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Pregnancy Timeline by SemestersDevelopmental TimelineFertilizationFirst TrimesterSecond TrimesterThird TrimesterFirst Thin Layer of Skin AppearsEnd of Embryonic PeriodEnd of Embryonic PeriodFemale Reproductive SystemBeginning Cerebral HemispheresA Four Chambered HeartFirst Detectable Brain WavesThe Appearance of SomitesBasic Brain Structure in PlaceHeartbeat can be detectedHeartbeat can be detectedFinger and toe prints appearFinger and toe prints appearFetal sexual organs visibleBrown fat surrounds lymphatic systemBone marrow starts making blood cellsBone marrow starts making blood cellsInner Ear Bones HardenSensory brain waves begin to activateSensory brain waves begin to activateFetal liver is producing blood cellsBrain convolutions beginBrain convolutions beginImmune system beginningWhite fat begins to be madeHead may position into pelvisWhite fat begins to be madePeriod of rapid brain growthFull TermHead may position into pelvisImmune system beginningLungs begin to produce surfactant
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Fetal Timeline      Maternal Timeline      News     News Archive    Aug 17, 2015 

In a study of brain circadian neurons that govern our daily sleep-wake cycle, researchers identified
the circadian clock as behaving like a "bicycle" mechanism - having an up and down cycle.
One pedal — Sodium: high during daylight; the other — Potassium: high at nighttime.
Image credit: The Visible Embryo





What controls our waking up and going to sleep?

A simple 2-cycle mechanism turns our key brain neurons 'off' and 'on' during a 24-hour day.

Fifteen years ago, an odd mutant fruit fly caught the attention and curiosity of Dr. Ravi Allada, a circadian rhythms expert at Northwestern University. His curiosity led the neuroscientist to recently discover how an animal's biological clock wakes us up in the morning and puts us to sleep at night.

The circadian clock it turns out, operates simply. In a study of circadian neurons that govern daily sleep-wake cycles, Allada and his research team found that high sodium activity in these neurons during the day, turns the cells on to waken an animal. While at night, high potassium channel activity turns them off, allowing the animal to sleep. Looking deeper, researchers were surprised to find the same sleep-wake cycle in both flies and mice.

"This suggests the underlying mechanism controlling our sleep-wake cycle is ancient. This oscillation mechanism appears to be conserved across several hundred million years of evolution. And if it's in the mouse, it is likely in humans, too."

Ravi Allada PhD, Professor and Chair, Neurobiology, Weinberg College of Arts and Sciences, Northwestern University, senior author of the study.

Better understanding this switch-like mechanism could lead to new drug targets to address problems related to jet lag, shift work and other clock-induced disorders. Eventually, it might be possible to reset a person's internal clock to suit his or her situation.

The researchers call this a "bicycle" mechanism: two pedals that go up and down across a 24-hour day, conveying important time information to the neurons. That the researchers found the two pedals — a sodium current and a potassium current — active in both the simple fruit fly and the more complex mouse was unexpected.

The findings were published in the August 13 issue of the journal Cell.

"What is amazing is finding the same mechanism for sleep-wake cycle control in an insect and a mammal," said Matthieu Flourakis, the lead author of the study. "Mice are nocturnal, and flies are diurnal, or active during the day, but their sleep-wake cycles are controlled in the same way."

When he joined Allada's team, Flourakis had wondered if the activity of the fruit fly's circadian neurons changed with the time of day. With the help of Indira M. Raman, the Bill and Gayle Cook Professor in the department of neurobiology, he found very strong rhythms: The neurons fired a lot in the morning and very little in the evening.

Researchers next wanted to learn 'why.' They discovered that when sodium current is high, neurons fire more — awakening the animal. When potassium current is high, neurons quiet down — causing the animal to sleep.

The balance between sodium and potassium currents controls an animal's circadian rhythms.

Flourakis, Allada and their colleagues then wondered if such a process was present in an animal closer to humans. They studied a small region of the mouse brain that controls the animal's circadian rhythms — the suprachiasmatic nucleus, made up of 20,000 neurons — and found the same mechanism there.

Allada: "Our starting point for this research was mutant flies missing a sodium channel who walked in a halting manner and had poor circadian rhythms. It took a long time, but we were able to pull everything – genomics, genetics, behavior studies and electrical measurements of neuron activity — together in this paper, in a study of two species. Now, of course, we have more questions about what's regulating this sleep-wake pathway, so there is more work to be done."

•Rhythmic sodium leak conductance depolarizes Drosophila circadian pacemaker neurons
•NCA localization factor 1 links the molecular clock to sodium leak channel activity
•Antiphase cycles in resting K+ and Na+ conductances drive membrane potential rhythms
•This “bicycle” mechanism is conserved in master clock neurons between flies and mice

Circadian clocks regulate membrane excitability in master pacemaker neurons to control daily rhythms of sleep and wake. Here, we find that two distinctly timed electrical drives collaborate to impose rhythmicity on Drosophila clock neurons. In the morning, a voltage-independent sodium conductance via the NA/NALCN ion channel depolarizes these neurons. This current is driven by the rhythmic expression of NCA localization factor-1, linking the molecular clock to ion channel function. In the evening, basal potassium currents peak to silence clock neurons. Remarkably, daily antiphase cycles of sodium and potassium currents also drive mouse clock neuron rhythms. Thus, we reveal an evolutionarily ancient strategy for the neural mechanisms that govern daily sleep and wake.

The paper is titled "A Conserved Bicycle Model for Circadian Clock Control of Membrane Excitability."

In addition to Allada and Flourakis, other authors of the paper are Elzbieta Kula-Eversole, Tae Hee Han and Indira M. Raman, of Northwestern; Alan L. Hutchison, Aaron R. Dinner and Kevin P. White, of the University of Chicago; Kimberly Aranda and Dejian Ren, of the University of Pennsylvania; Devon L. Moose and Bridget C. Lear, of the University of Iowa; and Casey O. Diekman, of the New Jersey Institute of Technology.

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