Molecular mechanisms of pain in the anterior cingulate cortex

Decreased GABAA receptors and benzodiazepine binding sites in the anterior cingulate cortex in autism

The anterior cingulate cortex (ACC; BA 24) via its extensive limbic and high order association cortical connectivity to prefrontal cortex is a key part of an important circuitry participating in executive function, affect, and socio-emotional behavior. Multiple lines of evidence, including genetic and imaging studies, suggest that the ACC and gamma-amino-butyric acid (GABA) system may be affected in autism. The benzodiazepine binding site on the GABA(A) receptor complex is an important target for pharmacotherapy and has important clinical implications. The present multiple-concentration ligand-binding study utilized (3)H-muscimol and (3)H-flunitrazepam to determine the number (B(max)), binding affinity (K(d)), and distribution of GABA(A) receptors and benzodiazepine binding sites, respectively, in the ACC in adult autistic and control cases. Compared to controls, the autistic group had significant decreases in the mean density of GABA(A) receptors in the supragranular (46.8%) and infragranular (20.2%) layers of the ACC and in the density of benzodiazepine binding sites in the supragranular (28.9%) and infragranular (16.4%) lamina [corrected]. These findings suggest that in the autistic group this downregulation of both benzodiazepine sites and GABA(A) receptors in the ACC may be the result of increased GABA innervation and/or release disturbing the delicate excitation/inhibition balance of principal neurons as well as their output to key limbic cortical targets. Such disturbances likely underlie the core alterations in socio-emotional behaviors in autism.

Morphine modulation of pain processing in medial and lateral pain pathways

Background

Despite the wide-spread use of morphine and related opioid agonists in clinic and their powerful analgesic effects, our understanding of the neural mechanisms underlying opioid analgesia at supraspinal levels is quite limited. The present study was designed to investigate the modulative effect of morphine on nociceptive processing in the medial and lateral pain pathways using a multiple single-unit recording technique. Pain evoked neuronal activities were simultaneously recorded from the primary somatosensory cortex (SI), ventral posterolateral thalamus (VPL), anterior cingulate cortex (ACC), and medial dorsal thalamus (MD) with eight-wire microelectrode arrays in awake rats.

Results

The results showed that the noxious heat evoked responses of single neurons in all of the four areas were depressed after systemic injection of 5 mg/kg morphine. The depressive effects of morphine included (i) decreasing the neuronal response magnitude; (ii) reducing the fraction of responding neurons, and (iii) shortening the response duration. In addition, the capability of cortical and thalamic neural ensembles to discriminate noxious from innocuous stimuli was decreased by morphine within both pain pathways. Meanwhile, morphine suppressed the pain-evoked changes in the information flow from medial to lateral pathway and from cortex to thalamus. These effects were completely blocked by pre-treatment with the opiate receptor antagonist naloxone.

Conclusion

These results suggest that morphine exerts analgesic effects through suppressing both sensory and affective dimensions of pain.

Presynaptic and postsynaptic cortical mechanisms of chronic pain

Long-term potentiation (LTP) is a cellular model for learning and memory and believed to be critical for plastic changes in the brain. Depending on the central nervous system region, LTP has been proposed to contribute to many key physiological functions and pathological conditions, such as learning/memory, chronic pain, and drug addiction. While the induction of LTP in general requires activation of postsynaptic glutamate receptors, the expression of LTP can be mediated by postsynaptic mechanisms and/or presynaptic enhancement of glutamate release. In this review, we will evaluate recent progress made in the mechanisms of LTP in the anterior cingulate cortex (ACC) and explore its functional significance in synaptic changes after peripheral injury. Recent findings suggest that while ACC LTP in brain slice preparations is postsynaptically induced and expressed, injury triggered synaptic potentiation in the ACC contains both presynaptic enhancement of glutamate release and postsynaptic potentiation of AMPA receptor-mediated responses. Understanding presynaptic and postsynaptic mechanisms for ACC potentiation may help us to treat chronic pain in near future.

Roles of the AMPA receptor subunit GluA1 but not GluA2 in synaptic potentiation and activation of ERK in the anterior cingulate cortex

Cortical areas including the anterior cingulate cortex (ACC) are important for pain and pleasure. Recent studies using genetic and physiological approaches have demonstrated that the investigation of basic mechanism for long-term potentiation (LTP) in the ACC may reveal key cellular and molecular mechanisms for chronic pain in the cortex. Glutamate N-methyl D-aspartate (NMDA) receptors in the ACC are critical for the induction of LTP, including both NR2A and NR2B subunits. However, cellular and molecular mechanisms for the expression of ACC LTP have been less investigated. Here, we report that the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit, GluA1 but not GluA2 contributes to LTP in the ACC using genetic manipulated mice lacking GluA1 or GluA2 gene. Furthermore, GluA1 knockout mice showed decreased extracellular signal-regulated kinase (ERK) phosphorylation in the ACC in inflammatory pain models in vivo. Our results demonstrate that AMPA receptor subunit GluA1 is a key mechanism for the expression of ACC LTP and inflammation-induced long-term plastic changes in the ACC.

Excitatory neurotransmitters in brain regions in interictal migraine patients

OBJECTIVE

To examine biochemical differences in the anterior cingulate cortex (ACC) and insula during the interictal phase of migraine patients. We hypothesized that there may be differences in levels of excitatory amino acid neurotransmitters and/or their derivatives in migraine group based on their increased sensitivity to pain.

METHODS

2D J-resolved proton magnetic resonance spectroscopy (1H-MRS) data were acquired at 4.0 Tesla (T) from the ACC and insula in 10 migraine patients (7 women, 3 men, age 43 +/- 11 years) and 8 age gender matched controls (7 women, 3 men, age 41 +/- 9 years).

RESULTS

Standard statistical analyses including analysis of variance (ANOVA) showed no significant metabolite differences between the two subject cohorts in the ACC nor the insula. However, linear discriminant analysis (LDA) introduced a clear separation between subject cohorts based on N-acetyl aspartylglutamate (NAAG) and glutamine (Gln) in the ACC and insula.

CONCLUSION

These results are consistent with glutamatergic abnormalities in the ACC and insula in migraine patients during their interictal period compared to healthy controls. An alteration in excitatory amino acid neurotransmitters and their derivatives may be a contributing factor for migraineurs for a decrease in sensitivity for migraine or a consequence of the chronic migraine state. Such findings, if extrapolated to other regions of the brain would offer new opportunities to modulate central system as interictal or preemptive medications in these patients.

rTMS for suppressing neuropathic pain: a meta-analysis

This pooled individual data (PID)-based meta-analysis collectively assessed the analgesic effect of repetitive transcranial magnetic stimulation (rTMS) on various neuropathic pain states based on their neuroanatomical hierarchy. Available randomized controlled trials (RCTs) were screened. PID was coded for age, gender, pain neuroanatomical origins, pain duration, and treatment parameters analyses. Coded pain neuroanatomical origins consist of peripheral nerve (PN); nerve root (NR); spinal cord (SC); trigeminal nerve or ganglion (TGN); and post-stroke supraspinal related pain (PSP). Raw data of 149 patients were extracted from 5 (1 parallel, 4 cross-over) selected (from 235 articles) RCTs. A significant (P < .001) overall analgesic effect (mean percent difference in pain visual analog scale (VAS) score reduction with 95% confidence interval) was detected with greater reduction in VAS with rTMS in comparison to sham. Including the parallel study (Khedr et al), the TGN subgroup was found to have the greatest analgesic effect (28.8%), followed by PSP (16.7%), SC (14.7%), NR (10.0%), and PN (1.5%). The results were similar when we excluded the parallel study with the greatest analgesic effect observed in TGN (33.0%), followed by SC (14.7%), PSP (10.5%), NR (10.0%), and PN (1.5%). In addition, multiple (vs single, P = .003) sessions and lower (>1 and < or =10 Hz) treatment frequency range (vs >10 Hz) appears to generate better analgesic outcome. In short, rTMS appears to be more effective in suppressing centrally than peripherally originated neuropathic pain states.

PERSPECTIVE

This is the first PID-based meta-analysis to assess the differential analgesic effect of rTMS on neuropathic pain based on the neuroanatomical origins of the pain pathophysiology and treatment parameters. The derived information serves as a useful resource in regards to treatment parameters and patient population selection for future rTMS-pain studies.

Ca2+/calmodulin-dependent protein kinase IV links group I metabotropic glutamate receptors to fragile X mental retardation protein in cingulate cortex

Fragile X syndrome is caused by a lack of fragile X mental retardation protein (FMRP) due to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the central nervous system contribute to higher brain functions including learning/memory, persistent pain, and mental disorders. Our recent study has shown that activation of Group I mGluR up-regulated FMRP in anterior cingulate cortex (ACC), a key region for brain cognitive and executive functions; Ca(2+) signaling pathways could be involved in the regulation of FMRP by Group I mGluRs. In this study we demonstrate that stimulating Group I mGluRs activates Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) in ACC neurons. In ACC neurons of adult mice overexpressing CaMKIV, the up-regulation of FMRP by stimulating Group I mGluR is enhanced. The enhancement occurs at the transcriptional level as the Fmr1 mRNA level was further elevated compared with wild-type mice. Using pharmacological approaches, we found that inhibition of CaMKIV could attenuate the up-regulation of FMRP by Group I mGluRs. CaMKIV contribute to the regulation of FMRP by Group I mGluRs probably through cyclic AMP-responsive element binding protein (CREB) activation, as manipulation of CaMKIV could simultaneously cause the change of CREB phosphorylation induced by Group I mGluR activation. Our study has provided strong evidence for CaMKIV as a molecular link between Group I mGluRs and FMRP in ACC neurons and may help us to elucidate the pathogenesis of fragile X syndrome.

Enhanced quantal release of excitatory transmitter in anterior cingulate cortex of adult mice with chronic pain

The anterior cingulate cortex (ACC) is a forebrain structure that plays important roles in emotion, learning, memory and persistent pain. Our previous studies have demonstrated that the enhancement of excitatory synaptic transmission was induced by peripheral inflammation and nerve injury in ACC synapses. However, little information is available on their presynaptic mechanisms, since the source of the enhanced synaptic transmission could include the enhanced probability of neurotransmitter release at existing release sites and/or increases in the number of available vesicles. The present study aims to perform quantal analysis of excitatory synapses in the ACC with chronic pain to examine the source of these increases. The quantal analysis revealed that both probability of transmitter release and number of available vesicles were increased in a mouse model of peripheral inflammation, whereas only probability of transmitter release but not number of available vesicles was enhanced in a mouse model of neuropathic pain. In addition, we compared the miniature excitatory postsynaptic potentials (mEPSCs) in ACC synapses with those in other pain-related brain areas such as the amygdala and spinal cord. Interestingly, the rate and amplitude of mEPSCs in ACC synapses were significantly lower than those in the amygdala and spinal cord. Our studies provide strong evidences that chronic inflammatory pain increases both probability of transmitter release and number of available vesicles, whereas neuropathic pain increases only probability of transmitter release in the ACC synapses.

Disinhibitory involvement of the anterior cingulate cortex in the descending antinociceptive effect induced by electroacupuncture stimulation in rats

The present study was conducted to clarify the role of the anterior cingulate cortex (ACCX) in acupuncture analgesia. Experiments were performed on 35 female Wistar albino rats weighing about 300 g. Single unit recordings were made from ACCX neurons with a tungsten microelectrode. Descending ACCX neurons were identified by antidromic activation from electrical shocks applied to the ventral part of the ipsilateral PAG through a concentric needle electrode. Cathodal electroacupuncture stimulation of Ho-Ku (0.1 ms in duration, 45 Hz) for 15 min was done by inserting stainless steel needles bilaterally. An anodal silver-plate electrode (30 mm x 30 mm) was placed on the center of the abdomen. Naloxone (1.0 mg/kg, i.v.) was used to test whether changes of ACCX activities were induced by the endogenous opioid system. Data were collected from a total of 73 ACCX neurons. Forty-seven neurons had descending projection to the PAG, and the other 26 had no projections to the PAG. A majority of descending ACCX neurons were inhibited by electroacupuncture stimulation. By contrast, non-projection ACCX neurons were mainly unaffected by electroacupuncture. Naloxone did not reverse acupuncture effects on the changes of ACCX neuronal activities. Acupuncture stimulation had predominantly inhibitory effects on the activities of descending ACCX neurons. Since the functional connection between ACCX and PAG is inhibitory, electroacupuncture caused disinhibition of PAG neurons, whose activity is closely related to descending antinociception to the spinal cord. This disinhibitory effect elicited by acupuncture stimulation is thought to play a significant role in acupuncture analgesia.

Electrical stimulation therapies for CNS disorders and pain are mediated by competition between different neuronal networks in the brain

CNS neuronal networks are known to control normal physiological functions, including locomotion and respiration. Neuronal networks also mediate the pathophysiology of many CNS disorders. Stimulation therapies, including localized brain and vagus nerve stimulation, electroshock, and acupuncture, are proposed to activate "therapeutic" neuronal networks. These therapeutic networks are dormant prior to stimulatory treatments, but when the dormant networks are activated they compete with pathophysiological neuronal networks, disrupting their function. This competition diminishes the disease symptoms, providing effective therapy for otherwise intractable CNS disorders, including epilepsy, Parkinson's disease, chronic pain, and depression. Competition between stimulation-activated therapeutic networks and pathophysiological networks is a major mechanism mediating the therapeutic effects of stimulation. This network interaction is hypothesized to involve competition for "control" of brain regions that contain high proportions of conditional multireceptive (CMR) neurons. CMR regions, including brainstem reticular formation, amygdala, and cerebral cortex, have extensive connections to numerous brain areas, allowing these regions to participate potentially in many networks. The participation of CMR regions in any network is often variable, depending on the conditions affecting the organism, including vigilance states, drug treatment, and learning. This response variability of CMR neurons is due to the high incidence of excitatory postsynaptic potentials that are below threshold for triggering action potentials. These subthreshold responses can be brought to threshold by blocking inhibition or enhancing excitation via the paradigms used in stimulation therapies. Participation of CMR regions in a network is also strongly affected by pharmacological treatments (convulsant or anesthetic drugs) and stimulus parameters (strength and repetition rate). Many studies indicate that treatment of unanesthetized animals with antagonists (bicuculline or strychnine) of inhibitory neurotransmitter (GABA or glycine) receptors can cause CMR neurons to become consistently responsive to external inputs (e.g., peripheral nerve, sensory, or electrical stimuli in the brain) to which these neurons did not previously respond. Conversely, agents that enhance GABA-mediated inhibition (e.g., barbiturates and benzodiazepines) or antagonize glutamate-mediated excitation (e.g., ketamine) can cause CMR neurons to become unresponsive to inputs to which they responded previously. The responses of CMR neurons exhibit extensive short-term and long-term plasticity, which permits them to participate to a variable degree in many networks. Short-term plasticity subserves termination of disease symptoms, while long-term plasticity in CMR regions subserves symptom prevention. This network interaction hypothesis has value for future research in CNS disease mechanisms and also for identifying therapeutic targets in specific brain networks for more selective stimulation and pharmacological therapies.

Altered synaptic transmission in rat anterior cingulate cortex following peripheral nerve injury

BACKGROUND

Patients with neuropathic pain present not only with persistent pain but also a complex set of additional symptoms, including mood disorders and cognitive disturbance. Given the important roles of the anterior thalamic nuclei (ATN) and anterior cingulate cortex (ACC) in the cognitive and emotional aspects of pain, investigation of the properties of ATN-ACC synapses will help us to understand the mechanisms underlying neuropathic pain.

METHODS

We studied changes in ATN-evoked ACC excitatory postsynaptic potentials (EPSPs) induced by neuropathic pain in a rat model under halothane anaesthesia.

RESULTS

Neuropathic pain caused significant suppression of EPSPs in the ACC compared with rats subjected to sham surgery. Similar to previous evidence, acute inflammatory pain induced by formalin injection into the hind paw significantly increased synaptic efficacy in the ACC compared with naive rats. Neither of the pain paradigms altered the paired-pulse responses.

CONCLUSIONS

A possible explanation for the neuropathic pain-related suppression of EPSPs is that the ACC was already sufficiently active at baseline as a result of neuropathic pain, and ATN stimulation could not further increase the already elevated level of ACC activity. This abnormal excitability of the ATN-ACC synapse may be important in understanding the mechanism underlying neuropathic pain, particularly with respect to the affective and cognitive aspects.

Presynaptic and postsynaptic amplifications of neuropathic pain in the anterior cingulate cortex

Neuropathic pain is caused by a primary lesion or dysfunction in the nervous system. Investigations have mainly focused on the spinal mechanisms of neuropathic pain, and less is known about cortical changes in neuropathic pain. Here, we report that peripheral nerve injury triggered long-term changes in excitatory synaptic transmission in layer II/III neurons within the anterior cingulate cortex (ACC). Both the presynaptic release probability of glutamate and postsynaptic glutamate AMPA receptor-mediated responses were enhanced after injury using the mouse peripheral nerve injury model. Western blot showed upregulated phosphorylation of GluR1 in the ACC after nerve injury. Finally, we found that both presynaptic and postsynaptic changes after nerve injury were absent in genetic mice lacking calcium-stimulated adenylyl cyclase 1 (AC1). Our studies therefore provide direct integrative evidence for both long-term presynaptic and postsynaptic changes in cortical synapses after nerve injury, and that AC1 is critical for such long-term changes. AC1 thus may serve as a potential therapeutic target for treating neuropathic pain.

Genetic enhancement of trace fear memory and cingulate potentiation in mice overexpressing Ca2+/calmodulin-dependent protein kinase IV

Long-term potentiation (LTP) is a key cellular model for studying mechanisms for learning and memory. Previous studies reported that the Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is critical for gene regulation, and behavioral learning and memory. Less is known about the roles of CaMKIV in cortical plasticity and trace fear memory. Here we have found that LTP was significantly enhanced in the anterior cingulate cortex (ACC) of the mice overexpressing CaMKIV. By contrast, neither alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated basal excitatory synaptic transmission nor N-methyl-d-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents were affected. Furthermore, paired-pulse ratio in the transgenic mice is normal. In behavioral tests, we found that the CaMKIV transgenic mice exhibited significant enhancement in trace fear memory, while the acute sensory thresholds were not affected. Our results provide strong evidence that forebrain CaMKIV contributes to trace fear memory by enhancing synaptic potentiation in the ACC.

Involvement of P311 in the affective, but not in the sensory component of pain

Pain is comprised of the sensory and affective components. Compared to the well-investigated mechanisms of the sensory pain, much less is known about the mechanisms underlying the affective pain. In recent years, accumulating evidence suggests that the anterior cingulate cortex (ACC) is a key structure for pain affection. To identify the molecules that may be involved in the affective component of pain, we have searched the Allen Brain Atlas expression database for genes whose expression is enriched in the ACC, and found that P311, an 8-kDa peptide, showed the strong expression in the ACC. P311 is also expressed in other areas associated with pain affection including the amygdala, insular cortex and thalamus. To understand the role of P311 in pain perception, we have examined the pain behaviors of the mice lacking P311. P311^-/- mice showed normal heat and mechanical sensitivity, as well as normal formalin-induced inflammatory pain. In contrast, the formalin-induced avoidance behavior, which reflects pain-related negative emotion, was significantly attenuated in P311^-/- mice relative to the control mice. These results suggest that P311 is involved in the affective, but not in the sensory component of pain. Our study thus provides the first evidence suggesting that the affective and sensory pain may be regulated by distinct molecular mechanisms.

Dual influence of the striatum on neuropathic hypersensitivity

We studied whether striatal alpha(2)-adrenoceptors or N-methyl-d-aspartate (NMDA) receptors influence descending regulation of neuropathic hypersensitivity in the rat by microinjecting an alpha(2)-adrenoceptor agonist or NMDA-receptor antagonist into the dorsal striatum in animals with a spinal nerve ligation-induced neuropathy. Hypersensitivity was assessed in the hind limb by monofilaments and paw pressure test. Various neurotransmitter receptor antagonists were administered into the striatum or intrathecally to determine striatal and spinal neurotransmitters mediating the modulatory influence. The results indicate that the striatum has a dual effect on neuropathic hypersensitivity via two distinct pathways descending to the spinal cord. First, hypersensitivity is reduced following activation of noradrenergic alpha(2)-adrenoceptors and downstream dopamine D2 receptors in the striatum. This antihypersensitivity effect is predominantly ipsilateral and it descends via parallel dopaminergic and serotoninergic pathways to act on spinal dopamine D2 and 5-HT(1A) receptors, respectively. Second, tonic activation of striatal NMDA receptors promotes hypersensitivity by suppressing spinal GABAergic inhibition. The antihypersensitivity actions induced by striatal drug administrations were not associated with motor effects as suggested by lack of effect on the threshold of the uninjured limb or amplitude of the innocuous H-reflex. Involvement of striatal dopamine D2 receptors in the noradrenergic pain inhibitory circuitry may explain why disorders causing hypofunction of the striatal dopaminergic system, such as in Parkinson's disease, have been associated with pain. Furthermore, our findings indicate that striatal NMDA receptors provide a tonic supramedullary drive for medullospinal facilitatory influence that is known to be of importance for neuropathic hypersensitivity.

Roles of calcium-stimulated adenylyl cyclase and calmodulin-dependent protein kinase IV in the regulation of FMRP by group I metabotropic glutamate receptors

The fragile X syndrome is caused by the lack of fragile X mental retardation protein (FMRP) attributable to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the CNS contribute to different brain functions, including learning/memory, mental disorders, drug addiction, and persistent pain. Most of the previous studies have been focused on downstream targets of FMRP in hippocampal neurons, and fewer studies have been reported for the second-messenger signaling pathways between group I mGluRs and FMRP. Furthermore, no molecular study has been performed in the anterior cingulate cortex (ACC), a key region involved in high brain cognitive and executive functions. In this study, we demonstrate that activation of group I mGluR upregulated FMRP in ACC neurons of adult mice through the Ca(2+)-dependent signaling pathways. Using genetic approaches, we found that Ca(2+)/calmodulin-stimulated adenylyl cyclase 1 (AC1) and calcium/calmodulin-dependent kinase IV (CaMKIV) contribute to the upregulation of FMRP induced by stimulating group I mGluRs. The upregulation of FMRP occurs at the transcriptional level. The cAMP-dependent protein kinase is activated by stimulating group I mGluRs through AC1 in ACC neurons. Both AC1 and CaMKIV contribute to the regulation of FMRP by group I mGluRs probably through cAMP response element-binding protein activation. Our study has provided the first evidence for a molecular link between group I mGluRs and FMRP in ACC neurons and may help us to understand the pathogenesis of fragile X syndrome.

Role for NMDA receptors in visceral nociceptive transmission in the anterior cingulate cortex of viscerally hypersensitive rats

We have identified colorectal distension (CRD)-responsive neurons in the anterior cingulate cortex (ACC) and demonstrated that persistence of a heightened visceral afferent nociceptive input to the ACC induces ACC sensitization. In the present study, we confirmed that rostral ACC neurons of sensitized rats [induced by chicken egg albumin (EA)] exhibit enhanced spike responses to CRD. Simultaneous in vivo recording and reverse microdialysis of single ACC neurons showed that a low dose of glutamate (50 microM) did not change basal ACC neuronal firing in normal rats but increased ACC neuronal firing in EA rats from 18 +/- 2 to 32 +/- 3.8 impulses/10 s. A high dose of glutamate (500 microM) produced 1.95-fold and a 4.27-fold increases of ACC neuronal firing in sham-treated rats and in EA rats, respectively, suggesting enhanced glutamatergic transmission in the ACC neurons of EA rats. Reverse microdialysis of the 3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainite receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) reduced basal and abolished CRD-induced ACC neuronal firing in normal rats. In contrast, microdialysis of N-methyl-d-aspartate (NMDA) receptor antagonist AP5 had no effect on ACC neuronal firing in normal rats. However, AP5 produced 86% inhibition of ACC neuronal firing evoked by 50 mmHg CRD in the EA rats. In conclusion, ACC nociceptive transmissions are mediated by glutamate AMPA receptors in the control rats. ACC responses to CRD are enhanced in viscerally hypersensitive rats. The enhancement of excitatory glutamatergic transmission in the ACC appears to mediate this response. Furthermore, NMDA receptors mediate ACC synaptic responses after the induction of visceral hypersensitivity.

Antidepressants for the treatment of chronic pain

Chronic pain represents one of the most important public health problems and, in addition to classical analgesics, antidepressants are an essential part of the therapeutic strategy. This article reviews available evidence on the efficacy and safety of antidepressants in major chronic pain conditions; namely, neuropathic pain, headaches, low back pain, fibromyalgia, irritable bowel syndrome (IBS) and cancer pain. Studies, reviews and meta-analyses published from 1991 to March 2008 were retrieved through MEDLINE, PsycINFO and the Cochrane database using numerous key words for pain and antidepressants. In summary, evidence supports the use of tricyclic antidepressants in neuropathic pain, headaches, low back pain, fibromyalgia and IBS. The efficacy of the newer serotonin and norepinephrine reuptake inhibitors is less supported by evidence, but can be recommended in neuropathic pain, migraines and fibromyalgia. To date, evidence does not support an analgesic effect of serotonin reuptake inhibitors, but beneficial effects on well-being were reported in several chronic pain conditions. These results are discussed in the light of current insights in the neurobiology of pain, the reciprocal relationship between pain and depression, and future developments in this field of research.

Downregulation of selective microRNAs in trigeminal ganglion neurons following inflammatory muscle pain

Active regulation of gene expression in the nervous system plays an important role in the development and/or maintenance of inflammatory pain. MicroRNA (miRNA) negatively regulates gene expression via posttranscriptional or transcriptional inhibition of specific genes. To explore the possible involvement of miRNA in gene regulation during inflammatory pain, we injected complete Freund's adjuvant (CFA) unilaterally into the rat masseter muscle and quantified changes in neuron-specific mature miRNAs in the trigeminal ganglion (TG). Real-time reverse-transcription polymerase chain reaction revealed significant, but differential, downregulation of mature miR-10a, -29a, -98, -99a, -124a, -134, and -183 in the ipsilateral mandibular division (V3) of the TG within 4 hr after CFA. In contrast, levels of tested miRNAs did not change significantly in the contralateral V3 or the ipsilateral ophthalmic and maxillary divisions of the TG from inflamed rats, nor in the ipsilateral V3 of saline-injected animals. The downregulated miRNAs recovered differentially to a level equal to or higher than that in naive animals. Full recovery time varied with miRNA species but was at least 4 days. Expression and downregulation of some miRNAs were further confirmed by in situ hybridization of TG neurons that innervate the inflamed muscle. Although neurons of all sizes expressed these miRNAs, their signals varied between neurons. Our results indicate that miRNA species specific to neurons are quickly regulated following inflammatory muscle pain.

Pharmacological isolation of postsynaptic currents mediated by NR2A- and NR2B-containing NMDA receptors in the anterior cingulate cortex

NMDA receptors (NMDARs) are involved in excitatory synaptic transmission and plasticity associated with a variety of brain functions, from memory formation to chronic pain. Subunit-selective antagonists for NMDARs provide powerful tools to dissect NMDAR functions in neuronal activities. Recently developed antagonist for NR2A-containing receptors, NVP-AAM007, triggered debates on its selectivity and involvement of the NMDAR subunits in bi-directional synaptic plasticity. Here, we re-examined the pharmacological properties of NMDARs in the anterior cingulate cortex (ACC) using NVP-AAM007 as well as ifenprodil, a selective antagonist for NR2B-containing NMDARs. By alternating sequence of drug application and examining different concentrations of NVP-AAM007, we found that the presence of NVP-AAM007 did not significantly affect the effect of ifenprodil on NMDAR-mediated EPSCs. These results suggest that NVP-AAM007 shows great preference for NR2A subunit and could be used as a selective antagonist for NR2A-containing NMDARs in the ACC.

TNF-alpha involves in altered prefrontal synaptic transmission in mice with persistent inflammatory pain

Tumor necrosis factor alpha (TNF-alpha) is implicated in the development of persistent pain. Its expression increases both spinally and supraspinally after peripheral inflammation. The anterior cingulate cortex (ACC) is a forebrain structure known for its roles in pain transmission and modulation. Prefrontal synaptic transmission is potentiated in mice with chronic pain through an enhancement of presynaptic transmitter release. However, it is not known if TNF-alpha expression is altered in the ACC in response to persistent pain and if synaptic transmission within this region is modulated by TNF-alpha. In the present study, we examined TNF-alpha expression in the mouse ACC following hind-paw administration of complete Freund's adjuvant (CFA) and examined the role of TNF-alpha in ACC synaptic transmission. Quantification of TNF-alpha at the protein level (by ELISA) revealed enhanced expression following CFA-induced peripheral inflammation. In vitro whole-cell patch-clamp recordings revealed that TNF-alpha significantly enhanced synaptic transmission through increased probability of neurotransmitter release in the ACC. Our findings provide evidence that presynaptic alterations caused by peripheral inflammation is partly attributable to the up-regulation of TNF-alpha in the ACC.