Neurons of the rat suprachiasmatic nucleus show a circadian rhythm in membrane properties that is lost during prolonged whole-cell recording

The suprachiasmatic nucleus is commonly considered to contain the main pace maker of behavioral and hormonal circadian rhythms. Using whole-cell patch- clamp recordings, the membrane properties of suprachiasmatic nucleus neuron s were investigated in order to get more insight in membrane physiological mechanisms underlying the circadian rhythm in firing activity. Circadian rh ythmicity could not be detected either in spontaneous firing rate or in oth er membrane properties when whole-cell measurements were made following an initial phase shortly after membrane rupture. However, this apparent lack o f rhythmicity was not due to an unhealthy slice preparation or to seal form ation, as a clear day/night difference in firing rate was found in cell-att ached recordings. Furthermore, in a subsequent series of whole-cell recordi ngs, membrane properties were assessed directly after membrane rupture, and in this series we did find a significant day/night difference in spontaneo us firing rate, input resistance and frequency adaptation. As concerns the participation of different subpopulations of suprachiasmatic nucleus neuron s expressing circadian rhythmicity, cluster I neurons exhibited strong rhyt hmicity, whereas no day/night differences were found in cluster II neurons. Vasopressin-containing cells form a subpopulation of cluster I neurons and showed a more pronounced circadian rhythmicity than the total population o f cluster I neurons. In addition to their strong rhythm in spontaneous firi ng rate they also displayed a day/night difference in membrane potential. ( C) 1999 Elsevier Science B.V. All rights reserved.