Differential activation of glutamate receptors by spontaneously released transmitter in slices of neocortex (original) (raw)

NMDA Receptor Activation by Spontaneous Glutamatergic Neurotransmission

Journal of Neurophysiology, 2008

Under physiological conditions N-methyl-d-aspartate (NMDA) receptor activation requires coincidence of presynaptic glutamate release and postsynaptic depolarization due to the voltage-dependent block of these receptors by extracellular Mg2+. Therefore spontaneous neurotransmission in the absence of action potential firing is not expected to lead to significant NMDA receptor activation. Here we tested this assumption in layer IV neurons in neocortex at their resting membrane potential (approximately −67 mV). In long-duration stable recordings, we averaged a large number of miniature excitatory postsynaptic currents (mEPSCs,…

Tonic activation of NMDA receptors by ambient glutamate of non-synaptic origin in the rat hippocampus

Journal of Physiology-london, 2006

In several neuronal types of the CNS, glutamate and GABA receptors mediate a persistent current which reflects the presence of a low concentration of transmitters in the extracellular space. Here, we further characterize the tonic current mediated by ambient glutamate in rat hippocampal slices. A tonic current of small amplitude (53.99 ± 6.48 pA at +40 mV) with the voltage dependency and the pharmacology of NMDA receptors (NMDARs) was detected in virtually all pyramidal cells of the CA1 and subiculum areas. Manipulations aiming at increasing D-serine or glycine extracellular concentrations failed to modify this current indicating that the glycine binding sites of the NMDARs mediating the tonic current were saturated. In contrast, non-transportable inhibitors of glutamate transporters increased the amplitude of this tonic current, indicating that the extracellular concentration of glutamate primarily regulates its magnitude. Neither AMPA/kainate receptors nor metabotropic glutamate receptors contributed significantly to this tonic excitation of pyramidal neurons. In the presence of glutamate transporter inhibitors, however, a significant proportion of the tonic conductance was mediated by AMPA receptors. The tonic current was unaffected when inhibiting vesicular release of transmitters from neurons but was increased upon inhibition of the enzyme converting glutamate in glutamine in glial cells. These observations indicate that ambient glutamate is mainly of glial origin. Finally, experiments with the use-dependent antagonist MK801 indicated that NMDARs mediating the tonic conductance are probably extra-synaptic NMDARs.

Spontaneous and Evoked Glutamate Release Activates Two Populations of NMDA Receptors with Limited Overlap

Journal of Neuroscience, 2008

In a synapse, spontaneous and action potential-driven neurotransmitter release are assumed to activate the same set of postsynaptic receptors. Here, we tested this assumption using MK-801, a well-characterized use-dependent blocker of NMDA receptors. NMDA receptor-mediated spontaneous miniature excitatory postsynaptic currents (NMDA-mEPSCs) were substantially decreased by MK-801 within 2-minutes in a use-dependent manner. In contrast, MK-801 application at rest for 10-minutes did not significantly impair the subsequent NMDA receptor-mediated evoked EPSCs (NMDA-eEPSCs). Brief stimulation in the presence of MK-801 significantly depressed evoked NMDA-eEPSCs but only mildly affected the spontaneous NMDA-mEPSCs detected on the same cell. Optical imaging of synaptic vesicle fusion showed that spontaneous and evoked release could occur at the same synapse albeit without correlation between their kinetics. In addition, modeling glutamate diffusion and NMDA receptor activation revealed that postsynaptic densities larger than ~0.2 μm 2 can accommodate two populations of NMDA receptors with largely nonoverlapping responsiveness. Collectively, these results support the premise that spontaneous and evoked neurotransmission activate distinct sets of NMDA receptors and signal independently to the postsynaptic side.

modulation ofNMDA currents bymetabotropic glutamate receptor subtypes 1/5incultured mousecortical neurons

2000

1. Modulation of NMDA receptors by metabotropic glutamate receptors (mGluRs) in cultured mouse cortical neurons was investigated using whole-cell and single-channel recordings. 2. NMDA whole-cell current was reversibly attenuated by selective mGluR1/5 agonists (S)-3hydroxyphenylglycine (3HPG; 10-200 ,UM), (S)-3,5-dihydroxyphenylglycine (S-DHPG; 100,UM) and other mGluR agonists: (lS,3R)-1-aminocyclopentane-1,3-decarboxylic acid (1S,3R-ACPD; 200 uM), quisqualate (10 /M) and (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I; 100 AM). 3. The attenuation of NMDA current by 3HPG was totally eliminated by the mGluR antagonist (RS)-a-methyl-4-carboxyphenylglycine (MCPG; 500,FM) and by the selective mGluR1/5 antagonist (S)4-carboxyphenylglycine (4CPG; 300 FM). 4. mGluR2/3 agonists (2S,1'R,2'R'3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; 3,M), (S)4-carboxy-3-hydroxyphenylglycine (4C3HPG; 100-200 FM) and (S)4-carboxyphenylglycine (4CPG; 300 FM) did not reduce NMDA current. 5. The NMDA-induced increase in intracellular free Ca2' measured by fura-2 Ca2P imaging was attenuated by 3HPG (300 FM). 6. The suppression of NMDA current by 3HPG was not affected by treatments that altered intracellular Ca2P or cAMP levels, or by the protein kinase inhibitor, staurosporine (0'1-0-5 FM). 7. The open probability (NPO) of the NMDA receptor channel in excised outside-out patches was attenuated by 3HPG but not by 4C3HPG. This 3HPG effect was blocked by MCPG. 8. The 3HPG-induced reduction of NMDA whole-cell and single-channel currents was prevented by GDP,8S (200-400 FM). Intracellular dialysis of GTPyS (100 FM) also reduced NMDA whole-cell current, and rendered irreversible further reduction induced by 3HPG. 9. These data suggest that a selective activation of mGluR1/5 downmodulates the NMDA receptor channel in a membrane-delimited manner, mediated by G proteins, but not by diffusible second messengers. Glutamate and related amino acids are the primary In addition to activating ion channel-linked receptors, excitatory neurotransmitters in the vertebrate CNS. glutamate activates G protein-coupled metabotropic receptors Glutamate acts on three families of ion channel-linked (mGluRs) which exist in at least eight major subtypes (Pin & receptors, classified by preferred agonists: N-methyl-Duvoisin, 1995). mGluR subtypes 1 and 5 (group I mGluRs) D-aspartate (NMDA), (RS)-a-amino-3-hydroxy-5-methylare positively coupled to phospholipase C (PLC), and trigger isoxazolepropionic acid (AMPA) and kainate (Hollmann & elevations in intracellular inositol trisphosphate (1P3) and Heinemann, 1994). These ionotropic glutamate receptors diacylglycerol (DAG), followed by mobilization of intraalso mediate synaptic plasticity and excitotoxic injury (Bencellular Ca2+ and activation of protein kinase C (PKC).

Distinct Quantal Features of AMPA and NMDA Synaptic Currents in Hippocampal Neurons: Implication of Glutamate Spillover and Receptor Saturation

Biophysical Journal, 2003

Excitatory postsynaptic currents (EPSCs) were studied in the CA1 pyramidal cells of rat hippocampal slices. Components mediated by a-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and by N-methyl-D-aspartate (NMDA) receptors were separated pharmacologically. Quantal parameters of AMPA and NMDA receptor-mediated EPSCs were obtained using both maximal likelihood and autocorrelation techniques. Enhancement of transmitter release with 4-aminopyridine caused a significant increase in quantal size of NMDA EPSC. This was accompanied by a slowing of the EPSC decay. The maximal number of quanta in the NMDA current was unchanged, while the probability of quantal event dramatically enhanced. In contrast, neither the quantal size nor the kinetics of AMPA EPSC was altered by 4-aminopyridine, while the maximal number of quanta increased. These changes in the quantal parameters are consistent with a transition to multivesicular release of the neurotransmitter. Spillover of excessive glutamate on the nonsynaptic areas of dendritic spines causes an increase in the quantal size of NMDA synaptic current. The difference in quantal behavior of AMPA and NMDA EPSCs implies that different mechanisms underlie their quantization: the additive response of nonsaturated AMPA receptors contrasts with the variable involvement of saturated intrasynaptic and nonsaturated extrasynaptic NMDA receptors.

Slow excitatory postsynaptic currents mediated by N-methyl-D-aspartate receptors on cultured mouse central neurones

The Journal of physiology, 1988

1. Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked between pairs of cultured neurones from either hippocampus or spinal cord were examined using the tight-seal whole-cell recording technique. 2. Using the selective N-methyl-D-aspartate (NMDA)-receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), two components of the EPSP could be resolved in cultures from both brain regions. The APV-sensitive (slow) component had the same latency, but a much slower time-to-peak and longer duration than the APV-resistant (fast) component. Other NMDA antagonists such as ketamine also selectively blocked the slow component of the EPSP. 3. In Mg2+-free medium, the dual-component EPSP had a duration lasting up to 500 ms, greatly exceeding the membrane time constant of the postsynaptic neurone, suggesting that persistent activation of NMDA receptors was responsible for the long duration of the APV-sensitive component. 4. Under voltage clamp the excitatory postsynaptic currents (EPSCs...

Changes in agonist concentration dependence that are a function of duration of exposure suggest N-methyl-D-aspartate receptor nonsaturation during synaptic stimulation

Molecular pharmacology, 2001

Evidence suggests that N-methyl-D-aspartate receptors (NMDARs) have a relatively high affinity for agonist compared with non-NMDA receptors. Dose-response curves constructed with sustained agonist application suggest that the 50% effective concentration (EC(50)) for peak glutamate-evoked current at NMDARs is 1 to 10 microM, whereas that of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors is approximately 500 microM. Given estimates of synaptic cleft glutamate concentration in the millimolar range, it would be expected that NMDARs would be saturated with agonist. However, studies of synaptic NMDAR responses indicate that these receptors may not be saturated during single release events at many synapses. To address this apparent contradiction, we have compared the glutamate dose-response curve for the peak NMDAR current generated by sustained glutamate application with that obtained during brief synaptic-like pulses of agonist. Our results using both recombinant ...

Intracellular demonstration of an N-methyl-d-aspartate receptor mediated component of synaptic transmission in the rat hippocampus

Neuroscience Letters, 1985

Key words: hippocampal slice -D-2-amino-5-phosphonovalerate -magnesium ion -N-methyI-D-aspartate receptor -long-term potentiation -excitatory postsynaptic potential -rat Rat hippocampal CA1 pyramidal neurones were monosynaptically activated via stimulation of the Schaffer collateral-commissural pathway. On changing from a I mM Mg2+-containing to a Mg2+-free medium there was a pronounced prolongation of the intracellularly recorded excitatory postsynaptic potential. This effect was reversibly abolished by the selective N-methyl-o-aspartate (NMDA) antagonist, o-2-amino-5-phosphonovalerate (APV). We propose that Mg 2+ normally prevents expression of NMDA receptor-mediated responses during low-frequency stimulation. During a period of tetanic stimulation, however, cells may depolarize sufficiently to allow a significant NMDA component of the response to be manifest. This could then initiate long-term potentiation.

Frequency-dependent N-methyl-D-aspartate receptor-mediated synaptic transmission in rat hippocampus

The Journal of physiology, 1988

1. The effects of the N-methyl-D-aspartate (NMDA) antagonist, D-2-amino-5-phosphonovalerate (APV) were examined on synaptic responses evoked by high-frequency stimulation of the Schaffer collateral-commissural pathway, in the presence of Mg2+ (1 or 2 mM) and functional synaptic inhibition. 2. The synaptic response evoked by 100 Hz stimulation comprised fast excitatory postsynaptic potentials (EPSPs) evoked by each shock and a slow depolarization. APV reduced the size of the depolarization without depressing the fast EPSPs. 3. The mean (+/- 1 S.E.) amplitude of the APV-sensitive component (3.0 +/- 0.3 mV), evoked by 100 Hz stimulation at membrane potentials near rest, was invariably smaller than the first fast EPSP (9.8 +/- 0.7 mV). Both of these synaptic components had similar thresholds and increased in amplitude as the stimulus intensity was raised. There was a positive correlation between the amplitude of the two components (r = 0.57, P less than 0.01). 4. The amplitude of the AP...