Effects of spaced learning in the water maze on development of dentate granule cells generated in adult mice

Sequential treadmill exercise and cognitive training synergistically increase adult hippocampal neurogenesis in mice

Combining physical and cognitive training has been suggested to promote further benefits on brain and cognition, which could include synergistic improvement of hippocampal neuroplasticity. In this paper, we investigated whether treadmill exercise followed by a working memory training in the water maze increase adult hippocampal neurogenesis to a greater extent than either treatment alone. Our results revealed that ten days of scheduled running enhance cell proliferation/survival in the short-term as well as performance in the water maze. Moreover, exercised mice that received working memory training displayed more surviving dentate granule cells compared to those untreated or subjected to only one of the treatments. According to these findings, we suggest that combining physical and cognitive stimulation yield synergic effects on adult hippocampal neurogenesis by extending the pool of newly-born cells and subsequently favouring their survival. Future research could take advantage from this non-invasive, multimodal approach to achieve substantial and longer-lasting enhancement in adult hippocampal neurogenesis, which might be relevant for improving cognition in healthy or neurologically impaired conditions.

Glucocorticoid-mediated mechanisms of hippocampal damage: Contribution of subgranular neurogenesis

A comprehensive overview of the interplay between glucocorticoids (GCs) and adult hippocampal neurogenesis (AHN) is presented, particularly, in the context of a diseased brain. The effectors of GCs in the dentate gyrus neurogenic niche of the hippocampal are reviewed, and the consequences of the GC signaling on the generation and integration of new neurons are discussed. Recent findings demonstrating how GC signaling mediates impairments of the AHN in various brain pathologies are overviewed. GC-mediated effects on the generation and integration of adult-born neurons in the hippocampal dentate gyrus depend on the nature, severity, and duration of the acting stress factor. GCs realize their effects on the AHN primarily via specific glucocorticoid and mineralocorticoid receptors. Disruption of the reciprocal regulation between the hypothalamic-pituitary-adrenal (HPA) axis and the generation of the adult-born granular neurons is currently considered to be a key mechanism implicating the AHN into the pathogenesis of numerous brain diseases, including those without a direct hippocampal damage. These alterations vary from reduced proliferation of stem and progenitor cells to increased cell death and abnormalities in morphology, connectivity, and localization of young neurons. Although the involvement of the mutual regulation between the HPA axis and the AHN in the pathogenesis of cognitive deficits and mood impairments is evident, several unresolved critical issues are stated. Understanding the details of GC-mediated mechanisms involved in the alterations in AHN could enable the identification of molecular targets for ameliorating pathology-induced imbalance in the HPA axis/AHN mutual regulation to conquer cognitive and psychiatric disturbances.

Adult hippocampal neurogenesis in Egyptian fruit bats from three different environments: Are interpretational variations due to the environment or methodology?

We quantified both proliferative (Ki-67 immunohistochemistry) and immature (doublecortin immunohistochemistry) cells within the dentate gyrus of adult Egyptian fruit bats from three distinct environments: (a) primary rainforest, (b) subtropical woodland, and (c) fifth-generation captive-bred. We used four different previously reported methods to assess the effect of the environment on proliferative and immature cells: (a) the comparison of raw totals of proliferative and immature cells; (b) these totals standardized to brain mass; (c) these totals expressed as a density using the volume of the granular cell layer (GCLv) for standardization; and (d) these totals expressed as a percentage of the total number of granule cells. For all methods, the numbers of proliferative cells did not differ statistically among the three groups, indicating that the rate of proliferation, while malleable to experimental manipulation or transiently in response to events of importance in the natural habitat, appears to occur, for the most part, at a predetermined rate within a species. For the immature cells, raw numbers and standardizations to brain mass and GCLv revealed no difference between the three groups studied; however, standardization to total granule cell numbers indicated that the two groups of wild-caught bats had significantly higher numbers of immature neurons than the captive-bred bats. These contrasting results indicate that the interpretation of the effect of the environment on the numbers of immature neurons appears method dependent. It is possible that current methods are not sensitive enough to reveal the effect of different environments on proliferative and immature cells.

Regulation of adult hippocampal neurogenesis exerted by sexual, cognitive and physical activity: An update

In 1962, Joseph Altman described that the brain generates neurons after the postnatal period, and this continues throughout your life (Altman, 1962). This was a breakthrough in the neuroscience field because before this the accepted paradigm was that the brain only generated neurons during the embryonal development. This discovery has been controversial ever since, especially since one of the areas of the brain with neurogenic properties is the hippocampus, which is the area involved in memory storage and neurodegenerative processes. The adult hippocampal neurogenesis modulates in response to different environmental factors. In this article, we review how exercise and cognitive and sexual activity can regulate the generation of new neurons in the hippocampal in an adult brain and the impact of these new neurons in the brain circuitry.

An unexpected improvement in spatial learning and memory ability in alpha-synuclein A53T transgenic mice

Growing evidence suggests, as Parkinson's disease (PD) progresses, that its non-motor symptoms appear prior to or in parallel with its motor deficits. Alpha-synuclein A53T transgenic mouse (A53T) is an essential tool to investigate the onsets and the extents of PD non-motor symptoms. Our aim is to investigate spatial learning and memory ability in A53T mice. In our rotarod tests, no motor coordination impairments were detected in mice of 3, 6, 9, and 12 months old. We then investigated their spatial learning and memory ability through Morris water maze in 3- and 9-month-old mice. No significant difference in escape latency was detected among the A53T mice and the control mice. However, an unexpected improvement in spatial learning and memory ability was observed in the probe session among the A53T mice. Reversal learning by Morris water maze also indicated that 3- and 9-month-old A53T mice exhibited a better cognitive flexibility compared to their littermate controls. Further studies by western blots showed that alpha-synuclein expressions in hippocampus of the A53T mice were noticeably up-regulated. The immunofluorescence staining of 5-bromo-2-deoxyuridine (Brdu) and doublecortin (DCX) demonstrated that neither the Brdu-positive neurons nor the Brdu/DCX positive neurons in hippocampus were significantly altered between the two groups. These results suggest that our A53T mice exhibit improved spatial learning and memory ability prior to their motor coordination deficits. These results are not induced by neurogenesis in the hippocampus.

Memory formation orchestrates the wiring of adult-born hippocampal neurons into brain circuits

During memory formation, structural rearrangements of dendritic spines provide a mean to durably modulate synaptic connectivity within neuronal networks. New neurons generated throughout the adult life in the dentate gyrus of the hippocampus contribute to learning and memory. As these neurons become incorporated into the network, they generate huge numbers of new connections that modify hippocampal circuitry and functioning. However, it is yet unclear as to how the dynamic process of memory formation influences their synaptic integration into neuronal circuits. New memories are established according to a multistep process during which new information is first acquired and then consolidated to form a stable memory trace. Upon recall, memory is transiently destabilized and vulnerable to modification. Using contextual fear conditioning, we found that learning was associated with an acceleration of dendritic spines formation of adult-born neurons, and that spine connectivity becomes strengthened after memory consolidation. Moreover, we observed that afferent connectivity onto adult-born neurons is enhanced after memory retrieval, while extinction training induces a change of spine shapes. Together, these findings reveal that the neuronal activity supporting memory processes strongly influences the structural dendritic integration of adult-born neurons into pre-existing neuronal circuits. Such change of afferent connectivity is likely to impact the overall wiring of hippocampal network, and consequently, to regulate hippocampal function.

Persistent Gliosis Interferes with Neurogenesis in Organotypic Hippocampal Slice Cultures

Neurogenesis in the adult hippocampus has become an intensively investigated research topic, as it is essential for proper hippocampal function and considered to bear therapeutic potential for the replacement of pathologically lost neurons. On the other hand, neurogenesis itself is frequently affected by CNS insults. To identify processes leading to the disturbance of neurogenesis, we made use of organotypic hippocampal slice cultures (OHSC), which, for unknown reasons, lose their neurogenic potential during cultivation. In the present study, we show by BrdU/Prox1 double-immunostaining that the generation of new granule cells drops by 90% during the first week of cultivation. Monitoring neurogenesis dynamically in OHSC from POMC-eGFP mice, in which immature granule cells are endogenously labeled, revealed a gradual decay of the eGFP signal, reaching 10% of initial values within 7 days of cultivation. Accordingly, reverse transcription quantitative polymerase chain reaction analysis showed the downregulation of the neurogenesis-related genes doublecortin and Hes5, a crucial target of the stem cell-maintaining Notch signaling pathway. In parallel, we demonstrate a strong and long-lasting activation of astrocytes and microglial cells, both, morphologically and on the level of gene expression. Enhancement of astroglial activation by treating OHSC with ciliary neurotrophic factor accelerated the loss of neurogenesis, whereas treatment with indomethacin or an antagonist of the purinergic P2Y12 receptor exhibited potent protective effects on the neurogenic outcome. Therefore, we conclude that OHSC rapidly lose their neurogenic capacity due to persistent inflammatory processes taking place after the slice preparation. As inflammation is also considered to affect neurogenesis in many CNS pathologies, OHSC appear as a useful tool to study this interplay and its molecular basis. Furthermore, we propose that modification of glial activation might bear the therapeutic potential of enabling neurogenesis under neuropathological conditions.

A Transgenic Rat for Specifically Inhibiting Adult Neurogenesis

The growth of research on adult neurogenesis and the development of new models and tools have greatly advanced our understanding of the function of newborn neurons in recent years. However, there are still significant limitations in the ability to identify the functions of adult neurogenesis in available models. Here we report a transgenic rat (TK rat) that expresses herpes simplex virus thymidine kinase in GFAP+ cells. Upon treating TK rats with the antiviral drug valganciclovir, granule cell neurogenesis can be completely inhibited in adulthood, in both the hippocampus and olfactory bulb. Interestingly, neurogenesis in the glomerular and external plexiform layers of the olfactory bulb was only partially inhibited, suggesting that some adult-born neurons in these regions derive from a distinct precursor population that does not express GFAP. Within the hippocampus, blockade of neurogenesis was rapid and nearly complete within 1 week of starting treatment. Preliminary behavioral analyses indicate that general anxiety levels and patterns of exploration are generally unaffected in neurogenesis-deficient rats. However, neurogenesis-deficient TK rats showed reduced sucrose preference, suggesting deficits in reward-related behaviors. We expect that TK rats will facilitate structural, physiological, and behavioral studies that complement those possible in existing models, broadly enhancing understanding of the function of adult neurogenesis.

Chronic retinoic acid treatment suppresses adult hippocampal neurogenesis, in close correlation with depressive-like behavior

Clinical studies have highlighted an association between retinoid treatment and depressive symptoms. As we had shown before that chronic application of all-trans retinoic acid (RA) potently activated the hypothalamus-pituitary-adrenal (HPA) stress axis, we here questioned whether RA also induced changes in adult hippocampal neurogenesis, a form of structural plasticity sensitive to stress and implicated in aspects of depression and hippocampal function. RA was applied intracerebroventricularly (i.c.v.) to adult rats for 19 days after which animals were subjected to tests for depressive-like behavior (sucrose preference) and spatial learning and memory (water maze) performance. On day 27, adult hippocampal neurogenesis and astrogliosis was quantified using BrdU (newborn cell survival), PCNA (proliferation), doublecortin (DCX; neuronal differentiation), and GFAP (astrocytes) as markers. RA was found to increase retinoic acid receptor-α (RAR-α) protein expression in the hippocampus, suggesting an activation of RA-induced signaling mechanisms. RA further potently suppressed cell proliferation, newborn cell survival as well as neurogenesis, but not astrogliosis. These structural plasticity changes were significantly correlated with scores for anhedonia, a core symptom of depression, but not with water maze performance. Our results suggest that RA-induced impairments in hippocampal neurogenesis correlate with depression-like symptoms but not with spatial learning and memory in this design. Thus, manipulations aimed to enhance neurogenesis may help ameliorate emotional aspects of RA-associated mood disorders. © 2016 Wiley Periodicals, Inc.

The relationship between behavior acquisition and persistence abilities: Involvement of adult hippocampal neurogenesis

The influence of the learning process on the persistence of the newly acquired behavior is relevant both for our knowledge of the learning/memory mechanisms and for the educational policy. However, it is unclear whether during an operant conditioning process with a continuous reinforcement paradigm, individual differences in acquisition are also associated to differences in persistence of the acquired behavior. In parallel, adult neurogenesis has been implicated in spatial learning and memory, but the specific role of the immature neurons born in the adult brain is not well known for this process. We have addressed both questions by analyzing the relationship between water maze task acquisition scores, the persistence of the acquired behavior, and the size of the different subpopulations of immature neurons in the adult murine hippocampus. We have found that task acquisition and persistence rates were negatively correlated: the faster the animals find the water maze platform at the end of acquisition stage, the less they persist in searching for it at the learned position in a subsequent non-reinforced trial; accordingly, the correlation in the number of some new neurons' subpopulations and the acquisition rate is negative while with persistence in acquired behavior is positive. These findings reveal an unexpected relationship between the efficiency to learn a task and the persistence of the new behavior after a non-reinforcement paradigm, and suggest that the immature neurons might be involved in different roles in acquisition and persistence/extinction of a learning task. © 2016 Wiley Periodicals, Inc.

Spatial learning and neurogenesis: Effects of cessation of wheel running and survival of novel neurons by engagement in cognitive tasks

Physical exercise stimulates cell proliferation in the adult dentate gyrus and facilitates acquisition and/or retention of hippocampal-dependent tasks. It is established that regular physical exercise improves cognitive performance. However, it is unclear for how long these benefits last after its interruption. Independent groups of rats received both free access to either unlocked (EXE Treatment) or locked (No-EXE Treatment) running wheels for 7 days, and daily injections of bromodeoxyuridine (BrdU) in the last 3 days. After a time delay period of either 1, 3, or 6 weeks without training, the animals were tested in the Morris water maze (MWM) either in a working memory task dependent on hippocampal function (MWM-HD) or in a visible platform searching task, independent on hippocampal function (MWM-NH). Data confirmed that exposure of rats to 7 days of spontaneous wheel running increases cell proliferation and neurogenesis. In contrast, neurogenesis was not accompanied by significant improvements of performance in the working memory version of the MWM. Longer time delays between the end of exercise and the beginning of cognitive training in the MWM resulted in lower cell survival; that is, the number of novel surviving mature neurons was decreased when this delay was 6 weeks as compared with when it was 1 week. In addition, data showed that while exposure to the MWM-HD working memory task substantially increased survival of novel neurons, exposure to the MWM-NH task did not, thus indicating that survival of novel dentate gyrus neurons depends on the engagement of this brain region in performance of cognitive tasks. © 2015 Wiley Periodicals, Inc.

Running per se stimulates the dendritic arbor of newborn dentate granule cells in mouse hippocampus in a duration-dependent manner

Laboratory rodents provided chronic unlimited access to running wheels display increased neurogenesis in the hippocampal dentate gyrus. In addition, recent studies indicate that such an access to wheels stimulates dendritic arborization in newly formed neurons. However, (i) the presence of the running wheel in the housing environment might also bear intrinsic influences on the number and shape of new neurons and (ii) the dendritic arborization of new neurons might be insensitive to moderate daily running activity (i.e., several hours). In keeping with these uncertainties, we have examined neurogenesis and dendritic arborization in newly formed granular cells in adult C57Bl/6N male mice housed for 3 weeks under standard conditions, with a locked wheel, with a running wheel set free 3 h/day, or with a running wheel set permanently free. The results indicate that the presence of a blocked wheel in the home cage increased cell proliferation, but not the number of new neurons while running increased in a duration-dependent manner the number of newborn neurons, as assessed by DCX labeling. Morphological analyses of the dendritic tree of newborn neurons, as identified by BrdU-DCX co-staining, revealed that although the presence of the wheel stimulated their dendritic architecture, the amplitude of this effect was lower than that elicited by running activity, and was found to be running duration-dependent.