Ovarian steroidogenic responsiveness to exogenous gonadotropin stimulation in young and middle-aged female rats

Changes of cyclic AMP levels and phosphodiesterase activities in the rat ovary

Cyclic AMP (cAMP) is a second messenger that plays a critical role in follicular recruitment, development and luteinization in the mammalian ovary. The cellular level of cAMP is largely dependent on the activity of phosphodiesterase (PDE), which degrades cAMP into 5'-AMP. The present study was conducted to investigate the level of cAMP and the activity of cAMP-PDE in postnatal rats; immature rats during gonadotropin-primed follicular development, ovulation and luteinization; adult rats during normal estrous cycling; and aged rats that spontaneously developed persistent estrous (PE) by radioimmunoassay (RIA). All four rat models were confirmed by histological examination of one ovary and assayed using the other ovary by RIA. In the postnatal rats, the ovarian cAMP level was high on day 10 after birth, while ovarian cAMP-PDE activity was highest at 21 days of age. In the immature female rats, both the ovarian cAMP level and cAMP-PDE activity increased remarkably after treatment with equine chorionic gonadotropin (eCG), increased continuously 24 h after injection of human chorionic gonadotropin (hCG) for induction of ovulation and luteinization, and then declined significantly. In the adult rats during the normal estrous cycle, the ovarian cAMP levels were low on the day of estrus, and there were no significant changes in ovarian cAMP-PDE activity throughout the estrous cycle. In the PE rats, the ovarian cAMP levels were similar to those of the adult rats on the day of estrus but were lower than those on the other days of the estrous cycle; ovarian cAMP-PDE activity was lower than that in the adult rats on any day of the estrous cycle. Together, these findings indicate that the ovarian cAMP level and cAMP-PDE activity were regulated in a stage-dependent manner during ovarian follicular development, atresia and luteinization and providing evidences that cAMP and cAMP-specific PDEs are involved in these physiological processes.

Site-specific decrease of progesterone receptor mRNA expression in the hypothalamus of middle-aged persistently estrus rats

Middle-aged females gradually become acyclic and spontaneously develop a persistently estrus (PE) state. PE rats, acyclic for 30 days (early PE), are unresponsive to the positive feedback action of estrogen, but respond to a progesterone challenge with a luteinizing hormone (LH) surge and ovulation; unlike long-term PE rats, acyclic for 90 days, neither estrogen nor estrogen plus progesterone will elicit an LH surge [10th International Congress of Endocrinology, San Francisco, P3 (1996) 1061]. We hypothesize that the PE state may develop due to a diminished level of estrogen-induced progesterone receptor (PR) expression in the hypothalamus that prevents progesterone from stimulating LH regulating circuits. To test this hypothesis, PR mRNA levels were measured in hypothalamic regions of young, proestrus (2-3 months of age), early PE (10-12 months) and long-term PE (13-15 months) rats. The anteroventral periventricular nucleus (AVPV), an important regulatory site of the LH surge, had decreased PR mRNA levels in early and long-term PE rats compared with proestrus rats. However, PR mRNA levels were reduced only in long-term PE rats in the ventromedial nucleus (VMH) and arcuate nucleus (ARH). In the medial preoptic nucleus (MPN), levels of PR mRNA did not change. A previous report showed that exogenous progesterone stimulates an LH surge in young and early PE animals, indicating that the expression of PR mRNA demonstrated in this study is sufficient to mediate progesterone facilitation of the LH surge in early PE rats. In acyclic, long-term PE rats, diminished estrogen-induced expression of progesterone receptors is correlated with a previously shown inability to respond to exogenous progesterone.

Selected contribution: Time-dependent hypoxic respiratory responses in female rats are influenced by age and by the estrus cycle

Age affects time-dependent respiratory responses to episodic hypoxia in male rats, particularly long-term facilitation (LTF), a serotonin-dependent respiratory "memory" [Zabka AG, Behan M, and Mitchell GS, J Physiol (Lond) 531: 509, 2001]. Because age and gender influence serotonergic function, we tested the hypotheses that the short-term hypoxic response (STHR), posthypoxia frequency decline (PHFD) and LTF of phrenic and hypoglossal (XII) motor output change with age and stage of the estrus cycle in female rats. Young (3-4 mo) and middle-aged (13 mo) female Sprague-Dawley rats were anesthetized, paralyzed, vagotomized, and ventilated. STHR was measured during and PHFD after the first of three 5-min episodes of isocapnic hypoxia (arterial P(O)(2) 35-45 Torr). LTF was assessed 60 min postepisodic hypoxia. Phrenic and XII STHR increased with age (P < 0.05). PHFD was unaffected by age or gender. Phrenic LTF increased with age in both estrus and diestrus (P < 0.05), whereas XII LTF increased in middle-aged female rats during diestrus only. Age and gender influence time-dependent hypoxic phrenic and XII responses in a complex manner.

Effect of bone morphogenetic protein-7 on folliculogenesis and ovulation in the rat

We have previously established the presence of a functional bone morphogenetic protein (BMP) system in the ovary by demonstrating the expression of BMP ligands and receptors as well as novel cellular functions. Specifically, BMP-4 and BMP-7 are expressed in theca cells, and their receptors by granulosa cells. These BMPs enhanced and attenuated the stimulatory action of FSH on estradiol and progesterone production, respectively. To investigate the underlying mechanism of the differential regulation, we analyzed mRNA levels for key regulators in the steroid biosynthetic pathways by RNase protection assay. BMP-7 enhanced P450 aromatase (P450(arom)) but suppressed steroidogenic acute regulatory protein (StAR) mRNAs induced by FSH, whereas mRNAs encoding further-downstream steroidogenic enzymes, including P450 side-chain cleavage enzyme and 3beta-hydroxysteroid dehydrogenase, were not significantly altered. These findings suggest that BMP-7 stimulation and inhibition of P450(arom) and StAR mRNA expression, respectively, may play a role in the mechanisms underlying the differential regulation of estradiol and progesterone production. To establish the physiological relevance of BMP functions, we investigated the in vivo effects of injections of recombinant BMP-7 into the ovarian bursa of rats. Ovaries treated with BMP-7 had decreased numbers of primordial follicles, yet had increased numbers of primary, preantral, and antral follicles, suggesting that BMP-7 may act to facilitate the transition of follicles from the primordial stage to the pool of primary, preantral, and antral follicles. In this regard, we have also found that BMP-7 caused an increase in DNA synthesis and proliferation of granulosa cells from small antral follicles in vitro. In contrast to the stimulatory activity, BMP-7 exhibited pronounced inhibitory effects on ovulation rate and serum progesterone levels. These findings establish important new biological activities of BMP-7 in the context of ovarian physiology, including folliculogenesis and ovulation.