A decreased survival of proliferated cells in the hippocampus is associated with a decline in spatial memory in aged rats

A decreased survival of proliferated cells in the hippocampus is associated with a decline in spatial memory in aged rats
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  Neuroscience Letters 399 (2006) 171–174 A decreased survival of proliferated cells in the hippocampus is associatedwith a decline in spatial memory in aged rats Henny Wati a , Koutaro Kudo a , b , Chunxiang Qiao a , Toshihide Kuroki c , Shigenobu Kanba c , ∗ a  Department of Neuropsychiatry, Interdisciplinary Graduate School of Medicine and Engineering, Yamanashi University, Yamanashi, Japan b  Department of Psychiatry, Graduate School of Medical Sciences, Tokyo University, Tokyo, Japan c  Department of Neuropsychiatry, Graduate School of Medical Sciences, Kyushu University, 3-1-1 Maidashi, Higashiku, Fukuoka 812-8582, Japan Received 16 November 2005; received in revised form 4 January 2006; accepted 28 January 2006 Abstract In aged rats, although learning and memory impairment is prominent, both the number of granular cells and the degree of neuronal progenitorproliferation in the hippocampus are known to be preserved. We examined the association between the survival of newly generated neurons inthe hippocampus and the learning ability in aged rats. By using BrdU, a cell proliferation marker to determine neurogenesis and contextual fearconditioning to determine learning ability, we found that in aged rats, along with memory impairment, the survival of both the proliferated cells atbaseline and those enhanced by contextual fear conditioning decreased remarkably. These results suggest that the integration of newly generatedneurons into hippocampal circuitry is decreased with aging, this phenomenon may, in part, explain the decline in learning and memory in agedrats.© 2006 Elsevier Ireland Ltd. All rights reserved. Keywords:  Aging; Neurogenesis; Learning Adultneurogenesisinmammalshasbeenwidelystudiedinrats,mice, three shrews, rhesus monkeys [1,7,10] and humans [4]. These studies revealed that hippocampal granular cells are con-tinuously produced throughout life. Neuronal progenitor cellsin the subgranular zone (SGZ) of dentate gyrus (DG) continueto proliferate, migrate to granular layer and become differenti-ated into granular cells [13] and thereafter become integratedinto the hippocampal circuitry [17]. Then after about 1–2 weeks the new cells will become functionally involved in the learn-ing process [21]. The survival of generated neurons is known to increase by spatial learning tasks [6] or environmental enrich-ment [13], while hippocampal injury [8] increases proliferation, and physical activity increases survival, proliferation, and dif-ferentiation [17]. On the other hand, acute social stress [7], the stimulation of NMDA receptors, or high adrenal steroidlevel [13] decreases neuronal progenitor proliferation in adultanimals.There is a vast amount of evidence on learning and mem-ory impairment in aged rats [3,4,11,14,19,20,23], nevertheless ∗ Corresponding author. Tel.: +81 92 642 5620; fax: +81 92 642 5644.  E-mail address: (S. Kanba). the roles of neurogenesis in the age-related decline in memoryremains to be elucidated. In aged rats, the proliferation is sig-nificantly decreased to about 10–17% below adult level [10],although the overall granular cell number is preserved [18]. In addition, the decrease in neuronal progenitor proliferation inaged rats is not reported to be related to the spatial learningability [11]. Therefore, it remains to be determined whether the survival of newly proliferated neurons decreases in the agingprocess, and if so, whether or not such a decrease in survival isrelated with a decline in memory.BrdU,athymidinanalogthatincorporateincell’sDNAintheS phase of cell cycle is considered to be suitable for the detec-tion of proliferation, migration and neuron age in the centralnervous system, due to its characteristics such as it is: perma-nently retained in the cell, it is non toxic to individual cellsand it has no effect on cell differentiation and migration [12].In our study, BrdU was administered one week before contex-tual fear conditioning (CFC), which is one of hippocampus-dependent tasks [2,9]. This gives a sufficient period for the newborn cells to be incorporated into the learning process[21].Therefore,toinvestigatetheeffectofagingonlearningabilityand cell survival in the hippocampus, we immunohistochem- 0304-3940/$ – see front matter © 2006 Elsevier Ireland Ltd. All rights reserved.doi:10.1016/j.neulet.2006.01.056  172  H. Wati et al. / Neuroscience Letters 399 (2006) 171–174 ically quantified the baseline number of BrdU positive cellsand the CFC-associated increase of neuron proliferation in hip-pocampal subregions of young and old rats.Fifteen-week-old (240–250g) and 120-week-old(600–900g) SD male rats (SLC; Shizuoka, Japan), weremaintained individually in a 12h light/dark, temperature-controlled room with free access to chow and tap water. Allexperimental procedures were conducted in accordance withthe guidelines of the Ethical Committee of Animal Experimentsof Yamanashi Medical University. All efforts were made tominimize the number of animals used and their suffering.BrdU (50mg/kg) was administered every 6h, by intraperi-toneal injection, for 7 days before CFC. CFC was conductedaccording to the method of Silva et al. [22]. In brief, in a clear Plexiglas conditioning chamber (28W × 21H × 22Dcm), thefloor was lined with 18 stainless steel bars (4mm diameter;1.5cm spacing), as a footshock grid to deliver scrambledshock from a stimulator. The aversive unconditioned stimuluswas 0.75mA footshock for 2s. The sound-attenuating box(48W × 48H × 48Dcm) accompanied with a 20W light anda ventilation fan for background white noise (74dB). Adiscrete tone conditioned stimulus (CS) (800Hz, 20-s duration,80dB) served as cue. Prior to the conditioning, all rats werehabituated by being placed in the conditioning chamber for 1minute once a day for consecutive 3 days. On the conditioningday, the rats were placed into the conditioning chamber andallowed to explore for 3 minutes. A footshock was delivered18s after the tone CS (a CFC group). In a control group, nofootshock was delivered. The rats were allowed to recoverfor 30s in the conditioning chamber and then were returnedto their home cages. Two hours later, the rats were againintroduced into the conditioning chamber and were observedfor 5min, without CS tone. The total time of freezing behavior(cessation of all but respiratory movement) was evaluated andexpressed as the percentage of freezing time. Immediatelyafter the CFC tests, the rats were deeply anesthetized withsodium pentobarbital and then were transcardially perfusedwith 4% PFA in 0.1M PB, and then the brains were quicklyremoved. After being post-fixed, 12 coronal sections (40um),160umintervalthroughoutthehippocampalformation(2.9mmposterior to bregma suture) were cut by a cryostat according tothe Paxinos & Watson brain map [15] and then were collected in PBS (0.1M; pH 7.4) for staining. After DNA denaturizing(formamide/SSC incubation in 65 ◦ C; SSC rinse; 2N HClincubation in 37 ◦ C; 0.1M Boric acid rinse), endogenousperoxidase was blocked by 3% H 2 O 2 . After blocking by10% normal goat serum (NGS), the sections were incubatedwith anti-BrdU (1:1000 Haralan Sera Lab. OBT0030) for24h at 4 ◦ C, followed by secondary antibody (biotinylatedgoat, anti-rat IgG, Vector BA9400), amplification with ABC,before color development with DAB nickel. BrdU positivecells were counted at  × 400 and  × 1000 under light microscopy(Olympus BX-60), by omitting the cells in the outermostforcal plane. Quantification was done by a researcher who isblind to the experimental conditions. A cell was consideredto be in the SGZ of DG if it was touching or in the SGZ,or in the hilar if it was located more than two cells away Fig. 1. Freezing behavior in response to context. In a control group, bothyoung and aged rats exhibited little or no freezing behavior in response tothe context during testing. In the CFC group, no difference was seen in thepercentage of freezing time in the young and aged rats. But during the test-ing period 2h after footshock, young and aged rats differed significantly incontext-dependent freezing behavior (  p <0.001); the young CFC rats ( n =6,mean ± S.E.M. 65.722 ± 10.234) exhibited a significantly longer percentage of freezing time than the young control rats ( n =6, mean ± S.E.M. 3.667 ± 2.914).The freezing in young CFC rats was also markedly superior to that in theaged CFC rats ( n =6, mean ± S.E.M. 13.111 ± 5.822) which was not signifi-cantly different to the aged control rats ( n =5, mean ± S.E.M. 3.133 ± 3.015). #  p <0.0001. from the SGZ. The average positive cell number per squaremillimeter of the DG area was counted. All data were analyzedby the Kruskal–Wallis and Mann–Whitney  U  -test based onBonfferoni’s correction. Differences of   p <0.05 were acceptedas statistically significant.At the first exposure to context-tone paired with footshock,the young and the aged rats showed no difference in freez-ing behavior (data not shown). However, when exposed to thecontext only 2h after the footshock, the aged rats showed a sig-nificant decrease in the percentage of freezing time comparedto their younger counterparts (Fig. 1). This implies that aging- relatedimpairmentofCFCisnotcausedbysensoryimpairment.A similar impairment in spatial-learning tasks has been previ-ously reported in aged rats [3,5,11,14,19,20,23].DG is the only site where adult-neurogenesis is observed inthe hippocampus [1,6,10,11,21]. When comparing the aged and theyoungratsinthecontrolgroups,itwasevidentthatneuroge-nesiswasremarkablyimpairedbyaging(Fig.2),asalsoreported by other studies [10,11]. Several mechanisms are thought to be responsible for the age-related reduction of neurogenesis: thedecrease in the neural precursor proliferation activity, the aber-rant or inhibited migratory mechanism that displaces newborncells, or decreases in neuronal survival [10].   H. Wati et al. / Neuroscience Letters 399 (2006) 171–174  173Fig. 2. BrdU positive cells in response to the context. To investigate the sur-vival of both the proliferated cells at baseline and of those stimulated byCFC in young and aged rats, we performed a BrdU injection a week beforeCFC training. BrdU positive cells were only found in DG in all four groups.There was marked reduction of neurogenesis both in the aged CFC rats ( n =6,mean ± S.E.M. 3.302 ± 1.206) as compared to the to young CFC rats ( n =6,mean ± S.E.M. 63.660 ± 13.349) (  p <0.0001) as well as in the aged controlrats ( n =5, mean ± S.E.M. 5.074 ± 2.463) as compared to the young controlrats ( n =6, mean ± S.E.M. 47.077 ± 4.206) (  p <0.0001). In the young rats, asignificant difference was observed between the CFC and the control groups(  p <0.0001). In the aged rats, no significant increase was observed in the BrdUpositive cells by CFC.  #  p <0.0001. WithCFC,theyoungratsshowedsignificantincreaseinBrdUpositive cells in DG. However, in the aged rats CFC did notincrease the number of BrdU positive cells. In adult rats, it isknown that either environmental enrichment or Morris watermaze increases the survival of newly generated neurons, but nottheir proliferation [6,13,16]. A recent study using the Morris water maze, demonstrated that aged rats had less proliferatingcellsthanyoungrats.However,intheagedrats,cellproliferationdid not correlate with the memory ability [11]. In their study, BrdU was injected after training, thus it labeled the neuronsnewly produced by the training. In addition, the ages of ratswere 21–24 months and the performances of some aged ratswere as good as those of young rats. After administering BrdUfor7dayspriortoCFC,weexaminedtheinfluenceoflearningonneuronal survival. In addition, we used much older rats (28–30months) and confirmed the performance of each aged rat to beinferior to that of the young rats. Added together, we proposedthat in aging both the proliferation and the survival of neuronsare impaired. However, since in both young and aged rats, thenumber of proliferating cells is not related to spatial memory[11], the survival of proliferating cells may play more importantroles in memory formation in the aged rats.In conclusion, the survival of the number of BrdU positivecellsatbaselineaswellasthoseinducedbyCFCdecreaseintheaged rats; the phenomenon may therefore explain, in part, theage-related decline in spatial learning. Acknowledgements This study was supported by the Target Oriented Brain Sci-encePromotionProgramandagrantfromtheJapaneseMinistryof Culture, Sports and Science (No. 16390321). This study wasalso supported by grants from the Japanese Ministry of Healthand Labor. References [1] J. Altman, G.D. Das, Autoradiographic and histological evidence of postnatal hippocampal neurogenesis in rats, J. Comp. Neurol. 124 (1965)319–336.[2] S.G. Anagnostaras, S. Maren, M.S. 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