CIRCADIAN-RHYTHM GENERATION IN THE CULTURED SUPRACHIASMATIC NUCLEUS

The suprachiasmatic nucleus (SCN) of the hypothalamus is involved in t he generation and entrainment of circadian rhythms. The results of a s eries of experiments in long-term cultured organotypic SCN slices sugg est that (1) some but not all SCN neurons display circadian rhythmicit y in their extracelluar discharges. To the extent they could be studie d, these neurons did not usually show synchronized high/low levels of activity; (2) simultaneous daily depolarization of these neurons (K+ p ulses) to some extent influenced the distribution of the firing rate o f SCN neurons around the time of expected daily pulses; (3) extracellu ar Ca++ and synaptic input is required for the pacemaker activity of t he SCN. We conclude that the mammalian biological clock is a heterogen eous neuronal system in which the circadian pacemaker rhythm is genera ted and entrained via complex interactions among SCN neurons.