Although contradicting evidence exists, the production of new neurons, or neurogenesis, has convincingly been shown in countless mammalian species up into adulthood, namely in the subventricular zone and hippocampal dentate gyrus. Stress has been shown to alter brain plasticity, and furthermore, inhibit neurogenesis in the hippocampus in which changes appear to be mediated not only by glucocorticoid hormones and their interaction with gonadal hormones, but also by the context in which stress occurs, and the valence and duration of the stressor. This chapter provides an overview of the role of stress in adult hippocampal neurogenesis and how these stress-induced changes may alter or change behavior and cognition, suggesting a potential target for better understanding stress-related disorders.

The medial prefrontal cortex (mPFC) is important for cognitive flexibility, the ability to switch between two task-relevant dimensions. Changes in neuronal oscillations and alterations in the coupling across frequency ranges have been correlated with attention and cognitive flexibility. Here we show that astrocytes in the mPFC of adult male Sprague Dawley rats, participate in cognitive flexibility through the astrocyte-specific Ca²⁺ binding protein S100β, which improves cognitive flexibility and increases phase amplitude coupling between theta and gamma oscillations. We further show that reduction of astrocyte number in the mPFC impairs cognitive flexibility and diminishes delta, alpha and gamma power. Conversely, chemogenetic activation of astrocytic intracellular Ca²⁺ signaling in the mPFC enhances cognitive flexibility, while inactivation of endogenous S100β among chemogenetically activated astrocytes in the mPFC prevents this improvement. Collectively, our work suggests that astrocytes make important contributions to cognitive flexibility and that they do so by releasing a Ca²⁺ binding protein which in turn enhances coordinated neuronal oscillations.

Increases in the number and/or the size of dendritic spines, sites of excitatory synapses, have been linked to different types of learning as well as synaptic plasticity in several brain regions, including the hippocampus, sensory cortex, motor cortex, and cerebellum. By contrast, a previous study reported that dorsal striatum-dependent maze learning has no effect on medium spiny neuron dendritic spines in the dorsal striatum. These findings might suggest brain region-specific differences in levels of plasticity as well as different cellular processes underlying different types of learning. No previous studies have investigated whether dendritic spine density changes may be localized to specific subpopulations of medium spiny neurons, nor have they examined dorsolateral striatum-dependent maze trained rats in comparison to an untrained maze-enriched control group. To address these questions further, we labeled medium spiny neurons with the lipophilic dye DiI and stained for the protein product of immediate early gene zif 268, an indirect marker of neuronal activation, in both trained and untrained maze groups. We found a small but significant increase in dendritic spine density on medium spiny neurons of the dorsolateral striatum after early intensive training, along with robust increases in the density of spines with mushroom morphology coincident with reductions in the density of spines with thin morphology. However, these results were not correlated with zif 268 expression. Our results suggest that short-term intensive training on a dorsolateral striatum-dependent learning paradigm induces rapid increases in dendritic spine density and maturation on medium spiny neurons of the dorsolateral striatum, an effect which may contribute to early acquisition of the learned response in maze training.

Autism spectrum disorder (ASD) is often associated with cognitive deficits and excessive anxiety. Neuroimaging studies have shown atypical structure and neural connectivity in the hippocampus, medial prefrontal cortex (mPFC) and striatum, regions associated with cognitive function and anxiety regulation. Adult hippocampal neurogenesis is involved in many behaviors that are disrupted in ASD including cognition, anxiety and social behaviors. Additionally, glial cells, such as astrocytes and microglia, are important for modulating neural connectivity during development, and glial dysfunction has been hypothesized to be a key contributor to the development of ASD. Cells with astroglial characteristics are known to serve as progenitor cells in the developing and adult brain. Here, we examined adult neurogenesis in the hippocampus, as well as astroglia and microglia in the hippocampus, mPFC and striatum of two models which display autism-like phenotypes, Cntnap2-/- and Shank3+/ΔC transgenic mice. We found a substantial decrease in the number of immature neurons and radial glial progenitor cells in the ventral hippocampus of both transgenic models compared to wild-type controls. No consistent differences were detected in the number or size of astrocytes or microglia in any other brain region examined. Future work is needed to explore the functional contribution of adult neurogenesis to autism-related behaviors as well as to temporally characterize glial plasticity as it is associated with ASD.

A variety of experiences have been shown to affect the production of neurons in the adult hippocampus. These effects may be mediated by experience-driven hormonal changes, which, in turn, interact with factors such as sex, age and life history to alter brain plasticity. Although the effects of physical experience and stress have been extensively characterized, various types of social experience across the lifespan trigger profound neuroendocrine changes in parallel with changes in adult neurogenesis. This review article focuses on the influence of specific social experiences on adult neurogenesis in the dentate gyrus and the potential role of hormones in these effects.

The purine nucleoside adenosine has the critical autacoid function of directly linking cellular excitability to energy availability. The mechanism is activated whenever the rate of adenosine triphosphate (ATP) utilization exceeds the rate of synthesis. In CNS neurons, adenosine is produced by the rapid intracellular hydrolysis of purine nucleotides during neural excitation and then is extruded into extracellular space. The nucleoside is also produced by the extracellular hydrolysis of ATP by ectonucleotidases. Extracellular adenosine interacts with G-protein linked stereospecific receptors to reestablish metabolic homeostasis by exerting extraordinarily potent inhibition of neural excitation via a number of mechanisms. This autacoid mechanism is directly linked to the production of a depression-like behavioral state termed conservation-withdrawal during times of physical stress or severe emotional distress. We review evidence here that adenosine produces a transition to conservation-withdrawal by activation of A2A receptors in the ventral-medial striatum.