Pain in neuropsychiatry: Insights from animal models

Spared nerve injury decreases motivation in long-access homecage-based operant tasks in mice

ABSTRACT

Neuropathic pain causes both sensory and emotional maladaptation. Preclinical animal studies of neuropathic pain-induced negative affect could result in novel insights into the mechanisms of chronic pain. Modeling pain-induced negative affect, however, is variable across research groups and conditions. The same injury may or may not produce robust negative affective behavioral responses across different species, strains, and laboratories. Here, we sought to identify negative affective consequences of the spared nerve injury model on C57BL/6J male and female mice. We found no significant effect of spared nerve injury across a variety of approach-avoidance conflict, hedonic choice, and coping strategy assays. We hypothesized these inconsistencies may stem in part from the short test duration of these assays. To test this hypothesis, we used the homecage-based Feeding Experimentation Device version 3 to conduct 12-hour, overnight progressive ratio testing to determine whether mice with chronic spared nerve injury had decreased motivation to earn palatable food rewards. Our data demonstrate that despite equivalent task learning, spared nerve injury mice are less motivated to work for a sugar pellet than sham controls. Furthermore, when we normalized behavioral responses across all the behavioral assays we tested, we found that a combined normalized behavioral score is predictive of injury state and significantly correlates with mechanical thresholds. Together, these results suggest that homecage-based operant behaviors provide a useful platform for modeling nerve injury-induced negative affect and that valuable pain-related information can arise from agglomerative data analyses across behavioral assays-even when individual inferential statistics do not demonstrate significant mean differences.

Environmental factors and their impact on chronic pain development and maintenance

It is more than recognized and accepted that the environment affects the physiological responses of all living things, from bacteria to superior vertebrates, constituting an important factor in the evolution of all species. Environmental influences range from natural processes such as sunlight, seasons of the year, and rest to complex processes like stress and other mood disorders, infections, and air pollution, being all of them influenced by how each creature deals with them. In this chapter, it will be discussed how some of the environmental elements affect directly or indirectly neuropathic pain, a type of chronic pain caused by a lesion or disease of the somatosensory nervous system. For that, it was considered the edge of knowledge in translational research, thus including data from human and experimental animals as well as the applicability of such findings.

Betaine eliminates CFA-induced depressive-like behaviour in mice may be through inhibition of microglia and astrocyte activation and polarization

Chronic pain can induce not only nociceptive but also depressive emotions. A previous study demonstrated that betaine, a commonly used nutrient supplement, has an anti-nociceptive effect, but whether betaine can alleviate chronic pain-induced depressive emotion is elusive. Our current study found that betaine administration significantly eliminated complete Freund's adjuvant (CFA)-induced pain-related depressive-like behaviour. Mechanistically, betaine treatment inhibited microglia and astrocyte activation. Furthermore, betaine significantly promoted the transition of microglia from the M1 to the M2 phenotype, as well as the transition of astrocytes from the A1 to the A2 phenotype. Additionally, the release of pro-inflammatory factors such as IL-18, IL-1β and IL-6 and anti-inflammatory factors such as IL-10 in the hippocampus induced by CFA were also reversed by betaine administration. Overall, betaine has therapeutic effects on pain-related depressive-like phenotypes caused by CFA, possibly through altering the polarization of microglia and astrocytes to reduce neuroinflammation.

Janus effect of the anterior cingulate cortex: Pain and emotion

Over the past 20 years, clinical and preclinical studies point to the anterior cingulate cortex (ACC) as a site of interest for several neurological and psychiatric conditions. The ACC plays a critical role in emotion, autonomic regulation, pain processing, attention, memory and decision making. An increasing number of studies have demonstrated the involvement of the ACC in the emotional component of pain and its comorbidity with emotional disorders such as anxiety and depression. Thanks to the development of animal models combined with state-of-the-art technologies, we now have a better mechanistic understanding of the functions of the ACC. Hence, the primary aim of this review is to compile the most recent preclinical studies on the role of ACC in the emotional component and consequences of chronic pain. Herein, we thus thoroughly describe the pain-induced electrophysiological, molecular and anatomical alterations in the ACC and in its related circuits. Finally, we discuss the next steps that are needed to strengthen our understanding of the involvement of the ACC in emotional and pain processing.

Prokineticin System Is a Pharmacological Target to Counteract Pain and Its Comorbid Mood Alterations in an Osteoarthritis Murine Model

Osteoarthritis (OA) is the most prevalent joint disease associated with chronic pain. OA pain is often accompanied by mood disorders. We addressed the role of the Prokineticin (PK) system in pain and mood alterations in a mice OA model induced with monosodium iodoacetate (MIA). The effect of a PK antagonist (PC1) was compared to that of diclofenac. C57BL/6J male mice injected with MIA in the knee joint were characterized by allodynia, motor deficits, and fatigue. Twenty-eight days after MIA, in the knee joint, we measured high mRNA of PK2 and its receptor PKR1, pro-inflammatory cytokines, and MMP13. At the same time, in the sciatic nerve and spinal cord, we found increased levels of PK2, PKR1, IL-1β, and IL-6. These changes were in the presence of high GFAP and CD11b mRNA in the sciatic nerve and GFAP in the spinal cord. OA mice were also characterized by anxiety, depression, and neuroinflammation in the prefrontal cortex and hippocampus. In both stations, we found increased pro-inflammatory cytokines. In addition, PK upregulation and reactive astrogliosis in the hippocampus and microglia reactivity in the prefrontal cortex were detected. PC1 reduced joint inflammation and neuroinflammation in PNS and CNS and counteracted OA pain and emotional disturbances.

Perspectives on pain in Down syndrome

Down syndrome (DS) or trisomy 21 is a genetic condition often accompanied by chronic pain caused by congenital abnormalities and/or conditions, such as osteoarthritis, recurrent infections, and leukemia. Although DS patients are more susceptible to chronic pain as compared to the general population, the pain experience in these individuals may vary, attributed to the heterogenous structural and functional differences in the central nervous system, which might result in abnormal pain sensory information transduction, transmission, modulation, and perception. We tried to elaborate on some key questions and possible explanations in this review. Further clarification of the mechanisms underlying such abnormal conditions induced by the structural and functional differences is needed to help pain management in DS patients.

Hydrogen Sulfide Interacting with Cannabinoid 2 Receptors during Sciatic Nerve Injury-Induced Neuropathic Pain

Hydrogen sulfide (H₂S) donors make opioids more effective in inhibiting nociception during inflammatory and neuropathic pain. We examined whether the analgesic, anxiolytic and/or antidepressant actions of the cannabinoid 2 receptor (CB2R) agonist, JWH-133, might be improved by pretreatment with H₂S donors, DADS and GYY4137 in mice with sciatic nerve injury-provoked neuropathy (CCI). The reversion of the antinociceptive effects of these treatments with the CB2R antagonist, AM630, and the regulatory actions of H₂S in the phosphorylation of NF-κB inhibitor alpha (IKBα) and in the brain-derived neurotrophic factor (BDNF), CB2R, Nrf2 and heme oxygenase 1 (HO-1) levels in prefrontal cortex (PFC), ventral hippocampus (vHIP) and periaqueductal gray matter (PAG), were examined. Data showed that the analgesic effects of JWH-133, systemically and locally administered, were improved by the DADS or GYY4137 pretreatment. The co-treatment of GYY4137 with JWH-133 also stopped anxiodepressive-like activities that concur with neuropathy. Our data likewise showed that both H₂S donors normalized the inflammatory (p-IKBα), neurotrophic (BDNF) variations caused by CCI, increased the expression of CB2R and activated the Nrf2/HO-1 antioxidant pathway in PFC, v-HIP and/or PAG of animals with neuropathic pain. In addition, the blockade of the analgesia produced by high doses of DADS and GYY4137 with AM630 indicated the contribution of the endocannabinoid system in the effects of H₂S during neuropathic pain, thus supporting the positive interaction between H₂S and CB2R. Therefore, this study demonstrates the potential use of CB2R agonists combined with H₂S donors as a possible treatment for peripheral nerve injury-caused neuropathic pain and the associated emotional disturbances.

Synergistic interaction between acetaminophen and L-carnosine improved neuropathic pain via NF-κB pathway and antioxidant properties in chronic constriction injury model

Background

Inflammation is known to underlie the pathogenesis in neuropathic pain. This study investigated the anti-inflammatory and neuroprotective mechanisms involved in antinociceptive effects of co-administration of acetaminophen and L-carnosine in chronic constriction injury (CCI)-induced peripheral neuropathy in male Wistar rats.

Methods

Fifty-six male Wistar rats were randomly divided into seven experimental groups (n = 8) treated with normal saline/acetaminophen/acetaminophen + L-carnosine. CCI was used to induce neuropathic pain in rats. Hyperalgesia and allodynia were assessed using hotplate and von Frey tests, respectively. Investigation of spinal proinflammatory cytokines and antioxidant system were carried out after twenty-one days of treatment.

Results

The results showed that the co-administration of acetaminophen and L-carnosine significantly (P < 0.001) increased the paw withdrawal threshold to thermal and mechanical stimuli in ligated rats compared to the ligated naïve group. There was a significant (P < 0.001) decrease in the levels of nuclear factor kappa light chain enhancer B cell inhibitor, calcium ion, interleukin-1-beta, and tumour necrotic factor-alpha in the spinal cord of the group coadministered with acetaminophen and L-carnosine compared to the ligated control group. Co-administration with acetaminophen and L-carnosine increased the antioxidant enzymatic activities and reduced the lipid peroxidation in the spinal cord.

Conclusions

Co-administration of acetaminophen and L-carnosine has anti-inflammatory effects as a mechanism that mediate its antinociceptive effects in CCI-induced peripheral neuropathy in Wistar rat.

Identifying genetic determinants of inflammatory pain in mice using a large-scale gene-targeted screen

ABSTRACT

Identifying the genetic determinants of pain is a scientific imperative given the magnitude of the global health burden that pain causes. Here, we report a genetic screen for nociception, performed under the auspices of the International Mouse Phenotyping Consortium. A biased set of 110 single-gene knockout mouse strains was screened for 1 or more nociception and hypersensitivity assays, including chemical nociception (formalin) and mechanical and thermal nociception (von Frey filaments and Hargreaves tests, respectively), with or without an inflammatory agent (complete Freund's adjuvant). We identified 13 single-gene knockout strains with altered nocifensive behavior in 1 or more assays. All these novel mouse models are openly available to the scientific community to study gene function. Two of the 13 genes (Gria1 and Htr3a) have been previously reported with nociception-related phenotypes in genetically engineered mouse strains and represent useful benchmarking standards. One of the 13 genes (Cnrip1) is known from human studies to play a role in pain modulation and the knockout mouse reported herein can be used to explore this function further. The remaining 10 genes (Abhd13, Alg6, BC048562, Cgnl1, Cp, Mmp16, Oxa1l, Tecpr2, Trim14, and Trim2) reveal novel pathways involved in nociception and may provide new knowledge to better understand genetic mechanisms of inflammatory pain and to serve as models for therapeutic target validation and drug development.

Glycinergic Modulation of Pain in Behavioral Animal Models

Animal models of human pain conditions allow for detailed interrogation of known and hypothesized mechanisms of pain physiology in awake, behaving organisms. The importance of the glycinergic system for pain modulation is well known; however, manipulation of this system to treat and alleviate pain has not yet reached the sophistication required for the clinic. Here, we review the current literature on what animal behavioral studies have allowed us to elucidate about glycinergic pain modulation, and the progress toward clinical treatments so far. First, we outline the animal pain models that have been used, such as nerve injury models for neuropathic pain, chemogenic pain models for acute and inflammatory pain, and other models that mimic painful human pathologies such as diabetic neuropathy. We then discuss the genetic approaches to animal models that have identified the crucial glycinergic machinery involved in neuropathic and inflammatory pain. Specifically, two glycine receptor (GlyR) subtypes, GlyRα1(β) and GlyRα3(β), and the two glycine transporters (GlyT), GlyT1 and GlyT2. Finally, we review the different pharmacological approaches to manipulating the glycinergic system for pain management in animal models, such as partial vs. full agonism, reversibility, and multi-target approaches. We discuss the benefits and pitfalls of using animal models in drug development broadly, as well as the progress of glycinergic treatments from preclinical to clinical trials.

Pharmacological Potential of Avicennia alba Leaf Extract: An Experimental Analysis Focusing on Antidiabetic, Anti-inflammatory, Analgesic, and Antidiarrheal Activity

Avicennia alba is a mangrove plant that is extensively used to treat severe health issues. This focus of this study was to investigate the antidiabetic, anti-inflammatory, analgesic, and antidiarrheal activities of methanolic extract of A. alba leaves in Swiss albino mouse model. The antidiabetic, anti-inflammatory, analgesic, and antidiarrheal activities of the leaf extract were performed using alloxan-monohydrate, carrageenan-induced paw edema, acetic acid-induced writhing test and the hot plate method, and castor oil-induced method, respectively. The extract was used at doses ranging from 200 to 500 mg/kg to conduct the investigation. Leaf extract at 400 and 500 mg/kg showed potent antidiabetic activity in alloxan-induced diabetic mice. Advanced research is needed to control blood glucose levels and carrageenan paw edema-based anti-inflammatory effects. Both tests showed statistically significant result in a dose-dependent manner. The maximum dose (500 mg/kg) demonstrated potent analgesic activity in both writhing test and hot plate method. The plant extract also showed significant antidiarrheal activity at 400 and 500 mg/kg in experimental mice. However, more research is needed to explore the possible mechanisms and isolate the compounds associated with these bioactivities from the leaf extract of A. alba.

The Antagonism of the Prokineticin System Counteracts Bortezomib Induced Side Effects: Focus on Mood Alterations

The development of neuropathy and of mood alterations is frequent after chemotherapy. These complications, independent from the antitumoral mechanism, are interconnected due to an overlapping in their processing pathways and a common neuroinflammatory condition. This study aims to verify whether in mice the treatment with the proteasome inhibitor bortezomib (BTZ), at a protocol capable of inducing painful neuropathy, is associated with anxiety, depression and supraspinal neuroinflammation. We also verify if the therapeutic treatment with the antagonist of the prokineticin (PK) system PC1, which is known to contrast pain and neuroinflammation, can prevent mood alterations. Mice were treated with BTZ (0.4 mg/kg three times/week for 4 weeks); mechanical allodynia and locomotor activity were evaluated over time while anxiety (dark light and marble burying test), depression (sucrose preference and swimming test) and supraspinal neuroinflammation were checked at the end of the protocol. BTZ treated neuropathic mice develop anxiety and depression. The presence of mood alterations is related to the presence of neuroinflammation and PK system activation in prefrontal cortex, hippocampus and hypothalamus with high levels of PK2 and PKR2 receptor, IL-6 and TNF-α, TLR4 and an upregulation of glial markers. PC1 treatment, counteracting pain, prevented the development of supraspinal inflammation and depression-like behavior in BTZ mice.

Pain and depression comorbidity causes asymmetric plasticity in the locus coeruleus neurons

There is strong comorbidity between chronic pain and depression, although the neural circuits and mechanisms underlying this association remain unclear. By combining immunohistochemistry, tracing studies and western-blotting, with the use of different DREADDs (Designer Receptor Exclusively Activated by Designer Drugs) and behavioural approaches in a rat model of neuropathic pain (chronic constriction injury), we explore how this comorbidity arises. To this end, we evaluated the time-dependent plasticity of noradrenergic-locus coeruleus (LC) neurons relative to the site of injury: ipsilateral (LCipsi) or contralateral (LCcontra) at three different time points: short- (2 days), mid- (7 days), and long-term (30-35 days from nerve injury). Nerve injury led to sensorial hypersensitivity from the onset of injury, whereas depressive-like behavior was only evident following long-term pain. Global chemogenetic blockade of the LCipsi system alone increased short-term pain sensitivity while the blockade of the LCipsi or LCcontra relieved pain-induced depression. The asymmetric contribution of LC-modules was also evident as neuropathy develops. Hence, chemogenetic blockade of the LCipsi→spinal cord projection, increased pain-related behaviours in the short-term. However, this lateralized circuit is not universal as the bilateral chemogenetic inactivation of the LC-rostral anterior cingulate cortex (rACC) pathway or the intra-rACC antagonism of alpha1- and alpha2-adrenoreceptors reversed long-term pain-induced depression. Furthermore, chemogenetic LC to spinal cord activation, mainly through LCipsi, reduced sensorial hypersensitivity irrespective of the time post-injury. Our results indicate that asymmetric activation of specific LC modules promotes early restorative-analgesia, as well as late depressive-like behavior in chronic pain and depression comorbidity.

Comorbid Chronic Pain and Depression: Shared Risk Factors and Differential Antidepressant Effectiveness

The bidirectional relationship between depression and chronic pain is well-recognized, but their clinical management remains challenging. Here we characterize the shared risk factors and outcomes for their comorbidity in the Australian Genetics of Depression cohort study (N = 13,839). Participants completed online questionnaires about chronic pain, psychiatric symptoms, comorbidities, treatment response and general health. Logistic regression models were used to examine the relationship between chronic pain and clinical and demographic factors. Cumulative linked logistic regressions assessed the effect of chronic pain on treatment response for 10 different antidepressants. Chronic pain was associated with an increased risk of depression (OR = 1.86 [1.37-2.54]), recent suicide attempt (OR = 1.88 [1.14-3.09]), higher use of tobacco (OR = 1.05 [1.02-1.09]) and misuse of painkillers (e.g., opioids; OR = 1.31 [1.06-1.62]). Participants with comorbid chronic pain and depression reported fewer functional benefits from antidepressant use and lower benefits from sertraline (OR = 0.75 [0.68-0.83]), escitalopram (OR = 0.75 [0.67-0.85]) and venlafaxine (OR = 0.78 [0.68-0.88]) when compared to participants without chronic pain. Furthermore, participants taking sertraline (OR = 0.45 [0.30-0.67]), escitalopram (OR = 0.45 [0.27-0.74]) and citalopram (OR = 0.32 [0.15-0.67]) specifically for chronic pain (among other indications) reported lower benefits compared to other participants taking these same medications but not for chronic pain. These findings reveal novel insights into the complex relationship between chronic pain and depression. Treatment response analyses indicate differential effectiveness between particular antidepressants and poorer functional outcomes for these comorbid conditions. Further examination is warranted in targeted interventional clinical trials, which also include neuroimaging genetics and pharmacogenomics protocols. This work will advance the delineation of disease risk indicators and novel aetiological pathways for therapeutic intervention in comorbid pain and depression as well as other psychiatric comorbidities.

Neuropathic pain increases spontaneous and noxious-evoked activity of locus coeruleus neurons

The noradrenergic locus coeruleus nucleus is an important station in both the ascending and descending pain regulatory pathways. These neurons discharge in tonic and phasic modes in response to sensory stimuli. However, few studies have set out to characterize the electrophysiological response of the locus coeruleus to noxious stimuli in conditions of neuropathic pain. Thus, the effects of mechanical nociceptive stimulation of the sciatic nerve area on spontaneous (tonic) and sensory-evoked (phasic) locus coeruleus discharge were studied by extracellular recording in anesthetized rats seven, fourteen and twenty-eight days after chronic constriction injury. Minor significant electrophysiological changes were found seven and fourteen days after nerve injury. However, alterations to the spontaneous activity in both the ipsilateral and contralateral locus coeruleus were found twenty-eight days after nerve constriction, as witnessed by an increase of burst firing incidence and irregular firing patterns. Furthermore, noxious-evoked responses were exacerbated in the contralateral and ipsilateral nucleus at twenty-eight days after injury, as were the responses evoked when stimulating the uninjured paw. In addition, mechanical stimulation of the hindpaw produced a significant sensitization of neuronal tonic activity after 28 days of neuropathy. In summary, long-term nerve injury led to higher spontaneous activity and exacerbated noxious-evoked responses in the locus coeruleus to stimulation of nerve-injured and even uninjured hindpaws, coinciding temporally with the development of depressive and anxiogenic-like behavior.

Pra-C exerts analgesic effect through inhibiting microglial activation in anterior cingulate cortex in complete Freund's adjuvant-induced mouse model

Chronic pain is highly prevalent worldwide and severely affects daily lives of patients and family members. Praeruptorin C (Pra-C) is a main active ingredient derived from Peucedanum praeruptorum Dunn, traditionally used as antibechic, anti-bronchitis and anti-hypertension drug. Here, we evaluated the effects of Pra-C in a chronic inflammatory pain mouse model induced by complete Freund's adjuvant (CFA) injection. Pra-C (3 mg/kg) treatment for just 3 days after CFA challenge relieved CFA-induced mechanical allodynia and hindpaw edema in mice. In the anterior cingulate cortex (ACC), Pra-C treatment inhibited microglia activation and reduced levels of proinflammatory cytokines, TNF-α and IL-1β, and suppressed upregulation of glutamate receptors caused by CFA injection. In addition, Pra-C attenuated neuronal hyperexcitability in ACC of CFA-injected mice. In vitro studies confirmed the analgesic effect of Pra-C was due to its inhibitory ability on microglial activation. In conclusion, Pra-C administration had a certain effect on relieving chronic pain by inhibiting microglial activation, attenuating proinflammatory cytokine releasing and regulating excitatory synaptic proteins in the ACC of the CFA-injected mice.

Long-lasting analgesic and neuroprotective action of the non-benzodiazepine anxiolytic etifoxine in a mouse model of neuropathic pain

Neuropathic pain is frequently associated with anxiety and major depressive disorders, which considerably impact the overall patient experience. Favoring GABAergic inhibition through the pain matrix has emerged as a promising strategy to restore proper processing of nociceptive and affective information in neuropathic pain states. In this context, the non-benzodiazepine anxiolytic etifoxine (EFX), known to amplify GABAergic inhibition through positive modulation of GABA_A receptors and neurosteroidogenesis, presents several advantages. Therefore, we sought to investigate the preclinical therapeutic potential of EFX on the somatosensory and affective components of neuropathic pain. Here, we used a murine model in which neuropathic pain was induced by the implantation of a compressive cuff around the sciatic nerve (mononeuropathy). We showed that the intraperitoneal EFX treatment for five consecutive days (50 mg/kg) relieved mechanical allodynia in a sustained manner. Besides its effect on evoked mechanical hypersensitivity, EFX also alleviated aversiveness of ongoing pain as well as anxiodepressive-like consequences of neuropathic pain following cuff-induced mononeuropathy. This effect was also seen 12 weeks after induction of the neuropathy when allodynia was no longer present. Analgesic and neuroprotective actions of EFX were also seen by the absence of neuropathic pain symptoms if a second sciatic nerve constriction injury was applied to the contralateral hindpaw. Mass spectrometry analysis revealed a normalization of brainstem serotonin levels in EFX-treated animals and an increase in norepinephrine. This study suggests that EFX presents promising therapeutic potential for the relief of both somatosensory and affective consequences of neuropathic pain, a beneficial effect that is likely to involve monoamine descending controls.