-induced mechanical hypersensitivity (paclitaxel: 32 or 64 mg/kg, cumulative; oxaliplatin: 30 mg/kg, cumulative) on behavioral depression, as measured with operant responding for palatable food during periods of food restriction and ad libitum chow, consumption of noncontingently available palatable food in the presence of ad libitum chow, and voluntary wheel running. The study employed two inbred mouse strains (C57BL/6J and Balb/cJ) and examined potential sex differences. All chemotherapeutic regimens caused profound mechanical hypersensitivity for the duration of the observation periods (up to 7 months), but no treatments changed voluntary wheel running or consumption of noncontingent palatable food. The high dose of paclitaxel temporarily reduced operant responding for palatable food in male C57BL/6J mice undergoing food restriction or maintained on ad libitum chow. However, paclitaxel failed to decrease operant responding for palatable food in free-feeding female C57BL/6J mice or Balb/cJ mice of either sex. Moreover, oxaliplatin did not significantly alter operant responding for palatable food in male or female C57BL/6J mice maintained on ad libitum chow. These findings demonstrate a dissociation between chemotherapy-induced mechanical hypersensitivity and behavioral depression. The transient effects of paclitaxel on operant responding in male C57BL/6J mice may represent a fleeting behavioral correlate of chemotherapy-associated pain-like behaviors.

Impact of Dose, Sex, and Strain on Oxaliplatin-Induced Peripheral Neuropathy in Mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common, dose limiting, and long-lasting side effect of chemotherapy treatment. Unfortunately, no treatment has proven efficacious for this side effect. Rodent models play a crucial role in the discovery of new mechanisms underlying the initiation, progression, and recovery of CIPN and the potential discovery of new therapeutics. However, there is limited consistency in the dose, the sex, age, and genetic background of the animal used in these studies and the outcome measures used in evaluation of CIPN rely primarily on noxious and reflexive measures. The main objective of this study was to provide a comprehensive and systematic characterization of oxaliplatin-induced peripheral neuropathy in mice by using a battery of behavioral, sensory, electrophysiological, and morphometric measures in both sexes of the two widely used strains of mice, C57BL/6J and BALB/cJ. Mice received intraperitoneal injections of 3 or 30 mg/kg cumulative doses of oxaliplatin over the course of 2 weeks. Both doses induced long-term and time-dependent mechanical and cold hypersensitivity. Our results show that 30 mg/kg oxaliplatin reduced the locomotor activity in C57BL/6J mice, and C57BL/6J females showed anxiety-like behavior one-week post completion of treatment. In the same dose group, BALB/cJ males and females sustained a larger decrease in sucrose preference than either male or female C57BL/6J mice. Both strains failed to show significant changes in burrowing and nesting behaviors. Two clinically relevant assessments of changes to the peripheral nerve fibers, nerve conduction and intraepidermal nerve fiber density (IENFD) were evaluated. Only BALB/cJ females showed significant reduction in the nerve conduction amplitude 1 week after 30 mg/kg oxaliplatin regimen. Moreover, this dose of the chemo agent reduced the IENF density in both sexes and strains. Our findings suggest that mouse strain, sex, and assay type should be carefully considered when assessing the effects of oxaliplatin and potential therapeutic interventions.

Long-term inflammatory pain does not impact exploratory behavior and stress coping strategies in mice

ABSTRACT

Pain puts patients at risk for developing psychiatric conditions such as anxiety and depression. Preclinical mouse models of pain-induced affective behavior vary widely in methodology and results, impairing progress towards improved therapeutics. To systematically investigate the effect of long-term inflammatory pain on exploratory behavior and stress coping strategy, we assessed male C57BL/6J mice in the forced swim test (FST), elevated zero maze, and open field test at 4 and 6 weeks postinjection of Complete Freund's Adjuvant, while controlling for testing order and combination. Inflammatory pain did not induce a passive stress coping strategy in the FST and did not reduce exploratory behavior in the elevated zero maze or the open field test. Using systematic correlational analysis and composite behavioral scores, we found no consistent association among measures for mice with or without inflammatory pain. A meta-analysis of similar studies indicated a modest, significant effect of Complete Freund's Adjuvant on exploratory behavior, but not immobility in the FST, and high heterogeneity among effect sizes in all 3 paradigms. Given the urgency for understanding the mechanisms of pain comorbidities and identifying novel therapies, these findings support the reallocation of our limited resources away from such unreliable assays and toward motivated and naturalistic behaviors. Future studies in pain and psychiatric translational research may benefit by considering outcomes beyond binary categorization, quantifying the associations between multiple measured behaviors, and agnostically identifying subtle yet meaningful patterns in behaviors.

Characterization of acute pain-induced behavioral passivity in mice: Insights from statistical modeling

Affective-motivational disturbances are highly inconsistent in animal pain models. The reproducibility of the open-field test in assessing anxiety, malaise or disability remains controversial despite its popularity. While traumatic, persistent or multiregional pain models are commonly considered more effective in inducing negative affect or functional impairment, the early psychobehavioral changes before pain chronification are often underexplored. Here, we aimed to clarify the fundamental relationship between hypernociception and passive distress-like behavior using a model of transient inflammatory pain. To minimize latent confounders and increase data consistency, male C57BL/6N mice were habituated to the open-field arena 6 times before receiving the unilateral intraplantar injection of prostaglandin E2 (PGE2) or vehicle. Open-field (40-min exploration) and nociceptive behavior were evaluated repeatedly along the course of hypernociception in both wild-type and transgenic mice with a known pronociceptive phenotype. To reduce subjectivity, multivariate open-field behavioral outcomes were analyzed by statistical modeling based on exploratory factor analyses, which yielded a 2-factor solution. Within 3 hr after PGE2 injection, mice developed significantly reduced center exploration (factor 1) and a marginally significant increase in their habituation tendency (factor 2), which were not apparent in vehicle-injected mice. The behavioral passivity generally improved as hypernociception subsided. Therefore, transient inflammatory irritation is sufficient to suppress mouse open-field exploratory activity. The apparent absence of late affective-motivational changes in some rodents with prolonged hypernociception may not imply a lack of preceding or underlying neuropsychological alterations. Procedural pain after invasive animal experiments, however small, should be assessed and adequately controlled as a potential research confounder.

Neurons in the Locus Coeruleus Modulate the Hedonic Effects of Sub-Anesthetic Dose of Propofol

Propofol is a worldwide-used intravenous general anesthetic with ideal effects, but hedonic effects of propofol have been reported and cause addictive issue. There is little known about the neurobiological mechanism of hedonic effects of propofol. Increasing researches have shown that the dopaminergic nervous system of the ventral tegmental area (VTA) and the noradrenergic system of locus coeruleus (LC) play a crucial role in hedonic experiences, which are putative sites for mediating the hedonic effects of propofol. In the present study, rat hedonic response scale and place conditioning paradigm were employed to examine the euphoric effects of propofol. In vivo GCaMP-based (AVV-hSyn-GCaMP6s) fiber photometry calcium imaging was used to monitor the real-time neuronal activity in VTA and LC area in rats exhibiting propofol-induced euphoric behaviors. Then DREADDs (designer receptors exclusively activated by designer drugs) modulation using rAAV-hSyn-hM4D(Gi)-EGFP was performed to confirm the neuronal substrate that mediates the euphoric effects of propofol. The score of hedonic facial responses was significantly increased in the 4 mg/kg group compared with that of the 0 mg/kg group. The locomotor activity in the propofol-paired compartment was significantly increased at the 4 mg/kg dose compared with that of the saline-paired group. When compared with the 0 mg/kg group, the place preference increased in the 4 mg/kg group. Administration of 4 mg/kg of propofol triggers reliable increases in GcaMP fluorescence. However, in the VTA GcaMP-expressing rats, administration of 4 mg/kg of propofol did not induce any change of GcaMP signals. The facial score and the place preference, which increased by 4 mg/kg propofol were abolished by chemogenetic inhibition of the neuronal activity in the LC area. Our results suggest that LC noradrenergic neurons, not VTA dopaminergic neurons, are directly involved in the hedonic effects of sub-anesthetic dose of propofol.

The partial saphenous nerve injury model of pain impairs reward-related learning but not reward sensitivity or motivation

ABSTRACT

Chronic pain is highly comorbid with affective disorders, including major depressive disorder. A core feature of major depressive disorder is a loss of interest in previously rewarding activities. Major depressive disorder is also associated with negative affective biases where cognitive processes are modulated by the affective state. Previous work from our laboratory has shown that reward-related learning and memory is impaired in rodent models of depression generated through a variety of different manipulations. This study investigated different aspects of reward-related behaviour in a rodent model of chronic pain, the partial saphenous nerve injury (PSNI). Using our reward-learning assay, an impairment in reward learning was observed with no difference in sucrose preference, consistent with a lack of effect on reward sensitivity and similar to the effects seen in depression models. In a successive negative contrast task, chronic pain was not associated with changes in motivation for reward either under normal conditions or when reward was devalued although both sham and PSNI groups exhibited the expected negative contrast effect. In the affective bias test, PSNI rats developed a positive affective bias when treated with gabapentin, an effect not seen in the controls suggesting an association with the antinociceptive effects of the drug inducing a relatively more positive affective state. Together, these data suggest that there are changes in reward-related cognition in this chronic pain model consistent with previous findings in rodent models of depression. The effects seen with gabapentin suggest that pain-associated negative affective state may be remediated by this atypical analgesic.

Comparison of chemotherapy effects on mechanical sensitivity and food-maintained operant responding in male and female rats

Chemotherapies of varying classes often cause neuropathy and debilitating chemotherapy-induced neuropathic pain sufficient to limit treatment and reduce quality of life for many patients battling cancer. There are currently no effective preventive or alleviative treatments for chemotherapy-induced neuropathic pain. Preclinical models have been developed to test candidate chemotherapy-induced neuropathic pain treatments; however, studies using these models rarely provide direct comparisons of effects of different chemotherapies or assess the degree to which chemotherapies produce clinically relevant signs of pain-depressed behavior. Male and female Sprague-Dawley rats received four injections of vehicle, paclitaxel, oxaliplatin, vincristine, or bortezomib on alternate days. Mechanical hypersensitivity, body weight, and food-maintained operant responding were evaluated before, during, and for up to 42 days after initiation of treatment. Morphine potency and effectiveness to reverse chemotherapy-induced effects were also evaluated. All four chemotherapies produced dose-dependent and sustained mechanical hypersensitivity in all rats. Vincristine and oxaliplatin produced transient weight loss and decreases in food-maintained operant responding in all rats, whereas paclitaxel and bortezomib produced lesser or no effect. At 4 weeks after treatment, operant responding was depressed only in paclitaxel-treated males. Morphine reversed mechanical hypersensitivity in all rats but failed to reverse paclitaxel-induced depression of operant responding in males. We conclude that chemotherapy treatments sufficient to produce sustained mechanical hypersensitivity failed to produce sustained or morphine-reversible behavioral depression in rats. Insofar as pain-related behavioral depression is a cardinal sign of chemotherapy-induced neuropathic pain in humans, these results challenge the presumption that these chemotherapy-dosing regimens are sufficient to model clinically relevant chemotherapy-induced neuropathic pain in rats.

Resistance of Food-Maintained Operant Responding to Mechanical Punishment in Rats: Further Evidence for Weak "Affective/Motivational Pain" in Rat Models of Inflammatory and Neuropathic Pain

Clinically relevant chronic pain is often associated with functional impairment and behavioral depression as an “affective/motivational” sign of pain; however preclinical animal models of inflammatory and neuropathic pain often produce weak evidence of impaired function. We hypothesized that hindpaw mechanical stimulation produced by a requirement to rear on a textured “NOX” plate would punish operant responding in rats treated with intraplantar complete Freund’s adjuvant (CFA, a model of inflammatory pain) or the chemotherapeutic paclitaxel (PTX, a model of neuropathic pain) and produce sustained pain-related depression of operant behavior. Male Sprague–Dawley rats were trained under a progressive-ratio (PR) schedule of food-maintained operant responding, then treated with CFA (100 µL in left hindpaw), PTX (2.0 mg/kg IP on alternate days for four total injections; 6.6 mg/kg IV on alternate days for three total injections), or saline vehicle. PR break points and mechanical thresholds for paw withdrawal from von Frey filaments were then tracked for 28 days. Subsequently, rats were tested with the opioid receptor antagonist naltrexone to assess latent sensitization and with the kappa opioid receptor (KOR) agonist U69593 to assess KOR function. CFA produced significant mechanical hypersensitivity for 3 weeks but decreased PR breakpoints for only 1 day. Both IP and IV PTX produced mechanical hypersensitivity for at least three weeks; however, only IV PTX decreased PR breakpoints, and this decrease was not alleviated by morphine. After recovery, naltrexone reinstated mechanical hypersensitivity in CFA- but not PTX-treated rats, and it did not reinstate depression of breakpoints in any group. U69593 dose-dependently decreased PR breakpoints in all groups with no difference between control vs. CFA/PTX groups. These results suggest that rearing on a textured NOX plate was not sufficient to punish operant responding in CFA- and PTX-treated rats despite the presence of sustained mechanical hypersensitivity. The rapid recovery of operant responding could not be attributed to latent sensitization, KOR downregulation, or behavioral tolerance. These results extend the range of conditions under which putative chronic pain manipulations produce weak evidence for depression of operant responding as a sign of the “affective/motivational” component of pain in rats.

The influence of rat strain on the development of neuropathic pain and comorbid anxio-depressive behaviour after nerve injury

Back-translating the clinical manifestations of human disease burden into animal models is increasingly recognized as an important facet of preclinical drug discovery. We hypothesized that inbred rat strains possessing stress hyper-reactive-, depressive- or anxiety-like phenotypes may possess more translational value than common outbred strains for modeling neuropathic pain. Rats (inbred: LEW, WKY, F344/ICO and F344/DU, outbred: Crl:SD) were exposed to Spared Nerve Injury (SNI) and evaluated routinely for 6 months on behaviours related to pain (von Frey stimulation and CatWalk-gait analysis), anxiety (elevated plus maze, EPM) and depression (sucrose preference test, SPT). Markers of stress reactivity together with spinal/brain opioid receptor expression were also measured. All strains variously developed mechanical allodynia after SNI with the exception of stress-hyporesponsive LEW rats, despite all strains displaying similar functional gait-deficits after injury. However, affective changes reflective of anxiety- and depressive-like behaviour were only observed for F344/DU in the EPM, and for Crl:SD in SPT. Although differences in stress reactivity and opioid receptor expression occurred, overall they were relatively unaffected by SNI. Thus, anxio-depressive behaviours did not develop in all strains after nerve injury, and correlated only modestly with degree of pain sensitivity or with genetic predisposition to stress and/or affective disturbances.

Kappa opioid receptors mediate an initial aversive component of paclitaxel-induced neuropathy

RATIONALE

Cancer patients receiving the antineoplastic drug paclitaxel report higher incidences and longer duration of treatment-resistant depression than patients receiving other classes of chemotherapeutics. Rodents treated with paclitaxel exhibit a suite of changes in affect-like behaviors. Further, paclitaxel causes chemotherapy-induced peripheral neuropathy (CIPN) in humans and rodents. Kappa opioid receptors (KOR) have a well-established role in depression and neuropathy. The contributions of KOR signaling to paclitaxel-induced aversive-like state and CIPN in rodents remain to be explored.

OBJECTIVES

We aimed to investigate whether dysregulation of the KOR/dynorphin system is associated with paclitaxel-mediated pain-like behavior and depression-like behavior.

METHODS

Cancer-free male C57BL/6J mice were treated with four injections of vehicle or paclitaxel (32 mg/kg cumulative). The effects of the selective KOR antagonist norbinaltorphimine (norBNI) on paclitaxel-induced sucrose preference deficits and mechanical hypersensitivity were measured. Prodynorphin mRNA and receptor-mediated G protein activation were measured at two time points following the last paclitaxel injection using quantitative real-time polymerase chain reaction and agonist-stimulated [³⁵S]guanosine-5'-O'-(γ-thio)-triphosphate ([³⁵S]GTPγS) binding, respectively, in the nucleus accumbens (NAc), caudate-putamen, amygdala, and spinal cord.

RESULTS

Paclitaxel produced a norBNI-reversible sucrose preference deficit, whereas mechanical hypersensitivity was not reversed by norBNI. Paclitaxel treatment increased the levels of mRNA for prodynorphin, a precursor for endogenous KOR agonists, in the NAc. Paclitaxel also had time-dependent effects on KOR-mediated G protein activation in the NAc.

CONCLUSIONS

These results suggest that KOR signaling mediates an initial aversive component of paclitaxel, but not necessarily paclitaxel-induced mechanical hypersensitivity.

The Operant Plantar Thermal Assay: A Novel Device for Assessing Thermal Pain Tolerance in Mice

Pain is a multidimensional experience of sensory-discriminative, cognitive, and affective processes; however, current basic research methods rely heavily on response to threshold stimuli, bypassing the supraspinal processing that ultimately gives rise to the pain experience. We developed the operant plantar thermal assay (OPTA), which utilizes a novel, conflict-based operant task requiring evaluation and active decision-making to obtain reward under thermally aversive conditions to quantify thermal pain tolerance. In baseline measures, male and female mice exhibited similar temperature preferences, however in the OPTA, female mice exhibited greater temperature-dependent tolerance, as defined by choice time spent in an adverse thermal condition to obtain reward. Increasing reward salience (4% vs 10% sucrose solution) led to increased thermal tolerance for males but not females. To determine whether neuropathic and inflammatory pain models alter thermal tolerance, animals with chronic constriction injury (CCI) or complete Freund's adjuvant (CFA), respectively, were tested in the OPTA. Surprisingly, neuropathic animals exhibited increased thermal tolerance, as shown by greater time spent in the reward zone in an adverse thermal condition, compared with sham animals. There was no effect of inflammation on thermal tolerance. Administration of clonidine in the CCI model led to increased thermal tolerance in both injured and sham animals. In contrast, the non-steroidal anti-inflammatory meloxicam was anti-hyperalgesic in the CFA model, but reduced thermal pain tolerance. These data support the feasibility of using the OPTA to assess thermal pain tolerance to gain new insights into complex pain behaviors and to investigate novel aspects of analgesic efficacy.

Attenuated dopamine receptor signaling in nucleus accumbens core in a rat model of chemically-induced neuropathy

Neuropathy is major source of chronic pain that can be caused by mechanically or chemically induced nerve injury. Intraplantar formalin injection produces local necrosis over a two-week period and has been used to model neuropathy in rats. To determine whether neuropathy alters dopamine (DA) receptor responsiveness in mesolimbic brain regions, we examined dopamine D₁-like and D₂-like receptor (D_1/2R) signaling and expression in male rats 14 days after bilateral intraplantar formalin injections into both rear paws. D₂R-mediated G-protein activation and expression of the D₂R long, but not short, isoform were reduced in nucleus accumbens (NAc) core, but not in NAc shell, caudate-putamen or ventral tegmental area of formalin- compared to saline-treated rats. In addition, D₁R-stimulated adenylyl cyclase activity was also reduced in NAc core, but not in NAc shell or prefrontal cortex, of formalin-treated rats, whereas D₁R expression was unaffected. Other proteins involved in dopamine neurotransmission, including dopamine uptake transporter and tyrosine hydroxylase, were unaffected by formalin treatment. In behavioral tests, the potency of a D₂R agonist to suppress intracranial self-stimulation (ICSS) was decreased in formalin-treated rats, whereas D₁R agonist effects were not altered. The combination of reduced D₂R expression and signaling in NAc core with reduced suppression of ICSS responding by a D₂R agonist suggest a reduction in D₂ autoreceptor function. Altogether, these results indicate that intraplantar formalin produces attenuation of highly specific DA receptor signaling processes in NAc core of male rats and suggest the development of a neuropathy-induced allostatic state in both pre- and post-synaptic DA receptor function.

Evidence and explanation for the involvement of the nucleus accumbens in pain processing

The nucleus accumbens (NAc) is a subcortical brain structure known primarily for its roles in pleasure, reward, and addiction. Despite less focus on the NAc in pain research, it also plays a large role in the mediation of pain and is effective as a source of analgesia. Evidence for this involvement lies in the NAc’s cortical connections, functions, pharmacology, and therapeutic targeting. The NAc projects to and receives information from notable pain structures, such as the prefrontal cortex, anterior cingulate cortex, periaqueductal gray, habenula, thalamus, etc. Additionally, the NAc and other pain-modulating structures share functions involving opioid regulation and motivational and emotional processing, which each work beyond simply the rewarding experience of pain offset. Pharmacologically speaking, the NAc responds heavily to painful stimuli, due to its high density of μ opioid receptors and the activation of several different neurotransmitter systems in the NAc, such as opioids, dopamine, calcitonin gene-related peptide, γ-aminobutyric acid, glutamate, and substance P, each of which have been shown to elicit analgesic effects. In both preclinical and clinical models, deep brain stimulation of the NAc has elicited successful analgesia. The multi-functional NAc is important in motivational behavior, and the motivation for avoiding pain is just as important to survival as the motivation for seeking pleasure. It is possible, then, that the NAc must be involved in both pleasure and pain in order to help determine the motivational salience of positive and negative events.

Reward Processing under Chronic Pain from the Perspective of "Liking" and "Wanting": A Narrative Review

The therapeutic goals of patients with chronic pain are not only to relieve pain but also to improve the quality of life. Chronic pain negatively affects various aspects of daily life, such as by decreasing the motivation to work and reward sensitivity, which may lead to difficulties in daily life or even unemployment. Human and animal studies have shown that chronic pain damages reward processing; the exploration of associated internal mechanisms may aid the development of treatments to repair this damage. Incentive salience theory, used widely to describe reward processing, divides this processing into “liking” (reward-induced hedonic sensory impact) and “wanting” (reward-induced motivation) components. It has been employed to explain pathological changes in reward processing induced by psychiatric disorders. In this review, we summarize the findings of studies of reward processing under chronic pain and examine the effects of chronic pain on “liking” and “wanting.” Evidence indicates that chronic pain compromises the “wanting” component of reward processing; we also discuss the neural mechanisms that may mediate this effect. We hope that this review aids the development of therapies to improve the quality of life of patients with chronic pain.

The α2,3-selective potentiator of GABAA receptors, KRM-II-81, reduces nociceptive-associated behaviors induced by formalin and spinal nerve ligation in rats

Clinical evidence indicates that positive allosteric modulators (PAMs) of GABAA receptors have analgesic benefit in addition to efficacy in anxiety disorders. However, the utility of GABAA receptor PAMs as analgesics is compromised by the central nervous system side effects of non-selective potentiators. A selective potentiator of GABAA receptors associated with α2/3 subunits, KRM-II-81(5-(8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepin-3-yl)oxazole), has demonstrated anxiolytic, anticonvulsant, and antinociceptive effects in rodents with reduced motoric side effects. The present study evaluated the potential of KRM-II-81 as a novel analgesic. Oral administration of KRM-II-81 attenuated formalin-induced flinching; in contrast, diazepam was not active. KRM-II-81 attenuated nociceptive-associated behaviors engendered by chronic spinal nerve ligation (L5/L6). Diazepam decreased locomotion of rats at the dose tested in the formalin assay (10 mg/kg) whereas KRM-II-81 produced small decreases that were not dose-dependent (10-100 mg/kg). Plasma and brain levels of KRM-II-81 were used to demonstrate selectivity for α2/3- over α1-associated GABAA receptors and to define the degree of engagement of these receptors. Plasma and brain concentrations of KRM-II-81 were positively-associated with analgesic efficacy. GABA currents from isolated rat dorsal-root ganglion cultures were potentiated by KRM-II-81 with an ED50 of 32 nM. Measures of respiratory depression were reduced by alprazolam whereas KRM-II-81 was either inactive or produced effects with lower potency and efficacy. These findings add to the growing body of data supporting the idea that α2/3-selective GABAA receptor PAMs will have efficacy and tolerability as pain medications including those for neuropathic pain. Given their predicted anxiolytic effects, α2/3-selective GABAA receptor PAMs offer an additional inroad into the management of pain.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

Pain-Induced Negative Affect Is Mediated via Recruitment of The Nucleus Accumbens Kappa Opioid System

Negative affective states affect quality of life for patients suffering from pain. These maladaptive emotional states can lead to involuntary opioid overdose and many neuropsychiatric comorbidities. Uncovering the mechanisms responsible for pain-induced negative affect is critical in addressing these comorbid outcomes. The nucleus accumbens (NAc) shell, which integrates the aversive and rewarding valence of stimuli, exhibits plastic adaptations in the presence of pain. In discrete regions of the NAc, activation of the kappa opioid receptor (KOR) decreases the reinforcing properties of rewards and induces aversive behaviors. Using complementary techniques, we report that in vivo recruitment of NAc shell dynorphin neurons, acting through KOR, is necessary and sufficient to drive pain-induced negative affect. Taken together, our results provide evidence that pain-induced adaptations in the kappa opioid system within the NAc shell represent a functional target for therapeutic intervention that could circumvent pain-induced affective disorders. VIDEO ABSTRACT.

Chronic pain impairs cognitive flexibility and engages novel learning strategies in rats

Cognitive flexibility, the ability to adapt behavior to changing outcomes, is critical to survival. The prefrontal cortex is a key site of cognitive control, and chronic pain is known to lead to significant morphological changes to this brain region. Nevertheless, the effects of chronic pain on cognitive flexibility and learning remain uncertain. We used an instrumental paradigm to assess adaptive learning in an experimental model of chronic pain induced by tight ligation of the spinal nerves L5/6 (spinal nerve ligation model). Naive, sham-operated, and spinal nerve ligation (SNL) rats were trained to perform fixed-ratio, variable-ratio, and contingency-shift behaviors for food reward. Although all groups learned an initial lever-reward contingency, learning was slower in SNL animals in a subsequent choice task that reversed reinforcement contingencies. Temporal analysis of lever-press responses across sessions indicated no apparent deficits in memory consolidation or retrieval. However, analysis of learning within sessions revealed that the lever presses of SNL animals occurred in bursts, followed by delays. Unexpectedly, the degree of bursting correlated positively with learning. Under a variable-ratio probabilistic task, SNL rats chose a less profitable behavioral strategy compared with naive and sham-operated animals. After extinction of behavior for learned preferences, SNL animals reverted to their initially preferred (ie, less profitable) behavioral choice. Our data suggest that in the face of uncertainty, chronic pain drives a preference for familiar associations, consistent with reduced cognitive flexibility. The observed burst-like responding may represent a novel learning strategy in animals with chronic pain.

Influence of neuropathic pain on nicotinic acetylcholine receptor plasticity and behavioral responses to nicotine in rats

Tobacco smoking is particularly evident in individuals experiencing chronic pain. This complex relationship is poorly understood at both molecular and behavioral levels. Here, we describe experiments aimed at understanding whether a chronic pain state induces neuroadaptations into the brain or peripheral nerves that involve nicotinic acetylcholine receptors (nAChRs) and whether these neuroadaptations directly lead to increased vulnerability to nicotine addiction or to the development of coping strategies to relieve pain symptoms. We found that ligation of the rat L5 spinal nerve led to a dramatic downregulation in the mRNA expression levels of all nAChR subunits examined in dorsal root ganglia and a time-dependent downregulation of discrete subunits, particularly in the cingulate cortex and the amygdala. Spinal nerve ligation and sham-operated rats showed minor or no changes in patterns of acquisition and motivation for nicotine taking. Spinal nerve ligation rats also showed similar vulnerability to nicotine seeking as sham animals when reinstatement was induced by nicotine-associated cues, but failed to reinstate lever pressing when relapse was induced by nicotine priming. Spinal nerve ligation and sham rats were equally sensitive to nicotine-induced anxiety-like behavior and antinociception; however, nicotine produced a potent and long-lasting antiallodynic effect in spinal nerve ligation rats. These results demonstrate that chronic pain leads to plasticity of nAChRs that do not directly facilitate nicotine addictive behaviors. Instead, nicotine potently decreases allodynia, an effect that could lead to increased nicotine consumption in chronic pain subjects.

Corticolimbic circuitry in the modulation of chronic pain and substance abuse

The transition from acute to chronic pain is accompanied by increased engagement of emotional and motivational circuits. Adaptations within this corticolimbic circuitry contribute to the cellular and behavioral maladaptations associated with chronic pain. Central regions within the corticolimbic brain include the mesolimbic dopamine system, the amygdala, and the medial prefrontal cortex. The evidence reviewed herein supports the notion that chronic pain induces significant changes within these corticolimbic regions that contribute to the chronicity and intractability of pain. In addition, pain-induced changes in corticolimbic circuitry are poised to impact motivated behavior and reward responsiveness to environmental stimuli, and may modulate the addiction liability of drugs of abuse, such as opioids.

Delta/mu opioid receptor interactions in operant conditioning assays of pain-depressed responding and drug-induced rate suppression: assessment of therapeutic index in male Sprague Dawley rats

RATIONALE AND OBJECTIVES

Although delta/mu receptor interactions vary as a function of behavioral endpoint, there have been no assessments of these interactions using assays of pain-depressed responding. This is the first report of delta/mu interactions using an assay of pain-depressed behavior.

METHODS

A mult-cycle FR10 operant schedule was utilized in the presence of (nociception) and in the absence of (rate suppression) a lactic acid inflammatory pain-like manipulation. SNC80 and methadone were used as selective/high efficacy delta and mu agonists, respectively. Both SNC80 and methadone alone produced a dose-dependent restoration of pain-depressed responding and dose-dependent response rate suppression. Three fixed ratio mixtures, based on the relative potencies of the drugs in the nociception assay, also produced dose-dependent antinociception and sedation. Isobolographic analysis indicated that all three mixtures produced supra-additive antinociceptive effects and simply additive sedation effects.

CONCLUSIONS

The therapeutic index (TI) inversely varied as a function of amount of SNC80 in the mixture, such that lower amounts of SNC80 produced a higher TI, and larger amounts produced a lower TI. Compared to literature using standard pain-elicited assays, the orderly relationship between SNC80 and TI reported here may be a unique function of assessing pain-depressed behavior.

Peripheral nerve injury impairs the ability to maintain behavioural flexibility following acute stress in the rat

Chronic neuropathic pain often leads to impaired cognition and reduced behavioural flexibility. This study used a rat model to investigate if a peripheral nerve injury, with or without an additional acute psychological stress, alters behavioural flexibility and goal directed behaviour as measured by sensitivity to devaluation. Neuropathic pain was induced by a chronic constriction injury (CCI) of the sciatic nerve. CCI, sham-injury and naïve rats were trained to press two levers for two rewards. In outcome devaluation tests, one of the rewards was devalued by pre-feeding it to satiety, immediately prior to an extinction test measuring responding on the two levers. The ability to preferentially direct responding toward the action earning the currently-valued reward was taken as evidence of goal-directed behaviour. To test the impact of acute stress, rats were subjected to 15min restraint following pre-feeding and prior to the devaluation test. Neither CCI surgery nor acute stress alone altered sensitivity to devaluation, but in combination CCI and acute stress significantly reduced sensitivity to devaluation. This Study demonstrates that relatively mild stressors that are without effect in uninjured populations can markedly impair cognition under conditions of chronic pain. It further suggests that overlapping neural substrates regulated by nerve injury and/or acute stress are having a cumulative effect on behavioural flexibility.

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aβ, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.

Neuropathic pain

Neuropathic pain is caused by a lesion or disease of the somatosensory system, including peripheral fibres (Aβ, Aδ and C fibres) and central neurons, and affects 7-10% of the general population. Multiple causes of neuropathic pain have been described and its incidence is likely to increase owing to the ageing global population, increased incidence of diabetes mellitus and improved survival from cancer after chemotherapy. Indeed, imbalances between excitatory and inhibitory somatosensory signalling, alterations in ion channels and variability in the way that pain messages are modulated in the central nervous system all have been implicated in neuropathic pain. The burden of chronic neuropathic pain seems to be related to the complexity of neuropathic symptoms, poor outcomes and difficult treatment decisions. Importantly, quality of life is impaired in patients with neuropathic pain owing to increased drug prescriptions and visits to health care providers, as well as the morbidity from the pain itself and the inciting disease. Despite challenges, progress in the understanding of the pathophysiology of neuropathic pain is spurring the development of new diagnostic procedures and personalized interventions, which emphasize the need for a multidisciplinary approach to the management of neuropathic pain.