Corticotropin-releasing hormone and the hypothalamic–pituitary–adrenal axis in psychiatric disease

Since the 1960s, both corticotropin-releasing hormone (CRH) and the hypothalamic–pituitary–adrenal (HPA) axis have been studied in detail across a range of psychiatric illnesses, leading to important contributions to our knowledge in this area. This research arose from the conceptualization of depression, in particular, as a stress-related disorder. However, stress is now regarded as an integral component of psychiatric illnesses in general, whether as an environmental trigger or in the initial pathogenesis, and there is evidence of altered HPA axis function across a range of mental disorders. The chapter will cover the extensive literature on HPA axis abnormalities in these disorders with a particular emphasis on the CRH system as it is very evident that this 41-amino acid-containing peptide is not only a major physiologic regulator of HPA axis activity but also important in the pathogenesis of mental disorders. In particular, we discuss the abundant reports pertaining to major depressive disorder, where hyperactivity of the HPA axis, of mild to moderate severity, has been demonstrated in 30–50% of cases. Also under consideration is the less extensively studied, but equally intriguing question of HPA axis integrity in bipolar affective disorder. In addition there will be a concise summary of recent findings in schizophrenia and anxiety disorders, with an emphasis on post-traumatic stress disorder (PTSD) in the latter case. Interestingly, in diametric opposition to the theory of HPA hyperactivity in depression, PTSD has features consistent with hypofunctioning of this system. Advances in animal and human studies have made it possible to synthesize these findings, and while much still remains unknown, we are gradually building up a clearer picture of this very important axis in health, at times of stress, and in chronic enduring mental illness.