Enhanced presynaptic neurotransmitter release in the anterior cingulate cortex of mice with chronic pain

Cuprizone-induced demyelination provokes abnormal intrinsic properties and excitatory synaptic transmission in the male mouse anterior cingulate cortex

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS). Demyelination in the CNS provokes hyperalgesia, negative emotions, and/or cognitive impairment. Cuprizone (CPZ)-induced demyelination is a major demyelinating disease model for rodents. The anterior cingulate cortex (ACC) is a brain region that is responsible for higher brain functions related to MS symptoms. However, little is known whether CPZ exposure induces demyelination in the ACC coincides with changes to intrinsic neuron properties and synaptic transmission. In this study, we first examined if CPZ exposure induces demyelination in the male mouse ACC. CPZ exposure induced demyelination in the ACC and decreased body weight. In addition, demyelination altered intrinsic properties and excitatory synaptic transmission in layer II/III pyramidal neurons from the ACC as indicated by whole-cell patch-clamp in brain slice preparations. CPZ exposure decreased the frequency of action potentials due to increasing rheobase. At the synapse level, CPZ exposure also suppressed evoked excitatory synaptic transmission to the ACC. Finally, CPZ exposure also changed the kinetics of AMPA and NMDA receptors. These results suggest that CPZ exposure induces demyelination in the ACC coinciding with changes in intrinsic properties, action potentials and excitatory synaptic transmission.

Glutamate transporter activator LDN-212320 prevents chronic pain-induced cognitive impairment and anxiety-like behaviors in a mouse model

The astroglial glutamate transporter in the hippocampus and anterior cingulate cortex (ACC) is critically involved in chronic pain-induced cognitive and psychiatric abnormalities. We have previously reported that LDN-212320, a glutamate transporter-1 (GLT-1) activator, attenuates complete Freund's adjuvant (CFA)-induced acute and chronic nociceptive pain. However, the cellular and molecular mechanisms underlying GLT-1 modulation in the hippocampus and ACC during chronic pain-induced cognitive deficit-like and anxiety-like behaviors remain unknown. Here, we have investigated the effects of LDN-212320 on CFA-induced chronic pain associated with cognitive deficit-like and anxiety-like behaviors in mice. We have evaluated the effects of LDN-212320 on CFA-induced impaired spatial, working, and recognition memory using Y-maze and object-place recognition tests. In addition, we have determined the effects of LDN-21230 on chronic pain-induced anxiety-like behaviors using elevated plus maze and marble burying test. We have also examined the effects of LDN-212320 on cAMP response element-binding protein (pCREB), brain-derived neurotrophic factor (BDNF), protein kinase A (PKA), and Ca2 +/calmodulin-dependent protein kinase II (CaMKII) expression in the hippocampus and ACC during CFA-induced cognitive deficit-like and anxiety-like behaviors using the Western blot analysis and immunofluorescence assay. Pretreatment with LDN-212320 (20 mg/kg) significantly attenuated CFA-induced impaired spatial, working, and recognition memory. Furthermore, LDN-212320 (20 mg/kg) significantly reduced CFA-induced anxiety-like behaviors. Additionally, LDN-212320 (20 mg/kg) significantly reversed CFA-induced decreased pCREB, BDNF, PKA and CaMKII expression in the hippocampus and ACC. Overall, these results suggest that the LDN-212320 prevents CFA-induced cognitive deficit-like and anxiety-like behaviors by activating CaMKII/CREB/BDNF signaling pathway in the hippocampus and ACC. Therefore, LDN-212320 could be a potential treatment for chronic pain associated with cognitive impairment and anxiety-like behaviors.

Zingerone alleviates inflammatory pain by reducing the intrinsic excitability of anterior cingulate cortex neurons in a mice model

Background

Zingiber officinale Roscoe has been shown to possess analgesic properties. Zingerone (ZO), a bioactive compound derived from Zingiber officinale Roscoe, exhibits a range of pharmacological effects, including anti-inflammatory, anti-cancer, antioxidant, antibacterial, and anti-apoptotic activities. However, the analgesic properties of zingerone remain unclear.

Methods

Complete Freund's adjuvant (CFA) was administered to the left hind paw of C57BL/6 mice to induce a model of inflammatory pain. The analgesic effects of zingerone were assessed using the Von Frey and Hargreaves tests. In vivo fiber photometry and whole-cell patch clamp techniques were employed to investigate the potential mechanisms.

Results

Both acute and long-term treatment with zingerone resulted in a significant increase in mechanical and thermal pain thresholds in mice experiencing CFA-induced inflammatory pain. Mechanical stimulation led to a pronounced increase in calcium levels within the anterior cingulate cortex (ACC) neurons of the inflammatory pain model, which was alleviated by zingerone administration. Furthermore, zingerone was found to modify synaptic transmission to ACC neurons and decrease their intrinsic excitability by prolonging the refractory period of these neurons.

Conclusion

Zingerone demonstrates potential for alleviating CFA-induced inflammatory pain by reducing the intrinsic excitability of ACC neurons in a mouse model.

Astrocyte neuronal metabolic coupling in the anterior cingulate cortex of mice with inflammatory pain

Chronic pain is a major global concern, with at least 1 in 5 people suffering from chronic pain worldwide. Mounting evidence indicates that neuroplasticity of the anterior cingulate cortex (ACC) is a critical step in the development of chronic pain. Previously, we found that chronic pain and fear learning are both associated with enhanced neuronal excitability and cause similar neuroplasticity-related gene expression changes in the ACC of male mice. However, neuroplasticity, imposes large metabolic demands. In the brain, neurons have the highest energy needs and interact with astrocytes, which extract glucose from blood, mobilize glycogen, and release lactate in response to neuronal activity. Here, we use chronic and continuous inflammatory pain models in female and male mice to investigate the involvement of astrocyte-neuronal lactate shuttling (ANLS) in the ACC of female and male mice experiencing inflammatory pain. We found that ANLS in the mouse ACC promotes the development of chronic inflammatory pain, and expresses sex specific patterns of activation. Specifically, whereas both male and female mice show similar levels of chronic pain hypersensitivity, only male mice show sustained increases in lactate levels. Accordingly, chronic pain alters the expression levels of proteins involved in lactate metabolism and shuttling in a sexually dimorphic manner. We found that disrupting astrocyte-neuronal lactate shuttling in the ACC prior to inflammatory injury prevents the development of pain hypersensitivity in female and male mice, but only reduces temporary pain in male mice. Furthermore, using a transgenic mouse model (itga1-null mice) that displays a naturally occurring form of spontaneous osteoarthritis (OA), a painful inflammatory pain condition, we found that whereas both female and male mice develop OA, only male mice show increases in mechanisms involved in astrocyte-neuronal lactate shuttling. Our findings thus indicate that there are sex differences in astrocyte-neuronal metabolic coupling in the mouse ACC during chronic pain development.

mGluR5-mediated astrocytes hyperactivity in the anterior cingulate cortex contributes to neuropathic pain in male mice

Astrocytes regulate synaptic transmission in healthy and pathological conditions, but their involvement in modulating synaptic transmission in chronic pain is unknown. Our study demonstrates that astrocytes in the anterior cingulate cortex (ACC) exhibit abnormal calcium signals and induce the release of glutamate in male mice. This leads to an elevation in extracellular glutamate concentration, activation of presynaptic kainate receptors, and an increase in synaptic transmission following neuropathic pain. We discovered that the abnormal calcium signals are caused by the reappearance of metabotropic glutamate receptor type 5 (mGluR5) in astrocytes in male mice. Importantly, when we specifically inhibit the Gq pathway using iβARK and reduce the expression of mGluR5 in astrocytes through shRNA, we observe a restoration of astrocytic calcium activity, normalization of synaptic transmission and extracellular concentration of glutamate, and improvement in mechanical allodynia in male mice. Furthermore, the activation of astrocytes through chemogenetics results in an overabundance of excitatory synaptic transmission, exacerbating mechanical allodynia in mice with neuropathic pain, but not in sham-operated male mice. In summary, our findings suggest that the abnormal calcium signaling in astrocytes, mediated by mGluR5, plays a crucial role in enhancing synaptic transmission in ACC and contributing to mechanical allodynia in male mice.

Action potential-independent spontaneous microdomain Ca2+ transients-mediated continuous neurotransmission regulates hyperalgesia

Neurotransmitters and neuromodulators can be released via either action potential (AP)-evoked transient or AP-independent continuous neurotransmission. The elevated AP-evoked neurotransmission in the primary sensory neurons plays crucial roles in hyperalgesia. However, whether and how the AP-independent continuous neurotransmission contributes to hyperalgesia remains largely unknown. Here, we show that primary sensory dorsal root ganglion (DRG) neurons exhibit frequent spontaneous microdomain Ca2+ (smCa) activities independent of APs across the cell bodies and axons, which are mediated by the spontaneous opening of TRPA1 channels and trigger continuous neurotransmission via the cyclic adenosine monophosphate-protein kinase A signaling pathway. More importantly, the frequency of smCa activity and its triggered continuous neurotransmission in DRG neurons increased dramatically in mice experiencing inflammatory pain, inhibition of which alleviates hyperalgesia. Collectively, this work revealed the AP-independent continuous neurotransmission triggered by smCa activities in DRG neurons, which may serve as a unique mechanism underlying the nociceptive sensitization in hyperalgesia and offer a potential target for the treatment of chronic pain.

The elevated open platform stress suppresses excitatory synaptic transmission in the layer V anterior cingulate cortex

There are various forms of stress including; physical, psychological and social stress. Exposure to physical stress can lead to physical sensations (e.g. hyperalgesia) and negative emotions including anxiety and depression in animals and humans. Recently, our studies in mice have shown that acute physical stress induced by the elevated open platform (EOP) can provoke long-lasting mechanical hypersensitivity. This effect appears to be related to activity in the anterior cingulate cortex (ACC) at the synaptic level. Indeed, EOP exposure induces synaptic plasticity in layer II/III pyramidal neurons from the ACC. However, it is still unclear whether or not EOP exposure alters intrinsic properties and synaptic transmission in layer V pyramidal neurons. This is essential because these neurons are known to be a primary output to subcortical structures which may ultimately impact the behavioral stress response. Here, we studied both intrinsic properties and excitatory/inhibitory synaptic transmission by using whole-cell patch-clamp method in brain slice preparations. The EOP exposure did not change intrinsic properties including resting membrane potentials and action potentials. In contrast, EOP exposure suppressed the frequency of miniature and spontaneous excitatory synaptic transmission with an alteration of kinetics of AMPA/GluK receptors. EOP exposure also reduced evoked synaptic transmission induced by electrical stimulation. Furthermore, we investigated projection-selective responses of the mediodorsal thalamus to the layer V ACC neurons. EOP exposure produced short-term depression in excitatory synaptic transmission on thalamo-ACC projections. These results suggest that the EOP stress provokes abnormal excitatory synaptic transmission in layer V pyramidal neurons of the ACC.

Targeting Prefrontal Cortex Dysfunction in Pain

The prefrontal cortex (PFC) has justifiably become a significant focus of chronic pain research. Collectively, decades of rodent and human research have provided strong rationale for studying the dysfunction of the PFC as a contributing factor in the development and persistence of chronic pain and as a key supraspinal mechanism for pain-induced comorbidities such as anxiety, depression, and cognitive decline. Chronic pain alters the structure, chemistry, and connectivity of PFC in both humans and rodents. In this review, we broadly summarize the complexities of reported changes within both rodent and human PFC caused by pain and offer insight into potential pharmacological and nonpharmacological approaches for targeting PFC to treat chronic pain and pain-associated comorbidities. SIGNIFICANCE STATEMENT: Chronic pain is a significant unresolved medical problem causing detrimental changes to physiological, psychological, and behavioral aspects of life. Drawbacks of currently approved pain therapeutics include incomplete efficacy and potential for abuse producing a critical need for novel approaches to treat pain and comorbid disorders. This review provides insight into how manipulation of prefrontal cortex circuits could address this unmet need of more efficacious and safer pain therapeutics.

Selective enhancement of fear extinction by inhibiting neuronal adenylyl cyclase 1 (AC1) in aged mice

Adenylyl cyclase 1 (AC1) is a selective subtype of ACs, which is selectively expressed in neurons. The activation of AC1 is activity-dependent, and AC1 plays an important role in cortical excitation that contributes to chronic pain and related emotional disorders. Previous studies have reported that human-used NB001 (hNB001, a selective AC1 inhibitor) produced analgesic effects in different animal models of chronic pain. However, the potential effects of hNB001 on learning and memory have been less investigated. In the present study, we found that hNB001 affected neither the induction nor the expression of trace fear, but selectively enhanced the relearning ability during the extinction in aged mice. By contrast, the same application of hNB001 did not affect recent, remote auditory fear memory, or remote fear extinction in either adult or aged mice. Furthermore, a single or consecutive 30-day oral administration of hNB001 did not affect acute nociceptive response, motor function, or anxiety-like behavior in either adult or aged mice. Our results are consistent with previous findings that inhibition of AC1 did not affect general sensory, emotional, and motor functions in adult mice, and provide strong evidence that inhibiting the activity of AC1 may be beneficial for certain forms of learning and memory in aged mice.

Hippocampal synaptic plasticity injury mediated by SIRT1 downregulation is involved in chronic pain-related cognitive dysfunction

AIMS

Cognitive dysfunction associated with chronic pain may be caused by impaired synaptic plasticity. Considering the impact of silent information regulator 1 (SIRT1) on synaptic plasticity, we explored the exact role of SIRT1 in cognitive impairment caused by chronic pain.

METHODS

We evaluated the memory ability of mice with the fear conditioning test (FCT) after spared nerve injury (SNI) model. Western blotting and immunofluorescence were used to analyze the expression levels of SIRT1. Hippocampal synaptic plasticity was detected with Golgi staining, transmission electron microscopy, and long-term potentiation (LTP). In the intervention study, AAV9-CaMKIIα-Cre-EGFP was injected to SIRT1flox/flox mice to knockdown the expression levels of SIRT1. Besides, SNI mice were injected with AAV2/9-CaMKIIα-SIRT1-3*Flag-GFP or SRT1720 to increase the expression levels or enzymatic activity of SIRT1.

RESULTS

Our current results indicated that cognitive function in SNI mice was impaired, SIRT1 expression in glutaminergic neurons in the hippocampal CA1 area was downregulated, and synaptic plasticity was altered. Selective knockdown of SIRT1 in hippocampus damaged synaptic plasticity and cognitive function of healthy mice. In addition, the impaired synaptic plasticity and cognitive dysfunction of SNI mice could be improved by the upregulation of SIRT1 expression or enzyme activity.

CONCLUSIONS

Reduced SIRT1 expression in hippocampus of SNI mice may induce cognitive impairment associated with chronic pain by mediating the impaired synaptic plasticity.

Inhibition of calcium-stimulated adenylyl cyclase subtype 1 (AC1) for the treatment of pain and anxiety symptoms in Parkinson's disease mice model

Pain and anxiety are two common and undertreated non-motor symptoms in Parkinson's disease (PD), which affect the life quality of PD patients, and the underlying mechanisms remain unclear. As an important subtype of adenylyl cyclases (ACs), adenylyl cyclase subtype 1 (AC1) is critical for the induction of cortical long-term potentiation (LTP) and injury induced synaptic potentiation in the cortical areas including anterior cingulate cortex (ACC) and insular cortex (IC). Genetic deletion of AC1 or pharmacological inhibition of AC1 improved chronic pain and anxiety in different animal models. In this study, we proved the motor deficit, pain and anxiety symptoms of PD in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice model. As a lead candidate AC1 inhibitor, oral administration (1 dose and seven doses) of NB001 (20 and 40 mg/kg) showed significant analgesic effect in MPTP-treated mice, and the anxiety behavior was also reduced (40 mg/kg). By using genetic knockout mice, we found that AC1 knockout mice showed reduced pain and anxiety symptoms after MPTP administration, but not AC8 knockout mice. In summary, genetic deletion of AC1 or pharmacological inhibition of AC1 improved pain and anxiety symptoms in PD model mice, but didn't affect motor function. These results suggest that NB001 is a potential drug for the treatment of pain and anxiety symptoms in PD patients by inhibiting AC1 target.

Recruitment of cortical silent responses by forskolin in the anterior cingulate cortex of adult mice

Recent studies using different experimental approaches demonstrate that silent synapses may exist in the adult cortex including the sensory cortex and anterior cingulate cortex (ACC). The postsynaptic form of long-term potentiation (LTP) in the ACC recruits some of these silent synapses and the activity of calcium-stimulated adenylyl cyclases (ACs) is required for such recruitment. It is unknown if the chemical activation of ACs may recruit silent synapses. In this study, we found that activation of ACs contributed to synaptic potentiation in the ACC of adult mice. Forskolin, a selective activator of ACs, recruited silent responses in the ACC of adult mice. The recruitment was long-lasting. Interestingly, the effect of forskolin was not universal, some silent synapses did not undergo potentiation or recruitment. These findings suggest that these adult cortical synapses are not homogenous. The application of a selective calcium-permeable AMPA receptor inhibitor 1-naphthyl acetyl spermine (NASPM) reversed the potentiation and the recruitment of silent responses, indicating that the AMPA receptor is required. Our results strongly suggest that the AC-dependent postsynaptic AMPA receptor contributes to the recruitment of silent responses at cortical LTP.

Pain-related cortico-limbic plasticity and opioid signaling

Neuroplasticity in cortico-limbic circuits has been implicated in pain persistence and pain modulation in clinical and preclinical studies. The amygdala has emerged as a key player in the emotional-affective dimension of pain and pain modulation. Reciprocal interactions with medial prefrontal cortical regions undergo changes in pain conditions. Other limbic and paralimbic regions have been implicated in pain modulation as well. The cortico-limbic system is rich in opioids and opioid receptors. Preclinical evidence for their pain modulatory effects in different regions of this highly interactive system, potentially opposing functions of different opioid receptors, and knowledge gaps will be described here. There is little information about cell type- and circuit-specific functions of opioid receptor subtypes related to pain processing and pain-related plasticity in the cortico-limbic system. The important role of anterior cingulate cortex (ACC) and amygdala in MOR-dependent analgesia is most well-established, and MOR actions in the mesolimbic system appear to be similar but remain to be determined in mPFC regions other than ACC. Evidence also suggests that KOR signaling generally serves opposing functions whereas DOR signaling in the ACC has similar, if not synergistic effects, to MOR. A unifying picture of pain-related neuronal mechanisms of opioid signaling in different elements of the cortico-limbic circuitry has yet to emerge. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

The anterior cingulate cortex is critical for acute stress-induced hypersensitivity in mice

Stress can be categorized according to physical, psychological and social factors. Exposure to stress produces stress-induced hypersensitivity and forms negative emotions such as anxiety and depression. For example, acute physical stress induced by the elevated open platform (EOP) causes prolonged mechanical hypersensitivity. The anterior cingulate cortex (ACC) is a cortical region involved in pain and negative emotions. Recently, we showed that mice exposed to the EOP changed spontaneous excitatory, but not inhibitory transmission in layer II/III pyramidal neurons of the ACC. However, it is still unclear whether the ACC is involved in the EOP induced mechanical hypersensitivity, and how the EOP alters evoked synaptic transmission on excitatory and inhibitory synaptic transmission in the ACC. In this study, we injected ibotenic acid into the ACC to examine if it was involved in stress-induced mechanical hypersensitivity induced by EOP exposure. Next, by using whole-cell patch-clamp recording from brain slice preparation, we analyzed action potentials and evoked synaptic transmission from layer II/III pyramidal neurons within the ACC. Lesion of the ACC completely blocked the stress-induced mechanical hypersensitivity induced by EOP exposure. Mechanistically, EOP exposure mainly altered evoked excitatory postsynaptic currents such as input-output and paired pulse ratio. Intriguingly, the mice exposed in the EOP also produced low-frequency stimulation induced short-term depression on excitatory synapses in the ACC. These results suggest that the ACC plays a critical role in the modulation of stress-induced mechanical hypersensitivity, possibly through synaptic plasticity on excitatory transmission.

Glial Glutamate Transporter Modulation Prevents Development of Complete Freund's Adjuvant-Induced Hyperalgesia and Allodynia in Mice

Glial glutamate transporter (GLT-1) modulation in the hippocampus and anterior cingulate cortex (ACC) is critically involved in nociceptive pain. The objective of the study was to investigate the effects of 3-[[(2-methylphenyl) methyl] thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, against microglial activation induced by complete Freund's adjuvant (CFA) in a mouse model of inflammatory pain. Furthermore, the effects of LDN-212320 on the protein expression of glial markers, such as ionized calcium-binding adaptor molecule 1 (Iba1), cluster of differentiation molecule 11b (CD11b), mitogen-activated protein kinases (p38), astroglial GLT-1, and connexin 43 (CX43), were measured in the hippocampus and ACC following CFA injection using the Western blot analysis and immunofluorescence assay. The effects of LDN-212320 on the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus and ACC were also assessed using an enzyme-linked immunosorbent assay. Pretreatment with LDN-212320 (20 mg/kg) significantly reduced the CFA-induced tactile allodynia and thermal hyperalgesia. The anti-hyperalgesic and anti-allodynic effects of LDN-212320 were reversed by the GLT-1 antagonist DHK (10 mg/kg). Pretreatment with LDN-212320 significantly reduced CFA-induced microglial Iba1, CD11b, and p38 expression in the hippocampus and ACC. LDN-212320 markedly modulated astroglial GLT-1, CX43, and, IL-1β expression in the hippocampus and ACC. Overall, these results suggest that LDN-212320 prevents CFA-induced allodynia and hyperalgesia by upregulating astroglial GLT-1 and CX43 expression and decreasing microglial activation in the hippocampus and ACC. Therefore, LDN-212320 could be developed as a novel therapeutic drug candidate for chronic inflammatory pain.

TRPA1 as a O2 sensor detects microenvironmental hypoxia in the mice anterior cingulate cortex

Transient receptor potential ankyrin 1 (TRPA1) is a member of the TRP channel family and is expressed in peripheral and central nervous systems. In the periphery, TRPA1 senses cold and pain. However, the functions of TRPA1 in the CNS are unclear. Here, we examined the roles of TRPA1 on neural activity and synaptic transmission in layer II/III pyramidal neurons from mice anterior cingulate cortex (ACC) by whole-cell patch-clamp recordings. The activation of Cinnamaldehyde (CA), which is TRPA1 agonist produced inward currents and these were blocked by the TRPA1 antagonists. Furthermore, activating TRPA1 changed the properties of action potentials such as the firing rate, rise time and decay time. In contrast, stimulating TRPA1 did not alter the spontaneous synaptic transmission. Finally, we examined the functional role of TRPA1 on neurons in a hypoxic environment. We induced an acute hypoxia by substituting nitrogen (N₂) gas for oxygen (O₂) in the external solution. N₂ produced biphasic effects that consisting of inward currents in the early phase and outward currents in the late phase. Importantly, blocking TRPA1 reduced inward currents, but not outward currents. In contrast, a K_ATP channel blocker completely inhibited outward currents. These results suggest that TRPA1 acts on postsynaptic neurons in the ACC as an acute O₂ sensor.

MiR-130/SNAP-25 axis regulate presynaptic alteration in anterior cingulate cortex involved in lead induced attention deficits

Brain volume decrease in the anterior cingulate cortex (ACC) after lead (Pb) exposure has been linked to persistent impairment of attention behavior. However, the precise structural change and molecular mechanism for the Pb-induced ACC alteration and its contribution to inattention have yet to be fully characterized. The present study determined the role of miRNA regulated synaptic structural and functional impairment in the ACC and its relationship to attention deficit disorder in Pb exposed mice. Results showed that Pb exposure induced presynaptic impairment and structural alterations in the ACC. Furthermore, we screened for critical miRNA targets responsible for the synaptic alteration. We found that miR-130, which regulates presynaptic vesicle releasing protein SNAP-25, was responsible for the presynaptic impairment in the ACC and attention deficits in mice. Blocking miR-130 function reversed the Pb-induced decrease in the expression of its presynaptic target SNAP-25, leading to the redistribution of presynaptic vesicles, as well as improved presynaptic function and attention in Pb exposed mice. We report, for the first time, that miR-130 regulating SNAP-25 mediates Pb-induced presynaptic structural and functional impairment in the ACC along with attention deficit disorder in mice.

Comparative investigation of analgesic tolerance to taurine, sodium salicylate and morphine: Involvement of peripheral muscarinic receptors

Nowadays various analgesic medications are used for the management of acute and chronic pain. Among these opioid and non-steroidal anti-inflammatory drugs stand in the first line of therapy, however, prolonged administration of these substance is generally challenged by development of analgesic tolerance in patients. Therefore, it is highly valuable to find new pharmacological strategies for prolonged therapeutic procedures. In this respect, Taurine, a free amino acid, has been shown to induce significant analgesia at both spinal and peripheral levels through cholinergic mechanisms. In the present study, we used hot-plate analgesic test to investigate how taurine either as a single medication or in combination with sodium salicylate and morphine may affect both acute response to pain and development of analgesic tolerance. The effect of taurine was also tested on morphine withdrawal syndrome. Hyoscine butyl bromide was used to assess the role of muscarinic receptors in taurine-mediated effects. Finally, biochemical assay was done to reveal how the activity of brain acetylcholinesterase may change in relation with muscarinic receptor activity. Results indicated that acute administration of taurine-sodium salicylate combination causes more potent analgesia compared to the use of tau (but not SS alone) and this seems to be mediated via activity of muscarinic receptors in peripheral nervous system. Furthermore, the effect of this combination undergoes less analgesic tolerance during time. Combination of taurine and morphine is an effective strategy to attenuate both morphine analgesic tolerance and dependence and this also seems to depend on activity of muscarinic receptors, however through differential cellular mechanisms.

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.

Activation of VIP interneurons in the prefrontal cortex ameliorates neuropathic pain aversiveness

While dysfunction of the medial prefrontal cortex (mPFC) has been implicated in chronic pain, the underlying neural circuits and the contribution of specific cellular populations remain unclear. Using in vivo Ca²⁺ imaging, we report that in both male and female mice, peripheral nerve injury-induced neuropathic pain causes a marked reduction of vasoactive intestinal polypeptide (VIP)-expressing interneuron activity in the prelimbic area of the mPFC, which contributes to decreased prefrontal cortical outputs. Moreover, prelimbic glutamatergic projections to GABAergic interneurons in the anterior cingulate cortex (ACC) are diminished, leading to loss of cortical-cortical inhibition and increased pyramidal neuron activity in the ACC. Chemogenetic activation of prelimbic VIP interneurons restores neuronal responses in the mPFC-ACC pathway and attenuates pain-like behaviors in mice. Furthermore, restoration of prelimbic outputs to the ACC reverses nerve injury-induced ACC hyperactivation. These findings reveal mPFC circuit changes associated with neuropathic pain and highlight VIP interneurons as potential therapeutic targets for pain treatment.

Down-regulation of NR2B receptors contributes to the analgesic and antianxiety effects of enriched environment mediated by endocannabinoid system in the inflammatory pain mice

Chronic pain states are highly prevalent and yet poorly controlled by currently available analgesics. It has been reported that enriched environment (EE), as a new way of endogenous pharmacotherapy, is effective in attenuating chronic inflammatory pain. However, the underlying molecular mechanisms are still not fully understood. NMDA NR2B receptor plays a critical role in pain transmission and modulation. Thus, in this study, we aimed at the effect of EE on the NR2B receptors expression in the prefrontal cortex, hippocampus and thalamus in the inflammatory pain mice. The results showed a significant increase of NR2B receptors in the thalamus of mice at 7 d following injection of CFA in the subcutaneous of the bottom of the left hind paw. EE significantly reduced the duration of mechanical hypersensitivity and anxiety-related behavior and the expression of NR2B receptors as compared to the standard condition. Furthermore, EE significantly increased 2-arachidonoylglycero (2-AG) levels at 7 d in the inflammatory pain mice as compared to the standard condition, and the effect of EE on the behavior and the expression of NR2B receptors was abolished by intraperitoneal injection of AM281 (a selective antagonist of CB1 receptor). Elevated 2-AG levels by intraperitoneal injection of JZL184 (a selective inhibitor of MAGL, the enzyme responsible for 2-AG hydrolysis) produced the same effect as EE. Results from this study provide the evidence that EE mimics endocannabinoids to take analgesic and anti-anxiety activities by decreasing the expression of the NR2B receptors via the CB1 receptor in the thalamus, pending further studies.

Inflammatory Pain Alters Dopaminergic Modulation of Excitatory Synapses in the Anterior Cingulate Cortex of Mice

Pain modulation of dopamine-producing nuclei is known to contribute to the affective component of chronic pain. However, pain modulation of pain-related cortical regions receiving dopaminergic inputs is understudied. The present study demonstrates that mice with chronic inflammatory injury of the hind paws develop persistent mechanical hypersensitivity and transient anxiety. Peripheral inflammation induced by injection of complete Freund's Adjuvant (CFA) induced potentiation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) currents with a presynaptic component in layer II/III of the ACC. After four days of inflammatory pain, the dopamine-mediated inhibition of AMPAR currents was significantly reduced in the ACC. Furthermore, dopamine enhanced presynaptic modulation of excitatory transmission, but only in mice with inflammatory pain. High-performance liquid chromatography (HPLC) analysis of dopamine tissue concentration revealed that dopamine neurotransmitter concentration in the ACC was reduced three days following CFA. Our results demonstrate that inflammatory pain induces activity-dependent changes in excitatory synaptic transmission and alters dopaminergic homeostasis in the ACC.

Topographic and widespread auditory modulation of the somatosensory cortex: potential for bimodal sound and body stimulation for pain treatment

OBJECTIVE

There has been growing interest in understanding multisensory integration in the cortex through activation of multiple sensory and motor pathways to treat brain disorders, such as tinnitus or essential tremors. For tinnitus, previous studies show that combined sound and body stimulation can modulate the auditory pathway and lead to significant improvements in tinnitus symptoms. Considering that tinnitus is a type of chronic auditory pain, bimodal stimulation could potentially alter activity in the somatosensory pathway relevant for treating chronic pain. As an initial step towards that goal, we mapped and characterized neuromodulation effects in the somatosensory cortex (SC) in response to sound and/or electrical stimulation of the body.

APPROACH

We first mapped the topographic organization of activity across the SC of ketamine-anesthetized guinea pigs through electrical stimulation of different body locations using subcutaneous needle electrodes or with broadband acoustic stimulation. We then characterized how neural activity in different parts of the SC could be facilitated or suppressed with bimodal stimulation.

MAIN RESULTS

The topography in the SC of guinea pigs in response to electrical stimulation of the body aligns consistently to that shown in previous rodent studies. Interestingly, auditory broadband noise stimulation primarily excited SC areas that typically respond to stimulation of lower body locations. Although there was only a small subset of SC locations that were excited by acoustic stimulation alone, all SC recording sites could be altered (facilitated or suppressed) with bimodal stimulation. Furthermore, specific regions of the SC could be modulated by stimulating an appropriate body region combined with broadband noise.

SIGNIFICANCE

These findings show that bimodal stimulation can excite or modulate firing across a widespread yet targeted population of SC neurons. This approach may provide a non-invasive method for altering or disrupting abnormal firing patterns within certain parts of the SC for chronic pain treatment.

Role of anterior cingulate cortex inputs to periaqueductal gray for pain avoidance

Although pain-related excessive fear is known to be a key factor in chronic pain disability, which involves the anterior cingulate cortex (ACC), little is known about the downstream circuits of the ACC for fear avoidance in pain processing. Using behavioral experiments and functional magnetic resonance imaging with optogenetics at 15.2 T, we demonstrate that the ACC is a part of the abnormal circuit changes in chronic pain and its downstream circuits are closely related to modulating sensorimotor integration and generating active movement rather than carrying sensory information. The projection from the ACC to the dorsolateral and lateral parts of the periaqueductal gray (dl/lPAG) especially enhances both reflexive and active avoidance behavior toward pain. Collectively, our results indicate that increased signals from the ACC to the dl/lPAG might be critical for excessive fear avoidance in chronic pain disability.

Physiological roles of mammalian transmembrane adenylyl cyclase isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors (GPCRs). The transmembrane ACs display varying expression patterns across tissues, giving the potential for them to have a wide array of physiological roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gαs, so it was long assumed that all ACs are activated by Gαs-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform-specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

Chronic facial inflammatory pain-induced anxiety is associated with bilateral deactivation of the rostral anterior cingulate cortex

Patients with chronic pain, especially orofacial pain, often suffer from affective disorders, including anxiety. Previous studies largely focused on the role of the caudal anterior cingulate cortex (cACC) in affective responses to pain, long-term potentiation (LTP) in cACC being thought to mediate the interaction between anxiety and chronic pain. But recent evidence indicates that the rostral ACC (rACC), too, is implicated in processing affective pain. However, whether such processing is associated with neuronal and/or synaptic plasticity is still unknown. We addressed this issue in a chronic facial inflammatory pain model (complete Freund's adjuvant model) in rats, by combining behavior, Fos protein immunochemistry and ex vivo intracellular recordings in rACC slices prepared from these animals. Facial mechanical allodynia occurs immediately after CFA injection, peaks at post-injection day 3 and progressively recovers until post-injection days 10-11, whereas anxiety is delayed, being present at post-injection day 10, when sensory hypersensitivity is relieved, but, notably, not at post-injection day 3. Fos expression reveals that neuronal activity follows a bi-phasic time course in bilateral rACC: first enhanced at post-injection day 3, it gets strongly depressed at post-injection day 10. Ex vivo recordings from lamina V pyramidal neurons, the rACC projecting neurons, show that both their intrinsic excitability and excitatory synaptic inputs have undergone long-term depression (LTD) at post-injection day 10. Thus chronic pain processing is associated with dynamic changes in rACC activity: first enhanced and subsequently decreased, at the time of anxiety-like behavior. Chronic pain-induced anxiety might thus result from a rACC deactivation-cACC hyperactivation interplay.

A transcriptomic analysis of neuropathic pain in the anterior cingulate cortex after nerve injury

The anterior cingulate cortex (ACC) is a core brain region processing pain emotion. In this study, we performed RNA sequencing analysis to reveal transcriptomic profiles of the ACC in a rat chronic constriction injury (CCI) model. A total of 1628 differentially expressed genes (DEGs) were identified by comparing sham-operated rats with rats of 12 hours, 1, 3, 7, and 14 days after surgery, respectively. Although these inflammatory-related DEGs were generally increased after CCI, different kinetics of time-series expression were observed with the development of neuropathic pain affection. Specifically, the expression of Ccl5, Cxcl9 and Cxcl13 continued to increase following CCI. The expression of Ccl2, Ccl3, Ccl4, Ccl6, and Ccl7 were initially upregulated after CCI and subsequently decreased after 12 hours. Similarly, the expression of Rac2, Cd68, Icam-1, Ptprc, Itgb2, and Fcgr2b increased after 12 hours but reduced after 1 day. However, the expression of the above genes increased again 7 days after CCI, when the neuropathic pain affection had developed. Furthermore, gene ontology analysis, Kyoto Encyclopedia of Genes and Genomes pathway enrichment and interaction network analyses further showed a high connectivity degree among these chemokine targeting genes. Similar expressional changes in these genes were found in the rat spinal dorsal horn responsible for nociception processing. Taken together, our results indicated chemokines and their targeting genes in the ACC may be differentially involved in the initiation and maintenance of neuropathic pain affection. These genes may be a target for not only the nociception but also the pain affection following nerve injury.

Differential Rearrangement of Excitatory Inputs to the Medial Prefrontal Cortex in Chronic Pain Models

Chronic pain patients suffer a disrupted quality of life not only from the experience of pain itself, but also from comorbid symptoms such as depression, anxiety, cognitive impairment, and sleep disturbances. The heterogeneity of these symptoms support the idea of a major involvement of the cerebral cortex in the chronic pain condition. Accordingly, abundant evidence shows that in chronic pain the activity of the medial prefrontal cortex (mPFC), a brain region that is critical for executive function and working memory, is severely impaired. Excitability of the mPFC depends on the integrated effects of intrinsic excitability and excitatory and inhibitory inputs. The main extracortical sources of excitatory input to the mPFC originate in the thalamus, hippocampus, and amygdala, which allow the mPFC to integrate multiple information streams necessary for cognitive control of pain including sensory information, context, and emotional salience. Recent techniques, such as optogenetic methods of circuit dissection, have made it possible to tease apart the contributions of individual circuit components. Here we review the synaptic properties of these main glutamatergic inputs to the rodent mPFC, how each is altered in animal models of chronic pain, and how these alterations contribute to pain-associated mPFC deactivation. By understanding the contributions of these individual circuit components, we strive to understand the broad spectrum of chronic pain and comorbid pathologies, how they are generated, and how they might be alleviated.

Synaptic potentiation of anterior cingulate cortex contributes to chronic pain of Parkinson's disease

Parkinson’s disease (PD) is a multi-system neurodegenerative disorder. Patients with PD often suffer chronic pain. In the present study, we investigated motor, sensory and emotional changes in three different PD mice models. We found that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treatment caused significant changes in all measurements. Mechanical hypersensitivity of PD model induced by MPTP peaked at 3 days and persisted for at least 14 days. Using Fos transgenic mice, we found that neurons in the anterior cingulate cortex (ACC) were activated after MPTP treatment. Inhibiting ACC by bilateral microinjection of muscimol significantly reduced mechanical hypersensitivity and anxiety-like responses. By contrast, MPTP induced motor deficit was not affected, indicating ACC activity is mostly responsible for sensory and emotional changes. We also investigated excitatory synaptic transmission and plasticity using brain slices of MPTP treated animals. While L-LTP was blocked or significantly reduced. E-LTP was not significantly affected in slices of MPTP treated animals. LTD induced by repetitive stimulation was not affected. Furthermore, we found that paired-pulse facilitation and spontaneous release of glutamate were also altered in MPTP treated animals, suggesting presynaptic enhancement of excitatory transmission in PD. Our results suggest that ACC synaptic transmission is enhanced in the animal model of PD, and cortical excitation may play important roles in PD related pain and anxiety.

Inhibition of calcium-stimulated adenylyl cyclase subtype 1 (AC1) for the treatment of neuropathic and inflammatory pain in adult female mice

Cortical long-term potentiation (LTP) serves as a cellular model for chronic pain. As an important subtype of adenylyl cyclases (ACs), adenylyl cyclase subtype 1 (AC1) is critical for the induction of cortical LTP in the anterior cingulate cortex (ACC). Genetic deletion of AC1 or pharmacological inhibition of AC1 blocked behavioral allodynia in animal models of neuropathic and inflammatory pain. Our previous experiments have identified a lead candidate AC1 inhibitor, NB001, which is highly selective for AC1 over other AC isoforms, and found that NB001 is effective in inhibiting behavioral allodynia in animal models of chronic neuropathic and inflammatory pain. However, previous experiments were carried out in adult male animals. Considering the potential gender difference as an important issue in researches of pain and analgesia, we investigated the effect of NB001 in female chronic pain animal models. We found that NB001, when administered orally, has an analgesic effect in female animal models of neuropathic and inflammatory pain without any observable side effect. Genetic deletion of AC1 also reduced allodynia responses in models of neuropathic pain and chronic inflammation pain in adult female mice. In brain slices of adult female mice, bath application of NB001(20 μM) blocked the induction of LTP in ACC. Our results indicate that calcium-stimulated AC1 is required for injury-related cortical LTP and behavioral allodynia in both sexes of adult animals, and NB001 can be used as a potential therapeutic drug for treating neuropathic and inflammatory pain in man and woman.

Role of the Anterior Cingulate Cortex in Translational Pain Research

As the most common symptomatic reason to seek medical consultation, pain is a complex experience that has been classified into different categories and stages. In pain processing, noxious stimuli may activate the anterior cingulate cortex (ACC). But the function of ACC in the different pain conditions is not well discussed. In this review, we elaborate the commonalities and differences from accumulated evidence by a variety of pain assays for physiological pain and pathological pain including inflammatory pain, neuropathic pain, and cancer pain in the ACC, and discuss the cellular receptors and signaling molecules from animal studies. We further summarize the ACC as a new central neuromodulation target for invasive and non-invasive stimulation techniques in clinical pain management. The comprehensive understanding of pain processing in the ACC may lead to bridging the gap in translational research between basic and clinical studies and to develop new therapies.

Acute elevated platform triggers stress induced hyperalgesia and alters glutamatergic transmission in the adult mice anterior cingulate cortex

Pain is composed of both physiological and affective/emotional components which potentiate one another. In addition, exposure to stress modulates pain and affective behaviors including, anxiety-like behavior and/or depression-like behaviors. Indeed, chronic exposure to stress has been known to enhance stress-induced hyperalgesia (SIH). The anterior cingulate cortex (ACC) is critically involved in pain sensation and emotions. Animal models of chronic pain, but not acute nociception have been found to induce synaptic plasticity on glutamatergic and GABAergic transmission in the rodent ACC. However, it is unclear whether acute stress exposure could produce SIH and cause synaptic plasticity in the ACC. Accordingly, we studied how acute exposure of stress by the elevated open platform (EOP) could affect mechanical threshold, thermal and cold latency in the adult mice. Thirty minutes of the EOP produced mechanical hypersensitivity lasting for 60 min and thermal hypersensitivity immediately after the exposure. Next, we tested whether the stress could alter the excitatory and inhibitory synaptic transmission in the ACC. We performed whole-cell patch-clamp recordings from layer II/III pyramidal neurons in the ACC and analyzed both glutamatergic and GABAergic transmission in mice following the EOP. Thirty minutes of the EOP altered the rise and decay time of spontaneous glutamatergic AMPA/GluK receptors mediated currents, but did not change the frequency or amplitude of excitatory transmission. By contrast, the kinetics of inhibitory synaptic currents were not altered by the EOP. These results suggest that acute stress by the elevated platform produces SIH and causes synaptic plasticity on excitatory transmission, but not inhibitory transmission in the ACC.

Paeonol ameliorates CFA-induced inflammatory pain by inhibiting HMGB1/TLR4/NF-κB p65 pathway

The enhanced release of inflammatory cytokines mediated by high mobility group box1 (HMGB1) leads to pain sensation, and has been implicated in the etiology of inflammatory pain. Paeonol (PAE), a major active phenolic component in Cortex Moutan, provides neuroprotective efficacy via exerting anti-inflammatory effect. However, the role and mechanism of PAE in inflammatory pain remain to be fully clarified. In this study, we showed that PAE treatment significantly ameliorated mechanical and thermal hyperalgesia of mice induced by complete Freund's adjuvant (CFA). The analgesic effect of PAE administration was associated with suppressing the enhanced expression of HMGB1 as well as the downstream signaling molecules including toll-like receptor 4 (TLR4), the nuclear NF-κB p65, TNF-α and IL-1β after CFA insult in the anterior cingulate cortex (ACC), a key brain region responsible for pain processing. Furthermore, inhibition of HMGB1 activity by glycyrrhizin (GLY), an HMGB1 inhibitor, alleviated CFA-induced pain and also facilitated PAE-mediated analgesic effect in mice along with the decreased expression of TLR4, NF-κB p65, TNF-α and IL-1β upon CFA injury. Collectively, we showed PAE exerted analgesic effect through inhibiting the HMGB1/TLR4/NF-κB p65 pathway and subsequent generation of cytokines TNF-α and IL-1β in the ACC.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H⁺] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca²⁺, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H⁺ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

Neuronal Correlates of Cognitive Control Are Altered in Women With Endometriosis and Chronic Pelvic Pain

Endometriosis is a debilitating women's health condition and is the most common cause of chronic pelvic pain. Impaired cognitive control is common in chronic pain conditions, however, it has not yet been investigated in endometriosis. The aim of this study was to explore the neuronal correlates of cognitive control in women with endometriosis. Using a cross-sectional study design with data collected at a single time-point, event-related potentials were elicited during a cued continuous performance test from 20 women with endometriosis (mean age = 28.5 ± 5.2 years) and 20 age- and gender-matched controls (mean age = 28.5 ± 5.2 years). Event-related potential components were extracted and P3 component amplitudes were derived with temporal principal components analysis. Behavioral and ERP outcomes were compared between groups and subjective pain severity was correlated with ERP component amplitudes. No significant behavioral differences were seen in task performance between the groups (all p > 0.094). Target P3b (all p < 0.034) and SW (all p < 0.040), and non-target early P3a (eP3a; all p < 0.023) and late P3a (lP3a; all p < 0.035) amplitudes were smaller for the endometriosis compared to the healthy control group. Lower non-target eP3a (p < 0.001), lP3a (p = 0.013), and SW (p = 0.019) amplitudes were correlated with higher pain severity scores. Findings suggest that endometriosis-associated chronic pelvic pain is linked to alterations in stimulus-response processing and inhibitory control networks, but not impaired behavioral performance, due to compensatory neuroplastic changes in overlapping cognitive control and pain networks.

Role of mu-opioid receptor in nociceptive modulation in anterior cingulate cortex of rats

Lots of studies have demonstrated that anterior cingulate cortex plays important roles in the pain perception and pain modulation. The present study explored the role of mu-opioid receptor in nociceptive modulation in anterior cingulate cortex of rats with neuropathic pain. Neuropathic pain model was set up by chronic constriction injury of the left sciatic nerve of rats. The hindpaw withdrawal latency to thermal and mechanical stimulation, by hot plate and Randall Selitto Test respectively, was used to evaluate the rat’s responses to noxious stimulation. Results showed that intra-anterior cingulate cortex injection of morphine could induce the antinociception dose-dependently. By intra-anterior cingulate cortex injection of opioid receptor antagonist, the morphine-induced antinociception could be attenuated by naloxone, as well as much significantly by the selective mu-opioid receptor antagonist β-funaltrexamine, indicating that mu-opioid receptor is involved in the morphine-induced antinociception in anterior cingulate cortex of rats with neuropathic pain. The morphine-induced antinociception was much more decreased in rats with neuropathic pain than that in normal rats, and there was a significant decrease in mu-opioid receptor messenger RNA levels in anterior cingulate cortex of rats with neuropathic pain, indicating that there may be a down-regulation in mu-opioid receptor expression in anterior cingulate cortex of rats with neuropathic pain. To further confirm the role of mu-opioid receptor in morphine-induced antinociception in anterior cingulate cortex, normal rats were received intra-anterior cingulate cortex administration of small interfering RNA targeting mu-opioid receptor and it was found that there was a down-regulation in mu-opioid receptor messenger RNA levels, as well as a down-regulation in mu-opioid receptor expression in anterior cingulate cortex tested by real-time polymerase chain reaction and western blotting. Furthermore, the morphine-induced antinociceptive effect decreased significantly in rats with small interfering RNA targeting mu-opioid receptor, which indicated that knockdown mu-opioid receptor in anterior cingulate cortex could also attenuate morphine-induced antinociceptive effect. These results strongly suggest that mu-opioid receptor plays a significant role in nociceptive modulation in anterior cingulate cortex of rats.

D1 receptors in the anterior cingulate cortex modulate basal mechanical sensitivity threshold and glutamatergic synaptic transmission

The release of dopamine (DA) into target brain areas is considered an essential event for the modulation of many physiological effects. While the anterior cingulate cortex (ACC) has been implicated in pain related behavioral processes, DA modulation of synaptic transmission within the ACC and pain related phenotypes remains unclear. Here we characterized a Crispr/Cas9 mediated somatic knockout of the D1 receptor (D1R) in all neuronal subtypes of the ACC and find reduced mechanical thresholds, without affecting locomotion and anxiety. Further, the D1R high-efficacy agonist SKF 81297 and low efficacy agonist (±)-SKF-38393 inhibit α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) currents in the ACC. Paradoxically, the D1R antagonists SCH-23390 and SCH 33961 when co-applied with D1R agonists produced a robust short-term synergistic depression of AMPAR currents in the ACC, demonstrating an overall inhibitory role for D1R ligands. Overall, our data indicate that absence of D1Rs in the ACC enhanced peripheral sensitivity to mechanical stimuli and D1R activation decreased glutamatergic synaptic transmission in ACC neurons.

Neuronal Adenylyl Cyclase Targeting Central Plasticity for the Treatment of Chronic Pain

Chronic pain is a major health problem and the effective treatment for chronic pain is still lacking. The recent crisis created by the overuse of opioids for pain treatment has clearly shown the need for non-addictive novel pain medicine. Conventional pain medicines usually inhibit peripheral nociceptive transmission and reduce central transmission, especially pain-related excitatory transmission. For example, both opioids and gabapentin produce analgesic effects by inhibiting the release of excitatory transmitters and reducing neuronal excitability. Here, we will review recent studies of central synaptic plasticity contributing to central sensitization in chronic pain. Neuronal selective adenylyl cyclase subtype 1 (AC1) is proposed to be a key intracellular protein that causes both presynaptic and postsynaptic forms of long-term potentiation (LTP). Inhibiting the activity of AC1 by selective inhibitor NB001 blocks behavioral sensitization and injury-related anxiety in animal models of chronic pain. We propose that inhibiting injury-related LTPs will provide new mechanisms for designing novel medicines for the treatment of chronic pain and its related emotional disorders.

Reduced behavioral withdrawal responses during fear retrieval in adult mice and rats

Pain triggers emotional changes in humans and animals, including fear and anxiety. Conversely, fear and anxiety may enhance suffering of patients with pain. However, in animal models of acute pain, it has been reported that fear may inhibit pain by activating endogenous inhibitory systems. In this study, we wanted to examine if behavioral withdrawal responses may be affected during fear retrieval, a condition where fear-associated tone is applied. We found that thermal pain thresholds were significantly increased during fear retrieval. Our results indicate that animals are suffering fear like-events, while their behavioral responses are inhibited. These results indicate that it will be important to evaluate both emotional and behavioral withdrawal responses for future development of new pain medicine.

The Medial Prefrontal Cortex as a Central Hub for Mental Comorbidities Associated with Chronic Pain

Chronic pain patients frequently develop and suffer from mental comorbidities such as depressive mood, impaired cognition, and other significant constraints of daily life, which can only insufficiently be overcome by medication. The emotional and cognitive components of pain are processed by the medial prefrontal cortex, which comprises the anterior cingulate cortex, the prelimbic, and the infralimbic cortex. All three subregions are significantly affected by chronic pain: magnetic resonance imaging has revealed gray matter loss in all these areas in chronic pain conditions. While the anterior cingulate cortex appears hyperactive, prelimbic, and infralimbic regions show reduced activity. The medial prefrontal cortex receives ascending, nociceptive input, but also exerts important top-down control of pain sensation: its projections are the main cortical input of the periaqueductal gray, which is part of the descending inhibitory pain control system at the spinal level. A multitude of neurotransmitter systems contributes to the fine-tuning of the local circuitry, of which cholinergic and GABAergic signaling are particularly emerging as relevant components of affective pain processing within the prefrontal cortex. Accordingly, factors such as distraction, positive mood, and anticipation of pain relief such as placebo can ameliorate pain by affecting mPFC function, making this cortical area a promising target region for medical as well as psychosocial interventions for pain therapy.

Ascending noradrenergic excitation from the locus coeruleus to the anterior cingulate cortex

Anterior cingulate cortex (ACC) plays important roles in sensory perception including pain and itch. Neurons in the ACC receive various neuromodulatory inputs from subcortical structures, including locus coeruleus noradrenaline (LC-NA) neurons. Few studies have been reported about synaptic and behavioral functions of LC-NA projections to the ACC. Using viral-genetic method (AAV-DIO-eYFP) on DBH-cre mice, we found that LC-NA formed synaptic connections to ACC pyramidal cells but not interneurons. This is further supported by the electron microscopic study showing NAergic fibers contact the presynaptic inputs and post-synaptic areas of the pyramidal cells. NA application produced both pre- and post-synaptic potentiation effects in ACC excitatory transmission in vivo and in vitro. Activation of LC-NA projection to the ACC by optogenetic method produced enhancement of excitatory transmission in vitro and induced scratching and behavioral sensitization for mechanical stimulation. Our results demonstrate that LC-NA projections enhance or facilitate brain responses to pain and itch by potentiating glutamatergic synaptic transmissions in the ACC.

Spinal hevin mediates membrane trafficking of GluA1-containing AMPA receptors in remifentanil-induced postoperative hyperalgesia in mice

INTRODUCTION

Hevin, a matricellular protein involved in tissue repair and remodeling, is crucial for initiation and development of excitatory synapses. Besides, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA) is an ionotropic transmembrane receptor for glutamate that mediates fast synaptic transmission in the central nervous system (CNS). This study aimed to investigate the correlation between spinal Hevin and AMPA receptors in remifentanil-induced postoperative hyperalgesia in mice.

METHODS

Remifentanil (1.33 μg/kg/min for 60 min) was subcutaneously injected into a mouse model of postoperative pain. The von Frey and hot plate tests were performed to assess mechanical and thermal hyperalgesia. The gene and protein expression of Hevin and the membrane trafficking of GluA1-containing AMPA receptors in the dorsal horn of spinal cord were detected by quantitative reverse transcription polymerase chain reaction (RT-qPCR) and Western blot analysis. In addition, Hevin-shRNA, exogenous Hevin, and 1-naphtylacetyl-spermine (NASPM) were administrated intrathecally to assess the relationship between spinal Hevin and AMPA receptors.

RESULTS

Perioperative administration of remifentanil can aggravate and prolong incision-induced mechanical and thermal hyperalgesia. Treatment with remifentanil increased the expression of spinal Hevin and the membrane trafficking of AMPA receptors. Additionally, knockdown of spinal Hevin attenuated pain hypersensitivity and downregulated membrane trafficking of AMPA receptors after treatment with remifentanil. Meanwhile, preadministration of NASPM reversed spontaneous pain and membrane trafficking of spinal GluA1-containing AMPA receptors induced by exogenous Hevin in naïve mice.

CONCLUSIONS

Spinal Hevin was involved in the maintenance of remifentanil-induced postoperative hyperalgesia via modulating membrane trafficking of AMPA receptors.

Electroacupuncture Alleviates Pain-Related Emotion by Upregulating the Expression of NPS and Its Receptor NPSR in the Anterior Cingulate Cortex and Hypothalamus

Objective

Electroacupuncture (EA) is reported effective in alleviating pain-related emotion; however, the underlying mechanism of its effects still needs to be elucidated. The NPS-NPSR system has been validated for the involvement in the modulation of analgesia and emotional behavior. Here, we aimed to investigate the role of the NPS-NPSR system in the anterior cingulate cortex (ACC), hypothalamus, and central amygdala (CeA) in the use of EA to relieve affective pain modeled by complete Freund's adjuvant- (CFA-) evoked conditioned place aversion (C-CPA). Materials and Methods. CFA injection combined with a CPA paradigm was introduced to establish the C-CPA model, and the elevated O-maze (EOM) was used to test the behavioral changes after model establishment. We further explored the expression of NPS and NPSR at the protein and gene levels in the brain regions of interest by immunofluorescence staining and quantitative real-time PCR.

Results

We observed that EA stimulation delivered to the bilateral Zusanli (ST36) and Kunlun (BL60) acupoints remarkably inhibited sensory pain, pain-evoked place aversion, and anxiety-like behavior. The current study showed that EA significantly enhanced the protein expression of this peptide system in the ACC and hypothalamus, while the elevated expression of NPSR protein alone was just confined to the affected side in the CeA. Moreover, EA remarkably upregulated the mRNA expression of NPS in CeA, ACC, and hypothalamus and NPSR mRNA in the hypothalamus and CeA.

Conclusions

These data suggest the effectiveness of EA in alleviating affective pain, and these benefits may at least partially be attributable to the upregulation of the NPS-NPSR system in the ACC and hypothalamus.

Cyclic AMP-dependent positive feedback signaling pathways in the cortex contributes to visceral pain

Cortical areas including the anterior cingulate cortex (ACC) play critical roles in different types of chronic pain. Most of previous studies focus on the sensory inputs from somatic areas, and less information about plastic changes in the cortex for visceral pain. In this study, chronic visceral pain animal model was established by injection with zymosan into the colon of adult male C57/BL6 mice. Whole cell patch-clamp recording, behavioral tests, western blot, and Cannulation and ACC microinjection were employed to explore the role of adenylyl cyclase 1 (AC1) in the ACC of C57/BL6 and AC1 knock out mice. Integrative approaches were used to investigate possible changes of neuronal AC1 in the ACC after the injury. We found that AC1, a key enzyme for pain-related cortical plasticity, was significantly increased in the ACC in an animal model of irritable bowel syndrome. Inhibiting AC1 activity by a selective AC1 inhibitor NB001 significantly reduced the up-regulation of AC1 protein in the ACC. Furthermore, we found that AC1 is required for NMDA GluN2B receptor up-regulation and increases of NMDA receptor-mediated currents. These results suggest that AC1 may form a positive regulation in the cortex during chronic visceral pain. Our findings demonstrate that the up-regulation of AC1 protein in the cortex may underlie the pathology of chronic visceral pain; and inhibiting AC1 activity may be beneficial for the treatment of visceral pain.

Presynaptic glutamatergic transmission and feedback system of oxytocinergic neurons in the hypothalamus of a rat model of adjuvant arthritis

The neurohypophysial hormone oxytocin (OXT) is synthesized in the hypothalamic paraventricular and supraoptic nuclei. Recently, some studies have considered OXT to be important in sensory modulation and that the OXT protein is upregulated by acute and chronic nociception. However, the mechanism by which OXT is upregulated in neurons is unknown. In this study, we examined the resting membrane potentials and excitatory postsynaptic currents in OXT-ergic neurons in the paraventricular nucleus in adjuvant arthritis rat model, a model of chronic inflammation, using whole-cell patch-clamping. Transgenic rats expressing OXT and monomeric red fluorescent protein 1 (mRFP1) fusion protein to visualize the OXT-ergic neurons were used, and the OXT-mRFP1 transgenic rat model of adjuvant arthritis was developed by injection of heat-killed Mycobacterium butyricum. Furthermore, the feedback system of synthesized OXT was also examined using the OXT receptor antagonist L-368,899. We found that the resting membrane potentials and frequency of miniature excitatory postsynaptic currents and spontaneous excitatory postsynaptic currents in OXT-monomeric red fluorescent protein 1 neurons in the paraventricular nucleus were significantly increased in adjuvant arthritis rats. Furthermore, L-368,899 dose-dependently increased the frequency of miniature excitatory postsynaptic currents and spontaneous excitatory postsynaptic currents in OXT-ergic neurons. Following bath application of the GABAA receptor antagonist picrotoxin and the cannabinoid receptor 1 antagonist AM 251, L-368,899 still increased the frequency of miniature excitatory postsynaptic currents. However, following bath application of the nitric oxide synthase inhibitor Nω-Nitro-L-arginine methyl ester hydrochloride, L-368,899 did not alter the miniature excitatory postsynaptic current frequency. Thus, it is suggested that OXT-ergic neuron activity is upregulated via an increase in glutamate release, and that the upregulated OXT neurons have a feedback system with released endogenous OXT. It is possible that nitric oxide, but not GABA, may contribute to the feedback system of OXT neurons in chronic inflammation.

Exposure to workplace bullying, microRNAs and pain; evidence of a moderating effect of miR-30c rs928508 and miR-223 rs3848900

Prolonged exposure to bullying behaviors may give rise to symptoms such as anxiety, depression and chronic pain. Earlier data suggest that these symptoms often are associated with stress-induced low-grade systemic inflammation. Here, using data from both animals and humans, we examined the moderating role of microRNAs (miRNAs, miRs) in this process. In the present study, a resident-intruder paradigm, blood samples, tissue harvesting and subsequent qPCR analyses were used to screen for stress-induced changes in circulating miRNAs in rats. The negative acts questionnaire (NAQ), TaqMan assays and a numeric rating scale (NRS) for pain intensity were then used to examine the associations among bullying behaviors, relevant miRNA polymorphisms and pain in a probability sample of 996 Norwegian employees. In rats, inhibited weight gain, reduced pituitary POMC expression, adrenal Nr3c1 mRNA downregulation, as well as increased miR-146a, miR-30c and miR-223 in plasma were observed following 1 week of repeated exposure to social stress. When following up the miRNA findings from the animal study in the human working population, a stronger relationship between NAQ and NRS scores was observed in subjects with the miR-30c GG genotype (rs928508) compared to other subjects. A stronger relationship between NAQ and NRS scores was also seen in men with the miR-223 G genotype (rs3848900) as compared to other men. Our findings show that social stress may induce many physiological changes including changed expression of miRNAs. We conclude that the miR-30c GG genotype in men and women, and the miR-223 G genotype in men, amplify the association between exposure to bullying behaviors and pain.Lay summaryUsing an animal model of social stress, we identified miR-146a, miR-30c and miR-223 as potentially important gene regulatory molecules that may be involved in the stress response. Interestingly, human genotypes affecting the expression of mature miR-30c and miR-223 had a moderating effect on the association between exposure to bullying and pain. Subjects with the miR-30c rs928508 GG genotype had a significantly stronger association between exposure to bullying behaviors and pain than other subjects. The same was observed in men with the miR-223 rs3848900 G genotype, as compared to other men.

Protein Kinase C Lambda Mediates Acid-Sensing Ion Channel 1a-Dependent Cortical Synaptic Plasticity and Pain Hypersensitivity

Chronic pain is a serious debilitating disease for which effective treatment is still lacking. Acid-sensing ion channel 1a (ASIC1a) has been implicated in nociceptive processing at both peripheral and spinal neurons. However, whether ASIC1a also contributes to pain perception at the supraspinal level remains elusive. Here, we report that ASIC1a in ACC is required for thermal and mechanical hypersensitivity associated with chronic pain. ACC-specific genetic deletion or pharmacological blockade of ASIC1a reduced the probability of cortical LTP induction and attenuated inflammatory thermal hyperalgesia and mechanical allodynia in male mice. Using cell type-specific manipulations, we demonstrate that ASIC1a in excitatory neurons of ACC is a major player in cortical LTP and pain behavior. Mechanistically, we show that ASIC1a tuned pain-related cortical plasticity through protein kinase C λ-mediated increase of membrane trafficking of AMPAR subunit GluA1 in ACC. Importantly, postapplication of ASIC1a inhibitors in ACC reversed previously established nociceptive hypersensitivity in both chronic inflammatory pain and neuropathic pain models. These results suggest that ASIC1a critically contributes to a higher level of pain processing through synaptic potentiation in ACC, which may serve as a promising analgesic target for treatment of chronic pain.SIGNIFICANCE STATEMENT Chronic pain is a debilitating disease that still lacks effective therapy. Ion channels are good candidates for developing new analgesics. Here, we provide several lines of evidence to support an important role of cortically located ASIC1a channel in pain hypersensitivity through promoting long-term synaptic potentiation in the ACC. Our results indicate a promising translational potential of targeting ASIC1a to treat chronic pain.

Epigenetic suppression of liver X receptor β in anterior cingulate cortex by HDAC5 drives CFA-induced chronic inflammatory pain

Background

Liver X receptors (LXRs), including LXRα and LXRβ, are key regulators of transcriptional programs for both cholesterol homeostasis and inflammation in the brain. Here, the modes of action of LXRs and the epigenetic mechanisms regulating LXRβ expression in anterior cingulate cortex (ACC) of chronic inflammatory pain (CIP) are investigated.

Methods

The deficit of LXR isoform and analgesic effect of LXR activation by GW3965 were evaluated using the mouse model of CIP induced by hindpaw injection of complete Freund’s adjuvant (CFA). The mechanisms involved in GW-mediated analgesic effects were analyzed with immunohistochemical methods, ELISA, co-immunoprecipitation (Co-IP), Western blot, and electrophysiological recording. The epigenetic regulation of LXRβ expression was investigated by chromatin immunoprecipitation, quantitative real-time PCR, and sequencing.

Results

We revealed that CFA insult led to LXRβ reduction in ACC, which was associated with upregulated expression of histone deacetylase 5 (HDAC5), and knockdown of LXRβ by shRNA led to thermal hyperalgesia. Co-IP showed that LXRβ interacted with NF-κB p65 physically. LXRβ activation by GW3965 exerted analgesic effects by inhibiting the nuclear translocation of NF-κB, reducing the phosphorylation of mitogen-activated protein kinases (MAPKs) in ACC, and decreasing the promoted input-output and enhanced mEPSC frequency in ACC neurons after CFA exposure. In vitro experiments confirmed that HDAC5 triggered histone deacetylation on the promoter region of Lxrβ, resulting in downregulation of Lxrβ transcription.

Conclusion

These findings highlight an epigenetic mechanism underlying LXRβ deficits linked to CIP, and LXRβ activation may represent a potential novel target for the treatment of CIP with an alteration in inflammation responses and synaptic transmission in ACC.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1507-3) contains supplementary material, which is available to authorized users.

Selective optogenetic inhibition of medial prefrontal glutamatergic neurons reverses working memory deficits induced by neuropathic pain

Stability of local medial prefrontal cortex (mPFC) network activity is believed to be critical for sustaining cognitive processes such as working memory (WM) and decision making. Dysfunction of the mPFC has been identified as a leading cause to WM deficits in several chronic pain conditions; however, the underlying mechanisms remain largely undetermined. Here, to address this issue, we implanted multichannel arrays of electrodes in the prelimbic region of the mPFC and recorded the neuronal activity during a food-reinforced delayed nonmatch to sample (DNMS) task of spatial WM. In addition, we used an optogenetic technique to selectively suppress the activity of excitatory pyramidal neurons that are considered the neuronal substrate for memory retention during the delay period of the behavioral task. Within-subject behavioral performance and pattern of neuronal activity were assessed after the onset of persistent pain using the spared nerve injury model of peripheral neuropathy. Our results show that the nerve lesion caused a disruption in WM and prelimbic spike activity and that this disruption was reversed by the selective inhibition of prelimbic glutamatergic pyramidal neurons during the delay period of the WM task. In spared nerve injury animals, photoinhibition of excitatory neurons improved the performance level and restored neural activity to a similar profile observed in the control animals. In addition, we found that selective inhibition of excitatory neurons does not produce antinociceptive effects. Together, our findings suggest that disruption of balance in local prelimbic networks may be crucial for the neurological and cognitive deficits observed during painful syndromes.