CA1 Field Research Articles

Three-dimensional analysis of synaptic organization in the hippocampal CA1field in Alzheimer's disease.

Alzheimer’s disease is the most common form of dementia, characterized by a persistent and progressive impairment of cognitive functions. Alzheimer’s disease is typically associated with extracellular deposits of amyloid-β peptide and accumulation of abnormally phosphorylated tau protein inside neurons (amyloid-β and neurofibrillary pathologies). It has been proposed that these pathologies cause neuronal degeneration and synaptic alterations, which are thought to constitute the major neurobiological basis of cognitive dysfunction in Alzheimer’s disease. The hippocampal formation is especially vulnerable in the early stages of Alzheimer’s disease. However, the vast majority of electron microscopy studies have been performed in animal models. In the present study, we performed an extensive 3D study of the neuropil to investigate the synaptic organization in the stratum pyramidale and radiatum in the CA1 field of Alzheimer’s disease cases with different stages of the disease, using focused ion beam/scanning electron microscopy (FIB/SEM). In cases with early stages of Alzheimer’s disease, the synapse morphology looks normal and we observed no significant differences between control and Alzheimer’s disease cases regarding the synaptic density, the ratio of excitatory and inhibitory synapses, or the spatial distribution of synapses. However, differences in the distribution of postsynaptic targets and synaptic shapes were found. Furthermore, a lower proportion of larger excitatory synapses in both strata were found in Alzheimer’s disease cases. Individuals in late stages of the disease suffered the most severe synaptic alterations, including a decrease in synaptic density and morphological alterations of the remaining synapses. Since Alzheimer’s disease cases show cortical atrophy, our data indicate a reduction in the total number (but not the density) of synapses at early stages of the disease, with this reduction being much more accentuated in subjects with late stages of Alzheimer’s disease. The observed synaptic alterations may represent a structural basis for the progressive learning and memory dysfunctions seen in Alzheimer’s disease cases.

Scaling of Network Excitability and Inhibition may Contribute to the Septotemporal Differentiation of Sharp Waves–Ripples in Rat Hippocampus In Vitro

The functional organization of the hippocampus along its longitudinal (septotemporal or dorsoventral) axis is conspicuously heterogeneous. This functional diversification includes the activity of sharp wave and ripples (SPW–Rs), a complex intrinsic network pattern involved in memory consolidation. In this study, using transverse slices from the ventral and the dorsal rat hippocampus and recordings of CA1 field potentials we studied the development of SPW–Rs and possible changes in local network excitability and inhibition, during in vitro maintenance of the hippocampal tissue. We found that SPW–Rs develop gradually in terms of magnitude and rate of occurrence in the ventral hippocampus. On the contrary, neither the magnitude nor the rate of occurrence significantly changed in dorsal hippocampal slices during their in vitro maintenance. The development of SPW–Rs was accompanied by an increase in local network excitability more in the ventral than in the dorsal hippocampus, and an increase in local network inhibition in the ventral hippocampus only. Furthermore, the amplitude of SPWs positively correlated with the level of maximum excitation of the local neuronal network in both segments of the hippocampus, and the local network excitability and inhibition in the ventral but not the dorsal hippocampus. Blockade of α5 subunit-containing GABAA receptor by L-655,708 significantly reduced the rate of occurrence of SPWs and enhanced the probability of their generation in the form of clusters in the ventral hippocampus without affecting activity in the dorsal hippocampus. The present evidence suggests that a dynamic upregulation of excitation and inhibition in the local neuronal network may significantly contribute to the generation of SPW–Rs, particularly in the ventral hippocampus.

Chronic lithium exposure attenuates ketamine-induced mania-like behavior and c-Fos expression in the forebrain of mice

Ketamine, a dissociative anaesthetic, has been used in the treatment of major depressive disorder (MDD) as a rapid acting antidepressant drug. Recent studies have shown that ketamine may increase the potential risk of treatment-induced mania in MDD patients. Lithium is a well-known mood stabilizer and has been widely used for the treatment of mania. It is not fully understood which forebrain regions are involved in ketamine- and lithium-induced expression of c-Fos. Therefore, our aim was to investigate the effect of chronic lithium treatment on mania-like behavior and c-Fos expression in the mouse forebrain activated by a single administration of ketamine. In the open field test, our results showed that ketamine significantly increased the total distance and total cumulative duration of movement in mice, while chronic lithium could attenuate these effects of ketamine. In addition, acute ketamine induced higher c-Fos expression in the lateral septal nucleus, hypothalamus, amygdala, and hippocampus of mice in the treatment group compared to those in the control group. However, chronic lithium inhibited the significant increase in c-Fos-immunoreactive neurons following acute ketamine administration in the dentate gyrus of the hippocampus, field CA1 of the hippocampus, dorsal subiculum, ventral subiculum, ventral subiculum, central amygdaloid nucleus and basolateral amygdaloid nucleus. In summary, our research shows that pretreatment with lithium moderates the effects of acute ketamine administration on mania-like behavior and c-Fos expression in the forebrain. These findings could be helpful in better understanding the episodes of mania related to ketamine treatment for MDD and bipolar disorder.

Neonatal proinflammatory challenge evokes a microglial response and affects the ratio between subtypes of GABAergic interneurons in the hippocampus of juvenile rats: sex-dependent and sex-independent effects.

Neonatal proinflammatory challenge (NPC) may contribute to the development of psychiatric disorders in adults. A double exposure of neonatal rats to lipopolysaccharide, a component of cellular wall of gram-negative bacteria, on postnatal days 3 and 5 provokes the development of depressive- and anxiety-like behaviors. NPC impairs neuroplasticity and cognition in adult animals, significant modifications of neuroplasticity being evident even in adolescence. We studied effects of NPC on microglia and GABAergic neuronal population of the dorsal hippocampus in juvenile male and female rats using immunofluorescent histochemistry. The expression of glutamic acid decarboxylase-67 (GAD67) and calcium-binding proteins calretinin, calbindin, and parvalbumin were used as quantitative markers of GABAergic interneurons and their specific subpopulations, respectively. NPC induced changes of microglial morphology indicating inflammatory activation mostly expressed in CA3 field; the effect was similar in males and females. The number of GAD67 expressing neurons was similar in the dorsal hippocampus of females and males independently on the NPC. The portion of calbindin-immunoreactive GAD67-positive neurons significantly increased while the portion of calretinin-immunoreactive GAD67-positive neurons significantly decreased in the CA1 field of rats exposed to NPC independently on their sex. NPC did not affect the parvalbumin-positive subpopulation of GABAergic neurons in the hippocampus of rats of either sex. These data suggest that NPC-induced modification of GABAergic neuronal population composition under the proinflammatory conditions is involved in the maintenance of excitation/inhibition homeostatic balance in the hippocampus.

Low dose of carbon ion irradiation induces early delayed cognitive impairments in mice.

People often encounter various sources of ionizing radiation, both in modern medicine and under various environmental conditions, such as space travel, nuclear power plants or in conditions of man-made disasters that may lead to long-term cognitive impairment. Whilst the effect of exposure to low and high doses of gamma and X-radiation on the central nervous system (CNS) has been well investigated, the consequences of protons and heavy ions irradiation are quite different and poorly understood. As for the assessment of long-term effects of carbon ions on cognitive abilities and neurodegeneration, very few data appeared in the literature. The main object of the research is to investigate the effects of accelerated carbon ions on the cognitive function. Experiments were performed on male SHK mice at an age of two months. Mice were irradiated with a dose of 0.7Gy of accelerated carbon ions with an energy of 450meV/n in spread-out Bragg peak (SOBP) on a U-70 particle accelerator (Protvino, Russia). Two months after the irradiation, mice were tested for total activity, spatial learning, as well as long- and short-term hippocampus-dependent memory. One month after the evaluation of cognitive activity, histological analysis of dorsal hippocampus was carried out to assess its morphological state and to reveal late neuronal degeneration. It was found that the mice irradiated with accelerated carbon ions develop an altered behavioral pattern characterized by anxiety and a shortage in hippocampal-dependent memory retention, but not in episodic memory. Nissl staining revealed a reduction in the number of cells in the dorsal hippocampus of irradiated mice, with the most pronounced reduction in cell density observed in the dentate gyrus (DG) hilus. Also, the length of the CA3 field of the dorsal hippocampus was significantly reduced, and the number of cells in it was moderately decreased. Experiments with the use of Fluoro-Jade B (FJB) staining revealed no FJB-positive regions in the dorsal hippocampus of irradiated and control animals 3 months after the irradiation. Thus, no morbid cells were detected in irradiated and control groups. The results obtained indicate that total irradiation with a low dose of carbon ions can produce a cognitive deficit in adult mice without evidence of neurodegenerative pathologic changes.

Prenatal Introduction of Dexamethasone Causes Disruption of Glucocorticoid Feedback Associated with a Change in the Amount of Corticosteroid Receptors in Extrahypothalamic Brain Structures of Adult Rats

As are other artificial glucocorticoids, dexamethasone is widely used in everyday obstetric practice. The threat of termination of pregnancy is an indication for the use of glucocorticoids. However, there is evidence that the introduction of glucocorticoids during pregnancy can lead to impaired brain development and behavior of offspring that can be caused by a change in the activity of the hypothalamic–pituitary–adrenocortical system (HPAS) due to past effects. The aim of the present work was to study the effect of the introduction of synthetic hormone dexamethasone at the 14th–16th (DM14–16) and 17th–19th (DM17–19) days of prenatal ontogenesis on the stress reactivity of the HPAS in adult 3-month-old rats, as well as on the level of gluco- (GR) and mineralocorticoid (MR) receptors in the most vulnerable areas of the brain (hippocampus and neocortex). Significant intergroup differences in the dynamics of a change in the level of corticosterone in the blood of adult rats in response to a weak stress effect (rapid stress reactivity test) were detected. Using the immunohistochemical method, changes in the expression (the amount of immunopositive cells) of GR and MR were studied in CA1 field and dentate gyrus (DG) of the hippocampus, as well as in second and fifth neocortex layers in adult male rats. In animals of the DM14–16 group, a significant decrease in the number of cells intensively stained using antibodies to GR in CA1 field, hippocampus DG, and neocortex layer V (to 24.5, 32.4, and 5.5%, respectively) as compared with the control were observed. The introduction of dexamethasone at the 17th–19th day of gestation also led to a decrease in the amount of intensively stained GR-immunopositive cells in the hippocampus CA1 field and neocortex layer V (to 31.9 and 35.7%, respectively), but to a lesser degree than in animals from the group DM14–16. A trend toward a decrease in the number of GR-positive cells is accompanied by an increase in immunoreactivity of the cells to MR. Thus, the introduction of dexamethasone at different periods of prenatal ontogenesis modifies the work of gluco- and mineralocorticoid receptors; however, the degree, localization, and direction of these changes differ depending on the time of the effect.

Cross-Talk of the Glutamate and Leptin Receptor Pathways

The hormone leptin is produced in adipocytes of white adipose tissue and crosses the blood-brain barrier. Leptin receptors are present in the brain regions that are involved in higher cognitive functions. In particular, leptin directly influences the glutamate receptor trafficking in CA3 → CA1 synapses to increase the phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) level, which is controlled by phosphoinositide 3-kinase (PI3K). It is well recognized that glutamate receptor trafficking involves at least some components of the insulin signaling cascade. However, the effects of leptin and insulin hormones differ at the cell and behavioral levels and often oppose each other. The domain organization of synaptic proteins was analyzed for CA1 field neurons. A molecular mechanism of leptin effects in the hippocampus was assumed to involve a cross-talk of the molecular pathways of the leptin receptors and NMDA-type glutamate receptors. Non-receptor protein kinases of the Src subfamily and, in particular, kinase Fyn are part of glutamate receptor macrocomplexes and are involved in regulating the efficiency of synaptic transmission. Fyn was assumed to utilize its SH2 domain to interact with leptin receptors directly or through other proteins and contribute to leptin signaling through the PI3K signaling pathway. The hypothesis explains experimental findings and sheds further light on the fine tuning of hormone-dependent modulation of hippocampal synaptic processes.

ДЕЙСТВИЕ ДВУХ НОВЫХ ТРИПЕПТИДОВ НА УРОВНЕ ГИППОКАМПА У КРЫС

Experiments on hippocampal slices in rats showed that, in contrast to piracetam (100 µM to 10 mM), two new tripeptides, i.e. H-asparagil-glutamyl-proline-OH and H-asparagil-glutamyl-arginin-OH (concentrations 500 µM, 1 and 2.5 mM respectively), strengthened the orthodrome population responses in field СА1. In all three concentrations the effect of H-asparagil-glutamyl-proline-OH was stronger than of the other tripeptide. That is both tripeptides facilitate synaptic transmission within the Schaffer collaterals-pyramidal neurons system in hippocumpal field CA1. Similarly to piracetam (100 µM, within 15 minutes) these tripeptides supported short-term potentiation of the hippocampal synaptic transfer by N-methyl-D-aspartate (NMDA). Specific NMDA-receptors antagonist D-AP5 cancelled the NMDA effect fully which infers that the observed effect is achieved through activation of NMDA-receptors. Effect from H-asparagil-glutamyl-proline-OH excelled equally piracetam and H-asparagil-glutamyl-arginin-OH; H-asparagil-glutamyl-arginin-OH excelled piracetam only.

Virus Injection to the Pituitary via Transsphenoidal Approach and the Innervation of Anterior and Posterior Pituitary of Rat.

The theory holds that the anterior pituitary in mammals receives humoral regulation. Previous studies have reported that the pars distalis of the anterior pituitary of several mammalian species contains substance P-, calcitonin gene-related peptide (CGRP)-, and galanin-like immunoreactive nerve fibers, but the origins of these nerve fibers are unclear. Removal of the pituitary gland, also called hypophysectomy, involves methods that access the pituitary gland via the transauricular or parapharyngeal pathways. However, these methods are not applicable for viral tracer injection to investigate the innervation of the anterior pituitary. The transauricular technique leads to inaccuracies in locating the pituitary gland, while the parapharyngeal approach causes high mortality in animals. Here, we introduce a protocol that accesses the pituitary gland in the rat via the transsphenoidal pathway. This method imitates surgical manipulations such as endotracheal intubation and sphenoid bone drilling, which involve the use of custom-made devices. Using the transsphenoidal pathway greatly improves the survival rate of rats because no additional dissection of blood vessels and nerves is required. Moreover, the pituitary gland can be viewed clearly and directly during the operation, making it possible to accurately inject pseudorabies virus (PRV) 152-expressing enhanced green fluorescent protein (EGFP) into the anterior or posterior pituitary, respectively. After injecting PRV 152 into the anterior pituitary, we found no evidence of direct innervation of the anterior pituitary in the rat brain. However, PRV 152 injection into the posterior pituitary revealed retrograde transneuronal cell bodies in many brain areas, including the CA1 field of the hippocampus, the basolateral amygdaloid nucleus, posterior part (BLP), the arcuate hypothalamic nucleus (Arc), the dorsal portion of the dorsomedial hypothalamic nucleus (DMD), the suprachiasmatic nucleus (SCh), and the subfornical organ (SFO). In the present study, we provide a description of a possible model of hypophysectomy or pituitary injection, and identify brain regions involved in regulating the rat pituitary gland using transneuronal retrograde cell body labeling with PRV.

Integrity of the uncinate fasciculus is associated with emotional pattern separation-related fMRI signals in the hippocampal dentate and CA3

Alterations in white matter integrity have been demonstrated in a number of psychiatric disorders involving emotional disruptions. One such pathway – the uncinate fasciculus – connects the orbitofrontal cortex (OFC) to the medial temporal lobes (MTL) and has been associated with early life adversity, maltreatment, anxiety, and depression. While it is purported to play a role in episodic memory and discrimination, its exact function remains poorly understood. We have previously described the role of the amygdala and dentate (DG)/CA3 fields of the hippocampus in the mnemonic discrimination of emotional experiences (i.e. emotional pattern separation). However, how this computation may be modulated by connectivity with the orbitofrontal cortex remains unknown. Here we asked if the uncinate fasciculus plays a role in influencing MTL subregional activity during emotional pattern separation. By combining diffusion imaging with high-resolution fMRI, we found that reduced integrity of the UF is related to elevated BOLD fMRI activation of the DG/CA3 subregions of the hippocampus during emotional lure discrimination. We additionally report that higher levels of DG/CA3 activity are associated with poorer memory performance, suggesting that greater activation in this network (possibly driven by CA3 recurrent collaterals) is associated with memory errors. Based on this work we suggest that the UF is one pathway that may allow the OFC to exert control on this network and improve discrimination of emotional experiences, although further work is necessary to fully evaluate this possibility. This work provides novel insight into the role of prefrontal interactions with the MTL, particularly in the context of emotional memory.

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.

Rhythmic Activity in the Hippocampus and Entorhinal Cortex is Impaired in a Model of Kainate Neurotoxicity in Rats in Free Behavior

The hippocampus and medial entorhinal cortex (MEC) interact by means of bidirectional connections and play an important role in the processing, memorization, and reproduction of information. Data obtained in healthy animals show that the θ and γ oscillations are critical activities necessary for the interaction of the hippocampus and the MEC in signal processing. At the same time, these structures are among the most vulnerable parts of the brain to hyperactivation leading to excitotoxic damage and neuron death. In the present study toxicity was provoked by systemic administration of kainic acid (KA), inducing the development of status epilepticus. In control rats given physiological saline and rats given injections of KA, local field potentials were recorded simultaneously in hippocampal field CA1 and the MEC during exploratory behavior in an open field. A clearly apparent θ rhythm (4–10 Hz) was observed, along with a slow γ rhythm (25–50 Hz) and a fast γ rhythm (55–100 Hz) in the hippocampus and MEC of animals of both groups. Movement of control animals to the center of the open field was accompanied by an increase in the frequency of the θ rhythm and a decrease in the frequency of the fast γ rhythm in the hippocampus; the MEC showed a decrease in the power of the slow γ rhythm. This was not seen in rats given KA. This group also showed impairment to the phase-amplitude modulation of MEC activity by the hippocampal θ rhythm: changes in this modulation on movement of animals from the peripheral zones to the center of the open field were significantly less marked than in controls. There was also a significant increase in θ coherence between the hippocampus and MEC for all locations of the animal in the open field. Changes in the characteristics of rhythms in hippocampus-entorhinal interactions are potential biomarkers for impairments to the coding of spatial information and its retrieval from memory due to status epilepticus and often leading to the development of a convulsive focus in the temporal structures of the brain.

3D Synaptic Organization of the Rat CA1 and Alterations Induced by Cocaine Self-Administration.

The hippocampus plays a key role in contextual conditioning and has been proposed as an important component of the cocaine addiction brain circuit. To gain knowledge about cocaine-induced alterations in this circuit, we used focused ion beam milling/scanning electron microscopy to reveal and quantify the three-dimensional synaptic organization of the neuropil of the stratum radiatum of the rat CA1, under normal circumstances and after cocaine-self administration (SA). Most synapses are asymmetric (excitatory), macular-shaped, and in contact with dendritic spine heads. After cocaine-SA, the size and the complexity of the shape of both asymmetric and symmetric (inhibitory) synapses increased but no changes were observed in the synaptic density. This work constitutes the first detailed report on the 3D synaptic organization in the stratum radiatum of the CA1 field of cocaine-SA rats. Our data contribute to the elucidation of the normal and altered synaptic organization of the hippocampus, which is crucial for better understanding the neurobiological mechanisms underlying cocaine addiction.

Activation of p38 MAPK participates in the sulbactam-induced cerebral ischemic tolerance mediated by glial glutamate transporter-1 upregulation in rats

Our previous studies have shown that sulbactam can play a neuroprotection role in hippocampal neurons by upregulating the expression and function of glial glutamate transporter-1 (GLT-1) during ischemic insult. Here, using rat global cerebral ischemia model, we studied in vivo the role of p38 mitogen-activated protein kinases (MAPK) in the sulbactam-induced GLT-1 upregulation and neuroprotection against ischemia. The hippocampal CA1 field was selected as observing target. The expressions of phosphorylated-p38 MAPK and GLT-1 were assayed with western blot analysis and immunohistochemistry. The condition of delayed neuronal death (DND) was assayed with neuropathological evaluation under thionin staining. It was shown that administration of sulbactam protected CA1 hippocampal neurons against ischemic insult accompanied with significantly upregulation in the expressions of phosphorylated-p38 MAPK and GLT-1. The time course analysis showed that sulbactam activated p38 MAPK before the GLT-1 upregulation in either normal or global cerebral ischemic rats. Furthermore, inhibiting p38 MAPK activation by SB203580 blocked the GLT-1 upregulation and neuroprotection induced by sulbactam. The above results suggested that p38 MAPK, at least partly, participated in the sulbactam-induced brain tolerance to ischemia mediated by GLT-1 upregulation in rats.

Neuropsychological and neuropathological observations of a long-studied case of memory impairment

We report neuropsychological and neuropathological findings for a patient (A.B.), who developed memory impairment after a cardiac arrest at age 39. A.B. was a clinical psychologist who, although unable to return to work, was an active participant in our neuropsychological studies for 24 y. He exhibited a moderately severe and circumscribed impairment in the formation of long-term, declarative memory (anterograde amnesia), together with temporally graded retrograde amnesia covering ∼5 y prior to the cardiac arrest. More remote memory for both facts and autobiographical events was intact. His neuropathology was extensive and involved the medial temporal lobe, the diencephalon, cerebral cortex, basal ganglia, and cerebellum. In the hippocampal formation, there was substantial cell loss in the CA1 and CA3 fields, the hilus of the dentate gyrus (with sparing of granule cells), and the entorhinal cortex. There was also cell loss in the CA2 field, but some remnants remained. The amygdala demonstrated substantial neuronal loss, particularly in its deep nuclei. In the thalamus, there was damage and atrophy of the anterior nuclear complex, the mediodorsal nucleus, and the pulvinar. There was also loss of cells in the medial and lateral mammillary nuclei in the hypothalamus. We suggest that the neuropathology resulted from two separate factors: the initial cardiac arrest (and respiratory distress) and the recurrent seizures that followed, which led to additional damage characteristic of temporal lobe epilepsy.

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.

The Effect of Allopregnanolone on Enzymatic Activity of the DNA Base Excision Repair Pathway in the Sheep Hippocampus and Amygdala under Natural and Stressful Conditions.

The neurosteroid allopregnanolone (AL) has many beneficial functions in the brain. This study tested the hypothesis that AL administered for three days into the third brain ventricle would affect the enzymatic activity of the DNA base excision repair (BER) pathway in the hippocampal CA1 and CA3 fields and the central amygdala in luteal-phase sheep under both natural and stressful conditions. Acute stressful stimuli, including isolation and partial movement restriction, were used on the last day of infusion. The results showed that stressful stimuli increased N-methylpurine DNA glycosylase (MPG), thymine DNA glycosylase (TDG), 8-oxoguanine glycosylase (OGG1), and AP-endonuclease 1 (APE1) mRNA expression, as well as repair activities for 1,N6-ethenoadenine (εA), 3,N4-ethenocytosine (εC), and 8-oxoguanine (8-oxoG) compared to controls. The stimulated events were lower in stressed and AL-treated sheep compared to sheep that were only stressed (except MPG mRNA expression in the CA1 and amygdala, as well as TDG mRNA expression in the CA1). AL alone reduced mRNA expression of all DNA repair enzymes (except TDG in the amygdala) relative to controls and other groups. DNA repair activities varied depending on the tissue—AL alone stimulated the excision of εA in the amygdala, εC in the CA3 and amygdala, and 8-oxoG in all tissues studied compared to controls. However, the excision efficiency of lesioned bases in the AL group was lower than in the stressed and stressed and AL-treated groups, with the exception of εA in the amygdala. In conclusion, the presented modulating effect of AL on the synthesis of BER pathway enzymes and their repair capacity, both under natural and stressful conditions, indicates another functional role of this neurosteroid in brain structures.

The relation between tau pathology and granulovacuolar degeneration of neurons

Neurofibrillary tangles arising from aggregated microtubule-associated protein tau occur in aged brains and are hallmarks of neurodegenerative diseases. A subset of neurons containing aggregated tau displays granulovacuolar degeneration (GVD) that is characterized by membrane-bound cytoplasmic vacuoles, each containing an electron-dense granule (GVB). Tau pathology induces GVBs in experimental models, but GVD does not generally follow tau pathology in the human brain. The entorhinal cortex, DRN, and LC are among the regions that display pathological changes of tau earliest, whereas neurons with GVBs occur first in the hippocampus and have been found in oral raphe nuclei only at the most advanced GVD stage. To date, there is no detailed report about neurons with GVD in aminergic nuclei. We studied the relation between tau pathology and GVD in field CA1 of the hippocampus, entorhinal cortex, dorsal (DRN) and median (MRN) raphe nucleus, and locus coeruleus from elderly subjects with Braak & Braak stages of tau pathology ranging from 0 to VI. Tau pathology and GVBs were visualized by means of immunolabeling and quantified. Percentages of neurons containing GVBs were significantly related to percentages of AT8-positive neurons in the regions examined. GVD and tau pathology were found together in neurons to a different extent in regions of the brain. 53.2% of AT8-immunoreactive neurons in CA1, 19.8% in layer II of the entorhinal cortex, 29.6% in the DRN, and 31.4% in the locus coeruleus contained GVBs. Age-related factors, the percentage of neurons with pretangles in a region of the brain, and the metabolism of a neuron possibly influence the prevalence of neurons with GVBs.

The hippocampal formation of two carnivore species: The feliform banded mongoose and the caniform domestic ferret.

Employing cyto-, myelo-, and chemoarchitectural staining techniques, we analyzed the structure of the hippocampal formation in the banded mongoose and domestic ferret, species belonging to the two carnivoran superfamilies, which have had independent evolutionary trajectories for the past 55 million years. Our observations indicate that, despite the time since sharing a last common ancestor, these species show extensive similarities. The four major portions of the hippocampal formation (cornu Ammonis, dentate gyrus, subicular complex, and entorhinal cortex) were readily observed, contained the same internal subdivisions, and maintained the topological relationships of these subdivisions that could be considered typically mammalian. In addition, adult hippocampal neurogenesis was observed in both species, occurring at a rate similar to that observed in other mammals. Despite the overall similarities, several differences to each other, and to other mammalian species, were observed. We could not find evidence for the presence of the CA2 and CA4 fields of the cornu Ammonis region. In the banded mongoose the dentate gyrus appears to be comprised of up to seven lamina, through the sublamination of the molecular and granule cell layers, which is not observed in the domestic ferret. In addition, numerous subtle variations in chemoarchitecture between the two species were observed. These differences may contribute to an overall variation in the functionality of the hippocampal formation between the species, and in comparison to other mammalian species. These similarities and variations are important to understanding to what extent phylogenetic affinities and constraints affect potential adaptive evolutionary plasticity of the hippocampal formation.

Possible Mechanisms of the Influence of the Supramillary Nucleus on the Functioning of the Dentate Gyrus and the CA2 Field of the Hippocamsus (Role of Disinhibition)

Abstract—We analyzed possible mechanisms of the influence of the supramammillary nucleus of the hypothalamus (SMN) on the activity of granule cells of the dentate gyrus and pyramidal neurons of the CA2 field of the hippocampus. We used known data on the functional organization of excitatory and inhibitory interactions in the neural network, which includes the SMN, hippocampus, septum, and entorhinal cortex. We propose that the induction of long-term potentiation in the perforant path inputs to granule cells and pyramidal neurons of the CA2 field during simultaneous stimulation of the SMN and the entorhinal cortex is promoted by disinhibition, which involves interconnected inhibitory neurons located in different fields of the hippocampus, the septum, and the SMN. In the waking state, the glutamatergic input from the SMN to the CA2 field promotes the potentiation of inputs from the entorhinal cortex and from the CA3 field to the pyramidal neurons of the CA2 field, which facilitates the propagation of signals through the trisynaptic hippocampal pathway. Taking into account the known data on the increased activity of the SMN during paradoxical sleep, data on the participation of the SMN in an increase in the activity of granule cells and pyramidal neurons of the CA2 field, and data on the high efficiency of input from the CA2 field into the CA1 field, we hypothesized that during paradoxical sleep, when the transmission of information through the classical trisynaptic pathway is suppressed, it is transferred to the CA1 field through the dentate gyrus and CA2 field. Our analysis suggests that the SMN-induced facilitation of signal transmission through different hippocampal fields as well as an increase in the activity of SMN and granule cells during paradoxical sleep, which promotes neurogenesis, may underlie the participation of SMN in memory processes.