Resumen
La enfermedad de Alzheimer (EA) es la forma más común de demencia y tiene una elevada morbilidad y mortalidad. La EA se caracteriza principalmente por la presencia de dos estructuras ...aberrantes en el cerebro de los pacientes, placas seniles formadas por péptido-β-amiloide (Aβ) y ovillos neurofibrilares cuyo principal componente es la proteína tau fosforilada. Aunque actualmente no se conoce bien la etiopatogenia, cada vez son más los estudios que demuestran un efecto causal del microbioma intestinal sobre la EA y las funciones cognitivas, a través del "eje microbiota intestino-cerebro". Las evidencias científicas sugieren un posible efecto protector de los polifenoles del vino frente a los trastornos neurodegenerativos aunque se desconocen los mecanismos y, hasta el momento, los estudios para evaluar de forma exhaustiva el efecto del vino sobre la etiopatogenia de la EA son muy escasos. El objetivo principal de la línea de investigación que enmarca este trabajo es entender cómo la dieta, y especialmente los polifenoles presentes en los alimentos vegetales, y otros factores del estilo de vida interactúan con el microbioma oral e intestinal, en relación con la salud digestiva y el deterioro cognitivo. Para ello, se está llevando a cabo una aproximación experimental que tiene como finalidad evaluar el posible efecto protector de los polifenoles del vino, mediante la suplementación de la dieta en dos modelos murinos de la EA (patología Aß y Tau), y, por otro lado, se está profundizando en el estudio de los mecanismos de protección mediante la evaluación de los efectos del ácido protocatéquico sobre la actividad eléctrica del cerebro.
Alzheimer's disease (AD) is the most common form of age-related dementia with high morbidity and mortality. AD is mainly characterized by the presence of two aberrant structures in the brain of patients, senile plaques formed by peptide-β-amyloid (Aβ) and neurofibrillary tangles whose main component is phosphorylated tau protein. Although the etiopathogenesis is currently not well understood, an increasing number of studies demonstrate a causal effect of the gut microbiome on AD and cognitive functions, through the "gut-brain microbiota axis". Scientific evidence suggests a possible protective effect of wine polyphenols against neurodegenerative disorders although the mechanisms are unknown and, so far, studies to evaluate comprehensively the effect of wine on the etiopathogenesis of AD are very scarce. The main objective of the research line that frames this work is to understand how diet, and especially the polyphenols present in plant foods, and other lifestyle factors interact with the oral and intestinal microbiome in relation to digestive health and cognitive impairment. To this end, an experimental approach is being carried out to evaluate the possible protective effect of wine polyphenols through dietary supplementation in two murine models of AD (Aß and Tau pathology), and, on the other hand, the study of the protective mechanisms is being deepened by evaluating the effects of protocatechuic acid on the electrical activity of the brain.
Ongoing network activity often manifests as irregular fluctuations in local field potentials (LFPs), a complex mixture of multicellular synaptic currents of varying locations and extensions. Among ...other conditions, for synchronously firing presynaptic units to generate sizable postsynaptic LFPs, their axonal territories should overlap. We have taken advantage of anatomical regularity of the rat hippocampus and combined multiple linear recordings and spatial discrimination techniques to separate pathway-specific LFPs with enough spatial resolution to discriminate postsynaptic regions of varying activation, and to investigate their presynaptic origin, chemical nature, and spatial extension. We identified 6 main excitatory and inhibitory LFP generators with different synaptic territories in principal cells and hippocampal subfields matching anatomical pathways. Some recognized pathways did not contribute notably to LFPs. Each showed different septo-temporal spatial modules over which the field potential fluctuations were synchronous. These modules were explained by either the strong overlap of synaptic territories of coactivated afferent neurons (e.g., CA3 clusters for CA1 Schaffer LFPs), or widespread coalescence of postsynaptic territories (granule cell somatic inhibition). We also show evidence that distinct modes of afferent synchronization generate stereotyped spatial patterns of synchronous LFPs in one pathway. Thus, studying spatial coherence of pathway-specific LFPs provides remote access to the dynamics of afferent populations.
Analysis of local field potentials (LFPs) helps understand the function of the converging neuronal populations that produce the mixed synaptic activity in principal cells. Recently, using independent ...component analysis (ICA), we resolved ongoing hippocampal activity into several major contributions of stratified LFP-generators. Here, using pathway-specific LFP reconstruction, we isolated LFP-generators that describe the activity of Schaffer-CA1 and Perforant-Dentate excitatory inputs in the anesthetized rat. First, we applied ICA and current source density analysis to LFPs evoked by electrical subthreshold stimulation of the pathways. The results showed that pathway specific activity is selectively captured by individual components or LFP-generators. Each generator matches the known distribution of axonal terminal fields in the hippocampus and recovers the specific time course of their activation. Second, we use sparse weak electrical stimulation to prime ongoing LFPs with activity of a known origin. Decomposition of ongoing LFPs yields a few significant LFP-generators with distinct spatiotemporal characteristics for the Schaffer and Perforant inputs. Both pathways convey an irregular temporal pattern in bouts of population activity of varying amplitude. Importantly, the contribution of Schaffer and Perforant inputs to the power of raw LFPs in the hippocampus is minor (7 and 5%, respectively). The results support the hypothesis on a sparse population code used by excitatory populations in the entorhino-hippocampal system, and they validate the separation of LFP-generators as a powerful tool to explore the computational function of neuronal circuits in real time.
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.
The mechanism of the propagation of spreading depression is unclear. Classical theories proposed a self-maintained cycle fed by elevated potassium and/or glutamate in the extracellular space. Earlier ...we found
in vivo a characteristic oscillatory field activity that is synchronous in a strip of tissue ahead of the oncoming wave of neuron depolarization and that occurs before the extracellular potassium level begins to rise Herreras O, Largo C, Ibarz JM, Somjen GG, Marrín del Río R (1994) Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus. J Neurosci 14:7087–7098. We investigated here the possible participation of glutamate and the role of glia in the prodromal field oscillations using extra and intracellular recordings and pharmacological manipulations in rat hippocampal slices. As earlier shown
in vivo, field oscillations propagated ahead of the negative potential shift covering distances of up to 1 mm. The oscillatory prodromals were initially subthreshold but then each wave became crowned by a population spike. The frequency of the oscillatory prodromals was variable among slices (80–115 Hz), but constant in individual slices. The blockade of ionotropic glutamate receptors decreased the frequency of prodromal oscillations, retarded spreading depression propagation, and shortened the duration of depolarization. Blocking the glutamate membrane transport increased the oscillatory frequency. The selective metabolic poisoning of astrocytes led to gradual disorganization of prodromal oscillations whose frequency first increased and then decreased. Also, the amplitude of the population spikes within the burst diminished as individual cells fired fewer action potentials, although still phase-locked with population spikes. The effects of glial metabolic impairment were observed within the period when neuron electrical properties were still normal, and were blocked by glutamate receptor antagonists. These data suggest that glutamate released from glial cells and possibly also from neurons has a role in the generation of oscillations and neuron firing synchronization that precede the spreading depression-related depolarization, but additional mechanisms are required to fully explain the onset and propagation of spreading depression.
Dendritic voltage-dependent currents and inhibition modulate the information flow between synaptic and decision areas. Subthreshold and spike currents are sequentially recruited by synaptic ...potentials in the apical shaft of pyramidal cells, which may also decide cell output. We studied the global role of proximal apical recruited currents on cell output in vitro and in the anesthetized rat after local blockade of Na+ currents in the axon initial segment (AIS) or the proximal apical shaft and their modulation by inhibition. Microejection of TTX, field potentials, and intrasomatic and intradendritic recordings were employed. Dendritic population spikes (PSs) were much smaller in vitro, but the gross relations between synaptic and active currents are similar to in vivo. Activation of Schaffer collaterals triggered PSs and action potentials (APs) in the apical shaft that fully propagated to the axon. However, the specific blockade of proximal Na+ currents avoided cell firing, although antidromic PSs and APs readily invaded somata. The somatic depolarization of subthreshold excitatory postsynaptic potentials (EPSPs) also decreased to about 50%. These results were not due to decreased excitatory input by TTX. However, when GABA(A) inhibition was locally removed, Schaffer synaptic currents skipped the proximal dendrite and fired somatic PSs, although initiated at the AIS. It is concluded that apical currents recruited en passant by Schaffer synaptic potentials in the apical shaft constitute a necessary amplifier for this input to cause output decision. Local inhibition decides when and where an AP will initiate, constituting an efficient mechanism to discriminate and weight different inputs.
1. The events leading to the Schaffer collateral-induced discharge of CA1 pyramidal neurons were investigated in the hippocampus of anesthetized rats by current source-density (CSD) analysis. 2. The ...earliest evoked currents detected shortly after a stimulus were a sink in the zone where synapses are known to be located (300-350 microns ventral to the somatic layer) flanked by two smaller sources in the distal portion of the apical dendrites and in the somatic layer. This synaptic sink (SyS) extended over 75-100 microns; it lasted for 15-20 ms, and it reached its maximum amplitude some milliseconds after the population spike (PS) and remained in the same location. Stimuli submaximal and supramaximal for evoking a PS yielded the same pattern of current distribution for the SyS. Presynaptic fiber volleys were not detected in these recordings. 3. During the rising phase of the SyS a second sink appeared in a more proximal portion of the apical dendrites. This late dendritic sink (LS) extended over 50-75 microns and was centered 100-150 microns ventral to the somatic layer. This proximal dendritic sink was of amplitude comparable with the SyS; it outlasted the latter and was not necessarily followed by a somatic PS. The LS was extinguished with the appearance of a PS, whereas the SyS persisted regardless of the presence of a PS. 4. After maximal stimuli the LS grew until it exceeded a threshold amplitude, and then, it started to move somatopetally as a continuously propagating sink (PrS). The average speed of propagation was approximately 0.2 m/s. In 0.5-0.7 ms the PrS reached the cell-body layer displacing the passive source that moved into the basal dendrites. The PrS then became the intensive sink corresponding to the main (negative) phase of the somatic PS. This was followed by the development of an active source in the soma layer, probably corresponding to the repolarization phase of the PS. 5. From these observations it appears that the LS and PrS are active dendritic responses. It may be inferred that, shortly after the synaptic currents enter the dendrites, depolarization of adjacent membranes causes the opening of low-threshold, voltage-dependent, slowly inactivating channels that generate the LS. If the depolarization resulting from the LS current is intense enough, another population of channels open that are also voltage-dependent but of higher threshold and faster inactivation.
Glutamic acid is an important excitatory neurotransmitter in the mammalian CNS. It has been established that synaptic transmission is mediated mostly by the ionotropic glutamate receptors AMPA and ...NMDA, with fast and slow kinetics, respectively. The recent demonstration in hippocampal neurones of a class of glutamate receptors that are activated by kainate and not by AMPA (that is, kainate-selective receptors) opens the possibility that receptors, others than those of the AMPA type, might also be involved in fast neurotransmission. The lack of specific pharmacological tools to dissect out AMPA from kainate receptors has hampered the functional study of kainate receptors. However, the recent finding that a 2,3-benzodiazepine (GYKI53655) behaves as a selective antagonist of AMPA receptors allows us to address the question of the role of rapidly inactivating kainate receptors in synaptic transmission.
The supporting role of glial cells in maintaining neurons and in ion homeostasis has been studied in situ by perfusing the gliotoxin fluorocitrate (FC) through a microdialysis fiber in the CA1 area ...of urethane-anesthetized rats. Extracellular direct current potential, extracellular potassium concentration (K+o) and amino acid levels, extracellular pH (pHo), and evoked field activity were studied. Histology verified the swelling of glial cells after 4 hr of FC treatment. Massive neuron damage was evident after 8 hr. FC dialysis caused the rapid decrease of glutamine, pHo became progressively more acid, and K+o moderately elevated. Orthodromic transmission was variably blocked within 30 min to 4 hr. After 4 hr, spreading depression (SD) waves that originated from the neocortex invaded hippocampal CA1, K+o increased to higher levels, pHo became very acid, and there were steep increases in taurine, glutamate, and GABA levels. Simultaneously, the antidromic population spike (a-PS) became depressed and eventually disappeared. When a shorter dialysis probe that spared cortex was used to sample CA1, no SD was seen, a-PS was not abolished, and ion homeostasis was altered less markedly. Repeated SD provoked in hippocampus in the absence of FC caused only mild depression of a-PS. Dialysis of high-K+ solution in healthy neocortex or hippocampus caused only slight elevation of K+o at distances of 200-400 microns from the dialysis membrane. After treatment with FC, similar high-K+ dialysis raised K+o much more. We conclude the following: (1) recurrent SD waves injure neurons if and only if glial function has failed; (2) neurons can regulate K+o, albeit imperfectly; (3) glia is required for the normal fine tuning of K+o and particularly for the recovery of pathologically elevated K+o; and (4) glia are required for the regulation of pHo. The similarities between glial poisoning by FC and the reported changes in the penumbra of ischemic infarcts suggest that the extension of neuron loss into the penumbral region might depend on failure of glial protection.
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