Effect of arousing stimuli on circulating corticosterone and the circadian rhythms of luteinizing hormone (LH) surges and locomotor activity in estradiol-treated ovariectomized (ovx + EB) Syrian hamsters

• Novel wheel suppresses LH surges better in intact than estrogen-treated ovariectomized hamsters.
• Toy arousal at ZT4, but not ZT5, suppresses the LH surge and resets the phase of the master pacemaker.
• Arousal with toys is not as effective as a novel wheel in decreasing LH surges.
• Plasma corticosterone increases transiently immediately after novel wheel exposure.
• Novel wheel exposure only blocks LH surges when the corticosterone increase is prolonged ~ 2 h.

In most proestrous hamsters, novel wheel exposure phase advances activity rhythms and blocks the preovulatory LH surge, which occurs 2 h earlier the next day. Because wheel immobilization does not prevent these effects we hypothesized that arousal alone blocks and phase advances the LH surge. Ovariectomized (ovx) hamsters received a jugular vein cannula and estradiol benzoate (EB) or vehicle was injected sc. The next day (Day 1), at zeitgeber time (ZT) 4–5 (ZT 12 = lights off), after obtaining a blood sample, each hamster was exposed to constant darkness (DD), and either remained in her home cage or was transferred to a new cage and exposed to a running wheel or a 2-hour arousal paradigm. Blood samples were obtained in dim red light and activity was recorded hourly until ~ ZT 10–11 on Days 1 and 2. For the next 1–2 weeks, activity was monitored in DD. Plasma LH and corticosterone were assessed by RIA. Novel wheel exposure or arousal at ZT 4 greatly attenuated the Day 1 LH surge in ovx + EB hamsters, and phase advanced the Day 2 LH surge by about 2 h. In proestrous hamsters, novel wheel exposure led to a prolonged (> 2 h) increase in corticosterone levels only when LH surges were blocked. Phase advances in activity rhythms were enhanced by estradiol and arousal. The results suggest that estradiol modulates the effectiveness of non-photic stimuli. The role of the increased activity of the hypothalamic–pituitary–adrenal axis associated with novel wheel-induced attenuation of LH surges in ovx + EB hamsters remains to be determined.