![]() ![]() Interestingly, these neurons did not change during the day, indicating that their behaviour was not the result of day-to-night changes in the environment, but rather a specific feature of that neuron. pumilio, one third of the neurons exhibited a prolonged ‘rest’ during which they could not become excited, thus delaying new action potentials ( Figure 1). When this type of stimulus was applied to the SCN of the diurnal R. An inhibitory stimulus is one that makes the membrane potential of a neuron more negative, making an action potential harder to trigger, and reducing the activity of downstream neurons. found that some neurons displayed a peculiar electrical behaviour that had not been observed in the SCN of nocturnal animals. This activity pattern is likely an adaptation for diurnality. pumilio overall (left panels, smaller halos) while maintaining distinct excitation patterns between day and night. This ‘brake’ to SCN excitability operates irrespective of the time of day, leading to reduced activity in the SCN of R. This resting phase delays action potential firing, the frequency of which is shown in a simplified scheme on the upper left corner of each panel. ![]() pumilio SCN (left side) – but not in the mouse SCN (right side) – one out of three neurons (in magenta) shows a prolonged resting phase after inhibitory stimuli. ![]() In both diurnal (left) and nocturnal (right) mammals, SCN activity (orange halos around neurons) is higher during the day (top, larger halos) than at night (bottom, smaller halos). pumilio reacted to excitatory electrical stimuli – stimuli that dissipate the membrane potential (which at rest is about –70 mV) and trigger an action potential that relays the signal to downstream neurons – similarly to the SCN neurons of nocturnal rodents ( Belle et al., 2009). This suggests that both diurnal and nocturnal species use similar mechanisms to generate daily rhythms in the brain, irrespectively of when the animals are active. This revealed that, like in nocturnal animals, single neurons in the SCN of diurnal animals were more excitable and active during the day than at night ( Allen et al., 2017 Figure 1). Now, in eLife, a group of scientists led by Robert Lucas, Casey Diekman, and Mino Belle – including Beatriz Bano-Otalora and Matthew Moye as joint first authors – report on the electrical properties of individual SCN neurons in the diurnal four-striped mouse Rhabdomys pumilio ( Bano-Otalora et al., 2021).įirst, the team (who are based at the Universities of Manchester and Exeter, the New Jersey Institute of Technology and Merck) recorded the electrical activity of single neurons using a technique called whole-cell patch clamping. However, information on single neurons is lost in these recordings, limiting our understanding of the cell-specific mechanisms that potentially determine whether an animal is nocturnal or diurnal. Surprisingly, such data suggest that the master clock neurons in diurnal and nocturnal animals have similar molecular oscillators and electrical properties ( Sato and Kawamura, 1984 Yan et al., 2020). In diurnal species, only recordings from the entire SCN are available. Most of the current knowledge about these neurons and how they synchronise derives from studies on nocturnal rodents, like mice, rats, and hamsters ( Carmona-Alcocer et al., 2020). Each neuron in the SCN times its electrical activity to the day-to-night cycle, ultimately generating rhythmic inputs that the body obeys ( Reppert and Weaver, 2002). ![]() In mammals, the conductor of this symphony is the ‘master circadian clock’ which resides in the suprachiasmatic nucleus (SCN), a cluster of around 20,000 neurons in a region of the brain called the hypothalamus. Like instruments in an orchestra, the concert of ‘ticks’ generated by these clocks, which are found throughout the body, must be harmonized to coordinate the activities of different organs. These activities are regulated by molecular oscillators called circadian clocks, which consist of positive and negative feedback loops of gene transcription and protein translation that allow processes to take place with a ~24 hour periodicity. Many physiological processes, such as wakefulness or sleep, are synced to the hours of daylight and darkness. So how do brains determine whether we are nocturnal or diurnal? However, this is not the case for many other animals, which enjoy nightlife and rest throughout the day. Most humans are diurnal, meaning they are usually awake during the day and asleep at night. ![]()
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