[Circadian oscillator in the suprachiasmatic nucleus: systemic electrophysiologic studies]

The suprachiasmatic nuclei are the neural substrate for a circadian oscillator in mammals. In the present paper we propose two systemic approaches concerning SCN functions. Firstly, we studied the effect of SCN lesions on the cortical EEG in rats. We could demonstrate that in particular the low frequency EEG (delta) power, a putative indicator for sleep homeostasis and sleep propensity, was dramatically reduced in SCN lesioned rats. Since SCN lesioned rats do not have circadian consolidated sleep and wake periods, sleep propensity may stay on a relatively low level as observed in the attenuated EEG slow wave activity. Secondly, integral electrophysiological activity of the SCN was investigated with a noise, analysis of the spontaneous membrane current of voltage clamped SCN neurons. This analysis revealed two components in spontaneous activity of SCN neurons: a low frequency, TTX resistant component (< 10 Hz) that showed higher activity levels in the nighttime SCN, and a high frequency component (50 to 500 Hz) that was more intense in the daytime SCN. We believe that the high frequency component manifests the synaptic bombardment of the single SCN cell embedded in a highly active neuronal population during daytime. On the other hand, the low frequency component may originate from either non-synaptic input (the putative neuronal coupling), or may be generated by the neuron itself.