The temporal dynamics of intrahippocampal corticosterone in response to stress-related stimuli with different emotional and physical load: an in vivo microdialysis study in C57BL/6 and DBA/2 inbred mice.

There is strong evidence for a pivotal interaction of corticosteroid signalling and behavioral adaptation to stress. To further elucidate this relation, we monitored the dynamics of free corticosterone in the murine hippocampus of two inbred mouse strains using in vivo microdialysis. C57BL/6JOlaHsd (C57BL/6) and DBA/2OlaHsd (DBA/2) inbred mouse strains have been shown to differ in their anxiety-related and depression-like behavior and provide, thus, an interesting animal model to study the stimulus-response profile of the hypothalamus-pituitary-adrenocortical (HPA) system as a function of emotional and physical load. We, first, compared peripheral and intracerebral concentration patterns of corticosterone by simultaneous microdialysis of the jugular vein and the hippocampus in anesthetized mice and found strain differences in blood versus intracerebral free corticosterone concentrations. C57BL/6 showed almost the same steroid levels in either compartment, whereas DBA/2 mice displayed higher glucocorticoid levels in the circulation than in the hippocampus. This data suggest a strain difference in the tissue environment influencing the amount of biological active corticosterone at the receptor site. Measurements of intrahippocampal corticosterone in freely moving mice revealed that DBA/2 display a prolonged glucocorticoid increase in response to a single forced swimming stress (FST), as compared to C57BL/6 mice indicating a reduced inhibitory HPA axis feedback. Exposure to a novel environment (NE) induced a desensitization of the HPA system in DBA/2 animals as they show an attenuated intracerebral corticosterone dynamics after a subsequent FST. Testing animals in an elevated plus-maze (EPM), however, did not significantly stimulate coriticosterone release in either strain. The analysis of the area under the curve revealed a high amount of corticosterone released through FST and a low glucocorticoid release after NE or EPM exposure that are independent of the strain. This data indicate a strong stimulus dependency of corticosterone secretion that is strain independent, whereas the dynamics and feedback of the HPA axis is different between both inbred strains. Behavioral phenotyping of animals revealed a strong impact of microdialysis procedure on FST and EPM performance. Innate emotionality differences of both strains, however, were not affected. Though descriptive in nature, the present results suggest an altered corticosteroid signalling in the DBA/2 strain compared to C57BL/6 mice. Whether this observation causally underlies the differences in anxiety-related and depression-like behavior has to be further experimentally validated. In addition, our study highlights the use of in vivo microdialysis to assess the neuroendocrine endophenotype of animal models via profiling of stimulus-response patterns of stress hormones.