The brain is continuously exposed to varying levels of adrenal corticosteroid hormones such as corticosterone in rodents and cortisol in humans. Natural fluctuations occur due to ultradian and circadian variations or are caused by exposure to stressful situations. Brain cells express two types of corticosteroid receptors, i.e. mineralocorticoid and glucocorticoid receptors, which differ in distribution and affinity. These receptors can mediate both rapid non-genomic and slow gene-mediated neuronal actions. As a consequence of these factors, natural (e.g. stress-induced) shifts in corticosteroid level are associated with a complex mosaic of time- and region-dependent changes in neuronal activity. A series of experiments in humans and rodents have revealed that these time- and region-dependent cellular characteristics are also reflected in distinct cognitive patterns after stress. Thus, directly after a peak of corticosteroids, attention and vigilance are increased, and areas involved in emotional responses and simple behavioral strategies show enhanced activity. In the aftermath of stress, areas involved in higher cognitive functions become activated allowing individuals to link stressful events to the specific context and to store information for future use. Both phases of the brain’s response to stress are important to face a continuously changing environment, promoting adaptation at the short as well as long term. We argue that a balanced response during the two phases is essential for resilience. This balance may become compromised after repeated stress exposure, particularly in genetically vulnerable individuals and aggravate disease manifestation. This not only applies to psychiatric disorders but also to neurological diseases such as epilepsy.

In 1968, Bruce McEwen discovered that ³H-corticosterone administered to adrenalectomised rats is retained in neurons of hippocampus rather than those of hypothalamus. This discovery signalled the expansion of endocrinology into the science of higher brain regions. With this in mind, our contribution highlights the saga of the brain mineralocorticoid receptor (MR) in three episodes. First, the precloning era dominated by the conundrum of two types of corticosterone-binding receptors in the brain, which led to the identification of the high-affinity corticosterone receptor as the ‘promiscuous’ MR cloned in 1987 by Jeff Arriza and Ron Evans in addition to the classical glucocorticoid receptor (GR). Then, the post-cloning period aimed to disentangle the function of the brain MR from that of the closely related GR on different levels of biological complexity. Finally, the synthesis section that highlights the two faces of brain MR: Salt and Stress. ‘Salt’ refers to the regulation of salt appetite, and reciprocal arousal, motivation and reward, by a network of aldosterone-selective MR-expressing neurons projecting from nucleus tractus solitarii (NTS) and circumventricular organs. ‘Stress’ is about the limbic-forebrain nuclear and membrane MRs, which act as a switch in the selection of the best response to cope with a stressor. For this purpose, activation of the limbic MR promotes selective attention, memory retrieval and the appraisal process, while driving emotional expressions of fear and aggression. Subsequently, rising glucocorticoid concentrations activate GRs in limbic-forebrain circuitry underlying executive functions and memory storage, which contribute in balance with MR-mediated actions to homeostasis, excitability and behavioural adaptation.

In response to a stressful encounter, the brain activates a comprehensive stress system that engages the organism in an adaptive response to the threatening situation. This stress system acts on multiple peripheral tissues and feeds back to the brain; one of its key players is the family of corticosteroid hormones. Corticosteroids affect brain functioning through both delayed, genomic and rapid, non-genomic mechanisms. The latter mode of action has long been known, but it is only in recent years that the physiological basis in the brain is beginning to be unravelled. We now know that corticosteroids exert rapid, non-genomic effects on the excitability and activation of neurons in (amongst others) the hypothalamus, hippocampus, amygdala and prefrontal cortex. In addition, corticosteroids affect cognition, adaptive behaviour and neuroendocrine output within minutes. Knowledge on the identity of the receptors and secondary pathways mediating the non-genomic effects of corticosteroids on a cellular level is accumulating. Interestingly, in many cases, an essential role for the ‘classical’ mineralocorticoid and glucocorticoid receptors in a novel membrane-associated mechanism is found. Here, we systematically review the recent literature on non-genomic actions of corticosteroids on neuronal activity and functioning in selected limbic brain targets. Further, we discuss the relevance of these permissive effects for cognition and neuroendocrine control, and the integration of this novel mode of action into the complex balanced pattern of stress effects in the brain.