Enriched environment, nitric oxide production and synaptic plasticity prevent the aging-dependent impairment of spatial cognition

In rodents, neuronal plasticity decreases and spatial learning and working memory deficits increase upon aging. Several authors have shown that rats reared in enriched environments have better cognitive performance in association with increased neuronal plasticity than animals reared in standard environments. We hypothesized that enriched environment could preserve animals from the age-associated neurological impairments, mainly through NO-dependent mechanisms of induction of neuronal plasticity. We present evidence that 27 months old rats from an enriched environment show a better performance in spatial working memory than standard reared rats of the same age. Both mtNOS and cytosolic nNOS activities were found significantly increased (73% and 155%, respectively) in female rats from enriched environment as compared with control animals kept in a standard environment. The enzymatic activity of complex I was 80% increased in rats from enriched environment as compared with control rats. We conclude that an extensively enriched environment prevents old rats from the aging-associated impairment of spatial cognition, synaptic plasticity and nitric oxide production.     

A decision-making model based on a spiking neural circuit and synaptic plasticity

To adapt to the environment and survive, most animals can control their behaviors by making decisions. The process of decision-making and responding according to cues in the environment is stable, sustainable, and learnable. Understanding how behaviors are regulated by neural circuits and the encoding and decoding mechanisms from stimuli to responses are important goals in neuroscience. From results observed in Drosophila experiments, the underlying decision-making process is discussed, and a neural circuit that implements a two-choice decision-making model is proposed to explain and reproduce the observations. Compared with previous two-choice decision making models, our model uses synaptic plasticity to explain changes in decision output given the same environment. Moreover, biological meanings of parameters of our decision-making model are discussed. In this paper, we explain at the micro-level (i.e., neurons and synapses) how observable decision-making behavior at the macro-level is acquired and achieved.     

Intracellular recording in behaving animals

Electrophysiological recordings from behaving animals provide an unparalleled view into the functional role of individual neurons. Intracellular approaches can be especially revealing as they provide information about a neuron's inputs and intrinsic cellular properties, which together determine its spiking output. Recent technical developments have made intracellular recording possible during an ever-increasing range of behaviors in both head-fixed and freely moving animals. These recordings have yielded fundamental insights into the cellular and circuit mechanisms underlying neural activity during natural behaviors in such areas as sensory perception, motor sequence generation, and spatial navigation, forging a direct link between cellular and systems neuroscience.     

Fruit fly behavior in response to chemosensory signals

An important question in contemporary sensory neuroscience is how animals perceive their environment and make appropriate behavioral choices based on chemical perceptions. The fruit fly Drosophila melanogaster exhibits robust tastant and odor-evoked behaviors. Understanding how the gustatory and olfactory systems support the perception of these contact and volatile chemicals and translate them into appropriate attraction or avoidance behaviors has made an unprecedented contribution to our knowledge of the organization of chemosensory systems. In this review, I begin by describing the receptors and signaling mechanisms of the Drosophila gustatory and olfactory systems and then highlight their involvement in the control of simple and complex behaviors. The topics addressed include feeding behavior, learning and memory, navigation behavior, neuropeptide modulation of chemosensory behavior, and I conclude with a discussion of recent work that provides insight into pheromone signaling pathways.     

The back and forth of dendritic plasticity

Synapses are located throughout the often-elaborate dendritic tree of central neurons. Hebbian models of plasticity require temporal association between synaptic input and neuronal output to produce long-term potentiation of excitatory transmission. Recent studies have highlighted how active dendritic spiking mechanisms control this association. Here, we review new work showing that associative synaptic plasticity can be generated without neuronal output and that the interplay between neuronal architecture and the active electrical properties of the dendritic tree regulates synaptic plasticity.     

Timing of Environmental Enrichment Affects Memory in the House Cricket,Acheta domesticus

Learning appears to be ubiquitous among animals, as it plays a key role in many behaviors including foraging and reproduction. Although there is some genetic basis for differences in learning ability and memory retention, environment also plays an important role, as it does for any other trait. For example, adult animals maintained in enriched housing conditions learn faster and remember tasks for longer than animals maintained in impoverished conditions. Such plasticity in adult learning ability has often been linked to plasticity in the brain, and studies aimed at understanding the mechanisms, stimuli, and consequences of adult behavioral and brain plasticity are numerous. However, the role of experiences during post-embryonic development in shaping plasticity in adult learning ability and memory retention remain relatively unexplored. Using the house cricket (Acheta domesticus) as a model organism, we developed a protocol to allow the odor preference of a large number of crickets to be tested in a short period of time. We then used this new protocol to examine how enrichment or impoverishment at two developmental stages (either the last nymphal instar or young adult) affected adult memory. Our results show that regardless of nymphal rearing conditions, crickets that experienced an enriched rearing condition as young adults performed better on a memory task than individuals that experienced an impoverished condition. Older adult crickets (more than 1 week post adult molt) did not demonstrate differences in memory of the odor task, regardless of rearing condition as a young adult. Our results suggest that environmentally-induced plasticity in memory may be restricted to the young adult stage.     

Dendrite remodeling in development and disease

One of the most important features of neuronal function is the capacity to dynamically adapt in response to changes in the environment and neuronal activity. Among cellular elements that show this kind of plasticity are dendrites, the components that receive and process neuronal inputs. Dendrite remodeling occurs during normal development of the nervous system as well as in response to injury or diseases in the adult. In either case, selective stabilization and/or elimination of dendritic branches is likely important to shape dendritic arbors. Here I review examples of the phenomena and consider potential cellular and molecular mechanisms that underlie dendrite remodeling and how they might relate in development and disease.     

Cellular bases of behavioral plasticity: establishing and modifying synaptic circuits in the Drosophila genetic system

Genetic malleability and amenability to behavioral assays make Drosophila an attractive model for dissecting the molecular mechanisms of complex behaviors, such as learning and memory. At a cellular level, Drosophila has contributed a wealth of information on the mechanisms regulating membrane excitability and synapse formation, function, and plasticity. Until recently, however, these studies have relied almost exclusively on analyses of the peripheral neuromuscular junction, with a smaller body of work on neurons grown in primary culture. These experimental systems are, by themselves, clearly inadequate for assessing neuronal function at the many levels necessary for an understanding of behavioral regulation. The pressing need is for access to physiologically relevant neuronal circuits as they develop and are modified throughout life. In the past few years, progress has been made in developing experimental approaches to examine functional properties of identified populations of Drosophila central neurons, both in cell culture and in vivo. This review focuses on these exciting developments, which promise to rapidly expand the frontiers of functional cellular neurobiology studies in Drosophila. We discuss here the technical advances that have begun to reveal the excitability and synaptic transmission properties of central neurons in flies, and discuss how these studies promise to substantially increase our understanding of neuronal mechanisms underlying behavioral plasticity.     

Role of delayed nonsynaptic neuronal plasticity in long-term associative memory

BACKGROUND: It is now well established that persistent nonsynaptic neuronal plasticity occurs after learning and, like synaptic plasticity, it can be the substrate for long-term memory. What still remains unclear, though, is how nonsynaptic plasticity contributes to the altered neural network properties on which memory depends. Understanding how nonsynaptic plasticity is translated into modified network and behavioral output therefore represents an important objective of current learning and memory research. RESULTS: By using behavioral single-trial classical conditioning together with electrophysiological analysis and calcium imaging, we have explored the cellular mechanisms by which experience-induced nonsynaptic electrical changes in a neuronal soma remote from the synaptic region are translated into synaptic and circuit level effects. We show that after single-trial food-reward conditioning in the snail Lymnaea stagnalis, identified modulatory neurons that are extrinsic to the feeding network become persistently depolarized between 16 and 24 hr after training. This is delayed with respect to early memory formation but concomitant with the establishment and duration of long-term memory. The persistent nonsynaptic change is extrinsic to and maintained independently of synaptic effects occurring within the network directly responsible for the generation of feeding. Artificial membrane potential manipulation and calcium-imaging experiments suggest a novel mechanism whereby the somal depolarization of an extrinsic neuron recruits command-like intrinsic neurons of the circuit underlying the learned behavior. CONCLUSIONS: We show that nonsynaptic plasticity in an extrinsic modulatory neuron encodes information that enables the expression of long-term associative memory, and we describe how this information can be translated into modified network and behavioral output.     

Epigenetic mechanisms in cognition

Although the critical role for epigenetic mechanisms in development and cell differentiation has long been appreciated, recent evidence reveals that these mechanisms are also employed in postmitotic neurons as a means of consolidating and stabilizing cognitive-behavioral memories. In this review, we discuss evidence for an "epigenetic code" in the central nervous system that mediates synaptic plasticity, learning, and memory. We consider how specific epigenetic changes are regulated and may interact with each other during memory formation and how these changes manifest functionally at the cellular and circuit levels. We also describe a central role for mitogen-activated protein kinases in controlling chromatin signaling in plasticity and memory. Finally, we consider how aberrant epigenetic modifications may lead to cognitive disorders that affect learning and memory, and we review the therapeutic potential of epigenetic treatments for the amelioration of these conditions.     

Neural strategies for learning during sensitive periods of development

Precise patterns of neural connectivity and synaptic communication are modified by experience during restricted "sensitive" periods of development, and the circuitry and associated behaviors that emerge during such periods are frequently preserved throughout the lifespan. In many neural systems, the expression of various molecules that influence synaptic transmission and neuronal morphology are developmentally regulated and may serve to constrain the timing of enhanced sensitivity to experiential inputs. In this highly selective review I concentrate on recent findings from the visual system and the song system that provide novel insights into the mechanisms that regulate sensitive period plasticity, and that raise new questions concerning what makes sensitive periods so sensitive.     

The role of syndecans in the regulation of body weight and synaptic plasticity

Body weight is tightly regulated by a feedback mechanism involving peripheral adiposity signals and multiple central nervous system neurotransmitter pathways. Despite the tight regulation of body weight there is an increase in the prevalence of obesity and overweight in Western society. Obesity and overweight are conditions of excess body weight stored as fat. Syndecan-3, a member of the syndecan family of type I transmembrane heparan sulfate proteoglycans is a novel a regulator of feeding behavior and body weight. Syndecans are extracellular matrix molecules (ECMs) that modulate cell adhesion, cell-cell interactions and ligand-receptor interactions. The finding that syndecan-3 can regulate body weight is novel and provides a unique link between the extracellular matrix and body weight regulatory mechanisms. Uniquely, hormones such as leptin previously thought only to regulate body weight by modulating neuropeptide levels, have now been demonstrated to regulate neuronal plasticity in the hypothalamus. ECMs and syndecans have long been recognized as regulators of plasticity. Therefore, this review will focus on highlighting the role of syndecans and in particular syndecan-3 in neuronal development and synaptic organization and how these processes may integrate body weight regulation. As part of this review, we will highlight how syndecan-3 can mediate the activity of adiposity signals, such as leptin, and facilitate changes in neuronal plasticity.     

Plasticity as a developing trait: exploring the implications

Individual differences in plasticity have been classically framed as genotype-by-environment interactions, with different genotypes showing different reaction norms in response to environmental conditions. However, research has shown that early experience can be a critical factor in shaping an individual's plasticity to later environmental factors. In other words, plasticity itself can be investigated as a developing trait that reflects the combined action of an individual's genes and previous interactions with the environment. In this paper I explore some implications of the idea that the early environment modulates long-term plasticity, with an emphasis on plasticity in behavioral traits. I begin by focusing on the mechanisms that mediate plasticity at the proximate level, and discussing the possibility that some traits may work as generalized mediators of plasticity by affecting the sensitivity of multiple phenol types across developmental contexts. I then tackle the complex problem of the evolution of reaction norms for plasticity. Next, I consider a number of potential implications for research on parental effects and phenotypic matching, and conclude by discussing how plasticity may become a target of evolutionary conflict between parents and offspring. In total, I aim to show how the idea of plasticity as a developing trait offers a rich source of questions and insights that may inform future research in this area.     

Regulation of neurogenesis by neurotrophins: implications in hippocampus-dependent memory

Neurogenesis, the generation of new neurons from neural precursor cells (NPCs), is a multi-step process that includes the proliferation of NPCs, fate determination, migration, and neuronal maturation. Neurogenesis is regulated by several extrinsic factors,such as enriched environment, physical exercise, hormones and stress, many of which also induce the expression of neurotrophins.In this review, we summarize studies on the role of neurotrophins in neurogenesis during development and in adults.We discuss the functional significance of neurogenesis in learning and memory, and how neurotrophins regulate this process.In this context, we describe recent experiments linking adult neurogenesis to long-term synaptic plasticity in the hippocampal dentate gyrus. Further study of the relationship between neurotrophins, adult neurogenesis and dentate synaptic plasticity might provide new insights into the mechanisms by which gene-environment interactions control cognition and brain plasticity.     

Are latitudinal clines in body size adaptive?

Body size of animals often increases with increasing latitude. These latitudinal clines in body size have interested biologists for over 150 years. However, the mechanisms that generate these clines in size are still unclear, though latitudinal gradients in temperature appear to play an important role. More importantly, many studies that examine latitudinal clines in body size and the mechanisms responsible for these clines use phenotypic data, confounding genetic (adaptive) and non‐genetic (plasticity) sources of variation. Yet, most of these studies make adaptive conclusions based on phenotypic measures of size. Here I show the dangers of making adaptive inferences from phenotypic measures of size. In addition, I use a specific form of plasticity in body size of ectotherms, called the temperature–size rule, to illustrate how confusion about genetic and non‐genetic contributions to phenotypic variation has hampered progress in understanding the evolution of latitudinal clines in size. Field‐based measurements of body size can no doubt be influenced by plasticity, but demonstrating that latitudinal clines have a genetic basis is necessary to show that these patterns are adaptive.     

Historical and ecological controls on phylogenetic diversity in Californian plant communities

An unresolved controversy regarding social behaviors is exemplified when natural selection might lead to behaviors that maximize fitness at the social-group level but are costly at the individual level. Except for the special case of groups of clones, we do not have a general understanding of how and when group-optimal behaviors evolve, especially when the behaviors in question are flexible. To address this question, we develop a general model that integrates behavioral plasticity in social interactions with the action of natural selection in structured populations. We find that group-optimal behaviors can evolve, even without clonal groups, if individuals exhibit appropriate behavioral responses to each other's actions. The evolution of such behavioral responses, in turn, is predicated on the nature of the proximate behavioral mechanisms. We model a particular class of proximate mechanisms, prosocial preferences, and find that such preferences evolve to sustain maximum group benefit under certain levels of relatedness and certain ecological conditions. Thus, our model demonstrates the fundamental interplay between behavioral responses and relatedness in determining the course of social evolution. We also highlight the crucial role of proximate mechanisms such as prosocial preferences in the evolution of behavioral responses and in facilitating evolutionary transitions in individuality.     

How stabilizing selection and nongenetic inheritance combine to shape the evolution of phenotypic plasticity

Relatively little is known about whether and how nongenetic inheritance interacts with selection to impact the evolution of phenotypic plasticity. Here, we empirically evaluated how stabilizing selection and a common form of nongenetic inheritance—maternal environmental effects—jointly influence the evolution of phenotypic plasticity in natural populations of spadefoot toads. We compared populations that previous fieldwork has shown to have evolved conspicuous plasticity in resource‐use phenotypes (“resource polyphenism”) with those that, owing to stabilizing selection favouring a narrower range of such phenotypes, appear to have lost this plasticity. We show that: (a) this apparent loss of plasticity in nature reflects a condition‐dependent maternal effect and not a genetic loss of plasticity, that is “genetic assimilation,” and (b) this plasticity is not costly. By shielding noncostly plasticity from selection, nongenetic inheritance generally, and maternal effects specifically, can preclude genetic assimilation from occurring and consequently impede adaptive (genetic) evolution.     

The biological principles of swarm intelligence

The roots of swarm intelligence are deeply embedded in the biological study of self-organized behaviors in social insects. From the routing of traffic in telecommunication networks to the design of control algorithms for groups of autonomous robots, the collective behaviors of these animals have inspired many of the foundational works in this emerging research field. For the first issue of this journal dedicated to swarm intelligence, we review the main biological principles that underlie the organization of insects’ colonies. We begin with some reminders about the decentralized nature of such systems and we describe the underlying mechanisms of complex collective behaviors of social insects, from the concept of stigmergy to the theory of self-organization in biological systems. We emphasize in particular the role of interactions and the importance of bifurcations that appear in the collective output of the colony when some of the system’s parameters change. We then propose to categorize the collective behaviors displayed by insect colonies according to four functions that emerge at the level of the colony and that organize its global behavior. Finally, we address the role of modulations of individual behaviors by disturbances (either environmental or internal to the colony) in the overall flexibility of insect colonies. We conclude that future studies about self-organized biological behaviors should investigate such modulations to better understand how insect colonies adapt to uncertain worlds.