The morphology of bi-directional experience-dependent cortical plasticity: a meta-analysis

The environmental sculpting hypothesis of juvenile and adult hippocampal neurogenesis

We propose that a major contribution of juvenile and adult hippocampal neurogenesis is to allow behavioral experience to sculpt dentate gyrus connectivity such that sensory attributes that are relevant to the animal's environment are more strongly represented. This "specialized" dentate is then able to store a larger number of discriminable memory representations. Our hypothesis builds on accumulating evidence that neurogenesis declines to low levels prior to adulthood in many species. Rather than being necessary for ongoing hippocampal function, as several current theories posit, we argue that neurogenesis has primarily a prospective function, in that it allows experience to shape hippocampal circuits and optimize them for future learning in the particular environment in which the animal lives. Using an anatomically-based simulation of the hippocampus (BACON), we demonstrate that environmental sculpting of this kind would reduce overlap among hippocampal memory representations and provide representation cells with more information about an animal's current situation; consequently, it would allow more memories to be stored and accurately recalled without significant interference. We describe several new, testable predictions generated by the sculpting hypothesis and evaluate the hypothesis with respect to existing evidence. We argue that the sculpting hypothesis provides a strong rationale for why juvenile and adult neurogenesis occurs specifically in the dentate gyrus and why it declines significantly prior to adulthood.

Structural Components of Synaptic Plasticity and Memory Consolidation

Consolidation of implicit memory in the invertebrate Aplysia and explicit memory in the mammalian hippocampus are associated with remodeling and growth of preexisting synapses and the formation of new synapses. Here, we compare and contrast structural components of the synaptic plasticity that underlies these two distinct forms of memory. In both cases, the structural changes involve time-dependent processes. Thus, some modifications are transient and may contribute to early formative stages of long-term memory, whereas others are more stable, longer lasting, and likely to confer persistence to memory storage. In addition, we explore the possibility that trans-synaptic signaling mechanisms governing de novo synapse formation during development can be reused in the adult for the purposes of structural synaptic plasticity and memory storage. Finally, we discuss how these mechanisms set in motion structural rearrangements that prepare a synapse to strengthen the same memory and, perhaps, to allow it to take part in other memories as a basis for understanding how their anatomical representation results in the enhanced expression and storage of memories in the brain.

Motherhood induces and maintains behavioral and neural plasticity across the lifespan in the rat

Maternal behavior is multidimensional, encompassing many facets beyond the direct care of the young. Formerly unfamiliar activities are required of the mother. These include behaviors such as retrieving, grouping, crouching-over, and licking young, and protecting them against predators, together with enhancements in other behaviors, such as nest building, foraging, and aggression (inter/intra-species, predatory, etc.). When caring for young, the mother must strike a seemingly lose-lose bargain: leave the relative safety of the nest and her helpless offspring to forage for food and resources where predators await both mother and her vulnerable young, or remain entrenched and safe, thereby ensuring a slow and inexorable fate. Two predictions thus arise from this maternal cost-benefit ratio: first, there may be enhancements in behaviors on which the female relies, for example, predation and spatial ability, used for acquiring food and resources and for navigating her environment. Second, there may be reductions in the fear and anxiety inherent to the decision to leave the nest and to forage in an unforgiving environment where encounters with predators or reluctant/resistant prey await. There is overwhelming support for both hypotheses, with improvements in learning and memory accompanied by a diminution in stress responses and anxiety. The current review will examine the background for the phenomenon that is the maternal brain, and recent relevant data. In sum, the data indicate a remarkable set of changes that take place in the maternal (and, to a lesser extent, the paternal), brain, arguably, for the natural, simple but singular experience of reproduction.

Ultrastructural plasticity associated with hippocampal-dependent learning: A meta-analysis

In order to develop a profile of how individual synapses in the hippocampal formation alter their structure following learning experience, a meta-analysis synthesized the available literature on morphological change following hippocampal-dependent learning. Analysis of the 132 calculated effect sizes suggest a consistent profile of morphological change in the hippocampus following learning experience. Across the hippocampal formation, dendritic complexity, spine density, and the size of perforated postsynaptic densities showed consistent increases following training. Both the density of synapses in general and perforated synapses in particular showed unique responses to training, depending on the duration of training and/or different cell layers of the hippocampal formation. Most importantly, it seems that this profile, while consistent, is small and specific--only a select few of the morphological parameters typically measured in anatomical studies of plasticity showed significant change following training. Collectively, these data suggest that the distinct electrophysiological properties of neocortical versus hippocampal synapses may be at least partially mediated by distinct morphological cascades. That is, on the basis of theory, and with the support of the current data, it seems that synaptogenesis correlates with enduring neocortical plasticity, while structural changes correlate with more transient hippocampal plasticity. To be able to state these conclusions with conviction, however, more data are needed in several key areas for continued pursuit of the morphological correlates of hippocampal-dependent learning.