The brain contains 'clock' neurons that mold circadian behaviors and link them to cues from the environment, like light and seasonal changes. In fruit flies, the clock releases the peptide Pigment-dispersing factor (PDF) to synchronize the activity of the clock neurons and drive time-based behaviors like mating and sleep. PDF may also underlie memory formation, explaining the cognitive dysfunction that occurs when the clock is desynchronized from the environment.

Flyer-Adams et al. tested how well fruit flies with a functioning core clock but lacking the PDF output signal could learn. Flies without PDF had severely impaired memory . However, memory regulation by PDF likely occurs without direct signaling to the main memory structure of flies. These results suggest that PDF from the clock may promote normal memory throughout the day by acting as a timestamp to learning. The VIP pathway in humans may play a similar role.

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Journal Reference:

1. Johanna G. Flyer-Adams, Emmanuel J. Rivera-Rodriguez, Junwei Yu, Jacob D. Mardovin, Martha L. Reed, Leslie C. Griffith. Regulation of olfactory associative memory by the circadian clock output signal Pigment-dispersing factor (PDF). The Journal of Neuroscience, 2020; JN-RM-0782-20 DOI: 10.1523/JNEUROSCI.0782-20.2020