Chronic Exposure to Low Levels of Oestradiol‐17β Affects Oestrous Cyclicity, Hypothalamic Norepinephrine and Serum Luteinising Hormone in Young Intact Rats

Abstract

Chronic exposure to oestrogens is known to inhibit the secretion of luteinising hormone (LH) in rats causing anovulation. Hypothalamic catecholamines, norepinephrine (NE) and dopamine (DA) play an important role in LH regulation. However, the effects of chronic exposure to low levels of oestradiol on hypothalamic catecholamines have not been investigated thoroughly. To study this, adult female Sprague-Dawley rats were either sham implanted or implanted with oestradiol-17ß (E2) pellets (20 ng/day) for 30 (E-30), 60 (E-60) or 90 (E-90) days. E2 exposure affected oestrous cyclicity and ovarian morphology in a duration-dependent manner. There was no change in oestrous cyclicity in E-30 rats, however, 75% of E-60 and 95% of E-90 rats were acyclic (p<0.05). Cycling rats from E-30 or control group were sacrificed at different time points on the afternoon of pro-oestrous. E-30 rats in oestrous, constant oestrous rats in the E-60 and E-90 groups and a group of old constant oestrous (OCE) rats were sacrificed at 1200 h. LH was measured in the serum by RIA. Individual hypothalamic nuclei that are involved in LH regulation were microdissected and analysed for NE and DA levels using HPLC-EC. NE levels in the hypothalamic nuclei increased significantly in control and E-30 groups during the afternoon of pro-oestrous which was accompanied by a rise in LH levels (p<0.05). On the day of oestrous, NE concentrations in hypothalamic nuclei and serum LH were significantly lower in E-60, E-90 and OCE rats compared to E-30 and control rats. On the other hand, DA levels declined significantly in one hypothalamic nucleus. These results indicate that chronic E2 exposure affects hypothalamic catecholamine and serum LH levels in a duration-dependent manner. This coincides well with the loss of cyclicity observed in these animals. These results suggest that repeated exposure to endogenous oestrogens could play a role in reproductive senescence.

Chronic estradiol exposure induces oxidative stress in the hypothalamus to decrease hypothalamic dopamine and cause hyperprolactinemia

“…However, E 2 exposure for 60 or 90 days resulted in a significant increase in serum E 2 levels (67.2 Ϯ 8.4 and 90.2 Ϯ 9.2 in E-60 and E-90 groups, respectively) (P Ͻ 0.05). This is most likely due to the presence of cystic follicles in the ovaries of these animals, as previously reported (24). Similarly, serum E 2 levels in OCE rats (94.2 Ϯ 22) were significantly higher compared with control and E-30 rats (P Ͻ 0.05).…”

“…Rats were provided food and water ad libitum. Estrous cyclicity was determined in young (3-4 mo old) and old (16 -18 mo old) rats by vaginal cytology (24). Young animals showing 4-to 5-day regular estrous cycles and old animals that exhibited estrous smears for more than 10 consecutive days (old constant estrous, OCE) were used in the experiments.…”

“…The PRL tracer was obtained from Amersham Pharmacia Biotech (Waukesha, WI). The assay was performed as described previously (24,30).…”

Chronic exposures to low levels of estradiol and their effects on the ovaries and reproductive hormones: Comparison with aging

“…On the other hand, repeated exposures to periodic increases in E2 during the course of normal reproductive cycles probably contribute to a reduction in hypothalamic catecholamines that causes a decrease in LH secretion and failure of ovulation leading to reproductive senescence 10 . We have found that a similar state of reproductive senescence can be induced in young animals that are exposed to low levels of E2 on a daily basis for 60 or 90 days 11 . Ovaries from old constant estrous (OCE) rats that are exposed to endogenous E2 during the course of their estrous cycles appear to be cystic and very similar to ovaries from young, E2-exposed rats.…”

Estrogen upregulates inflammatory signals through NF-κB, IFN-γ, and nitric oxide via Akt/mTOR pathway in the lymph node lymphocytes of middle-aged female rats

“…Initially, the middle-aged female rats were either sham operated or bilaterally OVX and after a week of recovery, 30-day placebo or estrogen pellets [17β-estradiol (0.6 μg or 300 μg), Innovative Research America, Florida, USA] were implanted subcutaneously in the nape of the neck. The doses were selected based on a preliminary study in our laboratory and a published study [28,29]. At the end of the treatment period, the animals were sacrificed by decapitation between 08:00 to 10:00 h. Trunk blood was collected in Falcon tubes, serum was isolated and used for hormone assay.…”