The electrical activity of the rat suprachiasmatic nucleus (SCN) was examined in anesthetized rats in vivo using single-unit electrophysiological techniques. The present data confirm the daily variation in the electrical activity of the SCN previously reported in vitro and in vivo using multiple-unit recording techniques. They further suggest that subpopulations of suprachiasmatic neurons with different neural connections have a different daily rhythm of activity. Neurons in the SCN region showed a significant rhythm of activity (p = 0.034; Kruskall-Wallis analysis of variance [KW-ANOVA]). The greatest activity occurred during the second part of the light period (ZT 10-12), and the lowest activity occurred in the early part of the light period (ZT 0-2). The subgroup of cells in the suprachiasmatic region with output projections to the arcuate nucleus (ARC) and/or supraoptic nucleus (SON) regions also showed a significant rhythm (p = 0.001; K-W ANOVA). Their activity appeared to show two peaks near the light-dark (ZT 10-12) and dark-light (ZT 22-24) transition periods with the lowest activity at ZT 16-18. This rhythm was significantly different (p = 0.016) from that of neurons without an output projection to the ARC and/or SON. Retinorecipient suprachiasmatic neurons appeared to have a less robust daily rhythm in their activity. The change in the firing behavior of the cells was not reflected simply by changes in mean firing rate. Examination of the coefficient of variation of the interspike interval distribution of cells at different times of day revealed changes in the firing pattern of cells in the SCN region that did not have output projections (p = 0.032; K-W ANOVA). The present results thus suggest that the SCN is composed of a heterogeneous population of neurons and that different rhythms of activity are expressed by neurons with different neural connections. There were changes in both firing pattern and firing rate.
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