Stress response and the neuroendocrinologic factors through which it is mediated are disturbed in anxiety and in affective disorders. While acute stress is thought to result in hypothalamus-pituitary-adrenal- (HPA) axis hyperactivity (Varghese 2001), chronic stress may result in decreased HPA-response (Booji 2013). Antidepressant treatment, on the other hand, is thought to realign HPA–axis activity (Schüle 2007).

On the other hand, dysregulation within the serotonergic neurotransmitter system is understood as a central moderator in the pathophysiology of affective and anxiety disorders. Serotonergic transmission both regulates- and is regulated by- glucocortocoids. Cortisol results in an increase in serotonin synthesis and release while serotonergic transmission is thought to downregulate HPA-axis activity (Lanfumey, 2008). Positron emission tomography (PET) studies have demonstrated the link between the serotonergic system and the HPA-axis in humans in vivo. For example, a negative correlation between cortisol and 5HT1A receptor levels in various brain regions has been shown (Lanzenberger, 2010). SERT expression, on the other hand, was shown using PET to be positively related to HPA-axis reactivity (Frokjaer 2013).

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Available literature on interactions between the HPA-axis and the serotonergic system will be discussed with a focus on data acquired via PET studies.

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The interaction between the serotonergic system and the HPA-axis is likely bilateral and may be understood as a neurobiological link by which stress may foster the development of depression and anxiety.

M. Spies has received travel grants from AOP Orphan Pharmaceuticals AG, Janssen-Cilag Pharma Gmbh, and Eli Lilly, workshop participation from Eli Lilly, and speaker honoraria from Janssen-Cilag Pharma Gmbh. R. Lanzenberger received travel grants and conference speaker honoraria from AstraZeneca, Lundbeck A/S, Roche Austria GmbH, Dr. Willmar Schwabe GmbH & Co. KG, AOP Orphan Pharmaceuticals, and Janssen-Cilag Pharma Gmbh.