Neurobiology of the Circadian Clock

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PROJECT SUMMARY A fundamental question in neuroscience is how changes in gene expression are translated into changes in neuronal physiology, and ultimately into changes in behavior. The brain’s 24-hour timing mechanism, or biological clock, is a system that is uniquely suited to the study of neural plasticity and the genes-to-behavior problem. The neural network that generates and drives circadian rhythms in physiology and behavior is located within the suprachiasmatic nuclei (SCN) of the hypothalamus. SCN neurons exhibit endogenous circadian rhythms in spontaneous spike frequency, even as single cells in isolation, and within these neurons is a defined network of “clock genes” that forms autoregulatory transcription/translation feedback loops (TTFLs) to generate near 24-hour rhythms. There are key gaps in our knowledge regarding the mechanisms of SCN entrainment and the pacemaker plasticity it induces. Unlike rhythms generation, real-time dynamic SCN molecular and neural network responses during entrainment have been previously un-observable, and thus knowledge is limited regarding the actual dynamic topologies of entrainment at those levels. We have developed and instituted novel methods enabling direct observation of SCN molecular and neural network entrainment topologies using an approach that combines ChrimsonR optogenetic manipulation of clock neuron electrical activity with PER2::LUC real- time reporting of clock gene activation (EX vivo CIrcadian Timing and Entrainment, EXCITE). EXCITE provides precise timing, duration and intensity of recurring input stimulation to the isolated SCN, and tracks SCN clock molecular rhythms at high temporal and spatial resolution for 3-5 weeks ex vivo. The isolated SCN in this system strikingly recapitulates canonical features of circadian clock entrainment in intact animals, including light-like phase responses with period after-effects, period matching and systematic phase angle differences to stimuli that deviate from 24 hours, and differential entrainment to photoperiods with a minimum tolerable night. We will use EXCITE to examine - (1) Molecular and Neural Network Topologies for SCN Entrainment and Plasticity, (2) SCN Neural Network Topology of Entrainment, and (3) Molecular Mechanisms of Photoperiod- Induced SCN Network Plasticity. Successful completion of these aims will provide novel insight into SCN entrainment and plasticity - how the SCN molecular and neural networks are modified by light input to result in behavioral plasticity. Defining the mechanisms by which the SCN encodes light history and photoperiod will open the way for manipulation of SCN neural and transcriptional networks to ameliorate circadian disorders.