During the transition to menopause, women experience a variety of physical and psychological symptoms that are directly or indirectly linked to changes in hormone secretion. Establishing animal models with intact ovaries is essential for understanding these interactions and finding new therapeutic targets. In this study, we assessed the endocrine profile, as well as the estrous cycle, in the 4-vinylcyclohexene diepoxide (VCD)-induced follicular depletion rat model in 10-day intervals over 1 month to accurately establish the best period for studies of the transition period. Twenty-eight-day-old female rats were injected daily with VCD or oil s.c. for 15 days and euthanized in the diestrus phase approximately 70, 80, 90 and 100 days after the onset of treatment. The percentage of rats showing irregular cycles and the plasma level of FSH increased only in the 100-day VCD group. Plasma anti-Müllerian hormone (AMH) and progesterone were lower in all VCD groups compared to control groups, while estradiol remained unchanged or higher. As in control groups, dihydrotestosterone (DHT) progressively decreased in the 70-90-day VCD groups; however, it was followed by a sharp increase only in the 100-day VCD group. No changes were found in plasma corticosterone, prolactin, thyroid hormones or luteinizing hormone. Based on the estrous cycle and endocrine profile, we conclude that 1) the time window from 70 to 100 days is suitable to study a perimenopause-like state in this model, and 2) regular cycles with low progesterone and AMH and normal FSH can be used as markers of the early/mid-transition period, whereas irregular cycles associated with higher FSH and DHT can be used as markers of the late transition period to estropause.

Cold stress-induced ovarian sympathetic activation is associated with the development of ovarian cysts in rats. Although we have hypothesised that polycystic ovary (PCO) features induced by cold stress, as prevented by lesion of the noradrenergic nucleus locus coeruleus (LC), were a result of the increased activity of the ovarian norepinephrine (NE) system, this was not evident after 8 weeks of stress. In the present study, we investigated the temporal changes in LC and ovarian NE activities and steroid secretion in rats exposed to single (SS) or repeated (RS) cold stress. SS and 4 week (4W)-RS but not 8 week (8W)-RS increased c-Fos expression in the LC and ovarian NE release. Plasma oestradiol, testosterone and progesterone levels tended to increase in 4W-RS and were elevated in 8W-RS rats, which displayed PCO morphology. β-adrenergic receptor agonist increased steroid hormone release from the ovary of unstressed (US) but not from 8W-RS rats. To determine whether increased activity of noradrenergic system during the initial 4 weeks of RS would be sufficient to promote PCO, rats were exposed to 4 weeks of cold stress and kept in ambient temperature for the next 4 weeks (4W-RS/4W-US). Accordingly, PCO morphology, increased steroid secretion and decreased ovulation rate were found in 4W-RS/4W-US rats, strengthening the hypothesis that the initial increase in NE release triggers the development of PCO. The correlated activity of LC neurones and ovarian noradrenergic terminals and the induction of PCO in 4W-RS/4W-US rats provide functional evidence for a major role of NE in disrupting follicular development and causing the long-lasting endocrine abnormalities found in stress-induced PCO.