Hyperactivity and cortical disinhibition in mice with restricted expression of mutant huntingtin to parvalbumin-positive cells.

Recent evidence suggests that interneurons are involved in the pathophysiology of Huntington Disease (HD). Abnormalities in the function of interneurons expressing the calcium buffer parvalbumin (PV) have been observed in multiple mouse models of HD, although it is not clear how PV-positive interneuron dysfunction contributes to behavioral and synaptic deficits. Here, we use the cre-lox system to drive expression of mutant huntingtin (mthtt) in parvalbumin (PV)-positive neurons and find that mutant mice exhibit diffuse mthtt immunoreactivity in PV-rich areas at 10months of age and mthtt aggregates in PV-positive processes at 24months of age. At midlife, mutant mice are hyperactive and display impaired GABA release in the motor cortex, characterized by reduced miniature inhibitory events and severely blunted responses to gamma frequency stimulation, without a loss of PV-positive interneurons. In contrast, 24month-old mutant mice show normalized behavior and responses to gamma frequency stimulation, possibly due to compensatory changes in pyramidal neurons or the formation of inclusions with age. These data indicate that mthtt expression in PV-positive neurons is sufficient to drive a hyperactive phenotype and suggest that mthtt-mediated dysfunction in PV-positive neuronal populations could be a key factor in the hyperkinetic behavior observed in HD. Further clarification of the roles for specific PV-positive populations in this phenotype is warranted to definitively identify cellular targets for intervention.

A Role for PGC-1α in Transcription and Excitability of Neocortical and Hippocampal Excitatory Neurons.

The transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) is a critical regulator of genes involved in neuronal metabolism, neurotransmission, and morphology. Reduced PGC-1α expression has been implicated in several neurological and psychiatric disorders. An understanding of PGC-1α's roles in different cell types will help determine the functional consequences of PGC-1α dysfunction and/or deficiency in disease. Reports from our laboratory and others suggest a critical role for PGC-1α in inhibitory neurons with high metabolic demand such as fast-spiking interneurons. Here, we document a previously unrecognized role for PGC-1α in maintenance of gene expression programs for synchronous neurotransmitter release, structure, and metabolism in neocortical and hippocampal excitatory neurons. Deletion of PGC-1α from these neurons caused ambulatory hyperactivity in response to a novel environment and enhanced glutamatergic transmission in neocortex and hippocampus, along with reductions in mRNA levels from several PGC-1α neuron-specific target genes. Given the potential role for a reduction in PGC-1α expression or activity in Huntington Disease (HD), we compared reductions in transcripts found in the neocortex and hippocampus of these mice to that of an HD knock-in model; few of these transcripts were reduced in this HD model. These data provide novel insight into the function of PGC-1α in glutamatergic neurons and suggest that it is required for the regulation of structural, neurosecretory, and metabolic genes in both glutamatergic neuron and fast-spiking interneuron populations in a region-specific manner. These findings should be considered when inferring the functional relevance of changes in PGC-1α gene expression in the context of disease.

Potassium-coupled chloride cotransport controls intracellular chloride in rat neocortical pyramidal neurons.

Chloride (Cl(-)) homeostasis is critical for many cell functions including cell signaling and volume regulation. The action of GABA at GABA(A) receptors is primarily determined by the concentration of intracellular Cl(-). Developmental regulation of intracellular Cl(-) results in a depolarizing response to GABA in immature neocortical neurons and a hyperpolarizing or shunting response in mature neocortical neurons. One protein that participates in Cl(-) homeostasis is the neuron-specific K(+)-Cl(-) cotransporter (KCC2). Thermodynamic considerations predict that in the physiological ranges of intracellular Cl(-) and extracellular K(+) concentrations, KCC2 can act to either extrude or accumulate Cl(-). To test this hypothesis, we examined KCC2 function in pyramidal cells from rat neocortical slices in mature (18-28 d postnatal) and immature (3-6 d postnatal) rats. Intracellular Cl(-) concentration was estimated from the reversal potential of whole-cell currents evoked by local application of exogenous GABA. Both increasing and decreasing the extracellular K(+) concentration resulted in a concomitant change in intracellular Cl(-) concentration in neurons from mature rats. KCC2 inhibition by furosemide caused a change in the intracellular Cl(-) concentration that depended on the concentration of pipette Cl(-); in recordings with low pipette Cl(-), furosemide lowered intracellular Cl(-), whereas in recordings with elevated pipette Cl(-), furosemide raised intracellular Cl(-). In neurons from neonatal rats, manipulation of extracellular K(+) had no effect on intracellular Cl(-) concentration, consistent with the minimal KCC2 mRNA levels observed in neocortical neurons from immature animals. These data demonstrate a physiologically relevant and developmentally regulated role for KCC2 in Cl(-) homeostasis via both Cl(-) extrusion and accumulation.

Ectopic action potential generation in cortical interneurons during synchronized GABA responses.

In the presence of 4-aminopyridine and excitatory amino acid receptor antagonists, individual neurons in brain slice preparations exhibit large gamma aminobutyric acid (GABA)-mediated responses as a consequence of synchronous GABA release from a network of interneurons. These synchronized GABA responses are frequently associated with ectopic action potentials (EAPs), which are thought to be action potentials initiated in distal axon terminals which subsequently travel antidromically toward the soma. Ectopic action potentials feature prominently in some models of epilepsy. Neocortical synchronized GABA responses propagate across the cortex, predominantly in superficial layers. The role that EAPs may play in contributing to laminar differences in the synchronized GABA response has not been addressed. Here we examined the occurrence of EAPs during synchronized GABA responses in neurons within layers I and II/III. EAPs occurred in 78% of layer I interneurons and in 25% of layer II/III interneurons (including chandelier cells). EAPs were not observed in layer II/III pyramidal neurons. The prevalence of EAPs in layer I interneurons provides a mechanism by which layer I can support both the initiation and propagation of synchronized GABA responses. Thus, layer I interneurons are a critical component of a network capable of synchronizing a propagating wave of GABA release across the neocortex.

Morphological properties of intracellularly labeled layer I neurons in rat neocortex.

The morphology of neurons in layer I of rat neocortex, including Cajal-Retzius (CR) cells, was studied by using intracellular biocytin staining in brain slices obtained from rats during the first 22 postnatal days. Within the first postnatal week, horizontal bipolar neurons or CR cells were prominent in layer I. Typically, CR cells had one main dendrite and one axon originating from opposite poles of the somata. Even though the main dendrites and axons could be quite long, complex dendritic or axonal arbors were not observed. Starting around postnatal day 6 (PN 6), CR cells were less frequently observed. From PN 10 to PN 21, nonpyramidal neurons with diverse morphologies became the main neuronal component in layer I. The somata of layer I nonpyramidal neurons were quite variable in size and shape. Dendrites were smooth or sparsely spiny, and the dendritic trees were mainly restricted to layer I, covering an area with a diameter of about 200 microns. Axon collaterals of these cells formed elaborate arbors with diameters of around 700 microns in layer I and extending, in many cases, to layer II/III and even layer IV. This extensive axonal plexus provides a rich anatomical base on which layer I neurons, functioning as local circuit elements, may interact with each other and with neurons in other layers.

G-protein activation by metabotropic glutamate receptors reduces spike frequency adaptation in neocortical neurons.

Intracellular recordings were obtained from neocortical brain slices of adult rats maintained in vitro. The effect of metabotropic glutamate receptor activation on spike frequency adaptation in regular spiking layer II and III neurons was determined. Putative metabotropic glutamate receptor agonists and antagonists, as well as inhibitors of intracellular signaling systems, were tested. Activation of metabotropic glutamate receptors by bath applied (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (1S,3R-ACPD; 50-200 microM) reduced the first interspike interval and increased action potential frequency at all current intensities. This effect was not blocked by ionotropic glutamate receptor antagonists. Under these recording conditions, quisqualate (1-10 microM) similarly reduced spike frequency adaptation. Neither 1R,3S-ACPD, L-2-carboxycyclopropylglycine-I nor the putative presynaptic metabotropic glutamate receptor agonist, L-2-amino-4-phosphonobutyrate, mimicked the effects of 1S,3R-ACPD or quisqualate. Bath application of the putative metabotropic glutamate receptor antagonist, alpha-methyl-4-carboxyphenylglycine, competitively antagonized the excitatory actions of 1S,3R-ACPD. Another putative antagonist, L-2-amino-3-phosphonopropionate, failed to antagonize the reduction in spike frequency adaptation. Intracellular injection of guanosine-5'-O-(2-thiodiphosphate), a non-hydrolysable analog of GTP, inhibited the postsynaptic metabotropic glutamate receptor-mediated effects. However, the depression of synaptic transmission by 1S,3R-ACPD was not antagonized by this compound. The decrease in spike frequency adaptation by 1S,3R-ACPD was not prevented by prior exposure to the non-specific protein kinase inhibitors H-7 or H-8 (10 microM), the protein kinase A inhibitor H-89 (0.25 microM) or the protein kinase C inhibitor staurosporine (0.10 microM). These data suggest that the metabotropic glutamate receptor-mediated reduction in spike adaptation requires the activation of specific G-protein-coupled metabotropic glutamate receptor subtypes located on postsynaptic sites. The increase in neuronal excitability observed in the adult neocortex may be mediated either by an unidentified G-protein-coupled second messenger or via a membrane-delimited G-protein action.

N-methyl-D-aspartate receptor-mediated calcium accumulation in neocortical neurons.

Calcium imaging and patch-clamp recording techniques were used to investigate the relationship between membrane properties and intracellular calcium changes in response to the excitatory amino acid neurotransmitter glutamate. Application of glutamate to cultured neocortical neurons produced concentration-dependent increases in intracellular calcium, membrane depolarization and transmembrane current. At a low concentration (3 microM), glutamate induced only a small depolarization (< 10 mV), yet produced a substantial increase in intracellular calcium. The calcium increase was observed in the presence of extracellular magnesium, was dependent on extracellular calcium, was blocked by an N-methyl-D-aspartate receptor antagonist, and was not affected by manipulation of intracellular calcium stores. This low concentration of glutamate also induced membrane currents that exhibited an N-methyl-D-aspartate-like unconventional voltage dependence. When glutamate was increased to a concentration known to produce excitotoxicity (500 microM), large depolarizations and membrane currents were induced, which rapidly reversed following prolonged glutamate applications. Changes in intracellular calcium in response to 500 microM glutamate had both voltage-sensitive and -insensitive components, and consistently remained elevated following removal of glutamate. These results indicate that low concentrations of glutamate can preferentially activate N-methyl-D-aspartate receptors, leading to increases in intracellular calcium. Functionally this may be involved in N-methyl-D-aspartate receptor responses to ambient extracellular glutamate. In addition, N-methyl-D-aspartate receptor-mediated calcium influx and subsequent depolarization induced by high glutamate concentrations can produce alterations in intracellular calcium homeostasis, which may play an important role in excitotoxicity.

Glutamate transporters regulate excitability in local networks in rat neocortex.

Excitatory postsynaptic currents (EPSCs) in the neocortex are principally mediated by glutamate receptors. Termination of excitation requires rapid removal of glutamate from the synaptic cleft following release. Glutamate transporters are involved in EPSC termination but the effect of uptake inhibition on excitatory neurotransmission varies by brain region. Epileptiform activity is largely mediated by a synchronous synaptic activation of cells in local cortical circuits, presumably associated with a large release of glutamate. The role of glutamate transporters in regulating epileptiform activity has not been addressed. Here we examine the effect of glutamate transport inhibition on EPSCs and epileptiform events in layer II/III pyramidal cells in rat neocortex. Inhibiting glutamate transporters with DL-threo-beta-benzyloxyaspartic acid (TBOA; 30 microM) had no effect on the amplitude or decay time of evoked, presumably alpha-amino-3-hydroxyl-5-methyl-isoxazolepropionic acid-mediated, EPSCs. In contrast, the amplitude and duration of epileptiform discharges were significantly enhanced. TBOA resulted also in a decreased threshold for evoking epileptiform activity and an increased probability of occurrence of spontaneous epileptiform discharges. TBOA's effects were not inhibited by the group I and II metabotropic glutamate receptors antagonist (S)-alpha-methyl-4-carboxyphenylglycine or the kainate receptor antagonist [(3S,4aR, 6S, 8aR)-6-((4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid]. D-(-)-2-amino-5-phosphonovaleric acid could both prevent excitability changes by TBOA and block already induced changes. Dihydrokainate (300 microM) had effects similar to TBOA suggesting involvement of the glial transporter GLT-1. Inhibiting glutamate transport increases local network excitability under conditions where there is an enhanced release of glutamate. Our results indicate that uptake inhibition produces an elevation of extracellular glutamate levels and activation of N-methyl-D-aspartate receptors.

Chloride accumulation and depletion during GABA(A) receptor activation in neocortex.

The response of neocortical neurons to GABA is strongly influenced by the intracellular chloride concentration. We tested the hypothesis that activation of GABA(A) receptors can result in either depletion or accumulation of intracellular chloride. The measured reversal potentials of currents evoked by exogenously applied or synaptically released GABA were not significantly different. During GABA responses, voltage steps to the reversal potential revealed prominent tail-like currents. The polarity of these currents was opposite to that of the GABA-evoked currents, consistent with either accumulation or depletion of intracellular chloride. These results demonstrate that currents evoked by exogenously applied and synaptically released GABA share similar ionic dependencies. Current fluxes during GABA(A) receptor activation can be sufficiently large to change the intracellular chloride concentration.

Reactive astrocytes show enhanced inwardly rectifying K+ currents in situ.

Injury and diseases of the nervous system can induce astrocytes to form tenacious glial scars. We induced focal cortical freeze-lesions in neonatal rats and examined scars histologically and electrophysiologically in tissue slices isolated 2-3 weeks after lesioning. Lesions displayed marked gliosis, characterized by upregulation of GFAP labeling. Reactive astrocytes surrounding the scar showed marked hypertrophy, enlarged cell bodies and extended processes frequently terminating with endfeet-like structures on blood vessels. These reactive astrocytes showed enhanced expression of inwardly rectifying K+ (K(IR)) channels, widely believed to be an important pathway for astrocytic K+ buffering. These results suggest that a subpopulation of reactive astrocytes along a glial scar might be instrumental in buffering K+ away from the lesion.

Nystatin-perforated patch recordings disclose NMDA-induced outward currents in cultured neocortical neurons.

Excitatory amino acid-induced responses were studied in cultured rat neocortical neurons using two types of whole-cell patch-clamp recordings. Conventional recording methods, using either KCl or CsCl in the patch pipet, showed N-methyl-D-aspartate (NMDA) currents to be biphasic, consisting of peak and steady-state currents with similar reversal potentials. Recordings obtained with the nystatin-perforated patch method and KCl as the principal intracellular cation disclosed an NMDA-evoked outward current. Outward currents were not seen with either recording method in response to kainate or quisqualate, nor in response to NMDA when CsCl was the major intracellular cation. The NMDA-evoked outward current is attributed to activation of a potassium current by calcium entering through the NMDA channel. This outward current may serve to limit neuronal depolarization during excessive NMDA-mediated excitation.

Extracellular K+ and Ca2+ changes during epileptiform discharges in the immature rat neocortex.

Picrotoxin-induced epileptiform activity was examined in neocortical slices prepared from 8- to 15-day-old rats. This activity consisted of spontaneous bursts of 3-5 discharges that resembled interictal spikes and were interspersed with ictal-like paroxysms lasting 10-30 s. Measurements of extracellular potassium ([K+]o) and calcium ([Ca2+]o) were made during these spontaneous epileptiform events, using ion-sensitive electrodes. Individual interictal spikes were associated with [Ca2+]o decreases of 0.1-0.2 mM, whereas sustained ictal-like discharges were accompanied by decreases of 0.3-0.4 mM. Measurement of [K+]o showed that individual interictal spikes were associated with increases in [K+]o up to 12 mM, whereas increases to more than 20 mM accompanied long-lasting ictal-like discharges. Maximum increases in [K+]o were observed ca. 600 microns below the pial surface. [K+]o increases were followed by undershoots of the resting [K+]o level. The unusually high [K+]o levels associated with epileptiform discharges in the immature neocortex suggest that disturbances in [K+]o regulation may contribute to the generation of the picrotoxin-induced, spontaneous, prolonged ictal-like discharges observed in the 8- to 15-day age group.

Quisqualate induces an inward current via mGluR activation in neocortical pyramidal neurons.

Activation of metabotropic glutamate receptors (mGluRs) has multiple effects on the excitability of pyramidal neurons in rat frontal neocortex. Synaptic transmission and intrinsic excitability are both affected. During studies of the effects of quisqualate on synaptic activity, it was observed that quisqualate also induced a slow inward current. Whole-cell patch clamp recordings were obtained from layer II/III pyramidal neurons of neocortical slices in vitro. The bath solution contained APV, CNQX and bicuculline to block ionotropic glutamate and GABA(A) receptors. At a holding potential of -70 mV, quisqualate (2 microM) induced an inward current of about 60 pA. The response was reversible upon washing. This current was associated with an increase in membrane conductance and was still seen in the presence of TTX (0.5 microM). Bath application of the nonselective mGluR antagonist, (R, S)-alpha-methyl-4-carboxyphenyglycine (MCPG, 200-500 microM) reduced the current by 70%. Other mGluR agonists (ACPD, DHPG, L-CCG-1 and L-AP4) did not induce a significant inward current at the concentrations tested. The current-voltage relation of the quisqualate-induced current was linear with a reversal potential near 0 mV suggesting involvement of nonselective cation channels. The quisqualate-induced inward current was markedly reduced (72%) with 200 microM GDP-beta-S in the pipette solution, indicating that it is a postsynaptic phenomenon mediated by a G-protein dependent mechanism. These results suggest that mGluRs can directly increase the postsynaptic excitability of pyramidal cells.

AMPA receptor-mediated EPSCs in rat neocortical layer II/III interneurons have rapid kinetics.

The properties of spontaneous and miniature (m) AMPA receptor-mediated excitatory postsynaptic currents (EPSCs) were studied in rat neocortical layer II/III fast spiking interneurons. Under optimal recording conditions, averaged mEPSCs had a 10-90% rise time of about 0.3 ms. The decay of averaged mEPSCs was double exponential with time constants of about 1 and 4 ms. Kinetics were observed to slow as series resistance increased. The amplitudes of mEPSCs were much smaller at +50 mV than at -50 mV indicating that the currents were inwardly rectifying. These results suggest that synaptic AMPA receptors on neocortical inhibitory interneurons have a deactivation time constant less than 1 ms which largely determines the decay of the synaptic currents. The receptors appear to lack GluR-2 subunits and may be Ca2+ permeable.

Selective amplitude histograms: a statistical approach to EEG-single unit relationships in generalized epilepsy.

Statistical analysis techniques that permit detection and quantification of EEG-single unit correlations were employed in a study of generalized penicillin epilepsy in the cat. Single unit activity was recorded in cerebral and cerebellar cortices and compared to the locally recorded EEG. It was found that during generalized paroxysmal activity the background EEG was significantly related to the time of occurrence of unit discharge. The degree of synchrony of various units differed but tended to increase as the duration and extent of paroxysmal activity increased. These relationships were usually not evident upon visual inspection of the original unprocessed data but were consistently detected by the analysis techniques described here. The results demonstrate the usefulness of a statistical approach to the analysis of single unit data and suggest that, not only is there a significant relationship between paroxysmal events and neuronal activity in generalized penicillin epilepsy, but also an overall trend to closer synchronization of EEG and single unit discharges during nonparoxysmal periods.

Chronic exposure of developing cortical neurons to GABA down-regulates GABA/benzodiazepine receptors and GABA-gated chloride currents.

Cultures of cerebral neurons were prepared from chick embryos, 8.5 days in ovo, and maintained in vitro. Following chronic exposure of these cells to GABA, the levels of [3H]flunitrazepam binding in situ and electrophysiological responsiveness to gamma-aminobutyric acid (GABA) was examined. Treatment with 100 microM GABA for 7 days reduced [3H]flunitrazepam binding in situ by 70 +/- 8% compared to untreated controls. The binding of [3H]N-methylscopolamine was unaffected by this treatment. The reduction in [3H]flunitrazepam binding was prevented by concomitant exposure of developing neurons to the GABA antagonist R 5135, suggesting that GABAA receptor occupancy is required. The loss of bezodiazepine receptors was dependent on the GABA concentration in the culture medium and a half-saturation (IC50) value of 11.2 +/- 3.7 microM was estimated. Whole-cell patch-clamp recordings were obtained to assess the functional properties of the labile receptor pool observed in the binding studies. Neurons cultured with 100 microM GABA for 7 days showed a 60-70% reduction in the peak current amplitudes observed in response to application of 10-100 microM GABA. However, the rate of rapid desensitization, quantified by measuring changes in input conductance, was unchanged by chronic GABA exposure, yielding decay time constants of 27.1 +/- 2.1 and 34.7 +/- 4.7 s for control and treated cells, respectively. The results are consistent with a GABA modulation of the GABAA/benzodiazepine receptor complex by means of down-regulation.

Nicotinic acetylcholine receptor-mediated synaptic potentials in rat neocortex.

In the neocortex, fast excitatory synaptic transmission can typically be blocked by using excitatory amino acid (EAA) receptor antagonists. In recordings from layer II/III neocortical pyramidal neurons, we observed an evoked excitatory postsynaptic potential (EPSP) or current (EPSC) in the presence of EAA receptor antagonists (40-100 microM D-APV+20 microM CNQX, or 5 mM kynurenic acid) plus the GABA(A)-receptor antagonist bicuculline (BIC, 20 microM). This EAA-antagonist resistant EPSC was observed in about 70% of neurons tested. It had a duration of approximately 20 ms and an amplitude of 61.5+/-6.8 pA at -70 mV (n=35). The EAA-antagonist resistant EPSC current-voltage relation was linear and reversed near 0 mV (n=23). The nonselective nicotinic acetylcholine receptor (nAChR) antagonists dihydro-beta-erythroidine (DH beta E, 100 microM) or mecamylamine (50 microM) reduced EPSC amplitudes by 42 (n=20) and 33% (n=9), respectively. EPSC kinetics were not significantly changed by either antagonist. Bath application of 10 microM neostigmine, a potent acetylcholinesterase inhibitor, prolonged the EPSC decay time. EAA-antagonist resistant EPSCs were observed in the presence of antagonists of metabotropic glutamate, serotonergic (5-HT(3)) and purinergic (P2) receptors. The EAA-antagonist resistant EPSC appears to be due in part to activation of postsynaptic nAChRs. These results suggest the existence of functional synaptic nAChRs on pyramidal neurons in rat neocortex.

Depletion of intracellular Ca2+ stores or lowering extracellular calcium alters intracellular Ca2+ changes during cerebral energy deprivation.

Cytoplasmatic calcium concentrations are elevated two to three fold during cerebral ischemia. In order to determine the role of calcium-release from intracellular stores vs. calcium entry from the extracellular space, intracellular stores were depleted by the use of thapsigargin and calcium was removed from the incubation fluid prior to energy deprivation (ED). CA 1 pyramidal neurons in hippocampal rat slices were filled with a 1:2 mixture of Fluo-3 and Fura Red by intracellular injection. The neurons were visualized in a Confocal Laser Scanning Microscope (CLSM) and the fluorescence ratio from the probe mixture was used to quantify the calcium concentration. Intracellular calcium concentration was monitored before and during ED. The intracellular calcium concentration was 55 nM prior to ED and increased to 25 microM during ED. The resting levels were the same in the experimental groups, but the increase during ED was significantly lower in the intervention groups. The increase in the calcium free group was to 1 microM and in the thapsigargin group to 5 microM. In the last experimental group, thapsigargin treatment and removal of extracellular calcium, the intracellular calcium increased to 630 nM. These results demonstrate that the increased intracellular calcium seen during ED originates from several sources. Calcium-release from intracellular stores may be of major importance in calcium-related neuronal injury during cerebral ischemia.

Glutamate-induced ionic currents in cultured neurons from the rat superior colliculus.

The ionic currents induced in cultured rat superior colliculus neurons by rapid application of glutamate (Glut) and the glutamate receptor agonists quisqualate (Quis) and N-methyl-D-aspartate (NMDA) were examined using the whole-cell patch clamp technique. Dissociated cell cultures consisting exclusively of superficial gray layer neurons from rats aged E21-P2 were used. After 7-10 days in vitro, all neurons responded to Glut and the selective agonists, NMDA and Quis. Glut was a mixed agonist, and a variable fraction (10-100%) of Glut-activated currents was due to involvement of NMDA receptors. The NMDA response was strongly regulated by extracellular Ca and Mg levels and modified by exposure to Quis. Quis transiently removed the block of NMDA-activated currents by D-amino-phosphonovaleric acid (APV).

Developmental changes in the voltage-dependence of neocortical NMDA responses.

A change in the response of neocortical neurons to N-methyl-D-aspartate (NMDA) was observed during the first 2 weeks of postnatal development. When NMDA was bath applied, the membrane current-voltage relationship recorded in neurons from postnatal day (PN) 3-5 rats displayed a region of decreased inward current at hyperpolarized membrane potentials. By PN 9-14, the net inward current at hyperpolarized potentials was significantly less than that recorded in PN 3-5 neurons. These results indicate that a developmental increase in the voltage-dependence of NMDA responses exists, which may be due to changes in magnesium sensitivity of the NMDA receptor.