Differential loss and preservation of glutamate receptor function in bipolar cells in the rd10 mouse model of retinitis pigmentosa

Photoreceptor degenerations can trigger morphological alterations in second-order neurons, however, the functional implications of such changes are not well known. We conducted a longitudinal study, using whole-cell patch-clamp, immunohistochemistry and electron microscopy to correlate physiological with anatomical changes in bipolar cells of the rd10 mouse – a model of autosomal recessive retinitis pigmentosa. Rod bipolar cells (RBCs) showed progressive changes in mGluR6-induced currents with advancing rod photoreceptor degeneration. Significant changes in response amplitude and kinetics were observed as early as postnatal day (P)20, and by P45 the response amplitudes were reduced by 91%, and then remained relatively stable until 6 months. These functional changes correlated with the loss of rod photoreceptors and mGluR6 receptor expression. Moreover, we showed that RBCs make transient ectopic connections with cones during progression of the disease. At P45, ON-cone bipolar cells (ON-CBCs) retain mGluR6 responses for longer periods than the RBCs, but by about 6 months these cells also strongly downregulate mGluR6 expression. We propose that the relative longevity of mGluR6 responses in CBCs is due to the slower loss of the cones. In contrast, ionotropic glutamate receptor expression and function in OFF-CBCs remains normal at 6 months despite the loss of synaptic input from cones. Thus, glutamate receptor expression is differentially regulated in bipolar cells, with the metabotropic receptors being absolutely dependent on synaptic input. These findings define the temporal window over which bipolar cells may be receptive to photoreceptor repair or replacement.     

An Interaction of Oxytocin Receptors with Metabotropic Glutamate Receptors in Hypothalamic Astrocytes

Hypothalamic astrocytes play a critical role in the regulation and support of many different neuroendocrine events, and are affected by oestradiol. Both nuclear and membrane oestrogen receptors (ERs) are expressed in astrocytes. Upon oestradiol activation, membrane‐associated ER signals through the type 1a metabotropic glutamate receptor (mGluR1a) to induce an increase of free cytoplasmic calcium concentration ([Ca²⁺]_i). Because the expression of oxytocin receptors (OTRs) is modulated by oestradiol, we tested whether oestradiol also influences oxytocin signalling. Oxytocin at 1, 10, and 100 nm induced a [Ca²⁺]_i flux measured as a change in relative fluorescence [ΔF Ca²⁺ = 330 ± 17 relative fluorescent units (RFU), ΔF Ca²⁺ = 331 ± 22 RFU, and ΔF Ca²⁺ = 347 ± 13 RFU, respectively] in primary cultures of female post‐pubertal hypothalamic astrocytes. Interestingly, OTRs interacted with mGluRs. The mGluR1a antagonist, LY 367385 (20 nm ), blocked the oxytocin (1 nm )‐induced [Ca²⁺]_i flux (ΔF Ca²⁺ = 344 ± 19 versus 127 ± 11 RFU, P < 0.001). Conversely, the mGluR1a receptor agonist, (RS)‐3,5‐dihydroxyphenyl‐glycine (100 nm ), increased the oxytocin (1 nm )‐induced [Ca²⁺]_i response (ΔF Ca²⁺ = 670 ± 31 RFU) compared to either compound alone (P < 0.001). Because both oxytocin and oestradiol rapidly signal through the mGluR1a, we treated hypothalamic astrocytes sequentially with oxytocin and oestradiol to determine whether stimulation with one hormone affected the subsequent [Ca²⁺]_i response to the second hormone. Oestradiol treatment did not change the subsequent [Ca²⁺]_i flux to oxytocin (P > 0.05) and previous oxytocin exposure did not affect the [Ca²⁺]_i response to oestradiol (P > 0.05). Furthermore, simultaneous oestradiol and oxytocin stimulation failed to yield a synergistic [Ca²⁺]_i response. These results suggest that the OTR signals through the mGluR1a to release Ca²⁺ from intracellular stores and rapid, nongenomic oestradiol stimulation does not influence OTR signalling in astrocytes.     

Photothrombosis ischemia stimulates a sustained astrocytic Ca2+ signaling in vivo

Although there is significant information concerning the consequences of cerebral ischemia on neuronal function, relatively little is known about functional responses of astrocytes, the predominant glial-cell type in the central nervous system. In this study, we asked whether focal ischemia would impact astrocytic Ca(2+) signaling, a characteristic form of excitability in this cell type. In vivo Ca(2+) imaging of cortical astrocytes was performed using two-photon (2-P) microscopy during the acute phase of photothrombosis-induced ischemia initiated by green light illumination of circulating Rose Bengal. Although whisker evoked potentials were reduced by over 90% within minutes of photothrombosis, astrocytes in the ischemic core remained structurally intact for a few hours. In vivo Ca(2+) imaging showed that an increase in transient Ca(2+) signals in astrocytes within 20 min of ischemia. These Ca(2+) signals were synchronized and propagated as waves amongst the glial network. Pharmacological manipulations demonstrated that these Ca(2+) signals were dependent on activation of metabotropic glutamate receptor 5 (mGluR5) and metabotropic gamma-aminobutyric acid receptor (GABA(B)R) but not by P2 purinergic receptor or A1 adenosine receptor. Selective inhibition of Ca(2+) in astrocytes with BAPTA significantly reduced the infarct volume, demonstrating that the enhanced astrocytic Ca(2+) signal contributes to neuronal damage presumably through Ca(2+)-dependent release of glial glutamate. Because astrocytes offer multiple functions in close communication with neurons and vasculature, the ischemia-induced increase in astrocytic Ca(2+) signaling may represent an initial attempt for these cells to communicate with neurons or provide feed back regulation to the vasculature.     

铝电解工人神经行为功能及mGluR1表达的改变

[目的]探讨铝电解作业工人神经行为功能与外周血淋巴细胞中代谢性谷氨酸受体I亚型（mGluR1）表达的变化。[方法]①从太原铝厂选取电解车间退休工人35名作为接触组,选取无职业接触铝的该铝厂退休工人32名为对照组,询问调查对象的一般状况;②采用世界卫生组织推荐的神经行为测试组合（NCTB）对两组工人神经行为功能进行测试;③用蛋白印迹（Western blot）检测外周血淋巴细胞mGluR1蛋白的表达水平。[结果]①接触组和对照组的一般情况（年龄、性别、工龄、教育程度、吸烟、饮酒和退休年数）均衡,差别没有统计学意义（P〉0.05）;②接触组情感状态指标愤怒-敌意（POMSA）和疲惫-惰性（POMSF）得分较对照组明显增加（P〈0.05）;困惑-迷茫（POMSC）、忧郁-一沮丧（POMSD）、紧张-焦虑（POMST）和有力-好动（POMSV）的得分没有明显改变（P〉0.05）;数字广度（DSP）包括顺序数字广度（DSPF）和倒序数字广度（DSPB）、数字译码数（DSY）和目标追踪的正确打点数（PAC）均比对照组降低（P〈0.05）,简单反应时（SRT）、最快反应时（SRTF）、最慢反应时（SRTS）、非利手提转捷度（SANN）、利手提转捷度（SANP）、Benton视觉保留（BVR）和打点总数（PA）的得分两组差异无统计学意义（P〉0.05）,提示电解接触铝对退休的作业工人的情感状态和神经行为均产生了一定的负面影响;③Western blot结果显示接触组mGluR1蛋白表达量高于对照组,差异有统计学意义（P〈0.01）。[结论]铝电解工人神经行为功能受损,淋巴细胞mGluRl蛋白表达水平上调。     

Functional interactions between presynaptic NMDA receptors and metabotropic glutamate receptors co-expressed on rat and human noradrenergic terminals

BACKGROUND AND PURPOSE: Electrophysiological studies described potentiation of NMDA receptor function by metabotropic glutamate receptors (mGluRs) of group I occurring postsynaptically. Since release-enhancing NMDA receptors exist on noradrenergic terminals and group I mGluRs have recently been identified on these nerve endings, we have investigated if NMDA receptor-mGluR interactions also can occur at the presynaptic level. EXPERIMENTAL APPROACH: Rat hippocampus and human neocortex synaptosomes were labelled with [(3)H]noradrenaline and superfused with mGluR agonists and antagonists. NMDA-evoked [(3)H]noradrenaline release was produced by removal of external Mg(2+) or by simultaneous application of NMDA and AMPA in Mg(2+)-containing solutions. KEY RESULTS: The mGluR1/5 agonist 3,5-DHPG, inactive on its own, potentiated both the release of [(3)H]noradrenaline elicited by AMPA/NMDA/glycine and that evoked by NMDA/glycine following Mg(2+) removal. The effect of 3,5-DHPG on the AMPA/NMDA/glycine-induced release was insensitive to the mGluR1 antagonist CPCCOEt, but it was abolished by the mGluR5 antagonist MPEP; moreover, it was potentiated by the mGluR5 positive allosteric modulator DFB. When NMDA receptors were activated by Mg(2+) removal, both mGluR5 and mGluR1 contributed to the evoked release, the mGluR-mediated release being blocked only by CPCCOEt and MPEP in combination. Experiments with human neocortex synaptosomes show NMDA receptor-mGluR interactions qualitatively similar to those observed in rodents. CONCLUSIONS AND IMPLICATIONS: Group I mGluRs, both of the mGluR1 and mGluR5 subtypes, co-localize with NMDA receptors on noradrenergic terminals of rat hippocampus and human neocortex. Depending on the mode of activation, NMDA receptors exert differential permissive roles on the activation of presynaptic mGluR1 and mGluR5.     

Emerging drugs for gastroesophageal reflux disease

Proton pump inhibitors (PPIs) are very effective and safe drugs for the treatment of erosive and non-erosive gastroesophageal reflux disease (GERD). Nevertheless, a significant proportion of GERD patients (30 – 40%) continue to suffer from symptoms during PPI treatment, which has stimulated the search for better drugs. Improvement of PPI pharmacokinetics and pharmacodynamics has been the main focus of drug development in the past decade with the ultimate goal of optimizing acid inhibition. New inhibitors of the proton pump with a longer half-life, acting faster and longer, have been developed, including potassium-competitive acid blockers. Recent data, however, suggest that the therapeutic efficacy of acid suppression may have reached its maximum and other mechanisms may have to be targeted to further improve symptom control. Potential drugs interacting with different targets are reflux inhibitors such as GABA_B receptor agonists and mGluR5 antagonists. These agents reduce the number of transient lower esophageal sphincter relaxation thereby reducing both acid and non-acid reflux. Theoretically, visceral analgesics to modulate visceral perception or even growth factors to enhance mucosal healing may be other emerging drugs to treat GERD.     

Inhibition of transient lower esophageal sphincter relaxation and gastroesophageal reflux by metabotropic glutamate receptor ligands

BACKGROUND & AIMS: Transient lower esophageal sphincter relaxation (TLESR) is the major mechanism of gastroesophageal acid reflux. TLESR is mediated via vagal pathways, which may be modulated by metabotropic glutamate receptors (mGluRs). Group I mGluRs (mGluR1 and 5) have excitatory effects on neurons, whereas group II (mGluR2 and 3) and group III (mGluR4, 6, 7, and 8) are inhibitory. This study determined the effect of mGluRs on triggering of TLESR and reflux in an established conscious ferret model. METHODS: Esophageal manometric/pH studies were performed in ferrets with chronic esophagostomies. TLESR were induced by a gastric load of 25 mL glucose (pH 3.5) and 30 mL air. RESULTS: In control treated animals, gastric load induced 3.52 +/- 0.46 TLESRs per 47-minute study, 89.7% of which were associated with reflux episodes (n = 16). The mGluR5 antagonist MPEP inhibited TLESR dose dependently, with maximal 71% +/- 7% inhibition at 35 micromol/kg (n = 9; P < .0001). MPEP also significantly reduced reflux episodes (P < .001) and increased basal lower esophageal sphincter pressure (P < .05). MPEP inhibited swallowing dose dependently, suggesting a common action on trigger mechanisms for swallowing and TLESR. The more selective analogue, MTEP, had more potent effects (90% +/- 6% inhibition TLESR at 40 micromol/kg; n = 8; P < .0001). In contrast, the group I agonist DHPG tended to increase TLESR. The group II agonist (2R, 4R)-APDC was ineffective, whereas the group III agonist L-(AP4 slightly reduced TLESR (33% at 11 micromol/kg; P < .05). The selective mGluR8 agonist (S)-3, 4-DCPG inhibited TLESR by 54% at 15 micromol/kg (P < .01). CONCLUSIONS: mGluR5 antagonists potently inhibit TLESR and reflux in ferrets, implicating mGluR5 in the mechanism of TLESR. mGluR5 antagonists are therefore promising as therapy for patients with GERD.     

Recovery of rod-mediated a-wave during light-adaptation in mGluR6-deficient mice

The purpose of this study was to compare the a-waves of mGluR6-deficient mice (KO) to that of wild-type mice (WT), and to determine whether the light-adapted electroretinogram of the KO mice originate exclusively from cones. Dark-adapted a-waves were recorded under the same conditions from both types of mice. With a 96-cd/m(2) background, the a-wave from both types of mice showed a rapid recovery over a 50-min period. The analysis of the a-waves in KO mice indicated that the recovery was determined mainly by the rod component. The light-adapted b-wave of WT mice showed no corresponding recovery. We conclude that rod contribution must be considered in the analyses of the light-adapted a-waves of KO mice.     

Nicotine-induced conditioned place preference in rats: Sex differences and the role of mGluR5 receptors

To elucidate sex differences in nicotine addiction and the underlying mechanisms of the conditioning aspects of nicotine, nicotine-induced conditioned place preference (CPP) was evaluated in male and female Sprague Dawley rats using a three-chambered CPP apparatus and a biased design. In a series of experiments, the dose-response curve was obtained, pairings between the drug and initially non-preferred versus preferred compartments were compared, and the involvement of mGluR5 receptors in nicotine-induced CPP was evaluated. Modulation of nicotine-induced CPP with mGluR5 inhibition was obtained by MPEP (2-methyl-6-(phenylethynyl)-pyridine hydrochloride). Our results show that nicotine induces CPP dose-dependently in male rats but not in female rats. The comparison of the biased protocol, pairing nicotine with the initially preferred and non-preferred chambers, indicated that nicotine-induced CPP in male rats under both conditions, but the effect was stronger when nicotine was paired with the initially non-preferred side. The selective mGluR5 antagonist MPEP inhibited nicotine-induced CPP in male rats. In conclusion, the results of the current study in rats demonstrate that the conditioning effect of nicotine is more important in males than in females. Furthermore, in line with reported findings, our results suggest that mGluR5 antagonism may be therapeutically useful in smoking cessation during the maintenance of smoking behavior when conditioning plays an important role, notwithstanding the fact that this effect is observed only in male rats, not in females.     

Repetitive Self-Grooming Behavior in the BTBR Mouse Model of Autism is Blocked by the mGluR5 Antagonist MPEP

Autism is a neurodevelopmental disorder characterized by abnormal reciprocal social interactions, communication deficits, and repetitive behaviors with restricted interests. BTBR T+tf/J (BTBR) is an inbred mouse strain that shows robust behavioral phenotypes with analogies to all three of the diagnostic symptoms of autism, including well-replicated deficits in reciprocal social interactions and social approach, unusual patterns of ultrasonic vocalization, and high levels of repetitive self-grooming. These phenotypes offer straightforward behavioral assays for translational investigations of pharmacological compounds. Two suggested treatments for autism were evaluated in the BTBR mouse model. Methyl-6-phenylethynyl-pyridine (MPEP), an antagonist of the mGluR5 metabotropic glutamate receptor, blocks aberrant phenotypes in the Fmr1 mouse model of Fragile X, a comorbid neurodevelopmental disorder with autistic features. Risperidone has been approved by the United States Food and Drug Administration for the treatment of irritability, tantrums, and self-injurious behavior in autistic individuals. We evaluated the actions of MPEP and risperidone on two BTBR phenotypes, low sociability and high repetitive self-grooming. Open field activity served as an independent control for non-social exploratory activity and motor functions. C57BL/6J (B6), an inbred strain with high sociability and low self-grooming, served as the strain control. MPEP significantly reduced repetitive self-grooming in BTBR, at doses that had no sedating effects on open field activity. Risperidone reduced repetitive self-grooming in BTBR, but only at doses that induced sedation in both strains. No overall improvements in sociability were detected in BTBR after treatment with either MPEP or risperidone. Our findings suggest that antagonists of mGluR5 receptors may have selective therapeutic efficacy in treating repetitive behaviors in autism.