A temporal perspective on stress hormones and memory

Life is seldom devoid of stress. Stress is caused by external or internal environmental stimuli that challenge the homeostasis of the organism. In coping with these challenges, the body is mobilised to restate the inner balance. Among others, the central nervous system (CNS) is a key effector in this adaptive process (in dealing with the challenges by employing cognitive capabilities) as well as a major target of the stress-initiated physiological alterations. Correspondingly, the functionality of learning and memory is undergoing constant and dynamic modification in a stressful context. This leads to commonly observed phenomena in which human memory formation and retention are susceptible to the influence of perceived stress. This has constituted the central interest of this research that aims to identify certain of the underlying mechanisms and definable rules of the stress’s impacts on learning and memory, and more specifically, to decipher the intriguing phenomenon that memory is not uniformly affected by stress, but often with opposing ends (i.e. enhanced versus impaired memory). Perceived stress stimulates the sympatho-adrenomedullar system and the hypothalamic-pituitaryadrenal (HPA) axis, both of which eventually activate the adrenal gland which can, in response, release two categories of hormones: (nor)adrenaline and glucocorticoids. These are, among others, the neuromodulators that can, through their active forms in the CNS, manipulate the brain functions by molecular actions. Thus, they are the two major types of stress hormones on which we have focused our research interest. We have also narrowed down our observation of memory-related functionality to two main structures: the hippocampus and the amygdala; the former was selected due to its well-recognised role in declarative memory, and the latter for its renowned involvement in emotional memory - as negative emotion is often linked to stress. The entire research was conducted with a view to identifying functional relevances; and multiple levels of study were achieved - as in rodents and humans. In the animal, we adopted an electrophysiologically functional model at the network level - long-term potentiation (LTP), which represents a long-lasting increase in synaptic strength for neuronal contacts that have undergoneactivation and transmission events (activity-dependent synaptic plasticity). It shares various key characteristics with memory and is widely acknowledged as the best model to date to account for memory. In the human, we observed overall memory behaviour and employed fMRI technology to monitor real-time brain functional activities during memory encoding. Bearing our questions, focuses and approaches in mind, we first distilled insights from several previous studies and established a theoretical model in which the relationship between the stress hormones and memory is viewed from a new perspective that is centred on the time-dependency consideration of hormonal actions. We considered this as the starting-point to subsequent experimental attempts. In Chapter 2, we elaborated on the proposed theory by elucidating several variables that can drive stress actions towards defined modulation of memory. Here, we highlighted the significance of convergence in time and space of the "stress factors" and the learning and memory activity to which they are intrinsically linked. Stress is part of the context or history of this learning activity, it modifies the way how new information is acquired and retained. Functional regulation relies upon the molecular actions of modulatory factors. In our theory, the best example is given by glucocorticoids. This major type of stress hormones, by employing different, timedependent mechanisms - genomic and nongenomic, can result in opposite regulations of neuronal physiology. The fast, nongenomic action, in combination with the effects from other fast-acting factors (e.g. noradrenaline, CRH), can facilitate synaptic transmission while the slow-onset, genemediate mechanism simply impairs it. In the following, we aimed to acquire more substantial evidence in support of this view. We extended our observations concerning (inter)actions of corticosteroids and β-adrenoceptor agonists from the hippocampal CA1 area to the dentate gyrus. As shown in Chapter 3, experiments were performed in in vitro rats brain slices. In the dentate gyrus, we were unable to demonstrate a corticosterone-mediated effect on LTP per se; however, a certain effect was revealed during inhibition of GABAergic transmission. A β-adrenergic agonist, isoproterenol, was capable of facilitating the induction of LTP after brief administration. The major finding was that, if corticosterone was supplied in unison with isoproterenol and LTP induction, synaptic potentiation was accelerated (or enhanced during the early stage of potentiation), while a pretreatment of corticosterone hours in advance of isoproterenol and synaptic stimulation unequivocally hampered the LTP induced later on. This finding indicates that the time-dependent mechanisms of corticosteroids can enable bidirectional modifications of the interaction between the glucocorticoid and β-adrenergic systems, with direct impacts on activity-dependent synaptic plasticity within the hippocampus. Reported in Chapter 4, we were able to replicate this study in the rat basolateral amygdala. Similarly, the β-adrenergic agonist, isoproterenol was shown to facilitate the induction of LTP within the amygdala, representing the efficacy of a fast-acting factor. However, corticosterone application predominantly displayed a suppressive effect on LTP, which was demonstrated by a gradual reversal of β-adrenergic-facilitated LTP in the case of co-application of corticosterone and isoproterenol around LTP induction, and by a full suppression of the isoproterenol-mediated LTP in the case of corticosterone pretreatment hours in advance. All these represent a slow-onset suppressive effect of corticosteroids on synaptic plasticity in the amygdala - a structure highly associated with emotional memory. We eventually elevated our research level to human study (Chapter 5), in which we asked our subjects to view and memorise both neutral and (negatively) emotional pictures and had their real-time brain activity monitored through fMRI. We concentrated on the time-dependent effects of glucocorticoids by administering an identical stress dose of cortisol to subjects at different time-points. In this study, we have identified a general enhanced memory of negative stimuli. Moreover, a prior adminiA temporal perspective on stress hormones and memory stration of cortisol (several hours in advance) was found to diminish the proportion of the negative pictures among all pictures remembered, without altering the absolute numbers of the remembered pictures, indicating a weight shift in memory encoding for emotional information. Neuroimaging has located this effect to the left hippocampus. This demonstrates a delayed, presumably genomic, glucocorticoid effect on human emotional memory. In conclusion, we have investigated two types of stress hormones, focusing on measurement of functional outcomes and exploitation of multiple levels of research (i.e. in the rodent and human brains). Our results support the notion that a focus on the molecular mechanisms of stress hormones is valid in the exploration of stress-mediated regulation of cognitive functions. These molecular actions can drive diverse regulatory patterns, best exemplified by the effects of glucocorticoids; indeed, we have demonstrated their facilitative effects and suppressive actions in specific contexts. Above all, we have proposed a new perspective to take in examination of stress influences, which is fundamentally based upon the understanding of the time-dependency of the hormonal functions; by doing so, we can add significant value to established theories. Especially from human-level research, our results indicate a significant relevance to the control of the emotional bias reflected in memory - likely achievable through glucocorticoids manipulation; this may lend support to newly developed therapeutic approaches in the field

Magnetic field configuration and field-aligned acceleration of energetic ions during substorm onsets

International audienceIn this paper, we present an interpretation of the observed field-aligned acceleration events measured by GEOS-2 near the night-side synchronous orbit at substorm onsets (Chen et al., 2000). We show that field-aligned acceleration of ions (with pitch angle asymmetry) is closely related to strong short-lived electric fields in the Ey direction. The acceleration is associated with either rapid dipolarization or further stretching of local magnetic field lines. Theoretical analysis suggests that a centrifugal mechanism is a likely candidate for the parallel energization. Equatorward or anti-equatorward energization occurs when the tail current sheet is thinner tailward or earthward of the spacecraft, respectively. The magnetic field topology leading to anti-equatorward energization corresponds to a situation where the near-Earth tail undergoes further compression and the inner edge of the plasma sheet extends inwards as close as the night-side geosynchronous altitudes

Density oscillations in trapped dipolar condensates

We investigated the ground state wave function and free expansion of a trapped dipolar condensate. We find that dipolar interaction may induce both biconcave and dumbbell density profiles in, respectively, the pancake- and cigar-shaped traps. On the parameter plane of the interaction strengths, the density oscillation occurs only when the interaction parameters fall into certain isolated areas. The relation between the positions of these areas and the trap geometry is explored. By studying the free expansion of the condensate with density oscillation, we show that the density oscillation is detectable from the time-of-flight image.Comment: 7 pages, 9 figure

Finite Element Simulation of Residual Stresses in Cryogenic Machining of AZ31B Mg Alloy

AbstractMagnesium alloys are lightweight materials primarily used in transportation industry, and are also emerging as a potential material for biodegradable fixation implants. However, unsatisfactory corrosion resistance largely limits the application of these materials. Residual stresses were reported to have significant influence on corrosion resistance of Mg alloys. In this study, a finite element model was developed to simulate the residual stresses in cryogenic machining of AZ31B Mg alloy. After calibration using experimental data, numerical simulations were conducted to study the influence of cutting edge radius and cooling method (dry vs. cryogenic) on residual stresses. The model can be used to establish proper cutting conditions to induce compressive residual stresses to enhance the corrosion resistance of Mg alloys

Structural phase transitions of vortex matter in an optical lattice

We consider the vortex structure of a rapidly rotating trapped atomic Bose-Einstein condensate in the presence of a co-rotating periodic optical lattice potential. We observe a rich variety of structural phases which reflect the interplay of the vortex-vortex and vortex-lattice interactions. The lattice structure is very sensitive to the ratio of vortices to pinning sites and we observe structural phase transitions and domain formation as this ratio is varied.Comment: 4 pages, 3 figure

Learning Complex Motor Skills for Legged Robot Fall Recovery

Falling is inevitable for legged robots in challenging real-world scenarios, where environments are unstructured and situations are unpredictable, such as uneven terrain in the wild. Hence, to recover from falls and achieve all-terrain traversability, it is essential for intelligent robots to possess the complex motor skills required to resume operation. To go beyond the limitation of handcrafted control, we investigated a deep reinforcement learning approach to learn generalized feedback-control policies for fall recovery that are robust to external disturbances. We proposed a design guideline for selecting key states for initialization, including a comparison to the random state initialization. The proposed learning-based pipeline is applicable to different robot models and their corner cases, including both small-/large-size bipeds and quadrupeds. Further, we show that the learned fall recovery policies are hardware-feasible and can be implemented on real robots

Exact solution of gyration radius of individual's trajectory for a simplified human mobility model

Gyration radius of individual's trajectory plays a key role in quantifying human mobility patterns. Of particular interests, empirical analyses suggest that the growth of gyration radius is slow versus time except the very early stage and may eventually arrive to a steady value. However, up to now, the underlying mechanism leading to such a possibly steady value has not been well understood. In this Letter, we propose a simplified human mobility model to simulate individual's daily travel with three sequential activities: commuting to workplace, going to do leisure activities and returning home. With the assumption that individual has constant travel speed and inferior limit of time at home and work, we prove that the daily moving area of an individual is an ellipse, and finally get an exact solution of the gyration radius. The analytical solution well captures the empirical observation reported in [M. C. Gonz`alez et al., Nature, 453 (2008) 779]. We also find that, in spite of the heterogeneous displacement distribution in the population level, individuals in our model have characteristic displacements, indicating a completely different mechanism to the one proposed by Song et al. [Nat. Phys. 6 (2010) 818].Comment: 4 pages, 4 figure

Consistent response of Indian summer monsoon to Middle East dust in observations and simulations

The response of the Indian summer monsoon (ISM) circulation and precipitation to Middle East dust aerosols on sub-seasonal timescales is studied using observations and the Weather Research and Forecasting model coupled with online chemistry (WRF-Chem). Satellite data show that the ISM rainfall in coastal southwest India, central and northern India, and Pakistan is closely associated with the Middle East dust aerosols. The physical mechanism behind this dust–ISM rainfall connection is examined through ensemble simulations with and without dust emissions. Each ensemble includes 16 members with various physical and chemical schemes to consider the model uncertainties in parameterizing short-wave radiation, the planetary boundary layer, and aerosol chemical mixing rules. Experiments show that dust aerosols increase rainfall by about 0.44 mm day−1 (~10 % of the climatology) in coastal southwest India, central and northern India, and north Pakistan, a pattern consistent with the observed relationship. The ensemble mean rainfall response over India shows a much stronger spatial correlation with the observed rainfall response than any other ensemble members. The largest modeling uncertainties are from the boundary layer schemes, followed by short-wave radiation schemes. In WRF-Chem, the dust aerosol optical depth (AOD) over the Middle East shows the strongest correlation with the ISM rainfall response when dust AOD leads rainfall response by about 11 days. Further analyses show that increased ISM rainfall is related to enhanced southwesterly monsoon flow and moisture transport from the Arabian Sea to the Indian subcontinent, which are associated with the development of an anomalous low-pressure system over the Arabian Sea, the southern Arabian Peninsula, and the Iranian Plateau due to dust-induced heating in the troposphere. The dust-induced heating in the mid-upper troposphere is mainly located in the Iranian Plateau rather than the Tibetan Plateau. This study demonstrates a thermodynamic mechanism that links remote desert dust emissions in the Middle East to ISM circulation and precipitation variability on sub-seasonal timescales, which may have implications for ISM rainfall forecasts

Dynamic overview techniques for image retrieval

One difficulty often overlooked in information retrieval systems is that search criteria themselves are often poorly defined. People describe their information needs in many different ways and frequently change their goals depending on the current results of their search. We have investigated the hypothesis that overviews of the space of available solutions are a good way to remedy this situation. Our overview techniques allow users to get a feel for the meaning of categories through randomly chosen examples, find similar images using content search, and to inspect the global distribution of images according to certain criteria. Users thus organize the retrieval task into an iterative browsing process that makes them specify their queries more accurately. As a result they are more satisfied with what the system retrieve