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PubMedDate not supplied

Six billion neurons lost in AIDS. A stereological study of the neocortex.

Human immunodeficiency virus type 1 (HIV1) is neurotropic. One of the morphological changes that is seen in patients with acquired immunodeficiency syndrome (AIDS) is cerebral atrophy affecting various structures including the neocortex. The cause of atrophy is not known. The total number of neocortical neurons was estimated in formalin fixed brains of 12 males with AIDS and 12 male controls matched for age and height. The mean number of neocortical neurons was 16.0 x 10(9) (coefficient of variation = 0.11) in the AIDS patients compared with 21.9 x 10(9) (coefficient of variation = 0.22) in the controls, a difference of approximately six billion (p < 0.005, 2-tailed). The global neuronal loss was 37%, and affected all four neocortical lobes. Ten patients did not have a history of central nervous system symptoms; two patients had a history of dementia. The number of neurons in the AIDS cases was not associated with dementia. AIDS is the first disease in which a global loss of neocortical neurons has been demonstrated using unbiased stereological methods. The loss of more than one third of the neurons may partly explain the cortical atrophy. Focal neuron loss has been reported by several authors, but none have been based on unbiased methods. In this group of AIDS patients the severe loss of neurons did not correspond to neurological deficits.

Acquired Immunodeficiency Syndrome↗
PubMed1993

Basal forebrain grafts in the hippocampus and neocortex: regulation of acetylcholine release.

The regulation of acetylcholine (ACh) release from cholinergic neurons transplanted to the hippocampus or neocortex was studied by microdialysis in awake rats. Fetal basal forebrain tissue was implanted as a cell suspension or solid graft into the fimbria-fornix-lesioned hippocampus, or as a cell suspension into the frontal cortex after excitotoxic lesion of the nucleus basalis. Several months after transplantation, microdialysis probes were implanted in areas of the hippocampus or frontal cortex reinnervated by the grafts. The grafts restored lesion-induced deficits in steady-state ACh release up to normal or above normal levels in both hippocampus and frontal cortex. The responses to KCl and tetrodotoxin suggested that the ACh release exhibited normal firing-dependent properties. By applying various behaviorally arousing stimuli that normally activate the basal forebrain projection systems, we wished to investigate the functional integration of the grafts in the host brain. In the hippocampus, sensory stimulation, immobilization stress and motor activity all resulted in increased release of graft-derived ACh amounting to 25-65% of the normal response. Variations in ACh levels during the day-night cycle was, however, not observed in the grafted rats. In the frontal cortex, immobilization enhanced the graft-derived ACh release (60% of normal response), whereas the response to sensory stimulation did not reach significance. Since the activity of the normal basal forebrain projection systems is under influence of monoaminergic brainstem afferents, we investigated the effects of systemic administration of amphetamine or apomorphine on ACh release in the hippocampus. Both drugs produced increases in graft-derived ACh release although the response was variable and less pronounced than normal. In conclusion, the graft-derived ACh release was affected by behavioral manipulations and catecholaminergic drugs that normally modify cholinergic septo-hippocampal and basalo-cortical activity. This strongly suggests a high degree of functional integration of the graft in the host brain allowing for a regulated release of transmitter that can be adjusted during ongoing behavior.

Acetylcholine↗
PubMed1994

Relationships between morphology and physiology of pyramid-pyramid single axon connections in rat neocortex in vitro.

1. Double intracellular recordings were made from 1163 pairs of pyramidal neurones in layer V-VI of the rat somatomotor cortex in vitro using sharp electrodes filled with biocytin. Monosynaptically connected pairs of cells were identified when an action potential in one could elicit a constant latency excitatory postsynaptic potential (EPSP) in the other and the cells were filled with biocytin. Labelled cells were subsequently identified histologically with avidin-horseradish peroxidase. 2. Thirty-four pairs of cells were found to be monosynaptically connected. Fifteen of these pairs were sufficiently stable for electrophysiological recordings and three of these were recovered sufficiently to permit full morphological reconstruction. 3. The EPSP recorded between the first pair of pyramids varied in amplitude between 0 and 3 mV (mean 1.33 +/- 1.06 mV) and fluctuated considerably (coefficient of variation, 0.796). This was largely due to a high incidence of apparent failures of transmission. On reconstruction two boutons from the presynaptic pyramid axon were in close apposition to the proximal portions of basal dendrites of the postsynaptic cell. 4. In the second pair of pyramids the EPSP had a mean amplitude of 1.06 mV, and displayed a 10-90% rise time of 2.8 ms and a width at half-amplitude of 23 ms. This EPSP did not alter significantly with changes in membrane potential at the soma. The presynaptic axon closely apposed the distal apical dendrite of the postsynaptic cell in eight places. 5. In the third pair of pyramids, the EPSPs, recorded at a relatively depolarized membrane potential, were long lasting and could elicit slow dendritic spikes with long and variable latencies. These slow spikes suggested that the postsynaptic recording site was dendritic and on reconstruction a possible location was identified on the apical dendrite. A total of five presynaptic boutons closely apposed three separate, proximal branches of the postsynaptic apical dendrite. 6. These results provide the first illustration of a morphological basis for variations in functional properties of pyramid-pyramid connections in the neocortex.

Animals↗
PubMed1996

Single axon IPSPs elicited in pyramidal cells by three classes of interneurones in slices of rat neocortex.

1. Using dual intracellular recordings in slices of adult rat neocortex, twenty-four IPSPs activated by single presynaptic interneurones were studied in simultaneously recorded pyramidal cells. Fast spiking interneurones inhibited one in four or five of their close pyramidal neighbours. No reciprocal connections were observed. After recordings neurones were filled with biocytin. 2. Interneurones that elicited IPSPs were classified as classical fast spiking (n = 10), as non-classical fast spiking (n = 3, including one burst-firing interneurone), as unclassified, or slow interneurones (n = 8), or as regular spiking interneurones (n = 3), i.e. interneurones whose electrophysiological characteristics were indistinguishable from those of pyramidal cells. 3. All of the seven classical fast spiking cells anatomically fully recovered had aspiny, beaded dendrites. Their partially myelinated axons ramified extensively, varying widely in shape and extent, but randomly selected labelled axon terminals typically innervated somata and large calibre dendrites on electron microscopic examination. One 'autapse' was demonstrated. One presumptive regular spiking interneurone axon made four somatic and five dendritic connections with unlabelled targets. 4. Full anatomical reconstructions of labelled classical fast spiking interneurones and their postsynaptic pyramids (n = 5) demonstrated one to five boutons per connection. The two recorded IPSPs that were fully reconstructed morphologically (3 and 5 terminals) were, however, amongst the smallest recorded (< 0.4 mV). Some connections may therefore involve larger numbers of contacts. 5. Single axon IPSPs were between 0.2 and 3.5 mV in average amplitude at -55 to -60 mV. Extrapolated reversal potentials were between -70 and -82 mV. IPSP time course correlated with the type of presynaptic interneurone, but not with IPSP latency, amplitude, reversal potential, or sensitivity to current injected at the soma. 6. Classical fast spiking interneurones elicited the fastest IPSPs (width at half-amplitude 14.72 +/- 3.83 ms, n = 10) and unclassified, or slow interneurones the slowest (56.29 +/- 23.44 ms, n = 8). Regular spiking interneurone IPSPs had intermediate half-widths (27.3 +/- 3.68 ms, n = 3). 7. Increasingly brief presynaptic interspike intervals increased the peak amplitude of, but not the area under, the summed IPSP. Only at interspike intervals between 10 and 20 ms did IPSP integrals exhibit paired pulse facilitation. Paired pulse depression was apparent at < 10 and 20-60 ms. During longer spike trains, summing IPSPs decayed to a plateau potential that was relatively independent of firing rate (100-250 Hz). Thereafter, the voltage response could increase again. 8. Summed IPSPs elicited by two to fifteen presynaptic spike trains decayed as, or more rapidly than, single-spike IPSPs. Summed IPSPs elicited by > 20 spikes (> 150 Hz), however, resulted in an additional, more slowly decaying component (latency > 50 ms, duration > 200 ms). The possible involvement of GABAB receptors in this component is discussed. 9. It is suggested that three broad classes of interneurones may activate GABAA receptors on relatively proximal portions of neocortical pyramidal neurones. The different time courses of the IPSPs elicited by the three classes may reflect different types of postsynaptic receptor rather than dendritic location. An additional class, burst firing, spiny interneurones appear to activate GABAA receptors on more distal sites.

Animals↗
PubMed2004

Persistent gamma oscillations in superficial layers of rat auditory neocortex: experiment and model.

Persistent in vitro gamma oscillations, induced by bath application of carbachol and kainate (amongst other drugs), were discovered by Eberhard Buhl and collaborators in 1998. The oscillations are robust, in that they can continue for hours; but the oscillations are also intricate in their mechanisms: they depend upon phasic synaptic excitation and inhibition, upon electrical coupling between interneurones and between pyramidal neurones, and--at least in neocortex--they depend upon complex intrinsic properties of some of the neurones.

Action Potentials↗
PubMed1992

Neuronal domains in developing neocortex.

The mammalian neocortex consists of a mosaic of columnar units whose development is poorly understood. Optical recordings of brain slices labeled with the fluorescent calcium indicator fura-2 revealed that the neonatal rat cortex was partitioned into distinct domains of spontaneously coactive neurons. In tangential slices, these domains were 50 to 120 micrometers in diameter; in coronal slices they spanned several cortical layers and resembled columns found in the adult cortex. In developing somatosensory cortex, domains were smaller than, and distinct from, the barrels, which represent sensory input from a single vibrissa. The neurons within each domain were coupled by gap junctions. Thus, nonsynaptic communication during cortical development defines discrete multicellular patterns that could presage adult functional architecture.

Action Potentials↗
PubMed1991

Clusters of coupled neuroblasts in embryonic neocortex.

The neocortex of the brain develops from a simple germinal layer into a complex multilayer structure. To investigate cellular interactions during early neocortical development, whole-cell patch clamp recordings were made from neuroblasts in the ventricular zone of fetal rats. During early corticogenesis, neuroblasts are physiologically coupled by gap junctions into clusters of 15 to 90 cells. The coupled cells form columns within the ventricular zone and, by virtue of their membership in clusters, have low apparent membrane resistances and generate large responses to the inhibitory neurotransmitter gamma-aminobutyric acid. As neuronal migration out of the ventricular zone progresses, the number of cells within the clusters decreases. These clusters allow direct cell to cell interaction at the earliest stages of corticogenesis.

Animals↗
PubMed1991

Cell cycle dependence of laminar determination in developing neocortex.

The neocortex is patterned in layers of neurons that are generated in an orderly sequence during development. This correlation between cell birthday and laminar fate prompted an examination of how neuronal phenotypes are determined in the developing cortex. At various times after labeling with [3H]thymidine, embryonic progenitor cells were transplanted into older host brains. The laminar fate of transplanted neurons correlates with the position of their progenitors in the cell cycle at the time of transplantation. Daughters of cells transplanted in S-phase migrate to layer 2/3, as do host neurons. Progenitors transplanted later in the cell cycle, however, produce daughters that are committed to their normal, deep-layer fates. Thus, environmental factors are important determinants of laminar fate, but embryonic progenitors undergo cyclical changes in their ability to respond to such cues.

Animals↗
PubMed1981

Species-typical behavior of hamsters deprived from birth of the neocortex.

Hamsters deprived from birth of the neocortex developed normally and displayed the usual hamster-typical behavioral patterns. With the additional concurrent destruction of midline limbic convolutions (cingulate and underlying dorsal hippocampal), there were deficits in maternal behavior and a lack of development of play behavior. These findings demonstrate in a rodent (i) that the striatal complex and limbic system, along with the remaining neuraxis, are sufficient for giving expression to a wide range of unlearned forms of species-typical behavior and (ii) that midline limbic structures are required for the expression of play behavior and the integrated performance of maternal behavior.

Animals↗
PubMed1987

Spontaneous ictal-like discharges and sustained potential shifts in the developing rat neocortex.

1. Intra- and extracellular recording techniques were used to study epileptogenesis in in vitro slices of immature rat neocortex. Slices of sensorimotor cortex were prepared from animals 5-60 days old. Epileptiform activity was induced by bath application of 50 microM picrotoxin. 2. Convulsant-induced paroxysmal activity was observed only rarely in the youngest age group (5-7 days) and consisted of orthodromically evoked bursts of low-amplitude isolated discharges. This activity was labile and could be evoked only at long interstimulus intervals (greater than 10 s). 3. Extracellular recordings in slices from 8- to 15-day-old rats showed spontaneous epileptiform activity consisting of 10- to 30-s paroxysms of repetitive spike discharges superimposed on a 3- to 5-mV negative steady potential. This steady potential declined slightly during the course of the prolonged discharge and returned quickly to base line following the last spike discharge. 4. Laminar analysis of epileptiform activity in 8- to 15-day-old rats showed that the spike discharges were negative and superimposed on a positive slow wave in superficial cortical layers. At 100 micron below the pial surface, the slow potential reversed polarity and remained negative throughout the remainder of the cortex. Spike discharges reversed polarity 800 micron below the pial surface. 5. In intracellular recordings from slices obtained from 9- to 14-day-old animals, each paroxysm began with a sharply rising membrane depolarization (paroxysmal depolarizing shift, or PDS). A second PDS occurred before the cells repolarized to their resting potential. A series of PDSs then followed, superimposed on a sustained membrane depolarization. This was associated with a 33% decrease in input resistance. Afterhyperpolarizations (AHPs) following termination of the depolarization were low in amplitude or absent. 6. In the 16- to 30-day-old age group, extracellular recordings showed paroxysmal activity consisting of 3-10 initial spikes followed by a sustained, slow, negative-potential shift. This slow potential could be as great as 30 mV in amplitude and could last as long as 180 s. Paroxysmal events recurred spontaneously at intervals of 4-11 min. Spontaneous PDSs and slow, negative-potential shifts were not observed after 30 days of age, although PDSs could still be evoked by orthodromic stimulation. 7. Intracellular recordings in the 16- to 30-day-old group revealed that each paroxysmal event consisted of an initial period of increased synaptic activity and cellular firing, followed by a marked, long-lasting depolarization (LLD), culminating in an AHP.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1989

Horizontal spread of synchronized activity in neocortex and its control by GABA-mediated inhibition.

1. Suppression of GABAA receptor-mediated inhibition disrupts the neural activity of neocortex and can lead to synchronized discharges that mimic those of partial epilepsy. We have studied the role of GABAA-mediated inhibition in controlling the synchronization and horizontal (tangential) spread of cortical activity. 2. Slices of rat SmI were maintained in vitro and focally stimulated in layer VI while recording with a horizontal array of extracellular electrodes. Inhibition was slightly suppressed by adding low concentrations of the GABAA antagonists bicuculline or bicuculline methiodide to the bathing medium. Under control conditions neural activity was narrowly confined to a vertical strip of cortex. The horizontal spread of activity expanded about twofold in the presence of antagonist concentrations (less than or equal to 0.5 microM) that were expected to suppress GABAA function by no more than 10-20%. 3. At antagonist concentrations between 0.4 and 1.0 microM, evoked epileptiform activity appeared. These threshold-dose epileptiform events showed wide variations in size and duration (even at the same recording site), very variable distances of horizontal propagation, specific sites of propagation failure, reversals of propagation direction, and directional asymmetries in their probability of propagation. This contrasts with activity observed previously (Ref. 9) in high bicuculline concentrations (greater than or equal to 10 microM): large, stereotyped events that propagate reliably without decrement or reflection. 4. Intracellular recordings were obtained from pyramidal neurons in layers II/III in the presence of less than or equal to 1 microM bicuculline. Inhibitory postsynaptic potentials (IPSPs) were observed during both primary evoked responses and propagating epileptiform events and were often comparable in size and duration to those in untreated cortex. Epileptiform field potentials were always correlated with synaptic activity in single cells, but the pattern and type of PSPs varied with the form of the field potentials. Large amplitude epileptiform events coincided with an overwhelming inhibition of upper layer neurons. 5. We conclude that 1) the horizontal spread of normal cortical activity is strongly constrained by GABAA-mediated IPSPs, 2) a relatively small reduction in the efficacy of inhibition leads to a large increase in the spread of excitation, 3) initiation and propagation of synchronized epileptiform activity can occur even in the presence of robust cortical inhibition, and 4) the character of epileptiform activity is strongly affected by the influences of inhibition.

Animals↗
PubMed1989

Synchronized excitation and inhibition driven by intrinsically bursting neurons in neocortex.

1. The cellular mechanisms of synchronous synaptic activity were studied in isolated slices of rat SmI neocortex in which gamma-aminobutyric acid (GABA)-mediated inhibition was slightly suppressed. Intracellular measurements were made from single neurons, and extracellular recordings monitored the timing and intensity of population events. 2. Neurons in cortical layers II-VI were classified by the attributes of their single action potentials and repetitive firing patterns during injection of intracellular current pulses. Regular-spiking (RS) cells occurred in all layers and had relatively long-duration spikes and strong frequency adaptation. Intrinsically bursting (IB) cells occurred only in layers IV and V and generated bursts of greater than or equal to 3 spikes; some IB cells of lower-layer V produced repetitive bursts during long depolarizing pulses. Fast-spiking (FS) cells had brief spikes and little or no adaptation and fired at high frequencies. 3. When GABAA-mediated inhibition was slightly reduced with low doses of bicuculline methiodide (BMI, 0.8-1.0 microM), synchronous events were evoked by stimulating layer VI with single shocks. Synchronous events were characterized by prominent, often all-or-none extracellular field potentials that propagated horizontally for variable distances up to several millimeters. Large field potentials were invariably correlated with excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) in single neurons. Both PSPs and field potentials often had long (up to 250 ms) and variable latencies, and sometimes two or more events were generated by single stimuli. In all cases the PSPs and field potentials were synchronous. Both field potentials and single cells sometimes generated short epochs (3-7 peaks) of rhythmic events at 20-50 Hz. 4. The physiological class of single neurons was correlated with the relative dominance of excitation and inhibition during each synchronous event. In phase with each synchronous event, most RS cells were very strongly inhibited with only small amounts of concurrent excitation. By contrast, IB cells were strongly and consistently excited, with relatively little inhibition. FS cells were also phasically excited. 5. Anatomic studies have identified RS and IB cells as pyramidal cells and FS cells as GABAergic nonpyramidal cells. This implies that, during the synchronous events of the present study, the majority of pyramidal cells were dominated by IPSPs. Synchronous excitation of FS cells, the presumed inhibitory interneurons, is consistent with this. Only a subset of the pyramidal neurons, almost all of them IB cells of the middle layers, displayed strong, synchronous excitation and clusters of action potentials.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Three-dimensional analysis of auditory-evoked potentials in rat neocortex.

1. A 8 X 8-channel microelectrode array was used to map epicortical field potentials evoked by bilaterally presented click stimuli from a 8 X 8-mm2 area in the right parietotemporal neocortex of four rats. In two rats, a 16-channel microelectrode array was also inserted into primary auditory cortex to record the laminar profile of auditory evoked potentials (AEP). 2. The epicortical responses began with a positive-negative fast wave followed by a positive-negative slow wave, similar to the previously reported P1, N1, P2, N2 complex. Topographical distributions of the potentials at the peak of each of these waves were distinct, suggesting that they were produced by separate but overlapping populations of cells. 3. Laminar recording revealed the asynchronous participation of supragranular and infragranular pyramidal cells in the generation of the evoked-response complex. The surface-recorded P1 was primarily produced by supragranular cells and the N1, by infragranular cells. The P2 and N2 were produced by temporally overlapping contributions from both cell groups. 4. We conclude that middle-latency components of the AEP complex are produced by both sequential and parallel activation of subpopulations of pyramidal cells in primary auditory cortex.

Acoustic Stimulation↗
PubMed1991

Excitatory synaptic involvement in epileptiform bursting in the immature rat neocortex.

1. Neocortical brain slices were prepared from animals 8-15 days of age and maintained in vitro. Intracellular recordings were obtained from neurons in cortical layers 2-3. The role of synaptic activity and excitatory amino acid receptors in generation of picrotoxin-induced ictal-like epileptiform activity in the immature neocortex was investigated. D-2-amino-5-phosphonovaleric acid (D-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were used as selective antagonists of N-methyl-D-aspartate (NMDA) and non-NMDA receptors, respectively. 2. Ictal-like epileptiform discharges were induced by bath application of the GABAA-receptor antagonist picrotoxin. Paroxysmal discharges, 7-25 s in duration, occurred spontaneously or could be evoked by electrical stimulation. These events consisted of an initial paroxysmal depolarizing shift (PDS) followed by a long-duration depolarization (LLD) with superimposed late PDSs. 3. The amplitudes of the initial PDS, LLD, and late PDSs were linearly dependent on membrane potential, increasing with hyperpolarization and diminishing on depolarization. All responses reversed polarity near 0 mV. Under voltage-clamp conditions, both transient and sustained currents were observed, coincident with PDSs and the LLD, respectively. The duration of the ictal-like events was similar under current- and voltage-clamp conditions, suggesting activation of intrinsic membrane currents did not significantly prolong epileptiform discharges. 4. Bath application of D-APV (20 microM) decreased the amplitude and duration of both the initial PDS and LLD without affecting the time-to-onset of epileptiform activity.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1992

Local synaptic circuits and epileptiform activity in slices of neocortex from children with intractable epilepsy.

1. Single and dual intracellular recordings were performed in neocortical slices obtained from tissue samples surgically removed from children (8 mo to 15 yr) for the treatment of intractable epilepsy. Electrical stimulation and glutamate microapplication were used to study local synaptic inputs to pyramidal cells. 2. In recordings with potassium-acetate electrodes, activation of presynaptic neocortical neurons with glutamate microdrops did not elicit a clear increase in postsynaptic potentials (PSPs) but did suppress current-evoked repetitive spike firing in recorded neurons. Bicuculline (10 microM) blocked this effect, suggesting it was caused by the activation of presynaptic gamma-aminobutyric acid (GABA) cells. In recordings with KCl electrodes, glutamate microdrops elicited an increase in the frequency and amplitude of depolarizing PSPs. Bicuculline (5-10 microM) blocked the glutamate-evoked PSPs, suggesting they were reversed GABAA-receptor-mediated inhibitory postsynaptic potentials (IPSPs). In one cell recorded with a KCl electrode (total n = 8), current-evoked spike trains elicited afterdischarges of reversed IPSPs, thus revealing a recurrent inhibitory circuit. Therefore local inhibitory synaptic circuits were robust and could be observed in tissue from patients as young as 11 mo. 3. In addition to short-latency (10-25 ms), monosynaptic excitatory postsynaptic potentials (EPSPs), electrical stimulation at low intensities sometimes elicited delayed EPSPs (20-60 ms). When GABAA-receptor-mediated synaptic inhibition was partially reduced in bicuculline (5-10 microM), electrical stimulation evoked large EPSPs at long and variable latencies (100-300 ms). Glutamate microapplication caused an increase in the frequency and amplitude of EPSPs; preliminary results suggest that glutamate microdrops were less likely to evoke EPSPs in tissue from younger patients (8-12 mo) than in slices from patients greater than 4 yr. Evidence for local excitatory synaptic circuits was thus found when synaptic inhibition was partially reduced. 4. After further reduction of inhibition in bicuculline (5-50 microM), electrical stimulation elicited epileptiform bursts. In pairs of simultaneously recorded neurons, bursts were generated synchronously from long-latency EPSPs (100-300 ms) in slices from patients as young as 8 mo. Reflected EPSPs at very long and variable latencies (500-1,100 ms) and repetitive epileptiform bursts could be evoked synchronously in pairs of cells. Glutamate activation of local presynaptic neurons elicited robust epileptiform events in recorded cells. This was seen in slices from patients as young as 16 mo. 5. These data provide physiological evidence for the presence of local inhibitory and excitatory synaptic circuits in human neocortex at least as early as 11 and 8 mo, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Adolescent↗
PubMed1993

Developmental changes in NMDA and non-NMDA receptor-mediated synaptic potentials in rat neocortex.

1. In vitro slices of frontal neocortex were prepared from rat pups 3-14 days of age. Whole-cell patch-clamp recordings were obtained from layer II-III cortical neurons, and measurements of passive membrane properties were made. The development of evoked synaptic excitation and inhibition was also examined with the use of current- and voltage-clamp techniques. 2. Pharmacological separation of excitatory synaptic activity into both N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated components was accomplished by application of D(-)2-amino-5-phosphonovaleric acid (APV), D(-)2-amino-7-phosphonoheptanoic acid (AP7), and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Inhibitory synaptic events were described according to their reversal potentials and modulation by the GABAA receptor antagonist bicuculline methiodide (BMI). 3. Pups were grouped into three categories on the basis of age: postnatal day (PN) 3-5, PN 6-8, and PN 9-14. In slices from PN 3-5 pups, neurons exhibited high input resistances (Rn) and relatively low resting membrane potentials (RMP). Rns decreased, and RMPs became more negative with development. At all ages studied, current-voltage relationships measured in current clamp were relatively linear, with inward rectification observed in some neurons at hyperpolarized membrane potentials. Neurons in each group were capable of firing overshooting action potentials. 4. Local stimulation in layer IV-V at 0.033 Hz elicited depolarizing excitatory postsynaptic potentials (EPSPs) in neurons from all three age groups. In PN 3-5 neurons, EPSPs were characterized by a long duration and latency to peak. By PN 6-8, EPSPs had decreased significantly in both duration and latency-to-peak. Some neurons responded with a single-component EPSP, whereas others exhibited multicomponent EPSPs consisting of distinct early and late components. In PN 3-5 neurons, increasing the frequency of stimulation from 0.033 to 1 Hz resulted in an overall decrease in the amplitude of the entire EPSP, whereas in PN 6-8 neurons the main decrease was observed in the late EPSP. 5. Excitatory postsynaptic currents (EPSCs) recorded in both PN 3-5 and PN 6-8 neurons were shorter in duration than corresponding EPSPs and consisted of both early and late components. Early EPSCs routinely increased in amplitude with hyperpolarization at all ages. In PN 3-5 neurons, the voltage dependence of late EPSCs was variable. By PN 6-8, late EPSCs always exhibited a region of reduced amplitude from -45 to -90 mV. The reversal potential for both early and late EPSCs was near +10 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1993

NMDA receptor-mediated components of miniature excitatory synaptic currents in developing rat neocortex.

1. In vitro slices of frontal neocortex were prepared from rat pups at various ages after birth: postnatal days (PN) 3-5, 6-8, and 9-14. Using whole-cell patch-clamp techniques, both spontaneous and evoked excitatory postsynaptic currents (EPSCs) were recorded from voltage-clamped layer II-III pyramidal neurons. Developmental changes in EPSCs were examined. 2. Four properties of miniature EPSCs (mEPSCs) were studied: rise time, amplitude, decay time constant (tau), and frequency. These parameters were not tetrodotoxin sensitive and did not exhibit significant developmental changes during the first two postnatal weeks. 3. mEPSCs occurred approximately every 2-3 s and had peak amplitudes of 25-30 pA. Within each age group, certain parameters of mEPSCs were voltage dependent. mEPSC rise time and decay tau were significantly increased at depolarized potentials (-30 to -45 mV) relative to hyperpolarized (-75 to -90 mV) or resting membrane potential (RMP) (-60 to -70 mV). 4. At threshold stimulation intensity, EPSCs were evoked in an "all-or-none" manner. The amplitude and decay tau of evoked unitary EPSCs and mEPSCs were not significantly different. As stimulation intensity was increased, a late EPSC component appeared that was not seen in mEPSCs. At suprathreshold stimulus intensities, EPSC duration was significantly longer in PN 3-5 than in PN 9-14 neurons. 5. The N-methyl-D-aspartate (NMDA) receptor antagonist D(-)2-amino-5-phosphonovaleric acid (APV, 10 microM) significantly decreased mEPSC decay tau and frequency only at depolarized membrane potentials. Likewise, EPSCs were depressed by APV to a greater extent at depolarized potentials, and the depression was mainly of the late component. mEPSCs recorded at RMP were blocked by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline- 2,3-dione (5 microM). 6. Removal of extracellular Mg2+ reversibly increased the decay tau of mEPSCs at RMP but not at depolarized membrane potentials. The decay tau and duration of evoked EPSCs were also increased in zero Mg2+. These effects were reversible with application of APV. All effects of zero Mg2+ on mEPSCs and EPSCs were observed as early as PN 3-5. 7. These results indicate that the basic kinetic properties of mEPSCs are present by PN 3-5 and do not change significantly over the first two postnatal weeks. NMDA receptor activation contributes to mEPSCs and sensitivity to Mg2+ appears as early as PN 3-5. Unitary EPSCs resemble mEPSCs, but a late NMDA receptor-mediated component appears in EPSCs as stimulus intensity is increased.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1994

Spread of epileptiform activity in the immature rat neocortex studied with voltage-sensitive dyes and laser scanning microscopy.

1. Adult rats and rats with a postnatal age of 3-29 days (PN 3-29) were used for the preparation of in vitro slices of the frontal neocortex. Epileptiform activity was induced by bath application of the gamma-aminobutyric acid-A (GABAA) receptor antagonists bicuculline or picrotoxin. 2. The voltage-sensitive dye RH 414 and a laser scanning microscope were used for multiple-site optical recordings of membrane potential changes associated with epileptiform activity. Optical signals were compared with simultaneously measured extra-cellular field potentials. 3. Optical signals could be reliably recorded for the duration of the experiments (2-4 h). Extracellular recordings of convulsant-induced paroxysmal depolarizing shifts (PDSs) in slices stained with RH 414 were comparable with those obtained in unstained slices. Changes in dye signals in response to reductions in extracellular calcium, addition of tetrodotoxin (TTX), or application of excitatory amino acid receptor antagonists indicate that the fluorescence changes correlate well with established electrophysiological measures of epileptiform activity. 4. In slices from adult animals, dye signals were observed at all recording sites. The response with the shortest latency occurred invariably at the site of stimulation, and activity spread rapidly in both vertical and horizontal directions. Spread was significantly faster in the vertical than in the horizontal direction. 5. Epileptiform activity was absent or only weakly expressed in slices from PN 3-9 animals. Activity was detectable predominantly in upper cortical layers. 6. Dye signals were observed at all measurement points in slices from PN 10-19 animals. In this age group, peak amplitude increased with spread of activity from lower to upper cortical layers. There was no significant difference between the speed of propagation in the vertical and in the horizontal directions. Spontaneous epileptiform activity occurred at a high rate in the PN 10-19 age group, and signals associated with spontaneous epileptiform events were largest in upper layers. 7. In the PN 10-19 age group, optical signals were characterized by the repetitive occurrence of PDS discharges superimposed on a sustained response. The amplitude of the sustained response decreased with increasing distance from the site of stimulation. Analysis of the latencies revealed that the superimposed PDS-like events were generated at multiple sites within the scanning area. Amplitude and rate of rise were largest in slices from PN 10-19 animals. These values declined with ongoing development.(ABSTRACT TRUNCATED AT 400 WORDS)

Age Factors↗