Effects of acute and sustained pain manipulations on performance in a visual-signal detection task of attention in rats

Formalin-induced pain prolongs sub- to supra-second time estimation in rats

Background

Temporal estimation can be influenced by pain, which is a complex psychological and physiological phenomenon. However, the time range in which perception is most sensitive to pain remains unclear.

Methods

In the present study, we explored the effects of acute inflammatory pain on time perception in the sub- to supra-second (0.6–2.4-s) and supra-second (2–8-s) ranges in rats. Plantar formalin injection was used to induce acute inflammatory pain, and a temporal bisection task was used to measure time perception. Task test sessions were held for five consecutive days (one per day): the day before injection (baseline), immediately after injection, and the three post-injection days. The point of subjective equality (PSE, which reflects the subjective duration) and Weber fraction (which reflects temporal sensitivity) were calculated and analysed.

Results

In the 0.6–2.4-s range, the PSE was significantly lower, indicating prolonged subjective duration, in the formalin group relative to the saline group (p = 0.049) immediately after injection. Formalin-induced pain also tended to lengthened time perception in the 0.6–2.4-s range on post-injection days 2 (p = 0.06) and 3 (p = 0.054). In the 2–8-s range, formalin injection did not affect the PSE or Weber fraction.

Conclusions

The enhanced effect of pain on temporal perception in the sub- to supra-second range is observed in this study and this effect is attenuated with the prolongation of estimated time, even in rats.

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.'.

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.

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.

Effects of intraplantar administration of Complete Freund's Adjuvant (CFA) on rotarod performance in mice

BACKGROUND AND AIMS

Preclinical animal models are crucial to study pain mechanisms and assess antinociceptive effects of medications. One major problem with current animal behavioral models is their lack of face validity with human nociception and the vulnerability for false-positive results. Here, we evaluated the usefulness of rotarod as a new way to assess inflammatory nociception in rodents.

METHODS

Adult male mice were injected with saline or Complete Freund's Adjuvant (CFA) in the left hindpaws. Mechanical allodynia and rotarod performance were evaluated before and after the administration of CFA. Mechanical allodynia was measured using von Frey filaments. Long-term effect of CFA on rotarod performance was also assessed for 2 weeks.

RESULTS

Our results showed that CFA administration decreased pain threshold and increased sensitivity to von Frey filaments compared to control group. In rotarod experiments, the starting speed of the rod rotation started at four RPM, and accelerated until it reached 40 RPM in 5 min. Rotarod performance was enhanced from day to day in the control group. However, rotarod performance in CFA group was attenuated after CFA administration, which was significant after 24 h compared to vehicle. This attenuation was blocked by ibuprofen. Haloperidol administration (positive control) produced similar results to CFA administration. CFA did not produce significant attenuation of rotarod performance after 1 week post-injection.

CONCLUSIONS

Collectively, our findings could encourage the use of rotarod assay to measure acute (but not chronic) inflammatory nociception as a useful tool in rodents.

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.

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.

Lactic acid suppresses IgE-mediated mast cell function in vitro and in vivo

Mast cells have functional plasticity affected by their tissue microenvironment, which greatly impacts their inflammatory responses. Because lactic acid (LA) is abundant in inflamed tissues and tumors, we investigated how it affects mast cell function. Using IgE-mediated activation as a model system, we found that LA suppressed inflammatory cytokine production and degranulation in mouse peritoneal mast cells, data that were confirmed with human skin mast cells. In mouse peritoneal mast cells, LA-mediated cytokine suppression was dependent on pH- and monocarboxylic transporter-1 expression. Additionally, LA reduced IgE-induced Syk, Btk, and ERK phosphorylation, key signals eliciting inflammation. In vivo, LA injection reduced IgE-mediated hypothermia in mice undergoing passive systemic anaphylaxis. Our data suggest that LA may serve as a feedback inhibitor that limits mast cell-mediated inflammation.

Affect-Driven Attention Biases as Animal Welfare Indicators: Review and Methods

Attention bias describes the differential allocation of attention towards one stimulus compared to others. In humans, this bias can be mediated by the observer's affective state and is implicated in the onset and maintenance of affective disorders such as anxiety. Affect-driven attention biases (ADABs) have also been identified in a few other species. Here, we review the literature on ADABs in animals and discuss their utility as welfare indicators. Despite a limited research effort, several studies have found that negative affective states modulate attention to negative (i.e., threatening) cues. ADABs influenced by positive-valence states have also been documented in animals. We discuss methods for measuring ADAB and conclude that looking time, dot-probe, and emotional spatial cueing paradigms are particularly promising. Research is needed to test them with a wider range of species, investigate attentional scope as an indicator of affect, and explore the possible causative role of attention biases in determining animal wellbeing. Finally, we argue that ADABs might not be best-utilized as indicators of general valence, but instead to reveal specific emotions, motivations, aversions, and preferences. Paying attention to the human literature could facilitate these advances.

Intermittent stimulation in the nucleus basalis of meynert improves sustained attention in rhesus monkeys

Sustained attention is essential in important behaviors in daily life. Many neuropsychiatric disorders are characterized by a compromised ability to sustain attention, making this cognitive domain an important therapeutic target. In this study, we tested a novel method of improving sustained attention. Monkeys were engaged in a continuous performance task (CPT) while the nucleus basalis of Meynert (NB), the main source of cholinergic innervation of the neocortex, was stimulated. Intermittent NB stimulation improved the animals' performance by increasing the hit rate and decreasing the false alarm rate. Administration of the cholinesterase inhibitor donepezil or the muscarinic antagonist scopolamine alone impaired performance, whereas the nicotinic antagonist mecamylamine alone improved performance. Applying NB stimulation while mecamylamine or donepezil were administered impaired CPT performance. Methylphenidate, a monoaminergic psychostimulant, was applied in conjunction with intermittent stimulation as a negative control, as it does not directly modulate cholinergic output. Methylphenidate also improved performance, and it produced further improvement when combined with NB stimulation. The additive effect of the combination suggested NB stimulation altered behavior independently from methylphenidate effects. We conclude that basal forebrain projections contribute to sustained attention, and that intermittent NB stimulation is an effective way of improving performance.

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.

Analysis of inflammation-induced depression of home cage wheel running in rats reveals the difference between opioid antinociception and restoration of function

Opioids are effective at inhibiting responses to noxious stimuli in rodents, but have limited efficacy and many side effects in chronic pain patients. One reason for this disconnect is that nociception is typically assessed using withdrawal from noxious stimuli in animals, whereas chronic pain patients suffer from abnormal pain that disrupts normal activity. We hypothesized that assessment of home cage wheel running in rats would provide a much more clinically relevant method to assess opioid efficacy to restore normal behavior. Intraplantar injection of Complete Freund's Adjuvant (CFA) into the right hindpaw depressed wheel running and caused mechanical allodynia measured with the von Frey test in both male and female rats. Administration of an ED₅₀ dose of morphine (3.2mg/kg) reversed mechanical allodynia, but did not reverse CFA-induced depression of wheel running. In contrast, administration of a low dose of morphine (1.0mg/kg) restored running for one hour in both sexes, but had no effect on mechanical allodynia. Administration of the atypical opioid buprenorphine had no effect on inflammation-induced depression of wheel running in male or female rats, but attenuated mechanical allodynia in male rats. Administration of buprenorphine and higher doses of morphine depressed wheel running in non-inflamed rats, suggesting that the side effects of opioids interfere with restoration of function. These data indicate that restoration of pain-depressed function requires antinociception in the absence of disruptive side effects. The disruptive side effects of opioids are consistent with the major limitation of opioid use in human pain patients.

Cross-centre replication of suppressed burrowing behaviour as an ethologically relevant pain outcome measure in the rat: a prospective multicentre study

Burrowing, an ethologically relevant rodent behaviour, has been proposed as a novel outcome measure to assess the global impact of pain in rats. In a prospective multicentre study using male rats (Wistar, Sprague-Dawley), replication of suppressed burrowing behaviour in the complete Freund adjuvant (CFA)-induced model of inflammatory pain (unilateral, 1 mg/mL in 100 µL) was evaluated in 11 studies across 8 centres. Following a standard protocol, data from participating centres were collected centrally and analysed with a restricted maximum likelihood-based mixed model for repeated measures. The total population (TP-all animals allocated to treatment; n = 249) and a selected population (SP-TP animals burrowing over 500 g at baseline; n = 200) were analysed separately, assessing the effect of excluding "poor" burrowers. Mean baseline burrowing across studies was 1113 g (95% confidence interval: 1041-1185 g) for TP and 1329 g (1271-1387 g) for SP. Burrowing was significantly suppressed in the majority of studies 24 hours (7 studies/population) and 48 hours (7 TP, 6 SP) after CFA injections. Across all centres, significantly suppressed burrowing peaked 24 hours after CFA injections, with a burrowing deficit of -374 g (-479 to -269 g) for TP and -498 g (-609 to -386 g) for SP. This unique multicentre approach first provided high-quality evidence evaluating suppressed burrowing as robust and reproducible, supporting its use as tool to infer the global effect of pain on rodents. Second, our approach provided important informative value for the use of multicentre studies in the future.

Task- and Treatment Length-Dependent Effects of Vortioxetine on Scopolamine-Induced Cognitive Dysfunction and Hippocampal Extracellular Acetylcholine in Rats

Major depressive disorder (MDD) is a common psychiatric disorder that often features impairments in cognitive function, and these cognitive symptoms can be important determinants of functional ability. Vortioxetine is a multimodal antidepressant that may improve some aspects of cognitive function in patients with MDD, including attention, processing speed, executive function, and memory. However, the cause of these effects is unclear, and there are several competing theories on the underlying mechanism, notably including regionally-selective downstream enhancement of glutamate neurotransmission and increased acetylcholine (ACh) neurotransmission. The current work sought to evaluate the ACh hypothesis by examining vortioxetine’s ability to reverse scopolamine-induced impairments in rodent tests of memory and attention. Additionally, vortioxetine’s effects on hippocampal extracellular ACh levels were examined alongside studies of vortioxetine’s pharmacokinetic profile. We found that acute vortioxetine reversed scopolamine-induced impairments in social and object recognition memory, but did not alter scopolamine-induced impairments in attention. Acute vortioxetine also induced a modest and short-lived increase in hippocampal ACh levels. However, this short-term effect is at variance with vortioxetine’s moderately long brain half life (5.1 hours). Interestingly, subchronic vortioxetine treatment failed to reverse scopolamine-induced social recognition memory deficits and had no effects on basal hippocampal ACh levels. These data suggest that vortioxetine has some effects on memory that could be mediated through cholinergic neurotransmission, however these effects are modest and only seen under acute dosing conditions. These limitations may argue against cholinergic mechanisms being the primary mediator of vortioxetine′s cognitive effects, which are observed under chronic dosing conditions in patients with MDD.

Home cage wheel running is an objective and clinically relevant method to assess inflammatory pain in male and female rats

BACKGROUND

The assessment of nociception in preclinical studies is undergoing a transformation from pain-evoked to pain-depressed tests to more closely mimic the effects of clinical pain. Many inflammatory pain-depressed behaviors (reward seeking, locomotion) have been examined, but these tests are limited because of confounds such as stress and difficulties in quantifying behavior.

NEW METHOD

The present study evaluates home cage wheel running as an objective method to assess the magnitude and duration of inflammatory pain in male and female rats.

RESULTS

Injection of Complete Freund's Adjuvant (CFA) into the right hindpaw to induce inflammatory pain almost completely inhibited wheel running for 2 days in male and female rats. Wheel running gradually returned to baseline levels within 12 days despite persistent mechanical hypersensitivity (von Frey test).

COMPARISON WITH EXISTING METHODS

Continuously monitoring home cage wheel running improves on previous studies examining inflammatory pain-depressed wheel running because it is more sensitive to noxious stimuli, avoids the stress of removing the rat from its cage for testing, and provides a complete analysis of the time course for changes in nociception.

CONCLUSIONS

The present data indicate that home cage wheel running is a clinically relevant method to assess inflammatory pain in the rat. The decrease in activity caused by inflammatory pain and subsequent gradual recovery mimics the changes in activity caused by pain in humans. The tendency for pain-depressed wheel running to be greater in female than male rats is consistent with the tendency for women to be at greater risk of chronic pain than men.

Evaluation of a Postoperative Pain-Like State on Motivated Behavior in Rats: Effects of Plantar Incision on Progressive-Ratio Food-Maintained Responding

There has been recent interest in characterizing the effects of pain-like states on motivated behaviors in order to quantify how pain modulates goal-directed behavior and the persistence of that behavior. The current set of experiments assessed the effects of an incisional postoperative pain manipulation on food-maintained responding under a progressive-ratio (PR) operant schedule. Independent variables included injury state (plantar incision or anesthesia control) and reinforcer type (grain pellet or sugar pellet); dependent variables were tactile sensory thresholds and response breakpoint. Once responding stabilized on the PR schedule, separate groups of rats received a single ventral hind paw incision or anesthesia (control condition). Incision significantly reduced breakpoints in rats responding for grain, but not sugar. In rats responding for sugar, tactile hypersensitivity recovered within 24 hr, indicating a faster recovery of incision-induced tactile hypersensitivity compared to rats responding for grain, which demonstrated recovery at PD2. The NSAID analgesic, diclofenac (5.6 mg/kg) completely restored incision-depressed PR operant responding and tactile sensitivity at 3 hr following incision. The PR schedule differentiated between sucrose and grain, suggesting that relative reinforcing efficacy may be an important determinant in detecting pain-induced changes in motivated behavior.