Why bright light keeps us awake
October 15, 2019
Science Daily/Salk Institute
Researchers are reporting a novel technique for tracing the activity of individual nerve fibers known as axons, and determining how neurons communicate. The team used this technique to uncover details about how the brain responds to light signals received by the retina in mice.
In recent decades, scientists have learned a great deal about how different neurons connect and send signals to each other. But it's been difficult to trace the activity of individual nerve fibers known as axons, some of which can extend from the tip of the toe to the head. Understanding these connections is important for figuring out how the brain receives and responds to signals from other parts of the body.
Researchers at the Salk Institute and UC San Diego are reporting a novel technique for tracing these connections and determining how neurons communicate. The team used this technique to uncover details about how the brain responds to light signals received by the retina in mice, published October 15, 2019, in Cell Reports.
"This study is a breakthrough because no one could figure out how to study these connections before," says Salk Professor Satchidananda Panda, co-corresponding author of the paper. "This new technique has enabled us to go well beyond the limitations of electron microscopy."
The new method makes use of several different laboratory techniques to understand a type of neuron called intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells, which are found in the retina, in the back of the eye, express a protein called melanopsin that senses blue light.
The Salk and UCSD teams used a virus to deliver a protein called a mini-singlet oxygen-generating protein (mini-SOG) to the ipRGCs, so that the cells could be viewed in more detail under election microscopy. The system was designed to tether the mini-SOG to the membranes of the light-sensitive cells so that the entire neuron, including its long axons that reach out to different parts of the brain, can be easily tracked under both light and electron microscope.
"Thanks to development and application of new genetically introduced probes for correlated multiscale light and electron microscopic imaging, our Salk and UCSD-based research teams were able to follow the small processes emanating from nerve cells over centimeters, all the way from the retina to multiple places where they connect to brain regions critical to circadian rhythms, eye reflexes and vision," says Mark Ellisman, distinguished professor of neurosciences at UC San Diego and adjunct professor at Salk, who co-led the work. "We were able to obtain unprecedented three-dimensional information about the machinery required for these neuronal cells to signal the next neurons in the complex circuits."
Most of the previous work with mini-SOGs has been done in cell lines, and using them in mice, to map how neurons from the retina wire the brain, was a first, according the researchers. The method enabled them to glean new information about the connections between ipRGCs and different parts of the brain.
The ipRGCs are known to connect to many brain regions that regulate very different tasks. The cells tell one part of the brain how bright it is outside so that our pupil can rapidly close -- in less than a second. The same ipRGCs also connect to the master clock in the brain that regulates our sleep-wake cycle. "However, it takes several minutes of bright light to make us fully awake," Panda says. "How the same ipRGCs do these very different tasks with different time scales was not clear until now."
The investigators found that the difference has to do with the way that light detected by the retina reaches the brain. By delivering the mini-SOG to the eyes of the mice, they were able to trace the signal to the part of the brain that constricts the pupil in response to light.
"These connections were much stronger -- similar to water pouring out of a garden hose," Panda says. "Whereas the connection between the ipRGCs and the master clocks were weaker -- more like drip irrigation." Because the ipRGCs deliver the light signal to the circadian center through this slower drip system, it takes longer for any meaningful information to reach and reset the brain clock.
"This research helps explain why, when you get up in the night to get a drink of water and turn on the light for a few seconds, you're usually able to go right back to sleep," Panda says. "But if you hear a noise outside and end up walking around your house for half an hour with the lights on, it's much harder. There will be enough light signal reaching the master clock neurons in the brain that ultimately wakes up the rest of the brain."
Panda says that the new technique will be useful for studying other neural connections, as the researchers can essentially use the same viruses to express mini-SOGs in any neuron and ask how different neurons make connections to different appendages.
"These findings and methods open new opportunities for brain researchers studying the long-distance wiring of brains in normal and in animal models of human disease," adds Ellisman.
https://www.sciencedaily.com/releases/2019/10/191015131508.htm
A timekeeper for siesta
October 7, 2019
Science Daily/University of Würzburg
External stimuli can rearrange the hierarchy of neuronal networks and influence behavior. This was demonstrated by scientists using the circadian clock of the fruit fly as an example.
Circadian clocks must be flexible and they must be able to adapt to varying environmental conditions. Otherwise, it would be impossible for living beings to change their patterns of activity when the days get shorter again as is happening now. After all, Drosophila, also known as the common fruit fly, no longer needs a long siesta in autumn to protect itself from excessive heat and predators as in the middle of summer, at least in our latitudes. At the same time, the fly must shift its evening activity peak a few hours forward if it doesn't want to end up buzzing around in the dark.
For the fruit fly to adapt to changing day-and-night rhythms, its circadian clock must be able to process external cues, so-called zeitgebers, which are used to synchronise the molecular and physiological properties of the organism. Light is the most important zeitgeber the fly uses for this.
Publication in Current Biology
Scientists from the Department of Neurobiology and Genetics at the University of Würzburg have been researching the interaction of light, photoreceptors and circadian clocks in the fruit fly for some time. Chair holder, Charlotte Förster, together with her former colleague Matthias Schlichting, who presently works at Brandeis University (Massachusetts, USA), have now figured out new and surprising details of this interaction. They present the results of their research in the current issue of Current Biology.
"In mammals, a combination of the traditional photoreceptor pathway (rods and cones of the retina) and the circadian photoreceptor melanopsin in retinal ganglion cells enables the fine-tuning of clock synchronisation," Charlotte Förster explains. She says that there is a comparable mechanism in Drosophila: "The compound eyes, the extraretinal Hofbauer-Buchner eyelets and the circadian photoreceptor cryptochrome all work together in the light synchronisation process," the professor summarises the central result of the recently published study.
It is known from earlier studies how the photoreceptor cryptochrome works. Located in special nerve cells, the so-called clock neurons, it interacts with the timeless clock protein during light exposure, leading to the degradation of the protein. Figuratively speaking, it turns the clock back to zero. However, less is known about the exchange between the eyes of the fruit fly and the clock neurons and how the different day length is mediated.
Experiments with different day lengths
For their study, the scientists worked with different specimens of fruit flies. They used healthy flies, eyeless flies and flies lacking specific visual pigments of the eye, the so-called rhodopsins. During the laboratory experiments, the insects were exposed to different light conditions: At a constant day length of 24 hours, the researchers extended the period of light in two-hour increments from twelve to a maximum of 20 hours and observed the activity patterns of the respective fly groups.
It turned out that the activity of the insects changed with increasing length of the daylight period. When periods of light and darkness alternate regularly every twelve hours, which corresponds to a typical day at the equator, healthy flies become active twice: around the time of "sunrise" and before the simulated "sunset." As the days get longer, the evening activity is also delayed and the "siesta" -- the midday resting period -- is extended. It is striking though that as the periods of daylight increase, the activity peak in the evening deviates from the simulated sunset and is much earlier in some cases. The largest deviation occurs when the daylight period is 20 hours long, probably because the flies are never confronted with such conditions in their natural environment.
Discovery in the compound eye
While searching for the molecular and neuronal mechanisms which the fruit fly uses to "fine-tune" its circadian clock in a manner of speaking, the neurobiologists had to carry out numerous experiments. Experiments on fruit flies that lacked these eyes demonstrated that the compound eyes play a key role. Their activity peak was also delayed as the length of the daylight period increased, but much less so than in their seeing relatives. More experiments were conducted to pinpoint which receptor cell and visual pigment are responsible for this. After all, each facet of the fly's compound eye has eight receptor cells and five rhodopsins. So the scientists selectively switched off the individual cells until it was clear that receptor cell 8 and rhodopsins 5 and 6 which occur there were their targets.
The scientists next investigated how the light signal reaches the brain of the fruit fly and how it travels from there to the clock neurons. Surprisingly, they found that while the signal is transported via so-called "small lateral clock neurons" during "moderate" light periods, it travels through "large lateral clock neurons" in the 20-hour light experiments. "Although the circadian clock of the fruit fly is comparatively small with just 150 neurons, the overall system has high plasticity," Charlotte Förster recapitulates the results of the study and she explains that this neuronal plasticity is necessary to enable the animals to quickly adjust to varying conditions.
https://www.sciencedaily.com/releases/2019/10/191007103615.htm
Blue-enriched white light to wake you up in the morning
March 6, 2019
Science Daily/The Korea Advanced Institute of Science and Technology (KAIST)
Here is good news for those who have difficulty with morning alertness. A research team proposed that a blue-enriched LED light can effectively help people overcome morning drowsiness. This study will provide the basis for major changes in future lighting strategies and thereby help create better indoor environments.
Considerable research has been devoted to unmasking circadian rhythms. The 2017 Nobel Prize in Physiology or Medicine went to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for unveiling the molecular mechanisms that control circadian rhythms. In particular, the relationship between light and its physiological effects has been investigated since the discovery of a novel, third type of photoreceptor in the human retina in the early 2000s. Rods and cones regulate visual effects, while the third type, photosensitive retinal ganglion cells, regulate a large variety of biological and behavioral processes including melatonin and cortisol secretion, alertness, and functional magnetic resonance imaging (fMRI).
Initial studies on light sources have shown that blue monochromatic, fully saturated lights are effective for stimulating physiological responses, but the relative effectiveness of commercially available white light sources is less well understood. Moreover, the research was more focused on the negative effects of blue light; for instance, when people are exposed to blue light at night, they have trouble achieving deep sleep because the light restrains melatonin secretion.
However, Professor Hyeon-Jeong Suk and Professor Kyungah Choi from the Department of Industrial Design and their team argue that the effects of blue-enriched morning light on physiological responses are time dependent, and that it has positive effects on melatonin levels and the subjective perception of alertness, mood, and visual comfort compared with warm white light.
The team conducted an experiment with 15 university students. They investigated whether an hour of morning light exposure with different chromaticity would affect their physiological and subjective responses differently. The decline of melatonin levels was significantly greater after the exposure to blue-enriched white light in comparison with warm white light.
Professor Suk said, "Light takes a huge part of our lives since we spend most of our time indoors. Light is one of the most powerful tools to affect changes in how we perceive and experience the environment around us."
Professor Choi added, "When we investigate all of the psychological and physiological effects of light, we see there is much more to light than just efficient quantities. I believe that human-centric lighting strategies could be applied to a variety of environments, including residential areas, learning environments, and working spaces to improve our everyday lives."
This research was collaborated with Professor Hyun Jung Chung from the Graduate School of Nanoscience and Technology.
https://www.sciencedaily.com/releases/2019/03/190306100602.htm