Morning and evening circadian oscillations in the suprachiasmatic nucleus in vitro

Daily biological rhythms are governed by an innate timekeeping mechanism, o r 'circadian clock'. In mammals, a clock in the suprachiasmatic nucleus (SC N) comprises multiple autonomous single-cell oscillators, but it is unclear how SCN cells interact to form a tissue with coherent metabolic and electr ical rhythms that might account for circadian animal behaviors. Here we dem onstrate that the circadian rhythm of SCN electrophysiological activity, re corded as a single daytime peak in hamster hypothalamic coronal slices, sho ws two distinct peaks when slices are cut in the horizontal plane. Substant iating an idea initially derived from behavioral observations, the properti es of these two peaks indicate functional organization of SCN tissue as a c lock with two oscillating components.