Decreased nociceptive sensitization in mice lacking the fragile X mental retardation protein: role of mGluR1/5 and mTOR

Fragile X mental retardation protein controls synaptic vesicle exocytosis by modulating N-type calcium channel density

Fragile X syndrome (FXS), the most common heritable form of mental retardation, is characterized by synaptic dysfunction. Synaptic transmission depends critically on presynaptic calcium entry via voltage-gated calcium (Ca_V) channels. Here we show that the functional expression of neuronal N-type Ca_V channels (Ca_V2.2) is regulated by fragile X mental retardation protein (FMRP). We find that FMRP knockdown in dorsal root ganglion neurons increases Ca_V channel density in somata and in presynaptic terminals. We then show that FMRP controls Ca_V2.2 surface expression by targeting the channels to the proteasome for degradation. The interaction between FMRP and Ca_V2.2 occurs between the carboxy-terminal domain of FMRP and domains of Ca_V2.2 known to interact with the neurotransmitter release machinery. Finally, we show that FMRP controls synaptic exocytosis via Ca_V2.2 channels. Our data indicate that FMRP is a potent regulator of presynaptic activity, and its loss is likely to contribute to synaptic dysfunction in FXS.

Mutations in the fragile X mental retardation protein are implicated in synaptic dysfunction in fragile X syndrome. Here, Ferron et al. show that fragile X mental retardation protein maintains proper neurotransmission by regulating the density of N-type calcium channels in the presynaptic terminal.

Drug repositioning: playing dirty to kill pain

The number of approved new molecular entity drugs has been decreasing as the pharmaceutical company investment in research and development is increasing. As we face this painful crisis, called an innovation gap, there is increasing awareness that development of new uses of existing drugs may be a powerful tool to help overcome this obstacle because it takes too long, costs too much and can be risky to release drugs developed de novo. Consequently, drug repositioning is emerging in different therapeutic areas, including the pain research area. Worldwide, pain is the main reason for seeking healthcare, and pain relief represents an unmet global clinical need. Therefore, development of analgesics with better efficacy, safety and cost effectiveness is of paramount importance. Despite the remarkable advancement in research on cellular and molecular mechanisms underlying pain pathophysiology over the past three decades, target-based therapeutic opportunities have not been pursued to the same extent. Phenotypic screening remains a more powerful tool for drug development than target-based screening so far. It sounds somewhat heretical, but some multi-action drugs, rather than very selective ones, have been developed intentionally. In the present review, we first critically discuss the utility of drug repositioning for analgesic drug development and then show examples of 'old' drugs that have been successfully repositioned or that are under investigation for their analgesic actions. We conclude that drug repositioning should be more strongly encouraged to help build a bridge between basic research and pain relief worldwide.

The mTOR signaling pathway regulates pain-related synaptic plasticity in rat entorhinal-hippocampal pathways

Background

Our previous work demonstrated that persistent peripheral nociception (PPN) leads to synaptic plasticity and functional changes in the rat hippocampus. The protein kinase mTOR is a critical regulator of protein synthesis-dependent synaptic plasticity in the hippocampus as well as synaptic plasticity associated with central and peripheral pain sensitization. We examined the role of mTOR signaling in pain-associated entorhinal cortex (EC) - hippocampal synaptic plasticity to reveal possible cellular mechanisms underlying the effects of chronic pain on cognition and emotion.

Results

Subcutaneous injection of bee venom (BV) into one hind paw to induce PPN resulted in sustained (> 8 h) mTOR phospho-activation and enhanced phosphorylation of the mTOR target p70 S6 kinase (S6K) in the hippocampus. The magnitude and duration of long-term potentiation (LTP) in both EC - dentate gyrus (DG) and EC - CA1 synaptic pathways were elevated in BV-treated rats as measured by microelectrode array recording. Moreover, the number of potentiated synapses in the hippocampus was markedly upregulated by BV-induced PPN. Both elevated mTOR-S6K signaling and enhanced LTP induced by BV injection were reversed by systemic injection of the mTOR inhibitor rapamycin (RAPA). Rats injected with BV exhibited markedly reduced ambulation and exploratory activity in the open field (signs of depression and anxiety) compared to controls, and these effects were also reversed by RAPA.

Conclusion

We suggest that PPN-induced enhancement of synaptic plasticity in EC - hippocampal pathways and the behavioral effects of PPN are dependent on mTOR-S6K signaling.

Axonal protein synthesis and the regulation of primary afferent function

Local protein synthesis has been demonstrated in the peripheral processes of sensory primary afferents and is thought to contribute to the maintenance of the neuron, to neuronal plasticity following injury and also to regeneration of the axon after damage to the nerve. The mammalian target of rapamycin (mTOR), a master regulator of protein synthesis, integrates a variety of cues that regulate cellular homeostasis and is thought to play a key role in coordinating the neuronal response to environmental challenges. Evidence suggests that activated mTOR is expressed by peripheral nerve fibers, principally by A-nociceptors that rapidly signal noxious stimulation to the central nervous system, but also by a subset of fibers that respond to cold and itch. Inhibition of mTOR complex 1 (mTORC1) has shown that while the acute response to noxious stimulation is unaffected, more complex aspects of pain processing including the setting up and maintenance of chronic pain states can be disrupted suggesting a route for the generation of new drugs for the control of chronic pain. Given the role of mTORC1 in cellular homeostasis, it seems that systemic changes in the physiological state of the body such as occur during illness are likely to modulate the sensitivity of peripheral sensory afferents through mTORC1 signaling pathways.

Activation of mammalian target of rapamycin contributes to pain nociception induced in rats by BmK I, a sodium channel-specific modulator

The mammalian target of rapamycin (mTOR) pathway is essential for maintenance of the sensitivity of certain adult sensory neurons. Here, we investigated whether the mTOR cascade is involved in scorpion envenomation-induced pain hypersensitivity in rats. The results showed that intraplantar injection of a neurotoxin from Buthus martensii Karsch, BmK I (10 μg), induced the activation of mTOR, as well as its downstream molecules p70 ribosomal S6 protein kinase (p70 S6K) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), in lumbar 5-6 dorsal root ganglia neurons on both sides in rats. The activation peaked at 2 h and recovered 1 day after injection. Compared with the control group, the ratios of p-mTOR/p-p70 S6K/p-4EBP1 in three types of neurons changed significantly. The cell typology of p-mTOR/p-p70 S6K/p-4E-BP1 immuno-reactive neurons also changed. Intrathecal administration of deforolimus, a specific inhibitor of mTOR, attenuated BmK I-induced pain responses (spontaneous flinching, paroxysmal pain-like behavior, and mechanical hypersensitivity). Together, these results imply that the mTOR signaling pathway is mobilized by and contributes to experimental scorpion sting-induced pain.

Activation of mammalian target of rapamycin mediates rat pain-related responses induced by BmK I, a sodium channel-specific modulator

The mammalian target of rapamycin (mTOR) is known to regulate cell proliferation and growth by controlling protein translation. Recently, it has been shown that mTOR signaling pathway is involved in long-term synaptic plasticity. However, the role of mTOR under different pain conditions is less clear. In this study, the spatiotemporal activation of mTOR that contributes to pain-related behaviors was investigated using a novel animal inflammatory pain model induced by BmK I, a sodium channel-specific modulator purified from scorpion venom. In this study, intraplantar injections of BmK I were found to induce the activation of mTOR, p70 ribosomal S6 protein kinase (p70 S6K) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) in rat L5-L6 spinal neurons. In the spinal cord, mTOR, p70 S6K and 4E-BP1 were observed to be activated in the ipsilateral and contralateral regions, peaking at 1-2 h and recovery at 24 h post-intraplantar (i.pl.) BmK I administration. In addition, intrathecal (i.t.) injection of rapamycin - a specific inhibitor of mTOR - was observed to result in the reduction of spontaneous pain responses and the attenuation of unilateral thermal and bilateral mechanical hypersensitivity elicited by BmK I. Thus, these results indicate that the mTOR signaling pathway is mobilized in the induction and maintenance of pain-activated hypersensitivity.

Biological constraints limit the use of rapamycin-inducible FKBP12-Inp54p for depleting PIP2 in dorsal root ganglia neurons

BACKGROUND

Rapamycin-induced translocation systems can be used to manipulate biological processes with precise temporal control. These systems are based on rapamycin-induced dimerization of FK506 Binding Protein 12 (FKBP12) with the FKBP Rapamycin Binding (FRB) domain of mammalian target of rapamycin (mTOR). Here, we sought to adapt a rapamycin-inducible phosphatidylinositol 4,5-bisphosphate (PIP2)-specific phosphatase (Inp54p) system to deplete PIP2 in nociceptive dorsal root ganglia (DRG) neurons.

RESULTS

We genetically targeted membrane-tethered CFP-FRBPLF (a destabilized FRB mutant) to the ubiquitously expressed Rosa26 locus, generating a Rosa26-FRBPLF knockin mouse. In a second knockin mouse line, we targeted Venus-FKBP12-Inp54p to the Calcitonin gene-related peptide-alpha (CGRPα) locus. We hypothesized that after intercrossing these mice, rapamycin treatment would induce translocation of Venus-FKBP12-Inp54p to the plasma membrane in CGRP+ DRG neurons. In control experiments with cell lines, rapamycin induced translocation of Venus-FKBP12-Inp54p to the plasma membrane, and subsequent depletion of PIP2, as measured with a PIP2 biosensor. However, rapamycin did not induce translocation of Venus-FKBP12-Inp54p to the plasma membrane in FRBPLF-expressing DRG neurons (in vitro or in vivo). Moreover, rapamycin treatment did not alter PIP2-dependent thermosensation in vivo. Instead, rapamycin treatment stabilized FRBPLF in cultured DRG neurons, suggesting that rapamycin promoted dimerization of FRBPLF with endogenous FKBP12.

CONCLUSIONS

Taken together, our data indicate that these knockin mice cannot be used to inducibly deplete PIP2 in DRG neurons. Moreover, our data suggest that high levels of endogenous FKBP12 could compete for binding to FRBPLF, hence limiting the use of rapamycin-inducible systems to cells with low levels of endogenous FKBP12.

Activation of mTOR in the spinal cord is required for pain hypersensitivity induced by chronic constriction injury in mice

BACKGROUND

The mammalian target of rapamycin (mTOR) is known to regulate cell growth, and it also participates in pain transmission as has been recently verified in inflammatory and neuropathic pain models. The targeting of mTOR represents a new strategy for the control of chronic pain. In the present study, we investigated the effect of mTOR in the expression of PSD95 and NR2B-PSD95 or GluA2-PSD95 interaction ratio in a chronic constriction injury (CCI) mice model.

METHODS

Paw mechanical withdrawal threshold (PMWT) and paw withdrawal thermal latency (PWTL) were respectively used to assess mechanical allodynia and thermal hyperalgesia after CCI operation and intrathecal injection of rapamycin. Western blot and co-immunoprecipitation were used to investigate the effects of rapamycin on the expression of PSD95 and interaction ratio of NR2B-PSD95 or GluA2-PSD95 in the spinal dorsal horn of mice.

RESULTS

Our study demonstrated that the inhibition of spinal mTOR with intrathecal injections of rapamycin (1 μg/5 μL) for days 1-6 after CCI surgery led to an obvious decrease in CCI-induced neuropathic pain. Rapamycin significantly reduced the PMWT of CCI mice, whereas there was no significant effect on PWTL. The active form of the mTOR signaling pathway (p-mTOR, p-4EBP1 and p-p70S6k) at the spinal level remarkably increased in CCI mice, and rapamycin could inhibit this up-regulation. The increased expression of PSD95 and the interaction ratio of GluA2-PSD95 or NR2B-PSD95 could also be inhibited by intrathecal injection of rapamycin.

CONCLUSION

These data suggest that the mTOR pathway is activated in the spinal dorsal horn in CCI-induced neuropathic pain, and the intrathecal injection of rapamycin can reduce mechanical allodynia. Our findings indicate that spinal mTOR is an important component of CCI-induced neuropathic pain, and mTOR may be a potential target for chronic pain therapy.

AMPK: An emerging target for modification of injury-induced pain plasticity

Chronic pain is a critical medical problem afflicting hundreds of millions of people worldwide with costly effects on society and health care systems. Novel therapeutic avenues for the treatment of pain are needed that are directly targeted to the molecular mechanisms that promote and maintain chronic pain states. Recent evidence suggests that peripheral pain plasticity is promoted and potentially maintained via changes in translation control that are mediated by mTORC1 and MAPK pathways. While these pathways can be targeted individually, stimulating the AMPK pathway with direct or indirect activators achieves inhibition of these pathways via engagement of a single kinase. Here we review the form, function and pharmacology of AMPK with special attention to its emerging role as a potential target for pain therapeutics. We present the existing evidence supporting a role of AMPK activation in alleviating symptoms of peripheral nerve injury- and incision-induced pain plasticity and the blockade of the development of chronic pain following surgery. We argue that these preclinical findings support a strong rationale for clinical trials of currently available AMPK activators and further development of novel pharmacological strategies for more potent and efficacious manipulation of AMPK in the clinical setting. Finally, we posit that AMPK represents a unique opportunity for drug development in the kinase area for pain because it is pharmacologically manipulated via activation rather than inhibition potentially offering a wider therapeutic window with interesting additional pharmacological opportunities. Altogether, the physiology, pharmacology and therapeutic opportunities surrounding AMPK make it an attractive target for novel intervention for chronic pain and its prevention.

Peripheral administration of translation inhibitors reverses increased hyperalgesia in a model of chronic pain in the rat

Chronic pain is extremely difficult to manage, in part due to lack of progress in reversing the underlying pathophysiology. Since translation of messenger ribonucleic acids (mRNAs) in the peripheral terminal of the nociceptor plays a role in the transition from acute to chronic pain, we tested the hypothesis that transient inhibition of translation in the peripheral terminal of the nociceptor could reverse hyperalgesic priming, a model of transition from acute to chronic pain. We report that injection of translation inhibitors rapamycin and cordycepin, which inhibit translation by different mechanisms, at the peripheral terminal of the primed nociceptor produces reversal of priming in the rat that outlasted the duration of action of these drugs to prevent the development of priming. These data support the suggestion that interruption of translation in the nociceptor can reverse a preclinical model of at least 1 form of chronic pain.

PERSPECTIVE

This study provides evidence that ongoing protein translation in the sensory neuron terminals is involved in pain chronification, and local treatment that transiently interrupts this translation may be a useful therapy to chronic pain.

Protein kinase B/Akt is required for complete Freund's adjuvant-induced upregulation of Nav1.7 and Nav1.8 in primary sensory neurons

Voltage-gated sodium channels (Nav) are essential for the generation and conduction of action potentials. Peripheral inflammation increases the expression of Nav1.7 and Nav1.8 in dorsal root ganglion (DRG) neurons, suggesting that they participate in the induction and maintenance of chronic inflammatory pain. However, how Nav1.7 and Nav1.8 are regulated in the DRG under inflammatory pain conditions remains unclear. Using a complete Freund's adjuvant (CFA)-induced chronic inflammatory pain model and Western blot analysis, we found that phosphorylated Akt (p-Akt) was significantly increased in the ipsilateral L4/5 DRGs of rats on days 3 and 7 after intraplantar CFA injection. Immunohistochemistry showed that the percentage of p-Akt-positive neurons in the DRG was also significantly increased in the ipsilateral L4/5 DRGs at these time points. Moreover, CFA injection increased the colocalization of p-Akt with Nav1.7 and Nav1.8 in L4/5 DRG neurons. Pretreatment of rats with an intrathecal injection of Akt inhibitor IV blocked CFA-induced thermal hyperalgesia and CFA-induced increases in Nav1.7 and Nav1.8 in the L4/5 DRGs on day 7 after CFA injection. Our findings suggest that the Akt pathway participates in inflammation-induced upregulation of Nav1.7 and Nav1.8 expression in DRG neurons. This participation might contribute to the maintenance of chronic inflammatory pain.

PERSPECTIVE

This article presents that inhibition of Akt blocks CFA-induced thermal hyperalgesia and CFA-induced increases in dorsal root ganglion Nav1.7 and Nav1.8. These findings have potential implications for use of Akt inhibitors to prevent and/or treat persistent inflammatory pain.

Resilience to audiogenic seizures is associated with p-ERK1/2 dephosphorylation in the subiculum of Fmr1 knockout mice

Young, but not adult, fragile X mental retardation gene (Fmr1) knockout (KO) mice display audiogenic seizures (AGS) that can be prevented by inhibiting extracellular signal-regulated kinases 1/2 (ERK1/2) phosphorylation. In order to identify the cerebral regions involved in these phenomena, we characterized the response to AGS in Fmr1 KO mice and wild type (WT) controls at postnatal day (P) 45 and P90. To characterize the diverse response to AGS in various cerebral regions, we evaluated the activity markers FosB/ΔFosB and phosphorylated ERK1/2 (p-ERK1/2). Wild running (100% of tested mice) followed by clonic/tonic seizures (30%) were observed in P45 Fmr1 KO mice, but not in WT mice. In P90 Fmr1 KO mice, wild running was only present in 25% of tested animals. Basal FosB/ΔFosB immunoreactivity was higher (P < 0.01 vs. WT) in the CA1 and subiculum of P45 Fmr1 KO mice. Following the AGS test, FosB/ΔFosB expression consistently increased in most of the analyzed regions in both groups at P45, but not at P90. Interestingly, FosB/ΔFosB immunoreactivity was significantly higher in P45 Fmr1 KO mice in the medial geniculate body (P < 0.05 vs. WT) and CA3 (P < 0.01). Neurons presenting with immunopositivity to p-ERK1/2 were more abundant in the subiculum of Fmr1 KO mice in control condition (P < 0.05 vs. WT, in both age groups). In this region, p-ERK1/2-immunopositive cells significantly decreased (–75%, P < 0.01) in P90 Fmr1 KO mice exposed to the AGS test, but no changes were found in P45 mice or in other brain regions. In both age groups of WT mice, p-ERK1/2-immunopositive cells increased in the subiculum after exposure to the acoustic test. Our findings illustrate that FosB/ΔFosB markers are overexpressed in the medial geniculate body and CA3 in Fmr1 KO mice experiencing AGS, and that p-ERK1/2 is markedly decreased in the subiculum of Fmr1 KO mice resistant to AGS induction. These findings suggest that resilience to AGS is associated with dephosphorylation of p-ERK1/2 in the subiculum of mature Fmr1 KO mice.

mTOR and its downstream pathway are activated in the dorsal root ganglion and spinal cord after peripheral inflammation, but not after nerve injury

Protein translation controlled through activation of mammalian target of rapamycin (mTOR) participates in many physiological and pathological processes. However, whether such activation is required for chronic pain is still unknown. Here, we examined activation of the mTOR signaling pathway during complete Freund's adjuvant (CFA)-induced chronic inflammatory pain and L5 spinal nerve ligation (SNL)-induced neuropathic pain in rats. Western blot analysis showed significantly increased levels of phosphorylated mTOR (p-mTOR) and phosphorylated p70S6 kinase 1 (p-S6K1, a downstream effector of mTOR) in the ipsilateral L4/5 spinal cord 2h, 1 day, 3 days, and 7 days after intraplantar CFA injection and in the ipsilateral L4/5 dorsal root ganglions (DRGs) 1 and 3 days after CFA injection. Immunohistochemistry also demonstrated increases in number of p-mTOR-labeled neurons in the ipsilateral L4/5 DRGs and in density of p-mTOR-labeled immunoreactivity in the ipsilateral L4/5 superficial dorsal horn 1 day after CFA injection. Moreover, intrathecal administration of rapamycin, a selective inhibitor of mTOR, significantly blocked CFA-induced mechanical allodynia and thermal hyperalgesia 1 day post-CFA injection. Interestingly, expression of neither p-mTOR nor p-S6K1 was markedly altered on days 3, 7, or 14 after L5 SNL in L5 spinal cord or DRG. These findings indicate that in DRG and spinal cord, mTOR and S6K1 are activated during chronic inflammatory pain, but not during neuropathic pain. Our results strongly suggest that mTOR and its downstream pathway contribute to the development of chronic inflammatory pain.

Fragile X mental retardation protein and synaptic plasticity

Loss of the translational repressor FMRP causes Fragile X syndrome. In healthy neurons, FMRP modulates the local translation of numerous synaptic proteins. Synthesis of these proteins is required for the maintenance and regulation of long-lasting changes in synaptic strength. In this role as a translational inhibitor, FMRP exerts profound effects on synaptic plasticity.

Proteomic and functional annotation analysis of injured peripheral nerves reveals ApoE as a protein upregulated by injury that is modulated by metformin treatment

Background

Peripheral nerve injury (PNI) results in a fundamental reorganization of the translational machinery in the injured peripheral nerve such that protein synthesis is increased in a manner linked to enhanced mTOR and ERK activity. We have shown that metformin treatment, which activates adenosine monophosphate-activated protein kinase (AMPK), reverses tactile allodynia and enhanced translation following PNI. To gain a better understanding of how PNI changes the proteome of the sciatic nerve and ascertain how metformin treatment may cause further change, we conducted a range of unbiased proteomic studies followed by biochemical experiments to confirm key results.

Results

We used multidimensional protein identification technology (MUDPIT) on sciatic nerve samples taken from rats with sham surgery, spinal nerve ligation (SNL) surgery or SNL + 200 mg/kg metformin treatment. MUDPIT analysis on these complex samples yielded a wide variety of proteins that were sorted according to their peptide counts in SNL and SNL + metformin compared to sham. These proteins were then submitted to functional annotation analysis to identify potential functional networks altered by SNL and SNL + metformin treatment. Additionally, we used click-chemistry-based labeling and purification of nascently synthesized proteins followed by MUDPIT to further identify peptides that were synthesized within the injured nerve. With these methods, we identified apolipoprotein E (ApoE) as a protein profoundly increased by PNI and further increased by PNI and metformin. This result was confirmed by Western Blot of samples from SNL rats and spared nerve injury (SNI) mice. Furthermore, we show that 7-day treatment with metformin in naïve mice leads to an increase in ApoE expression in the sciatic nerve.

Conclusions

These proteomic findings support the hypothesis that PNI leads to a fundamental reorganization of gene expression within the injured nerve. Our data identify a key association of ApoE with PNI that is regulated by metformin treatment. We conclude from the known functions of ApoE in the nervous system that ApoE may be an intrinsic factor linked to nerve regeneration after PNI, an effect that is further enhanced by metformin treatment.

BDNF regulates atypical PKC at spinal synapses to initiate and maintain a centralized chronic pain state

Background

Chronic pain is an important medical problem affecting hundreds of millions of people worldwide. Mechanisms underlying the maintenance of chronic pain states are poorly understood but the elucidation of such mechanisms have the potential to reveal novel therapeutics capable of reversing a chronic pain state. We have recently shown that the maintenance of a chronic pain state is dependent on an atypical PKC, PKMζ, but the mechanisms involved in controlling PKMζ in chronic pain are completely unknown. Here we have tested the hypothesis that brain derived neurotrophic factor (BDNF) regulates PKMζ, and possibly other aPKCs, to maintain a centralized chronic pain state.

Results

We first demonstrate that although other kinases play a role in the initiation of persistent nociceptive sensitization, they are not involved in the maintenance of this chronic pain state indicating that a ZIP-reversible process is responsible for the maintenance of persistent sensitization. We further show that BDNF plays a critical role in initiating and maintaining persistent nociceptive sensitization and that this occurs via a ZIP-reversible process. Moreover, at spinal synapses, BDNF controls PKMζ and PKCλ nascent synthesis via mTORC1 and BDNF enhances PKMζ phosphorylaton. Finally, we show that BDNF signaling to PKMζ and PKCλ is conserved across CNS synapses demonstrating molecular links between pain and memory mechanisms.

Conclusions

Hence, BDNF is a key regulator of aPKC synthesis and phosphorylation and an essential mediator of the maintenance of a centralized chronic pain state. These findings point to BDNF regulation of aPKC as a potential therapeutic target for the permanent reversal of a chronic pain state.

mTORC1 inhibition induces pain via IRS-1-dependent feedback activation of ERK

Mammalian target of rapamycin complex 1 (mTORC1) inhibitors are extensively used as immunosuppressants to prevent transplant rejection and in treatment of certain cancers. In patients, chronic treatment with rapamycin or its analogues (rapalogues) has been reported to lead to sensory hypersensitivity and pain conditions via an unknown mechanism. Here, we show that pharmacological or genetic inhibition of mTORC1 activates the extracellular signal-regulated kinase (ERK) pathway in sensory neurons via suppression of S6K1 to insulin receptor substrate 1 negative feedback loop. As a result, increased ERK activity induces sensory neuron sensitization, mechanical hypersensitivity, and spontaneous pain. The clinically available adenosine monophosphate-activated protein kinase activator, metformin, which is an antidiabetic drug, prevents rapamycin-induced ERK activation and the development of mechanical hypersensitivity and spontaneous pain. Taken together, our findings demonstrate that activation of the ERK pathway in sensory neurons as a consequence of mTORC1 inhibition leads to the development of pain. Importantly, this effect is abolished by co-treatment with metformin, thus providing a potential treatment option for rapalogue-evoked pain. Our findings highlight the physiological relevance of feedback signaling through mTORC1 inhibition and have important implications for development of pain therapeutics that target the mTOR pathway.

Epigenetic regulation of BDNF expression in the primary sensory neurons after peripheral nerve injury: implications in the development of neuropathic pain

Brain-derived neurotrophic factor (BDNF) is known to be up-regulated in the dorsal root ganglion (DRG) after peripheral nerve injury, and to contribute to neuropathic pain. Here, we found that thermal hyperalgesia and mechanical allodynia at day 7 post-injury were inhibited only when anti-BDNF antibody was intrathecally administrated at day 2 post-injury. Consistent with behavioral results, Western blot analysis showed that the expression levels of BDNF protein in the spinal dorsal horn were markedly induced during early stage post-injury. Moreover, the maximal increase in BDNF mRNA expression in the DRG was observed at day 1 post-injury, and significantly elevated levels were sustained for at least 14 days. Four of five BDNF mRNA transcripts were up-regulated after nerve injury, and the most inducible transcript was exon I. Using a chromatin immunoprecipitation (ChIP) assay, we found that nerve injury promotes histone H3 and H4 acetylation, transcriptionally active modifications, at BDNF promoter I at day 1 post-injury, and the levels of histone acetylation remain elevated for at least 7 days. Taken together, our findings suggest that an initial increase in BDNF exon I expression controlled by epigenetic mechanisms might have a crucial role in the development of neuropathic pain.

ZIPping to pain relief: the role (or not) of PKMζ in chronic pain

Chronic pain remains a significant clinical problem despite substantial advances in our understanding of how persistent nociceptor stimulation drives plasticity in the CNS. A major theme that has emerged in this area of work is the strong similarity between plasticity involved in learning and memory in CNS regions such as cortex and hippocampus with mechanisms underlying chronic pain development and maintenance in the spinal dorsal horn and other CNS areas such as anterior cingulate cortex (ACC). We, and others have recently implicated an atypical PKC (aPKC), called PKMζ, in the maintenance of pain plasticity based on biochemical assays and the use of a peptide pseudosubstrate inhibitor called ZIP. These studies indicate remarkable parallels between the potential role of PKMζ as a key molecule for the maintenance of long-term memory and long-term potentiation (LTP) and the maintenance of a chronic pain state. On the other hand, very recent studies have disputed the specificity of ZIP and called into question the role of PKMζ as a memory maintenance molecule. Here we critically review the evidence that PKMζ might represent a new target for the reversal of certain chronic pain states. Furthermore, we consider whether ZIP might have other aPKC or even non-aPKC targets and the significance of such off-target effects for evaluating maintenance mechanisms of chronic pain. We conclude that, current controversies aside, utilization of ZIP as a tool to interrogate maintenance mechanisms of chronic pain and further investigations into the potential role of PKMζ, and other aPKCs, in pain plasticity are likely to lead to further insights with the potential to unravel the enigma that is the disease of chronic pain.

Maladaptive dendritic spine remodeling contributes to diabetic neuropathic pain

Diabetic neuropathic pain imposes a huge burden on individuals and society, and represents a major public health problem. Despite aggressive efforts, diabetic neuropathic pain is generally refractory to available clinical treatments. A structure-function link between maladaptive dendritic spine plasticity and pain has been demonstrated previously in CNS and PNS injury models of neuropathic pain. Here, we reasoned that if dendritic spine remodeling contributes to diabetic neuropathic pain, then (1) the presence of malformed spines should coincide with the development of pain, and (2) disrupting maladaptive spine structure should reduce chronic pain. To determine whether dendritic spine remodeling contributes to neuropathic pain in streptozotocin (STZ)-induced diabetic rats, we analyzed dendritic spine morphology and electrophysiological and behavioral signs of neuropathic pain. Our results show changes in dendritic spine shape, distribution, and shape on wide-dynamic-range (WDR) neurons within lamina IV-V of the dorsal horn in diabetes. These diabetes-induced changes were accompanied by WDR neuron hyperexcitability and decreased pain thresholds at 4 weeks. Treatment with NSC23766 (N(6)-[2-[[4-(diethylamino)-1-methylbutyl]amino]-6-methyl-4-pyrimidinyl]-2-methyl-4,6-quinolinediamine trihydrochloride), a Rac1-specific inhibitor known to interfere with spine plasticity, decreased the presence of malformed spines in diabetes, attenuated neuronal hyperresponsiveness to peripheral stimuli, reduced spontaneous firing activity from WDR neurons, and improved nociceptive mechanical pain thresholds. At 1 week after STZ injection, animals with hyperglycemia with no evidence of pain had few or no changes in spine morphology. These results demonstrate that diabetes-induced maladaptive dendritic spine remodeling has a mechanistic role in neuropathic pain. Molecular pathways that control spine morphogenesis and plasticity may be promising future targets for treatment.

Self-injurious behaviour in intellectual disability syndromes: evidence for aberrant pain signalling as a contributing factor

BACKGROUND

In most individuals, injury results in activation of peripheral nociceptors (pain-sensing neurons of the peripheral nervous system) and amplification of central nervous system (CNS) pain pathways that serve as a disincentive to continue harmful behaviour; however, this may not be the case in some developmental disorders that cause intellectual disability (ID). Moreover, individuals affected by ID disorders may initiate self-injurious behaviour to address irritating or painful sensations. In normal individuals, a negative feedback loop decreases sensation of pain, which involves descending inhibitory neurons in the CNS that attenuate spinal nociceptive processing. If spinal nociceptive signalling is impaired in these developmental disorders, an exaggerated painful stimulus may be required in order to engage descending anti-nociceptive signals.

METHODS

Using electronic databases, we conducted a review of publications regarding the incidence of chronic pain or altered pain sensation in ID patients or corresponding preclinical models.

RESULTS

There is a body of evidence indicating that individuals with fragile X mental retardation and/or Rett syndrome have altered pain sensation. These findings in humans are supported by mechanistic studies using genetically modified mice harbouring mutations consistent with the human disease. Thus, once self-injurious behaviour is initiated, the signal to stop may be missing. Several developmental disorders that cause ID are associated with increased incidence of gastroesophageal reflux disease (GERD), which can cause severe visceral pain. Individuals affected by these disorders who also have GERD may self-injure as a mechanism to engage descending inhibitory circuits to quell visceral pain. In keeping with this hypothesis, pharmacological treatment of GERD has been shown to be effective for reducing self-injurious behaviour in some patients. Hence, multiple lines of evidence suggest aberrant nociceptive processing in developmental disorders that cause ID.

CONCLUSIONS

There is evidence that pain pathways and pain amplification mechanisms are altered in several preclinical models of developmental disorders that cause ID. We present hypotheses regarding how impaired pain pathways or chronic pain might contribute to self-injurious behaviour. Studies evaluating the relationship between pain and self-injurious behaviour will provide better understanding of the mechanisms underlying self-injurious behaviour in the ID population and may lead to more effective treatments.

Reduction of BDNF expression in Fmr1 knockout mice worsens cognitive deficits but improves hyperactivity and sensorimotor deficits

Fragile X syndrome (FXS) is a common cause of inherited intellectual disability and a well-characterized form of autism spectrum disorder. As brain-derived neurotrophic factor (BDNF) is implicated in the pathophysiology of FXS we examined the effects of reduced BDNF expression on the behavioral phenotype of an animal model of FXS, Fmr1 knockout (KO) mice, crossed with mice carrying a deletion of one copy of the Bdnf gene (Bdnf(+/-)). Fmr1 KO mice showed age-dependent alterations in hippocampal BDNF expression that declined after the age of 4 months compared to wild-type controls. Mild deficits in water maze learning in Bdnf(+/-) and Fmr1 KO mice were exaggerated and contextual fear learning significantly impaired in double transgenics. Reduced BDNF expression did not alter basal nociceptive responses or central hypersensitivity in Fmr1 KO mice. Paradoxically, the locomotor hyperactivity and deficits in sensorimotor learning and startle responses characteristic of Fmr1 KO mice were ameliorated by reducing BNDF, suggesting changes in simultaneously and in parallel working hippocampus-dependent and striatum-dependent systems. Furthermore, the obesity normally seen in Bdnf(+/-) mice was eliminated by the absence of fragile X mental retardation protein 1 (FMRP). Reduced BDNF decreased the survival of newborn cells in the ventral part of the hippocampus both in the presence and absence of FMRP. Since a short neurite phenotype characteristic of newborn cells lacking FMRP was not found in cells derived from double mutant mice, changes in neuronal maturation likely contributed to the behavioral phenotype. Our results show that the absence of FMRP modifies the diverse effects of BDNF on the FXS phenotype.

Spinal cord NMDA receptor-mediated activation of mammalian target of rapamycin is required for the development and maintenance of bone cancer-induced pain hypersensitivities in rats

Mammalian target of rapamycin (mTOR) controls mRNA translation and is critical for neuronal plasticity. However, how it participates in central sensitization underlying chronic pain is unclear. Here, we show that NMDA receptors are required for the functional role of spinal cord mTOR in bone cancer pain induced by injecting prostate cancer cells (PCCs) into the tibia. Intrathecal rapamycin, a specific mTOR inhibitor, dose dependently attenuated the development and maintenance of PCC-induced mechanical allodynia and thermal hyperalgesia. Rapamycin alone did not affect locomotor activity and acute responses to thermal or mechanical stimuli. Phosphorylation of mTOR and p70S6K (a downstream effector) was increased time dependently in L(4-5) dorsal horn and transiently in L(4-5) dorsal root ganglions on the ipsilateral side after PCC injection, although total expression of mTOR or p70S6K was not changed in these regions. The increases in dorsal horn were abolished by intrathecal infusion of DL-AP5, an NMDA receptor antagonist. Moreover, NMDA receptor subunit NR1 colocalized with mTOR and p70S6K in dorsal horn neurons. These findings suggest that PCC-induced dorsal horn activation of the mTOR pathway participates in NMDA receptor-triggered dorsal central sensitization under cancer pain conditions.

PERSPECTIVE

The present study shows that inhibition of spinal mTOR blocks cancer-related pain without affecting acute pain and locomotor function. Given that mTOR inhibitors are FDA-approved drugs, mTOR in spinal cord may represent a potential new target for preventing and/or treating cancer-related pain.

Resveratrol engages AMPK to attenuate ERK and mTOR signaling in sensory neurons and inhibits incision-induced acute and chronic pain

Background

Despite advances in our understanding of basic mechanisms driving post-surgical pain, treating incision-induced pain remains a major clinical challenge. Moreover, surgery has been implicated as a major cause of chronic pain conditions. Hence, more efficacious treatments are needed to inhibit incision-induced pain and prevent the transition to chronic pain following surgery. We reasoned that activators of AMP-activated protein kinase (AMPK) may represent a novel treatment avenue for the local treatment of incision-induced pain because AMPK activators inhibit ERK and mTOR signaling, two important pathways involved in the sensitization of peripheral nociceptors.

Results

To test this hypothesis we used a potent and efficacious activator of AMPK, resveratrol. Our results demonstrate that resveratrol profoundly inhibits ERK and mTOR signaling in sensory neurons in a time- and concentration-dependent fashion and that these effects are mediated by AMPK activation and independent of sirtuin activity. Interleukin-6 (IL-6) is thought to play an important role in incision-induced pain and resveratrol potently inhibited IL-6-mediated signaling to ERK in sensory neurons and blocked IL-6-mediated allodynia in vivo through a local mechanism of action. Using a model of incision-induced allodynia in mice, we further demonstrate that local injection of resveratrol around the surgical wound strongly attenuates incision-induced allodynia. Intraplantar IL-6 injection and plantar incision induces persistent nociceptive sensitization to PGE₂ injection into the affected paw after the resolution of allodynia to the initial stimulus. We further show that resveratrol treatment at the time of IL-6 injection or plantar incision completely blocks the development of persistent nociceptive sensitization consistent with the blockade of a transition to a chronic pain state by resveratrol treatment.

Conclusions

These results highlight the importance of signaling to translation control in peripheral sensitization of nociceptors and provide further evidence for activation of AMPK as a novel treatment avenue for acute and chronic pain states.

Therapeutic strategies in fragile X syndrome: dysregulated mGluR signaling and beyond

Fragile X syndrome (FXS) is an inherited neurodevelopmental disease caused by loss of function of the fragile X mental retardation protein (FMRP). In the absence of FMRP, signaling through group 1 metabotropic glutamate receptors is elevated and insensitive to stimulation, which may underlie many of the neurological and neuropsychiatric features of FXS. Treatment of FXS animal models with negative allosteric modulators of these receptors and preliminary clinical trials in human patients support the hypothesis that metabotropic glutamate receptor signaling is a valuable therapeutic target in FXS. However, recent research has also shown that FMRP may regulate diverse aspects of neuronal signaling downstream of several cell surface receptors, suggesting a possible new route to more direct disease-targeted therapies. Here, we summarize promising recent advances in basic research identifying and testing novel therapeutic strategies in FXS models, and evaluate their potential therapeutic benefits. We provide an overview of recent and ongoing clinical trials motivated by some of these findings, and discuss the challenges for both basic science and clinical applications in the continued development of effective disease mechanism-targeted therapies for FXS.

Fragile X mental retardation protein (FMRP) and the spinal sensory system

The purpose of this chapter is to discuss the role of the fragile X mental retardation protein (FMRP) in the spinal sensory system and the potential for use of the mouse model of fragile X syndrome to better understand some aspects of the human syndrome as well as advance knowledge in other areas of investigation, such as pain amplification, an important aspect of clinical pain disorders. We describe how the Fmr1 knockout mouse can be used to better understand the role of Fmrp in axons using cultures of sensory neurons and using manipulations to these neurons in vivo. We also discuss the established evidence for a role of Fmrp in nociceptive sensitization and how this evidence relates to an emerging role of translation control as a key process in pain amplification. Finally, we explore opportunities centered on the Fmr1 KO mouse for gaining further insight into the role of translation control in pain amplification and how this model may be used to identify novel therapeutic targets. We conclude that the study of the spinal sensory system in the Fmr1 KO mouse presents several unique prospects for gaining better insight into the human disorder and other clinical issues, such as chronic pain disorders, that affect millions of people worldwide.

Timing in the absence of supraspinal input III: regularly spaced cutaneous stimulation prevents and reverses the spinal learning deficit produced by peripheral inflammation

In the absence of brain input, spinal systems can adapt to new environmental relations. For example, spinally transected rats given a legshock each time the leg is extended exhibit a progressive increase in flexion duration that minimizes net shock exposure, a simple form of instrumental learning. This capacity for learning is modulated by prior stimulation; both variable shock and inflammation produce a lasting inhibition of learning. An extended exposure to fixed spaced shock has no adverse effect on learning and opposes the consequences of variable shock. The present studies expand on these findings and demonstrate that fixed stimulation ameliorates the impact of peripheral inflammation. Spinally transected rats were administered 900 fixed spaced legshocks before (Experiment 1) or 1,800 legshocks after (Experiment 2) a subcutaneous hindpaw injection of capsaicin. Learning was assessed 24 hr later. Treatment with fixed shock attenuated the capsaicin-induced inhibition of learning. These findings suggest that fixed stimulation promotes adaptive plasticity and may foster recovery after injury.

Genetic reduction of muscarinic M4 receptor modulates analgesic response and acoustic startle response in a mouse model of fragile X syndrome (FXS)

INTRODUCTION

The G-protein coupled muscarinic acetylcholine receptors, widely expressed in the CNS, have been implicated in fragile X syndrome (FXS). Recent studies have reported an overactive signaling through the muscarinic receptors in the Fmr1KO mouse model. Hence, it was hypothesized that reducing muscarinic signaling might modulate behavioral phenotypes in the Fmr1KO mice. Pharmacological studies from our lab have provided evidence for this hypothesis, with subtype-preferring muscarinic M1 and M4 receptor antagonists modulating select behaviors in the Fmr1KO mice. Since the pharmacological antagonists were not highly specific, we investigated the specific role of M4 receptors in the Fmr1KO mouse model, using a genetic approach.

METHODS

We created a double mutant heterozygous for the M4 receptor gene and hemizygous for the Fmr1 gene and examined the mutants on various behaviors. Each animal was tested on a behavior battery comprising of open-field activity (activity), light-dark (anxiety), marble burying (perseverative behavior), prepulse inhibition (sensorimotor gating), rotarod (motor coordination), passive avoidance (learning and memory) and hotplate (analgesia). Animals were also tested on the audiogenic seizure protocol and testis weights were measured.

RESULTS

Reduction of M4 receptor expression in the heterozygotes completely rescued the analgesic response and partly rescued the acoustic startle response phenotype in the Fmr1KO mice. However, no modulation was observed in a number of behaviors including learning and memory, activity, perseverative behavior and audiogenic seizures.

CONCLUSION

Reducing M4 receptor signaling altered only select behavioral phenotypes in the Fmr1KO mouse model, suggesting that other targets are involved in the modulation of fragile X behaviors.

Axonal protein synthesis: a potential target for pain relief?

Research on the role of axonal protein synthesis in the regulation of nociceptive mechanisms has grown significantly over the past four years. Recent advances include evidence that local translation of mRNA can occur in adult primary afferents under the control of the mammalian target of rapamycin (mTOR) and the extracellular signal-regulated kinase (ERK) signaling pathways. Studies investigating the effect of mTOR and ERK pathway inhibitors in a number of pain models suggest that these signaling pathways may act independently, depending on the type of sensory afferents studied. The evidence that nociception can be regulated at the level of mRNA translation in nociceptors has important implications for the understanding of the mechanisms of nociceptive plasticity and therefore for therapeutic interventions in chronic pain conditions.

Targeting adenosine monophosphate-activated protein kinase (AMPK) in preclinical models reveals a potential mechanism for the treatment of neuropathic pain

Neuropathic pain is a debilitating clinical condition with few efficacious treatments, warranting development of novel therapeutics. We hypothesized that dysregulated translation regulation pathways may underlie neuropathic pain. Peripheral nerve injury induced reorganization of translation machinery in the peripheral nervous system of rats and mice, including enhanced mTOR and ERK activity, increased phosphorylation of mTOR and ERK downstream targets, augmented eIF4F complex formation and enhanced nascent protein synthesis. The AMP activated protein kinase (AMPK) activators, metformin and A769662, inhibited translation regulation signaling pathways, eIF4F complex formation, nascent protein synthesis in injured nerves and sodium channel-dependent excitability of sensory neurons resulting in a resolution of neuropathic allodynia. Therefore, injury-induced dysregulation of translation control underlies pathology leading to neuropathic pain and reveals AMPK as a novel therapeutic target for the potential treatment of neuropathic pain.

FMRP stalls ribosomal translocation on mRNAs linked to synaptic function and autism

FMRP loss of function causes Fragile X syndrome (FXS) and autistic features. FMRP is a polyribosome-associated neuronal RNA-binding protein, suggesting that it plays a key role in regulating neuronal translation, but there has been little consensus regarding either its RNA targets or mechanism of action. Here, we use high-throughput sequencing of RNAs isolated by crosslinking immunoprecipitation (HITS-CLIP) to identify FMRP interactions with mouse brain polyribosomal mRNAs. FMRP interacts with the coding region of transcripts encoding pre- and postsynaptic proteins and transcripts implicated in autism spectrum disorders (ASD). We developed a brain polyribosome-programmed translation system, revealing that FMRP reversibly stalls ribosomes specifically on its target mRNAs. Our results suggest that loss of a translational brake on the synthesis of a subset of synaptic proteins contributes to FXS. In addition, they provide insight into the molecular basis of the cognitive and allied defects in FXS and ASD and suggest multiple targets for clinical intervention.

Systemic inhibition of the mammalian target of rapamycin (mTOR) pathway reduces neuropathic pain in mice

The management of neuropathic pain is unsatisfactory, and new treatments are required. Because the sensitivity of a subset of fast-conducting primary afferent nociceptors is thought to be regulated by the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway, selectively targeting mTORC1 represents a new strategy for the control of chronic pain. Here we show that activated mTOR was expressed largely in myelinated sensory fibers in mouse and that inhibiting the mTORC1 pathway systemically alleviated mechanical hypersensitivity in mouse models of inflammatory and neuropathic pain. Specifically, systemic administration of mTORC1 inhibitor temsirolimus (CCI-779), both acutely (25 mg/kg i.p.) and chronically (4 daily 25 mg/kg i.p.), inhibited the mTORC1 pathway in sensory axons and the spinal dorsal horn and reduced mechanical and cold hypersensitivity induced by nerve injury. Moreover, systemic treatment with CCI-779 also reduced mechanical but not heat hypersensitivity in an inflammatory pain state. This treatment did not influence nociceptive thresholds in naive or sham-treated control animals. Also, there was no evidence for neuronal toxicity after repeated systemic treatment with CCI-779. Additionally, we show that acute and chronic i.p. administration of Torin1 (20 mg/kg), a novel ATP-competitive inhibitor targeting both mTORC1 and mTORC2 pathways, reduced the response to mechanical and cold stimuli in neuropathic mice. Our findings emphasize the importance of the mTORC1 pathway as a regulator of nociceptor sensitivity and therefore as a potential target for therapeutic intervention, particularly in chronic pain.

Spinal protein kinase M ζ underlies the maintenance mechanism of persistent nociceptive sensitization

Sensitization of the pain pathway is believed to promote clinical pain disorders. We hypothesized that the persistence of a sensitized state in the spinal dorsal horn might depend on the activity of protein kinase M ζ (PKMζ), an essential mechanism of late long-term potentiation (LTP). To test this hypothesis, we used intraplantar injections of interleukin-6 (IL-6) in mice to elicit a transient allodynic state that endured ∼3 d. After the resolution of IL-6-induced allodynia, a subsequent intraplantar injection of prostaglandin E(2) (PGE(2)) or intrathecal injection of the metabotropic glutamate receptor 1/5 (mGluR1/5) agonist DHPG (dihydroxyphenylglycol) precipitated allodynia and/or nocifensive responses. Intraplantar injection of IL-6 followed immediately by intrathecal injection of a PKMζ inhibitor prevented the expression of subsequent PGE(2)-induced allodynia. Inhibitors of protein translation were effective in preventing PGE(2)-induced allodynia when given immediately after IL-6, but not after the initial allodynia had resolved. In contrast, spinal PKMζ inhibition completely abolished both prolonged allodynia to hindpaw PGE(2) and enhanced nocifensive behaviors evoked by intrathecal mGluR1/5 agonist injection after the resolution of IL-6-induced allodynia. Moreover, spinal PKMζ inhibition prevented the enhanced response to subsequent stimuli following resolution of hypersensitivity induced by plantar incision. The present findings demonstrate that the spinal cord encodes an engram for persistent nociceptive sensitization that is analogous to molecular mechanisms of late LTP and suggest that spinally directed PKMζ inhibitors may offer therapeutic benefit for injury-induced pain states.

Fragile X mental retardation protein regulates protein expression and mRNA translation of the potassium channel Kv4.2

A prominent characteristic of the inherited intellectual impairment disease fragile X syndrome (FXS) is neuronal hyperexcitability, resulting in a variety of symptoms, such as hyperactivity, increased sensitivity to sensory stimuli, and a high incidence of epileptic seizures. These symptoms account for a significant part of the disease pattern, but the underlying molecular mechanisms of neuronal hyperexcitability in FXS remain poorly understood. FXS is caused by loss of expression of fragile X mental retardation protein (FMRP), which regulates synaptic protein synthesis and is a key player to limit signaling pathways downstream of metabotropic glutamate receptors 1/5 (mGlu1/5). Recent findings suggest that FMRP might also directly regulate voltage-gated potassium channels. Here, we show that total and plasma membrane protein levels of Kv4.2, the major potassium channel regulating hippocampal neuronal excitability, are reduced in the brain of an FXS mouse model. Antagonizing mGlu5 activity with 2-methyl-6-(phenylethynyl)-pyridine (MPEP) partially rescues reduced surface Kv4.2 levels in Fmr1 knock-out (KO) mice, suggesting that excess mGlu1/5 signal activity contributes to Kv4.2 dysregulation. As an additional mechanism, we show that FMRP is a positive regulator of Kv4.2 mRNA translation and protein expression and associates with Kv4.2 mRNA in vivo and in vitro. Our results suggest that absence of FMRP-mediated positive control of Kv4.2 mRNA translation, protein expression, and plasma membrane levels might contribute to excess neuronal excitability in Fmr1 KO mice, and thus imply a potential mechanism underlying FXS-associated epilepsy.

Genetic reduction of group 1 metabotropic glutamate receptors alters select behaviors in a mouse model for fragile X syndrome

INTRODUCTION

Genetic heterogeneity likely contributes to variability in the symptoms among individuals with fragile X syndrome (FXS). Studies in the Fmr1 knockout (KO) mouse model for FXS suggest that excessive signaling through group I metabotropic glutamate receptors (Gp1 mGluRs), comprised of subtypes mGluR1 and mGluR5, may play a role. Hence, Gp1 mGluRs may act as modifiers of FXS. Currently no studies have addressed whether manipulation of mGluR1 activity may alter Fmr1 KO behavioral responses, and only a few have reported the effects of mGluR5 manipulation. Therefore, the goals for this study were to extend our understanding of the effects of modulating Gp1 mGluR activity on Fmr1 KO behavioral responses.

METHODS

The present study determined if genetically reducing mGluR1 or mGluR5 by 50% affects an extensive array of behaviors in the Fmr1 KO.

RESULTS

Reduction of mGluR1 moderately decreased Fmr1 KO activity. Reduction of mGluR5 caused an analgesic response in the Fmr1 KO and decreased active social behavior. Modulation of either mGluR1 or mGluR5 did not significantly alter audiogenic seizures, anxiety- and perseverative-related responses, sensorimotor gating, memory, or motor responses.

CONCLUSIONS

Genetic reduction of mGluR1 or mGluR5 modified a few select Fmr1 KO behaviors, although these modifications appeared to be subtle in nature and/or limited to select behaviors. This may indicate that 50% reduction of either mGluR1 or mGluR5 is insufficient to produce behavioral changes, and therefore, these receptors may not be dominant modifiers of a number of Fmr1 KO behavioral phenotypes.

Dysregulation of fragile × mental retardation protein and metabotropic glutamate receptor 5 in superior frontal cortex of individuals with autism: a postmortem brain study

Background

Fragile X syndrome is caused by loss of function of the fragile X mental retardation 1 (FMR1) gene and shares multiple phenotypes with autism. We have previously found reduced expression of the protein product of FMR1 (FMRP) in vermis of adults with autism.

Methods

In the current study, we have investigated levels of FMRP in the superior frontal cortex of people with autism and matched controls using Western blot analysis. Because FMRP regulates the translation of multiple genes, we also measured protein levels for downstream molecules metabotropic glutamate receptor 5 (mGluR5) and γ-aminobutyric acid (GABA) A receptor β3 (GABRβ3), as well as glial fibrillary acidic protein (GFAP).

Results

We observed significantly reduced levels of protein for FMRP in adults with autism, significantly increased levels of protein for mGluR5 in children with autism and significantly increased levels of GFAP in adults and children with autism. We found no change in expression of GABRβ3. Our results for FMRP, mGluR5 and GFAP confirm our previous work in the cerebellar vermis of people with autism.

Conclusion

These changes may be responsible for cognitive deficits and seizure disorder in people with autism.

Spinal phosphinositide 3-kinase-Akt-mammalian target of rapamycin signaling cascades in inflammation-induced hyperalgesia

Phosphinositide 3-kinase (PI3K), Akt, and their downstream kinase, mammalian target of rapamycin (mTOR), are implicated in neural plasticity. The functional linkages of this signaling cascade in spinal dorsal horn and their role in inflammatory hyperalgesia have not been elucidated. In the present work, we identified the following characteristics of this cascade. (1) Local inflammation led to increase in rat dorsal horn phosphorylation (activation) of Akt (pAkt) and mTOR (pmTOR), as assessed by Western blotting and immunocytochemistry. (2) Increased pAkt and pmTOR were prominent in neurons in laminae I, III, and IV, whereas pmTOR and its downstream targets (pS6, p4EBP) were also observed in glial cells. (3) Intrathecal treatment with inhibitors to PI3K or Akt attenuated Formalin-induced second-phase flinching behavior, as well as carrageenan-induced thermal hyperalgesia and tactile allodynia. (4) Intrathecal rapamycin (an mTORC1 inhibitor) displayed anti-hyperalgesic effect in both inflammatory pain models. Importantly, intrathecal wortmannin at anti-hyperalgesic doses reversed the evoked increase not only in Akt but also in mTORC1 signaling (pS6/p4EBP). (5) pAkt and pmTOR are expressed in neurokinin 1 receptor-positive neurons in laminae I-III after peripheral inflammation. Intrathecal injection of Substance P activated this cascade (increased phosphorylation) and resulted in hyperalgesia, both of which effects were blocked by intrathecal wortmannin and rapamycin. Together, these findings reveal that afferent inputs trigged by peripheral inflammation initiate spinal activation of PI3K-Akt-mTOR signaling pathway, a component of which participates in neuronal circuits of facilitated pain processing.

Modifying behavioral phenotypes in Fmr1KO mice: genetic background differences reveal autistic-like responses

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability in humans. In addition to cognitive impairment, patients may exhibit hyperactivity, attention deficits, social difficulties and anxiety, and autistic-like behaviors. The degree to which patients display these behaviors varies considerably and is influenced by family history, suggesting that genetic modifiers play a role in the expression of behaviors in FXS. Several studies have examined behavior in a mouse model of FXS in which the Fmr1 gene has been ablated. Most of those studies were done in Fmr1 knockout mice on a pure C57BL/6 or FVB strain background. To gain a better understanding of the effects of genetic background on behaviors resulting from the loss of Fmr1 gene expression, we generated F1 hybrid lines from female Fmr1 heterozygous mice on a pure C57BL/6J background bred with male Fmr1 wild-type (WT) mice of various background strains (A/J, DBA/2J, FVB/NJ, 129S1/SvImJ and CD-1). Male Fmr1 knockout and WT littermates from each line were examined in an extensive behavioral test battery. Results clearly indicate that multiple behavioral responses are dependent on genetic background, including autistic-like traits that are present on limited genetic backgrounds. This approach has allowed us to identify improved models for different behavioral symptoms present in FXS including autistic-like traits.

IL-6- and NGF-induced rapid control of protein synthesis and nociceptive plasticity via convergent signaling to the eIF4F complex

Despite the emergence of translational control pathways as mediators of nociceptive sensitization, effector molecules and mechanisms responsible for modulating activity in these pathways in pain conditions are largely unknown. We demonstrate that two major algogens, the cytokine interleukin 6 (IL-6) and the neurotrophin nerve growth factor (NGF), which are intimately linked to nociceptive plasticity across preclinical models and human pain conditions, signal primarily through two distinct pathways to enhance translation in sensory neurons by converging onto the eukaryotic initiation factor (eIF) eIF4F complex. We directly demonstrate that the net result of IL-6 and NGF signaling is an enhancement of eIF4F complex formation and an induction of nascent protein synthesis in primary afferent neurons and their axons. Moreover, IL-6- and NGF-induced mechanical nociceptive plasticity is blocked by inhibitors of general and cap-dependent protein synthesis. These results establish IL-6- and NGF-mediated cap-dependent translation of local proteins as a new model for nociceptive plasticity.

The role of fragile X mental retardation protein in major mental disorders

Fragile X mental retardation protein (FMRP) is highly enriched in neurons and binds to approximately 4% of mRNAs in mammalian brain. Its loss is a hallmark of fragile X syndrome (FXS), the most common form of mental retardation. In this review we discuss the mutation in the fragile X mental retardation-1 gene (FMR1), that leads to FXS, the role FMRP plays in neuronal cells, experiments from our own laboratory that demonstrate reductions of FMRP in additional psychiatric disorders (autism, schizophrenia, bipolar disorder, and major depressive disorder), and potential therapies to ameliorate the loss of FMRP. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Descending serotonergic facilitation mediated by spinal 5-HT3 receptors engages spinal rapamycin-sensitive pathways in the rat

We have recently reported the importance of spinal rapamycin-sensitive pathways in maintaining persistent pain-like states. A descending facilitatory drive mediated through spinal 5-HT3 receptors (5-HT3Rs) originating from superficial dorsal horn NK1-expressing neurons and that relays through the parabrachial nucleus and the rostroventral medial medulla to act on deep dorsal horn neurons is known be important in maintaining these pain-like states. To determine if spinal rapamycin-sensitive pathways are activated by a descending serotonergic drive, we investigated the effects of spinally administered rapamycin on responses of deep dorsal horn neurons that had been pre-treated with the selective 5-HT3R antagonist ondansetron. We also investigated the effects of spinally administered cell cycle inhibitor (CCI)-779 (a rapamycin ester analogue) on deep dorsal horn neurons from rats with carrageenan-induced inflammation of the hind paw. Unlike some other models of persistent pain, this model does not involve an altered 5-HT3R-mediated descending serotonergic drive. We found that the inhibitory effects of rapamycin were significantly reduced for neuronal responses to mechanical and thermal stimuli when the spinal cord was pre-treated with ondansetron. Furthermore, CCI-779 was found to be ineffective in attenuating spinal neuronal responses to peripheral stimuli in carrageenan-treated rats. Therefore, we conclude that 5-HT3R-mediated descending facilitation is one requirement for activation of rapamycin-sensitive pathways that contribute to persistent pain-like states.

Mammalian target of rapamycin in spinal cord neurons mediates hypersensitivity induced by peripheral inflammation

mTOR, the mammalian target of rapamycin, is a serine-threonine kinase known to regulate cell proliferation and growth. mTOR has also been implicated in neuronal synaptic plasticity as well as in pain transmission in models of chemically induced and neuropathic pain. To date, the role of mTOR as a modulator of inflammatory pain has not been examined. In this study, we investigated the role of mTOR in Sprague-Dawley rats using the carrageenan model of inflammatory pain. mRNA of Ras homolog enriched in brain (Rheb), a GTPase that positively regulates mTOR activation, was significantly increased 2 h following carrageenan injection. Four hours after induction of inflammation phosphorylation (p) of p70S6 kinase (S6K), ribosomal protein S6 (S6) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) was increased, indicating mTOR activation. Inhibition of spinal mTOR with intrathecal (i.t.) injection of rapamycin (0.1-3 microg) led to a dose-dependent decrease in carrageenan-induced thermal hyperalgesia and a reduction of mechanical allodynia. In vitro studies confirmed rapamycin inhibition of the mTOR pathway. Carrageenan-induced activation of the mTOR pathway in rats was localized predominantly to dorsal horn neurons in the superficial lamina. Taken together, these data show that the mTOR pathway is activated in dorsal horn neurons during inflammatory pain, and that inhibition of spinal mTOR attenuates inflammation-induced thermal and tactile hypersensitivity. Hence, our study indicates that spinal mTOR is an important regulator of spinal sensitization and suggests that targeting mTOR may provide a new avenue for pain therapy.

Mammalian target of rapamycin signaling in the spinal cord is required for neuronal plasticity and behavioral hypersensitivity associated with neuropathy in the rat

The protein kinase mammalian target of rapamycin (mTOR) regulates mRNA translation and is inhibited by rapamycin. Signaling pathways involving mTOR are implicated in physiological and pathophysiological processes. We determined the spinal effects of the rapamycin analogue cell cycle inhibitor (CCI)-779 on neuronal responses and behavioral hypersensitivity in a model of persistent neuropathic pain. We also assessed the anatomical distribution of spinal mTOR signaling pathways. Specifically, we ligated rat spinal nerves L5 and L6 to produce a model of neuropathic pain. After confirming neuropathy with behavioral testing, we obtained in vivo single-unit extracellular stimulus-evoked recordings from deep dorsal horn spinal neurons. We applied CCI-779 spinally in electrophysiological and behavioral studies and assessed its effects accordingly. We also used immunohistochemistry to probe for mTOR signaling pathways in dorsal root ganglia (DRG) and the spinal cord. We found that spinally administered CCI-779 rapidly attenuated calibrated mechanically but not thermally evoked neuronal responses and mechanically evoked behavioral responses. Immunohistochemistry showed presence of mTOR signaling pathways in nociceptive-specific C-fiber DRG and in neurons of inner lamina II of the spinal cord. We conclude that alterations in the activity of spinal mTOR signaling pathways are crucial to the full establishment of spinal neuronal plasticity and behavioral hypersensitivity associated with nerve injury.

Perspective

This study is consistent with growing evidence implicating mTOR signaling pathways as important modulators of persistent pain, providing novel insights into the molecular mechanisms of pain maintenance and potential for novel approaches into treating chronic pain.

The effect of dentine-bonding agents on substance P release in human dental pulp

AIM

To quantify the effect of dentine-bonding agents on Substance P (SP) release in healthy human dental pulp tissue.

METHODOLOGY

Forty pulp samples were obtained from healthy pre-molars where extraction was indicated for orthodontic reasons. In thirty of these pre-molars, a standardized Class V cavity preparation was performed, and teeth were divided equally into three groups: (i) Unetched-cavity control group: Class V cavities only; (ii) Experimental Group I: 'One-step' self-etch bonding agent was placed in the cavity; and (iii) Experimental Group II: 'Two-step' total-etch bonding agent was placed in the cavity. The remaining ten healthy pre-molars where extracted without treatment and served as an intact-teeth control group. SP was measured by radioimmunoassay.

RESULTS

Greater SP release was found in the 'one-step' bonding agent group, followed by the 'two-step' bonding agent group and the unetched-cavity control group. The lower SP values were for the intact-teeth control group. anova showed statistically significant differences between groups (P = 0.0001). Tukey HSD post hoc tests showed statistically significant differences in SP release between the intact-teeth control group and the three other groups (P < 0.01) and between the unetched-cavity control group and the 'one-step' bonding agent group (P < 0.05). No significant difference was found between the 'two-step' bonding agent and the unetched-cavity control group.

CONCLUSION

Dentine-bonding agents placed over Class V cavity preparations increased SP release. One-step dentine-bonding agents increased SP release most.