The Role of Circadian Rhythm in Memory Formation—Is it Necessary?

    The core molecular clock is responsible for maintaining circadian rhythms, our body's endogenous 24-hour cycle. Importantly, the circadian clock controls behaviors like sleep-wake cycles in adulthood—the regulation of sleep is critical in our everyday lives and learning more about the cellular basis of behaviors like sleep can help us better understand related complex processes, like memory formation. An excellent animal model used to study circadian rhythm are Drosophila, commonly known as fruit flies; sharing many homologous genes and conserved molecular pathways with humans, these organisms can help us grasp a better understanding of our endogenous clock.

    Circadian control of sleep-wake patterns is not necessarily present in young animals—it isn't present in human infants. In "Developmental emergence of sleep rhythms enables long-term memory capabilities in Drosophila" by Dr. Cavanaugh et al., 2nd (L2) and 3rd instar (L3) Drosophila larvae are examined to find out at which development time point do sleep rhythms emerge, what is the role of Dh44 in sleep- wake activity, and how does sleep play a role in memory consolidation. When measuring sleep across the day, L3 larvae showed an increase in sleep bouts in the subjective nighttime compared to L2, which showed mostly equal distributions of bout length across the day. Testing which neurons affected sleep bout length, Dh44 was discovered to decrease sleep duration in L2 upon activation. This was further validated through neuronal silencing of Dh44 through Kir 2.1 and knockdown of Dh44 through RNAi, with both experiments yielding increased sleep duration, bout length, and bout number and therefore proving the Dh44 was indeed responsible for maintaining arousal. To find out if Dh44 has circadian regulation and if it was connected to the clock, ATP-gated receptors were expressed in DN1a neurons (a subset of clock cells) and GCaMP6 in Dh44 neurons to measure levels of calcium in both L2 and L3. The findings indicated an increase in Ca2+ levels only in L3 larvae, thus proving how Dh44 neurons form connections with the DN1a clock neurons later in development and that arousal is controlled by the clock. Another validation of this finding is when testing for CCHa1 signaling (a signaling molecule released by DN1a neurons with its receptors in Dh44), manipulating the release of CCHa1 and knocking down CCHa1-R only affects sleep in L3 larvae; Dh44 neurons in L2 larvae haven’t yet formed the connection to DN1a clock cells and therefore they don’t respond to manipulations of CCHa1.

    Connecting their findings of the circadian rhythmicity of sleep, they next asked whether the rhythmicity of sleep behavior is necessary for short and long-term memory (LTM). When testing for short-term memory (STM), both L2 and L3 larvae exhibited similar performance in the aversive memory test, indicating that having rhythmic sleep is not necessary to form STM. When testing LTM, L2 larvae failed to show memory of the aversive cue and further manipulations using clock mutants, tim0 and clkJRK validated the findings with flies unable to produce LTM. To test whether deep sleep is necessary for LTM, sleep rhythm was disrupted through the knockdown of CCHa1-R in Dh44 neurons—results showed both L2 and L3 larvae unable to form LTM, and L3 larvae exhibiting a lowered arousal threshold at night, being easily woken up. Finally, to test if sleep itself was necessary, L3 larvae were deprived of sleep using a light stimulus and they concluded the same results—sleep, especially deep sleep, is necessary for the formation of LTM.

    A related question was asked in “Chronic sleep deprivation altered the expression of circadian clock genes and aggravated Alzheimer’s disease neuropathology” by Niu et al., studying the effect of disrupting circadian rhythms through chronic sleep deprivation (CSD) on learning and memory as is indicated in Alzheimer’s disease using mice as the model organism. In behavioral tests including the Morris water maze and the Y maze, the CSD mice declined in their cognitive ability, both in the AD (Alzheimer’s disease) and wild-type phenotypes compared to the non-sleep deprived mice. The CSD mice had a significantly longer escape latency, crossed the target position significantly fewer number of times, and spent less time in the target quadrant, showing how CSD can severely impair learning and memory.

    Both articles manipulate an organism’s circadian rhythm through disrupting sleep behavior, then they find the effects on memory formation. In both, we can see that suppressing sleep prevents the formation of long-term memories, and as the circadian clock controls sleep behavior, manipulations of clock genes result in consequences in sleep. Future directions could include manipulating the time of sleep deprivation as to induce short bouts of sleep during the daytime and observe the effect on learning and memory formation, similar to taking naps. As college students, we oftentimes find ourselves in this situation of having to choose to stay up to work or wake up early—it would be interesting to see concrete evidence in which method yields better retention. We can use these results to think about and better our own habits and behaviors—sleep is incredibly important, and it is critical we get enough of it.


References:

Niu, L., Zhang, F., Xu, X., Yang, Y., Li, S., Liu, H., & Le, W. (2022). Chronic sleep deprivation altered the expression of circadian clock genes and aggravated Alzheimer's disease neuropathology. Brain pathology (Zurich, Switzerland), 32(3), e13028. https://doi.org/10.1111/bpa.13028


Poe, A. R., Zhu, L., Szuperak, M., McClanahan, P. D., Anafi, R. C., Scholl, B., Thum, A. S., Cavanaugh, D. J., & Kayser, M. S. (2023). Developmental emergence of sleep rhythms enables long-term memory in Drosophila. Science advances, 9(36), eadh2301. https://doi.org/10.1126/sciadv.adh2301