Major depressive disorder and bistability in an HPA-CNS toggle switch

Area of Science:

• Neuroscience
• Mathematical Biology
• Psychiatry

Background:

• Major depressive disorder (MDD) is a common, complex psychiatric disorder with treatments often showing delayed effects.
• The hypothalamic-pituitary-adrenal (HPA) axis and its dysregulation, involving cortisol, are strongly linked to MDD.
• Current mathematical models of the HPA axis operate on hourly timescales, insufficient for understanding MDD's weekly dynamics.

Purpose of the Study:

• To develop a novel mathematical model of MDD dynamics operating on a weekly timescale.
• To investigate the role of HPA axis hormone gland growth and CNS interactions in MDD.
• To explain the delayed onset of antidepressant effects and the persistence of depressive states.

Main Methods:

• Proposed a mathematical model incorporating HPA axis hormone gland growth on a weekly timescale.
• Included mutual inhibition between the HPA axis and central nervous system (CNS) regions, forming a toggle switch.
• Analyzed the model's bistability between euthymic and depressed states.

Main Results:

• The model demonstrates bistability between euthymic and depressed states with dynamics occurring over weeks.
• It explains how prolonged stress, but not acute stress, can initiate self-sustaining depressive episodes.
• The model accounts for the week-long therapeutic lag of antidepressant medications and HPA axis dysregulation via gland mass changes.

Conclusions:

• The developed model provides a new framework for understanding MDD dynamics on a weekly timescale.
• It elucidates the mechanisms behind stress-induced persistent depression and delayed treatment efficacy.
• This understanding may inform and guide the development of more effective MDD treatment strategies.