Day differences in the cortisol awakening response predict day differences in synaptic plasticity in the brain

Area of Science:

• Neuroscience
• Chronobiology
• Endocrinology

Background:

• The cortisol awakening response (CAR) is a significant, variable diurnal cortisol pattern with an unknown function.
• Aberrant CAR patterns link to health issues and cognitive deficits, implying a crucial role.

Purpose of the Study:

• To investigate the relationship between the CAR and motor cortex synaptic plasticity.
• To explore the CAR's potential role in mediating circadian regulation of brain function.

Main Methods:

• Assessed temporal covariation between CAR magnitude and rapid transcranial magnetic stimulation (rTMS)-induced long-term depression (LTD)-like responses.
• Evaluated motor cortex plasticity across multiple time points and sessions in healthy participants.
• Used mixed regression modeling to analyze the association between CAR and plasticity.

Main Results:

• Days with higher-than-average CARs were associated with greater-than-average rTMS-induced plasticity.
• Variation in CAR magnitude significantly predicted variation in rTMS-induced responses (p=0.002).

Conclusions:

• The CAR may act as a mediator between central and peripheral circadian systems.
• This suggests the CAR plays a role in entraining daily levels of synaptic plasticity, potentially preparing the brain for daily challenges.