Complex interactions between mGluRs, intracellular Ca2+ stores and ion channels in neurons

Calcium-calmodulin signalling pathway up-regulates glutamatergic synaptic function in non-pyramidal, fast spiking rat hippocampal CA1 neurons

1. The role of Ca(2+)-calmodulin (CaM) signalling cascades in modulating glutamatergic synaptic transmission on CA1 non-pyramidal fast-spiking neurons was investigated using whole-cell recording and perfusion in rat hippocampal slices. 2. Paired stimuli (PS), consisting of postsynaptic depolarization to 0 mV and presynaptic stimulation at 1 Hz for 30 s, enhanced excitatory postsynaptic currents (EPSCs) on non-pyramidal neurons in the stratum pyramidale (SP). The potentiation was reduced by the extracellular application of D-amino-5-phosphonovaleric acid (DAP-5, 40 microM), and blocked by the postsynaptic perfusion of 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM), a CaM-binding peptide (100 microM) or CaMKII (281-301) (an autoinhibitory peptide of CaM-dependent protein kinases, 100 microM). 3. The application of adenophostin, an agonist of inositol trisphosphate receptors (IP(3)Rs) that evokes Ca(2+) release, into SP non-pyramidal neurons via the patch pipette (1 microM) enhanced EPSCs and occluded PS-induced synaptic potentiation. The co-application of BAPTA (10 mM) with adenophostin blocked synaptic potentiation. In addition, Ca(2+)-CaM (40:10 microM) induced synaptic potentiation, which occluded PS-induced potentiation and was attenuated by introducing CaMKII (281-301) (100 microM). EPSCs were sensitive to an antagonist of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR). 4. Application of Ca(2+)-CaM into SP non-pyramidal neurons induced the emergence of AMPAR-mediated EPSCs that were not evoked by low stimulus intensity before perfusion. Ca(2+)-CaM also increased the amplitude and frequency of spontaneous EPSCs. A scavenger of nitric oxide, carboxy-PTIO (30 microM in slice-perfusion solution), did not affect these increases in sEPSCs. 5. The magnitude of PS-, adenophostin- or Ca(2+)-CaM-induced synaptic potentiation in SP non-pyramidal neurons increased during postnatal development. 6. These results indicate that Ca(2+)-CaM signalling pathways in CA1 SP non-pyramidal neurons up-regulate glutamatergic synaptic transmission probably through the conversion of inactive-to-active synapses.

Postsynaptic Modulation of AMPA Receptor-Mediated Synaptic Responses and LTP by the Type 3 Ryanodine Receptor

The precise function of ryanodine receptors (RyRs) in synaptic transmission is unknown, but three of their subtypes are expressed in the brain. We examined the roleof RyRs in excitatory synaptic transmission in hippocampal slices, using type 3 RyR (RyR3)-deficient mice. The alpha-amino-3-hydroxy-5-methyl-4-isoxozolepropionic acid (AMPA) receptor-mediated basal synaptic responses in the CA1 region of mutant mice were smaller than those of wild-type mice, while there was no difference in N-methyl-d-aspartate receptor-mediated responses, suggesting selective postsynaptic modification of AMPA receptors by RyR3. The expression of synaptic AMPA receptor subunits examined by Western blotting or immunohistochemistry was indistinguishable, suggesting that the smaller AMPA synaptic responses in mutant mice were not due to the reduced number of synaptic AMPA receptors. Although the initial potentiation following tetanic stimulation of afferent fibers was similar, long-term potentiation (LTP) was smaller in mutant mice. There were no differences in presynaptic electrophysiological properties. We thus conclude that RyR3 postsynaptically regulates the properties of AMPA receptors and LTP.

Overview on the structure, composition, function, development, and plasticity of hippocampal dendritic spines

There has been an explosion of new information on the neurobiology of dendritic spines in synaptic signaling, integration, and plasticity. Novel imaging and analytical techniques have provided important new insights into dendritic spine structure and function. Results are accumulating across many disciplines, and a step toward consolidating some of this work has resulted in Dendritic Spines of the Hippocampus. Leaders in the field provide a discussion at the level of advanced under-graduates, with sufficient detail to be a contemporary resource for research scientists. Critical reviews are presented on topics ranging from spine structure, formation, and maintenance, to molecular composition, plasticity, and the role of spines in learning and memory. Dendritic Spines of the Hippocampus provides a timely discussion of our current understanding of form and function at these excitatory synapses. We asked authors to include areas of controversy in their papers so as to distinguish results that are generally agreed upon from those where multiple interpretations are possible. We thank the contributors for their insights and thoughtful discussions. In this paper we provide background on the structure, composition, function, development, plasticity, and pathology of hippocampal dendritic spines. In addition, we highlight where each of these subjects will be elaborated upon in subsequent papers of this special issue of Hippocampus.

Cajal-Retzius cells in early postnatal mouse cortex selectively express functional metabotropic glutamate receptors

Glutamate receptors have been linked to the regulation of several developmental events in the CNS. By using cortical slices of early postnatal mice, we show that in layer I cells, glutamate produces intracellular calcium ([Ca(2+)](i)) elevations mediated by ionotropic and metabotropic glutamate receptors (mGluRs). The contribution of mGluRs to these responses was demonstrated by application of tACPD, an agonist to groups I and II mGluRs, which evoked [Ca(2+)](i) increases that could be reversibly blocked by MCPG, an antagonist to groups I and II mGluRs. In the absence of extracellular Ca(2+), repetitive applications of tACPD or quisqualate, an agonist to group I mGluRs, elicited decreasing [Ca(2+)](i) responses that were restored by refilling a thapsigargin-sensitive Ca(2+) store. The use of specific group I mGluR agonists CHPG and DHPG indicated that the functional mGluR in layer I was of the mGluR1 subtype. Subtype specific antibodies confirmed the presence of mGlur1 alpha, but not mGluR5, in Cajal-Retzius (Reelin-immunoreactive) neurons.

Molecular determinants of metabotropic glutamate receptor signaling

Metabotropic glutamate (mglu) receptors are implicated in the regulation of many physiological and pathological processes in the CNS, including synaptic plasticity, learning and memory, motor coordination, pain transmission and neurodegeneration. Several recent studies have elucidated the molecular determinants of mglu receptor signaling and show that several mechanisms acting at different steps in signal propagation are involved. We attempt to offer an integrated view on how homologous and heterologous mechanisms regulate the initial steps of signal propagation, mainly at the level of mglu-receptor-G-protein coupling. Particular emphasis is placed on the role of phosphorylation mechanisms mediated by protein kinase C and G-protein-coupled receptor kinases, and on the emerging importance of some members of the regulators of G-protein signaling family, such as RGS2 and RGS4, which facilitate the GTPase activity that is intrinsic to the alpha-subunits of G(q) and G(i).

Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

Characterization of the metabotropic glutamate receptors mediating phospholipase C activation and calcium release in cerebellar granule cells: calcium-dependence of the phospholipase C response

In this study we have determined the metabotropic glutamate receptors (mGluRs) involved in the glutamate activation of phospholipase C (PLC) and Ca(2+) mobilization in cerebellar granule cells at 9 days in vitro; and studied the Ca(2+) modulation of the PLC response. Both PLC activation and Ca(2+) signalling were found to be mediated exclusively by the mGluR1 subtype, although both group I mGluRs, mGluR1 alpha and mGluR5, could be detected in cell extracts. Exposure of cells to medium devoid of Ca(2+) for various times before agonist stimulation reduced the PLC response, which was quickly recovered following the re-exposure of cells to Ca(2+)-containing medium. The extent of the glutamate response correlated well with changes in the cytosolic Ca(2+) concentration. On the other hand, loading of the intracellular Ca(2+) stores by a transient depolarization followed by washing in nondepolarizing buffer, allowed glutamate to release stored Ca(2+) in the majority of cells and enhanced glutamate activation of PLC. Under such conditions, the absence of extracellular Ca(2+) during stimulation and the chelation of cytosolic Ca(2+) with BAPTA/AM inhibited both glutamate-elicited Ca(2+) response and PLC activation. Overall, these results indicate that the mGluR-mediated activation of PLC depends on the presence of extracellular Ca(2+) and can be modulated by moderate changes of cytosolic Ca(2+). Furthermore, ryanodine reduced PLC stimulation by glutamate in predepolarized cells but not in control cells, suggesting that ryanodine receptors could play a role in the potentiation of the mGluR-mediated activation of PLC by Ca(2+) release in predepolarized cells.

Calcium stores regulate the polarity and input specificity of synaptic modification

Activity-induced synaptic modification is essential for the development and plasticity of the nervous system. Repetitive correlated activation of pre- and postsynaptic neurons can induce persistent enhancement or decrement of synaptic efficacy, commonly referred to as long-term potentiation or depression (LTP or LTD). An important unresolved issue is whether and to what extent LTP and LTD are restricted to the activated synapses. Here we show that, in the CA1 region of the hippocampus, reduction of postsynaptic calcium influx by partial blockade of NMDA (N-methyl-D-aspartate) receptors results in a conversion of LTP to LTD and a loss of input specificity normally associated with LTP, with LTD appearing at heterosynaptic inputs. The induction of LTD at homo- and heterosynaptic sites requires functional ryanodine receptors and inositol triphosphate (InsP3) receptors, respectively. Functional blockade or genetic deletion of type 1 InsP3 receptors led to a conversion of LTD to LTP and elimination of heterosynaptic LTD, whereas blocking ryanodine receptors eliminated only homosynaptic LTD. Thus, postsynaptic Ca2+, deriving from Ca2+ influx and differential release of Ca2+ from internal stores through ryanodine and InsP3 receptors, regulates both the polarity and input specificity of activity-induced synaptic modification.

Coupled intra- and extracellular Ca2+ dynamics in recurrent seizure-like events

We considered the evolution of Ca2+ oscillation dynamics in recurrent seizure-like events. Dynamic system behaviour was characterized in the state space reconstructed from intra- and extracellular [Ca2+] fluctuations simultaneously measured in cultured rat hippocampal slices under low-[Mg2+] conditions. When associated in the seizure-like event, these fluctuations occurred on a restricted set, the attractor, embedded in the full state space with less than five degrees of freedom. Instantaneous relative phase differences indicated field potential-driven phase jumps locked onto seizure-like events. To account for recurrent dynamics, calculations were performed on different extensions of a model for Ca2+ oscillation. These identified bidirectional, asymmetrical coupling of extracellular with intracellular (cytosolic, Ca2+ store, mitochondrial) Ca2+ dynamics as critical in its development.

Expression of the neuronal calcium sensor protein family in the rat brain

The neuronal calcium sensor proteins are members of the calcium-binding protein superfamily. They control localized calcium signalling on membranes and may make G-protein cascades sensitive to cytosolic calcium. The family members are recoverin (visinin, S-modulin), neuronal calcium sensor-1 (frequenin), hippocalcin, neuronal visinin-like protein-1 (visinin-like protein, neurocalcin-alpha), neuronal visinin-like protein-2 and neuronal visinin-like protein-3. Recoverin is expressed only in the retina and pineal gland. Using in situ hybridization, we mapped the expression of the other neuronal calcium sensor protein genes in the adult rat brain. Neuronal visinin-like protein-1 messenger RNA has a widespread distribution and is abundant in all brain areas except the caudate-putamen. Neuronal calcium sensor-1 gene expression is pan-neuronal. Neuronal calcium sensor-1 messenger RNA is present in the dendrites of hippocampal pyramidal and granule cells, suggesting a specific role in dendritic function. Hippocalcin and neuronal visinin-like protein-2 are mainly expressed in the forebrain and have similar expression patterns (neocortex, hippocampus and caudate-putamen). Neuronal visinin-like protein-3 has the most restricted expression; its highest expression level is in the cerebellum (Purkinje and granule cells). However, the neuronal visinin-like protein-3 gene is also expressed in many ventral nuclei throughout the fore- and midbrain, in the medial habenulae, and in the superior and inferior colliculi. The neuronal calcium sensor proteins are a relatively unexplored family of Ca(2+)-binding proteins. They are likely to be involved in many diverse areas of neuronal signalling. In this paper, we describe their expression in the rat brain as determined by in situ hybridization. As all five neuronal calcium sensor protein genes have distinctive expression patterns, they probably perform specific functions.

Synbindin, A novel syndecan-2-binding protein in neuronal dendritic spines

Dendritic spines are small protrusions on the surface of dendrites that receive the vast majority of excitatory synapses. We previously showed that the cell-surface heparan sulfate proteoglycan syndecan-2 induces spine formation upon transfection into hippocampal neurons. This effect requires the COOH-terminal EFYA sequence of syndecan-2, suggesting that cytoplasmic molecules interacting with this sequence play a critical role in spine morphogenesis. Here, we report a novel protein that binds to the EFYA motif of syndecan-2. This protein, named synbindin, is expressed by neurons in a pattern similar to that of syndecan-2, and colocalizes with syndecan-2 in the spines of cultured hippocampal neurons. In transfected hippocampal neurons, synbindin undergoes syndecan-2-dependent clustering. Synbindin is structurally related to yeast proteins known to be involved in vesicle transport. Immunoelectron microscopy localized synbindin on postsynaptic membranes and intracellular vesicles within dendrites, suggesting a role in postsynaptic membrane trafficking. Synbindin coimmunoprecipitates with syndecan-2 from synaptic membrane fractions. Our results show that synbindin is a physiological syndecan-2 ligand on dendritic spines. We suggest that syndecan-2 induces spine formation by recruiting intracellular vesicles toward postsynaptic sites through the interaction with synbindin.

Microtubules regulate local Ca2+ spiking in secretory epithelial cells

The role of the cytoskeleton in regulating Ca(2+) release has been explored in epithelial cells. Trains of local Ca(2+) spikes were elicited in pancreatic acinar cells by infusion of inositol trisphosphate through a whole cell patch pipette, and the Ca(2+)-dependent Cl(-) current spikes were recorded. The spikes were only transiently inhibited by cytochalasin B, an agent that acts on microfilaments. In contrast, nocodazole (5-100 micrometer), an agent that disrupts the microtubular network, dose-dependently reduced spike frequency and decreased spike amplitude leading to total blockade of the response. Consistent with an effect of microtubular disruption, colchicine also inhibited spiking but neither Me(2)SO nor beta-lumicolchicine, an inactive analogue of colchicine, had any effect. The microtubule-stabilizing agent, taxol, also inhibited spiking. The nocodazole effects were not due to complete loss of function of the Ca(2+) signaling apparatus, because supramaximal carbachol concentrations were still able to mobilize a Ca(2+) response. Finally, as visualized by 2-photon excitation microscopy of ER-Tracker, nocodazole promoted a loss of the endoplasmic reticulum in the secretory pole region. We conclude that microtubules specifically maintain localized Ca(2+) spikes at least in part because of the local positioning of the endoplasmic reticulum.

The metabotropic glutamate receptor subtype mGluR 2/3 is located at extrasynaptic loci in rat spinal dorsal horn synapses

The position of neurotransmitter receptors relative to active neurotransmitter release sites may be a major factor influencing neuronal responses. The location of the metabotropic glutamate receptor subtype mGluR2/3 was investigated in synaptic structures in the rat superficial spinal dorsal horn laminae by using a pre-embedding immunogold technique. Immunostaining for mGluR2/3 occurred in laminae I through III. Gold particles were encountered both in the cytosol and along the plasma membrane. Distinctive plasmalemmal immunodeposits were detected in vesicle-containing profiles, where they were located to membrane compartments distant from active release sites rather than in the close vicinity of synaptic specialisations. No distinct immunolabelling was observed in profiles meeting characteristics of primary afferent terminals. The extrasynaptic occurrence of mGluR2/3 suggests a presynaptic heteroreceptor role for these receptor subtypes in the spinal dorsal horn.

Homer: a link between neural activity and glutamate receptor function

The proteins of the Homer family bind to proline-rich sequences in group I metabotropic glutamate receptors, inositol trisphosphate receptors, ryanodine receptors, and Shank family proteins. Homer proteins also self associate and function as adaptors to couple interacting proteins. Recent observations indicate a role for Homer complexes in signal transduction, synaptogenesis and receptor trafficking.