Nociception- and anxiety-like behavior in rats submitted to different periods of restraint stress

Sex-based differences in the prevention of stress-induced anxiety by Resolvin D5 and its precursor docosahexaenoic acid: A comparative study

Acute stress can cause emotional dysregulation and trigger various molecular changes, including increased neuroinflammation in limbic regions. These changes have the potential to induce anxiety by disrupting brain physiology and functional connectivity. In this study, we investigated whether an 8-day treatment with inflammation-resolving compounds, specifically Resolvin D5 (RvD5) and its precursor, the omega-3 fatty acid docosahexaenoic acid (DHA), could alleviate anxiety induced by acute restraint stress (ARS) in male and female rats. Additionally, we assessed whether these effects persisted one week after treatment cessation. Serum corticosterone levels and proinflammatory cytokine levels in the hippocampus (HIP) were also assessed. Our results confirmed that ARS induced significant anxiety-like behavior in both the short and long term, with females displaying greater exploratory activity than males. Both RvD5 and DHA prevented the development of pronounced anxiety-like behavior in stressed rats, without affecting anxiety levels in non-stressed rats. Notably, the effect persisted for at least one-week post-treatment in females. The treatments also prevented the elevation of TNF alpha and interleukin-1 beta levels in the HIP and serum corticosterone levels in stressed animals. In conclusion, our findings confirm the neuroprotective profile of these compounds and indicate that the continuous use of DHA or RvD5 may have promising effects in preventing anxiety responses triggered by acute stressful event, regardless of sex. Furthermore, this study is the first to demonstrate that RvD5 can downregulate corticosterone levels in stressed animals.

Concurrent stress modulates the acute and post-acute effects of psilocybin in a sex-dependent manner

There is renewed interest in psychedelics, such as psilocybin, as therapies for multiple difficult-to-treat psychiatric disorders. Even though psychedelics can induce highly pleasant or aversive experiences, depending on multiple personal and environmental factors, there is little research into how such experiences impact post-acute mood-altering actions. Here we aimed at offsetting this gap. First, we tested whether acute psilocybin effects differed between sexes. Adult male and female C57BL/6J mice received saline or psilocybin (5 mg/kg; i.p.), and head-twitch response (HTR) frequency was quantified. Notably, while psilocybin increased HTR frequency in both sexes, the effect was greater in females. We then tested if stress exposure during acute drug effects impacted post-acute psilocybin actions. Following drug treatment, mice were returned to their homecage or restrained for 1 h. Anxiety- and depression-like behaviors were assessed starting 24 h following drug administration, using the marble burying, novelty-suppressed feeding, and splash tests. Psilocybin induced anxiolytic-, but not antidepressant-like, which were fully blocked by stress in males, but only partially so in females. Lastly, we assessed the acute stress-psilocybin interaction on plasma corticosterone levels in a separate cohort of mice, treated as above. Both stress and psilocybin independently increased corticosterone levels, without additive or interactive effects being observed for either sex. Our data reveals the role of sex and peri-acute negative experiences in the acute and post-acute actions of psilocybin. These findings underline the importance of non-pharmacological factors, such as the quality of the psychedelic experience, in the mood-altering effects of psychedelics, holding significant for both their therapeutic and recreational use.

Rat Models in Post-Traumatic Stress Disorder Research: Strengths, Limitations, and Implications for Translational Studies

Post-Traumatic Stress Disorder (PTSD) is a multifaceted psychiatric disorder triggered by traumatic events, leading to prolonged psychological distress and varied symptoms. Rat models have been extensively used to explore the biological, behavioral, and neurochemical underpinnings of PTSD. This review critically examines the strengths and limitations of commonly used rat models, such as single prolonged stress (SPS), stress-re-stress (S-R), and predator-based paradigms, in replicating human PTSD pathology. While these models provide valuable insights into neuroendocrine responses, genetic predispositions, and potential therapeutic targets, they face challenges in capturing the full complexity of PTSD, particularly in terms of ethological relevance and translational validity. We assess the degree to which these models mimic the neurobiological and behavioral aspects of human PTSD, highlighting areas where they succeed and where they fall short. This review also discusses future directions in refining these models to improve their utility for translational research, aiming to bridge the gap between preclinical findings and clinical applications.

Effects of Stress Exposure to Pain Perception in Pre-Clinical Studies: Focus on the Nociceptin/Orphanin FQ-NOP Receptor System

Exposure to physical and psychological stress modulates pain transmission in a dual manner. Stress-induced analgesia (SIA) refers to the reduction in pain sensitivity that can occur in response to acute stress. On the contrary, chronic stress exposure may lead to a phenomenon named stress-induced hyperalgesia (SIH). SIH is a clinically relevant phenomenon since it has been well documented that physical and psychological stress exacerbates pain in patients with several chronic pain syndromes, including migraine. The availability of animal models of SIA and SIH is of high importance for understanding the biological mechanisms leading to these phenomena and for the identification of pharmacological targets useful to alleviate the burden of stress-exacerbated chronic pain. Among these targets, the nociceptin/orphanin FQ (N/OFQ)-N/OFQ peptide (NOP) receptor system has been identified as a key modulator of both pain transmission and stress susceptibility. This review describes first the experimental approaches to induce SIA and SIH in rodents. The second part of the manuscript summarizes the scientific evidence that suggests the N/OFQ-NOP receptor system as a player in the stress-pain interaction and candidates NOP antagonists as useful drugs to mitigate the detrimental effects of stress exposure on pain perception.

Restraint stress-induced antinociceptive effects in acute pain: Involvement of orexinergic system in the nucleus accumbens

The complicated relevance between stress and pain has been identified. Neurotransmitters and neuropeptides of various brain areas play a role in this communication. Pain inhibitory response is known as stress-induced analgesia (SIA). The studies demonstrated that the nucleus accumbens (NAc) is critical in modulating pain. As a neuropeptide, orexin is crucially involved in initiating behavioral and physiological responses to threatening and unfeeling stimuli. However, the role of the orexin receptors of the NAc area after exposure to restraint stress (RS) as acute physical stress in the modulation of acute pain is unclear. One hundered twenty adult male albino Wistar rats (230-250 g) were used. Animals were unilaterally implanted with cannulae above the NAc. The SB334867 and TCS OX2 29 were used as antagonists for OX1r and OX2r, respectively. Different doses of the antagonists (1, 3, 10, and 30 nmol/0.5 µl DMSO) were microinjected intra-NAc five minutes before exposure to RS (3 hours). Then, the tail-flick test as a model of acute pain was performed, and the nociceptive threshold (Tail-flick latency; TFL) was measured in 60-minute time set intervals. According to this study's findings, the antinociceptive effects of RS in the tail-flick test were blocked during intra-NAc administration of SB334867 or TCS OX2 29. The RS as acute stress increased TFL and deceased pain-like behavior responses. The 50 % effective dose values of the OX1r and OX2r antagonists were 12.82 and 21.64 nmol, respectively. The result demonstrated contribution of the OX1r into the NAc was more remarkable than that of the OX2r on antinociceptive responses induced by the RS. Besides, in the absence of RS, the TFL was attenuated. The current study's data indicated that OX1r and OX2r into the NAc induced pain modulation responses during RS in acute pain. In conclusion, the findings revealed the involvement of intra-NAc orexin receptors in improving SIA.

Osteoporosis under psychological stress: mechanisms and therapeutics

Psychological stress has been associated with the onset of several diseases, including osteoporosis. However, the underlying pathogenic mechanism remains unknown, and effective therapeutic strategies are still unavailable. Growing evidence suggests that the sympathetic nervous system regulates bone homeostasis and vascular function under psychological stress, as well as the coupling of osteogenesis and angiogenesis in bone development, remodeling, and regeneration. Furthermore, extracellular vesicles (EVs), particularly mesenchymal stem cell extracellular vesicles (MSC-EVs), have emerged as prospecting therapies for stimulating angiogenesis and bone regeneration. We summarize the role of sympathetic regulation in bone homeostasis and vascular function in response to psychological stress and emphasize the relationship between vessels and bone. Finally, we suggest using MSC-EVs as a promising therapeutic method for treating osteoporosis in psychological stress.

Chronic restraint stress and social transfer of stress produce tactile allodynia mediated by the HMGB1/TNFα/TNFR1 pathway in female and male rats

Previous studies have shown the relevance of high mobility group box 1 protein (HMGB1) and tumor necrosis factor α (TNFα) in nerve or tissue injury-induced nociception. However, the role of these proteins in chronic stress and social transfer of stress (STS)-induced dysfunctional pain is not entirely known. The aim of this study was to determine the participation of the spinal HMGB1-TNFα signaling pathway and TNFα receptor 1 (TNFR1) in rats subjected to chronic restraint stress (CRS) and STS. Non-stressed female and male rats in contact with CRS rats increased sniffing behavior of the anogenital area, behavior related to STS. Rats subjected to CRS and STS reduced 50 % withdrawal threshold and reached the value of tactile allodynia after 21 days of stress. Rats return to the basal withdrawal threshold after 30 days without stress and return to allodynia values in only 5 days of stress sessions (priming). Female and male rats subjected to 28 days of CRS or STS were intrathecal injected with glycyrrhizin (inhibitor of HMGB1), thalidomide (inhibitor of the TNFα synthesis), and R7050 (TNFR1 antagonist), in all the cases, an antiallodynic effect was observed. Rats under CRS or STS enhanced HMGB1 and TNFR1 protein expression in DRG and dorsal spinal cord. Data suggest that the spinal HMGB1/TNFα/TNFR1 signaling pathway plays a relevant role in the maintenance of CRS and STS-induced nociceptive hypersensitivity in rats. These proteins could be helpful in developing pain treatments for fibromyalgia in humans.

A prolonged stress rat model recapitulates some PTSD-like changes in sleep and neuronal connectivity

Chronic post-traumatic stress disorder (PTSD) exhibits psychological abnormalities during fear memory processing in rodent models. To simulate long-term impaired fear extinction in PTSD patients, we constructed a seven-day model with multiple prolonged stress (MPS) by modifying manipulation repetitions, intensity, and unpredictability of stressors. Behavioral and neural changes following MPS conveyed longitudinal PTSD-like effects in rats for 6 weeks. Extended fear memory was estimated through fear retrieval induced-freezing behavior and increased long-term serum corticosterone concentrations after MPS manipulation. Additionally, memory retrieval and behavioral anxiety tasks continued enhancing theta oscillation activity in the prefrontal cortex-basal lateral amygdala-ventral hippocampus pathway for an extended period. Moreover, MPS and remote fear retrieval stimuli disrupted sleep-wake activities to consolidate fear memory. Our prolonged fear memory, neuronal connectivity, anxiety, and sleep alteration results demonstrated integrated chronic PTSD symptoms in an MPS-induced rodent model.

Restraint stress induced the antinociceptive responses via the dopamine receptors within the hippocampal CA1 area in animal model of persistent inflammatory pain

It has been declared that dopamine receptors within the hippocampal formation are involved in emotion, memory, and pain processing. Remarkably, both CA1 and dentate gyrus (DG) areas of the hippocampal formation are involved in persistent peripheral nociceptive perception. A prior study showed that dopamine receptors within the hippocampal DG have a critical role in antinociception induced by forced swim stress (FSS), as a physical stressor, in the presence of formalin irritation. The present experiments were designed to assess whether dopaminergic receptors within the CA1 have any role in antinociceptive responses induced by restraint stress (RS) as a psychological stressor after applying the formalin test as an animal model of persistent inflammatory pain. The D1- and D2-like dopamine receptor antagonists, SCH23390 and Sulpiride (0.25, 1, and 4 μg/0.5 μl), were injected into the CA1 areas of ninety-six male albino Wistar rats 5 min before a 3-h period of restraint stress. Ten min after stress termination, a 50-μl formalin 2.5 % was subcutaneously injected into the plantar surface of the rat's hind paw to induce persistent inflammatory pain. Nociceptive behaviors in both phases of the formalin test were analyzed in the 5-min blocks for a 60-min period. The obtained results demonstrate that although RS could induce an antinociceptive response in both phases of the formalin test, microinjection of D1- and D2-like dopamine receptors, antagonists attenuated RS-induced analgesia. These results support the hypothesis that acute restraint stress could induce analgesia via dopaminergic projection to the CA1 region of the hippocampal formation.

Chronic Restraint Stress Alters Rat Behavior Depending on Sex and Duration of Stress

Chronic restraint stress (CRS) can have different behavioral effects depending on variables associated with the stressor and the organism. This study aimed to verify the effect of the interaction between sex and duration of the CRS protocol in rats. Sprague-Dawley rats were divided by sex, intervention (CRS; control), and CRS duration (11 days; 22 days). Rats exposed to CRS showed better spatial learning than controls in the Morris water maze test, regardless of sex and stress duration. Males exposed to CRS for 11 days showed a higher rate of behaviors associated with anxiety than males exposed to 22 days of CRS at the elevated plus maze test, but the same was not observed in females. The weight gain of animals exposed to stress decreased in the first 11 days, showing a recovery from day 11 to day 22 of intervention. No effects of CRS were observed on behaviors associated with depression in the sucrose preference test. The results suggest habituation to the protocol, with a progressive decrease in the harmful effects of stress on and maintenance of the beneficial effects. It is possible that females are more resistant to the harmful effects of CRS on anxiety.

The degeneration-pain relationship in the temporomandibular joint: Current understandings and rodent models

Temporomandibular disorders (TMD) represent a group of musculoskeletal conditions involving the temporomandibular joints (TMJ), the masticatory muscles and associated structures. Painful TMD are highly prevalent and conditions afflict 4% of US adults annually. TMD include heterogenous musculoskeletal pain conditions, such as myalgia, arthralgia, and myofascial pain. A subpopulations of TMD patients show structural changes in TMJ, including disc displacement or degenerative joint diseases (DJD). DJD is a slowly progressing, degenerative disease of the TMJ characterized by cartilage degradation and subchondral bone remodeling. Patients with DJD often develop pain (TMJ osteoarthritis; TMJ OA), but do not always have pain (TMJ osteoarthrosis). Therefore, pain symptoms are not always associated with altered TMJ structures, which suggests that a causal relationship between TMJ degeneration and pain is unclear. Multiple animal models have been developed for determining altered joint structure and pain phenotypes in response to various TMJ injuries. Rodent models of TMJOA and pain include injections to induce inflammation or cartilage destruction, sustained opening of the oral cavity, surgical resection of the articular disc, transgenic approaches to knockout or overexpress key genes, and an integrative approach with superimposed emotional stress or comorbidities. In rodents, TMJ pain and degeneration occur during partially overlapping time periods in these models, which suggests that common biological factors may mediate TMJ pain and degeneration over different time courses. While substances such as intra-articular pro-inflammatory cytokines commonly cause pain and joint degeneration, it remains unclear whether pain or nociceptive activities are causally associated with structural degeneration of TMJ and whether structural degeneration of TMJ is necessary for producing persistent pain. A thorough understanding of the determining factors of pain-structure relationships of TMJ during the onset, progression, and chronification by adopting novel approaches and models should improve the ability to simultaneously treat TMJ pain and TMJ degeneration.

Fluoxetine reverses hyperactivity of anterior cingulate cortex and attenuates chronic stress-induced hyperalgesia

Somatic symptom disorder (SSD), which occurs in about 5-7 percent of the adult population, involves heightened physical and emotional sensitivity to pain. However, its neural mechanism remains elusive and thus hinders effective clinical intervention. In this study, we employed chronic restraint stress (CRS)-induced hyperalgesia as a mouse model to investigate the neural mechanism underlying SSD and its pharmacological treatment. We found that CRS induced hyperactivity of anterior cingulate cortex (ACC), whereas chemogenetic inhibition of such hyperactivity could prevent CRS-induced hyperalgesia. Systematic application and ACC local infusion of fluoxetine alleviated CRS-induced hyperalgesia. Moreover, we found that fluoxetine exerted its anti-hyperalgesic effects through inhibiting the hyperactivity of ACC and upregulating 5-HT1A receptors. Our study thus uncovers the functional role of 5-HT signaling in modulating pain sensation and provides a neural basis for developing precise clinical intervention for SSD.

The Neurobiological Links between Stress and Traumatic Brain Injury: A Review of Research to Date

Neurological dysfunctions commonly occur after mild or moderate traumatic brain injury (TBI). Although most TBI patients recover from such a dysfunction in a short period of time, some present with persistent neurological deficits. Stress is a potential factor that is involved in recovery from neurological dysfunction after TBI. However, there has been limited research on the effects and mechanisms of stress on neurological dysfunctions due to TBI. In this review, we first investigate the effects of TBI and stress on neurological dysfunctions and different brain regions, such as the prefrontal cortex, hippocampus, amygdala, and hypothalamus. We then explore the neurobiological links and mechanisms between stress and TBI. Finally, we summarize the findings related to stress biomarkers and probe the possible diagnostic and therapeutic significance of stress combined with mild or moderate TBI.

SHORT-TERM STRESS SIGNIFICANTLY DECREASES MORPHINE ANALGESIA IN TRIGEMINAL BUT NOT IN SPINAL INNERVATED AREAS IN RATS

Plenty information exists regarding the effects of chronic stress, although few data exist on the effects of short-lasting stressors, which would mimic daily challenges. Differences in craniofacial and spinal nociception have been observed, thus those observations obtained in spinally innervated areas cannot be directly applied to the orofacial region. Although, opioids are considered amongst the most effective analgesics, their use is sometimes hampered by the constipation they induce. Thus, our aims were to study if a short-lasting stressor, forced swim stress (FSS), modifies nociception, morphine antinociception and constipation in rats. Animals were submitted to 10-20min of FSS for three days, nociception and gastrointestinal transit were studied 24h after the last swimming session. Nociception and morphine (0.6-5mg/kg) antinociception were evaluated in the formalin and hypertonic saline tests in the orofacial area and limbs. Morphine-induced modifications in the GI transit were studied through radiographic techniques. Naloxone was administered, before each swimming session, to analyse the involvement of the endogenous opioid system on the effect of stress. Overall, stress did not alter nociception, although interestingly it reduced the effect of morphine in the orofacial tests and in the inflammatory phase of the formalin tests. Naloxone antagonized the effect of stress and normalized the effect of morphine. Stress did not modify the constipation induced by morphine. Opioid treatment may be less effective under a stressful situation, whilst adverse effects, such as constipation, are maintained. The prevention of stress may improve the level of opioid analgesia. Keywords.

The Impact of Increase in the Vertical Dimension of Occlusion on Nociception in Rats - A Preliminary Report

Since the change in vertical dimension of occlusion (VDO) is extremely important in prosthetic dentistry, the aim of the study was to examine the effect of VDO increase on nociception parameters in rodent experimental model. The study was performed on seven experimental groups (6 animals per group) on male Wistaralbino rats: sham; 0.6/3, 0.9/3, and 1.2/3 groups where VDO was increased by 0.6, 0.9, and 1.2 mm (respectively), for three days; 0.6/20, 0.9/20, and 1.2/20 groups where VDO was increased by 0.6, 0.9, and 1.2 mm (respectively), for twenty days. The VDO raising protocols were performed as follows: on a day 1, following anaesthesia, a two-phase impression was taken with addition silicones; on a day 3, the cementing process for both maxillary incisors and inside crowns preparation was performed, and cementing zirconium crowns, manufactured using CAD-CAM technology, were applied. The behavioural testing (the tail flick and hot plate test) was performed on day 3 and 20. The results obtained in the tail flick test suggest that the raise in VDO in the early phase induced increased sensitivity to pain in a stepwise manner, while this hyperalgesic effect was diminished in a timedependent manner. The stepwise increase in VDO also resulted in significant decline in the pain tolerance with the higher VDO (0.9 and 1.2 mm) in the hot plate test that persisted after twenty days in 1.2/20 group. It seems that VDO elevation is sufficient to produce hyperalgesic effect in this experimental model, which may be attenuated in time-dependent manner.

Innovations and advances in modelling and measuring pain in animals

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.

Central role of intestinal epithelial glucocorticoid receptor in alcohol- and corticosterone-induced gut permeability and systemic response

Corticosterone, the stress hormone, exacerbates alcohol-associated tissue injury, but the mechanism involved is unknown. We examined the role of the glucocorticoid receptor (GR) in corticosterone-mediated potentiation of alcohol-induced gut barrier dysfunction and systemic response. Hepatocyte-specific GR-deficient (GR^ΔHC ) and intestinal epithelial-specific GR-deficient (GR^ΔIEC ) mice were fed ethanol, combined with corticosterone treatment. Intestinal epithelial tight junction integrity, mucosal barrier function, microbiota dysbiosis, endotoxemia, systemic inflammation, liver damage, and neuroinflammation were assessed. Corticosterone potentiated ethanol-induced epithelial tight junction disruption, mucosal permeability, and inflammatory response in GR^ΔHC mouse colon; these effects of ethanol and corticosterone were absent in GR^ΔIEC mice. Gut microbiota compositions in ethanol-fed GR^ΔHC and GR^ΔIEC mice were similar to each other. However, corticosterone treatment in ethanol-fed mice shifted the microbiota composition to distinctly different directions in GR^ΔHC and GR^ΔIEC mice. Ethanol and corticosterone synergistically elevated the abundance of Enterobacteriaceae and Escherichia coli and reduced the abundance of Lactobacillus in GR^ΔHC mice but not in GR^ΔIEC mice. In GR^ΔHC mice, corticosterone potentiated ethanol-induced endotoxemia and systemic inflammation, but these effects were absent in GR^ΔIEC mice. Interestingly, ethanol-induced liver damage and its potentiation by corticosterone were observed in GR^ΔHC mice but not in GR^ΔIEC mice. GR^ΔIEC mice were also resistant to ethanol- and corticosterone-induced inflammatory response in the hypothalamus. These data indicate that the intestinal epithelial GR plays a central role in alcohol- and corticosterone-induced gut barrier dysfunction, microbiota dysbiosis, endotoxemia, systemic inflammation, liver damage, and neuroinflammation. This study identifies a novel target for potential therapeutic for alcohol-associated tissue injury.

Excessive energy expenditure due to acute physical restraint disrupts Drosophila motivational feeding response

To study the behavior of Drosophila, it is often necessary to restrain and mount individual flies. This requires removal from food, additional handling, anesthesia, and physical restraint. We find a strong positive correlation between the length of time flies are mounted and their subsequent reflexive feeding response, where one hour of mounting is the approximate motivational equivalent to ten hours of fasting. In an attempt to explain this correlation, we rule out anesthesia side-effects, handling, additional fasting, and desiccation. We use respirometric and metabolic techniques coupled with behavioral video scoring to assess energy expenditure in mounted and free flies. We isolate a specific behavior capable of exerting large amounts of energy in mounted flies and identify it as an attempt to escape from restraint. We present a model where physical restraint leads to elevated activity and subsequent faster nutrient storage depletion among mounted flies. This ultimately further accelerates starvation and thus increases reflexive feeding response. In addition, we show that the consequences of the physical restraint profoundly alter aerobic activity, energy depletion, taste, and feeding behavior, and suggest that careful consideration is given to the time-sensitive nature of these highly significant effects when conducting behavioral, physiological or imaging experiments that require immobilization.

Preclinical models of deep craniofacial nociception and temporomandibular disorder pain

Chronic pain in temporomandibular disorder (TMD) is a common health problem. Cumulating evidence indicates that the etiology of TMD pain is complex with multifactorial experience that could hamper the developments of treatments. Preclinical research is a resource to understand the mechanism for TMD pain, whereas limitations are present as a disease-specific model. It is difficult to incorporate multiple risk factors associated with the etiology that could increase pain responses into a single animal. This article introduces several rodent models which are often employed in the preclinical studies and discusses their validities for TMD pain after the elucidations of the neural mechanisms based on the clinical reports. First, rodent models were classified into two groups with or without inflammation in the deep craniofacial tissues. Next, the characteristics of each model and the procedures to identify deep craniofacial pain were discussed. Emphasis was directed on the findings of the effects of chronic psychological stress, a major risk factor for chronic pain, on the deep craniofacial nociception. Preclinical models have provided clinically relevant information, which could contribute to better understand the basis for TMD pain, while efforts are still required to bridge the gap between animal and human studies.

Neuroinflammation, body temperature and behavioural changes in CD1 male mice undergoing acute restraint stress: An exploratory study

BACKGROUND

Animal models used to study pathologies requiring rehabilitation therapy, such as cardiovascular and neurologic disorders or oncologic disease, must be as refined and translationally relevant as possible. Sometimes, however, experimental procedures such as those involving restraint may generate undesired effects which may act as a source of bias. However, the extent to which potentially confounding effects derive from such routine procedures is currently unknown. Our study was therefore aimed at exploring possible undesirable effects of acute restraint stress, whereby animals were exposed to a brightly lit enclosed chamber (R&L) similar to those that are commonly used for substance injection. We hypothesised that this would induce a range of unwanted physiological alterations [such as neuroinflammatory response and changes in body weight and in brown adipose tissue (BAT)] and behavioural modification, and that these might be mitigated via the use of non-aversive handling methods: Tunnel Handling (NAH-T) and Mechanoceptive Handling (NAH-M)) as compared to standard Tail Handling (TH).

METHODS

Two indicators of physiological alterations and three potentially stress sensitive behavioural parameters were assessed. Physiological alterations were recorded via body weight changes and assessing the temperature of Brown Adipose Tissue (BAT) using infra-red thermography (IRT), and at the end of the experiment we determined the concentration of cytokines CXCL12 and CCL2 in bone marrow (BM) and activated microglia in the brain. Nest complexity scoring, automated home-cage behaviour analysis (HCS) and Elevated Plus Maze testing (EPM) were used to detect any behavioural alterations. Recordings were made before and after a 15-minute period of R&L in groups of mice handled via TH, NAH-T or NAH-M.

RESULTS

BAT temperature significantly decreased in all handling groups following R&L regardless of handling method. There was a difference, at the limit of significance (p = 0.06), in CXCL12 BM content among groups. CXCL12 content in BM of NAH-T animals was similar to that found in Sentinels, the less stressed group of animals. After R&L, mice undergoing NAH-T and NAH-M showed improved body-weight maintenance compared to those exposed to TH. Mice handled via NAH-M spent a significantly longer time on the open arms of the EPM. The HCS results showed that in all mice, regardless of handling method, R&L resulted in a significant reduction in walking and rearing, but not in total distance travelled. All mice also groomed more. No difference among the groups was found in Nest Score, in CCL2 BM content or in brain activated microglia.

CONCLUSIONS

Stress induced by a common restraint procedure caused metabolic and behavioural changes that might increase the risk of unexpected bias. In particular, the significant decrease in BAT temperature could affect the important metabolic pathways controlled by this tissue. R&L lowered the normal frequency of walking and rearing, increased grooming and probably carried a risk of low-grade neuro-inflammation. Some of the observed alterations can be mitigated by Non-aversive handlings.

Temporomandibular Myofascial Pain Syndrome-Aetiology and Biopsychosocial Modulation. A Narrative Review

This review elaborates on the aetiology, diagnosis, and treatment of temporomandibular (TMD) myofascial pain syndrome (MPS) regulated by psychosocial factors. MPS impairs functioning in society due to the accompanying pain. Directed and self-directed biopsychosocial profile modulation may be beneficial in the treatment of MPS. Moreover, nutrition is also a considerable part of musculoskeletal system health. A fruit and vegetable diet contributes to a reduction in chronic pain intensity because of its anti-inflammatory influence. Cannabidiol (CBD) oils may also be used in the treatment as they reduce stress and anxiety. A promising alternative treatment may be craniosacral therapy which uses gentle fascia palpation techniques to decrease sympathetic arousal by regulating body rhythms and release fascial restrictions between the cranium and sacrum. MPS is affected by the combined action of the limbic, autonomic, endocrine, somatic, nociceptive, and immune systems. Therefore, the treatment of MPS should be deliberated holistically as it is a complex disorder.

Animal models of fibromyalgia: What is the best choice?

Fibromyalgia (FM) is a complex syndrome, with an indefinite aetiology and intricate pathophysiology that affects 2 - 3% of the world population. From the beginning of the 2000s, experimental animal models have been developed to mimic clinical FM and help obtain a better understanding of the relevant neurobiology. These animal models have enabled a broad study of FM symptoms and mechanisms, as well as new treatment strategies. Current experimental FM models include the reserpine-induced systemic depletion of biogenic amines, muscle application of acid saline, and stress-based (cold, sound, or swim) approaches, among other emerging models. FM models should: (i) mimic the cardinal symptoms and complaints reported by FM patients (e.g., spontaneous nociception, muscle pain, hypersensitivity); (ii) mimic primary comorbidities that can aggravate quality of life and lead to worse outcomes (e.g., fatigue, sleep disturbance, depression, anxiety); (iii) mimic the prevalent pathological mechanisms (e.g., peripheral and central sensitization, inflammation/neuroinflammation, change in the levels of the excitatory and inhibitory neurotransmitters); and (iv) demonstrate a pharmacological profile similar to the clinical treatment of FM. However, it is difficult for any one of these models to include the entire spectrum of clinical FM features once even FM patients are highly heterogeneous. In the past six years (2015 - 2020), a wide range of experimental FM studies has amounted to the literature reinforcing the need for an updated review. Here we have described, in detail, several approaches used to experimentally study FM, with a focus on recent studies in the field and in previously less discussed mechanisms. We highlight each model's challenges, limitations, and future directions, intending to help preclinical researchers establish the correct experimental FM model to use depending on their goals.

Understanding stress: Insights from rodent models

Through incorporating both physical and psychological forms of stressors, a variety of rodent models have provided important insights into the understanding of stress physiology. Rodent models also have provided significant information with regards to the mechanistic basis of the pathophysiology of stress-related disorders such as anxiety disorders, depressive illnesses, cognitive impairment and post-traumatic stress disorder. Additionally, rodent models of stress have served as valuable tools in the area of drug screening and drug development for treatment of stress-induced conditions. Although rodent models do not accurately reproduce the biochemical or physiological parameters of stress response and cannot fully mimic the natural progression of human disorders, yet, animal research has provided answers to many important scientific questions. In this review article, important studies utilizing a variety of stress models are described in terms of their design and apparatus, with specific focus on their capabilities to generate reliable behavioral and biochemical read-out. The review focusses on the utility of rodent models by discussing examples in the literature that offer important mechanistic insights into physiologically relevant questions. The review highlights the utility of rodent models of stress as important tools for advancing the mission of scientific research and inquiry.

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Stressful life events may lead to the onset of severe psychopathologies in humans.

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Rodents may model many features of stress exposure in human populations.

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Induction of stress via pharmacological and psychological manipulations alter rodent behavior.

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Mechanistic rodent studies reveal key molecular targets critical for new therapeutic targets.

Animal models for the discovery of novel drugs for post-traumatic stress disorder

INTRODUCTION

Existing pharmacological treatments for PTSD are limited and have been used primarily because of their effectiveness in other psychiatric conditions. To generate novel, PTSD specific pharmacotherapy, researchers must utilize animal models to assess the efficacy of experimental drugs.

AREAS COVERED

This review includes a discussion of factors that should be considered when developing an animal model of PTSD, as well as descriptions of the most commonly used models. Researchers have utilized physical stressors, psychological stressors, or a combination of the two to induce PTSD-like physiological and behavioral sequelae in animals. Such models have provided researchers with a valuable tool to examine the neurobiological mechanisms underlying the condition.

EXPERT OPINION

PTSD is a heterogeneous disorder that manifests as different symptom clusters in different individuals. Thus, there cannot be a one-size-fits-all approach to modeling the disorder in animals. Preclinical investigators must adopt a concentrated effort aimed at modeling specific PTSD subtypes and the distinct symptom profiles that result from specific types of human trauma. Moreover, researchers have focused so much on modeling a single PTSD syndrome in animals that studies examining only specific facets of the disorder are largely ignored. Future research employing animal models of PTSD requires greater focus on the nuances of PTSD.

Social stress as a trigger for depressive-like behavior and persistent hyperalgesia in mice: study of the comorbidity between depression and chronic pain

BACKGROUND

There is great comorbidity and similarity between chronic pain and major depressive disorders. We have recently shown that 10 days of social defeat stress (SDS) induces hyperalgesia regardless depressive-like behavior in mice. Here we aimed to investigate whether social stress predisposes to chronic pain and, inversely, whether chronic pain predisposes to stress-induced depression.

METHODS

Firstly, we used the 10 days SDS paradigm in mice followed by a mild protocol of repetitive inflammatory stimulus to evaluate if SDS would predispose to persistent hyperalgesia development. Secondly, we used the intense protocol of repetitive inflammatory stimulus followed by a subthreshold SDS to evaluate if persistent hyperalgesia would predispose to depressive-like behavior of social avoidance.

RESULTS

Our results showed that SDS predispose to chronic pain, since stressed mice injected with PGE2 for 7 days (mild protocol), stimuli normally not sufficient to trigger chronic pain, showed persistent hyperalgesia. Also, we showed that persistent hyperalgesia induced by repetitive inflammatory stimuli predispose to long-lasting depressive-like behavior of social avoidance induced by subthreshold SDS.

LIMITATIONS

We did not analyze molecular mechanism associated with chronic pain and depressive-like behavior induced by SDS. However, we hypothesized that SDS and 14 days of PGE2 would generate neuroplasticity on brain areas shared by chronic pain and depression, predisposing to pain chronification and depressive-like behavior, respectively.

CONCLUSIONS

We can conclude social stress as a key and a common factor for chronic pain and depression. We can also conclude that SDS predisposes to chronic pain and, inversely, chronic pain predisposes to depressive-like behavior.

Overview of Neurological Mechanism of Pain Profile Used for Animal "Pain-Like" Behavioral Study with Proposed Analgesic Pathways

Pain is the most common sensation installed in us naturally which plays a vital role in defending us against severe harm. This neurological mechanism pathway has been one of the most complex and comprehensive topics but there has never been an elaborate justification of the types of analgesics that used to reduce the pain sensation through which specific pathways. Of course, there have been some answers to curbing of pain which is a lifesaver in numerous situations-chronic and acute pain conditions alike. This has been explored by scientists using pain-like behavioral study methodologies in non-anesthetized animals since decades ago to characterize the analgesic profile such as centrally or peripherally acting drugs and allowing for the development of analgesics. However, widely the methodology is being practiced such as the tail flick/Hargreaves test and Von Frey/Randall-Selitto tests which are stimulus-evoked nociception studies, and there has rarely been a complete review of all these methodologies, their benefits and its downside coupled with the mechanism of the action that is involved. Thus, this review solely focused on the complete protocol that is being adapted in each behavioral study methods induced by different phlogogenic agents, the different assessment methods used for phasic, tonic and inflammatory pain studies and the proposed mechanism of action underlying each behavioral study methodology for analgesic drug profiling. It is our belief that this review could significantly provide a concise idea and improve our scientists' understanding towards pain management in future research.

Pain in neuropsychiatry: Insights from animal models

Pain is the most common symptom reported in clinical practice, meaning that it is associated with many pathologies as either the origin or a consequence of other illnesses. Furthermore, pain is a complex emotional and sensorial experience, as the correspondence between pain and body damage varies considerably. While these issues are widely acknowledged in clinical pain research, until recently they have not been extensively considered when exploring animal models, important tools for understanding pain pathophysiology. Interestingly, chronic pain is currently considered a risk factor to suffer psychiatric disorders, mainly stress-related disorders like anxiety and depression. Conversely, pain appears to be altered in many psychiatric disorders, such as depression, anxiety and schizophrenia. Thus, pain and psychiatric disorders have been linked in epidemiological and clinical terms, although the neurobiological mechanisms involved in this pathological bidirectional relationship remain unclear. Here we review the evidence obtained from animal models about the co-morbidity of pain and psychiatric disorders, placing special emphasis on the different dimensions of pain.

Rodent models of post-traumatic stress disorder: behavioral assessment

Although the etiology and expression of psychiatric disorders are complex, mammals show biologically preserved behavioral and neurobiological responses to valent stimuli which underlie the use of rodent models of post-traumatic stress disorder (PTSD). PTSD is a complex phenotype that is difficult to model in rodents because it is diagnosed by patient interview and influenced by both environmental and genetic factors. However, given that PTSD results from traumatic experiences, rodent models can simulate stress induction and disorder development. By manipulating stress type, intensity, duration, and frequency, preclinical models reflect core PTSD phenotypes, measured through various behavioral assays. Paradigms precipitate the disorder by applying physical, social, and psychological stressors individually or in combination. This review discusses the methods used to trigger and evaluate PTSD-like phenotypes. It highlights studies employing each stress model and evaluates their translational efficacies against DSM-5, validity criteria, and criteria proposed by Yehuda and Antelman's commentary in 1993. This is intended to aid in paradigm selection by informing readers about rodent models, their benefits to the clinical community, challenges associated with the translational models, and opportunities for future work. To inform PTSD model validity and relevance to human psychopathology, we propose that models incorporate behavioral test batteries, individual differences, sex differences, strain and stock differences, early life stress effects, biomarkers, stringent success criteria for drug development, Research Domain Criteria, technological advances, and cross-species comparisons. We conclude that, despite the challenges, animal studies will be pivotal to advances in understanding PTSD and the neurobiology of stress.

Animal models of post-traumatic stress disorder and novel treatment targets

Understanding the neurobiological basis of post-traumatic stress disorder (PTSD) is fundamental to accurately diagnose this neuropathology and offer appropriate treatment options to patients. The lack of pharmacological effects, too often observed with the most currently used drugs, the selective serotonin reuptake inhibitors (SSRIs), makes even more urgent the discovery of new pharmacological approaches. Reliable animal models of PTSD are difficult to establish because of the present limited understanding of the PTSD heterogeneity and of the influence of various environmental factors that trigger the disorder in humans. We summarize knowledge on the most frequently investigated animal models of PTSD, focusing on both their behavioral and neurobiological features. Most of them can reproduce not only behavioral endophenotypes, including anxiety-like behaviors or fear-related avoidance, but also neurobiological alterations, such as glucocorticoid receptor hypersensitivity or amygdala hyperactivity. Among the various models analyzed, we focus on the social isolation mouse model, which reproduces some deficits observed in humans with PTSD, such as abnormal neurosteroid biosynthesis, changes in GABAA receptor subunit expression and lack of pharmacological response to benzodiazepines. Neurosteroid biosynthesis and its interaction with the endocannabinoid system are altered in PTSD and are promising neuronal targets to discover novel PTSD agents. In this regard, we discuss pharmacological interventions and we highlight exciting new developments in the fields of research for novel reliable PTSD biomarkers that may enable precise diagnosis of the disorder and more successful pharmacological treatments for PTSD patients.

Daily administration of Sake Lees (Sake Kasu) reduced psychophysical stress-induced hyperalgesia and Fos responses in the lumbar spinal dorsal horn evoked by noxious stimulation to the hindpaw in the rats

We tested whether Sake Lees (SL) had inhibitory effects on hyperalgesia in the hindpaw under psychophysical stress conditions. Male rats were subjected to repeated forced swim stress treatments (FST) from Day −3 to Day −1. Intraperiotoneal administration of SL which contained low concentration of ethanol (SLX) was conducted after each FST. On Day 0, formalin-evoked licking behaviors and Fos responses in the lumbar spinal cord (DH) and several areas within the rostral ventromedial medulla (RVM) were quantified as nociceptive responses. FST-induced hyperalgesia in the hindpaw was prevented by repeated SL and SLX treatments. Fos expression was significantly increased in DH and some areas within the RVM under FST, which was prevented by repeated SL or SLX. These findings indicated that daily administration of SL had the potential to alleviate stress-induced hyperalgesia.

Hypertrophic Scar Severity at Autograft Sites Is Associated With Increased Pain and Itch After Major Thermal Burn Injury

Approximately three quarters of major thermal burn injury (MThBI) survivors suffer from hypertrophic scarring (HTS) and over half experience chronic pain or itch. In survivors of MThBI, HTS and chronic pain or itch are considered one of the greatest unmet challenges of postburn injury care and psychosocial reintegration. Although scarring, itch, and pain have been clinically associated, there are no prospective, multisite studies examining tissue autograft site pain or itch and scar outcomes. The authors collected a representative cohort (n = 56) of MThBI survivors who received autografting within 14 days of injury and evaluated graft-site pain or itch severity (0-10 Numeric Rating Scale) and HTS using a validated scar photograph assessment scale 6 months following MThBI. Given that stress is known to influence wound healing, the authors also assessed the relationship between previous trauma exposure, peritraumatic stress, preburn overall health (SF-12), scarring, and chronic pain or itch severity using Spearman's correlation. Association between HTS and chronic pain or itch was significant in a linear regression model adjusted for age, sex, and ethnicity (β = 0.2, P = .033 for pain, β = 0.2, P = .019 for itch). Results indicate that prior trauma exposure is inversely correlated (r = -.363, P = .030) with scar severity, but not pain or itch severity 6 months after MThBI. Study results suggest that preburn chronic pain or itch is associated with pathological scarring 6 months following MThBI. Results also indicate that stress may improve scarring after MThBI. Further work to understand the mechanisms that underlie both HTS and chronic pain or itch and their relationship to chronic stress is critical to the development of novel therapies to assist burn survivors recover.

PGE2/EP4 receptor and TRPV1 channel are involved in repeated restraint stress-induced prolongation of sensitization pain evoked by subsequent PGE2 challenge

Prevalence of prior stressful experience is linked to high incidence of chronic pain. Stress, particularly repeated stress, is known to induce maladaptive neuroplasticity along peripheral and central pain transmission pathways. These maladaptive neuroplastic events facilitate sensitization of nociceptive neurons and transition from acute to chronic pain. Pro-inflammatory and pain mediators are involved in inducing neuroplasticity. Pain mediators such as prostaglandin E2 (PGE2), EP4 receptor and transient receptor potential vanilloid-1 (TRPV1) contribute to the genesis of chronic pain. In this study, we examined the role of PGE2/EP4 signaling and TRPV1 signaling in repeated restraint stress-induced prolongation of sensitization pain, a model for transition from acute to chronic pain, in both in vivo and in vitro models. We found that pre-exposure to single restraint stress induced analgesia that masked sensitization pain evoked by subsequent PGE2 challenge. However, pre-exposure to 3d consecutive restraint stress not only prolonged sensitization pain, but also increased stress hormone corticosterone (CORT) in serum, COX2 levels in paw skin, and EP4 and TRPV1 levels in dorsal root ganglion (DRG) and paw skin. Pre-exposure to CORT for 3d, not 1d, also prolonged sensitization pain evoked by PGE2. Co-injection of glucocorticoid receptor (GR) antagonist RU486, COX2 inhibitor NS-398, EP4 receptor antagonist L161,982 or TRPV1 antagonist capsazepine prevented 3d restraint stress prolonged sensitization pain evoked by PGE2. In DRG cultures, CORT increased EP4 and TRPV1 protein levels through GR activation. These data suggest that PGE2/EP4 signaling and TRPV1 signaling in peripheral pain pathway contribute to repeated stress-predisposed transition from acute to chronic pain.

The effects of trauma on brain and body: A unifying role for the midbrain periaqueductal gray

Post-traumatic stress disorder (PTSD), a diagnosis that may follow the experience of trauma, has multiple symptomatic phenotypes. Generally, individuals with PTSD display symptoms of hyperarousal and of hyperemotionality in the presence of fearful stimuli. A subset of individuals with PTSD; however, elicit dissociative symptomatology (i.e., depersonalization, derealization) in the wake of a perceived threat. This pattern of response characterizes the dissociative subtype of the disorder, which is often associated with emotional numbing and hypoarousal. Both symptomatic phenotypes exhibit attentional threat biases, where threat stimuli are processed preferentially leading to a hypervigilant state that is thought to promote defensive behaviors during threat processing. Accordingly, PTSD and its dissociative subtype are thought to differ in their proclivity to elicit active (i.e., fight, flight) versus passive (i.e., tonic immobility, emotional shutdown) defensive responses, which are characterized by the increased and the decreased expression of the sympathetic nervous system, respectively. Moreover, active and passive defenses are accompanied by primarily endocannabinoid- and opioid-mediated analgesics, respectively. Through critical review of the literature, we apply the defense cascade model to better understand the pathological presentation of defensive responses in PTSD with a focus on the functioning of lower-level midbrain and extended brainstem systems.

A refinement to the formalin test in mice

The constant refinement of tests used in animal research is crucial for the scientific community. This is particularly true for the field of pain research, where ethical standards are notably sensitive. The formalin test is widely used in pain research and some of its mechanisms resemble those underlying clinical pain in humans. Immediately upon injection, formalin triggers two waves (an early and a late phase) of strong, nociceptive behaviour, characterised by licking, biting, lifting and shaking the injected paw of the animal. Although well characterised at the behaviour level, since its proposal over four decades ago, there has not been any significant refinement to the formalin test, especially those combining minimisation of animal distress and preservation of behavioural outcomes of the test.  Here, we propose a modified and improved method for the formalin test. We show that anaesthetising the animal with the inhalable anaesthetic sevoflurane at the time of the injection can produce reliable, robust and reproducible results whilst animal distress during the initial phase is reduced. Importantly, our results were validated by pharmacological suppression of the behaviour during the late phase of the test with gabapentin, the anaesthetic showing no interference with the drug. In addition, we demonstrate that this is also a useful method to screen for changes in pain behaviour in response to formalin in transgenic lines.

A refinement to the formalin test in mice

The constant refinement of tests used in animal research is crucial for the scientific community. This is particularly true for the field of pain research, where ethical standards are notably sensitive. The formalin test is widely used in pain research and some of its mechanisms resemble those underlying clinical pain in humans. Immediately upon injection, formalin triggers two waves (an early and a late phase) of strong, nociceptive behaviour, characterised by licking, biting, lifting and shaking the injected paw of the animal. Although well characterised at the behaviour level, since its proposal over four decades ago, there has not been any significant refinement to the formalin test, especially those combining minimisation of animal distress and preservation of behavioural outcomes of the test.  Here, we propose a modified and improved method for the formalin test. We show that anaesthetising the animal with the inhalable anaesthetic sevoflurane at the time of the injection can produce reliable, robust and reproducible results whilst animal distress during the initial phase is reduced. Importantly, our results were validated by pharmacological suppression of the behaviour during the late phase of the test with gabapentin, the anaesthetic showing no interference with the drug. In addition, we demonstrate that this is also a useful method to screen for changes in pain behaviour in response to formalin in transgenic lines.

Animal Models of PTSD: The Socially Isolated Mouse and the Biomarker Role of Allopregnanolone

Post-traumatic stress disorder (PTSD) is a debilitating undertreated condition that affects 8%-13% of the general population and 20%-30% of military personnel. Currently, there are no specific medications that reduce PTSD symptoms or biomarkers that facilitate diagnosis, inform treatment selection or allow monitoring drug efficacy. PTSD animal models rely on stress-induced behavioral deficits that only partially reproduce PTSD neurobiology. PTSD heterogeneity, including comorbidity and symptoms overlap with other mental disorders, makes this attempt even more complicated. Allopregnanolone, a neurosteroid that positively, potently and allosterically modulates GABAA receptors and, by this mechanism, regulates emotional behaviors, is mainly synthesized in brain corticolimbic glutamatergic neurons. In PTSD patients, allopregnanolone down-regulation correlates with increased PTSD re-experiencing and comorbid depressive symptoms, CAPS-IV scores and Simms dysphoria cluster scores. In PTSD rodent models, including the socially isolated mouse, decrease in corticolimbic allopregnanolone biosynthesis is associated with enhanced contextual fear memory and impaired fear extinction. Allopregnanolone, its analogs or agents that stimulate its synthesis offer treatment approaches for facilitating fear extinction and, in general, for neuropsychopathologies characterized by a neurosteroid biosynthesis downregulation. The socially isolated mouse model reproduces several other deficits previously observed in PTSD patients, including altered GABAA receptor subunit subtypes and lack of benzodiazepines pharmacological efficacy. Transdiagnostic behavioral features, including expression of anxiety-like behavior, increased aggression, a behavioral component to reproduce behavioral traits of suicidal behavior in humans, as well as alcohol consumption are heightened in socially isolated rodents. Potentials for assessing novel biomarkers to predict, diagnose, and treat PTSD more efficiently are discussed in view of developing a precision medicine for improved PTSD pharmacological treatments.

'Dis-comfort eating': An investigation into the use of food as a coping strategy for the management of chronic pain

OBJECTIVES

Chronic pain and obesity are major public health concerns. Animal and human models have demonstrated that eating high-sugar nutrient-dense foods confers analgesic effects. Moreover, recent research suggests that people with chronic pain may "comfort eat" to cope with their pain. Given the harmful impact of obesity on chronic pain, it is critical to determine whether pain elicits comfort eating amongst individuals with chronic pain to ensure that this potentially maladaptive pain coping strategy is not overlooked in chronic pain treatment. Therefore, this study aimed to: determine whether chronic pain intensity predicts pain-induced comfort eating and identify mediators of this relationship; to determine whether pain-induced comfort eating predicts elevated BMI; and to establish whether BMI predicts chronic pain interference.

METHODS

This study utilised a cross-sectional online survey design and a sample of 151 adults with chronic pain.

RESULTS

Over three-quarters of this chronic pain sample reported engaging in pain-induced comfort eating. Chronic pain intensity did not significantly predict pain-induced comfort eating. However, there was a significant indirect effect of chronic pain intensity on pain-induced comfort eating through stress-but not experiential avoidance or pain catastrophising. As predicted, pain-induced comfort eating significantly predicted increased BMI, and BMI in turn significantly predicted greater chronic pain interference.

DISCUSSION

This study indicates that pain-induced comfort eating is both common and harmful amongst individuals with chronic pain, across the entire BMI spectrum. Pain-induced comfort eating and stress have emerged as promising chronic pain treatment targets. The findings are discussed and interpreted in light of extant research and theory, as well as limitations of the current study. Future research directions and clinical implications are also considered.

Short-term, low-dose fluoxetine prevents oestrous cycle-linked increase in anxiety-like behaviour in female rats

BACKGROUND AND AIMS

We sought a robust behavioural test that evoked increased anxiety-like behaviour during the late dioestrus phase of the oestrous cycle (similar to the premenstrual period in women) and tested whether this could be prevented by acute low-dose fluoxetine (FLX).

METHODS

Female Wistar rats in different stages of their cycle were exposed to four different tests of anxiety-like behaviour.

RESULTS

No oestrous cycle differences were detected in fear potentiated startle or conditioned freezing to an aversive context. In a light switch-off test where rats move from one compartment of a shuttle-box to the other to turn off an aversive light, females displayed enhanced responding in late dioestrus. During isolation restraint stress females in late dioestrus emitted three times more 22 kHz ultrasound vocalisations (USV) than at other cycle stages. Using the USV test, short-term administration of low-dose FLX (1.75 mg kg^-1, i.p.) designed to blunt the sharp fall in brain allopregnanolone concentration during late dioestrus but without affecting 5-HT systems, prevented the increase in isolation stress-evoked USVs.

CONCLUSIONS

The light switch-off and isolation restraint-induced USV tests evoke unconditioned adverse emotional responses that are ethologically relevant and sensitive to oestrous cycle stage. The USV test fulfils many criteria required of a model for premenstrual syndrome in women. Using the USV test, short-term administration of FLX to increase brain allopregnanolone concentration without affecting 5-HT systems prevented the increased USV responding in late dioestrus. Short-term low-dose FLX treatment may have potential to alleviate development of adverse premenstrual symptoms in women.

Involvement of brain-derived neurotrophic factor (BDNF) in chronic intermittent stress-induced enhanced mechanical allodynia in a rat model of burn pain

BACKGROUND

Reports show that stressful events before injury exacerbates post-injury pain. The mechanism underlying stress-induced heightened thermal pain is unclear. Here, we examined the effects of chronic intermittent stress (CIS) on nociceptive behaviors and brain-derived nerve growth factor (BDNF) system in the prefrontal cortex (PFC) and hypothalamus of rats with and without thermal injury.

RESULTS

Unstressed rats showed transient mechanical allodynia during stress exposure. Stressed rats with thermal injury displayed persistent exacerbated mechanical allodynia (P < 0.001). Increased expression of BDNF mRNA in the PFC (P < 0.05), and elevated TrkB and p-TrkB (P < 0.05) protein levels in the hypothalamus were observed in stressed rats with thermal injury but not in stressed or thermally injured rats alone. Furthermore, administration of CTX-B significantly reduced stress-induced exacerbated mechanical allodynia in thermally injured rats (P < 0.001).

CONCLUSION

These results indicate that BDNF-TrkB signaling in PFC and hypothalamus contributes to CIS-induced exacerbated mechanical allodynia in thermal injury state.

Exodontia-induced muscular hypofunction by itself or associated to chronic stress impairs masseter muscle morphology and its mitochondrial function

Stress is associated with orofacial pain sensitivity and is qualified as a temporomandibular disorder risk factor. During stressful periods, painful thresholds of masticatory muscles in individuals suffering muscle facial pain are significantly lower than in controls, but the exact physiologic mechanism underlying this relation remains unclear. Our hypothesis is that chronic unpredictable stress and masticatory hypofunction induce morphologic and metabolic masseter muscle changes in rats. For test this hypothesis, adult Wistar rats were submitted to chronic unpredictable stress and/or exodontia of left molars and the left masseter muscle was removed for analysis. The parameters evaluated included ultrastructure, oxidative level, metabolism activity and morphological analysis in this muscle. Our data show by histological analysis, that stress and exodontia promoted a variation on diameters and also angled contours in masseter fibers. The masticatory hypofunction increased oxidative metabolism as well as decreased reactive species of oxygen in masseter muscle. The ultrastructural analysis of muscle fibers showed disruption of the sarcoplasmic reticulum cisterns in certain regions of the fiber in stress group, and the disappearance of the sarcoplasmic reticulum membrane in group with association of stress and exodontia. Our findings clarify mechanisms by which chronic stress and masticatory hypofunction might be involved in the pathophysiology of muscular dysfunctions. Masticatory hypofunction influenced oxidative stress and induced oxidative metabolism on masseter muscle, as well as altered its fiber morphology. Chronic stress presented malefic effect on masseter morphology at micro and ultra structurally. When both stimuli were applied, there were atrophic fibers and a complete mitochondrial derangement.

Updates in PTSD Animal Models Characterization

Post-traumatic stress disorder (PTSD) is a chronic, debilitating mental disorder afflicting more than 7% of the US population and 12% of military service members. Since the Afghanistan and Iraq wars, thousands of US service members have returned home with PTSD. Despite recent progress, the molecular mechanisms underlying the pathology of PTSD are poorly understood. To promote research on PTSD (especially its molecular mechanisms) and to set a molecular basis for discovering novel medications for this disorder, well-validated animal models are needed. However, to develop PTSD animal models is a challenging process, due to predisposing factors such as physiological, behavioral, emotional, and cognitive changes that emerge after trauma. Currently, there is no well-validated animal model of PTSD, although several stress paradigms mimic the behavioral symptoms and neurological alterations seen in PTSD. In this chapter, we will provide an overview of animal models of PTSD including learned helplessness, footshock, restraint stress, inescapable tail shock, single-prolonged stress, underwater trauma, social isolation, social defeat, early-life stress, and predator-based stress. We emphasize rodent models because they reproduce some of the behavioral and biotical phenotypes seen in PTSD. We will also present data showing that homologous biological measures are increasingly incorporated in studies to assess markers of risk and therapeutic response in these models. Therefore, PTSD animal models may be refined in hopes of capitalizing on the understanding of the molecular mechanisms and delivering tools in order to develop new and more efficacious treatments for PTSD.

The Unpredictable Chronic Mild Stress Protocol for Inducing Anhedonia in Mice

Depression is a highly prevalent and debilitating condition, only partially addressed by current pharmacotherapies. The lack of response to treatment by many patients prompts the need to develop new therapeutic alternatives and to better understand the etiology of the disorder. Pre-clinical models with translational merits are rudimentary for this task. Here we present a protocol for the unpredictable chronic mild stress (UCMS) method in mice. In this protocol, adolescent mice are chronically exposed to interchanging unpredictable mild stressors. Resembling the pathogenesis of depression in humans, stress exposure during the sensitive period of mice adolescence instigates a depressive-like phenotype evident in adulthood. UCMS can be used for screenings of antidepressants on the variety of depressive-like behaviors and neuromolecular indices. Among the more prominent tests to assess depressive-like behavior in rodents is the sucrose preference test (SPT), which reflects anhedonia (core symptom of depression). The SPT will also be presented in this protocol. The ability of UCMS to induce anhedonia, instigate long-term behavioral deficits and enable reversal of these deficits via chronic (but not acute) treatment with antidepressants strengthens the protocol's validity compared to other animal protocols for inducing depressive-like behaviors.

Pituitary Hormones and Orofacial Pain

Clinical and basic research on regulation of pituitary hormones, extra-pituitary release of these hormones, distribution of their receptors and cell signaling pathways recruited upon receptor binding suggests that pituitary hormones can regulate mechanisms of nociceptive transmission in multiple orofacial pain conditions. Moreover, many pituitary hormones either regulate glands that produce gonadal hormones (GnH) or are regulated by GnH. This implies that pituitary hormones may be involved in sex-dependent mechanisms of orofacial pain and could help explain why certain orofacial pain conditions are more prevalent in women than men. Overall, regulation of nociception by pituitary hormones is a relatively new and emerging area of pain research. The aims of this review article are to: (1) present an overview of clinical conditions leading to orofacial pain that are associated with alterations of serum pituitary hormone levels; (2) discuss proposed mechanisms of how pituitary hormones could regulate nociceptive transmission; and (3) outline how pituitary hormones could regulate nociception in a sex-specific fashion. Pituitary hormones are routinely used for hormonal replacement therapy, while both receptor antagonists and agonists are used to manage certain pathological conditions related to hormonal imbalance. Administration of these hormones may also have a place in the treatment of pain, including orofacial pain. Hence, understanding the involvement of pituitary hormones in orofacial pain, especially sex-dependent aspects of such pain, is essential to both optimize current therapies as well as provide novel and sex-specific pharmacology for a diversity of associated conditions.

Social defeat stress induces hyperalgesia and increases truncated BDNF isoforms in the nucleus accumbens regardless of the depressive-like behavior induction in mice

Epidemiological studies have shown a close association between pain and depression. There is evidence showing this association as patients with depression show a high chronic pain prevalence and vice versa. Considering that social stress is critical for the development of depression in humans, we used a social defeat stress (SDS) model which induces depressive-like behavior in mice. In this model, mice are exposed to an aggressor mouse for ten days, suffering brief periods of agonistic contact and long periods of sensory contact. Some mice display social avoidance, a depressive-like behavior, and are considered susceptible, while some mice do not, and are considered resilient. Thus, we investigated the nociceptive behavior of mice submitted to SDS and the neuroplastic changes in dopaminergic mesolimbic system. Our results showed that the stressed mice (resilient and susceptible) presented a higher sensitivity to pain than the control mice in chemical and mechanical tests. We also verified that susceptible mice have higher Bdnf mRNA in the VTA compared to the resilient and control mice. The stressed mice had less mature BDNF and more truncated BDNF protein in the NAc compared with control mice. Although social stress may trigger the development of depression and hyperalgesia, these two conditions may manifest independently as social stress induced hyperalgesia even in mice that did not display depressive-like behavior. Also, increased Bdnf in the VTA seems to be associated with depressive-like behavior, whereas high levels of truncated BDNF and low mature BDNF appear to be associated with hyperalgesia induced by social defeat stress.

Preclinical Animal Models for Temporomandibular Joint Tissue Engineering

There is a paucity of in vivo studies that investigate the safety and efficacy of temporomandibular joint (TMJ) tissue regeneration approaches, in part due to the lack of established animal models. Review of disease models for study of TMJ is presented herein with an attempt to identify relevant preclinical animal models for TMJ tissue engineering, with emphasis on the disc and condyle. Although degenerative joint disease models have been mainly performed on mice, rats, and rabbits, preclinical regeneration approaches must employ larger animal species. There remains controversy regarding the preferred choice of larger animal models between the farm pig, minipig, goat, sheep, and dog. The advantages of the pig and minipig include their well characterized anatomy, physiology, and tissue properties. The advantages of the sheep and goat are their easier surgical access, low cost per animal, and its high tissue availability. The advantage of the dog is that the joint space is confined, so migration of interpositional devices should be less likely. However, each species has limitations as well. For example, the farm pig has continuous growth until about 18 months of age, and difficult surgical access due to the zygomatic arch covering the lateral aspect of joint. The minipig is not widely available and somewhat costly. The sheep and the goat are herbivores, and their TMJs mainly function in translation. The dog is a carnivore, and the TMJ is a hinge joint that can only rotate. Although no species provides the gold standard for all preclinical TMJ tissue engineering approaches, the goat and sheep have emerged as the leading options, with the minipig as the choice when cost is less of a limitation; and with the dog and farm pig serving as acceptable alternatives. Finally, naturally occurring TMJ disorders in domestic species may be harnessed on a preclinical trial basis as a clinically relevant platform for translation.

Suppression of Urinary Voiding by Conditional High Frequency Stimulation of the Pelvic Nerve in Conscious Rats

Female Wistar rats were instrumented to record bladder pressure and to stimulate the left pelvic nerve. Repeated voids were induced by continuous infusion of saline into the bladder (11.2 ml/h) via a T-piece in the line to the bladder catheter. In each animal tested (n = 6) high frequency pelvic nerve stimulation (1-3 kHz, 1-2 mA sinusoidal waveform for 60 s) applied within 2 s of the onset of a sharp rise in bladder pressure signaling an imminent void was able to inhibit micturition. Voiding was modulated in three ways: (1) Suppression of voiding (four rats, n = 13 trials). No fluid output or a very small volume of fluid expelled (<15% of the volume expected based on the mean of the previous 2 or 3 voids). Voiding suppressed for the entirety of the stimulation period (60 s) and resumed within 37 s of stopping stimulation. (2) Void deferred (four rats, n = 6 trials). The imminent void was suppressed (no fluid expelled) but a void occurred later in the stimulation period (12-44 s, mean 24.5 ± 5.2 s after the onset of the stimulation). (3) Reduction in voided volume (five rats, n = 20 trials). Voiding took place but the volume of fluid voided was 15-80% (range 21.8-77.8%, mean 45.3 ± 3.6%) of the volume expected from the mean of the preceding two or three voids. Spontaneous voiding resumed within 5 min of stopping stimulation. Stimulation during the filling phase in between voids had no effect. The experiments demonstrate that conditional high frequency stimulation of the pelvic nerve started at the onset of an imminent void can inhibit voiding. The effect was rapidly reversible and was not accompanied by any adverse behavioral side effects.