**3.1 Proestrous LH profiles**

The general profile of the LH surge was comparable between ages. Basal plasma LH levels averaged 0.3 to 0.4 ng/ml and onset of the surge occurred around ZT9.5, where after LH levels increased rapidly and reached peak levels around ZT12. Subsequently, LH levels gradually declined. We found a significant decrease in LH peak and total LH levels with age (*P*=0.041 and *P* = 0.035, respectively) (Table 1). Pearson correlation tests showed that the magnitude of the LH surge (i.e. total LH levels) correlated with onset and height of the LH surge (LH onset time: r=-0.527 with *P* < 0.001; LH peak levels: r=0.924 with *P* < 0.001).


Table 1. Proestrous LH and P surge characteristics of young and middle-aged 4-day cyclic WU rats. Basal levels are depicted as the average concentration ZT 5.5-7.5, onset time of the LH surge (ZT at which LH levels exceeded basal levels plus 3xSD), peak time of the LH surge (ZT at which the highest concentration was measured), peak height of the LH surge (highest concentration measured), preovulatory P surge levels (cumulative value from ZT 8.5-14.5), and the total amount of LH or P released during the surge (cumulative value during the complete sampling period). All data are expressed as group means SEM in ng/ml (concentrations) or h:min (time). Significant differences (*P* < 0.05) between young and middle aged with ages are indicated with an asterisk.

Somatostatin in the Periventricular Nucleus of the Female Rat:

ZT 5 on day 2 (Figure 2D).

Age Specific Effects of Estrogen and Onset of Reproductive Aging 77

patterns at other time points were in general comparable with those found in the young rats, i.e. maximal numbers of SOM-ir cells in PeVN section 7 at ZT 11 and in PeVN section 8 at

Fig. 1. Total number of SOM-ir cells (sum of PeVN sections 4-8) in young (4.5 months old) and middle aged (9 mo old) Wistar rats at different time points after E2 treatment. ZT 5: 2 (day 1) or 26 (day 2) h after E2 treatment, ZT 11: 8 (day 1) or 32 (day 2) h after E2 treatment. n=5 for each young age group, numbers within base of bars indicate the number of animals.

Fig. 2. Rostral to caudal distribution of SOM-ir cells in the PeVN of young (4.5 months old)

(A and B) and middle aged (C and D) OVX females at different time points after E2

treatment. Numbers within base of bars indicate the number of animals.
