Consideration of circadian rhythm in neurophysiology
By Andrea Meredith, University of Maryland School of Medicine
Some scientists are early birds, some are night owls. Does this affect the results you obtain? Here are some considerations for time-of-day and the underlying circadian process in neurophysiological investigations.
1. The clock mechanism
Your internal time keeper, the body’s biological clock, is located in the hypothalamus of the brain. Neurons of the suprachiasmatic nucleus (SCN) organise the daily timing of the body’s physiology and behavioural states, phase-locked to the solar day-night cycle. These SCN clock neurons undergo periodic changes in intracellular state, produced by an evolutionary conserved molecular pathway regulated by ‘clock genes'. First revealed in Drosophila, this discovery was recognised by the 2017 Nobel Prize for Physiology or Medicine, awarded to Jeffrey Hall, Michael Rosbash and Michael Young. This central clock regulates diurnal rhythms for everything from sleep and arousal to locomotor activity, hormone secretion, body temperature, metabolism and cardiovascular function.
2. Consider entrainment
Most laboratory animals are housed on a 12 hour light:12 hour dark cycle, entraining the circadian rhythm to a 24-hour cycle. Nocturnal mice and rats are the preclinical model of choice for most neurophysiology research. Yet for convenience, most experimentation is performed during the day, the normal resting period for nocturnal rodents. How closely these rodent ‘resting’ brain states relate to diurnal human brain states has not been well-studied, but may have important implications for relating findings in animal models to human neurophysiology. Could this contribute to the failure of some animal models as predictive platforms in preclinical research? Given that the timing of at least 50 physiological parameters in mice show distinct circadian periodicity, it may be important to consider which systems are ‘tuned up’ or ‘tuned down’ at different times of day in a translationally-relevant manner.
3. Environmental stimuli influence circadian phase
Did your colleague leave the light on in the animal facility when they checked their mice? Worse yet, did they check the mice after the lights went off, exposing the animals to light during the dark phase? Light is the most potent entraining cue (zeitgeber) for the circadian clock. Even short pulses of light can entrain the phase of the SCN clock, influencing brain regions and peripheral tissues. Other environmental cues that can influence rhythmicity include the availability of food, temperature, exercise and stress, which may vary in typical animal facilities.
4. Firing round the clock
Circadian regulation of physiology is based on daily changes in cellular excitability and signalling. The best example is in the SCN itself, where circadian differences in membrane excitability shape the daily oscillation in spontaneous action potential activity. These neurons signal time to the rest of the brain, including the all-important Hypothalamic-Pituitary-Adrenal (HPA) axis. More recently, clock genes have been shown to regulate rhythms in long-term potentiation, receptor trafficking, synaptic physiology and neuronal activity in other brain regions. The function of the circadian clock has been implicated in several neurological disorders, such as mood disorders, drug and alcohol addiction, stress and anxiety, eating disorders, schizophrenia and neurocognitive dysfunction.
5. Reproducibility
Why does the work-all-night graduate student’s data differ from the at-it-early postdoc’s? Neglect for consideration of the circadian regulation neurophysiological processes research can be a contributor to poor experimental reproducibility and rigor. Just like you would standardise your studies by sex, strain, and age, don’t forget time-of-day.
References
Bittman et al. Animal Care Practices in Experiments on Biological Rhythms and Sleep. Journal of the American Association for Laboratory Animal Science. Vol 52, No 4 July 2013 Pages 437–443
Colwell. Circadian Medicine. Wiley Blackwell 2015.
Matveyenko. Consideration for Circadian Physiology in Rodent Research. PHYSIOLOGY 33: 250–251, 2018.
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