Daily biological rhythms are governed by an innate timekeeping mechanism,
or 'circadian clock'. In mammals, a clock in the suprachiasmatic
nucleus (SCN) comprises multiple autonomous single-cell oscillators, but it
is unclear how SCN cells interact to form a tissue with coherent metabolic
and electrical rhythms that might account for circadian animal behaviors.
Here we demonstrate that the circadian rhythm of SCN electrophysiological
activity, recorded as a single daytime peak in hamster hypothalamic coronal
slices, shows two distinct peaks when slices are cut in the horizontal plane.
Substantiating an idea initially derived from behavioral observations, the
properties of these two peaks indicate functional organization of SCN tissue
as a clock with two oscillating components.
