Circadian difference in firing rate of isolated rat suprachiasmatic nucleus neurons.

The suprachiasmatic nucleus (SCN) of the hypothalamus contains the primary circadian clock in mammals. Dissociated SCN neurons in long-term culture exhibit a circadian modulation of spontaneous electrical activity. To evaluate the presence of circadian differences in spontaneous activity of isolated SCN neurons without synaptic connections, dissociated rat SCN neurons were studied with on-cell recording 3-4 days after preparation, before the formation of dendrites, axons and synapses. A day-night difference in spontaneous electrical firing rate was found in acutely dissociated SCN neurons. During the first subjective day, the average firing rate (0.87+/-0.12 Hz) was significantly higher than during the first subjective night (0.24+/-0.06 Hz), while the firing rate on the next day (0.68+/-0.11 Hz) was significantly higher than during the preceding night. These data suggest that populations of isolated SCN neurons with no synaptic interactions contain a functioning circadian clock, and are particularly amenable to biophysical experiments.