Most of us know more about sleeping babies than snoozin’ fruitflies, but most young animals need buckets of sleep. Today, a study published in Science offers a comprehensive picture of why newborns sleep so much, and what difference it makes — at least to a fruit fly.
The goal “was to show that sleep early in development is required for normal structural growth of the brain,” says first author Matthew Kayser, a physician in the department of psychiatry at the University of Pennsylvania.
The basic plan was to reduce sleep among fruit flies that had just emerged from the pupal stage, and then look at their brains and behavior eight or nine days later. (Technically, fruit flies become adults immediately after the pupal stage, so we’ll call the day 1 flies “youngsters” and the day 8, 9, or 10 flies “oldsters.”)
Before starting, the researchers knew that
Dopamine, a common neurotransmitter, causes waking in the fruit fly
The brain’s dorsal fan-shaped body promotes sleep in the fruit fly
Fruit flies that were kept awake just after emerging from the pupal stage failed to mate normally as mature adults
If there’s one thing that concerns evolution, it’s mating, says Kayser, a specialist in the neurobiology of sleep. This abundance of sleep in young animals is found “across the board, in humans, other mammals, even fruit flies. Things that are conserved like that tend to be important.”
But exactly what mechanism causes extended dormancy among young flies?
First, Kayser and senior author Amita Sehgal, of the Howard Hughes Medical Institute and the Perelman School of Medicine at the University of Pennsylvania, measured sleep. They found that the youngsters slept almost 17 hours a day, versus about 12 for oldsters. Sleep was also “deeper”: only about 25 percent of the young awoke after 10 seconds of light, versus 65 percent of oldsters.
Those drowsy, young flies also had 30 percent less dopamine than the older flies.
Better than coffee?
When Sehgal and Kayser raised dopamine levels in the young flies, their increased wakefulness demonstrated that the normal condition — increased sleep and resistance to waking when young — results from a low dopamine level.
Moving on, the researchers found that the normal low-dopamine condition in the young flies was due to decreased dopamine production among specific neurons that connect to a sleep-promoting structure called the dorsal fan-shaped body. “If you want to disrupt early sleep, the best way to do it is by activating these dopamine neurons,” to slow the sleep-promoter, Sehgal told us. “The animal begins to resemble a mature adult,” and spends more time awake (maybe finishing its tax returns or pressing buttons on a phone?).
In other words, more dopamine translates to less sleep.
Looking further, the researchers found that a structure in the brain’s olfactory (smell) system was stunted in the sleep-deprived oldsters. Normally, this structure grows for a while during adulthood, “But sleep loss is stunting the growth of this structure, and so we are saying that sleep is required for these actively growing regions,” Sehgal says.
It turns out that the structure in question detects pheromones — chemicals that flies use to communicate about mating and other necessities.
To recap, dopamine inhibits the sleep-promoting fan-shaped body, and excess dopamine at day 1 reduces sleep, which interferes with growth in a part of the brain linked to mating. Sehgal and Kayser already knew that young flies that got less sleep caused a significant reduction in courting and mating behavior. And the same result appeared in their dopamine-rich, sleep-deprived flies.
In science, every good answer raises a question, and Sehgal mentions these:
What is the ultimate reason that dopamine is less abundant at day one?
How does sleep assist the growth of some brain structures?
What other parts of the brain are harmed by a lack of sleep in early life?
Ultimately, Sehgal says, research on early sleep abnormalities could shed light on problems like sudden infant death syndrome. “There are data showing that SIDS is somehow connected to sleep. We are being very speculative, but if sleep is important for circuits developing in the brain, deprivation of sleep could be quite harmful.”
– David J. Tenenbaum