Fig. 4.

The circadian timing of the LH surge is coupled to an oscillator within the dorsomedial SCN. A and B, Double-plotted actograms (top) and hourly LH profiles (bottom) of two representative LD22 desynchronized OVX+E₂ rats bled during either an aligned day (A) or a misaligned day (B). Arrowhead on left indicates day of ovariectomy for both animals. On actograms, black bars represent locomotor activity, single diagonal lines indicate onset of dmSCN-associated locomotor activity bout with the angle set to match the LD-dissociated period indicated in the periodogram (D), double diagonal bands indicate the time of lights-off. Note that in A the two trend lines cross between the day of bleeding and the preceding day, with the LD-dissociated locomotor activity onset roughly overlapping with the time of lights-off, whereas in B the LD-dissociated locomotor activity onset beings approximately 7 h after lights-off. The day of bleeding is highlighted as in Fig. 3. Thin black lines expand the 24-h time scale on the bleeding day to easily visualize the plasma LH profiles shown below. Squares indicate the time of the LH surge onset both in the actogram and on the LH profile. Black arrows indicate the onset of the LD-dissociated locomotor activity in both actograms and LH plots. X-axes are in clock time, to match the actograms. C, Plasma LH profiles in a representative LD24 animal. The gray circle represents the LH surge onset; black arrow indicates the onset of locomotor activity, as in A and B. C, The x-axis is in Zeitgeber time (ZT) with ZT12 = time of lights-off. *, A high-LH point not considered part of the surge due lack of contiguity (see Materials and Methods). D, Periodograms of animals shown in A (top) and B (bottom).