Increased levels of Homer1b/c and Shank1a in the post-synaptic density of spinal dorsal horn neurons are associated with neuropathic pain in rats

Association between TRP channels and glutamatergic synapse gene polymorphisms and migraine and the comorbidities anxiety and depression in a Chinese population

Background: Genetic and environmental factors contribute to migraine and the comorbidities of anxiety and depression. However, the association between genetic polymorphisms in the transient receptor potential (TRP) channels and glutamatergic synapse genes with the risk of migraine and the comorbidities of anxiety and depression remain unclear. Methods: 251 migraine patients containing 49 comorbidities with anxiety and 112 with depression and 600 controls were recruited. A customized 48-plex SNPscan kit was used for genotyping 13 SNPs of nine target genes. Logistic regression was conducted to analyze these SNPs' association with the susceptibility of migraine and comorbidities. The generalized multifactor dimension reduction (GMDR) was applied to analyze the SNP-SNP and gene-environment interactions. The GTEx database was used to examine the effects of the significant SNPs on gene expressions. Results: The TRPV1 rs8065080 and TRPV3 rs7217270 were associated with an increased risk of migraine in the dominant model [OR_adj (95% CI): 1.75 (1.09-2.90), p = 0.025; 1.63 (1.02-2.58), p = 0.039, respectively]. GRIK2 rs2227283 was associated with migraine in the edge of significance [OR_adj (95% CI) = 1.36 (0.99-1.89), p = 0.062]. In migraine patients, TRPV1 rs222741 was associated with both anxiety risk and depression risk in the recessive model [OR_adj (95% CI): 2.64 (1.24-5.73), p = 0.012; 1.97 (1.02-3.85), p = 0.046, respectively]. TRPM8 rs7577262 was associated with anxiety (OR_adj = 0.27, 95% CI = 0.10-0.76, p = 0.011). TRPV4 rs3742037, TRPM8 rs17862920 and SLC17A8 rs11110359 were associated with depression in dominant model [OR_adj (95% CI): 2.03 (1.06-3.96), p = 0.035; 0.48 (0.23-0.96), p = 0.042; 0.42 (0.20-0.84), p = 0.016, respectively]. Significant eQTL and sQTL signals were observed for SNP rs8065080. Individuals with GRS (Genetic risk scores) of Q4 (14-17) had a higher risk of migraine and a lower risk of comorbidity anxiety than those with Genetic risk scores scores of Q1 (0-9) groups [OR_adj (95% CI): 2.31 (1.39-3.86), p = 0.001; 0.28 (0.08-0.88), p = 0.034, respectively]. Conclusion: This study suggests that TRPV1 rs8065080, TRPV3 rs7217270, and GRIK2 rs2227283 polymorphism may associate with migraine risk. TRPV1 rs222741 and TRPM8 rs7577262 may associate with migraine comorbidity anxiety risk. rs222741, rs3742037, rs17862920, and rs11110359 may associate with migraine comorbidity depression risk. Higher GRS scores may increase migraine risk and decrease comorbidity anxiety risk.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

RNA-seq of spinal cord from nerve-injured rats after spinal cord stimulation

Spinal cord stimulation has become an important modality in pain treatment especially for neuropathic pain conditions refractory to pharmacotherapy. However, the molecular control of inhibitory and excitatory mechanisms observed after spinal cord stimulation are poorly understood. Here, we used RNA-seq to identify differences in the expression of genes and gene networks in spinal cord tissue from nerve-injured rats with and without repetitive conventional spinal cord stimulation treatment. Five weeks after chronic constrictive injury to the left sciatic nerve, male and female rats were randomized to receive repetitive spinal cord stimulation or no treatment. Rats receiving spinal cord stimulation underwent epidural placement of a miniature stimulating electrode and received seven sessions of spinal cord stimulation (50 Hz, 80% motor threshold, 0.2 ms, constant current bipolar stimulation, 120 min/session) over four consecutive days. Within 2 h after the last spinal cord stimulation treatment, the L4-L6 spinal segments ipsilateral to the side of nerve injury were harvested and used to generate libraries for RNA-seq. Our RNA-seq data suggest further increases of many existing upregulated immune responses in chronic constrictive injury rats after repetitive spinal cord stimulation, including transcription of cell surface receptors and activation of non-neuronal cells. We also demonstrate that repetitive spinal cord stimulation represses transcription of several key synaptic signaling genes that encode scaffold proteins in the post-synaptic density. Our transcriptional studies suggest a potential relationship between specific genes and the therapeutic effects observed in patients undergoing conventional spinal cord stimulation after nerve injury. Furthermore, our results may help identify new therapeutic targets for improving the efficacy of conventional spinal cord stimulation and other chronic pain treatments.

Immunostaining for Homer reveals the majority of excitatory synapses in laminae I-III of the mouse spinal dorsal horn

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Identifying glutamatergic synapses is important for tracing synaptic circuits.

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Most proteins at glutamatergic synapses are masked by tissue fixation.

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Homer can reveal glutamatergic synapses without the need for antigen retrieval.

The spinal dorsal horn processes somatosensory information before conveying it to the brain. The neuronal organization of the dorsal horn is still poorly understood, although recent studies have defined several distinct populations among the interneurons, which account for most of its constituent neurons. All primary afferents, and the great majority of neurons in laminae I–III are glutamatergic, and a major factor limiting our understanding of the synaptic circuitry has been the difficulty in identifying glutamatergic synapses with light microscopy. Although there are numerous potential targets for antibodies, these are difficult to visualize with immunocytochemistry, because of protein cross-linking following tissue fixation. Although this can be overcome by antigen retrieval methods, these lead to difficulty in detecting other antigens. The aim of this study was to test whether the postsynaptic protein Homer can be used to reveal glutamatergic synapses in the dorsal horn. Immunostaining for Homer gave punctate labeling when viewed by confocal microscopy, and this was restricted to synapses at the ultrastructural level. We found that Homer puncta were colocalized with the AMPA receptor GluR2 subunit, but not with the inhibitory synapse-associated protein gephyrin. We also examined several populations of glutamatergic axons and found that most boutons were in contact with at least one Homer punctum. These results suggest that Homer antibodies can be used to reveal the great majority of glutamatergic synapses without antigen retrieval. This will be of considerable value in tracing synaptic circuits, and also in investigating plasticity of glutamatergic synapses in pain states.

Intracellular mGluR5 plays a critical role in neuropathic pain

Spinal mGluR5 is a key mediator of neuroplasticity underlying persistent pain. Although brain mGluR5 is localized on cell surface and intracellular membranes, neither the presence nor physiological role of spinal intracellular mGluR5 is established. Here we show that in spinal dorsal horn neurons >80% of mGluR5 is intracellular, of which ∼60% is located on nuclear membranes, where activation leads to sustained Ca(2+) responses. Nerve injury inducing nociceptive hypersensitivity also increases the expression of nuclear mGluR5 and receptor-mediated phosphorylated-ERK1/2, Arc/Arg3.1 and c-fos. Spinal blockade of intracellular mGluR5 reduces neuropathic pain behaviours and signalling molecules, whereas blockade of cell-surface mGluR5 has little effect. Decreasing intracellular glutamate via blocking EAAT-3, mimics the effects of intracellular mGluR5 antagonism. These findings show a direct link between an intracellular GPCR and behavioural expression in vivo. Blockade of intracellular mGluR5 represents a new strategy for the development of effective therapies for persistent pain.

Cell adhesion downregulates the expression of Homer1b/c and contributes to drug resistance in multiple myeloma cells

Previous studies have demonstrated that Homer1b/c plays an important pro-apoptotic role through classical mitochondrial apoptotic pathway. The present study was undertaken to determine the expression and functional significance of Homer1b/c in multiple myeloma (MM). We found that Homer1b/c was lowly expressed in MM cell apoptotic model induced by doxorubicin. The positive role of Homer1b/c in cell apoptosis was further confirmed by knocking down Homer1b/c. Further study confirmed that Homer1b/c was able to affect the CAM-DR via pro-apoptotic activity regulating the ability of cell adhesion. Collectively, these data indicate that Homer1b/c may represent a good candidate for pursuing clinical trial in MM.

Upregulation of Homer1a Promoted Retinal Ganglion Cell Survival After Retinal Ischemia and Reperfusion via Interacting with Erk Pathway

Retinal ischemia and reperfusion (I/R) is extensively involved in ocular diseases, causing retinal ganglion cell (RGCs) death resulting in visual impairment and blindness. Homer1a is considered as an endogenous neuroprotective protein in traumatic brain injury. However, the roles of Homer1a in RGCs I/R injury have not been elucidated. The present study investigated the changes in expression and effect of Homer1a in RGCs both in vitro and in vivo after I/R injury using Western blot, TUNEL assay, gene interference and overexpression, and gene knockout procedures. The levels of Homer1a and phosphorylated Erk (p-Erk) increased in RGCs and retinas after I/R injury. Upregulation of Homer1a in RGCs after I/R injury decreased the level of p-Erk, and mitigated RGCs apoptosis. Conversely, downregulation of Homer1a increased the level of p-Erk, and augmented RGCs apoptosis. Furthermore, inhibition of the p-ERK reduced RGCs apoptosis, and increased the expression of Homer 1a after I/R injury. Finally, the retinas of Homer1a KO mice treated with I/R injury had significantly less dendrites and RGCs, compared with Homer1a WT mice. These findings demonstrated that Homer1a may contribute to RGCs survival after I/R injury by interacting with Erk pathway.

Downregulation of Homer1b/c improves neuronal survival after traumatic neuronal injury

Homer protein, a member of the post-synaptic density protein family, plays an important role in the neuronal synaptic activity and is extensively involved in neurological disorders. The present study investigates the role of Homer1b/c in modulating neuronal survival by using an in vitro traumatic neuronal injury model, which was achieved by using a punch device that consisted of 28 stainless steel blades joined together and produced 28 parallel cuts. Downregulation of Homer1b/c by specific siRNA significantly (p<0.05) alleviated the cytoplasmic calcium levels and neuron lactate dehydrogenase release, and ultimately decreased the apoptotic rate after traumatic neuronal injury compared with non-targeting siRNA control treatment in cultured rat cortical neurons. Moreover, the expression of metabotropic glutamate receptor 1a (mGluR1a) was significantly (p<0.05) reduced in the Homer1b/c siRNA-transfected neurons after injury. Therefore, Homer1b/c not only modulated the mGluR1a-inositol 1,4,5-triphosphate receptors-Ca(2+) signal transduction pathway, but also regulated the expression of mGluR1a in mechanical neuronal injury. These findings indicate that the suppression of Homer1b/c expression potentially protects neurons from glutamate excitotoxicity after injury and might be an effective intervention target in traumatic brain injury.

Spinal synaptic scaffolding protein Homer 1b/c regulates CREB phosphorylation and c-fos activation induced by inflammatory pain in rats

Previous studies have shown that spinal Homer 1b/c plays an important role in the maintenance of chronic inflammatory pain induced by complete Freund's adjuvant (CFA). This study investigated the possible mechanism underlying Homer 1b/c mediating CFA-induced inflammatory pain. Chronic inflammation was induced by CFA injection into the left hind ankle of the rat. Homer 1b/c antisense or missense oligonucleotides were administered intrathecally (10μg/10μl) from 5 to 8 days following the onset of inflammation. Immunohistochemistry was conducted to detect the expression of phosphorylated cAMP response element binding protein (pCREB) and Fos protein in the spinal dorsal horn. Intrathecal administration of Homer 1b/c antisense oligonucleotides not only markedly reduced the expression of Homer 1b/c protein, but also attenuated CFA-induced inflammation, spinal CREB phosphorylation, and Fos expression. These results demonstrate for the first time that Homer 1b/c regulates CREB phosphorylation and c-fos activation in the spinal dorsal horn during the maintenance of chronic inflammatory pain, suggesting that Homer 1b/c may be involved in the development of CFA-induced inflammation.

Nerve injury-induced changes in Homer/glutamate receptor signaling contribute to the development and maintenance of neuropathic pain

While group 1 metabotropic glutamate receptors (mGluRs) and ionotropic N-methyl-d-aspartate (NMDA) receptors regulate nociception, the precise molecular mechanism(s) contributing to glutamate signaling in chronic pain remain unclear. Here we not only confirmed the key involvement of Homer proteins in neuropathic pain, but also distinguished between the functional roles for different Homer family members and isoforms. Chronic constriction injury (CCI) of the sciatic nerve induced long-lasting, time-dependent increases in the postsynaptic density expression of the constitutively expressed (CC) isoforms Homer1b/c and/or Homer2a/b in the spinal dorsal horn and supraspinal structures involved in nociception (prefrontal cortex, thalamus), that co-occurred with increases in their associated mGluRs, NR2 subunits of the NMDA receptor, and the activation of downstream kinases. Virus-mediated overexpression of Homer1c and Homer2b after spinal (intrathecal) virus injection exacerbated CCI-induced mechanical and cold hypersensitivity, however, Homer1 and Homer2 gene knockout (KO) mice displayed no changes in their neuropathic phenotype. In contrast, overexpression of the immediate early gene (IEG) Homer1a isoform reduced, while KO of Homer1a gene potentiated neuropathic pain hypersensitivity. Thus, nerve injury-induced increases in CC-Homers expression promote pain in pathological states, but IEG-Homer induction protects against both the development and maintenance of neuropathy. Additionally, exacerbated pain hypersensitivity in transgenic mice with reduced Homer binding to mGluR5 supports also an inhibitory role for Homer interactions with mGluR5 in mediating neuropathy. Such data indicate that nerve injury-induced changes in glutamate receptor/Homer signaling contribute in dynamic but distinct ways to neuropathic pain processing, which has relevance for the etiology of chronic pain symptoms and its treatment.

Homers at the Interface between Reward and Pain

Pain alters opioid reinforcement, presumably via neuroadaptations within ascending pain pathways interacting with the limbic system. Nerve injury increases expression of glutamate receptors and their associated Homer scaffolding proteins throughout the pain processing pathway. Homer proteins, and their associated glutamate receptors, regulate behavioral sensitivity to various addictive drugs. Thus, we investigated a potential role for Homers in the interactions between pain and drug reward in mice. Chronic constriction injury (CCI) of the sciatic nerve elevated Homer1b/c and/or Homer2a/b expression within all mesolimbic structures examined and for the most part, the Homer increases coincided with elevated mGluR5, GluN2A/B, and the activational state of various down-stream kinases. Behaviorally, CCI mice showed pain hypersensitivity and a conditioned place-aversion (CPA) at a low heroin dose that supported conditioned place-preference (CPP) in naïve controls. Null mutations of Homer1a, Homer1, and Homer2, as well as transgenic disruption of mGluR5-Homer interactions, either attenuated or completely blocked low-dose heroin CPP, and none of the CCI mutant strains exhibited heroin-induced CPA. However, heroin CPP did not depend upon full Homer1c expression within the nucleus accumbens (NAC), as CPP occurred in controls infused locally with small hairpin RNA-Homer1c, although intra-NAC and/or intrathecal cDNA-Homer1c, -Homer1a, and -Homer2b infusions (to best mimic CCI's effects) were sufficient to blunt heroin CPP in uninjured mice. However, arguing against a simple role for CCI-induced increases in either spinal or NAC Homer expression for heroin CPA, cDNA infusion of our various cDNA constructs either did not affect (intrathecal) or attenuated (NAC) heroin CPA. Together, these data implicate increases in glutamate receptor/Homer/kinase activity within limbic structures, perhaps outside the NAC, as possibly critical for switching the incentive motivational properties of heroin following nerve injury, which has relevance for opioid psychopharmacology in individuals suffering from neuropathic pain.

Pattern of acute induction of Homer1a gene is preserved after chronic treatment with first- and second-generation antipsychotics: effect of short-term drug discontinuation and comparison with Homer1a-interacting genes

Homer1a is a glutamate-related gene whose expression is induced by antipsychotics acutely (i.e. 90 min after treatment). Acute Homer1a expression is preserved after prolonged antipsychotic treatments, while the effects of short-term discontinuation after chronic antipsychotic treatment have not yet been assessed. Here, we studied early and long-term effects on gene expression by antipsychotics for Homer1a and other components of glutamatergic synapses. In the first paradigm, we evaluated Homer1a acute expression by single administration of antipsychotics (haloperidol 0.8 mg/kg, ziprasidone 10 and 4 mg/kg, clozapine 15 mg/kg). Haloperidol and ziprasidone induced Homer1a in the striatum. Induction by ziprasidone was dose-dependent. These results suggest that acute Homer1a expression correlates with dopaminergic affinity and motor side effects of antipsychotics. In the second paradigm, we studied antipsychotic-mediated long-term changes in Homer1a and glutamate-related genes. Rats were treated (21 days) with haloperidol 0.8 mg/kg, ziprasidone 4 mg/kg, or vehicle, and then sacrificed at 90 min (early time-point) or 24 h (delayed time-point) after last injection. Gene expression at these two time-points was compared. Homer1a preserved its pattern of expression at the early but not at the delayed time-point. Significant changes were also observed for PSD-95. The results suggest that Homer1a preserves its expression profile after chronic antipsychotics.

The role of glutamate receptors in traumatic brain injury: implications for postsynaptic density in pathophysiology

Traumatic brain injury (TBI) is the major cause of death and disability, and the incidence of TBI continues to increase rapidly. In recent years, increasing attention has been paid to an important structure at the postsynaptic membrane: the postsynaptic density (PSD). Glutamate receptors, as major components of the PSD, are highly responsive to alterations in the glutamate concentration at excitatory synapses and activate intracellular signal transduction via calcium and other second messengers following TBI. PSD scaffold proteins (PSD-95, Homer, and Shank), which anchor glutamate receptors and form a network structure, also have potential effects on these downstream signaling pathways. The changes in the function and structure of these major PSD proteins are also induced by TBI, indicating that there is a more complicated mechanism associated with PSD proteins in the pathophysiological process of TBI.

Loose ligation of the rat sciatic nerve elicits early accumulation of Shank1 protein in the post-synaptic density of spinal dorsal horn neurons

Plasticity in the spinal dorsal horn may contribute to the development of pain following peripheral nerve injury. Shank proteins are a constituent family of the post-synaptic density (PSD), and they may play a role in synaptic plasticity through activity-dependent synaptic remodeling and growth. In this study we examined the early consequences of the loose ligation of the sciatic nerve on Shank1 protein and message levels in the PSD of spinal dorsal horn neurons. Four hours after sciatic ligation, the protein levels of Shank1 increased in the ipsilateral PSD of ligated animals. In contrast, no changes were detected in the contralateral PSD of these ligated animals, or either the ipsilateral or contralateral PSD of sham-operated animals. Shank1 was linked to the PSD marker protein PSD-95 and the NR2B subunit of NMDA receptors. The ligated animals also exhibited two early signs of pain behavior, a shift in weight distribution and thermal hyperalgesia. There was no overall change in Shank1 message in either ligated or sham-operated animals. The accumulation of Shank1 in the PSD was abolished by intrathecal pre-treatment with anisomycin or Shank1 siRNA, but not with non-target siRNA. The same pre-treatment prevented both the early signs of pain behavior. Intrathecal pre-treatment with either MK-801 or U0126 similarly prevented the Shank1 accumulation and alleviated both the behavioral signs of pain. The early accumulation of Shank1 in the PSD of dorsal horn neurons may be a necessary step in the injury-associated plasticity that in time leads to the development of persistent pain.

Early changes in Homer1 proteins in the spinal dorsal horn are associated with loose ligation of the rat sciatic nerve

BACKGROUND

Plasticity in the spinal dorsal horn is thought to underlie, at least in part, pain behavior after peripheral nerve injury. Homer1 proteins play an important role in synaptic plasticity through an activity-dependent remodeling of the postsynaptic density (PSD). In this study, we examined the early consequences of the loose ligation of the sciatic nerve on the levels of Homer1a and Homer1b/c proteins in the PSD of spinal dorsal horn neurons.

METHODS

Male rats were randomly assigned to control, sham-operated, or ligated groups. Four hours after sciatic exposure or ligation, the animals were anesthetized and killed. Dorsal horn ipsilateral and contralateral quadrants were homogenized and centrifuged to obtain a PSD-containing LP1 fraction. Homer1 isoforms were identified in Western immunoblots. In some animals, Homer1 small interfering RNA (siRNA), nontarget siRNA, MK-801, or U01026 was injected intrathecally before surgery to assess the effects of this treatment on the levels of Homer1 isoforms and on 2 signs of injury-associated pain behavior, a shift in weight-bearing distribution and thermal hyperalgesia.

RESULTS

In ligated animals, the protein levels of Homer1a increased and those of Homer1b/c decreased in the ipsilateral LP1 fraction of the spinal dorsal horn. In contrast, no changes were detected in the contralateral LP1 fraction of ligated animals or the ipsilateral or contralateral LP1 fraction of sham-operated animals. Intrathecal injections of Homer1 siRNA, but not nontarget siRNA, 2 h before the ligation prevented the accumulation of Homer1a and loss of Homer1b/c in the ipsilateral LP1 fraction. The same pretreatment with Homer1 siRNA also alleviated both a shift in weight-bearing behavior and thermal hyperalgesia in the ligated animals. Intrathecal injections of MK-801 or U0126 15 min before the ligation similarly prevented the injury-associated changes in Homer1 protein levels and the behavioral signs of pain.

CONCLUSION

The ligation-associated changes in the protein levels of Homer1a and Homer1b/c in the ipsilateral PSD of spinal dorsal horn neurons may be an important early reflection of the injury-associated plasticity that in time leads to the development of persistent pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Developmental roles for Homer: more than just a pretty scaffold

Homer proteins are best known as scaffold proteins at the post-synaptic density where they facilitate synaptic signalling and are thought to be required for learning and memory. Evidence implicating Homer proteins in the development of the nervous system is also steadily accumulating. Homer is highly conserved and is expressed at key developmental time points in the nervous system of several species. Homer regulates intracellular calcium homeostasis, clustering and trafficking of receptors and proteins at the cytosolic surface of the plasma membrane, transcription and translation, and cytoskeletal organization. Each of these functions has obvious potential to regulate neuronal development, and indeed Homer is implicated in several pathologies associated with the developing nervous system. Current data justify more critical experimental approaches to the role of Homer in the developing nervous system and related neurological disorders.

Loose ligation of the sciatic nerve is associated with TrkB receptor-dependent decreases in KCC2 protein levels in the ipsilateral spinal dorsal horn

Significant decreases in the protein levels of potassium-chloride co-transporter 2 (KCC2) were detected in the ipsilateral spinal dorsal horn 4h following loose ligation of the sciatic nerve. These decreases were associated with a change in hindlimb weight distribution suggestive of pain behavior. In contrast, no changes in GABA-A receptor subunit alpha-1 levels were detected. The decreases in KCC2 coincided with a significant ipsilateral increase in BDNF protein levels. Both the decreases in KCC2 levels and the early pain behavior were prevented by intrathecal pre-treatment with the BDNF-sequestering TrkB/Fc chimera protein or the tyrosine kinase blocker K252a. The ligation-associated decreases in KCC2 levels were transient. In the ipsilateral spinal dorsal horn of ligated animals exhibiting weight-bearing pain behavior 7 days after the ligation the KCC2 levels were identical to those in control or sham-operated animals. These data suggested that TrkB-dependent reduction in KCC2 protein levels in the spinal dorsal horn was an early consequence of peripheral nerve injury. This decrease in KCC2 may have elicited an early increase in overall dorsal horn neuronal excitability perhaps through a loss of GABA inhibition which is critically dependent on KCC2 activity. The increased neuronal excitability may in turn have caused enhanced and exaggerated communication between primary afferents and dorsal horn neurons to contribute to the early behavioral signs of pain.

TRPC channels: interacting proteins

TRP channels, in particular the TRPC and TRPV subfamilies, have emerged as important constituents of the receptor-activated Ca2+ influx mechanism triggered by hormones, growth factors, and neurotransmitters through activation ofphospholipase C (PLC). Several TRPC channels are also activated by passive depletion of endoplasmic reticulum (ER) Ca2+. Although in several studies the native TRP channels faithfully reproduce the respective recombinant channels, more often the properties of Ca2+ entry and/or the store-operated current are strikingly different from that of the TRP channels expressed in the same cells. The present review aims to discuss this disparity in the context of interaction of TRPC channels with auxiliary proteins that may alter the permeation and regulation of TRPC channels.

Loose ligation of the sciatic nerve in rats elicits transient up-regulation of Homer1a gene expression in the spinal dorsal horn

Changes in the expression of many genes underlie injury-elicited plasticity in the spinal dorsal horn. Homer1 is a recently identified gene that appears to play a critical role in the expression of synaptic plasticity in several brain regions, including the hippocampus. In this study we investigated the early consequences of chronic constriction injury of the sciatic nerve on Homer1 gene expression in the spinal dorsal horn. Significant increases in Homer1a mRNA levels in the ipsilateral dorsal horn were detected at 4h post-ligation, and these levels remained elevated at 8h before returning to baseline values by 24h after the ligation. In contrast, the levels of Homer1b/c mRNA did not change at any of these selected post-ligation times. The ligation-associated induction of Homer1a was dependent on activation of NMDA receptors and the extracellular signal-regulated kinase 1 and 2 (ERK1/2) pathway. The non-competitive NMDA-receptor antagonist, MK-801, and a specific inhibitor of the ERK1/2 pathway, U0126, significantly attenuated the injury-elicited increases in Homer1a mRNA when compared to saline-treated animals. These data provide the first evidence for a potential role of Homer1a in peripheral nerve injury-elicited plasticity in the spinal dorsal horn. These data also imply that the early and transient up-regulation of Homer1a gene expression may be an important contributor to the eventual development of neuropathic pain.