The potential role of periovarian fat and leptin in initiation of puberty in the immature rat.

Twenty-four-day old female rats were injected with 25 microg recombinant leptin twice daily for 24, 48, 72, or 93 h. Periovarian fat contained significantly greater amounts of aromatase mRNA at 72 and 93 h and estradiol at 72 h after leptin treatment as compared to the ovary. On the other hand, there was an increase in testosterone at 72 h and progesterone at 72 and 93 h after leptin in the ovary as compared to periovarian fat. Cholesterol side chain cleavage and 17alpha-hydroxylase mRNA were found only in the ovary and not periovarian fat. Thus, periovarian fat has a greater capacity to synthesize estrogen than the ovary in the immature rat and adipose tissue may serve as the primary source of estrogen in the prepubertal period which results in the onset of puberty.

Duration of estrogen exposure prior to follicle-stimulating hormone stimulation is critical to granulosa cell growth and differentiation in rats.

Estrogens have been reported to exert both stimulatory and inhibitory effects on granulosa cell function. Previous studies from our laboratory showed that 12 h after administration of diethylstilbestrol (DES; a synthetic estrogen), FSH-stimulated granulosa cell proliferation and aromatase activity were increased; however, 48 h after DES, FSH stimulation of both parameters was inhibited. In other experiments, exposure of rats to DES for a period of 26 h blocked ovulation in response to eCG and hCG administration, whereas the same treatment regimen without DES caused ovulation in all treated rats. Thus, DES may in some cases actually interfere with maturation and development of ovulatory follicles. The present study was designed 1) to confirm that the duration of estrogen pre-exposure determines the way granulosa cells respond to FSH and 2) to investigate the underlying mechanisms involved. While DES was used in preliminary experiments, the majority of the studies were conducted with estradiol, a natural estrogen, in order to conform as closely as possible to the normal physiology. In the experimental protocol, immature female rats received injections of DES or implants of estradiol pellets 12 h (short exposure) or 36 h (long exposure) before 36 h of FSH treatment. Rats were killed, ovaries removed, and granulosa cells collected at the end of the FSH treatment period. The results demonstrate that exposure to either of these estrogens for 12 h allowed the subsequent FSH stimulation to produce high cellular proliferation, high aromatase enzyme activity, and large amounts of FSH receptor and aromatase mRNA. Estrogen exposure for 36 h, however, resulted in significantly decreased FSH stimulation of all these parameters. These findings confirm that short exposure to estrogen enhances the response of granulosa cells to FSH while longer exposure makes granulosa cells refractory to FSH. This differential sensitivity of granulosa cells to estrogen exposure could help explain how dominant follicles survive to ovulate while others are lost to atresia during ovarian cycles.