Activation and desensitization of N-methyl-D-aspartate receptors in nucleated outside-out patches from mouse neurones

High I(h) channel density in the distal apical dendrite of layer V pyramidal cells increases bidirectional attenuation of EPSPs

Despite the wealth of recent research on active signal propagation along the dendrites of layer V neocortical pyramidal neurons, there is still little known regarding the traffic of subthreshold synaptic signals. We present a study using three simultaneous whole cell recordings on the apical dendrites of these cells in acute rat brain slices to examine the spread and attenuation of spontaneous excitatory postsynaptic potentials (sEPSPs). Equal current injections at each of a pair of sites separated by approximately 500 microm on the apical dendrite resulted in equal voltage transients at the other site ("reciprocity"), thus disclosing linear behavior of the neuron. The mean apparent "length constants" of the apical dendrite were 273 and 446 microm for somatopetal and somatofugal sEPSPs, respectively. Trains of artificial EPSPs did not show temporal summation. Blockade of the hyperpolarization-activated cation current (I(h)) resulted in less attenuation by 17% for somatopetal and by 47% for somatofugal sEPSPs. A pronounced location-dependent temporal summation of EPSP trains was seen. The subcellular distribution and biophysical properties of I(h) were studied in cell-attached patches. Within less than approximately 400 microm of the soma, a low density of approximately 3 pA/microm(2) was found, which increased to approximately 40 pA/microm(2) in the apical distal dendrite. I(h) showed activation and deactivation kinetics with time constants faster than 40 ms and half-maximal activation at -95 mV. These findings suggest that integration of synaptic input to the apical tuft and the basal dendrites occurs spatially independently. This is due to a high I(h) channel density in the apical tuft that increases the electrotonic distance between these two compartments in comparison to a passive dendrite.

Modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor desensitization by extracellular protons

The interstitial milieu of the brain is buffered to an average pH of 7.3, but synaptic activation produces a temporal sequence of events that can affect pH in the synaptic cleft. Furthermore, pathophysiological processes such as ischemia and seizures produce global and prolonged acidification of interstitial pH. Changes in pH, in turn, can affect neuronal excitability by modulating receptors and channels. Patch-clamp recordings were made from cultured rat hippocampal neurons to determine whether physiologically relevant changes in interstitial pH (6.5-7.8) significantly affect AMPA receptor function. Acidic pH, such as that typically associated with ischemia (pH 6.5), significantly inhibited AMPA receptor-mediated responses in neurons. The effect of pH was agonist-dependent, with 2-fold greater inhibition of responses evoked by the strongly desensitizing agonists glutamate and quisqualate than the weakly desensitizing agonist kainate. Additional experiments tested the hypothesis that protons modulate AMPA receptor desensitization. In the presence of drugs that block AMPA receptor desensitization, pH 6. 5 had no effect on glutamate-evoked responses. In neuronal macropatches, protons increased equilibrium desensitization without affecting macroscopic desensitization or deactivation kinetics. The mechanisms and molecular determinants of pH-mediated effects were further investigated using human embryonic kidney 293 cells expressing recombinant AMPA receptors. Inhibition of kainate-evoked responses varied with subunit and isoform composition, ranging from 10% to >40%. Flop isoforms, which exhibit faster and more extensive desensitization, were most strongly inhibited. These findings suggest that interstitial acidification can modulate AMPA receptor-mediated synaptic transmission and that differences in receptor sensitivity to proton modulation may underlie the selective vulnerability of certain neuronal populations to ischemia.

Slow desensitization regulates the availability of synaptic GABA(A) receptors

At central synapses, a large and fast spike of neurotransmitter efficiently activates postsynaptic receptors. However, low concentrations of transmitter can escape the cleft and activate presynaptic and postsynaptic receptors. We report here that low concentrations of GABA reduce IPSCs in hippocampal neurons by preferentially desensitizing rather than opening GABA(A) channels. GABA transporter blockade also caused desensitization by locally elevating GABA to approximately 1 microm. Recovery of the IPSC required several seconds, mimicking recovery of the channel from slow desensitization. These results indicate that low levels of GABA can regulate the amplitude of IPSCs by producing a slow form of receptor desensitization. Accumulation of channels in this absorbing state allows GABA(A) receptors to detect even a few molecules of GABA in the synaptic cleft.

Fast removal of synaptic glutamate by postsynaptic transporters

Glutamate transporters are believed to remove glutamate from the synaptic cleft only slowly because they cycle slowly. However, we show that when glutamate binds to postsynaptic transporters at the cerebellar climbing fiber synapse, it evokes a conformation change and inward current that reflect glutamate removal from the synaptic cleft within a few milliseconds, a time scale much faster than the overall cycle time. Contrary to present models, glutamate removal does not require binding of an extracellular proton, and the time course of transporter anion conductance activation differs from that of glutamate removal. The charge movement associated with glutamate removal is consistent with the majority of synaptically released glutamate being removed from the synaptic cleft by postsynaptic transporters.

AMPA receptor current density, not desensitization, predicts selective motoneuron vulnerability

Spinal motoneurons are more susceptible to AMPA receptor-mediated injury than are other spinal neurons, a property that has been implicated in their selective degeneration in amyotrophic lateral sclerosis (ALS). The aim of this study was to determine whether this difference in vulnerability between motoneurons and other spinal neurons can be attributed to a difference in AMPA receptor desensitization and/or to a difference in density of functional AMPA receptors. Spinal motoneurons and dorsal horn neurons were isolated from embryonic rats and cultured on spinal astrocytes. Single-cell RT-PCR quantification of the relative abundance of the flip and flop isoforms of the AMPA receptor subunits, which are known to affect receptor desensitization, did not reveal any difference between the two cell populations. Examination of AMPA receptor desensitization by patch-clamp electrophysiological measurements on nucleated and outside-out patches and in the whole-cell mode also yielded similar results for the two cell groups. However, AMPA receptor current density was two- to threefold higher in motoneurons than in dorsal horn neurons, suggesting a higher density of functional AMPA receptors in motoneuron membranes. Pharmacological reduction of AMPA receptor current density in motoneurons to the level found in dorsal horn neurons eliminated selective motoneuron vulnerability to AMPA receptor activation. These results suggest that the greater AMPA receptor current density of spinal motoneurons may be sufficient to account for their selective vulnerability to AMPA receptor agonists in vitro.

Corticosteroid actions in hippocampus require DNA binding of glucocorticoid receptor homodimers

Glucocorticoids are secreted from the adrenal gland in very high amounts after stress. In the brain, these stress hormones potently modulate ionic currents, monoaminergic transmission, synaptic plasticity and cellular viability, most notably in the hippocampus where corticosteroid receptors are highly enriched. Here we show that at least some of these actions require DNA binding of glucocorticoid receptor (GR) homodimers.

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.

The trapping block of NMDA receptor channels in acutely isolated rat hippocampal neurones

N-methyl-D-aspartate (NMDA) receptor responses were recorded from acutely isolated rat hippocampal neurones using the whole-cell patch-clamp technique. A rapid perfusion system was used to study the voltage-dependent block of NMDA channels by Mg2+, amantadine (AM) and N-2-(adamantyl)-hexamethylenimine (A-7). Mg2+, AM and A-7-induced stationary blockade of NMDA channels increased with the blocker concentration but did not depend on the agonist (aspartate; Asp) concentration. Blockade by AM and A-7, but not Mg2+, was weakly use dependent. 'Hooked' tail currents were observed after coapplication of Asp and Mg2+, AM or A-7. The hooked tail current kinetics, amplitude and carried charge indicated that Mg2+, AM and A-7 did not prevent closure and desensitization of NMDA channels nor agonist dissociation. Tail currents following Asp application in the absence and continuous presence of Mg2+, AM or A-7 had similar kinetics. Application of multiple stationary and kinetic criteria to the Mg2+, AM and A-7 blockade led us to conclude that their effects on NMDA channels can be described in terms of a 'trapping' model, which is fully symmetrical with respect to the blocking transition. In general, the apparent blocking/recovery kinetics predicted by the fully symmetrical trapping model differ significantly from the microscopic kinetics and depend on the rate of binding and unbinding of the blocker, the NMDA channel open probability and the rate of solution exchange.

In CA1 pyramidal neurons of the hippocampus protein kinase C regulates calcium-dependent inactivation of NMDA receptors

The NMDA subtype of the glutamate-gated channel exhibits a high permeability to Ca(2+). The influx of Ca(2+) through NMDA channels is limited by a rapid and Ca(2+)/calmodulin (CaM)-dependent inactivation that results from a competitive displacement of cytoskeleton-binding proteins from the NR1 subunit of the receptor by Ca(2+)/CaM (Zhang et al., 1998; Krupp et al., 1999). The C terminal of this subunit can be phosphorylated by protein kinase C (PKC) (Tingley et al., 1993). The present study sought to investigate whether PKC regulates Ca(2+)-dependent inactivation of the NMDA channel in hippocampal neurons. Activation of endogenous PKC by 4beta-phorbol 12-myristate 13-acetate enhanced peak (I(p)) and depressed steady-state (I(ss)) NMDA-evoked currents, resulting in a reduction in the ratio of these currents (I(ss)/I(p)). We demonstrated previously that PKC activity enhances I(P) via a sequential activation of the focal adhesion kinase cell adhesion kinase beta/proline-rich tyrosine kinase 2 (CAKbeta/Pyk2) and the nonreceptor tyrosine kinase Src (Huang et al., 1999; Lu et al., 1999). Here, we report that the PKC-induced depression of I(ss) is unrelated to the PKC/CAKbeta/Src-signaling pathway but depends on the concentration of extracellular Ca(2+). Intracellular applications of CaM reduced I(ss)/I(p) and occluded the Ca(2+)-dependent effect of phorbol esters on I(ss.) Moreover, increasing the concentration of intracellular Ca(2+) buffer or intracellular application of the inhibitory CaM-binding peptide (KY9) greatly reduced the phorbol ester-induced depression of I(ss). Taken together, these results suggest that PKC enhances Ca(2+)/CaM-dependent inactivation of the NMDA channel, most likely because of a phosphorylation-dependent regulation of interactions between receptor subunits, CaM, and other postsynaptic density proteins.

C-Terminal truncation of NR2A subunits impairs synaptic but not extrasynaptic localization of NMDA receptors

NMDA receptors interact via the extended intracellular C-terminal domain of the NR2 subunits with constituents of the postsynaptic density for purposes of retention, clustering, and functional regulation at central excitatory synapses. To examine the role of the C-terminal domain of NR2A in the synaptic localization and function of NR2A-containing NMDA receptors in hippocampal Schaffer collateral-CA1 pyramidal cell synapses, we analyzed mice which express NR2A only in its C-terminally truncated form. In CA1 cell somata, the levels, activation, and deactivation kinetics of extrasynaptic NMDA receptor channels were comparable in wild-type and mutant NR2A(Delta)(C/)(Delta)(C) mice. At CA1 cell synapses, however, the truncated receptors were less concentrated than their full-length counterparts, as indicated by immunodetection in cultured neurons, synaptosomes, and postsynaptic densities. In the mutant, the NMDA component of evoked EPSCs was reduced in a developmentally progressing manner and was even more reduced in miniature EPSCs (mEPSCs) elicited by spontaneous glutamate release. Moreover, pharmacologically isolated NMDA currents evoked by synaptic stimulation had longer latencies and displayed slower rise and decay times, even in the presence of an NR2B-specific antagonist. These data strongly suggest that the C-terminal domain of NR2A subunits is important for the precise synaptic arrangement of NMDA receptors.

Properties of voltage-gated potassium currents in nucleated patches from large layer 5 cortical pyramidal neurons of the rat

Voltage-gated potassium currents were studied in nucleated outside-out patches obtained from large layer 5 pyramidal neurons in acute slices of sensorimotor cortex from 13- to 15-day-old Wistar rats (22-25 C). Two main types of current were found, an A-current (IA) and a delayed rectifier current (IK), which were blocked by 4-aminopyridine (5 mM) and tetraethylammonium (30 mM), respectively. Recovery from inactivation was mono-exponential (for IA) or bi-exponential (for IK) and strongly voltage dependent. Both IA and IK could be almost fully inactivated by depolarising prepulses of sufficient duration. Steady-state inactivation curves were well fitted by the Boltzmann equation with half-maximal voltage (V ) and slope factor (k) values of -81.6 mV and -6.7 mV for IA, and -66.6 mV and -9.2 mV for IK. Peak activation curves were described by the Boltzmann equation with V and k values of -18.8 mV and 16.6 mV for IA, and -9.6 mV and 13.2 mV for IK. IA inactivated mono-exponentially during a depolarising test pulse, with a time constant ( approximately 7 ms) that was weakly dependent on membrane potential. IK inactivated bi-exponentially with time constants ( approximately 460 ms, approximately 4.2 s) that were also weakly voltage dependent. The time to peak of both IA and IK depended strongly on membrane potential. The kinetics of IA and IK were described by a Hodgkin-Huxley-style equation of the form mNh, where N was 3 for IA and 1 for IK. These results provide a basis for understanding the role of voltage-gated potassium currents in the firing properties of large layer 5 pyramidal neurons of the rat neocortex.

Voltage-gated K+ channels in layer 5 neocortical pyramidal neurones from young rats: subtypes and gradients

We investigated the types and distribution of voltage-gated K+ channels in the soma and apical dendrite of layer 5 (L5) neocortical pyramidal neurones, of young rats (postnatal days 13-15), in acute brain slices. A slow inactivating outward K+ current and a fast inactivating outward K+ current were detected in nucleated patches. The slow K+ current was completely blocked by tetraethylammonium (TEA) with an IC50 of 5 +/- 1 mM (mean +/- s.e.m.) and was partially blocked by 4-aminopyridine (4-AP). The fast K+ current was blocked by 4-AP with an IC50 of 4.2 +/- 0.5 mM, but was not blocked by TEA. The activation kinetics of the slow K+ current were described by a second order Hodgkin-Huxley model. The slow K+ current displayed bi-exponential inactivation. A fourth order Hodgkin-Huxley model for activation and first order for inactivation described the kinetics of the fast K+ current. In somatic cell-attached recordings, three classes of single K+ channels could be differentiated based on their unitary conductance and inactivation kinetics, a fast inactivating channel having a conductance of 13 +/- 1 pS, a slow inactivating channel having a conductance of 9.5 +/- 0.5 pS, and a very slowly inactivating channel having a conductance of 16 +/- 1 pS. The inactivation time constants of the slow and of the very slow K+ channel corresponded to the two inactivation time constants of the slow K+ current observed in nucleated patches. This suggested that two distinct K+ channels mediated the slow K+ current in nucleated patches. The three subtypes of K+ channels that were observed in somatic recordings were present along the apical dendrite. The amplitude of ensemble K+ currents in cell-attached patches decreased along the apical dendrite as the distance from the soma increased, with a slope of -0.9 +/- 0.3 pA per 100 microm. The results suggest that the decrease of the voltage-gated K+ channel density from the soma along the apical dendrite of L5 pyramidal neurones helps to define a distal, low threshold region for the initiation of dendritic regenerative potentials.

In CA1 pyramidal neurons of the hippocampus protein kinase C regulates calcium-dependent inactivation of NMDA receptors

The NMDA subtype of the glutamate-gated channel exhibits a high permeability to Ca(2+). The influx of Ca(2+) through NMDA channels is limited by a rapid and Ca(2+)/calmodulin (CaM)-dependent inactivation that results from a competitive displacement of cytoskeleton-binding proteins from the NR1 subunit of the receptor by Ca(2+)/CaM (Zhang et al., 1998; Krupp et al., 1999). The C terminal of this subunit can be phosphorylated by protein kinase C (PKC) (Tingley et al., 1993). The present study sought to investigate whether PKC regulates Ca(2+)-dependent inactivation of the NMDA channel in hippocampal neurons. Activation of endogenous PKC by 4beta-phorbol 12-myristate 13-acetate enhanced peak (I(p)) and depressed steady-state (I(ss)) NMDA-evoked currents, resulting in a reduction in the ratio of these currents (I(ss)/I(p)). We demonstrated previously that PKC activity enhances I(P) via a sequential activation of the focal adhesion kinase cell adhesion kinase beta/proline-rich tyrosine kinase 2 (CAKbeta/Pyk2) and the nonreceptor tyrosine kinase Src (Huang et al., 1999; Lu et al., 1999). Here, we report that the PKC-induced depression of I(ss) is unrelated to the PKC/CAKbeta/Src-signaling pathway but depends on the concentration of extracellular Ca(2+). Intracellular applications of CaM reduced I(ss)/I(p) and occluded the Ca(2+)-dependent effect of phorbol esters on I(ss.) Moreover, increasing the concentration of intracellular Ca(2+) buffer or intracellular application of the inhibitory CaM-binding peptide (KY9) greatly reduced the phorbol ester-induced depression of I(ss). Taken together, these results suggest that PKC enhances Ca(2+)/CaM-dependent inactivation of the NMDA channel, most likely because of a phosphorylation-dependent regulation of interactions between receptor subunits, CaM, and other postsynaptic density proteins.

C-Terminal truncation of NR2A subunits impairs synaptic but not extrasynaptic localization of NMDA receptors

NMDA receptors interact via the extended intracellular C-terminal domain of the NR2 subunits with constituents of the postsynaptic density for purposes of retention, clustering, and functional regulation at central excitatory synapses. To examine the role of the C-terminal domain of NR2A in the synaptic localization and function of NR2A-containing NMDA receptors in hippocampal Schaffer collateral-CA1 pyramidal cell synapses, we analyzed mice which express NR2A only in its C-terminally truncated form. In CA1 cell somata, the levels, activation, and deactivation kinetics of extrasynaptic NMDA receptor channels were comparable in wild-type and mutant NR2A(Delta)(C/)(Delta)(C) mice. At CA1 cell synapses, however, the truncated receptors were less concentrated than their full-length counterparts, as indicated by immunodetection in cultured neurons, synaptosomes, and postsynaptic densities. In the mutant, the NMDA component of evoked EPSCs was reduced in a developmentally progressing manner and was even more reduced in miniature EPSCs (mEPSCs) elicited by spontaneous glutamate release. Moreover, pharmacologically isolated NMDA currents evoked by synaptic stimulation had longer latencies and displayed slower rise and decay times, even in the presence of an NR2B-specific antagonist. These data strongly suggest that the C-terminal domain of NR2A subunits is important for the precise synaptic arrangement of NMDA receptors.

Anabolic steroids induce region- and subunit-specific rapid modulation of GABA(A) receptor-mediated currents in the rat forebrain

Anabolic-androgenic steroids (AAS) have become significant drugs of abuse in recent years with the highest increase reported in adolescent girls. In spite of the increased use of AAS, the CNS effects of these steroids are poorly understood. We report that in prepubertal female rats, three commonly abused AAS, 17alpha-methyltestosterone, stanozolol, and nandrolone, induced rapid and reversible modulation of GABAergic currents in neurons of two brain regions known to be critical for the expression of reproductive behaviors: the ventromedial nucleus of the hypothalamus (VMN) and the medial preoptic area (mPOA). All three AAS significantly enhanced peak synaptic current amplitudes and prolonged synaptic current decays in neurons of the VMN. Conversely all three AAS significantly diminished peak current amplitudes of synaptic currents from neurons of the mPOA. The endogenous neuroactive steroids, 3alpha-hydroxy-5alpha-pregnan-20-one and 5alpha-androstane-3alpha,17beta-diol, potentiated currents in the VMN as did the AAS. In contrast to the negative modulation induced by AAS in the mPOA, the endogenous steroids potentiated responses in this region. To determine the concentration response relationships, modulation by the AAS, 17alpha-methyltestosterone (17alpha-meT), was assessed for currents evoked by ultrafast perfusion of brief pulses of GABA to acutely isolated neurons. Half-maximal effects on currents elicited by 1 mM GABA were elicited by submicromolar concentrations of AAS for neurons from both brain regions. In addition, the efficacy of 10(-5) to 10(-2) M GABA was significantly increased by 1 microM 17alpha-meT. Previous studies have demonstrated a striking dichotomy in receptor composition between the VMN and the mPOA with regard to gamma subunit expression. To determine if the preferential expression of gamma(2) subunit-containing receptors in the VMN and of gamma(1) subunit-containing receptors in the mPOA could account for the region-specific effects of AAS in the two regions, responses elicited by ultrafast perfusion of GABA to human embryonic kidney 293 cells transfected with alpha(2), beta(3), and gamma(2) or alpha(2), beta(3), and gamma(1) subunit cDNAs were analyzed. As with native VMN neurons, positive modulation of GABA responses was elicited for alpha(2)beta(3)gamma(2) recombinant receptors, while negative modulation was induced at alpha(2)beta(3)gamma(1) receptors as in the mPOA. Our data demonstrate that AAS in doses believed to occur in steroid abusers can induce significant modulation of GABAergic transmission in brain regions essential for neuroendocrine function. In addition, the effects of these steroids can vary significantly between brain regions in a manner that appears to depend on the subunit composition of GABA(A) receptors expressed.

Cyclic GMP-dependent feedback inhibition of AMPA receptors is independent of PKG

In central neurons, the second messenger cGMP is believed to induce long-term changes in efficacy at glutamatergic synapses through activation of protein kinase G (PKG). Stimulating nitric oxide synthase, activating soluble guanylyl cyclase or elevating concentrations of intracellular cGMP depressed excitatory synaptic transmission in CA1 hippocampal neurons. Unexpectedly, intracellular cGMP depressed responses of AMPA receptors and inhibited excitatory postsynaptic currents in hippocampal neurons independently of phosphorylation. Our findings demonstrate that cGMP's modulation of excitatory transmission may involve a coupling of AMPA channel activity to levels of cGMP.

Selective depression of low-release probability excitatory synapses by sodium channel blockers

Sodium channels (NaChs) play a central role in action potential generation and are uniquely poised to influence the efficacy of transmitter release. We evaluated the effect of partial NaCh blockade on two aspects of synaptic efficacy First, we evaluated whether NaCh blockade accounts for the ability of certain drugs to selectively depress glutamate release. Second, we evaluated the contribution of NaChs to intraneuronal variability in glutamate release probability (p(r)). The antiglutamate drug riluzole nearly completely depresses glutamate excitatory postsynaptic currents (EPSCs) at concentrations that barely affect GABAergic inhibitory postsynaptic currents (IPSCs). NaCh inhibition explains the selective depression. Unlike other presynaptic depressants, partial NaCh blockade increases paired-pulse EPSC depression. This result is explained by selective depression of low-p(r) synapses. We conclude that local variations in the action potential contribute to p(r) variability among excitatory synapses.

Pregnenolone sulfate modulates inhibitory synaptic transmission by enhancing GABA(A) receptor desensitization

We examined the effects of the neurosteroid pregnenolone sulfate (PS) on GABA(A) receptor-mediated synaptic currents and currents elicited by rapid applications of GABA onto nucleated outside-out patches in cultured postnatal rat hippocampal neurons. At 10 microm, PS significantly depressed peak responses and accelerated the decay of evoked inhibitory synaptic currents. In nucleated outside-out patches, PS depressed peak currents and speeded deactivation after 5 msec applications of a saturating concentration of GABA. PS also increased the rate and degree of macroscopic GABA receptor desensitization during prolonged GABA applications. In a paired GABA application paradigm, PS slowed the rate of recovery from desensitization. In contrast to its prominent effects on currents produced by saturating GABA concentrations, PS had only small effects on peak currents and failed to alter deactivation after brief applications of the weakly desensitizing GABA(A) receptor agonists taurine and beta-alanine. However, when beta-alanine was applied for a sufficient duration to promote receptor desensitization, PS augmented macroscopic desensitization and slowed deactivation. These results suggest that PS inhibits GABA-gated chloride currents by enhancing receptor desensitization and stabilizing desensitized states. This contention is supported by kinetic modeling studies in which increases in the rate of entry into doubly liganded desensitized states mimic most effects of PS.

Dysfunctions in mice by NMDA receptor point mutations NR1(N598Q) and NR1(N598R)

NMDA receptors in mice were mutated by gene targeting to substitute asparagine (N) in position 598 of the NR1 subunit to glutamine (Q) or arginine (R). Animals expressing exclusively the mutated NR1 alleles, NR1(Q/Q) and NR1(-/R) mice, developed a perinatally lethal phenotype mainly characterized by respiratory failure. The dysfunctions were partially rescued in heterozygous mice by the presence of pure wild-type receptors. Thus, NR1(+/Q) mice exhibited reduced life expectancy, with females being impaired in nurturing; NR1(+/R) mice displayed signs of underdevelopment such as growth retardation and impaired righting reflex, and died before weaning. We analyzed the key properties of NMDA receptors, high Ca(2+) permeability, and voltage-dependent Mg(2+) block, in the mutant mice. Comparison of the complex physiological and phenotypical changes observed in the different mutants indicates that properties controlled by NR1 subunit residue N598 are important for autonomic brain functions at birth and during postnatal development. We conclude that disturbed NMDA receptor signaling mediates a variety of neurological phenotypes.

Pregnenolone sulfate modulates inhibitory synaptic transmission by enhancing GABA(A) receptor desensitization

We examined the effects of the neurosteroid pregnenolone sulfate (PS) on GABA(A) receptor-mediated synaptic currents and currents elicited by rapid applications of GABA onto nucleated outside-out patches in cultured postnatal rat hippocampal neurons. At 10 microm, PS significantly depressed peak responses and accelerated the decay of evoked inhibitory synaptic currents. In nucleated outside-out patches, PS depressed peak currents and speeded deactivation after 5 msec applications of a saturating concentration of GABA. PS also increased the rate and degree of macroscopic GABA receptor desensitization during prolonged GABA applications. In a paired GABA application paradigm, PS slowed the rate of recovery from desensitization. In contrast to its prominent effects on currents produced by saturating GABA concentrations, PS had only small effects on peak currents and failed to alter deactivation after brief applications of the weakly desensitizing GABA(A) receptor agonists taurine and beta-alanine. However, when beta-alanine was applied for a sufficient duration to promote receptor desensitization, PS augmented macroscopic desensitization and slowed deactivation. These results suggest that PS inhibits GABA-gated chloride currents by enhancing receptor desensitization and stabilizing desensitized states. This contention is supported by kinetic modeling studies in which increases in the rate of entry into doubly liganded desensitized states mimic most effects of PS.

Acute decrease in net glutamate uptake during energy deprivation

The extracellular glutamate concentration ([glu](o)) rises during cerebral ischemia, reaching levels capable of inducing delayed neuronal death. The mechanisms underlying this glutamate accumulation remain controversial. We used N-methyl-D-aspartate receptors on CA3 pyramidal neurons as a real-time, on-site, glutamate sensor to identify the source of glutamate release in an in vitro model of ischemia. Using glutamate and L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) as substrates and DL-threo-beta-benzyloxyaspartate (TBOA) as an inhibitor of glutamate transporters, we demonstrate that energy deprivation decreases net glutamate uptake within 2-3 min and later promotes reverse glutamate transport. This process accounts for up to 50% of the glutamate accumulation during energy deprivation. Enhanced action potential-independent vesicular release also contributes to the increase in [glu](o), by approximately 50%, but only once glutamate uptake is inhibited. These results indicate that a significant rise in [glu](o) already occurs during the first minutes of energy deprivation and is the consequence of reduced uptake and increased vesicular and nonvesicular release of glutamate.

D-serine is an endogenous ligand for the glycine site of the N-methyl-D-aspartate receptor

Functional activity of N-methyl-D-aspartate (NMDA) receptors requires both glutamate binding and the binding of an endogenous coagonist that has been presumed to be glycine, although D-serine is a more potent agonist. Localizations of D-serine and it biosynthetic enzyme serine racemase approximate the distribution of NMDA receptors more closely than glycine. We now show that selective degradation of d-serine with D-amino acid oxidase greatly attenuates NMDA receptor-mediated neurotransmission as assessed by using whole-cell patch-clamp recordings or indirectly by using biochemical assays of the sequelae of NMDA receptor-mediated calcium flux. The inhibitory effects of the enzyme are fully reversed by exogenously applied D-serine, which by itself did not potentiate NMDA receptor-mediated synaptic responses. Thus, D-serine is an endogenous modulator of the glycine site of NMDA receptors and fully occupies this site at some functional synapses.

Dysfunctions in mice by NMDA receptor point mutations NR1(N598Q) and NR1(N598R)

NMDA receptors in mice were mutated by gene targeting to substitute asparagine (N) in position 598 of the NR1 subunit to glutamine (Q) or arginine (R). Animals expressing exclusively the mutated NR1 alleles, NR1(Q/Q) and NR1(-/R) mice, developed a perinatally lethal phenotype mainly characterized by respiratory failure. The dysfunctions were partially rescued in heterozygous mice by the presence of pure wild-type receptors. Thus, NR1(+/Q) mice exhibited reduced life expectancy, with females being impaired in nurturing; NR1(+/R) mice displayed signs of underdevelopment such as growth retardation and impaired righting reflex, and died before weaning. We analyzed the key properties of NMDA receptors, high Ca(2+) permeability, and voltage-dependent Mg(2+) block, in the mutant mice. Comparison of the complex physiological and phenotypical changes observed in the different mutants indicates that properties controlled by NR1 subunit residue N598 are important for autonomic brain functions at birth and during postnatal development. We conclude that disturbed NMDA receptor signaling mediates a variety of neurological phenotypes.

Kinetic differences between synaptic and extrasynaptic GABA(A) receptors in CA1 pyramidal cells

GABA(A)-mediated IPSCs typically decay more rapidly than receptors in excised patches in response to brief pulses of applied GABA. We have investigated the source of this discrepancy in CA1 pyramidal neurons. IPSCs in these cells decayed rapidly, with a weighted time constant tau(Decay) of approximately 18 msec (24 degrees C), whereas excised and nucleated patch responses to brief pulses of GABA (2 msec, 1 mM) decayed more than three times as slowly (tau(Decay), approximately 63 msec). This discrepancy was not caused by differences between synaptic and exogenous transmitter transients because (1) there was no dependence of tau(Decay) on pulse duration for pulses of 0.6-4 msec, (2) responses to GABA at concentrations as low as 10 microM were still slower to decay (tau(Decay), approximately 41 msec) than IPSCs, and (3) responses of excised patches to synaptically released GABA had decay times similar to brief pulse responses. These data indicate that the receptors mediating synaptic versus brief pulse responses have different intrinsic properties. However, synaptic receptors were not altered by the patch excision process, because fast, spontaneous IPSCs could still be recorded in nucleated patches. Elevated calcium selectively modulated patch responses to GABA pulses, with no effect on IPSCs recorded in nucleated patches, demonstrating the presence of two receptor populations that are differentially regulated by intracellular second messengers. We conclude that two receptor populations with distinct kinetics coexist in CA1 pyramidal cells: slow extrasynaptic receptors that dominate the responses of excised patches to exogenous GABA applications and fast synaptic receptors that generate rapid IPSCs.

Kinetic differences between synaptic and extrasynaptic GABA(A) receptors in CA1 pyramidal cells

GABA(A)-mediated IPSCs typically decay more rapidly than receptors in excised patches in response to brief pulses of applied GABA. We have investigated the source of this discrepancy in CA1 pyramidal neurons. IPSCs in these cells decayed rapidly, with a weighted time constant tau(Decay) of approximately 18 msec (24 degrees C), whereas excised and nucleated patch responses to brief pulses of GABA (2 msec, 1 mM) decayed more than three times as slowly (tau(Decay), approximately 63 msec). This discrepancy was not caused by differences between synaptic and exogenous transmitter transients because (1) there was no dependence of tau(Decay) on pulse duration for pulses of 0.6-4 msec, (2) responses to GABA at concentrations as low as 10 microM were still slower to decay (tau(Decay), approximately 41 msec) than IPSCs, and (3) responses of excised patches to synaptically released GABA had decay times similar to brief pulse responses. These data indicate that the receptors mediating synaptic versus brief pulse responses have different intrinsic properties. However, synaptic receptors were not altered by the patch excision process, because fast, spontaneous IPSCs could still be recorded in nucleated patches. Elevated calcium selectively modulated patch responses to GABA pulses, with no effect on IPSCs recorded in nucleated patches, demonstrating the presence of two receptor populations that are differentially regulated by intracellular second messengers. We conclude that two receptor populations with distinct kinetics coexist in CA1 pyramidal cells: slow extrasynaptic receptors that dominate the responses of excised patches to exogenous GABA applications and fast synaptic receptors that generate rapid IPSCs.

The general anesthetic propofol slows deactivation and desensitization of GABA(A) receptors

Propofol (2,6-di-isopropylphenol) has multiple actions on GABA(A) receptor function that act in concert to potentiate GABA-evoked currents. To understand how propofol influences inhibitory IPSCs, we examined the effects of propofol on responses to brief applications of saturating concentrations of GABA (1-30 mM). GABA was applied using a fast perfusion system to nucleated patches excised from hippocampal neurons. In this preparation, propofol (10 microM) had no detectable agonist effect but slowed the decay, increased the charge transfer (62%), and enhanced the peak amplitude (8%) of currents induced by brief pulses (3 msec) of GABA. Longer pulses (500 msec) of GABA induced responses that desensitized with fast (tau(f) = 1.5-4.5 msec) and slow (tau(s) = 1-3 sec) components and, after the removal of GABA, deactivated exponentially (tau(d) = 151 msec). Propofol prolonged this deactivation (tau(d) = 255 msec) and reduced the development of both fast and slow desensitization. Recovery from fast desensitization, assessed using pairs of brief pulses of GABA, paralleled the time course of deactivation, indicating that fast desensitization traps GABA on the receptor. With repetitive applications of pulses of GABA (0.33 Hz), the charge transfer per pulse declined exponentially (tau approximately 15 sec) to a steady-state value equal to approximately 40% of the initial response. Despite the increased charge transfer per pulse with propofol, the time course of the decline was unchanged. These experimental data were interpreted using computer simulations and a kinetic model that assumed fast and slow desensitization, as well as channel opening developed in parallel from a pre-open state. Our results suggest that propofol stabilizes the doubly liganded pre-open state without affecting the isomerization rate constants to and from the open state. Also, the rate constants for agonist dissociation and entry into the fast and slow desensitization states were reduced by propofol. The recovery rate constant from fast desensitization was slowed, whereas that from slow desensitization appeared to be unchanged. Taken together, the effects of propofol on GABA(A) receptors enhance channel opening, particularly under conditions that promote desensitization.

Models of calmodulin trapping and CaM kinase II activation in a dendritic spine

Activation of calcium/calmodulin-dependent protein kinase II (CaMKII) by calmodulin following calcium entry into the cell is important for long-term potentiation (LTP). Here a model of calmodulin binding and trapping by CaMKII in a dendritic spine was used to estimate levels and durations of CaMKII activation following LTP-inducing tetani. The calcium signal was calcium influx through NMDA receptor channels computed in a highly detailed dentate granule cell model. Calcium could bind to calmodulin and calmodulin to CaMKII. CaMKII subunits were either free, bound with calmodulin, trapped, autonomous, or capped. Strong low-frequency tetanic input produced little calmodulin trapping or CaMKII activation. Strong high-frequency tetanic input caused large numbers of CaMKII subunits to become trapped, and CaMKII was strongly activated. Calmodulin trapping and CaMKII activation were highly dependent on tetanus frequency (particularly between 10 and 100 Hz) and were highly sensitive to relatively small changes in the calcium signal. Repetition of a short high-frequency tetanus was necessary to achieve high levels of CaMKII activation. Three stages of CaMKII activation were found in the model: a short, highly activated stage; an intermediate, moderately active stage; and a long-lasting third stage, whose duration depended on dephosphorylation rates but whose decay rate was faster at low CaMKII activation levels than at high levels. It is not clear which of these three stages is most important for LTP.

The general anesthetic propofol slows deactivation and desensitization of GABA(A) receptors

Propofol (2,6-di-isopropylphenol) has multiple actions on GABA(A) receptor function that act in concert to potentiate GABA-evoked currents. To understand how propofol influences inhibitory IPSCs, we examined the effects of propofol on responses to brief applications of saturating concentrations of GABA (1-30 mM). GABA was applied using a fast perfusion system to nucleated patches excised from hippocampal neurons. In this preparation, propofol (10 microM) had no detectable agonist effect but slowed the decay, increased the charge transfer (62%), and enhanced the peak amplitude (8%) of currents induced by brief pulses (3 msec) of GABA. Longer pulses (500 msec) of GABA induced responses that desensitized with fast (tau(f) = 1.5-4.5 msec) and slow (tau(s) = 1-3 sec) components and, after the removal of GABA, deactivated exponentially (tau(d) = 151 msec). Propofol prolonged this deactivation (tau(d) = 255 msec) and reduced the development of both fast and slow desensitization. Recovery from fast desensitization, assessed using pairs of brief pulses of GABA, paralleled the time course of deactivation, indicating that fast desensitization traps GABA on the receptor. With repetitive applications of pulses of GABA (0.33 Hz), the charge transfer per pulse declined exponentially (tau approximately 15 sec) to a steady-state value equal to approximately 40% of the initial response. Despite the increased charge transfer per pulse with propofol, the time course of the decline was unchanged. These experimental data were interpreted using computer simulations and a kinetic model that assumed fast and slow desensitization, as well as channel opening developed in parallel from a pre-open state. Our results suggest that propofol stabilizes the doubly liganded pre-open state without affecting the isomerization rate constants to and from the open state. Also, the rate constants for agonist dissociation and entry into the fast and slow desensitization states were reduced by propofol. The recovery rate constant from fast desensitization was slowed, whereas that from slow desensitization appeared to be unchanged. Taken together, the effects of propofol on GABA(A) receptors enhance channel opening, particularly under conditions that promote desensitization.

Subunit- and site-specific pharmacology of the NMDA receptor channel

N-Methyl-D-aspartate (NMDA) receptor channels play important roles in various physiological functions such as synaptic plasticity and synapse formation underlying memory, learning and formation of neural networks during development. They are also important for a variety of pathological states including acute and chronic neurological disorders, psychiatric disorders, and neuropathic pain syndromes. cDNA cloning has revealed the molecular diversity of NMDA receptor channels. The identification of multiple subunits with distinct distributions, properties and regulation, implies that NMDA receptor channels are heterogeneous in their pharmacological properties, depending on the brain region and the developmental stage. Furthermore, mutation studies have revealed a critical role for specific amino acid residues in certain subunits in determining the pharmacological properties of NMDA receptor channels. The molecular heterogeneity of NMDA receptor channels as well as their dual role in physiological and pathological functions makes it necessary to develop subunit- and site-specific drugs for precise and selective therapeutic intervention. This review summarizes from a molecular perspective the recent advances in our understanding of the pharmacological properties of NMDA receptor channels with specific references to agonists binding sites, channel pore regions, allosteric modulation sites for protons, polyamines, redox agents, Zn2+ and protein kinases, phosphatases.

Inhibition of uptake unmasks rapid extracellular turnover of glutamate of nonvesicular origin

Maintaining glutamate at low extracellular concentrations in the central nervous system is necessary to protect neurons from excitotoxic injury and to ensure a high signal-to-noise ratio for glutamatergic synaptic transmission. We have used DL-threo-beta-benzyloxyaspartate (TBOA), an inhibitor of glutamate uptake, to determine the role of glutamate transporters in the regulation of extracellular glutamate concentration. By using the N-methyl-D-aspartate receptors of patched CA3 hippocampal neurons as "glutamate sensors," we observed that application of TBOA onto organotypic hippocampal slices led to a rapid increase in extracellular glutamate concentration. This increase was Ca(2+)-independent and was observed in the presence of tetrodotoxin. Moreover, prevention of vesicular glutamate release with clostridial toxins did not affect the accumulation of glutamate when uptake was inhibited. Inhibition of glutamine synthase, however, increased the rate of accumulation of extracellular glutamate, indicating that glial glutamate stores can serve as a source in this process. TBOA blocked synaptically evoked transporter currents in astrocytes without inducing a current mediated by the glutamate transporter. This indicates that this inhibitor is not transportable and does not release glutamate by heteroexchange. These results show that under basal conditions, the activity of glutamate transporters compensates for the continuous, nonvesicular release of glutamate from the intracellular compartment. As a consequence, acute disruption of transporter activity immediately results in significant accumulation of extracellular glutamate.

Ca2+ permeability and kinetics of glutamate receptors in rat medial habenula neurones: implications for purinergic transmission in this nucleus

1. We have previously investigated P2X receptor-mediated synaptic currents in medial habenula neurones and shown that they can be calcium permeable. We now investigate the receptor properties of glutamate, the other, more abundant excitatory transmitter, to determine its receptor subtypes and their relative calcium permeability. This may have implications for the physiological role of the P2X receptors which mediate synaptic currents. 2. Using fast application of ATP, L-glutamate or kainate to nucleated patches, glutamate receptors were determined to be of the AMPA subtype but no functional P2X receptors were detected. 3. The deactivation and desensitization rates of the AMPA channel were determined to have time constants of 1.77 +/- 0.21 ms (n = 10) and 4.01 +/- 0.85 ms (n = 9) at -60 mV, respectively. AMPA receptors recovered from desensitization with two exponential components with time constants of 21.08 +/- 2.95 and 233.60 +/- 51.1 ms (n = 3). None of the deactivation or desensitization properties of the GluR channels depended on membrane potential. 4. The current-voltage relationship under different ionic conditions revealed that the GluR channel was equally permeable to Cs+ and Na+ but relatively impermeable to Ca2+ (PCa/PCs = 0.13, n = 6). 5. For both synaptic currents and somatic currents activated by fast application of L-glutamate to nucleated patches, decay time constants were similar at +/-60 mV in the presence of Mg2+ ions. Thus GluR channels appear to be of the AMPA subtype and not the NMDA subtype. 6. Thus, under the conditions of this study, neurones of the medial habenula lack functional NMDA receptors and possess AMPA receptors that have low permeability to Ca2+. We conclude that the P2X receptor-mediated synaptic currents are the only calcium-permeable fast-transmitter gated currents in these neurones which may be important for their physiological function.

Active NMDA glutamate receptors are expressed by mammalian osteoclasts

1. The N-methyl-D-aspartate (NMDA) glutamate receptor, widely distributed in the mammalian nervous system, has recently been identified in bone. In this study, we have investigated whether NMDA receptors expressed by osteoclasts have an electrophysiological activity. 2. Using the patch clamp technique two agonists of the NMDA receptor, L-glutamate (Glu) and NMDA, were shown to activate whole-cell currents recorded in isolated rabbit osteoclasts. 3. The current-voltage (I-V ) relationships of the currents induced by Glu (IGlu) and NMDA (INMDA) were studied using Mg2+-free solutions. The agonist-induced currents had a linear I-V relationship with a reversal potential near 0 mV, as expected for a voltage independent and non-selective cationic current. 4. IGlu and INMDA were sensitive to specific blockers of NMDA subtype glutamate receptors, such as magnesium ions, (5R, 10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten -5,10-imine (MK-801) and 1-(1,2-diphenylethyl) piperidine (DEP). The block of IGlu and INMDA by these specific antagonists was voltage dependent, strong for negative potentials (inward current) and absent for positive potentials (outward current). 5. These results demonstrate that NMDA receptors are functional in rabbit osteoclasts, and that their electrophysiological and pharmacological properties in these cells are similar to those documented for neuronal cells. Active NMDA receptors expressed by osteoclasts may represent a new target for regulating bone resorption.

Lack of AMPA receptor desensitization during basal synaptic transmission in the hippocampal slice

Excitatory postsynaptic currents in the CA1 region of rat hippocampal slices are mediated primarily by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in response to synaptically released glutamate. Outside-out patches from pyramidal cells in this region have shown that AMPA receptors are desensitized by short (1 ms) pulses of glutamate. We have taken a number of approaches to ask whether synaptic receptors desensitize in response to synaptically released glutamate in the slice. Recordings with paired pulses and minimal stimulation conditions that are presumably activating only a single release site do not show evidence for desensitization. Furthermore, cyclothiazide, a drug that blocks desensitization, does not alter paired-pulse ratios even under conditions of high probability of release, which should maximize desensitization. These results suggest that synaptic receptors do not desensitize in response to synaptically released glutamate during basal synaptic transmission.

GABA- and glutamate-activated channels in green fluorescent protein-tagged gonadotropin-releasing hormone neurons in transgenic mice

Mice were generated expressing green fluorescent protein (GFP) under the control of the gonadotropin-releasing hormone (GnRH) promoter. Green fluorescence was observed in, and restricted to, GnRH-immunopositive neuronal somata in the olfactory bulb, ganglion terminale, septal nuclei, diagonal band of Broca (DBB), preoptic area (POA), and caudal hypothalamus, as well as GnRH neuronal dendrites and axons, including axon terminals in the median eminence and organum vasculosum of the lamina terminalis (OVLT). Whole-cell recordings from GFP-expressing GnRH neurons in the OVLT-POA-DBB region revealed a firing pattern among GFP-expressing GnRH neurons distinct from that of nonfluorescent neurons. Nucleated patches of GFP-expressing GnRH neurons exhibited pronounced responses to fast application of GABA and smaller responses to L-glutamate and AMPA. One-fifth of the nucleated patches responded to NMDA. The GABA-A, AMPA, and NMDA receptor channels on GnRH neurons mediating these responses may play a role in the modulation of GnRH secretory oscillations.

Developmental study of miniature IPSCs of CA3 hippocampal cells: modulation by midazolam

Maturation of GABAA/benzodiazepine receptors is associated with changes in their subunit composition. We have investigated whether these changes are accompanied by a developmental modification in the kinetic properties of miniature IPSCs (mIPSCs) and sensitivity to midazolam, a benzodiazepine agonist. In the presence of TTX (10 microM) and excitatory amino acid antagonists, AP5 (50 microM) and CNQX (50 microM), we whole-cell recorded mIPSCs in CA3 cells of hippocampal slices of adult and young (4-8 days) rats. mIPSCs were mediated by GABAA receptors as they were suppressed by bicuculline (10 microM). In both the adult and young rats, mIPSCs were similar in amplitude and kinetic properties. However, the mIPSCs frequency markedly increased with age from 4+/-3 Hz in the young rats to 20+/-9 Hz in the adult rats. In both age groups, midazolam (0.01 microM(-10) microM) and pentobarbital (30 microM) did not affect the amplitude, frequency and rise time of the mIPSCs but they increased to a similar extent their decay time constant. The current responses to isoguvacine, a GABAA agonist, were potentiated by 0.1 microM midazolam in both age groups. It is concluded that in immature and adult rats, synaptic GABAA receptors of CA3 were not different in their kinetic properties and sensitivity to midazolam.

GABA- and glutamate-activated channels in green fluorescent protein-tagged gonadotropin-releasing hormone neurons in transgenic mice

Mice were generated expressing green fluorescent protein (GFP) under the control of the gonadotropin-releasing hormone (GnRH) promoter. Green fluorescence was observed in, and restricted to, GnRH-immunopositive neuronal somata in the olfactory bulb, ganglion terminale, septal nuclei, diagonal band of Broca (DBB), preoptic area (POA), and caudal hypothalamus, as well as GnRH neuronal dendrites and axons, including axon terminals in the median eminence and organum vasculosum of the lamina terminalis (OVLT). Whole-cell recordings from GFP-expressing GnRH neurons in the OVLT-POA-DBB region revealed a firing pattern among GFP-expressing GnRH neurons distinct from that of nonfluorescent neurons. Nucleated patches of GFP-expressing GnRH neurons exhibited pronounced responses to fast application of GABA and smaller responses to L-glutamate and AMPA. One-fifth of the nucleated patches responded to NMDA. The GABA-A, AMPA, and NMDA receptor channels on GnRH neurons mediating these responses may play a role in the modulation of GnRH secretory oscillations.

Modulation of the channel activity of the epsilon2/zeta1-subtype N-methyl D-aspartate receptor by PSD-95

A channel-associated protein PSD-95 has been shown to induce clustering of N-methyl D-aspartate (NMDA) receptors, interacting with the COOH terminus of the epsilon subunit of the receptors. The effects of PSD-95 on the channel activity of the epsilon2/zeta1 heteromeric NMDA receptor were examined by injection of PSD-95 cRNA into Xenopus oocytes expressing the NMDA receptors. Expression of PSD-95 decreased the sensitivity of the NMDA receptor channels to L-glutamate. Mutational studies showed that the interaction between the COOH terminus of the epsilon2 subunit of the NMDA receptor and the second PSD-95/Dlg/Z0-1 domain of PSD-95 is critical for the decrease in glutamate sensitivity. It is known that protein kinase C markedly potentiates the channel activity of the NMDA receptor expressed in oocytes. PSD-95 inhibited the protein kinase C-mediated potentiation of the channels. Thus, we demonstrated that PSD-95 functionally modulates the channel activity of the epsilon2/zeta1 NMDA receptor. PSD-95 makes signal transmission more efficient by clustering the channels at postsynaptic sites. In addition to this, our results suggest that PSD-95 plays a protective role against neuronal excitotoxicity by decreasing the glutamate sensitivity of the channels and by inhibiting the protein kinase C-mediated potentiation of the channels.

Distal versus proximal inhibitory shaping of feedback excitation in the electrosensory lateral line lobe: implications for sensory filtering

Distal versus proximal inhibitory shaping of feedback excitation in the electrosensory lateral line lobe: implications for sensory filtering. J. Neurophysiol. 80: 3214-3232, 1998. The inhibition controlling the indirect descending feedback (parallel fibers originating from cerebellar granule cells in the eminentia posterior pars granularis) to electrosensory lateral line lobe (ELL) pyramidal cells was studied using intracellular recording techniques in vitro. Parallel fibers (PF) contact stellate cells and dendrites of ventral molecular layer (VML) GABAergic interneurons. Stellate cells provide local input to pyramidal cell distal dendrites, whereas VML cells contact their somata and proximal dendrites. Single-pulse stimulation of PF evoked graded excitatory postsynaptic potentials (EPSPs) that were blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl--aspartate (NMDA) antagonists. The EPSPs peaked at 6.4 +/- 1.8 ms (mean +/- SE; n = 11) but took >50 ms to decay completely. Tetanic stimulation (100 ms, 100 Hz) produced a depolarizing wave with individual EPSPs superimposed. The absolute amplitude of the individual EPSPs decreased during the train. Spike rates, established by injected current, mostly were increased, but in some cells were decreased, by tetanic stimulation. Global application of a gamma-aminobutyric acid-A (GABAA) antagonist to the recorded cell's soma and apical dendritic region increased the EPSP peak and decay phase amplitudes. Tetanic stimulation always increased current-evoked spike rates after GABAA blockade during, and for several hundred milliseconds after, the stimulus. Application of a GABAB antagonist did not have any significant effects on the PF-evoked response. This, and the lack of any long hyperpolarizing inhibitory postsynaptic potentials, suggests that VML and stellate cell inhibition does not involve GABAB receptors. Focal GABAA antagonist applications to the dorsal molecular layer (DML) and pyramidal cell layer (PCL) had contrasting effects on PF-evoked EPSPs. DML GABAA blockade significantly increased the EPSP peak amplitude but not the decay phase of the EPSP, whereas PCL GABAA-blockade significantly increased the decay phase, but not the EPSP peak, amplitude. The order of antagonist application did not affect the outcome. On the basis of the known circuitry of the ELL, we conclude that the distal inhibition originated from GABAergic molecular layer stellate cells and the proximal inhibition originated from GABAergic cells of the ventral molecular layer (VML cells). Computer modeling of distal and proximal inhibition suggests that intrinsic differences in IPSP dynamics between the distal and proximal sites may be amplified by voltage-dependent NMDA receptor and persistent sodium currents. We propose that the different time courses of stellate cell and VML cell inhibition allows them to act as low- and high-pass filters respectively on indirect descending feedback to ELL pyramidal cells.

Opposite modulation of NMDA receptors by lysophospholipids and arachidonic acid: common features with mechanosensitivity

1. Two classes of amphiphilic compounds, lysophospholipids and arachidonic acid, have been suggested to produce opposite deformations of the lipid bilayer. We have found that their effects on N-methyl-D-aspartate (NMDA) responses are opposite, and resemble those of mechanical deformations of the plasma membrane. 2. Lysophospholipids inhibited NMDA responses both in nucleated patches taken from cultured neurons and in cells expressing recombinant NMDA receptors. This inhibition was reversible, voltage independent and stronger at non-saturating doses of agonist. It was not linked to the charge of the polar head, and was not mimicked by lysophosphatidic acid or phosphatidylcholine. In outside-out patches, lysophospholipids reduced the open probability of NMDA-activated channels without changing their single-channel conductance. 3. The inhibition produced by lysophospholipids occluded that produced by a mechanical compression induced by changes in osmotic or hydrostatic pressure. 4. The potentiation of NMDA responses by arachidonic acid was observed both in native and recombinant receptors, including those in which the putative 'fatty acid binding domain' had been deleted. This suggests that, like lysophospholipids, arachidonic acid alters the NMDA receptor by insertion into the lipid bilayer. 5. Recombinant receptors in which the cytoplasmic tails had been modified or deleted were still sensitive to mechanical deformation. A linkage to the cytoskeleton is therefore not required for NMDA receptor mechanosensitivity. 6. The fact that the NMDA responses are depressed similarly by compression and lysophospholipids, and potentiated similarly by stretch and arachidonic acid supports the notion that the modulation of NMDA receptor activity by asymmetrical amphiphilic compounds involves pressure changes transmitted through the lipid bilayer. Compounds with a large hydrophilic head mimic the effects of a compression, and compounds with a small hydrophilic head mimic the effects of stretch.

The AMPA receptor subunit GluR-B in its Q/R site-unedited form is not essential for brain development and function

Calcium permeability of L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) in excitatory neurons of the mammalian brain is prevented by coassembly of the GluR-B subunit, which carries an arginine (R) residue at a critical site of the channel pore. The codon for this arginine is created by site-selective adenosine deamination of an exonic glutamine (Q) codon at the pre-mRNA level. Thus, central neurons can potentially control the calcium permeability of AMPARs by the level of GluR-B gene expression as well as by the extent of Q/R-site editing, which in postnatal brain, positions the R codon into >99% of GluR-B mRNA. To study whether the small amount of unedited GluR-B is of functional relevance, we have generated mice carrying GluR-B alleles with an exonic arginine codon. We report that these mutants manifest no obvious deficiencies, indicating that AMPAR-mediated calcium influx into central neurons can be solely regulated by the levels of Q/R site-edited GluR-B relative to other AMPAR subunits. Notably, a targeted GluR-B gene mutant with 30% reduced GluR-B levels had 2-fold higher AMPAR-mediated calcium permeability in hippocampal pyramidal cells with no sign of cytotoxicity. This constitutes proof in vivo that elevated calcium influx through AMPARs need not generate pathophysiological consequences.

Multiple sites of action of neomycin, Mg2+ and spermine on the NMDA receptors of rat hippocampal CA1 pyramidal neurones

1. The effects of neomycin on NMDA-evoked currents in isolated CA1 hippocampal pyramidal neurones were investigated and single channel activity was examined in outside-out patches taken from cultured hippocampal neurones. The effects of neomycin on two combinations of NMDA receptor subunits (NR1a-NR2A and NR1a-NR2B) expressed in human embryonic kidney (HEK293) cells were also studied. 2. Neomycin (0. 01-1 mM) caused a potentiation of NMDA-activated currents in all neurones examined. No evidence of a voltage-dependent depression was observed in whole-cell recordings. 3. In outside-out patch recordings relatively low concentrations (30 and 100 microM) of neomycin caused a voltage-dependent reduction in single channel current amplitude as well as a large increase in the frequency of channel opening. 4. In saturating concentrations of glycine, neomycin enhanced NMDA-activated currents and this glycine-independent enhancement was confirmed using recombinant NR1a-NR2B receptors. Neomycin substantially increased the potency of glycine for the receptor by reducing the rate of dissociation of glycine from the receptor. Neomycin demonstrated a glycine-dependent enhancement of currents mediated by the NR1a-NR2A combination of subunits but a paradoxical depression was observed in saturating concentrations of glycine. 5. Neomycin increased the rate of deactivation of glutamate-activated currents consistent with neomycin causing a reduction in the affinity of the receptor for agonist. 6. These results indicate that neomycin has multiple and complex effects on NMDA receptors.

Interactions of GYKI 52466 and NBQX with cyclothiazide at AMPA receptors: experiments with outside-out patches and EPSCs in hippocampal neurones

In outside-out patches from cultured hippocampal neurones, glutamate (1 mM) applied for 1 ms evoked currents which rose rapidly (tau(on) 451 +/- 31 micros) to a peak and then deactivated with slower kinetics (1.95 +/- 0.13 ms). Offset time constants were significantly slower with longer application durations (tau(off) 3.10 +/- 0.19, 3.82 +/- 0.25, 4.80 +/- 0.65 and 7.56 +/- 0.65 ms with 10, 20, 100 and 500 ms applications respectively). Desensitization was complete within 100 ms with a similar rate for all application durations (4.74 +/- 0.34 ms with 100 ms applications). GYKI 52466 reduced inward peak currents with an IC50 of 11.7 +/- 0.6 microM and had similar potency on steady-state currents to longer glutamate applications. GYKI 52466 had no significant effect on desensitization or deactivation time constants but caused a modest and significant prolongation of onset kinetics at higher concentrations. Cyclothiazide (100 microM) potentiated steady-state currents 25-fold at 100 ms and caused a modest but significant slowing in onset kinetics (601 +/- 49 micros with 1 ms applications) but a more pronounced prolongation of deactivation time constants (5.55 +/- 0.66 ms with 1 ms applications). In 50% of neuronal patches cyclothiazide completely eliminated desensitization. In those patches with residual desensitization, the rate was not significantly different to control (5.36 +/- 0.43 ms with 100 ms applications). Following 100 ms applications of glutamate, GYKI 52466 had IC50s of 11.7 +/- 1.1 microM and 75.1 +/- 7.0 microM in the absence and presence of cyclothiazide (100 microM) respectively. Onset kinetics were slowed from 400 +/- 20 micros to 490 +/- 30 micros by cyclothiazide (100 microM) and then further prolonged by GYKI 52466 (100 microM) to a double exponential function (tau(on1) 1.12 +/- 0.13 ms and tau(on2) 171.5 +/- 36.5 ms). GYKI 52466 did not re-introduce desensitization but concentration-dependently weakened cyclothiazide's prolongation of deactivation time constants (1 ms applications: 5.01 +/- 0.71, 4.47 +/- 0.80 and 2.28 +/- 0.64 ms with GYKI 52466 30, 100 and 300 microM respectively). NBQX reduced peak current responses with an IC50 of 28.2 +/- 1.3 nM. Paradoxically, steady-state currents with 500 ms applications of glutamate were potentiated from 3.3 +/- 1.2 pA to 29.4 +/- 6.4 pA by NBQX (1 nM). Higher concentrations of NBQX then antagonized this potentiated response. The potency of NBQX in antagonizing steady-state currents to 500 ms applications of glutamate (IC50 120.9 +/- 30.2 nM) was 2-fold less than following 100 ms applications (IC50 67.7 +/- 2.6 nM). NBQX had no effect on rapid onset, desensitization or deactivation time constants. However, a slow relaxation of inhibition was seen with longer applications. NBQX was 2-5-fold less potent against inward currents in the presence of cyclothiazide (100 microM) depending on the application duration but had no effect on the rapid onset, desensitization or deactivation time constants. The same relaxation of inhibition was seen as with NBQX alone. NBQX (1 microM) reduced AMPA receptor-mediated EPSC amplitude to 7 +/- 1% of control with no effect on kinetics. Cyclothiazide (330 microM) caused a 2.8-fold prolongation of the decay time constant (control 26.6 +/- 2.2 ms, cyclothiazide 74.2 +/- 7.6 ms, n = 9). Additional application of NBQX (1 microM) partly reversed this prolongation to 1.9 fold (47.7 +/- 2.5 ms, n = 5). These results support previous findings that cyclothiazide also allosterically influences AMPA receptor agonist/antagonist recognition sites. There were no interactions between NBQX and cyclothiazide on desensitization or deactivation time constants of glutamate-induced currents but clear interactions on EPSC deactivation kinetics. This raises the possibility that the interactions of NBQX, GYKI 52466 and cyclothiazide on AMPA-receptor-mediated EPSC kinetics observed are due to modulation of glutamate-release at presynaptic AMPA receptors.

Facilitation of currents through rat Ca2+-permeable AMPA receptor channels by activity-dependent relief from polyamine block

1. In outside-out patches excised from human embryonic kidney (HEK) 293 cells expressing Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptor (AMPAR) channels, currents activated by 1 ms glutamate pulses at negative membrane potentials facilitated during and following a repetitive (2 to 100 Hz) agonist application. The degree of facilitation depended on subunit type, membrane potential and stimulation frequency being antagonized by a slow recovery from desensitization. 2. Activity-dependent current facilitation occurred in Ca2+-permeable but not in Ca2+-impermeable AMPAR channels. Current facilitation, however, does not depend on Ca2+ flux. Rather it reflects a relief from the block of Ca2+-permeable AMPARs by intracellular polyamines since facilitation occurred only in the presence of polyamines and since facilitated currents had a nearly linear current-voltage relation (I-V). 3. Relief from polyamine block was use dependent and occurred mainly in open channels. The relief mechanism was determined primarily by membrane potential rather than by current flow. 4. In closed channels the degree of polyamine block was independent of membrane potential. The voltage dependence of the rate of relief from the block in open channels rather than the voltage dependence of the block underlies the inwardly rectifying shape of the I-V at negative potentials. 5. Currents through native Ca2+-permeable AMPAR channels in outside-out or nucleated patches from either hippocampal basket cells or a subtype of neocortical layer II nonpyramidal cells also showed facilitation. 6. It is concluded that a use-dependent relief from polyamine block during consecutive AMPAR channel openings underlies current facilitation. This polyamine-AMPAR interaction may represent a new activity-dependent postsynaptic mechanism for control of synaptic signalling.

Potentiation of GABAergic synaptic transmission by AMPA receptors in mouse cerebellar stellate cells: changes during development

1. The effects of low concentrations of domoate, an agonist at both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and kainate receptors (AMPARs and KARs, respectively), were investigated in stellate cells in slices of mouse cerebellum at two developmental stages (postnatal day (PN) 11-13 and PN21-25). 2. Low concentrations of domoate enhanced the frequency of miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin (TTX) at PN11-13 but not at PN21-25. 3. The effects of low concentrations of domoate on synaptic activity were probably mediated by the activation of AMPARs and not KARs, since they were blocked by GYKI 53655 (LY300168), a selective AMPAR antagonist. 4. Domoate increased mIPSC frequency in part by activation of presynaptic voltage-dependent Ca2+ channels since potentiation was reduced by 60 % in the presence of Cd2+. AMPARs in stellate cells were found to be permeable to Ca2+. The residual potentiation in the presence of Cd2+ could thus be due to a direct entry of Ca2+ through AMPAR channels. 5. In the presence of TTX, potentiation of synaptic activity by focal application of domoate was not restricted to the region of the cell body, but was observed within distances of 120 micro(m). These experiments also revealed a strong spatial correlation between the location of the presynaptic effects of domoate and the activation of postsynaptic AMPARs. 6. Our data show a developmentally regulated presynaptic potentiation of synaptic transmission between cerebellar interneurones mediated by AMPARs. We discuss the possibility that the developmental switch could be due to a shift in the localization of AMPARs from the axonal to the somato-dendritic compartment.

Neuropeptide Y-mediated enhancement of NMDA-stimulated [3H]dopamine release from rat prefrontal cortex is reversed by sigma1 receptor antagonists

Sigma (sigma) receptors are located in limbic areas, including the prefrontal cortex, where decreased dopamine levels have been linked to negative symptoms. Although the endogenous ligands for sigma receptors are unknown, neuropeptide Y (NPY) has been named as the potential endogenous agonist at these receptors. NPY enhanced NMDA-stimulated [3H]dopamine release in rat prefrontal cortex. This was in contrast to the inhibition produced by the sigma agonists (+)pentazocine and BD737. However, four sigma antagonists, including one which is sigma1 selective, that reverse (+)pentazocine- or BD737-mediated inhibition all reversed the NPY-mediated enhancement. In addition, PYX-1, a Y receptor antagonist, reversed both the (+)pentazocine- and BD737-mediated inhibition and the NPY-mediated enhancement of release. Peptide YY (PYY), [Leu31,Pro34]NPY and NPY(13-36) did not mimic the effect of NPY. Our findings are consistent with NPY acting as an endogenous ligand for a subtype of sigma receptor with characteristics different from Y1, Y2 and Y3 receptors but sensitive to PYX-1. These findings suggest a role for NPY, via sigma receptors, as a modulator of dopamine levels in the prefrontal cortex.

Ca2+-permeable non-NMDA glutamate receptors in rat magnocellular basal forebrain neurones

1. Ionotropic glutamate receptor-mediated responses were recorded from rat magnocellular basal forebrain neurones under voltage clamp from a somatically located patch-clamp pipette. Currents were recorded from both acutely dissociated neurones and neurones maintained in culture for up to 6 weeks. 2. Non-NMDA and NMDA receptor-mediated events could be distinguished pharmacologically using the selective agonists (S)-alpha-amino-3-hydroxy-5-methyl-isoxazolepropionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA), and antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonopentanoic acid (AP5). 3. Responses to rapid application of AMPA displayed pronounced and rapid desensitization. Responses to kainate showed no desensitization. Steady-state EC50 values for AMPA and kainate were 2.7 +/- 0.4 microM (n = 5) and 138 +/- 25 microM (n = 10), respectively. Cyclothiazide markedly increased current amplitude of responses to both agonists, whereas concanavalin A had no clear effect on either response. The selective AMPA receptor antagonist GYKI 53655 inhibited responses to kainate with an IC50 of 1.2 +/- 0.08 microM (n = 5) at -70 mV. These data strongly suggest that AMPA receptors are the predominant non-NMDA receptors expressed by basal forebrain neurones. 4. At -70 mV, approximately 6 % of control current amplitude remained, at a maximally effective concentration of GYKI 53655. This residual response displayed desensitization, was insensitive to cyclothiazide and was potentiated by concanavalin A, suggesting that it was mediated by a kainate receptor. 5. Current-voltage relationships for non-NMDA receptor-mediated currents were obtained from both nucleated patches pulled from neurones in culture and from acutely dissociated neurones. With 30 microM spermine in the recording pipette, currents frequently displayed double-rectification characteristic of non-NMDA receptors with high Ca2+ permeabilities. Ca2+ permeability, relative to Na+ and Cs+, was investigated using constant field theory. The measured Ca2+ to Na+ permeability coefficient ratio was 0.26-3.6; median, 1.27 (n = 15). 6. Current flow through non-NMDA receptors was inhibited by Ca2+, Cd2+ and Co2+ ions. At a holding potential of -70 mV, a maximally effective concentration of Cd2+ (> 30 mM) reduced current amplitude by approximately 90 %, with an IC50 of 44 microM. In six out of seven cells tested, block by Cd2+ was voltage sensitive. 7. Ca2+ permeability of many of the non-NMDA receptors expressed by magnocellular basal forebrain neurones may underlie the unusual sensitivity of cholinergic basal forebrain neurones to non-NMDA receptor-mediated excitotoxicity.

Dentate gyrus basket cell GABAA receptors are blocked by Zn2+ via changes of their desensitization kinetics: an in situ patch-clamp and single-cell PCR study

Although GABA type A receptors (GABAARs) in principal cells have been studied in detail, there is only limited information about GABAARs in interneurons. We have used the patch-clamp technique in acute rat hippocampal slices in combination with single-cell PCR to determine kinetic, pharmacological, and structural properties of dentate gyrus basket cell GABAARs. Application of 1 mM GABA (100 msec) to nucleated patches via a piezo-driven fast application device resulted in a current with a fast rise and a marked biexponential decay (time constants 2.4 and 61.8 msec). This decay could be attributed to strong receptor desensitization. Dose-response curves for the peak and the slow component yielded EC50 values of 139 and 24 microM, respectively. Zn2+ caused a marked blocking effect on both the peak and the slow component via a noncompetitive mechanism (IC50 values of 8 and 16 microM). This led to an acceleration of the slow component as well as a prolongation of recovery from desensitization. Zn2+ sensitivity was suggested to depend on the absence of gamma-subunits in GABAARs. To test this hypothesis we performed single-cell reverse transcription PCR that revealed primarily the presence of alpha2-, beta2-, beta3-, gamma1-, and gamma2-subunit mRNAs. In addition, flunitrazepam increased the receptor affinity for its agonist, indicating the presence of functional benzodiazepine binding sites, i.e., gamma-subunits. Thus, additional factors seem to co-determine the Zn2+ sensitivity of native GABAARs. The modulatory effects of Zn2+ on GABAAR desensitization suggest direct influences on synaptic integration via changes in inhibition and shunting at GABAergic synapses.

Dentate gyrus basket cell GABAA receptors are blocked by Zn2+ via changes of their desensitization kinetics: an in situ patch-clamp and single-cell PCR study

Although GABA type A receptors (GABAARs) in principal cells have been studied in detail, there is only limited information about GABAARs in interneurons. We have used the patch-clamp technique in acute rat hippocampal slices in combination with single-cell PCR to determine kinetic, pharmacological, and structural properties of dentate gyrus basket cell GABAARs. Application of 1 mM GABA (100 msec) to nucleated patches via a piezo-driven fast application device resulted in a current with a fast rise and a marked biexponential decay (time constants 2.4 and 61.8 msec). This decay could be attributed to strong receptor desensitization. Dose-response curves for the peak and the slow component yielded EC50 values of 139 and 24 microM, respectively. Zn2+ caused a marked blocking effect on both the peak and the slow component via a noncompetitive mechanism (IC50 values of 8 and 16 microM). This led to an acceleration of the slow component as well as a prolongation of recovery from desensitization. Zn2+ sensitivity was suggested to depend on the absence of gamma-subunits in GABAARs. To test this hypothesis we performed single-cell reverse transcription PCR that revealed primarily the presence of alpha2-, beta2-, beta3-, gamma1-, and gamma2-subunit mRNAs. In addition, flunitrazepam increased the receptor affinity for its agonist, indicating the presence of functional benzodiazepine binding sites, i.e., gamma-subunits. Thus, additional factors seem to co-determine the Zn2+ sensitivity of native GABAARs. The modulatory effects of Zn2+ on GABAAR desensitization suggest direct influences on synaptic integration via changes in inhibition and shunting at GABAergic synapses.