Light signalling in cryptochrome-deficient mice.

The mammalian master clock driving circadian rhythmicity in physiology, metabolism, and behaviour resides within the suprachiasmatic nuclei (SCN) of the anterior hypothalamus and is composed of intertwined negative and positive autoregulatory transcription-translation feedback loops. The Cryptochrome 1 and 2 gene products act in the negative feedback loop and are indispensable for molecular core oscillator function, as evident from the arrhythmic wheel running behaviour and absence of cyclic clock gene expression in mCry1/mCry2 double mutant mice in constant darkness. Recently, we have measured real-time multi-unit electrode activity recordings in hypothalamic slices from mCry-deficient mice kept in constant darkness and observed a complete lack of circadian oscillations in firing patterns. This proves that CRY proteins, and thus an intact circadian clock, are prerequisite for circadian rhythmicity in membrane excitability in SCN neurons. Strikingly, when mCry-deficient mice are housed in normal light-dark cycles, a single non-circadian peak in neuronal activity can be detected in SCN slices prepared two hours after the beginning of the day. This light-induced increase in electric activity of the SCN suggests that deletion of the mCry genes converts the core oscillator in an hour-glass-like timekeeper and may explain why in normal day-night cycles mCry-deficient mice show apparently normal behaviour.