Chronic stress: implications for neuronal morphology, function and neurogenesis

Functional actions of corticosteroids in the hippocampus

Corticosteroid hormones are released in high amounts after stress. The hormones enter the brain compartment and bind to high affinity mineralocorticoid receptors--particularly enriched in limbic regions--as well as to lower affinity glucocorticoid receptors which are more ubiquitous. Shortly after the stressful event, corticosteroids (in concert with specific monoamines and neuropeptides) have the potential to increase cellular excitability in subfields of the hippocampus, like the CA1 area. These effects are rapid in onset and occur via a nongenomic pathway. At the same time, however, the hormones also start slower, gene-mediated processes. These cause attenuation of excitatory information flow through the CA1 hippocampal area. Induction of long-term potentiation at that time is impaired. This may help to normalize hippocampal activity some hours after the stressful event and preserve information encoded within the context of the event. These adaptational effects of the hormones may become maladaptive if the stressful event is associated with other challenges of the network (like ischemic insults) or when stress occurs repetitively, in an uncontrollable and unpredictable manner. In that case, i) normalization of activity seems to be less efficient (particularly when other limbic areas like the amygdala nuclei are activated during stress), ii) induction of long-term potentiation is hampered at all times and iii) serotonin responses are attenuated. This may contribute to the precipitation of clinical symptoms in stress-related disorders such as major depression. A better understanding of the corticosteroid actions could lead to a more rational treatment strategy of stress-related disorders.

Brief treatment with the glucocorticoid receptor antagonist mifepristone normalizes the reduction in neurogenesis after chronic stress

In rodents, stress suppresses adult neurogenesis. This is thought to involve activation of glucocorticoid receptors in the brain. In the present study, we therefore questioned whether glucocorticoid receptor blockade by mifepristone can normalize the effects of chronic stress on adult neurogenesis. Rats received mifepristone on the last 4 days of a 21-day chronic unpredictable and inescapable stress regimen. Neurogenesis was analysed by stereological quantification of adult-generated cell survival (bromodeoxyuridine), young neuronal survival (doublecortin) and cell proliferation (Ki-67). The results show that only 4 days of mifepristone treatment normalized the stress-induced reductions in neurogenesis. Importantly, mifepristone by itself had no effect on neurogenesis. We conclude that, contrary to other compounds interfering with the effects of chronic stress on neurogenesis, like antidepressants, the normalizing effects of mifepristone on neurogenesis are rapid and particularly potent in a high stress environment. This neurogenic action of mifepristone could potentially contribute to its clinical mechanism of action.

The coming out of the brain mineralocorticoid receptor

Corticosteroids - secreted after stress - have profound effects on brain and behavior. These effects are mediated by mineralocorticoid and glucocorticoid receptors, which are abundantly expressed in limbic neurons. The role of mineralocorticoid receptors in higher brain functions has never been well understood. Here we argue that the recently discovered low-affinity membrane version of the mineralocorticoid receptor contributes to the initial phase of the stress reaction; this is complemented by the glucocorticoid receptor which terminates the stress response. This concept may explain why human carriers of a mineralocorticoid receptor gene variant display enhanced neuroendocrine and autonomic responsiveness to a psychological stressor.

Functional implications of decreases in neurogenesis following chronic mild stress in mice

Numerous data from human and animal studies suggest that hippocampal plasticity might be a key element in depression. However, the connection remains loose at best and further data are needed. Human studies are of necessity limited, but animal models can help providing further insight. Unpredictable chronic mild stress (UCMS) is a commonly used model because it mimics depression-like phenotypes satisfactorily. Its rationale is based on the underlying stress-induced difficulties found in many depressed patients. We therefore studied learning and hippocampal neurogenesis in mice from three different inbred strains subjected to UCMS. Learning was assessed in different hippocampus-dependent and independent tasks. The rate of survival of newly generated brain cells was determined in behaviorally-naive animals. Results demonstrated a dramatic reduction of surviving new brain cells in both the hippocampus and the subventricular zone of UCMS-treated animals. This reduction was observed both for neurons and for other cells of the hippocampus. Behavioral data demonstrated an impairment of hippocampus-dependent learning, whereas hippocampus-independent learning was spared. However, the specific results were strongly dependent on strain and sex so that there does not appear to be a direct causative relationship between the deficits in neurogenesis and behavior.