Hormone oscillations in the HPA axis: dynamical diseases and beyond

Aim: Hormone pulsatility is an important aspect of the hypothalamic-pituitary-adrenal (HPA) axis in health and disease. We use the properties of simple mathematical models to determine whether hormone pulsatility reflects the presence of a time delay in the production of hormones and/or is related to the impulsive nature of hormone secretion. Methods: The predictions of two models for HPA pulsatility are compared. The first model assumes pulsatility arises because of a time delay in the synthesis of glucocorticoids (GCs). The second model suggests that pulsatility reflects the impulsive nature of hormone secretion. The generation of oscillations by the second mechanism does not require a time delay. Results: The time delay for the synthesis of GC (0–10 minutes) may not be long enough to account for the oscillations in adrenocorticotrophin (ACTH) and GC observed with constant corticotrophin-releasing hormone (CRH) infusion in rats. A simple mechanism for hormone release, illustrated using an integrate-and-fire mechanism, reproduces the observed hormone pulsatility. Conclusions: The water solubility of CRH and ACTH draws attention to the role played by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in the calcium-dependent exocytosis of peptide hormones. Abnormalities in SNARE proteins are anticipated to cause changes in the amplitude modulation of ACTH and CRH hormone pulses. In mice, a mutation in a SNARE protein causes abnormalities in the HPA axis. Mutations in SNARE proteins occur in many neurodegenerative and neuropsychiatric diseases. Abnormalities in HPA function also occur in these disorders. The identification of SNARE protein mutations in exosomes in serum and cerebrospinal fluid in humans may make it possible to determine whether there exists a causal relationship between an SNARE protein mutation and abnormalities in the HPA axis in this patient group.