Localization of nyctalopin in the mammalian retina

Complete X-linked congenital stationary night blindness (CSNB1) is a hereditary visual disease characterized by abnormalities in both the dark- and light-adapted electroretinogram, consistent with a defect in synaptic transmission between photoreceptors and ON-bipolar cells. The gene responsible for CSNB1, NYX, encodes a novel, leucine-rich repeat protein, nyctalopin. Consistent with its predicted glycosylphosphatidylinositol linkage, we show that recombinant nyctalopin is targeted to the extracellular cell surface in transfected HEK293 cells. Within the retina, strong nyctalopin immunoreactivity is present in the outer plexiform layer, the site of the photoreceptor to bipolar cell synapses. Double labelling of nyctalopin and known synaptic proteins in the outer plexiform layer indicate that nyctalopin is associated with the ribbon synapses of both rod and cone terminals. In the inner plexiform layer, nyctalopin immunoreactivity is associated with rod bipolar cell terminals. Our findings support a role for nyctalopin in synaptic transmission and/or synapse formation at ribbon synapses in the retina.     

Membrane-Localised Oestrogen Receptor α and β Influence Neuronal Activity Through Activation of Metabotropic Glutamate Receptors

Until recently, the idea that oestradiol could affect cellular processes independent of nuclear oestrogen receptors (ERs) was controversial. This was despite the large number of carefully controlled studies performed both within and outside the nervous system demonstrating that oestrogens regulate various intracellular signalling pathways by acting at the membrane surface of cells and/or at biological rates incompatible with the time course of genomic-initiated events. At present, it is far less controversial that oestradiol acts at surface membrane receptors to regulate nervous system function. Recent studies have demonstrated that the classical intracellular ERs, ERα and ERβ, are major players in mediating the actions of oestradiol on the membrane surface. This review focuses on one potential mechanism by which surface-localised ERα and ERβ stimulate intracellular signalling events in cells of the nervous system. After oestradiol treatment, both ERα and ERβ are capable of activating different classes of metabotropic glutamate receptors (mGluRs). Oestradiol activation of mGluRs is independent of glutamate, but requires expression of several different caveolin proteins to compartmentalise the different ERs with mGluRs into functional signalling microdomains. ER/mGluR signalling is a potential means by which oestrogens can both rapidly and for extended periods, influence a variety of intracellular signalling processes and behaviours.     

Membrane estradiol signaling in the brain

While the physiology of membrane-initiated estradiol signaling in the nervous system has remained elusive, a great deal of progress has been made toward understanding the activation of cell signaling. Membrane-initiated estradiol signaling activates G proteins and their downstream cascades, but the identity of membrane receptors and the proximal signaling mechanism(s) have been more difficult to elucidate. Mounting evidence suggests that classical intracellular estrogen receptor-α (ERα) and ERβ are trafficked to the membrane to mediate estradiol cell signaling. Moreover, an interaction of membrane ERα and ERβ with metabotropic glutamate receptors has been identified that explains the pleomorphic actions of membrane-initiated estradiol signaling. This review focuses on the mechanism of actions initiated by membrane estradiol receptors and discusses the role of scaffold proteins and signaling cascades involved in the regulation of nociception, sexual receptivity and the synthesis of neuroprogesterone, an important component in the central nervous system signaling.     

Epilepsy and autism spectrum disorders: Are there common developmental mechanisms?

Autistic spectrum disorders (ASD) and epilepsies are heterogeneous disorders that have diverse etiologies and pathophysiologies. The high rate of co-occurrence of these disorders suggest potentially shared underlying mechanisms. A number of well-known genetic disorders share epilepsy and autism as prominent phenotypic features, including tuberous sclerosis, Rett syndrome, and fragile X. In addition, mutations of several genes involved in neurodevelopment, including ARX, DCX, neuroligins and neuropilin2 have been identified in children with epilepsy, ASD or often both. Finally, in animal models, early-life seizures can result in cellular and molecular changes that could contribute to learning and behavioral disabilities as seen in ASD. Increased understanding of the common genetic, molecular and cellular mechanisms of ASD and epilepsy may provide insight into their underlying pathophysiology and elucidate new therapeutic approaches of both conditions.     

Reduced function of L-AP4-sensitive metabotropic glutamate receptors in human epileptic sclerotic hippocampus

Human temporal lobe epilepsy is characterized by strong synaptic reorganization that leads to abnormal recurrent excitatory synaptic connections among hippocampal neurons. In addition, electrophysiological studies show that synaptic activity of the main afferent input to the hippocampus, the perforant path, is prolonged and amplified by changes in postsynaptic glutamate receptors. The current view is that these morphological and physiological abnormalities contribute significantly to the hyperexcitability seen in the hippocampus of temporal lobe epilepsy. Recently, it was found that presynaptic inhibitory metabotropic glutamate receptors are an important negative feedback mechanism that controls synaptic release of glutamate in the hippocampus. In this study, we assessed the functionality of this feedback system by investigating the metabotropic glutamate receptor mediated depression of excitatory synaptic transmission in surgically removed hippocampi from patients with marked synaptic reorganization (Ammon's horn sclerosis group) and from patients without detectable reorganization (lesion group). We report here that this control of synaptic transmission is lost in hippocampi from the Ammon's horn sclerosis group whereas this control is preserved in hippocampi from the lesion group. The data presented here suggest that the loss of feedback inhibition mediated by metabotropic glutamate receptors could be a further, previously not recognized, mechanism in the pathophysiology of temporal lobe epilepsy.     

Time-dependent induction of depotentiation in the dentate gyrus of freely moving rats: involvement of group 2 metabotropic glutamate receptors

Depotentiation comprises a reversal of tetanization-induced long-term potentiation (LTP) which occurs following low-frequency stimulation (LFS) in the hippocampus in vivo. Although depotentiation has been consistently demonstrated in the CA1 region, no positive reports of the existence of depotentiation in the dentate gyrus in vivo have occurred. This study therefore investigated whether depotentiation is possible in the dentate gyrus in vivo. We found that depotentiation can be induced, but it is very tightly dependent on the interval between tetanization and LFS. Thus, LFS given 2 or 5 min following tetanization produced significant depotentiation, whereas LFS given 10-30 min following tetanization had no significant effect on the expression of LTP. Depotentiation occurred in two phases: a transient depression of evoked responses to below pre-tetanization values, which occurred in the first 60 min following LFS, and a recovery of this response to a stable level of synaptic transmission which comprised a significant reduction in the magnitude of LTP. Group 2 metabotropic glutamate receptors (mGluRs) play an important role in the expression of long-term depression in vivo. We therefore investigated whether group 2 mGluRs contribute to depotentiation. The group 2 antagonist (2S)-alpha-ethylglutamic acid (EGLU) inhibited the early transient depression at a concentration which inhibits LTD in vivo, but did not block the expression of depotentiation. EGLU also inhibited the transient depression induced by 5 Hz given alone. Increasing the concentration of EGLU prevented depotentiation, however. The group 2 agonist (S)-4-carboxy-3-hydroxyphenyl- glycine (4C3HPG) inhibited LTP and enhanced depotentiation. These data suggest a role for group 2 mGluRs in depotentiation.     

The mGluR5 antagonist MPEP selectively inhibits the onset and maintenance of ethanol self-administration in C57BL/6J mice

Rationale Many of the biochemical, physiological, and behavioral effects of ethanol are known to be mediated by ionotropic glutamate receptors. Emerging evidence implicates metabotropic glutamate receptors (mGluRs) in the biobehavioral effects of ethanol and other drugs of abuse, but there is little information regarding the role of mGluRs in the reinforcing effects of ethanol. Materials and methods Male C57BL/6J mice were trained to lever-press on a concurrent fixed ratio 1 schedule of ethanol (10% v/v) vs water reinforcement during 16-h sessions. Effects of mGluR1, mGluR2/3, and mGluR5 antagonists were then tested on parameters of ethanol self-administration behavior. Results The mGluR5 antagonist MPEP (1–10 mg/kg, i.p.) dose-dependently reduced ethanol-reinforced responding but had no effect on concurrent water-reinforced responding. Analysis of the temporal pattern of responding showed that MPEP reduced ethanol-reinforced responding during peak periods of behavior occurring during the early hours of the dark cycle. Further analysis showed that MPEP reduced the number of ethanol response bouts and bout-response rate. MPEP also produced a 13-fold delay in ethanol response onset (i.e., latency to the first response) with no corresponding effect on water response latency or locomotor activity. The mGluR1 antagonist CPCCOEt (1–10 mg/kg, i.p.) or the mGluR2/3 antagonist LY 341495 (1–30 mg/kg, i.p.) failed to alter ethanol- or water-reinforced responding. Conclusions These data indicate that mGlu5 receptors selectively regulate the onset and maintenance of ethanol self-administration in a manner that is consistent with reduction in ethanol’s reinforcement function.     

Both mGluR1 and mGluR5 mediate Ca2+ release and inward currents in hippocampal CA1 pyramidal neurons

Using combined whole-cell voltage-clamp recording and Ca2+ imaging we have investigated further the characteristics and pharmacology of group I metabotropic l-glutamate receptor (mGluR)-mediated responses in CA1 pyramidal neurons of the rat hippocampus. The selective group I mGluR agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG), evoked a transient increase in intracellular Ca2+ levels ([Ca2+]i), within neuronal somas and apical dendrites, together with a relatively long lasting inward current (I(DHPG)). Both types of response were enhanced by depolarisation (-30 mV), and this condition was used for their characterisation. The DHPG-induced [Ca2+]i rise was much more sensitive to manipulations of Ca2+ homeostasis, such as using the Ca2+ store depleting agent, cyclopiazonic acid (50-100 microM), the fast Ca2+ buffer, BAPTA (intracellular; 20-40 mM) and Ca(2+)-free/EGTA (1 mM) bath solution, than I(DHPG), suggesting that these responses are, in the main part, mediated by distinct processes. The selective mGluR1 and mGluR5 antagonists, (S)-(+)-alpha-amino-a-methylbenzeneacetic acid (LY367385; 100 microM) and 2-methyl-6-(phenylethynyl)-pyridine (MPEP;10 microM), respectively, markedly inhibited both I(DHPG) and the DHPG-evoked increase in [Ca2+]i. Moreover, these antagonists inhibited the Ca2+ response by more than 50% suggesting a synergistic interaction between mGluR1 and mGluR5. This study demonstrates that in CA1 pyramidal neurons group I mGluR-mediated inward currents and Ca2+ release from intracellular stores are enhanced under depolarising conditions and that mGluR1 and mGluR5 both contribute to these phenomena.