Figure 2.
In rats housed in a 12 h LD cycle, the 24 h rhythm of PER2 immunoreactivity in the dorsal striatum (a) is opposite in phase to the rhythm of PER2 expression in the SCN (b). The daily fluctuations in extracellular striatal DA levels (c) and wheel-running activity (d) peak at night, preceding the peak of PER2 in the striatum. In each panel, the gray block indicates the time of day when lights were off (between ZT12–ZT24). a, In the dorsal striatum, PER2 expression is greatest at ZT1, near the beginning of the light phase, and is lowest during the dark phase (expressed as mean number of PER2-IR nuclei ± SEM at each ZT; n = 4 per ZT). b, In the SCN, PER2 expression is greatest at ZT13, near the beginning of the dark phase, and is lowest during the light phase. c, In vivo microdialysis sampling in the dorsal striatum shows that extracellular DA levels are lowest during the middle of the light phase and highest in the middle of the dark phase (expressed as mean picograms of DA/microliter of dialysate ± SEM, samples taken every 30 min; n = 3). d, Wheel-running activity is also greatest during the dark phase and is virtually absent during the light phase (expressed as mean wheel revolutions/10 min bin over 24 h ± SEM; n = 24).