Longitudinal depolarization gradients along the somatodendritic axis of CA1 pyramidal cells: a novel feature of spreading depression.

We studied the subcellular correlates of spreading depression (SD) in the CA1 rat hippocampus by combining intrasomatic and intradendritic recordings of pyramidal cells with extracellular DC and evoked field and unitary activity. The results demonstrate that during SD only specific parts of the dendritic membranes are deeply depolarized and electrically shunted. Somatic impalements yielded near-zero membrane potential (V(m)) and maximum decrease of input resistance (R(in)) whether the accompanying extracellular negative potential (V(o)) moved along the basal, the apical or both dendritic arbors. However, apical intradendritic recordings showed a different course of local V(m) that is hardly detected from the soma. A decreasing depolarization gradient was observed from the edge of SD-affected fully depolarized subcellular regions toward distal dendrites. Within apical dendrites, the depolarizing front moved toward and stopped at proximal dendrites during the time course of SD so that distal dendrites had repolarized in part or in full by the end of the wave. The drop of local R(in) was initially maximal at any somatodendritic loci and also recovered partially before the end of SD. This recovery was stronger and faster in far dendrites and is best explained by a wave-like somatopetal closure of membrane conductances. Cell subregions far from SD-affected membranes remain electrically excitable and show evoked unitary and field activity. We propose that neuronal depolarization during SD is caused by current flow through extended but discrete patches of shunted membranes driven by uneven longitudinal depolarization.

Structural inhomogeneities differentially modulate action currents and population spikes initiated in the axon or dendrites.

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.

Activity-dependent changes of tissue resistivity in the CA1 region in vivo are layer-specific: modulation of evoked potentials.

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 Omega cm, respectively). In the stratum pyramidale we found a sharply delimited high-resistivity (643+/-35 Omega cm) band approximately 20 microm wide. Resistivity in slices was approximately 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 (approximately 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 approximately 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.

Macroscopic and subcellular factors shaping population spikes.

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 approximately 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.

Modulation of dendritic action currents decreases the reliability of population spikes.

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 approximately 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.

Effects of the gliotoxin fluorocitrate on spreading depression and glial membrane potential in rat brain in situ.

DC extracellular potential shifts (deltaVo) associated with spreading depression (SD) reflect massive cell depolarization, but their cellular generators remain obscure. We have recently reported that the glial specific metabolic poison fluorocitrate (FC) delivered by microdialysis in situ caused a rapid impairment of glial function followed some hours later by loss of neuronal electrogenic activity and neuron death. We have used the time windows for selective decay of cell types so created to study the relative participation of glia and neurons in SD, and we report a detailed analysis of the effects of FC on evoked SD waves and glial membrane potential (Vm). Extracellular potential (Vo), interstitial potassium concentration ([K+]o), evoked potentials, and transmembrane glial potentials were monitored in the CA1 area before, during, and after administration of FC with or without elevated K+ concentration in the dialysate. SD waves propagated faster and lasted longer during FC treatment. DeltaVo in stratum pyramidale, which normally are much shorter and of smaller amplitude than those in stratum radiatum, expanded during FC treatment to match those in stratum radiatum. The coalescing SD waves that develop late during prolonged high-K+ dialysis and are typically limited to stratum radiatum, also expanded into stratum pyramidale under the influence of FC. SD provoked in neocortex normally does not spread to the CA1, but during FC treatment it readily reached CA1 via entorhinal cortex. Once neuronal function began to deteriorate, SD waves became smaller and slower, and eventually failed to enter the region around the FC source. Slow, moderately negative deltaVo that mirrored [K+]o increments could still be recorded well after neuronal function and SD-associated Vo had disappeared. Glial cell Vm gradually depolarized during FC administration, beginning much before depression of neuronal antidromic action potentials. Calculations based on the results predict a large decrease in glial potassium content during FC treatment. The results are compatible with neurons being the major generator of the deltaVo associated with SD. We conclude that energy shortage in glial cells makes brain tissue more susceptible to SD and therefore it may increase the risk of neuron damage.

[Quality of life after radical prostatectomy]. / Calidad de vida tras prostatectomía radical.

OBJECTIVE: To evaluate the quality of life of our prostatectomized patients relative to the following factors: continence, mictional quality, sexual potency and psychological repercussion. MATERIAL AND METHODS: The study includes a series of 204 patients undergoing radical prostatectomy between June 1986 and October 1996, where a personal questionnaire was administered to 112 of them. The questionnaire consisted of 25 questions dealing with various aspects related to their quality of life. RESULTS: The overall rating on continence shows the following results: total continence 59.8%, minimal incontinence grade I 17.8%, moderate incontinence grade II 13.3% and total incontinence grade III 8%. Only 2.6% retains sexual potency after surgery. 29.3% of impotent patients consulted for their dysfunction. 91% declared to be satisfied with the results of the surgical procedure. CONCLUSIONS: In our experience, continence (total + grade I incontinence) is acceptable for 77.6%, the level of mictional satisfaction being very high. There is a high index of impotence after surgery. However, most patients appear to be impervious to this fact. Overall, quality of life of our patients has not changed significantly as a result of the intervention.

Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus.

To detect what initiates spreading depression (SD), the early prodromal events were investigated in hippocampal CA1 of urethane-anesthetized rats. SD was provoked by microdialysis or focal microinjection of high-K+ solution. Extracellular DC potentials and extracellular potassium concentration ([K+]o) were recorded, and spontaneous and evoked potentials analyzed for current source-density (CSD). In the front of an approaching SD wave, several seconds before the onset of the typical sustained negative potential shift (delta Vo) and the increased [K+]o, fast electrical activity was detected. This consisted initially of small rhythmic (60-70 Hz) sawtooth wavelets, which then gave way to a shower of population spikes (PSs) of identical frequency. Prodromal wavelets and PSs were synchronized over considerable distances in the tissue. Sawtooth wavelets were identified as pacemakers of the prodromal PS burst. Simultaneous recording at three depths revealed that the spontaneous prodromal PSs occurred exactly in phase in dendrites and somata whereas synaptically transmitted PSs arose first in the proximal dendrites and were conducted from there into the soma membrane. During a spike burst, stratum (st.) pyramidale served as current sink, while in the proximal sublayer of st. radiatum spike-sinks gave way to spike sources that grew larger as the sinks in st. pyramidale began to subside. Blocking synaptic transmission did not abolish the prodromal spike burst, yet repetitive orthodromic activation inhibited it without altering the subsequent SD waveform. Complex changes in cell excitability were detected even before fast spontaneous activities. We concluded that, in the initial evolution of SD, changes in neuron function precede the regenerating depolarization by several seconds. We propose that the opening of normally closed electric junctions among neurons can best explain the long-distance synchronization and the flow current that occurs in the leading edge of a propagating wave of SD.

Rectification properties and Ca2+ permeability of glutamate receptor channels in hippocampal cells.

Excitatory amino acids exert a depolarizing action on central nervous system cells through an increase in cationic conductances. Non-NMDA receptors have been considered to be selectively permeable to Na+ and K+, while Ca2+ influx has been thought to occur through the NMDA receptor subtype. Recently, however, the expression of cloned non-NMDA receptor subunits has shown that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are permeable to Ca2+ whenever the receptor lacks a particular subunit (edited GluR-B). The behaviour of recombinant glutamate receptor channels predicts that Ca2+ would only permeate through receptors that show strong inward rectification and vice versa, i.e. AMPA receptors with linear current-voltage relationships would be impermeable to Ca2+. Using the whole-cell configuration of the patch-clamp technique, we have studied the Ca2+ permeability and the rectifying properties of AMPA receptors, when activated by kainate, in hippocampal neurons kept in culture or acutely dissociated from differentiated hippocampus. Cells were classified according to whether they showed outward rectifying (type I), inward rectifying (type II) or almost linear (type III) current-voltage relationships for kainate-activated responses. AMPA receptors of type I cells (52.2%) were mostly Ca(2+)-impermeable (PCa/PCs = 0.1), while type II cells (6.5%) expressed Ca(2+)-permeable receptors (PCa/PCs = 0.9). Type III cells (41.3%) showed responses with low but not negligible Ca2+ permeability (PCa/PCs = 0.18). The degree of Ca2+ permeability and inward rectification were well correlated in cultured cells, i.e. more inward rectification corresponded to higher Ca2+ permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectral estimation of temporal series at unequal intervals.

Many biological variables present rhythmic oscillations at different frequencies. Most common techniques, which statistically characterize temporal series and permit the study of these rhythms, require equidistant sampling. However, it is not always possible to register at regular intervals many of the variables under study, either because of the nature of the phenomenon which generates them or because of the difficulty in obtaining the samples. This paper proposes a method for spectral estimation by means of fitting to the cosine functions of sampled variables using a nonuniform point process.

Cortical visual evoked potentials during the sleep-wakefulness cycle in the freely moving cat. A dynamic study.

The dynamic features of visual evoked potentials, elicited by a light emitting diode chronically implanted in the frontal sinus of the freely moving cat, were studied during sleep stages by means of calculation of the power spectra of the EEG prior to the stimulus and the poststimulus EEG which contain the single evoked potential. The statistical analysis between mean frequencies from both EEG prior to the stimulus and single evoked potential, within each sleep stage, showed that the main changes occurred during slow sleep, with a significant increase in the percentage of the alpha band. These results are interpreted as a partial desynchronization evoked by the visual stimulation during the synchronized phase of sleep.

Cortical visual evoked potentials during the sleep wakefulness cycle of the freely moving cat. Characterization and statistical comparisons.

Visual evoked potentials (VEPs) were obtained during the stages of wakefulness (W), slow sleep (SS) and paradoxical sleep (PS) by means of a light-emitting diode chronically implanted in the frontal sinus of the freely moving cat. Statistical analysis of the variables: latencies, latency intervals and amplitudes, between each of the mentioned stages shows that, for the first components, variations occurred only in the first interval of latency during SS vs. W. Lengthening of VEP latencies and increase of VEP amplitudes were observed for all secondary components in the comparisons between both SS and W, and SS and PS. PS-VEPs vs. W-VEPs showed shortening of latencies and decrease of amplitudes of all secondary components of the former case. The results confirm that in the freely moving cat, the secondary VEP response is more intensely affected by sleep than the primary VEP response, but indicate that there are different mechanisms in the generation of the VEP during SS and PS.

[Statistical analysis of the parameters defining the cortical visual evoked potentials in the phases of the sleep-wake cycle]. / Análisis estadístico de los parámetros que definen a los potenciales evocados visuales corticales durante las fases del ciclo vigilia-sueño.

Visual evoked potential parameters (latencies, intervals of latencies and amplitudes) obtained by photic stimulation using a light-emitting diode implanted in the frontal sinus of cats were studied by statistical methods (analysis of variance) during the stages of wakefulness, slow sleep and paradoxical sleep. The results show: a) greater intraindividual homogeneity in all cases with special emphasis on the latencies; b) the greatest homogeneity of responses was found during slow sleep and paradoxical sleep stages; c) in relation to the influences exerted by the sleep-wakefulness cycle on the visual evoked potentials, the parameters most affected were those closely related to the secondary complex. We conclude, that latency, due to its great homogeneity, is the most useful parameter in this kind of experiments and secondly, that it is the secondary complex of the visual evoked potentials that is affected by the endogenous conditions of the subject (in our case the sleep-wakefulness cycle stage).

Wire-wrapped, dual-in-line chronic electrode connector system.

A connector system is described, which consists of a modified integrated circuit used as a terminal for chronic electrodes within the brain of animals, allowing easy connection to the awake subject without soldering.