Cingulate Alpha-2A Adrenoceptors Mediate the Effects of Clonidine on Spontaneous Pain Induced by Peripheral Nerve Injury

Revealing a role of brainstem monoaminergic nuclei on the pronociceptive effect of sleep restriction

Sleep disturbances and persistent pain conditions are public health challenges worldwide. Although it is well-known that sleep deficit increases pain sensitivity, the underlying mechanisms remain elusive. We have recently demonstrated the involvement of nucleus accumbens (NAc) and anterior cingulate cortex (ACC) in the pronociceptive effect of sleep restriction. In this study, we found that sleep restriction increases c-Fos expression in NAc and ACC, suggesting hyperactivation of these regions during prolonged wakefulness in male Wistar rats. Blocking adenosine A2A receptors in the NAc or GABAA receptors in the ventral tegmental area (VTA), dorsal raphe nucleus (DRN), or locus coeruleus (LC) effectively mitigated the pronociceptive effect of sleep restriction. In contrast, the blockade of GABAA receptors in each of these nuclei only transiently reduced carrageenan-induced hyperalgesia. Pharmacological activation of dopamine D2, serotonin 5-HT1A and noradrenaline alpha-2 receptors within the ACC also prevented the pronociceptive effect of sleep restriction. While pharmacological inhibition of these same monoaminergic receptors in the ACC restored the pronociceptive effect which had been prevented by the GABAergic disinhibition of the of the VTA, DRN or LC. Overall, these findings suggest that the pronociceptive effect of sleep restriction relies on increased adenosinergic activity on NAc, heightened GABAergic activity in VTA, DRN, and LC, and reduced inhibitory monoaminergic activity on ACC. These findings advance our understanding of the interplay between sleep and pain, shedding light on potential NAc-brainstem-ACC mechanisms that could mediate increased pain sensitivity under conditions of sleep impairment.

Deconstruction the feedforward inhibition changes in the layer III of anterior cingulate cortex after peripheral nerve injury

The anterior cingulate cortex (ACC) is one of the critical brain areas for processing noxious information. Previous studies showed that peripheral nerve injury induced broad changes in the ACC, contributing to pain hypersensitivity. The neurons in layer 3 (L3) of the ACC receive the inputs from the mediodorsal thalamus (MD) and form the feedforward inhibition (FFI) microcircuits. The effects of peripheral nerve injury on the MD-driven FFI in L3 of ACC are unknown. In our study, we record the enhanced excitatory synaptic transmissions from the MD to L3 of the ACC in mice with common peroneal nerve ligation, affecting FFI. Chemogenetically activating the MD-to-ACC projections induces pain sensitivity and place aversion in naive mice. Furthermore, chemogenetically inactivating MD-to-ACC projections decreases pain sensitivity and promotes place preference in nerve-injured mice. Our results indicate that the peripheral nerve injury changes the MD-to-ACC projections, contributing to pain hypersensitivity and aversion.

Coordination between midcingulate cortex and retrosplenial cortex in pain regulation

Introduction

The cingulate cortex, with its subregions ACC, MCC, and RSC, is key in pain processing. However, the detailed interactions among these regions in modulating pain sensation have remained unclear.

Methods

In this study, chemogenetic tools were employed to selectively activate or inhibit neuronal activity in the MCC and RSC of rodents to elucidate their roles in pain regulation.Results: Our results showed that chemogenetic activation in both the RSC and MCC heightened pain sensitivity. Suppression of MCC activity disrupted the RSC's regulation of both mechanical and thermal pain, while RSC inhibition specifically affected the MCC's regulation of thermal pain.

Discussion

The findings indicate a complex interplay between the MCC and RSC, with the MCC potentially governing the RSC's pain regulatory mechanisms. The RSC, in turn, is crucial for the MCC's control over thermal sensation, revealing a collaborative mechanism in pain processing.

Conclusion

This study provides evidence for the MCC and RSC's collaborative roles in pain regulation, highlighting the importance of their interactions for thermal and mechanical pain sensitivity. Understanding these mechanisms could aid in developing targeted therapies for pain disorders.

Nerve Injury Triggers Time-dependent Activation of the Locus Coeruleus, Influencing Spontaneous Pain-like Behavior in Rats

BACKGROUND

Dynamic changes in neuronal activity and in noradrenergic locus coeruleus (LC) projections have been proposed during the transition from acute to chronic pain. Thus, the authors explored the cellular cFos activity of the LC and its projections in conjunction with spontaneous pain-like behavior in neuropathic rats.

METHODS

Tyrosine hydroxylase:Cre and wild-type Long-Evans rats, males and females, were subjected to chronic constriction injury (CCI) for 2 (short-term, CCI-ST) or 30 days (long-term, CCI-LT), evaluating cFos and Fluoro-Gold expression in the LC, and its projections to the spinal cord (SC) and rostral anterior cingulate cortex (rACC). These tests were carried out under basal conditions (unstimulated) and after noxious mechanical stimulation. LC activity was evaluated through chemogenetic and pharmacologic approaches, as were its projections, in association with spontaneous pain-like behaviors.

RESULTS

CCI-ST enhanced basal cFos expression in the LC and in its projection to the SC, which increased further after noxious stimulation. Similar basal activation was found in the neurons projecting to the rACC, although this was not modified by stimulation. Strong basal cFos expression was found in CCI-LT, specifically in the projection to the rACC, which was again not modified by stimulation. No cFos expression was found in the CCI-LT LCipsilateral (ipsi)/contralateral (contra)→SC. Chemogenetics showed that CCI-ST is associated with greater spontaneous pain-like behavior when the LCipsi is blocked, or by selectively blocking the LCipsi→SC projection. Activation of the LCipsi or LCipsi/contra→SC dampened pain-like behavior. Moreover, Designer Receptor Exclusively Activated by Designer Drugs (DREADDs)-mediated inactivation of the CCI-ST LCipsi→rACC or CCI-LT LCipsi/contra→rACC pathway, or intra-rACC antagonism of α-adrenoreceptors, also dampens pain-like behavior.

CONCLUSIONS

In the short term, activation of the LC after CCI attenuates spontaneous pain-like behaviors via projections to the SC while increasing nociception via projections to the rACC. In the long term, only the projections from the LC to the rACC contribute to modulate pain-like behaviors in this model.

EDITOR’S PERSPECTIVE

Single-cell RNA sequencing uncovers the cell type-dependent transcriptomic changes in the retrosplenial cortex after peripheral nerve injury

The retrosplenial cortex (RSC) is a vital area for storing remote memory and has recently been found to undergo broad changes after peripheral nerve injury. However, little is known about the role of RSC in pain regulation. Here, we examine the involvement of RSC in the pain of mice with nerve injury. Notably, reducing the activities of calcium-/calmodulin-dependent protein kinase type II-positive splenial neurons chemogenetically increases paw withdrawal threshold and extends thermal withdrawal latency in mice with nerve injury. The single-cell or single-nucleus RNA-sequencing results predict enhanced excitatory synaptic transmissions in RSC induced by nerve injury. Local infusion of 1-naphthyl acetyl spermine into RSC to decrease the excitatory synaptic transmissions relieves pain and induces conditioned place preference. Our data indicate that RSC is critical for regulating physiological and neuropathic pain. The cell type-dependent transcriptomic information would help understand the molecular basis of neuropathic pain.

Cingulate protein arginine methyltransferases 1 regulates peripheral hypersensitivity via fragile X messenger ribonucleoprotein

The deficit of fragile X messenger ribonucleoprotein (FMRP) leads to intellectual disability in human and animal models, which also leads to desensitization of pain after nerve injury. Recently, it was shown that the protein arginine methyltransferases 1 (PRMT1) regulates the phase separation of FMRP. However, the role of PRMT1 in pain regulation has been less investigated. Here we showed that the downregulation of PRMT1 in the anterior cingulate cortex (ACC) contributes to the development of peripheral pain hypersensitivity. We observed that the peripheral nerve injury decreased the expression of PRMT1 in the ACC; knockdown of the PRMT1 via shRNA in the ACC decreased the paw withdrawal thresholds (PWTs) of naïve mice. Moreover, the deficits of FMRP abolished the effects of PRMT1 on pain sensation. Furthermore, overexpression of PRMT1 in the ACC increased the PWTs of mice with nerve injury. These observations indicate that the downregulation of cingulate PRMT1 was necessary and sufficient to develop peripheral hypersensitivity after nerve injury. Thus, we provided evidence that PRMT1 is vital in regulating peripheral pain hypersensitivity after nerve injury via the FMRP.

Neurotransmitter receptor densities are associated with changes in regional Cerebral blood flow during clinical ongoing pain

Arterial spin labelling (ASL) plays an increasingly important role in neuroimaging pain research but does not provide molecular insights regarding how regional cerebral blood flow (rCBF) relates to underlying neurotransmission. Here, we integrate ASL with positron emission tomography (PET) and brain transcriptome data to investigate the molecular substrates of rCBF underlying clinically relevant pain states. Two data sets, representing acute and chronic ongoing pain respectively, were utilised to quantify changes in rCBF; one examining pre-surgical versus post-surgical pain, and the second comparing patients with painful hand Osteoarthritis to a group of matched controls. We implemented a whole-brain spatial correlation analysis to explore associations between change in rCBF (ΔCBF) and neurotransmitter receptor distributions derived from normative PET templates. Additionally, we utilised transcriptomic data from the Allen Brain Atlas to inform distributions of receptor expression. Both datasets presented significant correlations of ΔCBF with the μ-opioid and dopamine-D2 receptor expressions, which play fundamental roles in brain activity associated with pain experiences. ΔCBF also correlated with the gene expression distributions of several receptors involved in pain processing. Overall, this is the first study illustrating the molecular basis of ongoing pain ASL indices and emphasises the potential of rCBF as a biomarker in pain research.

Synaptic Plasticity in the Pain-Related Cingulate and Insular Cortex

Cumulative animal and human studies have consistently demonstrated that two major cortical regions in the brain, namely the anterior cingulate cortex (ACC) and insular cortex (IC), play critical roles in pain perception and chronic pain. Neuronal synapses in these cortical regions of adult animals are highly plastic and can undergo long-term potentiation (LTP), a phenomenon that is also reported in brain areas for learning and memory (such as the hippocampus). Genetic and pharmacological studies show that inhibiting such cortical LTP can help to reduce behavioral sensitization caused by injury as well as injury-induced emotional changes. In this review, we will summarize recent progress related to synaptic mechanisms for different forms of cortical LTP and their possible contribution to behavioral pain and emotional changes.

Modulation of Pathological Pain by Epidermal Growth Factor Receptor

Chronic pain has been widely recognized as a major public health problem that impacts multiple aspects of patient quality of life. Unfortunately, chronic pain is often resistant to conventional analgesics, which are further limited by their various side effects. New therapeutic strategies and targets are needed to better serve the millions of people suffering from this devastating disease. To this end, recent clinical and preclinical studies have implicated the epidermal growth factor receptor signaling pathway in chronic pain states. EGFR is one of four members of the ErbB family of receptor tyrosine kinases that have key roles in development and the progression of many cancers. EGFR functions by activating many intracellular signaling pathways following binding of various ligands to the receptor. Several of these signaling pathways, such as phosphatidylinositol 3-kinase, are known mediators of pain. EGFR inhibitors are known for their use as cancer therapeutics but given recent evidence in pilot clinical and preclinical investigations, may have clinical use for treating chronic pain. Here, we review the clinical and preclinical evidence implicating EGFR in pathological pain states and provide an overview of EGFR signaling highlighting how EGFR and its ligands drive pain hypersensitivity and interact with important pain pathways such as the opioid system.

Dexmedetomidine attenuates sepsis-associated inflammation and encephalopathy via central α2A adrenoceptor

Sepsis-associated encephalopathy (SAE) is a significant clinical issue that is associated with increased mortality and cost of health care. Dexmedetomidine, an α2 adrenoceptor agonist that is used to provide sedation, has been shown to induce neuroprotection under various conditions. This study was designed to determine whether dexmedetomidine protects against SAE and whether α2 adrenoceptor plays a role in this protection. Six- to eight-week old CD-1 male mice were subjected to cecal ligation and puncture (CLP). They were treated with intraperitoneal injection of dexmedetomidine in the presence or absence of α2 adrenoceptor antagonists, atipamezole or yohimbine, or an α2A adrenoceptor antagonist, BRL-44408. Hippocampus and blood were harvested for measuring cytokines. Mice were subjected to Barnes maze and fear conditioning 14 days after CLP to evaluate their learning and memory. CLP significantly increased the proinflammatory cytokines including tumor necrosis factor α, interleukin (IL)-6 and IL-1β in the blood and hippocampus. CLP also increased the permeability of blood-brain barrier (BBB) and impaired learning and memory. These CLP detrimental effects were attenuated by dexmedetomidine. Intracerebroventricular application of atipamezole, yohimbine or BRL-44408 blocked the protection of dexmedetomidine on the brain but not on the systemic inflammation. Astrocytes but not microglia expressed α2A adrenoceptors. Microglial depletion did not abolish the protective effects of dexmedetomidine. These results suggest that dexmedetomidine reduces systemic inflammation, neuroinflammation, injury of BBB and cognitive dysfunction in septic mice. The protective effects of dexmedetomidine on the brain may be mediated by α2A adrenoceptors in the astrocytes.

Evodiamine reduced peripheral hypersensitivity on the mouse with nerve injury or inflammation

Management of chronic pain is still hard, and new analgesic drugs are needed. Evodiamine (Evo) and rutaecarpine (Rut) are two major active components of Evodia rutaecarpa, a Chinese traditional medicine that has been used as an analgesic for a long time. However, their effects on peripheral hypersensitivity remain unknown. Similar to capsaicin, the Evo and Rut were docked to the transient receptor potential cation channel subfamily V member 1 (TRPV1) in molecular simulation experiments. Moreover, Evo (10 µM) and Rut (50 µM) activated TRPV1 on human embryonic kidney 293 (HEK293) cells in electrophysiological recording experiments. Behaviorally, the application of Evo and Rut reduced peripheral hypersensitivity in a dose-dependent manner, which was blocked by capsazepine (a selective inhibitor of TRPV1). Furthermore, both Evo and Rut increased time in the open arms of the elevated plus maze on mice with nerve injury. These observations suggested that Evo and Rut reduced peripheral hypersensitivity and anxiety in mice with nerve injury or inflammation via TRPV1.

Restoration of Cingulate Long-Term Depression by Enhancing Non-apoptotic Caspase 3 Alleviates Peripheral Pain Hypersensitivity

Nerve injury in somatosensory pathways may lead to neuropathic pain, which affects the life quality of ∼8% of people. Long-term enhancement of excitatory synaptic transmission along somatosensory pathways contributes to neuropathic pain. Caspase 3 (Casp3) plays a non-apoptotic role in the hippocampus and regulates internalization of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits. Whether Casp3-AMPAR interaction is involved in the maintenance of peripheral hypersensitivity after nerve injury remained unknown. Here, we show that nerve injury suppresses long-term depression (LTD) and downregulates Casp3 in the anterior cingulate cortex (ACC). Interfering with interactions between Casp3 and AMPAR subunits or reducing Casp3 activity in the ACC suppresses LTD induction and causes peripheral hypersensitivity. Overexpression of Casp3 restores LTD and reduces peripheral hypersensitivity after nerve injury. We reveal how Casp3 is involved in the maintenance of peripheral hypersensitivity. Our findings suggest that restoration of LTD via Casp3 provides a therapeutic strategy for neuropathic pain management.

Synaptic alterations and immune response are sexually dimorphic in a non-pertussis toxin model of experimental autoimmune encephalomyelitis

Multiple sclerosis is an autoimmune disorder of the central nervous system (CNS) characterized by locomotor impairments, cognitive deficits, affective disorders, and chronic pain. Females are predominately affected by MS compared to males and develop motor symptoms earlier. However, key symptoms affect all patients regardless of sex. Previous studies have shown that demyelination and axonal damage play key roles in symptom development, but it is unclear why sex differences exist in MS onset, and effective symptom treatment is still lacking. We here used a non-pertussis toxin (nPTX) experimental autoimmune encephalomyelitis (EAE) model in C57BL/6 mice, to explore chronic symptoms and sex differences in CNS autoimmunity. We observed that, like in humans, female mice developed motor disease earlier than males. Further, changes in pre- and post-synaptic protein expression levels were observed in a sexually dimorphic manner with an overall shift towards excitatory signaling. Our data suggest that this shift towards excitatory signaling is achieved through different mechanisms in males and females. Altogether, our study helps to better understand sex-specific disease mechanisms to ultimately develop better diagnostic and treatment tools.

Skin/Muscle Incision and Retraction Induces Evoked and Spontaneous Pain in Mice

Background

Surgery is a frequent cause of persistent pain. Unrelieved chronic postsurgical pain causes unnecessary patient suffering and discomfort and usually leads to psychological complications. The rat model of skin/muscle incision and retraction (SMIR) with decreased paw withdrawal thresholds developed by Flatters was usually used to investigate the underlying mechanism of chronic postsurgical pain.

Objectives

The aim of our study was to develop a new mice model of SMIR for further investigation with transgenic mice and so on and to evaluate the analgesic effects of clonidine and gabapentin on pain behavior with this new mice model.

Methods

Male C57BL/6 mice were anesthetized, and a 1.0-1.3 cm incision was made in the skin of the medial thigh approximately 3 mm medial to the saphenous vein to reveal the muscle of the thigh. The paw withdrawal threshold (PWT) to mechanical stimuli and the paw withdrawal latency to heat stimuli were measured before and after SMIR. Furthermore, the PWT to mechanical stimuli and conditioned place preference (CPP) was measured before and after the systemic injection of clonidine and gabapentin.

Results

SMIR-evoked mechanical hypersensitivity in mice began on day 1 after the procedure, prominent between days 1 and 10 after the procedure, persisted at least until day 14, and disappeared on day 18 after the procedure. However, the mice model of SMIR did not evoke significant heat hypersensitivity. Systemic injection of clonidine and gabapentin raised the PWT in the SMIR mice dose-dependently. Compared with the mice that underwent the sham operation, mice of SMIR spent a longer time in the clonidine-paired chamber than those of NS, while the gabapentin-paired chamber has no difference with that of NS in the CPP paradigm.

Conclusion

These data suggested that the mice model of SMIR demonstrated a persistent pain syndrome, including evoked pain and spontaneous pain. Clonidine and gabapentin could relieve mechanical hypersensitivity dose-dependently simultaneously. However, clonidine but not gabapentin could alleviate the spontaneous pain of SMIR in the mice model.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.