Number Of Hippocampal Neurons Research Articles

Trans-Synaptic Spread of Tau Pathology In Vivo

Tauopathy in the brain of patients with Alzheimer's disease starts in the entorhinal cortex (EC) and spreads anatomically in a defined pattern. To test whether pathology initiating in the EC spreads through the brain along synaptically connected circuits, we have generated a transgenic mouse model that differentially expresses pathological human tau in the EC and we have examined the distribution of tau pathology at different timepoints. In relatively young mice (10–11 months old), human tau was present in some cell bodies, but it was mostly observed in axons within the superficial layers of the medial and lateral EC, and at the terminal zones of the perforant pathway. In old mice (>22 months old), intense human tau immunoreactivity was readily detected not only in neurons in the superficial layers of the EC, but also in the subiculum, a substantial number of hippocampal pyramidal neurons especially in CA1, and in dentate gyrus granule cells. Scattered immunoreactive neurons were also seen in the deeper layers of the EC and in perirhinal and secondary somatosensory cortex. Immunoreactivity with the conformation-specific tau antibody MC1 correlated with the accumulation of argyrophilic material seen in old, but not young mice. In old mice, axonal human tau immunoreactivity, especially at the endzones of the perforant pathway, was greatly reduced. Relocalization of tau from axons to somatodendritic compartments and propagation of tauopathy to regions outside of the EC correlated with mature tangle formation in neurons in the EC as revealed by thioflavin-S staining. Our data demonstrate propagation of pathology from the EC and support a trans-synaptic mechanism of spread along anatomically connected networks, between connected and vulnerable neurons. In general, the mouse recapitulates the tauopathy that defines the early stages of AD and provides a model for testing mechanisms and functional outcomes associated with disease progression.

Mild traumatic brain injury (MTBI) leads to spatial learning deficits

Primary objective: The aim of this study was to investigate the effect of mild and severe TBI on young male Wistar rats’ spatial learning.Research design: Randomized repeated measure experimental design was used to examine spatial learning in three independent animal groups.Methods and procedures: Twenty-four (severe n = 9, mild n = 8, sham n = 7) male rats were included in the study. Animals received controlled mild (1.5 mm), severe (2.5 mm) cortical impact injury or sham surgery. Spatial learning was assessed daily using a modified Morris water maze test, 20 days post-trauma, for 5 consecutive days. Percentage time travelled within each quadrant and escape latency were calculated. All animals’ hippocampal brain regions were examined post-injury using neuron (MAP2) and pre-synaptic protein (Synaptophysin) biomarkers.Main outcomes and results: It took the animals with mild injury until day 3 to reach the platform; and animals with mild and severe injury spent significantly less time in the target quadrant than the sham. The hippocampal neuron numbers differed proportionately between animals with severe and mild injury, but the percentage of synaptophysin density was significantly less in the dentate gyrus of both animals with mild and severe injury than sham group.Conclusion: Persistent spatial learning deficits exist after mild TBI; these deficits appear equivalent to deficits exhibited after a more severe injury.

Transgenic Expression of Intraneuronal Aβ42But Not Aβ40Leads to Cellular Aβ Lesions, Degeneration, and Functional Impairment without Typical Alzheimer's Disease Pathology

An early role of amyloid-β peptide (Aβ) aggregation in Alzheimer's disease pathogenesis is well established. However, the contribution of intracellular or extracellular forms of Aβ to the neurodegenerative process is a subject of considerable debate. We here describe transgenic mice expressing Aβ1-40 (APP47) and Aβ1-42 (APP48) with a cleaved signal sequence to insert both peptides during synthesis into the endoplasmic reticulum. Although lower in transgene mRNA, APP48 mice reach a higher brain Aβ concentration. The reduced solubility and increased aggregation of Aβ1-42 may impair its degradation. APP48 mice develop intracellular Aβ lesions in dendrites and lysosomes. The hippocampal neuron number is reduced already at young age. The brain weight decreases during aging in conjunction with severe white matter atrophy. The mice show a motor impairment. Only very few Aβ1-40 lesions are found in APP47 mice. Neither APP47 nor APP48 nor the bigenic mice develop extracellular amyloid plaques. While intracellular membrane expression of Aβ1-42 in APP48 mice does not lead to the AD-typical lesions, Aβ aggregates develop within cells accompanied by considerable neurodegeneration.

The Effects of Bilateral Vestibular Loss on Hippocampal Volume, Neuronal Number, and Cell Proliferation in Rats

Previous studies in humans have shown that bilateral loss of vestibular function is associated with a significant bilateral atrophy of the hippocampus, which correlated with the patients’ spatial memory deficits. More recently, patients who had recovered from unilateral vestibular neuritis have been reported to exhibit a significant atrophy of the left posterior hippocampus. Therefore, we investigated whether bilateral vestibular deafferentation (BVD) would result in a decrease in neuronal number or volume in the rat hippocampus, using stereological methods. At 16 months post-BVD, we found no significant differences in hippocampal neuronal number or volume compared to sham controls, despite the fact that these animals exhibited severe spatial memory deficits. By contrast, using bromodeoxyuridine (BrdU) as a marker of cell proliferation, we found that the number of BrdU-labeled cells significantly increased in the dentate gyrus of the hippocampus between 48 h and 1 week following BVD. Although a substantial proportion of these cells survived for up to 1 month, the survival rate was significantly lower in BVD animals when compared with that in sham animals. These results suggest a dissociation between the effects of BVD on spatial memory and hippocampal structure in rats and humans, which cannot be explained by an injury-induced increase in cell proliferation.

Presentation of the change in the number of hippocampal neurons by stereological method in surviving cases of mechanical asphyxia: An experimental rat study

Asphyxia is the most common death cause in Forensic Medicine. Strangulations do not always result in death. Cases recovering from asphyxia have many disturbances in different organs because of ischemia. In this study, using stereological methods, after asphyxia was performed by strangulation, the mean number of pyramidal neurons in hippocampus was aimed to evaluate. Rats are divided into two groups. The first one was the control group and the other one was asphyxiated by strangulation. The brains of strangulated group rats were extracted after seven days. Sections were taken by cryostat with the method of Systematic Randomised Sample Strategy and stained with hematoxylen-eosin. In these sections, total pyramidal neuronal numbers in hippocampus were determined by the Optical Fractioning Method. In our study, the mean cell number of asphyxiated group (158985,2±17439,8) was significantly less in the control group (265689,4±39009,5) (p=0.009). In conclusion, strangulation-induced asphyxia is resulted in hippocampal neuronal lost.

Ceftriaxone improves spatial learning and memory in chronic cerebral hypoperfused rats

Cerebral hypoperfusion is associated with cognitive decline in ageing, mild cognitive impairment, vascular type dementia, and Alzheimer's disease. The mechanisms leading to such neurological impairments are still uncertain. Although several mechanisms have been proposed as contributing factors leading to neuronal injury, glutamate excitotoxicity seems to be the relevant one. Recently, it was found that -lactam antibiotics such as ceftriaxone show a neuroprotective effect by upregulation of glutamate transporter and reduction of glutamate excitotoxicity. To study the contribution of glutamate excitotoxicity and the effects of ceftriaxone on spatial learning and memory in chronic cerebral hypoperfusion, we conducted experiments in rats subjected to permanent bilateral common carotid artery occlusion. Ceftriaxone (200 mg/kg) was injected daily in rats for 5 days after the onset of the arterial ligation. A Morris water maze was used to assess learning and memory two months after arterial ligation. Hippocampal histology and glutamate transporter 1 (GLT-1) expression were also studied. Results showed that ceftriaxone improved learning and memory performance. Consistently, the histological study showed an increase hippocampal CA1 and CA3 neuronal numbers in the ceftriaxone-treated group compared with the vehicle-treated group. Although not statistically significant, the GLT-1 protein level in the hippocampus was 86% higher than the sham group. We conclude that ceftriaxone treatment can attenuate neuronal injury and improve spatial learning and memory after chronic cerebral hypoperfusion and that glutamate excitotoxicity may play an important role in the pathophysiology of chronic cerebral hypoperfusion.

Hypoxic preconditioning attenuates global cerebral ischemic injury following asphyxial cardiac arrest through regulation of delta opioid receptor system

This study was designed to investigate whether delta opioid receptor (DOR) is involved in the neuroprotective effect induced by hypoxic preconditioning (HPC) in the asphyxial cardiac arrest (CA) rat model. Twenty-four hours after the end of 7-day HPC, the rats were subjected to 8-min asphyxiation and resuscitated with a standardized method. In the asphyxial CA rat model, HPC improved the neurological deficit score (NDS), inhibited neuronal apoptosis, and increased the number of viable hippocampal CA1 neurons at 24 h, 72 h, or 7 days after restoration of spontaneous circulation (ROSC); however, the above-mentioned neuroprotection of HPC was attenuated by naltrindole (a selective DOR antagonist). The expression of hypoxia-inducible factor-1α (HIF-1α) and DOR, and the content of leucine enkephalin (L-ENK) in the brain were also investigated after the end of 7-day HPC. HPC upregulated the neuronal expression of HIF-1α and DOR, and synchronously elevated the content of L-ENK in the rat brain. HIF-1α siRNA was used to further elucidate the relationship between HIF-1α and DOR in the HPC-treated brain. Knockdown of HIF-1α by siRNA markedly abrogated the HPC induced upregulation of HIF-1α and DOR. The present study demonstrates that the expression of DOR in the rat brain is upregulated by HIF-1α following exposure to 7-day HPC, at the same time, HPC also increases the production of endogenous DOR ligand L-ENK in the brain. DOR activation after HPC results in prolonged neuroprotection against subsequent global cerebral ischemic injury, suggesting a new mechanism of HPC-induced neuroprotection on global cerebral ischemia following CA and resuscitation.

Involvement of neuronal nitric oxide synthase in cognitive impairment in isoflurane-treated rats

The underlying causes of post-operative cognitive dysfunction (POCD) in elderly patients remain to be elucidated. In order to explore possible contributory mechanisms, we tested the effects of isoflurane anesthesia on (i) expression of hippocampal neuronal nitric oxide synthase (nNOS) and (ii) the relationship of changes in nNOS expression to cognitive dysfunction in isoflurane-treated aged rats. Our results indicate that isoflurane treatment leads to significant changes in correct reactions (F=28.35, p<0.001), initiative avoidances (F=29.33, p<0.001), and total reaction time (TRT) (F=6.99, p<0.05) of treated rats in the Y-maze test. Isoflurane-treated rats had fewer correct reactions and initiative avoidances in the Y-maze test 24 and 48h after 2h of isoflurane anesthesia compared with control group rats (p<0.05). TRTs to complete 20 trials of the Y-maze test increased significantly 48h after 2h anesthesia. The number of nNOS-positive hippocampal neurons decreased 24h after anesthesia, corresponding to an increased mean immunostaining grey-scale value. These data show that isoflurane causes a transient decrease in expression of hippocampal nNOS in aged rats during early post-anesthesia stages, and that the transient decrease of nNOS is closely correlated with cognitive impairment in isoflurane-treated aged rats.

Luteolin reduces primary hippocampal neurons death induced by neuroinflammation

Objectives: This study examined whether luteolin may exert an anti-inflammatory effect in microglia and may be neuroprotective by regulating microglia activation.Methods: We treated BV2 microglia with 1·0 μg/ml lipopolysaccharide (LPS) after incubation with luteolin for 1 hour, the nitric oxide (NO) levels were determined by a Griess reaction, the inducible NO synthase (iNOS), cyclooxygenase-2 (COX-2), tumor necrosis factor-alpha (TNF-alpha), and interleukin 1beta (IL-1beta) mRNA expression were determined by real-time PCR analysis, the iNOS and COX-2 protein induction were determined by Western blot analysis, and the levels of prostaglandin E2 (PGE2), TNF-alpha, and IL-1beta were determined by enzyme-linked immunosorbent assay (ELISA) kits. Rat primary hippocampal neurons were co-cultured with LPS-activated BV2 microglia with 20 μM luteolin for 24 hours, the hippocampal neurons viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and the number of apoptotic hippocampal neurons was determined by immunofluorescence detection.Results: Luteolin significantly inhibited the expression of iNOS and COX-2 in LPS-induced BV2 microglia. Moreover, the compound down-regulated the proinflammatory cytokines (TNF-alpha and IL-1beta) as well as the production of NO and PGE2 in these cells. When hippocampal neurons were co-cultured with LPS-stimulated BV2 microglia, the administration of 20 μM luteolin increased the neurons viability and reduced the number of apoptotic neurons.Conclusion: These data demonstrate that anti-inflammatory activity of luteolin in microglia contributes to its neuroprotective effect and suggest that it may have a potential therapeutic application in the treatment of neurodegenerative diseases.

Nutrient restriction during early life reduces cell proliferation in the hippocampus at adulthood but does not impair the neuronal differentiation process of the new generated cells

Maternal malnutrition results in learning deficits and predisposition to anxiety and depression in the offspring that extend into adulthood. At the cellular level, learning and memory rely on the production of new neurons in the dentate gyrus (DG) of the hippocampus, and hippocampal neurogenesis has been associated with the etiology and treatment of depression, but whether adult neurogenesis is affected by malnutrition during early life is not known. To investigate the effects of perinatal undernutrition on neurogenesis at adulthood, pregnant Sprague-Dawley rats were fed either ad libitum (C) or were undernourished by reducing their daily food intake by 50% in relation to the C group during gestation and lactation (FR/FR). At birth, one subset of control pups was cross-fostered to food-restricted dams to constitute a third group of animals that were undernourished during the lactation period only (AdLib/FR). At 90 days of age, pups were injected with bromodeoxyuridine (BrdU) and sacrificed 2 h, 1 week, or 3 weeks later. The number of BrdU-labeled cells in the DG was significantly reduced in the offspring of FR/FR dams in relation to controls at all the time points examined. However, the proportion of new cells exhibiting a neuronal phenotype was higher in FR/FR rats than in controls as revealed by the colabeling at 3 weeks of the BrdU-labeled cells with neuron-specific nuclear protein (NeuN). AdLib/FR animals exhibited also reduced BrdU labeling at 2 h and 1 week. Nevertheless, we found no significant differences at 3 weeks in either the number of BrdU-labeled cells or in the proportion of new neurons between controls and AdLib/FR rats. These results indicate that the decreased number of hippocampal neurons in perinatally undernourished rats is due to the deleterious effects of early nutrient restriction on cell proliferation but not on the neuronal differentiation process of the new generated cells.

Hippocampal interneurons are abnormal in schizophrenia

Objective The cellular substrate of hippocampal dysfunction in schizophrenia remains unknown. We tested the hypothesis that hippocampal interneurons are abnormal in schizophrenia, but that the total number of hippocampal neurons in the pyramidal cell layer is normal. Methods We collected whole hippocampal specimens of 13 subjects with schizophrenia and 20 matched healthy control subjects to study the number of all neurons, the somal volume of neurons, the number of somatostatin- and parvalbumin-positive interneurons and the messenger RNA levels of somatostatin, parvalbumin and glutamic acid decarboxylase 67. Results The total number of hippocampal neurons in the pyramidal cell layer was normal in schizophrenia, but the number of somatostatin- and parvalbumin-positive interneurons, and the level of somatostatin, parvalbumin and glutamic acid decarboxylase mRNA expression were reduced. Conclusions The study provides strong evidence for a specific defect of hippocampal interneurons in schizophrenia and has implications for emerging models of hippocampal dysfunction in schizophrenia.

Diabetes impairs learning performance and affects the mitochondrial function of hippocampal pyramidal neurons

Previous research has demonstrated that diabetes induces learning and memory deficits. However, the mechanism of memory impairment induced by diabetes is poorly understood. The present study investigated the effect of streptozotocin (STZ)-induced diabetes on spatial learning and memory using the Morris Water Maze. The effects of diabetes on CA1 pyramidal neurons in hippocampus were also examined. Diabetes impaired spatial learning and memory of rats. Diabetes induced the apoptosis of neurons and translocation of Bax from cytoplasm to mitochondria. On the contrary, diabetes induced cytochrome c release into the cytoplasm from mitochondria. Release of Cyt-c from mitochondria into cytoplasm may play a role in apoptosis of the CA1 pyramidal neurons, which resulted in a decrease of the number of neurons in hippocampus and impaired the performance function. These results partially explain the mechanism of the effect of diabetes on learning and memory. To protect mitochondrial function is possible candidate for preventing the impairments of diabetes on hippocampal function.

Contextual learning requires synaptic AMPA receptor delivery in the hippocampus

The hippocampus plays a central role in learning and memory. Although synaptic delivery of AMPA-type glutamate receptors (AMPARs) contributes to experience-dependent synaptic strengthening, its role in hippocampus-dependent learning remains elusive. By combining viral-mediated in vivo gene delivery with in vitro patch-clamp recordings, we found that the inhibitory avoidance task, a hippocampus-dependent contextual fear-learning paradigm, delivered GluR1-containing AMPARs into CA3-CA1 synapses of the dorsal hippocampus. To block the synaptic delivery of endogenous AMPARs, we expressed a fragment of the GluR1-cytoplasmic tail (the 14-aa GluR1 membrane-proximal region with two serines mutated to phospho-mimicking aspartates: MPR-DD). MPR-DD prevented learning-driven synaptic AMPAR delivery in CA1 neurons. Bilateral expression of MPR-DD in the CA1 region of the rat impaired inhibitory avoidance learning, indicating that synaptic GluR1 trafficking in the CA1 region of the hippocampus is required for encoding contextual fear memories. The fraction of CA1 neurons that underwent synaptic strengthening positively correlated with the performance in the inhibitory avoidance fear memory task. These data suggest that the robustness of a contextual memory depends on the number of hippocampal neurons that participate in the encoding of a memory trace.

Neuroprotective effects of quercetin, rutin and okra (Abelmoschus esculentus Linn.) in dexamethasone-treated mice

The administration of dexamethasone, a synthetic glucocorticoid receptor agonist, causes neuronal death in the CA3 layer of the hippocampus, which has been associated with learning and memory impairments. This study aimed to examine the ability of okra (Abelmoschus esculentus Linn.) extract and its derivatives (quercetin and rutin) to protect neuronal function and improve learning and memory deficits in mice subjected to dexamethasone treatment. Learning and memory functions in mice were examined using the Morris water maze test. The results showed that the mice treated with dexamethasone had prolonged water maze performance latencies and shorter time spent in the target quadrant while mice pretreated with quercetin, rutin or okra extract prior to dexamethasone treatment showed shorter latencies and longer time spent in target quadrant. Morphological changes in pyramidal neurons were observed in the dexamethasone treated group. The number of CA3 hippocampal neurons was significantly lower while pretreated with quercetin, rutin or okra attenuated this change. Prolonged treatment with dexamethasone altered NMDA receptor expression in the hippocampus. Pretreatment with quercetin, rutin or okra extract prevented the reduction in NMDA receptor expression. Dentate gyrus (DG) cell proliferation was examined using the 5-bromo-2-deoxyuridine (BrdU) immunohistochemistry technique. The number of BrdU-immunopositive cells was significantly reduced in dexamethasone-treated mice compared to control mice. Pretreatment with okra extract, either quercetin or rutin was found to restore BrdU-immunoreactivity in the dentate gyrus. These findings suggest that quercetin, rutin and okra extract treatments reversed cognitive deficits, including impaired dentate gyrus (DG) cell proliferation, and protected against morphological changes in the CA3 region in dexamethasone-treated mice. The precise mechanism of the neuroprotective effect of these plant extracts should be further investigated.

Variation in memory and the hippocampus across populations from different climates: a common garden approach

Selection for enhanced cognitive traits is hypothesized to produce enhancements to brain structures that support those traits. Although numerous studies suggest that this pattern is robust, there are several mechanisms that may produce this association. First, cognitive traits and their neural underpinnings may be fixed as a result of differential selection on cognitive function within specific environments. Second, these relationships may be the product of the selection for plasticity, where differences are produced owing to an individual's experiences in the environment. Alternatively, the relationship may be a complex function of experience, genetics and/or epigenetic effects. Using a well-studied model species (black-capped chickadee, Poecile atricapillus), we have for the first time, to our knowledge, addressed these hypotheses. We found that differences in hippocampal (Hp) neuron number, neurogenesis and spatial memory previously observed in wild chickadees persisted in hand-raised birds from the same populations, even when birds were raised in an identical environment. These findings reject the hypothesis that variation in these traits is owing solely to differences in memory-based experiences in different environments. Moreover, neuron number and neurogenesis were strikingly similar between captive-raised and wild birds from the same populations, further supporting the genetic hypothesis. Hp volume, however, did not differ between the captive-raised populations, yet was very different in their wild counterparts, supporting the experience hypothesis. Our results indicate that the production of some Hp factors may be inherited and largely independent of environmental experiences in adult life, regardless of their magnitude, in animals under high selection pressure for memory, while traits such as volume may be more plastic and modified by the environment.

ESTIMATION OF THE NUMBER OF NEURONS IN THE HIPPOCAMPUS OF RATS WITH PENICILLIN INDUCED EPILEPSY

Epilepsy is a neurological disease arising from strong and uncontrollable electrical firings of a group of neurons in the central nervous system. Experimental epileptic models have been developed to assess the physiopathology of epileptic seizures. This study was undertaken to estimate the number of neurons in the rat hippocampus with penicillin induced epilepsy, using a stereological method, "the optical fractionator". In the experimental group, 500 IU penicillin-G was injected intra-cortically, and in the control group, the same volume of saline was administered. A week later, the animals were decapitated and their brains were removed by craniatomy. Frozen brains were cut with a thickness of 150 ěm in a cryostat. Sections were collected by systematic random sampling and stained with hematoxylen-eosin. Microscopic images of pyramidal cell layers from hippocampus CA1, CA2 and CA3 subfields were then transferred to a monitor, using a 100x objective (N.A. = 1.25). Using the optical disector method, the neurons were counted in the frames and determined with a fractionator sampling scheme. The total pyramidal neuron number was then estimated using the optical fractionator method. The total pyramidal neuron number was found to be statistically lower in the experimental group (mean = 142,888 ± 11,745) than in the control group (mean = 177,953 ± 10,907) (p &lt; 0.05). The results suggest that a decrease in the hippocampal neuronal number in a penicillin model of epilepsy can be determined objectively and efficiently using the optical fractionator method.

Hippocampal neuron number loss in rats exposed to ingested sulfite

Sulfite, which is continuously formed in the body during metabolism of sulfur-containing amino acids, is commonly used in preservatives. It has been shown that there are toxic effects of sulfite on many cellular components. The aim of this study was to investigate the possible toxic effects of sulfite on pyramidal neurons by counting cell numbers in CA1 and CA2-CA3 subdivisions of the rat hippocampus. For this purpose, male albino rats were divided into a control group and a sulfite group (25 mg/kg). Sulfite was administered to the animals via drinking water for 8 weeks. At the end of the experimental period, brains were removed and neurons were estimated in total and in a known fraction of CA1 and CA2-CA3 subdivisions of the left hippocampus by using the optical fractionator method--a stereological method. Results showed that sulfite treatment caused a significant decrease in the total number of pyramidal neurons in three subdivisions of the hippocampus (CA1 and CA2-CA3) in the sulfite group compared with the control group (p < 0.05, Mann Whitney U test). It was concluded that exogenous administration of sulfite causes loss of pyramidal neurons in CA1 and CA2-CA3 subdivisions of the rat hippocampus.

Mechanism of isoflavone aglycone’s effect on cognitive performance of senescence-accelerated mice

This study investigated the effect of isoflavone aglycone (IA) on the learning and memory performance of senescence-accelerated mice, and explored its neural protective mechanism. Results showed that SAM-P/8 senescence-accelerated mice treated with IA performed significantly better in the Y-maze cognitive test than the no treatment control ( P < 0.05). The cortex AchE activity, serum SOD and GSH-Px activities were notably higher ( P < 0.05). MDA concentration and the β-secretase activity in the hippocampal tissue were both lower ( P < 0.05). Additionally, the number of hippocampal neurons was increased and cell morphology was significantly improved. Data suggested that IA could indirectly increase concentration of the cholinergic neural transmitter Ach through regulation of AchE, therefore improving the central cholinergic function and enhancing the learning and memory ability. By reducing the β-secretase activity, IA could decrease the formation and deposition of insoluble Adebris, relieve the resulted toxicity and damage to neurons, and thereby effectively protect the nervous system.

The effect of environmental harshness on neurogenesis: A large‐scale comparison

Harsh environmental conditions may produce strong selection pressure on traits, such as memory, that may enhance fitness. Enhanced memory may be crucial for survival in animals that use memory to find food and, thus, particularly important in environments where food sources may be unpredictable. For example, animals that cache and later retrieve their food may exhibit enhanced spatial memory in harsh environments compared with those in mild environments. One way that selection may enhance memory is via the hippocampus, a brain region involved in spatial memory. In a previous study, we established a positive relationship between environmental severity and hippocampal morphology in food-caching black-capped chickadees (Poecile atricapillus). Here, we expanded upon this previous work to investigate the relationship between environmental harshness and neurogenesis, a process that may support hippocampal cytoarchitecture. We report a significant and positive relationship between the degree of environmental harshness across several populations over a large geographic area and (1) the total number of immature hippocampal neurons, (2) the number of immature neurons relative to the hippocampal volume, and (3) the number of immature neurons relative to the total number of hippocampal neurons. Our results suggest that hippocampal neurogenesis may play an important role in environments where increased reliance on memory for cache recovery is critical.

Age-related changes in apoptosis in rat hippocampus induced by oxidative stress

Also known as programmed cell death, apoptosis is a sequence of events that leads to elimination of cells without releasing harmful substances into the surrounding area. Apoptosis may be induced by intracellular or extracellular signals. Certain apoptotic signals activate mitochondrial pro-apoptotic events and increase reactive oxygen species (ROS). Increased ROS production may lead to oxidative stress. The goal of our study was to characterize age-related changes in apoptosis induced by oxidative stress in the hippocampus. Rats 2, 7, 21 and 38 days old, and adult rats were used for our study. Hippocampal CA1, CA2, CA3 and dentate gyrus apoptosis, and hippocampal superoxide dismutase (SOD), glutathione peroxidase (GPx) enzyme activities and thiobarbituric acid reactive substances (TBARS) levels were measured. We found that numbers of hippocampal neurons were low in rats 2, 7 and 21 days old (CA1, p < 0.001; CA3, p < 0.05; gyrus dentatus, p < 0.001). The terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) positive cell count was highest in the CA1 and dentate gyrus of 21-day-old rats. Among 21-day-old rats, the hippocampal TBARS levels and SOD enzyme activity were high, whereas GPx activity was low. These results demonstrate that the hippocampal CA1 and dentate gyrus of 21-day-old rats are more prone to damage by oxidative stress.