Homer1a signaling in the amygdala counteracts pain-related synaptic plasticity, mGluR1 function and pain behaviors

Repeated social defeat stress differently affects arthritis-associated hypersensitivity in male and female mice

Chronic stress enhances the risk of neuropsychiatric disorders and contributes to the aggravation and chronicity of pain. The development of stress-associated diseases, including pain, is affected by individual vulnerability or resilience to stress, although the mechanisms remain elusive. We used the repeated social defeat stress model promoting susceptible and resilient phenotypes in male and female mice and induced knee mono-arthritis to investigate the impact of stress vulnerability on pain and immune system regulation. We analyzed different pain-related behaviors, measured blood cytokine and immune cell levels, and performed histological analyses at the knee joints and pain/stress-related brain areas. Stress susceptible male and female mice showed prolonged arthritis-associated hypersensitivity. Interestingly, hypersensitivity was exacerbated in male but not female mice. In males, stress promoted transiently increased neutrophils and Ly6Chigh monocytes, lasting longer in susceptible than resilient mice. While resilient male mice displayed persistently increased levels of the anti-inflammatory interleukin (IL)-10, susceptible mice showed increased levels of the pro-inflammatory IL-6 at the early- and IL-12 at the late arthritis stage. Although joint inflammation levels were comparable among groups, macrophage and neutrophil infiltration was higher in the synovium of susceptible mice. Notably, only susceptible male mice, but not females, presented microgliosis and monocyte infiltration in the prefrontal cortex at the late arthritis stage. Blood Ly6Chigh monocyte depletion during the early inflammatory phase abrogated late-stage hypersensitivity and the associated histological alterations in susceptible male mice. Thus, recruitment of blood Ly6Chigh monocytes during the early arthritis phase might be a key factor mediating the persistence of arthritis pain in susceptible male mice. Alternative neuro-immune pathways that remain to be explored might be involved in females.

Chemogenetic Manipulation of Amygdala Kappa Opioid Receptor Neurons Modulates Amygdala Neuronal Activity and Neuropathic Pain Behaviors

Neuroplasticity in the central nucleus of the amygdala (CeA) plays a key role in the modulation of pain and its aversive component. The dynorphin/kappa opioid receptor (KOR) system in the amygdala is critical for averse-affective behaviors in pain conditions, but its mechanisms are not well understood. Here, we used chemogenetic manipulations of amygdala KOR-expressing neurons to analyze the behavioral consequences in a chronic neuropathic pain model. For the chemogenetic inhibition or activation of KOR neurons in the CeA, a Cre-inducible viral vector encoding Gi-DREADD (hM4Di) or Gq-DREADD (hM3Dq) was injected stereotaxically into the right CeA of transgenic KOR-Cre mice. The chemogenetic inhibition of KOR neurons expressing hM4Di with a selective DREADD actuator (deschloroclozapine, DCZ) in sham control mice significantly decreased inhibitory transmission, resulting in a shift of inhibition/excitation balance to promote excitation and induced pain behaviors. The chemogenetic activation of KOR neurons expressing hM3Dq with DCZ in neuropathic mice significantly increased inhibitory transmission, decreased excitability, and decreased neuropathic pain behaviors. These data suggest that amygdala KOR neurons modulate pain behaviors by exerting an inhibitory tone on downstream CeA neurons. Therefore, activation of these interneurons or blockade of inhibitory KOR signaling in these neurons could restore control of amygdala output and mitigate pain.

Mutations in Metabotropic Glutamate Receptor 1 Contribute to Natural Short Sleep Trait

Sufficient and efficient sleep is crucial for our health. Natural short sleepers can sleep significantly shorter than the average population without a desire for more sleep and without any obvious negative health consequences. In searching for genetic variants underlying the short sleep trait, we found two different mutations in the same gene (metabotropic glutamate receptor 1) from two independent natural short sleep families. In vitro, both of the mutations exhibited loss of function in receptor-mediated signaling. In vivo, the mice carrying the individual mutations both demonstrated short sleep behavior. In brain slices, both of the mutations changed the electrical properties and increased excitatory synaptic transmission. These results highlight the important role of metabotropic glutamate receptor 1 in modulating sleep duration.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Synapse development organized by neuronal activity-regulated immediate-early genes

Classical studies have shown that neuronal immediate-early genes (IEGs) play important roles in synaptic processes critical for key brain functions. IEGs are transiently activated and rapidly upregulated in discrete neurons in response to a wide variety of cellular stimuli, and they are uniquely involved in various aspects of synapse development. In this review, we summarize recent studies of a subset of neuronal IEGs in regulating synapse formation, transmission, and plasticity. We also discuss how the dysregulation of neuronal IEGs is associated with the onset of various brain disorders and pinpoint key outstanding questions that should be addressed in this field.

Immediate-early genes (IEGs), genes that are rapidly and transiently activated by cellular stimuli, regulate the interactions between neurons and key brain functions. Ji Won Um and colleagues at Daegu Gyeongbuk Institute of Science and Technology in South Korea review recent studies on three IEGs that are activated by neuronal activity and highlight their contribution to neuronal excitability and cognitive behaviors. These genes rely on different molecular mechanisms to regulate neuronal receptors and the structure of synapses. Research in mice lacking any one of these IEGs reveals their contribution to learning and memory as well as to some behavioral abnormalities associated with neuropsychiatric disorders. Further research into the activity of IEGs will advance our understanding of how a neuron’s environment influences brain development and disease.

Amygdala Plasticity and Pain

The amygdala is a limbic brain region that plays a key role in emotional processing, neuropsychiatric disorders, and the emotional-affective dimension of pain. Preclinical and clinical studies have identified amygdala hyperactivity as well as impairment of cortical control mechanisms in pain states. Hyperactivity of basolateral amygdala (BLA) neurons generates enhanced feedforward inhibition and deactivation of the medial prefrontal cortex (mPFC), resulting in pain-related cognitive deficits. The mPFC sends excitatory projections to GABAergic neurons in the intercalated cell mass (ITC) in the amygdala, which project to the laterocapsular division of the central nucleus of the amygdala (CeLC; output nucleus) and serve gating functions for amygdala output. Impairment of these cortical control mechanisms allows the development of amygdala pain plasticity. Mechanisms of abnormal amygdala activity in pain with particular focus on loss of cortical control mechanisms as well as new strategies to correct pain-related amygdala dysfunction will be discussed in the present review.

Sustained Activity of Metabotropic Glutamate Receptor: Homer, Arrestin, and Beyond

When activated, metabotropic glutamate receptors (mGlus) exert long-lasting changes within the glutamatergic synapses. One mechanism is a tonic effect of downstream signal transduction pathways via sustained activation of mGlu itself. Like many other G protein-coupled receptors (GPCRs), mGlu can exist in a constitutively active state, which persists agonist independently. In this paper, we review the current knowledge of the mechanisms underlying the constitutive activity of group I mGlus. The issues concerning Homer1a mechanism in the constitutive activity of group I mGlus and recent findings regarding the significant role of β-arrestin in sustained GPCR activity are also discussed. We propose that once in a state of sustained activation, the mGlu persistently activates downstream signaling pathways, including various adaptor proteins and kinases, such as β-arrestin and mitogen-activated protein kinases. In turn, these effector molecules bind to or phosphorylate the mGlu C-terminal binding domains and consequently regulate the activation state of the mGlu.

Preclinical anaylses of [18F]cEFQ as a PET tracer for imaging metabotropic glutamate receptor type 1 (mGluR1)

Metabotropic glutamate receptor type 1 (mGluR1) is related with various neurological and psychiatric diseases, such as anxiety, depression, epilepsy, Parkinson's disease, and neuropathic pain. Hence, mGluR1 is an important target for drug development and imaging. We synthesized [¹⁸F]cEFQ (3-ethyl-2-[¹⁸F]fluoroquinolin-6-yl cis-(4-methoxycyclohexyl)methanone) as a PET tracer for selective mGluR1 imaging and evaluated its properties in rodents. A chloroquinoline precursor was labeled by a nucleophilic substitution reaction, and the resulting [¹⁸F]cEFQ was obtained with high radiochemical purity (>99%) and specific activity (63-246 GBq/µmol). The log D value was 3.24, and the initial brain uptake at 10 min was over 4% of injected dose per gram in BALB/c mice. According to PET/CT and autoradiography in SD rats, [¹⁸F]cEFQ showed wide distribution in the whole brain and the highest uptake in the cerebellum. Pre-treatment with unlabeled cEFQ or the mGluR1-specific antagonist JNJ16259685 blocked the uptake of [¹⁸F]cEFQ. However, the uptake was not blocked by pre-treatment with the mGluR5-specific antagonist ABP688. The trans isomer [¹⁸F]tEFQ did not show high uptake in the mGluR1-rich region. [¹⁸F]cEFQ was straightforwardly prepared using a chloro-derivative precursor. Its feasibility as a specific and selective PET agent for imaging mGluR1 was proved by in vitro and in vivo experiments using rodents.

Mechanisms of radiotherapy-associated cognitive disability in patients with brain tumours

Standard treatment of primary and metastatic brain tumours includes high-dose megavoltage-range radiation to the cranial vault. About half of patients survive >6 months, and many attain long-term control or cure. However, 50-90% of survivors exhibit disabling cognitive dysfunction. The radiation-associated cognitive syndrome is poorly understood and has no effective prevention or long-term treatment. Attention has primarily focused on mechanisms of disability that appear at 6 months to 1 year after radiotherapy. However, recent studies show that CNS alterations and dysfunction develop much earlier following radiation exposure. This finding has prompted the hypothesis that subtle early forms of radiation-induced CNS damage could drive chronic pathophysiological processes that lead to permanent cognitive decline. This Review presents evidence of acute radiation-triggered CNS inflammation, injury to neuronal lineages, accessory cells and their progenitors, and loss of supporting structure integrity. Moreover, injury-related processes initiated soon after irradiation could synergistically alter the signalling microenvironment in progenitor cell niches in the brain and the hippocampus, which is a structure critical to memory and cognition. Progenitor cell niche degradation could cause progressive neuronal loss and cognitive disability. The concluding discussion addresses future directions and potential early treatments that might reverse degenerative processes before they can cause permanent cognitive disability.

Exploring novel paths towards protein signatures of chronic pain

Pain is a major symptom of many medical conditions and the worldwide number one reason for people to seek medical assistance. It affects the quality of life of patients and poses a heavy financial burden on society with high costs of treatment and lost productivity. Furthermore, the treatment of chronic pain presents a big challenge as pain therapeutics often lack efficacy and exhibit minimal safety profiles. The latter can be largely attributed to the fact that current therapies target molecules with key physiological functions throughout the body. In light of these difficulties, the identification of proteins specifically involved in chronic pain states is of paramount importance for designing selective interventions. Several profiling efforts have been employed with the aim to dissect the molecular underpinnings of chronic pain, both on the level of the transcriptome and proteome. However, generated results are often inconsistent and non-overlapping, which is largely due to inherent technical constraints. A potential solution may be offered by emerging strategies capable of performing standardized and reproducible proteome analysis, such as data-independent acquisition-mass spectrometry (DIA-MS). We have recently demonstrated the applicability of DIA-MS to interrogate chronic pain-related proteome alterations in mice. Based on our results, we aim to provide an overview on DIA-MS and its potential to contribute to the comprehensive characterization of molecular signatures underlying pain pathologies.

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.

Shifting towards a model of mGluR5 dysregulation in schizophrenia: Consequences for future schizophrenia treatment

Metabotropic glutamate receptor subtype 5 (mGluR5), encoded by the GRM5 gene, represents a compelling novel drug target for the treatment of schizophrenia. mGluR5 is a postsynaptic G-protein coupled glutamate receptor strongly linked with several critical cellular processes that are reported to be disrupted in schizophrenia. Accordingly, mGluR5 positive allosteric modulators show encouraging therapeutic potential in preclinical schizophrenia models, particularly for the treatment of cognitive dysfunctions against which currently available therapeutics are largely ineffective. More work is required to support the progression of mGluR5-targeting drugs into the clinic for schizophrenia treatment, although some obstacles may be overcome by comprehensively understanding how mGluR5 itself is involved in the neurobiology of the disorder. Several processes that are necessary for the regulation of mGluR5 activity have been identified, but not examined, in the context of schizophrenia. These processes include protein-protein interactions, dimerisation, subcellular trafficking, the impact of genetic variability or mutations on protein function, as well as epigenetic, post-transcriptional and post-translational processes. It is essential to understand these aspects of mGluR5 to determine whether they are affected in schizophrenia pathology, and to assess the consequences of mGluR5 dysfunction for the future use of mGluR5-based drugs. Here, we summarise the known processes that regulate mGluR5 and those that have already been studied in schizophrenia, and discuss the consequences of this dysregulation for current mGluR5 pharmacological strategies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Modulation of nociceptive ion channels and receptors via protein-protein interactions: implications for pain relief

In the last 2 decades biomedical research has provided great insights into the molecular signatures underlying painful conditions. However, chronic pain still imposes substantial challenges to researchers, clinicians and patients alike. Under pathological conditions, pain therapeutics often lack efficacy and exhibit only minimal safety profiles, which can be largely attributed to the targeting of molecules with key physiological functions throughout the body. In light of these difficulties, the identification of molecules and associated protein complexes specifically involved in chronic pain states is of paramount importance for designing selective interventions. Ion channels and receptors represent primary targets, as they critically shape nociceptive signaling from the periphery to the brain. Moreover, their function requires tight control, which is usually implemented by protein-protein interactions (PPIs). Indeed, manipulation of such PPIs entails the modulation of ion channel activity with widespread implications for influencing nociceptive signaling in a more specific way. In this review, we highlight recent advances in modulating ion channels and receptors via their PPI networks in the pursuit of relieving chronic pain. Moreover, we critically discuss the potential of targeting PPIs for developing novel pain therapies exhibiting higher efficacy and improved safety profiles.

CART treatment improves memory and synaptic structure in APP/PS1 mice

Major characteristics of Alzheimer’s disease (AD) include deposits of β-amyloid (Aβ) peptide in the brain, loss of synapses, and cognitive dysfunction. Cocaine- and amphetamine-regulated transcript (CART) has recently been reported to attenuate Aβ-induced toxicity. In this study, CART localization in APP/PS1 mice was characterized and the protective effects of exogenous CART treatment were examined. Compared to age-matched wild type mice, 8-month-old APP/PS1 mice had significantly greater CART immunoreactivity in the hippocampus and cortex. A strikingly similar pattern of Aβ plaque-associated CART immunoreactivity was observed in the cortex of AD cases. Treatment of APP/PS1 mice with exogenous CART ameliorated memory deficits; this effect was associated with improvements in synaptic ultrastructure and long-term potentiation, but not a reduction of the Aβ plaques. Exogenous CART treatment in APP/PS1 mice prevented depolarization of the mitochondrial membrane and stimulated mitochondrial complex I and II activities, resulting in an increase in ATP levels. CART treatment of APP/PS1 mice also reduced reactive oxygen species and 4-hydroxynonenal, and mitigated oxidative DNA damage. In summary, CART treatment reduced multiple neuropathological measures and improved memory in APP/PS1 mice, and may therefore be a promising and novel therapy for AD.

Impact of breathing 100% oxygen on radiation-induced cognitive impairment

Future space missions are expected to include increased extravehicular activities (EVAs) during which astronauts are exposed to high-energy space radiation while breathing 100% oxygen. Given that brain irradiation can lead to cognitive impairment, and that oxygen is a potent radiosensitizer, there is a concern that astronauts may be at greater risk of developing cognitive impairment when exposed to space radiation while breathing 100% O(2) during an EVA. To address this concern, unanesthetized, unrestrained, young adult male Fischer 344 × Brown Norway rats were allowed to breathe 100% O(2) for 30 min prior to, during and 2 h after whole-body irradiation with 0, 1, 3, 5 or 7 Gy doses of 18 MV X rays delivered from a medical linear accelerator at a dose rate of ~425 mGy/min. Irradiated and unirradiated rats breathing air (~21% O(2)) served as controls. Cognitive function was assessed 9 months postirradiation using the perirhinal cortex-dependent novel object recognition task. Cognitive function was not impaired until the rats breathing either air or 100% O(2) received a whole-body dose of 7 Gy. However, at all doses, cognitive function of the irradiated rats breathing 100% O(2) was improved over that of the irradiated rats breathing air. These data suggest that astronauts are not at greater risk of developing cognitive impairment when exposed to space radiation while breathing 100% O(2) during an EVA.

How anchoring proteins shape pain

Cellular responsiveness to external stimuli can be altered by extracellular mediators which activate membrane receptors, in turn signalling to the intracellular space via calcium, cyclic nucleotides, membrane lipids or enzyme activity. These signalling events trigger a cascade leading to an effector which can be a channel, an enzyme or a transcription factor. The effectiveness of these intracellular events is enhanced when they are maintained in close proximity by anchoring proteins, which assemble complexes of signalling molecules such as kinases together with their targets, and in this way enhance both the speed and the precision of intracellular signalling. The A kinase anchoring protein (AKAP) family are adaptor proteins originally named for their ability to associate Protein Kinase A and its targets, but several other enzymes bound by AKAPs have now been found and a wide variety of target structures has been described. This review provides an overview of anchoring proteins involved in pain signalling. The key anchoring proteins and their ion channel targets in primary sensory neurons responding to painful stimuli (nociceptors) are discussed.

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.

Differential expression of Homer1a in the hippocampus and cortex likely plays a role in radiation-induced brain injury

Fractionated partial or whole-brain irradiation is the primary treatment for metastatic brain tumors. Despite reducing tumor burden and increasing lifespan, progressive, irreversible cognitive impairment occurs in >50% of the patients who survive >6 months after fractionated whole-brain irradiation. The exact mechanism(s) responsible for this radiation-induced brain injury are unknown; however, preclinical studies suggest that radiation modulates the extracellular receptor kinase signaling pathway, which is associated with cognitive impairment in many neurological diseases. In the study reported here, we demonstrated that the extracellular receptor kinase transcriptionally-regulated early response gene, Homer1a, was up-regulated transiently in the hippocampus and down-regulated in the cortex of young adult male Fischer 344 X Brown Norway rats at 48 h after 40 Gy of fractionated whole-brain irradiation. Two months after fractionated whole-brain irradiation, these changes in Homer1a expression correlated with a down-regulation of the hippocampal glutamate receptor 1 and protein kinase Cγ, and an up-regulation of cortical glutamate receptor 1 and protein kinase Cγ. Two drugs that prevent radiation-induced cognitive impairment in rats, the angiotensin type-1 receptor blocker, L-158,809, and the angiotensin converting enzyme inhibitor, ramipril, reversed the fractionated whole-brain irradiation-induced Homer1a expression at 48 h in the hippocampus and cortex and restored glutamate receptor 1 and protein kinase Cγ to the levels in sham-irradiated controls at 2 months after fractionated whole-brain irradiation. These data indicate that Homer1a is, (1) a brain region specific regulator of radiation-induced brain injury, including cognitive impairment and (2) potentially a druggable target for preventing it.

Alleviation of chronic pain following rat spinal cord compression injury with multimodal actions of huperzine A

Diverse mechanisms including activation of NMDA receptors, microglial activation, reactive astrogliosis, loss of descending inhibition, and spasticity are responsible for ∼40% of cases of intractable neuropathic pain after spinal cord injury (SCI). Because conventional treatments blocking individual mechanisms elicit only short-term effectiveness, a multimodal approach with simultaneous actions against major pain-related pathways may have value for clinical management of chronic pain. We hypothesize that [-]-huperzine A (HUP-A), an alkaloid isolated from the club moss Huperzia serrata, that is a potent reversible inhibitor of acetylcholinesterase and NMDA receptors, could mitigate pain without invoking drug tolerance or dependence by stimulating cholinergic interneurons to impede pain signaling, inhibiting inflammation via microglial cholinergic activation, and blocking NMDA-mediated central hypersensitization. We tested our hypothesis by administering HUP-A i.p. or intrathecally to female Sprague-Dawley rats (200-235 g body weight) after moderate static compression (35 g for 5 min) of T10 spinal cord. Compared with controls, HUP-A treatment demonstrates significant analgesic effects in both regimens. SCI rats manifested no drug tolerance following repeated bolus i.p. or chronic intrathecal HUP-A dosing. The pain-ameliorating effect of HUP-A is cholinergic dependent. Relative to vehicle treatment, HUP-A administration also reduced neural inflammation, retained higher numbers of calcium-impermeable GluR2-containing AMPA receptors, and prevented Homer1a up-regulation in dorsal horn sensory neurons. Therefore, HUP-A may provide safe and effective management for chronic postneurotrauma pain by reestablishing homeostasis of sensory circuits.

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.

Scaffold protein Homer 1: implications for neurological diseases

Homer proteins are commonly known as scaffold proteins at postsynaptic density. Homer 1 is a widely studied member of the Homer protein family, comprising both synaptic structure and mediating postsynaptic signaling transduction. Both an immediate-early gene encoding a Homer 1 variant and a constitutively expressed Homer 1 variant regulate receptor clustering and trafficking, intracellular calcium homeostasis, and intracellular molecule complex formation. Substantial preclinical investigations have implicated that each of these Homer 1 variants are associated with the etiology of many neurological diseases, such as pain, mental retardation syndromes, Alzheimer's disease, schizophrenia, drug-induced addiction, and traumatic brain injury.

Transient focal cerebral ischemia differentially decreases Homer1a and 1b/c contents in the postsynaptic density

Homer is a scaffold protein in the postsynaptic density (PSD) and binds to the intracellular tail of group I metabotropic glutamate receptors (mGluRs). Although Homer contributes to the regulation of physiological function in synapses, the role of Homer proteins under pathophysiological conditions, such as cerebral ischemia, is still not fully clear. In the present study, we sought to determine whether transient focal cerebral ischemia would affect the level of Homer1 in the isolated-PSD fraction from rats. We showed that Homer1a (short form) and Homer1b/c (long form) as well as group I mGluR were localized in the cortical PSD. Cerebral ischemia decreased the content of Homer1a, which is a dominant-negative inhibitor of the long form of Homer proteins, in the PSD at 4 h of reperfusion without changing the level of Homer1a in cortical homogenates. On the other hand, the levels of Homer1b/c in the both PSD and homogenates were decreased at 24 h of reperfusion. These results suggest that these decreases in the level of Homer1 proteins after cerebral ischemia may contribute to the disturbance of synaptic function and subsequent development of cerebral ischemia.

Somatosensory scaffolding structures

Dynamic changes in somatosensory perception occur as a result of multiple signaling events. In many instances, over-activation of sensory receptors results in the desensitization and subsequent increased threshold for activation of receptors. In other cases, receptor sensitization can occur following tissue injury and/or inflammation. In both cases, signaling mechanisms that control alterations in receptor activities can significantly affect organism response to sensory stimuli, including thermal, mechanical, and chemical. Due to the homeostatic nature of somatosensory recognition, dynamic changes in receptor response can negatively affect an individual's way of life, as well as alert individuals to tissue damage. Here, we will focus on scaffolding structures that regulate somatosensory neuronal excitability.

Metabotropic glutamate receptors and the control of chronic pain

Over the past two decades metabotropic glutamate (mGlu) receptor ligands have been investigated for their potential therapeutic effects in different disorders of the central nervous system (CNS), including anxiety, depression, schizophrenia, and neurodegenerative diseases. In addition, it has been widely demonstrated that mGlu receptors are able to modulate pain transmission both in inflammatory and neuropathic pain models. A large number of preclinical studies combining the use of selective ligands with the knockout strategy have revealed more details about the role of the different mGlu receptor subtypes in the modulation of pain information. This review will address the role of mGlu receptors in pain sensitivity focusing on different strategies to achieve pain control by targeting specific mGlu receptor subtypes. Specifically, pharmacological interventions aimed at inhibiting group I mGlu receptor-mediated signaling and/or potentiating groups II and III mGlu receptor signaling together with an epigenetic approach leading to an increased expression of mGlu2 receptors will be discussed.