Deborah N. Alfarez

Effects of chronic stress on structure and cell function in rat hippocampus and hypothalamus

It has become increasingly clear that the increase in corticosteroid levels, e.g. after a brief stressor induce molecular and cellular changes in brain, including the hippocampal formation. These effects eventually result in behavioral adaptation. Prolonged exposure to stress, though, may lead to mal-adaptation and even be a risk factor for diseases like major depression in genetically predisposed individuals. We conducted a series of experiments where changes in brain function were examined after 3 weeks of unpredictable stress. After unpredictable stress, inhibitory input to neurons involved in the hypothalamus-pituitary-adrenal (HPA) axis regulation was suppressed, which may dysregulate the axis and lead to overexposure of the brain to glucocorticoids. Furthermore, glutamate transmission in the dentate gyrus (DG) was enhanced, possibly through transcriptional regulation of receptor subunits. Combined with enhanced calcium channel expression this could increase vulnerability to cell death. Neurogenesis and apoptosis in the dentate were diminished. Synaptic plasticity was suppressed both in the dentate and CA1 area. Collectively, these effects may give rise to deficits in memory formation. Finally, we observed reduced responses to serotonin in the CA1 area, which could contribute to the onset of symptoms of depression in predisposed individuals. All of these endpoints provide potential targets for novel treatment strategies of stress-related brain disorders.

Corticosterone and stress reduce synaptic potentiation in mouse hippocampal slices with mild stimulation

Elevation of circulating corticosterone levels, either through exogenous administration of the hormone or following stress exposure, is known to reduce hippocampal synaptic potentiation in rodents. It is presently debated whether this reduction is due to activation of hippocampal glucocorticoid receptors or is primarily caused in other brain structures projecting to the hippocampus. To address this issue, we examined whether synaptic potentiation in hippocampal slices from mice with low basal corticosterone levels was altered 1-4 h after a brief in vitro administration of 100 nM corticosterone. Population spike and field excitatory postsynaptic potential (fEPSP) were recorded in the cell and dendritic layers, respectively, of the CA1 area, in response to Schaffer collateral/commissural fiber stimulation. Basal characteristics of the stimulus-response relationship were not affected by corticosterone treatment, except that after corticosterone treatment the maximal fEPSP slope was reduced while the excitability ratio was increased. For studies on potentiation of the fEPSP and population spike, stimulus intensities were chosen to evoke half maximal responses before potentiation; this intensity was significantly lower for the fEPSP than for the population spike. Primed burst potentiation of the fEPSP but not population spike was significantly attenuated after corticosterone treatment. When using a more rigorous stimulation paradigm, i.e. theta burst potentiation, synaptic potentiation was not affected by corticosterone. Raising corticosterone levels in mice by exposure to a psychosocial stressor led to comparable results in subsequent in vitro experiments; stress reduced primed burst potentiation only of the fEPSP. These data support that corticosterone affects synaptic potentiation in the mouse via direct activation of hippocampal glucocorticoid receptors but only when using mild stimulation conditions.

Corticosterone reduces dendritic complexity in developing hippocampal CA1 neurons

Although prolonged stress and corticosteroid exposure induce morphological changes in the hippocampal CA3 area, the adult CA1 area is quite resistant to such changes. Here we addressed the question whether elevated corticosteroid hormone levels change dendritic complexity in young, developing CA1 cells. In organotypic cultures (prepared from P5 rats) that were 14–21 days cultured in vitro, two doses of corticosterone (30 and 100 nM) were tested. Dendritic morphology of CA1 neurons was established by imaging neurons filled with the fluorescent dye Alexa. Application of 100 nM corticosterone for 20 minutes induced atrophy of the apical dendritic tree 1–4 hours later. Fractal analysis showed that total neuronal complexity was reduced twofold when compared with vehicle‐treated neurons. Exposing organotypic slices to 30 nM corticosterone reduced apical length in a more delayed manner: only neurons examined more than 2 hours after exposure to corticosterone showed atrophy of the apical dendritic tree. Neither dose of corticosterone affected the length of basal dendrites or spine density. Corticosterone was ineffective in changing morphology of the apical dendrites when tested in the presence of the glucocorticoid receptor antagonist RU38486. These results suggest that high physiological levels of corticosterone, via activation of the glucocorticoid receptor, can, during the course of only a few hours, reduce the dendritic complexity of CA1 pyramidal neurons in young, developing hippocampal tissue. These findings suggest that it is relevant to maintain plasma corticosterone levels low during hippocampal development.

Stress, corticosteroid hormones and hippocampal synaptic function.

Exposure to stressful events has profound impact on hippocampus-dependent learning and memory processes. Traumatic and stressful experiences are remembered well in general, but have also been reported to suppress learning and memory processes. These bi-directional effects are, at least in part, modulated by corticosteroid hormones that are released during exposure to stressful experiences. An important question that remains to be addressed is how exactly exposure to stressful situations and elevated corticosteroid hormone levels affect learning and memory processes. Evidence is accumulating that exposure to stressful situations and elevated corticosteroid hormone levels modulates fast excitatory amino acid mediated synaptic transmission and synaptic plasticity, which are considered to underlie learning and memory processes in the hippocampus. In particular, exposure to stressful events has been reported to facilitate synaptic plasticity when delivered shortly before or after high frequency stimulation. By contrast, stressful events and elevated corticosteroid hormones suppress synaptic potentiation when stress precedes high frequency stimulation. From the mechanistic point of view, it is potentially important that exposure to stressful events and elevated corticosteroid hormone levels target key mechanisms that are involved in synaptic plasticity, i.e. AMPA receptors and NMDA receptors.