There are few students who are unfamiliar with the practice of cramming: staying up the night before an exam, studying until the early hours of the morning. Sometimes, this works for students in the short-term. However, problems arise when the student then tries to recall the information a month later. This difficulty is due to the difference between short-term and long-term memorization. The process of committing information to the long-term memory is called memory consolidation.
Two graduate students working at the Griffith Lab, Paula Haynes and Bethany Christmann, have recently published a paper in the life science and biomedicine scientific journal, eLife. Working with the lab’s principal investigator and Brandeis professor, Dr. Leslie Griffith, the group sought to understand the physiological and anatomical relationship between sleep and memory.
As the paper, titled, “A single pair of neurons links sleep to memory consolidation in Drosophila melanogaster,” elaborates that “evidence suggests that sleep plays a role in promoting the consolidation of memory … It is not, however, clear exactly how sleep promotes memory consolidation.”
Prior to this experiment, there were two popular hypotheses in the scientific community. As the paper states, “…[sleep] may simply be a permissive state generated by other brain regions that prevents sensory interference with memory circuits, or alternatively the memory circuitry itself may actively participate in sleep promotion as an integral aspect of the consolidation process.” Simply put, either sleep is a period of time when the senses stop interfering with the memory consolidation process, or the process of memorization may cause people to fall asleep.
The primary purpose of their research was to study the role of DPM neurons and their products in the regulation of sleep in Drosophila melanogaster (the fruit fly). DPM stands for dorsal paired medial neurons. These cells are essential for memory consolidation in fruit flies because they release an inhibitory neurotransmitter product called GABA onto a part of the brain called the mushroom body. The absence of GABA in the mushroom body manifests as patterns of nighttime sleep loss in fruit flies.
The researchers experimented with fruit flies due to the simplicity of their brain structure. As explained in the paper, “Although the anatomy involved in memory consolidation in mammals is highly complex and distributed, in the fly it is quite compact.”
Ultimately, the researchers proposed a new, slightly more complicated model for the relationship between sleep and memory: Certain neurons in the mushroom body, called α’/β’ neurons, are responsible for the process of waking up. DPM neurons release the chemicals GABA and 5HT during memory consolidation, and inhibit the α’/β’ neurons, which results in sleep.
The researchers concluded, “The specific involvement of neurons required for memory consolidation … in the regulation of sleep suggests that generation of sleep by activation of learning circuits is an intrinsic property of the circuit, not an extrinsically imposed phenomenon. Further, our finding that the memory-consolidation specific DPM neurons are inhibitory suggests that inhibitory neurotransmitters may play an as-of-yet uncharacterized role in memory consolidation in Drosophila.”
In other words, the data suggests that sleep and memory are inherently linked, as opposed to being influenced together by an unidentified third party. Additionally, this research proposes a potential novel role for inhibitory neurons in the process of long-term memory storage of fruit flies.
