Internal Mg2+ block of recombinant NMDA channels mutated within the selectivity filter and expressed in Xenopus oocytes

Synaptic dendritic activity modulates the single synaptic event

Synaptic transmission is the key system for the information transfer and elaboration among neurons. Nevertheless, a synapse is not a standing alone structure but it is a part of a population of synapses inputting the information from several neurons on a specific area of the dendritic tree of a single neuron. This population consists of excitatory and inhibitory synapses the inputs of which drive the postsynaptic membrane potential in the depolarizing (excitatory synapses) or depolarizing (inhibitory synapses) direction modulating in such a way the postsynaptic membrane potential. The postsynaptic response of a single synapse depends on several biophysical factors the most important of which is the value of the membrane potential at which the response occurs. The concurrence in a specific time window of inputs by several synapses located in a specific area of the dendritic tree can, consequently, modulate the membrane potential such to severely influence the single postsynaptic response. The degree of modulation operated by the synaptic population depends on the number of synapses active, on the relative proportion between excitatory and inbibitory synapses belonging to the population and on their specific mean firing frequencies. In the present paper we show results obtained by the simulation of the activity of a single Glutamatergic excitatory synapse under the influence of two different populations composed of the same proportion of excitatory and inhibitory synapses but having two different sizes (total number of synapses). The most relevant conclusion of the present simulations is that the information transferred by the single synapse is not and independent simple transition between a pre- and a postsynaptic neuron but is the result of the cooperation of all the synapses which concurrently try to transfer the information to the postsynaptic neuron in a given time window. This cooperativeness is mainly operated by a simple mechanism of modulation of the postsynaptic membrane potential which influences the amplitude of the different components forming the postsynaptic excitatory response.

Electrophysiological-mechanical coupling in the neuronal membrane and its role in ultrasound neuromodulation and general anaesthesia

The current understanding of the role of the cell membrane is in a state of flux. Recent experiments show that conventional models, considering only electrophysiological properties of a passive membrane, are incomplete. The neuronal membrane is an active structure with mechanical properties that modulate electrophysiology. Protein transport, lipid bilayer phase, membrane pressure and stiffness can all influence membrane capacitance and action potential propagation. A mounting body of evidence indicates that neuronal mechanics and electrophysiology are coupled, and together shape the membrane potential in tight coordination with other physical properties. In this review, we summarise recent updates concerning electrophysiological-mechanical coupling in neuronal function. In particular, we aim at making the link with two relevant yet often disconnected fields with strong clinical potential: the use of mechanical vibrations-ultrasound-to alter the electrophysiogical state of neurons, e.g., in neuromodulation, and the theories attempting to explain the action of general anaesthetics. STATEMENT OF SIGNIFICANCE: General anaesthetics revolutionised medical practice; now an apparently unrelated technique, ultrasound neuromodulation-aimed at controlling neuronal activity by means of ultrasound-is poised to achieve a similar level of impact. While both technologies are known to alter the electrophysiology of neurons, the way they achieve it is still largely unknown. In this review, we argue that in order to explain their mechanisms/effects, the neuronal membrane must be considered as a coupled mechano-electrophysiological system that consists of multiple physical processes occurring concurrently and collaboratively, as opposed to sequentially and independently. In this framework the behaviour of the cell membrane is not the result of stereotypical mechanisms in isolation but instead emerges from the integrative behaviour of a complexly coupled multiphysics system.

Multisynaptic cooperation shapes single glutamatergic synapse response

The activity of thousands of excitatory synapse in the dendritic tree produces variations of membrane potential which, while can produce the spike generation at soma (hillock), can also influence the output of a single glutamatergic synapse. We used a model of synaptic diffusion and EPSP generation to simulate the effect of different number of active synapses on the output of a single one. Our results show that, also in subthreshold conditions, the excitatory dendritic activity can influence several parameters of the single synaptic output such as its amplitude, its time course, the NMDA-component activation and consequently phenomena like STP and LTP.

Functional Maturation of Human Stem Cell-Derived Neurons in Long-Term Cultures

Differentiated neurons can be rapidly acquired, within days, by inducing stem cells to express neurogenic transcription factors. We developed a protocol to maintain long-term cultures of human neurons, called iNGNs, which are obtained by inducing Neurogenin-1 and Neurogenin-2 expression in induced pluripotent stem cells. We followed the functional development of iNGNs over months and they showed many hallmark properties for neuronal maturation, including robust electrical and synaptic activity. Using iNGNs expressing a variant of channelrhodopsin-2, called CatCh, we could control iNGN activity with blue light stimulation. In combination with optogenetic tools, iNGNs offer opportunities for studies that require precise spatial and temporal resolution. iNGNs developed spontaneous network activity, and these networks had excitatory glutamatergic synapses, which we characterized with single-cell synaptic recordings. AMPA glutamatergic receptor activity was especially dominant in postsynaptic recordings, whereas NMDA glutamatergic receptor activity was absent from postsynaptic recordings but present in extrasynaptic recordings. Our results on long-term cultures of iNGNs could help in future studies elucidating mechanisms of human synaptogenesis and neurotransmission, along with the ability to scale-up the size of the cultures.

AMPA/NMDA cooperativity and integration during a single synaptic event

Coexistence of AMPA and NMDA receptors in glutamatergic synapses leads to a cooperative effect that can be very complex. This effect is dependent on many parameters including the relative and absolute number of the two types of receptors and biophysical parameters that can vary among synapses of the same cell. Herein we simulate the AMPA/NMDA cooperativity by using different number of the two types of receptors and considering the effect of the spine resistance on the EPSC production. Our results show that the relative number of NMDA with respect to AMPA produces a different degree of cooperation which depends also on the spine resistance.

A model of cooperative effect of AMPA and NMDA receptors in glutamatergic synapses

Glutamatergic synapses play a pivotal role in brain excitation. The synaptic response is mediated by the activity of two receptor types (AMPA and NMDA). In the present paper we propose a model of glutamatergic synaptic activity where the fast current generated by the AMPA conductance produces a local depolarization which activates the voltage- and [Mg(2+)]-dependent NMDA conductance. This cooperative effect is dependent on the biophysical properties of the synaptic spine which can be considered a high input resistance specialized compartment. Herein we present results of simulations where different values of the spine resistance and of the Mg(2+) concentrations determine different levels of cooperativeness between AMPA and NMDA receptors in shaping the post-synaptic response.

A model of dopamine regulation of glutamatergic synapse on medium size spiny neurons

Spiny neurons of striatum receive glutamatergic synapses on dendritic spines on the neck of which project dopaminergic synapses. Dopamine modulates, by D1 type receptors, the glutamatergic synapses by inducing the phosphorylation of AMPA and NMDA receptors which produces an increased amplitude response. Herein we present a model where, in addition to phosphorylation, the direct modulation by dopamine of the spine resistance can cooperate in producing the observed effect on some of these synapses.

A model of dopamine modulated glutamatergic synapse

The dopamine neurotransmitter regulates important neural pathways and its action in the brain is very complex. When dopaminergic neurons make synapses on spiny neurons of the striatum nucleus, they tune the responsiveness of glutamatergic synapses by means of the dopamine D1 and D2 receptors. We studied the effect of dopamine D1 receptors on glutamatergic synapse of GABAergic spiny neurons in striatum nucleus where they are located on the neck of the same spine. The action of dopamine consists essentially in promoting the phosphorylation of AMPA and NMDA receptors thus increasing the Excitatory Post Synaptic Current peak amplitude. The consequence is a cooperative effect of glutamatergic and dopaminergic synapses for the regulation of the GABAergic neuronal code. The mechanisms by which the phosphorylation induces the increase of the EPSC amplitude still remain unclear although the lack of this regulation can be involved in several pathologies as, for example, the Parkinson's disease. We tested, by computational experiments based on our model of glutamatergic synapse, three parameters of the synaptic function that could be involved in dopamine action: (a) time binding of glutamate to receptors; (b) open probability of the receptors; and (c) single receptor conductance. For different reasons, any of the three parameters could be responsible of the increased EPSC-dopamine-dependent. Our computational results were compared and discussed with experimental results found in literature. Although for our model both the open probability and the single receptor conductance can reproduce the phosphorylation effect of dopamine, we argue that the dopamine effect consists essentially in an increase of the single receptor conductance due to a 3D rearrangement of the phosphorylated receptors.

The potential of targeting NMDA receptors outside the CNS

INTRODUCTION

NMDA receptor (NMDAR) is an ionotropic glutamate receptor with a high permeability to calcium and a unique feature of controlling numerous calcium-dependent processes. Apart from being widely distributed in the CNS, the presence of NMDAR and its potential significance in a variety of non-neuronal cells and tissues has become an interesting research topic.

AREAS COVERED

The current review summarizes prevailing knowledge on the role of NMDARs in the kidney, bone and parathyroid gland, three main organs responsible for calcium homeostasis, as well as in the heart, an organ whose function is highly dependable on balanced intracellular calcium concentrations. The review also examines studies that have advanced our understanding of the therapeutic potential of NMDAR agonists and antagonists in renal, cardiovascular and bone pathologies.

EXPERT OPINION

NMDARs have a preeminent role in many physiological and pathological processes outside the CNS. In certain organs and/or disease conditions, activating the NMDAR leads to beneficial effects for the target organ, whereas in other diseases cell signaling downstream of NMDAR activation can exacerbate their pathology. Therefore, targeting NMDARs therapeutically is rather intricate work, and surely requires more extensive investigation in order to properly tune up the diverse NMDAR's actions translating them into beneficial cellular responses.

Glutamate receptor pores

Glutamate receptors are ligand-gated ion channels that mediate fast excitatory synaptic transmission throughout the central nervous system. Functional receptors are homo- or heteromeric tetramers with each subunit contributing a re-entrant pore loop that dips into the membrane from the cytoplasmic side. The pore loops form a narrow constriction near their apex with a wide vestibule toward the cytoplasm and an aqueous central cavity facing the extracellular solution. This article focuses on the pore region, reviewing how structural differences among glutamate receptor subtypes determine their distinct functional properties.

Data-driven modeling of synaptic transmission and integration

In this chapter, we describe how to create mathematical models of synaptic transmission and integration. We start with a brief synopsis of the experimental evidence underlying our current understanding of synaptic transmission. We then describe synaptic transmission at a particular glutamatergic synapse in the mammalian cerebellum, the mossy fiber to granule cell synapse, since data from this well-characterized synapse can provide a benchmark comparison for how well synaptic properties are captured by different mathematical models. This chapter is structured by first presenting the simplest mathematical description of an average synaptic conductance waveform and then introducing methods for incorporating more complex synaptic properties such as nonlinear voltage dependence of ionotropic receptors, short-term plasticity, and stochastic fluctuations. We restrict our focus to excitatory synaptic transmission, but most of the modeling approaches discussed here can be equally applied to inhibitory synapses. Our data-driven approach will be of interest to those wishing to model synaptic transmission and network behavior in health and disease.

The discovery of human of GLUD2 glutamate dehydrogenase and its implications for cell function in health and disease

While the evolutionary changes that led to traits unique to humans remain unclear, there is increasing evidence that enrichment of the human genome through DNA duplication processes may have contributed to traits such as bipedal locomotion, higher cognitive abilities and language. Among the genes that arose through duplication in primates during the period of increased brain development was GLUD2, which encodes the hGDH2 isoform of glutamate dehydrogenase expressed in neural and other tissues. Glutamate dehydrogenase GDH is an enzyme central to the metabolism of glutamate, the main excitatory neurotransmitter in mammalian brain involved in a multitude of CNS functions, including cognitive processes. In nerve tissue GDH is expressed in astrocytes that wrap excitatory synapses, where it is thought to play a role in the metabolic fate of glutamate removed from the synaptic cleft during excitatory transmission. Expression of GDH rises sharply during postnatal brain development, coinciding with nerve terminal sprouting and synaptogenesis. Compared to the original hGDH1 (encoded by the GLUD1 gene), which is potently inhibited by GTP generated by the Krebs cycle, hGDH2 can function independently of this energy switch. In addition, hGDH2 can operate efficiently in the relatively acidic environment that prevails in astrocytes following glutamate uptake. This adaptation is thought to provide a biological advantage by enabling enhanced enzyme catalysis under intense excitatory neurotransmission. While the novel protein may help astrocytes to handle increased loads of transmitter glutamate, dissociation of hGDH2 from GTP control may render humans vulnerable to deregulation of this enzyme's function. Here we will retrace the cloning and characterization of the novel GLUD2 gene and the potential implications of this discovery in the understanding of mechanisms that permitted the brain and other organs that express hGDH2 to fine-tune their functions in order to meet new challenging demands. In addition, the potential role of gain-of-function of hGDH2 variants in human neurodegenerative processes will be considered.

Mechanistic and structural determinants of NMDA receptor voltage-dependent gating and slow Mg2+ unblock

NMDA receptor (NMDAR)-mediated currents depend on membrane depolarization to relieve powerful voltage-dependent NMDAR channel block by external magnesium (Mg(o)(2+)). Mg(o)(2+) unblock from native NMDARs exhibits a fast component that is consistent with rapid Mg(o)(2+) -unbinding kinetics and also a slower, millisecond time scale component (slow Mg(o)(2+) unblock). In recombinant NMDARs, slow Mg(o)(2+) unblock is prominent in GluN1/2A (an NMDAR subtype composed of GluN1 and GluN2A subunits) and GluN1/2B receptors, with slower kinetics observed for GluN1/2B receptors, but absent from GluN1/2C and GluN1/2D receptors. Slow Mg(o)(2+) unblock from GluN1/2B receptors results from inherent voltage-dependent gating, which increases channel open probability with depolarization. Here we examine the mechanisms responsible for NMDAR subtype dependence of slow Mg(o)(2+) unblock. We demonstrate that slow Mg(o)(2+) unblock from GluN1/2A receptors, like GluN1/2B receptors, results from inherent voltage-dependent gating. Surprisingly, GluN1/2A and GluN1/2B receptors exhibited equal inherent voltage dependence; faster Mg(o)(2+) unblock from GluN1/2A receptors can be explained by voltage-independent differences in gating kinetics. To investigate the absence of slow Mg(o)(2+) unblock in GluN1/2C and GluN1/2D receptors, we examined the GluN2 S/L site, a site responsible for several NMDAR subtype-dependent channel properties. Mutating the GluN2 S/L site of GluN2A subunits from serine (found in GluN2A and GluN2B subunits) to leucine (found in GluN2C and GluN2D) greatly diminished both voltage-dependent gating and slow Mg(o)(2+) unblock. Therefore, the residue at the GluN2 S/L site governs the expression of both slow Mg(o)(2+) unblock and inherent voltage dependence.

How Nox2-containing NADPH oxidase affects cortical circuits in the NMDA receptor antagonist model of schizophrenia

SIGNIFICANCE

Schizophrenia is a complex neuropsychiatric disorder affecting around 1% of the population worldwide. Its mode of inheritance suggests a multigenic neurodevelopmental disorder with symptoms appearing during late adolescence/early adulthood, with its onset strongly influenced by environmental stimuli. Many neurotransmitter systems, including dopamine, glutamate, and gamma-aminobutyric acid, show alterations in affected individuals, and the behavioral and physiological characteristics of the disease can be mimicked by drugs that produce blockade of N-methyl-d-aspartate glutamate receptors (NMDARs).

RECENT ADVANCES

Mounting evidence suggests that drugs that block NMDARs specifically impair the inhibitory capacity of parvalbumin-expressing (PV+) fast-spiking neurons in adult and developing rodents, and alterations in these inhibitory neurons is one of the most consistent findings in the schizophrenic postmortem brain. Disruption of the inhibitory capacity of PV+ inhibitory neurons will alter the functional balance between excitation and inhibition in prefrontal cortical circuits producing impairment of working memory processes such as those observed in schizophrenia.

CRITICAL ISSUES

Mechanistically, the effect of NMDAR antagonists can be attributed to the activation of the Nox2-dependent reduced form of nicotinamide adenine dinucleotide phosphate oxidase pathway in cortical neurons, which is consistent with the emerging role of oxidative stress in the pathogenesis of mental disorders, specifically schizophrenia. Here we review the mechanisms by which NMDAR antagonists produce lasting impairment of the cortical PV+ neuronal system and the roles played by Nox2-dependent oxidative stress mechanisms.

FUTURE DIRECTIONS

The discovery of the pathways by which oxidative stress leads to unbalanced excitation and inhibition in cortical neural circuits opens a new perspective toward understanding the biological underpinnings of schizophrenia.

NMDA receptors with incomplete Mg²⁺ block enable low-frequency transmission through the cerebellar cortex

The cerebellar cortex coordinates movements and maintains balance by modifying motor commands as a function of sensory-motor context, which is encoded by mossy fiber (MF) activity. MFs exhibit a wide range of activity, from brief precisely timed high-frequency bursts, which encode discrete variables such as whisker stimulation, to low-frequency sustained rate-coded modulation, which encodes continuous variables such as head velocity. While high-frequency MF inputs have been shown to activate granule cells (GCs) effectively, much less is known about sustained low-frequency signaling through the GC layer, which is impeded by a hyperpolarized resting potential and strong GABA(A)-mediated tonic inhibition of GCs. Here we have exploited the intrinsic MF network of unipolar brush cells to activate GCs with sustained low-frequency asynchronous MF inputs in rat cerebellar slices. We find that low-frequency MF input modulates the intrinsic firing of Purkinje cells, and that this signal transmission through the GC layer requires synaptic activation of Mg²⁺-block-resistant NMDA receptors (NMDARs) that are likely to contain the GluN2C subunit. Slow NMDAR conductances sum temporally to contribute approximately half the MF-GC synaptic charge at hyperpolarized potentials. Simulations of synaptic integration in GCs show that the NMDAR and slow spillover-activated AMPA receptor (AMPAR) components depolarize GCs to a similar extent. Moreover, their combined depolarizing effect enables the fast quantal AMPAR component to trigger action potentials at low MF input frequencies. Our results suggest that the weak Mg²⁺ block of GluN2C-containing NMDARs enables transmission of low-frequency MF signals through the input layer of the cerebellar cortex.

Ionic flow enhances low-affinity binding: a revised mechanistic view into Mg2+ block of NMDA receptors

The N-methyl-d-aspartate receptor (NMDAR) channel is one of the major excitatory amino acid receptors in the mammalian brain. Since external Mg(2+) blocks the channel in an apparently voltage-dependent fashion, this ligand-gated channel displays intriguing voltage-dependent control of Na(+) and Ca(2+) permeability and thus plays an important role in synaptic physiology. We found that the essential features of Mg(2+) block could not be solely envisaged by binding of a charged blocker in the membrane electric field. Instead, the blocking effect of Mg(2+) is critically regulated by, and quantitatively correlated with, the relative tendency of outward and inward ionic fluxes. The 'intrinsic' affinity of Mg(2+) to the binding sites, however, is low (in the millimolar range) in the absence of net ionic flow at 0 mV. Besides, extracellular and intracellular Mg(2+) blocks the channel at distinct sites of electrical distances 0.7 and 0.95 from the outside, respectively. The two sites are separated by a high energy barrier for the movement of Mg(2+) (but not Na(+) or the other ions), and functionally speaking, each could accommodate 1.1 and 0.8 coexisting permeating ions, respectively. Mg(2+) block of the ionic flow thus is greatly facilitated by the flux-coupling effect or the ionic flow (the preponderant direction of permeant ion movement) per se, as if the poorly permeable Mg(2+) is 'pushed' against a high energy barrier by the otherwise permeating ions. Extracellular and intracellular Mg(2+) block then is in essence 'use dependent', more strongly inhibiting both Na(+) and Ca(2+) fluxes with stronger tendencies of influx and efflux, respectively. In conclusion, although permeant ions themselves could compete with Mg(2+), the flow or the tendency of movement of the permeant ions may actually enhance rather than interfere with Mg(2+) block, making the unique current-voltage relationship of NMDAR and the molecular basis of many important neurobiological phenomena.

Liposome reconstitution and modulation of recombinant N-methyl-D-aspartate receptor channels by membrane stretch

In this study, the heteromeric N-methyl-D-aspartate (NMDA) receptor channels composed of NR1a and NR2A subunits were expressed, purified, reconstituted into liposomes, and characterized by using the patch clamp technique. The protein exhibited the expected electrophysiological profile of activation by glutamate and glycine and internal Mg2+ blockade. We demonstrated that the mechanical energy transmitted to membrane-bound NMDA receptor channels can be exerted directly by tension developed in the lipid bilayer. Membrane stretch and application of arachidonic acid potentiated currents through NMDA receptor channels in the presence of intracellular Mg2+. The correlation of membrane tension induced by either mechanical or chemical stimuli with the physiological Mg2+ block of the channel suggests that the synaptic transmission can be altered if NMDA receptor complexes experience local changes in bilayer thickness caused by dynamic targeting to lipid microdomains, electrocompression, or chemical modification of the cell membranes. The ability to study gating properties of NMDA receptor channels in artificial bilayers should prove useful in further study of structure-function relationships and facilitate discoveries of new therapeutic agents for treatment of glutamate-mediated excitotoxicity or analgesic therapies.

Mutations within the selectivity filter of the NMDA receptor-channel influence voltage dependent block by 5-hydroxytryptamine

BACKGROUND AND PURPOSE

Voltage-dependent block by Mg2+ is a cardinal feature of NMDA receptors which acts as a coincidence detector to prevent the receptor from over-activation. Inhibition of NMDA receptor currents by 5-hydroxytryptamine (5-HT) indicated that 5-HT, similar to Mg2+, binds within the membrane electric field. In the present study, we assessed whether point mutations of critical asparagine residues located within the selectivity filter of NR1 and NR2A subunits of NMDA receptor-channel affect voltage-dependent block by 5-HT.

EXPERIMENTAL APPROACH

The mode of action of 5-HT and Mg2+ on wild-type and mutated NMDA receptor-channels expressed in Xenopus oocytes was investigated using the two-electrode voltage clamp recording technique.

KEY RESULTS

The mutation within the NR1 subunit NR1(N0S or N0Q) strongly reduced the voltage dependent block by 5-HT and increased the IC50. The corresponding mutations within the NR2 subunits NR2A(N0Q or N+1Q) reduced the block by 5-HT to a lesser extent. This is in contrast to the block produced by external Mg2+ where a substitution at the NR2A(N0) and NR2A(N+1) sites but not at the NR1(N0) site significantly reduced Mg2+ block.

CONCLUSION AND IMPLICATIONS

The block of NMDA receptor-channels by 5-HT depends on the NR1-subunit asparagine residue and to a lesser extent on the NR2A-subunit asparagine residues. These data suggest that the interaction of 5-HT with functionally important residues in a narrow constriction of the pore of the NMDA receptor-channel provides a significant barrier to ionic fluxes through the open channel due to energetic factors governed by chemical properties of the binding site and the electric field.

Voltage-dependent inhibition of recombinant NMDA receptor-mediated currents by 5-hydroxytryptamine

The effect of 5-HT and related indolealkylamines on heteromeric recombinant NMDA receptors expressed in Xenopus oocytes was investigated using the two-electrode voltage-clamp recording technique. In the absence of external Mg(2+) ions, 5-HT inhibited NMDA receptor-mediated currents in a concentration-dependent manner. The inhibitory effect of 5-HT was independent of the NR1a and NR2 subunit combination. The inhibition of glutamate-evoked currents by 5-HT was use- and voltage-dependent. The voltage sensitivity of inhibition for NR1a+NR2 subunit combinations by 5-HT was similar, exhibiting an e-fold change per approximately 20 mV, indicating that 5-HT binds to a site deep within the membrane electric field. The inhibition of the open NMDA receptor by external Mg(2+) and 5-HT was not additive, suggesting competition between Mg(2+) and 5-HT for a binding site in the NMDA receptor channel. The concentration-dependence curves for 5-HT and 5-methoxytryptamine (5-MeOT) inhibition of NMDA receptor-mediated currents are shifted to the right in the presence of external Mg(2+). The related indolealkylamines inhibited glutamate-evoked currents with the following order of inhibitory potency: 5-MeOT=5-methyltryptamine>tryptamine>7-methyltryptamine>5-HT>>tryptophan=melatonin. Taken together, these data suggest that 5-HT and related compounds can attenuate glutamate-mediated excitatory synaptic responses and may provide a basis for drug treatment of excitoxic neurodegeneration.

High-affinity Zn block in recombinant N-methyl-D-aspartate receptors with cysteine substitutions at the Q/R/N site

In ionotropic glutamate receptors, many channel properties (e.g., selectivity, ion permeation, and ion block) depend on the residue (glutamine, arginine, or asparagine) located at the tip of the pore loop (the Q/R/N site). We substituted a cysteine for the asparagine present at that position in both NR1 and NR2 N-methyl-D-aspartate (NMDA) receptor subunits. Under control conditions, receptors containing mutated NR1 and NR2 subunits show much smaller glutamate responses than wild-type receptors. However, this difference disappears upon addition of heavy metal chelators in the extracellular bath. The presence of cysteines at the Q/R/N site in both subunits of NR1/NR2C receptors results in a 220,000-fold increase in sensitivity of the inhibition by extracellular Zn. In contrast with the high-affinity Zn inhibition of wild-type NR1/NR2A receptors, the high-affinity Zn inhibition of mutated NR1/NR2C receptors shows a voltage dependence, which resembles very much that of the block by extracellular Mg. This indicates that the Zn inhibition of the mutated receptors results from a channel block involving Zn binding to the thiol groups introduced into the selectivity filter. Taking advantage of the slow kinetics of the Zn block, we show that both blocking and unblocking reactions require prior opening of the channel.

Free intracellular Mg(2+) concentration and inhibition of NMDA responses in cultured rat neurons

1. Intracellular Mg(2+) (Mg(2+)(i)) blocks single-channel currents and modulates the gating kinetics of NMDA receptors. However, previous data suggested that Mg(2+)(i) inhibits whole-cell current less effectively than predicted from excised-patch measurements. We examined the basis of this discrepancy by testing three hypothetical explanations. 2. To test the first hypothesis, that control of free Mg(2+)(i) concentration ([Mg(2+)](i)) during whole-cell recording was inadequate, we measured [Mg(2+)](i) using mag-indo-1 microfluorometry. The [Mg(2+)](i) measured in cultured neurons during whole-cell recording was similar to the pipette [Mg(2+)] measured in vitro, suggesting that [Mg(2+)](i) was adequately controlled. 3. To test the second hypothesis, that open-channel block by Mg(2+)(i) was modified by patch excision, we characterised the effects of Mg(2+)(i) using cell-attached recordings. We found the affinity and voltage dependence of open-channel block by Mg(2+)(i) similar in cell-attached and outside-out patches. Thus, the difference between Mg(2+)(i) inhibition of whole-cell and of patch currents cannot be attributed to a difference in Mg(2+)(i) block of single-channel current. 4. The third hypothesis tested was that the effect of Mg(2+)(i) on channel gating was modified by patch excision. Results of cell-attached recording and modelling of whole-cell data suggest that the Mg(2+)(i)-induced stabilisation of the channel open state is four times weaker after patch excision than in intact cells. This differential effect of Mg(2+)(i) on channel gating explains why Mg(2+)(i) inhibits whole-cell NMDA responses less effectively than patch responses.

Permeant ion regulation of N-methyl-D-aspartate receptor channel block by Mg(2+)

Block of the channel of N-methyl-D-aspartate (NMDA) receptors by external Mg(2+) (Mg(o)(2+)) has broad implications for the many physiological and pathological processes that depend on NMDA receptor activation. An essential property of channel block by Mg(o)(2+) is its powerful voltage dependence. A widely cited explanation for the strength of the voltage dependence of block is that the Mg(o)(2+)-binding site is located deep in the channel of NMDA receptors; Mg(o)(2+) then would sense most of the membrane potential field during block. However, recent electrophysiological and mutagenesis studies suggest that the blocking site cannot be deep enough to account for the voltage dependence of Mg(o)(2+) block. Here we describe the basis for this discrepancy: the magnitude and voltage dependence of channel block by Mg(o)(2+) are strongly regulated by external and internal permeant monovalent cations. Our data support a model in which access to the channel by Mg(o)(2+) is prevented when permeant ion-binding sites at the external entrance to the channel are occupied. Mg(o)(2+) can block the channel only when the permeant ion-binding sites are unoccupied and then can either unblock back to the external solution or permeate the channel. Unblock to the external solution is prevented if external permeant ions bind while Mg(2+) blocks the channel, although permeation is still permitted. The model provides an explanation for the strength of the voltage dependence of Mg(o)(2+) block and quantifies the interdependence of permanent and blocking ion binding to NMDA receptors.

Shaping excitation at glutamatergic synapses

Glutamatergic synapses vary, exhibiting EPSCs of widely different magnitudes and timecourses. The main contributors to this variability are: presynaptic factors, including release probability, quantal content and vesicle composition; factors that modulate the concentration and longevity of glutamate in the cleft, including diffusion and the actions of glutamate transporters; and postsynaptic factors, including the types and locations of ionotropic glutamate receptors, their numbers, and the nature and locations of associated intracellular signalling systems.

Target cell-specific modulation of transmitter release at terminals from a single axon

In the hippocampus, a CA3 pyramidal cell forms excitatory synapses with thousands of other pyramidal cells and inhibitory interneurons. By using sequential paired recordings from three connected cells, we show that the presynaptic properties of CA3 pyramidal cell terminals, belonging to the same axon, differ according to the type of target cell. Activation of presynaptic group III metabotropic glutamate receptors decreases transmitter release only at terminals contacting CA1 interneurons but not CA1 pyramidal cells. Furthermore, terminals contacting distinct target cells show different frequency facilitation. On the basis of these results, we conclude that the pharmacological and physiological properties of presynaptic terminals are determined, at least in part, by the target cells.