Studies on the distinct control of free corticosterone levels in the brain

Lead Research Organisation: University of Bristol
Department Name: Henry Wellcome LINE


Glucocorticoids (in humans cortisol) are hormones which are crucial for the health and wellbeing of an organism. They play a vital role in many body functions including metabolism and growth. Importantly, glucocorticoid hormones also modulate brain functions such as mood, emotion and memory. Glucocorticoids are best known for their release during a stressful situation. Therefore, they are often called 'stress hormones'. During and after stress they are essential for the mobilisation of energy and for a wide variety of processes to support so-called stress coping and stress adaptation strategies. It is therefore not surprising that glucocorticoid hormones play an important role in psychiatric diseases such as depression and anxiety; diseases which seem to involve disturbed coping with stressful events. Glucocorticoid hormones are secreted by the adrenal glands (situated just above the kidneys) and show a clear rhythm over the 24-hour day/night cycle with highest levels reached just before the start of the active period in animals and man. Interestingly, we now know that glucocorticoids are not continuously secreted from the adrenal glands into the bloodstream but rather in a series of pulses, with every pulse lasting about one hour. Through the circulation the glucocorticoids can reach all organs and tissues in the body. However, the situation is even more complex as the largest part of the hormones is bound to proteins present in the plasma. Only a minor fraction of hormone is unbound (free) and, importantly, it is only this free fraction that is 'seen' by the tissues and is therefore biologically active. Thus, it is highly surprising that there is hardly any information available on the regulation of free corticosterone levels in the body. Furthermore, it is not known whether the brain, a principal target organ for glucocorticoid action, is exposed to the same levels of free glucocorticoid hormone as those present in the circulation. In this respect, we recently have made an important discovery. Measuring the levels of free glucocorticoid hormone directly in the brain of rats, i.e. free corticosterone - with a technique called in vivo microdialysis- we found that the pulsatile rhythm of blood corticosterone is maintained in the brain. However, we also found that the response of corticosterone to stress is profoundly delayed in the brain as compared to the circulation. We have therefore hypothesised that free glucocorticoid levels in the brain are regulated distinctly from those in the circulation having fundamental consequences for the wide variety of glucocorticoid-modulated processes in the brain. We therefore have designed the here proposed comprehensive research plan in which we will study the regulation of free corticosterone in the brain of rats and mice using state-of-the-art techniques including genetic approaches. By performing dual microdialysis we will be able to monitor free corticosterone levels and stress responses in the brain and circulation simultaneously. Thus, our project will benefit the basic knowledge of the neurobiology of stress and, therefore, has without doubt the potential to make a profound contribution to the improvement of both human and animal wellbeing.

Technical Summary

Glucocorticoid hormones (cortisol in humans, corticosterone in rodents) affect numerous processes throughout the body including the brain, over the circadian cycle and after stress. A tight control of glucocorticoid secretion is vital as chronic hypo- or hypersecretion can cause disease. Circulating glucocorticoid levels show a diurnal rhythm and an ultradian rhythm reflecting pulsatile secretion from the adrenal gland. Only a minor, 'free' fraction of these glucocorticoids act in the tissues as most hormone is bound to plasma proteins and is therefore biologically inactive. Recently, we found that free glucocorticoid levels in the extracellular space of the rat brain shows a diurnal and an ultradian rhythm. However, stress-induced free glucocorticoid levels in the brain peaked approximately 25 min later than total glucocorticoid levels in the circulation. Nevertheless, brain and blood hormone levels returned to baseline concomitantly suggesting an accelerated elimination of hormone from the brain. Thus, it appears that glucocorticoid exposure of the brain after stress covers a shorter time span than the time one would predict based on circulating glucocorticoid levels. Therefore, we have hypothesised that free glucocorticoid levels in the brain are regulated distinctly from those in the circulation precipitating in different glucocorticoid receptor (GR)-binding profiles in brain versus peripheral tissues. We have planned to directly compare baseline and stress-induced free glucocorticoid hormone profiles in brain, blood and subcutaneous tissue and to discern if differential hormone profiles result in distinct GR occupancy and nuclear translocation patterns in brain versus peripheral tissues. Moreover, we will clarify the role of the ATP-binding cassette transporter P-glycoprotein (ABCB1, MDR1), and of hippocampal mineralocorticoid receptors and GRs in these profiles and patterns by genetic deletion and pharmacological approaches.
Description 1. Using newly-established peripheral microdialysis, we identified virtual identical circadian and ultradian free corticosterone rhythms in different body compartments including the brain. These data presented the basis for a successful translation of our microdialysis method into humans.

2. We found that after stress the rise in free corticosterone is 20-30 min delayed when compared to the rise in plasma total glucocorticoid levels, irrespective of compartment. Such delayed increase inherently results in a postponed glucocorticoid hormone action including postponed negative feedback. This will lead to stress-induced, glucocorticoid-sensitive processes (e.g. memory consolidation) going on for longer than previously thought on the basis of the time course of plasma total hormone levels.

3. We identified that a rapid release of corticosteroid-binding globulin (CBG) from the liver into the circulation causes the delayed free hormone response. Thus a new, highly dynamic role for CBG in the regulation of glucocorticoid hormone physiology after acute stress was uncovered.
Exploitation Route It will be important for both basic and clinical researchers to take into account that corticosteroid-binding globulin may play an important role in the regulation of glucocorticoid hormone responses to stimuli. Peripheral microdialysis for glucocorticoid hormones could be of particular interest to these researchers.
Sectors Healthcare,Pharmaceuticals and Medical Biotechnology

Description Project Grant
Amount £306,885 (GBP)
Funding ID 092947/Z/10/Z 
Organisation Wellcome Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 06/2010 
End 10/2014
Title Peripheral microdialysis for glucocorticoid hormones 
Description We have developed peripheral microdialysis for the measurement of free levels of corticosterone in freely behaving rats. A thin, flexible microdialysis probe is inserted into the jugular vein or under the skin. Free corticosterone levels in the dialysates are measured using a highly sensitive radioimmunoassay method. This technique replaces the need for continuous blood sampling for specific research questions, in particular for studies on glucocorticoid hormone circadian rhythms and stress responsiveness. 
Type Of Material Physiological assessment or outcome measure 
Provided To Others? No  
Impact We have translated this method into humans and have studied glucocorticoid rhythms and responses using subcutaneous and intravenous microdialysis in volunteers.