On the multiple-conductance single channels activated by excitatory amino acids in large cerebellar neurones of the rat

Single-channel mechanisms underlying the function, diversity and plasticity of AMPA receptors

The functional properties of AMPA receptors shape many of the essential features of excitatory synaptic signalling in the brain, including high-fidelity point-to-point transmission and long-term plasticity. Understanding the behaviour and regulation of single AMPAR channels is fundamental in unravelling how central synapses carry, process and store information. There is now an abundance of data on the importance of alternative splicing, RNA editing, and phosphorylation of AMPAR subunits in determining central synaptic diversity. Furthermore, auxiliary subunits have emerged as pivotal players that regulate AMPAR channel properties and add further diversity. Single-channel studies have helped reveal a fascinating picture of the unique behaviour of AMPAR channels - their concentration-dependent single-channel conductance, the basis of their multiple-conductance states, and the influence of auxiliary proteins in controlling many of their gating and conductance properties. Here we summarize basic hallmarks of AMPAR single-channels, in relation to function, diversity and plasticity. We also present data that reveal an unexpected feature of AMPAR sublevel behaviour. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

A structurally derived model of subunit-dependent NMDA receptor function

Key points

The kinetics of NMDA receptor (NMDAR) signalling are a critical aspect of the physiology of excitatory synaptic transmission in the brain.

Here we develop a mechanistic description of NMDAR function based on the receptor tetrameric structure and the principle that each agonist‐bound subunit must undergo some rate‐limiting conformational change after agonist binding, prior to channel opening.

By fitting this mechanism to single channel data using a new MATLAB‐based software implementation of maximum likelihood fitting with correction for limited time resolution, rate constants were derived for this mechanism that reflect distinct structural changes and predict the properties of macroscopic and synaptic NMDAR currents.

The principles applied here to develop a mechanistic description of the heterotetrameric NMDAR, and the software used in this analysis, can be equally applied to other heterotetrameric glutamate receptors, providing a unifying mechanistic framework to understanding the physiology of glutamate receptor signalling in the brain.

Abstract

NMDA receptors (NMDARs) are tetrameric complexes comprising two glycine‐binding GluN1 and two glutamate‐binding GluN2 subunits. Four GluN2 subunits encoded by different genes can produce up to 10 different di‐ and triheteromeric receptors. In addition, some neurological patients contain a de novo mutation or inherited rare variant in only one subunit. There is currently no mechanistic framework to describe tetrameric receptor function that can be extended to receptors with two different GluN1 or GluN2 subunits. Here we use the structural features of glutamate receptors to develop a mechanism describing both single channel and macroscopic NMDAR currents. We propose that each agonist‐bound subunit undergoes some rate‐limiting conformational change after agonist binding, prior to channel opening. We hypothesize that this conformational change occurs within a triad of interactions between a short helix preceding the M1 transmembrane helix, the highly conserved M3 motif encoded by the residues SYTANLAAF, and the linker preceding the M4 transmembrane helix of the adjacent subunit. Molecular dynamics simulations suggest that pre‐M1 helix motion is uncorrelated between subunits, which we interpret to suggest independent subunit‐specific conformational changes may influence these pre‐gating steps. According to this interpretation, these conformational changes are the main determinants of the key kinetic properties of NMDA receptor activation following agonist binding, and so these steps sculpt their physiological role. We show that this structurally derived tetrameric model describes both single channel and macroscopic data, giving a new approach to interpreting functional properties of synaptic NMDARs that provides a logical framework to understanding receptors with non‐identical subunits.

The kinetics of NMDA receptor (NMDAR) signalling are a critical aspect of the physiology of excitatory synaptic transmission in the brain.

Here we develop a mechanistic description of NMDAR function based on the receptor tetrameric structure and the principle that each agonist‐bound subunit must undergo some rate‐limiting conformational change after agonist binding, prior to channel opening.

By fitting this mechanism to single channel data using a new MATLAB‐based software implementation of maximum likelihood fitting with correction for limited time resolution, rate constants were derived for this mechanism that reflect distinct structural changes and predict the properties of macroscopic and synaptic NMDAR currents.

The principles applied here to develop a mechanistic description of the heterotetrameric NMDAR, and the software used in this analysis, can be equally applied to other heterotetrameric glutamate receptors, providing a unifying mechanistic framework to understanding the physiology of glutamate receptor signalling in the brain.

Retour aux sources: defining the structural basis of glutamate receptor activation

Ionotropic glutamate receptors (iGluRs) are the major excitatory neurotransmitter receptor in the vertebrate CNS and, as a result, their activation properties lie at the heart of much of the neuronal network activity observed in the developing and adult brain. iGluRs have also been implicated in many nervous system disorders associated with postnatal development (e.g. autism, schizophrenia), cerebral insult (e.g. stroke, epilepsy), and disorders of the ageing brain (e.g. Alzheimer's disease, Parkinsonism). In view of this, an emphasis has been placed on understanding how iGluRs activate and desensitize in functional and structural terms. Early structural models of iGluRs suggested that the strength of the agonist response was primarily governed by the degree of closure induced in the ligand-binding domain (LBD). However, recent studies have suggested a more nuanced role for the LBD with current evidence identifying the iGluR LBD interface as a "hotspot" regulating agonist behaviour. Such ideas remain to be consolidated with recently solved structures of full-length iGluRs to account for the global changes that underlie channel activation and desensitization.

Modes of glutamate receptor gating

The time course of excitatory synaptic currents, the major means of fast communication between neurons of the central nervous system, is encoded in the dynamic behaviour of post-synaptic glutamate-activated channels. First-pass attempts to explain the glutamate-elicited currents with mathematical models produced reaction mechanisms that included only the most basic functionally defined states: resting vs. liganded, closed vs. open, responsive vs. desensitized. In contrast, single-molecule observations afforded by the patch-clamp technique revealed an unanticipated kinetic multiplicity of transitions: from microseconds-lasting flickers to minutes-long modes. How these kinetically defined events impact the shape of the synaptic response, how they relate to rearrangements in receptor structure, and whether and how they are physiologically controlled represent currently active research directions. Modal gating, which refers to the slowest, least frequently observed ion-channel transitions, has been demonstrated for representatives of all ion channel families. However, reaction schemes have been largely confined to the short- and medium-range time scales. For glutamate receptors as well, modal gating has only recently come under rigorous scrutiny. This article reviews the evidence for modal gating of glutamate receptors and the still developing hypotheses about the mechanism(s) by which modal shifts occur and the ways in which they may impact the time course of synaptic transmission.

GABA increases Ca2+ in cerebellar granule cell precursors via depolarization: implications for proliferation

The amino acids glutamate and gamma-aminobutyric acid (GABA) have primarily been characterized as the most prevalent excitatory and inhibitory, respectively, neurotransmitters in the vertebrate central nervous system. However, the role of these signaling molecules extends far beyond the synapse. GABA, glutamate, and their complement of receptors are essential signaling molecules that regulate developmental processes in both embryonic and young adult mammals. In this review, we describe the current knowledge on the role of GABA and glutamate in development, focusing on the perinatal cerebellum. We will then present novel data suggesting that GABA depolarizes granule cell precursors via GABA(A) receptors, which leads to calcium increases in these cells. Finally, we will consider the role of GABA and glutamate signaling on cell proliferation and perhaps neural cancers. From our review of the literature and these data, we hypothesize that GABA(A) receptors and metabotropic glutamate receptors may be a novel target for the pharmacological regulation of the cerebellar tumors, medulloblastomas.

Activation of recombinant NR1/NR2C NMDA receptors

The N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptors comprises both NR1 and NR2 subunits, and plays numerous roles in both physiological and pathophysiological processes in the central nervous system (CNS). NR2C-containing NMDA receptors are most abundant in cerebellum, thalamus and olfactory bulb, and are also expressed in oligodendrocytes and hippocampal interneurons. We have used patch clamp recording to explore the activation properties of recombinant NR1/NR2C receptors expressed in HEK293 cells. NR1/NR2C receptors activated by a maximally effective concentration of glutamate and glycine had two main conductance levels of 45 pS and 28 pS when the extracellular Ca(2+) concentration was 0.5 mm and the holding potential was -80 mV. The occurrence of the lower subconductance state was reduced in the absence of extracellular Ca(2+). The distribution of closed durations recorded from patches with a high probability of containing only one active channel were best fitted by five exponential functions; the apparent open duration histogram could be fitted by two exponential functions (n = 10 patches). The apparent mean open time of NR1/NR2C receptors was brief (0.52 +/- 0.04 ms), suggesting that the stability of the open state of the NR1/NR2C receptors is lower than other NR2-containing receptors. NR1/NR2C open probability was exceptionally low, being 0.011 +/- 0.002 in patches containing a single active receptor (n = 8). Fast agonist concentration jumps were performed on outside out patches with multiple NR1/NR2C channels, which activated with a 10-90% rise time of 3.9 +/- 0.4 ms, faster than other NR2-containing receptors. The deactivation time constant after a brief (5-8 ms) application of a maximally effective concentration of agonists was 319 +/- 34 ms. The majority of the patches also showed a modest level of desensitization that could be described by either a single or a double exponential time course with the fastest time constant between 15 and 47 ms. Conceptual models of activation were fitted using the maximum interval likelihood (MIL) method to the sequence of open and closed durations recorded from outside-out patches that contained one active NR1/NR2C channel. NR1/NR2C receptor properties including modest desensitization and low open probability could be described by gating schemes similar to those previously proposed for other NMDA receptor subunit combinations.

The acetylcholine receptor: a model of an allosteric membrane protein mediating intercellular communication

Over the past 20 years the nicotinic acetylcholine receptor has become the prototype of a superfamily of ligand-gated ion channels. As a single macromolecular entity of M(r) about 300,000, the receptor protein mediates, altogether, the activation and the desensitization of the associated ion channel and the regulation of these processes by extracellular and intracellular signals. The notion is discussed that the acetylcholine receptor is a membrane-bound allosteric protein which possesses several categories of specific sites for neurotransmitters and for regulatory ligands, and undergoes conformational transitions which link these diverse sites together. At this elementary molecular level, interactions between signalling pathways may be mediated by membrane-bound allosteric receptors and/or by other categories of cytoplasmic allosteric proteins.

Climbing Fibre Responses in Olivo-cerebellar Slice Cultures. I. Microelectrode Recordings from Purkinje Cells

Cerebellar slices prepared from newborn rats were co-cultured with slices derived from the inferior olive of 4-day-old rats. After several weeks in vitro olivary fibres projecting into the cerebellar tissue could be assessed by anterograde labelling with the fluorescent dye 1,1-dioctadecyl-3,3,3,3'-tetramethylindocarbocyanine perchlorate (Dil). Following electrical field stimulation of the olivary tissue, all-or-nothing complex spikes were generated in Purkinje cells, which closely resembled climbing fibre responses as seen in situ. These responses were completely and reversibly abolished by 6-cyano-7-nitroquinoxaline-2-3-dione (CNQX, 5 microM), an antagonist of non-N-methyl-d-aspartate excitatory amino acid receptors. Wash in of smaller concentrations of CNQX (0.5 - 2 microM) resulted in a graded dose-dependent depression of the climbing fibre-induced postsynaptic potentials and in a consecutive failure of distinct active components of the complex spikes. With climbing fibre synaptic transmission blocked by CNQX, complex spike-like potentials could, however, still be evoked by intrasomatic injection of depolarizing current pulses. Increasing the concentration of Mg2+ in the bathing solution from 0.5 to up to 8 mM depressed regenerative complex-spike components. Olivary stimulation elicited only monophasic postsynaptic potentials in Purkinje cells under these conditions. These observations indicate that voltage-gated conductances which are substantially involved in the generation of the complex spike, are gated by the climbing fibre synaptic depolarization rather than directly by the climbing fibre transmitter.

In Vitro Development of Rat Cerebellar Neurons of Early Embryonic Origin. An Anatomical and Electrophysiological Study

The development of the major morphological and electrophysiological properties of presumptive Purkinje cells (PCs) was studied in primary cultures of rat cerebellum dissociated on the 14th embryonic day, when PCs are minimally differentiated and migrate in vivo. PCs were identified with a specific antibody to calbindin D-28K (CaBP), which allowed visualization of the different morphological types of PCs between 3 and 29 days in vitro (DIV). CaBP-immunopositive cells were first detected at 3 DIV. Thereafter, the shape of these cells resembled some of those described in vivo. After 20 DIV, 95% of the CaBP-immunopositive cells had characteristic PC dendritic trees, although they were very atrophic. Glial cells immunopositive for the glial fibrillary acidic protein (GFAP) were first seen at 3 DIV. Thereafter GFAP-immunopositive cells resembled Bergmann cells or velate astrocytes. Neurons regarded as PCs were studied electrophysiologically using the patch-clamp whole-cell configuration. Voltage-dependent, tetrodotoxin-sensitive fast inward currents were virtually absent at 2 - 4 DIV, but increased between 7 and 14 DIV to reach two-thirds of the amplitude obtained after 15 DIV. These currents were large enough to give rise to overshooting spikes as early as 7 DIV in the current-clamp mode. This time schedule is in keeping with that of PCs developed in situ. The tetraethylammonium-sensitive, slowly inactivating outward currents had reached two-thirds of the amplitude obtained after 15 DIV by 3 - 4 DIV. Their amplitude remained stable between 4 and 7 DIV, and increased to their maximal value during 7 - 14 DIV, with a marked shortening of action potentials. 4-Aminopyridine-sensitive, fast-inactivating outward currents might also be associated with development, since they were present in 66% of the cells between 7 and 14 DIV but in only 39% from 15 to 29 DIV; however, their amplitude did not vary with time. Presumptive PCs bore l-glutamate-activated receptors, which preceded the emergence of kynurenate-sensitive, spontaneous synaptic currents at 7 DIV. These currents were sometimes intermingled with inhibitory currents, although presumptive PCs were sensitive to gamma-aminobutyrate at 7 DIV. The present model represents some unequivocal features of PC development, although the PCs used had undergone minimal differentiation in vivo and were cultured in a very disturbed cellular environment.

Blocker studies of the functional architecture of the NMDA receptor channel

Blockade of ion channels passing through the NMDA receptors of isolated rat hippocampus pyramidal neurons with tetraalkylammonium compounds, 9-aminoacridine, and Mg2+ was studied using patch-clamp methods in the whole-cell configuration. Currents through NMDA channels were evoked by application of 100 microM aspartate in magnesium-free medium containing glycine (3 microM) to neurons. Analysis of the kinetics, charge transfer, and relationships between the extent of suppression of stationary currents on the one hand and membrane potential, agonist concentration, and blocker concentration on the other showed that blockers had different effects on the closing, desensitization, and agonist dissociation of NMDA channels. The size of the blocker was found to be the decisive factor determining its action on the gating functions of NMDA channels: larger blockers prevented closure and/or desensitization of the channel; smaller blockers only had partial effects on these processes, while the smallest blockers had no effect at all. These experiments showed that the apparent affinity of the blocker for the channel (1/IC50) depended not only on the microscopic equilibrium dissociation constant (Kd), but also on the number of blocker binding sites, their mutual influences, and, of particular importance, the interaction of the blocker with the gating structures of the channel. These data led us to propose hypotheses relating to the geometry of the NMDA channel and the structure of its gating mechanism. The channel diameter at the level of activated gates was estimated to be 11 A.

Quantitative analysis of tetrapentylammonium-induced blockade of open N-methyl-D-aspartate channels

The blockade of open N-methyl-d-aspartate (NMDA) channels by tetrapentylammonium (TPentA) in acutely isolated rat hippocampal neurons was studied using whole-cell patch-clamp techniques. TPentA prevented the closure of the NMDA channel following what is known as the foot-in-the-door mechanism. Hooked tail currents appearing after termination of the agonist (aspartate) and TPentA coapplication were analyzed quantitatively according to the corresponding sequential kinetic model. Studies of the hooked tail current amplitude and the degree of the stationary current inhibition dependence on the blocker concentration led to a new method for estimation of fast foot-in-the-door blocker binding/unbinding rate constants. The application of this method to the NMDA channel blockade by TPentA allowed finding the values of its binding (1.48 microM(-1)s(-1)) and unbinding (14 s(-1)) rate constants. An analysis of the dependence of the electric charge carried during the hooked tail current on the blocker concentration led to a new method for estimation of the maximum NMDA channel open probability, P(0). The value of P(0) found in experiments with TPentA was 0.04.

Two-component blocking kinetics of open NMDA channels by organic cations

NMDA receptor channel responses were recorded from acutely isolated rat hippocampal neurons, using whole-cell patch-clamp techniques. In the continuous presence of aspartate, tetraethylammonium, tetrabutylammonium, 1-amino-3-propyl-adamantane and 9-aminoacridine caused changes in the current through NMDA channels, which were described by two-exponential functions. It was established that depending on the behavior of the amplitude of the fast component for the recovery kinetics, the blocker action can be assigned to one of five types described by the simplest models. The effects of tetraethylammonium, tetrabutylammonium and 1-amino-3-propyl-adamantane were well described by these models. Using 9-aminoacridine as an example, it was shown that the simplest models cannot describe all possible types of the blocker-channel interaction. In such cases, the method of the simplest models combination can be used. The application of the simplest kinetic models analysis allowed to make the following conclusions: at least two molecules of 1-amino-3-propyl-adamantane or 9-aminoacridine can simultaneously bind to the open channel and block it; the occupation of 9-aminoacridine blocking sites in the channel can proceed in at least two different ways; the binding of tetrabutylammonium and 9-aminoacridine prevented the closure of the activation and/or desensitization gates of the channel, while that of tetraethylammonium did not.

NMDA receptor diversity in the cerebellum: identification of subunits contributing to functional receptors

Recent studies of N-methyl-D-aspartate (NMDA) receptors have led to the suggestion that there are two distinct classes of native NMDA receptors, identifiable from their single-channel conductance properties. 'High-conductance' openings arise from NR2A- or NR2B-containing receptors, and 'low-conductance' openings arise from NR2C- or NR2D-containing receptors. In addition, the low-conductance channels show reduced sensitivity to block by Mg2+. The readily identified cell types and simple architecture of the cerebellum make it an ideal model system in which to determine the contribution of specific subunits to functional NMDA receptors. Furthermore, mRNA for all of these four NR2 subunits are represented in this brain region. We have examined NMDA channels in Purkinje cells, deep cerebellar nuclei (DCN) neurons and Golgi cells. First we find that NR2D-containing NMDA receptors give rise to low-conductance openings in cell-attached recordings from Purkinje cells. The characteristic conductance of these events cannot, therefore, be ascribed to patch excision. Second, patches from some DCN neurons exhibit mixed populations of high- and low-conductance openings. Third, Golgi cells also exhibit a mixed population of high- and low-conductance NMDA receptor openings. The features of these low-conductance openings are consistent with the presence of NR2D-containing NMDA receptors, as suggested by in situ hybridization data. On the other hand the existence of high-conductance channels, with properties typical of NR2B-containing receptors, was not expected. Our results provide new evidence about the subunit composition of NMDA receptors in identified cerebellar cells, and suggest that examination of single-channel properties is a potentially powerful approach for determining the possible subunit composition of native NMDA receptors.

Analysis of excitatory and inhibitory spontaneous synaptic activity in mouse retinal ganglion cells

Spontaneous inhibitory and excitatory postsynaptic currents (sIPSCs and sEPSCs) were identified and characterized with whole cell and perforated patch voltage-clamp recordings in adult mouse retinal ganglion cells. Pharmacological dissection revealed that all cells were driven by spontaneous synaptic inputs mediated by glutamate and gamma-aminobutyric acid-A (GABAA) receptors. One-half (7/14) of the cells also received glycinergic spontaneous synaptic inputs. Both GABAA and glycine receptor-mediated sIPSCs had rise times (10-90%) of < 1 ms. The decay times of the GABAA receptor-mediated sIPSCs were comparable with those of the glycine receptor-mediated sIPSCs. The average decay time constant for monoexponentially fitted sIPSCs was 63.2 +/- 74.1 ms (mean +/- SD, n = 3278). Glutamate receptor-mediated sEPSCs had an average rise time of 0.50 +/- 0.20 ms (n = 109) and an average monoexponential decay time constant of 5.9 +/- 8.6 ms (n = 2705). Slightly more than two-thirds of the spontaneous synaptic events were monoexponential (68% for sIPSCs and 76% for sEPSCs). The remainder of the events was biexponential. The amplitudes of the spontaneous synaptic events were not correlated with rise times, suggesting that the electrotonic filtering properties of the neurons and/or differences in the spatial location of synaptic inputs could not account for the difference between the decay time constants of the glutamate and GABAA/glycine receptor-mediated spontaneous synaptic events. The amplitudes of sEPSCs were similar to those recorded in tetrodotoxin (TTX), consistent with the events measured in control saline being the response to the release of a single quantum of transmitter. The range of the sIPSC amplitudes in control saline was wider than that recorded in TTX, consistent with some sIPSCs being evoked by presynaptic spikes having an average quantal size greater than one. The rates of sIPSCs and sEPSCs were determined under equivalent conditions by recording with perforated patch electrodes at potentials at which both types of event could be identified. Two groups of ganglion cell were observed; one group had an average sEPSCs/sIPSCs frequency ratio of 0.96 +/- 0.77 (n = 28) and another group had an average ratio of 6.63 +/- 0.82 (n = 7). These findings suggest that a subset of cells is driven much more strongly by excitatory synaptic inputs. We propose that this subset of cells could be OFF ganglion cells, consistent with the higher frequency of spontaneous action potentials found in OFF ganglion cells in other studies.

Glutamate receptors in the mammalian central nervous system

Glutamate receptors (GluRs) mediate most of the excitatory neurotransmission in the mammalian central nervous system (CNS). In addition, they are involved in plastic changes in synaptic transmission as well as excitotoxic neuronal cell death that occurs in a variety of acute and chronic neurological disorders. The GluRs are divided into two distinct groups, ionotropic and metabotropic receptors. The ionotropic receptors (iGluRs) are further subdivided into three groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) receptor channels. The metabotropic receptors (mGluRs) are coupled to GTP-binding proteins (G-proteins), and regulate the production of intracellular messengers. The application of molecular cloning technology has greatly advanced our understanding of the GluR system. To date, at least 14 cDNAs of subunit proteins constituting iGluRs and 8 cDNAs of proteins constituting mGluRs have been cloned in the mammalian CNS, and the molecular structure, distribution and developmental change in the CNS, functional and pharmacological properties of each receptor subunit have been elucidated. Furthermore, the obtained clones have provided valuable tools for conducting studies to clarify the physiological and pathophysiological significances of each subunit. For example, the generation of gene knockout mice has disclosed critical roles of some GluR subunits in brain functions. In this article, we review recent progress in the research for GluRs with special emphasis on the molecular diversity of the GluR system and its implications for physiology and pathology of the CNS.

Two blocking sites of amino-adamantane derivatives in open N-methyl-D-aspartate channels

Using whole-cell patch-clamp techniques, we studied the blockade of open N-methyl-D-aspartate (NMDA) channels by amino-adamantane derivatives (AADs) in rat hippocampal neurons acutely isolated by the vibrodissociation method. The rapid concentration-jump technique was used to replace superfusion solutions. A kinetic analysis of the interaction of AAD with open NMDA channels revealed fast and slow components of their blockade and recovery. Mathematical modeling showed that these kinetic components are evidence for two distinct blocking sites of AADs in open NMDA channels. A comparative analysis of different simplest models led us to conclude that these AAD blocking sites can be simultaneously occupied by two blocker molecules. The voltage dependence of the AAD block suggested that both sites were located deep in the channel pore.

Expression and heteromeric interactions of non-N-methyl-D-aspartate glutamate receptor subunits in the developing and adult cerebellum

The localization and expression of ionotropic non-N-methyl-D-aspartate glutamate receptors (GluR) were investigated in the developing and adult rat cerebellum using subunit-specific polyclonal antibodies for immunocytochemical, immunoblot and immunoprecipitation studies. In P7 animals, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor immunoreactivity was detected in all layers of the cerebellar cortex with the exception of the external granule cell layer. Antibodies against the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunits GluR1 and GluR4 gave strong immunoreactive staining of Bergmann glia in both young and adult animals, and both antibodies showed prominent staining of the molecular layer in the adult cerebellum. Dense immunoreactive staining of Purkinje cell somata and dendrites was obtained with anti-GluR2/3/4c in both the developing and adult cerebellum. Whereas each of the three alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor antibodies stained the internal, but not the external, granule cell layer, immunostaining for the kainate-type subunits GluR6/7 and KA2 was detected in both the external and internal granule cell layers. as well as in the molecular layer in both P7 and adult cerebellum. Immunoblot analysis of total cerebellar protein indicated that the level of GluR4 expression increased 15-fold from P1 to P18, whereas the expression of the KA2 subunit protein was nine-fold lower in adult cerebellum than it was at P1. The expression of GluR1 increased moderately (two-fold) from P1 to adult. Subunit interactions between GluR1 and GluR4, as well as between GluR6/7 and KA2, were demonstrated in immunoprecipitation experiments; and the GluR4 and KA2 subunits appear to be present exclusively in heteromeric assemblies with GluR1 and GluR6/7, respectively. The results show that the various alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate- and kainate-type subunits are differentially expressed during cerebellar development and further define the possible subunit composition of non-N-methyl-D-aspartate receptors in the major cerebellar cell types.

Digital processing of the current noise evoked by kainate in cerebellar granule cells

Current induced in cultured cerebellar granule cells by the bath application of kainate (500 microM) was measured using the conventional patch-clamp technique. Two different kinds of responses were observed after the agonist perfusion. Some cells exhibited small inward whole-cell currents: 116 +/- 40 pA (7 cells) at a clamp potential of -60 mV; in other cells, the agonist induced significantly larger currents: 420 +/- 35 pA (6 cells) at a clamp potential of -60 mV. The current flowing in the agonist-activated ionic channels was indirectly estimated by processing the fluctuations of whole-cell current by means of an original parametric method. Mean conductance of the underlying channels was then determined from the single-channel current estimated at different clamp potentials. In the cells exhibiting small inward currents, the mean conductance was equal to 0.5 +/- 0.2 pS (7 cells), whereas in the cells with large inward currents it was 3 +/- 0.4 pS (6 cells). This result gives a coherent explanation of the different kinds of responses observed at macroscopic level in the whole-cell current and confirms that kainate-activated channels can exhibit different levels of conductance.

Subconductance states of a mutant NMDA receptor channel kinetics, calcium, and voltage dependence

The kinetic properties of main and subconductance states of a mutant mouse N-methyl-D-aspartate (NMDA) receptor channel were examined. Recombinant receptors made of zeta-epsilon 2 (NR1-NR2B) subunits having asparagine-to-glutamine mutations in the M2 segment (zeta N598Q/epsilon 2N589Q) were expressed in Xenopus oocytes. Single channel currents recorded from outside-out patches were analyzed using hidden Markov model techniques. In Ca(2+)-free solutions, an open receptor channel occupies a main conductance (93 pS) and a subconductance (62 pS) with about equal probability. There are both brief and long-lived subconductance states, but only a single main level state. At -80 mV, the lifetime of the main and the longer-lived sub level are both approximately 3.3 ms. The gating of the pore and the transition between conductance levels are essentially independent processes. Surprisingly, hyperpolarization speeds both the sub-to-main and main-to-sub transition rate constants (approximately 120 mV/e-fold change), but does not alter the equilibrium occupancies. Extracellular Ca2+ does not influence the transition rate constants. We conclude that the subconductance levels arise from fluctuations in the energetics of ion permeation through a single pore, and that the voltage dependence of these fluctuations reflects the modulation by the membrane potential of the barrier between the main and subconductance conformations of the pore.

Modification of NMDA receptor channels and synaptic transmission by targeted disruption of the NR2C gene

A novel strain of mutant mouse has been generated with a deletion of the gene encoding the NR2C subunit of the NMDA receptor, which is primarily expressed in cerebellar granule cells. Patch-clamp recordings from granule cells in thin cerebellar slices were used to assess the consequences of the gene deletion. In granule cells of wild-type animals, a wide range of single-channel conductances were observed (19-60 pS). The disruption of the NR2C gene results in the disappearance of low-conductance NMDA receptor channels ( < 37 pS) normally expressed in granule cells during developmental maturation. The NMDA receptor-mediated synaptic current is markedly potentiated in amplitude, but abbreviated in duration (with no net difference in total charge), and the non-NMDA component of the synaptic current was reduced. We conclude that the NR2C subunit contributes to functional heteromeric NMDA receptor-subunit assemblies at the mossy fiber synapse and extrasynaptic sites during maturation, and the conductance level exhibited by a given receptor macromolecule may reflect the stochiometry of subunit composition.

Modification of NMDA receptor channels and synaptic transmission by targeted disruption of the NR2C gene

A novel strain of mutant mouse has been generated with a deletion of the gene encoding the NR2C subunit of the NMDA receptor, which is primarily expressed in cerebellar granule cells. Patch-clamp recordings from granule cells in thin cerebellar slices were used to assess the consequences of the gene deletion. In granule cells of wild-type animals, a wide range of single-channel conductances were observed (19-60 pS). The disruption of the NR2C gene results in the disappearance of low-conductance NMDA receptor channels ( < 37 pS) normally expressed in granule cells during developmental maturation. The NMDA receptor-mediated synaptic current is markedly potentiated in amplitude, but abbreviated in duration (with no net difference in total charge), and the non-NMDA component of the synaptic current was reduced. We conclude that the NR2C subunit contributes to functional heteromeric NMDA receptor-subunit assemblies at the mossy fiber synapse and extrasynaptic sites during maturation, and the conductance level exhibited by a given receptor macromolecule may reflect the stochiometry of subunit composition.

Identification of a native low-conductance NMDA channel with reduced sensitivity to Mg2+ in rat central neurones

1. We have identified a new type of NMDA channel in rat central neurones that express mRNA for the NR2D subunit. We have examined single NMDA channels in cerebellar Purkinje cells (which possess NR1 and 2D), deep cerebellar nuclei (NR1, 2A, 2B and 2D) and spinal cord dorsal horn neurones (NR1, 2B and 2D). 2. In Purkinje cells, NMDA opened channels with a main conductance of 37.9 +/- 1.1 pS and a subconductance of 17.8 +/- 0.7 pS, with frequent transitions between the two levels. 3. NMDA activated low-conductance ('38/18 pS') events (along with high-conductance--'50/40 pS'--openings) in some patches from deep cerebellar nuclei and dorsal horn neurones. Our evidence suggests that 38/18 pS and 50/40 pS events arose from distinct types of NMDA receptors. 4. The transitions for 38/18 pS events were asymmetrical: steps from 38 to 18 pS were more frequent (72.2%) than steps from 18 to 38 pS. This feature appeared common to the 38/18 pS events in all three cell types, suggesting similarity in the low-conductance channels. 5. The 38/18 pS channels in Purkinje cells exhibited characteristic NMDA receptor properties, including requirement for glycine, antagonism by D-2-amino-5-phosphonopentanoic acid (D-AP5) and 7-chlorokynurenic acid, and voltage-dependent block by extracellular Mg2+. 6. The mean open time for the 38 pS state (0.74 +/- 0.07 ms) was significantly briefer than that for the 18 pS state (1.27 +/- 0.18 ms). 7. Mg2+ block of low-conductance NMDA channels in Purkinje cells was less marked than block of 50/40 pS channels in cerebellar granule cells. 8. The time course of appearance of 38/18 pS NMDA channels matched the expression of mRNA for the NR2D subunit. Thus 38/18 pS events were present in > 70% of Purkinje cell patches in 0- to 8-day-old animals, and absent by postnatal day 12. 9. We propose that the 38/18 pS NMDA channels identified here (associated with the NR2D subunit), and the other low-conductance NMDA channel associated with the NR2C subunit, may together constitute a functionally distinct subclass of native NMDA receptors.

Coupled Markov chain model: characterization of membrane channel currents with multiple conductance sublevels as partially coupled elementary pores

A parameterized Markov chain model is developed to represent the characteristics of channel currents that either are the superposition of many single channels or show multiple conductance sublevels. The simplified model takes the form of a set of binary chains that are interdependent according to a simple lumped coupling parameter. When varied, this parameter realizes a range of behaviors from tight coupling to complete independence. Other model parameters describe the intrinsic characteristics of the binary chains. An identification procedure for the model parameters is developed based on hidden Markov modeling ideas but incorporating a novel parameter estimation. The usefulness of the model in analyzing certain types of data is demonstrated with examples of real channel currents.

Pentobarbitone modulation of NMDA receptors in neurones isolated from the rat olfactory brain

1. The action of pentobarbitone on the N-methyl-D-aspartate (NMDA) receptors of neurones freshly dissociated from the olfactory bulb and olfactory tubercle has been studied using patch-clamp techniques. 2. Pentobarbitone produced a concentration-dependent depression of the currents evoked by NMDA with an IC50 value of c. 250 microM. 3. Analysis of the NMDA-evoked noise produced power spectra that could be fitted by the sum of two Lorentzians with corner frequencies of 17 and 82 Hz. Pentobarbitone increased the corner frequency of the high frequency component but did not alter the apparent single channel conductance estimated from the noise. 4. Single channel recordings in either the cell-attached or outside-out patch configurations revealed that NMDA (20 or 50 microM) opened channels with a main conductance level around 55 pS and a principal subconductance around 44 pS. The uncorrected mean open time of the channels was 3.4 ms and mean burst length was 6.0 ms. Mean cluster length was about 12 ms. 5. Pentobarbitone produced a concentration-dependent reduction in both mean open time and burst length. Mean cluster length was much less affected. Pentobarbitone did not decrease unitary current amplitude or bias the open-state current amplitude distribution in favour of a particular substate. 6. From these data it appears that pentobarbitone depresses the inward current evoked by NMDA by reducing the probability of channel opening and this results from a shortening of the lifetime of the channel open state and by decreasing burst length.

Analysis of synaptic transmission at single identified boutons on rat spinal neurons in culture

The spatial organization of receptor channels has a major influence on the speed and possible plasticity of synaptic signal transmission. We have studies glutamatergic synapses on neurons in organotypic cultures of rat spinal cord. In order to avoid the problems related to the analysis of currents of unknown origin within a neuron, we chose to examine the functional properties of single identified synapses. Iontophoretic mapping of the cell surface revealed hot spots of high glutamate sensitivity coincident with presynaptic boutons stained with the dye FM 1-43. Local application of KCl to these sites caused bursts of synaptic release. Hot spots typically consisted of 330 receptors with an average single-channel conductance of 8.3 pS. Evoked synaptic currents involved only about 40-50 receptors and nevertheless showed characteristics of saturation. This suggests that glutamate receptor clusters at sites of presynaptic terminals are organized into well separated subclusters opposite release sites.

Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells

1. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage-clamp conditions. Consistent with activation of non-NMDA-type glutamate receptors, the sEPSCs reversed at potentials above 0 mV, were blocked by 1 microM CNQX and prolonged by 2 mM aniracetam. 2. The peak conductance of the averaged sEPSCs (n = 70-400) was 130 +/- 60 pS (mean +/- S.D.; 17 cells, ranging from 70 to 290 pS). Amplitude distributions were skewed towards larger amplitudes. 3. The decay of individual and mean sEPSCs was exponential with a mean time constant (tau d) of 3.75 +/- 0.84 ms (n = 13), which was voltage independent. The 10-90% rise time of the sEPSCs was 1.30 +/- 0.44 ms (n = 13). There was no correlation between sEPSC rise time and tau d suggesting that dendritic filtering alone did not shape the time course of sEPSCs. 4. Light-evoked EPSCs in these retinal ganglion cells are mediated by concomitant activation of NMDA and non-NMDA receptors; however, no NMDA component was discerned in the sEPSCs, even when recording at -96 mV in Mg(2+)-free solutions. The decay time course was not altered by 20 microM AP7, an NMDA antagonist, nor was an NMDA component unmasked by adding glycine or D-serine. These results suggest that NMDA and non-NMDA receptors are not coactivated by a single vesicle of transmitter during spontaneous release, and thus are probably not colocalized in the postsynaptic membrane at the sites of spontaneous release. 5. The sEPSCs were an order of magnitude faster than the non-NMDA receptor-mediated EPSCs evoked by light stimuli, and it is proposed that the EPSC time course is determined largely by the extended time course of release of synaptic vesicles from bipolar cells. The quantal content of a light-evoked non-NMDA receptor-mediated EPSC in an on-off cell is about 200 quanta.

Ion permeation properties of the cloned mouse epsilon 2/zeta 1 NMDA receptor channel

The heteromeric mouse epsilon 2/zeta 1 N-methyl-D-aspartate (NMDA) receptor was expressed in Xenopus oocytes, and its channel properties were studied using both the outside-out patch-clamp and two-microelectrode voltage-clamp techniques. In the cloned receptor channel, permeation properties of monovalent and divalent cations, and voltage-dependent block by Mg2+ were similar to those reported previously in the native NMDA receptor channels. The sequence of single-channel conductances for alkali metals was Rb+ > Cs+ approximately K+ > Na+ > Li+, whereas the sequence of relative permeabilities was Cs+ > Rb+ > K+ approximately Na+ > Li+. The single-channel conductances measured in isotonic Ca2+, Sr2+ and Ba2+ solutions were almost equal, and approximately one-fifth of the value in the Na+ solution, although the permeabilities for these alkaline earth cations were higher than for Na+. It is likely that Ca2+, Sr2+ and Ba2+ would enter the NMDA receptor channel more easily than Na+, but would bind to a site in the channel more tightly, the net effect being a reduced value of the current.

A comparison of non-NMDA receptor channels in type-2 astrocytes and granule cells from rat cerebellum

1. Patch-clamp recording methods have been used to compare the pharmacological properties and single-channel characteristics of non-NMDA receptor channels in cerebellar type-2 astrocytes and granule cells. 2. In type-2 astrocytes whole-cell concentration-response curves for glutamate, quisqualate, AMPA and kainate gave EC50 values of 5.8, 3.8, 7.6 and 160 microM and Hill slopes of 1.65, 1.18, 1.64 and 1.65, respectively, resembling estimates for granule cell receptors. 3. The non-NMDA receptor antagonists CNQX and diCl-HQC (see Methods) inhibited whole-cell kainate currents in both cell types. The IC50 for CNQX antagonism of the kainate response was 536 nM in type-2 astrocytes, and 500 nM in granule cells. The IC50 for diCl-HQC was 3.5 microM in astrocytes and 3.7 microM in granule cells. 4. CNQX acted as a competitive antagonist of whole-cell kainate responses in type-2 astrocytes and granule cells giving Schild plots with a slope near 1. The equilibrium constant, K, for CNQX binding was 524 nM in astrocytes and 489 nM in granule cells. 5. Quisqualate and AMPA responses showed rapid desensitization in type-2 astrocytes with a ratio of steady-state to peak response of 0.09. Concanavalin A reduced this desensitization. 6. Non-NMDA channels in type-2 astrocytes and granule cells showed a low permeability to Ca2+ ions with a reversal potential, for kainate-activated whole-cell currents in isotonic Ca2+, of approximately -25 mV for astrocytes and -45 mV for granule cells. 7. Outside-out patches from type-2 astrocytes exhibited a range of single-channel conductances that were superficially similar to the glutamate-activated conductances in granule cells. However, the type-2 astrocytes were devoid of NMDA receptors, hence all of these conductances originated from non-NMDA channels. Their slope conductances were approximately 11, 21, 32, 42 and 52 pS. Amplitudes were verified with mean low-variance plots and single-channel current-voltage curves, which were linear. 8. There was also evidence of lower conductance kainate-activated channels in astrocyte patches. From noise analysis their estimated mean conductance was 1.9 pS, as described for the 'low-conductance' type kainate responses in cerebellar neurones. 9. Apparent open times, shut times and burst lengths of AMPA-activated (3-10 microM) channels were examined in patches from type-2 astrocytes, and kinetic properties of the 40 and 50 pS levels were compared with the lower levels. 10. Our results indicate some marked pharmacological similarities between non-NMDA receptor channels in type-2 astrocytes and granule cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Facilitatory effect of docosahexaenoic acid on N-methyl-D-aspartate response in pyramidal neurones of rat cerebral cortex

1. The effect of docosahexaenoic acid (DHA) on N-methyl-D-aspartic acid (NMDA) responses in the presence of glycine was investigated in pyramidal neurons acutely dissociated from rat cerebral cortex in whole-cell and single channel configurations. 2. DHA potentiated the NMDA-induced response but reduced the non-NMDA (kainate-induced) response in a concentration-dependent manner at a holding potential of -60 mV under voltage-clamp conditions. 3. Arachidonic acid (AA) also potentiated the NMDA-induced response in a manner similar to DHA. Oleic acid caused a slight potentiation. However, other polyunsaturated and saturated fatty acids had no such effects. 4. The facilitatory action of DHA on the NMDA-induced response was not affected by adding inhibitors of cyclo-oxygenase, lipoxygenase or phospholipase A2, suggesting that DHA may exert its facilitatory effect directly on the NMDA receptor. 5. The facilitatory action of DHA was observed in the presence of a saturating dose of NMDA. Moreover, a detailed analysis of the NMDA receptor-operated single channel currents revealed that, in the presence of DHA, the open probability of the channel increased without changing the conductance, indicating that DHA may act by binding directly to a novel site on the NMDA receptor or by altering the lipid environment of the NMDA receptor and thereby potentiating the response to NMDA. 6. The results are discussed in terms of the possibility that DHA may play an important role in the genesis of long-term potentiation, at least that involving the activation of NMDA receptors.

Modelling the cerebellar Purkinje cell: experiments in computo

Detailed compartmental models of neurons are useful tools for investigating neuronal properties and mechanisms that are not accessible to experimental procedures. If a rigorous approach is used in building the model, simulation studies can be as valuable as laboratory experimentation. As such, modelling becomes an additional method for exploring the function of neurons and nervous systems. As an example, a complex compartmental model with active dendritic membrane of a Purkinje cell is described. The response properties of the model to parallel fiber inputs were investigated. The model fired simple spikes in patterns comparable with those recorded from Purkinje cells in vivo. Synchronous activation of only 20 granule cell inputs was sufficient to generate a measurable response in simulated peri-stimulus histograms. This sensitivity to small excitatory inputs was caused by P-type Ca2+ channels in the dendritic membrane. Such P channels may also be present in the spine heads. Simulations suggest, however, that Ca2+ channels in spine heads cannot be activated by single parallel fiber inputs.

Glutamate receptor-mediated currents and toxicity in embryonal carcinoma cells

While primary neuronal cell cultures have been used to investigate excitotoxicity, development of cell lines exhibiting glutamate receptor-mediated death is desirable. P19 mouse embryonal carcinoma cells, exposed to retinoic acid and plated onto a layer of cultured mouse cortical glial cells, differentiated into neuron-like elements immunoreactive for neurofilaments and neuron-specific enolase. Whole-cell recordings revealed inward currents in response to extracellular application of either NMDA or kainate. The NMDA-induced currents exhibited a voltage-dependent blockade by magnesium, required glycine for maximal activation, and were blocked by the NMDA antagonist dizocilpine. Kainate-induced currents were blocked by the AMPA/kainate receptor antagonist CNQX. Exposure to 500 microM NMDA for 24 h destroyed most P19 cells (EC50 approximately 70 microM); death was prevented by dizocilpine or D-APV. Exposure to 500 microM kainate also resulted in widespread death reduced by CNQX. Thus differentiated P19 cells exhibited both excitatory amino acid responses and vulnerability to excitotoxicity, characteristic of CNS neurons. These cells may provide a genetically open system useful for studying glutamate receptor-mediated phenomena at a molecular level.

Estimated conductance of glutamate receptor channels activated during EPSCs at the cerebellar mossy fiber-granule cell synapse

We have analyzed the variance associated with the decay of the non-NMDA receptor component of synaptic currents, recorded from mossy fiber-granule cell synapses in cerebellar slices, to obtain a conductance estimate for the synaptic channel. Current fluctuations arising from the random channel gating properties were separated from those arising from the fluctuations in the population of channels by subtracting the mean excitatory postsynaptic current (EPSC) waveform scaled to the EPSC peak amplitude. A weighted mean single-channel conductance of approximately 20 pS was determined from the relationship between the mean current and the variance around the mean during the decay of evoked and spontaneous synaptic currents. This result suggests that high conductance non-NMDA channels, such as the 10-30 pS glutamate receptor channel previously characterized in granule cells, carry the majority of the fast component of the EPSC at this synapse. In addition, our data are consistent with the activation of surprisingly few (approximately 10) non-NMDA channels by a single packet of transmitter.

The modulation of N-methyl-D-aspartate receptors by redox and alkylating reagents in rat cortical neurones in vitro

1. The properties of sulfhydryl redox modulation of the N-methyl-D-aspartate (NMDA) receptor have been examined in rat cortical neurones in culture. Electrophysiological measurements were performed with the whole-cell and outside-out patch variants of the patch-clamp technique. 2. The disulphide reducing agent dithiothreitol (DTT; 0.1-10 mM) potentiated 10 microM NMDA-mediated whole-cell currents when applied slowly alone via the superfusate. The initial rate of reduction, as well as the degree of potentiation, was dependent on the concentration of DTT although the process was complicated by the fact that a second, large component appeared at a concentration of 10 mM of this agent. 3. DTT (0.1-10 mM) was also rapidly applied together with the agonist from a perfusion pipette. With this method, the second component was not readily apparent, and the concentration of DTT producing a half-maximal potentiation of the NMDA response was 1.9 +/- 0.3 mM. Two other disulphide reducing agents, ethylene glycol bisthioglycolate and meso-bis(N,N-dimethyl)adipamide-2,5-dithiol, also potentiated NMDA responses, but were not as effective as DTT. 4. Following a 4 mM DTT treatment, we observed that the NMDA receptor underwent spontaneous oxidation with a half-time of 1.9 min. In contrast, the sulfhydryl oxidizing agent 5,5'-dithio-bis-(2-nitro-benzoic acid) (DTNB; 500 microM) produced a more rapid reversal of the effects of DTT (t1/2 = 0.6 min). The spontaneously oxidized receptor could be further oxidized with DTNB and fully reduced with DTT. 5. After receptor oxidation with 500 microM DTNB, NMDA produced whole-cell responses with an EC50 of 68.4 +/- 9.4 microM, whereas after reduction with 4 mM DTT the EC50 for NMDA was 32.5 +/- 3.4 microM. In addition, the maximum response after reduction with DTT was substantially increased over that observed after oxidation. 6. Single channel measurements performed on outside-out patches revealed that reduction produced a dramatic increase in the number of NMDA-induced channel openings. We observed a 2.1 +/- 0.2-fold increase in the frequency of openings during reduction with 500 microM DTT when compared to patches which had been exposed to 500 microM DTNB. Small but significant differences were observed in the single channel conductance for the oxidized (34.6 +/- 1.1 pS) and reduced (37.6 +/- 1.5 pS) states of the receptor. In contrast, no significant changes were seen in the arithmetic mean channel open time between the two redox conditions (5.4 +/- 0.3 ms after oxidation, 6.0 +/- 0.7 ms after reduction).(ABSTRACT TRUNCATED AT 400 WORDS)

Use-dependent pentobarbital block of kainate and quisqualate currents

The effects of pentobarbital on whole-cell excitatory amino acid-induced currents were studies in cultured rat cortical neurons. Currents evoked by 40 microM kainate were reversibly inhibited by pentobarbital with an IC50 value of 50 microM. The block of the kainate response by pentobarbital was use dependent, requiring kainate stimulation. In the absence of kainate activation, 10 min perfusions of 100 microM pentobarbital inhibited kainate-induced currents less than 10%. Recovery from pentobarbital block also exhibited use dependence, reversing in 5-10 s with kainate stimulation, while persisting 10 min or more in the absence of agonist. Pentobarbital inhibition of the kainate response was not voltage dependent. Responses evoked by 10 microM quisqualate consisted of a peak current desensitizing to a smaller steady-state current. The co-application of 100 microM pentobarbital reduced the steady-state current by 49 +/- 5%. The peak current before desensitization, however, was inhibited less than 10%. Currents evoked by 25 microM N-methyl-D-aspartate were not significantly inhibited by co-application of 100 microM pentobarbital. The results suggest that the pentobarbital-induced inhibition of kainate responses involves open channel block and that the block of quisqualate currents primarily involve non-desensitizing receptor channels that generate steady-state currents.

Evidence for more than one type of non-NMDA receptor in outside-out patches from cerebellar granule cells of the rat

1. Application of non-NMDA (non-N-methyl-D-aspartate) receptor agonists onto outside-out patches of cerebellar granule cells gave two characteristic types of response (in different patches) which we have referred to as 'high conductance' and 'low conductance' responses. At a qualitative level these patches could be readily distinguished by the size of the noise increase accompanying their membrane currents. 2. In high conductance patches both AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) and kainate gave discrete single-channel conductances (10-30 pS), while in low conductance patches, AMPA produced small discrete events (6-10 pS), and kainate opened channels with conductances too small to be directly resolved. All patches examined contained NMDA receptor channels with characteristic 50 and 40 pS conductance levels. 3. Despite the marked differences in single-channel conductances, kainate dose-response curves constructed for high and low conductance patches had similar EC50 values of approximately 150 microM. 4. Spectral analysis of low conductance kainate responses gave an estimated channel conductance of approximately 1.5 pS. In these same low conductance patches AMPA produced discrete openings with two conductance levels; their mean conductances (and relative proportions) were 6 (87%) and 10 pS (13%). 5. In high conductance patches, glutamate (10-30 microM), AMPA (3-10 microM), and kainate (10-30 microM), each activated non-NMDA channels with three multiple conductance levels. The amplitudes of these conductance levels (approximately 10, 20 and 30 pS) were similar for each of the agonists, and their relative proportions (i.e. areas in the amplitude histograms) were constant for all three agonists. In addition, the relative proportion of levels was constant between patches, and all three levels were invariably present. These observations are all consistent with the idea that the three multiple conductances originate from a single receptor channel, activated by AMPA, kainate and glutamate. 6. Non-NMDA single-channel current-voltage (I-V) plots showed outward rectification in high conductance patches. For all three multiple conductance levels the ratio of outward to inward single-channel slope conductance was 1.8 +/- 0.1 and this rectification remained present in symmetrical Na+ solutions. 7. In high conductance patches, the events produced by a rapid application of 20-50 microM glutamate were compared with those activated during steady-state application.(ABSTRACT TRUNCATED AT 400 WORDS)

Single channel properties at the synaptic site

The properties of single channels activated during spontaneous postsynaptic currents in small cultured rat hippocampal neurons were investigated in low-noise whole cell recordings. The technique of nonstationary fluctuation analysis, which has previously been applied to sodium currents, was modified so that fluctuations were measured around the least-squares scaled fit of the ensemble average to individual synaptic currents. This had the effect of separating channel gating fluctuations from the quantal fluctuations of scale from event to event. Single channel amplitude was estimated from the variance--current distribution, and the kinetics of channel gating fluctuations were studied. Channels involved in the non-N-methyl-D-aspartate (non-NMDA) phase of the excitatory glutamatergic postsynaptic current showed a single channel amplitude of 1.5 pS, while those in the NMDA phase had a conductance of 42 pS. The single channel conductance estimated for the inhibitory chloride synaptic current was 14 pS. In addition, NMDA phase channel openings could be resolved directly in the whole cell current against the low noise level afforded by the small cells. Single channel lifetime and amplitude distributions of the channels activated during the postsynaptic current were measured, and confirmed the accuracy of the fluctuation method.

Single-channel conductances of NMDA receptors expressed from cloned cDNAs: comparison with native receptors

To cast light on the subunit composition of native NMDA-type glutamate receptors, four cloned subunits of the NMDA receptor have been expressed, in pairs, in Xenopus oocytes, and their single-channel properties have been measured. The conductances of the channels, and their characteristic patterns of sublevel transitions, turn out to be useful diagnostic criteria for subunit composition. The NR1-NR2A and NR1-NR2B combinations (which have identical TM2 sequences) are very similar to each other. Both have 50 pS openings and brief 40 pS sublevels (in 1 mM external Ca2+), with similar mean lifetimes and frequencies. They also show close quantitative resemblance to the channels of hippocampal CA1 and dentate gyrus cells and of cerebellar granule cells, except that the NR1-NR2A combination has a lower glycine sensitivity than the native channels. In contrast, the NR1-NR2C combination produces a channel with 36 pS and 19 pS conductances of similar (brief) duration; these closely resemble the 38-18 pS channels that have previously been observed in large cerebellar neurons in culture (together with 50 pS channels).

The action of anaesthetics on stimulus-secretion coupling and synaptic activity

1. Anaesthetics are known to depress excitatory synaptic transmission and the mechanism of this inhibition has been investigated using bovine adrenal chromaffin cells as an experimental model. 2. These cells are homologous with post-ganglionic sympathetic neurons and have well characterized receptor and secretory mechanisms. They are amenable both to the direct measurement of evoked secretion with its associated ion fluxes, and to electrophysiological investigation using the patch clamp technique. 3. These approaches have been used to study the influence of anaesthetics on pre- and post-synaptic mechanisms involved in stimulus-secretion coupling. 4. A variety of agents inhibited secretion evoked by direct depolarization, and this was shown to be due to a reduction in calcium influx. 5. Direct inhibition of voltage-gated calcium currents was confirmed by whole-cell patch clamp measurements. 6. In addition, anaesthetics powerfully modulated nicotinic receptor mediated events: carbachol-evoked secretion was more sensitive to anaesthetics than that stimulated by high potassium. 7. The mechanism of anaesthetic action on the nAChR was examined in more detail with patch-clamp experiments. 8. These showed that anaesthetics reduced the probability of channels being in the open state, largely as a result of reduction in mean channel open time. 9. The data are discussed in relation to excitatory synaptic transmission.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).

Ligand-activated ion channels may share common gating mechanisms with the Shaker potassium channel

This paper proposes a detailed gating mechanism for the N-methyl-D-aspartate (NMDA) channel. In the NMDAR1 subunit, the signal of agonist binding may be carried from Y456 to W590 through an electron transport chain, including W480 which could be the glycine modulatory site. The channel's opening may arise from repulsion between negatively charged W590s, analogous to W435s of the Shaker K+ channel. The cyclic nucleotide-gated channels may be activated by a similar mechanism, but the opening of nicotinic acetylcholine receptor (nAChR) channels is likely to be initiated by the formation of tyrosine radicals. The role of disulfide-bonded cysteines in the redox modulation can also be explained.

Activation of N-methyl-D-aspartate receptors by L-glutamate in cells dissociated from adult rat hippocampus

1. Single channel recording techniques were used to study the ion channel openings resulting from activation of N-methyl-D-aspartate (NMDA) receptors by the agonist glutamate. Patches were from cells acutely dissociated from adult rat hippocampus (CA1). Channel activity was studied at low glutamate concentrations (20-100 nM) with 1 microM-glycine, in the absence of extracellular divalent cations. 2. Channel openings were to two main conductance levels corresponding to 50 pS and 40 pS openings in extracellular solution with 1 mM-Ca2+. Around 80% of openings were to the large conductance level. The single channel conductances increased as extracellular Ca2+ was reduced. 3. Distributions of channel open times were described by three exponential components of 87 microseconds, 0.91 ms and 4.72 ms (relative areas of 51, 31 and 18%). Most long openings were to the large conductance level. 4. The channel closed time distribution was complex, requiring five exponential components to describe it adequately. Of these five components, at least three, with time constants of 68 microseconds, 0.72 ms and 7.6 ms (relative areas of 38, 12 and 17%) represent gaps within single activations of the receptor. The presence of a component with a mean of 7.6 ms is notable because gaps of this length have not previously been identified as being within single NMDA receptor channel activations. 5. Channel activations were identified as including gaps underlying at least the first three closed time components. Activations consisted of clusters of channel openings. Distributions of the length of these clusters had mean time constants of 88 microseconds, 3.4 ms and 32 ms (relative areas of 45, 25 and 30%). Long clusters contained short, intermediate and long duration openings as well as subconductance openings. The open probability within clusters averaged 0.62. Three components were evident in distributions of the number of openings per cluster. These had mean values of 1.22, 3.2 and 11 openings per cluster. 6. An inverse correlation was evident between the length of adjacent open and closed times. When open intervals were separated into groups based on the length of adjacent gaps, the time constants of the exponential components in these conditional open time distributions were independent of the length of the adjacent gap. This supports the idea that the NMDA receptor channel gating has the properties of a discrete Markov process. 7. The long duration of NMDA receptor channel clusters suggests that they contribute to the slow time course of the NMDA receptor-mediated synaptic current.

The polyamine diaminodecane (DA-10) produces a voltage-dependent flickery block of single NMDA receptor channels

Receptor binding assays have shown that diaminodecane (DA-10) reduced binding of open channel blockers to the N-methyl-D-aspartate (NMDA) subtype of postsynaptic glutamate receptor through an interaction with the polyamine regulatory site. Because the action of DA-10 was opposite to that of the polyamine agonist spermine and was reversed by polyamine antagonists, DA-10 has been classified as an inverse agonist at the polyamine site. Using whole-cell voltage-clamp and single-channel recordings from cultured rat cortical neurons, we show that at negative holding potentials DA-10 (1-300 microM) reduced NMDA receptor whole cell current (IC50 = 34 microM) and produced a flickery block of NMDA single-channel currents. The flickery block of NMDA single channels was voltage-dependent and not reversed by the polyamine antagonist diethylenetriamine (DET). Potential mechanisms for the flickery block of NMDA single channel currents are discussed.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Cooperative gating of chloride channel subunits in endothelial cells

New methods are described to detect subconductance levels and to analyse ion channel gating. These methods are applied to simulated and experimental data. Single chloride channel records from inside-out membrane patches excised from human umbilical venous endothelial cells (HUVEC) exhibit, in addition to the full closed and full open configurations, intermediate subconductance levels which are multiple of an elementary conductance of 112.5 pS. Analysis of transitions from one state to another and the comparison of real data with simulated data leads to the proposal of a cooperative model of gating for the observed subunits of a chloride channel.

A novel modulatory binding site for zinc on the GABAA receptor complex in cultured rat neurones

1. The properties of gamma-aminobutyric acidA (GABAA) receptor-ion channel complexes and the interaction with the transition metal zinc, were studied on rat sympathetic and cerebellar neurones in dissociated culture using patch clamp recording techniques. 2. The antagonism of GABA-induced membrane currents by zinc on sympathetic neurones was subject to developmental influence. Using embryonic sympathetic neurones acutely cultured for 24-72 h, GABA responses were more depressed by zinc when compared to responses evoked on adult neurones cultured for the same period. For neurones developing in vivo, the percentage inhibition of GABA responses produced by zinc in embryonic neurones was estimated to decline by 50% after 48.2 days following birth. 3. Embryonic sympathetic neurones maintained in culture for prolonged periods (40-50 days in vitro, DIV) became less sensitive to zinc when compared to neurones cultured for shorter periods (10-20 DIV). The decrease in the zinc inhibition for neurones maintained in vitro proceeded at an apparent rate of 0.55% per day. 4. Activation of the GABA receptor by muscimol (0.2-2 microM) was also antagonized by zinc (50-100 microM). 5. Lowering the pH of the perfusing Krebs solution did not affect the inhibition of GABA responses by zinc on sympathetic neurones. 6. Modulation of the GABAA receptor by some benzodiazepines, a barbiturate, a steroid based on pregnanolone, or antagonists bicuculline and picrotoxinin, did not interfere with the antagonism exerted by zinc on sympathetic neurones. A novel binding site for zinc on the GABAA receptor is proposed. 7. Analysis of the GABA-activated current noise on sympathetic neurones revealed two kinetic components to the power spectra requiring a double Lorentzian fit. The time constant describing the fast component (tau 2, 2.1 ms) was unaffected by zinc, whereas the slow component time constant (tau 1, 21.7 ms) was slightly reduced to 17.1 ms. 8. The apparent single-channel conductance for GABA-activated ion channels was determined from the power spectra (gamma s = 22.7 pS) and also from the relationship between the mean GABA-induced inward current and the variance of the current (gamma v = 24 pS). Zinc (25-100 microM) did not affect the single-channel conductance. 9. Single GABA-activated ion channels were recorded from outside-out patches taken from the soma of large cerebellar neurones. Single GABA channels were capable of activation to multiple current amplitudes which were assessed into the following conductance levels: 8, 18, 23, 29 and 34 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Uneven distribution of excitatory amino acid receptors on ventral horn neurones of newborn rat spinal cord

1. The distribution of excitatory amino acid receptors on ventral horn neurones was investigated using slices of newborn rat spinal cord. 2. The neurone and the tip of the pipette used to inject amino acids were visualized using Lucifer Yellow under a fluorescent microscope. The pipette was precisely located on the soma and dendrite of the neurone under visual control, and L-glutamate (Glu), L-aspartate (Asp), N-methyl-D-aspartate (NMDA), kainate (KA) and quisqualate (Quis) were ionophoretically applied with a short pulse. The potential changes were intracellularly recorded from the soma. 3. Sensitivity to Glu as tested with short pulses (1-2 ms) was almost the same at the soma and along dendrites. 4. The amplitude of the responses to NMDA produced at the soma and the proximal part of the dendrite was about the same as that of Glu, but smaller than that of Glu at the distal part of the dendrite. Suppression of the Glu potential by an NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), was greater at the soma than at the dendrite, suggesting that the contribution of NMDA receptors to the Glu potential was greater at the soma. 5. Sensitivity to Asp was about one-half that to Glu sensitivity on the soma and even less on the dendrite. Sensitivity to KA was high at the soma and low at the dendrite. However, Quis responses were produced throughout the neurone. 6. The Quis response induced by the application of a short pulse showed two phases: a fast response followed by a very slow depolarization that lasted more than 10 s. 7. The fast Quis response was easily desensitized and insensitive to APV. The time course of the fast Quis potential was shorter than that of Glu. 8. The slow Quis response was more pronounced at the dendrites than at the soma and was reduced by the intracellular injection of EGTA, suggesting the contribution of Ca2+ in the cell, possibly mediated by a second messenger system. 9. Experimental results suggest that the distribution of excitatory amino acid receptors differs between the soma and the dendrites of spinal neurones.

Excitatory amino acid receptor-channels in Purkinje cells in thin cerebellar slices

Glutamate receptors of the N-methyl-D-aspartate (NMDA) and non-NMDA type serve different functions during excitatory synaptic transmission. Although many central neurons bear both types of receptor, the evidence concerning the sensitivity of cerebellar Purkinje cells to NMDA is contradictory. To investigate the receptor types present in Purkinje cells, we have used whole-cell and outside-out patch-clamp methods to record from cells in thin cerebellar slices from young rats. At a holding potential of -70 mV (in nominally Mg(2+)-free medium, with added glycine) NMDA caused a whole-cell current response which consisted of a dramatic increase in the frequency of synaptic currents. In the presence of tetrodotoxin (TTX) and the gamma-aminobutyric acidA (GABAA) receptor antagonist bicuculline, spontaneous synaptic currents and responses to NMDA were inhibited. In a proportion of cells a small polysynaptic response to NMDA persisted, which was further reduced by the non-NMDA receptor antagonist 6-cyano-2,3-dihydro-7-nitroquinoxalinedione (CNQX). The non-NMDA glutamate receptor agonists kainate (KA), quisqualate (QA) and s-alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (s-AMPA), evoked large inward currents due to the direct activation of receptors in Purkinje cells. NMDA applied to excised membrane patches failed to evoke any single-channel currents, whereas s-AMPA and QA caused small inward currents accompanied by marked increases in current noise. Spectral analysis of the s-AMPA noise in patches gave an estimated mean channel conductance of approximately 4 pS.(ABSTRACT TRUNCATED AT 250 WORDS)

Subconductance states in calcium-activated potassium channels from canine airway smooth muscle

The single-channel patch clamp technique was used to analyze subconductance states in the 260 pS calcium-activated potassium channel from canine airway smooth muscle. More than sixty minutes of single channel data (greater than 87,000 events) from five excised patches were analyzed. Six subconductance amplitudes were clearly established to be 17, 33, 41, 52, 63 and 72% of the full conductance. Subconductance openings were usually brief (milliseconds) and represented less than 5% of the total channel open time, but they also persisted for several seconds on rare occasions. They appeared to be unaffected by voltage or time after seal formation, but may have increased in occurrence with decreasing calcium concentration. Irregular amplitude intervals, and the presence of ramp-like, analog transitions between conductance states, suggest a model for maxi-K subconductance states in which the channel protein undergoes random conformational changes causing a variable pore size.

Kinetic analysis of interactions between kainate and AMPA: evidence for activation of a single receptor in mouse hippocampal neurons

AMPA but not kainate produces a rapidly desensitizing response in mouse hippocampal neurons. The characteristic action of these agonists appears to arise from activation of a single receptor with active and desensitized states, for which AMPA and kainate have different relative affinity. The equilibrium potency of a series of five agonists that produce rapidly desensitizing responses at non-NMDA receptors (EC50 1 microM to 4 mM) was similar to their equilibrium potency for block of kainate responses. Increasing the concentration of kainate overcame such block, but in the presence of AMPA the rate of activation of responses to kainate was slowed. Conversely, in the presence of kainate the amplitude of rapidly desensitizing responses evoked by AMPA was reduced, and the rate of onset of desensitization was slowed.