Entrainment memories: What does stable mean?

Area of Science:

• Chronobiology
• Systems Biology
• Biophysics

Background:

• Circadian rhythms are synchronized to the environment by diurnal factors (zeitgebers) through complex molecular mechanisms.
• Traditional models describe entrainment as an acute response, but evidence for longer-term adaptations is emerging.
• The interplay between light, internal clock phase, and molecular machinery (zeitnehmer) is crucial for daily time-keeping.

Purpose of the Study:

• To investigate how the structure and strength of zeitgebers influence entrainment trajectories.
• To explore the role of transitions between different entraining conditions on circadian clock adaptation.
• To propose a new theoretical framework for circadian entrainment incorporating a long-range memory component.

Main Methods:

• Utilized *Neurospora* as a model organism to study circadian entrainment.
• Analyzed diverging entrainment pathways under varying zeitgeber conditions and transition types.
• Developed a theoretical model incorporating a third descriptive parameter related to consecutive periodic situations.

Main Results:

• Observed distinct entrainment routes that become progressively earlier or later, indicating a 'memory' effect.
• Demonstrated that entrainment is not solely an acute reaction but involves multi-cycle adaptations.
• Identified a potential link between entrainment pathways and Gibbs free energy gradients.

Conclusions:

• Circadian entrainment involves a long-range memory component, extending beyond current phase and zeitgeber conditions.
• A novel theoretical framework is proposed, suggesting that the differential between consecutive periodic situations is key.
• Entrainment may be understood not just as time-keeping but as a form of biological homeostasis regulated by energy gradients.