幼年和老年大鼠学习中海马CA_3区长时程突触增强的年龄特征

研究发现幼年和老年大鼠在条件性饮水反应的建立、消退和再建立过程中,海马CA_3区有习得性长时程突触增强(LTP)的形成、消退和再形成现象。在它的形成和再形成以及每实验日训练作业后习得性LTP的发展上,幼年鼠明显快于老年鼠,而习得性LTP的消退,在两组间无明显差异。这既表明海马CA_3区的习得性LTP具年龄特征,也为论证习得性LTP可能是学习和记忆的神经基础之一提供了新的证据。     

Long-Term Survival of Intrastriatal Dopaminergic Grafts: Modulation of Acetylcholine Release by Graft-Derived Dopamine

The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12-14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of D-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12-14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 microM) and nomifensine (10 microM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 microM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating supersensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor supersensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 microM) and release of endogenous DA by DL-amphetamine (3 microM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.     

Evaluation ofChaoborus predation on natural populations of herbivorous zooplankton in a eutrophic lake

Hydrobiologia - The effect ofChaoborus predation on natural zooplankton communities was tested in enclosure experiments during June, July and August in the eutrophic Frederiksborg Slotssø,...     

Comparative and Experimental Approaches to the Study of Microbial Food Webs

ABSTRACT The study of microbial food webs is dominated by field measurements of microbial standing stocks and rate processes and to a lesser extent by laboratory studies. These approaches reflect the concerns of microbial ecologists to assess accurately the capabilities of microorganisms and to compare microbial processes to other ecosystem parameters. These approaches have led to enormous advances in understanding microbial food webs. Reconciling our expanding knowledge with general questions about the significance and representation of microbial food webs in ecosystem studies requires additional approaches including comparative studies and field experiments. Comparative studies, analyses of microbial stocks or rates across a wide range of ecosystems, lead to quantitative models of microbial processes. These models facilitate testing of hypotheses at a very general level, allow the comparison of different stocks or rate processes across a gradient of systems, and detect unusual situations or outlier systems. Field experimental manipulations offer the advantages of working with intact natural communities, of direct evaluation of results with statistical methods, and of testing important qualitative hypotheses. Both comparative and field manipulation studies have led to important advances in the study of microbial food webs and should be expanded.     

Fire regimes and phytomass growth dynamics in a Quercus coccifera garrigue

An experiment was set up in a Quercus coccifera garrigue in southern France to analyze the effect of burning frequency and season on phytomass production. Fire regimes consisted of late spring or early autumn burns, every 6 yr, every 3 yr, or every 2 yr. The experiment started in 1969 and lasted for 19 yr. In May 1981 and May 1987, 10 samples, each 1 m², were harvested per treatment. Fire frequency had an effect on the quantity of phytomass which was produced: above-ground phytomass decreased with increasing fire frequency. This was mainly due to the lower biomass of woody plants. In all burning treatments the phytomass of herbs was higher than in the unburned vegetation. Within each burning frequency, the total phytomass of the spring-burned vegetation was always higher than that of the autumn-burned community. Generally, the herb phytomass produced was higher in the autumn-burned plots. There were two fairly distinct phases in the period following fire, each with a different level of annual phytomass production. For the first six years it was about 300 g m^?2 yr^?1, falling thereafter to about 50 g m^?2 yr^?1. This and other studies on Q. coccifera garrigue indicate that this community is very resilient with respect to fire, but possesses a low productive capacity and does not show any sign of degeneration up to 30 yr old.     

Food and feeding of aquatic larvae of the midge chaoborus flavicans (meigen) (diptera : chaoboridae) in the laboratory

Laboratory observations on the feeding of fourth instar larvae of Chaoborus flavicans (Meigen) show that features relating to the prey, such as swimming behaviour, size and shape, play a major role in determining the kind of food eaten by the predator.In tests with two interacting limnetic cyclopoid copepods, Cyclops abyssorum out-competed Mesocyclops leuckarti when Chaoborus was absent but did not do so when Chaoborus was present.     

Synthesis and hybridizing properties of isoDNAs including 3′-O,4′-C-ethyleneoxy-bridged 5-methyluridine derivatives

3′,4′-Ethyleneoxy-bridged 5-methyluridine derivatives with methyl groups in the bridge, (R)-Me-3′,4′-EoNA-T and (S)-Me-3′,4′-EoNA-T, were synthesized, and these two analogs and unsubstituted 3′,4′-EoNA-T were successfully incorporated into a 2′,5′-linked oligonucleotide (isoDNA). Their duplex-forming ability with complementary DNA and complementary RNA, and triplex-forming ability with double-stranded DNA, were evaluated by UV-melting experiments. The results indicated that isoDNAs, including these 3′,4′-EoNA analogs, could hybridize exclusively with complementary RNA. In particular, 3′,4′-EoNA-T and (R)-Me-3′,4′-EoNA-T modifications within isoDNA could stabilize the duplexes with complementary RNA compared with unmodified or 3′,4′-BNA-modified isoDNAs.     

Responses of nitrogen and phosphorus resorption from leaves and branches to long-term nitrogen deposition in a Chinese fir plantation北大核心CSCD

Aims Our objectives were to investigate differences in nutrient resorption between different plant organs (leaf and branch), among plants with different life spans (one-year old, two-year old and senesced), and under different duration of nitrogen (N) deposition treatments in a Chinese fir (Cunninghamia lanceolata) plantation. Methods The long-term N deposition experiment was conducted in a 12-year-old fir plantation of subtropical China. N deposition treatment was initiated in January 2004 until now, up-going 14 years. N deposition were designed at 4 levels of 0, 60, 120, and 240 kg·hm–2·a–1, indicated as N0, N1, N2, and N3, respectively, with 3 replicates for each treatment. The solution of CO(NH2)2 was sprayed on the forest floor each month. In the study, we measured N and phosphorus (P) concentrations and analyzed the pattern of nutrient resorption of mature and senescing leaves and branches. The different responses of needles N and P resorption after 7- and 14-year N deposition treatments were also compared. Important findings After 14 years of N deposition, (1) during the senescing process, leaf and branch C, N, and P content gradually decreased with increasing treatment duration, with higher content in leaf than in branch. N content decreased in the order of one-year old green leaf > two-year old green leaf > senescent leaf > one-year old living branch > two-year old living branch > senescent branch, and N3 > N2 > N1 > N0, with C:N showing the opposite trend. Senescent organs had higher C:N, N:P, and C:P than mature living organs. N deposition increased N, N:P, and C:P of mature living organs (except for the two-year old green leaf), while decreased P and C:N. (2) N resorption efficiency (REN) and P resorption efficiency (REP) of leaves and branches decreased gradually with increasing life span. REP was typically higher in leaf and branch than REN. Leaf had lower REN (28.12%) than branch (30.00%), but higher REP (45.82%) than branch (30.42%). A highly significant linear correlation existed between N:P and REN:REP in leaves and branches. (3) REN decreased but REP increased with the treatment duration of N deposition. The longer experimental duration (14 years) reduced REN by 9.85%, 3.17%, 11.71% under N1, N2, and N3 treatments, respectively, and increased REP by 71.98%, 42.25%, 9.60%, respectively, than the shorter treatment duration (7 years). In summary, the responses of essential nutrients resorption efficiency for different plant organs and life span varied with the levels and duration of N deposition treatment. REN:REP in leaf and branch were mostly driven by N:P of leaf and branch. The results highlight that nutrients resorption is significantly influenced by long-term N deposition. © Chinese Journal of Plant Ecology.     

Global‐scale impacts of nitrogen deposition on tree carbon sequestration in tropical,temperate, and boreal forests: A meta‐analysis

Elevated nitrogen (N) deposition may increase net primary productivity in N‐limited terrestrial ecosystems and thus enhance the terrestrial carbon (C) sink. To assess the magnitude of this N‐induced C sink, we performed a meta‐analysis on data from forest fertilization experiments to estimate N‐induced C sequestration in aboveground tree woody biomass, a stable C pool with long turnover times. Our results show that boreal and temperate forests responded strongly to N addition and sequestered on average an additional 14 and 13 kg C per kg N in aboveground woody biomass, respectively. Tropical forests, however, did not respond significantly to N addition. The common hypothesis that tropical forests do not respond to N because they are phosphorus‐limited could not be confirmed, as we found no significant response to phosphorus addition in tropical forests. Across climate zones, we found that young forests responded more strongly to N addition, which is important as many previous meta‐analyses of N addition experiments rely heavily on data from experiments on seedlings and young trees. Furthermore, the C–N response (defined as additional mass unit of C sequestered per additional mass unit of N addition) was affected by forest productivity, experimental N addition rate, and rate of ambient N deposition. The estimated C–N responses from our meta‐analysis were generally lower that those derived with stoichiometric scaling, dynamic global vegetation models, and forest growth inventories along N deposition gradients. We estimated N‐induced global C sequestration in tree aboveground woody biomass by multiplying the C–N responses obtained from the meta‐analysis with N deposition estimates per biome. We thus derived an N‐induced global C sink of about 177 (112–243) Tg C/year in aboveground and belowground woody biomass, which would account for about 12% of the forest biomass C sink (1,400 Tg C/year).     

Grazing enhances belowground carbon allocation,microbial biomass,and soil carbon in a subtropical grassland

Despite the large contribution of rangeland and pasture to global soil organic carbon (SOC) stocks, there is considerable uncertainty about the impact of large herbivore grazing on SOC, especially for understudied subtropical grazing lands. It is well known that root system inputs are the source of most grassland SOC, but the impact of grazing on partitioning of carbon allocation to root tissue production compared to fine root exudation is unclear. Given that different forms of root C have differing implications for SOC synthesis and decomposition, this represents a significant gap in knowledge. Root exudates should contribute to SOC primarily after microbial assimilation, and thus promote microbial contributions to SOC based on stabilization of microbial necromass, whereas root litter deposition contributes directly as plant‐derived SOC following microbial decomposition. Here, we used in situ isotope pulse‐chase methodology paired with plant and soil sampling to link plant carbon allocation patterns with SOC pools in replicated long‐term grazing exclosures in subtropical pasture in Florida, USA. We quantified allocation of carbon to root tissue and measured root exudation across grazed and ungrazed plots and quantified lignin phenols to assess the relative contribution of microbial vs. plant products to total SOC. We found that grazing exclusion was associated with dramatically less overall belowground allocation, with lower root biomass, fine root exudates, and microbial biomass. Concurrently, grazed pasture contained greater total SOC, and a larger fraction of SOC that originated from plant tissue deposition, suggesting that higher root litter deposition under grazing promotes greater SOC. We conclude that grazing effects on SOC depend on root system biomass, a pattern that may generalize to other C4‐dominated grasslands, especially in the subtropics. Improved understanding of ecological factors underlying root system biomass may be the key to forecasting SOC and optimizing grazing management to enhance SOC accumulation.