Metabotropic Glutamate Receptor mGluR1 Research Articles

Overexpression of Homer1a in the basal and lateral amygdala impairs fear conditioning and induces an autism-like social impairment.

BackgroundAutism spectrum disorders (ASDs) represent a heterogeneous group of disorders with a wide range of behavioral impairments including social and communication deficits. Apart from these core symptoms, a significant number of ASD individuals display higher levels of anxiety, and some studies indicate that a subset of ASD individuals have a reduced ability to be fear conditioned. Deciphering the molecular basis of ASD has been considerably challenging and it currently remains poorly understood. In this study we examined the molecular basis of autism-like impairments in an environmentally induced animal model of ASD, where pregnant rats are exposed to the known teratogen, valproic acid (VPA), on day 12.5 of gestation and the subsequent progeny exhibit ASD-like symptoms. We focused our analysis on the basal and lateral nucleus of the amygdala (BLA), a region of the brain found to be associated with ASD pathology.MethodsWe performed whole genome gene expression analysis on the BLA using DNA microarrays to examine differences in gene expression within the amygdala of VPA-exposed animals. We validated one VPA-dysregulated candidate gene (Homer1a) using both quantitative PCR (qRT-PCR) and western blot. Finally, we overexpressed Homer1a within the basal and lateral amygdala of naïve animals utilizing adeno-associated viruses (AAV) and subsequently examined these animals in a battery of behavioral tests associated with ASD, including auditory fear conditioning, social interaction and open field.ResultsOur microarray data indicated that Homer1a was one of the genes which exhibited a significant upregulation within the amygdala. We observed an increase in Homer1a messenger RNA (mRNA) and protein in multiple cohorts of VPA-exposed animals indicating that dysregulation of Homer1a levels might underlie some of the symptoms exhibited by VPA-exposed animals. To test this hypothesis, we overexpressed Homer1a within BLA neurons utilizing a viral-mediated approach and found that overexpression of Homer1a impaired auditory fear conditioning and reduced social interaction, while having no influence on open-field behavior.ConclusionsThis study indicates that dysregulation of amygdala Homer1a might contribute to some autism-like symptoms induced by VPA exposure. These findings are interesting in part because Homer1a influences the functioning of Shank3, metabotropic glutamate receptors (mGluR5), and Homer1, and these proteins have previously been associated with ASD, indicating that these differing models of ASD may have a similar molecular basis.Electronic supplementary materialThe online version of this article (doi:10.1186/s13229-016-0077-9) contains supplementary material, which is available to authorized users.

Are Type 1 metabotropic glutamate receptors a viable therapeutic target for the treatment of cerebellar ataxia?

The cerebellum is a key brain structure for accurate coordination of sensory and motor function. Compared with other brain regions, the cerebellum expresses a particularly high level of Type 1 metabotropic glutamate receptors (mGluR1). In this review we aim to explore the significance of these receptors for cerebellar synapse function and their potential for treating cerebellar ataxia, a poorly treated degenerative motor disorder that is often hereditary. We find a significant and historical literature showing pivotal mechanisms linking mGluR1 activity with healthy cerebellar synaptic function and motor coordination. This is best illustrated by the impaired motor behaviour in mGluR1 knockout mice that bears strong resemblance to human ataxias. More recent literature also indicates that an imbalance of mGluR1 signalling is as critical as its removal. Too much, as well as too little, mGluR1 activity contributes to ataxia in several clinically relevant mouse models, and perhaps also in humans. Given the availability and ongoing refinement of selective pharmacological tools to either reduce (negative allosteric modulation) or boost (positive allosteric modulation) mGluR1 activity, our findings suggest that pharmacological manipulation of these receptors should be explored as an exciting new approach for the treatment of a variety of human cerebellar ataxias.

The TRPM1 channel in ON-bipolar cells is gated by both the α and the βγ subunits of the G-protein Go.

Transmission from photoreceptors to ON bipolar cells in mammalian retina is mediated by a sign-inverting cascade. Upon binding glutamate, the metabotropic glutamate receptor mGluR6 activates the heterotrimeric G-protein Gαoβ3γ13, and this leads to closure of the TRPM1 channel (melastatin). TRPM1 is thought to be constitutively open, but the mechanism that leads to its closure is unclear. We investigated this question in mouse rod bipolar cells by dialyzing reagents that modify the activity of either Gαo or Gβγ and then observing their effects on the basal holding current. After opening the TRPM1 channels with light, a constitutively active mutant of Gαo closed the channel, but wild-type Gαo did not. After closing the channels by dark adaptation, phosducin or inactive Gαo (both sequester Gβγ) opened the channel while the active mutant of Gαo did not. Co-immunoprecipitation showed that TRPM1 interacts with Gβ3 and with the active and inactive forms of Gαo. Furthermore, bioluminescent energy transfer assays indicated that while Gαo interacts with both the N- and the C- termini of TRPM1, Gβγ interacts only with the N-terminus. Our physiological and biochemical results suggest that both Gαo and Gβγ bind TRPM1 channels and cooperate to close them.

Territories of heterologous inputs onto Purkinje cell dendrites are segregated by mGluR1-dependent parallel fiber synapse elimination

In Purkinje cells (PCs) of the cerebellum, a single "winner" climbing fiber (CF) monopolizes proximal dendrites, whereas hundreds of thousands of parallel fibers (PFs) innervate distal dendrites, and both CF and PF inputs innervate a narrow intermediate domain. It is unclear how this segregated CF and PF innervation is established on PC dendrites. Through reconstruction of dendritic innervation by serial electron microscopy, we show that from postnatal day 9-15 in mice, both CF and PF innervation territories vigorously expand because of an enlargement of the region of overlapping innervation. From postnatal day 15 onwards, segregation of these territories occurs with robust shortening of the overlapping proximal region. Thus, innervation territories by the heterologous inputs are refined during the early postnatal period. Intriguingly, this transition is arrested in mutant mice lacking the type 1 metabotropic glutamate receptor (mGluR1) or protein kinase Cγ (PKCγ), resulting in the persistence of an abnormally expanded overlapping region. This arrested territory refinement is rescued by lentivirus-mediated expression of mGluR1α into mGluR1-deficient PCs. At the proximal dendrite of rescued PCs, PF synapses are eliminated and free spines emerge instead, whereas the number and density of CF synapses are unchanged. Because the mGluR1-PKCγ signaling pathway is also essential for the late-phase of CF synapse elimination, this signaling pathway promotes the two key features of excitatory synaptic wiring in PCs, namely CF monoinnervation by eliminating redundant CF synapses from the soma, and segregated territories of CF and PF innervation by eliminating competing PF synapses from proximal dendrites.

The mGluR5 antagonist MPEP suppresses the expression and reinstatement, but not the acquisition, of the ethanol-conditioned place preference in mice

The glutamatergic system may play a vital role in regulating neurobehavioral effects of various drugs of abuse. In the present study, we evaluated the effects of 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective antagonist of the type 5 metabotropic glutamate receptor (mGluR5) on the acquisition, expression and reinstatement of ethanol conditioned place preference (CPP). In the ethanol acquisition study, mice were conditioned with saline or ethanol (20% v/v, 2g/kg) on alternating days for 8 consecutive days and were given MPEP 10min before ethanol conditioning. In another experiment, animals were conditioned with 2g/kg ethanol and MPEP was administered 10min prior to the post-conditioning test. In a reinstatement study, following the extinction phase, animals were pretreated with MPEP 10 min prior to a priming injection of 1.0g/kg ethanol. The mGluR5 antagonist MPEP significantly reduced the expression and the reinstatement in dose-dependent manner, but not acquisition of ethanol-induced CPP. These results indicate that mGluR5 may be involved in the expression and reinstatement of conditioned rewarding effects of ethanol, but not the acquisition of ethanol, which provide an evidence that mGluR5 blockade might make dissociable contributions during the training (acquisition phase), the performance of behavior (expression phase) and reinstatement.

Gain-of-function nature of Cav1.4 L-type calcium channels alters firing properties of mouse retinal ganglion cells

Proper function of Cav1.4 L-type calcium channels is crucial for neurotransmitter release in the retina. Our understanding about how different levels of Cav1.4 channel activity affect retinal function is still limited. In the gain-of-function mouse model Cav1.4-IT we expected a reduction in the photoreceptor dynamic range but still transmission toward retinal ganglion cells. A fraction of Cav1.4-IT ganglion cells responded to light stimulation in multielectrode array recordings from whole-mounted retinas, but showed a significantly delayed response onset. Another significant number of cells showed higher activity in darkness. In addition to structural remodeling observed at the first retinal synapse of Cav1.4-IT mice the functional data suggested a loss of contrast enhancement, a fundamental feature of our visual system. In fact, Cav1.4-IT mouse retinas showed a decline in spatial response and changes in their contrast sensitivity profile. Photoreceptor degeneration was obvious from the nodular structure of cone axons and enlarged pedicles which partly moved toward the outer nuclear layer. Loss of photoreceptors was also expressed as reduced expression of proteins involved in chemical and electrical transmission, as such metabotropic glutamate receptor mGluR6 and the gap junction protein Connexin 36. Such gross changes in retinal structure and function could also explain the diminished visual performance of CSNB2 patients. The expression pattern of the plasma-membrane calcium ATPase 1 which participates in the maintenance of the intracellular calcium homeostasis in photoreceptors was changed in Cav1.4-IT mice. This might be part of a protection mechanism against increased calcium influx, as this is suggested for Cav1.4-IT channels.

Inhibition of Group I Metabotropic Glutamate Receptors Reverses Autistic-Like Phenotypes Caused by Deficiency of the Translation Repressor eIF4E Binding Protein 2.

Exacerbated mRNA translation during brain development is associated with several autism spectrum disorders (ASDs). We recently demonstrated that the deletion of a negative regulator of mRNA translation initiation, the eukaryotic initiation factor 4E-binding protein 2, leads to ASD-like behaviors and increased excitatory synaptic activity. Here we demonstrated that autistic behavioral and electrophysiological phenotypes can be treated in adult mice with antagonists of group I metabotropic glutamate receptors (mGluRs), which have been previously used in other ASD models (i.e., fragile X syndrome). These findings support the use of group I mGluR antagonists as a potential therapy that extends to autism models involving exacerbated mRNA translation initiation.

Complex formation and functional interaction between adenosine A1 receptor and type-1 metabotropic glutamate receptor

The adenosine A1 receptor (A1R) is a G protein-coupled receptor (GPCR) for adenosine, a ubiquitous neuromodulator, and thus regulates neuronal excitability, as well as arousal and sensitivity to pain. In addition, we have previously described a new mode of action for A1R: in cerebellar Purkinje cells, its activation attenuates neuronal responses to glutamate, as mediated by the type-1 metabotropic glutamate receptor (mGluR1). mGluR1 is also a GPCR, and elicits such responses as long-term depression of the postsynaptic response to glutamate, a cellular basis for cerebellar motor learning. Here, we explore in greater detail the interaction between A1R and mGluR1 using non-neuronal cells. Co-immunoprecipitation and Förster resonance energy transfer (FRET) analysis reveal that A1R and mGluR1 form a complex. Furthermore, we found that mGluR1 activation inhibits A1R signaling, as measured by changes in intracellular cAMP. These findings demonstrate that A1R and mGluR1 have the intrinsic ability to form a heteromeric complex and mutually modulate signaling. This interaction may represent a new form of intriguing GPCR-mediated cellular responses.

Dual target strategy: combining distinct non-dopaminergic treatments reduces neuronal cell loss and synergistically modulates L-DOPA-induced rotational behavior in a rodent model of Parkinson's disease.

The glutamate metabotropic receptor 5 (mGluR5) and the adenosine A2A receptor (A2A R) represent major non-dopaminergic therapeutic targets in Parkinson's disease (PD) to improve motor symptoms and slow down/revert disease progression. The 6-hydroxydopamine rat model of PD was used to determine/compare the neuroprotective and behavioral impacts of single and combined administration of one mGluR5 antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), and two A2A R antagonists, (E)-phosphoric acid mono-[3-[8-[2-(3-methoxyphenyl)vinyl]-7-methyl-2,6-dioxo-1-prop-2-ynyl-1,2,6,7-tetrahydropurin-3-yl]propyl] (MSX-3) and 8-ethoxy-9-ethyladenine (ANR 94). Chronic treatment with MPEP or MSX-3 alone, but not with ANR 94, reduced the toxin-induced loss of dopaminergic neurons in the substantia nigra pars compacta. Combining MSX-3 and MPEP further improved the neuroprotective effect of either antagonists. At the behavioral level, ANR 94 and MSX-3 given alone significantly potentiated L-DOPA-induced turning behavior. Combination of either A2A R antagonists with MPEP synergistically increased L-DOPA-induced turning. This effect was dose-dependent and required subthreshold drug concentration, which per se had no motor stimulating effect. Our findings suggest that co-treatment with A2A R and mGluR5 antagonists provides better therapeutic benefits than those produced by either drug alone. Our study sheds some light on the efficacy and advantages of combined non-dopaminergic PD treatment using low drug concentration and establishes the basis for in-depth studies to identify optimal doses at which these drugs reach highest efficacy. Combined treatment with low concentrations of known adenosine A2A receptor (A2A R) and metabotropic glutamate receptor (mGluR5) antagonists results in a therapeutic benefit and provides better results than those produced by either drug given alone, both in terms of motor performance and neuroprotection. Future trials should involve careful optimization of drug combinations and concentrations that may avoid the emergence of debilitating side effects and slow-down/revert disease progression.

MGluR5 Positive and Negative Allosteric Modulators Differentially Affect Dendritic Spine Density and Morphology in the Prefrontal Cortex.

Positive and negative allosteric modulators (PAMs and NAMs, respectively) of type 5 metabotropic glutamate receptors (mGluR5) are currently being investigated as novel treatments for neuropsychiatric diseases including drug addiction, schizophrenia, and Fragile X syndrome. However, only a handful of studies have examined the effects of mGluR5 PAMs or NAMs on the structural plasticity of dendritic spines in otherwise naïve animals, particularly in brain regions mediating executive function. In the present study, we assessed dendritic spine density and morphology in pyramidal cells of the medial prefrontal cortex (mPFC) after repeated administration of either the prototypical mGluR5 PAM 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5- yl)benzamide (CDPPB, 20 mg/kg), the clinically utilized mGluR5 NAM 1-(3-chlorophenyl)-3-(3-methyl-5-oxo-4Himidazol- 2-yl)urea (fenobam, 20 mg/kg), or vehicle in male Sprague-Dawley rats. Following once daily treatment for 10 consecutive days, coronal brain sections containing the mPFC underwent diolistic labeling and 3D image analysis of dendritic spines. Compared to vehicle treated animals, rats administered fenobam exhibited significant increases in dendritic spine density and the overall frequency of spines with small (<0.2 μm) head diameters, decreases in frequency of spines with medium (0.2-0.4 μm) head diameters, and had no changes in frequency of spines with large head diameters (>0.4 μm). Administration of CDPPB had no discernable effects on dendritic spine density or morphology, and neither CDPPB nor fenobam had any effect on spine length or volume. We conclude that mGluR5 PAMs and NAMs differentially affect mPFC dendritic spine structural plasticity in otherwise naïve animals, and additional studies assessing their effects in combination with cognitive or behavioral tasks are needed.

Restoring the ON Switch in Blind Retinas: Opto-mGluR6, a Next-Generation, Cell-Tailored Optogenetic Tool.

Photoreceptor degeneration is one of the most prevalent causes of blindness. Despite photoreceptor loss, the inner retina and central visual pathways remain intact over an extended time period, which has led to creative optogenetic approaches to restore light sensitivity in the surviving inner retina. The major drawbacks of all optogenetic tools recently developed and tested in mouse models are their low light sensitivity and lack of physiological compatibility. Here we introduce a next-generation optogenetic tool, Opto-mGluR6, designed for retinal ON-bipolar cells, which overcomes these limitations. We show that Opto-mGluR6, a chimeric protein consisting of the intracellular domains of the ON-bipolar cell–specific metabotropic glutamate receptor mGluR6 and the light-sensing domains of melanopsin, reliably recovers vision at the retinal, cortical, and behavioral levels under moderate daylight illumination.

Local group I mGluR antagonists reduce TMJ-evoked activity of trigeminal subnucleus caudalis neurons in female rats

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are functionally linked to estrogen receptors and play a key role in the plasticity of central neurons. Estrogen status strongly influences sensory input from the temporomandibular joint (TMJ) to neurons at the spinomedullary (Vc/C1–2) region. This study tested the hypothesis that TMJ input to trigeminal subnucleus caudalis/upper cervical cord (Vc/C1–2) neurons involved group I mGluR activation and depended on estrogen status. TMJ-responsive neurons were recorded in superficial laminae at the Vc/C1–2 region in ovariectomized (OvX) female rats treated with low-dose estradiol (2μg/day, LE) or high-dose estradiol (20μg/day, HE) for 2days. TMJ-responsive units were activated by adenosine triphosphate (ATP, 1mM) injected into the joint space. Receptor antagonists selective for mGluR1 (CPCCOEt) or mGluR5 (MPEP) were applied topically to the Vc/C1–2 surface at the site of recording 10min prior to the intra-TMJ ATP stimulus. In HE rats, CPCCOEt (50 and 500μM) markedly reduced ATP-evoked unit activity. By contrast, in LE rats, a small but significant increase in neural activity was seen after 50μM CPCCOEt, while 500μM caused a large reduction in activity that was similar in magnitude as that seen in HE rats. Local application of MPEP produced a significant inhibition of TMJ-evoked unit activity independent of estrogen status. Neither mGluR1 nor mGluR5 antagonism altered the spontaneous activity of TMJ units in HE or LE rats. High-dose MPEP caused a small reduction in the size of the convergent cutaneous receptive field in HE rats, while CPCCOEt had no effect. These data suggest that group I mGluRs play a key role in sensory integration of TMJ nociceptive input to the Vc/C1–2 region and are largely independent of estrogen status.

Group II mGluRs modulate baseline and arthritis pain-related synaptic transmission in the rat medial prefrontal cortex

The medial prefrontal cortex (mPFC) serves executive control functions that are impaired in neuropsychiatric disorders and pain. Therefore, restoring normal synaptic transmission and output is a desirable goal. Group II metabotropic glutamate receptors mGluR2 and mGluR3 are highly expressed in the mPFC, modulate synaptic transmission, and have been targeted for neuropsychiatric disorders. Their pain-related modulatory effects in the mPFC remain to be determined. Here we evaluated their ability to restore pyramidal output in an arthritis pain model.Whole-cell patch-clamp recordings of layer V mPFC pyramidal cells show that a selective group II mGluR agonist (LY379268) decreased synaptically evoked spiking in brain slices from normal and arthritic rats. Effects were concentration-dependent and reversed by a selective antagonist (LY341495). LY379268 decreased monosynaptic excitatory postsynaptic currents (EPSCs) and glutamate-driven inhibitory postsynaptic currents (IPSCs) in the pain model. Effects on EPSCs preceded those on IPSCs and could explain the overall inhibitory effect on pyramidal output. LY379268 decreased frequency, but not amplitude, of miniature EPSCs without affecting miniature IPSCs. LY341495 alone increased synaptically evoked spiking under normal conditions and in the pain model.In conclusion, group II mGluRs act on glutamatergic synapses to inhibit direct excitatory transmission and feedforward inhibition onto pyramidal cells. Their net effect is decreased pyramidal cell output. Facilitatory effects of a group II antagonist suggest the system may be tonically active to control pyramidal output. Failure to release the inhibitory tone and enhance mPFC output could be a mechanism for the development or persistence of a disease state such as pain.

MGluR1‐β‐arrestin 2 Signaling Mediates Induction of Excitatory Synaptic Plasticity

A subset of seven‐transmembrane receptors (7TMRs) couple to both G protein‐ and β‐arrestin (βarr)‐mediated signaling pathways. Our objective is to determine if group I metabotropic glutamate receptors (mGluR1 and mGluR5) engage βarr signaling during the induction of synaptic plasticity in the CNS. In these experiments we tested the role of βarr‐Src‐MAPK signaling in the induction of plasticity at the hippocampal mossy fiber‐CA3 synapse.&#x0D; We compared mGluR1‐dependent synaptic plasticity in patch clamp recordings from CA3 pyramidal neurons in hippocampal slices prepared from wildtype, βarr 1 and βarr 2 knockout mice. Potentiation of EPSCs induced by low frequency, paired stimulation of mossy fiber inputs to CA3 pyramidal neurons was absent in mice lacking βarr 2 (91 ± 7% n=9 for βarr 2‐/‐, 125 ± 9% n=18 for wildtype, p=0.01) but intact in mice lacking βarr 1 (128 ± 11% n=8 for βarr 1‐/‐, 122 ± 13% n=17 for wildtype, p=0.74). In contrast, high‐frequency stimulation that induced long‐term potentiation was unaffected by gene targeting of either βarr isoform (190 ± 23% n=9 for βarr 1‐/‐, 153 ± 18% n=6 for wildtype, p=0.22; 163 ± 18% n=7 for βarr 2‐/‐, 167 ± 14% n=6 for wildtype, p=0.89). Pharmacological dissection implicated ERK1/2 of the MAPK pathway (104 ± 8% n=6 for u0126‐treated slices, 139 ± 9% n=15 for vehicle slices, p=0.01), whereas the role of Src tyrosine kinases was equivocal (114 ± 8% n=9 for PP2‐treated slices, 139 ± 9% n=15 for vehicle slices, p=0.06). &#x0D; These data reveal a novel signaling function for βarrs when coupled to mGluR1 and therefore yield insight into new strategies for therapeutic targeting of group I mGluRs, which in recent years has been pursued for a number of neuropathologies.&#x0D; Research supported by R21 NS088916‐01 from NINDS to GTS.

Cocaine Decreases Metabotropic Glutamate Receptor mGluR1 Currents in Dopamine Neurons by Activating mGluR5.

Midbrain dopamine neurons are important mediators of reward and movement and are sensitive to cocaine-induced plasticity. After even a single injection of cocaine, there is an increase in AMPA-dependent synaptic transmission. The present study examines cocaine-induced plasticity of mGluR-dependent currents in dopamine neurons in the substantia nigra. Activation of mGluR1 and mGluR5 resulted in a mixture of inward and outward currents mediated by a nonselective cation conductance and a calcium-activated potassium conductance (SK), respectively. A single injection of cocaine decreased the current activated by mGluR1 in dopamine neurons, and it had no effect on the size of the mGluR5-mediated current. When the injection of cocaine was preceded by treatment of the animals with a blocker of mGluR5 receptors (MPEP), cocaine no longer decreased the mGluR1 current. Thus, the activation of mGluR5 was required for the cocaine-mediated suppression of mGluR1-mediated currents in dopamine neurons. The results support the hypothesis that mGluR5 coordinates a reduction in mGluR1 functional activity after cocaine treatment.

Transient receptor potential channels and their role in modulating radial glial-neuronal interaction: a signaling pathway involving mGluR5.

The guidance of developing neurons to the right position in the central nervous system is of central importance in brain development. Canonical transient receptor potential (TRPC) channels are thought to mediate turning responses of growth cones to guidance cues through fine control of calcium transients. Proliferating and 1- to 5-day-differentiated neural progenitor cells (NPCs) showed expression of Trpc1 and Trpc3 mRNA, while Trpc4-7 was not clearly detected. Time-lapse imaging showed that the motility pattern of neuronal cells was phasic with bursts of rapid movement (>60 μm/h), changes in direction, and intermittent slow phases or stallings (<40 μm/h), which frequently occurred in close contact with radial glial processes. Genetic interference with the TRPC3 and TRPC1 channel enhanced the motility of NPCs (burst frequency/stalling frequency). TRPC3-deficient cells or cells treated with the TRPC3 blocker pyr3 infrequently changed direction and seldom contacted radial glial processes. TRPC channels are also activated by group I metabotropic glutamate receptors (mGluR1 and mGluR5). As shown here, pyr3 blocked the calcium response mediated through mGluR5 in radial glial processes. Furthermore, 2-methyl-6-(phenylethynyl)pyridine, a blocker of mGluR5, affected the motility pattern in a similar way as TRPC3/6 double knockout or pyr3. The results suggest that radial glial cells exert attractant signals to migrating neuronal cells, which alter their motility pattern. Our results suggest that mGluR5 acting through TRPC3 is of central importance in radial glial-mediated neuronal guidance.

Relevance of the metabotropic glutamate receptor (mGluR5) in the regulation of NREM-REM sleep cycle and homeostasis: Evidence from mGluR5 (−/−) mice

Sleep is a homeostatically regulated behavior and sleep loss evokes a proportional increase in sleep time and delta slow wave activity. Glutamate and pharmacological modulation of the metabotropic glutamate receptors (mGluR) signaling have been implicated in the organization of vigilance states. Here, the role of the mGluR5 on homeostatic regulation of sleep-wake cycle and electroencephalographic (EEG) activity was examined in mGluR5 (−/−) mice. We first characterized the sleep-wake EEG phenotype in mGluR5 (−/−) and wild-type (WT) littermates mice by continuous recording for 72h of EEG, body temperature (BT) and locomotor activity (LMA). Next, we investigated the influence of sleep deprivation on the recovery sleep and EEG slow wave activity (1–4Hz) during NREM sleep to assess whether mGluR5 deletion affects the sleep homeostasis process. Like the control animals, mGluR5 (−/−) mice exhibited a clear-cut circadian sleep-wake architecture, however they showed reduced REM sleep time during the light phase with shorter REM sleep bouts and reduced state transitions in the NREM sleep-REM sleep cycle during the first and last 24h of the spontaneous 72h recording period. In addition, mGluR5 (−/−) mice had decreased slow EEG delta power during NREM sleep and enhanced LMA associated with elevated BT during the dark phase. Moreover, mGluR5 (−/−) mice exhibited reduced slow wave activity and sleep drive after sleep deprivation, indicating altered sleep homeostatic processes. The findings strongly indicate that mGluR5 is involved in shaping the stability of NREM sleep-REM sleep state transitions, NREM slow wave activity and homeostatic response to sleep loss.

Enhancement of Short-Term Memory by Methyl-6-(Phenylethynyl)-Pyridine in the BTBR T+tf/J Mouse Model of Autism Spectrum Disorder.

BackgroundAutism spectrum disorder (ASD) encompasses a range of disorders that are characterized by social and communication deficits and repetitive behaviors. This study evaluated the effect of methyl-6-(phenylethynyl)-pyridine (MPEP), an antagonist of the mGluR5 metabotropic glutamate receptor, on memory enhancement in the BTBR T+tf/J (BTBR) mouse strain, which has been recognized as a model of ASD.MethodsThe pharmacological effects of MPEP on memory and motor coordination were assessed using the Morris water maze and rotarod tests in BTBR and C57BL/6J (B6) mice. Furthermore, we performed morphological analyses of cerebellar foliation in BTBR and B6 mice using hematoxylin and eosin staining.ResultsMPEP-treated BTBR mice exhibited improved learning and memory in the Morris water maze test. MPEP administration also improved motor coordination in the rotarod test. However, no significant difference was observed regarding the numbers of Purkinje cells in the cerebella of BTBR versus normal B6 mice.ConclusionThis study suggests that the mGluR5 antagonist MPEP has the potential to ameliorate learning and memory dysfunction and impaired motor coordination in BTBR mice. These results further suggest that the BTBR mouse model may be useful in pharmacological studies investigating drugs that could potentially alleviate cognitive dysfunction in ASD.

Purkinje neuron Ca2+ influx reduction rescues ataxia in SCA28 model.

Spinocerebellar ataxia type 28 (SCA28) is a neurodegenerative disease caused by mutations of the mitochondrial protease AFG3L2. The SCA28 mouse model, which is haploinsufficient for Afg3l2, exhibits a progressive decline in motor function and displays dark degeneration of Purkinje cells (PC-DCD) of mitochondrial origin. Here, we determined that mitochondria in cultured Afg3l2-deficient PCs ineffectively buffer evoked Ca²⁺ peaks, resulting in enhanced cytoplasmic Ca²⁺ concentrations, which subsequently triggers PC-DCD. This Ca²⁺-handling defect is the result of negative synergism between mitochondrial depolarization and altered organelle trafficking to PC dendrites in Afg3l2-mutant cells. In SCA28 mice, partial genetic silencing of the metabotropic glutamate receptor mGluR1 decreased Ca²⁺ influx in PCs and reversed the ataxic phenotype. Moreover, administration of the β-lactam antibiotic ceftriaxone, which promotes synaptic glutamate clearance, thereby reducing Ca²⁺ influx, improved ataxia-associated phenotypes in SCA28 mice when given either prior to or after symptom onset. Together, the results of this study indicate that ineffective mitochondrial Ca²⁺ handling in PCs underlies SCA28 pathogenesis and suggest that strategies that lower glutamate stimulation of PCs should be further explored as a potential treatment for SCA28 patients.

Retrograde signaling for climbing fiber synapse elimination.

Neurons form exuberant synapses with target cells early in development. Then, necessary synapses are selectively strengthened whereas unnecessary connections are weakened and eventually eliminated during postnatal development. This process is known as synapse elimination and is a crucial step for shaping immature neural circuits into functionally mature versions. Accumulating evidence suggests that retrograde signaling from postsynaptic cells regulates synapse elimination, but the underlying mechanisms remain unknown. Here, we show that semaphorin3A (Sema3A) and semaphorin7A (Sema7A) mediate retrograde signals for elimination of redundant climbing fiber (CF) to Purkinje cell (PC) synapses in the developing cerebellum, a representative model of synapse elimination in the central nervous system. We picked up candidate retrograde signaling molecules that are expressed in PCs during the period of CF synapse elimination and the receptors of these candidate molecules that are present in CFs. We then assessed the effects of lentivirus-mediated RNAi-knockdown of these molecules on CF synapse elimination. By this systematic screening, we found that knockdown of Sema3A in PCs or its co-receptor, plexinA4 (PlxnA4), in CFs accelerated CF synapse elimination and decreased CF-mediated synaptic inputs. Conversely, knockdown of Sema7A in PCs or either of the two receptors for Sema7A, plexinC1 (PlxnC1) and integrinB1 (ItgB1), in CFs impaired CF synapse elimination. Importantly, the effect of Sema7A involves signaling by type 1 metabotropic glutamate receptor (mGluR1), a canonical pathway in PCs for the final stage of CF synapse elimination. These results demonstrate that specific semaphorins act as retrograde signaling molecules and regulate distinct processes of CF synapse elimination during postnatal cerebellar development.