Endogenous NMDA-Receptor Activation Regulates Glutamate Release in Cultured Spinal Neurons

1998 ◽  
Vol 80 (1) ◽  
pp. 196-208 ◽  
Author(s):  
Antoine Robert ◽  
Joel A. Black ◽  
Stephen G. Waxman
Keyword(s):  
Nmda Receptor ◽  
Nmda Receptors ◽  
Single Channel ◽  
Glutamate Release ◽  
Receptor Activation ◽  
Receptor Activity ◽  
Spinal Neurons ◽  
Quantal Size ◽  
Endogenous Activity ◽  
Nmda Receptor Activation

Robert, Antoine, Joel A. Black, and Stephen G. Waxman. Endogenous NMDA-receptor activation regulates glutamate release in cultured spinal neurons. J. Neurophysiol. 80: 196–208, 1998. N-methyl-d-aspartate (NMDA) receptor activation plays a fundamental role in the genesis of electrical activity of immature neurons and may participate in activity-dependent aspects of CNS development. A recent study has suggested that NMDA-receptor–mediated glutamatergic neurotransmission might occur in the developing spinal cord via activation of nonsynaptic receptors, but the details of NMDA-receptor activation in the developing CNS are not yet well understood. We describe here a model of cultured spinal neurons that display ongoing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor activity characterized by spontaneous excitatory postsynaptic currents (EPSCs), with NMDA-receptor activity detectable only as single channel events. dl-2-amino-5-phosphonovaleric acid (100 μM) and tetrodotoxin (TTX) 100 nM each reduced the occurrence of spontaneous AMPA EPSCs; quantal analysis showed a decrease in the number of released quanta but no changes in quantal size, indicating that NMDA-receptor activation and Na+ channel activity affect the generation of spontaneous AMPA EPSCs, at least in part, via mechanisms that impinge on the presynaptic terminal. Once the Mg2+-block was released, activity of NMDA receptors dramatically increased the release of quantal and multiquantal amounts of glutamate, indicating that the NMDA receptors are physiologically coupled to glutamate release. In Mg2+-free solution, TTX application elicited an increase in the number of quantal AMPA EPSCs and a reduction in the number of multiquantal EPSCs, consistent with an effect of NMDA-receptor activation on presynaptic terminals. Our results suggest that endogenous activity at a small number of NMDA receptors can regulate the release of neurotransmitters at developing AMPA synapses.

scholarly journals NMDA Receptor Activation by Spontaneous Glutamatergic Neurotransmission

2009 ◽  
Vol 101 (5) ◽  
pp. 2290-2296 ◽  
Author(s):  
Felipe Espinosa ◽  
Ege T. Kavalali
Keyword(s):  
Nmda Receptor ◽  
Nmda Receptors ◽  
Ampa Receptor ◽  
Cortical Neurons ◽  
Glutamate Release ◽  
Receptor Activation ◽  
Receptor Activity ◽  
Resting Membrane Potential ◽  
Nmda Current ◽  
Nmda Receptor Activation

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, >100) before or after application of dl-2 amino 5-phosphonovaleric acid, a specific blocker of NMDA receptors. The difference between the two mEPSC waveforms showed that the NMDA current component comprises ∼20% of the charge transfer during an average mEPSC detected at rest. Importantly, the contribution of the NMDA component was markedly enhanced at membrane potentials expected for the depolarized up states (approximately −50 mV) that cortical neurons show during slow oscillations in vivo. In addition, partial block of the α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor component of the mEPSCs did not cause a significant reduction in the NMDA component, indicating that potential AMPA receptor-driven local depolarizations did not drive NMDA receptor activity at rest. Collectively these results indicate that NMDA receptors significantly contribute to signaling at rest in the absence of dendritic depolarizations or concomitant AMPA receptor activity.

scholarly journals Probing the activation sequence of NMDA receptors with lurcher mutations

2012 ◽  
Vol 140 (3) ◽  
pp. 267-277 ◽  
Author(s):  
Swetha E. Murthy ◽  
Tamer Shogan ◽  
Jessica C. Page ◽  
Eileen M. Kasperek ◽  
Gabriela K. Popescu
Keyword(s):  
Nmda Receptor ◽  
Nmda Receptors ◽  
Single Channel ◽  
Receptor Activation ◽  
Dissociation Rate ◽  
Activation Mechanism ◽  
Structural Determinants ◽  
Ionic Flux ◽  
Single Channel Currents ◽  
Nmda Receptor Activation

N-methyl-d-aspartate (NMDA) receptor activation involves a dynamic series of structural rearrangements initiated by glutamate binding to glycine-loaded receptors and culminates with the clearing of the permeation pathway, which allows ionic flux. Along this sequence, three rate-limiting transitions can be quantified with kinetic analyses of single-channel currents, even though the structural determinants of these critical steps are unknown. In inactive receptors, the major permeation barrier resides at the intersection of four M3 transmembrane helices, two from each GluN1 and GluN2 subunits, at the level of the invariant SYTANLAAF sequence, known as the lurcher motif. Because the A7 but not A8 residues in this region display agonist-dependent accessibility to extracellular solutes, they were hypothesized to form the glutamate-sensitive gate. We tested this premise by examining the reaction mechanisms of receptors with substitutions in the lurcher motifs of GluN1 or GluN2A subunits. We found that, consistent with their locations relative to the proposed activation gate, A8Y decreased open-state stability, whereas A7Y dramatically stabilized open states, primarily by preventing gate closure; the equilibrium distribution of A7Y receptors was strongly shifted toward active states and resulted in slower microscopic association and dissociation rate constants for glutamate. In addition, for both A8- and A7-substituted receptors, we noticed patterns of kinetic changes that were specific to GluN1 or GluN2 locations. This may be a first indication that the sequence of discernible kinetic transitions during NMDA receptor activation may reflect subunit-dependent movements of M3 helices. Testing this hypothesis may afford insight into the activation mechanism of NMDA receptors.

The effect of varying stimulus intensity on NMDA-receptor activity in cat visual cortex

1990 ◽  
Vol 64 (5) ◽  
pp. 1413-1428 ◽  
Author(s):  
K. Fox ◽  
H. Sato ◽  
N. Daw
Keyword(s):  
Visual Cortex ◽  
Nmda Receptor ◽  
Nmda Receptors ◽  
Receptor Activation ◽  
Visual Response ◽  
Background Luminance ◽  
Stimulus Contrast ◽  
Cat Visual Cortex ◽  
Low Levels ◽  
Nmda Receptor Activation

1. A study was made of the relative contribution of N-methyl-D-aspartate (NMDA) and non-NMDA receptors to the visual responses of cells in different layers of the cat visual cortex at different levels of excitatory drive (which was varied by altering the stimulus contrast). 2. Receptive fields were mapped for 121 cells in area 17 of cat cortex. Cells were characterized to determine the optimal visual stimulus, the brightness of which was then varied relative to background luminance to construct a contrast-response (C-R) curve for each cell. Curves were made during control conditions and during application of agonists (NMDA and quisqualate) and/or antagonists [(D)-2-amino-5-phosphonovaleric acid (D-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)] to examine the excitatory amino acid components of the visual response. 3. Threshold responses were obtained with stimuli between 1/60 and 1.8 X background luminance. The cell response, measured by firing rate, was linearly related to stimulus contrast over 1-2 decades and saturated at higher contrasts. 4. Application of APV reduced the slope of the linear portion of the C-R curve for cells located in layers II and III (average reduction, 59% of control). APV did not decrease the threshold to stimulation. The "just suprathreshold" responses to stimulation were reduced by the same proportion as the saturation responses for individual cells. The principal effect was therefore to reduce the gain of the C-R curve in these layers. 5. Application of APV reduced the spontaneous activity of cells located in layers IV, V, and VI with little if any effect on the gain of the C-R curve. This suggests a tonic background level of NMDA-receptor activation in these layers, which is not directly related to the visual response. 6. Low levels of NMDA increased the gain of the C-R curve in layers II/III and V/VI. On the other hand, low levels of quisqualate increased the overall level of firing without affecting the gain of the C-R curve. NMDA did not increase the gain of the curve in layer IV. 7. These experiments show that visual stimuli that produce just suprathreshold responses activate NMDA receptors. The degree of activation is proportionally the same for small responses and large responses for an individual cell. Rather than finding a threshold for NMDA-receptor activation, a continuous range of NMDA-receptor influence was observed over the entire response range.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of spinal visceral nociceptive transmission by NMDA receptor activation in the rat

1996 ◽  
Vol 75 (6) ◽  
pp. 2344-2353 ◽  
Author(s):  
R. Kolhekar ◽  
G. F. Gebhart
Keyword(s):  
Nmda Receptor ◽  
Nmda Receptors ◽  
Receptive Fields ◽  
Receptor Activation ◽  
Nmda Receptor Antagonist ◽  
Response Threshold ◽  
Sprague Dawley ◽  
Spinal Neurons ◽  
Nociceptive Transmission ◽  
Stimulus Response

1. Thirty-three neurons in the L6-Sl spinal cord of 30 adult male Sprague-Dawley rats were characterized for responses to colorectal distention (CRD, 20-80 mmHg, 20 s) and convergent cutaneous receptive fields in the presence and absence of N-methyl-D-aspartate (NMDA; 1 microM) or D-serine (1 microM) administered locally by pressure ejection. 2. NMDA ejected locally increased the resting (spontaneous) activity, responses to CRD, postdistention afterdischarges, encoding of visceral nociception, and the size of convergent cutaneous receptive fields of some neurons. Facilitation of responses to noxious intensities of CRD (> or = 40 mmHg) was apparent between 30 s and 4 min after drug ejection. The slope of stimulus-response functions to graded intensities of CRD was increased significantly by NMDA, although mean response threshold was not significantly altered after NMDA ejection. 3. Facilitatory effects of NMDA on responses to CRD and increases in size of convergent cutaneous receptive fields were blocked or reversed by administration of the NMDA receptor antagonist, 5-amino-2-phosphono-valeric acid. 4. D-serine, an agonist at the glycine modulatory site on the NMDA receptor complex, generally mimicked the effects of NMDA on neurons responsive to CRD. The effects of D-serine were blocked by the glycine site antagonist 7-chloro-kynurenic acid (7-CK). 7-CK also blocked NMDA-produced effects on responses to CRD and increases in size of cutaneous receptive fields. 5. No differences were found between spinal neurons with and without documented long ascending projections with respect to effects of NMDA or D-serine. 6. These findings demonstrate involvement of spinal NMDA receptors in mediating hyperexcitability of spinal neurons to visceral nociceptive input and suggest an important contribution of spinal NMDA receptors in visceral hyperalgesic syndromes.

scholarly journals Glycine-dependent activation of NMDA receptors

2015 ◽  
Vol 145 (6) ◽  
pp. 513-527 ◽  
Author(s):  
Kirstie A. Cummings ◽  
Gabriela K. Popescu
Keyword(s):  
Nmda Receptor ◽  
Nmda Receptors ◽  
Time Course ◽  
Glutamate Release ◽  
Receptor Activation ◽  
Reaction Scheme ◽  
Glutamate Binding ◽  
Receptor Isoforms ◽  
Sufficient Detail ◽  
The Central Nervous System

N-methyl-d-aspartate (NMDA) receptors are the only neurotransmitter receptors whose activation requires two distinct agonists. Heterotetramers of two GluN1 and two GluN2 subunits, NMDA receptors are broadly distributed in the central nervous system, where they mediate excitatory currents in response to synaptic glutamate release. Pore opening depends on the concurrent presence of glycine, which modulates the amplitude and time course of the glutamate-elicited response. Gating schemes for fully glutamate- and glycine-bound NMDA receptors have been described in sufficient detail to bridge the gap between microscopic and macroscopic receptor behaviors; for several receptor isoforms, these schemes include glutamate-binding steps. We examined currents recorded from cell-attached patches containing one GluN1/GluN2A receptor in the presence of several glycine-site agonists and used kinetic modeling of these data to develop reaction schemes that include explicit glycine-binding steps. Based on the ability to match a series of experimentally observed macroscopic behaviors, we propose a model for activation of the glutamate-bound NMDA receptor by glycine that predicts apparent negative agonist cooperativity and glycine-dependent desensitization in the absence of changes in microscopic binding or desensitization rate constants. These results complete the basic steps of an NMDA receptor reaction scheme for the GluN1/GluN2A isoform and prompt a reevaluation of how glycine controls NMDA receptor activation. We anticipate that our model will provide a useful quantitative instrument to further probe mechanisms and structure–function relationships of NMDA receptors and to better understand the physiological and pathological implications of endogenous fluctuations in extracellular glycine concentrations.

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