Voltage sensitivity of NMDA-receptor mediated postsynaptic currents

Tonic NMDAR Currents of NR2A-Containing NMDARs Represent Altered Ambient Glutamate Concentration in the Supraoptic Nucleus

NMDA receptors (NMDARs) modulate glutamatergic excitatory tone in the brain via two complementary modalities: a phasic excitatory postsynaptic current and a tonic extrasynaptic modality. Here, we demonstrated that the tonic NMDAR-current (I NMDA) mediated by NR2A-containing NMDARs is an efficient biosensor detecting the altered ambient glutamate level in the supraoptic nucleus (SON). I NMDA of magnocellular neurosecretory cells (MNCs) measured by nonselective NMDARs antagonist, AP5, at holding potential (V holding) -70 mV in low concentration of ECF Mg2+ ([Mg2+]o) was transiently but significantly increased 1-week post induction of a DOCA salt hypertensive model rat which was compatible with that induced by a NR2A-selective antagonist, PEAQX (I PEAQX) in both DOCA-H2O and DOCA-salt groups. In agreement, NR2B antagonist, ifenprodil, or NR2C/D antagonist, PPDA, did not affect the holding current (I holding) at V holding -70 mV. Increased ambient glutamate by exogenous glutamate (10 mM) or excitatory amino acid transporters (EAATs) antagonist (TBOA, 50 mM) abolished the I PEAQX difference between two groups, suggesting that attenuated EAATs activity increased ambient glutamate concentration, leading to the larger I PEAQX in DOCA-salt rats. In contrast, only ifenprodil but not PEAQX and PPDA uncovered I NMDA at V holding +40 mV under 1.2 mM [Mg2+]o condition. I ifenprodil was not different in DOCA-H2O and DOCA-salt groups. Finally, NR2A, NR2B, and NR2D protein expression were not different in the SON of the two groups. Taken together, NR2A-containing NMDARs efficiently detected the increased ambient glutamate concentration in the SON of DOCA-salt hypertensive rats due to attenuated EAATs activity.

Activity-induced spontaneous spikes in GABAergic neurons suppress seizure discharges: an implication of computational modeling

Background

Epilepsy, a prevalent neurological disorder, appears self-termination. The endogenous mechanism for seizure self-termination remains to be addressed in order to develop new strategies for epilepsy treatment. We aim to examine the role of activity-induced spontaneous spikes at GABAergic neurons as an endogenous mechanism in the seizure self-termination.

Methods and Results

Neuronal spikes were induced by depolarization pulses at cortical GABAergic neurons from temporal lobe epilepsy patients and mice, in which some of these neurons fired activity-induced spontaneous spikes. Neural networks including excitatory and inhibitory neurons were computationally constructed, and their functional properties were based on our studies from whole-cell recordings. With the changes in the portion and excitability of inhibitory neurons that generated activity-induced spontaneous spike, the efficacies to suppress synchronous seizure activity were analyzed, such as its onset time, decay slope and spike frequency. The increases in the proportion and excitability of inhibitory neurons that generated activity-induced spontaneous spikes effectively suppressed seizure activity in neural networks. These factors synergistically strengthened the efficacy of seizure activity suppression.

Conclusion

Our study supports a notion that activity-induced spontaneous spikes in GABAergic neurons may be an endogenous mechanism for seizure self-termination. A potential therapeutic strategy for epilepsy is to upregulate the cortical inhibitory neurons that generate activity-induced spontaneous spikes.

Mechanistic and structural determinants of NMDA receptor voltage-dependent gating and slow Mg2+ unblock

NMDA receptor (NMDAR)-mediated currents depend on membrane depolarization to relieve powerful voltage-dependent NMDAR channel block by external magnesium (Mg(o)(2+)). Mg(o)(2+) unblock from native NMDARs exhibits a fast component that is consistent with rapid Mg(o)(2+) -unbinding kinetics and also a slower, millisecond time scale component (slow Mg(o)(2+) unblock). In recombinant NMDARs, slow Mg(o)(2+) unblock is prominent in GluN1/2A (an NMDAR subtype composed of GluN1 and GluN2A subunits) and GluN1/2B receptors, with slower kinetics observed for GluN1/2B receptors, but absent from GluN1/2C and GluN1/2D receptors. Slow Mg(o)(2+) unblock from GluN1/2B receptors results from inherent voltage-dependent gating, which increases channel open probability with depolarization. Here we examine the mechanisms responsible for NMDAR subtype dependence of slow Mg(o)(2+) unblock. We demonstrate that slow Mg(o)(2+) unblock from GluN1/2A receptors, like GluN1/2B receptors, results from inherent voltage-dependent gating. Surprisingly, GluN1/2A and GluN1/2B receptors exhibited equal inherent voltage dependence; faster Mg(o)(2+) unblock from GluN1/2A receptors can be explained by voltage-independent differences in gating kinetics. To investigate the absence of slow Mg(o)(2+) unblock in GluN1/2C and GluN1/2D receptors, we examined the GluN2 S/L site, a site responsible for several NMDAR subtype-dependent channel properties. Mutating the GluN2 S/L site of GluN2A subunits from serine (found in GluN2A and GluN2B subunits) to leucine (found in GluN2C and GluN2D) greatly diminished both voltage-dependent gating and slow Mg(o)(2+) unblock. Therefore, the residue at the GluN2 S/L site governs the expression of both slow Mg(o)(2+) unblock and inherent voltage dependence.

NMDA receptors with incomplete Mg²⁺ block enable low-frequency transmission through the cerebellar cortex

The cerebellar cortex coordinates movements and maintains balance by modifying motor commands as a function of sensory-motor context, which is encoded by mossy fiber (MF) activity. MFs exhibit a wide range of activity, from brief precisely timed high-frequency bursts, which encode discrete variables such as whisker stimulation, to low-frequency sustained rate-coded modulation, which encodes continuous variables such as head velocity. While high-frequency MF inputs have been shown to activate granule cells (GCs) effectively, much less is known about sustained low-frequency signaling through the GC layer, which is impeded by a hyperpolarized resting potential and strong GABA(A)-mediated tonic inhibition of GCs. Here we have exploited the intrinsic MF network of unipolar brush cells to activate GCs with sustained low-frequency asynchronous MF inputs in rat cerebellar slices. We find that low-frequency MF input modulates the intrinsic firing of Purkinje cells, and that this signal transmission through the GC layer requires synaptic activation of Mg²⁺-block-resistant NMDA receptors (NMDARs) that are likely to contain the GluN2C subunit. Slow NMDAR conductances sum temporally to contribute approximately half the MF-GC synaptic charge at hyperpolarized potentials. Simulations of synaptic integration in GCs show that the NMDAR and slow spillover-activated AMPA receptor (AMPAR) components depolarize GCs to a similar extent. Moreover, their combined depolarizing effect enables the fast quantal AMPAR component to trigger action potentials at low MF input frequencies. Our results suggest that the weak Mg²⁺ block of GluN2C-containing NMDARs enables transmission of low-frequency MF signals through the input layer of the cerebellar cortex.

Distinct modes of AMPA receptor suppression at developing synapses by GluN2A and GluN2B: single-cell NMDA receptor subunit deletion in vivo

During development there is an activity-dependent switch in synaptic N-Methyl-D-aspartate (NMDA) receptor subunit composition from predominantly GluN2B to GluN2A, though the precise role of this switch remains unknown. By deleting GluN2 subunits in single neurons during synaptogenesis, we find that both GluN2B and GluN2A suppress AMPA receptor expression, albeit by distinct means. Similar to GluN1, GluN2B deletion increases the number of functional synapses, while GluN2A deletion increases the strength of unitary connections without affecting the number of functional synapses. We propose a model of excitatory synapse maturation in which baseline activation of GluN2B-containing receptors prevents premature synapse maturation until correlated activity allows induction of functional synapses. This activity also triggers the switch to GluN2A, which dampens further potentiation. Furthermore, we analyze the subunit composition of synaptic NMDA receptors in CA1 pyramidal cells, provide electrophysiological evidence for a large population of synaptic triheteromeric receptors, and estimate the subunit-dependent open probability.

Triheteromeric NR1/NR2A/NR2B receptors constitute the major N-methyl-D-aspartate receptor population in adult hippocampal synapses

NMDA receptors (NMDARs), fundamental to learning and memory and implicated in certain neurological disorders, are heterotetrameric complexes composed of two NR1 and two NR2 subunits. The function of synaptic NMDARs in postnatal principal forebrain neurons is typically attributed to diheteromeric NR1/NR2A and NR1/NR2B receptors, despite compelling evidence for triheteromeric NR1/NR2A/NR2B receptors. In synapses, the properties of triheteromeric NMDARs could thus far not be distinguished from those of mixtures of diheteromeric NMDARs. To find a signature of NR1/NR2A/NR2B receptors, we have employed two gene-targeted mouse lines, expressing either NR1/NR2A or NR1/NR2B receptors without NR1/NR2A/NR2B receptors, and compared their synaptic properties with those of wild type. In acute hippocampal slices of mutants older than 4 weeks we found a distinct voltage dependence of NMDA R-mediated excitatory postsynaptic current (NMDA EPSC) decay time for the two diheteromeric NMDARs. In wild-type mice, NMDA EPSCs unveiled the NR1/NR2A characteristic for this voltage-dependent deactivation exclusively, indicating that the contribution of NR1/NR2B receptors to evoked NMDA EPSCs is negligible in adult CA3-to-CA1 synapses. The presence of NR1/NR2A/NR2B receptors was obvious from properties that could not be explained by a mixture of diheteromeric NR1/NR2A and NR1/NR2B receptors or by the presence of NR1/NR2A receptors alone. The decay time for NMDA EPSCs in wild type was slower than that for NR1/NR2A receptors, and the sensitivity of NMDA EPSCs to NR2B-directed NMDAR antagonists was 50%. Thus, NR2B is prominent in adult hippocampal synapses as an integral part of NR1/NR2A/NR2B receptors.

Synaptic NR2A- but not NR2B-Containing NMDA Receptors Increase with Blockade of Ionotropic Glutamate Receptors

NMDA receptors (NMDAR) are key molecules involved in physiological and pathophysiological brain processes such as plasticity and excitotoxicity. Neuronal activity regulates NMDA receptor levels in the cell membrane. However, little is known on which time scale this regulation occurs and whether the two main diheteromeric NMDA receptor subtypes in forebrain, NR1/NR2A and NR1/NR2B, are regulated in a similar fashion. As these differ considerably in their electrophysiological properties, the NR2A/NR2B ratio affects the neurons’ reaction to NMDA receptor activation. Here we provide evidence that the basal turnover rate in the cell membrane of NR2A- and NR2B-containing receptors is comparable. However, the level of the NR2A subtype in the cell membrane is highly regulated by NMDA receptor activity, resulting in a several-fold increased insertion of new receptors after blocking NMDAR for 8 h. Blocking AMPA receptors also increases the delivery of NR2A-containing receptors to the cell membrane. In contrast, the amount of NR2B-containing receptors in the cell membrane is not affected by ionotropic glutamate receptor block. Moreover, electrophysiological analysis of synaptic currents in hippocampal cultures and CA1 neurons of hippocampal slices revealed that after 8 h of NMDA receptor blockade the NMDA EPSCs increase as a result of augmented NMDA receptor-mediated currents. In conclusion, synaptic NR2A- but not NR2B-containing receptors are dynamically regulated, enabling neurons to change their NR2A/NR2B ratio within a time scale of hours.

Four excitatory postsynaptic ionotropic receptors coactivated at the motoneuron-Renshaw cell synapse

Renshaw cells (RCs) are spinal interneurons excited by collaterals of the axons of motoneurons (MNs). They respond to a single motoneuronal volley by a surprisingly long (tens of milliseconds) train of action potentials. We have analyzed this synaptic response in spinal cord slices of neonatal mice in light of recent observations suggesting that the MN axons release both acetylcholine and glutamate. We found that the RC synaptic current involves four components of similar amplitudes mediated by two nicotinic receptors (nAChRs, tentatively identified as alpha(7) homomers and alpha(4)beta(2) heteromers) and two glutamate receptors (AMPARs and NMDARs). The decay time constants of the four components cover a wide range: from 3.6 +/- 2.2 ms (alpha(7) nAChRs) to 54.6 +/- 19.5 ms (NMDARs, at -45 mV). The RC discharge can be separated into an initial doublet of high-frequency action potentials followed by later spikes with a variable latency and longer interspike intervals. The initial doublet involves the four ionotropic receptors as well as endogenous voltage-dependent conductances. The late discharge depends on NMDARs, but these receptors must be primed by the initial depolarization. The activation of the NMDARs is prolonged by the fact that their slow deactivation is further slowed by depolarization. The formation of the initial doublet is favored by hyperpolarization, whereas the late discharge is favored by depolarization. This suggests that in physiological conditions the pattern of discharge of the RC in response to a MN input may alternate between a phasic and a tonic response.

Voltage-dependent gating of NR1/2B NMDA receptors

Ligand-gated ion channels are activated by agonist binding, but may also be modulated by membrane voltage. N-Methyl-d-aspartate receptors (NMDARs) exhibit especially strong voltage dependence due to channel block by external Mg(2+) (Mg(o)(2+)). Here we demonstrate that activity of NMDARs composed of NR1 and NR2B subunits (NR1/2B receptors) is enhanced by depolarization even in 0 Mg(o)(2+), causing slow current relaxations in response to rapid voltage changes. We present a kinetic model of receptor activation that incorporates voltage-dependent gating-associated NR2B subunit conformational changes. The model accurately reproduces current relaxations during depolarizations and subsequent repolarizations in 0 Mg(o)(2+). Model simulations in physiological Mg(o)(2+) concentrations show that voltage-dependent receptor gating also underlies the slow component of Mg(o)(2+) unblock, a phenomenon that previously was shown to influence Mg(o)(2+) unblock kinetics during dendritic spikes. We propose that voltage-dependent gating of NR1/2B receptors confers enhanced voltage and time dependence on NMDAR-mediated signalling.

The synaptic activation of NMDA receptors and Ca2+ signalling in neurons

Long-term potentiation (LTP) in the hippocampus is a model system for understanding the synaptic basis of learning and memory. We have studied the mechanism of induction of LTP using voltage-clamp techniques and confocal imaging of Ca2+ in rat hippocampal slices. In the Schaffer collateral-commissural pathway the neurotransmitter L-glutamate activates two classes of ionotropic receptor, named after the selective ligands AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) and NMDA (N-methyl-D-aspartate). During low frequency transmission the excitatory postsynaptic potential (EPSP) is mediated predominantly by AMPA receptors. NMDA receptors play a minor role because their ion channels are substantially blocked by Mg2+, and this block is intensified by GABA-mediated synaptic inhibition. During high frequency transmission the GABA-mediated inhibition is depressed, by mechanisms initiated by GABAB autoreceptors. This allows a greater contribution from the NMDA receptors, through which Ca2+ enters the dendrites of the postsynaptic neurons to initiate a cascade of biochemical processes which ultimately result in enhanced synaptic efficiency.

Fast and slow voltage-dependent dynamics of magnesium block in the NMDA receptor: the asymmetric trapping block model

The NMDA receptor (NMDAR) produces a long-lasting component of the glutamatergic EPSC in mammalian central neurons. The current through NMDARs is voltage dependent as a result of block by extracellular magnesium, which has recently been shown to give rise to a complex time dependence, with fast and slow components of responses to changes in membrane potential. Here, we studied the dynamics of block and unblock by measuring voltage step responses in conjunction with fast perfusion of agonist in nucleated patches isolated from rat cortical pyramidal neurons. We found that slow unblock shows a progressive onset during synaptic-like responses to brief pulses of agonist. Repolarizing briefly from +40 to -70 mV revealed that slow unblock is reestablished with a time constant of approximately 5 msec at room temperature. Also, the time course of deactivation, in response to a pulse of agonist, slows twofold over the potential range -30 to +40 mV. An asymmetric "trapping block" model in which the voltage-independent closing rate constant of the blocked channel is approximately three times that of the unblocked channel accounts quantitatively for all of these phenomena and for responses to action potential waveform clamp. This model allows much more accurate prediction of NMDAR current in physiological conditions of magnesium concentration and changing membrane potential than previously possible. It suggests a positive allosteric link between occupation of the NMDAR pore by magnesium and closure of the permeation gate.

Kinetics of Mg2+ unblock of NMDA receptors: implications for spike-timing dependent synaptic plasticity

The time course of Mg(2+) block and unblock of NMDA receptors (NMDARs) determines the extent they are activated by depolarization. Here, we directly measure the rate of NMDAR channel opening in response to depolarizations at different times after brief (1 ms) and sustained (4.6 s) applications of glutamate to nucleated patches from neocortical pyramidal neurons. The kinetics of Mg(2+) unblock were found to be non-instantaneous and complex, consisting of a prominent fast component (time constant approximately 100 micros) and slower components (time constants 4 and approximately 300 ms), the relative amplitudes of which depended on the timing of the depolarizing pulse. Fitting a kinetic model to these data indicated that Mg(2+) not only blocks the NMDAR channel, but reduces both the open probability and affinity for glutamate, while enhancing desensitization. These effects slow the rate of NMDAR channel opening in response to depolarization in a time-dependent manner such that the slower components of Mg(2+) unblock are enhanced during depolarizations at later times after glutamate application. One physiological consequence of this is that brief depolarizations occurring earlier in time after glutamate application are better able to open NMDAR channels. This finding has important implications for spike-timing-dependent synaptic plasticity (STDP), where the precise (millisecond) timing of action potentials relative to synaptic inputs determines the magnitude and sign of changes in synaptic strength. Indeed, we find that STDP timing curves of NMDAR channel activation elicited by realistic dendritic action potential waveforms are narrower than expected assuming instantaneous Mg(2+) unblock, indicating that slow Mg(2+) unblock of NMDAR channels makes the STDP timing window more precise.

NMDA receptor-mediated currents in rat cerebellar granule and unipolar brush cells

The properties of N-methyl-D-aspartate (NMDA) receptor-mediated currents at the giant cerebellar mossy-fiber unipolar brush cell (UBC) synapse were compared with those of adjacent granule cells using patch-clamp recording methods in thin slices of rat cerebellar nodulus. In UBCs, NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) decayed as a single exponential whose time constant was independent of membrane potential. The EPSC was reduced in all cells by the NR1/NR2B-selective antagonist ifenprodil, and the Zn(2+) chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) produced a transient potentiation in 50% of cells. In contrast, the NMDA EPSC in granule cells decayed as a double exponential that dramatically switched to a slower rate at positive membrane potentials. The synaptic response in some granule cells also displayed a late second peak at positive potentials, and in others, activation of mossy fibers produced repetitive trains of EPSCs indicating they may be postsynaptic to the UBC network. Single-channel recordings of outside-out somatic patches from UBCs in magnesium-free solution revealed only high-conductance (50 pS) channels whose open time was increased with depolarization, but the opening frequency was decreased to yield a low (p(o) = 0.0298), voltage-independent opening probability. Lowering extracellular calcium (2.5-0.25 mM) had no effects on channel gating, although an increase of single-channel conductance was observed at lower calcium concentrations. Taken together, the data support the notion that the NMDA receptor in UBCs may comprise both NR1/NR2A and NR1/NR2B receptors. Furthermore, the properties of the EPSC in these two classes of feedforward glutamatergic interneurons display fundamental differences that may relate to their roles in synaptic integration.

Corticostriatal paired-pulse potentiation produced by voltage-dependent activation of NMDA receptors and L-type Ca(2+) channels

AMPA and N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses expressed differential paired-pulse plasticity when examined in the same cell using intracellular or whole cell voltage-clamp recordings. Electrical stimulation of corticostriatal afferents in brain slices bathed in artificial cerebrospinal fluid containing bicuculline produces excitatory postsynaptic potentials and excitatory postsynaptic currents (EPSCs) mediated primarily by AMPA receptors. Cell-to-cell variation existed in AMPA receptor paired-pulse plasticity, but within-cell plasticity was stable over a range of stimulation intensities. Addition of 6-cyano-7-nitroquinoxalene-2,3-dione blocked most of the synaptic response leaving behind a small AP-5-sensitive component. Increasing the stimulation intensity produced large, long-lasting NMDA receptor-mediated responses. In contrast to AMPA receptor-mediated responses, NMDA receptor responses consistently showed an increase in paired-pulse potentiation with increasing stimulation intensity. This relationship was restricted to interstimulus intervals shorter than 100 ms. Paired-pulse potentiation of NMDA receptor responses was voltage-dependent and reduced by removal of extracellular Mg(2+). Block of postsynaptic L-type Ca(2+) channels with nifedipine produced a voltage-dependent reduction of NMDA receptor excitatory postsynaptic currents (EPSCs) and a voltage-dependent reduction of NMDA receptor paired-pulse potentiation. These data indicate depolarization during the first NMDA receptor response causes facilitation of the second by removing voltage-dependent block of NMDA receptors by Mg(2+) and by activating voltage-dependent Ca(2+) channels.

Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.

Survival promotion of mesencephalic dopaminergic neurons by depolarizing concentrations of K+ requires concurrent inactivation of NMDA or AMPA/kainate receptors

The death of dopaminergic neurons that occurs spontaneously in mesencephalic cultures was prevented by depolarizing concentrations of K+ (20-50 mM). However, unlike that observed previously in other neuronal populations of the PNS or CNS, promotion of survival required concurrent blockade of either NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptors by the specific antagonists, MK-801 and GYKI-52466, respectively. Rescued neurons appeared to be healthy and functional because the same treatment also dramatically enhanced their capacity to accumulate dopamine. The effects on survival and uptake were rather specific to dopaminergic neurons, rapidly reversible and still observed when treatment was delayed after plating. Glutamate release increased substantially in the presence of elevated concentrations of K+, and chronic treatment with glutamate induced a loss of dopaminergic neurons that was prevented by MK-801 or GYKI-52466 suggesting that an excitotoxic process interfered with survival when only the depolarizing treatment was applied. The effects of the depolarizing stimulus in the presence of MK-801 were mimicked by BAY K-8644 and abolished by nifedipine, suggesting that neuroprotection resulted from Ca(2+) influx through L-type calcium channels. Measurement of intracellular calcium revealed that MK-801 or GYKI-52466 were required to maintain Ca(2+) levels within a trophic range, thus preventing K+-induced excitotoxic stress and Ca(2+) overload. Altogether, our results suggest that dopaminergic neurons may require a finely tuned interplay between glutamatergic receptors and calcium channels for their development and maturation.

Electrophysiological and morphological characterization of neurons within neocortical ectopias

Focal developmental abnormalities in neocortex, including ectopic collections of neurons in layer I (ectopias), have been associated with behavioral and neurological deficits. In this study, we used infrared differential interference contrast microscopy and whole cell patch-clamp to complete the first characterization of neurons within and surrounding neocortical ectopias. Current-clamp recordings revealed that neurons within ectopias display multiple types of action potential firing patterns, and biocytin labeling indicated that approximately 20% of the cells in neocortical ectopias can be classified as nonpyramidal cells and the rest as atypically oriented pyramidal cells. All cells had spontaneous excitatory (glutamatergic) and inhibitory (GABAergic) postsynaptic currents. Exhibitory postsynaptic currents consisted of both N-methyl-D-aspartate (NMDA) receptor-mediated and AMPA/kainate (A/K) receptor-mediated currents. The NMDA receptor-mediated component had decay time constants of 15.35 +/- 2.2 (SE) ms, while the A/K component had faster decay kinetics of 7.6 +/- 1.7 ms at -20 mV. GABA(A) receptor-mediated synaptic currents in ectopic cells reversed at potentials near the Cl- equilibrium potential and had decay kinetics of 16.65 +/- 1.3 ms at 0 mV. Furthermore we show that cells within ectopias receive direct excitatory and inhibitory input from adjacent normatopic cortex and can display a form of epileptiform activity.

Postsynaptic variability of firing in rat cortical neurons: the roles of input synchronization and synaptic NMDA receptor conductance

Neurons in the functioning cortex fire erratically, with highly variable intervals between spikes. How much irregularity comes from the process of postsynaptic integration and how much from fluctuations in synaptic input? We have addressed these questions by recording the firing of neurons in slices of rat visual cortex in which synaptic receptors are blocked pharmacologically, while injecting controlled trains of unitary conductance transients, to electrically mimic natural synaptic input. Stimulation with a Poisson train of fast excitatory (AMPA-type) conductance transients, to simulate independent inputs, produced much less variability than encountered in vivo. Addition of NMDA-type conductance to each unitary event regularized the firing but lowered the precision and reliability of spikes in repeated responses. Independent Poisson trains of GABA-type conductance transients (reversing at the resting potential), which simulated independent activity in a population of presynaptic inhibitory neurons, failed to increase timing variability substantially but increased the precision of responses. However, introduction of synchrony, or correlations, in the excitatory input, according to a nonstationary Poisson model, dramatically raised timing variability to in vivo levels. The NMDA phase of compound AMPA-NMDA events conferred a time-dependent postsynaptic variability, whereby the reliability and precision of spikes degraded rapidly over the 100 msec after the start of a synchronous input burst. We conclude that postsynaptic mechanisms add significant variability to cortical responses but that substantial synchrony of inputs is necessary to explain in vivo variability. We suggest that NMDA receptors help to implement a switch from precise firing to random firing during responses to concerted inputs.

NMDA and non-NMDA receptors are co-localized at excitatory synapses of rat hypoglossal motoneurons

We used whole-cell patch clamp recordings in a rat brainstem slice preparation to characterize the properties of miniature excitatory postsynaptic currents (mEPSCs) in hypoglossal motoneurons. The distinct kinetic characteristics of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated synaptic responses allowed us to study dual component mEPSCs mediated by the two receptor types. Using this approach, NMDA and non-NMDA receptors were found to be co-localized at the same synaptic locations. In addition, some sites contain only NMDA receptors since a large proportion of mEPSCs were apparently mediated by NMDA receptors only. Furthermore, the amplitudes of pharmacologically isolated NMDA receptor-mediated mEPSCs were highly variable in individual cells and their decay kinetics were modulated by membrane potential.

Rapid kinetics and inward rectification of miniature EPSCs in layer I neurons of rat neocortex

With the use of the whole cell patch-clamp technique combined with visualization of neurons in brain slices, we studied the properties of miniature excitatory postsynaptic currents (mEPSCs) in rat neocortical layer I neurons. At holding potentials (-50 to -70 mV) near the resting membrane potential (RMP), mEPSCs had amplitudes of 5-100 pA and were mediated mostly by alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) receptors. Amplitude histograms were skewed toward large events. An N-methyl-D-aspartate (NMDA) component was revealed by depolarization to -30 mV or by the use of a Mg2+-free bathing solution. At RMP, averaged AMPA mEPSCs had a 10-90% rise time of approximately 0.3 ms (uncorrected for instrument filtering). The decay of averaged mEPSCs was best fit by double-exponential functions in most cases. The fast, dominating component had a decay time constant of approximately 1.2 ms and comprised approximately 80% of the total amplitude. A small slow component had a decay time constant of approximately 4 ms. Positive correlations were found between rise and decay times of both individual and averaged mEPSCs, indicative of dendritic filtering. Some large-amplitude mEPSCs and spontaneous EPSCs (recorded in the absence of tetrodotoxin) had slower kinetics, suggesting a role of asynchronous transmitter release in shaping EPSCs. The amplitudes of mEPSCs were much smaller at +60 mV than at -60 mV, indicating that synaptic AMPA-receptor-mediated currents were inwardly rectifying. These results suggest that neocortical layer I neurons receive both NMDA- and AMPA-receptor-mediated synaptic inputs. The rapid decay of EPSCs appears to be largely determined by AMPA receptor deactivation. The observed rectification of synaptic responses suggests that synaptic AMPA receptors in layer I neurons may lack GluR-2 subunits and may be Ca2+ permeable.

Synaptic NMDA receptors in developing mouse hippocampal neurones: functional properties and sensitivity to ifenprodil

1. Whole-cell patch-clamp techniques were used to record pharmacologically isolated NMDA receptor-mediated EPSCs (NMDA EPSCs) from CA1 pyramidal cells (PCs) in hippocampal slices from 4-day-old to 36-week-old mice, in order to characterize developmental changes in functional properties and subunit composition of synaptic NMDA receptors. 2. During the first postnatal weeks the dendritic tree of CA1 PCs stained with biocytin increased both in size and in complexity. This was associated with an increase in amplitude of the focally evoked NMDA EPSCs recorded either in nominally Mg(2+)-free or Mg(2+)-containing saline. In adult PCs (> 5 weeks old) EPSC amplitude was 4-fold larger than in very young (up to 2 weeks old) neurones. 3. The sensitivity of NMDA EPSCs to blockade by Mg2+ did not change with age. In very young, intermediate and adult PCs the EPSC-voltage relation displayed an area of negative slope conductance at membrane potentials more negative than -30 mV. The apparent Kd values of the NMDA receptors for Mg2+ at 0 mV were 7.8 +/- 6.4, 10.4 +/- 14.1 and 6.5 +/- 4.7 mM in very young, intermediate and adult neurones, respectively. 4. The decay of the NMDA EPSC in both young and adult neurones could be described by the sum of a fast and a slow exponential function. Both EPSC rise time and fast and slow decay time constants measured at -60 mV, decreased with age. 5. The decay of NMDA EPSCs in young versus adult PCs was differentially modulated by membrane voltage. In young PCs depolarization slowed both the fast and the slow EPSC components. In adult PCs depolarization slightly accelerated the initial EPSC decay, though the overall duration of the EPSC did not change. The rise time of the EPSCs was not affected by voltage at any age. 6. The subunit-selective NMDA receptor antagonist ifenprodil similarly blocked iontophoretic NMDA-induced currents and NMDA EPSCs. In both young and adult PCs, the concentration-response curves for this effect disclosed distinct low and high affinity binding sites for ifenprodil. 7. In young PCs, low and high affinity binding sites for ifenprodil were about equally expressed (57 versus 43%, respectively), whereas in adult PCs, synaptic NMDA receptors expressed a majority (78%) of low affinity binding sites for ifenprodil. 8. The long duration of NMDA EPSCs (and by implication, of Ca2+ transfer through NMDA receptor channels) and its further prolongation by depolarization in young PCs are consistent with heightened NMDA-dependent neuronal plasticity early in development. The age-related changes in these properties may result from a developmental change in NMDA receptor subunit composition.

Regulation of EPSPs by the synaptic activation of GABAB autoreceptors in rat hippocampus

1. Intracellular recording was used to study the influence of GABAB autoreceptor-mediated regulation of monosynaptic GABAA and GABAB receptor-mediated hyperpolarizing inhibitory postsynaptic potentials (IPSPAs and IPSPBs, respectively) on alpha-amino-3-hydroxy-5-methyl -4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potentials (EPSPAs and EPSPNs, respectively) in the CA1 region of rat hippocampal slices. To achieve this, synaptic potential were evoked monosynaptically by near stimulation following blockade of either EPSPNs, by the NMDA receptor antagonist (R)-2-amino-5-phosphonopentanoate (AP5; 0.05 mM), or EPSPAs, by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.01 mM). 2. Paired-pulse stimulation at 3-50 Hz caused an increase in the duration (paired-pulse widening) of EPSPAs, which paralleled the time course of paired-pulse depression of monosynaptic IPSCs, and a potentiation of the amplitude (paired-pulse potentiation) of EPSPAs, which did not. Paired-pulse stimulation also caused frequency-dependent changes in EPSPNs. At frequencies > 40 Hz it produced paired-pulse depression of EPSPNs, along with marked summation of IPSPS, and at frequencies < 40 Hz it caused paired-pulsed enlargement of EPSPNs, concomitant with a reduction in IPSPS. 3. Paired-pulse potentiation of EPSPAs at 50 Hz was enhanced by picrotoxin (0.1 mM) but was not significantly affected by 3-amino-propyl(diethoxymethyl)phosphinic acid (CGP 35348; 1 mM). Paired-pulse depression of EPSPNs at 50 Hz was converted to paired-pulse enlargement by picrotoxin but was unaffected by CGP 35348. These effects can be explained by block of IPSPAs by picrotoxin. 4. Paired-pulsed widening of EPSPAs at 5 Hz was occluded by picrotoxin and abolished by CGP 35348. Similarly, paired-pulsed enlargement of EPSPNs at 5 Hz was occluded, and in some cases converted to paired-pulse depression, by picrotoxin. The effects of CGP 35348 were more complex in that this antagonist reduced paired-pulse enlargement of EPSPNs in control medium whereas it eliminated paired-pulsed depression of EPSPNs in the presence of picrotoxin, effects consistent with its block of GABAB autoreceptors and IPSPBS, respectively. 5. 'Priming' using a 'priming stimulation protocol' (a single 'priming stimulus' followed at 1-50 Hz ('priming frequency') by a 'primed burst' of four shocks at 20-100 Hz ('burst frequency')) caused an increase in both 'primed' EPSPAs and EPSPNs compared with 'unprimed' EPSPAs and EPSPNs. This effect was optimal when the respective priming and burst frequencies were 5 and 100 Hz. 6. In the presence of either picrotoxin or CGP 35348 the primed EPSPAs and EPSPNs resembled unprimed EPSPAs and EPSPNs, respectively. This was because picrotoxin occluded whereas CGP 35348 blocked the effect of priming on EPSPS. 7. CGP 35348 had only modest effects on EPSPAs but enhanced EPSPNs evoked by a tetanus (20 stimuli at 100 Hz), in either the presence or absence of picrotoxin. In the absence of picrotoxin, CGP 35348 also promoted depolarization by enhancing a depolarizing GABAA receptor-mediated component (IPSPD). These effects can all be attributed to block of IPSPBS by CGP 35348. 8. CGP 35348 blocked the induction of long-term potentiation (LTP) of extracellularly recorded field EPSPs elicited by a priming stimulation protocol in control medium but was ineffective in the presence of picrotoxin. CGP 35348 was also ineffective at preventing tetanus-induced LTP (100 Hz, 1 s) in both the absence and presence of picrotoxin. 9. These data demonstrate the complex regulation of AMPA and NMDA receptor-mediated EPSPs during various patterns of synaptic activation caused by the dynamic changes in GABA-mediated synaptic inhibition, which are orchestrated by GABAA autoreceptors in a frequency-dependent

Evidence that heterosynaptic depolarization underlies associativity of long-term potentiation in rat hippocampus

1. Whole-cell patch-clamp recording has been used to study the effect of heterosynaptic depolarization on pure N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission in the CA1 region of rat hippocampal slices. 2. In neurones voltage clamped at -60 mV, paired-pulse stimulation of one set of Schaffer collateral-commissural fibres resulted in homosynaptic paired-pulse facilitation of the NMDA receptor-mediated excitatory postsynaptic current (EPSCN). In contrast, stimulation of one set of fibres prior to stimulation of a second set of fibres (i.e. heterosynaptic paired-pulse stimulation) did not result in any heterosynaptic interactions. 3. However, under current-clamp conditions, heterosynaptic paired-pulse stimulation resulted in heterosynaptic 'paired-pulse facilitation' of the NMDA receptor-mediated excitatory postsynaptic potential (EPSPN). 4. In neurones held at -50 or -40 mV, perfusion of nominally Mg(2+)-free medium converted the response to heterosynaptic paired-pulse stimulation from 'heterosynaptic facilitation' to 'heterosynaptic depression' of EPSPN. 5. When neurones were held at potentials of between -30 and +40 mV then heterosynaptic paired-pulse stimulation, in normal Mg(2+)-containing medium, resulted in 'paired-pulse depression' of EPSPN. Under voltage-clamp conditions (tested at +40 mV) no heterosynaptic interactions were seen. 6. The time course of 'heterosynaptic facilitation' at -60 mV and of 'heterosynaptic depression' at +40 mV of EPSPN was similar to the time course of EPSCN. 7. We conclude, firstly, that the voltage clamp is able to prevent any voltage breakthrough associated with the synaptic activation of NMDA receptors from influencing neighbouring synapses. Secondly, when the neurone is not voltage clamped these same synapses are strongly influenced by the spreading depolarization generated by the synaptic activation of their neighbours. The time course and direction of this influence are compatible with the hypothesis that spreading synaptic depolarization, leading to a reduction of the voltage-dependent Mg2+ block of synaptic NMDA receptor channels, underlies the property of associativity.

Characterization of spontaneous excitatory synaptic currents in salamander retinal ganglion cells

1. Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage-clamp conditions. Consistent with activation of non-NMDA-type glutamate receptors, the sEPSCs reversed at potentials above 0 mV, were blocked by 1 microM CNQX and prolonged by 2 mM aniracetam. 2. The peak conductance of the averaged sEPSCs (n = 70-400) was 130 +/- 60 pS (mean +/- S.D.; 17 cells, ranging from 70 to 290 pS). Amplitude distributions were skewed towards larger amplitudes. 3. The decay of individual and mean sEPSCs was exponential with a mean time constant (tau d) of 3.75 +/- 0.84 ms (n = 13), which was voltage independent. The 10-90% rise time of the sEPSCs was 1.30 +/- 0.44 ms (n = 13). There was no correlation between sEPSC rise time and tau d suggesting that dendritic filtering alone did not shape the time course of sEPSCs. 4. Light-evoked EPSCs in these retinal ganglion cells are mediated by concomitant activation of NMDA and non-NMDA receptors; however, no NMDA component was discerned in the sEPSCs, even when recording at -96 mV in Mg(2+)-free solutions. The decay time course was not altered by 20 microM AP7, an NMDA antagonist, nor was an NMDA component unmasked by adding glycine or D-serine. These results suggest that NMDA and non-NMDA receptors are not coactivated by a single vesicle of transmitter during spontaneous release, and thus are probably not colocalized in the postsynaptic membrane at the sites of spontaneous release. 5. The sEPSCs were an order of magnitude faster than the non-NMDA receptor-mediated EPSCs evoked by light stimuli, and it is proposed that the EPSC time course is determined largely by the extended time course of release of synaptic vesicles from bipolar cells. The quantal content of a light-evoked non-NMDA receptor-mediated EPSC in an on-off cell is about 200 quanta.

Inhibitory mechanisms in epileptiform activity induced by low magnesium

In rat hippocampal slices epileptiform activity was induced by superfusion with Mg(2+)-free artificial cerebrospinal fluid (ACSF). Paroxysmal depolarization shifts (PDS) were evoked by electrical stimulation of Schaffer collaterals. To investigate the afterpotentials that follow PDS, intracellular recordings were made from CA1 pyramidal cells. The experiments revealed that several components are engaged in the generation of PDS afterpotentials in Mg(2+)-free ACSF. A long lasting component which determined the overall duration of the PDS afterhyperpolarization was blocked by intracellular application of ethylenebis(oxonitrilo)-tetraacetate (EGTA); concomitantly, the afterhyperpolarizations following depolarizing current injections were blocked. This indicated that the long lasting component was due to a slow Ca(2+)-activated K+ current. The block of Ca(2+)-activated K+ current uncovered a depolarizing PDS afterpotential with an N-shaped voltage dependence, suggesting that this depolarizing afterpotential component may be due to an N-methyl D-aspartate (NMDA) conductance. Intracellular injection of Cl- revealed that the PDS were followed by Cl- currents lasting about 500 ms. This component could be blocked by application of bicuculline suggesting that it is due to a synaptically GABA-mediated (i.e. gamma-aminobutyric acid) Cl- current. A comparison of PDS afterpotentials in Mg(2+)-free ACSF and those in other models of epileptiform activity suggests that similar sequences of inhibitory components are activated in spite of different pharmacological alterations of membrane conductances which induce the epileptiform discharges.

A comparison of paired-pulsed facilitation of AMPA and NMDA receptor-mediated excitatory postsynaptic currents in the hippocampus

Paired-pulse facilitation of excitatory synaptic transmission was investigated in the CA1 region of rat hippocampal slices using whole-cell patch-clamp recording. To optimise the measurement of excitatory synaptic transmission, gamma-amino-butyric acid (GABA)-mediated synaptic inhibition was eliminated using both GABAA and GABAB antagonists. Pure alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSCs) were then isolated pharmacologically. Paired-pulse facilitation of either AMPA or NMDA receptor-mediated EPSCs (EPSCA and EPSCN, respectively) was investigated using two stimuli of identical strength delivered at intervals of between 25 and 1000 ms. The paired-pulse facilitation profiles of both EPSCA and EPSCN were similar. Paired-pulse facilitation of EPSCA was independent of holding potential. In contrast paired-pulse facilitation of EPSCN was markedly voltage-dependent; maximum facilitation was recorded at hyperpolarised membrane potentials. At positive membrane potentials there was little or no paired-pulse facilitation and, in most neurones, paired-pulse depression was observed. Voltage-dependence of paired-pulse facilitation of EPSCN was similar in the presence of nominal absence of Mg2+ in the bathing medium, and was unaffected by extensive dialysis of neurones with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). These data are consistent with a presynaptic locus for paired-pulse facilitation of EPSCA. However, paired-pulse facilitation of EPSCN involves postsynaptic factors.

Effects of an N-methyl-D-aspartate antagonist and a GABAergic antagonist on entorhinal tetanic responses during the early stages of amygdala kindling in rats

Changes in synaptic potentials during each train stimulation (tetanic responses) have been suggested to intimately relate to the development of kindling. We examined the effects of an NMDA antagonist, carboxypiperazinephosphonate (CPP), and a GABAergic antagonist, picrotoxin, on entorhinal tetanic responses evoked by train stimuli (10 Hz, 100 pulses) at the developmental stage (seizure stage; 0-2) of amygdala kindling in conscious rats, to clarify the significance of facilitation in tetanic responses and the roles of NMDA and GABA receptors in the development of kindling. Facilitation of tetanic responses was noted as a progressive increase in both amplitude and duration of negative potentials in the tetanic responses, especially during the later half of train pulses (51-100). The negative potential area (mV x ms) of the averaged tetanic responses was used as an estimate of the magnitudes of excitatory synaptic activity in the tetanic responses, and correlated significantly (P < 0.001) with the duration of afterdischarges (AD). CPP (10 mg/kg) reversibly blocked AD in association with a significant decrease (P < 0.05) in the negative potential area. The CPP-sensitive component consisted of a slow negative potential with a duration longer than 60 ms and was greater in the later tetanic responses (51-100) than the earlier ones (1-50). Picrotoxin (2-3 mg/kg), which did not produce convulsions, significantly (P < 0.005) increased the negative potential area in the tetanic responses in association with a reversible decrease in the AD threshold. Although positive potentials ascribable to inhibitory synaptic activity were often negligible in the tetanic responses in controls, picrotoxin further decreased the positive potentials of tetanic responses, if any.(ABSTRACT TRUNCATED AT 250 WORDS)

Voltage-dependent kinetics of N-methyl-D-aspartate synaptic currents in rat cerebellar granule cells

Decay kinetics of N-methyl-D-aspartate excitatory postsynaptic currents (NMDA-EPSCs) have been voltage-dependent in some, but not all neurons studied so far, and almost no information has been available on the voltage-dependence of the rising phase. In this work we investigated the effect of membrane potential on rising and decay kinetics of the NMDA-EPSC in cerebellar granule cells using the tight-seal whole-cell recording technique. NMDA-EPSCs were evoked by electrical mossy fibre stimulation in the presence of 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione, 1.2 mM Mg2+ and 5 microM glycine. The rate of rise of NMDA-EPSCs remained substantially unchanged when the cell was depolarized, indicating that the limiting step of channel opening was voltage-insensitive. The NMDA-EPSC, however, flattened around the peak and the time-to-peak increased. This observation was explained by the influence of decay. Decay was biphasic and slowed down with membrane depolarization. Moreover, the fast component of decay increased less than the slow component. This complex voltage-dependence may extend the integrative role of the NMDA current during synaptic transmission.

Activation of NMDA receptors in hippocampal area CA1 by low and high frequency orthodromic stimulation and their contribution to induction of long-term potentiation

N-methyl-D-aspartate (NMDA) receptors are important in many instances of synaptic plasticity. In hippocampal area CA1, long-term potentiation (LTP) can be induced by both NMDA receptor-dependent and -independent mechanisms. Using intracellular recordings and single-electrode voltage clamp, we isolated and characterized NMDA receptor-mediated synaptic responses. NMDA receptor-mediated responses evoked by low frequency orthodromic stimulation were inhibited in a dose-dependent manner by the competitive antagonist D,L-2-amino-5-phosphonovaleric acid (APV). High frequency (tetanic) stimulation, which facilitates synaptic release of glutamate, failed to overcome the blockade of NMDA receptors by APV. Using extracellular recordings of field potentials, we studied the contribution of NMDA receptors to LTP induced by different patterns of tetanic stimulation. LTP was inhibited in a dose-dependent manner by APV, but was more sensitive to APV than were NMDA receptor-mediated synaptic responses. This most likely reflects a threshold for NMDA receptor activation in LTP induction. A component of LTP that resisted blockade by APV was induced by high (200 Hz), but not low (25 Hz), frequency tetanization. This NMDA receptor-independent component of LTP persisted for > 4 hours and accounted for approximately half the potentiation induced by 200 Hz tetanization. Procedures necessary to induce LTP at the Schaffer collateral/commissural synapses in area CA1 by both NMDA receptor-dependent and -independent mechanisms are now well characterized. Using the same neuronal population, it will be possible to ask if processes involved in the maintenance of LTP are shared even when LTP is induced through two different mechanisms.

The role of Ca2+ entry via synaptically activated NMDA receptors in the induction of long-term potentiation

Influx of Ca2+ through the NMDA subtype of glutamate receptor is widely accepted as a trigger for many forms of neural plasticity. However, direct support for this model has been elusive, since indirect activation of dendritic voltage-sensitive Ca2+ channels is difficult to exclude. We have optically measured synaptically induced changes in cytoplasmic free Ca2+ concentration in pyramidal cell dendrites in hippocampal slices. Steady postsynaptic depolarization to the synaptic reversal potential eliminated the effect of voltage-sensitive Ca2+ channels. Under these conditions, synaptically induced Ca2+ transients were observed, which were blocked by the NMDA receptor antagonist APV. In addition, the magnitude of LTP was diminished when induced with the postsynaptic membrane held at progressively more positive potentials. LTP could be completely suppressed at potentials near +100 mV. These results provide important experimental support for a role for Ca2+ influx through NMDA receptors in synaptic plasticity.

Characterization of Ca2+ signals induced in hippocampal CA1 neurones by the synaptic activation of NMDA receptors

1. A combination of confocal microscopy, whole-cell patch-clamp recording, intracellular dialysis and pharmacological techniques have been employed to study Ca2+ signalling in CA1 pyramidal neurones, within rat hippocampal slices. 2. In the soma of CA1 neurones, depolarizing steps applied through the patch-pipette resulted in transient increases in the fluorescence emitted by the Ca2+ indicator fluo-3. The intensity of the fluorescence transients was proportional to the magnitude of the Ca2+ currents recorded through the pipette. Both the somatic fluorescence transients and the voltage-activated Ca2+ currents ran down in parallel over a period of between approximately 15-45 min. The fluorescence transients were considered, therefore, to be caused by increases in cytosolic free Ca2+. 3. Under current-clamp conditions, high-frequency (tetanic) stimulation (100 Hz, 1 s) of the Schaffer collateral-commissural pathway led to compound excitatory postsynaptic potentials (EPSPs) and somatic Ca2+ transients. The somatic Ca2+ transients were sensitive to the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonopentanoate (AP5; 100 microM). These transients, but not the EPSPs, disappeared with a time course similar to that of the run-down of voltage-gated Ca2+ currents. Tetanus-induced somatic Ca2+ transients could not be elicited under voltage-clamp conditions. 4. Fluorescence images were obtained from the dendrites of CA1 pyramidal neurones starting at least 30 min after obtaining whole-cell access to the neurone. Measurements were obtained only after voltage-gated Ca2+ channel activity had run down completely. 5. Tetanic stimulation of the Schaffer collateral-commissural pathway resulted in compound EPSPs and excitatory postsynaptic currents (EPSCs), under current- and voltage-clamp, respectively. In both cases, these were invariably associated with dendritic Ca2+ transients. In cells voltage-clamped at -35 mV, the fluorescent signal increased on average 2-fold during the tetanus and decayed to baseline values with a half-time (t1/2) of approximately 5 s. 6. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) partially reduced the tetanus-induced EPSC without affecting the Ca2+ transients. In contrast, AP5, which also depressed the EPSC, substantially reduced or eliminated the Ca2+ transients. 7. In normal (i.e. 1 mM Mg(2+)-containing) medium, NMDA receptor-mediated synaptic currents displayed the typical region of negative slope conductance in the peak I-V relationship (between -90 and -35 mV). The dendritic tetanus-induced Ca2+ transients also displayed a similar anomalous voltage dependence, decreasing in size from -35 to -90 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Fractional contribution of calcium to the cation current through glutamate receptor channels

The Ca2+ fraction of the ion current flowing through glutamatergic NMDA and AMPA/kainate receptor channels was determined in forebrain neurons of the medial septum. The neurons were overloaded with the Ca2+ indicator dye fura-2 (1 mM) via the recording patch pipettes. This approach allowed the direct determination of the Ca2+ influx from changes in the Ca(2+)-sensitive fura-2 fluorescence. We found that, at negative membrane potentials and at an extracellular free Ca2+ concentration of 1.6 mM, the Ca2+ fraction of the current through the NMDA receptor channels is only 6.8%, about 2-fold lower than previously estimated from reversal potential measurements. Interestingly, a quite high fractional Ca2+ current of 1.4% was determined for the linearly conducting AMPA/kainate receptor channels found in these neurons.

Kinetic properties of NMDA receptor-mediated synaptic currents in rat hippocampal pyramidal cells versus interneurones

1. Whole-cell tight-seal recordings were obtained from visually identified pyramidal cells (PCs) and interneurones (INs) in the CA1 field of thin hippocampal slices from 13- to 23-day-old rats. The INs sampled were classified according to their location either in the molecular layer (M-INs) or in the oriens layer and alveus (OA-INs). PCs and INs differed in their mode of firing when depolarized by a prolonged current pulse. Whereas PCs fired a single action potential, most INs responded with non-accommodating high frequency spike firing. 2. In the presence of 1 microM tetrodotoxin (TTX), bath application of either 50 microM L-glutamate with 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 2.5 microM N-methyl-D-aspartate (NMDA), induced a similar conductance increase in PCs and INs that was completely blocked by 200 microM DL-2-amino-5-phosphonovaleric acid (APV). The NMDA receptor-mediated currents reversed around 4 mV and exhibited an area of negative slope conductance at potentials more negative than -20 to -30 mV in the presence of 1-2 mM Mg2+. 3. Dual-component excitatory postsynaptic currents (EPSCs) were evoked in PCs and INs by stimulating afferent fibres close to the neurone. The NMDA receptor-mediated component of the EPSCs (NMDA EPSC) was isolated by adding 10 microM CNQX to block non-NMDA receptors. The NMDA EPSCs in all cell types reversed around 1.5 mV and were abolished by 50 microM APV. 4. In saline containing 1 mM Mg2+, the peak current-voltage (I-V) relationship of NMDA EPSCs in PCs and INs showed an area of negative slope conductance at voltages more negative than -20 to -30 mV. In nominally Mg(2+)-free saline, the peak I-V relation was linear over a much wider voltage range in both cell types. 5. The 10-90% rise times of NMDA EPSCs at -60 mV ranged from 4.5 to 16 ms in PCs (mean 8.7 ms; n = 25) and in M-INs (mean 9.1 ms; n = 10). Their decay could be best fitted with the sum of two exponentials. The decay of NMDA EPSCs in PCs was significantly slower than that recorded in INs. The average fast (tau f) and slow (tau s) time constants of decay were, respectively, 66.5 and 353.9 ms in PCs, and 34.4 and 212.5 ms in M-INs.(ABSTRACT TRUNCATED AT 400 WORDS)

Different proportions of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor currents at the mossy fibre-granule cell synapse of developing rat cerebellum

The mossy fibre-granule cell synapse undergoes major developmental changes during the second and third weeks after birth. We investigated synaptic transmission during postnatal days 10-22 by means of whole-cell patch-clamp recordings from granule cells in situ. Parasagittal slices were cut from rat cerebellar vermis, and excitatory postsynaptic currents were evoked in granule cells by mossy fibre stimulation with 1.2 mM Mg++ in the extracellular solution. In the majority of granule cells recorded at postnatal days 16-22, excitatory currents were characterized by a fast initial peak followed by a slower component, while in many of the cells recorded at more immature stages, the fast peak was virtually absent. Pharmacological and kinetic data indicated that the fast and slow components were mediated by non-N-methyl-D-aspartate and N-methyl-D-aspartate receptor activation, respectively. The magnitude of the non-N-methyl-D-aspartate current increased with developmental age, while the magnitude of the NMDA current did not change markedly. The age-dependent change of the non-N-methyl-D-aspartate currents could not be accounted for by changes in recording conditions or granule cell electrotonic properties. Furthermore, from postnatal day 11 to 16 the extent of Mg++ block on the N-methyl-D-aspartate receptor did not change, and could not explain the increasing non-N-methyl-D-aspartate/N-methyl-D-aspartate current ratio. We concluded therefore that the age-dependent increase of the non-N-methyl-D-aspartate current was the main cause of the different postsynaptic current waveforms observed at different ages. The developmental change in the proportion of N-methyl-D-aspartate and non-N-methyl-D-aspartate currents may be relevant to the processes regulating granule cell maturation and excitability.

A model of NMDA receptor-mediated activity in dendrites of hippocampal CA1 pyramidal neurons

1. The role of synaptic activation of NMDA (N-methyl-D-aspartate) receptor-mediated conductances on CA1 hippocampal pyramidal cells in short-term excitability changes was studied with the use of a computational model. Model parameters were based on experimental recordings from dendrites and somata and previous hippocampal simulations. Representation of CA1 neurons included NMDA and non-NMDA excitatory dendritic synapses, dendritic and somatic inhibition, five intrinsic membrane conductances, and provision for activity-dependent intracellular and extracellular ion concentration changes. 2. The model simulated somatic and dendritic potentials recorded experimentally. The characteristic CA1 spike afterdepolarization was a consequence of the longitudinal spread of dendritic charge, reactivation of slow Ca(2+)-dependent K+ conductances, slow synaptic processes (NMDA-dependent depolarizing and gamma-aminobutyric acid-mediated hyperpolarizing currents) and was sensitive to extracellular potassium accumulation. Calcium currents were found to be less important in generating the spike afterdepolarization. 3. Repetitive activity was influenced by the cumulative activation of the NMDA-mediated synaptic conductances, the frequency-dependent depression of inhibitory synaptic responses, and a shift in the potassium reversal potential. NMDA receptor activation produced a transient potentiation of the excitatory postsynaptic potential (EPSP). The frequency dependence of EPSP potentiation was similar to the experimental data, reaching a maximal value near 10 Hz. 4. Although the present model did not have compartments for dendritic spines, Ca2+ accumulation was simulated in a restricted space near the intracellular surface of the dendritic membrane. The simulations demonstrated that the Ca2+ component of the NMDA-operated synaptic current can be a significant factor in increasing the Ca2+ concentration at submembrane regions, even in the absence of Ca2+ spikes. 5. Elevation of the extracellular K+ concentration enhanced the dendritic synaptic response during repetitive activity and led to an increase in intracellular Ca2+ levels. This increase in dendritic excitability was partly mediated by NMDA receptor-mediated conductances. 6. Blockade of Ca(2+)-sensitive K+ conductances in the dendrites increased the size of EPSPs leading to a facilitation of dendritic and somatic spike activity and increased [Ca2+]i. NMDA receptor-mediated conductances appeared as an amplifying component in this mechanism, activated by the relatively depolarized membrane potential. 7. The results suggest that dendritic NMDA receptors, by virtue of their voltage-dependency, can interact with a number of voltage-sensitive conductances to increase the dendritic excitatory response during periods of repetitive synaptic activation. These findings support experimental results that implicate NMDA receptor-mediated conductances in the short-term response plasticity of the CA1 hippocampal pyramidal neuron.

Excitatory synaptic potentials in neurons of the deep nuclei in olivo-cerebellar slice cultures

Excitatory postsynaptic potentials evoked in neurons of the deep cerebellar nuclei, either by electrical stimulation within the nuclei in cerebellar slice cultures or by electrical stimulation of olivary explants in olivo-cerebellar co-cultures, were investigated in the rat by means of intracellular recordings. In neurons of the deep cerebellar nuclei, stimulation of the nuclear tissue, as well as stimulation of the olivary tissue, induced a fast rising excitatory postsynaptic potential, followed by an inhibitory postsynaptic potential and a long-lasting excitation. The fast rising excitatory postsynaptic potential and the following inhibitory postsynaptic potential were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The remaining depolarization was abolished by D-(-)-2-amino-5-phosphonovalerate, suggesting that this potential was mediated by N-methyl-D-aspartate receptors. With only D-(-)-2-amino-5-phosphonovalerate added to the bath, the slow excitation was depressed, whereas the fast excitatory and inhibitory postsynaptic potentials were not affected. In the presence of bicuculline, the 6-cyano-7-nitroquinoxaline-2,3-dione- and the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials elicited by stimulation of the olivary tissue had the same latency, and were both graded with stimulation strength. The time-to-peak and the duration of the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials were considerably longer than those of the 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive excitatory postsynaptic potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental regulation of NMDA receptor-mediated synaptic currents at a central synapse

The central nervous system has extraordinary plasticity in early life. This is thought to involve N-methyl-D-aspartate (NMDA) receptors which, along with the non-NMDA receptors, mediate fast excitatory synaptic transmission. Although NMDA receptors may be transiently enhanced early in life, it has not been possible to demonstrate directly a functional change in the NMDA receptor-mediated synaptic response because of the voltage-dependence of the NMDA conductance and the overlapping inhibitory synaptic conductances. Here I report that the duration of evoked NMDA-receptor-mediated excitatory postsynaptic currents (e.p.s.cs) in the superior colliculus is several times longer at early developmental stages compared to that measured in older animals. In contrast, the amplitude of NMDA-receptor-mediated miniature e.p.s.cs does not change during development. The kinetic response of excised membrane patches to a brief activation of NMDA receptors is similar to that of the NMDA e.p.s.c, which suggests that the time course of the NMDA e.p.s.c. in the superior colliculus reflects slow NMDA channel properties as in the hippocampus. Therefore, these data indicate that the molecular properties of NMDA receptors are developmentally regulated and thus may be controlling the ability of synapses to change in early life.

Slow voltage-dependent changes in channel open-state probability underlie hysteresis of NMDA responses in Mg2+-free solutions

Many single-channel studies rely on the assumption that the channels are functioning under steady-state conditions. In examining the basis for nonlinear whole-cell current-voltage curves in Mg(2+)-free solutions we discovered that N-methyl-D-aspartate (NMDA) channels in excised patches reversibly shifted their open-state probability (Po) in a voltage-dependent way, exhibiting approximately 3- to 4-fold greater Po at positive potentials than at rest. Changes in Po were mainly attributable to shifts in frequency of channel opening. Po changed remarkably slowly (2-15 min), explaining the hysteresis of whole-cell current-voltage curves obtained in nonequilibrium conditions. The slow increase in Po provides a mechanism by which NMDA channels can substantially increase Ca2+ influx in cells depolarized for prolonged periods of time and may play a role in excitotoxicity.

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

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

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

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

Patch clamp analysis of excitatory synapses in mammalian spinal cord slices

Excitatory synaptic transmission to visually identified alpha-moto neurones was studied in thin slice preparations of the neonatal rat spinal cord. Excitatory postsynaptic currents (EPSCs) elicited by stimulation of intraspinal presynaptic fibres were recorded using the whole-cell patch clamp technique, following blockade of inhibitory transmission by bath application of strychnine and bicuculline. The EPSCs could be separated pharmacologically into N-methyl-D-aspartate- (NMDA) and non-NMDA-receptor-mediated components, where the contribution of the NMDA-mediated component was significant only at holding potentials more positive than -50 mV. Graded stimulation of intraspinal fibres showed that the NMDA- and the non-NMDA-mediated EPSCs were evoked by activation of presynaptic fibres with similar sensitivities to the stimulation intensity, suggesting that the same presynaptic fibres released the excitatory amino-acid (EAA) activating the two sub-sets of receptors. Studies of the amplitude fluctuations of EPSCs elicited by stimulation of a presumed single fibre revealed similar proportions of transmission failures and similar distributions of both the NMDA- and the non-NMDA-mediated components. These similarities suggest that the EAA transmitter activating the two sub-types of receptors is released from the same set of synaptic boutons and that the receptors are therefore post-synaptically co-localized. In addition the gamma aminobutyric acidB (GABAB) receptor agonist L-baclofen, which is known to decrease transmitter release, changed the amplitude distributions of non-NMDA- and NMDA-receptor-mediated EPSCs into unimodal distributions without affecting the amplitude of the presumed unitary event. The similarity between the transmitter release profiles of the two EAA components further supports the notion of postsynaptic receptor co-localization.