Whole-cell current noise produced by excitatory and inhibitory amino acids in large cerebellar neurones of the rat

Glutamate Activates AMPA Receptor Conductance in the Developing Schwann Cells of the Mammalian Peripheral Nerves

Schwann cells (SCs) are myelinating cells of the PNS. Although SCs are known to express different channels and receptors on their surface, little is known about the activation and function of these proteins. Ionotropic glutamate receptors are thought to play an essential role during development of SC lineage and during peripheral nerve injury, so we sought to study their functional properties. We established a novel preparation of living peripheral nerve slices with preserved cellular architecture and used a patch-clamp technique to study AMPA-receptor (AMPAR)-mediated currents in SCs for the first time. We found that the majority of SCs in the nerves dissected from embryonic and neonatal mice of both sexes respond to the application of glutamate with inward current mediated by Ca²⁺-permeable AMPARs. Using stationary fluctuation analysis (SFA), we demonstrate that single-channel conductance of AMPARs in SCs is 8-11 pS, which is comparable to that in neurons. We further show that, when SCs become myelinating, they downregulate functional AMPARs. This study is the first to demonstrate AMPAR-mediated conductance in SCs of vertebrates, to investigate elementary properties of AMPARs in these cells, and to provide detailed electrophysiological and morphological characterization of SCs at different stages of development.SIGNIFICANCE STATEMENT We provide several important conceptual and technical advances in research on the PNS. We pioneer the first description of AMPA receptor (AMPAR)-mediated currents in the PNS glia of vertebrates and provide new insights into the properties of AMPAR channels in peripheral glia; for example, their Ca²⁺ permeability and single-channel conductance. We describe for the first time the electrophysiological and morphological properties of Schwann cells (SCs) at different stages of development and show that functional AMPARs are expressed only in developing, not mature, SCs. Finally, we introduce a preparation of peripheral nerve slices for patch-clamp recordings. This preparation opens new possibilities for studying the physiology of SCs in animal models and in surgical human samples.

Increased cell-intrinsic excitability induces synaptic changes in new neurons in the adult dentate gyrus that require Npas4

Electrical activity regulates the manner in which neurons mature and form connections to each other. However, it remains unclear whether increased single-cell activity is sufficient to alter the development of synaptic connectivity of that neuron or whether a global increase in circuit activity is necessary. To address this question, we genetically increased neuronal excitability of in vivo individual adult-born neurons in the mouse dentate gyrus via expression of a voltage-gated bacterial sodium channel. We observed that increasing the excitability of new neurons in an otherwise unperturbed circuit leads to changes in both their input and axonal synapses. Furthermore, the activity-dependent transcription factor Npas4 is necessary for the changes in the input synapses of these neurons, but it is not involved in changes to their axonal synapses. Our results reveal that an increase in cell-intrinsic activity during maturation is sufficient to alter the synaptic connectivity of a neuron with the hippocampal circuit and that Npas4 is required for activity-dependent changes in input synapses.

Modifying spiking precision in conductance-based neuronal models

The temporal precision of a neuron's spiking can be characterized by calculating its "jitter," defined as the standard deviation of the timing of individual spikes in response to repeated presentations of a stimulus. Sub-millisecond jitters have been measured for neurons in a variety of experimental systems and appear to be functionally important in some instances. We have investigated how modifying a neuron's maximal conductances affects jitter using the leaky integrate-and-fire (LIF) model and an eight-conductance Hodgkin-Huxley type (HH8) model. We observed that jitter can be largely understood in the LIF model in terms of the neuron's filtering properties. In the HH8 model we found the role of individual conductances in determining jitter to be complicated and dependent on the model's spiking properties. Distinct behaviors were observed for populations with slow (<11.5 Hz) and fast (>11.5 Hz) spike rates and appear to be related to differences in a particular channel's activity at times just before spiking occurs.

Tonic NMDA receptor-mediated current in prefrontal cortical pyramidal cells and fast-spiking interneurons

Tonically activated neuronal currents mediated by N-methyl-d-aspartate receptors (NMDARs) have been hypothesized to contribute to normal neuronal function as well as to neuronal pathology resulting from excessive activation of glutamate receptors (e.g., excitotoxicity). Whereas cortical excitatory cells are very vulnerable to excitotoxic insult, the data regarding resistance of inhibitory cells (or interneurons) are inconsistent. Types of neurons with more pronounced tonic NMDAR current potentially associated with the activation of extrasynaptic NMDARs could be expected to be more vulnerable to excessive activation by glutamate. In this study, we compared tonic activation of NMDARs in excitatory pyramidal cells and inhibitory fast-spiking interneurons in prefrontal cortical slices. We assessed tonic NMDAR current by measuring holding current shift as well as noise reduction following NMDAR blockade after removal of spontaneous glutamate release. In addition, we compared NMDAR miniature excitatory postsynaptic currents (EPSCs) in both cell types. We have demonstrated for the first time that tonic NMDAR currents are present in inhibitory fast-spiking interneurons. We found that the magnitude of tonic NMDAR current is similar in pyramidal cells and fast-spiking interneurons, and that quantal release of glutamate does not significantly impact tonic NMDAR current.

Modulation of proton-induced current fluctuations in the human nicotinic acetylcholine receptor channel

The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that switches upon activation from a closed state to a full conducting state. We found that the mutation delta S268K, located at 12' position of the second transmembrane domain of the delta subunit of the human nAChR generates a long-lived intermediate conducting state, from which openings to a wild-type like conductance level occur on a submillisecond time scale. Aiming to understand the interplay between structural changes near the 12' position and channel gating, we investigated the influence of various parameters: different ligands (acetylcholine, choline and epibatidine), ligand concentrations, transmembrane voltages and both fetal and adult nAChRs. Since sojourns in the high conductance state are not fully resolved in time, spectral noise analysis was used as a complement to dwell time analysis to determine the gating rate constants. Open channel current fluctuations are described by a two-state Markov model. The characteristic time of the process is markedly influenced by the ligand and the receptor type, whereas the frequency of openings to the high conductance state increases with membrane hyperpolarization. Conductance changes are discussed with regard to reversible transfer reaction of single protons at the lysine 12' side chain.

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.

GABA-Induced Cl- current in cultured embryonic human dorsal root ganglion neurons

gamma-Aminobutyric acid (GABA)-activated channels in embryonic (5-8 wk old) human dorsal root ganglion (DRG) neurons in dissociated culture were characterized by whole cell and single-channel techniques. All DRG neurons when held at negative holding membrane potentials displayed inward current to micromolar concentrations of GABA applied by pressure pulses from closely positioned micropipettes. The current was directly proportional to the concentration of GABA (EC50, 111 microM; Hill coefficient, 1.7). DRG neurons also responded to micromolar concentrations of pentobarbital and alphaxalone but not to cis-4-aminocrotonic acid (CACA), glycine, or taurine. Baclofen (100 microM) affected neither the holding currents nor K+ conductance (when patch pipettes were filled with 130 mM KCl) caused by depolarizing pulses. Whole cell GABA-currents were blocked by bicuculline, picrotoxin, and t-butylbicyclophosphorothionate (TBPS; all at 100 microM). The reversal potential of whole cell GABA-currents was close to the theoretical Cl- equilibrium potential, shifting with changes in intracellular Cl- concentration in a manner expected for Cl--selective channels. The whole cell I-V curve for GABA-induced currents demonstrated slight outward rectification with nearly symmetrical outside and inside Cl- concentrations. Spectral analysis of GABA-induced membrane current fluctuations showed that the kinetic components were best fitted by a triple Lorentzian function. The apparent elementary conductance for GABA-activated Cl- channels determined from the power spectra was 22.6 pS. Single-channel recordings from cell-attached patches with pipettes containing 10 microM GABA indicated that GABA-activated channels have a main and a subconductance level with values of 30 and 19 pS, respectively. Mean open and closed times of the channel were characterized by two or three exponential decay functions, suggesting two or three open channel states and two closed states. Single channels showed a lack of rectification. The actions of GABA on cultured human embryonic DRG neurons are mediated through the activation of GABAA receptors with properties corresponding to those found in the CNS of human and other mammalian species but differing from those of cultured human adult DRG neurons.

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.

Distribution of glutamate receptors GluR 2/3 and NR1 in the developing rat cerebellum

The distribution of glutamate receptors GluR2/3 and NR1 was analysed immunohistochemically during development of the rat cerebellum. GluR2/3 immunoreactivity appeared by postnatal day P0 in somata of Purkinje cells. Throughout P7, P15, P20 and adulthood, GluR2/3 immunoreactivity was found in the entire Purkinje cell dendritic arbor reaching to the external granular layer and, by P15, the surface of the cerebellum. By P7, the granular layer revealed scattered, mildly reactive, cells. NR-1 immunoreactivity first gained prominence about P7 in the region of the multi-layered Purkinje cell somata. By P15, NR1 was prominent in Purkinje cell somata and Golgi cells. The reaction product extended into the primary main dendrite of Purkinje cells. By P21, stellate and basket cells had intense reactivity throughout the molecular layer and reactive large-diameter dendrites of Golgi cells projected toward the molecular layer. Granule cells remained very weak among strongly reactive Golgi cell somata and dendrites. Ultrastructural immunohistochemistry revealed NR1 reaction product in Purkinje cell somata, in stellate cell somata and dendrites and on postsynaptic membranes of scattered spines throughout the molecular layer. The later appearance and restricted location of NR1 in somata and proximal dendrites of Purkinje cells contrasted markedly with GluR2/3 which appeared before birth and remained prominent throughout Purkinje cell dendritic arbors of adults. The time of NR1 expression correlated with the generation of granule cells, their synaptogenesis on Purkinje cells, the formation of stellate/baske cells and the shift of climbing fibre synapses from distal to proximal dendrites. The developmental appearance of stellate/basket cells and Golgi cells as well as their high reactivity remaining into adulthood suggest that these inhibitory molecular and granular layer interneurons are the principal targets of glutamate axons serving NR1 synaptic properties while Purkinje cells and brush type granule cells are targets for glutamate connections with GluR2/3 characteristics.

Whole-cell and single-channel currents activated by GABA and glycine in granule cells of the rat cerebellum

1. Patch-clamp methods have been used to characterize GABA-and glycine-activated channels and spontaneous synaptic currents in granule cells in thin cerebellar slices from 7- to 20-day-old rats. 2. All granule cells responded to 10 microM GABA, while approximately 60% responded to 100 microM glycine. With repeated against application, whole-cell responses to GABA, but not those to glycine, declined over a period of minutes unless the pipette solution contained Mg-ATP. 3. Whole-cell concentration-response curves gave EC50 values at 45.2 and 99.6 microM and Hill slopes of 0.94 and 2.6 for GABA and glycine, respectively. At saturating concentrations, currents evoked by GABA were fivefold larger than those evoked by glycine. 4. Whole-cell current-voltage (I-V) relationships of GABA- and glycine-activated currents reversed close to the predicted Cl- equilibrium potential. Partial replacement of intracellular Cl- with F- shifted the GABA reversal potential to a more negative value. 'Instantaneous' I-V relationships produced by ionophoretic application of GABA were linear, while 'steady-state' I-V relationships produced by ramp changes in potential showed outward rectification. For glycine, 'steady-state' I-V plots were linear. 5. Responses to GABA were blocked by the GABAA receptor antagonists bicuculline (15 microM), SR-95531 (10 microM) and picrotoxinin (100 microM) while responses to glycine were selectively blocked by strychnine (200 nM), indicating the presence of two separate receptor types. 6. In outside-out membrane patches, GABA opened channels with conductances of 16 and 28 pS. The proportion of openings to each of the conductances varied between patches, possibly indicating the activation of two distinct channel types. Glycine-activated single-channel currents had conductances of 32, 55 and 104 pS. Single-channel I-V relationships were linear. 7. Spontaneous synaptic currents with a rapid rise time and biexponential decay were present in more than half of the cells examined. These currents were eliminated by bicuculline (15 microM) or SR-95331 (10 microM) and were greatly reduced in frequency by tetrodotoxin (TTX; 300 nM), suggesting that they were mediated by GABA and arose from spontaneous activity in Golgi interneurones. In granule cells where this spontaneous synaptic activity was apparent, glycine and low concentrations of GABA increased the frequency of the synaptic currents.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of intracellular Ca2+ on GABA-activated currents in cerebellar granule cells in culture

The patch clamp technique was used to study the effects of intracellular free calcium ([Ca2+]i) on GABAA-evoked whole-cell and single channel currents of cultured cerebellar granule cells. Changes in [Ca2+]i were obtained by adding to the extracellular solution the calcium ionophore A23187 (2 microM). The relationship between [Ca2+]i and [Ca2+]o in the presence or absence of A23187 was assessed using fluorimetric measurements from Fura-2 loaded cells. In 2 mM [Ca2+]o and A23187, [Ca2+]i was about 1.5 microM, whereas in the absence of A23187 it was about 250 nM. In whole-cell experiments (symmetrical chloride concentrations) at -50 mV, GABA (0.5 microM) evoked inward currents that did not desensitize. Bath application of A23187 significantly reduced the steady-state amplitude of GABA currents by 37 +/- 6%. Single channel currents activated by GABA (0.5 microM) were also recorded in the outside-out configuration of the patch clamp technique. Kinetic analysis of single channel events revealed that A23187 significantly increased the long closed time constant (tau c3) without affecting the open time constants (tau o1 and tau o2) or the short and medium closed time constants (tau c1 and tau c2). Moreover, application of A23187 induced a significant reduction of burst duration (tau b). We conclude that a rise in [Ca2+]i by A23187 may decrease the binding affinity of GABA for the GABAA receptor.

Currents activated by GABA and their modulation by Zn2+ in cerebellar granule cells in culture

Whole-cell and single-channel currents evoked by gamma-aminobutyric acid (GABA) were recorded from rat cerebellar granule cells in culture. The electrophysiological properties of these currents were studied in control condition and in the presence of external Zn2+ (10-30 microM). GABA (10 microM) induced bicuculline-sensitive whole-cell currents which desensitized. The desensitization was more rapid for higher concentrations of GABA (30-300 microM). The current-voltage relation of GABA currents was linear from -70 to +50 mV. Two different types of cells were found with respect to the stoichiometry for agonist binding, one with Hill coefficient 1.5 and another one with coefficient 1. The half-maximum concentration displayed more variability, with values varying from 10 to 50 microM. The time constant of recovery from desensitization (tau r) was estimated to be 36 s. Zn2+ (30 microM) blocked GABA-activated whole-cell currents in a non-competitive and voltage-independent way without a significant change in the current kinetics. In excised outside-out patches, GABA (0.5 microM) activated single-channel events of 19 and 31 pS. Kinetic analysis yielded two mean shut times (tau c1 = 2.70 ms, tau c2 = 205 ms) and one mean open time (tau o = 3.64 ms). Zn2+ (10 microM) did not affect single-channel conductances and mean open and shut times, but significantly reduced the probability of opening from 0.17 to 0.06. It is probable that Zn2+ binds to a site located on the extracellular part of the GABAA receptor channel complex.

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.

Excitatory and inhibitory synaptic currents and receptors in rat medial septal neurones

1. A thin-slice preparation was used to study the postsynaptic potentials and the underlying currents of visually identified rat medial septal (MS) neurones under tight-seal voltage- and current-clamp conditions. 2. Upon stimulation of the afferent fibres, all MS neurones exhibited a sequence of excitatory-inhibitory postsynaptic potentials (EPSP-IPSP). Under voltage clamp, with potassium glutamate as internal solution and at negative holding potentials (Vh), this synaptic pattern appeared as an initial inward current followed by a longer lasting outward current. 3. The inward postsynaptic current was completely abolished by 5 microM-6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) whereas the outward current disappeared in the presence of 10 microM-bicuculline. Thus the major excitatory and inhibitory synaptic inputs were identified as being due to activation of quisqualate/kainate glutamatergic and gamma-aminobutyric acid (GABAA) receptors, respectively. 4. At positive Vh a CNQX-resistant component of the excitatory postsynaptic current (EPSC) was revealed. This component was slower than the one mediated by the quisqualate receptor and was abolished by 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), indicating that N-methyl-D-aspartate (NMDA) receptors are involved in excitatory synaptic transmission in MS cells. The existence of the two main subtypes (NMDA and non-NMDA) of glutamatergic receptors in MS neurones was also confirmed by the responses of the neurones to bath application of the different agonists (glutamate, quisqualate, kainate and NMDA). 5. The CNQX-sensitive EPSC had a reversal potential near 0 mV. The fast rise time (approximately 0.7 ms) indicates a somatic location of the excitatory synapses. The relaxation kinetics of the fast EPSC were fitted by a single exponential function with a time constant of 1.13 +/- 0.1 ms. This parameter was independent of Vh. Fast EPSCs were blocked by CNQX in a dose-dependent manner (dissociation constant, KD = 0.2 microM). 6. Inhibitory postsynaptic currents (IPSCs) were studied in symmetrical chloride solutions after blockade of the excitatory receptors. The current-voltage relation was linear and reversed at 0 mV. The IPSCs had a fast rise time and their decay was best fitted by the sum of two exponentials with time constant of approximately 20 and 50 ms (Vh = -60 mV). The IPSCs were abolished by bicuculline (KD = 1 microM), a selective antagonist of GABAA receptors. As expected, bath application of GABA produced large whole-cell currents. 7. In many cells, in addition to the usual EPSP-IPSP sequence, failures of either the EPSP or the IPSP were frequently observed during the experimental protocol.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate receptor channels in isolated patches from CA1 and CA3 pyramidal cells of rat hippocampal slices

1. Currents activated by glutamate receptor (GluR) agonists were recorded from outside-out patches isolated from the soma of visually identified pyramidal neurones of the CA3 and CA1 region of rat hippocampal slices. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), L-glutamate (L-Glu), and kainate (KA) were delivered either by bath application through perfusion of the recording chamber or by rapid application via a piezo-driven two-barrelled fast application system. 2. Bath application of each of the three agonists activated inward currents in all patches (n = 134) at holding potentials of -50 or -60 mV. The current amplitude increased in size between 3 to 30 microM-AMPA and 100 microM to 1 mM-KA. With this slow mode of bath application, the responses showed no apparent desensitization even at saturating concentrations of AMPA (30 microM) and KA (1 mM). 3. The ratio of currents activated by 30 microM-AMPA and 300 microM-KA showed a characteristic difference between CA3 and CA1 neurones. The ratio was 0.242 +/- 0.028 (mean +/- S.E.M., n = 16) for CA3 cell patches and 0.097 +/- 0.012 (n = 8) for CA1 cell patches indicating that GluRs in the two cell populations are different. 4. The steady-state current-voltage relations (I-Vs) for AMPA- and KA-activated currents showed pronounced outward rectification for both cell types (when the main cations are Na+ in the bath and Cs+ in the pipette solution). The current reversed close to 0 mV and the ratio of chord conductances 80 mV on either side of the reversal potential was 2.66 for KA-activated currents in CA3 cell patches and 2.60 in CA1 cell patches. AMPA-activated currents showed a time-dependent increase after steps to positive membrane potentials and a decrease after steps to negative voltages, indicating that a gating process is responsible for outward rectification of the steady-state I-V. 5. The permeability (P) of GluR channels was high for Na+ as compared to Cs+ for both cell types (PNa/PCs = 0.88 and 0.84). The permeability was low for N-methyl-D-glucamine+ (PNMG/PCs < or = 0.03) and Ca2+ (PCa/PCs < or = 0.05). 6. The current noise level increased during application of AMPA or KA. Apparent single-channel conductances obtained from fluctuation analysis were higher for AMPA than for KA, but similar for both cell types. In CA3 cell patches, AMPA activated channels with an apparent chord conductance of 7.2 pS, KA of 3.0 pS conductance.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

The minimal inhibitory synaptic currents evoked in neonatal rat motoneurones

1. Tight-seal whole-cell recordings were made from lumbar motoneurones visually identified in thin slices of neonatal rat spinal cord. The inhibitory postsynaptic currents (IPSCs) were evoked by extracellular stimulation of a neighbouring internuncial neurone in the presence of glutamate receptor antagonists. 2. Glycinergic IPSCs were recorded in the presence of bicuculline. The IPSCs appeared in an all-or-none manner as the graded stimulus intensity exceeded a certain threshold. Their latencies showed a unimodal distribution with a mean of 0.81 ms at 37 degrees C. Thus, the observed IPSCs are suggested to be monosynaptically evoked unitary IPSCs. The mean conductance of unitary IPSCs was 2.9 +/- 1.2 nS (+/- S.D.). 3. When the external Ca2+ concentration ([Ca2+]o) was reduced, the number of failures in response to stimulation increased, thereby reducing the mean amplitude of IPSCs. The mean amplitude of IPSCs was linearly related to the [Ca2+]o (0.35-1.4 mM) with a mean slope of 3.1 +/- 0.67 on double logarithmic co-ordinates. 4. The amplitude of individual IPSCs decreased with decrease in [Ca2+]o. However, below 0.7 mM [Ca2+]o, the mean amplitude of IPSCs (excluding failures) reached a steady minimum level. The mean conductance of these IPSCs measured in 0.5 mM [Ca2+]o was 657 +/- 281 pS. 5. The minimal IPSCs had a coefficient of variation of 0.50 +/- 0.13. No clear correlation was observed between the rise time and the amplitude of minimal IPSCs evoked in individual motoneurones, indicating that the amplitude variability is not due to the different synaptic locations. 6. Spontaneous miniature IPSCs were recorded from motoneurones in the presence of tetrodotoxin. The miniature IPSCs had a mean conductance of 739 +/- 278 pS, being comparable to the minimal evoked IPSCs. 7. Under various internal and external Cl- concentration, the reversal potential of the IPSCs (EIPSC) approximately coincided with the Cl- equilibrium potential. A 730-fold change in the potassium concentration gradient across the membrane did not affect the EIPSC. The permeability ratio of K+ to Cl- (Pk/PCl) was less than 0.05. 8. It is concluded that the IPSCs are carried almost exclusively by Cl- and that the minimal evoked IPSCs represent the quantal response of the transmitter.

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)

On the mechanism of action of picrotoxin on GABA receptor channels in dissociated sympathetic neurones of the rat

1. The mechanism of action of picrotoxin on GABA receptor channels in rat sympathetic neurones has been investigated with whole-cell clamp. In addition, the action of picrotoxin on single GABA channels has been examined in outside-out membrane patches from these cells. 2. Picrotoxin, at concentrations which dramatically reduced the amplitude of whole-cell GABA currents, did not alter the spectral time constants or single-channel conductance estimated by analysis of GABA-activated current noise. This was observed at potentials both negative and positive to the GABA reversal potential (i.e. for both inward and outward GABA currents). In control conditions, the slow and fast time constants from GABA noise were 40 +/- 14 ms and 2 +/- 0.4 ms, while the estimated single-channel conductance was 14 +/- 2 pS. In the presence of picrotoxin, the time constants and estimated single-channel conductance were 41 +/- 5 ms, 2.7 +/- 0.6 ms and 15 +/- 2.3 pS. 3. Picrotoxin did not alter the shape of the whole-cell GABA current-voltage relationship, indicating that the steady-state block was not voltage dependent. The lack of effect of picrotoxin on the GABA noise spectra and the lack of outward rectification makes it unlikely that picrotoxin acts by a simple voltage-dependent (or voltage-independent) channel blocking mechanism. In the presence of picrotoxin the reversal potential for GABA remained at approximately 0 mV in symmetrical chloride. 4. Distributions of total burst durations, obtained from single-channel records with low concentrations of GABA, were fitted with three or four exponential components. Picrotoxin had no consistent effect on the time constants of the total burst length distributions. It also did not alter the amplitude of the main conductance state. However, picrotoxin did reduce the frequency of channel openings. 5. The application of brief ionophoretic pulses of GABA, to cells under whole-cell voltage clamp, revealed that the rate of onset of block by picrotoxin was accelerated in the presence of GABA. In the absence of agonist, picrotoxin produced a more slowly equilibrating block. 6. Our data are consistent with a mechanism whereby picrotoxin binds preferentially to an agonist bound form of the receptor and stabilizes an agonist-bound shut state. This could, for example, mean that picrotoxin enhances the occurrence of a desensitized state or an allosterically blocked state.

Integrity of perforant path fibers and the frequency of action potential independent excitatory and inhibitory synaptic events in dentate gyrus granule cells

Whole-cell voltage clamp recordings in 400 microns thick hippocampal slices revealed discrete excitatory and inhibitory postsynaptic currents which persisted at synapses on granule cells following abolition of action potentials with 1 microM tetrodotoxin (TTX). The conductances associated with excitatory amino acid and GABAA receptor mediated events had mean peaks of 200 and 800 pS, and decayed monoexponentially with time constants of 5.6 and 5.3 ms. At a holding potential close to the normal resting membrane potential of granule cells (-80 to -90 mV), the frequency of glutamate/aspartate mediated spontaneous excitatory postsynaptic currents (sEPSCs) was decreased from 2.04 Hz in slices cut parallel to the plane of the perforant path to 0.87 Hz in slices cut in a plane that disrupted the distal perforant path fibres, suggesting that presynaptic integrity influences the rate of action potential independent neurotransmitter release. The orientation of the slicing had no effect on the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs).

N-methyl-D-aspartate and aspartate raise the cytosolic free calcium concentration by acting upon receptors transiently expressed on immature cerebellar Purkinje cells

N-Methyl-D-aspartate (NMDA) or aspartate (Asp) increased the cytosolic free calcium concentration ([Ca]in) in some populations of Purkinje cells dissociated from immature rat cerebellum. The NMDA- and Asp-induced rise in [Ca]in was affected only a little by adding glycine or NMDA antagonists, but was reduced either by adding Mg2+, Gallopamil hydrochloride (D-600) and gamma-amino-butyric acid, or by removing external Na+. The results suggest that stimulation of the NMDA-sensitive receptors transiently expressed on immature Purkinje cell soma results in a rise in [Ca]in through the activation of voltage-dependent Ca2+ channels.

Effects of pentobarbitone on the properties of nicotinic channels of chromaffin cells

We have used the whole cell patch clamp technique to investigate the action of pentobarbitone on the nicotinic channels of bovine chromaffin cells. Application of agonists induced an inward current associated with a large increase in current noise. The noise could be fitted by Lorentzian functions with time constants of 17 +/- 2 ms for 10 microM acetylcholine and 10 +/- 1 ms for 10 microM carbachol. The single channel conductance estimated from the current variance was about 25 pS in each case. Pentobarbitone decreased the time constants in a concentration-dependent fashion, but the unit conductances were unaffected. Single channel events were recorded in chromaffin cells held under voltage clamp. Pentobarbitone did not reduce the amplitude of channel openings or the probability of channel opening but reduced the mean channel open time. This reduction was sufficient to account for the decrease in inward current produced by pentobarbitone.

N-methyl-D-aspartate receptor-mediated spontaneous activity in cerebellar granule cells in culture

An outside-out configuration of the patch-clamp method was used to study the properties of spontaneous and glutamate-evoked single channel activity in cerebellar granule cells in culture. Both spontaneous occurring events and glutamate-evoked single channel currents had similar conductances (17.7, 36.5; 17.3, 35.8 pS) and mean open times (0.7, 7.1; 0.6, 7.8 ms). In magnesium free solution, the spontaneous channels were blocked by the NMDA-receptor antagonist APV (0.1 mM) and by the NMDA-channel blocker MK-80 (0.1 mM). The spontaneous activity disappeared at negative holding potentials when the bathing solution contained magnesium (5 mM). Maximum conductance of spontaneous channels increased with days in culture (DIC) from 9 pS at 1 DIC to 59 pS at 10 DIC. These experiments suggest that endogenous transmitter activates NMDA-receptors in cerebellar granule cells in culture.

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)

Perpetual inhibitory activity in mammalian brain slices generated by spontaneous GABA release

Miniature spontaneous inhibitory postsynaptic currents (sIPSCs) mediated by GABAA receptors were recorded using whole-cell patch clamp recordings in rat brain slices maintained in vitro at 34 +/- 1 degree C. We have found that firing of action potentials by principal neurons or by GABAergic interneurons is not necessary to the generation of sIPSCs since they persist in the presence of 1-5 microM tetrodotoxin (TTX). The average frequency of the discrete sIPSCs exhibits a large cell-to-cell variability and is between 5-15 Hz. The amplitudes of the sIPSCs depend on the difference between the membrane potential and the equilibrium potential for Cl- (ECl). Generally, 70-80 mV away from ECl, sIPSCs have a mean amplitude of 30-80 pA (i.e. peak conductance of 400-1000 pS) with an average decay time constant of 5.8 ms. Accordingly, unitary single sIPSCs arise from the simultaneous activation of no more than 20 GABAA receptor/channels. The perpetual barrage of spontaneous GABAergic activity is very likely to be a critical factor in the regulation of neuronal excitability and the mechanism of action of several neuroactive compounds.

Patch clamp analysis of excitatory synaptic currents in granule cells of rat hippocampus

1. Excitatory postsynaptic potentials (EPSPs) and their underlying currents (EPSCs) were recorded from dentate granule cells in thin hippocampal slices of rats using the tight-seal whole-cell recording technique. 2. At resting membrane potentials (ca -60 to -70 mV), the EPSCs clearly consisted of a dominant fast and a smaller slow component. The slow EPSC component markedly increased with depolarization. This resulted in a region of negative slope conductance (between -50 and -30 mV) in the peak current-voltage (I-V) relation of the dual-component EPSC in most neurones. The EPSCs reversed entirely at -1.2 +/- 2.8 mV (n = 15). 3. Using selective antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptors, two pharmacologically distinct components of the natural EPSCs were isolated. The non-NMDA EPSCs displayed a linear I-V relation. Their rise times (0.5-1.9 ms) were independent of membrane voltage but seemed to depend critically on the precise dendritic location of the synapse. Their decay was approximated by a single exponential with a time constant ranging from 3 to 9 ms. The time course of these EPSCs was independent of changes in extracellular Mg2+. 4. The NMDA EPSCs displayed a non-linear I-V relation. At resting membrane potentials their peak amplitudes were 20 pA and increased steadily with depolarization to -30 mV. At membrane voltages positive to -30 mV the peak I-V relation was linear. The rise times of NMDA EPSCs ranged from 4 to 9 ms and were insensitive to membrane voltage. 5. The NMDA EPSCs decayed biexponentially. Both time constants, tau f and tau s, increased with depolarization in an exponential manner, tau s being more voltage dependent than tau f. Lowering extracellular Mg2+ slightly reduced both rate constants but did not completely abolish their voltage sensitivity. 6. Bath application of NMDA to outside-out patches from granule cells induced single channel currents of 52 pS in nominally Mg(2+)-free solutions. They displayed a burst-like single-channel activity with clusters of bursts lasting several hundreds of milliseconds. Currents through single NMDA receptor channels reversed around 0 mV. 7. The fractional contributions of NMDA and non-NMDA components to peak currents and synaptic charge transfer were assessed. At resting membrane potential the NMDA EPSC component accounted for 23% of the peak current and for 64% of the synaptic charge transfer. The contribution of the NMDA EPSC component to the synaptic charge transfer strongly increased with small depolarizations from rest.

Synaptic- and agonist-induced excitatory currents of Purkinje cells in rat cerebellar slices

1. Postsynaptic currents originating from activation of the two major excitatory inputs to Purkinje cells were studied in thin slices of rat cerebellum, using the tight-seal whole-cell recording technique. Two types of excitatory postsynaptic currents were analysed: those evoked by stimulation of the granule cell-parallel fibre system (PF-EPSC) and those elicited by stimulation of the climbing fibres (CF-EPSC). 2. Both types of postsynaptic currents had a linear current-voltage relation, reversing at membrane potentials close to 0 mV. Their time course of activation was independent of the membrane potential. 3. For both types of postsynaptic currents, the time course of decay was well described by a single exponential function, with a time constant which increased as the membrane potential was made more positive. 4. Postsynaptic currents arising from stimulation of the climbing fibre generally had a slightly faster time course of onset and decay than those associated with stimulation of the granule cell-parallel fibre system. The average values of the 10-90% rise time were 1.8 +/- 0.4 ms (means +/- S.D., n = 7) for PF-EPSCs and 0.8 +/- 0.3 ms (n = 9) for CF-EPSCs. Time constants of decay, at a holding potential of -60 mV, had values of 8.3 +/- 1.6 ms (n = 7) and 6.4 +/- 1.1 ms (n = 9) for PF-EPSCs and CF-EPSCs respectively. 5. CF-EPSCs and PF-EPSCs had the characteristics described above in slices derived from animals aged 9-22 days old and 9-15 days old, respectively. The PF-EPSCs in animals older than 15 days had very slow time courses and positive apparent reversal potentials, suggesting that they originated from distal locations, not under accurate voltage control. 6. In order to assess the quality of the voltage clamp, responses to hyperpolarizing pulses from -70 mV were analysed. The capacitive currents could be fitted by the sum of two exponentials, and were interpreted with an equivalent electrical circuit comprising two main compartments (soma and proximal dendrites on one hand, distal dendrites on the other). Analysis of synaptic currents in terms of this model suggested that the recorded time course of decay was approximately correct. 7. CF-EPSCs as well as PF-EPSCs were insensitive to the NMDA receptor antagonist 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), but were blocked in a dose-dependent reversible manner by the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX).(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction of 5-bromowillardiine with non-NMDA receptors expressed in Xenopus laevis oocytes injected with chick brain mRNA

Excitatory amino acid (EAA) receptors were studied in Xenopus laevis oocytes microinjected with 1-day-old chick brain mRNA using a two-electrode voltage clamp technique. EAA agonists could be differentially characterised according to their current/voltage (I/V) relationships; I/V plots to kainate (KA) and domoate (Dom) inwardly rectified at positive membrane potentials whereas quisqualate (QA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and L-glutamate (L-Glu) produced linear I/V plots. Interestingly, the non-NMDA (N-methyl-D-aspartate) agonist, 5-bromowillardiine (BrW), displayed properties similar to KA and Dom. It is proposed that BrW acts as a KA-like agonist on chick EAA receptors expressed in Xenopus oocytes.

GABA release triggered by the activation of neuron-like non-NMDA receptors in cultured type 2 astrocytes is carrier-mediated

Kainate (KA), quisqualate (QA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) stimulated gamma-aminobutyric acid [3H]gamma-aminobutyric acid (GABA) release from cultured cerebellar type 2 astrocytes and from their bipotential precursors. The evoked release was prevented by the antagonist 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX). AMPA and QA applied together with KA at concentrations around or above their EC50S (20-50 microM) antagonized the stimulatory effect of KA on [3H]GABA release. On the other hand, the releasing action of KA was potentiated by concentrations of QA in the low micromolar range (2-5 microM), particularly when the concentration of KA was at the borderline of effectiveness (10 microM). KA and QA did not elevate intracellular cyclic GMP levels in astrocyte cultures, although guanylate cyclase was present in both type 2 and type 1 astrocytes. The inability of KA to elevate cyclic GMP levels in astrocytes was the only major difference in the behavior of this glutamate agonist between astroglial and neuronal cultures. The GABA transport inhibitor nipecotic acid or replacement of NaCl with LiCl abolished [3H]GABA uptake and also KA- and QA-induced release of preaccumulated [3H]GABA. Therefore, [3H]GABA was released from type 2 astrocytes and their progenitors through its Na(+)-dependent transport system, operating in an outward direction when the cells were depolarized by non-NMDA receptor agonists.

Quisqualate agonists occlude kainate-induced current in cultured striatal neurons

We employed the whole cell patch-clamp technique to examine the ionic currents induced via activation of kainate/quisqualate receptors on striatal neurons in primary culture when N-methyl-D-aspartate receptors were blocked by selective antagonists. Bath perfusion of 10 microM-1 mM each of quisqualate, glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (a selective quisqualate agonist) or kainate, induced only a sustained current, but more rapid application by pressure ejection of each of the first three agonists (but not kainate) also activated a rapidly desensitizing current. The current induced by a near-saturating concentration of kainate (1 mM) was, on average, 16-fold larger than the maximum sustained current induced by quisqualate (10 microM), or 7.5-fold larger than that induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (100 microM) or glutamate (100 microM). When kainate (100 microM-10 mM) was co-applied with each of the agonists (1 microM-1 mM), the sustained current was not the algebraic sum of the currents activated by kainate or the other agonist alone; rather, the kainate-induced current was increasingly occluded by co-application with increasing concentrations of another agonist. The potency to occlude kainate-induced current had a rank order of quisqualate greater than alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate approximately glutamate; although at sufficiently high concentrations all three agonists could occlude the kainate-induced current completely. When kainate and quisqualate were co-applied during the continued presence of quisqualate, the onset of the kainate-induced sustained current was dramatically slowed. However, the steady-state occlusion by quisqualate could be abolished when the ratio kainate to quisqualate was raised to 100:1; therefore, the occlusion appears to involve a competition between kainate and quisqualate at some shared receptor binding sites which have a higher affinity for quisqualate than kainate.

Responses to excitatory amino acids of Purkinje cells' and neurones of the deep nuclei in cerebellar slice cultures

1. The actions of the endogenous excitatory amino acids (EAAS) glutamate (Glu), aspartate (Asp) and homocysteate (HCA) on Purkinje cells and neurones of the deep nuclei in cerebellar slice cultures were investigated using intracellular recordings in the single-electrode voltage-clamp mode and the whole-cell configuration of the patch-clamp technique. 2. Purkinje cells and neurones of deep cerebellar nuclei were identified according to their localization in the living cultures, their morphology as revealed by intracellular injections of Lucifer Yellow and their immunoreactivity to antibodies to the 28 kDa Ca2(+)-binding protein. 3. When Purkinje cells were voltage-clamped near their resting membrane potential in a TTX-containing salt solution, Glu, Asp and HCA induced inward currents which were abolished by 6-cyano-7-nitroxaline-2,3-dione (CNQX), a selective antagonist of the non-N-methyl-D-aspartate (NMDA) subtype of EAA receptors. The selective antagonist of NMDA receptors, D-(-)-2-amino-5-phosphonovaleric acid (D-APV), was ineffective in blocking the responses induced by these three amino acids. NMDA, even at high concentrations and in magnesium-free bathing solution, had no detectable effect on membrane properties of Purkinje cells grown in culture during 11-34 days. 4. In magnesium-containing saline, the amplitude of the responses induced by Glu, Asp and HCA was a linear function of the membrane potential. 5. In contrast, neurones of the deep cerebellar nuclei were responsive to NMDA and the inward currents induced by Glu, Asp and HCA were partially blocked both by CNQX and by D-APV. 6. In magnesium-containing saline, the amplitude of the currents induced by NMDA as well as by the three endogenous EAAs decreased at hyperpolarizing holding potentials whereas the current-voltage relation of the responses induced by quisqualate (QA) was strictly linear. 7. It is concluded that Purkinje cells in cerebellar slice cultures do not express NMDA receptors and that excitation of these neurones by the endogenous amino acids Glu, Asp and HCA is mediated exclusively through the activation of non-NMDA receptors. In the same preparation, neurones of the deep cerebellar nuclei possess NMDA and non-NMDA receptors which can be both activated by the three endogenous excitatory amino acids.

Glutamate receptor channels in rat DRG neurons: activation by kainate and quisqualate and blockade of desensitization by Con A

Primary afferent C fibers in rat dorsal roots are depolarized by the excitatory amino acids kainate and domoate. Under whole-cell voltage clamp, kainate and domoate increase membrane conductance in a subpopulation of freshly dissociated DRG neurons. In contrast to kainate currents observed in CNS neurons, responses to kainate and domoate in DRG cells desensitize with prolonged agonist exposure. Half-maximal activation is achieved with much lower concentrations of kainate and domoate in sensory neurons than in CNS neurons from cerebral cortex. Rapid applications of glutamate, quisqualate, and AMPA evoke a transient current in DRG neurons and desensitize cells to subsequent applications of kainate or domoate. Brief incubation with the lectin concanavalin A eliminates desensitization to excitatory amino acids; after treatment with concanavalin A, all five agonists gate sustained currents of similar amplitude via the same receptor.

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

1. Single-channel currents evoked by excitatory amino acids have been examined in outside-out patches from large cerebellar neurones (including Purkinje cells) in tissue culture. L-Glutamate (3-10 microM), L-aspartate (3-10 microM), NMDA (N-methyl-D-aspartate, 10-50 microM), ibotenate (50 microM), quisqualate (3-50 microM), and kainate (3-50 microM) all produced single-channel currents with multiple amplitudes. 2. Single-channel currents recorded over a range of patch potentials had a mean interpolated reversal potential of -3.8 +/- 0.5 mV. The directly resolvable multiple conductance levels could be classified into five main groups, with mean values (averaged for all agonists) of: 47.9 +/- 0.7, 38.5 +/- 0.8, 27.8 +/- 1.4, 18.2 +/- 0.5 and 8.3 +/- 0.6 pS. 3. From the relative areas under current amplitude histograms it was estimated that the percentage of openings with conductances greater than 30 pS was about 83% with NMDA, 79% with glutamate and 78% with aspartate. In some patches, the majority of greater than 30 pS events evoked by these agonists were to the maximum conductance of 48 pS, whereas in other patches there were more 38 pS openings than 48 pS openings. Only 27% of quisqualate openings, and about 10% of kainate openings, were greater than 30 pS. 4. Of the small amplitude (less than 20 pS) events, 93% of quisqualate openings were to the 8 pS level whereas approximately 87% of less than 20 pS currents produced by NMDA, glutamate and aspartate were to the 18 pS level (the remainder being 8 pS). Direct transitions could occur between certain levels (including events above and below 30 pS) suggesting that these are sublevels of multiple-conductance channels. The most frequently occurring transitions were between the 48 and 38 pS levels, and the 38 and 18 pS levels. 5. Channel openings occurred in bursts, within which individual openings were separated either by brief closures (gaps), or by direct transitions between the multiple conductance levels. The briefest of these gaps (less than 200-400 microseconds) could represent a mixture of transitions to lower conductance levels as well as partially resolved complete shuttings. The mean duration of the longer gaps within bursts, thought to represent complete but partially resolved shuttings was 1.05 +/- 0.25 ms (pooled for all agonists). 6. Burst-length distributions could be fitted with the sum of three exponentials. The briefest component may have arisen from brief single openings. The two slower components probably reflect the existence of two kinetically distinct open states.(ABSTRACT TRUNCATED AT 400 WORDS)