Enduring attentional deficits in rats treated with a peripheral nerve injury

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.

Chronic Pain-Related Cognitive Deficits: Preclinical Insights into Molecular, Cellular, and Circuit Mechanisms

Cognitive impairment is a common comorbidity of chronic pain, significantly disrupting patients' quality of life. Despite this comorbidity being clinically recognized, the underlying neuropathological mechanisms remain unclear. Recent preclinical studies have focused on the fundamental mechanisms underlying the coexistence of chronic pain and cognitive decline. Pain chronification is accompanied by structural and functional changes in the neural substrate of cognition. Based on the developments in electrophysiology and optogenetics/chemogenetics, we summarized the relevant neural circuits involved in pain-induced cognitive impairment, as well as changes in connectivity and function in brain regions. We then present the cellular and molecular alternations related to pain-induced cognitive impairment in preclinical studies, mainly including modifications in neuronal excitability and structure, synaptic plasticity, glial cells and cytokines, neurotransmitters and other neurochemicals, and the gut-brain axis. Finally, we also discussed the potential treatment strategies and future research directions.

P2X4 signalling contributes to hyperactivity but not pain sensitization comorbidity in a mouse model of attention deficit/hyperactivity disorder

Introduction: Attention deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder characterized by hyperactivity, inattention, and impulsivity that often persist until adulthood. Frequent comorbid disorders accompany ADHD and two thirds of children diagnosed with ADHD also suffer from behavioural disorders and from alteration of sensory processing. We recently characterized the comorbidity between ADHD-like symptoms and pain sensitisation in a pharmacological mouse model of ADHD, and we demonstrated the implication of the anterior cingulate cortex and posterior insula. However, few studies have explored the causal mechanisms underlying the interactions between ADHD and pain. The implication of inflammatory mechanisms has been suggested but the signalling pathways involved have not been explored. Methods: We investigated the roles of purinergic signalling, at the crossroad of pain and neuroinflammatory pathways, by using a transgenic mouse line that carries a total deletion of the P2X4 receptor. Results: We demonstrated that P2X4 deletion prevents hyperactivity in the mouse model of ADHD. In contrast, the absence of P2X4 lowered thermal pain thresholds in sham conditions and did not affect pain sensitization in ADHD-like conditions. We further analysed microglia reactivity and the expression of inflammatory markers in wild type and P2X4KO mice. Our results revealed that P2X4 deletion limits microglia reactivity but at the same time exerts proinflammatory effects in the anterior cingulate cortex and posterior insula. Conclusion: This dual role of P2X4 could be responsible for the differential effects noted on ADHD-like symptoms and pain sensitization and calls for further studies to investigate the therapeutic benefit of targeting the P2X4 receptor in ADHD patients.

Excitatory neurons in the lateral parabrachial nucleus mediate the interruptive effect of inflammatory pain on a sustained attention task

BACKGROUND

Attentional deficits are among the most common pain-induced cognitive disorders. Pain disrupts attention and may excessively occupy attentional resources in pathological states, leading to daily function impairment and increased disability. However, the neural circuit mechanisms by which pain disrupts attention are incompletely understood.

METHODS

We used a three-choice serial reaction time task (3CSRTT) to construct a sustained-attention task model in male C57BL/6J mice. Formalin or complete Freund's adjuvant was injected into a paw to establish an inflammatory pain model. We measured changes in 3CSRTT performance in the two inflammatory pain models, and investigated the neural circuit mechanisms of pain-induced attentional deficits.

RESULTS

Acute inflammatory pain impaired 3CSRTT performance, while chronic inflammatory pain had no effect. Either inhibition of the ascending pain pathway by blockade of the conduction of nociceptive signals in the sciatic nerve using the local anesthetic lidocaine or chemogenetic inhibition of Ca2+/calmodulin-dependent protein kinase IIα (CaMKIIα) neurons in the lateral parabrachial nucleus (LPBN) attenuated the acute inflammatory pain-induced impairment of 3CSRTT performance, while chemogenetic activation of CaMKIIα neurons in the LPBN disrupted the 3CSRTT. Furthermore, the activity of CaMKIIα neurons in the LPBN was significantly lower on Day 2 after complete Freund's adjuvant injection than on the day of injection, which correlated with the recovery of 3CSRTT performance during chronic inflammatory pain.

CONCLUSIONS

Activation of excitatory neurons in the LPBN is a mechanism by which acute inflammatory pain disrupts sustained attention. This finding has implications for the treatment of pain and its cognitive comorbidities.

Pain hypersensitivity in a pharmacological mouse model of attention-deficit/hyperactivity disorder

Clinical evidence suggests that pain hypersensitivity develops in patients with attention-deficit/hyperactivity disorder (ADHD). However, the mechanisms and neural circuits involved in these interactions remain unknown because of the paucity of studies in animal models. We previously validated a mouse model of ADHD obtained by neonatal 6-hydroxydopamine (6-OHDA) injection. Here, we have demonstrated that 6-OHDA mice exhibit a marked sensitization to thermal and mechanical stimuli, suggesting that phenotypes associated with ADHD include increased nociception. Moreover, sensitization to pathological inflammatory stimulus is amplified in 6-OHDA mice as compared to shams. In this ADHD model, spinal dorsal horn neuron hyperexcitability was observed. Furthermore, ADHD-related hyperactivity and anxiety, but not inattention and impulsivity, are worsened in persistent inflammatory conditions. By combining in vivo electrophysiology, optogenetics, and behavioral analyses, we demonstrated that anterior cingulate cortex (ACC) hyperactivity alters the ACC-posterior insula circuit and triggers changes in spinal networks that underlie nociceptive sensitization. Altogether, our results point to shared mechanisms underlying the comorbidity between ADHD and nociceptive sensitization. This interaction reinforces nociceptive sensitization and hyperactivity, suggesting that overlapping ACC circuits may be targeted to develop better treatments.

Chronic Pain Produces Reversible Memory Deficits That Depend on Task Difficulty in Rats

Cognitive impairment associated with chronic pain remains relatively poorly understood. Use of analgesic drugs and often present co-morbidities in patients can preclude conclusions of causative relationships between chronic pain and cognitive deficits. Here, the impact of pain resulting from spinal nerve ligation (SNL) injury in rats on short and long-term memory was assessed in the novel object recognition task. To understand if chronic pain seizes the limited cognitive resources that are available at any given time, task difficulty was varied by using either very different (ie, easy task) or similar (ie, difficult task) pairs of objects. Nerve-injured, male rats exhibited no short or long-term memory deficits under easy task conditions. However, unlike sham-operated controls, injured rats showed deficits in both short and long-term memory by failing to differentiate similar objects in the difficult task version. In SNL rats, duloxetine produced anti-allodynic effects and ameliorated long-term memory deficits in the difficult task suggesting benefits of pain relief possibly complemented by noradrenergic mediated cognitive enhancement. Together these data suggest chronic pain reversibly takes up a significant amount of limited cognitive resources, leaving sufficient available for easy, but not difficult, tasks. PERSPECTIVE: Memory deficits in a rat model of chronic pain were only seen when the cognitive load was high, ie, in a difficult task. Acute treatment with duloxetine was sufficient to relieve memory deficits, suggesting chronic pain induces memory deficits by seizing limited cognitive resources to the detriment of task-related stimuli.

Neuroinflammation as a possible link between attention-deficit/hyperactivity disorder (ADHD) and pain

Attention-deficit/hyperactivity disorder (ADHD) and pathological pain are two complex syndromes of multifactorial origin. Despite their prevalence and broad impacts, these conditions are seldom recognized and managed simultaneously. The co-existence of neuropsychiatric conditions (such as ADHD) and altered pain perception and chronic pain has been noted in children, and the comorbidity of ADHD and chronic pain is well documented in adults. Pathophysiological studies have suggested dysfunction of the dopaminergic system as a common neurochemical basis for comorbid ADHD and pain. Considerable evidence supports the role of neuroinflammation in the pathophysiology of both. We suggest that central neuroinflammation underlies altered pain perception and pain sensitization in persons with ADHD. Based on our hypothesis, targeting neuroinflammation may serve as a potential new therapeutic intervention to treat ADHD and comorbid pain in children and adolescents and a preventive strategy for the development of chronic pain in adults with ADHD.

Cognition in the Chronic Pain Experience: Preclinical Insights

Acutely, pain is protective. It promotes escape from, and future avoidance of, noxious stimuli through strong and often lifetime associative memories. However, with persistent acute pain or when pain becomes chronic, these memories can promote negative emotions and poor decisions often associated with deleterious behaviors. In this review, we discuss how preclinical studies can provide insights into the relationship between cognition and chronic pain. We also discuss the concept of pain as a cognitive disorder and new strategies for treating chronic pain that emphasize inhibiting the formation of pain memories or promoting 'forgetting' of established pain memories.

Factors mediating pain-related risk for opioid use disorder

Pain is a complex experience with far-reaching organismal influences ranging from biological factors to those that are psychological and social. Such influences can serve as pain-related risk factors that represent susceptibilities to opioid use disorder. This review evaluates various pain-related risk factors to form a consensus on those that facilitate opioid abuse. Epidemiological findings represent a high degree of co-occurrence between chronic pain and opioid use disorder that is, in part, driven by an increase in the availability of opioid analgesics and the diversion of their use in a non-medical context. Brain imaging studies in individuals with chronic pain that use/abuse opioids suggest abuse-related mechanisms that are rooted within mesocorticolimbic processing. Preclinical studies suggest that pain states have a limited impact on increasing the rewarding effects of opioids. Indeed, many findings indicate a reduction in the rewarding and reinforcing effects of opioids during pain states. An increase in opioid use may be facilitated by an increase in the availability of opioids and a decrease in access to non-opioid reinforcers that require mobility or social interaction. Moreover, chronic pain and substance abuse conditions are known to impair cognitive function, resulting in deficits in attention and decision making that may promote opioid abuse. A better understanding of pain-related risk factors can improve our knowledge in the development of OUD in persons with pain conditions and can help identify appropriate treatment strategies. This article is part of the special issue on 'Vulnerabilities to Substance Abuse.'.

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.

Preclinical Neuropathic Pain Assessment; the Importance of Translatability and Bidirectional Research

For patients suffering with chronic neuropathic pain the need for suitable novel therapies is imperative. Over recent years a contributing factor for the lack of development of new analgesics for neuropathic pain has been the mismatch of primary neuropathic pain assessment endpoints in preclinical vs. clinical trials. Despite continuous forward translation failures across diverse mechanisms, reflexive quantitative sensory testing remains the primary assessment endpoint for neuropathic pain and analgesia in animals. Restricting preclinical evaluation of pain and analgesia to exclusively reflexive outcomes is over simplified and can be argued not clinically relevant due to the continued lack of forward translation and failures in the clinic. The key to developing new analgesic treatments for neuropathic pain therefore lies in the development of clinically relevant endpoints that can translate preclinical animal results to human clinical trials. In this review we discuss this mismatch of primary neuropathic pain assessment endpoints, together with clinical and preclinical evidence that supports how bidirectional research is helping to validate new clinically relevant neuropathic pain assessment endpoints. Ethological behavioral endpoints such as burrowing and facial grimacing and objective measures such as electroencephalography provide improved translatability potential together with currently used quantitative sensory testing endpoints. By tailoring objective and subjective measures of neuropathic pain the translatability of new medicines for patients suffering with neuropathic pain will hopefully be improved.

Sensory, Affective, and Cognitive Effects of Trigeminal Injury in Mice

PURPOSE

To characterize adequate study of chronic neuropathic orofacial pain induced by a mental nerve injury in a mouse model, we propose a behavioral assessment of its dimensions: sensory, affective, and cognitive.

MATERIALS AND METHODS

Trigeminal injury was induced by a chronic mental nerve constriction (MnC). Behavioral tests were conducted to assess the different dimensions of pain and to evaluate the general well-being of mice.

RESULTS

Rodents who went through MnC showed signs of mechanical hyperalgesia and increased escape/avoidance behavior. They showed no alterations in general well-being behaviors, yet the injury was sufficient to induce impairment in the ability to adapt to the environmental requirements.

CONCLUSIONS

MnC injury is an efficient model for the study of orofacial pain in mice, capable of inducing impairment in the different dimensions of pain. Intensity and temporality of its effects make our model less aggressive, yet effective to generate cognitive impairment. This work provides a solid foundation for the study of the neural circuits involved in the processing of neuropathic orofacial pain.

Behavioral, Biochemical and Electrophysiological Changes in Spared Nerve Injury Model of Neuropathic Pain

Neuropathic pain is a pathological condition induced by a lesion or disease affecting the somatosensory system, with symptoms like allodynia and hyperalgesia. It has a multifaceted pathogenesis as it implicates several molecular signaling pathways involving peripheral and central nervous systems. Affective and cognitive dysfunctions have been reported as comorbidities of neuropathic pain states, supporting the notion that pain and mood disorders share some common pathogenetic mechanisms. The understanding of these pathophysiological mechanisms requires the development of animal models mimicking, as far as possible, clinical neuropathic pain symptoms. Among them, the Spared Nerve Injury (SNI) model has been largely characterized in terms of behavioral and functional alterations. This model is associated with changes in neuronal firing activity at spinal and supraspinal levels, and induces late neuropsychiatric disorders (such as anxious-like and depressive-like behaviors, and cognitive impairments) comparable to an advanced phase of neuropathy. The goal of this review is to summarize current findings in preclinical research, employing the SNI model as a tool for identifying pathophysiological mechanisms of neuropathic pain and testing pharmacological agent.

Nociceptive input after peripheral nerve injury results in cognitive impairment and alterations in primary afferent physiology in rats

Pain alters cognitive performance through centrally mediated effects in the brain. In this study, we hypothesized that persistent activation of peripheral nociceptors after injury would lead to the development of a chronic pain state that impairs attention-related behavior and results in changes in peripheral neuron phenotypes. Attentional performance was measured in rats using the 5-choice serial reaction time titration variant to determine the initial impact of partial L5 spinal nerve ligation and the effect of persistent nociceptor activation on the resolution of injury. The changes in peripheral neuronal sensibilities and phenotypes were determined in sensory afferents using electrophysiologic signatures and receptive field properties from dorsal root ganglion recordings. Partial spinal nerve injury impaired attentional performance, and this was further impaired in a graded fashion by nociceptive input through an engineered surface. Impairment in attention persisted for only up to 4 days initially, followed by a second phase 7 to 10 weeks after injury in animals exposed to nociceptive input. In animals with prolonged impairment in behavior, the mechanonociceptors displayed a persistent hypersensitivity marked by decreased threshold, increased activity to a given stimulus, and spontaneous activity. Nerve injury disrupts attentional performance acutely and is worsened with peripheral mechanonociceptor activation. Acute impairment resolves, but persistent nociceptive activation produces re-emergence of impairment in the attention-related task associated with electrophysiological abnormalities in peripheral nociceptors. This is consistent with the development of a chronic pain state marked by cognitive impairment and related to persistently abnormal peripheral input.

The locus coeruleus noradrenergic system gates deficits in visual attention induced by chronic pain

Despite years of research on pain comorbidity with affective disorders and cognitive deficits, it is still unclear how deficit in attention co-occurs with chronic pain. It is likely that altered neuroplasticity and or dysregulated neurotransmitters induced by chronic pain, at which pain and cognitive processing systems overlap, may have a negative effect on cognitive processing such as attention. One of the main common networks involved in attentional and pain processing is the noradrenergic system originating from the locus coeruleus (LC). We hypothesized that heightened noradrenaline release from LC induced by chronic pain could cause a deficit in visual attention. For this purpose, performance on the 5-choice serial reaction time test (5-CSRTT) was tested in animals with and without a chronic constriction injury and a selective depletion of noradrenaline in the LC. In addition, pain sensitivity was measured via mechanical allodynia and thermal hyperalgesia. We found that the increase in pain sensitivity following chronic pain correlates with a decline in executive functions as measured by 5-CSRTT. This was true in conditions of both low and high attentional demand. Interestingly, a selective depletion of noradrenaline in LC improved the attentional deficits caused by chronic pain. We argue that changes to the noradrenergic system originating in LC can improve deficits in visual attention induced by chronic pain. Deficit in attention is a common comorbidity among patients with chronic pain which adversely affects them in their family and work lives. Patients struggle with functional impairment due to pain, and deficite in attention adds to this dysfunction. Our findings identify the NE-LC system as a key mediator between chronic pain and the attentional deficits associated with this. This finding calls for further investigations concerning treatments related to the noradrenergic system to reduce the malicious effects of chronic pain.

Formalin-induced and neuropathic pain altered time estimation in a temporal bisection task in rats

Time perception is an important ability that is related closely to humans’ and animals’ daily activities. It can be distorted by various emotional states. In human studies, experimental pain has been shown to prolong the perception of time. However, related animal studies are lacking. In this study, we used a temporal bisection task to investigate how acute inflammatory pain (induced by hind-paw formalin injection) and chronic neuropathic pain [induced by spinal nerve ligation (SNL)] affected time perception in rats. Rats were trained to recognize “short” (1200-ms) and “long” (2400-ms) anchor-duration pure tones and were rewarded for corresponding lever presses. During testing, rats perceived a series of intermediate-duration and anchor-duration pure tones, and selected levers corresponding to the “short” and “long” tones. After formalin injection, rats gave more “long” lever-press responses than after saline injection. The point of subjective equality after formalin injection also increased, suggesting that formalin-induced acute pain extended time perception. In contrast, rats that had undergone SNL gave fewer “long” lever-press responses compared with the sham surgery group. This animal study suggests that formalin-induced pain and neuropathic pain may have different effects on time perception.

High trait impulsivity potentiates the effects of chronic pain on impulsive behavior

•

We explored a potential relation between trait and chronic pain-induced impulsivity.

•

Low trait impulsivity rats with neuropathic pain perform similarly to controls.

•

High trait impulsivity rats are delay intolerant in chronic pain conditions.

•

Trait characteristics influence chronic pain comorbid manifestations.

Preclinical studies on impulsive decision-making in chronic pain conditions are sparse and often contradictory. Outbred rat populations are heterogeneous regarding trait impulsivity manifestations and therefore we hypothesized that chronic pain-related alterations depend on individual traits. To test this hypothesis, we used male Wistar-Han rats in two independent experiments. Firstly, we tested the impact of spared nerve injury (SNI) in impulsive behavior evaluated by the variable delay-to-signal test (VDS). In the second experiment, SNI impact on impulsivity was again tested, but in groups previously categorized as high (HI) and low (LI) trait impulsivity in the VDS.

Results showed that in an heterogenous population SNI-related impact on motor impulsivity and delay intolerance cannot be detected. However, when baseline impulsivity was considered, HI showed a significantly higher delay intolerance than the respective controls more prevalent in left-lesioned animals and appearing to result from a response correction on prematurity from VDS I to VDS II, which was present in Sham and right-lesioned animals.

In conclusion, baseline differences should be more often considered when analyzing chronic pain impact. While this study pertained to impulsive behavior, other reports indicate that this can be generalized to other behavioral dimensions and that trait differences can influence not only the manifestation of comorbid behaviors but also pain itself in a complex and not totally understood manner.

Incisional Nociceptive Input Impairs Attention-related Behavior and Is Associated with Reduced Neuronal Activity in the Prefrontal Cortex in Rats

WHAT WE ALREADY KNOW ABOUT THIS TOPIC

WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Cognitive capacity may be reduced from inflammation, surgery, anesthesia, and pain. In this study, we hypothesized that incision-induced nociceptive input impairs attentional performance and alters neuronal activity in the prefrontal cortex.

METHODS

Attentional performance was measured in rats by using the titration variant of the 5-choice serial reaction time to determine the effect of surgical incision and anesthesia in a visual attention task. Neuronal activity (single spike and local field potentials) was measured in the medial prefrontal cortex in animals during the task.

RESULTS

Incision significantly impaired attention postoperatively (area under curve of median cue duration-time 97.2 ± 56.8 [n = 9] vs. anesthesia control 25.5 ± 14.5 s-days [n = 9], P = 0.002; effect size, η = 0.456). Morphine (1 mg/kg) reduced impairment after incision (area under curve of median cue duration-time 31.6 ± 36.7 [n = 11] vs. saline 110 ± 64.7 s-days [n = 10], P < 0.001; η = 0.378). Incision also decreased cell activity (n = 24; 1.48 ± 0.58 vs. control, 2.93 ± 2.02 bursts/min; P = 0.002; η = 0.098) and local field potentials (n = 28; η = 0.111) in the medial prefrontal cortex.

CONCLUSIONS

These results show that acute postoperative nociceptive input from incision reduces attention-related task performance and decreases neuronal activity in the medial prefrontal cortex. Decreased neuronal activity suggests nociceptive input is more than just a distraction because neuronal activity increases during audiovisual distraction with similar behavioral impairment. This suggests that nociceptive input and the medial prefrontal cortex may contribute to attentional impairment and mild cognitive dysfunction postoperatively. In this regard, pain may affect postoperative recovery and return to normal activities through attentional impairment by contributing to lapses in concentration for routine and complex tasks.

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.

Up-Down Reader: An Open Source Program for Efficiently Processing 50% von Frey Thresholds

Most pathological pain conditions in patients and rodent pain models result in marked alterations in mechanosensation and the gold standard way to measure this is by use of von Frey fibers. These graded monofilaments are used to gauge the level of stimulus-evoked sensitivity present in the affected dermal region. One of the most popular methods used to determine von Frey thresholds is the up–down testing paradigm introduced by Dixon for patients in 1980 and by Chapman and colleagues for rodents in 1994. Although the up–down method is very accurate, leading to its widespread use, defining the 50% threshold from primary data is complex and requires a relatively time-consuming analysis step. We developed a computer program, the Up–Down Reader (UDReader), that can locate and recognize handwritten von Frey assessments from a scanned PDF document and translate these measurements into 50% pain thresholds. Automating the process of obtaining the 50% threshold values negates the need for reference tables or Microsoft Excel formulae and eliminates the chance of a manual calculation error. Our simple and straightforward method is designed to save research time while improving data collection accuracy and is freely available at https://sourceforge.net/projects/updownreader/ or in supplementary files attached to this manuscript.

Assessment of Behavioral Disruption in Rats with Abdominal Inflammation Using Visual Cue Titration and the Five-choice Serial-reaction Time Task

BACKGROUND

Both acute and chronic pain result in a number of behavioral symptoms in patients, including cognitive effects such as decreased attention and working memory. Intraperitoneal administration of dilute lactic acid in rodents has been used to induce abdominal inflammation and produce effects in behavioral assays of both sensory-discriminative and affective pain modalities.

METHODS

Intraperitoneal injection of dilute lactic acid was used to study the impact of abdominal inflammation on an operant task requiring sustained visual attention in rats (N = 7 to 15/group) that adapts dynamically to performance ability. The effects of ketoprofen and morphine on lactic acid-induced impairment were compared with those on the disruptive effects of scopolamine.

RESULTS

Lactic acid impaired performance in a concentration-dependent manner, increasing the duration of cue presentation required to maintain optimal performance from 0.5 ± 0.2 s (mean ± SD) to 17.2 ± 11.4 s after the administration of 1.8% (v/v) (N = 13). The latency to emit correct responses and to retrieve the food reward were both increased by lactic acid. All effects of lactic acid injection were reversed by both ketoprofen and morphine in a dose-dependent manner. Scopolamine, however, produced dose-dependent, nonpain-related disruption in sustained attention that was not altered by either ketoprofen or morphine.

CONCLUSIONS

These data demonstrate that abdominal inflammation induced by lactic acid produces robust disruption in a visual attention-based operant task and that this disruption is reversed by analgesics. Future studies will focus on pain-related circuitry and its impact on both limbic forebrain and frontal cortical mechanisms.

Nuclear factor erythroid 2-related factor 2 antibody attenuates thermal hyperalgesia in the dorsal root ganglion: Neurochemical changes and behavioral studies after sciatic nerve-pinch injury

Oxidative stress is generated in several peripheral nerve injury models.Nuclear factor erythroid 2-related factor 2 (Nrf2) is activated to have a role in antioxidant effect. After nerve injury, the severely painful behavior is also performed. However, little has been explored regarding the function of Nrf2 in this painful process. Therefore, in this study, we compared the effects of Nrf2 antibody administration following sciatic nerve-pinch injury on painful behavior induced in young mice and neurochemical changes in dorsal root ganglion neurons. After pinch nerve injury, we found that the magnitude of the thermal allodynia was significantly decreased after application of Nrf2 antibody (5ul, 1mg/ml) in such injured animals and phosphorylated ERK(p-ERK) as well as the apoptotic protein (i.e., Bcl-6) in DRG neurons were also down-regulated in the anti-Nrf2-treated injured groups compared to the saline-treated groups. Taken collectively, these data suggested that the Nrf2 antibody reduced thermal hyperalgesia via ERK pathway and the down regulation of Bcl-6 protein from the apoptosis pathway might be protecting against the protein deletions caused by anti-Nrf2 effect and suggested the new therapeutic strategy with Nrf2 inhibitor following nerve injury.