Intrinsic mechanisms of pain inhibition: activation by stress

Neuroanatomical and neurochemical substrates mediating fear-induced antinociception: A systematic review of rodent preclinical studies

Fear-induced antinociception (FIA), an instinctive defensive response producing pain suppression in stressful and/or dangerous situations, has been the subject of extensive research to elucidate the mechanisms involved in triggering and controlling pain during emotional disorders. In this systematic review, we synthesized pre-clinical studies that demonstrated the neural hodology and the neurochemical bases of FIA in laboratory animals. The literature search in PubMed, Web of Science, Science Direct, and Scopus, from inception up to July 2022, retrieved 797 articles from which 50 studies were included in this review. This review highlights key encephalic regions implicated in the modulation of FIA, such as the prefrontal cortex, the amygdaloid complex, the hippocampus, the hypothalamus, the corpora quadrigemina, the periaqueductal gray matter, and some reticular formation nuclei. FIA-related neural pathways, neurotransmitters and neuromodulators such as glutamatergic, serotonergic, norepinephrine, GABAergic, nitrergic, opioidergic and endocannabinoid connections across these encephalic regions were also addressed. Understanding these neural circuits and molecular neural mediation sheds light on the complex interplay between fear, anxiety, and pain modulation, offering potential avenues for therapeutic interventions targeting pain management in the context of heightened emotional states.

Effects of stress on pain in females using a mobile health app in the Russia-Ukraine conflict

The chronic and acute effects of stress can have divergent effects on health; long-term effects are associated with detrimental physical and mental health sequelae, while acute effects may be advantageous in the short-term. Stress-induced analgesia, the attenuation of pain perception due to stress, is a well-known phenomenon that has yet to be systematically investigated under ecological conditions. Using Flo, a women's health and wellbeing app and menstrual cycle tracker, with a world-wide monthly active usership of more than 57 million, women in Ukraine were monitored for their reporting of stress, pain and affective symptoms before, and immediately after, the onset of the Russian-Ukrainian conflict. To avoid potential selection (attrition) or collider bias, we rely on a sample of 87,315 users who were actively logging multiple symptoms before and after the start of the war. We found an inverse relationship between stress and pain, whereby higher reports of stress predicted lower rates of pain. Stress did not influence any other physiological symptoms with a similar magnitude, nor did any other symptom have a similar effect on pain. This relationship generally decreased in magnitude in countries neighbouring and surrounding Ukraine, with Ukraine serving as the epicentre. These findings help characterise the relationship between stress and health in a real-world setting.

Fibromyalgia Animal Models Using Intermittent Cold and Psychological Stress

Fibromyalgia (FM) is a chronic pain condition characterized by widespread musculoskeletal pain and other frequent symptoms such as fatigue, sleep disturbance, cognitive impairment, and mood disorder. Based on the view that intermittent stress would be the most probable etiology for FM, intermittent cold- and intermittent psychological stress-induced generalized pain (ICGP and IPGP) models in mice have been developed and validated as FM-like pain models in terms of the patho-physiological and pharmacotherapeutic features that are shared with clinical versions. Both models show long-lasting and generalized pain and female-predominant sex differences after gonadectomy. Like many other neuropathic pain models, ICGP and IPGP were abolished in lysophosphatidic acid receptor 1 (LPAR1) knock-out mice or by LPAR1 antagonist treatments, although deciding the clinical importance of this mechanism depends on waiting for the development of a clinically available LPAR1 antagonist. On the other hand, the nonsteroidal anti-inflammatory drug diclofenac with morphine did not suppress hyperalgesia in these models, and this is consistent with the clinical findings. Pharmacological studies suggest that the lack of morphine analgesia is associated with opioid tolerance upon the stress-induced release of endorphins and subsequent counterbalance through anti-opioid NMDA receptor mechanisms. Regarding pharmacotherapy, hyperalgesia in both models was suppressed by pregabalin and duloxetine, which have been approved for FM treatment in clinic. Notably, repeated treatments with mirtazapine, an α2 adrenergic receptor antagonist-type antidepressant, and donepezil, a drug for treating Alzheimer's disease, showed potent therapeutic actions in these models. However, the pharmacotherapeutic treatment should be carried out 3 months after stress, which is stated in the FM guideline, and many preclinical studies, such as those analyzing molecular and cellular mechanisms, as well as additional evidence using different animal models, are required. Thus, the ICGP and IPGP models have the potential to help discover and characterize new therapeutic medicines that might be used for the radical treatment of FM, although there are several limitations to be overcome.

Therapeutic exercise interventions in rat models of arthritis

Arthritis is the leading cause of musculoskeletal pain and disability worldwide. Nearly 50% of individuals over the age of 65 have arthritis, which contributes to limited function, articular pain, physical inactivity, and diminished quality of life. Therapeutic exercise is often recommended in clinical settings for patients experiencing arthritic pain, however, there is little practical guidance regarding the use of therapeutic exercise to alleviate arthritic musculoskeletal pain. Rodent models of arthritis allow researchers to control experimental variables, which cannot be done with human participants, providing an opportunity to test therapeutic approaches in preclinical models. This literature review provides a summary of published findings in therapeutic exercise interventions in rat models of arthritis as well as gaps in the existing literature. We reveal that preclinical research in this field has yet to adequately investigate the impact of experimental variables in therapeutic exercise including their modality, intensity, duration, and frequency on joint pathophysiology and pain outcomes.

Stress-induced analgesia: an evaluation of effects on temporal summation of pain and the role of endogenous opioid mechanisms

Acute stress reduced the initial pain rating in a temporal summation protocol via nonopioid mechanisms but did not affect temporal summation slope, an indicator of central sensitization.

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Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Immune cell-mediated opioid analgesia

Pathological pain is regulated by a balance between pro-algesic and analgesic mechanisms. Interactions between opioid peptide-producing immune cells and peripheral sensory neurons expressing opioid receptors represent a powerful intrinsic pain control in animal models and in humans. Therefore, treatments based on general suppression of immune responses have been mostly unsuccessful. It is highly desirable to develop strategies that specifically promote neuro-immune communication mediated by opioids. Promising examples include vaccination-based recruitment of opioid-containing leukocytes to painful tissue and the local reprogramming of pro-algesic immune cells into analgesic cells producing and secreting high amounts of opioid peptides. Such approaches have the potential to inhibit pain at its origin and be devoid of central and systemic side effects of classical analgesics. In support of these concepts, in this article, we describe the functioning of peripheral opioid receptors, migration of opioid-producing immune cells to inflamed tissue, opioid peptide release, and the consequent pain relief. Conclusively, we provide clinical evidence and discuss therapeutic opportunities and challenges associated with immune cell-mediated peripheral opioid analgesia.

Endogenous antinociceptive system and potential ways to influence It

The biological significance of pain is to protect the organism from possible injury. However, there exists a situation, where, in the interest of survival, it is more important not to perceive pain. Spontaneous suppression of pain or weakening of nociception is mediated by an endogenous antinociceptive (analgesic) system. Its anatomical substrate ranges from the periaqueductal gray matter of the midbrain, through the noradrenergic and serotonergic nuclei of the brain stem to the spinal neurons, which receive "pain" information from nociceptors. Moreover, the activity of this system is under significant control of emotional and cognitive circuits. Pain can be moderated primarily through stimulation of positive emotions, while negative emotions increase pain. Paradoxically, one pain can also suppress another pain. Analgesia can be induced by stress, physical exercise, orosensory stimulation via a sweet taste, listening to music, and after placebo, i.e. when relief from pain is expected. Since pain has sensory, affective, and cognitive components, it turns out that activation of these entire systems can, in specific ways, contribute to pain suppression.

Endogenous cannabinoid modulation of restraint stress-induced analgesia in thermal nociception

It is thought that endogenous cannabinoids have a role in the analgesia induced by specific forms of stress. We examined if the role of endogenous cannabinoids is also dependent upon the mode of nociception, and whether this could be altered by drugs which block their enzymatic degradation. In C57BL/6 mice, restraint stress produced analgesia in the hot-plate and plantar tests, two thermal pain assays that engage distinct supraspinal and spinal nociceptive pathways. Stress-induced analgesia in the hot-plate test was abolished by pre-treatment with the opioid receptor antagonist naltrexone but was unaffected by the cannabinoid receptor antagonist 1-(2,4-Dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281). By contrast, stress-induced analgesia in the plantar test was abolished by pre-treatment with naltrexone plus AM281, but not by either antagonist individually. Remarkably, inhibiting the breakdown of endocannabinoids, with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195, rescued stress-induced analgesia in the hotplate test when endogenous opioid signalling was blocked by naltrexone. Furthermore, JZL195 recruited analgesia induced by sub-threshold restraint stress in both thermal pain assays. These findings indicate the role of endocannabinoids in stress-induced analgesia differs with the type of thermal pain behaviour. However, by inhibiting their breakdown, endocannabinoids can be recruited to substitute for endogenous opioid signalling when their activity is blocked, indicating a degree of redundancy between opioid and cannabinoid systems. Together these data suggest targeting endocannabinoid breakdown could provide an alternative, or adjuvant to mainstream analgesics such as opioids.

The interaction between stress and chronic pain through the lens of threat learning

Stress and pain are interleaved at multiple levels - interacting and influencing each other. Both are modulated by psychosocial factors including fears, beliefs, and goals, and are served by overlapping neural substrates. One major contributing factor in the development and maintenance of chronic pain is threat learning, with pain as an emotionally-salient threat - or stressor. Here, we argue that threat learning is a central mechanism and contributor, mediating the relationship between stress and chronic pain. We review the state of the art on (mal)adaptive learning in chronic pain, and on effects of stress and particularly cortisol on learning. We then provide a theoretical integration of how stress may affect chronic pain through its effect on threat learning. Prolonged stress, as may be experienced by patients with chronic pain, and its resulting changes in key brain networks modulating stress responses and threat learning, may further exacerbate these impairing effects on threat learning. We provide testable hypotheses and suggestions for how this integration may guide future research and clinical approaches in chronic pain.

Exploiting endogenous opioids: Lessons learned from endomorphin 2 in the female rat

Effective management of chronic pain is demanded by ethical as well as medical considerations. Although opioid analgesics remain among the most effective pharmacotherapies for ameliorating many types of pain, their use is clouded by concerns regarding their addictive properties, underscored by the current epidemic of prescription opioid abuse and attendant deaths. Medicinal harnessing of endogenous opioid antinociception could provide a strategy for continuing to take advantage of the powerful antinociceptive properties of opioids while avoiding their abuse potential. Based on our studies of endogenous mechanism that suppress and facilitate spinal endomorphin 2 antinociception over the rat reproductive cycle, we identified multiple signaling molecules that could serve as targets for activating endogenous opioid analgesia for chronic pain management in women. Our findings emphasize the need for a precision medicine approach that includes stage of menstrual cycle as an important determinant of drug targets for (activating/harnessing) endogenous opioid antinociceptive systems/ capabilities. Utilization of drugs that harness endogenous opioid antinociception in accordance with varying physiological states represents a novel approach for effective pain management.

Pain inhibition through transplantation of fetal neuronal progenitors into the injured spinal cord in rats

Neuropathic pain after spinal cord injury (SCI) is a complex condition that responds poorly to usual treatments. Cell transplantation represents a promising therapy; nevertheless, the ideal cell type in terms of neurogenic potential and effectiveness against pain remains largely controversial. Here, we evaluated the ability of fetal neural stem cells (fNSC) to relieve chronic pain and, secondarily, their effects on motor recovery. Adult Wistar rats with traumatic SCI were treated, 10 days after injury, with intra-spinal injections of culture medium (sham) or fNSCs extracted from telencephalic vesicles (TV group) or the ventral medulla (VM group) of E/14 embryos. Sensory (von Frey filaments and hot plate) and motor (the Basso, Beattie, Bresnahan locomotor rating scale and inclined plane test) assessments were performed during 8 weeks. Thereafter, spinal cords were processed for immunofluorescence and transplanted cells were quantified by stereology. The results showed improvement of thermal hyperalgesia in the TV and VM groups at 4 and 5 weeks after transplantation, respectively. Moreover, mechanical allodynia improved in both the TV and VM groups at 8 weeks. No significant motor recovery was observed in the TV or VM groups compared with sham. Stereological analyses showed that ~70% of TV and VM cells differentiated into NeuN⁺ neurons, with a high proportion of enkephalinergic and GABAergic cells in the TV group and enkephalinergic and serotoninergic cells in the VM group. Our study suggests that neuronal precursors from TV and VM, once implanted into the injured spinal cord, maturate into different neuronal subtypes, mainly GABAergic, serotoninergic, and enkephalinergic, and all subtypes alleviate pain, despite no significant motor recovery. The study was approved by the Animal Ethics Committee of the Medical School of the University of São Paulo (protocol number 033/14) on March 4, 2016.

Yin-and-yang bifurcation of opioidergic circuits for descending analgesia at the midbrain of the mouse

In the descending analgesia pathway, opioids are known to disinhibit the projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), leading to suppression of pain signals at the spinal cord level. The locus coeruleus (LC) has been proposed to engage in the descending pathway through noradrenergic inputs to the spinal cord. Nevertheless, how the LC is integrated in the descending analgesia circuit has remained unknown. Here, we show that the opioidergic analgesia pathway is bifurcated in structure and function at the PAG. A knockout as well as a PAG-specific knockdown of phospholipase C β4 (PLCβ4), a signaling molecule for G protein-coupled receptors, enhanced swim stress-induced and morphine-induced analgesia in mice. Immunostaining after simultaneous retrograde labeling from the RVM and the LC revealed two mutually exclusive neuronal populations at the PAG, each projecting either to the LC or the RVM, with PLCβ4 expression only in the PAG-LC projecting cells that provide a direct synaptic input to LC-spinal cord (SC) projection neurons. The PAG-LC projection neurons in wild-type mice turned quiescent in response to opiates, but remained active in the PLCβ4 mutant, suggesting a possibility that an increased adrenergic function induced by the persistent PAG-LC activity underlies the enhanced opioid analgesia in the mutant. Indeed, the enhanced analgesia in the mutant was reversed by blocking α2-noradrenergic receptors. These findings indicate that opioids suppress descending analgesia through the PAG-LC pathway, while enhancing it through the PAG-RVM pathway, i.e., two distinct pathways with opposing effects on opioid analgesia. These results point to a therapeutic target in pain control.

Analgesic Effect of 5-(3,4-Dihydroxyphenyl)-3-hydroxy-1-(2-hydroxyphenyl)penta-2,4-dien-1-one in Experimental Animal Models of Nociception

Curcuminoids derived from turmeric rhizome have been reported to exhibit antinociceptive, antioxidant and anti-inflammatory activities. We evaluated the peripheral and central antinociceptive activities of 5-(3,4-dihydroxyphenyl)-3-hydroxy-1-(2-hydroxyphenyl)penta-2,4-dien-1-one (DHHPD), a novel synthetic curcuminoid analogue at 0.1, 0.3, 1 and 3 mg/kg (intraperitoneal), through chemical and thermal models of nociception. The effects of DHHPD on the vanilloid and glutamatergic systems were evaluated through the capsaicin- and glutamate-induced paw licking tests. Results showed that DHHPD significantly (p < 0.05) attenuated the writhing response produced by the 0.8% acetic acid injection. In addition, 1 and 3 mg/kg of DHHPD significantly (p < 0.05) reduced the licking time spent by each mouse in both phases of the 2.5% formalin test and increased the response latency of mice on the hot-plate. However, the effect produced in the latter was not reversed by naloxone, a non-selective opioid receptor antagonist. Despite this, DHHPD decreased the licking latency of mice in the capsaicin- and glutamate-induced paw licking tests in a dose response manner. In conclusion, DHHPD showed excellent peripheral and central antinociceptive activities possibly by attenuation of the synthesis and/or release of pro-inflammatory mediators in addition to modulation of the vanilloid and glutamatergic systems without an apparent effect on the opioidergic system.

Interplay between 5-HT2C and 5-HT1A receptors in the dorsal periaqueductal gray in the modulation of fear-induced antinociception in mice

The confinement of rodents to the open arm of the elevated-plus maze provokes antinociception (OAA). As a type of defensive reaction, the OAA has been investigated through systemic and intramesencephalic (e.g., dorsal portion of the periaqueductal gray - dPAG) injections of anxiolytic-like drugs [e.g., serotonergic (5-HT) receptor agonists or antagonists]. Here we investigated the effects of (i) intra-dPAG injections of a 5HT_2C receptor agonist (MK-212; 0.21 or 0.63 nmol) and antagonist (SB 242084; 0.01, 0.1 or 1.0 nmol); (ii) combined injections of SB 242084 and MK-212 into the dPAG; (iii) combined injections of SB 242084 with 8-OHDPAT (10 nmol) into the dPAG on the OAA in male Swiss mice. Nociception was assessed with the writhing test induced by acetic acid injection. Results showed that (i) intra-dPAG injection of MK-212 (0.63 nmol) increased the OAA; (ii) intra-dPAG SB 242084 (1.0 nmol) prevented the OAA; (iii) SB 242084 (0.1 nmol, a dose devoid of intrinsic effect on nociception) blocked the OAA enhancement provoked by MK-212 and enabled 8-OH-DPAT to prevent the OAA. These results suggest that OAA is mediated by 5-HT_2C receptors within the dPAG. Intra-dPAG SB242084 administration provoked similar results on the effects produced by MK-212 and 8-OH-DPAT on OAA. In addition, the dPAG 5-HT_1A and 5-HT_2C receptors interact each other in the modulation of OAA.

Behavioral characteristics of capsaicin mediated cutaneous, myogenic, and arthrogenic orofacial nociception in rats

OBJECTIVE

To assess changes in orofacial tactile sensitivity and gnawing related to capsaicin-mediated cutaneous, myogenic, and arthrogenic nociception in the rat.

DESIGN

After recovery from anesthesia, orofacial tactile sensitivity and gnawing were assessed using operant testing methods following capsaicin application. Twenty female CD-Hairless rats were tested with bilateral capsaicin cream application to the cheek or with isoflurane anesthesia alone. Following several weeks of recovery, animals (n = 20) received either 10 μL unilateral masseter injections of vehicle, or phosphate buffered saline (PBS) to assess injection sensitization. After several weeks, masseter capsaicin (1.0%) injections (10 μL) were assessed compared to vehicle and PBS (n = 13). Weeks later capsaicin TMJ injections were evaluated. Animals (n = 11) received either 10 μL unilateral TMJ injections of capsaicin solution (1%) or vehicle.

RESULTS

Capsaicin cream to the skin significantly altered gnawing activity (increased puncture time by 248 s (p = 0.0002)) and tactile sensitivity (decreased tolerated bottle distance by 0.980 cm compared to isoflurane only (p = 0.0001)). Similarly, capsaicin masseter injection increased puncture time (339.6 s, p = 0.07) and decreased tolerated bottle distance (1.04 cm, p = 0.005) compared to vehicle. However, intra-articular capsaicin in the TMJ only modified gnawing (increased puncture time by 133 s), with no changes found in tactile sensitivity compared to vehicle.

CONCLUSION

Application of capsaicin to the skin and masseter had similar behavioral effects; however, intra-articular injections to the TMJ only affected gnawing. These data indicate the behavioral changes in rodent models of myogenic and cutaneous pain may be markedly different than models of arthrogenic pain originating from the TMJ.

Stress and Addiction: When a Robust Stress Response Indicates Resiliency

OBJECTIVE

Stress reactivity research has traditionally focused on the idea that exaggerated responses to stress may have adverse effects on health. Accumulating evidence suggests that attenuated responses to stress and delayed recovery may also be problematic.

METHODS

This review focuses on the role of the stress response of the hypothalamic-pituitary-adrenocortical axis, the endogenous opioid system, and the cardiovascular system in hypertension, pain perception, and addictive behaviors. Results from multiple methods of assessment and stress paradigms conducted in our laboratory over the past two decades are integrated with research from other investigators and with existing theories.

RESULTS

Research indicates that exaggerated biological and physiological responses to stress and attenuated pain perception are associated with hypertension and risk for cardiovascular diseases. This research complements work linking reduced stress responses with enhanced pain sensitivity and discomfort. Multiple studies have also demonstrated that an attenuated stress response is linked to exacerbation of withdrawal symptoms and relapse in nicotine addiction. Evidence indicates important moderators (i.e., sex, personality traits, and early life adversity) and hypothalamic-pituitary-adrenocortical- and endogenous opioid system-related mechanisms in the altered response to stress. I integrate these findings in a conceptual model emphasizing that robust stress responses in the context of addiction and relapse should be considered as a marker of resiliency.

CONCLUSIONS

A blunted stress response may indicate long-term physiological dysregulation that could usher harmful consequences for cardiovascular disease, pain perception, and addictive disorders. The impact of dysregulation is influenced by multiple individual and situational factors that should be considered in evaluating the clinical significance of stress response dysregulation.

Challenges and opportunities for brainstem neuroimaging with ultrahigh field MRI

The human brainstem plays a central role in connecting the cerebrum, the cerebellum and the spinal cord to one another, hosting relay nuclei for afferent and efferent signaling, and providing source nuclei for several neuromodulatory systems that impact central nervous system function. While the investigation of the brainstem with functional or structural magnetic resonance imaging has been hampered for years due to this brain structure's physiological and anatomical characteristics, the field has seen significant advances in recent years thanks to the broader adoption of ultrahigh-field (UHF) MRI scanning. In the present review, we focus on the advantages offered by UHF in the context of brainstem imaging, as well as the challenges posed by the investigation of this complex brain structure in terms of data acquisition and analysis. We also illustrate how UHF MRI can shed new light on the neuroanatomy and neurophysiology underlying different brainstem-based circuitries, such as the central autonomic network and neurotransmitter/neuromodulator systems, discuss existing and foreseeable clinical applications to better understand diseases such as chronic pain and Parkinson's disease, and explore promising future directions for further improvements in brainstem imaging using UHF MRI techniques.

Voluntary and evoked behavioral correlates in inflammatory pain conditions under different social housing conditions

Most preclinical pain models rely on short-duration stimulus-evoked hind paw measurements even though chronic pain is usually a day and night experience. Pain is a debilitating condition that influences the sociability and the ability for voluntary tasks, but the relevant behavioral readouts for these aspects are mostly underrepresented in the literature. Moreover, we lack standardization in most behavioral paradigms. Important aspects are herewith the combination and duration of particular behavioral tasks and the effects of social environment. We aimed at thoroughly investigating stimulus-evoked and voluntary behavioral parameters in the Complete Freund's Adjuvant model of unilateral hind paw inflammation in male mice. Moreover, we analyzed the impact of different social housing conditions. We used a portfolio of classical response measurements, detailed gait analysis, using 2 different measuring systems (Dynamic weight bearing and CatWalk), as well as observer-independent voluntary wheel running and homecage monitoring in a longitudinal time frame. The impact of grouped or isolated housing was investigated in all behavioral paradigms. We observed that unilateral hind paw inflammation provoked changes in several behaviors. Among these were wheel running activity and different homecage activity parameters. Stimulus-evoked hypersensitivity lasted much longer than gait abnormalities and decreased voluntary wheel running activity. Similar effects were monitored in both social housing conditions. This is the first longitudinal study providing detailed insights into various voluntary behavioral parameters related to pain in a unilateral inflammatory model. Stimulus-evoked behavioral changes lasted longer than changes in voluntary behavioral parameters, and the social environment hardly affects these changes.

EXPRESS: Voluntary and evoked behavioral correlates in neuropathic pain states under different housing conditions

Background

There is an urgent need to develop and incorporate novel behavioral tests in classically used preclinical pain models. Most rodent studies are based upon stimulus-evoked hindpaw measurements even though chronic pain is usually a day and night experience. Chronic pain is indeed a debilitating condition that influences the sociability and the ability for voluntary tasks, but the relevant behavioral readouts for these aspects are mostly under-represented in the literature. Moreover, we lack standardization in most behavioral paradigms to guarantee reproducibility and ensure adequate discussion between different studies. This concerns not only the combination, application, and duration of particular behavioral tasks but also the effects of different housing conditions implicating social isolation.

Results

Our aim was to thoroughly characterize the classically used spared nerve injury model for 12 weeks following surgery. We used a portfolio of classical stimulus-evoked response measurements, detailed gait analysis with two different measuring systems (Dynamic weight bearing (DWB) system and CatWalk), as well as observer-independent voluntary wheel running and home cage monitoring (Laboras system). Additionally, we analyzed the effects of social isolation in all behavioral tasks. We found that evoked hypersensitivity temporally matched changes in static gait parameters, whereas some dynamic gait parameters were changed in a time-dependent manner. Interestingly, voluntary wheel running behavior was not affected in spared nerve injury mice but by social isolation. Besides a reduced climbing activity, spared nerve injury mice did not showed tremendous alterations in the home cage activity.

Conclusion

This is the first longitudinal study providing detailed insights into various voluntary behavioral parameters related to pain and highlights the importance of social environment on spontaneous non-evoked behaviors in a mouse model of chronic neuropathy. Our results provide fundamental considerations for future experimental planning and discussion of pain-related behavioral changes.

Dynamic weight bearing as a non-reflexive method for the measurement of abdominal pain in mice

BACKGROUND

Chronic pelvic pain (CPP) is a high burden for patients and society. It affects 15-24% of women in reproductive age and is an area of high unmet medical need. CPP can be caused by a wide range of visceral diseases such as abdominal infections, gastrointestinal or gynaecological diseases like endometriosis. Despite the high medical need for this condition, pharmacological approaches are hampered by the limited number of available methods for the behavioural evaluation of pain in inflammation-driven animal models of pelvic pain.

METHODS

The dynamic weight bearing (DWB) system was used for the evaluation of spontaneous behaviour changes in the zymosan-induced peritonitis mouse model. Inflammatory mediator levels were evaluated in peritoneal lavage and their correlation with the behavioural endpoints was assessed. We evaluated the effect on behavioural endpoints of the selective cyclooxygenase-2 (COX-2) inhibitor celecoxib and the Nav 1.8 blocker A-803467.

RESULTS

The presence of a relief posture, characterized by a significantly increased weight distribution towards the front paws, was observed following intraperitoneal injection of zymosan. A positive correlation was detected between PGE2 levels in the peritoneal lavage and DWB endpoints. In addition, zymosan-induced weight bearing changes were reverted by celecoxib and A-803467.

CONCLUSIONS

This study described for the first time the use of DWB as a non-subjective and non-reflexive method for the evaluation of inflammatory-driven abdominal pain in a mouse model.

Preclinical Assessment of Inflammatory Pain

While acute inflammation is a natural physiological response to tissue injury or infection, chronic inflammation is maladaptive and engenders a considerable amount of adverse pain. The chemical mediators responsible for tissue inflammation act on nociceptive nerve endings to lower neuronal excitation threshold and sensitize afferent firing rate leading to the development of allodynia and hyperalgesia, respectively. Animal models have aided in our understanding of the pathophysiological mechanisms responsible for the generation of chronic inflammatory pain and allowed us to identify and validate numerous analgesic drug candidates. Here we review some of the commonly used models of skin, joint, and gut inflammatory pain along with their relative benefits and limitations. In addition, we describe and discuss several behavioral and electrophysiological approaches used to assess the inflammatory pain in these preclinical models. Despite significant advances having been made in this area, a gap still exists between fundamental research and the implementation of these findings into a clinical setting. As such we need to characterize inherent pathophysiological pathways and develop new endpoints in these animal models to improve their predictive value of human inflammatory diseases in order to design safer and more effective analgesics.

Automated assessment of pain in rats using a voluntarily accessed static weight-bearing test

The weight-bearing test is one method to assess pain in rodent animal models; however, the acceptance of this convenient method is limited by the low throughput data acquisition and necessity of confining the rodents to a small chamber.

NEW METHODS

We developed novel data acquisition hardware and software, data analysis software, and a conditioning protocol for an automated high throughput static weight-bearing assessment of pain. With this device, the rats voluntarily enter the weighing chamber, precluding the necessity to restrain the animals and thereby removing the potential stress-induced confounds as well as operator selection bias during data collection. We name this device the Voluntarily Accessed Static Incapacitance Chamber (VASIC).

RESULTS

Control rats subjected to the VASIC device provided hundreds of weight-bearing data points in a single behavioral assay. Chronic constriction injury (CCI) surgery and paw pad injection of complete Freund's adjuvant (CFA) or carrageenan in rats generated hundreds of weight-bearing data during a 30 minute recording session. Rats subjected to CCI, CFA, or carrageenan demonstrated the expected bias in weight distribution favoring the un-operated leg, and the analgesic effect of i.p. morphine was demonstrated. In comparison with existing methods, brief water restriction encouraged the rats to enter the weighing chamber to access water, and an infrared detector confirmed the rat position with feet properly positioned on the footplates, triggering data collection. This allowed hands-off measurement of weight distribution data reducing operator selection bias.

CONCLUSION

The VASIC device should enhance the hands-free parallel collection of unbiased weight-bearing data in a high throughput manner, allowing further testing of this behavioral measure as an effective assessment of pain in rodents.

MeCP2 plays an analgesic role in pain transmission through regulating CREB / miR-132 pathway

Background

The Methyl CpG binding protein 2 gene (MeCP2 gene) encodes a critical transcriptional repressor and is widely expressed in mammalian neurons. MeCP2 plays a critical role in neuronal differentiation, neural development, and synaptic plasticity. Mutations and duplications of the human MECP2 gene lead to severe neurodevelopmental disorders, such as Rett syndrome and autism. In this study we investigate the role of MeCP2 in the spinal cord and found that MeCP2 plays an important role as an analgesic mediator in pain circuitry.

Findings

Experiments using MeCP2 transgenic mice showed that overexpression of MeCP2 weakens both acute mechanical pain and thermal pain, suggesting an analgesic role of MeCP2 in acute pain transduction. We found that through p-CREB/miR-132 signaling cascade is involved in MeCP2-mediated pain transduction. We also examined the role of MeCP2 in chronic pain formation using spared nerve injury (SNI) model. Strikingly, we found that development of neuropathic pain attenuates in MeCP2 transgenic mice comparing to wild type (WT) mice.

Conclusions

Our study shows that MeCP2 plays an analgesic role in both acute pain transduction and chronic pain formation through regulating CREB-miR-132 pathway. This work provides a potential therapeutic target for neural pathologic pain, and also sheds new lights on the abnormal sensory mechanisms associated with autism spectrum orders.

Trigeminal Inflammatory Compression (TIC) injury induces chronic facial pain and susceptibility to anxiety-related behaviors

Our laboratory previously developed a novel neuropathic and inflammatory facial pain model for mice referred to as the Trigeminal Inflammatory Compression (TIC) model. Rather than inducing whole nerve ischemia and neuronal loss, this injury induces only slight peripheral nerve demyelination triggering long-term mechanical allodynia and cold hypersensitivity on the ipsilateral whisker pad. The aim of the present study is to further characterize the phenotype of the TIC injury model using specific behavioral assays (i.e. light-dark box, open field exploratory activity, and elevated plus maze) to explore pain- and anxiety-like behaviors associated with this model. Our findings determined that the TIC injury produces hypersensitivity 100% of the time after surgery that persists at least 21 weeks post injury (until the animals are euthanized). Three receptive field sensitivity pattern variations in mice with TIC injury are specified. Animals with TIC injury begin displaying anxiety-like behavior in the light-dark box preference and open field exploratory tests at week eight post injury as compared to sham and naïve animals. Panic anxiety-like behavior was shown in the elevated plus maze in mice with TIC injury if the test was preceded with acoustic startle. Thus, in addition to mechanical and cold hypersensitivity, the present study identified significant anxiety-like behaviors in mice with TIC injury resembling the clinical symptomatology and psychosocial impairments of patients with chronic facial pain. Overall, the TIC injury model's chronicity, reproducibility, and reliability in producing pain- and anxiety-like behaviors demonstrate its usefulness as a chronic neuropathic facial pain model.

Significance of neuronal cytochrome P450 activity in opioid-mediated stress-induced analgesia

Stressful environmental changes can suppress nociceptive transmission, a phenomenon known as "stress-induced analgesia". Depending on the stressor and the subject, opioid or non-opioid mechanisms are activated. Brain μ opioid receptors mediate analgesia evoked either by exogenous agents (e.g. morphine), or by the release of endogenous opioids following stressful procedures. Recent work with morphine and neuronal cytochrome P450 (P450)-deficient mice proposed a signal transduction role for P450 enzymes in µ analgesia. Since µ opioid receptors also mediate some forms of stress-induced analgesia, the present studies assessed the significance of brain P450 activity in opioid-mediated stress-induced analgesia. Two widely-used models of opioid stress-induced analgesia (restraint and warm water swim) were studied in both sexes of wild-type control and P450-deficient (Null) mice. In control mice, both stressors evoked moderate analgesic responses which were blocked by pretreatment with the opioid antagonist naltrexone, confirming the opioid nature of these responses. Consistent with literature, sex differences (control female>control male) were seen in swim-induced, but not restraint-induced, analgesia. Null mice showed differential responses to the two stress paradigms. As compared with control subjects, Null mice showed highly attenuated restraint-induced analgesia, showing a critical role for neuronal P450s in this response. However, warm water swim-induced analgesia was unchanged in Null vs. control mice. Additional control experiments confirmed the absence of morphine analgesia in Null mice. These results are the first to show that some forms of opioid-mediated stress-induced analgesia require brain neuronal P450 activity.

Learning without Memory

After a CS-US learning trial, powerful feedback (FB) selectively modulates the hedonic value of the US, perhaps affecting the CS as well. FB. operates regardless of the subject's attributions or awareness, often influencing the subject's unconscious motives in a single trial. Most of the evidence comes from feeding research where the taste US and the internal FB are independently manipulated, but similar influences are at work with painful, thermal and sexual USs in a wide variety of species including humans. Implications for cognitive theorizing and neural research on mechanisms of learning are discussed.

Burrowing as a non-reflex behavioural readout for analgesic action in a rat model of sub-chronic knee joint inflammation

BACKGROUND

Innate responses against spontaneous pain are proposed to improve the predictive validity of preclinical analgesia models. Therefore, development and validation of novel readouts is necessary. To investigate whether innate rodent burrowing is a useful alternative behavioural readout for assessment of analgesic efficacy, a complete Freund's adjuvant (CFA)-induced model of sub-chronic inflammation was used to compare the effects of naproxen, ibuprofen and pregabalin in weight-bearing (WB), open-field (OF) and burrowing assays.

METHODS

Male Sprague Dawley rats were injected with 150 μL of CFA (2 mg/mL) into the knee (hind leg) 3 days before testing. Naproxen, ibuprofen and pregabalin were administered at different doses 30, 90 and 60 min, respectively, before testing. WB was determined using a rat incapacitance tester; horizontal distance moved and vertical rearings were recorded in an OF; and burrowing was measured by the weight of gravel remaining in a hollow tube after 60 min.

RESULTS

CFA-induced arthritis reduced WB, OF activity and burrowing. Naproxen, pregabalin and ibuprofen treatment normalized WB; however, horizontal OF activity was not improved by any treatment; rearing behaviour was moderately reinstated by ibuprofen (100 mg/kg). In burrowing, naproxen (100 mg/kg), ibuprofen (31.6 and 100 mg/kg) and pregabalin (10 mg/kg) reversed CFA-induced deficits.

CONCLUSIONS

Burrowing performance is an alternative non-reflex readout relying on innate rodent behaviour that is affected by nociceptive behaviour and can be pharmacologically manipulated. The burrowing assay appears to be more sensitive than OF assays and is as sensitive as WB assays at distinguishing between analgesic doses and doses that impair locomotion.

The dorsomedial hypothalamus mediates stress-induced hyperalgesia and is the source of the pronociceptive peptide cholecystokinin in the rostral ventromedial medulla

While intense or highly arousing stressors have long been known to suppress pain, relatively mild or chronic stress can enhance pain. The mechanisms underlying stress-induced hyperalgesia (SIH) are only now being defined. The physiological and neuroendocrine effects of mild stress are mediated by the dorsomedial hypothalamus (DMH), which has documented connections with the rostral ventromedial medulla (RVM), a brainstem region capable of facilitating nociception. We hypothesized that stress engages both the DMH and the RVM to produce hyperalgesia. Direct pharmacological activation of the DMH increased sensitivity to mechanical stimulation in awake animals, confirming that the DMH can mediate behavioral hyperalgesia. A behavioral model of mild stress also produced mechanical hyperalgesia, which was blocked by inactivation of either the DMH or the RVM. The neuropeptide cholecystokinin (CCK) acts in the RVM to enhance nociception and is abundant in the DMH. Using a retrograde tracer and immunohistochemical labeling, we determined that CCK-expressing neurons in the DMH are the only significant supraspinal source of CCK in the RVM. However, not all neurons projecting from the DMH to the RVM contained CCK, and microinjection of the CCK2 receptor antagonist YM022 in the RVM did not interfere with SIH, suggesting that transmitters in addition to CCK play a significant role in this connection during acute stress. While the RVM has a well-established role in facilitation of nociception, the DMH, with its well-documented role in stress, may also be engaged in a number of chronic or abnormal pain states. Taken as a whole, these findings establish an anatomical and functional connection between the DMH and RVM by which stress can facilitate pain.

Temporal regularity determines the impact of electrical stimulation on tactile reactivity and response to capsaicin in spinally transected rats

Nociceptive plasticity and central sensitization within the spinal cord depend on neurobiological mechanisms implicated in learning and memory in higher neural systems, suggesting that the factors that impact brain-mediated learning and memory could modulate how stimulation affects spinal systems. One such factor is temporal regularity (predictability). The present paper shows that intermittent hindleg shock has opposing effects in spinally transected rats depending upon whether shock is presented in a regular or irregular (variable) manner. Variable intermittent legshock (900 shocks) enhanced mechanical reactivity to von Frey stimuli (hyperreactivity), whereas 900 fixed-spaced legshocks produced hyporeactivity. The impact of fixed-spaced shock depended upon the duration of exposure; a brief exposure (36 shocks) induced hyperreactivity whereas an extended exposure (900 shocks) produced hyporeactivity. The enhanced reactivity observed after variable shock was most evident 60-180 min after treatment. Fixed and variable intermittent stimulation applied to the sciatic nerve, or the tail, yielded a similar pattern of results. Stimulation had no effect on thermal reactivity. Exposure to fixed-spaced shock, but not variable shock, attenuated the enhanced mechanical reactivity (EMR) produced by treatment with hindpaw capsaicin. The effect of fixed-spaced stimulation lasted 24h. Treatment with fixed-spaced shock also attenuated the maintenance of capsaicin-induced EMR. The results show that variable intermittent shock enhances mechanical reactivity, while an extended exposure to fixed-spaced shock has the opposite effect on mechanical reactivity and attenuates capsaicin-induced EMR.

Experimental hypoglycemia is a human model of stress-induced hyperalgesia

Hypoglycemia is a physiological stress that leads to the release of stress hormones, such as catecholamines and glucocorticoids, and proinflammatory cytokines. These factors, in euglycemic animal models, are associated with stress-induced hyperalgesia. The primary aim of this study was to determine whether experimental hypoglycemia in humans would lead to a hyperalgesic state. In 2 separate 3-day admissions separated by 1 to 3 months, healthy study participants were exposed to two 2-hour euglycemic hyperinsulinemic clamps or two 2-hour hypoglycemic hyperinsulinemic clamps. Thermal quantitative sensory testing and thermal pain assessments were measured the day before and the day after euglycemia or hypoglycemia. In contrast to prior euglycemia exposure, prior hypoglycemia exposure resulted in enhanced pain sensitivity to hot and cold stimuli as well as enhanced temporal summation to repeated heat-pain stimuli. These findings suggest that prior exposure to hypoglycemia causes a state of enhanced pain sensitivity that is consistent with stress-induced hyperalgesia. This human model may provide a framework for hypothesis testing and targeted, mechanism-based pharmacological interventions to delineate the molecular basis of hyperalgesia and pain susceptibility.

Restoration of electric footshock-induced immunosuppression in mice by Gynostemma pentaphyllum components

The immunomodulatory effects of the ethanol extract of Gynostemma pentaphyllum (GP-EX) were examined in electric footshock (EFS)-stressed mice. The mice were orally administered various doses of GP-EX for 7 days before exposure to EFS (duration: 3 min, interval: 10 s, intensity: 2 mA) once a day from day 8 for 14 days with continuous daily feeding of GP-EX. Oral administration of GP-EX to mice prevented EFS stress-induced immunosuppression as determined by the lymphoid organ (thymus and spleen) weight and cellularity. In addition, oral administration of GP-EX restored EFS-suppressed functional properties of mature lymphocytes in terms of concanavalin A-induced proliferation of splenocytes and lipopolysaccharide-induced cytokine production (TNF-α, IL-1β). Furthermore, we found that mice that were orally administered with GP-EX generated much more potent ovalbumin-specific cytotoxic T lymphocyte responses upon intravenous ovalbumin injection compared to the untreated controls. These results demonstrate that oral administration of the ethanol extract of Gynostemma pentaphyllum could increase host defense in immunocompromised situations such as stress-induced immunosuppression.