The mammalian master clock driving circadian rhythmicity in physiology and 
behavior resides within the suprachiasmatic nuclei (SCN) of the anterior 
hypothalamus. Circadian rhythms are generated by a set of clock genes via 
intertwined negative and positive autoregulatory transcription-translation 
feedback loops. The Cryptochrome 1 and 2 genes are indispensable for molecular 
core oscillator function, as evident from the arrhythmic wheel-running behavior 
and lack of rhythmic clock gene expression in mCry1/mCry2 double-mutant mice in 
constant darkness. In the present study, using real-time multiunit electrode 
activity recordings in hypothalamic slices, we show that SCN neurons from 
mCry-deficient mice kept in constant darkness lack circadian oscillations in 
firing patterns. This proves that cryptochromes, and thus an intact circadian 
clockwork, are prerequisites for circadian electrical activity in SCN neurons. 
Interestingly, when mCry-deficient mice were kept in normal light-dark 
conditions and SCN slices were prepared 2 hr after the beginning of the day, a 
single noncircadian peak in neuronal activity was detected. This light-induced 
rise in electrical activity of the SCN may explain why mCry-deficient mice lack 
the arrhythmic short bouts of wheel-running activity and instead show apparently 
normal behavior in normal day-night cycles.