LTP after Stress: Up or Down?

When an organism is exposed to a stressful situation, corticosteroid levels in the brain rise. This rise has consequences for behavioral performance, including memory formation. Over the past decades, it has become clear that a rise in corticosteroid level is also accompanied by a reduction in hippocampal long-term potentiation (LTP). Recent studies, however, indicate that stress does not lead to a universal suppression of LTP. Many factors, including the type of stress, the phase of the stress response, the area of investigation, type of LTP, and the life history of the organism determine in which direction LTP will be changed

Hormonal Regulation of AMPA Receptor Trafficking and Memory Formation

Humans and rodents retain memories for stressful events very well. The facilitated retention of these memories is normally very useful. However, in susceptible individuals a variety of pathological conditions may develop in which memories related to stressful events remain inappropriately present, such as in post-traumatic stress disorder. The memory enhancing effects of stress are mediated by hormones, such as norepinephrine and glucocorticoids which are released during stressful experiences. Here we review recently identified molecular mechanisms that underlie the effects of stress hormones on synaptic efficacy and learning and memory. We discuss AMPA receptors as major target for stress hormones and describe a model in which norepinephrine and glucocorticoids are able to strengthen and prolong different phases of stressful memories

Interactions between noradrenaline and corticosteroids in the brain: from electrical activity to cognitive performance

One of the core reactions in response to a stressful situation is the activation of the hypothalamus–pituitary–adrenal axis which increases the release of glucocorticoid hormones from the adrenal glands. In concert with other neuro-modulators, such as (nor)adrenaline, these hormones enable and promote cognitive adaptation to stressful events. Recent studies have demonstrated that glucocorticoid hormones and noradrenaline, via their receptors, can both rapidly and persistently regulate the function of excitatory synapses which are critical for storage of information. Here we will review how glucocorticoids and noradrenaline alone and in synergy dynamically tune these synapses in the hippocampus and amygdala, and discuss how these hormones interact to promote behavioral adaptation to stressful situations

Regulation of Excitatory Synapses and Fearful Memories by Stress Hormones

Memories for emotionally arousing and fearful events are generally well retained. From the evolutionary point of view this is a highly adaptive behavioral response aimed to remember relevant information. However, fearful memories can also be inappropriately and vividly (re)expressed, such as in posttraumatic stress disorder. The memory formation of emotionally arousing events is largely modulated by hormones, peptides, and neurotransmitters which are released during and after exposure to these conditions. One of the core reactions in response to a stressful situation is the rapid activation of the autonomic nervous system, which results in the release of norepinephrine in the brain. In addition, stressful events stimulate the hypothalamus–pituitary–adrenal axis which slowly increases the release of glucocorticoid hormones from the adrenal glands. Here we will review how glucocorticoids and norepinephrine regulate the formation of fearful memories in rodents and humans and how these hormones can facilitate the storage of information by regulating excitatory synapses

Chronic Stress Effects on Hippocampal Structure and Synaptic Function: Relevance for Depression and Normalization by Anti-Glucocorticoid Treatment

Exposure of an organism to environmental challenges activates two hormonal systems that help the organism to adapt. As part of this adaptational process, brain processes are changed such that appropriate behavioral strategies are selected that allow optimal performance at the short term, while relevant information is stored for the future. Over the past years it has become evident that chronic uncontrollable and unpredictable stress also exerts profound effects on structure and function of limbic neurons, but the impact of chronic stress is not a mere accumulation of repeated episodes of acute stress exposure. Dendritic trees are reduced in some regions but expanded in others, and cells are generally exposed to a higher calcium load upon depolarization. Synaptic strengthening is largely impaired. Neurotransmitter responses are also changed, e.g., responses to serotonin. We here discuss: (a) the main cellular effects after chronic stress with emphasis on the hippocampus, (b) how such effects could contribute to the development of psychopathology in genetically vulnerable individuals, and (c) their normalization by brief treatment with anti-glucocorticoids

Application of a pharmacological transcriptome filter identifies a shortlist of mouse glucocorticoid receptor target genes associated with memory consolidation

Glucocorticoids regulate memory consolidation, facilitating long-term storage of relevant information to adequately respond to future stressors in similar conditions. This effect of glucocorticoids is well-established and is observed in multiple types of behaviour that depend on various brain regions. By and large, higher glucocorticoid levels strengthen event-related memory, while inhibition of glucocorticoid signalling impairs consolidation. The mechanism underlying this glucocorticoid effect remains unclear, but it likely involves the transcriptional effects of the glucocorticoid receptor (GR). We here used a powerful paradigm to investigate the transcriptional effects of GR in the dorsal hippocampus of mice after training in an auditory fear conditioning task, aiming to identify a shortlist of GR target genes associated to memory consolidation. Therefore, we utilized in an explorative study the properties of selective GR modulators (CORT108297 and CORT118335), alongside the endogenous agonist corticosterone and the classical GR antagonist RU486, to pinpoint GR-dependent transcriptional changes. First, we confirmed that glucocorticoids can modulate memory strength via GR activation. Subsequently, by assessing the specific effects of the available GR-ligands on memory strength, we established a pharmacological filter which we imposed on the hippocampal transcriptome data. This identified a manageable shortlist of eight genes by which glucocorticoids may modulate memory consolidation, warranting in-depth follow-up. Overall, we showcase the strength of the concept of pharmacological transcriptome filtering, which can be readily applied to other research topics with an established role of glucocorticoids

An emerging role for microglia in stress-effects on memory

Stressful experiences evoke, among others, a rapid increase in brain (nor)epinephrine (NE) levels and a slower increase in glucocorticoid hormones (GCs) in the brain. Microglia are key regulators of neuronal function and contain receptors for NE and GCs. These brain cells may therefore potentially be involved in modulating stress effects on neuronal function and learning and memory. In this review, we discuss that stress induces (1) an increase in microglial numbers as well as (2) a shift toward a pro-inflammatory profile. These microglia have (3) impaired crosstalk with neurons and (4) disrupted glutamate signaling. Moreover, microglial immune responses after stress (5) alter the kynurenine pathway through metabolites that impair glutamatergic transmission. All these effects could be involved in the impairments in memory and in synaptic plasticity caused by (prolonged) stress, implicating microglia as a potential novel target in stress-related memory impairments