Contribution of amygdala CRF neurons to chronic pain

Disrupted stress-induced analgesia in a neuropathic pain state is rescued by the endocannabinoid degradation inhibitor JZL195

Acute stress normally engages descending brain pathways to produce an antinociceptive response, known as stress-induced analgesia. Paradoxically, these descending pain modulatory pathways are also involved in the maintenance of the abnormal pain associated with chronic neuropathic pain. It remains unclear how stress-induced analgesia is affected by neuropathic pain states. We therefore examined the impact of a chronic constriction nerve-injury (CCI) model of neuropathic pain on restraint stress-induced analgesia in C57BL/6 mice. Thirty minutes of restraint stress produced analgesia in the hotplate thermal nociceptive assay that was less in CCI compared to control mice who underwent a sham-surgery. In sham but not CCI mice, stress-induced analgesia was reduced by the opioid receptor antagonist naltrexone. The cannabinoid CB1 receptor antagonist AM281 did not affect stress-induced analgesia in either sham or CCI mice. Low-dose pre-treatment with the dual fatty acid amide hydrolase and monoacylglycerol lipase inhibitor JZL195 increased stress-induced analgesia in CCI but not sham mice. The JZL195 enhancement of stress-induced analgesia in CCI mice was abolished by AM281 but was unaffected by naltrexone. These findings indicate that the acute opioid-mediated analgesic response to a psychological stressor is disrupted in a nerve-injury model of neuropathic pain. Importantly, this impairment of stress-induced analgesia was rescued by blockade of endocannabinoid breakdown via a cannabinoid CB1 receptor dependent mechanism. These findings suggest that subthreshold treatment with endocannabinoid degradation blockers could be used to alleviate the disruption of endogenous pain control systems in a neuropathic pain state.

Parabrachial neurons promote nociplastic pain

The parabrachial nucleus (PBN) in the dorsal pons responds to bodily threats and transmits alarm signals to the forebrain. Parabrachial neuron activity is enhanced during chronic pain, and inactivation of PBN neurons in mice prevents the establishment of neuropathic, chronic pain symptoms. Chemogenetic or optogenetic activation of all glutamatergic neurons in the PBN, or just the subpopulation that expresses the Calca gene, is sufficient to establish pain phenotypes, including long-lasting tactile allodynia, that scale with the extent of stimulation, thereby promoting nociplastic pain, defined as diffuse pain without tissue inflammation or nerve injury. This review focuses on the role(s) of molecularly defined PBN neurons and the downstream nodes in the brain that contribute to establishing nociplastic pain.

Mu opioid receptors gate the locus coeruleus pain generator

The locus coeruleus (LC) plays a paradoxical role in chronic pain. Although largely known as a potent source of endogenous analgesia, increasing evidence suggests injury can transform the LC into a chronic pain generator. We sought to clarify the role of this system in pain. Here, we show optogenetic inhibition of LC activity is acutely antinociceptive. Following long-term spared nerve injury, the same LC inhibition is analgesic - further supporting its pain generator function. To identify inhibitory substrates that may naturally serve this function, we turned to endogenous LC mu opioid receptors (LC-MOR). These receptors provide powerful LC inhibition and exogenous activation of LC-MOR is antinociceptive. We therefore hypothesized that endogenous LC-MOR-mediated inhibition is critical to how the LC modulates pain. Using cell type-selective conditional knockout and rescue of LC-MOR receptor signaling, we show these receptors bidirectionally regulate thermal and mechanical hyperalgesia - providing a functional gate on the LC pain generator.

Activation of CRF/CRFR1 Signaling in the Central Nucleus of the Amygdala Contributes to Chronic Stress-Induced Exacerbation of Neuropathic Pain by Enhancing GluN2B-NMDA Receptor-Mediated Synaptic Plasticity in Adult Male Rats

Exacerbation of pain by chronic stress and comorbidity of pain with stress-related disorders such as depression and post-traumatic stress disorder, represent significant clinical challenges. Previously we have documented that chronic forced swim (FS) stress exacerbates neuropathic pain in spared nerve injury (SNI) rats, associated with an up-regulation of GluN2B-containing N-methyl-D-aspartate receptors (GluN2B-NMDARs) in the central nucleus of the amygdala (CeA). However, the molecular mechanisms underlying chronic FS stress (CFSS)-mediated exacerbation of pain sensitivity in SNI rats still remain unclear. In this study, we demonstrated that exposure of CFSS to rats activated the corticotropin-releasing factor (CRF)/CRF receptor type 1 (CRFR1) signaling in the CeA, which was shown to be necessary for CFSS-induced depressive-like symptoms in stressed rats, and as well, for CFSS-induced exacerbation of pain hypersensitivity in SNI rats exposed to chronic FS stress. Furthermore, we discovered that activation of CRF/CRFR1 signaling in the CeA upregulated the phosphorylation of GluN2B-NMDARs at tyrosine 1472 (pGluN2BY1472) in the synaptosomal fraction of CeA, which is highly correlated to the enhancement of synaptic GluN2B-NMDARs expression that has been observed in the CeA in CFSS-treated SNI rats. In addition, we revealed that activation of CRF/CRFR1 signaling in the CeA facilitated the CFSS-induced reinforcement of long-term potentiation as well as the enhancement of NMDAR-mediated excitatory postsynaptic currents in the basolateral amygdala (BLA)-CeA pathway in SNI rats. These findings suggest that activation of CRF/CRFR1 signaling in the CeA contributes to chronic stress-induced exacerbation of neuropathic pain by enhancing GluN2B-NMDAR-mediated synaptic plasticity in rats subjected to nerve injury. PERSPECTIVE: Our present study provides a novel mechanism for elucidating stress-induced hyperalgesia and highlights that the CRF/CRFR1 signaling and the GluN2B-NMDAR-mediated synaptic plasticity in the CeA may be important as potential therapeutic targets for chronic stress-induced pain exacerbation in human neuropathic pain. DATA AVAILABILITY: The data that support the findings of this study are available from the corresponding author upon reasonable request.

CRF regulates pain sensation by enhancement of corticoaccumbal excitatory synaptic transmission

Both peripheral and central corticotropin-releasing factor (CRF) systems have been implicated in regulating pain sensation. However, compared with the peripheral, the mechanisms underlying central CRF system in pain modulation have not yet been elucidated, especially at the neural circuit level. The corticoaccumbal circuit, a structure rich in CRF receptors and CRF-positive neurons, plays an important role in behavioral responses to stressors including nociceptive stimuli. The present study was designed to investigate whether and how CRF signaling in this circuit regulated pain sensation under physiological and pathological pain conditions. Our studies employed the viral tracing and circuit-, and cell-specific electrophysiological methods to label the CRF-containing circuit from the medial prefrontal cortex to the nucleus accumbens shell (mPFCCRF-NAcS) and record its neuronal propriety. Combining optogenetic and chemogenetic manipulation, neuropharmacological methods, and behavioral tests, we were able to precisely manipulate this circuit and depict its role in regulation of pain sensation. The current study found that the CRF signaling in the NAc shell (NAcS), but not NAc core, was necessary and sufficient for the regulation of pain sensation under physiological and pathological pain conditions. This process was involved in the CRF-mediated enhancement of excitatory synaptic transmission in the NAcS. Furthermore, we demonstrated that the mPFCCRF neurons monosynaptically connected with the NAcS neurons. Chronic pain increased the protein level of CRF in NAcS, and then maintained the persistent NAcS neuronal hyperactivity through enhancement of this monosynaptic excitatory connection, and thus sustained chronic pain behavior. These findings reveal a novel cell- and circuit-based mechanistic link between chronic pain and the mPFCCRF → NAcS circuit and provide a potential new therapeutic target for chronic pain.

Pain in Parkinson's disease: a neuroanatomy-based approach

Parkinson's disease is a progressive neurodegenerative disorder characterized by the deposition of misfolded alpha-synuclein in different regions of the central and peripheral nervous system. Motor impairment represents the signature clinical expression of Parkinson's disease. Nevertheless, non-motor symptoms are invariably present at different stages of the disease and constitute an important therapeutic challenge with a high impact for the patients' quality of life. Among non-motor symptoms, pain is frequently experienced by patients, being present in a range of 24-85% of Parkinson's disease population. Moreover, in more than 5% of patients, pain represents the first clinical manifestation, preceding by decades the exordium of motor symptoms. Pain implies a complex biopsychosocial experience with a downstream complex anatomical network involved in pain perception, modulation, and processing. Interestingly, all the anatomical areas involved in pain network can be affected by a-synuclein pathology, suggesting that pathophysiology of pain in Parkinson's disease encompasses a 'pain spectrum', involving different anatomical and neurochemical substrates. Here the various anatomical sites recruited in pain perception, modulation and processing are discussed, highlighting the consequences of their possible degeneration in course of Parkinson's disease. Starting from peripheral small fibres neuropathy and pathological alterations at the level of the posterior laminae of the spinal cord, we then describe the multifaceted role of noradrenaline and dopamine loss in driving dysregulated pain perception. Finally, we focus on the possible role of the intertwined circuits between amygdala, nucleus accumbens and habenula in determining the psycho-emotional, autonomic and cognitive experience of pain in Parkinson's disease. This narrative review provides the first anatomically driven comprehension of pain in Parkinson's disease, aiming at fostering new insights for personalized clinical diagnosis and therapeutic interventions.

Chemogenetic Signaling in Space and Time: Considerations for Designing Neuroscience Experiments Using DREADDs

The use of designer receptors exclusively activated by designer drugs (DREADDs) has led to significant advances in our understanding of the neural circuits that govern behavior. By allowing selective control over cellular activity and signaling, DREADDs have become an integral tool for defining the pathways and cellular phenotypes that regulate sleep, pain, motor activity, goal-directed behaviors, and a variety of other processes. In this review, we provide a brief overview of DREADDs and discuss notable discoveries in the neurosciences with an emphasis on circuit mechanisms. We then highlight methodological approaches to achieve pathway specific activation of DREADDs. Finally, we discuss spatial and temporal constraints of DREADDs signaling and how these features can be incorporated into experimental designs to precisely dissect circuits of interest.

Inhibiting CRF Projections from the Central Amygdala to Lateral Hypothalamus and Amygdala Deletion of CRF Alters Binge-Like Ethanol Drinking in a Sex-Dependent Manner

Background

Binge alcohol drinking is a dangerous pattern of consumption that can contribute to the development of more severe alcohol use disorders (AUDs). Importantly, the rate and severity of AUDs has historically differed between men and women, suggesting that there may be sex differences in the central mechanisms that modulate alcohol (ethanol) consumption. Corticotropin releasing factor (CRF) is a centrally expressed neuropeptide that has been implicated in the modulation of binge-like ethanol intake, and emerging data highlight sex differences in central CRF systems.

Methods

In the present report we characterized CRF+ neurocircuitry arising from the central nucleus of the amygdala (CeA) and innervating the lateral hypothalamus (LH) in the modulation of binge-like ethanol intake in male and female mice.

Results

Using chemogenetic tools we found that silencing the CRF+ CeA to LH circuit significantly blunted binge-like ethanol intake in male, but not female, mice. Consistently, genetic deletion of CRF from neurons of the CeA blunted ethanol intake exclusively in male mice. Furthermore, pharmacological blockade of the CRF type-1 receptor (CRF1R) in the LH significantly reduced binge-like ethanol intake in male mice only, while CRF2R activation in the LH failed to alter ethanol intake in either sex. Finally, a history of binge-like ethanol drinking blunted CRF mRNA in the CeA regardless of sex.

Conclusions

These observations provide novel evidence that CRF+ CeA to LH neurocircuitry modulates binge-like ethanol intake in male, but not female mice, which may provide insight into the mechanisms that guide known sex differences in binge-like ethanol intake.

Intra-accumbal orexinergic system contributes to the stress-induced antinociceptive behaviors in the animal model of acute pain in rats

Stress and pain are interleaved at numerous levels - influencing each other. Stress can increase the nociception threshold in animals, long-known as stress-induced analgesia (SIA). Orexin is known as a neuropeptide that modulates pain. The effect of stress on the mesolimbic system in the modulation of pain is known. The role of the intra-accumbal orexin receptors in the modulation of acute pain by forced swim stress (FSS) is unclear. In this study, 117 adult male albino Wistar rats (270-300 g) were used. The animals were unilaterally implanted with cannulae above the NAc. The antagonist of the orexin-1 receptor (OX1r), SB334867, and antagonist of the orexin-2 receptor (OX2r), TCS OX2 29, were microinjected into the NAc in different doses (1, 3, 10, and 30 nmol/0.5 µl DMSO) before exposure to FSS for a 6-min period. The tail-flick test was carried out as an assay nociception of acute pain, and the nociceptive threshold [tail-flick latency (TFL)] was measured for 60-minute. The findings demonstrated that exposure to acute stress could remarkably increase the TFLs and antinociceptive responses. Moreover, intra-accumbal microinjection of SB334867 or TCS OX2 29 blocked the antinociceptive effect of stress in the tail-flick test. The contribution of orexin receptors was almost equally modulating SIA. The present study's findings suggest that OX1r and OX2r within the NAc modulate stress-induced antinociceptive responses. The intra-accumbal microinjection of orexin receptors antagonists declares inducing antinociceptive responses by FSS in acute pain. Proposedly, intra-accumbla orexinergic receptors have a role in the development of SIA.

Altered volume of the amygdala subregions in patients with chronic low back pain

Background

Neuroimaging studies have suggested a pivotal role for the amygdala involvement in chronic low back pain (CLBP). However, the relationship between the amygdala subregions and CLBP has not yet been delineated. This study aimed to analyze whether the amygdala subregions were linked to the development of CLBP.

Methods

A total of 45 patients with CLBP and 45 healthy controls (HCs) were included in this study. All subjects were asked to complete a three-dimensional T1-weighted magnetic resonance imaging (3D-T1 MRI) scan. FreeSurfer 7.3.2 was applied to preprocess the structural MRI images and segment the amygdala into nine subregions. Afterwards, comparisons were made between the two groups in terms of the volumes of the amygdala subregions. Correlation analysis is utilized to examine the relationship between the amygdala subregion and the scale scores, as well as the pain duration in patients with CLBP. Additionally, logistic regression was used to explore the risk of the amygdala and its subregions for CLBP.

Results

In comparison to HCs, patients with CLBP exhibited a significant enlargement of the left central nucleus (Ce) and left cortical nucleus (Co). Furthermore, the increased volume of the left Ce was associated with a higher risk of CLBP.

Conclusion

Our study suggests that the left Ce and left Co may be involved in the pathophysiological processes of CLBP. Moreover, the volume of the left Ce may be a biomarker for detecting the risk of CLBP.

The integrative role of orexin-1 and orexin-2 receptors within the hippocampal dentate gyrus in the modulation of the stress-induced antinociception in the formalin pain test in the rat

The stressful experiences, by triggering a cascade of hormonal and neural changes, can produce antinociception commonly referred to as stress-induced antinociception (SIA). Orexin neuropeptides have an essential role in stress responses and pain modulation. The dentate gyrus receives orexinergic projections and has been shown to be involved in pain processing. The current study investigated the possible role of orexin-1 and orexin-2 receptors (OX1r and OX2r, respectively) within the dentate gyrus in SIA in a rat model of formalin-induced pain behavior in one hind paw. Male Wistar rats weighing 230-250 g underwent stereotaxic surgery and a cannula was implanted in their brains, above the dentate gyrus region. Either SB334867 or TCS OX2 29 (OX1r and OX2r antagonists, respectively) was microinjected into the dentate gyrus region at a range of doses at 1, 3, 10, and 30 nmol (control group received DMSO 12% as vehicle), 5 min before the forced swim stress (FSS) exposure. The formalin test was performed to assess pain-related behaviors. The results indicated that FSS exposure relieves pain-related behavior in the early and late phases of the formalin test. Blockade of intra-dentate gyrus OX1 or OX2 receptors reduced the antinociceptive responses induced by FSS in the formalin test, with more impact during the late phase. Our findings support the potential role of intra-dentate gyrus orexin receptors as target sites of orexin neurons in painful and stressful situations. Therefore, understanding the exact mechanisms of SIA and the role of the orexinergic system in this phenomenon can lead to identifying the strategies to guide future research and offer a new approach to discovering new pain therapeutic agents.

Methylphenidate attenuates signs of evoked neuropathic pain in animal model

Methylphenidate is a psychostimulant that increases dopamine and noradrenaline levels. Recent studies have shown that methylphenidate potentiates the effect of morphine and together suppress acute and chronic pain. In clinical practice, methylphenidate has been used as a treatment for ADHD and changes of pain threshold have been noted in these patients. The aim of this study was to determine the effect of methylphenidate in an animal model of peripheral neuropathic pain. Neuropathic pain was modeled by the chronic constriction of the sciatic nerve (CCI) in Wistar rats. We evaluated the effect of methylphenidate (1 mg/kg, s.c.) on evoked pain (reflex tests - plantar test, vonFrey test and operant test - thermal place preference) and on spontaneous pain (conditioned place preference). CCI induced thermal, mechanical and cold hyperalgesia/allodynia. Methyphenidate suppressed mechanical and cold hyperalgesia/allodynia, while had no effect on thermal one. Therefore, methylphenidate seems to be a new potential pharmacotherapy for the treatment of neuropathic pain.

Perinatal N(G)-Nitro-L-arginine methyl ester administration decreases anxiety- and depression-like behaviors in adult mice

OBJECTIVE

We hypothesized that perinatal manipulations of the nitrergic system would affect adult animal behaviors.

METHODS

We tested this hypothesis by perinatally administering N(G)-Nitro-L-arginine methyl ester (L-NAME), a non-specific antagonist of nitric oxide synthase for 15 days and assessed anxiety- and depression-like behaviors in adult mice. At 70 days of age, the mice were subjected to a battery of tests consisting of the open-field, light/dark box, forced swim, and tail-flick tests. The tests were performed at two-day intervals, and the order of the tests within the battery was determined according to the progressive invasiveness degree.

RESULTS

L-NAME-treated animals exhibited decreased anxiety-like behavior in the light/dark box and open field tests, with no change in locomotor activity. Additionally, they demonstrated decreased depression-like behavior in the forced swim test and no change in pain perception in the tail-flick test.

CONCLUSION

The nitrergic system is possibly involved in neural circuitry development that regulates behaviors since blocking perinatal nitric oxide production decreases anxiety- and depression-like behaviors in adult mice.

Hmgb1 Silencing in the Amygdala Inhibits Pain-Related Behaviors in a Rat Model of Neuropathic Pain

Chronic pain presents a therapeutic challenge due to the highly complex interplay of sensory, emotional-affective and cognitive factors. The mechanisms of the transition from acute to chronic pain are not well understood. We hypothesized that neuroimmune mechanisms in the amygdala, a brain region involved in the emotional-affective component of pain and pain modulation, play an important role through high motility group box 1 (Hmgb1), a pro-inflammatory molecule that has been linked to neuroimmune signaling in spinal nociception. Transcriptomic analysis revealed an upregulation of Hmgb1 mRNA in the right but not left central nucleus of the amygdala (CeA) at the chronic stage of a spinal nerve ligation (SNL) rat model of neuropathic pain. Hmgb1 silencing with a stereotaxic injection of siRNA for Hmgb1 into the right CeA of adult male and female rats 1 week after (post-treatment), but not 2 weeks before (pre-treatment) SNL induction decreased mechanical hypersensitivity and emotional-affective responses, but not anxiety-like behaviors, measured 4 weeks after SNL. Immunohistochemical data suggest that neurons are a major source of Hmgb1 in the CeA. Therefore, Hmgb1 in the amygdala may contribute to the transition from acute to chronic neuropathic pain, and the inhibition of Hmgb1 at a subacute time point can mitigate neuropathic pain.

The stress-induced antinociceptive responses to the persistent inflammatory pain involve the orexin receptors in the nucleus accumbens

Stress suppresses the sense of pain, a physiological phenomenon known as stress-induced analgesia (SIA). Brain orexin peptides regulate many physiological functions, including wakefulness and nociception. The contribution of the orexinergic system within the nucleus accumbens (NAc) in the modulation of antinociception induced by forced swim stress (FSS) remains unclear. The present study addressed the role of intra-accumbal orexin receptors in the antinociceptive responses induced by FSS during the persistent inflammatory pain model in the rat. Stereotaxic surgery was performed unilaterally on 106 adult male Wistar rats weighing 250-305 g. Different doses (1, 3, 10, and 30 nmol/ 0.5 μl DMSO) of orexin-1 receptor (OX1r) antagonist (SB334867) or OX2 receptor antagonist (TCS OX2 29) were administered into the NAc five minutes before exposure to FSS for a 6-min period. The formalin test was carried out using formalin injection (50 μl; 2.5%) into the rat's hind paw plantar surface, which induces biphasic pain-related responses. The first phase begins immediately after formalin infusion and takes 3-5 min. Subsequently, the late phase begins 15-20 min after formalin injection and takes 20-40 min. The findings demonstrated that intra-accumbal microinjection of SB334867 or TCS OX2 29 attenuated the FSS-induced antinociception in both phases of the formalin test, with the TCS OX2 29 showing higher potency. Moreover, the effect of TCS OX2 29 was more significant during the early phase of the formalin test. The results suggest that OX1 and OX2 receptors in the NAc might modulate the antinociceptive responses induced by the FSS.

A Parabrachial-to-Amygdala Circuit That Determines Hemispheric Lateralization of Somatosensory Processing

BACKGROUND

The central amygdala (CeA) is a bilateral hub of pain and emotional processing with well-established functional lateralization. We reported that optogenetic manipulation of neural activity in the left and right CeA has opposing effects on bladder pain.

METHODS

To determine the influence of calcitonin gene-related peptide (CGRP) signaling from the parabrachial nucleus on this diametrically opposed lateralization, we administered CGRP and evaluated the activity of CeA neurons in acute brain slices as well as the behavioral signs of bladder pain in the mouse.

RESULTS

We found that CGRP increased firing in both the right and left CeA neurons. Furthermore, we found that CGRP administration in the right CeA increased behavioral signs of bladder pain and decreased bladder pain-like behavior when administered in the left CeA.

CONCLUSIONS

These studies reveal a parabrachial-to-amygdala circuit driven by opposing actions of CGRP that determines hemispheric lateralization of visceral pain.

Chemogenetic inhibition of corticotropin-releasing factor neurons in the central amygdala alters binge-like ethanol consumption in male mice

Repetitive bouts of binge drinking can lead to neuroplastic events that alter ethanol's pharmacologic effects and perpetuate excessive consumption. The corticotropin-releasing factor (CRF) system is an example of ethanol-induced neuroadaptations that drive excessive ethanol consumption. Our laboratory has previously shown that CRF antagonist, when infused into the central amygdala (CeA), reduces binge-like ethanol consumption. The present study extends this research by assessing the effects of silencing CRF-producing neurons in CeA on binge-like ethanol drinking stemming from "Drinking in the Dark" (DID) procedures. CRF-ires-Cre mice underwent surgery to infuse Gi/o-coupled Designer Receptors Exclusively Activated by Designer Drugs (DREADD) virus or a control virus into either the CeA or basolateral amygdala (BLA). Gi/o-DREADD-induced CRF-neuronal inhibition in the CeA resulted in a 33% decrease in binge-like ethanol consumption. However, no effect on ethanol consumption was seen after DREADD manipulation in the BLA. Moreover, CeA CRF-neuronal inhibition had no effect on sucrose consumption. The effects of silencing CRF neurons in the CeA on ethanol consumption are not secondary to changes in motor function or anxiety-like behaviors as assessed in the open-field test (OFT). Finally, the DREADD construct's functional ability to inhibit CRF-neuronal activity was demonstrated by reduced ethanol-induced c-Fos following DREADD activation. Together, these data suggest that the CRF neurons in the CeA play an important role in binge ethanol consumption and that inhibition of the CRF-signaling pathway remains a viable target for manipulating binge-like ethanol consumption. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

Antagonism of the orexin receptors in the ventral tegmental area diminished the stress-induced analgesia in persistent inflammatory pain

As a part of descending pain inhibitory system, orexin (OXs) in the ventral tegmental area (VTA) are implicated in nociceptive responses. The current study aimed to evaluate the role of OX receptors (OXRs) in the VTA in stress-induced analgesia in persistent inflammatory pain. Ninety-nine adult male Wistar rats underwent forced swim stress (FSS) following intra-VTA infusion of various doses of SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol/0.3 μL) as an OX1R or OX2R antagonist, respectively. The nociceptive threshold was evaluated using the formalin test as an animal model of persistent inflammatory pain. Current results demonstrated FSS as acute stress produced analgesic responses in the persistent inflammatory pain. Moreover, either OX1R or OX2R antagonist infusion in the VTA hindered the FSS-induced analgesia in both early and late phases. The inhibitory effect of SB334768 in the FSS-induced analgesia was stronger than TCS OX2 29 in both early and late phases of the formalin test. Neither SB334768 nor TCS OX2 29 alone affects pain-related behaviors in formalin tests. Intra-VTA microinjection of each treatment could not modify locomotion in rats. The findings suggest that OX1R and OX2R in the VTA are implicated in FSS-induced analgesia mechanisms.

Blocking the dopaminergic receptors in the hippocampal dentate gyrus reduced the stress-induced analgesia in persistent inflammatory pain in the rat

Although the dentate gyrus (DG) as a component of the hippocampal formation has been well known for its role in memory, various studies showed a diverse population of unique cell types and various inputs and outputs in this region. Besides, brain dopamine is known for its roles in reward, motivation, pleasure, and being involved in the pain process. Further, previous studies demonstrated the participation of DG dopaminergic receptors in antinociception induced by lateral hypothalamus stimulation. This study aimed to investigate the role of DG dopaminergic receptors (D1- and D2-like dopamine receptors) in stress-induced analgesia (SIA) using the formalin test as a persistent inflammatory pain model. One hundred two male Wistar rats were unilaterally implanted with a cannula into the DG. Animals received an intra-DG infusion of SCH23390 (0.25, 1, and 4 μg/rat), or Sulpiride (0.25, 1, and 4 μg/rat) as D1- and D2-like dopamine receptor antagonists, respectively, five min before exposure to forced swim stress (FSS). Ten minutes after FSS termination, 2.5% formalin solution as an inflammatory agent was subcutaneously injected into the plantar surface of the hind paw, and the pain score was quantified for one hour. The findings revealed that exposure to FSS produced SIA, though this FSS-induced analgesia was attenuated in the early and late phase of the formalin test by intra-DG microinjection of SCH23390 or Sulpiride. These results suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in analgesia induced by FSS.

Superior control of inflammatory pain by corticotropin-releasing factor receptor 1 via opioid peptides in distinct pain-relevant brain areas

Background

Under inflammatory conditions, the activation of corticotropin-releasing factor (CRF) receptor has been shown to inhibit pain through opioid peptide release from immune cells or neurons. CRF’s effects on human and animal pain modulation depend, however, on the distribution of its receptor subtypes 1 and 2 (CRF-R1 and CRF-R2) along the neuraxis of pain transmission. The objective of this study is to investigate the respective role of each CRF receptor subtype on centrally administered CRF-induced antinociception during inflammatory pain.

Methods

The present study investigated the role of intracerebroventricular (i.c.v.) CRF receptor agonists on nociception and the contribution of cerebral CRF-R1 and/or CRF-R2 subtypes in an animal model of Freund’s complete adjuvant (FCA)-induced hind paw inflammation. Methods used included behavioral experiments, immunofluorescence confocal analysis, and reverse transcriptase-polymerase chain reaction.

Results

Intracerebroventricular, but systemically inactive, doses of CRF elicited potent, dose-dependent antinociceptive effects in inflammatory pain which were significantly antagonized by i.c.v. CRF-R1-selective antagonist NBI 27914 (by approximately 60%) but less by CRF-R2-selective antagonist K41498 (by only 20%). In line with these findings, i.c.v. administration of CRF-R1 agonist stressin I produced superior control of inflammatory pain over CRF-R2 agonist urocortin-2. Intriguingly, i.c.v. opioid antagonist naloxone significantly reversed the CRF as well as CRF-R1 agonist-elicited pain inhibition. Consistent with existing evidence of high CRF concentrations in brain areas such as the thalamus, hypothalamus, locus coeruleus, and periaqueductal gray following its i.c.v. administration, double-immunofluorescence confocal microscopy demonstrated primarily CRF-R1-positive neurons that expressed opioid peptides in these pain-relevant brain areas. Finally, PCR analysis confirmed the predominant expression of the CRF-R1 over CRF-R2 in representative brain areas such as the hypothalamus.

Conclusion

Taken together, these findings suggest that CRF-R1 in opioid-peptide-containing brain areas plays an important role in the modulation of inflammatory pain and may be a useful therapeutic target for inflammatory pain control.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02498-8.

Developmental-Based Classification of Enkephalin and Somatostatin Containing Neurons of the Chicken Central Extended Amygdala

The central extended amygdala, including the lateral bed nucleus of the stria terminalis and the central amygdala, plays a key role in stress response. To understand how the central extended amygdala regulates stress it is essential to dissect this structure at molecular, cellular and circuit levels. In mammals, the central amygdala contains two distinct cell populations that become active (on cells) or inactive (off cells) during the conditioned fear response. These two cell types inhibit each other and project mainly unidirectionally to output cells, thus providing a sophisticated regulation of stress. These two cell types express either protein kinase C-delta/enkephalin or somatostatin, and were suggested to originate in different embryonic domains of the subpallium that respectively express the transcription factors Pax6 or Nkx2.1 during development. The regulation of the stress response by the central extended amygdala is poorly studied in non-mammals. Using an evolutionary developmental neurobiology approach, we previously identified several subdivisions in the central extended amygdala of chicken. These contain Pax6, Islet1 and Nkx2.1 cells that originate in dorsal striatal, ventral striatal or pallidopreoptic embryonic divisions, and also contain neurons expressing enkephalin and somatostatin. To know the origin of these cells, in this study we carried out multiple fluorescent labeling to analyze coexpression of different transcription factors with enkephalin or somatostatin. We found that many enkephalin cells coexpress Pax6 and likely derive from the dorsal striatal division, resembling the off cells of the mouse central amygdala. In contrast, most somatostatin cells coexpress Nkx2.1 and derive from the pallidal division, resembling the on cells. We also found coexpression of enkephalin and somatostatin with other transcription factors. Our results show the existence of multiple cell types in the central extended amygdala of chicken, perhaps including on/off cell systems, and set the basis for studying the role of these cells in stress regulation.

Generation of a CRF1-Cre transgenic rat and the role of central amygdala CRF1 cells in nociception and anxiety-like behavior

Corticotropin-releasing factor type-1 (CRF1) receptors are critical to stress responses because they allow neurons to respond to CRF released in response to stress. Our understanding of the role of CRF1-expressing neurons in CRF-mediated behaviors has been largely limited to mouse experiments due to the lack of genetic tools available to selectively visualize and manipulate CRF1+ cells in rats. Here, we describe the generation and validation of a transgenic CRF1-Cre-tdTomato rat. We report that Crhr1 and Cre mRNA expression are highly colocalized in both the central amygdala (CeA), composed of mostly GABAergic neurons, and in the basolateral amygdala (BLA), composed of mostly glutamatergic neurons. In the CeA, membrane properties, inhibitory synaptic transmission, and responses to CRF bath application in tdTomato+ neurons are similar to those previously reported in GFP+ cells in CRFR1-GFP mice. We show that stimulatory DREADD receptors can be targeted to CeA CRF1+ cells via virally delivered Cre-dependent transgenes, that transfected Cre/tdTomato+ cells are activated by clozapine-n-oxide in vitro and in vivo, and that activation of these cells in vivo increases anxiety-like and nocifensive behaviors. Outside the amygdala, we show that Cre-tdTomato is expressed in several brain areas across the brain, and that the expression pattern of Cre-tdTomato cells is similar to the known expression pattern of CRF1 cells. Given the accuracy of expression in the CRF1-Cre rat, modern genetic techniques used to investigate the anatomy, physiology, and behavioral function of CRF1+ neurons can now be performed in assays that require the use of rats as the model organism.

Single-cell reconstruction reveals input patterns and pathways into corticotropin-releasing factor neurons in the central amygdala in mice

Corticotropin-releasing factor (CRF) neurons are one of the most densely distributed cell types in the central amygdala (CeA), and are involved in a wide range of behaviors including anxiety and learning. However, the fundamental input circuits and patterns of CeA-CRF neurons are still unclear. Here, we generate a monosynaptic-input map onto CeA-CRF neurons at single-cell resolution via a retrograde rabies-virus system. We find all inputs are located in 44 nested subregions that directly innervate CeA-CRF neurons; most of them are top-down convergent inputs expressing Ca²⁺/calmodulin-dependent protein kinase II, and are centralized in cortex, especially in the layer 4 of the somatosensory cortex, which may directly relay information from the thalamus. While the bottom-up divergent inputs have the highest proportion of glutamate decarboxylase expression. Finally, en passant structures of single input neuron are revealed by in-situ reconstruction in a modified 3D-reference atlas, represented by a Periaqueductal gray-Subparafascicular nucleus-Subthalamic nucleus-Globus pallidus-Caudoputamen-CeA pathway. Taken together, our findings provide morphological and connectivity properties of inputs onto CeA-CRF neurons, which may provide insights for future studies interrogating circuit mechanisms of CeA-CRF neurons in mediating various functions.

Viral retrograde tracing identifies input regions and patterns into the corticotropin releasing factor-expressing neurons in central amygdala, providing an important resource to disentangle the role of these cells in fear and anxiety.

Orexin receptors in the CA1 region of hippocampus modulate the stress-induced antinociceptive responses in an animal model of persistent inflammatory pain

Stress activates multiple neural pathways and neurotransmitters that often suppress pain perception, the phenomenon called stress-induced analgesia (SIA). Orexin neurons from the lateral hypothalamus project to entire brain structures such as the hippocampus. The present study examined this hypothesis that orexinergic receptors in the CA1 region of the hippocampus may play a modulatory role in the development of SIA in formalin test as an animal model of persistent inflammatory pain. One hundred-two adult male Wistar rats were administered with intra-CA1 orexin-1 receptor (OX1r) antagonist, SB334867, at the doses of 3, 10, 30, and 100 nmol or TCS OX2 29 as orexin-2 receptor (OX2r) antagonist at the doses of 1, 3, 10, and 30 nmol. Five min later, rats were exposed to forced swim stress (FSS) for a 6-min period. Then, pain-related behaviors induced by formalin injection were measured at the 5-min blocks during a 60-min period of formalin test. The current study indicated that solely stress exposure elicits antinociception in the early and late phases of the formalin test. The FSS-induced analgesia was prevented by intra-CA1 administration of SB334867 or TCS OX2 29 during either phase of the formalin test. Moreover, the contribution of the OX2r in the mediation of analgesic effect of stress was more prominent than that of the OX1r during both phases of the formalin test. It is suggested that OX1r and OX2r in the CA1 region of the hippocampus are involved in stress-induced analgesia in the animal model of persistent inflammatory pain.

Modulatory role of the orexin system in stress-induced analgesia: Involvement of the ventral tegmental area

BACKGROUND

Exposure to stressful experiences is often accompanied by suppressing pain perception, referred to as stress-induced analgesia. The neuropeptides orexins are essential in regulating the mechanism that responds to stressful and painful stimuli. Meanwhile, the ventral tegmental area (VTA), as a part of descending pain inhibitory system, responds to noxious stimuli. This study aimed to investigate the role of intra-VTA administration of orexin receptor antagonists on stress-induced antinociceptive responses in the animal model of acute pain.

METHOD

Ninety-three adult Wistar rats weighing 230-250 g were unilaterally implanted by a cannulae above the VTA. Animals were pretreated with different doses (1, 3, 10 and 30 nM/0.3 μl) of SB334867 as the orexin-1 receptor antagonist and TCS OX2 29 as the orexin-2 receptor antagonist into the VTA, just 5 min before 6 min exposure to forced swim stress (FSS). Nociceptive threshold was measured using the tail-flick test as a model of acute pain.

RESULTS

The results showed that exposure to FSS could significantly increase analgesic responses. Moreover, intra-VTA administration of SB334768 and TCS OX2 29 blocked the antinociceptive effect of FSS in the tail-flick test.

CONCLUSION

The findings suggest that OX1 and OX2 receptors in the VTA might modulate the antinociceptive behaviours induced by FSS in part.

SIGNIFICANCE

Acute exposure to physical stress suppresses pain-related behaviors in the animal model of acute pain. Blockade of the OX1 and OX2 receptors in the VTA attenuates antinociceptive responses induced by FSS. The contribution of the OX2 receptors in the VTA is more predominant than OX1 receptors in stress-induced analgesia.

The role of medial prefrontal cortex projections to locus ceruleus in mediating the sex differences in behavior in mice with inflammatory pain

We tested the hypothesis that the cognitive impairment associated with inflammatory pain may result from dysregulation of the top-down control of locus ceruleus's (LC) activity by the medial prefrontal cortex (mPFC). Injection of complete Freund's adjuvant (CFA) served as a model for inflammatory pain. The CFA injection decreased the thermal thresholds in both sexes but only the male mice showed increased anxiety-like behavior and diminished cognition, while the females were not affected. Increased calcium fluorescence, a marker for neuronal activity, was detected by photometry in the mPFC of males but not in females with CFA. Next, while chemogenetic inhibition of the projections from the mPFC to the LC improved the object recognition memory of males with pain, the inhibition of the mPFC to LC pathway in female mice produced anxiolysis and spatial memory deficits. The behavior results prompted us to compare the reciprocal innervation of mPFC and LC between the sexes. We used an anterograde transsynaptic tagging technique, which relies on postsynaptic cre transfer, to assess the innervation of LC by mPFC efferents. The males showed a higher rate of postsynaptic cre transfer into LC neurons from mPFC efferents than the females. And vice versa, a retrograde tracing experiment demonstrated that LC to mPFC projection neurons were more numerous in females when compared to males. In conclusion, we provide evidence that subtle differences in the reciprocal neuronal circuit between the LC and mPFC may contribute to sex differences associated with the adverse cognitive effects of inflammatory pain.

Optogenetic Manipulations of Amygdala Neurons Modulate Spinal Nociceptive Processing and Behavior Under Normal Conditions and in an Arthritis Pain Model

The amygdala is an important neural substrate for the emotional–affective dimension of pain and modulation of pain. The central nucleus (CeA) serves major amygdala output functions and receives nociceptive and affected–related information from the spino-parabrachial and lateral–basolateral amygdala (LA–BLA) networks. The CeA is a major site of extra–hypothalamic expression of corticotropin releasing factor (CRF, also known as corticotropin releasing hormone, CRH), and amygdala CRF neurons form widespread projections to target regions involved in behavioral and descending pain modulation. Here we explored the effects of modulating amygdala neurons on nociceptive processing in the spinal cord and on pain-like behaviors, using optogenetic activation or silencing of BLA to CeA projections and CeA–CRF neurons under normal conditions and in an acute pain model. Extracellular single unit recordings were made from spinal dorsal horn wide dynamic range (WDR) neurons, which respond more strongly to noxious than innocuous mechanical stimuli, in normal and arthritic adult rats (5–6 h postinduction of a kaolin/carrageenan–monoarthritis in the left knee). For optogenetic activation or silencing of CRF neurons, a Cre–inducible viral vector (DIO–AAV) encoding channelrhodopsin 2 (ChR2) or enhanced Natronomonas pharaonis halorhodopsin (eNpHR_3.0) was injected stereotaxically into the right CeA of transgenic Crh–Cre rats. For optogenetic activation or silencing of BLA axon terminals in the CeA, a viral vector (AAV) encoding ChR2 or eNpHR_3.0 under the control of the CaMKII promoter was injected stereotaxically into the right BLA of Sprague–Dawley rats. For wireless optical stimulation of ChR2 or eNpHR_3.0 expressing CeA–CRF neurons or BLA–CeA axon terminals, an LED optic fiber was stereotaxically implanted into the right CeA. Optical activation of CeA–CRF neurons or of BLA axon terminals in the CeA increased the evoked responses of spinal WDR neurons and induced pain-like behaviors (hypersensitivity and vocalizations) under normal condition. Conversely, optical silencing of CeA–CRF neurons or of BLA axon terminals in the CeA decreased the evoked responses of spinal WDR neurons and vocalizations, but not hypersensitivity, in the arthritis pain model. These findings suggest that the amygdala can drive the activity of spinal cord neurons and pain-like behaviors under normal conditions and in a pain model.

Capsazepine decreases corneal pain syndrome in severe dry eye disease

Background

Dry eye disease (DED) is a multifactorial disease of the ocular surface accompanied by neurosensory abnormalities. Here, we evaluated the effectiveness of transient receptor potential vanilloid-1 (TRPV1) blockade to alleviate ocular pain, neuroinflammation, and anxiety-like behavior associated with severe DED.

Methods

Chronic DED was induced by unilateral excision of the Harderian and extraorbital lacrimal glands of adult male mice. Investigations were conducted at 21 days after surgery. The mRNA levels of TRPV1, transient receptor potential ankyrin-1 (TRPA1), and acid-sensing ion channels 1 and 3 (ASIC1 and ASIC3) in the trigeminal ganglion (TG) were evaluated by RNAscope in situ hybridization. Multi-unit extracellular recording of ciliary nerve fiber activity was used to monitor spontaneous and stimulated (cold, heat, and acid) corneal nerve responsiveness in ex vivo eye preparations. DED mice received topical instillations of the TRPV1 antagonist (capsazepine) twice a day for 2 weeks from d7 to d21 after surgery. The expression of genes involved in neuropathic and inflammatory pain was evaluated in the TG using a global genomic approach. Chemical and mechanical corneal nociception and spontaneous ocular pain were monitored. Finally, anxiety-like behaviors were assessed by elevated plus maze and black and white box tests.

Results

First, in situ hybridization showed DED to trigger upregulation of TRPV1, TRPA1, ASIC1, and ASIC3 mRNA in the ophthalmic branch of the TG. DED also induced overexpression of genes involved in neuropathic and inflammatory pain in the TG. Repeated instillations of capsazepine reduced corneal polymodal responsiveness to heat, cold, and acidic stimulation in ex vivo eye preparations. Consistent with these findings, chronic capsazepine instillation inhibited the upregulation of genes involved in neuropathic and inflammatory pain in the TG of DED animals and reduced the sensation of ocular pain, as well as anxiety-like behaviors associated with severe DED.

Conclusion

These data provide novel insights on the effectiveness of TRPV1 antagonist instillation in alleviating abnormal corneal neurosensory symptoms induced by severe DED, opening an avenue for the repositioning of this molecule as a potential analgesic treatment for patients suffering from chronic DED.

Amygdalar Corticotropin-Releasing Factor Signaling Is Required for Later-Life Behavioral Dysfunction Following Neonatal Pain

Neonatal pain such as that experienced by infants in the neonatal intensive care unit is known to produce later-life dysfunction including heightened pain sensitivity and anxiety, although the mechanisms remain unclear. Both chronic pain and stress in adult organisms are known to influence the corticotropin-releasing factor (CRF) system in the Central Nucleus of the Amygdala, making this system a likely candidate for changes following neonatal trauma. To examine this, neonatal rats were subjected to daily pain, non-painful handling or left undisturbed for the first week of life. Beginning on postnatal day, 24 male and female rats were subjected to a 4-day fear conditioning and sensory testing protocol. Some subjects received intra-amygdalar administration of either Vehicle, the CRF receptor 1 (CRF₁) receptor antagonist Antalarmin, or the CRF receptor 2 (CRF₂) receptor antagonist Astressin 2B prior to fear conditioning and somatosensory testing, while others had tissue collected following fear conditioning and CRF expression in the CeA and BLA was assessed using fluorescent in situ hybridization. CRF₁ antagonism attenuated fear-induced hypersensitivity in neonatal pain and handled rats, while CRF₂ antagonism produced a general antinociception. In addition, neonatal pain and handling produced a lateralized sex-dependent decrease in CRF expression, with males showing a diminished number of CRF-expressing cells in the right CeA and females showing a similar reduction in the number of CRF-expressing cells in the left BLA compared to undisturbed controls. These data show that the amygdalar CRF system is a likely target for alleviating dysfunction produced by early life trauma and that this system continues to play a major role in the lasting effects of such trauma into the juvenile stage of development.

The Role of Corticotropin-Releasing Hormone at Peripheral Nociceptors: Implications for Pain Modulation

Peripheral nociceptors and their synaptic partners utilize neuropeptides for signal transmission. Such communication tunes the excitatory and inhibitory function of nociceptor-based circuits, eventually contributing to pain modulation. Corticotropin-releasing hormone (CRH) is the initiator hormone for the conventional hypothalamic-pituitary-adrenal axis, preparing our body for stress insults. Although knowledge of the expression and functional profiles of CRH and its receptors and the outcomes of their interactions has been actively accumulating for many brain regions, those for nociceptors are still under gradual investigation. Currently, based on the evidence of their expressions in nociceptors and their neighboring components, several hypotheses for possible pain modulations are emerging. Here we overview the historical attention to CRH and its receptors on the peripheral nociception and the recent increases in information regarding their roles in tuning pain signals. We also briefly contemplate the possibility that the stress-response paradigm can be locally intrapolated into intercellular communication that is driven by nociceptor neurons. Such endeavors may contribute to a more precise view of local peptidergic mechanisms of peripheral pain modulation.

Manganese-enhanced MRI reveals changes within brain anxiety and aversion circuitry in rats with chronic neuropathic pain- and anxiety-like behaviors

Chronic pain often predicts the onset of psychological distress. Symptoms including anxiety and depression after pain chronification reportedly are caused by brain remodeling/recruitment of the limbic and reward/aversion circuitries. Pain is the primary precipitating factor that has caused opioid overprescribing and continued overuse of opioids leading to the current opioid epidemic. Yet experimental pain therapies often fail in clinical trials. Better understanding of underlying pathologies contributing to pain chronification is needed to address these chronic pain related issues. In the present study, a chronic neuropathic pain model persisting 10 weeks was studied. The model develops both anxiety- and pain-related behavioral measures to mimic clinical pain. The manganese-enhanced magnetic resonance imaging (MEMRI) utilized improved MRI signal contrast in brain regions with higher neuronal activity in the rodent chronic constriction trigeminal nerve injury (CCI-ION) model. T1-weighted MEMRI signal intensity was increased compared to controls in supraspinal regions of the anxiety and aversion circuitry, including anterior cingulate gyrus (ACC), amygdala, habenula, caudate, ventrolateral and dorsomedial periaqueductal gray (PAG). Despite continuing mechanical hypersensitivity, MEMRI T1 signal intensity as the neuronal activity measure, was not significantly different in thalamus and decreased in somatosensory cortex (S1BF) of CCI-ION rats compared to naïve controls. This is consistent with decreased fMRI BOLD signal intensity in thalamus and cortex of patients with longstanding trigeminal neuropathic pain reportedly associated with gray matter volume decrease in these regions. Significant increase in MEMRI T2 signal intensity in thalamus of CCI-ION animals was indication of tissue water content, cell dysfunction and/or reactive astrogliosis. Decreased T2 signal intensity in S1BF cortex of rats with CCI-ION was similar to findings of reduced T2 signals in clinical patients with chronic orofacial pain indicating prolonged astrocyte activation. These findings support use of MEMRI and chronic rodent models for preclinical studies and therapeutic trials to reveal brain sites activated only after neuropathic pain has persisted in timeframes relevant to clinical pain and to observe treatment effects not possible in short-term models which do not have evidence of anxiety-like behaviors. Potential improvement is predicted in the success rate of preclinical drug trials in future studies with this model.

Inhibition of P2X7R in the amygdala ameliorates symptoms of neuropathic pain after spared nerve injury in rats

The amygdala circuitry and P2X7 receptor (P2X7R) have both been shown to play important roles in the modulation of neuropathic pain (NP). However, little is known about the functional role of P2X7R in the amygdala for the regulation of NP. This study aims to evaluate the alleviative effect of intra-amygdala microinfusion of a pharmacological antagonist of P2X7R (A-438079) on NP and explore its possible mechanism of action. Male Sprague-Dawley rats were used to construct the animal model of NP through spared nerve injury (SNI). The SNI rats randomly received chronic bilateral microinjection of A-438079 (100 pmol/side) or saline into the amygdalae via cannulas. Mechanical paw withdrawal threshold (MWT) and thermal withdrawal duration (TWD) were measured by von Frey monofilaments. Besides, tail suspension test (TST), forced swimming test (FST), open field test (OFT) and sucrose preference test (SPT) were performed to assess depression- and anxiety-like behaviors. Immunofluorescence assay was employed to determine the levels of glial fibrillary acidic protein (GFAP), ionized calcium binding adaptor molecule 1 (IBA-1) and connexin 43 (Cx43) in the spinal cord. In addition, the change of growth associated protein 43 (GAP43) level in the spinal cord was assessed by Western blot. Our data showed that chronic treatment with A-438079 increased MWT and decreased TWD on days 11-21 post-SNI while decreased depression-like and anxiety-like behaviors. A-438079 administration significantly attenuated the elevated immunoreactivities of IBA-1 and GFAP in microglia and astrocytes after SNI. Furthermore, the decreased expression of GAP-43 in the spinal cord due to SNI was significantly attenuated by A-438079. However, when A-438079 and a pharmacological agonist (BzATP) of P2X7R were given simultaneously, all the effects caused by A-438079 alone were reversed. In brief, our study revealed the protective role of inhibiting P2X7R in the amygdala against symptoms associated with NP, possibly attributing to its inhibitory effects on spinal microglia and astrocytes.

Sex differences in the development of anxiodepressive-like behavior of mice subjected to sciatic nerve cuffing

We investigated the contribution of nucleus locus ceruleus (LC) to the development of pain-associated affective behavior. Mice of both sexes were subjected to sciatic nerve cuffing, a model of peripheral nerve injury, and monitored for 45 days. Although the thermal and mechanical thresholds were equally decreased in both males and females, only the male mice developed anxiodepressive-like behavior, which was complemented by suppressed hippocampal neurogenesis. Furthermore, the LC activity was lower in males when compared with females subjected to sciatic cuffing. Next, we used a chemogenetic approach to modulate the activity of LC projections to the dentate gyrus of the hippocampus in females without cuffs and in males with sciatic cuffs. Sustained inhibition of the LC projections to the dentate gyrus for 15 days induced anxiodepressive-like behavior and reduced the hippocampal neurogenesis in females. Activation of the LC projections to the dentate gyrus for 15 days prevented the development of anxiodepressive-like behavior and increased the hippocampal neurogenesis in males with cuffs. In sum, we demonstrated that the LC projections to the hippocampus link the sensory to the affective component of neuropathic injury and that the female mice are able to dissociate the nociception from affect by maintaining robust LC activity. The work provides evidence that sex differences in LC response to pain determine the sex differences in the development of pain phenotype.

Left and right hemispheric lateralization of the amygdala in pain

Hemispheric asymmetries within the brain have been identified across taxa and have been extensively studied since the early 19th century. Here, we discuss lateralization of a brain structure, the amygdala, and how this lateralization is reshaping how we understand the role of the amygdala in pain processing. The amygdala is an almond-shaped, bilateral brain structure located within the limbic system. Historically, the amygdala was known to have a role in the processing of emotions and attaching emotional valence to memories and other experiences. The amygdala has been extensively studied in fear conditioning and affect but recently has been shown to have an important role in processing noxious information and impacting pain. The amygdala is composed of multiple nuclei; of special interest is the central nucleus of the amygdala (CeA). The CeA receives direct nociceptive inputs from the parabrachial nucleus (PBN) through the spino-parabrachio-amygdaloid pathway as well as more highly processed cortical and thalamic input via the lateral and basolateral amygdala. Although the amygdala is a bilateral brain region, most data investigating the amygdala's role in pain have been generated from the right CeA, which has an overwhelmingly pro-nociceptive function across pain models. The left CeA has often been characterized to have no effect on pain modulation, a dampened pro-nociceptive function, or most recently an anti-nociceptive function. This review explores the current literature on CeA lateralization and the hemispheres' respective roles in the processing and modulation of different forms of pain.

Cellular Circuits in the Brain and Their Modulation in Acute and Chronic Pain

Chronic, pathological pain remains a global health problem and a challenge to basic and clinical sciences. A major obstacle to preventing, treating, or reverting chronic pain has been that the nature of neural circuits underlying the diverse components of the complex, multidimensional experience of pain is not well understood. Moreover, chronic pain involves diverse maladaptive plasticity processes, which have not been decoded mechanistically in terms of involvement of specific circuits and cause-effect relationships. This review aims to discuss recent advances in our understanding of circuit connectivity in the mammalian brain at the level of regional contributions and specific cell types in acute and chronic pain. A major focus is placed on functional dissection of sub-neocortical brain circuits using optogenetics, chemogenetics, and imaging technological tools in rodent models with a view towards decoding sensory, affective, and motivational-cognitive dimensions of pain. The review summarizes recent breakthroughs and insights on structure-function properties in nociceptive circuits and higher order sub-neocortical modulatory circuits involved in aversion, learning, reward, and mood and their modulation by endogenous GABAergic inhibition, noradrenergic, cholinergic, dopaminergic, serotonergic, and peptidergic pathways. The knowledge of neural circuits and their dynamic regulation via functional and structural plasticity will be beneficial towards designing and improving targeted therapies.

Canine Adenovirus 2: A Natural Choice for Brain Circuit Dissection

Canine adenovirus-2 (CAV) is a canine pathogen that has been used in a variety of applications, from vaccines against more infectious strains of CAV to treatments for neurological disorders. With recent engineering, CAV has become a natural choice for neuroscientists dissecting the connectivity and function of brain circuits. Specifically, as a reliable genetic vector with minimal immunogenic and cytotoxic reactivity, CAV has been used for the retrograde transduction of various types of projection neurons. Consequently, CAV is particularly useful when studying the anatomy and functions of long-range projections. Moreover, combining CAV with conditional expression and transsynaptic tracing results in the ability to study circuits with cell- and/or projection-type specificity. Lastly, with the well-documented knowledge of viral transduction, new innovations have been developed to increase the transduction efficiency of CAV and circumvent its tropism, expanding the potential of CAV for circuit analysis.

Amygdala, neuropeptides, and chronic pain-related affective behaviors

Neuropeptides play important modulatory roles throughout the nervous system, functioning as direct effectors or as interacting partners with other neuropeptide and neurotransmitter systems. Limbic brain areas involved in learning, memory and emotions are particularly rich in neuropeptides. This review will focus on the amygdala, a limbic region that plays a key role in emotional-affective behaviors and pain modulation. The amygdala is comprised of different nuclei; the basolateral (BLA) and central (CeA) nuclei and in between, the intercalated cells (ITC), have been linked to pain-related functions. A wide range of neuropeptides are found in the amygdala, particularly in the CeA, but this review will discuss those neuropeptides that have been explored for their role in pain modulation. Calcitonin gene-related peptide (CGRP) is a key peptide in the afferent nociceptive pathway from the parabrachial area and mediates excitatory drive of CeA neurons. CeA neurons containing corticotropin releasing factor (CRF) and/or somatostatin (SOM) are a source of long-range projections and serve major output functions, but CRF also acts locally to excite neurons in the CeA and BLA. Neuropeptide S (NPS) is associated with inhibitory ITC neurons that gate amygdala output. Oxytocin and vasopressin exert opposite (inhibitory and excitatory, respectively) effects on amygdala output. The opioid system of mu, delta and kappa receptors (MOR, DOR, KOR) and their peptide ligands (β-endorphin, enkephalin, dynorphin) have complex and partially opposing effects on amygdala function. Neuropeptides therefore serve as valuable targets to regulate amygdala function in pain conditions. This article is part of the special issue on Neuropeptides.

Corticotropin-Releasing Factor in the Brain and Blocking Spinal Descending Signals Induce Hyperalgesia in the Latent Sensitization Model of Chronic Pain

Latent sensitization is a model of chronic pain in which an injury triggers a period of hyperalgesia followed by an apparent recovery, but in which pain sensitization persists but is suppressed by opioid and adrenergic receptors. One important characteristic of latent sensitization is that hyperalgesia can be triggered by acute stress. To determine whether the effect of stress is mimicked by the activation of corticotropin-releasing factor (CRF) signaling in the brain, rats with latent sensitization induced by injecting complete Freund's adjuvant (CFA, 50 μl) in one hind paw were given an intracerebroventricular (i.c.v.) injection of CRF. The i.c.v. injection of CRF (0.6 μg, 10 μl), but not saline, induced bilateral mechanical hyperalgesia in rats with latent sensitization. In contrast, CRF i.c.v. did not induce hyperalgesia in rats without latent sensitization (injected with saline in the hind paw). To determine whether descending pain inhibition mediates the suppression of hyperalgesia in latent sensitization, rats with CFA-induced latent sensitization received an intrathecal injection of lidocaine (10%, 1 μl) at the cervical-thoracic spinal cord to produce a spinal block. Lidocaine-injected rats, but not rats injected intrathecally with saline, developed bilateral mechanical hyperalgesia. Intrathecal lidocaine did not induce hyperalgesia in rats without latent sensitization (injected with saline in the hind paw). These results show that i.c.v. CRF mimicked the hyperalgesic response triggered by stress during latent sensitization, possibly by blocking inhibitory spinal descending signals that suppress hyperalgesia.

Brain corticotropin-releasing factor signaling: Involvement in acute stress-induced visceral analgesia in male rats

BACKGROUND

Water avoidance stress (WAS) induces a naloxone-independent visceral analgesia in male rats under non-invasive conditions of monitoring. The objective of the study was to examine the role of brain CRF signaling in acute stress-induced visceral analgesia (SIVA).

METHODS

Adult male Sprague-Dawley rats were chronically implanted with an intracerebroventricular (ICV) cannula. The visceromotor response (VMR) to graded phasic colorectal distension (CRD: 10, 20, 40, 60 mm Hg, 20 seconds, 4 minutes intervals) was monitored using manometry. The VMR to a first CRD (baseline) was recorded 5 minutes after an ICV saline injection, followed 1 hour later by ICV injection of either CRF (30, 100, or 300 ng and 1, 3, or 5 μg/rat) or saline and a second CRD, 5 minutes later. Receptor antagonists against CRF₁ /CRF₂ (astressin-B, 30 μg/rat), CRF₂ (astressin₂ -B, 10 μg/rat), oxytocin (tocinoic acid, 20 μg/rat), or vehicle were injected ICV 5 minutes before CRF (300 ng/rat, ICV) or 15 minutes before WAS (1 hour).

KEY RESULTS

ICV CRF (100 and 300 ng) reduced the VMR to CRD at 60 mm Hg by -36.6% ± 6.8% and -48.7% ± 11.7%, respectively, vs baseline (P < 0.001), while other doses had no effect and IP CRF (10 µg/kg) induced visceral hyperalgesia. Astressin-B and tocinoic acid injected ICV induced hyperalgesia and prevented the analgesic effect of ICV CRF (300 ng/rat) and WAS, while astressin₂ -B only blocked WAS-induced SIVA.

CONCLUSIONS & INFERENCES

These data support a role for brain CRF signaling via CRF₂ in SIVA in a model of WAS and CRD likely mediated by the activation of brain oxytocin pathway.

Activation of enkephalinergic (Enk) interneurons in the central amygdala (CeA) buffers the behavioral effects of persistent pain

Enk neurons in CeA modulate the activity of the amygdala projection neurons and it is very likely that changes of Enk signaling cause the heightened anxiety that accompanies chronic pain. We use chemogenetics and transgenic mice to investigate the effects of acute and continuous activation of the amygdala Enk neurons on persistent pain and anxiodepressive-like behavior in mice. Enk-cre mice were injected bilaterally into the CeA with cre-activated AAV-DREADD/Gq/mCherry, while neuropathic pain was induced by sciatic nerve constriction. A single injection of DREADD's ligand CNO decreased the anxiety-like behavior in both, uninjured mice and in mice with neuropathic pain and produced robust analgesia that lasted for 24 h. Furthermore, the activation of Enk neurons by the DREADD ligand led to increased c-Fos expression in PKC-δ interneurons of the CeA and in non-serotonergic neurons in the ventrolateral periaqueductal gray (vlPAG), a brain structure that is an essential part of the descending pain inhibitory system. Next, we added CNO to the drinking water of the experimental mice for 14 days in order to assess the effects of continuous activation of CeA Enk interneurons on anxiodepressive-like behavior, which is affected by chronic pain. The prolonged activation of the CeA Enk interneurons reduced neohypophagia in the novelty suppressed feeding test and increased ΔFosB (a marker for sustained neuronal activation) in the vlPAG of mice with chronic pain. All together, the results of our experiments point to an important role of the CeA Enk neurons in the control of both nociception and emotion. Activation of Enk neurons resulted in sustained analgesia accompanied by anxiolysis and antidepressant effects. Very likely, these effects of CeA Enk neurons are result of the activation of vlPAG, a brain region that is essential not only for descending inhibition of pain but it is also a core element in the resilience to stress.

A medial prefrontal cortex-nucleus acumens corticotropin-releasing factor circuitry for neuropathic pain-increased susceptibility to opioid reward

Recent studies have shown that persistent pain facilitates the response to morphine reward. However, the circuit mechanism underlying this process remains ambiguous. In this study, using chronic constriction injury (CCI) of the sciatic nerve in mice, we found that persistent neuropathic pain reduced the minimum number of morphine conditioning sessions required to induce conditioned place preference (CPP) behavior. This dose of morphine had no effect on the pain threshold. In the medial prefrontal cortex (mPFC), which is involved in both pain and emotion processing, corticotropin-releasing factor (CRF) expressing neuronal activity was increased in CCI mice. Chemogenetic inhibition of mPFC CRF neurons reversed CCI-induced morphine CPP facilitation. Furthermore, the nucleus acumens (NAc) received mPFC CRF functional projections that exerted excitatory effects on NAc neurons. Optogenetic inhibition of mPCF neuronal terminals or local infusion of the CRF receptor 1 (CRFR1) antagonist in the NAc restored the effects of neuropathic pain on morphine-induced CPP behavior, but not in normal mice. On a molecular level, in CCI mice, CRFR1 protein expression was increased in the NAc by a histone dimethyltransferase G9a-mediated epigenetic mechanism. Local G9a knockdown increased the expression of CRFR1 and mimicked CCI-induced hypersensitivity to acquiring morphine CPP. Taken together, these findings demonstrate a previously unknown and specific mPFC CRF engagement of NAc neuronal circuits, the sensitization of which facilitates behavioral responses to morphine reward in neuropathic pain states via CRFR1s.