Opiate self-administration as a measure of chronic nociceptive pain in arthritic rats

Unveiling the link between chronic pain and misuse of opioids and cannabis

Over 50 million Americans endure chronic pain where many do not receive adequate treatment and self-medicate to manage their pain by taking substances like opioids and cannabis. Research has shown high comorbidity between chronic pain and substance use disorders (SUD) and these disorders share many common neurobiological underpinnings, including hypodopaminergic transmission. Drugs commonly used for self-medication such as opioids and cannabis relieve emotional, bothersome components of pain as well as negative emotional affect that perpetuates misuse and increases the risk of progressing towards drug abuse. However, the causal effect between chronic pain and the development of SUDs has not been clearly established. In this review, we discuss evidence that affirms the proposition that chronic pain is a risk factor for the development of opioid and cannabis use disorders by outlining the clinical evidence and detailing neurobiological mechanisms that link pain and drug misuse. Central to the link between chronic pain and opioid and cannabis misuse is hypodopaminergic transmission and the modulation of dopamine signaling in the mesolimbic pathway by opioids and cannabis. Moreover, we discuss the role of kappa opioid receptor activation and neuroinflammation in the context of dopamine transmission, their contribution to opioid and cannabis withdrawal, along with potential new treatments.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Consciousness beyond the human case

There is growing interest in the relationship been AI and consciousness. Joseph LeDoux and Jonathan Birch thought it would be a good moment to put some of the big questions in this area to some leading experts. The challenge of addressing the questions they raised was taken up by Kristin Andrews, Nicky Clayton, Nathaniel Daw, Chris Frith, Hakwan Lau, Megan Peters, Susan Schneider, Anil Seth, Thomas Suddendorf, and Marie Vanderkerckhoeve.

Endogenous opioid systems alterations in pain and opioid use disorder

Decades of research advances have established a central role for endogenous opioid systems in regulating reward processing, mood, motivation, learning and memory, gastrointestinal function, and pain relief. Endogenous opioid systems are present ubiquitously throughout the central and peripheral nervous system. They are composed of four families, namely the μ (MOPR), κ (KOPR), δ (DOPR), and nociceptin/orphanin FQ (NOPR) opioid receptors systems. These receptors signal through the action of their endogenous opioid peptides β-endorphins, dynorphins, enkephalins, and nociceptins, respectfully, to maintain homeostasis under normal physiological states. Due to their prominent role in pain regulation, exogenous opioids-primarily targeting the MOPR, have been historically used in medicine as analgesics, but their ability to produce euphoric effects also present high risks for abuse. The ability of pain and opioid use to perturb endogenous opioid system function, particularly within the central nervous system, may increase the likelihood of developing opioid use disorder (OUD). Today, the opioid crisis represents a major social, economic, and public health concern. In this review, we summarize the current state of the literature on the function, expression, pharmacology, and regulation of endogenous opioid systems in pain. Additionally, we discuss the adaptations in the endogenous opioid systems upon use of exogenous opioids which contribute to the development of OUD. Finally, we describe the intricate relationship between pain, endogenous opioid systems, and the proclivity for opioid misuse, as well as potential advances in generating safer and more efficient pain therapies.

Practical Application of the Five Domains Animal Welfare Framework for Supply Food Animal Chain Managers

The author has worked as a consultant with global commercial supply managers for over 20 years. The focus of this commentary will be practical application of The Five Domains Model in commercial systems. Commercial buyers of meat need simple easy-to-use guidelines. They have to use auditors that can be trained in a workshop that lasts for only a few days. Auditing of slaughter plants by major buyers has resulted in great improvements. Supply chain managers need clear guidance on conditions that would result in a failed audit. Animal based outcome measures that can be easily assessed should be emphasized in commercial systems. Some examples of these key animal welfare indicators are: percentage of animals stunned effectively with a single application of the stunner, percentage of lame animals, foot pad lesions on poultry, and body condition scoring. A farm that supplies a buyer must also comply with housing specifications. The farm either has the specified housing or does not have it. It will be removed from the approved supplier list if housing does not comply. These types of easy to assess indicators can be easily evaluated within the four domains of nutrition, environment, health and behavioral interactions. The Five Domains Framework can also be used in a program for continuous improvement of animal welfare.

Pain, negative affective states and opioid-based analgesics: Safer pain therapies to dampen addiction

Across centuries and civilizations opioids have been used to relieve pain. In our modern societies, opioid-based analgesics remain one of the most efficient treatments for acute pain. However, the long-term use of opioids can lead to the development of analgesic tolerance, opioid-induced hyperalgesia, opioid use disorders, and overdose, which can ultimately produce respiratory depressant effects with fatal consequences. In addition to the nociceptive sensory component of pain, negative affective states arising from persistent pain represent a risk factor for developing an opioid use disorder. Several studies have indicated that the increase in prescribed opioid analgesics since the 1990s represents the root of our current opioid epidemic. In this review, we will present our current knowledge on the endogenous opioid system within the pain neuroaxis and the plastic changes occurring in this system that may underlie the occurrence of pain-induced negative affect leading to misuse and abuse of opioid medications. Dissecting the allostatic neuronal changes occurring during pain is the most promising avenue to uncover novel targets for the development of safer pain medications. We will discuss this along with current and potential approaches to treat pain-induced negative affective states that lead to drug misuse. Moreover, this chapter will provide a discussion on potential avenues to reduce the abuse potential of new analgesic drugs and highlight a basis for future research and drug development based on recent advances in this field.

Can Lambs in Pain Identify Medicated Feed?

Lambs in Australia undergo painful husbandry procedures as part of common husbandry. The magnitude and duration of pain are difficult to assess in lambs. Most currently used methods rely on behavioral expressions and physiological markers that may fail to detect the state of pain an animal experience. This study examined motivation of 12-week-old lambs experiencing chronic pain to self-medicate by consumption of feed containing an analgesic agent as an indicator of pain in lambs. In this study, 36 male Merino lambs were individually penned and acclimated to pelleted feed and two artificial odors: strawberry and banana. Once acclimated to odored feed, lambs were tested for their individual preference for the odors. Lambs were then assigned to one of two groups: Sham—sham handled day 0 and 7 or Ring—Ring castrated day 0 and tail docked day 7. To enable self-medication testing, lambs underwent a conditioning period (day 0–3) followed by the self-medication period (day 7–12). On day 0 lambs were castrated or sham handled, and then offered only medicated feed that contained an odor cue (either strawberry or banana). On day 7, lambs underwent tail-docking or sham handling and were offered both the conditioned medicated feed and non-medicated feed. Amount of each feed consumed was recorded 1 and 12 h after offer each day. Blood samples were taken for cortisol and white blood cell analysis and behavioral observations were recorded for 12 h following treatment. There was no difference in preference for medicated feed between Ring and Sham lambs during the self-medication phase (P = 0.18). Lambs in both groups displayed a significant preference for strawberry cued medicated feed during the self-medicated period when compared to the other testing periods (P = 0.05). Ring lambs displayed more active pain behaviors (mean = 15.1) than Sham (mean = 0.4, P < 0.05). Following castration, Ring lambs had a higher neutrophil/lymphocyte ratio at 6, 24, 48, and 72 h. This study was not able to demonstrate that lambs can self-medicate for a state of pain.

Lack of effect of different pain-related manipulations on opioid self-administration, reinstatement of opioid seeking, and opioid choice in rats

RATIONALE AND OBJECTIVE

Pain-related factors increase the risk for opioid addiction, and pain may function as a negative reinforcer to increase opioid taking and seeking. However, experimental pain-related manipulations generally do not increase opioid self-administration in rodents. This discrepancy may reflect insufficient learning of pain-relief contingencies or confounding effects of pain-related behavioral impairments. Here, we determined if pairing noxious stimuli with opioid self-administration would promote pain-related reinstatement of opioid seeking or increase opioid choice over food.

METHODS

In Experiment 1, rats self-administered fentanyl in the presence or absence of repeated intraplantar capsaicin injections in distinct contexts to model context-specific exposure to cutaneous nociception. After capsaicin-free extinction in both contexts, we tested if capsaicin would reinstate fentanyl seeking. In Experiment 2, rats self-administered heroin after intraperitoneal (i.p.) lactic acid injections to model acute visceral inflammatory pain. After lactic acid-free extinction, we tested if lactic acid would reinstate heroin seeking. In Experiment 3, we tested if repeated i.p. lactic acid or intraplantar Complete Freund's Adjuvant (CFA; to model sustained inflammatory pain) would increase fentanyl choice over food.

RESULTS

In Experiments 1-2, neither capsaicin nor lactic acid reinstated opioid seeking after extinction, and lactic acid did not increase heroin-induced reinstatement. In Experiment 3, lactic acid and CFA decreased reinforcement rate without affecting fentanyl choice.

CONCLUSIONS

Results extend the range of conditions across which pain-related manipulations fail to increase opioid seeking in rats and suggest that enhanced opioid-addiction risk in humans with chronic pain involves factors other than enhanced opioid reinforcement and relapse.

The NLRP3-related inflammasome modulates pain behavior in a rat model of trigeminal neuropathic pain

AIMS

Nod-like receptor family pyrin domain containing 3 (NLRP3) may play an important role in neuropathic pain. Treatment for trigeminal neuropathic pain remains a challenge, as common drugs either do not demonstrate beneficial therapeutic effects or induce intolerance in patients.

MAIN METHODS

In a rat model of trigeminal neuropathic pain, pain caused by the malpositioning of dental implants is similar to that experienced by humans. We used masculine Sprague-Dawley rats with inferior alveolar nerve damage as a model to investigate the differential regulation of NLRP3. First, we confirmed the level of NLRP3 in the medullary dorsal horn and variation of pain response behavior after silencing the expression of NLRP3 inflammasome bodies in rats with trigeminal neuropathic pain. Second, under localized anesthesia, we extracted the lower left second molar, implanted a micro-dental implant, and deliberately injured the inferior alveolar nerve.

KEY FINDINGS

After nerve damage, the level of NLRP3-related inflammasomes was upregulated in microglia and the expression of a component of the inflammasome gradually increased during postoperative days 3-21. The suppression of adenovirus-shRNA-NLRP3 on postoperative day 1 markedly inhibited the expression of pro-inflammatory cytokines and the activation of the inflammasome and mechanical allodynia. Furthermore, it attenuated cell death in microglia, as evidenced by increased Bcl-2, Bcl-xL, Bax, and Bik expression.

SIGNIFICANCE

The level of NLRP3 in the dorsal horn is a pivotal factor in trigeminal neuropathic pain, and inhibition of the early expression of NLRP3 might serve as a potential therapeutic approach.

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.'.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

Anticipatory Behavior for a Mealworm Reward in Laying Hens Is Reduced by Opioid Receptor Antagonism but Not Standard Feed Intake

It is widely accepted that the absence of suffering no longer defines animal welfare and that positive affective experiences are imperative. For example, laying hens may be housed in environments that do not cause chronic stress but may lack particular resources that promote positive affective experiences, such as conspecifics or effective enrichment. Despite a consensus of how important positive affect is for animal welfare, they are difficult to identify objectively. There is a need for valid and reliable indicators of positive affect. Pharmacological interventions can be an effective method to provide insight into affective states and can assist with the investigation of novel indicators such as associated biomarkers. We aimed to validate a pharmacological intervention that blocks the subjective hedonistic phase associated with reward in laying hens via the administration of the non-selective (μ, δ, and κ) opioid receptor antagonist, nalmafene. We hypothesized that nonfood deprived, hens that did not experience a positive affective state when presented with a mealworm food reward due to the administration of nalmefene, would show minimal anticipatory and consummatory behavior when the same food reward was later presented. Hens (n = 80) were allocated to treatment groups, receiving either nalmefene or vehicle (0.9% saline) once or twice daily, for four consecutive days. An anticipatory test (AT) was performed on all days 30 min post-drug administration. Behavioral responses during the appetitive and consummatory phase were assessed on days 1, 3 and 4. Anticipatory behavior did not differ between treatment groups the first time hens were provided with mealworm food rewards. However, antagonism of opioid receptors reduced anticipatory and consummatory behavior on days 3 and 4. Feed intake of standard layer mash was not impacted by treatment, thus nalmefene reduced non-homeostatic food consumption but not homeostatic consumption. Behavioral observations during the AT provided no evidence that nalmefene treated hens were fearful, sedated or nauseous. The results suggest that we successfully blocked the hedonistic subjective component of reward in laying hens and provide evidence that this method could be used to investigate how hens perceive their environment and identify associated novel indicators to assess hen welfare.

Tell-tale TINT: Does the Time to Incorporate into Nest Test Evaluate Postsurgical Pain or Welfare in Mice?

Identifying early indicators of distress in mice is difficult using either periodic monitoring or current technology. Likewise, poor pain identification remains a barrier to providing appropriate pain relief in many mouse models. The Time to Incorporate to Nest Test (TINT), a binary measure of the presence or absence of nesting behavior, was developed as a species-specific method of identifying moderate to severe distress and pain in mice. The current study was designed to evaluate alterations in nesting behavior after routine surgery and to validate the TINT's ability to measure pain-related behavioral changes. CD1 mice undergoing carotid artery catheterization as part of a commercial surgical cohort were randomly assigned various nesting, surgery, and analgesia conditions. To provide context for the TINT outcomes, we measured other variables affected by pain, such as weight loss, food consumption, and scores derived from the Mouse Grimace Scale (MGS). Mice that had surgery were more likely to have a negative TINT score as compared with controls. All mice were more likely to fail the TINT after receiving postoperative buprenorphine, suggesting that buprenorphine may have contributed to the failures. The TINT, MGS live scoring, and scoring MGS images all loaded strongly on a single component in a principal component analysis, indicating strong convergent validity between these measures. These data indicate that the TINT can provide a quick, objective indicator of altered welfare in mice, with the potential for a wide range of uses.

Knock-down of JAK2 and PTEN on pain behavior in rat model of trigeminal neuropathic pain

Trigeminal neuropathic pain is seen as a huge clinical challenge. Although numerous drugs have been developed to treat the condition, some patients have shown intolerance to the drugs and thus continue to suffer. In the present study, a rat model of trigeminal neuropathic pain was established using incorrectly positioned dental implants, which had various manifestations that were similar to human trigeminal neuropathic pain. Using this model, we investigated the differential regulation of JAK2 and PTEN. Firstly, we examined the expression of JAK2 and PTEN in the medullary dorsal horn. After inhibiting JAK2/PTEN, we evaluated nociception-related behavioral alterations. The rat models were established by replacing the left lower second molar with a mini dental implant. Immunoblot assay and immunofluorescence experiments indicated high expression of JAK2 and PTEN in medullary dorsal horn after the nerve injury, which attained plateau levels on post-operative day (POD) 5-10 and 10-20. Administration of adenovirus-shRNA-JAK2 on POD 1 reduced mechanical allodynia and downstream STAT activation. Meanwhile, the administration of adenovirus-shRNA-PTEN on POD 1 attenuated mechanical allodynia while upregulating AKT. In addition to postoperative JAK2 and PTEN activation, dexmedetomidine treatment (10 mg/kg) also modulated the downstream sensors of these signaling molecules. These data suggest that JAK2 and PTEN are pivotal to the development of trigeminal neuropathic pain, and that JAK2 and PTEN suppression alleviates the neuropathic pain.

Sensitized brain response to acute pain in patients using prescription opiates for chronic pain: A pilot study

BACKGROUND

Chronic opiate use leads to a sensitized behavioral response to acute pain, which in turn, leads to escalating doses of opiates. This study was designed to test the hypothesis that chronic opiate usage is also associated with a sensitized neurobiological response to acute pain in individuals that have used prescription opiates for 6 or more months.

METHODS

Fourteen patients with non-alcoholic chronic pancreatitis that have been taking prescription opiates for 6 or more months and 14 gender matched, non-opiate using controls were enrolled. Functional neuroimaging data was acquired while participants received blocks of thermal stimulation to their wrist (individually-tailored to their pain threshold).

RESULTS

Self-reported pain was significantly greater in opiate using patients (3.4 ± 3.4) than controls (0.2 ± 0.8: Brief Pain Inventory p < 0.005), however no significant difference between groups was observed in the individually-tailored pain thresholds. Opiate using patients evidenced a significantly greater response to pain than controls in two established nodes of the "Pain Matrix": somatosensory cortex (p_FWE≤0.001) and anterior cingulate cortex (p ≤ 0.01). This response was positively correlated with prescribed morphine equivalent dosages (average: 133.5 ± 94.8 mg/day).

CONCLUSION

The findings suggest that in chronic pancreatitis patients, a dose of opiates that normalizes their behavioral response to acute pain is associated with an amplified neural response to acute pain. Further longitudinal studies are needed to determine if this neural sensitization hastens a behavioral tolerance to opiates or the development of an opioid use disorder.

Interactions between pain states and opioid reward assessed with intracranial self-stimulation in rats

Opioids are an essential component of current clinical treatments for pain, but they also produce side effects that include abuse liability. Recent media attention surrounding the use of opioids in the United States has elevated the discussion of their benefits and drawbacks to one of national concern, leading to increased scrutiny of prescribing practices. Regulatory agencies have responded by recommending stricter limits on the amount and duration of opioid prescriptions for pain treatment; however, the relationship between pain states and the abuse-related effects of opioids is still not completely understood. Intracranial self-stimulation (ICSS) is one preclinical procedure that can be used to study the abuse-related effects of opioids in naïve subjects over the course of initial opioid exposure and in the context of inferred pain states. The goal of this review is to provide a summary of evidence from our laboratory using ICSS to study the modulation of opioid reward by pain states and examine these results in the context of related studies from other groups. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

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.

Determinants of opioid abuse potential: Insights using intracranial self-stimulation

Intracranial self-stimulation (ICSS) is one procedure that can be used for preclinical abuse potential assessment. In ICSS procedures, subjects with microelectrodes implanted into a brain-reward region are trained to press an operant response lever for pulses of electrical brain stimulation, and drugs are evaluated for their effectiveness to increase or "facilitate" ICSS responding (an abuse-related effect) or to depress ICSS responding (an abuse-limiting effect). ICSS has been used for decades to evaluate determinants of opioid abuse potential, and this article reviews pharmacological and biological determinants of opioid abuse potential as revealed by ICSS studies in rodents. One of the most important observations from ICSS studies is that abused mu opioid receptor (MOR) agonists like morphine often fail to produce abuse-related ICSS facilitation in opioid-naïve subjects, but several days of repeated opioid exposure is sufficient for opioid-induced facilitation to emerge. Future studies with ICSS could help (a) to clarify mechanisms that increase MOR agonist abuse potential during early opioid exposure or during chronic exposure leading to dependence, (b) to evaluate novel opioids either developed as candidate analgesics with reduced abuse potential or identified as designer opioids being synthesized and distributed for illicit use, and (c) to test candidate pharmacotherapies for treatment of opioid abuse in non-dependent and dependent subjects.

Fentanyl: Receptor pharmacology, abuse potential, and implications for treatment

Opioid overdoses, many of which are attributed to use of illicit fentanyl, are currently one of the leading causes of death in the U.S. Although fentanyl has been used safely for decades in clinical settings, the widespread use of illicit fentanyl is a recent phenomenon. Starting in 2013, illicitly manufactured fentanyl and its analogs began to appear on the streets. These substances were added to or sold as heroin, often unbeknownst to the user. Because fentanyl is so potent, only small amounts are needed to produce pharmacological effects, but the margin between safe and toxic doses is narrow. Surprisingly little is known about the exact signaling mechanisms underlying fentanyl-related respiratory depression or the effectiveness of naloxone in reversing this effect. Similarly, little is known about the ability of treatment medications such as buprenorphine, methadone, or naltrexone to reduce illicit fentanyl use. The present article reviews the receptor, preclinical and clinical pharmacology of fentanyl, and how its pharmacology may predict the effectiveness of currently approved medications for treating illicit fentanyl use.

Approach-aversion in calves following injections

We assessed aversion to injections using an avoidance-learning paradigm. Holstein calves (n = 24) were randomly assigned to one of four routes of administration for 0.5 ml of saline: intramuscular (IM), intranasal (IN), subcutaneous (SC) and a null control. Calves were first trained to approach a milk reward of 1 L. Once the latency to approach the reward was consistent, calves received their assigned treatment when approaching the bottle. For the first 3 treatment sessions calves received a 1 L milk reward. This reward was then reduced to 500 mL, and then to 250 mL, and finally to 0 mL, each for 3 sessions. Compared to control calves, calves receiving the intramuscular injections showed a longer latency to approach the milk reward, but only when the milk reward was 0.25 L (P = 0.05) and 0 L (P < 0.01). Calves receiving the intranasal injections showed longer latencies relative to the controls only for the 0 L reward (P = 0.01). Calves receiving the subcutaneous injections did not differ from controls for any of the milk rewards (P > 0.2). We conclude that IM injections are aversive and that SC and IN routes are a refinement to be considered when feasible.

The Current State of Cephalopod Science and Perspectives on the Most Critical Challenges Ahead From Three Early-Career Researchers

Here, three researchers who have recently embarked on careers in cephalopod biology discuss the current state of the field and offer their hopes for the future. Seven major topics are explored: genetics, aquaculture, climate change, welfare, behavior, cognition, and neurobiology. Recent developments in each of these fields are reviewed and the potential of emerging technologies to address specific gaps in knowledge about cephalopods are discussed. Throughout, the authors highlight specific challenges that merit particular focus in the near-term. This review and prospectus is also intended to suggest some concrete near-term goals to cephalopod researchers and inspire those working outside the field to consider the revelatory potential of these remarkable creatures.

In search of evidence for the experience of pain in honeybees: A self-administration study

Despite their common use as model organisms in scientific experiments, pain and suffering in insects remains controversial and poorly understood. Here we explore potential pain experience in honeybees (Apis mellifera) by testing the self-administration of an analgesic drug. Foragers were subjected to two different types of injuries: (i) a clip that applied continuous pressure to one leg and (ii) amputation of one tarsus. The bees were given a choice between two feeders, one offering pure sucrose solution, the other sucrose solution plus morphine. We found that sustained pinching had no effect on the amount of morphine consumed, and hence is unlikely to be experienced as painful. The amputated bees did not shift their relative preference towards the analgesic either, but consumed more morphine and more solution in total compared to intact controls. While our data do not provide evidence for the self-administration of morphine in response to pain, they suggest that injured bees increase their overall food intake, presumably to meet the increased energy requirements for an immune response caused by wounding. We conclude that further experiments are required to gain insights into potential pain-like states in honeybees and other insects.

Differential drug effects on spontaneous and evoked pain behavior in a model of trigeminal neuropathic pain

PURPOSE

Baclofen and morphine have shown efficacy against mechanical allodynia after infraorbital nerve chronic constriction injury (IoN-CCI). No drug effects have yet been reported on spontaneous trigeminal neuropathic pain. It has been proposed that the directed face grooming behavior that also develops following IoN-CCI offers a measure of spontaneous trigeminal neuropathic pain.

SUBJECTS AND METHODS

We examined the effects of a continuous 1-week infusion of 30 mg/day carbamazepine (the first-line drug treatment for trigeminal neuralgia), 1.06 mg/day baclofen, 4.18 mg/day clomipramine, and 5 mg/day morphine on spontaneous and mechanically evoked pain behavior (ie, directed face grooming and von Frey testing) in IoN-CCI rats.

RESULTS

Isolated face grooming was significantly reduced in rats receiving carbamazepine and baclofen but not in clomipramine- or morphine-treated rats. All drugs showed significant antiallodynic effects; carbamazepine showed the strongest effects, whereas clomipramine had only minor efficacy.

CONCLUSION

The tested drugs have differential effects in the IoN-CCI model, and different neuropathological mechanisms may underlie the different somatosensory symptoms in this model. A mechanism-based approach may be needed to treat (trigeminal) neuropathic pain. The present data support IoN-CCI as a model of trigeminal neuralgia in which isolated face grooming is used as a measure of spontaneous neuropathic pain.

Effects of paclitaxel on mechanical sensitivity and morphine reward in male and female C57Bl6 mice

This study evaluated the hypothesis that a paclitaxel treatment regimen sufficient to produce mechanical allodynia would alter sensitivities of male and female mice to the conditioned rewarding and reinforcing effects of morphine. Saline or paclitaxel were administered on Days 1, 3, 5, and 7 in male and female C57Bl/6 mice to induce morphine-reversible mechanical allodynia as measured by the Von Frey filament test. Paclitaxel treatment did not change sensitivity to morphine conditioned place preference (CPP) relative to saline treatment in either male or female mice. Morphine produced peak self-administration under a fixed ratio-1 (FR1) schedule of reinforcement for 0.03 mg/kg morphine per infusion in female mice and 0.1 mg/kg morphine per infusion in male mice. During the progressive ratio experiments, saline treatment in male mice decreased the number of morphine infusions for 12 days whereas the paclitaxel-treated male mice maintained responding for morphine similar to baseline levels during the same time period. However, paclitaxel did not have an overall effect on the reinforcing efficacy of morphine assessed over a limited dose range during the course of the repeated self-administration. These results suggest that the reward-related behavioral effects of morphine are overall not robustly altered by the presence of paclitaxel treatment under the current dosing regimen, with the exception of maintaining a small yet significant higher baseline than saline treatment during the development of allodynia in male mice. (PsycINFO Database Record

Sex-dependent effects of cannabis-induced analgesia

BACKGROUND

Preclinical studies demonstrate that cannabinoid-mediated antinociceptive effects vary according to sex; it is unknown if these findings extend to humans.

METHODS

This retrospective analysis compared the analgesic, subjective and physiological effects of active cannabis (3.56-5.60% THC) and inactive cannabis (0.00% THC) in male (N=21) and female (N=21) cannabis smokers under double-blind, placebo-controlled conditions. Pain response was measured using the Cold-Pressor Test (CPT). Participants immersed their hand in cold water (4°C); times to report pain (pain sensitivity) and withdraw the hand (pain tolerance) were recorded. Subjective drug ratings were also measured.

RESULTS

Among men, active cannabis significantly decreased pain sensitivity relative to inactive cannabis (p<0.01). In women, active cannabis failed to decrease pain sensitivity relative to inactive. Active cannabis increased pain tolerance in both men women immediately after smoking (p<0.001); a trend was observed for differences between men and women (p<0.10). Active cannabis also increased subjective ratings of cannabis associated with abuse liability ('Take again,' 'Liking,' 'Good drug effect'), drug strength, and 'High' relative to inactive in both men and women (p<0.01).

CONCLUSIONS

These results indicate that in cannabis smokers, men exhibit greater cannabis-induced analgesia relative to women. These sex-dependent differences are independent of cannabis-elicited subjective effects associated with abuse-liability, which were consistent between men and women. As such, sex-dependent differences in cannabis's analgesic effects are an important consideration that warrants further investigation when considering the potential therapeutic effects of cannabinoids for pain relief.

Toward an evolutionary basis for resilience to drug addiction

According to Müller & Schumann (M&S), people would have evolved adaptations for learning to use psychoactive plants and drugs as instruments that reveal particularly advantageous in modern urban environments. Here I "instrumentalize" this framework to propose an evolutionary basis for the existence of a biological resilience to drug addiction in people.

Changes in saccharin preference behavior as a primary outcome to evaluate pain and analgesia in acetic acid-induced visceral pain in mice

Reflex-based procedures are important measures in preclinical pain studies that evaluate stimulated behaviors. These procedures, however, are insufficient to capture the complexity of the pain experience, which is often associated with the depression of several innate behaviors. While recent studies have made efforts to evidence the suppression of some positively motivated behaviors in certain pain models, they are still far from being routinely used as readouts for analgesic screening. Here, we characterized and compared the effect of the analgesic ibuprofen (Ibu) and the stimulant, caffeine, in assays of acute pain-stimulated and pain-depressed behavior. Intraperitoneal injection of acetic acid (AA) served as a noxious stimulus to stimulate a writhing response or depress saccharin preference and locomotor activity (LMA) in mice. AA injection caused the maximum number of writhes between 5 and 20 minutes after administration, and writhing almost disappeared 1 hour later. AA-treated mice showed signs of depression-like behaviors after writhing resolution, as evidenced by reduced locomotion and saccharin preference for at least 4 and 6 hours, respectively. Depression-like behaviors resolved within 24 hours after AA administration. A dose of Ibu (40 mg/kg) – inactive to reduce AA-induced abdominal writhing – administered before or after AA injection significantly reverted pain-induced saccharin preference deficit. The same dose of Ibu also significantly reverted the AA-depressed LMA, but only when it was administered after AA injection. Caffeine restored locomotion – but not saccharin preference – in AA-treated mice, thus suggesting that the reduction in saccharin preference – but not in locomotion – was specifically sensitive to analgesics. In conclusion, AA-induced acute pain attenuated saccharin preference and LMA beyond the resolution of writhing behavior, and the changes in the expression of hedonic behavior, such as sweet taste preference, can be used as a more sensitive and translational model to evaluate analgesics.

Effects of repeated morphine on intracranial self-stimulation in male rats in the absence or presence of a noxious pain stimulus

Research on opioid analgesics such as morphine suggests that expression of abuse-related effects increases with repeated exposure. Repeated exposure to opioids often occurs clinically in the context of pain management, and a major concern for clinicians is the risk of iatrogenic addiction and dependence in patients receiving opioids for treatment of pain. This study compared abuse-related morphine effects in male rats in an intracranial self-stimulation (ICSS) procedure after repeated treatment either with morphine alone or with morphine in combination with a repeated noxious stimulus (intraperitoneal administration of dilute acid). The study also permitted comparison of morphine potency and effectiveness to block acid-induced depression of ICSS (antinociception) and to produce enhanced facilitation of ICSS (abuse-related effect). There were 3 main findings. First, initial morphine exposure to drug naïve rats did not produce abuse-related ICSS facilitation. Second, repeated daily treatment with 3.2 mg/kg/day morphine for 6 days increased expression of ICSS facilitation. This occurred whether morphine was administered in the absence or presence of the noxious stimulus. Finally, a lower dose of 1.0 mg/kg/day morphine was sufficient to produce antinociception during repeated acid treatment, but this lower dose did not reliably increase abuse-related morphine effects. Taken together, these results suggest that prior morphine exposure can increase abuse liability of subsequent morphine treatments even when that morphine exposure occurs in the context of a pain state. However, it may be possible to relieve pain with relatively low morphine doses that do not produce increases in abuse-related morphine effects.

The Mouse Grimace Scale: A Clinically Useful Tool?

Medical research has a heavy and continuing demand for rodent models across a range of disciplines. Behavioural assessment of pain in such models is highly time consuming, thus limiting the number of models and analgesics that can be studied. Facial expressions are widely used to assess pain in human infants. Recently the mouse grimace scale (MGS) has been developed and shown to be accurate and reliable, requiring only a short amount of training for the observer. This system therefore has the potential to become a highly useful tool both in pain research and clinical assessment of mouse pain. To date, the MGS has only been used as a research tool, however there is increasing interest in its use in cage-side clinical assessment. It is often wrongly assumed that MGS scores of animals not in pain (i.e. at baseline) are zero. Here, we aimed to assess the variability in baseline MGS scores between cohorts, sexes and strains of mice. Establishing the presence of a consistent baseline MGS score could lead to a valuable clinical pain assessment tool for mice when baseline information from the individual mouse may not be available as a comparator. Results demonstrated a significant difference in baseline MGS scores between both sexes (males > females) and strains of mice. The method used to score the facial action units (Live vs. retrospectively from still images) demonstrated significant differences in scores with live scores being significantly lower than retrospective scoring from images. The level of variation shown demonstrates the need for further research to be undertaken with regard to establishing baseline MGS scores for specific strains and sexes of mice, taking into account the method of scoring, prior to considering clinical implementation of this method in pain assessment.

Applying refinement to the use of mice and rats in rheumatoid arthritis research

Rheumatoid arthritis (RA) is a painful, chronic disorder and there is currently an unmet need for effective therapies that will benefit a wide range of patients. The research and development process for therapies and treatments currently involves in vivo studies, which have the potential to cause discomfort, pain or distress. This Working Group report focuses on identifying causes of suffering within commonly used mouse and rat ‘models’ of RA, describing practical refinements to help reduce suffering and improve welfare without compromising the scientific objectives. The report also discusses other, relevant topics including identifying and minimising sources of variation within in vivo RA studies, the potential to provide pain relief including analgesia, welfare assessment, humane endpoints, reporting standards and the potential to replace animals in RA research.

Persistent pain maintains morphine-seeking behavior after morphine withdrawal through reduced MeCP2 repression of GluA1 in rat central amygdala

As long-term opioids are increasingly used for control of chronic pain, how pain affects the rewarding effect of opioids and hence risk of prescription opioid misuse and abuse remains a healthcare concern and a challenging issue in current pain management. In this study, using a rat model of morphine self-administration, we investigated the molecular mechanisms underlying the impact of pain on operant behavior of morphine intake and morphine seeking before and after morphine withdrawal. We found that rats with persistent pain consumed a similar amount of daily morphine to that in control rats without pain, but maintained their level-pressing behavior of morphine seeking after abstinence of morphine at 0.2 mg/kg, whereas this behavior was gradually diminished in control rats. In the central nucleus of amygdala (CeA), a limbic structure critically involved in the affective dimension of pain, proteins of GluA1 subunits of glutamate AMPA receptors were upregulated during morphine withdrawal, and viral knockdown of CeA GluA1 eliminated the morphine-seeking behavior in withdrawn rats of the pain group. Chromatin immunoprecipitation analysis revealed that the methyl CpG-binding protein 2 (MeCP2) was enriched in the promoter region of Gria1 encoding GluA1 and this enrichment was significantly attenuated in withdrawn rats of the pain group. Furthermore, viral overexpression of CeA MeCP2 repressed the GluA1 level and eliminated the maintenance of morphine-seeking behavior after morphine withdrawal. These results suggest direct MeCp2 repression of GluA1 function as a likely mechanism for morphine-seeking behavior maintained by long-lasting affective pain after morphine withdrawal.

Lost but making progress--Where will new analgesic drugs come from?

There is a critical need for effective new pharmacotherapies for pain. The paucity of new drugs successfully reaching the clinic calls for a reassessment of current analgesic drug discovery approaches. Many points early in the discovery process present significant hurdles, making it critical to exploit advances in pain neurobiology to increase the probability of success. In this review, we highlight approaches that are being pursued vigorously by the pain community for drug discovery, including innovative preclinical pain models, insights from genetics, mechanistic phenotyping of pain patients, development of biomarkers, and emerging insights into chronic pain as a disorder of both the periphery and the brain. Collaborative efforts between pharmaceutical, academic, and public entities to advance research in these areas promise to de-risk potential targets, stimulate investment, and speed evaluation and development of better pain therapies.

Neurobiological studies of chronic pain and analgesia: Rationale and refinements

Chronic pain is a complex condition for which the need for specialized research and therapies has been recognized internationally. This review summarizes the context for the international call for expansion of pain research to improve our understanding of the mechanisms underlying pain in order to achieve improvements in pain management. The methods for conducting sensory assessment in animal models are discussed and the development of animal models of chronic pain is specifically reviewed, with an emphasis on ongoing refinements to more closely mimic a variety of human pain conditions. Pharmacological correspondences between pre-clinical pain models and the human clinical experience are noted. A discussion of the 3Rs Framework (Replacement, Reduction, Refinement) and how each may be considered in pain research is featured. Finally, suggestions are provided for engaging principal investigators, IACUC reviewers, and institutions in the development of strong partnerships to simultaneously expand our knowledge of the mechanisms underlying pain and analgesia while ensuring the humane use of animals in research.

Comparison of operant escape and reflex tests of nociceptive sensitivity

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

MeCP2 repression of G9a in regulation of pain and morphine reward

Opioids are commonly used for pain relief, but their strong rewarding effects drive opioid misuse and abuse. How pain affects the liability of opioid abuse is unknown at present. In this study, we identified an epigenetic regulating cascade activated by both pain and the opioid morphine. Both persistent pain and repeated morphine upregulated the transcriptional regulator MeCP2 in mouse central nucleus of the amygdala (CeA). Chromatin immunoprecipitation analysis revealed that MeCP2 bound to and repressed the transcriptional repressor histone dimethyltransferase G9a, reducing G9a-catalyzed repressive mark H3K9me2 in CeA. Repression of G9a activity increased expression of brain-derived neurotrophic factor (BDNF). Behaviorally, persistent inflammatory pain increased the sensitivity to acquiring morphine-induced, reward-related behavior of conditioned place preference in mice. Local viral vector-mediated MeCP2 overexpression, Cre-induced G9a knockdown, and CeA application of BDNF mimicked, whereas MeCP2 knockdown inhibited, the pain effect. These results suggest that MeCP2 directly represses G9a as a shared mechanism in central amygdala for regulation of emotional responses to pain and opioid reward, and for their behavioral interaction.

Persistent pain facilitates response to morphine reward by downregulation of central amygdala GABAergic function

Opioid-based analgesics are widely used for treating chronic pain, but opioids are highly addictive when repeatedly used because of their strong rewarding effects. In recent years, abuse of prescription opioids has dramatically increased, including incidences of misuse of opioid drugs prescribed for pain control. Despite this issue in current clinical pain management, it remains unknown how pain influences the abuse liability of prescription opioids. Pain as aversive experience may affect opioid reward of positive emotion through common brain sites involved in emotion processing. In this study, on a rat model of chronic pain, we determined how persistent pain altered behavioral responses to morphine reward measured by the paradigm of unbiased conditioned place preference (CPP), focusing on GABAergic synaptic activity in neurons of the central nucleus of the amygdala (CeA), an important brain region for emotional processing of both pain and reward. We found that pain reduced the minimum number of morphine-conditioning sessions required for inducing CPP behavior. Both pain and morphine conditioning that elicited CPP inhibited GABA synaptic transmission in CeA neurons. Pharmacological activation of CeA GABAA receptors reduced the pain and inhibited CPP induced both by an effective dose of morphine and by a sub-threshold dose of morphine under pain condition. Furthermore, inhibition of CeA GABAA receptors mimicked the pain effect, rendering the sub-threshold dose of morphine effective in CPP induction. These findings suggest that pain facilitates behavioral responses to morphine reward by predisposing the inhibitory GABA function in the CeA circuitry involved in the behavior of opioid reward.

A leptin-mediated central mechanism in analgesia-enhanced opioid reward in rats

Opioid analgesics are commonly used in chronic pain management despite a potential risk of rewarding. However, it remains unclear whether opioid analgesia would enhance the opioid rewarding effect thereby contributing to opioid rewarding. Utilizing a rat paradigm of conditioned place preference (CPP) combined with ankle monoarthritis as a condition of persistent nociception, we showed that analgesia induced by either morphine or the nonsteroid anti-inflammatory drug ibuprofen increased CPP scores in arthritic rats, suggesting that analgesia itself had a rewarding effect. However, arthritic rats exhibited a significantly higher CPP score in response to morphine than ibuprofen. Thus, the rewarding effect of morphine was enhanced in the presence of persistent nociception, producing a phenomenon of analgesia-enhanced opioid reward. At the cellular level, administration of morphine activated a cascade of leptin expression, glial activation, and dopamine receptor upregulation in the nucleus accumbens (NAc), while administration of ibuprofen decreased glial activation with no effect on leptin expression in the NAc. Furthermore, the morphine rewarding effect was blocked in leptin deficient ob/ob mice or by neutralizing leptin or interleukin-1β in the NAc without diminishing morphine analgesia. The data indicate that systemic opioid can activate a leptin-mediated central mechanism in the NAc that led to the enhanced opioid rewarding effect. These findings provide evidence for an interaction between opioid analgesia and opioid rewarding, which may have implications in clinical opioid dose escalation in chronic pain management.

"Bedside-to-Bench" Behavioral Outcomes in Animal Models of Pain: Beyond the Evaluation of Reflexes

Despite the myriad promising new targets and candidate analgesics recently identified in preclinical pain studies, little translation to novel pain medications has been generated. The pain phenotype in humans involves complex behavioral alterations, including changes in daily living activities and psychological disturbances. These behavioral changes are not reflected by the outcome measures traditionally used in rodents for preclinical pain testing, which are based on reflexes evoked by sensory stimuli of different types (mechanical, thermal or chemical). These measures do not evaluate the impact of the pain experience on the global behavior or disability of the animals, and therefore only consider a limited aspect of the pain phenotype. The development of relevant new outcomes indicative of pain to increase the validity of animal models of pain has been increasingly pursued over the past few years. The aim has been to translate “bedside-to-bench” outcomes from the human pain phenotype to rodents, in order to complement traditional pain outcomes by providing a closer and more realistic measure of clinical pain in rodents. This review summarizes and discusses the most important nonstandard outcomes for pain assessment in preclinical studies. The advantages and drawbacks of these techniques are considered, and their potential impact on the validation of potential analgesics is evaluated.

The Self-administration of Analgesic Drugs in Experimentally Induced Chronic Pain

Systemically and centrally delivered opioids have been comprehensively studied for their effects both in analgesic and addiction models for many decades, primarily in subjects with presumptive normal sensory thresholds. The introduction of disease-based models of persistent hypersensitivity enabled chronic evaluation of opioid analgesic pharmacology under the specific state of chronic pain. These studies have largely (but not uniformly) reported reduced opioid analgesic potency and efficacy under conditions of chronic pain. A comparatively limited set of studies has evaluated the impact of experimentally induced chronic pain on self-administration patterns of opioid and non-opioid analgesics. Similarly, these studies have primarily (but not exclusively) found that responding for opioids is reduced under conditions of chronic pain. Additionally, such experiments have also demonstrated that the condition of chronic pain evokes self-administration or conditioned place preference for non-opioid analgesics. The consensus is that the chronic pain alters responding for opioid and non-opioid analgesics in a manner seemingly related to their respective antiallodynic/antihyperalgesic properties under the specific state of chronic pain.

Effect of chronic pain on fentanyl self-administration in mice

The development of opioid addiction in subjects with established chronic pain is an area that is poorly understood. It is critically important to clearly understand the neurobiology associated with propensity toward conversion to addiction under conditions of chronic pain. To pose the question whether the presence of chronic pain influences motivation to self-administer opioids for reward, we applied a combination of rodent models of chronic mechanical hyperalgesia and opioid self-administration. We studied fentanyl self-administration in mice under three conditions that induce chronic mechanical hyperalgesia: inflammation, peripheral nerve injury, and repeated chemotherapeutic injections. Responding for fentanyl was compared among these conditions and their respective standard controls (naïve condition, vehicle injection or sham surgery). Acquisition of fentanyl self-administration behavior was reduced or absent in all three conditions of chronic hyperalgesia relative to control mice with normal sensory thresholds. To control for potential impairment in ability to learn the lever-pressing behavior or perform the associated motor tasks, all three groups were evaluated for acquisition of food-maintained responding. In contrast to the opioid, chronic hyperalgesia did not interfere with the reinforcing effect of food. These studies indicate that the establishment of chronic hyperalgesia is associated with reduced or ablated motivation to seek opioid reward in mice.

An overview of animal models of pain: disease models and outcome measures

Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience, including reflexive hyperalgesia measures, sensory and affective dimensions of pain, and impact of pain on function and quality of life. In this review, we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes as well as the main behavioral tests for assessing pain in each model.

PERSPECTIVE

Understanding animal models and outcome measures in animals will assist in translating data from basic science to the clinic.

Analgesics as reinforcers with chronic pain: Evidence from operant studies

Previously preclinical pain research has focused on simple behavioral endpoints to assess the efficacy of analgesics in acute and chronic pain models, primarily reflexive withdrawal from an applied mechanical or thermal stimulus. However recent research has been aimed at investigating other behavioral states in the presence of pain, including spontaneous, non-elicited pain. One approach is to investigate the reinforcing effects of analgesics in animals with experimental pain, which should serve as reinforcers by virtue of their ability to alleviate the relevant subjective states induced by pain. The gold standard for assessing drug reinforcement is generally accepted to be drug self-administration, and this review highlights the ability of drugs to serve as reinforcers in animals with experimental neuropathic pain, and the extent to which this behavior is altered in chronic pain states. Additionally, intracranial self-stimulation is an operant procedure that has been used extensively to study drug reinforcement mechanisms and the manner in which neuropathic pain alters the ability of drugs to serve as reinforcers in this paradigm will also be discussed. Drug self-administration and intracranial self-stimulation have promise as tools to investigate behavioral effects of analgesics in animals with chronic pain, particularly regarding the mechanisms through which these drugs motivate consumption in a chronic pain state.

Imaging opioid analgesia in the human brain and its potential relevance for understanding opioid use in chronic pain

Opioids play an important role for the management of acute pain and in palliative care. The role of long-term opioid therapy in chronic non-malignant pain remains unclear and is the focus of much clinical research. There are concerns regarding analgesic tolerance, paradoxical pain and issues with dependence that can occur with chronic opioid use in the susceptible patient. In this review, we discuss how far human neuroimaging research has come in providing a mechanistic understanding of pain relief provided by opioids, and suggest avenues for further studies that are relevant to the management of chronic pain with opioids.

This article is part of the Special Issue Section entitled ‘Neuroimaging in Neuropharmacology’.

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Brain mechanisms are crucial to opioid analgesia in humans.

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Opioids can have a direct effect on brain mechanisms for pain perception.

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Opioids can also engage descending inhibition of spinal nociception.

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Drug-induced tolerance, dependence and paradoxical pain may limit chronic opioid analgesic therapy.

Operant self-administration of a sigma ligand improves nociceptive and emotional manifestations of neuropathic pain

BACKGROUND

The treatment of neuropathic pain is unsatisfactory at the present moment and the sigma 1 receptor has been identified as a new potential target for neuropathic pain. The aim of this study was to use an operant self-administration model to reveal the potential interest of a new sigma 1 receptor antagonist, S1RA, in chronic pain that was developed in mice by a partial ligation of the sciatic nerve.

METHODS

Once that chronic pain had reached a steady state, mice were trained to maintain an operant behaviour to self-administer S1RA. The possible abuse liability of the analgesic compound was determined by evaluating operant self-administration in sham-operated mice. The influence of S1RA on the anhedonic state related to chronic pain was also evaluated by measuring the preference for palatable drink (2% sucrose solution) using a recently validated and highly sensitive behavioural device.

RESULTS

Nerve-injured mice, but not sham-operated animals, acquired the operant responding to obtain S1RA (6 mg/kg/infusion). After 10 days of S1RA self-administration, neuropathic pain was significantly reduced in nerve-injured mice. In addition, an anhedonic state was revealed in nerve-injured mice by a decreased consumption of palatable drink, which was significantly attenuated by S1RA (25 mg/kg).

CONCLUSIONS

These results reveal the analgesic efficacy of the sigma antagonist, S1RA, in neuropathic pain associated with an improvement of the emotional negative state and that was devoided of reinforcing effects. The operant responses evaluated in this new mouse model can have a high predictive value to estimate the clinical benefit/risk ratio of new analgesic compounds to treat chronic pain, such as S1RA.

Effect of anchoring 4-anilidopiperidines to opioid peptides

We report here the design, synthesis, and in vitro characterization of new opioid peptides featuring a 4-anilidopiperidine moiety. Despite the fact that the chemical structures of fentanyl surrogates have been found suboptimal per se for the opioid activity, the corresponding conjugates with opioid peptides displayed potent opioid activity. These studies shed an instructive light on the strategies and potential therapeutic values of anchoring the 4-anilidopiperidine scaffold to different classes of opioid peptides.