The Neurophysiology of the Mammalian Suprachiasmatic Nucleus and Its Visual Afferents

Abstract

In 1967 Richter concluded that the circadian clock controlling the rest-activity cycle of the rat is probably located “somewhere in the hypothalamus” (Richter 1967). He added: “...we assume that (the clock) is constituted of many cells, each one of which is programmed to function at a rate of 24 or nearly 24 h, that under ordinary conditions these cells all function together but that under certain conditions they may become desyn-chronized at least to some extent”. These statements represent perhaps the earliest speculation about the localization and neurophysiological organization of a circadian pacemaker in the mammalian hypothalamus. More recently Enright developed a more sophisticated neural model which includes specific mechanisms both for the generation of circadian oscillations and their photic entrainment. This model for the circadian pacemaker is primarily theoretical in nature. In order to construct a physiological model for the circadian pacemaker, a study of the functional properties of a “real” biological clock is essential. With the identification of the suprachiasmatic nuclei (SCN) as a putative pacemaker in the mammalian hypothalamus such an experimental program has become possible.