Neurons In Field CA1 Research Articles

Involvement of Pannexin-1 in the Mechanism of Deprivation Potentiation of Population Spikes of Neurons in Rat Hippocampal Field CA1

Testing of the connections of Schaffer collaterals with field CA1 neurons in living slices of rat hippocampus was used to study the mechanism of deprivation potentiation (DeP) of population spikes developing as a result of a 60-min pause in stimulation (deprivation). Previous studies have shown that DeP has the property of input specificity and the ability to persist for prolonged periods; it consists of two components with independent mechanisms of induction: an initial short-term “peak” of presynaptic origin and a long-lasting “plateau” with a Ca2+-dependent postsynaptic mechanism mediated by P2 purine receptors. Studies of the properties of the input specificity of DeP were run using stimulation of two different populations of Schaffer collaterals and recording of pop-spikes in the general population of neurons in field CA1. These experiments showed that changes in synaptic efficiency in the deprived input depend only on a presynaptic mechanism responsible for the development of the short-term component of DeP. Studies of the postsynaptic mechanism of induction of the long-term component of DeP demonstrated that the function of the adenosine triphosphate (ATP) source required for activation of P2 purine receptors is mediated by pannexin-1, which forms ATP-conducting channels on the postsynapse. A working model of the cyclic mechanism of induction of the long-term component of DeP is presented.

Formation and Retrieval of Associative Memory for a Complex Signal in Mice: Specific Involvement of Hippocampal Field CA1 Neurons

Most important events in the environment are complex in sensory terms. Integration of multimodal information relating to such complex signals by the brain is regarded by many investigators as a biological function of very early forms of consciousness. We report here use of a model of associative learning in mice in which electrocutaneous stimulation was combined with a complex conditioned signal consisting of simultaneous presentation of light and sound stimuli. These studies showed that mice successfully learned and formed long-term memory in a task consisting of conditioned reflex freezing in response to a complex conditioned stimulus. Immunohistochemical mapping of the neuronal expression of the c-fos gene in the hippocampus and associative retrosplenial cortex was performed to study the neural substrates of this memory. We found that only hippocampal field CA1 and not other areas of the hippocampus or the retrosplenial cortex underwent activation on retrieval of the associative memory for the complex conditioned signal, while its components produced no hippocampal activation. In addition, field CA1 showed a strong positive correlation between the level of freezing on retrieval of the memory and the number of neurons activated by retrieval of the memory. Thus, it can be suggested that hippocampal field CA1 is specifically involved in forming and retrieving associative memory for complex signals in the natural environment.

Morphological Changes and Characteristics of the Expression of Serine Racemase in the Hippocampus of Rats Exposed to Multiple Negative Gravitational Overloads

Objectives. To study the morphometric parameters and levels of expression of serine racemase in the hippocampus of rats repeatedly exposed to gravitational overloads in the caudocranial direction (–9 Gz). Materials and methods. Rat aged 12 months were subjected to 9-g forces for 5 min twice a day with 12-h intervals for 28 days (group 2, gravity group, n = 10); controls were not exposed to hypergravitation (group 1, controls, n = 10). Morphometric mean areas of the nuclei and perikarya of pyramidal cells were determined, along with the specific number of neurons with signs of damage, the relative areas of pyramidal neuron and neuroglial cell perikarya. Immunohistochemical studies addressed the level of expression of serine racemase in all zones of the hippocampus. Results. Signs of impaired blood supply in the microcirculatory component were seen in the form of spongiosis, thickening of capillary walls, and diapedesis of erythrocytes. All hippocampal zones in experimental rats showed sharp increases in the numbers of pyramidal neurons with signs of damage. Atrophic changes consisting of decreases in the mean and relative areas of neuron perikarya were identified. Immunohistochemical studies showed increased expression of serine racemase in field CA1, along with translocation of immunoreactive material into the processes of pyramidal neurons. Fields CA3 and CA4 showed a tendency to decreases in serine racemase content. Conclusions. Prolonged gravitational exposure in the caudocranial direction led to the appearance of signs of severe damage in all fields of the rat hippocampus, combined with increased expression and accumulation of serine racemase in the dendrites of pyramidal neurons in field CA1, which may be evidence that excitotoxicity is involved in the mechanisms of secondary damage in this zone.

Interhemisphere Asymmetry in the Structure of the Hippocampus in Men

Objectives. To study the cytoarchitectonic structure of the hippocampus in men in the left and right hemispheres. Materials and methods. Continuous series of total frontal sections of the cerebral hemispheres were studied in the brains of four men aged 19–30 years. Specimens were stained with cresyl violet by the Nissl method and analyzed using image analysis software. Results. The characteristics of the field profile of pyramidal neurons in field CA1 were determined, along with the distribution density of neurons and glial cells in the left and right hemispheres of male brains. The dimensions of the field profile of pyramidal neurons in hippocampal field CA1 in the right hemisphere were larger and their distribution density was lower as compared with the left hemisphere. Field CA1 in the right hemisphere of men’s brains had greater density of total glial cells and a higher ratio of satellite glial cells to the total number of glial cells as compared with the left hemisphere. Conclusions. The gender-related characteristics of the structural organization of the hippocampus in men evidently correlates with the properties of memory in men.

Morphological Changes and Characteristics of the Expression of Serine Racemase in the Rat Hippocampus in Combined Stress

Structural changes in the ventral segment of the hippocampus and the characteristics of the expression of serine racemase were studied in rats aged 12 and 24 months exposed to combined stress. Four groups of 10 animals were formed: group 1 were control rats aged 12 months; group 2 were control rats aged 24 months; group 3 were 12-month rats subjected to 30-min stress daily for seven days; group 4 were 24-month rats subjected to stress. Stress was applied in a special apparatus consisting of six isolated sectors of identical volume and allowing the combination of different types of stress to be applied (pulsing light, loud noise, vibration). The ventral segment of the hippocampus of experimental animals showed an increase in the specific number of shrunken hyperchromic neurons in field CA3 and decreases in neuron density, accompanied by pericellular edema, cytoplasmic vacuolization, and decreased serine racemase expression in the neuropil of the radial layer of fields CA1 and CA3. Stressed animals showed changes in neurons in the pyramidal layer of field CA3, which were combined with a decrease in the level of expression of serine racemase in dendrites in the radial layer of fields CA1 and CA3, which is taken as a sign of impairment to NMDA receptor-mediated transmission of nerve spikes in the hippocampus.

Structural reorganization of neurocytes of CA1 field of hippocampus in dynamic after experimental thermal trauma and application of lyophilized xenograft

The aim of the research was to establish the peculiarities of CA1 field of hippocampus neurocytes structural reorganization of animals in dynamics after experimental thermal injury and use of lyophilized xenograft.Materials and Methods. In the experiment on 35 mature white male rats microscopic, electronmicroscopic and morphometric study of animals’ hippocampus were made after severe thermal injury in terms of early necrectomy of affected area and closure by lyophilized xenograft. Experimental animals of the third experimental group were decapitated on the 7th, 14th and 21st days of experiment.Sections of the brain tissue have been taken from the hippocampus area for histological studies, fixed in 96o alcohol and 10 % neutral formalin and embedded in the paraffin blocks. Obtained on microtome sections were stained with hematoxylin and eosin and toluidine blue with Nissl method. Ultrathin sections were contrasted by uranyl acetate and lead citrate according to Reynolds method and were studied in the electron microscope PEM-125K. Morphometric study was performed using system of visual analysis of histological specimens.Results. It has been established that on the 7th day of the experiment with the usage of corrective factor the number of destroyed neurons in CA1 field of hippocampus was less, but still not significant compared to burned untreated animals. Neurocytes were in state of peripheral or segmental tigrolysis, and there was an increase in the square of hypochromic cells nuclei, some of them contained large nucleoli.Microscopically and electronmicroscopically on the 14th and especially on the 21st days of the experiment, there was found a significant improvement of histological condition of neurocytes, numerical density of the nerve cells in the CA1 field of hippocampus was significantly 1.25 times higher, and the number of normochromic cells 5.52 times more than in the animals with burns.Conclusions. Thus, the application of lyophilized xenograft after early necrectomy of burned skin earlier (the 7th day of experiment) after burn injury prevents deep changes of hippocampus CA1 field neurocytes, reduces the number of damaged cells.The positive impact of lyophilized xenograft application is the most expressive at the later stages after burn (the 14th and the 21st days of the experiment). Histologically there is a relative normalization of microscopic and ultrastructural composition of neurocytes, improvement in indices of density and proportion of nerve cells different types in the CA1 field of the hippocampus.

Latency of Repeated Responses to Single-Pulse Stimulation of Schaffer Collaterals Recorded in Hippocampal Field CA1 in Rats during Sleep

It has been suggested that hippocampal neurons activated via the neocortex by a novel stimulus during waking and storing memories of the stimulus must undergo repeated activation during sleep in order for the neocortex to form the corresponding permanent memory trace. This impelled us to study the possibility that reverberation of excitation occurs in neural networks connecting the hippocampus and the entorhinal cortex. Double and triple responses to single-pulse stimuli applied to Schaffer collaterals with previously potentiated synapses were recorded in the proximal part of field CA1 in the dorsal hippocampus in sleeping rats. Analysis of the latent periods of repeated responses suggested that the wave of excitation initiated in hippocampal field CA1, returning directly from the entorhinal cortex to the recording zone in field CA1 via fibers of the perforant pathway and also via field CA2, may again elicit discharges in field CA1 neurons, though not those which had been activated initially on stimulation of Schaffer collaterals. Thus, the neuronal circuits connecting the hippocampus and entorhinal cortex do not mediate reactivation of “trained” neurons during subsequent sleep periods.

Changes in the Expression of Bcl-2 Protein in Neurons in the Hippocampal Fields after Use of Ischemic Postconditioning of the Brain

The expression of Bcl-2 protein in pyramidal neurons in hippocampal fields CA1, CA2, CA3, and CA4 was studied in Mongolian gerbils (Meriones unguiculatus) in the early (two days) and late (seven days) reperfusion periods after 7-min ischemia of the forebrain, after use of ischemic postconditioning (IPostC) and in sham-operated animals (n = 60). The highest level of Bcl-2 expression in the latter group was seen in neurons in field CA4 and the lowest in neurons in field CA1 (p < 0.01). Reversible ischemic injury led to an increase in the deficit of morphologically unaltered neurons in the hippocampus at the later period of reperfusion and a significant decrease in neuronal Bcl-2 expression at the early reperfusion period, this decrease being significantly smaller in the late reperfusion period. IPostC consisting of three episodes of reperfusion-ischemia (15/15 sec) promoted a significant increase in the number of morphologically unaltered neurons in fields CA1 and CA3 in the early reperfusion period. An increase in the level of Bcl-2 expression was seen in the cytoplasm of morphologically unaltered neurons in all hippocampal fields. In the late reperfusion period after IPostC, the number of unaltered neurons was increased in fields CA1, CA3, and CA4 (p < 0.05); only hippocampal field CA1 neurons showed a significant increase in Bcl-2 expression (by 12.7%, p < 0.01). These results lead to the conclusion that the cytoporotective effect of IPostC for hippocampal field CA1 is mediated by a mechanism leading to an increase in Bcl-2 expression, i.e., via blockade of apoptosis.

Dynamics of changes in the structure of the hippocampus in a myocardial dysfunction.

Background. It is well known that cardiovascular disease lead to the disruption of integrative functions of the brain, provided by hippocampus. Objective. To investigate the dynamics of spatial and quantitative pathological changes of hippocampal neurons under experimental myocardial dysfunction. Methods. Brain of male Wistar rats were divided into experimental (injected with doxorubicin) and control (injected with 0.9% solution of NaCl) groups. Results. Brains of rats of the experimental group underwent significant pathological changes in glia, as well as vascular system. In the superficial layers cerebral cortex looked as porous, spongy mesh structure. The vessels of the brain at all levels have demonstrated the phenomena of circulatory disorders in the form of congestion and stasis. Analysis of cytoarchitectonics of the hippocampus of rats in the control group showed significant differences in the quantitative composition of neurons in field CA1, CA3 and dentate gyrus (50,3 ± 1,1; 66,6 ± 0,6 and 60,4 ± 1,1 in R≤ 0.05, respectively). Conclusion. In conditions of experimental myocardial dysfunction in the hippocampus a gradual loss of individual neurons, and rows of cells was revealed. The most sensitive areas of the hippocampus in those conditions was a field CA3, where the total loss of the cellular composition for a period of follow-up was 45.1%. Except quantitative changes of a qualitative change in the form of loss of order neurons, loosening of rows and a violation of the blood supply to the brain vessels was detected.

The Key Role of Calcium in the Mechanism of Deprivation Potentiation of Population Responses of Neurons in Hippocampal Field CA1

In vitro studies on living rat hippocampus slices addressed the role of Ca2+ in the mechanism of deprivation potentiation of population responses (pop spikes) of neurons in field CA1 induced by prolonged (60 min) interruption of low-frequency test stimulation of Schaffer collaterals. Two phases were seen in deprivation potentiation, with presumptively different origins: an initial short-lived “peak” (about 12 min) and a longer-lasting “plateau” (more than 1 h). The experiments reported here showed that the presence of a penetrating Ca2+ chelator (BAPTA-AM), decreasing the Ca2+ concentration in the solution, and depletion of the intracellular calcium depot (presence of thapsigargin/cyclopiazonic acid in the solution) led to reductions in the transient phase and blockade of the longer-lasting phase of deprivation potentiation. These studies thus demonstrate the key roles of both extracellular and stored intracellular calcium in the mechanism of development of deprivation potentiation.

Morphofunctional Changes in Hippocampal Field CA1 in Mongolian Gerbils Using Ischemic Postconditioning

The aim of the present work was to study the effects of ischemic postconditioning (IPC) on neuron viability in hippocampal field CA1 and cytoplasmic lactate dehydrogenase (LDH) activity in these cells in 30 male Mongolian gerbils (Meriones unguiculatus). Ischemic brain damage was modeled by bilateral occlusion of the common carotid arteries for 7 min. IPC was produced using three episodes of reperfusion-ischemia, 15 sec/15 sec. Morphometric analysis and histoenzymological LDH assay in the cytoplasm of hippocampal field CA1 neurons were performed 48 h after reperfusion, with quantitative cytometric assessment of enzyme activity. The results showed that 7-min ischemia led to a decrease in the number of viable neurons (to 24%) and decreased their LDH activity (from 0.260 ± 0.009 to 0.190 ± 0.006 units). Use of IPC led to a significant increase in the number of viable hippocampal field CA1 neurons (to 52.9%, p < 0.01), which was accompanied by an increase in their LDH activity (to 0.240 ± 0.008 units, p < 0.001).

Excitation Waves Returning to the Hippocampus via the Entorhinal Cortex Can Reactivate Populations of “Trained” Field CA1 Neurons during Deep Sleep

It is suggested that information on new stimuli from the neocortex is transmitted to the hippocampus, where temporal traces persist in the form of mosaics of modified synapses. During sleep, populations of neurons storing these traces are reactivated and return the information required for consolidation of a permanent memory trace to the neocortex. A possible mechanism for the reactivation of “trained” hippocampal neurons during memory consolidation consists of the reverberation of excitation in the neuron circuits linking the hippocampus and entorhinal cortex. Our studies in rats included recording of responses in hippocampal field CA1 to stimulation of Schaffer collaterals with potentiated synapses during waking and sleep. During deep sleep, discharges of field CA1 neurons were followed by waves of excitation which passed through the entorhinal cortex and reached the hippocampus and dentate gyrus via fibers of the perforant path, evoking neuron discharges in the latter. Repeated neuron discharges in field CA1 occurred on interaction of the early excitation wave returning directly via perforant path fibers and the late wave returning via Schaffer collaterals, not via the trisynaptic path via the dentate gyrus and hippocampal field CA3 but probably via field CA2.

Comparison of the Behavior of Rats after Prolonged Immobilization with Structural Changes in the Motor Cortex and Hippocampus

Behavioral and neuron-glial ratios in the motor neocortex and hippocampus after stress induced by discontinuous (7–8 h/day for three weeks) immobilization were compared in Wistar rats (n = 23). Immobilization led to suppression of motor and orientational activity in the open field test and increases in the numbers and durations of episodes of freezing. Morphometric measurements demonstrated significant increases in the density of neurons with hypoxic changes in hippocampal field CA3 and a three-fold increase in this value in the motor zone of the cortex in both hemispheres of experimental animals as compared with controls. No clear glial cell reaction was seen in the motor cortex. Increases in the density of glial elements and the number of multinucleolar neurons in field CA3 provide evidence of compensatory processes occurring in the brain. Hypoxic changes to neurons were functional in nature.

Intracellular Tetanization with Hyperpolarizing Currents Potentiates Synapses Formed by Mossy Fibers on Pyramidal Cells in Hippocampal Field CA3 in Rats

Studies on living rat hippocampal slices using point recording in the whole cell configuration addressed the efficiency of the synaptic responses of pyramidal neurons in field CA3 in conditions of minimal stimulation of mossy fibers. Paired-pulse responses were recorded before and after intracellular tetanizing hyperpolarization of pyramidal neurons. In these conditions, potentiation of excitatory synaptic transmission lasting at least 20 min was seen. This phenomenon, termed hyperpolarizing tetanization-induced long-term potentiation, could arise without simultaneous mossy fiber stimulation and showed signs of having a presynaptic origin. Administration of a Ca2+ chelator into pyramidal neurons completely suppressed this potentiation. The results obtained from these experiments suggest that induction of long-term potentiation evoked by hyperpolarizing tetanization was postsynaptic, while its expression appeared to be presynaptic. These results provide evidence of the importance of gamma-rhythm hyperpolarizing oscillations in altering the efficiency of synaptic inputs and the role of its network organization in the mechanisms of cellular plasticity.

Acquisition of an Active Avoidance Reaction in Rats and Morphological Changes in the Hippocampus in Pentylenetetrazol Kindling

The relationship between measures of active avoidance behavior and morphological changes in the hippocampus was studied in rats after kindling induced by administration of pentylenetetrazol. Pentylenetetrazol kindling impaired the acquisition of the avoidance reaction and increased the number of intersignal reactions without altering the acquisition of the avoidance reaction in the shuttle box in rats. The numbers of neurons in the hippocampus (fields CA1 and CA3) and dentate fascia decreased, while the numbers of damaged neurons increased. Inverse correlations between seizure severity and the numbers of neurons in hippocampal fields CA1 and CA3 and the dentate fascia were seen in rats subjected to kindling. Rats of this group also showed positive correlations between seizure severity and the numbers of damaged neurons in field CA1 and the dentate fascia. There were no correlations between measures of convulsive activity or the number of cells in hippocampal zones and measures of the acquired avoidance behavior. Control animals showed negative correlations between the numbers of damaged cells in field CA1 and the dentate fascia and the characteristics of avoidance behavior in the first and third training sessions.

Comparative Analysis of Spike Activity of Neurons in Hippocampal Field CA1 and CA3 in Rats of Different Typological Groups on Exposure to Emotional Stimuli

Spike activity from neurons in hippocampal field CA1 and CA3 was compared in rats avoiding ("altruists," group A) and not avoiding ("egoists," group E) the cries of a "victim" partner on testing using the emotional resonance method. Neuron discharge frequencies were compared in animals in the state of starvation, after satiation, and on exposure to emotionally positive and negative intracerebral electrical stimuli. These studies showed that hippocampal field CA1 was dominated by cells increasing their spike frequencies after satisfaction of the food motivation, while in field CA3, conversely, most cells decreased activity after eating. Exposure to emotionally significant stimuli identified interhemisphere differences in the activities of hippocampal neurons which were associated with the typological characteristics of the animals. In rats of group A, neurons in field CA1 were activated only in the left hippocampus, while the activity of cells in the right hippocampus was no different from that in baseline conditions. Field CA3 of rats of this group showed no asymmetry. In rats of group E, conversely, lateralization of activity was seen only in field CA3: discharge frequencies were significantly greater in the left hippocampus.

The Effects of Interleukin-10 on the Development of Epileptiform Activity in the Hippocampus Induced by Transient Hypoxia, Bicuculline, and Electrical Kindling

The comparative effects of the anti-inflammatory cytokine interleukin-10 on the development of epileptiform activity were studied in hippocampal field CA1 neurons in different models of epileptogenesis not accompanied by visible morphological lesions in brain cells: 1) a model of hypoxic kindling in rat hippocampal slices; 2) a disinhibitory model of epileptogenesis in rat hippocampal slices using the GABAA receptor blocker bicuculline; and 3) a partial electrical kindling model in intact rats. Interleukin-10 (1 ng/ml) blocked the development of post-hypoxic hyperexcitability of field CA1 pyramidal neurons in hippocampal slices, decreasing the effectiveness of hypoxia in suppressing neuron activity during the hypoxic episode. Interleukin-10 had no effect on the initiation of epileptiform activity in pyramidal neurons induced by the proconvulsant bicuculline. Single intrahippocampal injections of interleukin-10 at a dose of 1 ng in 5 microl suppressed the development of focal convulsions ("ictal" discharges) at the stimulation site in partial kindling in freely moving animals for several hours after administration. However, this cytokine had no effect on the duration of the "interictal" component of focal afterdischarges or on the severity of behavioral seizures. These results show that the anti-inflammatory cytokine interleukin-10, at the concentrations used here, has not only antihypoxic activity, but also a protective effect in relation to the initiation of the "ictal," but not the "interictal" component of epileptiform activity in hippocampal neurons.

Caspase-3 activity in hippocampal slices reflects changes in synaptic plasticity

Electrophysiological measures of the functional activity of neurons in field CA1 in conditions of paired-pulse stimulation of Schäffer collaterals were performed in relation to the involvement of caspase-3 in mediating neuroplasticity; the relationship between functional activity and caspase-3 activity in hippocampal slices from Wistar rats was addressed. Enzyme activity was assessed in each individual slice at the end of the electrophysiological experiment. The results obtained here showed that the highest level of enzyme activity was seen when the efficiency of interneuronal interactions decreased. Nerve cell excitability showed no changes; interactions increasing synaptic efficiency, particularly in paired-pulse stimulation, produced normal response amplitudes. Further deterioration of the functional state of slices and impairments in spike generation were accompanied by increases in caspase-3 activity to the normal level. Increases in the activity of another proteinase, cathepsin B, were generally seen in any deviation from normal functioning, though there was no correlation with any of the electrophysiological parameters. It is suggested that high caspase-3 activity in slices is linked with neuroplastic processes in synapses and has no direct relationship to nerve cell apoptosis.

Localization of the transcription factor, sterol regulatory element binding protein-2 (SREBP-2) in the normal rat brain and changes after kainate-induced excitotoxic injury

The transcription factor sterol regulatory element binding protein-2 (SREBP-2) has a key role in regulating cholesterol biosynthesis. In view of ongoing cholesterol biosynthesis in the brain, the present study was carried out to examine the distribution of the SREBP-2 in the normal rat brain, and possible changes after kainate-induced excitotoxicity. Western immunoblot analysis of SREBP-2 showed a dense band at 70 kDa corresponding to the transcriptionally active form of SREBP-2 in homogenates from the rat hippocampus, cortex and striatum. SREBP-2 immunolabeled sections showed dense labeling of pyramidal neurons in field CA1 and CA3 of the hippocampus, moderately dense labeling of pyramidal and non-pyramidal neurons of the cerebral neocortex, and moderate labeling of putative medium spiny neurons in the caudate nucleus and putamen. Label was observed in the somatic cytoplasm, nucleus and apical dendrites of pyramidal neurons, and dendritic shafts and spines in the neuropil in the hippocampus. The expression of SREBP-2 was also elucidated after excitotoxic neuronal injury induced by intracerebroventricular injections of kainate. Real time RT-PCR analyses showed that mRNA level of SREBP-2 was significantly reduced at both 1 day and 2 weeks post-kainate injection. Immunohistochemical analyses showed significantly reduced SREBP-2 immunoreactivity in the kainate-induced lesions, in support of the real time RT-PCR results. Taken together, the above results show that there is high level of SREBP-2 expression in the normal hippocampus, and that neuronal injury results in a significant reduction of SREBP-2 expression in the damaged areas.

The protective action of tumor necrosis factor α (TNF-α) on the hypersensitivity of pyramidal neurons in field CA1 in rat hippocampal slices induced by episodes of hypoxia

The protective action of tumor necrosis factor α (TNF-α) on the hypersensitivity of pyramidal neurons in field CA1 in rat hippocampal slices induced by episodes of hypoxia