The effect of ACPD on the responses to NMDA and AMPA varies with layer in slices of rat visual cortex

Membrane trafficking and positioning of mGluRs at presynaptic and postsynaptic sites of excitatory synapses

The plethora of functions of glutamate in the brain are mediated by the complementary actions of ionotropic and metabotropic glutamate receptors (mGluRs). The ionotropic glutamate receptors carry most of the fast excitatory transmission, while mGluRs modulate transmission on longer timescales by triggering multiple intracellular signaling pathways. As such, mGluRs mediate critical aspects of synaptic transmission and plasticity. Interestingly, at synapses, mGluRs operate at both sides of the cleft, and thus bidirectionally exert the effects of glutamate. At postsynaptic sites, group I mGluRs act to modulate excitability and plasticity. At presynaptic sites, group II and III mGluRs act as auto-receptors, modulating release properties in an activity-dependent manner. Thus, synaptic mGluRs are essential signal integrators that functionally couple presynaptic and postsynaptic mechanisms of transmission and plasticity. Understanding how these receptors reach the membrane and are positioned relative to the presynaptic glutamate release site are therefore important aspects of synapse biology. In this review, we will discuss the currently known mechanisms underlying the trafficking and positioning of mGluRs at and around synapses, and how these mechanisms contribute to synaptic functioning. We will highlight outstanding questions and present an outlook on how recent technological developments will move this exciting research field forward.

Modulation of excitatory neurotransmission by neuronal/glial signalling molecules: interplay between purinergic and glutamatergic systems

Glutamate is the main excitatory neurotransmitter of the central nervous system (CNS), released both from neurons and glial cells. Acting via ionotropic (NMDA, AMPA, kainate) and metabotropic glutamate receptors, it is critically involved in essential regulatory functions. Disturbances of glutamatergic neurotransmission can be detected in cognitive and neurodegenerative disorders. This paper summarizes the present knowledge on the modulation of glutamate-mediated responses in the CNS. Emphasis will be put on NMDA receptor channels, which are essential executive and integrative elements of the glutamatergic system. This receptor is crucial for proper functioning of neuronal circuits; its hypofunction or overactivation can result in neuronal disturbances and neurotoxicity. Somewhat surprisingly, NMDA receptors are not widely targeted by pharmacotherapy in clinics; their robust activation or inhibition seems to be desirable only in exceptional cases. However, their fine-tuning might provide a promising manipulation to optimize the activity of the glutamatergic system and to restore proper CNS function. This orchestration utilizes several neuromodulators. Besides the classical ones such as dopamine, novel candidates emerged in the last two decades. The purinergic system is a promising possibility to optimize the activity of the glutamatergic system. It exerts not only direct and indirect influences on NMDA receptors but, by modulating glutamatergic transmission, also plays an important role in glia-neuron communication. These purinergic functions will be illustrated mostly by depicting the modulatory role of the purinergic system on glutamatergic transmission in the prefrontal cortex, a CNS area important for attention, memory and learning.

Pre- and postsynaptic modulations of hypoglossal motoneurons by α-adrenoceptor activation in wild-type and Mecp2(-/Y) mice

Hypoglossal motoneurons (HNs) control tongue movement and play a role in maintenance of upper airway patency. Defects in these neurons may contribute to the development of sleep apnea and other cranial motor disorders including Rett syndrome (RTT). HNs are modulated by norepinephrine (NE) through α-adrenoceptors. Although postsynaptic mechanisms are known to play a role in this effect, how NE modulates the synaptic transmissions of HNs remains poorly understood. More importantly, the NE system is defective in RTT, while how the defect affects HNs is unknown. Believing that information of NE modulation of HNs may help the understanding of RTT and the design of new therapeutical interventions to motor defects in the disease, we performed these studies in which glycinergic inhibitory postsynaptic currents and intrinsic membrane properties were examined in wild-type and Mecp2(-/Y) mice, a mouse of model of RTT. We found that activation of α1-adrenoceptor facilitated glycinergic synaptic transmission and excited HNs. These effects were mediated by both pre- and postsynaptic mechanisms. The latter effect involved an inhibition of barium-sensitive G protein-dependent K(+) currents. The pre- and postsynaptic modulations of the HNs by α1-adrenoceptors were not only retained in Mecp2-null mice but also markedly enhanced, which appears to be a compensatory mechanism for the deficiencies in NE and GABAergic synaptic transmission. The existence of the endogenous compensatory mechanism is an encouraging finding, as it may allow therapeutical modalities to alleviate motoneuronal defects in RTT.

LTP and LTD vary with layer in rodent visual cortex

Mechanisms of plasticity in the visual cortex have been studied with long-term potentiation (LTP), long-term depression (LTD) and ocular dominance plasticity (ODP). It is now possible to compare results from these three forms of plasticity using knockout mice, and also by pharmacological manipulations. A review of the literature shows that if both LTP and LTD are completely abolished, then ODP will also be abolished. In other situations, there is little correlation. We hypothesize that this lack of correlation occurs because the mechanisms for LTP and LTD vary with layer in the visual cortex, and results show that they do.

Regulation of AMPA receptor activity, synaptic targeting and recycling: role in synaptic plasticity

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors for the neurotransmitter glutamate are oligomeric structures responsible for most fast excitatory responses in the central nervous system. The activity of AMPA receptors can be directly regulated by protein phosphorylation, which may also affect the interaction with intracellular proteins and, consequently, their recycling and localization to defined postsynaptic sites. This review focuses on recent advances in understanding the dynamic regulation of AMPA receptors, on a short- and long-term basis, and its implications in synaptic plasticity.

Long term potentiation varies with layer in rat visual cortex

Long term potentiation (LTP) in various layers of rat visual cortex was studied in 90 cells with visually identified, whole-cell recordings. LTP was induced in layer II/III, layer V or layer VI with theta burst stimulation (TBS), but was not observed in layer IV. In the presence of a NMDA antagonist, D-AP5, in the bath solution, potentiation was blocked in layer II/III, some depression was seen in layer V, and potentiation still remained in layer VI. After addition of a specific mGluR1 antagonist, LY367385, to the bath solution, LTP was reduced in layer II/III and layer V, and was blocked in layer VI. After a specific mGluR5 antagonist, MPEP was applied in the bath solution, LTP was enhanced in layer VI, and blocked in layer V. We conclude that: (1) LTP in layer VI is different from other layers, depending on mGluR1, but not NMDA receptors. (2) In layer II/III, LTP is NMDA-dependent and is not blocked by group I mGluR antagonists. (3) LTP in layer V is both NMDA receptor and mGluR5 receptor-dependent. (4) LTP was not induced in layer IV with TBS.

Two distinct signaling pathways upregulate NMDA receptor responses via two distinct metabotropic glutamate receptor subtypes

Molecular processes regulating the gain of NMDA receptors modulate diverse physiological and pathological responses in the CNS. Group I metabotropic glutamate receptors (mGluRs), which neighbor NMDA receptors and which can be coactivated by synaptically released glutamate, couple to several different second messenger pathways, each of which could target NMDA receptors. In CA3 pyramidal cells we show that the activation of mGluR1 potentiates NMDA current via a G-protein-independent mechanism involving Src kinase activation. In contrast, mGluR5-mediated enhancement of NMDA current requires G-protein activation, triggering a signaling cascade including protein kinase C and Src. These results indicate that one neurotransmitter, glutamate, can activate two distinct and independent signaling systems to target the same effector. These two pathways are likely to contribute significantly to the highly differentiated control of NMDA receptor function.

Interaction between metabotropic and NMDA subtypes of glutamate receptors in sprout suppression at young synapses

Recently, NMDA receptors (NMDARs) have been implicated in a cell contact-dependent suppression of sprouting in cultured Xenopus tectal neurons during an early period when neither AMPA/kainate (KA) receptors nor action potentials play a prominent role in cell-cell communication. We asked how the NMDA receptors function in the absence of the depolarizing effect of AMPA/KA receptor activity. We show that type II metabotropic glutamate receptors (mGluRs) can operate synergistically with NMDA receptors in the absence of AMPA/KA receptor function to suppress an early neurite sprouting response of the tectal neurons. Calcium imaging with fluo-3 AM and morphological analyses after exposure to glutamate receptor antagonists show that a combination of AMPA/KA receptor and type II mGluR blockers mimics the decrease in intracellular free calcium in response to glutamate and the structural effects produced by NMDA receptor antagonists in these cultures. Patch-clamp analyses confirmed a decrease in NMDA receptor-mediated currents with type II mGluR blockade, and 8-bromo cAMP application produced a decrease in NMDA receptor-mediated calcium influx. These data suggest that type II mGluRs potentiate NMDA receptor function by decreasing cAMP levels in tectal neurons. We also show that NMDARs exhibit low magnesium sensitivity in tectal neurons during the first few days in culture. Thus both metabotropic and ionotropic glutamate receptors can play a role in the contact-mediated suppression of ongoing sprouting at early neuron-neuron contacts before action potential activity.

Effect of the group I metabotropic glutamate agonist DHPG on the visual cortex

Metabotropic glutamate receptors have a variety of effects in visual cortex that depend on the age of the animal, the layer of the cortex, and the group of the receptor. Here we describe these effects for group I receptors, using both in vivo and in vitro preparations. The metabotropic group I glutamate receptor agonist 3,5 dihydroxyphenylglycine (DHPG) potentiates the responses to N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in slices of rat visual cortex. It also increases, initially, the visual response in the cat visual cortex. Both these effects are largest at 3-4 wk of age and decline to insignificance by 10 wk of age. Both are also largest in lower layers of cortex, which explains why the facilitatory effects found with the general metabotropic glutamate agonist 1S,3R aminocyclopentane-1,3-dicarboxylic acid (ACPD) are observed only in lower layers. Prolonged application of DHPG in the cat visual cortex, after the initial excitatory effect, produces depression. We also found that DHPG facilitates the NMDA response in fast-spiking cells, which are inhibitory, providing a partial explanation for this. Thus there are multiple effects of group I metabotropic glutamate receptors, which vary with layer and age in visual cortex.

2R,4R-4-Aminopyrrolidine-2,4-dicarboxylate (APDC) attenuates cortical EPSPs

We studied the effect of the Type II metabotropic glutamate receptor (mGluR 2,3) agonist APDC on the response of neurons in slices of rat visual cortex. In all cortical layers, APDC attenuated the EPSP produced by stimulation of the predominant excitatory input. This EPSP attenuation was seen in both younger and older rat slices and was present with G-protein blockade in the cell recorded, demonstrating that it was a presynaptic effect. Further, this EPSP attenuation was blocked by the mGluR 2,3 antagonist EGLU. A postsynaptic depressive effect of APDC on the NMDA response was seen in layers 2 and 3, but not in layers 5 and 6. Thus, the predominant action of Type II mGluRs in the visual cortex is a presynaptic reduction of glutamate release which persists through development. This regulation may be important in the setting of excitatory tone in visual cortex and in the extraction/processing of visual information.

Development and function of metabotropic glutamate receptors in cat visual cortex

Metabotropic glutamate receptors have been implicated in plasticity in the hippocampus and cerebellum. Are they also involved in plasticity in the visual cortex? This is a complicated question because of the diversity of metabotropic glutamate receptors and the variations in both receptors and plasticity with layer. Inhibition driven by group II metabotropic glutamate receptors is certainly correlated with ocular dominance segregation in layer IV of the cortex. Of the group I metabotropic glutamate receptors, mGluR5 may be involved in plasticity, but mGluR1 is unlikely to be. Both group I and group II receptors produce increases in cyclic adenosine monophosphate which are clearly related to plasticity. Further conclusions await the development of agonists and antagonists specific for individual metabotropic glutamate receptors, as opposed to groups of the receptors.