Winter could be behind us, but do you remember the challenge of waking up in those cold and dark days? Temperature influences the behavior of almost all living creatures, but there is still much to learn about the link between sensory neurons and neurons that control the sleep-wake cycle.
Northwestern University neurobiologists have discovered a clue to what is behind this behavior. In a fruit fly study, researchers identified a “thermometer” circuit that transmits information about the cold outside temperature from the fly antenna to the upper brain. They show how, through this circuit, cold and dark seasonal conditions can inhibit neurons within the brain of the fly that promote activity and wakefulness, particularly in the morning.
“This helps explain why – both for flies and for humans – it is so difficult to wake up in the winter morning,” said Marco Gallio, associate professor of neurobiology at the Weinberg College of Arts and Sciences. “By studying the behaviors in a fruit fly, we can better understand how and why temperature is so critical to regulating sleep.”
The study, conducted by Gallium and conducted in Drosophila melanogaster, was published today (May 21) in the magazine Current biology.
The article describes for the first time the “absolute cold” receptors residing in the fly antenna, which respond to temperature only below the “comfort zone” of the fly of about 77 degrees Fahrenheit. After identifying those neurons, the researchers followed them to their targets inside the brain. They found that the main recipients of this information are a small group of brain neurons that are part of a larger network that controls activity and sleep rhythms. When the cold circuit they discovered is active, the target cells, which are normally activated by the morning light, are turned off.
Drosophila is a classic model system for circadian biology, the area where researchers study the mechanisms that control our 24-hour rest and activity cycle. The focus of much of the current work is on how changes in external signals such as light and temperature affect the rhythms of activity and sleep and how the signals reach the specific brain circuits that control these responses.
While sensing ambient temperature is critical for small cold-blooded fruit flies, humans are still creatures of well-being and are constantly looking for ideal temperatures. Part of the reason humans seek optimal temperatures is that core and brain temperatures are intimately related to induction and sleep maintenance. Seasonal changes in daylight and temperature are also related to changes in sleep.
“Temperature detection is one of the most fundamental sensory modalities,” said Gallio, whose group is one of the few in the world who is systematically studying the detection of temperature in fruit flies. “The principles that we are finding in the brain of the fly – logic and organization – can be the same for all human beings. Whether it is to fly or to humans, sensory systems must solve the same problems, so often it they do the same way. “
Gallium is the corresponding author of the article. Michael H. Alpert, postdoctoral researcher in the Gallium laboratory, and Dominic D. Frank, former Ph.D. student in the Gallium laboratory, are co-first authors of the document.
“The ramifications of altered sleep are numerous: fatigue, reduced concentration, poor learning and alteration of a myriad of health parameters – yet we still do not fully understand how sleep is produced and regulated in the brain and how external conditions can change impact on sleep and about quality, “said Alpert.
The study, a collaborative effort of many years underway, was carried out in the Gallio laboratory by a series of scientists at different stages of their career, ranging from university students to the principal researcher.
“It is essential to study the brain in action,” said Frank. “Our results demonstrate the importance of functional studies to understand how the brain governs behavior.”
Overall, the study relied heavily on the ability to study both neuron activity and the role of these neurons on behavior. To do this, the researchers developed new tools and used a combination of functional and anatomical studies, neurogenetic and behavioral monitoring approaches to conduct these experiments both in the wild type and in transgenic flies.