Selective Phosphorylation of AMPA Receptor Contributes to the Network of Long-Term Potentiation in the Anterior Cingulate Cortex

Pregnancy ameliorates neuropathic pain through suppression of microglia and upregulation of the δ-opioid receptor in the anterior cingulate cortex in late-pregnant mice

PURPOSE

Pregnancy-induced analgesia develops in late pregnancy, but its mechanisms are unclear. The anterior cingulate cortex (ACC) plays a key role in the pathogenesis of neuropathic pain. The authors hypothesized that pregnancy-induced analgesia ameliorates neuropathic pain by suppressing activation of microglia and the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and by upregulating opioid receptors in the ACC in late-pregnant mice.

METHODS

Neuropathic pain was induced in non-pregnant (NP) or pregnant (P) C57BL/6JJmsSlc female mice by partial sciatic nerve ligation (PSNL). The nociceptive response was evaluated by mechanical allodynia and activation of microglia in the ACC was evaluated by immunohistochemistry. The expressions of phosphorylated AMPA receptors and opioid receptors in the ACC were evaluated by immunoblotting.

RESULTS

In von Frey reflex tests, NP-PSNL-treated mice showed a lower 50% paw-withdrawal threshold than NP-Naïve mice on experimental day 9. No difference in 50% paw-withdrawal threshold was found among the NP-Naïve, NP-Sham, P-Sham, and P-PSNL-treated mice. The number of microglia in the ACC was significantly increased in NP-PSNL-treated mice compared to NP-Sham mice. Immunoblotting showed significantly increased expression of phosphorylated AMPA receptor subunit GluR1 at Ser831 in NP-PSNL-treated mice compared to NP-Sham mice. Immunoblotting also showed significantly increased δ-opioid receptor in the ACC in P-Sham and P-PSNL-treated mice compared to NP-Sham mice.

CONCLUSION

Pregnancy-induced analgesia ameliorated neuropathic pain by suppressing activation of microglia and the expression of phosphorylated AMPA receptor subunit GluR1 at Ser831, and by upregulation of the δ-opioid receptor in the ACC in late-pregnant mice.

Increased GluK1 Subunit Receptors in Corticostriatal Projection from the Anterior Cingulate Cortex Contributed to Seizure-Like Activities

The corticostriatal connection plays a crucial role in cognitive, emotional, and motor control. However, the specific roles and synaptic transmissions of corticostriatal connection are less studied, especially the corticostriatal transmission from the anterior cingulate cortex (ACC). Here, a direct glutamatergic excitatory synaptic transmission in the corticostriatal projection from the ACC is found. Kainate receptors (KAR)-mediated synaptic transmission is increased in this corticostriatal connection both in vitro and in vivo seizure-like activities. GluK1 containing KARs and downstream calcium-stimulated adenylyl cyclase subtype 1 (AC1) are involved in the upregulation of KARs following seizure-like activities. Inhibiting the activities of ACC or its corticostriatal connection significantly attenuated pentylenetetrazole (PTZ)-induced seizure. Additionally, injection of GluK1 receptor antagonist UBP310 or the AC1 inhibitor NB001 both show antiepileptic effects. The studies provide direct evidence that KARs are involved in seizure activity in the corticostriatal connection and the KAR-AC1 signaling pathway is a potential novel antiepileptic strategy.

Enhanced long-term potentiation in the anterior cingulate cortex of tree shrew

Synaptic plasticity is a key cellular model for learning, memory and chronic pain. Most previous studies were carried out in rats and mice, and less is known about synaptic plasticity in non-human primates. In the present study, we used integrative experimental approaches to study long-term potentiation (LTP) in the anterior cingulate cortex (ACC) of adult tree shrews. We found that glutamate is the major excitatory transmitter and α-amino-3-hydroxy-5-methyl-4-isoxazole-propionicacid (AMPA) receptors mediate postsynaptic responses. LTP in tree shrews was greater than that in adult mice and lasted for at least 5 h. N-methyl-d-aspartic acid (NMDA) receptors, Ca2+ influx and adenylyl cyclase 1 (AC1) contributed to tree shrew LTP. Our results suggest that LTP is a major form of synaptic plasticity in the ACC of primate-like animals. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

The Pathway-Selective Dependence of Nitric Oxide for Long-Term Potentiation in the Anterior Cingulate Cortex of Adult Mice

Nitric oxide (NO) is a key diffusible messenger in the mammalian brain. It has been proposed that NO may diffuse in retrograde into presynaptic terminals, contributing to the induction of hippocampal long-term potentiation (LTP). Here, we present novel evidence that NO is selectively required for the synaptic potentiation of the interhemispheric projection in the anterior cingulate cortex (ACC). Unilateral low-frequency stimulation (LFS) induced a short-term synaptic potentiation on the contralateral ACC through the corpus callosum (CC). The use of the antagonists of the NMDA receptor (NMDAR), or the inhibitor of the L-type voltage-dependent Ca2+ channels (L-VDCCs), blocked the induction of this ACC-ACC potentiation. In addition, the inhibitor of NO synthase, or inhibitors for its downstream signaling pathway, also blocked this ACC-ACC potentiation. However, the application of the NOS inhibitor blocked neither the local electric stimulation-induced LTP nor the stimulation-induced recruitment of silent responses. Our results present strong evidence for the pathway-selective roles of NO in the LTP of the ACC.

Cortical synaptic basis of consciousness

Consciousness is one of final questions for humans to tackle in neuroscience. Due to a lack of understanding of basic brain networks and mechanisms of functions, our knowledge of consciousness mainly stays at a theoretical level. Recent studies using brain imaging in humans and modern neuroscience techniques in animal studies reveal the basic brain network for consciousness. The projection from the thalamus to different cortical regions forms a network of activities to maintain consciousness in humans and animals. These feedback and feedforward circuits maintain consciousness even in certain brain injury conditions. Pterions and ion channels that contribute to these circuit neural activities are targets for drugs and manipulations that affect consciousness such as anesthetic agents. Synaptic plasticity that trains synapses during learning and information recall modified the circuits and contributes to a high level of consciousness in a certain population.

An ACC-VTA-ACC positive-feedback loop mediates the persistence of neuropathic pain and emotional consequences

The central mechanisms underlying pain chronicity remain elusive. Here, we identify a reciprocal neuronal circuit in mice between the anterior cingulate cortex (ACC) and the ventral tegmental area (VTA) that mediates mutual exacerbation between hyperalgesia and allodynia and their emotional consequences and, thereby, the chronicity of neuropathic pain. ACC glutamatergic neurons (ACCGlu) projecting to the VTA indirectly inhibit dopaminergic neurons (VTADA) by activating local GABAergic interneurons (VTAGABA), and this effect is reinforced after nerve injury. VTADA neurons in turn project to the ACC and synapse to the initial ACCGlu neurons to convey feedback information from emotional changes. Thus, an ACCGlu-VTAGABA-VTADA-ACCGlu positive-feedback loop mediates the progression to and maintenance of persistent pain and comorbid anxiodepressive-like behavior. Disruption of this feedback loop relieves hyperalgesia and anxiodepressive-like behavior in a mouse model of neuropathic pain, both acutely and in the long term.

Phosphorylation of 4.1N by CaMKII Regulates the Trafficking of GluA1-containing AMPA Receptors During Long-term Potentiation in Acute Rat Hippocampal Brain Slices

OBJECTIVE

GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) inserted into postsynaptic membranes are key to the process of long-term potentiation (LTP). Some evidence has shown that 4.1N plays a critical role in the membrane trafficking of AMPARs. However, the underlying mechanism behind this is still unclear. We investigated the role of 4.1N-mediated membrane trafficking of AMPARs during theta-burst stimulation long-term potentiation (TBS-LTP), to illustrate the molecular mechanism behind LTP.

METHODS

LTP was induced by TBS in rat hippocampal CA1 neuron. Tat-GluA1 (MPR), which disrupts the association of 4.1N-GluA1, and autocamtide-2-inhibitory peptide, myristoylated (Myr-AIP), a CaMKII antagonist, were used to explore the role of 4.1N in the AMPARs trafficking during TBS-induced LTP. Immunoprecipitation (IP) and immunoblotting (IB)were used to detect protein expression, phosphorylation, and the interaction of p-CaMKII-4.1N-GluA1.

RESULTS

We found that Myr-AIP attenuated increases of p-CaMKII (T286), p-GluA1 (ser831), and 4.1N phosphorylation after TBS-LTP, and decreased the association of p-CaMKII-4.1N-GluA1, along with the expression of GluA1, at postsynaptic densities during TBS-LTP. We also designed interfering peptides to disrupt the interaction between 4.1N and GluA1, which showed that Tat-GluA1 (MPR) or Myr-AIP inhibited TBS-LTP and attenuated increases of GluA1 at postsynaptic sites, while Tat-GluA1 (MPR) or Myr-AIP had no effects on miniature excitatory postsynaptic currents (mEPSCs) in non-stimulated hippocampal CA1 neurons.

CONCLUSION

Active CaMKII enhanced the phosphorylation of 4.1N and facilitated the association of p-CaMKII with 4.1N-GluA1, which in turn resulted in GluA1 trafficking during TBS-LTP. The association of 4.1N-GluA1 is required for LTP, but not for basal synaptic transmission.

Long-term plasticity of NMDA GluN2B (NR2B) receptor in anterior cingulate cortical synapses

The anterior cingulate cortex (ACC) is a key cortical area for pain perception, emotional fear and anxiety. Cortical excitation is thought to be the major mechanism for chronic pain and its related emotional disorders such as anxiety and depression. GluN2B (or called NR2B) containing NMDA receptors play critical roles for such excitation. Not only does the activation of GluN2B contributes to the induction of the postsynaptic form of LTP (post-LTP), long-term upregulation of GluN2B subunits through tyrosine phosphorylation were also detected after peripheral injury. In addition, it has been reported that presynaptic NMDA receptors may contribute to the modulation of the release of glutamate from presynaptic terminals in the ACC. It is believed that inhibiting subtypes of NMDA receptors and/or downstream signaling proteins may serve as a novel therapeutic mechanism for future treatment of chronic pain, anxiety, and depression.

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.

Activation of the glutamatergic cingulate cortical-cortical connection facilitates pain in adult mice

The brain consists of the left and right cerebral hemispheres and both are connected by callosal projections. Less is known about the basic mechanism of this cortical-cortical connection and its functional importance. Here we investigate the cortical-cortical connection between the bilateral anterior cingulate cortex (ACC) by using the classic electrophysiological and optogenetic approach. We find that there is a direct synaptic projection from one side ACC to the contralateral ACC. Glutamate is the major excitatory transmitter for bilateral ACC connection, including projections to pyramidal cells in superficial (II/III) and deep (V/VI) layers of the ACC. Both AMPA and kainate receptors contribute to synaptic transmission. Repetitive stimulation of the projection also evoked postsynaptic Ca2+ influx in contralateral ACC pyramidal neurons. Behaviorally, light activation of the ACC-ACC connection facilitated behavioral withdrawal responses to mechanical stimuli and noxious heat. In an animal model of neuropathic pain, light inhibitory of ACC-ACC connection reduces both primary and secondary hyperalgesia. Our findings provide strong direct evidence for the excitatory or facilitatory contribution of ACC-ACC connection to pain perception, and this mechanism may provide therapeutic targets for future treatment of chronic pain and related emotional disorders.

Photic sensitization is mediated by cortico-accumbens pathway in rats with trigeminal neuropathic pain

Exposure to light stimuli may trigger or exacerbate perception of pain, also known as a common yet debilitating symptom of photophobia in patient with chronic orofacial pain. Mechanism underlying this phenomenon of photic sensitization in neuropathic condition remains elusive. Here, we found that rats developed hypersensitivity to normal light illumination after establishment of chronic constriction injury of infraorbital nerve (ION-CCI) model, which can be attenuated by blocking the exposure of photic stimulation. Additionally, this behavioral phenotype of light-sensitivity impairment was associated with overexpression of anterior cingulate cortex (ACC) c-fos positive neurons, enhancement of neural excitability in the ACC neurons and its excitatory synaptic transmission between nucleus accumbens (NAc). Optogenetic and chemogenic silencing of ACC-NAc pathway improved trigeminal sensitization in responses to light stimuli by decreasing spontaneous pain-like episodes in ION-CCI animals. In contrast, selective activation of ACC-to-NAc circuits enhanced photic hypersensitivity in dark environment. Thus, our data provided novel role of ACC and its projection to NAc in bidirectional modulation of photic sensation, which may contribute to the understanding of photic allodynia in trigeminal neuropathic pain status.

Inhibiting neuronal AC1 for treating anxiety and headache in the animal model of migraine

Migraines are a common medical condition. From a basic science point of view, the central mechanism for migraine and headache is largely unknown. In the present study, we demonstrate that cortical excitatory transmission is significantly enhanced in the anterior cingulate cortex (ACC)-a brain region which is critical for pain perception. Biochemical studies found that the phosphorylation levels of both the NMDA receptor GluN2B and AMPA receptor GluA1 were enhanced in ACC of migraine rats. Both the presynaptic release of glutamate and postsynaptic responses of AMPA receptors and NMDA receptors were enhanced. Synaptic long-term potentiation (LTP) was occluded. Furthermore, behavioral anxiety and nociceptive responses were increased, which were reversed by application of AC1 inhibitor NB001 within ACC. Our results provide strong evidence that cortical LTPs contribute to migraine-related pain and anxiety. Drugs that inhibit cortical excitation such as NB001 may serve as potential medicines for treating migraine in the future.

Genetic mechanisms for impaired synaptic plasticity in schizophrenia revealed by computational modelling

Schizophrenia phenotypes are suggestive of impaired cortical plasticity in the disease, but the mechanisms of these deficits are unknown. Genomic association studies have implicated a large number of genes that regulate neuromodulation and plasticity, indicating that the plasticity deficits have a genetic origin. Here, we used biochemically detailed computational modelling of post-synaptic plasticity to investigate how schizophrenia-associated genes regulate long-term potentiation (LTP) and depression (LTD). We combined our model with data from post-mortem mRNA expression studies (CommonMind gene-expression datasets) to assess the consequences of altered expression of plasticity-regulating genes for the amplitude of LTP and LTD. Our results show that the expression alterations observed post mortem, especially those in anterior cingulate cortex, lead to impaired PKA-pathway-mediated LTP in synapses containing GluR1 receptors. We validated these findings using a genotyped EEG dataset where polygenic risk scores for synaptic and ion channel-encoding genes as well as modulation of visual evoked potentials (VEP) were determined for 286 healthy controls. Our results provide a possible genetic mechanism for plasticity impairments in schizophrenia, which can lead to improved understanding and, ultimately, treatment of the disorder.

Caveolin-1-Mediated Cholesterol Accumulation Contributes to Exaggerated mGluR-Dependent Long-Term Depression and Impaired Cognition in Fmr1 Knockout Mice

Fragile X syndrome (FXS) is one of the most common inherited mental retardation diseases and is caused by the loss of fragile X mental retardation protein (FMRP) expression. The metabotropic glutamate receptor (mGluR) theory of FXS states that enhanced mGluR-dependent long-term depression (LTD) due to FMRP loss is involved in aberrant synaptic plasticity and autistic-like behaviors, but little is known about the underlying molecular mechanism. Here, we found that only hippocampal mGluR-LTD was exaggerated in adolescent Fmr1 KO mice, while N-methyl-D-aspartate receptor (NMDAR)-LTD was intact in mice of all ages. This development-dependent alteration was related to the differential expression of caveolin-1 (Cav1), which is essential for caveolae formation. Knockdown of Cav1 restored the enhanced mGluR-LTD in Fmr1 KO mice. Moreover, hippocampal Cav1 expression in Fmr1 KO mice induced excessive endocytosis of the α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluA2. This process relied on mGluR1/5 activation rather than NMDAR. Interference with Cav1 expression reversed these changes. Furthermore, massive cholesterol accumulation contributed to redundant caveolae formation, which provided the platform for mGluR-triggered Cav1 coupling to GluA2. Importantly, injection of the cholesterol scavenger methyl-β-cyclodextrin (Mβ-CD) recovered AMPA receptor trafficking and markedly alleviated hyperactivity, hippocampus-dependent fear memory, and spatial memory defects in Fmr1 KO mice. Together, our findings elucidate the important role of Cav1 in mediating mGluR-LTD enhancement and further inducing AMPA receptor endocytosis and suggest that cholesterol depletion by Mβ-CD during caveolae formation may be a novel and safe strategy to treat FXS.

Early-life social stress induces permanent alterations in plasticity and perineuronal nets in the mouse anterior cingulate cortex

Child maltreatment disrupts trajectories of brain development, but the underlying pathways are unclear. Stressful stimuli in early life interfere with maturation of local inhibitory circuitry and deposition of perineuronal nets (PNNs), specialized extracellular matrix structures involved in the closure of critical periods of development. Alterations in cortical PNN and parvalbumin (PV) following early-life stress (ELS) have been detected in human and animal studies. Aberrations in the anterior cingulate cortex (ACC) are the most consistent neuroimaging findings in maltreated people, but the molecular mechanisms linking ELS with ACC dysfunctions are unknown. Here, we employed a mouse model of early social threat to test whether ELS experienced in a sensitive period for ACC maturation could induce long-term aberrations of PNN and PV development in the ACC, with consequences on plasticity and ACC-dependent behavior. We found that ELS increased PNN but not PV expression in the ACC of young adult mice. This was associated with reduced frequency of inhibitory postsynaptic currents and long-term potentiation impairments and expression of intense object phobia. Our findings provide information on the long-term effects of ELS on ACC functionality and PNN formation and present evidence for a novel neurobiological pathway underlying the impact of early adversity on the brain.

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.

Different Synaptic Plasticity After Physiological and Psychological Stress in the Anterior Insular Cortex in an Observational Fear Mouse Model

Post-traumatic stress disorder (PTSD) can be triggered not only in people who have personally experienced traumatic events but also in those who witness them. Physiological and psychological stress can have different effects on neural activity, but little is known about the underlying mechanisms. There is ample evidence that the insular cortex, especially the anterior insular cortex (aIC), is critical to both the sensory and emotional experience of pain. It is therefore worthwhile to explore the effects of direct and indirect stress on the synaptic plasticity of the aIC. Here, we used a mouse model of observational fear to mimic direct suffering (Demonstrator, DM) and witnessing (Observer, OB) of traumatic events. After observational fear training, using a 64-channel recording system, we showed that both DM and OB mice exhibited a decreased ratio of paired-pulse with intervals of 50 ms in the superficial layers of the aIC but not in the deep layers. We found that theta-burst stimulation (TBS)–induced long-term potentiation (LTP) in OB mice was significantly higher than in DM mice, and the recruitment of synaptic responses occurred only in OB mice. Compared with naive mice, OB mice showed stronger recruitment and higher amplitude in the superficial layers of the aIC. We also used low-frequency stimulation (LFS) to induce long-term depression (LTD). OB mice showed greater LTD in both the superficial and deep layers of the aIC than naive mice, but no significant difference was found between OB and DM mice. These results provide insights into the changes in synaptic plasticity in the aIC after physiological and psychological stress, and suggest that different types of stress may have different mechanisms. Furthermore, identification of the possible causes of the differences in stress could help treat stress-related disorders.

Regular Aerobic Exercise Attenuates Pain and Anxiety in Mice by Restoring Serotonin-modulated Synaptic Plasticity in the ACC

PURPOSE

Clinical studies found that regular aerobic exercise has analgesic and anti-anxiety effects; however, the underlying neural mechanisms remain unclear. Multiple studies have suggested that regular aerobic exercise may exert brain-protective effects by promoting the release of serotonin, which may be a pain modulator. The anterior cingulate cortex (ACC) is a key brain area for pain information processing, receiving dense serotonergic innervation. As a result, we hypothesized that exercise may increase the release of serotonin in the ACC, thus improving pain and anxiety behaviours.

METHODS

Integrative methods were used, including behavioural, electrophysiological, pharmacological, biochemical, and genetic approaches, to explore the effects of regular aerobic exercise and the underlying neural mechanisms.

RESULTS

Regular aerobic exercise in the form of voluntary wheel running for 30 minutes daily for 15 days showed significant effectiveness in relieving pain and concomitant anxiety in complete Freund's adjuvant (CFA)-induced chronic inflammation pain models. c-Fos staining and multielectrode array recordings revealed alterations in neuronal activities and synaptic plasticity in the ACC. Moreover, systemic pharmacological treatment with 4-chloro-DL-phenylalanine (PCPA) to deplete endogenous serotonin and local delivery of serotonin to the ACC revealed that exercise-related serotonin release in the ACC bidirectionally modulates pain sensitization and anxiety behaviours by modulating synaptic plasticity in the ACC. Furthermore, we found that 5-HT1A and 5-HT7 receptors mediated the serotonin modulation effects under conditions of regular aerobic exercise through local infusion of a selective antagonist and shRNA in the ACC.

CONCLUSIONS

Our results reveal that regular aerobic exercise can increase serotonin release and modulate synaptic plasticity in the ACC, ultimately improving pain and concomitant anxiety behaviours through the functions of the 5-HT1A and 5-HT7 receptors.

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.

Brain-derived neurotrophic factor produced long-term synaptic enhancement in the anterior cingulate cortex of adult mice

Previous studies have demonstrated that brain-derived neurotrophic factor (BDNF) is one of the diffusible messengers for enhancing synaptic transmission in the hippocampus. Less information is available about the possible roles of BDNF in the anterior cingulate cortex (ACC). In the present study, we used 64-electrode array field recording system to investigate the effect of BDNF on ACC excitatory transmission. We found that BDNF enhanced synaptic responses in a dose-dependent manner in the ACC in C57/BL6 mice. The enhancement was long-lasting, and persisted for at least 3 h. In addition to the enhancement, BDNF also recruited inactive synaptic responses in the ACC. Bath application of the tropomyosin receptor kinase B (TrkB) receptor antagonist K252a blocked BDNF-induced enhancement. L-type voltage-gated calcium channels (L-VGCC), metabotropic glutamate receptors (mGluRs), but not NMDA receptors were required for BDNF-produced enhancement. Moreover, calcium-stimulated adenylyl cyclase subtype 1 (AC1) but not AC8 was essential for the enhancement. A selective AC1 inhibitor NB001 completely blocked the enhancement. Furthermore, BDNF-produced enhancement occluded theta burst stimulation (TBS) induced long-term potentiation (LTP), suggesting that they may share similar signaling mechanisms. Finally, the expression of BDNF-induced enhancement depends on postsynaptic incorporation of calcium-permeable AMPA receptors (CP-AMPARs) and protein kinase Mζ (PKMζ). Our results demonstrate that cortical BDNF may contribute to synaptic potentiation in the ACC.

Multiple synaptic connections into a single cortical pyramidal cell or interneuron in the anterior cingulate cortex of adult mice

The ACC is an important brain area for the processing of pain-related information. Studies of synaptic connections within the ACC provide an understanding of basic cellular and molecular mechanisms for brain functions such as pain, emotion and related cognitive functions. Previous study of ACC synaptic transmission mainly focused on presumably thalamic inputs into pyramidal cells. In the present study, we developed a new mapping technique by combining single neuron whole-cell patch-clamp recording with 64 multi-channel field potential recording (MED64) to examine the properties of excitatory inputs into a single neuron in the ACC. We found that a single patched pyramidal neuron or interneuron simultaneously received heterogeneous excitatory synaptic innervations from different subregions (ventral, dorsal, deep, and superficial layers) in the ACC. Conduction velocity is faster as stimulation distance increases in pyramidal neurons. Fast-spiking interneurons (FS-IN) show slower inactivation when compared to pyramidal neurons and regular-spiking interneurons (RS-IN) while pyramidal neurons displayed the most rapid activation. Bath application of non-competitive AMPA receptor antagonist GYKI 53655 followed by CNQX revealed that both FS-INs and RS-INs have AMPA and KA mediated components. Our studies provide a new strategy and technique for studying the network of synaptic connections.

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.

5-HT6R null mutatrion induces synaptic and cognitive defects

Serotonin 6 receptor (5-HT6R) is a promising target for a variety of human diseases, such as Alzheimer's disease (AD) and schizophrenia. However, the detailed mechanism underlying 5-HT6R activity in the central nervous system (CNS) is not fully understood. In the present study, 5-HT6R null mutant (5-HT6R^-/- ) mice were found to exhibit cognitive deficiencies and abnormal anxiety levels. 5-HT6R is considered to be specifically localized on the primary cilia. We found that the loss of 5-HT6R affected the Sonic Hedgehog signaling pathway in the primary cilia. 5-HT6R^-/- mice showed remarkable alterations in neuronal morphology, including dendrite complexity and axon initial segment morphology. Neurons lacking 5-HT6R exhibited increased neuronal excitability. Our findings highlight the complexity of 5-HT6R functions in the primary ciliary and neuronal physiology, supporting the theory that this receptor modulates neuronal morphology and transmission, and contributes to cognitive deficits in a variety of human diseases, such as AD, schizophrenia, and ciliopathies.

NMDA Receptor-Dependent Synaptic Depression in Potentiated Synapses of the Anterior Cingulate Cortex of adult Mice

Long-term potentiation (LTP) is an important molecular mechanism for chronic pain in the anterior cingulate cortex (ACC), a key cortical region for pain perception and emotional regulation. Inhibiting ACC LTP via various manipulations or pharmacological treatments blocks chronic pain. Long-term depression (LTD) is another form of synaptic plasticity in the ACC, which is also proved to be involved in the mechanisms of chronic pain. However, less is known about the interactive relationship between LTP and LTD in the ACC. Whether the synaptic depression could be induced after synaptic LTP in the ACC is not clear. In the present study, we used multi-channel field potential recording systems to study synaptic depression after LTP in the ACC of adult mice. We found that low frequency stimulus (LFS: 1 Hz, 15 min) inhibited theta burst stimulation (TBS)-induced LTP at 30 min after the induction of LTP. However, LFS failed to induce depression at 90 min after the induction of LTP. Furthermore, NMDA receptor antagonist AP-5 blocked the induction of synaptic depression after potentiation. The GluN2B-selective antagonist Ro25-6981 also inhibited the phenomenon in the ACC, while the GluN2A-selective antagonist NVP-AAM077 and the GluN2C/D-selective antagonist PPDA and UBP145 had no any significant effect. These results suggest that synaptic LTP can be depressed by LTD in a time dependent manner, and GluN2B-containing NMDA receptors play important roles in this form of synaptic depression.

Cellular and synaptic mechanisms for Parkinson's disease-related chronic pain

Parkinson’s disease is the second most common neurodegenerative disorder after Alzheimer’s disease. Chronic pain is experienced by the vast majority of patients living with Parkinson’s disease. The degeneration of dopaminergic neuron acts as the essential mechanism of Parkinson’s disease in the midbrain dopaminergic pathway. The impairment of dopaminergic neurons leads to dysfunctions of the nociceptive system. Key cortical areas, such as the anterior cingulate cortex (ACC) and insular cortex (IC) that receive the dopaminergic projections are involved in pain transmission. Dopamine changes synaptic transmission via several pathway, for example the D2-adenly cyclase (AC)-cyclic AMP (cAMP)-protein kinase A (PKA) pathway and D1-G protein-coupled receptor kinase 2 (GRK2)-fragile X mental retardation protein (FMRP) pathway. The management of Parkinson’s disease-related pain implicates maintenance of stable level of dopaminergic drugs and analgesics, however a more selective drug targeting at key molecules in Parkinson’s disease-related pain remains to be investigated.

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.

A unified computational model for cortical post-synaptic plasticity

Signalling pathways leading to post-synaptic plasticity have been examined in many types of experimental studies, but a unified picture on how multiple biochemical pathways collectively shape neocortical plasticity is missing. We built a biochemically detailed model of post-synaptic plasticity describing CaMKII, PKA, and PKC pathways and their contribution to synaptic potentiation or depression. We developed a statistical AMPA-receptor-tetramer model, which permits the estimation of the AMPA-receptor-mediated maximal synaptic conductance based on numbers of GluR1s and GluR2s predicted by the biochemical signalling model. We show that our model reproduces neuromodulator-gated spike-timing-dependent plasticity as observed in the visual cortex and can be fit to data from many cortical areas, uncovering the biochemical contributions of the pathways pinpointed by the underlying experimental studies. Our model explains the dependence of different forms of plasticity on the availability of different proteins and can be used for the study of mental disorder-associated impairments of cortical plasticity.

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.

Phosphodiesterase Inhibitors for Alzheimer's Disease: A Systematic Review of Clinical Trials and Epidemiology with a Mechanistic Rationale

BACKGROUND

Preclinical studies, clinical trials, and reviews suggest increasing 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) with phosphodiesterase inhibitors is disease-modifying in Alzheimer's disease (AD). cAMP/protein kinase A (PKA) and cGMP/protein kinase G (PKG) signaling are disrupted in AD. cAMP/PKA and cGMP/PKG activate cAMP response element binding protein (CREB). CREB binds mitochondrial and nuclear DNA, inducing synaptogenesis, memory, and neuronal survival gene (e.g., brain-derived neurotrophic factor) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α). cAMP/PKA and cGMP/PKG activate Sirtuin-1, which activates PGC1α. PGC1α induces mitochondrial biogenesis and antioxidant genes (e.g.,Nrf2) and represses BACE1. cAMP and cGMP inhibit BACE1-inducing NFκB and tau-phosphorylating GSK3β.

OBJECTIVE AND METHODS

We review efficacy-testing clinical trials, epidemiology, and meta-analyses to critically investigate whether phosphodiesteraseinhibitors prevent or treat AD.

RESULTS

Caffeine and cilostazol may lower AD risk. Denbufylline and sildenafil clinical trials are promising but preliminary and inconclusive. PF-04447943 and BI 409,306 are ineffective. Vinpocetine, cilostazol, and nicergoline trials are mixed. Deprenyl/selegiline trials show only short-term benefits. Broad-spectrum phosphodiesterase inhibitor propentofylline has been shown in five phase III trials to improve cognition, dementia severity, activities of daily living, and global assessment in mild-to-moderate AD patients on multiple scales, including the ADAS-Cogand the CIBIC-Plus in an 18-month phase III clinical trial. However, two books claimed based on a MedScape article an 18-month phase III trial failed, so propentofylline was discontinued. Now, propentofylline is used to treat canine cognitive dysfunction, which, like AD, involves age-associated wild-type Aβ deposition.

CONCLUSION

Phosphodiesterase inhibitors may prevent and treat AD.

Neuronal Plasticity: Neuronal Organization is Associated with Neurological Disorders

Stimuli from stressful events, attention in the classroom, and many other experiences affect the functionality of the brain by changing the structure or reorganizing the connections between neurons and their communication. Modification of the synaptic transmission is a vital mechanism for generating neural activity via internal or external stimuli. Neuronal plasticity is an important driving force in neuroscience research, as it is the basic process underlying learning and memory and is involved in many other functions including brain development and homeostasis, sensorial training, and recovery from brain injury. Indeed, neuronal plasticity has been explored in numerous studies, but it is still not clear how neuronal plasticity affects the physiology and morphology of the brain. Thus, unraveling the molecular mechanisms of neuronal plasticity is essential for understanding the operation of brain functions. In this timeline review, we discuss the molecular mechanisms underlying different forms of synaptic plasticity and their association with neurodegenerative/neurological disorders as a consequence of alterations in neuronal plasticity.

Sex difference in synaptic plasticity in the anterior cingulate cortex of adult mice

Sex differences in certain types of pain sensitivity and emotional responses have been previously reported. Synaptic plasticity is a key cellular mechanism for pain perception and emotional regulation, including long-term potentiation (LTP) and long-term depression (LTD). However, it is unclear whether there is a sex difference at synaptic level. Recent studies indicate that excitatory transmission and plasticity in the anterior cingulate cortex (ACC) are critical in chronic pain and pain related emotional responses. In the present study, we used 64-channel multielectrode (MED64) system to record synaptic plasticity in the ACC of male and female adult mice. We found that there was no significant difference in theta-burst stimulation (TBS)-induced LTP between female and male mice. Furthermore, the recruitment of inactive channels was also not different. For LTD, we found that LTD was greater in slices of ACC in male mice than female mice. Our results demonstrate that LTP in the ACC does not show any sex-related difference.

Cortical plasticity as synaptic mechanism for chronic pain

Adult brain structures such as the hippocampus are highly plastic to learning and gaining new experiences. Recent studies reveal that cortical areas that respond to sensory noxious stimuli (stimuli that cause pain in humans) are also highly plastic, like the learning-related hippocampus. Long-term potentiation (LTP), a key cellular model for learning and memory, is reported in the anterior cingulate cortex (ACC) and insular cortex (IC), two key cortical areas for pain perception. ACC and IC LTP exist in at least two major forms: presynaptically expressed LTP, and postsynaptically expressed LTP (post-LTP). In this short review, I will review, recent progress made in cortical LTPs, and explore potential roles of other forms of LTPs such as synaptic tagging. Their contribution to chronic pain as well as emotional changes caused by injury will be discussed.

The AMPA Receptor Code of Synaptic Plasticity

Changes in the properties and postsynaptic abundance of AMPA-type glutamate receptors (AMPARs) are major mechanisms underlying various forms of synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), and homeostatic scaling. The function and the trafficking of AMPARs to and from synapses is modulated by specific AMPAR GluA1-GluA4 subunits, subunit-specific protein interactors, auxiliary subunits, and posttranslational modifications. Layers of regulation are added to AMPAR tetramers through these different interactions and modifications, increasing the computational power of synapses. Here we review the reliance of synaptic plasticity on AMPAR variants and propose "the AMPAR code" as a conceptual framework. The AMPAR code suggests that AMPAR variants will be predictive of the types and extent of synaptic plasticity that can occur and that a hierarchy exists such that certain AMPARs will be disproportionally recruited to synapses during LTP/homeostatic scaling up, or removed during LTD/homeostatic scaling down.

Calcitonin gene-related peptide potentiated the excitatory transmission and network propagation in the anterior cingulate cortex of adult mice

The neuropeptide of calcitonin gene-related peptide (CGRP) plays critical roles in chronic pain, especially in migraine. Immunohistochemistry and in situ hybridization studies have shown that CGRP and its receptors are expressed in cortical areas including pain perception-related prefrontal anterior cingulate cortex. However, less information is available for the functional roles of CGRP in cortical regions such as the anterior cingulate cortex (ACC). Recent studies have consistently demonstrated that long-term potentiation is a key cellular mechanism for chronic pain in the ACC. In the present study, we used 64-electrode array field recording system to investigate the effect of CGRP on excitatory transmission in the ACC. We found that CGRP induced potentiation of synaptic transmission in a dose-dependently manner (1, 10, 50, and 100 nM). CGRP also recruited inactive circuit in the ACC. An application of the calcitonin receptor-like receptor antagonist CGRP_8-37 blocked CGRP-induced chemical long-term potentiation and the recruitment of inactive channels. CGRP-induced long-term potentiation was also blocked by N-methyl-D-aspartate (NMDA) receptor antagonist AP-5. Consistently, the application of CGRP increased NMDA receptor-mediated excitatory postsynaptic currents. Finally, we found that CGRP-induced long-term potentiation required the activation of calcium-stimulated adenylyl cyclase subtype 1 (AC1) and protein kinase A. Genetic deletion of AC1 using AC1^−/− mice, an AC1 inhibitor NB001 or a protein kinase A inhibitor KT5720, all reduced or blocked CGRP-induced potentiation. Our results provide direct evidence that CGRP may contribute to synaptic potentiation in important physiological and pathological conditions in the ACC, an AC1 inhibitor NB001 may be beneficial for the treatment of chronic headache.

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.

Antinociceptive effects of the adenylyl cyclase inhibitor ST034307 on tooth-movement-induced nociception in rats

OBJECTIVE

This study aimed to investigate the antinociceptive effects of the selective adenylyl cyclase type 1 (AC1) inhibitor ST034307 on tooth movement nociception through orofacial nociceptive behavior tests and molecular examination.

METHODS

We placed fixed nickel-titanium alloy closed-coil springs around the incisors of male Sprague-Dawley rats to induce tooth movement. We subsequently administered ST034307 (3 mg/kg), for 2 days, intraperitoneally, and then subjected the rats to a battery of behavioral tests (n = 10/group) to assess orofacial nociception. The changes in the expression of key molecules in the anterior cingulate cortex were measured by ELISA (n = 8/group) and Western blotting (n = 8/group).

RESULTS

Tooth movement increased face-grooming activities and rat grimace scale scores. Tooth movement was also associated with enhanced cyclic adenosine monophosphate (cAMP) generation as well as protein kinase A (PKA) activation. Moreover, the phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and expression of N-methyl-d-aspartate (NMDA) receptors in the anterior cingulate cortex increased during tooth movement. ST034307 significantly decreased mouth wiping and rat grimace scale scores, accompanied by reductions in cAMP generation, PKA activation, AMPA receptor phosphorylation, and NMDA receptor expression in the anterior cingulate cortex.

CONCLUSIONS

These results suggest that adenylyl cyclase type 1 plays an important role in the development of orthodontic tooth movement nociception. Furthermore, ST034307 can be used as an effective pharmacotherapy for orthodontic nociception.

No requirement of interlukine-1 for long-term potentiation in the anterior cingulate cortex of adult mice

Background

The enhanced expression of cytokines in the pathological states suggests that they have important roles in the initiation or maintenance of disease states.

Findings: To determine the involvement of cytokines in chronic neuropathic pain, the expression of cytokines in the anterior cingulate cortex neurons in the ligation of the common peroneal nerve mice was investigated. We utilized a cytokine enzyme-linked immunosorbent assay plate array to detect 23 cytokines in total eight mice including a female, and no significant differences were found in those cytokines between the common peroneal nerve model and sham surgery mice. Quantification of TNF-α at protein level revealed the unvaried expression in the anterior cingulate cortex in both neuropathic pain and visceral pain, but enhanced expression in the insular cortex in the visceral pain. Furthermore, we found that the IL-Ira, a kind of IL-1 receptor antagonist, had no effect on the theta burst stimulation-induced long-term potentiation in the anterior cingulate cortex.

Conclusions

Cytokines are not involved in chronic neuropathic pain induced by nerve injury in the anterior cingulate cortex. Our findings suggested that cytokines may not be a viable drug target to treat chronic neuropathic pain in the anterior cingulate cortex.

NMDA Receptor Dependent Long-term Potentiation in Chronic Pain

Since the discovery of NMDA receptor (NMDAR) dependent long-term potentiation (LTP) in the hippocampus, many studies have demonstrated that NMDAR dependent LTP exists throughout central synapses, including those involved in sensory transmission and perception. NMDAR LTP has been reported in spinal cord dorsal horn synapses, anterior cingulate cortex and insular cortex. Behavioral, genetic and pharmacological studies show that inhibiting or reducing NMDAR LTP produced analgesic effects in animal models of chronic pain. Investigation of signalling mechanisms for NMDAR LTP may provide novel targets for future treatment of chronic pain.

Loss of Synaptic Tagging in the Anterior Cingulate Cortex after Tail Amputation in Adult Mice

Anterior cingulate cortex (ACC) is known to play important roles in key brain functions such as pain perception, cognition, and emotion. Different forms of homosynaptic plasticity such as long-term potentiation (LTP) and long-term depression have been studied in ACC synapses. However, heterosynaptic plasticity such as synaptic tagging has not been reported. Here, we demonstrate synaptic tagging in the ACC of adult male mice by using a 64-channel multielectrode array recording system. Weak theta burst stimulation (TBS), normally inducing early-phase LTP or No-LTP in most of the activated channels, produced late phase-LTP (L-LTP) in a majority of channels when a strong TBS was applied earlier to a separate input within a certain time window. Similar to hippocampus, synaptic tagging in the ACC depends on the synthesis of new proteins. Tail amputation-induced peripheral injury caused a loss of this heterosynaptic L-LTP and occluded strong TBS-evoked L-LTP as well. Together, we provide the first report of the synaptic tagging-like phenomenon in the ACC of adult mice, and the loss of synaptic tagging to amputation may contribute to injury-related cognitive changes and phantom limb sensation and pain.SIGNIFICANCE STATEMENT ACC is an important cortical region involved in many brain functions. Previous studies have dissected the molecular mechanism of multiple types of homosynaptic plasticity of ACC synapses. Here, we report a novel form of heterosynaptic plasticity occurring in the ACC. This newly identified, protein synthesis-dependent neocortical synaptic tagging is sensitive to peripheral tail amputation injury and may provide basic mechanisms for synaptic pathophysiology of phantom pain and related cognitive changes.

Hyperactivity of Anterior Cingulate Cortex Areas 24a/24b Drives Chronic Pain-Induced Anxiodepressive-like Consequences

Pain associates both sensory and emotional aversive components, and often leads to anxiety and depression when it becomes chronic. Here, we characterized, in a mouse model, the long-term development of these sensory and aversive components as well as anxiodepressive-like consequences of neuropathic pain and determined their electrophysiological impact on the anterior cingulate cortex (ACC, cortical areas 24a/24b). We show that these symptoms of neuropathic pain evolve and recover in different time courses following nerve injury in male mice. In vivo electrophysiological recordings evidence an increased firing rate and bursting activity within the ACC when anxiodepressive-like consequences developed, and this hyperactivity persists beyond the period of mechanical hypersensitivity. Whole-cell patch-clamp recordings also support ACC hyperactivity, as shown by increased excitatory postsynaptic transmission and contribution of NMDA receptors. Optogenetic inhibition of the ACC hyperactivity was sufficient to alleviate the aversive and anxiodepressive-like consequences of neuropathic pain, indicating that these consequences are underpinned by ACC hyperactivity.SIGNIFICANCE STATEMENT Chronic pain is frequently comorbid with mood disorders, such as anxiety and depression. It has been shown that it is possible to model this comorbidity in animal models by taking into consideration the time factor. In this study, we aimed at determining the dynamic of different components and consequences of chronic pain, and correlated them with electrophysiological alterations. By combining electrophysiological, optogenetic, and behavioral analyses in a mouse model of neuropathic pain, we show that the mechanical hypersensitivity, ongoing pain, anxiodepressive consequences, and their recoveries do not necessarily exhibit temporal synchrony during chronic pain processing, and that the hyperactivity of the anterior cingulate cortex is essential for driving the emotional impact of neuropathic pain.

Cortical LTP: A Synaptic Model for Chronic Pain

Cumulative evidence indicates that cortical synapses not only play important roles in pain perception and related emotional functions but also undergo long-term potentiation (LTP) and contribute to chronic pain. LTP is found at two key cortical regions such as the anterior cingulate cortex (ACC) and insular cortex (IC), and inhibition of cortical LTP produces analgesic effects as well as anxiolytic effects. In this chapter, I will summarize our work on ACC and IC and provide evidence for calcium-stimulated AC1 as a key molecule for cortical LTP and chronic pain.

Characterization of excitatory synaptic transmission in the anterior cingulate cortex of adult tree shrew

The tree shrew, as a primate-like animal model, has been used for studying high brain functions such as social emotion and spatial learning memory. However, little is known about the excitatory synaptic transmission in cortical brain areas of the tree shrew. In the present study, we have characterized the excitatory synaptic transmission and intrinsic properties of pyramidal neurons in the anterior cingulate cortex (ACC) of the adult tree shrew, a key cortical region for pain perception and emotion. We found that glutamate is the major excitatory transmitter for fast synaptic transmission. Excitatory synaptic responses induced by local stimulation were mediated by AMPA and kainate (KA) receptors. As compared with mice, AMPA and KA receptor mediated responses were significantly greater. Interestingly, the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and miniature excitatory postsynaptic currents (mEPSCs) in tree shrews was significantly less than that of mice. Moreover, both the ratio of paired-pulse facilitation (PPF) and the time of 50% decay for fast blockade of NMDA receptor mediated EPSCs were greater in the tree shrew. Finally, tree shrew neurons showed higher initial firing frequency and neuronal excitability with a cell type-specific manner in the ACC. Our studies provide the first report of the basal synaptic transmission in the ACC of adult tree shrew.