AMPA Receptor-mediated Component Research Articles

Persistent activation of histamine H1 receptors in the hippocampal CA1 region enhances NMDA receptor-mediated synaptic excitation and long-term potentiation in astrocyte- and D-serine-dependent manner

Behavioral studies using pharmacological tools have implicated histamine H1 receptors in cognitive function via their interactions with N-methyl-D-aspartate receptors (NMDARs) in the hippocampus. However, little is known about the neurophysiological mechanism that underlies the interaction between H1 receptors and NMDARs. To explore how H1 receptor activation affects hippocampal excitatory neurotransmission and synaptic plasticity, this study aimed to examine the effect of H1 receptor ligands on both NMDAR-mediated synaptic currents and long-term potentiation (LTP) at synapses between Schaffer collaterals and CA1 pyramidal neurons using acute mouse hippocampal slices. We found that the H1 receptor antagonist/inverse agonists, pyrilamine (0.1 μM) and cetirizine (10 μM), decreased the NMDAR-mediated component of stimulation-induced excitatory postsynaptic currents (EPSCs) recorded from CA1 pyramidal neurons without affecting the AMPA receptor-mediated component of EPSCs and its paired pulse ratio. Pretreatment of slices with either the glial metabolism inhibitor, fluoroacetate (5 mM), or D-serine (100 μM) diminished the pyrilamine- or cetirizine-induced attenuation of the NMDAR-mediated EPSCs. Furthermore, the LTP of field excitatory postsynaptic potentials induced following high frequency stimulation of Schaffer collaterals was attenuated with application of pyrilamine or cetirizine. Pretreatment with D-serine again attenuated the pyrilamine-induced suppression of LTP. Our data suggest that H1 receptors in the CA1 can undergo persistent activation induced by their constitutive receptor activity and/or tonic release of endogenous histamine, resulting in facilitation of the NMDAR activity in a manner dependent of astrocytes and the release of D-serine. This led to the enhancement of NMDA-component EPSC and LTP at the Schaffer collateral-CA1 pyramidal neuron synapses.

Co-activation of both 5-HT1A and 5-HT7 receptors induced attenuation of glutamatergic synaptic transmission in the rat visual cortex

It has been suggested that functional interactions between the 5-HT receptor subtypes may modulate glutamatergic synaptic transmission. In this study, we used whole-cell patch-clamp recordings to test the role of 5-HT receptors in mediating the AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) in layer II/III pyramidal neurons of the rat visual cortex. We found that the AMPA receptor-mediated component of mEPSCs could be inhibited by exogenously applied 5-HT. 5-HT significantly reduced the glutamatergic mEPSC amplitude and increased the inter-event interval of glutamatergic mEPSCs. Bath application of 5-CT or 8-OH-DPAT (the 5-HT1A and 5-HT7 receptor agonist) mimicked 5-HT in its effect on mEPSCs. Additionally, a selective antagonist for the 5-HT7 receptor, SB-269970, displayed no influence on the inhibition of glutamatergic synaptic transmission by 5-CT or 8-OH-DPAT. Similar results were obtained by exogenously applied WAY-100135, the selective 5-HT1A receptor antagonist. However, the inhibition of glutamatergic synaptic transmission by 5-CT or 8-OH-DPAT was completely blocked by co-application of WAY-100135 and SB-269970. Altogether, our results indicated that 5-HT suppressed glutamatergic synaptic transmission by co-activation of synaptic 5-HT1A receptors and 5-HT7 receptors in layer II/III pyramidal neurons of the rat visual cortex.

GSK-3β inhibitors reverse cocaine-induced synaptic transmission dysfunction in the nucleus accumbens.

Nucleus accumbens receives glutamatergic projection from the prefrontal cortex (PFC) and dopaminergic input from the Ventral tegmental area (VTA). Recent studies have suggested a critical role for serine/threonine kinase glycogen synthase kinase 3β (GSK3β) in cocaine-induced hyperactivity; however, the effect of GSK3β on the modulation of glutamatergic and dopaminergic afferents is unclear. In this study, we found that the GSK3 inhibitors, LiCl (100 mg/kg, i.p.) or SB216763 (2.5 mg/kg, i.p.), blocked the cocaine-induced hyperlocomotor activity in rats. By employing single-unit recordings in vivo, we found that pretreatment with either SB216763 or LiCl for 15 min reversed the cocaine-inhibited firing frequency of medium spiny neuron (MSN) in the nucleus accumbens (NAc). Preperfusion of SB216763 (5 μM) ameliorated the inhibitory effect of cocaine on both the α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) (up to 99 ± 6.8% inhibition) and N-methyl-D-aspartic acid receptor (NMDAR)-mediate EPSC (up to 73 ± 9.7% inhibition) in the NAc in brain slices. The effect of cocaine on AMPA and NMDA receptor-mediate excitatory postsynaptic current (EPSC) were mimicked by the D1 -like receptor agonist SKF 38393 and blocked by the D1 -like receptor antagonist SCH 23390, whereas D2 -like receptor agonist or antagonist failed to mimic or to block the action of cocaine. Preperfusion of SB216763 for 5 min also ameliorated the inhibitory effect of SKF38393 on both AMPA and NMDA receptor-mediated components of EPSC, indicate the effect of SB216763 on cocaine was via the D1 -like receptor. Moreover, cocaine inhibited the presynaptic release of glutamate in the NAc, and SB216763 reversed this effect. In conclusion, D1 receptor-GSK3β pathway, which mediates glutamatergic transmission in the NAc core through a presynaptic mechanism, plays an important role in acute cocaine-induced hyperlocomotion.

Distinct Roles of GABAergic Interneurons in the Regulation of Striatal Output Pathways

Striatal GABAergic microcircuits are critical for motor function, yet their properties remain enigmatic due to difficulties in targeting striatal interneurons for electrophysiological analysis. Here, we use Lhx6-GFP transgenic mice to identify GABAergic interneurons and investigate their regulation of striatal direct- and indirect-pathway medium spiny neurons (MSNs). We find that the two major interneuron populations, persistent low-threshold spiking (PLTS) and fast spiking (FS) interneurons, differ substantially in their excitatory inputs and inhibitory outputs. Excitatory synaptic currents recorded from PLTS interneurons are characterized by a small, nonrectifying AMPA receptor-mediated component and a NMDA receptor-mediated component. In contrast, glutamatergic synaptic currents in FS interneurons have a large, strongly rectifying AMPA receptor-mediated component, but no detectable NMDA receptor-mediated responses. Consistent with their axonal morphology, the output of individual PLTS interneurons is relatively weak and sparse, whereas FS interneurons are robustly connected to MSNs and other FS interneurons and appear to mediate the bulk of feedforward inhibition. Synaptic depression of FS outputs is relatively insensitive to firing frequency, and dynamic-clamp experiments reveal that these short-term dynamics enable feedforward inhibition to remain efficacious across a broad frequency range. Surprisingly, we find that FS interneurons preferentially target direct-pathway MSNs over indirect-pathway MSNs, suggesting a potential mechanism for rapid pathway-specific regulation of striatal output pathways.

NMDA Receptors Mediate Neuron-to-Glia Signaling in Mouse Cortical Astrocytes

Chemical transmission between neurons and glial cells is an important element of integration in the CNS. Here, we describe currents activated by NMDA in cortical astrocytes, identified in transgenic mice that express enhanced green fluorescent protein under control of the human glial fibrillary acidic protein promoter. Astrocytes were studied by whole-cell voltage clamp either in slices or after gentle nonenzymatic mechanical dissociation. Acutely isolated astrocytes showed a three-component response to glutamate. The initial rapid component was blocked by 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), which is an antagonist of AMPA receptors (IC50, 2 microM), and the NMDA receptor antagonist D-AP-5 blocked the later sustained component (IC50, 0.6 microM). The third component of glutamate application response was sensitive to D,L-threo-beta-benzyloxyaspartate, a glutamate transporter blocker. Fast application of NMDA evoked concentration-dependent inward currents (EC50, 0.3 microM); these showed use-dependent block by (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801). These NMDA-evoked currents were linearly dependent on membrane potential and were not affected by extracellular magnesium at concentrations up to 10 mM. Electrical stimulation of axons in layer IV-VI induced a complex inward current in astrocytes situated in the cortical layer II, part of which was sensitive to MK-801 at holding potential -80 mV and was not affected by the AMPA glutamate receptor antagonist NBQX. The fast miniature spontaneous currents were observed in cortical astrocytes in slices as well. These currents exhibited both AMPA and NMDA receptor-mediated components. We conclude that cortical astrocytes express functional NMDA receptors that are devoid of Mg2+ block, and these receptors are involved in neuronal-glial signal transmission.

Maturation of EPSCs and Intrinsic Membrane Properties Enhances Precision at a Cerebellar Synapse

The timing of action potentials is an important determinant of information coding in the brain. The shape of the EPSP has a key influence on the temporal precision of spike generation. Here we use dynamic clamp recording and passive neuronal models to study how developmental changes in synaptic conductance waveform and intrinsic membrane properties combine to affect the EPSP and action potential generation in cerebellar granule cells. We recorded EPSCs at newly formed and mature mossy fiber-granule cell synapses. Both quantal and evoked currents showed a marked speeding of the AMPA receptor-mediated component. We also found evidence for age- and activity-dependent changes in the involvement of NMDA receptors. Although AMPA and NMDA receptors contributed to quantal EPSCs at immature synapses, multiquantal release was required to activate NMDA receptors at mature synapses, suggesting a developmental redistribution of NMDA receptors. These changes in the synaptic conductance waveform result in a faster rising EPSP and reduced spike latency in mature granule cells. Mature granule cells also have a significantly decreased input resistance, contributing to a faster decaying EPSP and a reduced spike jitter. We suggest that these concurrent developmental changes, which increase the temporal precision of EPSP-spike coupling, will increase the fidelity with which sensory information is processed within the input layer of the cerebellar cortex.

Modulation of excitatory synaptic transmission by GABA(C) receptor-mediated feedback in the mouse inner retina.

In many vertebrate CNS synapses, the neurotransmitter glutamate activates postsynaptic non-N-methyl-D-aspartate (NMDA) and NMDA receptors. Since their biophysical properties are quite different, the time course of excitatory postsynaptic currents (EPSCs) depends largely on the relative contribution of their activation. To investigate whether the activation of the two receptor subtypes is affected by the synaptic interaction in the inner plexiform layer (IPL) of the mouse retina, we analyzed the properties of the light-evoked responses of ON-cone bipolar cells and ON-transient amacrine cells in a retinal slice preparation. ON-transient amacrine cells were whole cell voltage-clamped, and the glutamatergic synaptic input from bipolar cells was isolated by a cocktail of pharmacological agents (bicuculline, strychnine, curare, and atropine). Direct puff application of NMDA revealed the presence of functional NMDA receptors. However, the light-evoked EPSC was not significantly affected by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5), but suppressed by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) or 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). These results indicate that the light-evoked EPSC is mediated mainly by AMPA receptors under this condition. Since bipolar cells have GABA(C) receptors at their terminals, it has been suggested that bipolar cells receive feedback inhibition from amacrine cells. Application of (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), a specific blocker of GABA(C) receptors, suppressed both the GABA-induced current and the light-evoked feedback inhibition observed in ON-cone bipolar cells and enhanced the light-evoked EPSC of ON-transient amacrine cells. In the presence of TPMPA, the light-evoked EPSC of amacrine cells was composed of AMPA and NMDA receptor-mediated components. Our results suggest that photoresponses of ON-transient amacrine cells in the mouse retina are modified by the activation of presynaptic GABA(C) receptors, which may control the extent of glutamate spillover.

Theta-frequency facilitation of AMPA receptor-mediated synaptic currents in the principal cells of the medial septum.

Recent evidence suggests that Ca(2+)-permeable AMPA receptors display rapid, short-lasting current facilitation. In this study, we investigated the properties of AMPA receptor-mediated synaptic currents in medial septal neurons of the rat in an in vitro slice preparation. Immunocytochemistry with a selective antibody to the GluR2 subunit revealed that both choline acetyltransferase-containing and parvalbumin-containing neurons of the medial septum express no detectable GluR2 subunit immunoreactivity. We used whole cell voltage-clamp recordings to measure synaptically evoked AMPA receptor-mediated currents from medial septal neurons following stimulation of midline afferents. The GYKI 52466 (50 microM)- and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX) (20 microM)-sensitive AMPA receptor-mediated component of the synaptic response was isolated by blocking GABA(A)- and N-methyl-D-aspartate receptor-mediated currents with 30 microM bicuculline and 100 microM 2-amino-5-phosphonovaleric acid, respectively. In some cases, patched cells were filled with Lucifer yellow (0.1%) and imaged using 2-photon laser scanning microscopy. AMPA receptor-mediated currents that were observed in large medial septal neurons (20--30 microm) displayed rectification. These currents were sensitive to external application of philanthotoxin-343 (PhTx-343, 50 microM), a potent, high-affinity antagonist of Ca(2+)-permeable, GluR2-lacking AMPA receptors. Rectifying AMPA receptor-mediated currents also displayed a rapid increase in amplitude when evoked five times at low frequency such as 6 Hz. In contrast to currents observed in large medial septal neurons, AMPA-receptor mediated currents evoked in the remaining small (8--11 microm) neurons were nonrectifying and displayed rapid synaptic depression when stimulated five times at 6 Hz. The currents evoked in these cells were unaffected by external application of PhTx-343 and were therefore GluR2-containing AMPA receptors. The results of the present study demonstrate that the principal projection neurons of the medial septum contain PhTx-343-sensitive, GluR2-lacking AMPA receptors that display rapid current facilitation when stimulated at low frequencies.

Afferent high strength tetanizations favour potentiation of the NMDA vs. AMPA receptor-mediated component of field EPSP in CA1 hippocampal slices of rats

Long-term potentiation (LTP) of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl- d-aspartate (NMDA) receptor-mediated components of ‘dual-component’ field excitatory postsynaptic potentials (fEPSP-A and fEPSP-N) was studied in the CA1 stratum radiatum in hippocampal slices of rats. Relative degrees of LTP of these fEPSP components were compared for tetanizations with low and high strengths. Magnitudes of fEPSP-A and fEPSP-N were estimated in parallel with a least-square fitting of a short-latent (0.1–8.8 ms) fragment of evoked responses by a weighted sum of ‘basic’ fEPSP-A and fEPSP-N, obtained during a short preliminary application of d-2-amino-5-phosphonovalerate (APV). We found that low-strength tetanizations selectively potentiated fEPSP-A, while high strength tetanizations potentiated both fEPSP components. These results demonstrate in the experiments with parallel measurements of fEPSP-A and fEPSP-N that LTP of these components differ depending on the strength of afferent tetanization. Unequal potentiation of the commissural–collateral and excitatory local-circuit synapses, which presumably contain different amounts of the AMPA and NMDA receptors, is discussed as the most probable explanation for these results.

Distinct Roles of Synaptic and Extrasynaptic NMDA Receptors in Excitotoxicity

Excitatory synaptic activity governs excitotoxicity and modulates the distribution of NMDA receptors (NMDARs) among synaptic and extrasynaptic sites of central neurons. We investigated whether NMDAR localization was functionally linked to excitotoxicity by perturbing F-actin, a cytoskeletal protein that participates in targeting synaptic NMDARs in dendritic spines. Depolymerizing F-actin did not affect NMDA-evoked whole-cell currents. However, the number of dendritic NMDAR clusters and the NMDAR-mediated component of miniature spontaneous EPSCs were reduced, whereas the number of AMPA receptor clusters and AMPA receptor-mediated component of EPSCs was unchanged. This selective perturbation of synaptically activated NMDARs had no effect on neuronal death or the accumulation of (45)Ca(2+) evoked by applying exogenous NMDA or L-glutamate, which reach both synaptic and extrasynaptic receptors. However, it increased survival and decreased (45)Ca(2+) accumulation in neurons exposed to oxygen glucose deprivation, which causes excitotoxicity by glutamate release at synapses. Thus, synaptically and extrasynaptically activated NMDARs are equally capable of excitotoxicity. However, their relative contributions vary with the location of extracellular excitotoxin accumulation, a factor governed by the mechanism of extracellular neurotransmitter accumulation, not the synaptic activation of NMDARs.

Synaptic transmission at nicotinic acetylcholine receptors in rat hippocampal organotypic cultures and slices.

1. Whole-cell clamp recordings of the compound synaptic current elicited by afferent stimulation of Schaffer collaterals showed that blockade of the NMDA, AMPA and GABAA receptor-mediated components by 6-nitro-7-sulphamoyl- benzo(f)quinoxaline-2,3-dione (NBQX), 3-((R)-2-carboxypiperazine-4-yl)propyl-1-phosphonate (R-CPP) and picrotoxin, respectively, left a small residual current in 39 out of 41 CA1 pyramidal neurones in organotypic cultures and 9 out of 16 CA1 cells in acutely prepared slices. 2. This current represented 2. 9 +/- 0.4 % of the compound evoked synaptic response in organoypic cultures and 1.4 +/- 0.5 % in slices. It was characterized by a slightly rectifying I-V curve and a reversal potential of 3.4 +/- 5. 1 mV. 3. This residual current was insensitive to blockers of GABAB, purinergic, muscarinic and 5-HT3 receptors, but it was essentially blocked by the nicotinic receptor antagonist d-tubocurarine (91 +/- 4 % blockade; 20 microM), and partly blocked by alpha-bungarotoxin (200 nM) and methyllycaconitine (10 nM), two antagonists with a higher selectivity for alpha7 subunit-containing nicotinic receptors (48 +/- 3 % and 55 +/- 11 % blockade, respectively). 4. The residual current was of synaptic origin, since it occurred after a small delay; its amplitude depended upon the stimulation intensity and it was calcium dependent and blocked by the sodium channel antagonist tetrodotoxin. 5. We conclude that afferent stimulation applied in the stratum radiatum evokes in some hippocampal neurones a small synaptic current mediated by activation of neuronal nicotinic receptors.

Selective Abolition of the NMDA Component of Long-Term Potentiation in Mice Lacking mGluR5

The mechanisms underlying the differential expression of long-term potentiation (LTP) by AMPA and NMDA receptors, are unknown, but could involve G-protein-linked metabotropic glutamate receptors. To investigate this hypothesis we created mutant mice that expressed no metabotropic glutamate receptor 5 (mGluR5), but showed normal development. In an earlier study of these mice we analyzed field-excitatory postsynaptic potential (fEPSPs) in CA1 region of the hippocampus and found a small decrease; possibly arising from changes in the NMDAR-mediated component of synaptic transmission. In the present study we used whole-cell patch clamp recordings of evoked excitatory postsynaptic currents (EPSCs) in CA1 pyramidal neurons to identify the AMPAR- and NMDAR-mediated components of LTP. Recordings from control mice following tetanus, or agonist application (IS, 3R-1-amino-cyclopentane 1,3-dicarboxylic acid) (ACPD), revealed equal enhancement of the AMPA and NMDA receptor-mediated components. In contrast, CA1 neurons from mGluR5-deficient mice showed a complete loss of the NMDA-receptor-mediated component of LTP (LTP(NMDA)), but normal LTP of the AMPA-receptor-mediated component (LTP(AMPA)). This selective loss of LTP(NMDA) was seen in three different genotypic backgrounds and was apparent at all holding potentials (-70 mV to +20 mV). Furthermore, the LTP(NMDA) deficit in mGluR5 mutant mice could be rescued by stimulating protein kinase C (PKC) with 4beta-phorbol-12,13-dibutyrate (PDBu). These results suggest that PKC may couple the postsynaptic mGluR5 to the NMDA-receptor potentiation during LTP, and that this signaling mechanism is distinct from LTP(AMPA). Differential enhancement of AMPAR and NMDA receptors by mGluR5 also supports a postsynaptic locus for LTP.

Metabotropic glutamate receptors are involved in calcium-induced LTP of AMPA and NMDA receptor-mediated responses in the rat hippocampus

Effects of metabotropic glutamate (mGlu) receptors on calcium-induced long-term potentiation (LTP) of α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) and N-methyl- d-aspartate (NMDA) receptor-mediated components were investigated in rat hippocampal slices using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs). Calcium-induced LTP comprises a parallel, long-lasting increase of AMPA and NMDA receptor-mediated components. The calcium-induced LTP of the AMPA receptor-mediated component can be significantly attenuated by the use of a selective NMDA antagonist. (R.S)-α-methyl-4-carboxyphenylglycine (MCPG), a selective antagonist of mGlu receptors, abolished the long-lasting increase of both AMPA and NMDA receptor-mediated components observed in calcium-induced LTP. In current clamp mode, the application of a high calcium alone or Schaffer fiber stimulation alone (20 Hz) only generated a short-term increase in the firing rate of evoked action potentials. Conversely, a long-term increase in the firing rate was observed if Schaffer fiber stimulation (20 Hz) accompanied the perfusion of high calcium. These results suggest that calcium-induced LTP involves a parallel, long-lasting enhancement in ionotropic AMPA and NMDA receptor-mediated components. More importantly, the mGlu receptor plays a critical role in the establishment of both AMPA and NMDA receptor-mediated components underlying calcium-induced LTP. In addition, the present study also described an experimental condition in which the coapplication of the high calcium pulse and Schaffer fiber stimulation (20 Hz) can synergistically elicit a long-term increase of neuronal excitability.

Differential effects of phorbol ester on AMPA and NMDA components of excitatory postsynaptic currents in dentate neurons of rat hippocampal slices

Protein kinase C (PKC) is present abundantly in the mammalian central nervous system, and is involved in a variety of neuronal functions. Phorbol esters mimic the role of diacylglycerol, the physiological activator of PKC. We examined effects of phorbol 12,13-diacetate (PDAc) on excitatory synaptic transmission in neurons in the dentate granule cell layer of rat hippocampal slices using the whole-cell patch clamp technique. Excitatory postsynaptic currents (EPSCs) evoked by stimulation of the perforant path (pp) consisted of AMPA and NMDA receptor-mediated components. The application of PDAc potentiated both components of the EPSC, but the effect was more pronounced on the NMDA component. The potentiating effect of PDAc on the NMDA component was dependent on the membrane potential, being most prominent at −31 and −51 mV. ω-Agatoxin-IVA, a P-type Ca 2+ channel blocker, suppressed both AMPA and NMDA components to a similar extent by reducing transmitter release. However, when the PDAc-potentiated AMPA component was reduced to the control level by applying ω-agatoxin-IVA, a substantial potentiation on the NMDA component remained. These results suggest that the potentiation of the NMDA component of the EPSC by PDAc is caused partly by a postsynaptic mechanism in the dentate neurons.

The complementary nature of long-term depression and potentiation revealed by dual component excitatory postsynaptic potentials in hippocampal slices from young rats.

Homosynaptic long-term depression and long-term potentiation were studied in hippocampal slices from 12-18-day-old rats using field excitatory postsynaptic potentials recorded in the CA1 subfield (stratum radiatum). Independent estimates of the alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and the N-methyl-D-aspartate receptor-mediated components of the field excitatory postsynaptic potential were obtained in parallel using early and late measurements of a dual component excitatory postsynaptic potential in a solution containing low (0.1 mM) magnesium and 1 microM of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Long-term depression, induced by 2 Hz stimulation for 10 min, was observed as an equal relative depression of the AMPA and N-methyl-D-aspartate receptor-mediated components of the field excitatory postsynaptic potential, whereas long-term potentiation induced by single or repeated high-frequency stimulation, was seen initially as a predominant potentiation of the AMPA receptor-mediated component. Within the first 30-60 min, long-term potentiation gradually changed to more equal increases of the two components of the excitatory postsynaptic potential. During alternating induction of long-term depression and long-term potentiation, the AMPA and N-methyl-D-aspartate receptor-mediated components could both be repeatedly regulated up and down. Long-term depression and long-term potentiation also showed several signs of interaction with each other during such experiments; e.g., long-term depression removed the occlusive effect of large long-term potentiation on a subsequent long-term potentiation, and long-term potentiation applied after the induction of long-term depression was found to be more stable than otherwise. The results support the notion that long-term depression and long-term potentiation employ changes in a common synaptic property. A tentative mechanism for this modification, expressed as equal changes of AMPA and N-methyl-D-aspartate receptor-mediated components of the excitatory postsynaptic potential, is an alteration in transmitter release, while the initial asymmetric part of long-term potentiation indicates involvement of an additional short-term modification.

Tetanically induced LTP involves a similar increase in the AMPA and NMDA receptor components of the excitatory postsynaptic current: investigations of the involvement of mGlu receptors

Whole-cell patch-clamp recordings of evoked excitatory postsynaptic currents (EPSCs) were made from granule cells of the rat dentate gyrus in vitro. Tetanic stimulation in control media evoked a statistically identical long-term potentiation (LTP) of both the AMPA and NMDA receptor-mediated components of the dual component EPSC (AM-PAR and NMDAR EPSCs), as shown by a similar percentage increase in both components when measured at a holding potential of -30 mV, and also by an identical time course of the pre- and post-LTP induced EPSC at -30 mV and -70 mV. Application of the selective metabotropic glutamate receptor (mGluR) agonist 1S,3R-ACPD induced a transient depression followed by a rapid onset LTP of both the AMPAR and the NMDAR components of the dual component EPSC. The ACPD- and tetanically induced LTP of the AMPAR EPSC was NMDAR dependent, being abolished by the NMDAR antagonist AP5. Tetanic stimulation, and application of ACPD, also induced a relatively rapid onset LTP of the pharmacologically isolated NMDAR EPSC. Such tetanically and ACPD-induced LTP of the isolated NMDAR EPSC was also dependent on NMDAR activation, being strongly inhibited by AP5. The tetanically and the ACPD-induced LTP of the NMDAR EPSC were dependent on protein kinase C (PKC) stimulation, being strongly inhibited by the PKC inhibitor PKCI (19-31). The studies suggest that coactivation of the mGluR and NMDAR are required for induction of LTP of both the AMPAR- and NMDAR-mediated synaptic transmission. Moreover, LTP of the NMDAR-mediated synaptic transmission appears to be dependent on coincident activation of the NMDAR and mGluR.

Dendritic glutamate receptor channels in rat hippocampal CA3 and CA1 pyramidal neurons.

1. Properties of dendritic glutamate receptor (GluR) channels were investigated using fast application of glutamate to outside-out membrane patches isolated from the apical dendrites of CA3 and CA1 pyramidal neurons in rat hippocampal slices. CA3 patches were formed (15-76 microns from the soma) in the region of mossy fibre (MF) synapses, and CA1 patches (25-174 microns from the soma) in the region of Schaffer collateral (SC) innervation. 2. Dual-component responses consisting of a rapidly rising and decaying component followed by a second, substantially slower, component were elicited by 1 ms pulses of 1 mM glutamate in the presence of 10 microM glycine and absence of external Mg2+. The fast component was selectively blocked by 2-5 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the slow component by 30 microM D-2-amino-5-phosphonopentanoic acid (D-AP5), suggesting that the fast and slow components were mediated by the GluR channels of the L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and NMDA type, respectively. The peak amplitude ratio of the NMDA to AMPA receptor-mediated components varied between 0.03 and 0.62 in patches from both CA3 and CA1 dendrites. Patches lacking either component were rarely observed. 3. The peak current-voltage (I-V) relationship of the fast component was almost linear, whereas the I-V relationship of the slow component showed a region of negative slope in the presence of 1 mM external Mg2+. The reversal potential for both components was close to 0 mV. 4. Kainate-preferring GluR channels did not contribute appreciably to the response to glutamate. The responses to 100 ms pulses of 1 mM glutamate were mimicked by application of 1 mM AMPA, whereas 1 mM kainate produced much smaller, weakly desensitizing currents. This suggests that the fast component is primarily mediated by the action of glutamate on AMPA-preferring receptors. 5. The mean elementary conductance of AMPA receptor channels was about 10 pS, as estimated by non-stationary fluctuation analysis. The permeability of these channels to Ca2+ was low (approximately 5% of the permeability to Cs+). 6. The elementary conductance of NMDA receptor channels was larger, with a main conductance state of about 45 pS. These channels were 3.6 times more permeable to Ca2+ than to Cs+.(ABSTRACT TRUNCATED AT 400 WORDS)

Synaptic potentiation of dual-component excitatory postsynaptic currents in the rat hippocampus.

1. Whole-cell patch-clamp recording has been used to study tetanus-induced synaptic potentiation of dual-component excitatory postsynaptic currents (EPSCs) in the CA1 region of rat hippocampal slices, following blockade of GABAA and GABAB receptor-mediated synaptic inhibition. 2. At a holding potential of -60 mV, the initial slope of the EPSC (between 10 and 60% of maximum amplitude) provided an accurate measurement of the AMPA receptor-mediated component, and the amplitude of the EPSC at a latency of 100 ms provided the best estimate of the size of the NMDA receptor-mediated component. 3. Neurons were voltage clamped for at least 45 min prior to delivery of a tetanus (test intensity, 100 Hz, 1 s). Measurements at 10 and 30 min following the tetanus were used as indications of short-term potentiation (STP) and long-term potentiation (LTP), respectively. One set of neurons were voltage clamped at -60 mV throughout. These neurons could be subdivided into two populations on the basis of whether or not there was LTP (n = 9), or only STP (n = 6), of the AMPA receptor-mediated component. A second set of neurons were voltage clamped at -60 mV for 30 min and then at -50 mV for 15 min before, during and for 30 min following tetanization. In these experiments there was STP but not LTP (n = 8). 4. In all neurons (n = 23), the time course of the potentiation of the NMDA receptor-mediated component paralleled that of the AMPA receptor-mediated component. In addition, potentiation of the NMDA and AMPA receptor-mediated components were of a similar magnitude. 5. These data demonstrate that it is possible to induce LTP by high frequency stimulation after 45 min of whole-cell recording. Under these conditions, there is a parallel potentiation of the AMPA and NMDA receptor-mediated components of dual-component EPSCs. This constitutes the first evidence, from studies of dual-component synaptic responses, which is consistent with a presynaptic locus of expression of tetanus-induced STP and LTP in the hippocampus.

Rapid turnover rate of the hippocampal synaptic NMDA-R1 receptor subunits

The decrease in the number of synaptic NMDA receptors produced by prior i.c.v. injections of antisense oligonucleotides was examined in the CA1 stratum radiatum of hippocampal slices. Isolated NMDA and AMPA receptor-mediated components of extracellular synaptic responses were compared in slices obtained from control animals and from animals treated for 48 h with an 18-mer phosphorothioate antisense oligodeoxynucleotide directed against NMDA-R1. In slices ipsilateral to the injected hemisphere, the synaptic NMDA component was decreased by 29% compared with the contralateral side and by 34% compared with animals injected with vehicle alone. At the same time, synaptic AMPA receptor-mediated components were not affected. These results are consistent with a rapid turnover rate of synaptic NMDA-R1 and demonstrate the feasibility of antisense oligonucleotide techniques for the depletion of specific synaptic receptor channels.