We have investigated the spatial map of tissue resistivity across CA1 layers
in vivo, its modifications during repetitive orthodromic activity, and the influence of this factor on the shaping of ...population spikes. Measurement of tissue resistance was made by a high-spatial-resolution three-electrode method. A computer network of equivalent resistors aided theoretical analysis. Tissue resistivity was homogeneous within the basal and apical dendritic trees (260±4.5 and 287±2.6 Ωcm, respectively). In the stratum pyramidale we found a sharply delimited high-resistivity (643±35 Ωcm) band ∼20 μm wide. Resistivity in slices was ∼30% higher than
in vivo. Computer analysis indicated that the high-resistance somatic layer has a strong influence on the somatic and proximal dendritic contribution to the shaping of population spikes, and reduces volume propagation of currents between dendritic trees. Repetitive orthodromic activation at the theta frequency (4–5 Hz, 20–30 s) caused a stereotyped cycle of field potentials and layer-specific changes of resistivity. Initially (∼10 s), long-lasting field excitatory postsynaptic potentials and multiple somatodendritic population spikes developed, and resistivity gradually increased in all layers at a similar rate (period average: 11%). Subsequently, the long-lasting field excitatory synaptic potential subsided and dendritic spike generators were strongly reduced, but multiple somatic spikes remained. Concurrently, the resistivity reached a plateau in all dendritic layers but continued to increase in the somatic layer for about 10–15 s (20% average and up to 50% maximum). Recovery required ∼60 s. The orthodromic somatic population spike increased variably during stimulation (up to 60%). Using local resistivity changes for correction, supernormal increments of the population spikes were offset, but not totally, uncovering several sub- and supernormal phases that were partially related to changes in dendritic population spike. These resistivity-independent modulations of the somatic population spike are caused by variable volume spread from dendritic spike currents and changed somatic contribution of firing units.
This report demonstrates that the strong heterogeneity in the stratum pyramidale is an important factor shaping and modulating the population spike. The different regional rates of resistivity variation force the independent correction of local evoked potentials. We show that not doing so may cause bulk errors in the interpretation of, for instance, field potential ratios widely used to measure the population excitability. The present results underscore the importance of checking variations in recording conditions, which are inherent in most experimental protocols.
1 Departamento de Ingeniería
Informática, Universidad Autónoma de Madrid, 28049 Madrid;
and 2 Departamento Investigación, Hospital
Ramón y Cajal, 28034 Madrid, Spain
Varona, P.,
J. M. Ibarz,
L. ...López-Aguado, and
O. Herreras.
Macroscopic and Subcellular Factors Shaping Population Spikes. J. Neurophysiol. 83: 2192-2208, 2000. Population spikes (PS) are built by the extracellular summation of
action currents during synchronous action potential (AP) firing. In the
hippocampal CA1, active dendritic invasion of APs ensures mixed
contribution of somatic and dendritic currents to any extracellular
location. We investigated the macroscopic and subcellular factors
shaping the antidromic PS by fitting its spatiotemporal map with a
multineuronal CA1 model in a volume conductor. Decreased summation by
temporal scatter of APs reduced less than expected the PS peak in the
stratum pyramidale (st. pyr.) but strongly increased the relative
contribution of far dendritic currents. Increasing the number of firing
cells also augmented the relative dendritic contribution to the somatic
PS, an effect caused by the different waveform of somatic and dendritic
unitary transmembrane currents ( I m ). Those
from somata are short-lasting and spiky, having smaller temporal
summation than those from dendrites, which are smoother and longer. The
different shape of compartmental I m s is
imposed by the fitting of backpropagating APs, which are large and fast
at the soma and smaller and longer in dendrites. The maximum sodium
conductance ( Na ) strongly affects the
unitary APs at the soma, but barely the PS at the stratum pyramidale
(st. pyr.). This occurred because somatic I m
saturated at low Na due to the strong
reduction of driving force during somatic APs, limiting the current
contribution to the extracellular space. On the contrary,
Na effectively defined the PS
amplitude in the st. radiatum. The relative contribution of dendritic
currents to the st. pyr. increases during the time span of the PS, from ~30-40% at the peak up to 100% at its end, a pattern resultant from the timing of active inward currents along the somatodendritic axis, which delay during backpropagation. Extreme changes imposed on
dendritic currents caused only moderate effects on the st. pyr. due to
reciprocal shunting of active soma and dendrites that partially
counterbalance the net amount of instant current. The amplitude of the
PS follows an inverse relation to the internal resistance
( R i ), which turned out to be a most critical
factor. Low R i facilitated the spread of APs
into dendrites and accelerated their speed, increasing temporal
overlapping of inward currents along the somatodendritic axis and
yielding the best PS reproductions. Model reconstruction of field
potentials is a powerful tool to understand the interactions between
different levels of complexity. The potential use of this approach to
restrain the variability of some experimental measurements is discussed.
1 Departamento de Investigación,
Hospital Ramón y Cajal and 2 Departamento
de Ingeniería Informática, Universidad
Autónoma, Madrid 28034, Spain
López-Aguado, L.,
J. M. Ibarz,
P. Varona, and
O. ...Herreras.
Structural Inhomogeneities Differentially Modulate Action
Currents and Population Spikes Initiated in the Axon or
Dendrites. J. Neurophysiol. 88: 2809-2820, 2002. Action potentials (APs) in CA1 pyramidal cells propagate in
different directions along the somatodendritic axis depending on the
activation mode (synaptic or axonal). We studied how the geometrical
inhomogeneities along the apical shaft, soma, and initial axon modulate
the transmembrane current ( I m ) flow
underlying APs, using model and experimental techniques. The
computations obtained at the subcellular level during forward- and
backpropagation were extrapolated to macroscopic level (field
potentials) and compared with the basic in vivo features of the ortho-
and antidromic population spike (PS) that reflects the sum total of all
elementary currents from synchronously firing cells. The matching of
theoretical and experimental results supports the following
conclusions. Because the charge carried by axonal APs is almost
entirely drained into dendrites, the soma invasion is preceded by
little capacitive currents ( I cap ), the
ionic currents ( I ion ) dominating
I m and the depolarizing phase. The
subsequent invasion of the tapering apical shaft is preceded, however,
by significant I cap , while
I ion decayed gradually. A similar
pattern occurred during backpropagation of spikes synaptically
initiated in the axon. On the contrary, when the AP was apically
initiated, the dendritic I ion was
boosted by the apical flare, it was preceded by weak
I cap and spread forwardly at a slower
velocity. Soma invasion is reliable once the AP reached the main apical
shaft but less so distal to the primary bifurcation, where it may be
upheld by concurrent synaptic activity. The decreasing internal
resistance of the apical shaft guided most axial current into the soma,
causing its fast charging. There, I ion
began later in the depolarizing phase of the AP and the reduced driving
force made it smaller. This, in addition to a poor temporal overlapping of somatodendritic inward currents within individual cells, built a
smaller extracellular sink, i.e., a smaller PS. In both experiment and
model, the antidromic (axon-initiated) PS in the soma layer is
approximately 30% larger than an orthodromic (apical shaft-initiated) PS contributed by the same number of firing cells. We conclude that the
dominance of capacitive or ionic current components on I m is a distinguishing feature of
forward and backward APs that is predictable from the geometric
inhomogeneities between conducting subregions. Correspondingly,
experimental and model APs have a faster rising slope during ortho than
antidromic activation. The moderate flare of the apical shaft makes
forward AP conduction quite safe. This alternative trigger zone enables
two different processing modes for apical inputs.
The potential shifts (delta Vo) associated with spreading depression (SD) were analysed with the help of multiple extracellular recording and ion-selective microelectrodes in the CA1 region of the ...dorsal hippocampus of anesthetized rats. Recurrent waves of SD were induced by perfusing high K+ solution through microdialysis probes. SD-related delta Vo had a composite wave shape, consisting of an early, rapidly shifting part (phase I) followed by a slower shift to a second negative maximum (phase II). delta Vo shifts in stratum radiatum usually started earlier, always lasted longer and had larger amplitude than those recorded in stratum pyramidale. The delta Vo responses in stratum radiatum had an inverted saddle shape created by a transient relatively positive "hump" interposed between phases I and II. During this "hump", the potentials in the two layers transiently approached one another. During continuous high K+ dialysis, successive delta Vo waves episodes evolved according to a consistent pattern: while phase I remained unchanged, phase II increased in amplitude and duration with each episode. Eventually, a depressed state developed which lasted for many minutes, termed here prolonged unstable spreading depression. During phase I, delta Vo and extracellular K (K+o) changes were correlated. During phase II, K+o decreased even as delta Vo continued to increase. During SD, Ca2+o decreased to < 0.01 mM. During phases I and II, both Ca2+o and Na+o remained low. The recoveries of Ca2+o and Na+o had an initial fast and a later much slower phase and took several minutes longer than the recoveries of K+o and delta Vo. Depth profiles of delta Vo and delta K+o revealed strikingly steep gradients early and late during a wave; but voltage and ion gradients were not precisely correlated either in time or in space. We conclude that delta Vo of phases I and II are generated by different processes. Membrane ion currents cannot fully explain the delta Vo responses. The possible contributions by ion diffusion and by active ion transport are discussed. The extremely low level to which Ca2+o sinks during SD, and its two-phase recovery, indicate intracellular sequestration or binding of substantial amounts of Ca2+ ions. The residual deficit of Ca2+o following recovery of SP shifts may account for the persistent depression of synaptic transmission after repolarization of neurons.
The propagation of sustained potential shifts associated with spreading depression (SD) was studied by microelectrodes placed in diverse layers at different locations in hippocampus of anesthetized ...rats. SD was induced by raising interstitial potassium concentration (K+0) focally in the CA1 region of the dorsal hippocampus either by microdialysis or by microinjection. Recurrent waves of SD propagated from the dialysis site throughout the hippocampus. Potential shifts (delta V0) associated with SD usually began earlier and were always of larger amplitude and longer duration in stratum (st.) radiatum (layer of apical dendrites) than in st. pyramidale (layer of pyramidal cell somata). The velocity of propagation in the two layers differed and varied independently one from the other. When SD was provoked by orthodromic train stimuli, the apparent direction of propagation in st. pyramidale was opposite that in st. radiatum. Microinjection of high K+ solution was more likely to provoke SD when placed in the st. radiatum, 50-100 microns ventral to st. pyramidale, than in other cytoarchitectonic layers. In about half the trials after 30 to 90 min of high K+ dialysis, a prolonged depressed state developed during which the potential in st. radiatum shifted at irregular intervals between near-rest level and a strongly negative level, while delta V0 shifts in st. pyramidale were smaller and more irregular in amplitude. This state is termed prolonged unstable SD". When the NMDA receptor antagonist CPP was dialyzed together with high K+, the onset of SD was postponed and delta V0 waves propagated in st. pyramidale without corresponding waves in st. radiatum; less frequently the other way around.
Extracellular (EC) concentrations of amino acids were determined in the rat dentate gyrus by means of non-linear regression analysis of 'in vivo' brain dialysis data, considering a simple model of ...diffusion through a dialysis membrane. The apparent diffusion constants (K) of several amino acids were also calculated in the 'in vivo' situation. While putative amino acid neurotransmitters (glutamate, aspartate and gamma-aminobutyric acid (GABA) were present in the EC fluid at the low micromolar range (0.8-2.9 microM), glutamine was by far the most prominent (193.4 microM). The values of intra/extracellular concentration ratios formed 3 groups: high (greater than 2000) for putative neurotransmitters; low (less than 100) for serine, glutamine, arginine and alpha-alanine; and intermediate (about 400) for taurine. The 'in vivo' calculated K values proved useful for estimation of both basal and changing EC concentrations of amino acids in relatively brief perfusions. These data were evaluated in terms of the functional significance of absolute EC concentrations and tissue-EC fluid ratios. Present findings indicate the simultaneous existence of both an inhibitory and an excitatory tonus as well as the utility of high intra/extracellular concentration ratios in determination of the possible neurotransmitter role of specific amino acids.
Using microcultured neurons and hippocampal slices, we found that under conditions that completely block AMPA receptors, kainate induces a reduction in the effectiveness of GABAergic synaptic ...inhibition. Evoked inhibitory postsynaptic currents (IPSCs) were decreased by kainate by up to 90%, showing a bell-shaped dose-response curve similar to that of native kainate-selective receptors. The down-regulation of GABAergic inhibition was not affected by antagonism of metabotropic receptors, while it was attenuated by CNQX. Kainate increased synaptic failures and reduced the frequency of miniature IPSCs, indicating a presynaptic locus of action. In vivo experiments using brain dialysis demonstrated that kainate reversibly abolished recurrent inhibition and induced an epileptic-like electroencephalogram (EEG) activity. These results indicate that kainate receptor activation downregulates GABAergic inhibition by modulating the reliability of GABA synapses.
Departmento de Investigación, Hospital Ramón y Cajal,
28034 Madrid, Spain
López-Aguado, L.,
J. M. Ibarz, and
O. Herreras.
Modulation of Dendritic Action Currents Decreases the Reliability
of ...Population Spikes. J. Neurophysiol. 83: 1108-1114, 2000. During synchronous action potential (AP)
firing of CA1 pyramidal cells, a population spike (PS) is recorded in
the extracellular space, the amplitude of which is considered a
reliable quantitative index of the population output. Because the AP
can be actively conducted and differentially modulated along the soma
and dendrites, the extracellular part of the dendritic inward currents
variably contributes to the somatic PS by spreading in the volume
conductor to adjacent strata. This contribution has been studied by
current-source density analysis and intracellular recordings in vivo
during repetitive backpropagation that induces their selective fading.
Both the PS and the ensemble action currents declined during
high-frequency activation, although at different rates and timings. The
decline was much stronger in dendrites than in the somatic region. At specific frequencies and for a short number of impulses the decrease of
the somatic PS was neither due to fewer firing cells nor to decreased
somatic action currents but to the blockade of dendritic action
currents. The dendritic contribution to the peak of the somatic
antidromic PS was estimated at ~30-40% and up to 100% at later
times in the positive-going limb. The blockade of AP dendritic invasion
was in part due to a decreased transfer of current from the soma that
underwent a cumulative increase of conductance and slow depolarization
during the train that eventually extended into the axon. The
possibility of differential modulation of soma and dendritic action
currents during APs should be checked when using the PS as a
quantitative parameter.
If oxygen is withdrawn from rat hippocampal slices, a spreading depression-like response occurs earlier and is of larger amplitude in the CA1 area than in the dentate gyrus. After reoxygenation, ...recovery of synaptic transmission correlates inversely with the time spent in spreading depression. Recovery occurs more frequently in dentate gyrus than in CA1. Chlorpromazine and the gangliosides GM1 and AGF2 promote recovery from hypoxic depression of synaptic transmission in CA1. Prevention of irreversible loss of function correlates closely with a shortening of the time spent in spreading depression. If Ca2+ is withdrawn before hypoxia, then synaptic function recovers upon restoration of oxygen and Ca2+o, despite prolonged spreading depression. When spreading depression lasting more than 6-9 minutes is induced in fully oxygenated slices by superfusion with high-K+ solution, then transient recovery is followed by long-lasting loss of synaptic function. In intact brain of anesthetized rats, synaptic transmission in CA1 recovers after spreading depression-like depolarization lasting more than 30 minutes, but is lost irreversibly after 60 minutes. We conclude that entry of Ca2+ into neurons caused by spreading depression-like depolarization is important in the selective vulnerability of neurons; the duration of depolarization is critical to cell survival; and in the presence of a normal blood supply, neurons resist protracted spreading depression-like depolarization.
PDF
